Thin film growth of rod-like and disc-shaped organic molecules on ...
Thin film growth of rod-like and disc-shaped organic molecules on ...
Thin film growth of rod-like and disc-shaped organic molecules on ...
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EIDESSTATTLICHE ERKLÄRUNGIch erkläre an Eides statt, dass ich die vorliegende Arbeit selbständig verfasst, <str<strong>on</strong>g>and</str<strong>on</strong>g>ere als dieangegebenen Quellen/Hilfsmittel nicht benutzt, und die den benutzten Quellen wörtlich undinhaltlich entnommene Stellen als solche kenntlich gemacht habe.Graz, am ……………………………………………………………………………..(Unterschrift)Englische Fassung:STATUTORY DECLARATIONI declare that I have authored this thesis independently, that I have not used other than thedeclared sources / resources, <str<strong>on</strong>g>and</str<strong>on</strong>g> that I have explicitly marked all material which has beenquoted either literally or by c<strong>on</strong>tent from the used sources.……………………………date………………………………………………..(signature)2
AbstractOrganic materials are c<strong>on</strong>sidered as promising c<str<strong>on</strong>g>and</str<strong>on</strong>g>idates for next generati<strong>on</strong> electr<strong>on</strong>ics. Inthis c<strong>on</strong>text, a detailed underst<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>of</str<strong>on</strong>g> the fundamental properties <str<strong>on</strong>g>of</str<strong>on</strong>g> thin <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g>s isdesirable. For this purpose <str<strong>on</strong>g>of</str<strong>on</strong>g>ten model systems with well-defined substrates <str<strong>on</strong>g>and</str<strong>on</strong>g> pristine<str<strong>on</strong>g>organic</str<strong>on</strong>g> adsorbates are investigated. In many cases it turns out that the properties <str<strong>on</strong>g>of</str<strong>on</strong>g> the thin<str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g>s depend <strong>on</strong> their structure <str<strong>on</strong>g>and</str<strong>on</strong>g> morphology, which in turn can <str<strong>on</strong>g>of</str<strong>on</strong>g>ten be tailoredby altering the <str<strong>on</strong>g>growth</str<strong>on</strong>g> parameters.In the present work the thin <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> (morphology, structure) <str<strong>on</strong>g>and</str<strong>on</strong>g> the adsorpti<strong>on</strong> /desorpti<strong>on</strong> kinetics <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>rod</str<strong>on</strong>g>-<str<strong>on</strong>g>like</str<strong>on</strong>g> molecule hexaphenyl (p6P) <str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>disc</str<strong>on</strong>g>-<str<strong>on</strong>g>shaped</str<strong>on</strong>g>molecule hexaaza-triphenylene-hexacarb<strong>on</strong>itrile (HATCN) were investigated <strong>on</strong> varioussubstrates.The thin <str<strong>on</strong>g>film</str<strong>on</strong>g>s were prepared utilizing <str<strong>on</strong>g>organic</str<strong>on</strong>g> molecular beam epitaxy (OMBE) under welldefined ultra-high vacuum (UHV) c<strong>on</strong>diti<strong>on</strong>s. The substrate temperature could be varied from100 K up to 1000 K.The substrate surfaces were characterized by Auger electr<strong>on</strong> spectroscopy (AES), X-rayphotoelectr<strong>on</strong> spectroscopy (XPS) <str<strong>on</strong>g>and</str<strong>on</strong>g> low energy electr<strong>on</strong> diffracti<strong>on</strong> (LEED). For the initialstate <str<strong>on</strong>g>of</str<strong>on</strong>g> layer <str<strong>on</strong>g>growth</str<strong>on</strong>g>, thermal desorpti<strong>on</strong> spectroscopy (TDS) was used to search for a wettinglayer <str<strong>on</strong>g>and</str<strong>on</strong>g> to reck<strong>on</strong> the kinetic parameters <str<strong>on</strong>g>of</str<strong>on</strong>g> desorpti<strong>on</strong>. In this work, the term wetting layeris used for a m<strong>on</strong>omolecular c<strong>on</strong>tinuous layer built up by <str<strong>on</strong>g>molecules</str<strong>on</strong>g> lying flat <strong>on</strong> the surface,which are particularly str<strong>on</strong>gly b<strong>on</strong>ded to the substrate. For thicker <str<strong>on</strong>g>film</str<strong>on</strong>g>s, in-situ XPS incombinati<strong>on</strong> with TDS was applied to reveal the kinetics <str<strong>on</strong>g>of</str<strong>on</strong>g> the layer <str<strong>on</strong>g>growth</str<strong>on</strong>g>. The workfuncti<strong>on</strong> could be measured in-situ with a purchased Kelvin probe (KP). Ex-situ atomic forcemicroscopy (AFM) c<strong>on</strong>ducted under ambient atmosphere c<strong>on</strong>diti<strong>on</strong>s was used to analyze the<str<strong>on</strong>g>film</str<strong>on</strong>g> morphology.The first part <str<strong>on</strong>g>of</str<strong>on</strong>g> this work (chapter 5) deals with the <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> hexaphenyl <strong>on</strong> clean <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>on</strong>modified mica surfaces. As it turned out, the formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a wetting layer crucially influencesthe further <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>: On clean mica, a wetting layer exists which leads to the <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g>el<strong>on</strong>gated isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s (so-called needles) c<strong>on</strong>sisting <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> lying flat <strong>on</strong> the surface (l<strong>on</strong>gmolecular axis parallel to the substrate surface). If the formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the wetting layer is5
prohibited by an artificial chemical or structural modificati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the mica substrate, terracesdevelop which comprise <str<strong>on</strong>g>molecules</str<strong>on</strong>g> which are oriented with their l<strong>on</strong>g axis normal to thesurface, i.e. st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g>. Thus the wetting layer serves as a template for further <str<strong>on</strong>g>film</str<strong>on</strong>g><str<strong>on</strong>g>growth</str<strong>on</strong>g>, as these <str<strong>on</strong>g>rod</str<strong>on</strong>g>-<str<strong>on</strong>g>like</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> prefer π-stacking (parallel arrangement with respect toeach other).On KCl (chapter 6), no wetting layer forms, which brings about the <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> terraces(st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g>) together with needles (lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g>), which arrange with respect tothe four fold symmetry <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate surface.In c<strong>on</strong>trast to the <str<strong>on</strong>g>rod</str<strong>on</strong>g>-<str<strong>on</strong>g>like</str<strong>on</strong>g> molecule p6P, the <str<strong>on</strong>g>disc</str<strong>on</strong>g> <str<strong>on</strong>g>shaped</str<strong>on</strong>g> HATCN <str<strong>on</strong>g>molecules</str<strong>on</strong>g> avoid π-stacking,which leads to a completely different <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> behavior (chapter 6 & 7). On Au (111), thefirst two layers are made up <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> lying flat <strong>on</strong> the surface, <strong>on</strong> which then the bulkphase starts to grow. Interestingly, with increasing <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness the flat lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g> inthe first layers become incorporated into the bulk structure as the temperature is raised above350 K, which leads also to a change in <str<strong>on</strong>g>film</str<strong>on</strong>g> morphology from layer-by-layer <str<strong>on</strong>g>growth</str<strong>on</strong>g> to isl<str<strong>on</strong>g>and</str<strong>on</strong>g><str<strong>on</strong>g>growth</str<strong>on</strong>g>. The isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s formed up<strong>on</strong> this rearrangement feature the well-known bulk crystalstructure <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN. The HATCN crystals are oriented with their (143) plane parallel to theAu (111) surface. On Ag (111), at the beginning a str<strong>on</strong>gly b<strong>on</strong>ded lying m<strong>on</strong>olayer is formedwhich does not desorb up<strong>on</strong> heating. Interestingly, with increasing <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness thism<strong>on</strong>olayer transforms into a m<strong>on</strong>olayer built up <str<strong>on</strong>g>of</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g>. On this m<strong>on</strong>olayermetastable layers start growing which most probably c<strong>on</strong>sist <str<strong>on</strong>g>of</str<strong>on</strong>g> flat lying, but orderless<str<strong>on</strong>g>molecules</str<strong>on</strong>g>. Like <strong>on</strong> gold, the metastable flat lying layers get incorporated into the bulk phaseas the <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness is increased, thus forming crystalline isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s, again with the well-knownbulk crystal structure. Nevertheless, the thick <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> <strong>on</strong> silver is different from the <strong>on</strong>e<strong>on</strong> gold, because the first m<strong>on</strong>olayer <strong>on</strong> silver is a stable <strong>on</strong>e, i.e. does not get integrated intothe bulk phase. This leads to a significantly different <str<strong>on</strong>g>film</str<strong>on</strong>g> morphology. Additi<strong>on</strong>ally, theHATCN crystals <strong>on</strong> silver are orderless with respect to the surface. These findings werenecessary to underst<str<strong>on</strong>g>and</str<strong>on</strong>g> the work functi<strong>on</strong> change induced by HATCN adsorpti<strong>on</strong> <strong>on</strong> Au(111) <str<strong>on</strong>g>and</str<strong>on</strong>g> Ag (111).6
KurzfassungOrganische Materialien gelten als vielversprechende K<str<strong>on</strong>g>and</str<strong>on</strong>g>idaten für neuartige elektr<strong>on</strong>ischeBauteile. In diesem Zusammenhang ist ein genaues Verständnis der grundlegendenEigenschaften v<strong>on</strong> dünnen organischen Schichten wünschenswert. Zu diesem Zweck werden<str<strong>on</strong>g>of</str<strong>on</strong>g>t Modellsysteme mit wohldefinierten Substraten und hochreinen organischen Adsorbatenuntersucht. In vielen Fällen stellt es sich heraus, dass die Eigenschaften der dünnenorganischen Filme v<strong>on</strong> ihrer Struktur und Morphologie abhängen, welche wiederum <str<strong>on</strong>g>of</str<strong>on</strong>g>tmalsdurch Änderungen der Wachstumsparameter beeinflusst werden können.In der vorliegenden Arbeit wurden das Wachstum v<strong>on</strong> dünnen Schichten (Morphologie,Struktur) und die Adsorpti<strong>on</strong>s / Desorpti<strong>on</strong>skinetik v<strong>on</strong> dem stäbchenförmigen MolekülHexaphenyl (p6P) und dem scheibchenförmigen Molekül Hexaaza-triphenylenehexacarb<strong>on</strong>itril(HATCN) auf verschiedenen Substraten untersucht.Die dünnen Schichten wurden unter Ultrahochvakuumbedingungen (UHV) mit Hilfe v<strong>on</strong>Molekularstrahlepitaxie (OMBE) präpariert. Die Temperatur des Substrates k<strong>on</strong>nte zwischen100 K und 1000 K variiert werden.Die Substratoberflächen wurden mittels Augerelektr<strong>on</strong>enspektroskopie (AES),Röntgenphotoelektr<strong>on</strong>enspektroskopie (XPS) und Beugung niederenergetischer Elektr<strong>on</strong>en(LEED) untersucht. Um festzustellen, ob ein ‚Wettinglayer‘ (= benetzende erste Schicht)existiert und zur Bestimmung der kinetischen Parameter der Desorpti<strong>on</strong> wurde beiultradünnen Schichten die thermische Desorpti<strong>on</strong>sspektroskopie (TDS) verwendet. In dieserArbeit wird der Begriff ‚Wettinglayer‘ für eine m<strong>on</strong>omolekulare k<strong>on</strong>tinuierliche Schichtverwendet, die aus Molekülen besteht, die flach auf der Oberfläche liegen und bes<strong>on</strong>ders starkan das Substrat gebunden sind. Für dickere Schichten wurde eine Kombinati<strong>on</strong> aus XPS undTDS Messungen angewendet, um die Kinetik des Schichtwachstums zu untersuchen. DieAustrittsarbeit k<strong>on</strong>nte mit einer Kelvins<strong>on</strong>de im UHV gemessen werden. DieRasterkraftmikroskopmessungen (AFM) zur Analyse der Schichtmorphologie wurden unterUmgebungsluft durchgeführt.Der erste Teil dieser Arbeit (Kapitel 5) befasst sich mit dem Wachstum v<strong>on</strong> Hexaphenyl aufreinen und modifizierten Glimmeroberflächen. Es stellte sich heraus, dass die Ausbildung7
eines Wettinglayers das weitere Schichtwachstum entscheidend beeinflusst: Auf reinemGlimmer existiert ein Wettinglayer, der zum Wachstum v<strong>on</strong> länglichen Inseln (sogenanntenNadeln) führt. Diese Nadeln bestehen aus Molekülen, die flach auf der Oberfläche liegen (mitder langen Molekularachse parallel zur Substratoberfläche). Wenn die Ausbildung einerbenetzenden Schicht durch künstliche chemische oder strukturelle Veränderungen desGlimmersubstrates verhindert wird, bilden sich terrassenförmige Strukturen aus. DieseTerrassen bestehen aus Molekülen, die mit ihrer langen Molekularachse normal auf dieOberfläche stehen, d.h. aus stehenden Molekülen. Also dient der Wettinglayer als Vorlage fürdas weitere Schichtwachstum, da die p6P Moleküle sogenanntes π-stacking bevorzugen (=parallele Anordnung zuein<str<strong>on</strong>g>and</str<strong>on</strong>g>er).Auf KCl (Kapitel 6) bildet sich keine benetzende Schicht, was zum Wachstum v<strong>on</strong> Terrassen(stehende Moleküle) zusammen mit Nadeln (liegende Moleküle) führt, die sich bezüglich dervierfachen Symmetrie der Substratoberfläche ausrichten.Im Gegensatz zum stäbchenförmigen Molekül p6P vermeiden die scheibchenförmigenHATCN Moleküle π-stacking, was zu einem völlig <str<strong>on</strong>g>and</str<strong>on</strong>g>eren Verhalten des Schichtwachstumsführt (Kapitel 6 & 7). Die ersten beiden Lagen auf Au (111) bestehen aus Molekülen, dieflach auf der Oberfläche liegen. Auf diesen beiden Lagen beginnt mit zunehmenderSchichtdicke die Kristallphase zu wachsen. Bemerkenswerterweise werden die flachliegenden Moleküle der ersten beiden Lagen mit steigender Schichtdicke in die Kristallphaseeingebaut, wenn die Temperatur über 350 K erhöht wird. Dies führt auch zu einer Änderungder Schichtmorphologie v<strong>on</strong> Lagenwachstum zu Inselwachstum. Die Inseln, die bei dieserUmordnung gebildet werden, weisen die bekannte Kristallstruktur v<strong>on</strong> HATCN auf. DieHATCN Kristallite sind mit ihrer (143) Ebene parallel zur Au (111) Oberfläche ausgerichtet.Auf Ag (111) bildet sich zu Beginn eine stark gebundene liegende Schicht, die während desHochheizens nicht desorbiert. Interessanterweise w<str<strong>on</strong>g>and</str<strong>on</strong>g>elt sich diese Schicht mit steigenderSchichtdicke in eine Schicht um, die aus stehenden Molekülen besteht. Auf dieser Schichtwiederum beginnt das Wachstum v<strong>on</strong> metastabilen Schichten, die wahrscheinlich aus flachliegenden, aber ungeordneten Molekülen bestehen. Wie auf der Goldoberfläche werden auchhier die metastabilen Schichten aus flach liegenden Molekülen in die Kristallphase eingebautwenn die Schichtdicke erhöht wird. Auf diese Weise werden kristalline Inseln gebildet, diewieder die bekannte Kristallstruktur aufweisen. Nichtsdestoweniger ist das Wachstum derdicken Schichten auf Silber <str<strong>on</strong>g>and</str<strong>on</strong>g>ers als jenes auf Gold, da die erste Lage auf Silber stabil istund nicht in die Kristallphase eingebaut wird. Ausserdem weisen die HATCN Kristalle auf8
Silber keine bevorzugte Orientierung zur Oberfläche auf. Diese Erkenntnisse warennotwendig, um die durch HATCN verursachte Änderung der Austrittarbeit v<strong>on</strong> Au (111) undAg (111) zu verstehen.9
C<strong>on</strong>tentsList <str<strong>on</strong>g>of</str<strong>on</strong>g> Symbols <str<strong>on</strong>g>and</str<strong>on</strong>g> Abbreviati<strong>on</strong>s.................................................................. 131 Int<str<strong>on</strong>g>rod</str<strong>on</strong>g>ucti<strong>on</strong>................................................................................................. 142 Basic c<strong>on</strong>cepts <str<strong>on</strong>g>of</str<strong>on</strong>g> epitaxial <str<strong>on</strong>g>growth</str<strong>on</strong>g> ............................................................ 162.1 <str<strong>on</strong>g>Thin</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>........................................................................................................... 162.2 Nucleati<strong>on</strong>..................................................................................................................... 182.3 Nucleati<strong>on</strong> <strong>on</strong> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s: Step-edge barriers .................................................................... 193 Film-substrate systems ............................................................................... 223.1 A <str<strong>on</strong>g>rod</str<strong>on</strong>g>-<str<strong>on</strong>g>like</str<strong>on</strong>g> molecule: The c<strong>on</strong>jugated <str<strong>on</strong>g>organic</str<strong>on</strong>g> semic<strong>on</strong>ductorpara-hexaphenyl (p6P) .................................................................................................. 223.2 A <str<strong>on</strong>g>disc</str<strong>on</strong>g>-<str<strong>on</strong>g>shaped</str<strong>on</strong>g> molecule: The <str<strong>on</strong>g>organic</str<strong>on</strong>g> molecular acceptorhexaaza-triphenylene-hexacarb<strong>on</strong>trile (HATCN) ......................................................... 233.3 Substrates ......................................................................................................................253.3.1 The mica (001) surface...................................................................................................... 253.3.2 The KCl (001) surface....................................................................................................... 253.3.3 The Au (111) surface......................................................................................................... 263.3.4 The Ag (111) surface......................................................................................................... 264 Analytical methods & instrumentati<strong>on</strong>..................................................... 274.1 The UHV chamber ........................................................................................................ 274.2 <str<strong>on</strong>g>Thin</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> preparati<strong>on</strong> by <str<strong>on</strong>g>organic</str<strong>on</strong>g> molecular beam epitaxy (OMBE)............................ 274.3 Thermal desorpti<strong>on</strong> spectroscopy (TDS) ...................................................................... 294.3.1 Theoretical background................................................................................................. 294.3.2 Experimental implementati<strong>on</strong> ....................................................................................... 344.4 X-ray photoelectr<strong>on</strong> spectroscopy (XPS)...................................................................... 364.5 Auger electr<strong>on</strong> spectroscopy (AES).............................................................................. 3710
4.6 Atomic force microscopy (AFM).................................................................................. 374.7 Kelvin Probe (KP)......................................................................................................... 395 Hexaphenyl <strong>on</strong> mica (001)......................................................................... 415.1 Calibrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the TD spectra ........................................................................................ 415.2 Freshly air-cleaved mica (001)...................................................................................... 425.3 p6P <strong>on</strong> freshly cleaved mica (001) – m<strong>on</strong>olayer regime............................................... 455.4 p6P <strong>on</strong> freshly cleaved mica (001) – multilayer regime ............................................... 465.5 p6P <strong>on</strong> carb<strong>on</strong> c<strong>on</strong>taminated mica (001)....................................................................... 525.6 p6P <strong>on</strong> sputtered mica ................................................................................................... 565.6.1 Step-edge barrier in <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> ...................................................................... 606 Hexaphenyl <strong>on</strong> KCl (001) ........................................................................... 646.1 Searching for a p6P wetting layer <strong>on</strong> KCl (001)........................................................... 676.2 Investigati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> morphology by AFM...................................................... 706.3 The influence <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate temperature <strong>on</strong> the p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> morphology..................... 736.4 Thermal stability <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>s <strong>on</strong> KCl (001) ................................................................ 797 HATCN <strong>on</strong> Au (111) ................................................................................... 837.1 Purificati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <str<strong>on</strong>g>and</str<strong>on</strong>g> the cracking pattern ......................................................... 837.2 The initial layer <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <strong>on</strong> Au (111) ......................................................... 867.3 Further development <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> ....................................................................... 917.4 The influence <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate temperature ................................................................... 947.5 The bulk phase (3D-isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s) ......................................................................................... 997.6 Work functi<strong>on</strong> modificati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> Au (111) by HATCN ................................................ 1027.7 HATCN <strong>on</strong> CN-c<strong>on</strong>taminated Au............................................................................... 1048 HATCN <strong>on</strong> Ag (111) ................................................................................. 1128.1 Initial <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>: The lying m<strong>on</strong>olayer.................................................................... 1128.2 Intermediate <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>: The st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing m<strong>on</strong>olayer.................................................... 1148.3 The multilayer regime ................................................................................................. 1168.4 Work functi<strong>on</strong> modificati<strong>on</strong>........................................................................................ 12211
9 Summary.................................................................................................... 1269.1 p6P <strong>on</strong> mica (001) ....................................................................................................... 1269.2 p6P <strong>on</strong> KCl (001) ........................................................................................................ 1279.3 HATCN <strong>on</strong> Au (111)................................................................................................... 1289.4 HATCN <strong>on</strong> Ag (111)................................................................................................... 12910 Appendix ................................................................................................... 132A Tutorial <strong>on</strong> the use <str<strong>on</strong>g>of</str<strong>on</strong>g> the Nanosurf easyScan2 AFM ................................................. 132A.1 Setting up the measurement............................................................................................. 132A.2 Imaging............................................................................................................................ 135A.3 Image processing............................................................................................................. 138B Tutorial <strong>on</strong> the use <str<strong>on</strong>g>of</str<strong>on</strong>g> the UHV Kelvin Probe............................................................. 140B.1 Quick start <str<strong>on</strong>g>of</str<strong>on</strong>g> the s<str<strong>on</strong>g>of</str<strong>on</strong>g>tware .............................................................................................. 140B.2 Positi<strong>on</strong>ing the tip............................................................................................................ 143B.3 The work functi<strong>on</strong> measurement ..................................................................................... 144B.4 Hints & tips ..................................................................................................................... 146C List <str<strong>on</strong>g>of</str<strong>on</strong>g> publicati<strong>on</strong>s <str<strong>on</strong>g>and</str<strong>on</strong>g> c<strong>on</strong>ference c<strong>on</strong>tributi<strong>on</strong>s ...................................................... 148C.1 Publicati<strong>on</strong>s in internati<strong>on</strong>al scientific journals............................................................... 148C. 2 C<strong>on</strong>ference C<strong>on</strong>tributi<strong>on</strong>s................................................................................................ 151D Bibliography................................................................................................................ 16012
List <str<strong>on</strong>g>of</str<strong>on</strong>g> Symbols <str<strong>on</strong>g>and</str<strong>on</strong>g> Abbreviati<strong>on</strong>sList <str<strong>on</strong>g>of</str<strong>on</strong>g> Symbols <str<strong>on</strong>g>and</str<strong>on</strong>g> Abbreviati<strong>on</strong>sp6PαAESAFMamuβE desF4TCNQHATCNHIBHWELEEDOMBEQMSRTSEMSTMTCNETDSΘUHVXPSXRDpara-hexaphenyl, sexiphenylmultilayer desorpti<strong>on</strong> peakAuger electr<strong>on</strong> spectroscopyatomic force microscopyatomic mass unitm<strong>on</strong>olayer desorpti<strong>on</strong> peakdesorpti<strong>on</strong> energytetrafluoro-tetracyanoquinodimethanehexaaza-triphenylene-hexacarb<strong>on</strong>itrilehole injecti<strong>on</strong> barrierhot wall epitaxylow energy electr<strong>on</strong> diffracti<strong>on</strong><str<strong>on</strong>g>organic</str<strong>on</strong>g> molecular beam epitaxyquadrupole mass spectrometerroom temperaturescanning electr<strong>on</strong> microscopyscanning tunneling microscopytetracyanoethylenethermal desorpti<strong>on</strong> spectroscopycoverageultra-high vacuumX-ray photoelectr<strong>on</strong> spectroscopyX-ray diffracti<strong>on</strong>13
1 Int<str<strong>on</strong>g>rod</str<strong>on</strong>g>ucti<strong>on</strong>1 Int<str<strong>on</strong>g>rod</str<strong>on</strong>g>ucti<strong>on</strong>Novel <str<strong>on</strong>g>organic</str<strong>on</strong>g> materials are attractive c<str<strong>on</strong>g>and</str<strong>on</strong>g>idates as active layer materials for <str<strong>on</strong>g>organic</str<strong>on</strong>g>electr<strong>on</strong>ic devices <str<strong>on</strong>g>like</str<strong>on</strong>g> thin <str<strong>on</strong>g>film</str<strong>on</strong>g> transistors, light emitting diodes or <str<strong>on</strong>g>organic</str<strong>on</strong>g> lasers. One <str<strong>on</strong>g>of</str<strong>on</strong>g> thefrequently used materials is para-hexaphenyl (p6P), which is a very interesting modelcompound for studying the basic physical properties <str<strong>on</strong>g>of</str<strong>on</strong>g> c<strong>on</strong>jugated <str<strong>on</strong>g>organic</str<strong>on</strong>g> semic<strong>on</strong>ductors[1]. Devices using p6P such as <str<strong>on</strong>g>organic</str<strong>on</strong>g> light emitting diodes (OLED) [2–6], <str<strong>on</strong>g>organic</str<strong>on</strong>g> fieldeffect transistors (OFET) <str<strong>on</strong>g>and</str<strong>on</strong>g> solar cells [7–11] have already been realized. In order t<str<strong>on</strong>g>of</str<strong>on</strong>g>abricate such devices, the <str<strong>on</strong>g>growth</str<strong>on</strong>g> parameters <str<strong>on</strong>g>and</str<strong>on</strong>g> stability properties <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g>shave to be well understood. Which <str<strong>on</strong>g>growth</str<strong>on</strong>g> mode is favored depends <strong>on</strong> the type <str<strong>on</strong>g>of</str<strong>on</strong>g> substrate,the substrate structure, the chemical compositi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate <str<strong>on</strong>g>and</str<strong>on</strong>g> the substratetemperature. Additi<strong>on</strong>ally, if a str<strong>on</strong>gly bound first layer, a so-called wetting layer, exists, thislayer will influence the further <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>. Several substrates have been used to study thethin <str<strong>on</strong>g>film</str<strong>on</strong>g> formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> hexaphenyl, for example Al [12–15], TiO 2 [15-17], CuO[18, 19] <str<strong>on</strong>g>and</str<strong>on</strong>g>Au [20–22]. Apart from these, a particular suitable substrate material is mica, because <strong>on</strong>e caneasily obtain well-defined single crystal surfaces by cleaving. Therefore the <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <strong>on</strong>mica has been the subject <str<strong>on</strong>g>of</str<strong>on</strong>g> many publicati<strong>on</strong>s [15, 23-39]. It has been shown that p6P growsin a highly ordered fashi<strong>on</strong> <strong>on</strong> the mica (001) surface, resulting in l<strong>on</strong>g (up to the range)nano-fibres, which are oriented parallel to each other [24–26]. This type <str<strong>on</strong>g>of</str<strong>on</strong>g> nano-structures isimportant for nano-optics. Previous X-ray diffracti<strong>on</strong> (XRD) studies revealed in this case aquasi-epitaxial relati<strong>on</strong>ship between the p6P crystallites <str<strong>on</strong>g>and</str<strong>on</strong>g> the mica substrate [27, 28]. Onthe other h<str<strong>on</strong>g>and</str<strong>on</strong>g>, in some cases the formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> ring-<str<strong>on</strong>g>like</str<strong>on</strong>g> structured isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s has been observed[29]. Apparently, the substrate c<strong>on</strong>diti<strong>on</strong>s have an important influence <strong>on</strong> the <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>.Another very frequently used substrate is potassium chloride (KCl). In this c<strong>on</strong>text, <str<strong>on</strong>g>film</str<strong>on</strong>g>sprepared with physical vapor depositi<strong>on</strong> (PVD) [40–46] <str<strong>on</strong>g>and</str<strong>on</strong>g> with hot-wall epitaxy (HWE)[47–49] have already been investigated. The initial <str<strong>on</strong>g>growth</str<strong>on</strong>g> stage exhibits crystalline needleswhich c<strong>on</strong>sist <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> lying <strong>on</strong> the surface <str<strong>on</strong>g>and</str<strong>on</strong>g> form a rectangular network [49]. When acertain coverage is reached, terraces <str<strong>on</strong>g>of</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are formed. Using the right <str<strong>on</strong>g>growth</str<strong>on</strong>g>parameters <str<strong>on</strong>g>and</str<strong>on</strong>g> depending <strong>on</strong> the desired applicati<strong>on</strong>, layers c<strong>on</strong>sisting <str<strong>on</strong>g>of</str<strong>on</strong>g> lying or st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing<str<strong>on</strong>g>molecules</str<strong>on</strong>g> can be fashi<strong>on</strong>ed. For the electr<strong>on</strong>ic properties <str<strong>on</strong>g>of</str<strong>on</strong>g> devices apart from the molecular14
1 Int<str<strong>on</strong>g>rod</str<strong>on</strong>g>ucti<strong>on</strong>arrangement <str<strong>on</strong>g>of</str<strong>on</strong>g> the active <str<strong>on</strong>g>organic</str<strong>on</strong>g> layer material also the metal/<str<strong>on</strong>g>organic</str<strong>on</strong>g> interface is <str<strong>on</strong>g>of</str<strong>on</strong>g> crucialimportance. One <str<strong>on</strong>g>of</str<strong>on</strong>g> the most important properties that can be tailored by altering this interfaceintenti<strong>on</strong>ally is the charge injecti<strong>on</strong>. The charge injecti<strong>on</strong> can be enhanced by applying selfassembled m<strong>on</strong>olayers (SAMs) that carry an intrinsic dipole [50, 51] or by <str<strong>on</strong>g>molecules</str<strong>on</strong>g> thatundergo a charge transfer type reacti<strong>on</strong> with the elect<str<strong>on</strong>g>rod</str<strong>on</strong>g>e materials [52, 53, 54], or byapplying thin layers <str<strong>on</strong>g>of</str<strong>on</strong>g> high work functi<strong>on</strong> metals [55]. In all cases a dipole at the interface iscreated that changes the electr<strong>on</strong>ic level alignment <str<strong>on</strong>g>of</str<strong>on</strong>g> subsequently deposited c<strong>on</strong>jugated<str<strong>on</strong>g>organic</str<strong>on</strong>g> materials in such a way, that the hole injecti<strong>on</strong> barrier (HIB) can significantly bereduced. For example has it been shown that by the inserti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a thin layer <str<strong>on</strong>g>of</str<strong>on</strong>g> tetrafluorotetracyanoquinodimethane(F4-TCNQ) the HIB into p6P can be lowered by as much as 1.2 eV[56]. The major drawback <str<strong>on</strong>g>of</str<strong>on</strong>g> c<strong>on</strong>venti<strong>on</strong>al cyano-based molecular acceptors <str<strong>on</strong>g>like</str<strong>on</strong>g>tetracyanoethylene (TCNE), tetracyanoquinodimethane (TCNQ) <str<strong>on</strong>g>and</str<strong>on</strong>g> F4-TCNQ is their ratherlow molecular weight, which implicates poor thermal stability <str<strong>on</strong>g>of</str<strong>on</strong>g> the grown structures. ForF4-TCNQ this has already been shown to be a problem in devices [57]. Therefore molecularelectr<strong>on</strong> acceptors with higher thermal stability (which means to a first approximati<strong>on</strong> ahigher molecular weight) are desired. One <str<strong>on</strong>g>of</str<strong>on</strong>g> the novel cyano-based molecular acceptors witha significantly higher molecular weight <str<strong>on</strong>g>and</str<strong>on</strong>g> therefore with a higher thermal stability ishexaaza-triphenylene-hexacarb<strong>on</strong>itrile (HATCN). This <str<strong>on</strong>g>disc</str<strong>on</strong>g>oid molecule features a verycomplex bulk structure with a unit cell c<strong>on</strong>sisting <str<strong>on</strong>g>of</str<strong>on</strong>g> 18 <str<strong>on</strong>g>molecules</str<strong>on</strong>g> [58, 59]. The thin <str<strong>on</strong>g>film</str<strong>on</strong>g><str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN was investigated in the course <str<strong>on</strong>g>of</str<strong>on</strong>g> this thesis in collaborati<strong>on</strong> with othergroups in detail.The first part <str<strong>on</strong>g>of</str<strong>on</strong>g> the present work focuses <strong>on</strong> the influence <str<strong>on</strong>g>of</str<strong>on</strong>g> the intenti<strong>on</strong>al manipulati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g>the substrate <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>on</strong> the influence <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate temperature <strong>on</strong> the layer <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>rod</str<strong>on</strong>g><str<strong>on</strong>g>like</str<strong>on</strong>g>molecule p6P <strong>on</strong> mica (001) <str<strong>on</strong>g>and</str<strong>on</strong>g> KCl (001). In particular, the influence <str<strong>on</strong>g>of</str<strong>on</strong>g> the surfacec<strong>on</strong>taminati<strong>on</strong> by carb<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> the structure modificati<strong>on</strong> by sputtering <strong>on</strong> the p6P layer <str<strong>on</strong>g>growth</str<strong>on</strong>g>was investigated.The sec<strong>on</strong>d part deals with the <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <strong>on</strong> Au (111) <str<strong>on</strong>g>and</str<strong>on</strong>g> Ag (111) <str<strong>on</strong>g>and</str<strong>on</strong>g> withthe influence <str<strong>on</strong>g>of</str<strong>on</strong>g> the thin <str<strong>on</strong>g>film</str<strong>on</strong>g> structure <strong>on</strong> the work functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the surfaces. Important issueswere the thermal stability <str<strong>on</strong>g>and</str<strong>on</strong>g> the desorpti<strong>on</strong> kinetics <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g>s <str<strong>on</strong>g>and</str<strong>on</strong>g> the correlati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the<str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> to the HATCN-induced work functi<strong>on</strong> change <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate surfaces.The presented results have been published in internati<strong>on</strong>al scientific journals. Thecorresp<strong>on</strong>ding references are given in the reference list [60-68].15
2 Basic c<strong>on</strong>cepts <str<strong>on</strong>g>of</str<strong>on</strong>g> epitaxial <str<strong>on</strong>g>growth</str<strong>on</strong>g>2 Basic c<strong>on</strong>cepts <str<strong>on</strong>g>of</str<strong>on</strong>g> epitaxial <str<strong>on</strong>g>growth</str<strong>on</strong>g>In the following selected ideas established in in<str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> will be <str<strong>on</strong>g>disc</str<strong>on</strong>g>ussed whichcannot always be transferred to <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> <strong>on</strong>e to <strong>on</strong>e, but nevertheless can <str<strong>on</strong>g>of</str<strong>on</strong>g>tenhelp to underst<str<strong>on</strong>g>and</str<strong>on</strong>g> the fundamental mechanisms <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>organic</str<strong>on</strong>g> layer or isl<str<strong>on</strong>g>and</str<strong>on</strong>g> formati<strong>on</strong>.Epitaxy is the comm<strong>on</strong>ly used denominati<strong>on</strong> for the oriented <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> a crystalline material<strong>on</strong> a single crystal surface. The epitaxial orientati<strong>on</strong> is determined by the c<strong>on</strong>diti<strong>on</strong> for theminimum <str<strong>on</strong>g>of</str<strong>on</strong>g> the Gibbs free energy <str<strong>on</strong>g>of</str<strong>on</strong>g> the system, which is governed by the structure <str<strong>on</strong>g>and</str<strong>on</strong>g> theenergy <str<strong>on</strong>g>of</str<strong>on</strong>g> the epitaxial interface. The key parameter which is used to describe the equilibriumbetween two phases is the chemical potential µ (eq. 2.1), which represents the derivatives <str<strong>on</strong>g>of</str<strong>on</strong>g>the Gibbs free energies with respect to the number <str<strong>on</strong>g>of</str<strong>on</strong>g> particles in the system at c<strong>on</strong>stantpressure P <str<strong>on</strong>g>and</str<strong>on</strong>g> temperature T.(2.1)In other words, the chemical potential µ is a measure <str<strong>on</strong>g>of</str<strong>on</strong>g> how much work has to be d<strong>on</strong>e tochange the number <str<strong>on</strong>g>of</str<strong>on</strong>g> particles in a phase by <strong>on</strong>e while keeping the temperature <str<strong>on</strong>g>and</str<strong>on</strong>g> thepressure c<strong>on</strong>stant.The term ‘epitaxy’ is <str<strong>on</strong>g>of</str<strong>on</strong>g> course not limited to in<str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>. In the case <str<strong>on</strong>g>of</str<strong>on</strong>g> large<str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> this term is also frequently used if the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> form highly regularstructures <strong>on</strong> single-crystalline substrates <str<strong>on</strong>g>and</str<strong>on</strong>g> a defined relati<strong>on</strong>ship between the regularstructures <str<strong>on</strong>g>of</str<strong>on</strong>g> the adsorbate <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> the substrate surface exists.2.1 <str<strong>on</strong>g>Thin</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>Up<strong>on</strong> depositi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> material the chemical potential <str<strong>on</strong>g>of</str<strong>on</strong>g> the surface will change due to adhesive<str<strong>on</strong>g>and</str<strong>on</strong>g> cohesive forces acting between the substrate <str<strong>on</strong>g>and</str<strong>on</strong>g> the adsorbate. Additi<strong>on</strong>ally, a latticemisfit <str<strong>on</strong>g>of</str<strong>on</strong>g> the two species will create misfit dislocati<strong>on</strong>s <str<strong>on</strong>g>and</str<strong>on</strong>g> strains whose energies also16
2 Basic c<strong>on</strong>cepts <str<strong>on</strong>g>of</str<strong>on</strong>g> epitaxial <str<strong>on</strong>g>growth</str<strong>on</strong>g>influence the chemical potential. Therefore the chemical potential <str<strong>on</strong>g>of</str<strong>on</strong>g> the deposit will varyfrom m<strong>on</strong>olayer to m<strong>on</strong>olayer due to the interacti<strong>on</strong> with the substrate, which leads to theappearance <str<strong>on</strong>g>of</str<strong>on</strong>g> the three well-known mechanisms <str<strong>on</strong>g>of</str<strong>on</strong>g> epitaxial <str<strong>on</strong>g>growth</str<strong>on</strong>g> [83, 84]:• Frank-van der Merwe mechanism (layer-by-layer <str<strong>on</strong>g>growth</str<strong>on</strong>g>):if for all layers . The completi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the first layer is morefavorable than starting the sec<strong>on</strong>d layer → formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> ‘wetting layers’• Stranski-Krastanov mechanism (layer-by-layer followed by 3D isl<str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>):If for small , but for large . After the formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> <strong>on</strong>eor more wetting layers the adsorbate <str<strong>on</strong>g>film</str<strong>on</strong>g> will start to form 3D isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s.• Volmer-Weber mechanism (3D isl<str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>):If for all . Isl<str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> will occur immediately.Generally, isl<str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> is favored when the interfacial b<strong>on</strong>ding is weaker than the b<strong>on</strong>dingin the deposit itself. Additi<strong>on</strong>ally, high substrate temperatures <str<strong>on</strong>g>and</str<strong>on</strong>g> a large lattice misfit willboth promote the cluster formati<strong>on</strong>.These general statements can be extended <str<strong>on</strong>g>and</str<strong>on</strong>g> specified for <str<strong>on</strong>g>organic</str<strong>on</strong>g> thin <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> inparticular: The possible forces acting between the substrate <str<strong>on</strong>g>and</str<strong>on</strong>g> the adsorbate are defined bycharge transfer, covalent chemical b<strong>on</strong>ding, electrostatic multi-pole interacti<strong>on</strong>s <str<strong>on</strong>g>and</str<strong>on</strong>g> van derWaals interacti<strong>on</strong>s (physisorpti<strong>on</strong>) [71]. In many cases these interacti<strong>on</strong>s influence <strong>on</strong>ly the<str<strong>on</strong>g>molecules</str<strong>on</strong>g> within the first <str<strong>on</strong>g>organic</str<strong>on</strong>g> m<strong>on</strong>olayer, whereas the sec<strong>on</strong>d <str<strong>on</strong>g>and</str<strong>on</strong>g> subsequent layers areunaffected. Additi<strong>on</strong>ally, in the case <str<strong>on</strong>g>of</str<strong>on</strong>g> molecular crystals intermolecular interacti<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> van17
2 Basic c<strong>on</strong>cepts <str<strong>on</strong>g>of</str<strong>on</strong>g> epitaxial <str<strong>on</strong>g>growth</str<strong>on</strong>g>der Waals type <str<strong>on</strong>g>and</str<strong>on</strong>g>/or electrostatic multi-pole type will be present throughout the <str<strong>on</strong>g>organic</str<strong>on</strong>g> bulkmaterial. As a c<strong>on</strong>sequence, the relative strength <str<strong>on</strong>g>of</str<strong>on</strong>g> the involved interacti<strong>on</strong>s determines themolecular packing <str<strong>on</strong>g>and</str<strong>on</strong>g> the structure <str<strong>on</strong>g>of</str<strong>on</strong>g> the first adsorbed m<strong>on</strong>olayer [94], which in turn <str<strong>on</strong>g>of</str<strong>on</strong>g>teninfluences the structure <str<strong>on</strong>g>of</str<strong>on</strong>g> the subsequent layers. Therefore the questi<strong>on</strong> as to the existence <str<strong>on</strong>g>of</str<strong>on</strong>g>a c<strong>on</strong>tinuous first m<strong>on</strong>olayer, which is <str<strong>on</strong>g>of</str<strong>on</strong>g>ten denoted as ‘wetting layer’, is a key tounderst<str<strong>on</strong>g>and</str<strong>on</strong>g>ing the thin <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>organic</str<strong>on</strong>g> systems. In some cases the wetting layer evenacts as a template for further <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>, which is dem<strong>on</strong>strated for instance in chapter 5,which deals with the <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <strong>on</strong> differently treated mica (001) surfaces. There it isdem<strong>on</strong>strated that if a wetting layer comprising lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g> is formed, the subsequentlayers also comprise lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g>. However, in some rare cases the structure <str<strong>on</strong>g>of</str<strong>on</strong>g> thesubsequent layers influences the structure <str<strong>on</strong>g>of</str<strong>on</strong>g> the wetting layer: In chapter 7 the very unusuallayer <str<strong>on</strong>g>growth</str<strong>on</strong>g> behavior <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <strong>on</strong> Au (111) is investigated <str<strong>on</strong>g>and</str<strong>on</strong>g> it turns out that the firsttwo m<strong>on</strong>olayers are both incorporated into the bulk structure after a critical layer thicknesshas been reached.2.2 Nucleati<strong>on</strong>In the classical thermodynamic approach, the formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a new phase B requires as anecessary prerequisite the appearance <str<strong>on</strong>g>of</str<strong>on</strong>g> small clusters <str<strong>on</strong>g>of</str<strong>on</strong>g> building units (atoms or <str<strong>on</strong>g>molecules</str<strong>on</strong>g>)in the volume <str<strong>on</strong>g>of</str<strong>on</strong>g> the supersaturated phase A (vapors, melt or in soluti<strong>on</strong>). These clusters ornuclei are small complexes <str<strong>on</strong>g>of</str<strong>on</strong>g> atoms or <str<strong>on</strong>g>molecules</str<strong>on</strong>g> which have the same properties as thecorresp<strong>on</strong>ding bulk phases, but are much smaller [83]. In a homogeneous system near thephase equilibrium between the two phases, always small density fluctuati<strong>on</strong>s will be present.These fluctuati<strong>on</strong>s can lead to the formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> an aggregate <str<strong>on</strong>g>of</str<strong>on</strong>g> phase B in phase A <str<strong>on</strong>g>and</str<strong>on</strong>g> if theinitial phase A is instablethe tendency to grow prevails after exceeding a certaincritical radius . This critical radius is defined by the interplay between the energy gain fromcreating the 3D droplet <str<strong>on</strong>g>and</str<strong>on</strong>g> the energy loss from creating new interfaces between the twophases. The critical size <str<strong>on</strong>g>of</str<strong>on</strong>g> the nuclei is also influenced by so-called surfactants because theprocess <str<strong>on</strong>g>of</str<strong>on</strong>g> nucleati<strong>on</strong> is stimulated by the presence <str<strong>on</strong>g>of</str<strong>on</strong>g> impurities, i<strong>on</strong>s or defects in thesubstrate.In <str<strong>on</strong>g>organic</str<strong>on</strong>g> thin <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>, the c<strong>on</strong>cept <str<strong>on</strong>g>of</str<strong>on</strong>g> heterogeneous nucleati<strong>on</strong> could be an approach todescribe the formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a phase B out <str<strong>on</strong>g>of</str<strong>on</strong>g> phase A <str<strong>on</strong>g>of</str<strong>on</strong>g> material 1 (adsorbate) in the presence <str<strong>on</strong>g>of</str<strong>on</strong>g>18
2 Basic c<strong>on</strong>cepts <str<strong>on</strong>g>of</str<strong>on</strong>g> epitaxial <str<strong>on</strong>g>growth</str<strong>on</strong>g>material 2 (substrate). However, it turned out that the critical nucleus size in <str<strong>on</strong>g>organic</str<strong>on</strong>g> thin <str<strong>on</strong>g>film</str<strong>on</strong>g><str<strong>on</strong>g>growth</str<strong>on</strong>g> is so small that it is impossible to designate such a nucleus as solid or liquid as nol<strong>on</strong>g range order can be observed. Therefore an atomistic view <str<strong>on</strong>g>of</str<strong>on</strong>g> nucleati<strong>on</strong> is necessary inorder to underst<str<strong>on</strong>g>and</str<strong>on</strong>g> the processes involved.2.3 Nucleati<strong>on</strong> <strong>on</strong> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s: Step-edge barriersMichely et al. [86-88] present an atomistic nucleati<strong>on</strong> theory which describes the nucleati<strong>on</strong><strong>on</strong> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s in the presence <str<strong>on</strong>g>of</str<strong>on</strong>g> a step-edge barrier. Such a step-edge barrier for diffusingadatoms has been described for the first time by Ehrlich et al. [89] <str<strong>on</strong>g>and</str<strong>on</strong>g> Schwöbel et al. [90]<str<strong>on</strong>g>and</str<strong>on</strong>g> is therefore <str<strong>on</strong>g>of</str<strong>on</strong>g>ten referred to as Ehrlich-Schwöbel-Barrier. As a c<strong>on</strong>sequence, an adatomneeds to surmount an activati<strong>on</strong> barrier when crossing a step edge, which leads to the <str<strong>on</strong>g>growth</str<strong>on</strong>g><str<strong>on</strong>g>of</str<strong>on</strong>g> pyramidal multilayer mounds (see Fig. 2.1, sometimes denoted as ‘wedding cakes’ [91,92]). These mounds are formed by repeated two-dimensi<strong>on</strong>al nucleati<strong>on</strong> <strong>on</strong> the subjacentisl<str<strong>on</strong>g>and</str<strong>on</strong>g>, respectively <str<strong>on</strong>g>and</str<strong>on</strong>g> their shape depends <strong>on</strong> the height <str<strong>on</strong>g>of</str<strong>on</strong>g> the step-edge barrier.19
2 Basic c<strong>on</strong>cepts <str<strong>on</strong>g>of</str<strong>on</strong>g> epitaxial <str<strong>on</strong>g>growth</str<strong>on</strong>g>Fig. 2.1: STM image <str<strong>on</strong>g>of</str<strong>on</strong>g> pyramidal multilayer mounds which are formed due to the presence <str<strong>on</strong>g>of</str<strong>on</strong>g> a stepedgebarrier up<strong>on</strong> Pt depositi<strong>on</strong> <strong>on</strong> Pt (111) in a partial CO pressure. From [86].In the case <str<strong>on</strong>g>of</str<strong>on</strong>g> an infinitely high step-edge barrier preventing any interlayer mass transport, thesurface fracti<strong>on</strong>s in the individual layers follow a Poiss<strong>on</strong> distributi<strong>on</strong> [93]. By proper analysis<str<strong>on</strong>g>of</str<strong>on</strong>g> the shapes <str<strong>on</strong>g>of</str<strong>on</strong>g> the mounds it is possible to determine the height <str<strong>on</strong>g>of</str<strong>on</strong>g> the step-edge barrier( ) [65]: For this purpose, the previously menti<strong>on</strong>ed theoretical model <str<strong>on</strong>g>of</str<strong>on</strong>g> Michely et al. isc<strong>on</strong>sulted: The height <str<strong>on</strong>g>of</str<strong>on</strong>g> the step-edge barrier together with the <strong>on</strong>-terrace jump rategoverns the ratejump rate):at which <str<strong>on</strong>g>molecules</str<strong>on</strong>g> diffuse downwards across a terrace edge (= interlayer(2.2)with , being the energy barrier for <strong>on</strong>-terrace diffusi<strong>on</strong>, which is much smallerthan . The interlayer jump rate can be estimated from the top terrace diameter(gained from STM or AFM analysis) <str<strong>on</strong>g>and</str<strong>on</strong>g> the experimentally used depositi<strong>on</strong> flux :(2.3)The <strong>on</strong>-terrace jump rate can be evaluated using kinetic nucleati<strong>on</strong> theory [85]: If the stepedgebarrier is large enough to effectively suppress interlayer mass transport, the <strong>on</strong>-terracejump rate is in general related to the nucleati<strong>on</strong> density, which can be c<strong>on</strong>nected to themound separati<strong>on</strong> :(2.4)which can again be gained from STM or AFM image analysis.is the dimensi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> thediffusi<strong>on</strong> process (here ) <str<strong>on</strong>g>and</str<strong>on</strong>g> is the critical isl<str<strong>on</strong>g>and</str<strong>on</strong>g> size <str<strong>on</strong>g>of</str<strong>on</strong>g> the largest unstable cluster.For systems with str<strong>on</strong>g barriers the choice turned out to be reas<strong>on</strong>able [86], which20
2 Basic c<strong>on</strong>cepts <str<strong>on</strong>g>of</str<strong>on</strong>g> epitaxial <str<strong>on</strong>g>growth</str<strong>on</strong>g>means that dimers are already stable <str<strong>on</strong>g>and</str<strong>on</strong>g> immobile. As so<strong>on</strong> as two adatoms are present <strong>on</strong> theisl<str<strong>on</strong>g>and</str<strong>on</strong>g>, nucleati<strong>on</strong> will occur after a certain time provided that a large residence time <str<strong>on</strong>g>of</str<strong>on</strong>g> allatoms <strong>on</strong> the isl<str<strong>on</strong>g>and</str<strong>on</strong>g> is given (which is the case for systems with large step-edge barriers). Insecti<strong>on</strong> 5.6.1 Step-edge barrier in <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> it is shown that this well-knownmodel for in<str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> can also be used for <str<strong>on</strong>g>organic</str<strong>on</strong>g> systems to verify the existence<str<strong>on</strong>g>and</str<strong>on</strong>g> to calculate the height <str<strong>on</strong>g>of</str<strong>on</strong>g> the step-edge barrier. However, supplementary ideas have to betaken into account when applying a model, which was designed for atoms, to large <str<strong>on</strong>g>organic</str<strong>on</strong>g><str<strong>on</strong>g>molecules</str<strong>on</strong>g> with more degrees <str<strong>on</strong>g>of</str<strong>on</strong>g> freedom.21
3 Film‐substrate systems3 Film-substrate systemsThis chapter shortly describes the used <str<strong>on</strong>g>organic</str<strong>on</strong>g> adsorbate <str<strong>on</strong>g>molecules</str<strong>on</strong>g> as well as the usedsingle-crystalline substrates. The investigated systems can be divided into• The <str<strong>on</strong>g>rod</str<strong>on</strong>g>-<str<strong>on</strong>g>like</str<strong>on</strong>g> molecule p6P <strong>on</strong> insulating substrates (mica, KCl)• The <str<strong>on</strong>g>disc</str<strong>on</strong>g>-<str<strong>on</strong>g>shaped</str<strong>on</strong>g> molecule HATCN <strong>on</strong> noble metals (Au, Ag)3.1 A <str<strong>on</strong>g>rod</str<strong>on</strong>g>-<str<strong>on</strong>g>like</str<strong>on</strong>g> molecule: The c<strong>on</strong>jugated <str<strong>on</strong>g>organic</str<strong>on</strong>g> semic<strong>on</strong>ductorpara-hexaphenyl (p6P)Hexaphenyl (p6P, frequently denoted as sexiphenyl) is the term used for a molecule thatc<strong>on</strong>sists <str<strong>on</strong>g>of</str<strong>on</strong>g> six phenyl rings (chemical formula C 36 H 26 ). In the case <str<strong>on</strong>g>of</str<strong>on</strong>g> para-hexaphenyl thephenyl rings are aligned in a row, forming thus a <str<strong>on</strong>g>rod</str<strong>on</strong>g>-<str<strong>on</strong>g>like</str<strong>on</strong>g> molecule. The electr<strong>on</strong>ic <str<strong>on</strong>g>and</str<strong>on</strong>g> opticalproperties <str<strong>on</strong>g>of</str<strong>on</strong>g> this molecule are related to the properties <str<strong>on</strong>g>of</str<strong>on</strong>g> other <str<strong>on</strong>g>rod</str<strong>on</strong>g>-<str<strong>on</strong>g>like</str<strong>on</strong>g> oligophenylenes(pnP) <str<strong>on</strong>g>and</str<strong>on</strong>g> are governed by their c<strong>on</strong>jugati<strong>on</strong>, which is an intrinsic property <str<strong>on</strong>g>of</str<strong>on</strong>g> all phenylenes.Regarding hydrocarb<strong>on</strong>s, the term ‘c<strong>on</strong>jugati<strong>on</strong>’ describes the molecular property <str<strong>on</strong>g>of</str<strong>on</strong>g> havingalternating carb<strong>on</strong> single- <str<strong>on</strong>g>and</str<strong>on</strong>g> double b<strong>on</strong>ds, which causes hybridizati<strong>on</strong> [69, 70,71]. The carb<strong>on</strong> orbitals are exclusively involved in building up the backb<strong>on</strong>e <str<strong>on</strong>g>of</str<strong>on</strong>g> themolecule, which determines the geometric structure <str<strong>on</strong>g>and</str<strong>on</strong>g> mechanical properties. The carb<strong>on</strong>electr<strong>on</strong>s, however, are delocalized across the c<strong>on</strong>jugated area <str<strong>on</strong>g>of</str<strong>on</strong>g> the molecule to formextended π-orbitals [71]. Thus, the corresp<strong>on</strong>ding electr<strong>on</strong>s are <str<strong>on</strong>g>of</str<strong>on</strong>g>ten denoted as π-electr<strong>on</strong>s.A prototypical example <str<strong>on</strong>g>of</str<strong>on</strong>g> a c<strong>on</strong>jugated <str<strong>on</strong>g>organic</str<strong>on</strong>g> molecule is benzene. C<strong>on</strong>jugated materials<str<strong>on</strong>g>of</str<strong>on</strong>g>ten exhibit a low electr<strong>on</strong>ic b<str<strong>on</strong>g>and</str<strong>on</strong>g> gap between the HOMO <str<strong>on</strong>g>and</str<strong>on</strong>g> the LUMO, which gives riseto semic<strong>on</strong>ducting behavior. In the case <str<strong>on</strong>g>of</str<strong>on</strong>g> hexaphenyl, the b<str<strong>on</strong>g>and</str<strong>on</strong>g> gap is [72, 73],which is <str<strong>on</strong>g>of</str<strong>on</strong>g>ten described as wide-gap semic<strong>on</strong>ducting behavior. The van der Waalsdimensi<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> a single molecule are. The atomic mass <str<strong>on</strong>g>of</str<strong>on</strong>g> a p6Pmolecule is 458.59 amu [74]. Fig. 1a shows a single p6P molecule in the planar c<strong>on</strong>figurati<strong>on</strong>.The p6P bulk crystal structure at room temperature is m<strong>on</strong>oclinic [75, 76] <str<strong>on</strong>g>and</str<strong>on</strong>g> well-known as22
3 Film‐substrate systemsherringb<strong>on</strong>e structure, which is characteristic not <strong>on</strong>ly <str<strong>on</strong>g>of</str<strong>on</strong>g> the pnPs, but also <str<strong>on</strong>g>of</str<strong>on</strong>g> various otherc<strong>on</strong>jugated aromatic <str<strong>on</strong>g>molecules</str<strong>on</strong>g>. In Fig. 1b the arrangement <str<strong>on</strong>g>of</str<strong>on</strong>g> 10 p6P <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in the unitcell is depicted. A very detailed descripti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> oligophenylenes at large <str<strong>on</strong>g>and</str<strong>on</strong>g> in particular <str<strong>on</strong>g>of</str<strong>on</strong>g>p6P can be found in ref [71].Fig. 3.1: Single p6P molecule in the planar c<strong>on</strong>formati<strong>on</strong>. From [77].3.2 A <str<strong>on</strong>g>disc</str<strong>on</strong>g>-<str<strong>on</strong>g>shaped</str<strong>on</strong>g> molecule: The <str<strong>on</strong>g>organic</str<strong>on</strong>g> molecular acceptorhexaaza-triphenylene-hexacarb<strong>on</strong>trile (HATCN)HATCN is the term used for a <str<strong>on</strong>g>disc</str<strong>on</strong>g> <str<strong>on</strong>g>shaped</str<strong>on</strong>g> molecule with the chemical forumla C 18 N 12 (seescheme in Fig. 3.2). The c<strong>on</strong>jugated heterocycles determine the physical <str<strong>on</strong>g>and</str<strong>on</strong>g> chemicalproperties <str<strong>on</strong>g>of</str<strong>on</strong>g> the molecule. Therefore this molecule is a str<strong>on</strong>gly electr<strong>on</strong>-deficientheterocycle which as a c<strong>on</strong>sequence <str<strong>on</strong>g>of</str<strong>on</strong>g> that avoids π-stacking [58]. Thus the crystal structureis characterized by perpendicular CN-π interacti<strong>on</strong>s, leading to a very complicated 3Dhexag<strong>on</strong>al arrangement [58, 59] (Fig. 3.3) with 18 <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in the unit cell.HATCN is assigned to the group <str<strong>on</strong>g>of</str<strong>on</strong>g> cyano-based acceptor <str<strong>on</strong>g>molecules</str<strong>on</strong>g> <str<strong>on</strong>g>like</str<strong>on</strong>g> tetracyanoethylene(TCNE) [78] or tetracyanoquinodimethane (TCNQ) [79] <str<strong>on</strong>g>and</str<strong>on</strong>g> its derivates. The majordrawback <str<strong>on</strong>g>of</str<strong>on</strong>g> c<strong>on</strong>venti<strong>on</strong>al cyano-based molecular acceptors is their rather low molecularweight, which implicates poor thermal stability <str<strong>on</strong>g>of</str<strong>on</strong>g> the grown structures. For F4-TCNQ( ) this has already been shown to be a problem in devices [57]. Thereforemolecular electr<strong>on</strong> acceptors with higher thermal stability (which means to a firstapproximati<strong>on</strong> a higher molecular weight) are desired. Thus the <str<strong>on</strong>g>disc</str<strong>on</strong>g>-<str<strong>on</strong>g>shaped</str<strong>on</strong>g> moleculeHATCN () is a promising novel molecular acceptor material which couldbe used to reduce the hole injecti<strong>on</strong> barrier <str<strong>on</strong>g>of</str<strong>on</strong>g> metals by increasing the work functi<strong>on</strong> to agreat extent.23
3 Film‐substrate systemsFig. 3.2: Schematic views <str<strong>on</strong>g>of</str<strong>on</strong>g> the HATCN molecule. Left: Simple ball model <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN, carb<strong>on</strong>atoms in grey <str<strong>on</strong>g>and</str<strong>on</strong>g> nitrogen atoms in blue. Right: Chemical structure <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN showing thec<strong>on</strong>jugati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the heterocycles which defines the chemical <str<strong>on</strong>g>and</str<strong>on</strong>g> physical properties <str<strong>on</strong>g>of</str<strong>on</strong>g> this molecule. Inred: diameter including van der Waals radii.Fig. 3.3: View <str<strong>on</strong>g>of</str<strong>on</strong>g> the 3D bulk structure <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN. From [58].24
3 Film‐substrate systems3.3 Substrates3.3.1 The mica (001) surfaceFor all the experiments presented in chapter 5 Hexaphenyl <strong>on</strong> mica (001) muscovite micawith the chemical formulahas been used. A detailed descripti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the mica crystal structure can be found in refs [77,80, 81].The mica was available in sheets <str<strong>on</strong>g>of</str<strong>on</strong>g> several square centimeters. After cutting out a quadraticpiece <str<strong>on</strong>g>of</str<strong>on</strong>g>the mica was cleaved under ambient atmosphere c<strong>on</strong>diti<strong>on</strong>s with ascotch tape <str<strong>on</strong>g>and</str<strong>on</strong>g> quickly transferred into the ultra-high vacuum chamber. In the secti<strong>on</strong> 4.3.2Experimental implementati<strong>on</strong> the mounting <str<strong>on</strong>g>of</str<strong>on</strong>g> the mica substrates <strong>on</strong>to the sample holder isdescribed in detail.3.3.2 The KCl (001) surfaceFig. 3.4: Na-type KCl structure.25
3 Film‐substrate systemsPotassium chloride has a crystalline structure <str<strong>on</strong>g>like</str<strong>on</strong>g> many other salts. Its structure is facecenteredcubic (Fig. 3.4) with a lattice c<strong>on</strong>stant <str<strong>on</strong>g>of</str<strong>on</strong>g> about 6.3 Å [81]. The KCl specimens wereprepared by cleaving a KCl block into pieces <str<strong>on</strong>g>of</str<strong>on</strong>g>with a razor blade. Thesmall KCl plates were mounted <strong>on</strong>to the sample holder in a similar way as the mica samples.KCl is frequently used as a substrate in <str<strong>on</strong>g>organic</str<strong>on</strong>g> thin <str<strong>on</strong>g>film</str<strong>on</strong>g> investigati<strong>on</strong>s because it has anatomically well-defined surface. The surface has a four-fold symmetry which leads to a fourfoldsymmetry <str<strong>on</strong>g>of</str<strong>on</strong>g> the epitaxial relati<strong>on</strong>ships <str<strong>on</strong>g>of</str<strong>on</strong>g> the grown <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g>s.3.3.3 The Au (111) surfaceThe gold bulk crystal structure is face-centered-cubic (fcc) with a lattice c<strong>on</strong>stant <str<strong>on</strong>g>of</str<strong>on</strong>g>about 4.08 Å. The idealized Au (111) surface has a 6-fold symmetry (hexag<strong>on</strong>al).However, c<strong>on</strong>sidering the first <str<strong>on</strong>g>and</str<strong>on</strong>g> sec<strong>on</strong>d topmost atomic layers, the symmetry <str<strong>on</strong>g>of</str<strong>on</strong>g> the surfacestructure reduces to a 3-fold rotati<strong>on</strong>al symmetry. The inter-atomic distance <str<strong>on</strong>g>of</str<strong>on</strong>g> the ideal Au(111) surface is <str<strong>on</strong>g>and</str<strong>on</strong>g> the inter-row distance is .Gold is the <strong>on</strong>ly fcc metal whose (111) surface rec<strong>on</strong>structs [82]. The driving force behindthis rec<strong>on</strong>structi<strong>on</strong> is the relief <str<strong>on</strong>g>of</str<strong>on</strong>g> surface stress. Al<strong>on</strong>g the [1-10] directi<strong>on</strong> an average 4.2%c<strong>on</strong>tracti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the topmost surface layer with respect to the underlying layer is found. In otherwords, the topmost atomic layer features al<strong>on</strong>g the [1-10] directi<strong>on</strong> atomic rows where 23atoms align <strong>on</strong> the same length as 22 atoms in the underlying layer. This surfacerec<strong>on</strong>structi<strong>on</strong> can be explained by a rectangular (22 x √3) surface unit cell. More detailedinformati<strong>on</strong> <strong>on</strong> the rec<strong>on</strong>structi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the Au (111) surface can be found in refs [71, 82].3.3.4 The Ag (111) surfaceThe silver bulk crystal structure is very similar to gold, namely fcc with a lattice c<strong>on</strong>stant <str<strong>on</strong>g>of</str<strong>on</strong>g> ≈4.09 Å. The inter-atomic distance <str<strong>on</strong>g>of</str<strong>on</strong>g> the ideal Ag (111) surface is <str<strong>on</strong>g>and</str<strong>on</strong>g> theinter-row distance is. The idealized Ag (111) surface has a hexag<strong>on</strong>alsymmetry <str<strong>on</strong>g>and</str<strong>on</strong>g> does not exhibit a surface rec<strong>on</strong>structi<strong>on</strong>.26
4 Analytical methods & instrumentati<strong>on</strong>4 Analytical methods & instrumentati<strong>on</strong>Most <str<strong>on</strong>g>of</str<strong>on</strong>g> the experimental results shown in this work were p<str<strong>on</strong>g>rod</str<strong>on</strong>g>uced in our laboratory, exceptfor scanning electr<strong>on</strong> microscopy (SEM), X-ray diffracti<strong>on</strong> (XRD) <str<strong>on</strong>g>and</str<strong>on</strong>g> part <str<strong>on</strong>g>of</str<strong>on</strong>g> the atomicforce microscopy (AFM) measurements, which were carried out by our partner groups inBratislava, Graz <str<strong>on</strong>g>and</str<strong>on</strong>g> Leoben, respectively. While AFM was performed ex-situ under ambientatmosphere c<strong>on</strong>diti<strong>on</strong>s, thermal desorpti<strong>on</strong> spectroscopy (TDS), X-ray photoelectr<strong>on</strong>spectroscopy (XPS), Auger electr<strong>on</strong> spectroscopy (AES) <str<strong>on</strong>g>and</str<strong>on</strong>g> Kelvin probe (KP) experimentshad to be accomplished under ultra-high vacuum (UHV) c<strong>on</strong>diti<strong>on</strong>s. All these UHVexperiments were c<strong>on</strong>ducted in our laboratory.4.1 The UHV chamberA schematic view as well as a more detailed descripti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the chamber <str<strong>on</strong>g>and</str<strong>on</strong>g> the sample holdercan be found in ref [71]. The base pressure in the UHV chamber could reachafter bake-out. The UHV was generated <str<strong>on</strong>g>and</str<strong>on</strong>g> maintained by rotary pumps <str<strong>on</strong>g>and</str<strong>on</strong>g> turbo pumps<str<strong>on</strong>g>and</str<strong>on</strong>g> m<strong>on</strong>itored via a I<strong>on</strong>ivac IM 520 i<strong>on</strong>isati<strong>on</strong> gauge.The sample was placed <strong>on</strong> a turnable sample holder which was located in the center <str<strong>on</strong>g>of</str<strong>on</strong>g> thechamber. The sample holder could be cooled down to about via a liquid nitrogencooling finger attached to the sample holder. The above menti<strong>on</strong>ed surface probing devices<str<strong>on</strong>g>and</str<strong>on</strong>g> a sputter gun for specimen cleaning were arranged within a measuring plane <str<strong>on</strong>g>and</str<strong>on</strong>g> could beaddressed by rotating the sample holder.4.2 <str<strong>on</strong>g>Thin</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> preparati<strong>on</strong> by <str<strong>on</strong>g>organic</str<strong>on</strong>g> molecular beam epitaxy(OMBE)27
4 Analytical methods & instrumentati<strong>on</strong>The <str<strong>on</strong>g>organic</str<strong>on</strong>g> thin <str<strong>on</strong>g>film</str<strong>on</strong>g>s were prepared by <str<strong>on</strong>g>organic</str<strong>on</strong>g> molecular beam epitaxy (OMBE) within theUHV chamber described in the previous secti<strong>on</strong>. For this purpose a home-built evaporati<strong>on</strong>source c<strong>on</strong>taining a Knudsen-cell was applied, which has already been described in detail inrefs [71, 80, 95]. This source has so far <strong>on</strong>ly been used for para-quaterphenyl (p4P) <str<strong>on</strong>g>and</str<strong>on</strong>g> parahexaphenyl(p6P) evaporati<strong>on</strong>, but could also be used for the preparati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <str<strong>on</strong>g>film</str<strong>on</strong>g>sbecause the vaporizati<strong>on</strong> temperature <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <str<strong>on</strong>g>and</str<strong>on</strong>g> p6P did not differ too much from eachother. However, the HATCN powder as received c<strong>on</strong>tained residual solvents, which had to beeliminated before the HATCN source could be used for experiments. Details <strong>on</strong> the cleaningprocedure can be found in secti<strong>on</strong> 7.1 Purificati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <str<strong>on</strong>g>and</str<strong>on</strong>g> its cracking pattern.The <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness was m<strong>on</strong>itored via a quartz microbalance (Infic<strong>on</strong> Inc.) which waspermanently positi<strong>on</strong>ed about 30° <str<strong>on</strong>g>of</str<strong>on</strong>g>f the axis <str<strong>on</strong>g>of</str<strong>on</strong>g> the Knudsen source. The microbalanceallowed an in-situ c<strong>on</strong>trol <str<strong>on</strong>g>of</str<strong>on</strong>g> the evaporati<strong>on</strong> rate. For the calibrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the impingement rate<strong>on</strong> the sample, the latter had temporarily been replaced by a sec<strong>on</strong>d quartz microbalance, asdescribed in ref. [80, 95]. The geometric factor(4.1)( : frequency change <str<strong>on</strong>g>of</str<strong>on</strong>g> the microbalance replacing the sample, : frequency change <str<strong>on</strong>g>of</str<strong>on</strong>g>the permanently positi<strong>on</strong>ed microbalance) turned out to be , showing thatapproximately the same amount <str<strong>on</strong>g>of</str<strong>on</strong>g> material impinges <strong>on</strong> the sample as <strong>on</strong> the permanentlypositi<strong>on</strong>ed microbalance. There is a simple relati<strong>on</strong>ship between the absolute mass thickness<str<strong>on</strong>g>and</str<strong>on</strong>g> the frequency change <str<strong>on</strong>g>of</str<strong>on</strong>g> the microbalance(4.2)Which yields for given (integral mass sensitivity, given by the manufacturer [96]),(4.3)28
4 Analytical methods & instrumentati<strong>on</strong><str<strong>on</strong>g>and</str<strong>on</strong>g> a frequency <str<strong>on</strong>g>of</str<strong>on</strong>g>(4.4)(4.5)The numerical value in this simple relati<strong>on</strong> is slightly different from that in ref [80] becausethe frequency <str<strong>on</strong>g>of</str<strong>on</strong>g> the quartz microbalance has changed due to the huge number <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>organic</str<strong>on</strong>g>layers evaporated <strong>on</strong> the crystal during the last years. Using the molecular mass <str<strong>on</strong>g>of</str<strong>on</strong>g> a singlep6P or HATCN molecule ( certainmass thickness values inhave been obtained that are equivalent to a frequencychange <str<strong>on</strong>g>of</str<strong>on</strong>g> Based <strong>on</strong> the density values <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <str<strong>on</strong>g>and</str<strong>on</strong>g> HATCN( [58], <strong>on</strong>e can additi<strong>on</strong>ally calculatethe equivalent mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness. The corresp<strong>on</strong>ding values are summarized in Table 4.1.Moleculep6P 0.10HATCN 0.08Table 4.1: Values <str<strong>on</strong>g>of</str<strong>on</strong>g> mass thickness <str<strong>on</strong>g>and</str<strong>on</strong>g> mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness equivalent to a frequencychange <str<strong>on</strong>g>of</str<strong>on</strong>g> the quartz microbalance <str<strong>on</strong>g>of</str<strong>on</strong>g>up<strong>on</strong> depositi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a p6P or HATCN <str<strong>on</strong>g>film</str<strong>on</strong>g>.4.3 Thermal desorpti<strong>on</strong> spectroscopy (TDS)TDS is a surface science technique which can be used to study adsorpti<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> desorpti<strong>on</strong>properties <str<strong>on</strong>g>of</str<strong>on</strong>g> adsorbates <strong>on</strong> heated sample surfaces. After depositing a known amount <str<strong>on</strong>g>of</str<strong>on</strong>g> thematerial to be analyzed, the sample is heated with a c<strong>on</strong>stant heating rate <str<strong>on</strong>g>and</str<strong>on</strong>g> the partialpressures <str<strong>on</strong>g>of</str<strong>on</strong>g> atoms <str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> coming from the sample are measured.4.3.1 Theoretical background29
4 Analytical methods & instrumentati<strong>on</strong>The most comm<strong>on</strong>ly used approach for describing the thermal desorpti<strong>on</strong> rate <str<strong>on</strong>g>of</str<strong>on</strong>g> an adsorbedspecies is the Polanyi-Wigner equati<strong>on</strong> [97](4.6)This equati<strong>on</strong> describes the correlati<strong>on</strong> between the desorpti<strong>on</strong> rate (r des ) <str<strong>on</strong>g>and</str<strong>on</strong>g> the coverage(ϴ), temperature (T) <str<strong>on</strong>g>and</str<strong>on</strong>g> the so-called kinetic parameters ν, n <str<strong>on</strong>g>and</str<strong>on</strong>g> E des which will be explainedin detail in the following. Before that, it is necessary to menti<strong>on</strong> that this approach to thermaldesorpti<strong>on</strong> spectroscopy <strong>on</strong>ly holds if the pumping speed is c<strong>on</strong>stant <str<strong>on</strong>g>and</str<strong>on</strong>g> sufficiently high inorder to prohibit significant readsorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> desorbed particles <strong>on</strong> the sample.To start with, the parameter ν is the so-called pre-exp<strong>on</strong>ential factor or frequency factor (unitis s -1 ) <str<strong>on</strong>g>of</str<strong>on</strong>g> the desorpti<strong>on</strong> process with the order n. According to transiti<strong>on</strong> state theory (TST)[98, 99], this frequency factor can be expressed in terms <str<strong>on</strong>g>of</str<strong>on</strong>g> the molecular partiti<strong>on</strong> functi<strong>on</strong>sin the transiti<strong>on</strong> state ( <str<strong>on</strong>g>and</str<strong>on</strong>g> in the initial (adsorbed) phase for n<strong>on</strong>-associativemolecular desorpti<strong>on</strong>, calculated at temperature(4.7)Note that the degree <str<strong>on</strong>g>of</str<strong>on</strong>g> freedom al<strong>on</strong>g the reacti<strong>on</strong> coordinate (normal to surface) is omittedfrom the calculati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g>. In the case <str<strong>on</strong>g>of</str<strong>on</strong>g> a desorpti<strong>on</strong> process with no activati<strong>on</strong>barrier to adsorpti<strong>on</strong>, as it is usually the case for large <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g>, the transiti<strong>on</strong> state isequivalent to the final state <str<strong>on</strong>g>of</str<strong>on</strong>g> the molecule. The final state is the gas phase without this <strong>on</strong>edegree <str<strong>on</strong>g>of</str<strong>on</strong>g> freedom normal to the surface [100]. In comm<strong>on</strong> TDS experiments <str<strong>on</strong>g>of</str<strong>on</strong>g> small<str<strong>on</strong>g>molecules</str<strong>on</strong>g> (e.g. CO, N 2 , H 2 ), the ratio <str<strong>on</strong>g>of</str<strong>on</strong>g> the partiti<strong>on</strong> functi<strong>on</strong>s is comm<strong>on</strong>ly assumed to benearly unity (i.e. entropy is nearly the same in gas <str<strong>on</strong>g>and</str<strong>on</strong>g> adsorbed states), in which case theabove expressi<strong>on</strong> gives a pre-exp<strong>on</strong>ential factor <str<strong>on</strong>g>of</str<strong>on</strong>g> about for room-temperaturedesorpti<strong>on</strong>. However, for large <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> (in that case n-alkanes) recent moleculardynamics und transiti<strong>on</strong> state theory calculati<strong>on</strong>s [102] predict a significant increase <str<strong>on</strong>g>of</str<strong>on</strong>g> thepre-exp<strong>on</strong>ential factor with chain length, which is attributed to increasing differences betweenthe adsorbate <str<strong>on</strong>g>and</str<strong>on</strong>g> transiti<strong>on</strong> state entropies. This occurs because the adsorbed <str<strong>on</strong>g>molecules</str<strong>on</strong>g> arerelatively hindered in translati<strong>on</strong>al <str<strong>on</strong>g>and</str<strong>on</strong>g>/or rotati<strong>on</strong>al degrees <str<strong>on</strong>g>of</str<strong>on</strong>g> freedom compared to the gas30
4 Analytical methods & instrumentati<strong>on</strong>phase. Up<strong>on</strong> desorpti<strong>on</strong>, the vibrati<strong>on</strong>al modes associated with the hindered translati<strong>on</strong>s <str<strong>on</strong>g>and</str<strong>on</strong>g>rotati<strong>on</strong>s in the adsorbed state will be c<strong>on</strong>verted to free translati<strong>on</strong>al <str<strong>on</strong>g>and</str<strong>on</strong>g> rotati<strong>on</strong>al modes inthe gas phase. These free moti<strong>on</strong>s will c<strong>on</strong>tribute much more to the partiti<strong>on</strong> functi<strong>on</strong> than thevibrati<strong>on</strong>s in the adsorbed state <str<strong>on</strong>g>and</str<strong>on</strong>g> the ratio can be very large. An attempt to quantifythe translati<strong>on</strong>al <str<strong>on</strong>g>and</str<strong>on</strong>g> the rotati<strong>on</strong>al partiti<strong>on</strong> functi<strong>on</strong> for several n-alkanes (which can be seenas model systems for other large <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g>) has been undertaken by Tait et al. [100]:They assume that in the transiti<strong>on</strong> state the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> behave as a freely rotating 3D gas. Theyare c<strong>on</strong>sidered to be rigid rotors in the gas phase i.e. they have three translati<strong>on</strong>al <str<strong>on</strong>g>and</str<strong>on</strong>g> threerotati<strong>on</strong>al degrees <str<strong>on</strong>g>of</str<strong>on</strong>g> freedom. Up<strong>on</strong> adsorpti<strong>on</strong> at low temperature, all six degrees <str<strong>on</strong>g>of</str<strong>on</strong>g> freedombecome vibrati<strong>on</strong>al modes, the so-called hindered rotati<strong>on</strong>s. At the desorpti<strong>on</strong> temperatures,however, the corresp<strong>on</strong>ding energy barriers may be small compared to , so that thesemoti<strong>on</strong>s may be effectively ‘free’. This depends <strong>on</strong> the corrugati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the interacti<strong>on</strong>potential, a detailed knowledge <str<strong>on</strong>g>of</str<strong>on</strong>g> which would be required to determine the true nature <str<strong>on</strong>g>of</str<strong>on</strong>g> theadsorbate moti<strong>on</strong>. Since detailed knowledge <str<strong>on</strong>g>of</str<strong>on</strong>g> the interacti<strong>on</strong> potential is lacking, certainassumpti<strong>on</strong>s about the degrees <str<strong>on</strong>g>of</str<strong>on</strong>g> freedom <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in the adsorbed state have to bemade. Tait et al. calculate the pre-exp<strong>on</strong>ential factors for two extreme cases: byassuming immobile adsorbate <str<strong>on</strong>g>molecules</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> by assuming a corrugati<strong>on</strong>-free surface<str<strong>on</strong>g>and</str<strong>on</strong>g> find the experimentally obtained values for n-alkanes to lie between the calculated<str<strong>on</strong>g>and</str<strong>on</strong>g> , but outside the respective errors. The best fit to the experimental preexp<strong>on</strong>entialfactors is in fact gained if it is assumed that the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> behave translati<strong>on</strong>ally<str<strong>on</strong>g>like</str<strong>on</strong>g> a 2D gas, but all rotati<strong>on</strong> is prohibited. However, the authors <str<strong>on</strong>g>of</str<strong>on</strong>g> that work state that thismodel is not intended to provide an accurate physical picture <str<strong>on</strong>g>of</str<strong>on</strong>g> the desorpti<strong>on</strong> system since itis hard to imagine a real adsorbate/substrate system where the interacti<strong>on</strong> potential allowscompletely free translati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the adsorbate but no rotati<strong>on</strong> at the same temperature. Rathertheir aim is to illustrate that the large increase in pre-exp<strong>on</strong>ential factor <str<strong>on</strong>g>of</str<strong>on</strong>g> n-alkanes withchain length can be accounted for entirely by c<strong>on</strong>sidering the increase in rotati<strong>on</strong>al entropyavailable to the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in the gas-phase-<str<strong>on</strong>g>like</str<strong>on</strong>g> transiti<strong>on</strong> state.In c<strong>on</strong>clusi<strong>on</strong> <strong>on</strong>e can say that there is substantial experimental [21, 71, 100, 101] <str<strong>on</strong>g>and</str<strong>on</strong>g>theoretical [102, 103] evidence that the desorpti<strong>on</strong> rate <str<strong>on</strong>g>of</str<strong>on</strong>g> large <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> is in factdetermined by very large pre-exp<strong>on</strong>ential factors compared to the comm<strong>on</strong>ly used value <str<strong>on</strong>g>of</str<strong>on</strong>g>.31
4 Analytical methods & instrumentati<strong>on</strong>The desorpti<strong>on</strong> order, denoted as index n in eq. (4.6), determines the influence <str<strong>on</strong>g>of</str<strong>on</strong>g> the coverage<strong>on</strong> the shape <str<strong>on</strong>g>of</str<strong>on</strong>g> the TD-spectra. Typical values for n aren = 0: The desorpti<strong>on</strong> rate does not depend <strong>on</strong> the coverage, which is fulfilled for desorpti<strong>on</strong>from a homogeneous multilayer <str<strong>on</strong>g>film</str<strong>on</strong>g>. The desorpti<strong>on</strong> rate increases exp<strong>on</strong>entially with thetemperature. After all the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> from the multilayer have desorbed, there is a steepdescent in the spectrum. Therefore the peak maximum shifts to higher temperatures forincreasing coverage.n = 1: The desorpti<strong>on</strong> rate is proporti<strong>on</strong>al to the coverage ϴ. This holds for a limited reservoir<str<strong>on</strong>g>of</str<strong>on</strong>g> particles when single atoms or <str<strong>on</strong>g>molecules</str<strong>on</strong>g> desorb directly, separately <str<strong>on</strong>g>and</str<strong>on</strong>g> independentlyfrom their adsorpti<strong>on</strong> sites. The spectra feature a characteristic asymmetric peak shape. Inc<strong>on</strong>trast to zero order desorpti<strong>on</strong>, the peak maximum stays at the same temperature for anycoverage.n = 2: The desorpti<strong>on</strong> rate is proporti<strong>on</strong>al to ϴ 2 . This is a typical order for associativedesorpti<strong>on</strong> processes where the adsorbed atoms or <str<strong>on</strong>g>molecules</str<strong>on</strong>g> need to find a reacti<strong>on</strong> partner inorder to desorb from the surface (e.g. hydrogen <strong>on</strong> various substrates). The peak shape isnearly symmetric <str<strong>on</strong>g>and</str<strong>on</strong>g> the peak maximum shows a characteristic shifting to lowertemperatures with increasing coverage.The last kinetic parameter which has to be <str<strong>on</strong>g>disc</str<strong>on</strong>g>ussed is the desorpti<strong>on</strong> energy, denoted as E desin eq. (4.6). This is the energy that is needed to surmount the surface potential barrier <str<strong>on</strong>g>and</str<strong>on</strong>g> istherefore also <str<strong>on</strong>g>of</str<strong>on</strong>g>ten referred to as the activati<strong>on</strong> energy for desorpti<strong>on</strong>. The desorpti<strong>on</strong> energycan also be coverage-dependent, which is for instance the case if lateral interacti<strong>on</strong>s betweenthe adsorbed <str<strong>on</strong>g>molecules</str<strong>on</strong>g> exist. The interacti<strong>on</strong> energy between the adsorbate species can eitherbe repulsive or attractive, leading to a decrease or an increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the activati<strong>on</strong> energy,respectively. Examples for the influence <str<strong>on</strong>g>of</str<strong>on</strong>g> repulsive interacti<strong>on</strong>s can be found in theliterature <strong>on</strong> TDS <str<strong>on</strong>g>of</str<strong>on</strong>g> large <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> [71], where repulsive Van-der-Waalsinteracti<strong>on</strong>s are <str<strong>on</strong>g>like</str<strong>on</strong>g>ly to occur. The decrease <str<strong>on</strong>g>of</str<strong>on</strong>g> E des with increasing coverage leads to shifts inthe spectra to lower temperatures in the m<strong>on</strong>olayer regime. Although this is characteristic forsec<strong>on</strong>d order desorpti<strong>on</strong> kinetics, the desorpti<strong>on</strong> order in this case is still .As the last part <str<strong>on</strong>g>of</str<strong>on</strong>g> the theoretical secti<strong>on</strong> <strong>on</strong> TDS some st<str<strong>on</strong>g>and</str<strong>on</strong>g>ard procedures that were used todirectly obtain the kinetic parameters from a single or a series <str<strong>on</strong>g>of</str<strong>on</strong>g> measured spectra arepresented.32
4 Analytical methods & instrumentati<strong>on</strong>• Leading edge analysisThis method allows the determinati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> coverage- <str<strong>on</strong>g>and</str<strong>on</strong>g> temperature-dependentactivati<strong>on</strong> parameters. To keep the temperature <str<strong>on</strong>g>and</str<strong>on</strong>g> the coverage c<strong>on</strong>stant, a smallsecti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the spectrum <strong>on</strong> its leading edge is selected [104]. This part <str<strong>on</strong>g>of</str<strong>on</strong>g> the spectrumis then plotted asequati<strong>on</strong>vs. , according to the logarithmic Polanyi-Wigner(4.8)From the slope <str<strong>on</strong>g>of</str<strong>on</strong>g> this Arrhenius plot E des can be determined. From the intercept withthe ordinate the frequency factor can be determined provided that reacti<strong>on</strong> order <str<strong>on</strong>g>and</str<strong>on</strong>g>coverage are known. For this purpose it is necessary to calibrate the y-ordinate <str<strong>on</strong>g>of</str<strong>on</strong>g> thespectra correctly. This method is applied to zero order desorpti<strong>on</strong> spectra <str<strong>on</strong>g>and</str<strong>on</strong>g> can alsobe applied to first order desorpti<strong>on</strong> spectra in the regi<strong>on</strong>, where the desorpti<strong>on</strong> rateincreases exp<strong>on</strong>entially (analysis.), i.e. if <strong>on</strong>e <strong>on</strong>ly uses the leading edge for• Analysis according to RedheadA rather simple relati<strong>on</strong> between the kinetic parameters was proposed by Redhead[97]: If desorpti<strong>on</strong> follows first order kinetics <str<strong>on</strong>g>and</str<strong>on</strong>g> the parameters are independent <str<strong>on</strong>g>of</str<strong>on</strong>g>the coverage, the desorpti<strong>on</strong> energy can be approximated by(4.9)The sec<strong>on</strong>d part in the brackets is small relative to the first <str<strong>on</strong>g>and</str<strong>on</strong>g> is estimated as. is the temperature at the peak maximum <str<strong>on</strong>g>and</str<strong>on</strong>g> is thefrequency factor for first order desorpti<strong>on</strong>, which has to be known. If the frequencyfactor for first order desorpti<strong>on</strong> is unknown, customary approaches for are either to33
4 Analytical methods & instrumentati<strong>on</strong>take the comm<strong>on</strong>ly chosen value <str<strong>on</strong>g>of</str<strong>on</strong>g>, which is, as menti<strong>on</strong>ed before, notrecommendable for large <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g>, or to assume <str<strong>on</strong>g>and</str<strong>on</strong>g> insert the value<str<strong>on</strong>g>of</str<strong>on</strong>g> the frequency factor for zero order desorpti<strong>on</strong>, which is easily gained by the abovementi<strong>on</strong>ed leading edge analysis. However, in systems where the multilayer <str<strong>on</strong>g>growth</str<strong>on</strong>g> issignificantly different from the m<strong>on</strong>olayer <str<strong>on</strong>g>growth</str<strong>on</strong>g>, this assumpti<strong>on</strong> does not have to becorrect: In the case <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <strong>on</strong> Au (111) it turned out that the frequency factors <str<strong>on</strong>g>of</str<strong>on</strong>g>the first m<strong>on</strong>olayer <str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> the multilayer differ by six orders <str<strong>on</strong>g>of</str<strong>on</strong>g> magnitude (seesecti<strong>on</strong>s 5.2 & 5.3).• Heating rate variati<strong>on</strong> methodIf a reliable approximati<strong>on</strong> for is not available or possible, a more detailed analysisthan the above presented is inevitable. The heating rate variati<strong>on</strong> method is based <strong>on</strong>the collecti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a series <str<strong>on</strong>g>of</str<strong>on</strong>g> spectra for the same adsorbate coverage utilizing differentheating rates [97]. Starting with eq. (4.6), <strong>on</strong>e can get the following relati<strong>on</strong> for firstorder desorpti<strong>on</strong>:(4.10)Taking the logarithm <str<strong>on</strong>g>and</str<strong>on</strong>g> rearrangement yields(4.11)Plotting <str<strong>on</strong>g>of</str<strong>on</strong>g> vs. for a series <str<strong>on</strong>g>of</str<strong>on</strong>g> -values thus yields E des fromthe slope <str<strong>on</strong>g>and</str<strong>on</strong>g> from the intercept with the ordinate.4.3.2 Experimental implementati<strong>on</strong>After the preparati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> thin <str<strong>on</strong>g>film</str<strong>on</strong>g>s in ultra-high vacuum (UHV) with a well-known thickness<strong>on</strong> the used samples, they were heated by resistive heating. The single crystals Au (111) <str<strong>on</strong>g>and</str<strong>on</strong>g>34
4 Analytical methods & instrumentati<strong>on</strong>Ag (111) used in this work exhibited a circular groove (<str<strong>on</strong>g>like</str<strong>on</strong>g> shown in Fig. 4.1 <strong>on</strong> the right)running around their side walls, where the Ta wires could be wound around for mountingpurposes. The mica <str<strong>on</strong>g>and</str<strong>on</strong>g> KCl sheets were heated by pinning them to a steel plate (Fig. 4.1 <strong>on</strong>the left), which was also heated by resistive heating. The temperature was in all casesmeasured by a Ni/NiCr thermocouple spot-welded <strong>on</strong> the backside <str<strong>on</strong>g>of</str<strong>on</strong>g> the single crystals or <strong>on</strong>the back side <str<strong>on</strong>g>of</str<strong>on</strong>g> the steel plate <str<strong>on</strong>g>and</str<strong>on</strong>g> c<strong>on</strong>trolled by a LABVIEW program, which allowedmaintaining a c<strong>on</strong>stant temperature or a linear heating <str<strong>on</strong>g>of</str<strong>on</strong>g> the samples. Details <strong>on</strong> thepeculiarity <str<strong>on</strong>g>of</str<strong>on</strong>g> doing TDS with samples with poor thermal c<strong>on</strong>ductivity <str<strong>on</strong>g>like</str<strong>on</strong>g> mica can be foundin chapter 5 Hexaphenyl <strong>on</strong> mica (001).Fig. 4.1: Heating <str<strong>on</strong>g>of</str<strong>on</strong>g> the samples either by pinning to a heated steel plate (left) or by direct resistiveheating (right). Temperature in both cases measured with a Ni/NiCr thermocouple.TDS was performed by desorbing the adsorbed <str<strong>on</strong>g>molecules</str<strong>on</strong>g> into a multiplexed line-<str<strong>on</strong>g>of</str<strong>on</strong>g>-sightquadrupole mass spectrometer (QMS), which was tuned to the most relevant masses. TheQMS 400 (by Pfeiffer Inc.) was equipped with a magnetic field enhanced cross-beam i<strong>on</strong>source <str<strong>on</strong>g>and</str<strong>on</strong>g> could detect a mass range <str<strong>on</strong>g>of</str<strong>on</strong>g>. For both used <str<strong>on</strong>g>molecules</str<strong>on</strong>g> (p6P,HATCN) it was observed that the mass <str<strong>on</strong>g>of</str<strong>on</strong>g> the intact molecule did not show the highest QMSsignal <str<strong>on</strong>g>and</str<strong>on</strong>g> therefore not the best signal to noise ratio. Usually, better signals were found fordistinctive masses much smaller than the mass <str<strong>on</strong>g>of</str<strong>on</strong>g> the intact molecule. The reas<strong>on</strong> for that iseither cracking <str<strong>on</strong>g>of</str<strong>on</strong>g> the intact <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in the QMS during the i<strong>on</strong>izati<strong>on</strong> process (the usedi<strong>on</strong>izati<strong>on</strong> energy was rather high:, which led to a str<strong>on</strong>g dissociati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the large<str<strong>on</strong>g>molecules</str<strong>on</strong>g>) or cracking <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> <strong>on</strong> the surface during the desorpti<strong>on</strong> process. The35
4 Analytical methods & instrumentati<strong>on</strong>latter will occur if str<strong>on</strong>g interacti<strong>on</strong>s between the adsorbed <str<strong>on</strong>g>molecules</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> the substratesurface exist. In chapter 7 HATCN <strong>on</strong> Ag (111) an example <str<strong>on</strong>g>of</str<strong>on</strong>g> the observati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> bothcracking processes is given <str<strong>on</strong>g>and</str<strong>on</strong>g> it is shown how the two cracking processes can easily bedistinguished.4.4 X-ray photoelectr<strong>on</strong> spectroscopy (XPS)One <str<strong>on</strong>g>of</str<strong>on</strong>g> the most frequently used methods to probe the chemical compositi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a samplesurface is XPS, <str<strong>on</strong>g>of</str<strong>on</strong>g>ten also denoted as electr<strong>on</strong> spectroscopy for chemical analysis (ESCA).This method involves irradiati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a solid in the vacuum with m<strong>on</strong>oenergetic s<str<strong>on</strong>g>of</str<strong>on</strong>g>t X-rays <str<strong>on</strong>g>and</str<strong>on</strong>g>sorting the emitted photoelectr<strong>on</strong>s by their kinetic energy.(4.12)ϕ s is the work functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the spectrometer. The kinetic energy<str<strong>on</strong>g>of</str<strong>on</strong>g> photoelectr<strong>on</strong>semitted up<strong>on</strong> irradiati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the sample with phot<strong>on</strong>s with a known energy <str<strong>on</strong>g>of</str<strong>on</strong>g> depends <strong>on</strong>the binding energy <str<strong>on</strong>g>of</str<strong>on</strong>g> the involved photoelectr<strong>on</strong> state <str<strong>on</strong>g>and</str<strong>on</strong>g> is therefore characteristicfor each element <str<strong>on</strong>g>and</str<strong>on</strong>g> thus allows to identify the comp<strong>on</strong>ents <str<strong>on</strong>g>of</str<strong>on</strong>g> the sample surface. Thefundamental process <str<strong>on</strong>g>of</str<strong>on</strong>g> generating photoelectr<strong>on</strong>s is illustrated in Fig. 4.2. The spectrumobtained is a plot <str<strong>on</strong>g>of</str<strong>on</strong>g> the number <str<strong>on</strong>g>of</str<strong>on</strong>g> emitted electr<strong>on</strong>s as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> their kinetic energy.Further details <strong>on</strong> this technique can be found in the literature [105-107].36
4 Analytical methods & instrumentati<strong>on</strong>Fig. 4.2: Schematic view <str<strong>on</strong>g>of</str<strong>on</strong>g> the X-ray induced photoelectr<strong>on</strong> (left) <str<strong>on</strong>g>and</str<strong>on</strong>g> the electr<strong>on</strong> beam inducedAuger electr<strong>on</strong> (right) emissi<strong>on</strong>. From [108].For the realizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the XPS experiments, a combined XPS/AES system by Leybold Inc.c<strong>on</strong>taining a c<strong>on</strong>centric hemispherical analyzer (CHA) was used. The X-ray source could beswitched between Mg K α ( ) <str<strong>on</strong>g>and</str<strong>on</strong>g> Al K α ( ) radiati<strong>on</strong>. St<str<strong>on</strong>g>and</str<strong>on</strong>g>ard operatingparameters were a source voltage <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> an emissi<strong>on</strong> current <str<strong>on</strong>g>of</str<strong>on</strong>g> .4.5 Auger electr<strong>on</strong> spectroscopy (AES)The Auger electr<strong>on</strong> spectroscopy technique for chemical analysis <str<strong>on</strong>g>of</str<strong>on</strong>g> surfaces is based <strong>on</strong> theAuger radiati<strong>on</strong>less process. When a core level <str<strong>on</strong>g>of</str<strong>on</strong>g> a surface atom is i<strong>on</strong>ized by an impingingelectr<strong>on</strong> beam, the atom may decay to a lower energy state through an electr<strong>on</strong>icrearrangement. The energy difference <str<strong>on</strong>g>of</str<strong>on</strong>g> these two states is given to the ejected Augerelectr<strong>on</strong> which has a kinetic energy characteristic <str<strong>on</strong>g>of</str<strong>on</strong>g> the parent atom. The kinetic energy canbe approximated in the following way(4.13)ϕ s is the work functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the spectrometer. E K is the primarily i<strong>on</strong>ized electr<strong>on</strong> level, E L1 <str<strong>on</strong>g>and</str<strong>on</strong>g>E L2 are the higher energy levels that are involved in the process. An illustrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> this processis shown in Fig. 4.2. More details <strong>on</strong> this well-known technique can be found in refs [105-110].The electr<strong>on</strong> gun for AES was operated with an emissi<strong>on</strong> current <str<strong>on</strong>g>of</str<strong>on</strong>g><str<strong>on</strong>g>and</str<strong>on</strong>g> the beamvoltage was usually in the range <str<strong>on</strong>g>of</str<strong>on</strong>g> to . As menti<strong>on</strong>ed in secti<strong>on</strong> (XPS), aCHA was used to analyze the kinetic energy <str<strong>on</strong>g>of</str<strong>on</strong>g> the sec<strong>on</strong>dary electr<strong>on</strong>s coming from thesample. A lock-in amplifier was used to uncover the Auger electr<strong>on</strong>s <str<strong>on</strong>g>and</str<strong>on</strong>g> to attenuate theusually high background in the sec<strong>on</strong>dary electr<strong>on</strong> spectrum.4.6 Atomic force microscopy (AFM)The atomic force microscope is a very useful tool for imaging very small areas <str<strong>on</strong>g>of</str<strong>on</strong>g> all types <str<strong>on</strong>g>of</str<strong>on</strong>g>surfaces. Un<str<strong>on</strong>g>like</str<strong>on</strong>g> scanning tunneling microscopy (STM), AFM is not restricted to electricallyc<strong>on</strong>ducting surfaces [107, 111]. In this technique, the probe is a small sharp tip which is37
4 Analytical methods & instrumentati<strong>on</strong>scanned very closely across the sample surface. The distance between the tip <str<strong>on</strong>g>and</str<strong>on</strong>g> the sampleis so small that atomic-range forces act between them. In order to measure these forces, the tipis attached to the end <str<strong>on</strong>g>of</str<strong>on</strong>g> a cantilever. The deflecti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the cantilever, which depends <strong>on</strong> theforces acting <strong>on</strong> the tip, can be determined using a laser beam deflecti<strong>on</strong> system (see Fig. 4.3).AFMs can be used in so-called static <str<strong>on</strong>g>and</str<strong>on</strong>g> dynamic operating modes, which are also <str<strong>on</strong>g>of</str<strong>on</strong>g>tendenoted as c<strong>on</strong>tact <str<strong>on</strong>g>and</str<strong>on</strong>g> tapping mode, respectively. In order to image s<str<strong>on</strong>g>of</str<strong>on</strong>g>t <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g>s, then<strong>on</strong>-c<strong>on</strong>tact tapping mode was utilized, because the forces acting <strong>on</strong> the presumably s<str<strong>on</strong>g>of</str<strong>on</strong>g>t <str<strong>on</strong>g>film</str<strong>on</strong>g>sduring c<strong>on</strong>tact-mode AFM could influence their structure.In the tapping mode, the cantilever is driven to oscillate up <str<strong>on</strong>g>and</str<strong>on</strong>g> down at a frequency near itsres<strong>on</strong>ance frequency using a piezo element. The interacti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> forces acting <strong>on</strong> the cantileverwhen the tip comes close to the surface (van der Waals force, dipole-dipole interacti<strong>on</strong>,electrostatic forces, etc) causes the amplitude <str<strong>on</strong>g>of</str<strong>on</strong>g> this oscillati<strong>on</strong> to decrease as the tip getscloser to the sample. The vibrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the cantilever is detected using the laser beamdeflecti<strong>on</strong> system. The measured laser beam deflecti<strong>on</strong> or cantilever vibrati<strong>on</strong> amplitude cannow be used as an input for a feedback loop that keeps the tip-sample interacti<strong>on</strong> (i.e.cantilever oscillati<strong>on</strong> amplitude) c<strong>on</strong>stant by changing the tip height as the cantilever isscanned over the sample. The output <str<strong>on</strong>g>of</str<strong>on</strong>g> this feedback loop thus corresp<strong>on</strong>ds to the localsample height. Further details <strong>on</strong> the dynamic operating mode can be found in refs [112, 113].Fig. 4.3: The principle <str<strong>on</strong>g>of</str<strong>on</strong>g> atomic force microscopy (AFM). From [114].38
4 Analytical methods & instrumentati<strong>on</strong>The AFM measurements presented in this work were carried out in tapping mode underambient atmosphere c<strong>on</strong>diti<strong>on</strong>s, which inhibited even close to atomic resoluti<strong>on</strong>. However,the obtained resoluti<strong>on</strong> was good enough for imaging the rather large structures formed by the<str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g>. Most <str<strong>on</strong>g>of</str<strong>on</strong>g> the AFM investigati<strong>on</strong>s were d<strong>on</strong>e utilizing a Nanosurf easyScan2 AFM system. Early AFM measurements (all p6P <strong>on</strong> mica images) were performed with aNanoscope Multimode IIIa scanning probe microscope (Digital Instruments Inc., SantaBarbara, CA) equipped with an AS-130(J) scanner. This instrument was located at theInstitute <str<strong>on</strong>g>of</str<strong>on</strong>g> Physics at the University <str<strong>on</strong>g>of</str<strong>on</strong>g> Leoben. In both cases, c<strong>on</strong>venti<strong>on</strong>al Si-tips were used.4.7 Kelvin Probe (KP)The Kelvin Probe is a n<strong>on</strong>-c<strong>on</strong>tact, n<strong>on</strong>-destructive vibrating capacitor device used to measurethe work functi<strong>on</strong> difference between a c<strong>on</strong>ducting specimen <str<strong>on</strong>g>and</str<strong>on</strong>g> a vibrating tip [115-119].When two materials with different work functi<strong>on</strong>s are brought together, electr<strong>on</strong>s in thematerial with the lower work functi<strong>on</strong> flow to the <strong>on</strong>e with the higher work functi<strong>on</strong>. If thesematerials are made into a parallel plate capacitor, equal <str<strong>on</strong>g>and</str<strong>on</strong>g> opposite surface charges form.The voltage developed over this capacitor is called the c<strong>on</strong>tact potential, which is thedifference between the work functi<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> the participating materials, <str<strong>on</strong>g>and</str<strong>on</strong>g> measuring it is d<strong>on</strong>eby applying an external backing potential to the capacitor until the surface charges disappear.At that point the backing potential will equal the c<strong>on</strong>tact potential. The tip <str<strong>on</strong>g>and</str<strong>on</strong>g> the sample d<strong>on</strong>ot need to touch each other <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>on</strong>ly very weak electric fields are required, which are not<str<strong>on</strong>g>like</str<strong>on</strong>g>ly to influence the electrical or chemical structure <str<strong>on</strong>g>of</str<strong>on</strong>g> the material.As explained above, the output <str<strong>on</strong>g>of</str<strong>on</strong>g> a Kelvin Probe measurement is not the work functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> thesample itself, but the work functi<strong>on</strong> difference between the tip <str<strong>on</strong>g>and</str<strong>on</strong>g> the specimen. In order toobtain the absolute work functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the sample thus the work functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the tip has to beknown. A st<str<strong>on</strong>g>and</str<strong>on</strong>g>ard procedure to determine the work functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the vibrating tip utilizing agadolinium foil is presented in refs [120, 121].All the KP measurements were carried out under UHV c<strong>on</strong>diti<strong>on</strong>s with a built-in KelvinProbe (KP Technology Inc.) equipped with a stainless steel tip. A photograph <str<strong>on</strong>g>of</str<strong>on</strong>g> the UHVKelvin Probe is shown in Fig. 4.4.39
4 Analytical methods & instrumentati<strong>on</strong>Fig. 4.4: Photograph <str<strong>on</strong>g>of</str<strong>on</strong>g> the UHV Kelvin Probe.40
5 Hexaphenyl <strong>on</strong> mica (001)5 Hexaphenyl <strong>on</strong> mica (001)This chapter presents a detailed survey <str<strong>on</strong>g>of</str<strong>on</strong>g> the adsorpti<strong>on</strong>/desorpti<strong>on</strong> kinetics, structure, <str<strong>on</strong>g>growth</str<strong>on</strong>g><str<strong>on</strong>g>and</str<strong>on</strong>g> morphology <str<strong>on</strong>g>of</str<strong>on</strong>g> ultra-thin p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>s <strong>on</strong> muscovite mica (001). Frequently for thedescripti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the hexag<strong>on</strong>al mica surface instead <str<strong>on</strong>g>of</str<strong>on</strong>g> Miller indices the Bravais-Miller indicesare used. In that case four axes are used <str<strong>on</strong>g>and</str<strong>on</strong>g> four Miller indices (hkil → mica (0001)), inwhich the parameters h,k,i are cyclically permutable <str<strong>on</strong>g>and</str<strong>on</strong>g> related by. As itturned out, the existence <str<strong>on</strong>g>of</str<strong>on</strong>g> a so-called ‘wetting layer’ affects the further <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> in atremendous way. In this work, the term ’wetting layer‘ is used when there exists am<strong>on</strong>omolecular m<strong>on</strong>olayer <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> lying flat <strong>on</strong> the surface. A very comm<strong>on</strong> way toinspect the m<strong>on</strong>olayer regime <str<strong>on</strong>g>and</str<strong>on</strong>g> thus to check whether a m<strong>on</strong>olayer exists or not is TDS.Additi<strong>on</strong>al informati<strong>on</strong> <strong>on</strong> the initial thin <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> can be gained by combining XPS withTDS, i.e. m<strong>on</strong>itoring the change <str<strong>on</strong>g>of</str<strong>on</strong>g> the XPS signal during heating up the sample. Themorphology <str<strong>on</strong>g>of</str<strong>on</strong>g> thicker <str<strong>on</strong>g>film</str<strong>on</strong>g>s was investigated by AFM <str<strong>on</strong>g>and</str<strong>on</strong>g> scanning electr<strong>on</strong> microscopy(SEM).The chemical <str<strong>on</strong>g>and</str<strong>on</strong>g> structural c<strong>on</strong>diti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the mica substrates also has an enormous effect <strong>on</strong>the thin <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>. In the following, thin <str<strong>on</strong>g>film</str<strong>on</strong>g>s will be characterized which were grown <strong>on</strong>clean mica (5.2-5.4), <strong>on</strong> carb<strong>on</strong> c<strong>on</strong>taminated mica (5.5) <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>on</strong> sputtered mica (5.6). XPS <str<strong>on</strong>g>and</str<strong>on</strong>g>low energy electr<strong>on</strong> diffracti<strong>on</strong> (LEED) were used to characterize the mica surface before the<str<strong>on</strong>g>film</str<strong>on</strong>g> depositi<strong>on</strong>.5.1 Calibrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the TD spectraAs shown in secti<strong>on</strong> 4.3.2, the mica sheets were mounted <strong>on</strong> a heated steel plate, <strong>on</strong> the backside <str<strong>on</strong>g>of</str<strong>on</strong>g> which the temperature was measured. A drawback <str<strong>on</strong>g>of</str<strong>on</strong>g> this experimental setup was thatbecause <str<strong>on</strong>g>of</str<strong>on</strong>g> the poor thermal c<strong>on</strong>ductivity <str<strong>on</strong>g>of</str<strong>on</strong>g> mica, the measured temperature did not representthe temperature <strong>on</strong> the mica surface properly <str<strong>on</strong>g>and</str<strong>on</strong>g> therefore the temperature scale had to becorrected. An uncorrected TD-spectrum for a p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> with thickness is shown inFig. 5.1. There exist two desorpti<strong>on</strong> peaks, <strong>on</strong>e due to the desorpti<strong>on</strong> from the tantalum wires,which were used to fix the sample to the steel plate (see insert in Fig. 5.1), which are also41
5 Hexaphenyl <strong>on</strong> mica (001)covered with p6P during the evaporati<strong>on</strong> process, <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>on</strong>e due to the desorpti<strong>on</strong> from mica.As a first approximati<strong>on</strong>, it is assumed that the multilayer desorpti<strong>on</strong> peak should beindependent <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate <str<strong>on</strong>g>and</str<strong>on</strong>g> hence desorpti<strong>on</strong> from tantalum should occur in the sametemperature range as desorpti<strong>on</strong> from mica. Therefore the temperature scale was correctedutilizing a linear regressi<strong>on</strong> between two characteristic temperatures. At the adsorpti<strong>on</strong>temperature <str<strong>on</strong>g>of</str<strong>on</strong>g>both materials will be at the same temperature. When heatingup, the mica sample temperature will increase more slowly than that <str<strong>on</strong>g>of</str<strong>on</strong>g> the steel plate <str<strong>on</strong>g>and</str<strong>on</strong>g> thiswill result in a deviati<strong>on</strong> which increases during heating. The sec<strong>on</strong>d characteristictemperature is the starting point <str<strong>on</strong>g>of</str<strong>on</strong>g> the multilayer desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P from mica. Keeping inmind the assumpti<strong>on</strong> above, the measured(where desorpti<strong>on</strong> from mica starts) can beassigned to (where desorpti<strong>on</strong> from the tantalum wires starts). This fits quite well top6P multilayer desorpti<strong>on</strong> from another metal surface, namely Au (111) [20, 21, 71]. All TDspectrashown in the following secti<strong>on</strong>s are displayed with the corrected temperature scale.Fig. 5.1: TD-spectrum <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <strong>on</strong> mica (001), adsorpti<strong>on</strong> temperature . Thisspectrum was used to calibrate the temperature scale. The insert shows the sample mounting.5.2 Freshly air-cleaved mica (001)42
5 Hexaphenyl <strong>on</strong> mica (001)The surface compositi<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> structure <str<strong>on</strong>g>of</str<strong>on</strong>g> a freshly cleaved mica (001) surface, after quicklytransferring it into the UHV chamber <str<strong>on</strong>g>and</str<strong>on</strong>g> pumping down, was checked by XPS <str<strong>on</strong>g>and</str<strong>on</strong>g> LEED,respectively. LEED showed the well-known 6-fold symmetric pattern without spot-splitting(Fig. 5.2). Such a spot-splitting has been observed <strong>on</strong>ly for vacuum cleaved mica [122, 123].Fig. 5.2: LEED pattern <str<strong>on</strong>g>of</str<strong>on</strong>g> freshly cleaved mica; taken at .XPS revealed that the surface was not completely clean. The spectra shown in Fig. 5.3 <str<strong>on</strong>g>and</str<strong>on</strong>g>Fig. 5.4 stem from a freshly air-cleaved mica surface, <strong>on</strong> which, in agreement with previousAES investigati<strong>on</strong>s [124] <str<strong>on</strong>g>of</str<strong>on</strong>g> air-cleaved mica, some amount <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> can be observed. It wasfound that there is carb<strong>on</strong> <strong>on</strong> the surface whenever mica is cleaved in air. The supposed [124]reas<strong>on</strong> for that is an adsorpti<strong>on</strong> process which requires carb<strong>on</strong> compounds <str<strong>on</strong>g>and</str<strong>on</strong>g> water. Theamount <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> c<strong>on</strong>taminati<strong>on</strong> may depend <strong>on</strong> the time it takes to get the mica into (ultra)high vacuum c<strong>on</strong>diti<strong>on</strong>s after the cleavage in air. An attempt to quantify this amount will be<str<strong>on</strong>g>disc</str<strong>on</strong>g>ussed in secti<strong>on</strong> 5.5 p6P <strong>on</strong> carb<strong>on</strong> c<strong>on</strong>taminated mica (001).43
5 Hexaphenyl <strong>on</strong> mica (001)Fig. 5.3: XPS overview spectrum <str<strong>on</strong>g>of</str<strong>on</strong>g> mica after cleaving in air <str<strong>on</strong>g>and</str<strong>on</strong>g> subsequent pump-down <str<strong>on</strong>g>of</str<strong>on</strong>g> thechamber. The C1s peak can clearly be seen even in the overview.Fig. 5.4: Detail scan <str<strong>on</strong>g>of</str<strong>on</strong>g> Fig. 5.3 to enlarge the C1s peak.44
5 Hexaphenyl <strong>on</strong> mica (001)5.3 p6P <strong>on</strong> freshly cleaved mica (001) – m<strong>on</strong>olayer regimeBy using TDS it was possible to investigate the ultra thin p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>s in the sub m<strong>on</strong>olayerregime. In Fig. 5.5 <strong>on</strong>e can see a TDS series <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>s <strong>on</strong> mica (001) with different <str<strong>on</strong>g>film</str<strong>on</strong>g>thickness. Due to the high i<strong>on</strong>izati<strong>on</strong> energy <str<strong>on</strong>g>of</str<strong>on</strong>g> the QMS the QMS-signal intensity <str<strong>on</strong>g>of</str<strong>on</strong>g> thecracking p<str<strong>on</strong>g>rod</str<strong>on</strong>g>ucts is usually higher than the QMS-signal <str<strong>on</strong>g>of</str<strong>on</strong>g> the original mass <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <str<strong>on</strong>g>and</str<strong>on</strong>g>am<strong>on</strong>g all the cracking p<str<strong>on</strong>g>rod</str<strong>on</strong>g>ucts <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P the mass 61 yielded the highest signal <str<strong>on</strong>g>and</str<strong>on</strong>g> thereforethe best signal to noise ratio. Therefore in the m<strong>on</strong>olayer regime, where the QMS signals arevery small, the signal <str<strong>on</strong>g>of</str<strong>on</strong>g> the mass 61 is shown to represent the desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the p6P<str<strong>on</strong>g>molecules</str<strong>on</strong>g>. The amount <str<strong>on</strong>g>of</str<strong>on</strong>g> deposited <str<strong>on</strong>g>molecules</str<strong>on</strong>g> is denoted by means <str<strong>on</strong>g>of</str<strong>on</strong>g> the nominal mean <str<strong>on</strong>g>film</str<strong>on</strong>g>thickness, as measured by the quartz microbalance.Fig. 5.5: TDS series <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P grown <strong>on</strong> clean mica (001) at. The m<strong>on</strong>olayer <str<strong>on</strong>g>and</str<strong>on</strong>g>multilayer desorpti<strong>on</strong> peaks are denoted by β <str<strong>on</strong>g>and</str<strong>on</strong>g> α, respectively. The total amount <str<strong>on</strong>g>of</str<strong>on</strong>g> deposited p6Pfor each single TD-spectrum is represented as the mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness in .Starting at a nominal mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness <str<strong>on</strong>g>of</str<strong>on</strong>g>, a peak which is labelled β arises atapproximately. This peak keeps increasing with increasing p6P depositi<strong>on</strong> until at a45
5 Hexaphenyl <strong>on</strong> mica (001)<str<strong>on</strong>g>film</str<strong>on</strong>g> thickness <str<strong>on</strong>g>of</str<strong>on</strong>g> abouta saturati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the β peak can be observed <str<strong>on</strong>g>and</str<strong>on</strong>g> another peakarises around , which is labelled α. This α peak increases steadily with increasing <str<strong>on</strong>g>film</str<strong>on</strong>g>thickness while the β peak remains unchanged. Since the β peak reaches saturati<strong>on</strong> at a rathersmall <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness ( ) <str<strong>on</strong>g>and</str<strong>on</strong>g> from the asymmetric peak shape as well as from thecoverage independent peak maximum, <strong>on</strong>e can assign this peak to first order desorpti<strong>on</strong> fromthe m<strong>on</strong>olayer. In previous AFM investigati<strong>on</strong>s [61] an indicati<strong>on</strong> for the existence <str<strong>on</strong>g>of</str<strong>on</strong>g> am<strong>on</strong>olayer was stated. A significantly higher root mean square (RMS) roughness <str<strong>on</strong>g>of</str<strong>on</strong>g> areas <str<strong>on</strong>g>of</str<strong>on</strong>g> ap6P-covered sample in between the crystallites or chains compared to the clean mica substratewas observed. Compared to these AFM measurements, the TDS measurements represent adirect pro<str<strong>on</strong>g>of</str<strong>on</strong>g> for the existence <str<strong>on</strong>g>of</str<strong>on</strong>g> such a c<strong>on</strong>tinuous m<strong>on</strong>olayer (wetting layer) <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P<str<strong>on</strong>g>molecules</str<strong>on</strong>g> <strong>on</strong> mica (001).5.4 p6P <strong>on</strong> freshly cleaved mica (001) – multilayer regimeThicker p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>s were investigated by TDS, XPS, SEM <str<strong>on</strong>g>and</str<strong>on</strong>g> AFM. A TDS series startingfrom up to is shown in Fig. 5.6.Fig. 5.6: TDS series <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P grown <strong>on</strong> mica (001) atin the multilayer regime. The totalamount <str<strong>on</strong>g>of</str<strong>on</strong>g> deposited p6P for each single TD-spectrum is given as the mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness in .46
5 Hexaphenyl <strong>on</strong> mica (001)In the case <str<strong>on</strong>g>of</str<strong>on</strong>g> multilayer desorpti<strong>on</strong> the signal <str<strong>on</strong>g>of</str<strong>on</strong>g> the intact molecule (mass 458) is shownbecause there the QMS-signal is sufficiently high to yield a satisfying signal to noise ratio. Inthis spectrum the m<strong>on</strong>olayer peak is not visible due to its much smaller intensity. From theasymmetric shape <str<strong>on</strong>g>of</str<strong>on</strong>g> the α peak, the fact that the peak shifts to higher temperatures with thecoverage <str<strong>on</strong>g>and</str<strong>on</strong>g> the comm<strong>on</strong> leading edge <str<strong>on</strong>g>of</str<strong>on</strong>g> the peaks <strong>on</strong>e can assign these peaks to zero orderdesorpti<strong>on</strong> from the multilayer. However, the peak shape in this case resembles much less azero order desorpti<strong>on</strong> behavior than that for p6P desorpti<strong>on</strong> from metal surfaces, e.g. for Au(111) [20, 21, 71]. The reas<strong>on</strong> for this is most probably the n<strong>on</strong> uniform temperaturedistributi<strong>on</strong> <strong>on</strong> the fr<strong>on</strong>t surface <str<strong>on</strong>g>of</str<strong>on</strong>g> the mica sample due to its poor thermal c<strong>on</strong>ductivity. Themultilayer desorpti<strong>on</strong> peaks in this case should rather be seen as the superpositi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a number<str<strong>on</strong>g>of</str<strong>on</strong>g> desorpti<strong>on</strong> peaks with zero order desorpti<strong>on</strong> behavior, each <str<strong>on</strong>g>of</str<strong>on</strong>g> them slightly shifted intemperature with respect to each other.For all the different <str<strong>on</strong>g>film</str<strong>on</strong>g>s <str<strong>on</strong>g>of</str<strong>on</strong>g> Fig. 5.6 AFM measurements were carried out (Fig. 5.7). Theobtained AFM images were similar to the <strong>on</strong>es published in the literature previously [23-29].47
5 Hexaphenyl <strong>on</strong> mica (001)Fig. 5.7: AFM images <str<strong>on</strong>g>of</str<strong>on</strong>g> different p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>s <strong>on</strong> freshly cleaved mica. Lateral resoluti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> all imageswas . Films were grown at room temperature. Thickness <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g>s: a) , b), c) , d) .At initial <str<strong>on</strong>g>growth</str<strong>on</strong>g> stages, characteristic needle-<str<strong>on</strong>g>like</str<strong>on</strong>g> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s (nano-fibers) <str<strong>on</strong>g>and</str<strong>on</strong>g> crystallites coexist<strong>on</strong> the mica surface. Within the needles the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are oriented with their l<strong>on</strong>g axisparallel to the surface <str<strong>on</strong>g>and</str<strong>on</strong>g> nearly normal with respect to the needle directi<strong>on</strong> (see Fig. 5.8).48
5 Hexaphenyl <strong>on</strong> mica (001)Fig. 5.8: Alignment <str<strong>on</strong>g>of</str<strong>on</strong>g> the hexaphenyl <str<strong>on</strong>g>molecules</str<strong>on</strong>g> relative to the mica (001) surface for differentepitaxially aligned hexaphenyl crystallites. The two possible mica (001) cleavage planes α <str<strong>on</strong>g>and</str<strong>on</strong>g> β areaccounted by respective lattice directi<strong>on</strong>s <strong>on</strong> the substrate surface. The thick arrows <strong>on</strong> top give thedifferent directi<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> the needle <str<strong>on</strong>g>shaped</str<strong>on</strong>g> hexaphenyl crystallites. These arrows run approximatelyal<strong>on</strong>g high symmetry directi<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> the mica (001) surface, namely [110] mica in the case <str<strong>on</strong>g>of</str<strong>on</strong>g> α(001) mica<str<strong>on</strong>g>and</str<strong>on</strong>g> [1-10] mica in the case <str<strong>on</strong>g>of</str<strong>on</strong>g> β(001) mica . Relative to this high symmetry substrate directi<strong>on</strong>, the arrowsare rotated by 6° in case <str<strong>on</strong>g>of</str<strong>on</strong>g> B1 <str<strong>on</strong>g>and</str<strong>on</strong>g> B2 while the arrows are exactly parallel for C1 <str<strong>on</strong>g>and</str<strong>on</strong>g> C2. The unitcells <str<strong>on</strong>g>of</str<strong>on</strong>g> the two-dimensi<strong>on</strong>al surface lattices <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate as well as for the hexaphenyl crystals aredrawn by full thick lines. From [128].The nano-fibers are highly oriented over large surface domains <str<strong>on</strong>g>and</str<strong>on</strong>g> grow epitaxially <strong>on</strong> thesubstrate surface by two different crystal orientati<strong>on</strong>s with the crystallographic {11-1} (point<strong>on</strong> line coincidence I, orientati<strong>on</strong> B) <str<strong>on</strong>g>and</str<strong>on</strong>g> the {11-2} (coincidence, orientati<strong>on</strong> C) planesparallel to mica (001) [128]. Each crystal orientati<strong>on</strong> shows two crystal alignments that aremirrored to each other denoted as B1, B2 <str<strong>on</strong>g>and</str<strong>on</strong>g> C1, C2, respectively.As shown in Fig. 5.8 <str<strong>on</strong>g>and</str<strong>on</strong>g> also in the AFM images Fig. 5.7a <str<strong>on</strong>g>and</str<strong>on</strong>g> Fig. 5.7b, the grown needlesare sometimes not fully parallel, which is directly related to the differently alignedhexaphenyl crystals. The reas<strong>on</strong> for this more or less parallel orientati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the needles is theexistence <str<strong>on</strong>g>of</str<strong>on</strong>g> surface dipole fields <strong>on</strong> the mica surface, which are caused by the positive charge<strong>on</strong> the surface due to K + i<strong>on</strong>s <str<strong>on</strong>g>and</str<strong>on</strong>g> the negative net charge in the subsurface layer due to theAl 3+ i<strong>on</strong>s substituting for Si 4+ i<strong>on</strong>s [24]. With increasing <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness, the needles growlarger <str<strong>on</strong>g>and</str<strong>on</strong>g> higher by regrouping the crystallites. It is proposed that the driving force for theregrouping <str<strong>on</strong>g>of</str<strong>on</strong>g> the crystallites is the strain that is induced locally by the crystallites into thewetting layer [61]. The rearrangement process is performed by the crystallites as entities.49
5 Hexaphenyl <strong>on</strong> mica (001)Once the crystallites are incorporated into the chain they are trapped there. Besides the <str<strong>on</strong>g>film</str<strong>on</strong>g>thickness, the substrate temperature also plays an important role in layer <str<strong>on</strong>g>growth</str<strong>on</strong>g>. Fig. 5.9shows the morphology <str<strong>on</strong>g>of</str<strong>on</strong>g><str<strong>on</strong>g>film</str<strong>on</strong>g>s <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <strong>on</strong> mica (001) grown at different substratesurface temperatures, namely , <str<strong>on</strong>g>and</str<strong>on</strong>g> , measured with SEM (Fig. 5.8a)<str<strong>on</strong>g>and</str<strong>on</strong>g> AFM (Fig. 5.8b). Cross secti<strong>on</strong>s as obtained by AFM are shown in Fig. 5.9c.Fig. 5.9: SEM (a) <str<strong>on</strong>g>and</str<strong>on</strong>g> AFM (b) images <str<strong>on</strong>g>of</str<strong>on</strong>g>thick <str<strong>on</strong>g>film</str<strong>on</strong>g>s <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <strong>on</strong> mica (001) grown at differentadsorpti<strong>on</strong> temperatures,<str<strong>on</strong>g>and</str<strong>on</strong>g> cross secti<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> the AFM images (c). Please notethe different Z-scales for the AFM images <str<strong>on</strong>g>and</str<strong>on</strong>g> the cross secti<strong>on</strong>s.At low temperatures, the <str<strong>on</strong>g>film</str<strong>on</strong>g> c<strong>on</strong>sists <strong>on</strong>ly <str<strong>on</strong>g>of</str<strong>on</strong>g> small crystallites (about ),which cover almost the whole surface. An increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate temperature duringadsorpti<strong>on</strong> leads to the formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> needle-<str<strong>on</strong>g>like</str<strong>on</strong>g> structures (about - several inlength). A further increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the temperature leads to an increase <str<strong>on</strong>g>of</str<strong>on</strong>g> needle height <str<strong>on</strong>g>and</str<strong>on</strong>g> width,resulting in a decrease <str<strong>on</strong>g>of</str<strong>on</strong>g> needle density. In order to inspect the rearrangement <str<strong>on</strong>g>of</str<strong>on</strong>g> thecrystallites during sample heating ‘XPS vs. temperature’ measurements have been performed.50
5 Hexaphenyl <strong>on</strong> mica (001)The height <str<strong>on</strong>g>of</str<strong>on</strong>g> the C1s signal <str<strong>on</strong>g>of</str<strong>on</strong>g> the p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> gives informati<strong>on</strong> <strong>on</strong> how much <str<strong>on</strong>g>of</str<strong>on</strong>g> the surfacearea is covered with the <str<strong>on</strong>g>film</str<strong>on</strong>g>. This way it is possible to find out if the p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> is arranged insmall crystallites which almost cover the whole surface or in large <str<strong>on</strong>g>and</str<strong>on</strong>g> high needle-<str<strong>on</strong>g>like</str<strong>on</strong>g>isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s which cover less surface area at the same mean coverage. Various XPS vs temperaturespectra are depicted in Fig. 5.10 with the <str<strong>on</strong>g>film</str<strong>on</strong>g> formati<strong>on</strong> temperature as a parameter.Fig. 5.10: Top: Change <str<strong>on</strong>g>of</str<strong>on</strong>g> the C1s signal forsubstrate temperatures (thick p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>s grown <strong>on</strong> mica (001) at different), as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the temperature during heating thesample with . Bottom: Representative thermal desorpti<strong>on</strong> spectrum for a thick p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> <strong>on</strong>mica (001) at a heating rate <str<strong>on</strong>g>of</str<strong>on</strong>g> .In the upper part <str<strong>on</strong>g>of</str<strong>on</strong>g> Fig. 5.10 <strong>on</strong>e can see the C1s XPS signal heights (symbols + line). At thebottom a TD-spectrum is depicted for comparis<strong>on</strong> to show where desorpti<strong>on</strong> starts. Sevennanometer thick p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>s were prepared at different adsorpti<strong>on</strong> temperatures <str<strong>on</strong>g>and</str<strong>on</strong>g> then heatedby . One can see that the height <str<strong>on</strong>g>of</str<strong>on</strong>g> the XPS peaks str<strong>on</strong>gly depends <strong>on</strong> the <str<strong>on</strong>g>film</str<strong>on</strong>g> formati<strong>on</strong>temperature. The higher the adsorpti<strong>on</strong> temperature, the lower is the signal. This is c<strong>on</strong>sistentwith the AFM observati<strong>on</strong> that at lower temperatures () the <str<strong>on</strong>g>film</str<strong>on</strong>g> c<strong>on</strong>sists <str<strong>on</strong>g>of</str<strong>on</strong>g> small51
5 Hexaphenyl <strong>on</strong> mica (001)crystallites while at higher temperatures () it c<strong>on</strong>sists <str<strong>on</strong>g>of</str<strong>on</strong>g> larger <str<strong>on</strong>g>and</str<strong>on</strong>g> higherneedle-<str<strong>on</strong>g>like</str<strong>on</strong>g> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s which do not cover as much substrate surface area as the small crystallites.Utilizing this technique, not <strong>on</strong>ly the surface morphology at a certain preparati<strong>on</strong> temperaturecan be characterized, but also the crystallite rearrangement process during heating. The <str<strong>on</strong>g>film</str<strong>on</strong>g>sprepared at <str<strong>on</strong>g>and</str<strong>on</strong>g> do not change in the morphology (as seen by the c<strong>on</strong>stantXPS signal) until desorpti<strong>on</strong> starts. The XPS signal decrease coincides directly with the TDspectrum.Observing the <str<strong>on</strong>g>film</str<strong>on</strong>g> C1s signal, however, <strong>on</strong>e can see a decrease <str<strong>on</strong>g>of</str<strong>on</strong>g> thecarb<strong>on</strong> signal already before desorpti<strong>on</strong> actually starts. This indicates a rearrangement <str<strong>on</strong>g>of</str<strong>on</strong>g> thecrystallites, forming larger needles which keep getting higher with increasing temperature,uncovering larger areas <str<strong>on</strong>g>of</str<strong>on</strong>g> the mica substrate. The results <str<strong>on</strong>g>of</str<strong>on</strong>g> these low temperatureexperiments are interesting for two reas<strong>on</strong>s: First <str<strong>on</strong>g>of</str<strong>on</strong>g> all it shows that the <str<strong>on</strong>g>film</str<strong>on</strong>g> morphology canc<strong>on</strong>siderably be changed by the low preparati<strong>on</strong> temperature. Sec<strong>on</strong>d, the c<strong>on</strong>stant XPS signalbetween <str<strong>on</strong>g>and</str<strong>on</strong>g> shows that the <str<strong>on</strong>g>film</str<strong>on</strong>g> morphology does not change duringwarming up to room temperature. This means that <str<strong>on</strong>g>film</str<strong>on</strong>g>s prepared at these low temperaturescan be investigated at room temperature by ex-situ techniques, for instance SEM <str<strong>on</strong>g>and</str<strong>on</strong>g> AFM, asit was d<strong>on</strong>e in this work.5.5 p6P <strong>on</strong> carb<strong>on</strong> c<strong>on</strong>taminated mica (001)As shown in Fig. 5.3, air-cleaved mica c<strong>on</strong>tains carb<strong>on</strong>. The amount <str<strong>on</strong>g>of</str<strong>on</strong>g> the carb<strong>on</strong>c<strong>on</strong>taminati<strong>on</strong> may depend <strong>on</strong> the time it takes to get the mica into (ultra) high vacuumc<strong>on</strong>diti<strong>on</strong>s after the cleavage. In order to obtain informati<strong>on</strong> <strong>on</strong> the influence <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> <strong>on</strong> the<str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>, the amount <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> was intenti<strong>on</strong>ally increased. Un<str<strong>on</strong>g>like</str<strong>on</strong>g> <strong>on</strong> Au surfaces [21, 71]there was no dissociati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> hexaphenyl <strong>on</strong> mica during heating, which made it necessary toinduce p6P dissociati<strong>on</strong> by other means. Eventually, the carb<strong>on</strong> coverage was prepared by X-ray induced dissociati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P. For this purpose a rather thick <str<strong>on</strong>g>film</str<strong>on</strong>g>was grown atlow temperatures (around ) <str<strong>on</strong>g>and</str<strong>on</strong>g> then exposed to the Mg Kα radiati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the X-ray gunfor about 2 h. The X-ray irradiati<strong>on</strong> led to the dissociati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a certain amount <str<strong>on</strong>g>of</str<strong>on</strong>g> the p6P<str<strong>on</strong>g>molecules</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> carb<strong>on</strong> accumulated <strong>on</strong> the surface. After a subsequent TD-flash, which wasnecessary to get rid <str<strong>on</strong>g>of</str<strong>on</strong>g> the residual p6P <str<strong>on</strong>g>and</str<strong>on</strong>g> its cracking p<str<strong>on</strong>g>rod</str<strong>on</strong>g>ucts, <strong>on</strong>ly carb<strong>on</strong> remained <strong>on</strong> thesurface.52
5 Hexaphenyl <strong>on</strong> mica (001)That way four different carb<strong>on</strong> coverages were prepared <str<strong>on</strong>g>and</str<strong>on</strong>g> the amount <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> wasdetermined by XPS, as can be seen in Fig. 5.11. The amount <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> could be increased upto a certain saturati<strong>on</strong> coverage, which is used as a reference in this figure. All the Ccoverages are given in a fracti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the saturati<strong>on</strong> coverage .Fig. 5.11: XPS spectra <str<strong>on</strong>g>of</str<strong>on</strong>g> mica with different carb<strong>on</strong> coverages. C-coverage in units <str<strong>on</strong>g>of</str<strong>on</strong>g> the carb<strong>on</strong>saturati<strong>on</strong> layer. (a) Air-cleaved mica, (b)–(d) After dissociati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P.The increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the carb<strong>on</strong> coverage as a result <str<strong>on</strong>g>of</str<strong>on</strong>g> the p6P dissociati<strong>on</strong> can be followed byXPS, as seen in Fig. 5.11, b–d. This additi<strong>on</strong>al carb<strong>on</strong> influences the TDS behavior <str<strong>on</strong>g>of</str<strong>on</strong>g> p6Psignificantly. The associated TD-spectra are shown in Fig. 5.12. In order to compare thespectra <str<strong>on</strong>g>of</str<strong>on</strong>g> different carb<strong>on</strong> coverages with each other, the <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness () <str<strong>on</strong>g>and</str<strong>on</strong>g>adsorpti<strong>on</strong> temperature () were held c<strong>on</strong>stant.53
5 Hexaphenyl <strong>on</strong> mica (001)Fig. 5.12: TDS series <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P grown <strong>on</strong> partially C-covered mica (001) surfaces at .The m<strong>on</strong>olayer <str<strong>on</strong>g>and</str<strong>on</strong>g> multilayer desorpti<strong>on</strong> peaks are denoted by β <str<strong>on</strong>g>and</str<strong>on</strong>g> α, respectively. C-coverage isgiven in units <str<strong>on</strong>g>of</str<strong>on</strong>g> the carb<strong>on</strong> saturati<strong>on</strong> layer, in accordance with the XPS spectra in Fig. 5.11.The TD-spectrum obtained from the freshly cleaved mica surface (labeled a)), shows twopeaks, β <str<strong>on</strong>g>and</str<strong>on</strong>g> α, for the m<strong>on</strong>olayer <str<strong>on</strong>g>and</str<strong>on</strong>g> the beginning multilayer. As menti<strong>on</strong>ed before, theexistence <str<strong>on</strong>g>of</str<strong>on</strong>g> this β peak is a direct pro<str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> a c<strong>on</strong>tinuous m<strong>on</strong>olayer <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <strong>on</strong> the mica (001)surface, thus <str<strong>on</strong>g>of</str<strong>on</strong>g> the existence <str<strong>on</strong>g>of</str<strong>on</strong>g> a wetting layer. With increasing C-coverage (curves b–d) <strong>on</strong>eobserves a decrease <str<strong>on</strong>g>of</str<strong>on</strong>g> the m<strong>on</strong>olayer (β) peak until at the saturati<strong>on</strong> coverage the β peak iscompletely missing. This indicates a significant change in initial <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>, i.e. in them<strong>on</strong>olayer regime. From these TD-spectra it follows that the p6P multilayer no l<strong>on</strong>ger grows<strong>on</strong> mica covered with a p6P wetting layer (Stranski–Krastanov <str<strong>on</strong>g>growth</str<strong>on</strong>g>), but <strong>on</strong> the micasurface itself (Volmer-Weber <str<strong>on</strong>g>growth</str<strong>on</strong>g>).Another direct pro<str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> a change in <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <strong>on</strong> carb<strong>on</strong> c<strong>on</strong>taminated surfacescould be obtained from AFM measurements. In Fig. 5.13 a thick <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <strong>on</strong> a C-saturated mica surface is shown.54
5 Hexaphenyl <strong>on</strong> mica (001)Fig. 5.13: AFM image <str<strong>on</strong>g>of</str<strong>on</strong>g> athick p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> grown <strong>on</strong> carb<strong>on</strong> covered mica (001) surface at. Z-scale: .Fig. 5.14: Cross secti<strong>on</strong> analysis <str<strong>on</strong>g>of</str<strong>on</strong>g> the same <str<strong>on</strong>g>film</str<strong>on</strong>g> al<strong>on</strong>g the white line shown in Fig. 5.13.55
5 Hexaphenyl <strong>on</strong> mica (001)Fig. 5.15: 3D AFM image <str<strong>on</strong>g>of</str<strong>on</strong>g> the same <str<strong>on</strong>g>film</str<strong>on</strong>g> as shown in Fig. 5.13 ( p6P <strong>on</strong> C-mica at ).A cross-secti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> Fig. 5.13, as depicted in Fig. 5.14, shows that the height <str<strong>on</strong>g>of</str<strong>on</strong>g> such isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s isroughly , which corresp<strong>on</strong>ds to the length <str<strong>on</strong>g>of</str<strong>on</strong>g> a p6P molecule. This is an evidence that<strong>on</strong> the C-covered mica surface the m<strong>on</strong>olayer does not c<strong>on</strong>sist <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> lying flat <strong>on</strong> thesurface but <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing upright <str<strong>on</strong>g>and</str<strong>on</strong>g> that additi<strong>on</strong>al material forms a sec<strong>on</strong>d layer <str<strong>on</strong>g>of</str<strong>on</strong>g><str<strong>on</strong>g>molecules</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing upright <strong>on</strong> this first layer. Fig. 5.15 shows a 3D-view <str<strong>on</strong>g>of</str<strong>on</strong>g> the same <str<strong>on</strong>g>film</str<strong>on</strong>g>. Asimilar <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> (p6P <str<strong>on</strong>g>molecules</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing upright) has already been reported for p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>s<strong>on</strong> mica surfaces, which were exposed to ambient c<strong>on</strong>diti<strong>on</strong>s for two weeks after cleavage[39], p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>s <strong>on</strong> mica treated with water or methanol [29], <strong>on</strong> gold covered mica [125], aswell as <strong>on</strong> carb<strong>on</strong> covered gold surfaces [21].5.6 p6P <strong>on</strong> sputtered micaIn the previous secti<strong>on</strong>s the vast influence <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> <strong>on</strong> the <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> was dem<strong>on</strong>strated.The next step was to try to get rid <str<strong>on</strong>g>of</str<strong>on</strong>g> the carb<strong>on</strong> <strong>on</strong> the freshly cleaved mica specimen, whichwas expected to lead to some change in the <str<strong>on</strong>g>film</str<strong>on</strong>g> morphology <str<strong>on</strong>g>and</str<strong>on</strong>g> wetting layer behavior. Firstattempts to eliminate the carb<strong>on</strong> by simple degassing under UHV c<strong>on</strong>diti<strong>on</strong>s or by heatingunder oxygen atmosphere were not successful. Therefore a cleaning procedure was tried,which has proven to be successful for many other substrates, to wit sputtering. The effect <str<strong>on</strong>g>of</str<strong>on</strong>g>sputtering depends str<strong>on</strong>gly <strong>on</strong> the used beam voltage <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>on</strong> the sputtering dose. If highbeam voltages are used <str<strong>on</strong>g>and</str<strong>on</strong>g> l<strong>on</strong>g sputter times (), it is possible to get a carb<strong>on</strong>-56
5 Hexaphenyl <strong>on</strong> mica (001)free surface, i.e. no carb<strong>on</strong> peak in the XPS. The drawback <str<strong>on</strong>g>of</str<strong>on</strong>g> using these parameters is thatthe surface c<strong>on</strong>tains a lot <str<strong>on</strong>g>of</str<strong>on</strong>g> holes <str<strong>on</strong>g>and</str<strong>on</strong>g> humps after this kind <str<strong>on</strong>g>of</str<strong>on</strong>g> brute treatment (Fig. 5.16)Fig. 5.16: The mica surface after sputtering with a beam voltage <str<strong>on</strong>g>of</str<strong>on</strong>g>for 30 minutes,Ar-pressure .So as to prohibit morphological damage <str<strong>on</strong>g>of</str<strong>on</strong>g> the mica samples during sputtering, a lower beamvoltage ( ) was used. This ‘gentle sputtering’ though did not affect the chemicalcompositi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the mica specimen, i.e. the XPS spectra remained unchanged. However, theLEED pattern c<strong>on</strong>tinually deteriorated with the sputtering time until after some minutes <str<strong>on</strong>g>of</str<strong>on</strong>g>sputtering no LEED pattern could be observed anymore, which indicated a destructi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> theordered surface structure.These structural changes <str<strong>on</strong>g>of</str<strong>on</strong>g> the mica surface influence the p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> significantly, ascan be seen in the TDS, which is depicted in Fig. 5.17. In this figure several TD-spectra arecompiled, which were recorded after different sputtering times. In all the spectra, theadsorpti<strong>on</strong> temperature was <str<strong>on</strong>g>and</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness was .57
5 Hexaphenyl <strong>on</strong> mica (001)Fig. 5.17: TDS series <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P grown <strong>on</strong> sputtered mica (001) surfaces at RT. The m<strong>on</strong>olayer<str<strong>on</strong>g>and</str<strong>on</strong>g> multilayer desorpti<strong>on</strong> peaks are denoted by β <str<strong>on</strong>g>and</str<strong>on</strong>g> α, respectively. The sputtering time is denoted insec<strong>on</strong>ds.As already described in Secti<strong>on</strong> 5.2, <strong>on</strong> the untreated mica surface two peaks can be seen,which are labeled α <str<strong>on</strong>g>and</str<strong>on</strong>g> β. The α peak corresp<strong>on</strong>ds to the initial multilayer, the β peak to them<strong>on</strong>olayer. In Fig. 5.17 <strong>on</strong>e can see a decrease <str<strong>on</strong>g>of</str<strong>on</strong>g> the β peak with increasing sputtering time,which <strong>on</strong> the other h<str<strong>on</strong>g>and</str<strong>on</strong>g> leads to an increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the α peak. The total area <str<strong>on</strong>g>of</str<strong>on</strong>g> the individualdesorpti<strong>on</strong> spectra remains c<strong>on</strong>stant within the experimental error. This means that, <str<strong>on</strong>g>like</str<strong>on</strong>g> <strong>on</strong> thecarb<strong>on</strong> covered mica surface (Fig. 5.12), a significant change <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> in them<strong>on</strong>olayer regime takes place as a result <str<strong>on</strong>g>of</str<strong>on</strong>g> disordering the surface structure. However, thereare some differences between the TDS series <strong>on</strong> the carb<strong>on</strong> covered mica surface <str<strong>on</strong>g>and</str<strong>on</strong>g> the TDSseries <strong>on</strong> the sputtered mica surface. In Fig. 5.12, the β peak decreases in height withincreasing carb<strong>on</strong> coverage <str<strong>on</strong>g>and</str<strong>on</strong>g> the peak maximum remains nearly c<strong>on</strong>stant. By c<strong>on</strong>trast, inFig. 5.17 in additi<strong>on</strong> to the decrease <str<strong>on</strong>g>of</str<strong>on</strong>g> the β peaks, also a shift to lower temperatures can beobserved. A possible explanati<strong>on</strong> for this different behavior is as follows: In the case <str<strong>on</strong>g>of</str<strong>on</strong>g>carb<strong>on</strong> c<strong>on</strong>taminati<strong>on</strong> by p6P dissociati<strong>on</strong>, most probably isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s <str<strong>on</strong>g>of</str<strong>on</strong>g> higher carb<strong>on</strong> densitywill develop, where no m<strong>on</strong>olayer can exist. Therefore the decreased β state results from thedecreased area <str<strong>on</strong>g>of</str<strong>on</strong>g> the untreated surface. On the other h<str<strong>on</strong>g>and</str<strong>on</strong>g>, sputtering will yield a uniform58
5 Hexaphenyl <strong>on</strong> mica (001)disordering <str<strong>on</strong>g>of</str<strong>on</strong>g> the whole surface. With increasing surface disorder, the binding strength <str<strong>on</strong>g>of</str<strong>on</strong>g> the<str<strong>on</strong>g>molecules</str<strong>on</strong>g> in the m<strong>on</strong>olayer will decrease.The p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> <strong>on</strong> the sputtered mica surface was also investigated by AFM. In Fig.5.18 the morphology <str<strong>on</strong>g>of</str<strong>on</strong>g> a thick p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> grown at room temperature <strong>on</strong> a surface aftera sputtering time <str<strong>on</strong>g>of</str<strong>on</strong>g> sec<strong>on</strong>ds (equivalent to curve d) in Fig. 5.17) is shown. A crosssecti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> this <str<strong>on</strong>g>film</str<strong>on</strong>g> is displayed in Fig. 5.19 <str<strong>on</strong>g>and</str<strong>on</strong>g> a 3D-view in Fig. 5.20.Fig. 5.18: AFM image <str<strong>on</strong>g>of</str<strong>on</strong>g> athick p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> grown <strong>on</strong> a sputtered mica (001) surface (sputteringtime:) at room temperature.Fig. 5.19: Secti<strong>on</strong> analysis al<strong>on</strong>g the white line in Fig. 5.18.59
5 Hexaphenyl <strong>on</strong> mica (001)Fig. 5.20: 3D AFM image <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g> shown in Fig. 5.18 ( p6P <strong>on</strong> sputtered mica at ).One can see that <str<strong>on</strong>g>like</str<strong>on</strong>g> <strong>on</strong> the carb<strong>on</strong> covered surface, the m<strong>on</strong>olayer c<strong>on</strong>sists <str<strong>on</strong>g>of</str<strong>on</strong>g> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s <str<strong>on</strong>g>of</str<strong>on</strong>g><str<strong>on</strong>g>molecules</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing upright (isl<str<strong>on</strong>g>and</str<strong>on</strong>g> height approximately ). However, the density <str<strong>on</strong>g>of</str<strong>on</strong>g>the isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s is higher, maybe due to a different step-edge barrier, <str<strong>on</strong>g>and</str<strong>on</strong>g> therefore nearly nosec<strong>on</strong>d layer can be observed in this figure.5.6.1 Step-edge barrier in <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>A characterizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the step-edge barrier (Ehrlich-Schwoebel-Barrier) could be carried out[65] by using an atomistic nucleati<strong>on</strong> theory (see secti<strong>on</strong> 2.3 Nucleati<strong>on</strong> <strong>on</strong> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s: Stepedgebarriers) that is established in in<str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> [86]. For that purpose several <str<strong>on</strong>g>film</str<strong>on</strong>g>s<str<strong>on</strong>g>of</str<strong>on</strong>g> different <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness from up to were prepared. With increasing <str<strong>on</strong>g>film</str<strong>on</strong>g>thickness (Fig. 5.21, Fig. 5.22) stepped mounds with a step height <str<strong>on</strong>g>of</str<strong>on</strong>g>are formed<strong>on</strong> top <str<strong>on</strong>g>of</str<strong>on</strong>g> which small el<strong>on</strong>gated crystallites arise that do not exhibit the terrace structure <str<strong>on</strong>g>and</str<strong>on</strong>g>most probably c<strong>on</strong>sist <str<strong>on</strong>g>of</str<strong>on</strong>g> lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g>, since they resemble the well-known p6P needlesobserved <strong>on</strong> freshly cleaved mica or KCl. The single mounds are separated by deep crevicesthat do not close with <strong>on</strong>going depositi<strong>on</strong> (sometimes referred to as ‘Zeno effect’ in theliterature [129]).60
5 Hexaphenyl <strong>on</strong> mica (001)Fig. 5.21: Series <str<strong>on</strong>g>of</str<strong>on</strong>g> AFM images showing the evoluti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g> morphology withincreasing <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness. All <str<strong>on</strong>g>film</str<strong>on</strong>g>s were grown at RT <strong>on</strong> i<strong>on</strong> bombarded mica (001). (a), (b), (c) , <str<strong>on</strong>g>and</str<strong>on</strong>g> (d) <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness [65, 77].61
5 Hexaphenyl <strong>on</strong> mica (001)Fig. 5.22: Left: A typical mound in a thick p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>. A small needle is seen <strong>on</strong> top <str<strong>on</strong>g>of</str<strong>on</strong>g> themound. Right: Cross secti<strong>on</strong> al<strong>on</strong>g the line in the image <strong>on</strong> the left. One can see the characteristiclateral shape <str<strong>on</strong>g>of</str<strong>on</strong>g> the mound, characterized by a deep trench separating it from the neighboring mounds.The mounds have a clear tendency to a polyg<strong>on</strong>al shape, as can be seen in Fig. 5.22. Thesurface fracti<strong>on</strong> covered by the needle-<str<strong>on</strong>g>like</str<strong>on</strong>g> structures increases with increasing <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness.It is supposed [77] that these p6P needles form <strong>on</strong> ordered substrates <str<strong>on</strong>g>and</str<strong>on</strong>g> that the terraces <strong>on</strong>top <str<strong>on</strong>g>of</str<strong>on</strong>g> the mounds exhibit an order sufficiently high to allow the needle formati<strong>on</strong>. For theevaluati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the mounds the needles <strong>on</strong> top <str<strong>on</strong>g>of</str<strong>on</strong>g> the mounds had to be removed by applying aheight thresholding algorithm to the AFM images, which is in detail explained in ref. [77].Using eq. 2.2 - eq. 2.4 with (two-dimensi<strong>on</strong>al diffusi<strong>on</strong> process) <str<strong>on</strong>g>and</str<strong>on</strong>g> (criticalnucleus size) together with the experimentally obtained values <str<strong>on</strong>g>of</str<strong>on</strong>g>(moundseparati<strong>on</strong>) <str<strong>on</strong>g>and</str<strong>on</strong>g>(top terrace diameter) <strong>on</strong>e ends up with a value <str<strong>on</strong>g>of</str<strong>on</strong>g>for the step-edge barrier. The error <str<strong>on</strong>g>of</str<strong>on</strong>g> this value stems mainly from theuncertainty <str<strong>on</strong>g>of</str<strong>on</strong>g> the parameter , which is hard to measure due to the needle-<str<strong>on</strong>g>like</str<strong>on</strong>g> structures <strong>on</strong>top <str<strong>on</strong>g>of</str<strong>on</strong>g> the mounds. The value for the step-edge barrier was corroborated by moleculardynamics simulati<strong>on</strong>s [65], too. However, it turned out that several interesting features arisefor the large <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> (compared to the classical in<str<strong>on</strong>g>organic</str<strong>on</strong>g> picture where just singleatoms are the diffusing species) which have to be taken into account for the simulati<strong>on</strong>s. First,the calculati<strong>on</strong>s revealed that the diffusi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>rod</str<strong>on</strong>g>-<str<strong>on</strong>g>like</str<strong>on</strong>g> p6P molecule is favored al<strong>on</strong>g itsl<strong>on</strong>g molecular axis. As a c<strong>on</strong>sequence, the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> will arrive at the step-edge with theirl<strong>on</strong>g axis perpendicular to it <str<strong>on</strong>g>and</str<strong>on</strong>g> if they overcome the barrier, they will be in the correctorientati<strong>on</strong> to be incorporated into the underlying layer. Another important feature <str<strong>on</strong>g>of</str<strong>on</strong>g> large<str<strong>on</strong>g>molecules</str<strong>on</strong>g> comes into play during the diffusi<strong>on</strong> process over the edge, where the molecule iseither sticking out over the terrace (if it is rigid) or str<strong>on</strong>gly bent. In case <str<strong>on</strong>g>of</str<strong>on</strong>g> rigid <str<strong>on</strong>g>molecules</str<strong>on</strong>g>,nearly all the intermolecular b<strong>on</strong>ds have to break at the same time if the molecule overcomesthe barrier, which leads to a relatively high step-edge barrier. According to this, a bending <str<strong>on</strong>g>of</str<strong>on</strong>g>the molecule is more favorable if the bending itself is not too energy-c<strong>on</strong>suming. Indeed,there is experimental pro<str<strong>on</strong>g>of</str<strong>on</strong>g> that the p6P molecule exhibits a rotati<strong>on</strong>al flexibility al<strong>on</strong>g theinter-ring C-C b<strong>on</strong>ds [130]. This was shown for p6P adsorpti<strong>on</strong> <strong>on</strong> a Ni (110)(2x1)-O surface,where a bending <str<strong>on</strong>g>of</str<strong>on</strong>g> the phenyl rings <str<strong>on</strong>g>of</str<strong>on</strong>g> a single molecule with respect to each other wasobserved in STM images. The calculati<strong>on</strong>s yielded a step-edge barrier <str<strong>on</strong>g>of</str<strong>on</strong>g>for therigid molecule <str<strong>on</strong>g>and</str<strong>on</strong>g> for the bending molecule, whereas more than 70% <str<strong>on</strong>g>of</str<strong>on</strong>g> the lattervalue is due to the energy cost related to the bending <str<strong>on</strong>g>of</str<strong>on</strong>g> the molecule.62
5 Hexaphenyl <strong>on</strong> mica (001)Eventually, an evaluati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the first undermost terraces in the AFM images yielded anothersignificant deviati<strong>on</strong> from the in<str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> model. It turned out that the tilt angle <str<strong>on</strong>g>of</str<strong>on</strong>g> the<str<strong>on</strong>g>molecules</str<strong>on</strong>g> is not c<strong>on</strong>stant in each layer. The first few layers <str<strong>on</strong>g>of</str<strong>on</strong>g> terraces are influenced by thesubstrate <str<strong>on</strong>g>and</str<strong>on</strong>g> exhibit a higher tilt angle from the surface normal, which decreases withincreasing distance to the mica surface (illustrated in Fig. 5.23). In the bulk the tilt angle is17° from the surface normal, which is exactly the value also obtained for the bulk terraces <strong>on</strong>KCl (see Fig. 6.8). The energy that is needed for the bending <str<strong>on</strong>g>of</str<strong>on</strong>g> a molecule decreases with thedecreasing tilt angle <str<strong>on</strong>g>of</str<strong>on</strong>g> the underlying terrace. Thus the c<strong>on</strong>tributi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the energy cost for thebending to the step-edge barrier varies from layer to layer, resulting in different step-edgebarriers. The step-edge barrier for the first layer was calculated utilizing another independentmethod which was applied to the thick <str<strong>on</strong>g>film</str<strong>on</strong>g> [65]: The number fracti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> first layerisl<str<strong>on</strong>g>and</str<strong>on</strong>g>s with a certain size that have nucleated a sec<strong>on</strong>d layer isl<str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>on</strong> top yields the criticalisl<str<strong>on</strong>g>and</str<strong>on</strong>g> size, which in turn gives the interlayer jump rate that is used to calculate the step-edgebarrier (eq. 2.2). That way a value <str<strong>on</strong>g>of</str<strong>on</strong>g>was obtained.for the step-edge barrier in the first m<strong>on</strong>olayerFig. 5.23: Schematic view <str<strong>on</strong>g>of</str<strong>on</strong>g> the varying tilt angles in the first few layers. The step-edge barrierdecreases with increasing tilt angle.63
6 Hexaphenyl <strong>on</strong> KCl (001)6 Hexaphenyl <strong>on</strong> KCl (001)This chapter overviews the <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <strong>on</strong> KCl (001). Particular attenti<strong>on</strong> was drawn to thethermal stability <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g>s <str<strong>on</strong>g>and</str<strong>on</strong>g> to the temperature dependence <str<strong>on</strong>g>of</str<strong>on</strong>g> the p6P morphology. Inc<strong>on</strong>trast to mica, KCl exhibits a moderate thermal c<strong>on</strong>ductivity, which allowed a rati<strong>on</strong>alcalculati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> thermal desorpti<strong>on</strong> parameters. Again, TDS was used to get informati<strong>on</strong> <strong>on</strong> theexistence <str<strong>on</strong>g>of</str<strong>on</strong>g> a wetting layer <str<strong>on</strong>g>and</str<strong>on</strong>g> combined TDS vs XPS measurements enabled m<strong>on</strong>itoring thethermal stability <str<strong>on</strong>g>of</str<strong>on</strong>g> the grown <str<strong>on</strong>g>film</str<strong>on</strong>g>s in-situ. AFM was used to characterize the morphology exsitu.After the installati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the KCl (001) substrate into the UHV chamber, small amounts <str<strong>on</strong>g>of</str<strong>on</strong>g>carb<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> oxygen were detected <strong>on</strong> the surface by XPS. However, after annealing <str<strong>on</strong>g>of</str<strong>on</strong>g> thesample for 30 minutes at , no oxygen could be detected <strong>on</strong> the surface anymore. InFig. 6.1 the XPS spectrum <str<strong>on</strong>g>of</str<strong>on</strong>g> KCl right after installati<strong>on</strong> is shown, in Fig. 6.2 after annealing.Fig: 6.1: XPS spectrum taken directly after installati<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> pump down. The peak atwhich can be seen in both spectra could not be assigned to an element (ghost). The small peak at64
6 Hexaphenyl <strong>on</strong> KCl (001), which can also be seen in both spectra, is not unambiguous. It could be the C1s peak, butalso the Mg Kα 3 X-ray satellite <str<strong>on</strong>g>of</str<strong>on</strong>g> the K2p peak (displacement , relative height 8.0 %).Fig. 6.2: XPS spectrum <str<strong>on</strong>g>of</str<strong>on</strong>g> KCl after annealing at for 30 minutes. It can be seen that the oxygenis removed by annealing. The Auger peak <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> is very small <str<strong>on</strong>g>and</str<strong>on</strong>g> almost coincides with the Augerpeak <str<strong>on</strong>g>of</str<strong>on</strong>g> potassium (substrate.). Thus it is difficult to ascertain the amount <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> <strong>on</strong> the KClThe KCl (001) surface is a rather smooth surface which exhibits several steps <str<strong>on</strong>g>and</str<strong>on</strong>g> flat terraces<str<strong>on</strong>g>of</str<strong>on</strong>g> atomic height ( , see Fig. 6.3)65
6 Hexaphenyl <strong>on</strong> KCl (001)Fig. 6.3: AFM images <str<strong>on</strong>g>of</str<strong>on</strong>g> the clean KCl (001) surface. Top left:image, shadedmode. Z-scale: . Top right: 3-D AFM image <str<strong>on</strong>g>of</str<strong>on</strong>g> the KCl (001) surface, Z-Scale: .Bottom: Cross-secti<strong>on</strong> al<strong>on</strong>g the black line. Atomic steps with a height <str<strong>on</strong>g>of</str<strong>on</strong>g>(lattice c<strong>on</strong>stant<str<strong>on</strong>g>of</str<strong>on</strong>g>) can be seen.As shown in chapter 5 Hexaphenyl <strong>on</strong> mica (001), the p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g>ten cruciallydepends <strong>on</strong> the formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a wetting layer. Therefore the first step was to find out whetherthere existed a wetting layer or not.66
6 Hexaphenyl <strong>on</strong> KCl (001)6.1 Searching for a p6P wetting layer <strong>on</strong> KCl (001)A series <str<strong>on</strong>g>of</str<strong>on</strong>g> thermal desorpti<strong>on</strong> spectra obtained after evaporati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> different amounts <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P<strong>on</strong> KCl (001) at , ranging from to , are shown in Fig. 6.4. Thisadsorpti<strong>on</strong> temperature was chosen because it is frequently used in the literature.Fig. 6.4: Series <str<strong>on</strong>g>of</str<strong>on</strong>g> thermal desorpti<strong>on</strong> spectra for p6P from KCl (001). Adsorpti<strong>on</strong> temperature:, heating rate: . Parameter is the mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness prior to desorpti<strong>on</strong>. The arrow pointsto a peak in curve a) which originates from desorpti<strong>on</strong> from the Ta clamps (compare with Fig. 5.1 insecti<strong>on</strong> 5.1 Calibrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the TD-spectra). The inset shows the linear dependence <str<strong>on</strong>g>of</str<strong>on</strong>g> adsorbed <str<strong>on</strong>g>and</str<strong>on</strong>g>desorbed p6P for two different adsorpti<strong>on</strong> temperatures, <str<strong>on</strong>g>and</str<strong>on</strong>g> , respectively.The asymmetric peak shape, the comm<strong>on</strong> leading edge <str<strong>on</strong>g>of</str<strong>on</strong>g> the spectra <str<strong>on</strong>g>and</str<strong>on</strong>g> the shift <str<strong>on</strong>g>of</str<strong>on</strong>g> the peakmaxima to higher temperature with increasing coverage are characteristic <str<strong>on</strong>g>of</str<strong>on</strong>g> a zero orderdesorpti<strong>on</strong>, as expected for multilayer desorpti<strong>on</strong>. Similar desorpti<strong>on</strong> spectra were alsoobtained for multilayer desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P from Au (111) [20, 21, 71] <str<strong>on</strong>g>and</str<strong>on</strong>g> mica (001) (chapter5). However, in these cases additi<strong>on</strong>al desorpti<strong>on</strong> peaks at higher temperatures were observedat small coverage, which indicated the existence <str<strong>on</strong>g>of</str<strong>on</strong>g> the more str<strong>on</strong>gly bound wetting layer. InFig. 6.4 no evidence for a str<strong>on</strong>gly bound layer can be seen. Even for the smallest coverage <str<strong>on</strong>g>of</str<strong>on</strong>g>the peak is located at the comm<strong>on</strong> leading edge. The small peak (marked by an67
6 Hexaphenyl <strong>on</strong> KCl (001)arrow) which appears for theTD spectrum is an experimental artefact: It stems fromdesorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a thick p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> from the tantalum clamps, which are used to hold theKCl sample to the heating plate (see secti<strong>on</strong> 4.3.2 Experimental implementati<strong>on</strong> for details).It serves as a calibrati<strong>on</strong> point for the temperature scale since it can be assumed that thetemperature <str<strong>on</strong>g>of</str<strong>on</strong>g> the metal wires corresp<strong>on</strong>ds quite well to that <str<strong>on</strong>g>of</str<strong>on</strong>g> the steel plate where thetemperature is measured. The temperature maximum for the p6P multilayer desorpti<strong>on</strong> at theindividual coverages is known from the above menti<strong>on</strong>ed literature. From the comm<strong>on</strong>leading edge (after proper correcti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the temperature scale) <strong>on</strong>e can easily determine thedesorpti<strong>on</strong> energy (heat <str<strong>on</strong>g>of</str<strong>on</strong>g> evaporati<strong>on</strong>) <str<strong>on</strong>g>and</str<strong>on</strong>g> also the pre-exp<strong>on</strong>ential factor for zeroorder desorpti<strong>on</strong> by applying the Polanyi–Wigner equati<strong>on</strong> (eq. 4.6):This is in good agreement with the recently obtained values for p6P <strong>on</strong> Au (111) [71]. Therelatively high value <str<strong>on</strong>g>of</str<strong>on</strong>g> the pre-exp<strong>on</strong>ential factor is typical for the desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> large<str<strong>on</strong>g>molecules</str<strong>on</strong>g> [21, 71, 100-104]. However, most important is the fact that down to the smallestcoverage <str<strong>on</strong>g>of</str<strong>on</strong>g> mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness the p6P <str<strong>on</strong>g>molecules</str<strong>on</strong>g> show a multilayer desorpti<strong>on</strong>behavior, i.e. the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> desorb from small isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s <str<strong>on</strong>g>and</str<strong>on</strong>g> not from a more str<strong>on</strong>gly boundwetting layer. Quite similar desorpti<strong>on</strong> spectra were obtained after p6P adsorpti<strong>on</strong> at .This dem<strong>on</strong>strates that no wetting layer exists, independent <str<strong>on</strong>g>of</str<strong>on</strong>g> the adsorpti<strong>on</strong> temperature.In the insert <str<strong>on</strong>g>of</str<strong>on</strong>g> Fig. 6.4 the correlati<strong>on</strong> between the integral <str<strong>on</strong>g>of</str<strong>on</strong>g> the TDS peak areas(proporti<strong>on</strong>al to the desorbed amount) <str<strong>on</strong>g>and</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness as obtained from the quartzmicrobalance is shown. It is a straight line, both for an adsorpti<strong>on</strong> temperature <str<strong>on</strong>g>of</str<strong>on</strong>g>, as68
6 Hexaphenyl <strong>on</strong> KCl (001)well as <str<strong>on</strong>g>of</str<strong>on</strong>g> . This shows that the sticking coefficient is independent <str<strong>on</strong>g>of</str<strong>on</strong>g> the coverage <str<strong>on</strong>g>and</str<strong>on</strong>g><str<strong>on</strong>g>of</str<strong>on</strong>g> the sample temperature in this range. It can therefore safely be assumed that the stickingcoefficient is unity at <str<strong>on</strong>g>and</str<strong>on</strong>g> below . The sec<strong>on</strong>d important result from this experiment isthat all <str<strong>on</strong>g>of</str<strong>on</strong>g> the adsorbed material desorbs again, i.e. no partial dissociati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the p6P<str<strong>on</strong>g>molecules</str<strong>on</strong>g> takes place. Such partial dissociati<strong>on</strong> has been observed for p6P <str<strong>on</strong>g>and</str<strong>on</strong>g> p4P <strong>on</strong>polycrystalline gold <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>on</strong> stepped gold surfaces [21, 71]. One could argue that a wettinglayer very well might exist at the adsorpti<strong>on</strong> temperature but that during the temperatureincrease in TDS the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in the wetting layer might become mobile enough to formisl<str<strong>on</strong>g>and</str<strong>on</strong>g>s, from where they then desorb in form <str<strong>on</strong>g>of</str<strong>on</strong>g> the observed zero order reacti<strong>on</strong> kinetics. Inorder to check this possibility the change <str<strong>on</strong>g>of</str<strong>on</strong>g> the potassium K2p signal was followed as afuncti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness at <str<strong>on</strong>g>and</str<strong>on</strong>g> , respectively (Fig. 6.5).Fig. 6.5: K2p 3/2 XPS signal as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness, grown at <str<strong>on</strong>g>and</str<strong>on</strong>g> substratetemperature, respectively. For the K2p signal at the data points are fitted by an exp<strong>on</strong>entialfuncti<strong>on</strong>. In the other case the line is drawn as a guide to the eye.At the p6P <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are not very mobile <str<strong>on</strong>g>and</str<strong>on</strong>g> a ‘hit <str<strong>on</strong>g>and</str<strong>on</strong>g> stick’ adsorpti<strong>on</strong> behaviorcan be assumed. In fact, in this case the decrease <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate K2p signal can be69
6 Hexaphenyl <strong>on</strong> KCl (001)approximated quite well by an exp<strong>on</strong>ential functi<strong>on</strong>, as <strong>on</strong>e would expect for an amorphouslayer-by-layer <str<strong>on</strong>g>growth</str<strong>on</strong>g>:(6.1)A mean free path <str<strong>on</strong>g>of</str<strong>on</strong>g> for the K2p electr<strong>on</strong>s ( ) in the p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> wascalculated from this exp<strong>on</strong>ential fit. In c<strong>on</strong>trast, if the p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> was formed at a substratetemperature <str<strong>on</strong>g>of</str<strong>on</strong>g>the K2p signal remained nearly c<strong>on</strong>stant up to a mean thickness <str<strong>on</strong>g>of</str<strong>on</strong>g>about. If a wetting layer was formed at this temperature the XPS substrate signalshould decrease at least as fast as at . The little change <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate signal showsthat the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are already str<strong>on</strong>gly de-wetting in this low coverage range by forming highisl<str<strong>on</strong>g>and</str<strong>on</strong>g>s with small footprints. Interestingly, with a further increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the coverage thedecrease <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate signal becomes similar to that at , just shifted by somethickness value. This indicates that further <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> does not proceed in form <str<strong>on</strong>g>of</str<strong>on</strong>g> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>swith high aspect ratios (height vs. footprint), but that rather a c<strong>on</strong>tinuous layer <str<strong>on</strong>g>growth</str<strong>on</strong>g> exists.A detailed characterizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g>s grown at different substrate temperatures is presented inthe following secti<strong>on</strong>s.6.2 Investigati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> morphology by AFMFig. 6.6 shows a typical AFM image <str<strong>on</strong>g>of</str<strong>on</strong>g> athick <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <strong>on</strong> KCl (001) prepared at. The <str<strong>on</strong>g>film</str<strong>on</strong>g> c<strong>on</strong>sists <str<strong>on</strong>g>of</str<strong>on</strong>g> characteristic p6P needles <str<strong>on</strong>g>and</str<strong>on</strong>g> in between large irregularly<str<strong>on</strong>g>shaped</str<strong>on</strong>g> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s can be found.70
6 Hexaphenyl <strong>on</strong> KCl (001)Fig. 6.6: AFM image <str<strong>on</strong>g>of</str<strong>on</strong>g> a thick p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> <strong>on</strong> KCl (001) grown by PVD at substratetemperature.Fig. 6.7: Cross secti<strong>on</strong> al<strong>on</strong>g the line as depicted in Fig. 6.6.71
6 Hexaphenyl <strong>on</strong> KCl (001)A cross secti<strong>on</strong> al<strong>on</strong>g the blue line indicated in Fig. 6.6 is shown in Fig. 6.7. The height <str<strong>on</strong>g>of</str<strong>on</strong>g> thebroad terraces is, showing that these isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s are formed <str<strong>on</strong>g>of</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g>. Theneedle <str<strong>on</strong>g>like</str<strong>on</strong>g> isl<str<strong>on</strong>g>and</str<strong>on</strong>g> in this case has a footprint width <str<strong>on</strong>g>of</str<strong>on</strong>g> about<str<strong>on</strong>g>and</str<strong>on</strong>g> a height <str<strong>on</strong>g>of</str<strong>on</strong>g> about. These needles grow mainly al<strong>on</strong>g two orthog<strong>on</strong>al KCl directi<strong>on</strong>s, in accordance withthe surface symmetry <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate. This AFM result agrees quite well with previouslyobtained morphologies for p6P <strong>on</strong> KCl (001) grown by hot wall epitaxy (HWE) [47-49].From XRD <str<strong>on</strong>g>and</str<strong>on</strong>g> TEM investigati<strong>on</strong>s it is known that both, the needle <str<strong>on</strong>g>like</str<strong>on</strong>g> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s <str<strong>on</strong>g>and</str<strong>on</strong>g> theterrace-<str<strong>on</strong>g>like</str<strong>on</strong>g> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s, grow epitaxially <strong>on</strong> the KCl (001) surface [45, 47]. It has also been shownthat indeed the needles c<strong>on</strong>sist <str<strong>on</strong>g>of</str<strong>on</strong>g> side tilted <str<strong>on</strong>g>molecules</str<strong>on</strong>g> with their l<strong>on</strong>g axes being parallel tothe surface, whereas the terraced mounds c<strong>on</strong>sist <str<strong>on</strong>g>of</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g> with a tilt angle <str<strong>on</strong>g>of</str<strong>on</strong>g> 17°from the surface normal (see Fig. 6.8).Fig. 6.8: Side view (a) <str<strong>on</strong>g>and</str<strong>on</strong>g> view al<strong>on</strong>g the molecular axis (b) <str<strong>on</strong>g>of</str<strong>on</strong>g> a molecular layer <str<strong>on</strong>g>of</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing<str<strong>on</strong>g>molecules</str<strong>on</strong>g>. The upper picture shows that the l<strong>on</strong>g molecular axes are tilted by 17° from the surfacenormal. The bottom picture shows the p6P [1−10]-directi<strong>on</strong> (marked arrow) with rows <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g>,72
6 Hexaphenyl <strong>on</strong> KCl (001)which were found to be lined up with the KCl [110] azimuth. In-plane lattice vectors a <str<strong>on</strong>g>and</str<strong>on</strong>g> b <str<strong>on</strong>g>and</str<strong>on</strong>g> thehigh-symmetry azimuth p6P [010] are also indicated. From [45].6.3 The influence <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate temperature <strong>on</strong> the p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>morphologyAs already shown in the previous chapter about p6P <strong>on</strong> mica <str<strong>on</strong>g>and</str<strong>on</strong>g> many times in the literature,the p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> morphology in general str<strong>on</strong>gly depends <strong>on</strong> the substrate temperature. This is <str<strong>on</strong>g>of</str<strong>on</strong>g>course also the case for p6P <strong>on</strong> KCl. In Fig. 6.9, a compilati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> AFM images <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g>s grownat different substrate temperatures is presented. The adsorpti<strong>on</strong> temperatures range fromup to <str<strong>on</strong>g>and</str<strong>on</strong>g> the associated cross secti<strong>on</strong>s can be found in Fig. 6.10.Fig. 6.9a shows an AFM image <str<strong>on</strong>g>of</str<strong>on</strong>g> a thick p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> grown at room temperature. At thistemperature the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> form very short <str<strong>on</strong>g>and</str<strong>on</strong>g> mostly el<strong>on</strong>gated isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s which grow al<strong>on</strong>gpreferential rectangular directi<strong>on</strong>s according to the four-fold symmetry <str<strong>on</strong>g>of</str<strong>on</strong>g> the KCl substrate.The isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s have an average height <str<strong>on</strong>g>of</str<strong>on</strong>g> approximately (see Fig. 6.10a). No terraces <str<strong>on</strong>g>of</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing<str<strong>on</strong>g>molecules</str<strong>on</strong>g> can be seen under these c<strong>on</strong>diti<strong>on</strong>s. The formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> needle-<str<strong>on</strong>g>like</str<strong>on</strong>g> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s <strong>on</strong> mica(001) comes al<strong>on</strong>g with a wetting layer <str<strong>on</strong>g>of</str<strong>on</strong>g> lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g> covering the substrate <str<strong>on</strong>g>and</str<strong>on</strong>g> servingas a template for further <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>. Up to now, p6P <strong>on</strong> KCl is the first example whereisl<str<strong>on</strong>g>and</str<strong>on</strong>g>s <str<strong>on</strong>g>of</str<strong>on</strong>g> lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g> grow directly <strong>on</strong> the substrate, instead <str<strong>on</strong>g>of</str<strong>on</strong>g> growing <strong>on</strong> a wettinglayer. In Fig. 6.9b–d AFM images <str<strong>on</strong>g>of</str<strong>on</strong>g> thick p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>s grown at <str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g>a thick <str<strong>on</strong>g>film</str<strong>on</strong>g> grown at are shown. These <str<strong>on</strong>g>film</str<strong>on</strong>g>s are in particular suitable for thecomparis<strong>on</strong> between <str<strong>on</strong>g>film</str<strong>on</strong>g>s grown by hot-wall epitaxy (HWE) <str<strong>on</strong>g>and</str<strong>on</strong>g> molecular beam epitaxy(MBE), because the HWE <str<strong>on</strong>g>film</str<strong>on</strong>g>s are typically grown at adsorpti<strong>on</strong> temperatures which rangefrom up to . In general, it is known that the needles (lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g>) areformed at the initial <str<strong>on</strong>g>growth</str<strong>on</strong>g> stage <str<strong>on</strong>g>and</str<strong>on</strong>g> that the terraces (st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g>) develop after acertain coverage is reached [49]. In Fig. 6.9b we see that at the p6P <str<strong>on</strong>g>molecules</str<strong>on</strong>g> forml<strong>on</strong>g needles which are in principle arranged in a rectangular network, but most <str<strong>on</strong>g>of</str<strong>on</strong>g> the needlesare tilted <str<strong>on</strong>g>and</str<strong>on</strong>g> do not incline at an angle around 90° [132]. The height <str<strong>on</strong>g>of</str<strong>on</strong>g> the needles is between<str<strong>on</strong>g>and</str<strong>on</strong>g> (see Fig. 6.10b). Few terraces <str<strong>on</strong>g>of</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g> (step height )are observed. In Fig. 6.9c () fewer needles but more terraces are found. The height <str<strong>on</strong>g>of</str<strong>on</strong>g>the needles is around (see Fig. 6.10c), that <str<strong>on</strong>g>of</str<strong>on</strong>g> the terraces is (st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing<str<strong>on</strong>g>molecules</str<strong>on</strong>g>).73
6 Hexaphenyl <strong>on</strong> KCl (001)Fig. 6.9: AFM images <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>s <strong>on</strong> KCl (001): (a) 1 nm grown at , Z-scale: ; (b) 1nm grown at , Z-scale: ; (c) grown at , Z-scale: ; (d)grown at , Z-scale: . The horiz<strong>on</strong>tal trenches, which are in particular visible in (b), areartifacts due to the image processing which was applied.74
6 Hexaphenyl <strong>on</strong> KCl (001)Fig. 6.10: AFM cross secti<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g>s al<strong>on</strong>g the lines as shown in Fig. 6.9: (a)grown at; (b) grown at ; (c) grown at ; (d) grown at .Here, the needles are well aligned in a rectangular network <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>on</strong>ly a few <str<strong>on</strong>g>of</str<strong>on</strong>g> them are shorter<str<strong>on</strong>g>and</str<strong>on</strong>g> tilted. Finally, in Fig. 6.9d the AFM image <str<strong>on</strong>g>of</str<strong>on</strong>g> a thick p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> grown at isshown. What <strong>on</strong>e sees is a single needle (cross secti<strong>on</strong> shown in Fig. 6.10d), which issignificantly larger than the needles at lower temperatures in Fig. 6.9a–c. Here it seems thatall the material is used to build up huge needles, which results in a str<strong>on</strong>g decrease <str<strong>on</strong>g>of</str<strong>on</strong>g> theneedle density. It should be added here that <strong>on</strong> most parts <str<strong>on</strong>g>of</str<strong>on</strong>g> the surface no needles can befound at all. Instead, some areas covered <strong>on</strong>ly with st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are found.Figure 6.11 shows a statistical analysis <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g>s shown in Fig. 6.9. For this analysis fromfive to eight different AFM pictures <str<strong>on</strong>g>of</str<strong>on</strong>g> each <str<strong>on</strong>g>film</str<strong>on</strong>g> at the individual temperatures were used.The lateral sizes <str<strong>on</strong>g>of</str<strong>on</strong>g> the analyzed pictures varied between <str<strong>on</strong>g>and</str<strong>on</strong>g> .75
6 Hexaphenyl <strong>on</strong> KCl (001)Fig. 6.11: Evoluti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> morphology <str<strong>on</strong>g>and</str<strong>on</strong>g> mean needle height with increasing adsorpti<strong>on</strong>temperature for the<str<strong>on</strong>g>film</str<strong>on</strong>g>s shown in Fig. 6.9. The c<strong>on</strong>tinuous lines are just guides for the eye.The figure <strong>on</strong> top shows how much <str<strong>on</strong>g>of</str<strong>on</strong>g> the surface area is covered with needles <str<strong>on</strong>g>and</str<strong>on</strong>g> terraces,respectively. This data was acquired by investigating AFM pictures utilizing a very simpleMATLAB program. One can see that with increasing adsorpti<strong>on</strong> temperature the needledensity decreases, whereas the terrace density, <str<strong>on</strong>g>and</str<strong>on</strong>g> thus the area covered with st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing<str<strong>on</strong>g>molecules</str<strong>on</strong>g>, increases. The reas<strong>on</strong> for the rather weak increase <str<strong>on</strong>g>of</str<strong>on</strong>g> terrace density fromto is that the huge needles at already c<strong>on</strong>tain most <str<strong>on</strong>g>of</str<strong>on</strong>g> the p6P <str<strong>on</strong>g>molecules</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g>therefore not enough material is left to form the terraces. Regarding the <str<strong>on</strong>g>film</str<strong>on</strong>g>, it isworth menti<strong>on</strong>ing that <strong>on</strong> most <str<strong>on</strong>g>of</str<strong>on</strong>g> the AFM images either needles or terraces are found <str<strong>on</strong>g>and</str<strong>on</strong>g><strong>on</strong>ly very few images show needles coexisting with terraces. This makes a statistical analysis<str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g> difficult. The data <str<strong>on</strong>g>of</str<strong>on</strong>g> this <str<strong>on</strong>g>film</str<strong>on</strong>g> presented in Fig. 6.11 is the mean value <str<strong>on</strong>g>of</str<strong>on</strong>g> theresults gained from seven AFM images which were c<strong>on</strong>sidered to be representative.It is clear that there might be some error in the statistical analysis <str<strong>on</strong>g>of</str<strong>on</strong>g> this <str<strong>on</strong>g>film</str<strong>on</strong>g> since it is notpossible to investigate the whole sample, but <strong>on</strong>ly small parts <str<strong>on</strong>g>of</str<strong>on</strong>g> the surface, at <strong>on</strong>ce with an76
6 Hexaphenyl <strong>on</strong> KCl (001)AFM. This is taken into account by the error bars. The figure at the bottom presents theevoluti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the mean needle height with increasing temperature. The mean needle height wasdetermined by a careful manual evaluati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> numerous AFM cross secti<strong>on</strong>s. In the imagesobtained from the <str<strong>on</strong>g>film</str<strong>on</strong>g> grown at , <strong>on</strong>ly very few needles were found <str<strong>on</strong>g>and</str<strong>on</strong>g> therefore anuncertainty <str<strong>on</strong>g>of</str<strong>on</strong>g> the statistical evaluati<strong>on</strong> in this case has to be taken into account. Thisstatistical uncertainty is expressed by an error bar. One can see that the mean needle heightslightly increases from at to at . Between <str<strong>on</strong>g>and</str<strong>on</strong>g>a huge gain <str<strong>on</strong>g>of</str<strong>on</strong>g> needle height is observed (from to ), which is significantlystr<strong>on</strong>ger than the <strong>on</strong>e between <str<strong>on</strong>g>and</str<strong>on</strong>g> . This finding has been c<strong>on</strong>firmed bypreparati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> another <str<strong>on</strong>g>film</str<strong>on</strong>g> grown at with a higher <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness ( ). The meanneedle height <str<strong>on</strong>g>of</str<strong>on</strong>g> this<str<strong>on</strong>g>film</str<strong>on</strong>g> is slightly higher, but comparable with the mean needleheight <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g>. With the data given in Fig. 6.11, a calculati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the mean <str<strong>on</strong>g>film</str<strong>on</strong>g>thickness <str<strong>on</strong>g>of</str<strong>on</strong>g> the different <str<strong>on</strong>g>film</str<strong>on</strong>g>s was c<strong>on</strong>ducted. The error <str<strong>on</strong>g>of</str<strong>on</strong>g> the statistical evaluati<strong>on</strong> is takeninto account. The results are: with an error <str<strong>on</strong>g>of</str<strong>on</strong>g> for the <str<strong>on</strong>g>film</str<strong>on</strong>g>,with an error <str<strong>on</strong>g>of</str<strong>on</strong>g> for the <str<strong>on</strong>g>film</str<strong>on</strong>g>, with an error <str<strong>on</strong>g>of</str<strong>on</strong>g> for the<str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> with an error <str<strong>on</strong>g>of</str<strong>on</strong>g> for the <str<strong>on</strong>g>film</str<strong>on</strong>g>. By analysis <str<strong>on</strong>g>of</str<strong>on</strong>g> theAFM images a higher <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness than the nominal <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness, according to themicrobalance ( each, except for for the <str<strong>on</strong>g>film</str<strong>on</strong>g>), is obtained. This can beexplained by the c<strong>on</strong>voluti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the tip with the sample surface. The deviati<strong>on</strong> from thenominal mean thickness is most significant for the <str<strong>on</strong>g>film</str<strong>on</strong>g> because here all the isl<str<strong>on</strong>g>and</str<strong>on</strong>g>sare much smaller than at higher temperatures. Each steep needle wall causes an error <str<strong>on</strong>g>and</str<strong>on</strong>g>therefore the error increases with the needle density. The mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness calculati<strong>on</strong> forthe <str<strong>on</strong>g>film</str<strong>on</strong>g> grown athuge error <str<strong>on</strong>g>of</str<strong>on</strong>g> the result.is not reliable because <str<strong>on</strong>g>of</str<strong>on</strong>g> its inhomogeneity, which is expressed by aWhen the <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness is raised to , as expected [49] the formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> mounds madeup <str<strong>on</strong>g>of</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g> prevails for <str<strong>on</strong>g>film</str<strong>on</strong>g>s prepared above RT (Fig. 6.12). Comparing thethick <str<strong>on</strong>g>film</str<strong>on</strong>g> with the thick <str<strong>on</strong>g>film</str<strong>on</strong>g>, the density <str<strong>on</strong>g>and</str<strong>on</strong>g> the size <str<strong>on</strong>g>of</str<strong>on</strong>g> the needles changes<strong>on</strong>ly little, whereas most <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate which was uncovered in Fig. 6.9 is in Fig. 6.12covered with st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g>. These images show that the formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> needle-<str<strong>on</strong>g>like</str<strong>on</strong>g> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>sis favored at <str<strong>on</strong>g>and</str<strong>on</strong>g> below RT, whereas at elevated temperatures the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> tend to st<str<strong>on</strong>g>and</str<strong>on</strong>g> up<str<strong>on</strong>g>and</str<strong>on</strong>g> form mounds, which resemble the p6P mounds <strong>on</strong> sputtered mica.77
6 Hexaphenyl <strong>on</strong> KCl (001)Fig. 6.12: AFM images <str<strong>on</strong>g>of</str<strong>on</strong>g> the surface morphology <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>s grown <strong>on</strong> KClat different substrate temperatures: a) , b) , e) , f) . The insets showcorresp<strong>on</strong>dingAFM images. The cross secti<strong>on</strong>s al<strong>on</strong>g the white lines indicated in theinsets are presented in c), d), e) <str<strong>on</strong>g>and</str<strong>on</strong>g> f).78
6 Hexaphenyl <strong>on</strong> KCl (001)6.4 Thermal stability <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>s <strong>on</strong> KCl (001)So far, it is clear that the adsorpti<strong>on</strong> temperature has an important influence <strong>on</strong> themorphology <str<strong>on</strong>g>of</str<strong>on</strong>g> the p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>s. In additi<strong>on</strong>, it is interesting to find out what happens with <str<strong>on</strong>g>film</str<strong>on</strong>g>swhich are grown at a certain adsorpti<strong>on</strong> temperature <str<strong>on</strong>g>and</str<strong>on</strong>g> then heated. This is in particularimportant for possible applicati<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>s when grown at a certain temperature <str<strong>on</strong>g>and</str<strong>on</strong>g> thenexposed to a higher temperature. A further aspect c<strong>on</strong>cerns the stability <str<strong>on</strong>g>of</str<strong>on</strong>g> a <str<strong>on</strong>g>film</str<strong>on</strong>g> grown at lowtemperatures <str<strong>on</strong>g>and</str<strong>on</strong>g> then unfrozen to room temperature, where typically ex situ investigati<strong>on</strong>sare performed (AFM, XRD). In order to follow temperature-induced changes in situ,combined XPS <str<strong>on</strong>g>and</str<strong>on</strong>g> TDS measurements were carried out. The height <str<strong>on</strong>g>of</str<strong>on</strong>g> the XPS C1s signal isa measure <str<strong>on</strong>g>of</str<strong>on</strong>g> the area which is covered with the p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>. In Fig. 6.13 the XPS C1s peakheights <str<strong>on</strong>g>of</str<strong>on</strong>g> thick p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>s grown at different adsorpti<strong>on</strong> temperatures as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g>the sample temperature are shown <strong>on</strong> top. In additi<strong>on</strong>, the TDS spectrum <str<strong>on</strong>g>of</str<strong>on</strong>g> a representativethick p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> is shown at the bottom.Fig. 6.13: XPS C1s signal heights (top) during heating <str<strong>on</strong>g>and</str<strong>on</strong>g> TDS (bottom) <str<strong>on</strong>g>of</str<strong>on</strong>g>(001) grown at different adsorpti<strong>on</strong> temperatures.thick p6P <strong>on</strong> KCl79
6 Hexaphenyl <strong>on</strong> KCl (001)The comm<strong>on</strong> feature <str<strong>on</strong>g>of</str<strong>on</strong>g> all the different <str<strong>on</strong>g>film</str<strong>on</strong>g>s is the final decrease <str<strong>on</strong>g>of</str<strong>on</strong>g> the C1s signal around. As <strong>on</strong>e can see in the TDS below, this is the temperature regi<strong>on</strong> where the p6P<str<strong>on</strong>g>molecules</str<strong>on</strong>g> desorb from the surface. During desorpti<strong>on</strong>, first the height <str<strong>on</strong>g>of</str<strong>on</strong>g> the needles willdecrease <str<strong>on</strong>g>and</str<strong>on</strong>g>, since the mean free path <str<strong>on</strong>g>of</str<strong>on</strong>g> electr<strong>on</strong>s at kinetic energies <str<strong>on</strong>g>of</str<strong>on</strong>g> about inp6P is <strong>on</strong>ly (see secti<strong>on</strong> 6.1), the C1s signal will <strong>on</strong>ly change significantly when theheight <str<strong>on</strong>g>of</str<strong>on</strong>g> the needles is in the range <str<strong>on</strong>g>of</str<strong>on</strong>g> the mean free path. Comparing the initial XPS C1ssignals <str<strong>on</strong>g>of</str<strong>on</strong>g> all thethick p6P <str<strong>on</strong>g>film</str<strong>on</strong>g>s grown at different adsorpti<strong>on</strong> temperatures, <strong>on</strong>eobserves a significantly smaller signal for the <str<strong>on</strong>g>film</str<strong>on</strong>g> grown at. In order to underst<str<strong>on</strong>g>and</str<strong>on</strong>g>this, we have to look at Fig. 6.9a–d <str<strong>on</strong>g>and</str<strong>on</strong>g> at Fig. 6.14a, where an AFM image <str<strong>on</strong>g>of</str<strong>on</strong>g> athick<str<strong>on</strong>g>film</str<strong>on</strong>g> grown at is shown. In Fig. 6.14a, the whole surface is covered with short,r<str<strong>on</strong>g>and</str<strong>on</strong>g>omly oriented <str<strong>on</strong>g>and</str<strong>on</strong>g> distributed isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s. The lateral size <str<strong>on</strong>g>of</str<strong>on</strong>g> these isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s ranges fromup to . At this low temperature an almost ‘hit <str<strong>on</strong>g>and</str<strong>on</strong>g> stick’ behavior for theimpinging <str<strong>on</strong>g>molecules</str<strong>on</strong>g> can be assumed. The <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are not mobile enough to formel<strong>on</strong>gated <str<strong>on</strong>g>and</str<strong>on</strong>g> oriented needles <str<strong>on</strong>g>like</str<strong>on</strong>g> at higher temperatures. The height <str<strong>on</strong>g>of</str<strong>on</strong>g> these isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s islower; therefore, the isl<str<strong>on</strong>g>and</str<strong>on</strong>g> density is higher <str<strong>on</strong>g>and</str<strong>on</strong>g> more <str<strong>on</strong>g>of</str<strong>on</strong>g> the surface is covered with p6P thanat , where the isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s are already el<strong>on</strong>gated <str<strong>on</strong>g>and</str<strong>on</strong>g> slightly oriented. A similar behaviorhas already been observed for p6P <strong>on</strong> mica (Fig. 5.9) where the XPS C1s signal decreasedwith increasing adsorpti<strong>on</strong> temperature. On mica, the height <str<strong>on</strong>g>of</str<strong>on</strong>g> the p6P needles increased withincreasing adsorpti<strong>on</strong> temperatures, which came al<strong>on</strong>g with a decrease <str<strong>on</strong>g>of</str<strong>on</strong>g> the needle density.The difference <strong>on</strong> KCl is that the XPS C1s signal has a minimum at<str<strong>on</strong>g>and</str<strong>on</strong>g> thenincreases again with increasing adsorpti<strong>on</strong> temperatures (up to). This can be explainedby comparing Fig. 6.9a, 6.9b <str<strong>on</strong>g>and</str<strong>on</strong>g> 6.9c. At , all the p6P <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are lying flat <strong>on</strong> thesurface, forming high el<strong>on</strong>gated isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s. At temperatures above room temperature, patches <str<strong>on</strong>g>of</str<strong>on</strong>g>st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are formed. The <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in these patches cover a larger part <str<strong>on</strong>g>of</str<strong>on</strong>g> thesurface than the same amount <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in the form <str<strong>on</strong>g>of</str<strong>on</strong>g> needles would do. Therefore, thefree surface area decreases for adsorpti<strong>on</strong> temperatures abovealthough the needleskeep growing higher. At even more <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing upright than at .Regarding the stability <str<strong>on</strong>g>of</str<strong>on</strong>g> the different <str<strong>on</strong>g>film</str<strong>on</strong>g>s, <strong>on</strong>e sees in Fig. 6.13 that the <str<strong>on</strong>g>film</str<strong>on</strong>g>s grown at <str<strong>on</strong>g>and</str<strong>on</strong>g>above room temperature remain stable up<strong>on</strong> temperature changes, whereas <str<strong>on</strong>g>film</str<strong>on</strong>g>s grown belowroom temperature () exhibit a change in the XPS C1s signal height duringheating already before desorpti<strong>on</strong> starts. This indicates a recrystallizati<strong>on</strong> taking place80
6 Hexaphenyl <strong>on</strong> KCl (001)between <str<strong>on</strong>g>and</str<strong>on</strong>g> . On the other h<str<strong>on</strong>g>and</str<strong>on</strong>g>, these <str<strong>on</strong>g>film</str<strong>on</strong>g>s remain stable up to roomtemperature, which means that no informati<strong>on</strong> <strong>on</strong> these <str<strong>on</strong>g>film</str<strong>on</strong>g>s is lost during ‘unfreezing’. Toobtain informati<strong>on</strong> <strong>on</strong> the morphology <str<strong>on</strong>g>of</str<strong>on</strong>g> such a recrystallized structure, a <str<strong>on</strong>g>film</str<strong>on</strong>g> was preparedat <str<strong>on</strong>g>and</str<strong>on</strong>g> then heated to , a temperature where the recrystallizati<strong>on</strong> has alreadytaken place. An AFM image <str<strong>on</strong>g>of</str<strong>on</strong>g> this particular <str<strong>on</strong>g>film</str<strong>on</strong>g> is shown in Fig. 6.14b. For comparis<strong>on</strong> inFig. 6.14a a <str<strong>on</strong>g>film</str<strong>on</strong>g> grown at 90 K is shown which has not been heated.Fig. 6.14: AFM images <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <strong>on</strong> KCl (001): (a) grown at , image taken at roomtemperature, Z-scale: ; (b) grown at <str<strong>on</strong>g>and</str<strong>on</strong>g> then heated to , Z-scale: ; (c)cross secti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> (a); (d) cross secti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> (b)81
6 Hexaphenyl <strong>on</strong> KCl (001)Comparing the <str<strong>on</strong>g>film</str<strong>on</strong>g> ‘before’ (Fig. 6.14a) <str<strong>on</strong>g>and</str<strong>on</strong>g> ‘after’ (Fig. 6.14b) recrystallizati<strong>on</strong>, <strong>on</strong>eobserves a significant change <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g> morphology. While in Fig. 6.14a nearly all the KClsurface is covered with r<str<strong>on</strong>g>and</str<strong>on</strong>g>omly arranged <str<strong>on</strong>g>and</str<strong>on</strong>g> r<str<strong>on</strong>g>and</str<strong>on</strong>g>omly oriented isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s with an averageheight <str<strong>on</strong>g>of</str<strong>on</strong>g> (Fig. 6.14c), we see in Fig. 6.14b that parts <str<strong>on</strong>g>of</str<strong>on</strong>g> the surface remain uncovered <str<strong>on</strong>g>and</str<strong>on</strong>g>that the isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s with an average height <str<strong>on</strong>g>of</str<strong>on</strong>g> (Fig. 5d) are already oriented according to thefour-fold symmetry <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate. Thus, the result <str<strong>on</strong>g>of</str<strong>on</strong>g> the recrystallizati<strong>on</strong> is a <str<strong>on</strong>g>film</str<strong>on</strong>g>c<strong>on</strong>sisting <str<strong>on</strong>g>of</str<strong>on</strong>g> slightly el<strong>on</strong>gated <str<strong>on</strong>g>and</str<strong>on</strong>g> ordered isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s which are significantly higher than the<strong>on</strong>es before the recrystallizati<strong>on</strong>.82
7 HATCN <strong>on</strong> Au (111)7 HATCN <strong>on</strong> Au (111)In this chapter the purificati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the used HATCN <str<strong>on</strong>g>molecules</str<strong>on</strong>g> is explained in detail. The drychemical as received from the group <str<strong>on</strong>g>of</str<strong>on</strong>g> Klaus Müllen (Max Planck Institute <str<strong>on</strong>g>of</str<strong>on</strong>g> PolymerResearch in Mainz) c<strong>on</strong>tained to some extent residual solvent, namely acet<strong>on</strong>itrile (CH 3 CN),which could be eliminated by annealing the source. The layer <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> this <str<strong>on</strong>g>disc</str<strong>on</strong>g>oidmolecular acceptor is rather complicated. The same applies to the desorpti<strong>on</strong> kinetics, whichcould be elucidated by the applicati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the heating rate variati<strong>on</strong> method though, asdescribed in secti<strong>on</strong> 4.3.1, <str<strong>on</strong>g>and</str<strong>on</strong>g> by doing ‘XPS vs temperature’ measurements. SupplementalXRD <str<strong>on</strong>g>and</str<strong>on</strong>g> AFM investigati<strong>on</strong>s corroborated the <str<strong>on</strong>g>growth</str<strong>on</strong>g> model which arose from the thermaldesorpti<strong>on</strong> studies.7.1 Purificati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <str<strong>on</strong>g>and</str<strong>on</strong>g> the cracking patternIn order to determine the cracking pattern <str<strong>on</strong>g>and</str<strong>on</strong>g> to purify the HATCN dry chemical, the<str<strong>on</strong>g>molecules</str<strong>on</strong>g> were desorbed directly from the Knudsen cell into the QMS. A mass spectrum <str<strong>on</strong>g>of</str<strong>on</strong>g>the HATCN material as received atis shown in Fig. 7.1. The base pressure in thevacuum chamber was around, therefore the signal <str<strong>on</strong>g>of</str<strong>on</strong>g> residual gas comp<strong>on</strong>ents<str<strong>on</strong>g>like</str<strong>on</strong>g> CO, N 2 , O 2 or H 2 O was rather high. Nevertheless, also a high amount <str<strong>on</strong>g>of</str<strong>on</strong>g> the solventacet<strong>on</strong>itrile (CH 3 CN) was found. During l<strong>on</strong>g-time heating <str<strong>on</strong>g>of</str<strong>on</strong>g> the source () up to, the decrease <str<strong>on</strong>g>of</str<strong>on</strong>g> the acet<strong>on</strong>itrile signal could be followed until the signal eventuallydisappeared. After the purificati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the HATCN material the cracking pattern (Fig. 7.2) wasdetermined by heating the source to about . Due to the high i<strong>on</strong>izati<strong>on</strong> energy( ) <str<strong>on</strong>g>of</str<strong>on</strong>g> the QMS that was used, the signal <str<strong>on</strong>g>of</str<strong>on</strong>g> the intact molecule is relatively smallcompared to the signals <str<strong>on</strong>g>of</str<strong>on</strong>g> its cracking p<str<strong>on</strong>g>rod</str<strong>on</strong>g>ucts. A detailed view <str<strong>on</strong>g>of</str<strong>on</strong>g> the higher masses isshown in Fig. 7.3, where the interval from to <str<strong>on</strong>g>of</str<strong>on</strong>g> the same crackingpattern is shown.83
7 HATCN <strong>on</strong> Au (111)Fig. 7.1: Mass spectrum <str<strong>on</strong>g>of</str<strong>on</strong>g> the HATCN material as received, taken at. Apart from residualgas, the solvent acet<strong>on</strong>itrile (CH 3 CN,) was seen, which stems from the synthesis <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN.Fig. 7.2: Cracking pattern <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN, source heated topeaks with the highest intensity, are indicated <str<strong>on</strong>g>and</str<strong>on</strong>g> identified.. Selected peak positi<strong>on</strong>s, mostly <str<strong>on</strong>g>of</str<strong>on</strong>g> the84
7 HATCN <strong>on</strong> Au (111)Fig. 7.3: Detailed view <str<strong>on</strong>g>of</str<strong>on</strong>g> the cracking pattern for masses above<str<strong>on</strong>g>and</str<strong>on</strong>g> identificati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the peaks.plus peak positi<strong>on</strong>sThe cracking pattern c<strong>on</strong>tains a variety <str<strong>on</strong>g>of</str<strong>on</strong>g> different peaks, which can predominantly beidentified as C x N x fragments. The mass <str<strong>on</strong>g>of</str<strong>on</strong>g> the intact molecule is correctly found to be. Most <str<strong>on</strong>g>of</str<strong>on</strong>g> the peaks in the cracking pattern can be correlated to each other byadding or subtracting(C 2 N 2 ), which is <strong>on</strong>e <str<strong>on</strong>g>of</str<strong>on</strong>g> the most <str<strong>on</strong>g>like</str<strong>on</strong>g>ly formed crackingp<str<strong>on</strong>g>rod</str<strong>on</strong>g>ucts am<strong>on</strong>g <str<strong>on</strong>g>and</str<strong>on</strong>g> . For the multiplexed TDS experiments, the QMSwas tuned to the masses with the highest intensities <str<strong>on</strong>g>like</str<strong>on</strong>g><str<strong>on</strong>g>and</str<strong>on</strong>g>. In successive TD experiments <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g>s desorbing from Au (111) <str<strong>on</strong>g>and</str<strong>on</strong>g> Ag (111) it turnedout that except , all the other multiplexed masses proceeded proporti<strong>on</strong>al to eachother if no cracking <strong>on</strong> the substrate occurred, which was the case for the Au substrate.Therefore in the TDS images shown in this chapter <strong>on</strong>ly the mass with the highest intensity isshown, which was in our case . On Ag (111), it was observed that apart fromcracking in the QMS, also cracking <strong>on</strong> the surface occurred (more details see chapter 8).85
7 HATCN <strong>on</strong> Au (111)7.2 The initial layer <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <strong>on</strong> Au (111)A typical series <str<strong>on</strong>g>of</str<strong>on</strong>g> TD-spectra <str<strong>on</strong>g>of</str<strong>on</strong>g> the extended m<strong>on</strong>olayer regime (nominal mean <str<strong>on</strong>g>film</str<strong>on</strong>g>thickness) is shown in Fig. 7.4. This series was obtained from <str<strong>on</strong>g>film</str<strong>on</strong>g>s that were grown ata substrate temperature <str<strong>on</strong>g>of</str<strong>on</strong>g>. Equivalent spectra were obtained for <str<strong>on</strong>g>film</str<strong>on</strong>g>s grown fromup to . In order to get the correct values for the absolute desorpti<strong>on</strong> rate thatare shown in Fig. 7.4, a correlati<strong>on</strong> between the measured quartz frequency change <str<strong>on</strong>g>and</str<strong>on</strong>g> theadsorbed amount <strong>on</strong> the sample surface in terms <str<strong>on</strong>g>of</str<strong>on</strong>g> m<strong>on</strong>olayers had to be found. For thispurpose <str<strong>on</strong>g>disc</str<strong>on</strong>g>oid <str<strong>on</strong>g>molecules</str<strong>on</strong>g> (van der Waals diameterwere assumed. The number<str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in a densely packed layer <str<strong>on</strong>g>of</str<strong>on</strong>g> circular disks can be calculated to be. This corresp<strong>on</strong>ds to a quartz frequency change <str<strong>on</strong>g>of</str<strong>on</strong>g> for theexperimental setup (see table 4.1). With this informati<strong>on</strong>, a quantitative evaluati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> thedesorpti<strong>on</strong> spectra can be performed. A frequency change <str<strong>on</strong>g>of</str<strong>on</strong>g>at the quartz microbalancecorresp<strong>on</strong>ds to a thickness increase <str<strong>on</strong>g>of</str<strong>on</strong>g>(see again table 4.1). In Ref. [66] a differentvalue for the density <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN was used ( instead <str<strong>on</strong>g>of</str<strong>on</strong>g> [58]) which led to a differentc<strong>on</strong>versi<strong>on</strong> factor between the frequency change in<str<strong>on</strong>g>and</str<strong>on</strong>g> the mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness( instead <str<strong>on</strong>g>of</str<strong>on</strong>g> ).86
7 HATCN <strong>on</strong> Au (111)Fig. 7.4: Series <str<strong>on</strong>g>of</str<strong>on</strong>g> thermal desorpti<strong>on</strong> spectra <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <strong>on</strong> Au (111) as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> coverage.Adsorpti<strong>on</strong> temperature , heating rate: . The chemical structure <str<strong>on</strong>g>of</str<strong>on</strong>g> a single HATCNmolecule is depicted in the insert.The evoluti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the desorpti<strong>on</strong> spectra can be divided into four different regimes:• For coverages corresp<strong>on</strong>ding to less than, (as based <strong>on</strong> our coveragecalibrati<strong>on</strong> with the quartz microbalance) HATCN desorbs at. A small shiftto lower temperature (by ) with increasing coverage is observed. This peak islabeled as β 1 <str<strong>on</strong>g>and</str<strong>on</strong>g> it corresp<strong>on</strong>ds to a coverage <str<strong>on</strong>g>of</str<strong>on</strong>g> about ½ m<strong>on</strong>olayer <str<strong>on</strong>g>of</str<strong>on</strong>g> flat lying<str<strong>on</strong>g>molecules</str<strong>on</strong>g>.• For coverages abovethe desorpti<strong>on</strong> spectra exhibit a shoulder at the leadingedge <str<strong>on</strong>g>of</str<strong>on</strong>g> the β 1 peak which shifts c<strong>on</strong>siderably (by more than) when increasingthe coverage up to about(full m<strong>on</strong>olayer). This additi<strong>on</strong>al feature is designated87
7 HATCN <strong>on</strong> Au (111)as the β 2 peak <str<strong>on</strong>g>and</str<strong>on</strong>g> signifies the completi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the first layer <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> lying flat <strong>on</strong>the surface.• With further increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the coverage a third, sharp desorpti<strong>on</strong> peak appears at. This peak increases in intensity up to a coverage <str<strong>on</strong>g>of</str<strong>on</strong>g> , with a slightshift <str<strong>on</strong>g>of</str<strong>on</strong>g> the peak to lower temperature. This peak is designated as the γ peak, whichcorresp<strong>on</strong>ds to a 2 nd layer <str<strong>on</strong>g>of</str<strong>on</strong>g> flat lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g>.• Interestingly, a further increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the coverage leads to the disappearance <str<strong>on</strong>g>of</str<strong>on</strong>g> the γpeak <str<strong>on</strong>g>and</str<strong>on</strong>g> a new peak at a higher temperature ( ) emerges. This latter peak,designated as the α peak, shows the typical feature <str<strong>on</strong>g>of</str<strong>on</strong>g> zero order desorpti<strong>on</strong> (comm<strong>on</strong>leading edge for various initial coverages) <str<strong>on</strong>g>and</str<strong>on</strong>g> corresp<strong>on</strong>ds to desorpti<strong>on</strong> from themultilayer, which is going to be described in detail in the following secti<strong>on</strong>s. Thus, thesec<strong>on</strong>d layer <str<strong>on</strong>g>of</str<strong>on</strong>g> flat lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g> is integrated into the bulk structure.The calculati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the kinetic desorpti<strong>on</strong> parameters <str<strong>on</strong>g>of</str<strong>on</strong>g> the β 1 <str<strong>on</strong>g>and</str<strong>on</strong>g> the γ peak was performedusing the heating rate variati<strong>on</strong> method (described in secti<strong>on</strong> 4.3.1 Theoretical background).For that purpose five different heating rates β were applied. In Fig. 7.5, a series <str<strong>on</strong>g>of</str<strong>on</strong>g> TD-spectrawith approximately the same <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness ( ), but different heating rates is presented.The peak maxima <str<strong>on</strong>g>of</str<strong>on</strong>g> the particular peaks shift to higher temperatures with increasing heatingrates <str<strong>on</strong>g>and</str<strong>on</strong>g> the extent <str<strong>on</strong>g>of</str<strong>on</strong>g> this shift gives informati<strong>on</strong> <strong>on</strong> the desorpti<strong>on</strong> energy <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>on</strong> the preexp<strong>on</strong>entialfactor, as described in eq. 4.11.88
7 HATCN <strong>on</strong> Au (111)Fig. 7.5: Top: Series <str<strong>on</strong>g>of</str<strong>on</strong>g> TD-spectra obtained by applying different heating rates. Nominal mean <str<strong>on</strong>g>film</str<strong>on</strong>g>thickness in all cases . The shift <str<strong>on</strong>g>of</str<strong>on</strong>g> the peak maxima is used for the calculati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the kineticdesorpti<strong>on</strong> parameters. In this figure, the intensity <str<strong>on</strong>g>of</str<strong>on</strong>g> the individual spectra was normalized so as tovisualize the shift <str<strong>on</strong>g>of</str<strong>on</strong>g> the peak maxima. Bottom: Evaluati<strong>on</strong> for the γ peak <str<strong>on</strong>g>and</str<strong>on</strong>g> the β 1 peak. ParametersA <str<strong>on</strong>g>and</str<strong>on</strong>g> B <str<strong>on</strong>g>of</str<strong>on</strong>g> the linear fits can be used to calculate <str<strong>on</strong>g>and</str<strong>on</strong>g> : ,89
7 HATCN <strong>on</strong> Au (111)The shift <str<strong>on</strong>g>of</str<strong>on</strong>g> the peak maximum <str<strong>on</strong>g>of</str<strong>on</strong>g> the β 1 peak is, the shift <str<strong>on</strong>g>of</str<strong>on</strong>g> the γ peak <strong>on</strong>ly. From this finding <strong>on</strong>e could already guess that the kinetic parameters <str<strong>on</strong>g>of</str<strong>on</strong>g> these twoadsorpti<strong>on</strong>/desorpti<strong>on</strong> states must be very different. A quantitative evaluati<strong>on</strong> (Fig. 7.5,bottom) <str<strong>on</strong>g>of</str<strong>on</strong>g> various sets <str<strong>on</strong>g>of</str<strong>on</strong>g> desorpti<strong>on</strong> spectra, each set c<strong>on</strong>taining spectra with differentheating rates <str<strong>on</strong>g>and</str<strong>on</strong>g> same <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness, indeed yielded<str<strong>on</strong>g>and</str<strong>on</strong>g>Surprisingly, the binding energies <str<strong>on</strong>g>of</str<strong>on</strong>g> both states are calculated to be pretty similar, whereasapparently the difference <str<strong>on</strong>g>of</str<strong>on</strong>g> their desorpti<strong>on</strong> temperatures is <strong>on</strong>ly caused by different preexp<strong>on</strong>entialfactors. For the β 2 state the peak maxima are not defined well enough to performa quantitative evaluati<strong>on</strong>, but from the qualitative similarity to the β 1 peak behavior a similarpre-exp<strong>on</strong>ential factor as for the β 1 state is proposed. The quantitative error for this type <str<strong>on</strong>g>of</str<strong>on</strong>g>evaluati<strong>on</strong> is by nature larger than for a leading edge evaluati<strong>on</strong>, due to the limited range <str<strong>on</strong>g>of</str<strong>on</strong>g>the heating rate variati<strong>on</strong> (<strong>on</strong>ly five different heating rates were applied). The physical90
7 HATCN <strong>on</strong> Au (111)meaning <str<strong>on</strong>g>of</str<strong>on</strong>g> the values <str<strong>on</strong>g>of</str<strong>on</strong>g> these parameters will be <str<strong>on</strong>g>disc</str<strong>on</strong>g>ussed in detail in the next secti<strong>on</strong>, afterthe calculati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the kinetic parameters <str<strong>on</strong>g>of</str<strong>on</strong>g> the multilayer (α) peak.7.3 Further development <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>Having a closer look at Fig. 7.4, <strong>on</strong>e notices that the <strong>on</strong>set <str<strong>on</strong>g>of</str<strong>on</strong>g> the black curve ( ) is not‘smooth’, i.e. a certain amount <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> still resides in the γ state, <str<strong>on</strong>g>and</str<strong>on</strong>g> that in the rangefrom up to the black curve proceeds below the other <strong>on</strong>es, indicating a slightdecrease <str<strong>on</strong>g>of</str<strong>on</strong>g> the high-temperature β peaks with increasing <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness. The latterobservati<strong>on</strong> can also be followed explicitly in Fig. 7.6, which shows a series <str<strong>on</strong>g>of</str<strong>on</strong>g> thermaldesorpti<strong>on</strong> spectra from up to mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness.Fig. 7.6: Series <str<strong>on</strong>g>of</str<strong>on</strong>g> thermal desorpti<strong>on</strong> spectra <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <strong>on</strong> Au (111) for larger coverages.Adsorpti<strong>on</strong> temperature: , heating rate . a) – d): Different initial coverages.91
7 HATCN <strong>on</strong> Au (111)With increasing initial coverage the amount <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> desorbing from the β 1 <str<strong>on</strong>g>and</str<strong>on</strong>g> β 2peak decreases c<strong>on</strong>tinuously. At a coverage <str<strong>on</strong>g>of</str<strong>on</strong>g> less than 20 % <str<strong>on</strong>g>of</str<strong>on</strong>g> a m<strong>on</strong>olayer (flat lying<str<strong>on</strong>g>molecules</str<strong>on</strong>g>) desorb from the β 1 <str<strong>on</strong>g>and</str<strong>on</strong>g> β 2 states <str<strong>on</strong>g>and</str<strong>on</strong>g> no <str<strong>on</strong>g>molecules</str<strong>on</strong>g> from the γ state anymore. Thus,not <strong>on</strong>ly the low-temperature γ peak gets integrated into the bulk phase, but also the hightemperatureβ peaks. This finding is illustrated in Fig. 7.7, where the amount <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> inthe separate adsorpti<strong>on</strong> states is plotted as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the nominal mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness.Fig. 7.7: Integrals <str<strong>on</strong>g>of</str<strong>on</strong>g> the separated TDS peaks <str<strong>on</strong>g>and</str<strong>on</strong>g> their sum from measurements such as in Fig. 7.4<str<strong>on</strong>g>and</str<strong>on</strong>g> Fig. 7.6 as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness. Heating ratein all cases. The parallelslopes indicate equal sticking coefficients for the various states which then is very <str<strong>on</strong>g>like</str<strong>on</strong>g>ly equal tounity.The linear slope <str<strong>on</strong>g>of</str<strong>on</strong>g> the overall sum <str<strong>on</strong>g>of</str<strong>on</strong>g> the peaks indicates a c<strong>on</strong>stant sticking coefficientduring the whole adsorpti<strong>on</strong> process. The same holds for each <str<strong>on</strong>g>of</str<strong>on</strong>g> the various adsorpti<strong>on</strong> states<str<strong>on</strong>g>and</str<strong>on</strong>g> thus it is reas<strong>on</strong>able to assume a sticking coefficient <str<strong>on</strong>g>of</str<strong>on</strong>g> 1. The curve subtends the zero-92
7 HATCN <strong>on</strong> Au (111)point, which indicates that all the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> which were adsorbed at the surface subsequentlydesorbed from the surface up<strong>on</strong> heating. In other words: there is no dissociati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> theHATCN <str<strong>on</strong>g>molecules</str<strong>on</strong>g> <strong>on</strong> the Au (111) surface.The leading edge <str<strong>on</strong>g>of</str<strong>on</strong>g> the multilayer peak can be used to calculate the desorpti<strong>on</strong> energy <str<strong>on</strong>g>and</str<strong>on</strong>g> thepre-exp<strong>on</strong>ential factor:Comparing the desorpti<strong>on</strong> parameters <str<strong>on</strong>g>of</str<strong>on</strong>g> the three distinct adsorpti<strong>on</strong> states (α, β 1 , γ), whichhave been determined in this secti<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> in the previous <strong>on</strong>e, it attracts attenti<strong>on</strong> that all thedesorpti<strong>on</strong> energies are nearly the same (within the error). Actually, there is even an oppositetrend between the desorpti<strong>on</strong> peak maxima <str<strong>on</strong>g>and</str<strong>on</strong>g> the desorpti<strong>on</strong> energies. Hence, the difference<str<strong>on</strong>g>of</str<strong>on</strong>g> the desorpti<strong>on</strong> temperatures for the individual adsorpti<strong>on</strong> states is mainly determined by thepre-exp<strong>on</strong>ential factors. Both, the multilayer peak (α) as well as the sec<strong>on</strong>d layer peak (γ)exhibit the large pre-exp<strong>on</strong>ential factor expected for large <str<strong>on</strong>g>molecules</str<strong>on</strong>g>. Surprisingly, them<strong>on</strong>olayer peaks (β) show a very small pre-exp<strong>on</strong>ential factor, usually <strong>on</strong>ly observed forsmall particle (atom, diatomic molecule) desorpti<strong>on</strong>. According to the definiti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the preexp<strong>on</strong>entialfactor ν (eq. 4.7), the partiti<strong>on</strong> functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in the first m<strong>on</strong>olayer issimilar to that in the gaseous phase at the desorpti<strong>on</strong> temperature. In other words, thetranslati<strong>on</strong>al <str<strong>on</strong>g>and</str<strong>on</strong>g> rotati<strong>on</strong>al degrees <str<strong>on</strong>g>of</str<strong>on</strong>g> freedom are already highly excited in the adsorbedphase when desorpti<strong>on</strong> starts. Apparently, the activati<strong>on</strong> energy for surface diffusi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> thelarge HATCN <str<strong>on</strong>g>molecules</str<strong>on</strong>g> <strong>on</strong> the inert gold surface is very small <str<strong>on</strong>g>and</str<strong>on</strong>g> the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> behave <str<strong>on</strong>g>like</str<strong>on</strong>g>a two-dimensi<strong>on</strong>al gas. The high pre-exp<strong>on</strong>ential factors for desorpti<strong>on</strong> from the 2 nd layer <str<strong>on</strong>g>and</str<strong>on</strong>g>93
7 HATCN <strong>on</strong> Au (111)the bulk phase indicate that <strong>on</strong>ly little excitati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the various degrees <str<strong>on</strong>g>of</str<strong>on</strong>g> freedom takes placeprior to desorpti<strong>on</strong>. This is the case if the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are rather immobile.As a matter <str<strong>on</strong>g>of</str<strong>on</strong>g> fact all the informati<strong>on</strong> gained by TDS is <strong>on</strong>ly valid at or directly below thedesorpti<strong>on</strong> temperature. In order to find out whether the integrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the first two layers (β +γ) into the bulk phase α happens up<strong>on</strong> adsorpti<strong>on</strong> or during heating the substrate, it isnecessary to examine the temperature dependence <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g> morphology.7.4 The influence <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate temperatureXPS was used to analyze the <str<strong>on</strong>g>film</str<strong>on</strong>g> morphology in-situ. Fig. 7.8 shows the height <str<strong>on</strong>g>of</str<strong>on</strong>g> the C1s,N1s <str<strong>on</strong>g>and</str<strong>on</strong>g> the Au4f 7/2 XPS signals for a <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness <str<strong>on</strong>g>of</str<strong>on</strong>g>, which corresp<strong>on</strong>ds to, as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate temperature during <str<strong>on</strong>g>film</str<strong>on</strong>g> preparati<strong>on</strong>. The signalsremain more or less c<strong>on</strong>stant up to , but when going to higher temperatures thesubstrate signal increases while the adsorbate signals decrease. This indicates a change in <str<strong>on</strong>g>film</str<strong>on</strong>g>morphology above room temperature, which will be explained in detail in the following.94
7 HATCN <strong>on</strong> Au (111)Fig.7.8: XPS signal heights <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate (Au4f 7/2 ) <str<strong>on</strong>g>and</str<strong>on</strong>g> the HATCN <str<strong>on</strong>g>molecules</str<strong>on</strong>g> (C1s <str<strong>on</strong>g>and</str<strong>on</strong>g> N1s) as afuncti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> substrate temperature during <str<strong>on</strong>g>film</str<strong>on</strong>g> preparati<strong>on</strong>. The sharp bend at room temperature pointsout a change <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g> morphology. corresp<strong>on</strong>d to a mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness <str<strong>on</strong>g>of</str<strong>on</strong>g> .The next figures show the XPS signal heights against the <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness for two <str<strong>on</strong>g>film</str<strong>on</strong>g>s, <strong>on</strong>e <str<strong>on</strong>g>of</str<strong>on</strong>g>them prepared at a rather low temperature (, Fig. 7.9), the other <strong>on</strong>e prepared at arather high temperature ( , Fig. 7.10).Fig.7.9: XPS signal heights <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate (Au 4f 7/2 ) <str<strong>on</strong>g>and</str<strong>on</strong>g> the HATCN <str<strong>on</strong>g>molecules</str<strong>on</strong>g> (C1s <str<strong>on</strong>g>and</str<strong>on</strong>g> N1s) as afuncti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness. Substrate temperature:. Blue: Exp<strong>on</strong>ential decay fitted to theAu4f signal which yields for the inelastic mean free path <str<strong>on</strong>g>of</str<strong>on</strong>g> the Au4f electr<strong>on</strong>s .The substrate signal in Fig. 7.9 can be approximated by an exp<strong>on</strong>ential decay,95
7 HATCN <strong>on</strong> Au (111)(7.1)indicating that at this temperature the multilayer grows in a layer-by-layer <str<strong>on</strong>g>like</str<strong>on</strong>g> fashi<strong>on</strong> (hit<str<strong>on</strong>g>and</str<strong>on</strong>g> stick behavior). From the exp<strong>on</strong>ential signal decay an inelastic mean free path for theAu4f photoelectr<strong>on</strong>s ( ) <str<strong>on</strong>g>of</str<strong>on</strong>g> is determined. This value issignificantly lower than the inelastic mean free path <str<strong>on</strong>g>of</str<strong>on</strong>g> electr<strong>on</strong>s in p6P ( ) [60]<str<strong>on</strong>g>and</str<strong>on</strong>g> p4P ( ) [30].Fig. 7.10: XPS signal heights <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrate (Au4f 7/2 ) <str<strong>on</strong>g>and</str<strong>on</strong>g> the HATCN <str<strong>on</strong>g>molecules</str<strong>on</strong>g> (C1s <str<strong>on</strong>g>and</str<strong>on</strong>g> N1s) as afuncti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness. Substrate temperature: .The substrate signal in Fig. 7.10 decreases <strong>on</strong>ly at the beginning <str<strong>on</strong>g>and</str<strong>on</strong>g> remains almost c<strong>on</strong>stantabove . This is a sign <str<strong>on</strong>g>of</str<strong>on</strong>g> str<strong>on</strong>g dewetting, i.e. isl<str<strong>on</strong>g>and</str<strong>on</strong>g> formati<strong>on</strong>. Additi<strong>on</strong>al <str<strong>on</strong>g>molecules</str<strong>on</strong>g>tend to join the isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s instead <str<strong>on</strong>g>of</str<strong>on</strong>g> covering the substrate.96
7 HATCN <strong>on</strong> Au (111)In Fig. 7.11a <str<strong>on</strong>g>and</str<strong>on</strong>g> 7.11b AFM images are presented forthick HATCN <str<strong>on</strong>g>film</str<strong>on</strong>g>s grown <strong>on</strong>the Au (111) substrate at <str<strong>on</strong>g>and</str<strong>on</strong>g> at . The AFM image <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g> grown atshows that the Au surface is nearly completely covered with the <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g>, c<strong>on</strong>sisting <str<strong>on</strong>g>of</str<strong>on</strong>g>grains with an average diameter <str<strong>on</strong>g>of</str<strong>on</strong>g>. From a cross secti<strong>on</strong> analysis a mean grainheight <str<strong>on</strong>g>of</str<strong>on</strong>g>is obtained.Fig. 7.11: a) AFM image in ‘shaded mode’ <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <str<strong>on</strong>g>film</str<strong>on</strong>g>s <strong>on</strong> Au (111) with a mean thickness <str<strong>on</strong>g>of</str<strong>on</strong>g>, taken ex-situ at room temperature. Substrate temperature during HATCN adsorpti<strong>on</strong>: .The insert a1) shows an enlarged detail. The cross secti<strong>on</strong> is taken al<strong>on</strong>g the line depicted in the AFMimage. b) AFM image in ‘shaded mode’ <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <str<strong>on</strong>g>film</str<strong>on</strong>g>s <strong>on</strong> Au (111) with a mean thickness <str<strong>on</strong>g>of</str<strong>on</strong>g>97
7 HATCN <strong>on</strong> Au (111), taken ex-situ at room temperature. Substrate temperature during HATCN adsorpti<strong>on</strong>: .The cross secti<strong>on</strong> is taken al<strong>on</strong>g the line depicted in the AFM image.A significant difference was observed for the <str<strong>on</strong>g>film</str<strong>on</strong>g> prepared at a substrate temperature <str<strong>on</strong>g>of</str<strong>on</strong>g>. The HATCN <str<strong>on</strong>g>film</str<strong>on</strong>g> shown in Fig. 7.11b c<strong>on</strong>sists <str<strong>on</strong>g>of</str<strong>on</strong>g> large isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s <str<strong>on</strong>g>of</str<strong>on</strong>g> coalesced grains(typical size ). A cross secti<strong>on</strong> reveals a mean height <str<strong>on</strong>g>of</str<strong>on</strong>g> the grains <str<strong>on</strong>g>of</str<strong>on</strong>g> about .Thus the AFM images corroborate the results obtained by XPS.The thermal stability <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g> morphology was investigated in-situ by ‘XPS vs temperature’measurements. For that purpose several <str<strong>on</strong>g>film</str<strong>on</strong>g>s <str<strong>on</strong>g>of</str<strong>on</strong>g> different thickness were prepared at<str<strong>on</strong>g>and</str<strong>on</strong>g> the substrate Au4f signal was followed during heating <str<strong>on</strong>g>of</str<strong>on</strong>g> the sample (Fig. 7.12).Fig. 7.12: XPS Au4f substrate signal change during heating <str<strong>on</strong>g>of</str<strong>on</strong>g> the sample, for various initialcoverages <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN: a) , b) , c) , d) , e) , f) , g) , h) .Substrate temperature during <str<strong>on</strong>g>film</str<strong>on</strong>g> depositi<strong>on</strong>: , heating rate . The peak maxima <str<strong>on</strong>g>of</str<strong>on</strong>g> thecorresp<strong>on</strong>ding desorpti<strong>on</strong> peaks are indicated.98
7 HATCN <strong>on</strong> Au (111)Curve a) in Fig. 7.12 shows the XPS signal change vs. temperature without a <str<strong>on</strong>g>film</str<strong>on</strong>g>, indicatingthe signal attenuati<strong>on</strong> due to the Debye-Waller effect <str<strong>on</strong>g>and</str<strong>on</strong>g> other experimental reas<strong>on</strong>s. Forinitial HATCN coverages <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> the Au4f substrate signal intensity does notchange during sample heating between <str<strong>on</strong>g>and</str<strong>on</strong>g> , but then starts to increasecorresp<strong>on</strong>ding to the desorpti<strong>on</strong> from the β states (see Fig. 7.4). For the<str<strong>on</strong>g>film</str<strong>on</strong>g> the sharpsignal increase aroundcorresp<strong>on</strong>ds to the desorpti<strong>on</strong> from the γ state, followed by thesignal change between <str<strong>on</strong>g>and</str<strong>on</strong>g> due to desorpti<strong>on</strong> from the β states. At higherinitial coverages (), the Au4f signal intensity exhibits a first sharp increase already inthe range <str<strong>on</strong>g>of</str<strong>on</strong>g> to , signified by Tr. The further signal change up tocorrelates with the desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the multilayer (α peak in Fig. 7.4 & 7.6). The lowtemperatureAu4f intensity increase is, however, not correlated with any desorpti<strong>on</strong> (see Fig.7.4 & 7.6). This indicates that a morphological transiti<strong>on</strong> (Tr) from a layer-<str<strong>on</strong>g>like</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> to 3Disl<str<strong>on</strong>g>and</str<strong>on</strong>g>s occurs in the range <str<strong>on</strong>g>of</str<strong>on</strong>g> to . The Tr <strong>on</strong>set temperature slightly decreaseswith increasing coverage. It is proposed that at this transiti<strong>on</strong> temperature the integrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g>the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> that are lying in first two layers (β- <str<strong>on</strong>g>and</str<strong>on</strong>g> γ-state) into the 3-dimensi<strong>on</strong>al bulkstructure (α state) takes place. The driving force for the transiti<strong>on</strong> is presumably thataccording to Szalay et al. [58, 59] the HATCN <str<strong>on</strong>g>molecules</str<strong>on</strong>g> avoid π-stacking (parallelalignment). The crystal packing <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN is instead dominated by perpendicular CN-πinteracti<strong>on</strong>s (T-stacking). Therefore, small amounts <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in the 3 rd layer could act asnucleati<strong>on</strong> centers for the formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> 3-dimensi<strong>on</strong>al isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s which already exhibit the bulkcrystal structure.7.5 The bulk phase (3D-isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s)In order to verify the previous findings <str<strong>on</strong>g>and</str<strong>on</strong>g> the proposed <str<strong>on</strong>g>growth</str<strong>on</strong>g> model, the bulk structure hadto be examined. This was d<strong>on</strong>e by XRD measurements. For this purpose, thicker <str<strong>on</strong>g>film</str<strong>on</strong>g>s( ) were prepared. It is to menti<strong>on</strong> that the characteristics <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g>s shownpreviously in Fig. 7.11 do not change significantly up<strong>on</strong> adsorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> additi<strong>on</strong>al material.AFM images <str<strong>on</strong>g>of</str<strong>on</strong>g> much thicker <str<strong>on</strong>g>film</str<strong>on</strong>g>s ( ), prepared at RT <str<strong>on</strong>g>and</str<strong>on</strong>g> at , are shown inFig. 7.13 & Fig. 7.14.99
7 HATCN <strong>on</strong> Au (111)Fig. 7.13: AFM images in ‘shaded mode’ <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <str<strong>on</strong>g>film</str<strong>on</strong>g>s <strong>on</strong> Au (111) with a mean thickness <str<strong>on</strong>g>of</str<strong>on</strong>g>, taken ex-situ at room temperature. Substrate temperature during HATCN adsorpti<strong>on</strong>:. Z-Scale: left: , right: . The right image is a detail view taken from the center<str<strong>on</strong>g>of</str<strong>on</strong>g> the left image.Fig. 7.14: AFM images in ‘shaded mode’ <str<strong>on</strong>g>of</str<strong>on</strong>g> a HATCN <str<strong>on</strong>g>film</str<strong>on</strong>g> at different positi<strong>on</strong>s <strong>on</strong> the Au (111)surface with a mean thickness <str<strong>on</strong>g>of</str<strong>on</strong>g>, taken ex-situ at room temperature. Substrate temperatureduring HATCN adsorpti<strong>on</strong>: . Z-Scale <str<strong>on</strong>g>of</str<strong>on</strong>g> both images: .100
7 HATCN <strong>on</strong> Au (111)X-ray pole figure measurements which were performed at athick <str<strong>on</strong>g>film</str<strong>on</strong>g> grown atshow that this <str<strong>on</strong>g>film</str<strong>on</strong>g> is composed <str<strong>on</strong>g>of</str<strong>on</strong>g> small crystallites, which exhibit a structurecorresp<strong>on</strong>ding to the bulk structure <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN (see secti<strong>on</strong> 3.2, Fig. 3.3): The <str<strong>on</strong>g>molecules</str<strong>on</strong>g>crystallize in a hexag<strong>on</strong>al symmetry with 18 <str<strong>on</strong>g>molecules</str<strong>on</strong>g> per unit cell resulting in a complexthree-dimensi<strong>on</strong>al network formed by T-stacking <str<strong>on</strong>g>of</str<strong>on</strong>g> neighboring <str<strong>on</strong>g>molecules</str<strong>on</strong>g>. No significantline broadening <str<strong>on</strong>g>of</str<strong>on</strong>g> the diffracti<strong>on</strong> peaks is observed which reveals the str<strong>on</strong>g tendency <str<strong>on</strong>g>of</str<strong>on</strong>g>HATCN for crystallizati<strong>on</strong> with crystallite sizes <str<strong>on</strong>g>of</str<strong>on</strong>g> at least . From the pole figuremeasurements (Fig. 7.15) a preferential orientati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the crystallites with the (143)orientati<strong>on</strong> (Fig. 7.16) parallel to the substrate surface, but with r<str<strong>on</strong>g>and</str<strong>on</strong>g>om azimuthal orientati<strong>on</strong>is obtained. In the (143) orientati<strong>on</strong> <strong>on</strong>e third <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> is orientated parallel (flat-<strong>on</strong>)to the substrate. The residual <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are oriented perpendicular to the flat-<strong>on</strong> <str<strong>on</strong>g>molecules</str<strong>on</strong>g><str<strong>on</strong>g>and</str<strong>on</strong>g> in turn perpendicular with respect to each other. In all three molecular c<strong>on</strong>formati<strong>on</strong>s the<str<strong>on</strong>g>molecules</str<strong>on</strong>g> which are parallel to each other are stepwise <str<strong>on</strong>g>of</str<strong>on</strong>g>fset with respect to each other.Fig. 7.15: X-ray pole figure <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN measured at a angle <str<strong>on</strong>g>of</str<strong>on</strong>g> 40° to detect the 143 diffracti<strong>on</strong>peaks. The str<strong>on</strong>g intensity at the polar angle <str<strong>on</strong>g>of</str<strong>on</strong>g> 0° shows that the crystallites are aligned with their(143) plane parallel to the surface. Note that for this hexag<strong>on</strong>al system Miller-Bravaisindices can be used with <str<strong>on</strong>g>and</str<strong>on</strong>g> . A permutati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the indicesyields equivalent planes. The ring structures at 15° <str<strong>on</strong>g>and</str<strong>on</strong>g> 30° stem from diffracti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> theequivalent planes (-153) <str<strong>on</strong>g>and</str<strong>on</strong>g> (413) <str<strong>on</strong>g>of</str<strong>on</strong>g> the hexag<strong>on</strong>al structure, indicating that no preferred azimuthalalignment <str<strong>on</strong>g>of</str<strong>on</strong>g> the crystallites exists.101
7 HATCN <strong>on</strong> Au (111)Fig. 7.16: Top view <str<strong>on</strong>g>of</str<strong>on</strong>g> the HATCN (143) plane. The dark yellow <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are parallel to the flat-<strong>on</strong>blue/grey <str<strong>on</strong>g>molecules</str<strong>on</strong>g>, but significantly <str<strong>on</strong>g>of</str<strong>on</strong>g>fset. Neighboring <str<strong>on</strong>g>molecules</str<strong>on</strong>g> prefer perpendicular arrangementwith respect to each other (T-stacking).7.6 Work functi<strong>on</strong> modificati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> Au (111) by HATCNThe work functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the Au (111) surface can be modified by HATCN adsorpti<strong>on</strong>. Whenperforming work functi<strong>on</strong> measurements using the Kelvin probe, it is <str<strong>on</strong>g>of</str<strong>on</strong>g> uttermost importanceto maintain a low base pressure in order to prohibit water adsorpti<strong>on</strong> <strong>on</strong> the surface during themeasurement. The work functi<strong>on</strong> was measured as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness. However,the rep<str<strong>on</strong>g>rod</str<strong>on</strong>g>ucibility <str<strong>on</strong>g>of</str<strong>on</strong>g> these measurements <strong>on</strong> Au (111) was rather unsatisfying, in c<strong>on</strong>trast tothose <strong>on</strong> Ag (111) (see secti<strong>on</strong> 8.4). In Fig. 7.17, two representative measurements are shown.The red curve shows the results <str<strong>on</strong>g>of</str<strong>on</strong>g> a measurement which was performed in September ’08,when the base pressure was rather high, <str<strong>on</strong>g>and</str<strong>on</strong>g> the black curve shows a measurement, which wasd<strong>on</strong>e in February ’09, directly after a bake-out <str<strong>on</strong>g>of</str<strong>on</strong>g> the chamber <str<strong>on</strong>g>and</str<strong>on</strong>g> therefore at a lower basepressure. The substrate temperature was in both cases . Unfortunately, the workfuncti<strong>on</strong> could not be measured at other temperatures very well, because the sample washeated by resistive heating, which influences the work functi<strong>on</strong>.102
7 HATCN <strong>on</strong> Au (111)Fig. 7.17: Work functi<strong>on</strong> against <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness. Y = 0 corresp<strong>on</strong>ds to the WF <str<strong>on</strong>g>of</str<strong>on</strong>g> clean Au (111). Theresults <str<strong>on</strong>g>of</str<strong>on</strong>g> two different measurements performed at different base pressures are shown. Substratetemperature .They differ quantitatively from each other, but nevertheless, there exists a certain qualitativesimilarity. Up to a coverage <str<strong>on</strong>g>of</str<strong>on</strong>g>, the work functi<strong>on</strong> decreases. Interestingly, theminimum <str<strong>on</strong>g>of</str<strong>on</strong>g> the curve, which is located at a <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness <str<strong>on</strong>g>of</str<strong>on</strong>g> approximately for bothmeasurements, coincides roughly with the disappearance <str<strong>on</strong>g>of</str<strong>on</strong>g> the γ peak in Fig. 7.4 <str<strong>on</strong>g>and</str<strong>on</strong>g> Fig.7.12. For thicker <str<strong>on</strong>g>film</str<strong>on</strong>g>s, the work functi<strong>on</strong> increases with increasing <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness. A possibleexplanati<strong>on</strong> for the decrease <str<strong>on</strong>g>of</str<strong>on</strong>g> the work functi<strong>on</strong> for thin <str<strong>on</strong>g>film</str<strong>on</strong>g>s might be the formati<strong>on</strong><str<strong>on</strong>g>of</str<strong>on</strong>g> a <str<strong>on</strong>g>film</str<strong>on</strong>g> c<strong>on</strong>sisting <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> lying flat <strong>on</strong> the surface, whereas the increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the workfuncti<strong>on</strong> could be attributed to the formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the alpha phase <str<strong>on</strong>g>and</str<strong>on</strong>g> the integrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g>formerly lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g> into this multilayer phase. A more detailed <str<strong>on</strong>g>disc</str<strong>on</strong>g>ussi<strong>on</strong> about thework functi<strong>on</strong> change due to HATCN in general can be found in secti<strong>on</strong> 8.4, where the workfuncti<strong>on</strong> change <str<strong>on</strong>g>of</str<strong>on</strong>g> an Ag (111) single crystal is investigated. Thus, the presented workfuncti<strong>on</strong> measurements would agree with the TD-spectra shown in Fig. 7.4 <str<strong>on</strong>g>and</str<strong>on</strong>g> Fig. 7.6. Thereas<strong>on</strong> for the bad rep<str<strong>on</strong>g>rod</str<strong>on</strong>g>ucibility <str<strong>on</strong>g>of</str<strong>on</strong>g> the work functi<strong>on</strong> change could be explained by thedynamic process <str<strong>on</strong>g>of</str<strong>on</strong>g> the rearrangement <str<strong>on</strong>g>of</str<strong>on</strong>g> the first layer. The <str<strong>on</strong>g>film</str<strong>on</strong>g>s could be influenced by the103
7 HATCN <strong>on</strong> Au (111)time passing by during <str<strong>on</strong>g>film</str<strong>on</strong>g> preparati<strong>on</strong> (linked to the depositi<strong>on</strong> rate) <str<strong>on</strong>g>and</str<strong>on</strong>g> the time passing bybetween the preparati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g>s <str<strong>on</strong>g>and</str<strong>on</strong>g> the work functi<strong>on</strong> measurements. Hence the waitingtime at room temperature would influence the structure <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g>, especially the structure <str<strong>on</strong>g>of</str<strong>on</strong>g>the very mobile first layer which has the str<strong>on</strong>gest impact <strong>on</strong> the work functi<strong>on</strong>.7.7 HATCN <strong>on</strong> CN-c<strong>on</strong>taminated AuAlthough no dissociati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <strong>on</strong> Au (111) was observed, it was noticed that afterseveral TD experiments the characteristics <str<strong>on</strong>g>of</str<strong>on</strong>g> the obtained spectra changed, if the substratewas not sputtered after each TD-spectrum. Obviously the amount <str<strong>on</strong>g>of</str<strong>on</strong>g> dissociated <str<strong>on</strong>g>molecules</str<strong>on</strong>g>which remain <strong>on</strong> the surface during heating up to is very small <str<strong>on</strong>g>and</str<strong>on</strong>g> it needs severalcycles <str<strong>on</strong>g>of</str<strong>on</strong>g> adsorpti<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> subsequent heating until a noteworthy c<strong>on</strong>taminati<strong>on</strong> appears. Inorder to investigate the influence <str<strong>on</strong>g>of</str<strong>on</strong>g> c<strong>on</strong>taminants <strong>on</strong> the <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>, a saturati<strong>on</strong> layer wasprepared by X-ray induced dissociati<strong>on</strong>, analog to the preparati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> C-mica, which isdescribed in the related secti<strong>on</strong> (5.5 p6P <strong>on</strong> carb<strong>on</strong> c<strong>on</strong>taminated mica (001)). In order tocreate a uniform <str<strong>on</strong>g>and</str<strong>on</strong>g> homogeneous layer, a thick <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN was prepared at a lowtemperature. This <str<strong>on</strong>g>film</str<strong>on</strong>g> was irradiated with Mg Kα X-radiati<strong>on</strong> for several hours <str<strong>on</strong>g>and</str<strong>on</strong>g>subsequently the substrate was heated to get rid <str<strong>on</strong>g>of</str<strong>on</strong>g> n<strong>on</strong>-dissociated <str<strong>on</strong>g>molecules</str<strong>on</strong>g> by thermaldesorpti<strong>on</strong>. This procedure was repeated until a saturati<strong>on</strong> coverage was reached. Thiscoverage was checked by XPS (Fig. 7.18) <str<strong>on</strong>g>and</str<strong>on</strong>g> AES (Fig. 7.19).104
7 HATCN <strong>on</strong> Au (111)Fig. 7.18: XP-spectrum <str<strong>on</strong>g>of</str<strong>on</strong>g> Au + saturati<strong>on</strong> layer <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> nitrogen (Au + CN), as obtained by X-ray induced dissociati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN.105
7 HATCN <strong>on</strong> Au (111)Fig. 7.19: AE-spectrum <str<strong>on</strong>g>of</str<strong>on</strong>g> Au + saturati<strong>on</strong> layer <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> nitrogen (Au + CN), as obtained by X-ray induced dissociati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN. This spectrum was obtained from the same surface as the XPspectrumshown in Fig. 7.18.Comparing Fig. 7.18 <str<strong>on</strong>g>and</str<strong>on</strong>g> Fig. 7.19, it is remarkable that the c<strong>on</strong>taminati<strong>on</strong> layer, whichnaturally c<strong>on</strong>sists <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> nitrogen, can be seen clearly <strong>on</strong>ly in AES, while for instancethe carb<strong>on</strong> peak in the XP spectrum is hardly visible. Therefore XPS is not well suited tospecify the cleanliness <str<strong>on</strong>g>of</str<strong>on</strong>g> the surface in this case.This saturati<strong>on</strong> layer <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> nitrogen leads to a c<strong>on</strong>siderable change in <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>, ascan be seen in a TD-spectrum (Fig. 7.20). Again, the TD-spectra do not depend <strong>on</strong> theadsorpti<strong>on</strong> temperature. The presented spectrum was obtained from a <str<strong>on</strong>g>film</str<strong>on</strong>g> which wasprepared at. Quantitatively equivalent spectra were obtained for adsorpti<strong>on</strong>temperatures up to .106
7 HATCN <strong>on</strong> Au (111)Fig. 7.20: Series <str<strong>on</strong>g>of</str<strong>on</strong>g> thermal desorpti<strong>on</strong> spectra <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <strong>on</strong> Au + CN as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> coverage.Adsorpti<strong>on</strong> temperature: , heating rate .For initial coverages a small peak (β) arises which saturates already at or even below.With increasing mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness the γ peak is formed, which is accompanied by theformati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the multilayer peak (α). The maximum <str<strong>on</strong>g>of</str<strong>on</strong>g> the γ peak shifts to lower temperatureswhich can be explained by repulsive interacti<strong>on</strong>s. The γ peak reaches its highest intensity at acoverage <str<strong>on</strong>g>of</str<strong>on</strong>g> , which apparently corresp<strong>on</strong>ds to a critical <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness since up<strong>on</strong>further adsorpti<strong>on</strong> suddenly the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> from the γ phase shade <str<strong>on</strong>g>of</str<strong>on</strong>g>f into the bulk phase. At acoverage <str<strong>on</strong>g>of</str<strong>on</strong>g> , the γ peak has already completely disappeared. A comparable behavior isshown by the β peak, which can be <str<strong>on</strong>g>disc</str<strong>on</strong>g>overed in a detailed view <str<strong>on</strong>g>of</str<strong>on</strong>g> this spectrum (Fig. 7.21).107
7 HATCN <strong>on</strong> Au (111)Fig. 7.21: Detail <str<strong>on</strong>g>of</str<strong>on</strong>g> Fig. 7.20. The β peak decreases steadily with increasing <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness.In Fig. 7.22, the amount <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in the separate adsorpti<strong>on</strong> states is plotted as a functi<strong>on</strong><str<strong>on</strong>g>of</str<strong>on</strong>g> the nominal mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness.108
7 HATCN <strong>on</strong> Au (111)Fig. 7.22: Integrals <str<strong>on</strong>g>of</str<strong>on</strong>g> the separated TDS peaks <str<strong>on</strong>g>and</str<strong>on</strong>g> their sum from measurements such as in Fig. 7.20as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness. Heating ratein all cases.In order to explain the <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> <strong>on</strong> the modified Au surface, the TD-spectra shown in thissecti<strong>on</strong> have to be compared to the TD-spectra <strong>on</strong> the clean Au surface (Fig. 7.4 & Fig. 7.6).On the clean surface first two layers <str<strong>on</strong>g>of</str<strong>on</strong>g> flat lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are formed, which later <strong>on</strong> getintegrated into the bulk phase with increasing <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness. Keeping in mind this <str<strong>on</strong>g>growth</str<strong>on</strong>g>model, the TDS data presented in this secti<strong>on</strong> can be explained as follows. The very small βpeak, which ranges from up to , saturates even below . This peakc<strong>on</strong>tains much less <str<strong>on</strong>g>molecules</str<strong>on</strong>g> than the β peak <strong>on</strong> the clean surface, where <strong>on</strong>e m<strong>on</strong>olayer <str<strong>on</strong>g>of</str<strong>on</strong>g>lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g> corresp<strong>on</strong>ds to a mean thickness <str<strong>on</strong>g>of</str<strong>on</strong>g> . The range <str<strong>on</strong>g>of</str<strong>on</strong>g> the desorpti<strong>on</strong>temperature <str<strong>on</strong>g>of</str<strong>on</strong>g> this peak is significantly lower than the <strong>on</strong>e <str<strong>on</strong>g>of</str<strong>on</strong>g> the m<strong>on</strong>olayer peak <strong>on</strong> theclean Au surface which indicates either a reduced binding energy or a reduced mobility(higher pre-exp<strong>on</strong>ential factor) <strong>on</strong> the CN-modified surface. Unfortunately, the definitemorphology <str<strong>on</strong>g>of</str<strong>on</strong>g> the CN saturati<strong>on</strong> layer is unknown. It is assumed that up<strong>on</strong> dissociati<strong>on</strong> theremnants <str<strong>on</strong>g>of</str<strong>on</strong>g> the HATCN <str<strong>on</strong>g>molecules</str<strong>on</strong>g> form densely packed <str<strong>on</strong>g>and</str<strong>on</strong>g> uniformly distributed flat109
7 HATCN <strong>on</strong> Au (111)isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s c<strong>on</strong>sisting <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> nitrogen. Thus a possible explanati<strong>on</strong> for the lowerdesorpti<strong>on</strong> temperature is that either the binding energy <str<strong>on</strong>g>of</str<strong>on</strong>g> these <str<strong>on</strong>g>molecules</str<strong>on</strong>g> is reducedcompared to the clean Au surface, or that the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are immobilized by the roughness <str<strong>on</strong>g>of</str<strong>on</strong>g>the surface. As seen in Fig. 7.21, the destabilizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> this layer starts earlier (at) than<strong>on</strong> the clean Au surface (at ). It is assumed that the reas<strong>on</strong> for the decrease <str<strong>on</strong>g>of</str<strong>on</strong>g> the βpeak is in both cases the same, namely the integrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> into the multilayer <str<strong>on</strong>g>and</str<strong>on</strong>g>that this integrati<strong>on</strong> starts as so<strong>on</strong> as the multilayer phase exists. Hence the earlierdestabilizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the β peak <strong>on</strong> the modified surface can be explained by the earlier formati<strong>on</strong><str<strong>on</strong>g>of</str<strong>on</strong>g> the bulk (α) phase, which begins at(see Fig. 7.20 <str<strong>on</strong>g>and</str<strong>on</strong>g> Fig. 7.22). The γ peakincreases up to a <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> is most probably formed by <str<strong>on</strong>g>molecules</str<strong>on</strong>g> lying flatin the first <str<strong>on</strong>g>and</str<strong>on</strong>g> the sec<strong>on</strong>d layer. In c<strong>on</strong>trast to the clean surface, the γ <str<strong>on</strong>g>and</str<strong>on</strong>g> the α peak areseparated more clearly. Hence the destabilizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the assumedly flat lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in theγ peak <str<strong>on</strong>g>and</str<strong>on</strong>g> the subsequent integrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> these <str<strong>on</strong>g>molecules</str<strong>on</strong>g> into the bulk structure (α) is seenmore clearly in Fig. 7.20 than in Fig. 7.4 <str<strong>on</strong>g>and</str<strong>on</strong>g> Fig. 7.6. M<strong>on</strong>itoring the Au4f XPS signalduring the heating <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g>s <str<strong>on</strong>g>of</str<strong>on</strong>g> different <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness yielded the spectrum shown in Fig.7.23.110
7 HATCN <strong>on</strong> Au (111)Fig. 7.23: XPS Au4f substrate signal change during heating <str<strong>on</strong>g>of</str<strong>on</strong>g> the Au + CN sample, for various initialcoverages <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN. Substrate temperature during <str<strong>on</strong>g>film</str<strong>on</strong>g> depositi<strong>on</strong>: , heating rate . Thepeak maxima <str<strong>on</strong>g>of</str<strong>on</strong>g> the corresp<strong>on</strong>ding desorpti<strong>on</strong> peaks are indicated.This spectrum is very similar to the <strong>on</strong>e shown in Fig. 7.12 from the clean Au substrate. Inc<strong>on</strong>trast to Fig. 7.12, the substrate signal change caused by the desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the first layer (β)is too small to be seen above . However, for the coverages <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> there isa slight increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the signal between <str<strong>on</strong>g>and</str<strong>on</strong>g> , which can be correlated to thedesorpti<strong>on</strong> from the first layer.For the <str<strong>on</strong>g>film</str<strong>on</strong>g> the sharp signal increase around corresp<strong>on</strong>ds to the desorpti<strong>on</strong>from the γ state. At higher initial coverages (), the Au4f signal intensity exhibits a firstsharp increase already in the range <str<strong>on</strong>g>of</str<strong>on</strong>g> to , signified by Tr. The further signalchange up to correlates with the desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the multilayer (α peak in Fig. 7.20).Just <str<strong>on</strong>g>like</str<strong>on</strong>g> <strong>on</strong> the clean surface, this low-temperature Au4f intensity increase is not correlatedwith any desorpti<strong>on</strong> (see Fig. 7.20) <strong>on</strong> the CN-modified surface. This indicates that amorphological transiti<strong>on</strong> (Tr) from a layer-<str<strong>on</strong>g>like</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> to 3D isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s occurs in the range <str<strong>on</strong>g>of</str<strong>on</strong>g>to . The Tr <strong>on</strong>set temperature slightly decreases with increasing coverage. It isproposed that at this transiti<strong>on</strong> temperature the integrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> that are lying infirst two layers (β- <str<strong>on</strong>g>and</str<strong>on</strong>g> γ-state) into the 3-dimensi<strong>on</strong>al bulk structure (α state) takes place <str<strong>on</strong>g>like</str<strong>on</strong>g><strong>on</strong> the clean gold surface.111
8 HATCN <strong>on</strong> Ag (111)8 HATCN <strong>on</strong> Ag (111)The <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <strong>on</strong> Ag (111) is in the following subdivided into three regimes: Secti<strong>on</strong>8.1 addresses the initial <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> ( ), secti<strong>on</strong> 8.2 the <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> intermediate <str<strong>on</strong>g>film</str<strong>on</strong>g>sbetween <str<strong>on</strong>g>and</str<strong>on</strong>g> mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness <str<strong>on</strong>g>and</str<strong>on</strong>g> secti<strong>on</strong> 8.3 the <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> bey<strong>on</strong>d thatthickness. The thickness <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g>s shown in this chapter is the nominal mean thickness thathas been obtained by applying the density <str<strong>on</strong>g>of</str<strong>on</strong>g> the crystal structure (, see table4.1). Therefore in the m<strong>on</strong>olayer regime this nominal mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness does notcorresp<strong>on</strong>d to the ‘real’ thickness <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g>s, but has to be seen as a measure for the amount<str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> <strong>on</strong> the surface.8.1 Initial <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>: The lying m<strong>on</strong>olayerWhen <str<strong>on</strong>g>film</str<strong>on</strong>g>s with a <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness are heated, no desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> intact <str<strong>on</strong>g>molecules</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g>also no desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> fragments is observed in the TD-spectra. This is a sign <str<strong>on</strong>g>of</str<strong>on</strong>g> a very str<strong>on</strong>gb<strong>on</strong>d between the substrate <str<strong>on</strong>g>and</str<strong>on</strong>g> the HATCN <str<strong>on</strong>g>molecules</str<strong>on</strong>g>. The LEED images in Fig. 8.1 showthat this m<strong>on</strong>olayer is highly ordered <str<strong>on</strong>g>and</str<strong>on</strong>g> that the structure <str<strong>on</strong>g>of</str<strong>on</strong>g> the m<strong>on</strong>olayer is a (7x7)superstructure.112
8 HATCN <strong>on</strong> Ag (111)Fig. 8.1: LEED images <str<strong>on</strong>g>of</str<strong>on</strong>g> athick HATCN <str<strong>on</strong>g>film</str<strong>on</strong>g> <strong>on</strong> Ag (111). The image <strong>on</strong> the left was taken at, the <strong>on</strong>e <strong>on</strong> the right at . The images show the (7x7) superstructure <str<strong>on</strong>g>of</str<strong>on</strong>g> the HATCNm<strong>on</strong>olayer <strong>on</strong> the hexag<strong>on</strong>al Ag (111) substrate. The respective unit cells are drawn in red.The best images were obtained for thethick <str<strong>on</strong>g>film</str<strong>on</strong>g>s, which corresp<strong>on</strong>ds to a moleculardensity <str<strong>on</strong>g>of</str<strong>on</strong>g>according to the calibrated microbalance. With increasing<str<strong>on</strong>g>film</str<strong>on</strong>g> thickness the spots fade away <str<strong>on</strong>g>and</str<strong>on</strong>g> cannot be seen for <str<strong>on</strong>g>film</str<strong>on</strong>g>s thicker than . Thisobservati<strong>on</strong> could be an indicator <str<strong>on</strong>g>of</str<strong>on</strong>g> a rearrangement <str<strong>on</strong>g>of</str<strong>on</strong>g> the lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in the (7x7)structure with <strong>on</strong>going HATCN depositi<strong>on</strong>.Fig. 8.2: STM images <str<strong>on</strong>g>of</str<strong>on</strong>g> the HATCN m<strong>on</strong>olayer <strong>on</strong> Ag (111), taken from [126]. The image <strong>on</strong> the leftshows the h<strong>on</strong>eycomb structure <str<strong>on</strong>g>of</str<strong>on</strong>g> the lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g>. The inset resolves individual <str<strong>on</strong>g>molecules</str<strong>on</strong>g>; thered rhombus indicates the hexag<strong>on</strong>al unit cell with. The image <strong>on</strong> the right shows asimulated STM image <str<strong>on</strong>g>of</str<strong>on</strong>g> a HATCN m<strong>on</strong>olayer <strong>on</strong> Ag (111). The structures <str<strong>on</strong>g>of</str<strong>on</strong>g> the individual HATCN<str<strong>on</strong>g>molecules</str<strong>on</strong>g> are overlaid as a guide to the eye to be able to correlate the STM picture with the moleculararrangement.STM measurements (Fig. 8.2, left) show that the m<strong>on</strong>olayer <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <strong>on</strong> Ag (111) forms ah<strong>on</strong>eycomb structure, which is characterized by a hexag<strong>on</strong>al unit cell c<strong>on</strong>taining two<str<strong>on</strong>g>molecules</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> a lattice parameter <str<strong>on</strong>g>of</str<strong>on</strong>g>[126]. The areal density <str<strong>on</strong>g>of</str<strong>on</strong>g> this structurecan be estimated to be. In the inset individual <str<strong>on</strong>g>molecules</str<strong>on</strong>g> can be113
8 HATCN <strong>on</strong> Ag (111)<str<strong>on</strong>g>disc</str<strong>on</strong>g>erned. The right h<str<strong>on</strong>g>and</str<strong>on</strong>g> side <str<strong>on</strong>g>of</str<strong>on</strong>g> Fig. 8.2 shows a simulated STM image, which is inexcellent agreement with the measured images. Six HATCN <str<strong>on</strong>g>molecules</str<strong>on</strong>g> form <strong>on</strong>e closed ring<str<strong>on</strong>g>like</str<strong>on</strong>g>feature, leaving a hole in its center. Together with the lattice c<strong>on</strong>stant <str<strong>on</strong>g>of</str<strong>on</strong>g> Ag (111)( ) <strong>on</strong>e can dem<strong>on</strong>strate that this h<strong>on</strong>eycomb structure is in accordance with the(7x7) superstructure seen in LEED:8.2 Intermediate <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>: The st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing m<strong>on</strong>olayerThis secti<strong>on</strong> addresses the <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g>s between a nominal thickness <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> .These <str<strong>on</strong>g>film</str<strong>on</strong>g>s show no desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> intact <str<strong>on</strong>g>molecules</str<strong>on</strong>g> up<strong>on</strong> heating too, but in c<strong>on</strong>trast to the<str<strong>on</strong>g>film</str<strong>on</strong>g>s smaller than, in the intermediate regime the desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> (CN) 2 fragments isclearly seen (Fig. 8.3). The (CN) 2 desorpti<strong>on</strong> saturates at about, which corresp<strong>on</strong>ds to anareal density <str<strong>on</strong>g>of</str<strong>on</strong>g>. From this TD-data it follows that there is anapproximately thick layer, which is b<strong>on</strong>ded very str<strong>on</strong>gly to the substrate because the<str<strong>on</strong>g>molecules</str<strong>on</strong>g> dissociate prior to any desorpti<strong>on</strong>.A layer <str<strong>on</strong>g>of</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g> <strong>on</strong> a layer <str<strong>on</strong>g>of</str<strong>on</strong>g> lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g> should be thicker, c<strong>on</strong>sidering thediameter <str<strong>on</strong>g>of</str<strong>on</strong>g> a HATCN molecule to be <str<strong>on</strong>g>and</str<strong>on</strong>g> most probably such a layer would be aphysisorbed <strong>on</strong>e which then would desorb without dissociati<strong>on</strong> at the surface. A layer <str<strong>on</strong>g>of</str<strong>on</strong>g><str<strong>on</strong>g>molecules</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing in the holes <str<strong>on</strong>g>of</str<strong>on</strong>g> the h<strong>on</strong>eycomb structure <str<strong>on</strong>g>of</str<strong>on</strong>g> the lying layer is ruled outsince such a layer would comprise too few <str<strong>on</strong>g>molecules</str<strong>on</strong>g> ( times the number <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> inthe lying m<strong>on</strong>olayer, thus together with the lying m<strong>on</strong>olayer at most a nominal thickness <str<strong>on</strong>g>of</str<strong>on</strong>g>). Thus the most reas<strong>on</strong>able explanati<strong>on</strong> for the desorpti<strong>on</strong> behavior <str<strong>on</strong>g>of</str<strong>on</strong>g> the HATCNfragments is that after the saturati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the lying m<strong>on</strong>olayer the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> tend to st<str<strong>on</strong>g>and</str<strong>on</strong>g>upright directly <strong>on</strong> the surface as the molecular density <strong>on</strong> the surface is increased. As ac<strong>on</strong>sequence, the CN groups which are in direct c<strong>on</strong>tact with the Ag (111) surface remain inthis state even up<strong>on</strong> heating up to while the residual CN groups separate from the<str<strong>on</strong>g>molecules</str<strong>on</strong>g> (probably the <strong>on</strong>es <strong>on</strong> the top <str<strong>on</strong>g>of</str<strong>on</strong>g> the st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing layer) <str<strong>on</strong>g>and</str<strong>on</strong>g> desorb at elevatedtemperatures (). The driving force for this rearrangement <str<strong>on</strong>g>of</str<strong>on</strong>g> the lyingm<strong>on</strong>olayer into a st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing m<strong>on</strong>olayer is most probably a mixture <str<strong>on</strong>g>of</str<strong>on</strong>g> several mechanisms that114
8 HATCN <strong>on</strong> Ag (111)all favor st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g>: First the HATCN <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are known to avoid π-stacking [58,59] (compare also chapter 5), which argues against the formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a layer <str<strong>on</strong>g>of</str<strong>on</strong>g> lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g>growing <strong>on</strong> top <str<strong>on</strong>g>of</str<strong>on</strong>g> the lying m<strong>on</strong>olayer. Additi<strong>on</strong>ally, DFT modeling [133] suggests that the<str<strong>on</strong>g>molecules</str<strong>on</strong>g> in the lying m<strong>on</strong>olayer are distorted because the CN groups bend down towards thesurface. In the st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing m<strong>on</strong>olayer the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are no l<strong>on</strong>ger distorted.Fig. 8.3: TDS <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <strong>on</strong> Ag (111). This figure shows the change <str<strong>on</strong>g>of</str<strong>on</strong>g> the mass 52 (CN) 2 signal. Noincrease <str<strong>on</strong>g>of</str<strong>on</strong>g> any <str<strong>on</strong>g>of</str<strong>on</strong>g> the masses related to the intact molecule was noticed up to .The reas<strong>on</strong> for the appearance <str<strong>on</strong>g>of</str<strong>on</strong>g> two distinct (CN) 2 peaks in the TDS is not clear yet.Obviously not all the CN groups, which are not in direct c<strong>on</strong>tact with the Ag (111) surface,separate from the molecule at the same temperature, which indicates a different bindingstrength to the molecule. A possible explanati<strong>on</strong> for the two peaks could be that the CNgroups next to the <strong>on</strong>es which are in direct c<strong>on</strong>tact with the Ag (111) surface exhibit adifferent binding strength to the molecule than the <strong>on</strong>es farther away.115
8 HATCN <strong>on</strong> Ag (111)8.3 The multilayer regimeAfter the desorpti<strong>on</strong> peak <str<strong>on</strong>g>of</str<strong>on</strong>g> the HATCN fragments has saturated the desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> intact<str<strong>on</strong>g>molecules</str<strong>on</strong>g> starts to appear. For the sake <str<strong>on</strong>g>of</str<strong>on</strong>g> clarity the TDS series is split into two parts. Fig.8.4 shows a TDS series <str<strong>on</strong>g>of</str<strong>on</strong>g> the regime between <str<strong>on</strong>g>and</str<strong>on</strong>g> . Two peaks can be seen, γ<str<strong>on</strong>g>and</str<strong>on</strong>g> α’, which both increase nearly simultaneously with increasing <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness. In thisregime the γ peak is always higher than the α’ peak. Fig. 8.5 shows the further evoluti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g>this TDS series. The <str<strong>on</strong>g>film</str<strong>on</strong>g>s were prepared at room temperature <str<strong>on</strong>g>and</str<strong>on</strong>g> the heating rate was in allcases. Quantitatively similar spectra are obtained if the <str<strong>on</strong>g>film</str<strong>on</strong>g>s are prepared at differentadsorpti<strong>on</strong> temperatures .Fig. 8.4: TDS series <str<strong>on</strong>g>of</str<strong>on</strong>g> the beginning multilayer, . Up to a <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness <str<strong>on</strong>g>of</str<strong>on</strong>g> themetastable γ peak shows a higher intensity than the α’ peak.116
8 HATCN <strong>on</strong> Ag (111)Above a <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness <str<strong>on</strong>g>of</str<strong>on</strong>g> the γ peak <str<strong>on</strong>g>and</str<strong>on</strong>g> the α‘ peak start decreasing with increasing<str<strong>on</strong>g>film</str<strong>on</strong>g> thickness which leads to an excessive increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the α peak, which stems from themultilayer. This finding is in detail illustrated in Fig. 8.6, where the integrated peak areas <str<strong>on</strong>g>of</str<strong>on</strong>g>Fig. 8.4 <str<strong>on</strong>g>and</str<strong>on</strong>g> Fig. 8.5 are plotted versus the <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness. Starting at , the total quantity(‘overall’) <str<strong>on</strong>g>of</str<strong>on</strong>g> desorbing <str<strong>on</strong>g>molecules</str<strong>on</strong>g> increases linearly with the coverage, which indicates asticking coefficient <str<strong>on</strong>g>of</str<strong>on</strong>g> unity. In general, there is good reas<strong>on</strong> to assume a sticking coefficient<str<strong>on</strong>g>of</str<strong>on</strong>g> unity for c<strong>on</strong>densable <str<strong>on</strong>g>molecules</str<strong>on</strong>g>. However, the increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the integrated peak area <str<strong>on</strong>g>of</str<strong>on</strong>g> the αpeak in the range between <str<strong>on</strong>g>and</str<strong>on</strong>g> can be regarded as an almost exp<strong>on</strong>ential <strong>on</strong>e,which is together with the decrease <str<strong>on</strong>g>of</str<strong>on</strong>g> the γ peak an apparent sign that an incorporati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g><str<strong>on</strong>g>molecules</str<strong>on</strong>g> from <strong>on</strong>e adsorpti<strong>on</strong> state (γ) into another adsorpti<strong>on</strong> state (α) is taking place.Similar to the <str<strong>on</strong>g>growth</str<strong>on</strong>g> <strong>on</strong> gold, also <strong>on</strong> silver a metastable phase exists which gets incorporatedinto the bulk phase as the <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness is increased bey<strong>on</strong>d a certain critical <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness.In fact the spectra shown in Fig. 8.4 <str<strong>on</strong>g>and</str<strong>on</strong>g> Fig. 8.5 resemble the TD-spectra obtained <strong>on</strong> theCN-c<strong>on</strong>taminated Au (111) surface: in both cases no high-temperature m<strong>on</strong>olayer peak can beseen <str<strong>on</strong>g>and</str<strong>on</strong>g> the γ-peaks are clearly separated from the multilayer α-peak.Fig. 8.5: TDS series <str<strong>on</strong>g>of</str<strong>on</strong>g> the ‘true’ multilayer regime,. The black curve in this image isthe same as in Fig. 8.4. The metastable γ peak decreases with increasing <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness, similar to the117
8 HATCN <strong>on</strong> Ag (111)γ peak <strong>on</strong> Au (see Fig. 7.6) <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>on</strong> CN-covered Au (Fig. 7.20). To the same degree the multilayer (α)peak increases in this regime.The desorpti<strong>on</strong> temperature <str<strong>on</strong>g>of</str<strong>on</strong>g> the multilayer <strong>on</strong> Ag (111) is assumed to be the same as <strong>on</strong> Au(111). In fact, the <strong>on</strong>set <str<strong>on</strong>g>of</str<strong>on</strong>g> the multilayer desorpti<strong>on</strong> in Fig. 7.6 is at , whereas the<strong>on</strong>set <str<strong>on</strong>g>of</str<strong>on</strong>g> the α peak in Fig. 8.5 is at . This deviati<strong>on</strong> can be explained by the differentmounting <str<strong>on</strong>g>of</str<strong>on</strong>g> the thermocouple to the respective single crystals. On Au (111) the Ni/NiCrthermocouple was spot-welded directly <strong>on</strong>to the gold. The purchased silver single crystalfeatured a small hole <strong>on</strong> the back side where the thermocouple could be squeezed in.Obviously the thermal c<strong>on</strong>tact was poorer in the latter case, which led to a delay <str<strong>on</strong>g>of</str<strong>on</strong>g> themeasured temperature during heating.Fig. 8.6: Integrated peak areas from Fig. 8.4 <str<strong>on</strong>g>and</str<strong>on</strong>g> Fig. 8.5 as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness(adsorbed amount vs. desorbed amount). The dashed blue line is a linear fit <str<strong>on</strong>g>of</str<strong>on</strong>g> the integrated overallpeak area.The thermal stability <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g> morphology was investigated in-situ by ‘XPS vs temperature’measurements. For that purpose several <str<strong>on</strong>g>film</str<strong>on</strong>g>s <str<strong>on</strong>g>of</str<strong>on</strong>g> different thickness were prepared at118
8 HATCN <strong>on</strong> Ag (111)<str<strong>on</strong>g>and</str<strong>on</strong>g> the substrate Ag3d signal was followed during heating <str<strong>on</strong>g>of</str<strong>on</strong>g> the sample (Fig. 8.7). Thesignal <str<strong>on</strong>g>of</str<strong>on</strong>g> the clean substrate features a slight decrease due to the Debye-Waller effect <str<strong>on</strong>g>and</str<strong>on</strong>g>other experimental reas<strong>on</strong>s, which will not be <str<strong>on</strong>g>disc</str<strong>on</strong>g>ussed here. Thethick <str<strong>on</strong>g>film</str<strong>on</strong>g> is stable upto . Above the substrate signal increases, which fits to the desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> (CN) 2in this range. According to the desorpti<strong>on</strong> characteristics <str<strong>on</strong>g>disc</str<strong>on</strong>g>ussed in secti<strong>on</strong> 8.2, the increase<str<strong>on</strong>g>of</str<strong>on</strong>g> the signal is caused by the desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the HATCN fragments <strong>on</strong> top <str<strong>on</strong>g>of</str<strong>on</strong>g> the st<str<strong>on</strong>g>and</str<strong>on</strong>g>inglayer, i.e. the outermost parts <str<strong>on</strong>g>of</str<strong>on</strong>g> the HATCN <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in the m<strong>on</strong>olayer. In agreement withFig. 8.3, the signal increase induced by the desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN fragments is alreadysaturated at . There is no significant signal change between <str<strong>on</strong>g>and</str<strong>on</strong>g> . This canbe explained in two ways: either the st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing layer forms already below or therearrangement <str<strong>on</strong>g>of</str<strong>on</strong>g> approximately three layers <str<strong>on</strong>g>of</str<strong>on</strong>g> lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g> into <strong>on</strong>e layer <str<strong>on</strong>g>of</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing<str<strong>on</strong>g>molecules</str<strong>on</strong>g> taking possibly place somewhere between <str<strong>on</strong>g>and</str<strong>on</strong>g> cannot be seenbecause in either way the attenuati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the signal would be the same. As the <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness isincreased up tostr<strong>on</strong>g signal changes can be seen. In total agreement with the TDspectrapresented above (Fig. 8.4 <str<strong>on</strong>g>and</str<strong>on</strong>g> Fig. 8.5), the substrate signal increases atdue tothe desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the γ peak <str<strong>on</strong>g>and</str<strong>on</strong>g> arounddue to the desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the α’ peak.119
8 HATCN <strong>on</strong> Ag (111)Fig. 8.7: XPS Ag3d substrate signal change during heating <str<strong>on</strong>g>of</str<strong>on</strong>g> the sample, for various initial coverages<str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN. Films were prepared at <str<strong>on</strong>g>and</str<strong>on</strong>g> the heating rate was . The peak maxima <str<strong>on</strong>g>of</str<strong>on</strong>g> thecorresp<strong>on</strong>ding desorpti<strong>on</strong> peaks are indicated.A new feature (at least for the Ag substrate) is seen as the <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness is raised to<str<strong>on</strong>g>and</str<strong>on</strong>g> above. The substrate signal intensity rises already before the desorpti<strong>on</strong> starts,dem<strong>on</strong>strating a morphological rearrangement from a c<strong>on</strong>tinuous layer-by-layer <str<strong>on</strong>g>like</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> intoan isl<str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>like</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> around , which is definitely an expected behavior <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCNmultilayers (see chapter 7). This rearrangement is <strong>on</strong>ly seen above a critical <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness <str<strong>on</strong>g>of</str<strong>on</strong>g>. Films which have underg<strong>on</strong>e this rearrangement feature no XPS substrate signalincrease around <str<strong>on</strong>g>and</str<strong>on</strong>g> no (or <strong>on</strong>ly a very small) γ desorpti<strong>on</strong> peak in the TDS. Theseobservati<strong>on</strong>s allow the c<strong>on</strong>clusi<strong>on</strong> that the γ peak stems from a <str<strong>on</strong>g>film</str<strong>on</strong>g> which grows in a layer<str<strong>on</strong>g>like</str<strong>on</strong>g>fashi<strong>on</strong> (wetting layer) <str<strong>on</strong>g>and</str<strong>on</strong>g> that the α peak stems from an isl<str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>like</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> that theincorporati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> into the α state takes place around. This idea iscorroborated by AFM images <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g>s prepared at <str<strong>on</strong>g>and</str<strong>on</strong>g> at (Fig. 8.8):120
8 HATCN <strong>on</strong> Ag (111)Fig. 8.8: AFM images in ‘shaded mode’ <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <str<strong>on</strong>g>film</str<strong>on</strong>g>s <strong>on</strong> Ag (111) <str<strong>on</strong>g>and</str<strong>on</strong>g> related cross secti<strong>on</strong>sal<strong>on</strong>g the marked arrows. The left image shows athick <str<strong>on</strong>g>film</str<strong>on</strong>g> grown at room temperature, theright image shows a thick <str<strong>on</strong>g>film</str<strong>on</strong>g> grown at . Both images were taken ex-situ at roomtemperature.The <str<strong>on</strong>g>film</str<strong>on</strong>g> grown atc<strong>on</strong>sists <str<strong>on</strong>g>of</str<strong>on</strong>g> very small grains <str<strong>on</strong>g>and</str<strong>on</strong>g> can be characterized as an almostc<strong>on</strong>tinuous layer-by-layer <str<strong>on</strong>g>like</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g>, whereas the <str<strong>on</strong>g>film</str<strong>on</strong>g> grown atforms large isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s. AnXRD investigati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g> grown at (see Fig. 8.9) has shown that the isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s arecrystalline yielding a similar reflecti<strong>on</strong> signal as the HATCN powder. In c<strong>on</strong>trast to the hugecrystalline isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s <strong>on</strong> gold, <strong>on</strong> silver there is no preferential orientati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the crystallites withrespect to the surface.121
8 HATCN <strong>on</strong> Ag (111)Fig. 8.9: Specular XRD scan <str<strong>on</strong>g>of</str<strong>on</strong>g> a thick HATCN <str<strong>on</strong>g>film</str<strong>on</strong>g> prepared at <strong>on</strong> Ag (111). Thepeak positi<strong>on</strong>s fit very well to the simulated pattern <str<strong>on</strong>g>of</str<strong>on</strong>g> the HATCN powder.8.4 Work functi<strong>on</strong> modificati<strong>on</strong>The change <str<strong>on</strong>g>of</str<strong>on</strong>g> the work functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the Ag (111) surface relative to the clean surface as afuncti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the HATCN coverage is shown in Fig. 8.10. The <str<strong>on</strong>g>film</str<strong>on</strong>g>s were prepared <str<strong>on</strong>g>and</str<strong>on</strong>g>measured at RT. The first series that is shown here in black was taken in January 09, thesec<strong>on</strong>d series approximately <strong>on</strong>e m<strong>on</strong>th later. Un<str<strong>on</strong>g>like</str<strong>on</strong>g> the series <strong>on</strong> gold, both Ag-series exhibitsimilar characteristics. A possible reas<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the better rep<str<strong>on</strong>g>rod</str<strong>on</strong>g>ucibility <strong>on</strong> silver could be thatthe forces acting between the substrate <str<strong>on</strong>g>and</str<strong>on</strong>g> the HATCN <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are str<strong>on</strong>ger, leading to amore well defined <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> the first layers.In general, the work functi<strong>on</strong> modificati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a surface due to adsorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> material isregarded as an interplay <str<strong>on</strong>g>of</str<strong>on</strong>g> the Pauli push-back <str<strong>on</strong>g>and</str<strong>on</strong>g> the charge transfer between the material<str<strong>on</strong>g>and</str<strong>on</strong>g> the substrate. The push-back for large <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> <strong>on</strong> coinage metals is usually in122
8 HATCN <strong>on</strong> Ag (111)the range <str<strong>on</strong>g>of</str<strong>on</strong>g> 0.7 – 1.0 eV [131]. In the regime <str<strong>on</strong>g>of</str<strong>on</strong>g> the first lying m<strong>on</strong>olayer () thework functi<strong>on</strong> decreases by <strong>on</strong>ly, indicating that the charge transfer almostcompensates the push-back. Between <str<strong>on</strong>g>and</str<strong>on</strong>g> the work functi<strong>on</strong> increases by . Inthis regime, the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> start st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing upright. Thereup<strong>on</strong> the surface density <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN<str<strong>on</strong>g>molecules</str<strong>on</strong>g> increases by a factor <str<strong>on</strong>g>of</str<strong>on</strong>g> more than three ( lying vs.st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing). It is not clear yet to what extent the increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the work functi<strong>on</strong> is related to ahigher charge transfer. Preliminary DFT results [133] suggest that the charge transfer permolecule is smaller for st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g> than for lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g>, but the calculated valuesfor the charge transfer depend str<strong>on</strong>gly <strong>on</strong> the molecular arrangement in the st<str<strong>on</strong>g>and</str<strong>on</strong>g>ingm<strong>on</strong>olayer, which is not known yet.Fig. 8.10: Work functi<strong>on</strong> against <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness. Y = 0 corresp<strong>on</strong>ds to the WF <str<strong>on</strong>g>of</str<strong>on</strong>g> clean Ag (111).Results <str<strong>on</strong>g>of</str<strong>on</strong>g> two different series taken at different dates are shown. Substrate temperature during <str<strong>on</strong>g>film</str<strong>on</strong>g><str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> measurements: .123
8 HATCN <strong>on</strong> Ag (111)At the work functi<strong>on</strong> saturates <str<strong>on</strong>g>and</str<strong>on</strong>g> remains c<strong>on</strong>stant thereafter. A very interestingcomparis<strong>on</strong> can be made between the work functi<strong>on</strong> change characteristics <str<strong>on</strong>g>and</str<strong>on</strong>g> the evoluti<strong>on</strong><str<strong>on</strong>g>of</str<strong>on</strong>g> the mass 52 desorpti<strong>on</strong> (Fig. 8.11). In this figure the integrated thermal desorpti<strong>on</strong> peakareas <str<strong>on</strong>g>of</str<strong>on</strong>g> mass 52 <str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> the intact molecule are plotted against the <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness according tothe microbalance (desorbed <str<strong>on</strong>g>molecules</str<strong>on</strong>g> vs adsorbed <str<strong>on</strong>g>molecules</str<strong>on</strong>g>). (CN) 2 is a QMS crackingp<str<strong>on</strong>g>rod</str<strong>on</strong>g>uct <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN too <str<strong>on</strong>g>and</str<strong>on</strong>g> its signal is therefore proporti<strong>on</strong>al to the signal <str<strong>on</strong>g>of</str<strong>on</strong>g> the intactmolecule. No desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> intact <str<strong>on</strong>g>molecules</str<strong>on</strong>g> was observed above. Therefore theintegrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the mass 52 peaks was performed from up to as <strong>on</strong>ly to takeinto account the (CN) 2 <str<strong>on</strong>g>molecules</str<strong>on</strong>g> which are formed by the dissociati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN directly <strong>on</strong>the Ag surface.Fig. 8.11: Integrated peak areas <str<strong>on</strong>g>of</str<strong>on</strong>g> mass 52 ((CN) 2 ) <str<strong>on</strong>g>and</str<strong>on</strong>g> mass 384 (HATCN) from Fig. 8.3, Fig. 8.4<str<strong>on</strong>g>and</str<strong>on</strong>g> Fig. 8.5 as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the mean <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness (adsorbed amount vs. desorbed amount).It is remarkable that in all coverage regimes the desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the mass 52 shows similarfeatures as the work functi<strong>on</strong> change: The lying m<strong>on</strong>olayer exhibits no desorpti<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> almost124
8 HATCN <strong>on</strong> Ag (111)no work functi<strong>on</strong> change. As <str<strong>on</strong>g>disc</str<strong>on</strong>g>ussed in the previous secti<strong>on</strong>s, these lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g> tend tost<str<strong>on</strong>g>and</str<strong>on</strong>g> up <str<strong>on</strong>g>and</str<strong>on</strong>g> form a m<strong>on</strong>olayer <str<strong>on</strong>g>of</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g> as the molecular density <strong>on</strong> the surfaceis increased. These st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g> feature a partial desorpti<strong>on</strong> the intensity <str<strong>on</strong>g>of</str<strong>on</strong>g> which canbe followed in TDS <str<strong>on</strong>g>and</str<strong>on</strong>g> which fits nicely to the increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the work functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the surface.Both curves, the work functi<strong>on</strong> change <str<strong>on</strong>g>and</str<strong>on</strong>g> the integrated peak area <str<strong>on</strong>g>of</str<strong>on</strong>g> the mass 52 desorpti<strong>on</strong>,saturate at . At this coverage the st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing layer is completed <str<strong>on</strong>g>and</str<strong>on</strong>g> additi<strong>on</strong>al <str<strong>on</strong>g>molecules</str<strong>on</strong>g>start building up the multilayer, which desorbs intact. Thus these <str<strong>on</strong>g>molecules</str<strong>on</strong>g> do not dissociateat the sample surface <str<strong>on</strong>g>and</str<strong>on</strong>g> do not lead to an increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the desorpti<strong>on</strong> signal <str<strong>on</strong>g>of</str<strong>on</strong>g> the mass 52between <str<strong>on</strong>g>and</str<strong>on</strong>g> . The work functi<strong>on</strong> is also not influenced by the <str<strong>on</strong>g>molecules</str<strong>on</strong>g>growing <strong>on</strong> top <str<strong>on</strong>g>of</str<strong>on</strong>g> the m<strong>on</strong>olayer. C<strong>on</strong>sidering all the presented data, the increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the workfuncti<strong>on</strong> can be linked to the formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing m<strong>on</strong>olayer through the mass 52desorpti<strong>on</strong> signal.125
9 Summary9 SummaryIn this thesis the <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> the adsorpti<strong>on</strong>/desorpti<strong>on</strong> kinetics <str<strong>on</strong>g>of</str<strong>on</strong>g> two different<str<strong>on</strong>g>molecules</str<strong>on</strong>g>, namely hexaphenyl (p6P) <str<strong>on</strong>g>and</str<strong>on</strong>g> hexaaza-triphenylene-hexacarb<strong>on</strong>itrile (HATCN),<strong>on</strong> various substrates (mica, KCl, Au, Ag) have been investigated with several surface sciencetechniques. Particular attenti<strong>on</strong> was paid to the thermal stability <str<strong>on</strong>g>of</str<strong>on</strong>g> the grown <str<strong>on</strong>g>film</str<strong>on</strong>g>s <str<strong>on</strong>g>and</str<strong>on</strong>g> to theinfluence <str<strong>on</strong>g>of</str<strong>on</strong>g> surface modificati<strong>on</strong>s. In the following a short summary including the mostimportant results <strong>on</strong> the different systems is given.9.1 p6P <strong>on</strong> mica (001)On a surface which was cleaved under ambient c<strong>on</strong>diti<strong>on</strong>s <str<strong>on</strong>g>and</str<strong>on</strong>g> subsequently taken into thevacuum chamber, the typical needle-<str<strong>on</strong>g>like</str<strong>on</strong>g> isl<str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> takes place at room temperature.Thermal desorpti<strong>on</strong> spectroscopy revealed that prior to the formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s, a morestr<strong>on</strong>gly bound wetting layer <str<strong>on</strong>g>of</str<strong>on</strong>g> mean thickness is formed (Stranski–Krastanov<str<strong>on</strong>g>growth</str<strong>on</strong>g> mode). In both, the wetting layer <str<strong>on</strong>g>and</str<strong>on</strong>g> the needle-<str<strong>on</strong>g>like</str<strong>on</strong>g> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s, the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> areoriented with their l<strong>on</strong>g axes parallel to the surface. The initial adsorpti<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> ultra-thin <str<strong>on</strong>g>film</str<strong>on</strong>g>layer <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> hexaphenyl (p6P) <strong>on</strong> mica (001) can be significantly influenced by surfacemodificati<strong>on</strong>s <str<strong>on</strong>g>and</str<strong>on</strong>g> by altering the surface temperature. Evaporati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P at leads tosmall crystallites, which cover almost the whole surface. This <str<strong>on</strong>g>film</str<strong>on</strong>g> is stable up to roomtemperature, but recrystallizes to a needle-<str<strong>on</strong>g>like</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g> above, prior to desorpti<strong>on</strong> at about. From XPS <str<strong>on</strong>g>and</str<strong>on</strong>g> LEED measurements <strong>on</strong>e finds that the freshly cleaved mica substrateis well ordered, but covered with approximately half a m<strong>on</strong>olayer <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong>. By irradiati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g>a thick p6P <str<strong>on</strong>g>film</str<strong>on</strong>g> <strong>on</strong> mica with X-rays <strong>on</strong>e can dissociate a fracti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g>thereby increase the carb<strong>on</strong> amount <str<strong>on</strong>g>of</str<strong>on</strong>g> the mica surface up to a saturati<strong>on</strong> coverage <str<strong>on</strong>g>of</str<strong>on</strong>g> <strong>on</strong>em<strong>on</strong>olayer. This increased carb<strong>on</strong> coverage affects the p6P layer <str<strong>on</strong>g>growth</str<strong>on</strong>g> dramatically. Thewetting layer disappears completely <str<strong>on</strong>g>and</str<strong>on</strong>g> the ultra-thin <str<strong>on</strong>g>film</str<strong>on</strong>g> c<strong>on</strong>sists <str<strong>on</strong>g>of</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g>,forming circular <str<strong>on</strong>g>shaped</str<strong>on</strong>g> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s. In the same way the layer <str<strong>on</strong>g>growth</str<strong>on</strong>g> can be influenced by126
9 Summarysputtering the mica surface, which leads to a disorganizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the well ordered surfacestructure. Again, no wetting layer is formed <strong>on</strong> this surface <str<strong>on</strong>g>and</str<strong>on</strong>g> the initial layer <str<strong>on</strong>g>growth</str<strong>on</strong>g>proceeds in the form <str<strong>on</strong>g>of</str<strong>on</strong>g> circular <str<strong>on</strong>g>shaped</str<strong>on</strong>g> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s <str<strong>on</strong>g>of</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing p6P <str<strong>on</strong>g>molecules</str<strong>on</strong>g> (isl<str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>mode). This change <str<strong>on</strong>g>of</str<strong>on</strong>g> layer <str<strong>on</strong>g>growth</str<strong>on</strong>g> is attributed to changes <str<strong>on</strong>g>of</str<strong>on</strong>g> the dipole field <strong>on</strong> the surface,which exists <strong>on</strong> the freshly cleaved mica surface, but vanishes either due to a change <str<strong>on</strong>g>of</str<strong>on</strong>g> thechemical compositi<strong>on</strong> or due to a disordering <str<strong>on</strong>g>of</str<strong>on</strong>g> the surface structure.From these results <str<strong>on</strong>g>and</str<strong>on</strong>g> from results reported in the literature <strong>on</strong>e could deduce a ‘general rulefor p6P <str<strong>on</strong>g>growth</str<strong>on</strong>g>’: If a wetting layer exists, it serves as a template for further <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g>because p6P <str<strong>on</strong>g>molecules</str<strong>on</strong>g> favor π-stacking, i.e. parallel alignment <str<strong>on</strong>g>of</str<strong>on</strong>g> the l<strong>on</strong>g molecular axis, themultilayer will be made up <str<strong>on</strong>g>of</str<strong>on</strong>g> lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g>, too. Such a behavior is generally seen if theb<strong>on</strong>ding between the p6P <str<strong>on</strong>g>molecules</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> the substrate is quite str<strong>on</strong>g <str<strong>on</strong>g>and</str<strong>on</strong>g> if the substrate iswell ordered. In cases where the wetting layer is missing, the binding strength <str<strong>on</strong>g>of</str<strong>on</strong>g> the<str<strong>on</strong>g>molecules</str<strong>on</strong>g> to the substrate is so small that it is more favorable to st<str<strong>on</strong>g>and</str<strong>on</strong>g> up, thus minimizing thec<strong>on</strong>tact area to the substrate <str<strong>on</strong>g>and</str<strong>on</strong>g> maximizing the π-stacking. However, this general rule isdisproven by the results shown in the following secti<strong>on</strong> <strong>on</strong> p6P <strong>on</strong> KCl (001).9.2 p6P <strong>on</strong> KCl (001)Thermal desorpti<strong>on</strong> spectroscopy measurements showed that no wetting layer <str<strong>on</strong>g>of</str<strong>on</strong>g> lying p6P<str<strong>on</strong>g>molecules</str<strong>on</strong>g> exists <strong>on</strong> KCl (001). On the other h<str<strong>on</strong>g>and</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g> starts to grow in form <str<strong>on</strong>g>of</str<strong>on</strong>g> needle<str<strong>on</strong>g>like</str<strong>on</strong>g> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s made up <str<strong>on</strong>g>of</str<strong>on</strong>g> lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g>. This is a quite unique result <str<strong>on</strong>g>and</str<strong>on</strong>g> c<strong>on</strong>tradicts the‘general rule for p6P <str<strong>on</strong>g>growth</str<strong>on</strong>g>’ <str<strong>on</strong>g>disc</str<strong>on</strong>g>ussed in the end <str<strong>on</strong>g>of</str<strong>on</strong>g> the previous secti<strong>on</strong>. To the best <str<strong>on</strong>g>of</str<strong>on</strong>g> myknowledge, the p6P-KCl (001) system is the first example where needle <str<strong>on</strong>g>like</str<strong>on</strong>g> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s withlying <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are formed without the existence <str<strong>on</strong>g>of</str<strong>on</strong>g> a wetting layer. From the very similar<str<strong>on</strong>g>growth</str<strong>on</strong>g> behavior <str<strong>on</strong>g>of</str<strong>on</strong>g> p6P <strong>on</strong> other alkali halide substrates, <str<strong>on</strong>g>like</str<strong>on</strong>g> NaCl <str<strong>on</strong>g>and</str<strong>on</strong>g> KBr [127], <strong>on</strong>e canspeculate that in general no wetting layers may exist <strong>on</strong> alkali halide substrates.The p6P <str<strong>on</strong>g>growth</str<strong>on</strong>g> <strong>on</strong> KCl (001) was thereup<strong>on</strong> investigated as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> temperature.Utilizing AFM, two different morphologies were found: the already menti<strong>on</strong>ed needles whichmost probably c<strong>on</strong>sist <str<strong>on</strong>g>of</str<strong>on</strong>g> lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> terraces c<strong>on</strong>sisting <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ingupright. These <str<strong>on</strong>g>film</str<strong>on</strong>g> morphologies str<strong>on</strong>gly depend <strong>on</strong> the adsorpti<strong>on</strong> temperature <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g>s.At room temperature <strong>on</strong>ly isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s <str<strong>on</strong>g>of</str<strong>on</strong>g> lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are formed, whereas at highertemperatures the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> tend to form terraces <str<strong>on</strong>g>of</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g>. Additi<strong>on</strong>ally, the127
9 Summaryneedle size increases with increasing temperature. The most significant change is observedbetween <str<strong>on</strong>g>and</str<strong>on</strong>g> , where the height <str<strong>on</strong>g>of</str<strong>on</strong>g> the needles increases nearly by an order <str<strong>on</strong>g>of</str<strong>on</strong>g>magnitude. Additi<strong>on</strong>ally, <str<strong>on</strong>g>film</str<strong>on</strong>g>s grown at temperatures far below room temperature ( )were investigated in situ with combined XPS <str<strong>on</strong>g>and</str<strong>on</strong>g> TDS measurements <str<strong>on</strong>g>and</str<strong>on</strong>g> ex-situ with AFM.At an adsorpti<strong>on</strong> temperature <str<strong>on</strong>g>of</str<strong>on</strong>g>the <str<strong>on</strong>g>film</str<strong>on</strong>g> c<strong>on</strong>sists <str<strong>on</strong>g>of</str<strong>on</strong>g> small crystallites, which coveralmost the whole surface. This <str<strong>on</strong>g>film</str<strong>on</strong>g> remains stable up to, but recrystallizes above. The result <str<strong>on</strong>g>of</str<strong>on</strong>g> the recrystallisati<strong>on</strong> is a <str<strong>on</strong>g>film</str<strong>on</strong>g> c<strong>on</strong>sisting <str<strong>on</strong>g>of</str<strong>on</strong>g> slightly el<strong>on</strong>gated <str<strong>on</strong>g>and</str<strong>on</strong>g>ordered isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s which are significantly higher than the <strong>on</strong>es before the recrystallizati<strong>on</strong>.9.3 HATCN <strong>on</strong> Au (111)The HATCN <str<strong>on</strong>g>film</str<strong>on</strong>g>s <strong>on</strong> Au (111) exhibit a quite unusual temperature stability which depends<strong>on</strong> the initially adsorbed material. At <str<strong>on</strong>g>and</str<strong>on</strong>g> below room temperature, the first two layers grow ina layer-<str<strong>on</strong>g>like</str<strong>on</strong>g> fashi<strong>on</strong> resembling wetting layers <str<strong>on</strong>g>and</str<strong>on</strong>g> are made up <str<strong>on</strong>g>of</str<strong>on</strong>g> lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g>. Suchlayers are meta-stable since an increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the coverage bey<strong>on</strong>d two m<strong>on</strong>olayers leads to adestabilizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the sec<strong>on</strong>d layer which leads to a transiti<strong>on</strong> to an isl<str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>like</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>proceeding <strong>on</strong> the first m<strong>on</strong>olayer. The temperature at which this transiti<strong>on</strong> takes placedepends slightly <strong>on</strong> the initial <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness <str<strong>on</strong>g>and</str<strong>on</strong>g> lies in the range <str<strong>on</strong>g>of</str<strong>on</strong>g> (thick <str<strong>on</strong>g>film</str<strong>on</strong>g>s) –(thin <str<strong>on</strong>g>film</str<strong>on</strong>g>s). The molecular arrangement within the isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s corresp<strong>on</strong>ds to that <str<strong>on</strong>g>of</str<strong>on</strong>g> theHATCN bulk structure <str<strong>on</strong>g>and</str<strong>on</strong>g> features a preferential orientati<strong>on</strong> with respect to the Au (111)surface. At the substrate-isl<str<strong>on</strong>g>and</str<strong>on</strong>g> interface, the primarily flat lying <str<strong>on</strong>g>molecules</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> the firstm<strong>on</strong>olayer adapt to the bulk structure <str<strong>on</strong>g>of</str<strong>on</strong>g> the isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s. With increasing initial coverage theaverage isl<str<strong>on</strong>g>and</str<strong>on</strong>g> size also increases <str<strong>on</strong>g>and</str<strong>on</strong>g> thus the amount <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>molecules</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> the first layerincorporated into the 3D isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s increases too. The <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in the first layer which are notcovered by isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s remain stable up to a higher temperature than those which have beenincorporated into the isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s. The reas<strong>on</strong> for that is that the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in the first layer exhibitthe same binding energy as the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in the isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s, but a smaller pre-exp<strong>on</strong>ential factorwhich is a sign for a high mobility <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>molecules</str<strong>on</strong>g>. Therefore these <str<strong>on</strong>g>molecules</str<strong>on</strong>g> desorb athigher temperatures because prior to desorpti<strong>on</strong> more degrees <str<strong>on</strong>g>of</str<strong>on</strong>g> freedom can be excited. Ifthe substrate surface is modified by a C-N layer prepared by intenti<strong>on</strong>al HATCN dissociati<strong>on</strong>,the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in the first layer desorb at smaller temperatures, indicating either a lowerbinding energy or a reduced mobility <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> <strong>on</strong> the roughened surface. Keeping in128
9 Summarymind that the binding energies <str<strong>on</strong>g>of</str<strong>on</strong>g> all adsorpti<strong>on</strong> states <strong>on</strong> the clean surface are very similar<str<strong>on</strong>g>and</str<strong>on</strong>g> that the desorpti<strong>on</strong> temperature <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <strong>on</strong> Au is mostly governed by the preexp<strong>on</strong>entialfactor, the latter explanati<strong>on</strong> seems more <str<strong>on</strong>g>like</str<strong>on</strong>g>ly.The work functi<strong>on</strong> change due to HATCN adsorpti<strong>on</strong> has turned out to be rep<str<strong>on</strong>g>rod</str<strong>on</strong>g>uciblequalitatively, but not quantitatively. For thin layers the work functi<strong>on</strong> decreases until ata minimum is reached which liesbelow that <strong>on</strong> clean Au.Subsequent depositi<strong>on</strong> leads to an increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the work functi<strong>on</strong> which happens to go <strong>on</strong> up to<str<strong>on</strong>g>and</str<strong>on</strong>g> above. During that increase the work functi<strong>on</strong> reaches values which range from thework functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the clean Au up toabove that.9.4 HATCN <strong>on</strong> Ag (111)The <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <strong>on</strong> Ag (111) is more complex than that <strong>on</strong> gold <str<strong>on</strong>g>and</str<strong>on</strong>g> features twocoverage-dependent rearrangements <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> which in additi<strong>on</strong> take place at differentsubstrate temperatures. The initial <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> is characterized by <str<strong>on</strong>g>molecules</str<strong>on</strong>g> lying flat <strong>on</strong> thesubstrate forming a highly ordered (7x7) (so-called ‘h<strong>on</strong>eycomb’) superstructure, as seen inLEED <str<strong>on</strong>g>and</str<strong>on</strong>g> STM. This layer is stable up to which means that neither desorpti<strong>on</strong> nordissociati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> was observed. Therefore the binding strength <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>molecules</str<strong>on</strong>g>in the first lying m<strong>on</strong>olayer could not be determined by TDS, but must be very high.Interestingly, the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> in this temperature-stable layer rearrange as the coverage isincreased above<str<strong>on</strong>g>and</str<strong>on</strong>g> form a m<strong>on</strong>olayer made up <str<strong>on</strong>g>of</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing <str<strong>on</strong>g>molecules</str<strong>on</strong>g> which exhibitsdesorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN fragments at , indicating a dissociati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>molecules</str<strong>on</strong>g>.However, no intact <str<strong>on</strong>g>molecules</str<strong>on</strong>g> desorb which shows that the Ag-CN binding is very str<strong>on</strong>g inthis layer, too. The rearrangement <str<strong>on</strong>g>of</str<strong>on</strong>g> the lying m<strong>on</strong>olayer into the st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing m<strong>on</strong>olayer wasalready seen at room temperature. When this layer has saturated at , meta-stable layersform which wet the st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing m<strong>on</strong>olayer <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>on</strong> these layers again the <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> proceeds inform <str<strong>on</strong>g>of</str<strong>on</strong>g> isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s. The meta-stable layers get incorporated into the isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s as the coverage isincreased, which is similar to the <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> HATCN <strong>on</strong> Au (111). Again, this rearrangementtakes place between <str<strong>on</strong>g>and</str<strong>on</strong>g> depending <strong>on</strong> the <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness.The work functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> Ag can be tailored by HATCN adsorpti<strong>on</strong> in a well-defined way. Up to, i.e. as l<strong>on</strong>g as the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> are lying flat <strong>on</strong> the surface, the work functi<strong>on</strong> somewhat129
9 Summarydecreases by up to . With the <str<strong>on</strong>g>molecules</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ing up the density <str<strong>on</strong>g>of</str<strong>on</strong>g> Ag-CN b<strong>on</strong>ds isincreased which leads to a higher charge transfer <str<strong>on</strong>g>and</str<strong>on</strong>g> hence to an increase <str<strong>on</strong>g>of</str<strong>on</strong>g> the workfuncti<strong>on</strong> by from to . Subsequent adsorpti<strong>on</strong> does not affect the work functi<strong>on</strong>.130
9 SummaryIn c<strong>on</strong>clusi<strong>on</strong> it has been shown in this work that the tailoring <str<strong>on</strong>g>of</str<strong>on</strong>g> thin <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g>s is possible<str<strong>on</strong>g>and</str<strong>on</strong>g> can be achieved by altering the <str<strong>on</strong>g>growth</str<strong>on</strong>g> parameters <str<strong>on</strong>g>and</str<strong>on</strong>g> by chemical <str<strong>on</strong>g>and</str<strong>on</strong>g> structuralmodificati<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> the substrates. It was found that there exists a delicate dependence <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g>structure <str<strong>on</strong>g>and</str<strong>on</strong>g> morphology <strong>on</strong> the substrate temperature during <str<strong>on</strong>g>and</str<strong>on</strong>g> after adsorpti<strong>on</strong>, whichdetermine to a large extent their physical <str<strong>on</strong>g>and</str<strong>on</strong>g> electr<strong>on</strong>ic characteristics. Thus the insightsgained in the course <str<strong>on</strong>g>of</str<strong>on</strong>g> this thesis could help to improve the performance <str<strong>on</strong>g>of</str<strong>on</strong>g> novel devicesbased <strong>on</strong> thin <str<strong>on</strong>g>organic</str<strong>on</strong>g> <str<strong>on</strong>g>film</str<strong>on</strong>g>s.131
10 Appendix10 AppendixATutorial <strong>on</strong> the use <str<strong>on</strong>g>of</str<strong>on</strong>g> the Nanosurf easyScan2 AFMIn this secti<strong>on</strong> it will be shortly explained how to h<str<strong>on</strong>g>and</str<strong>on</strong>g>le a properly installed <str<strong>on</strong>g>and</str<strong>on</strong>g> workingnanosurf easyScan2 AFM system. Detailed informati<strong>on</strong> <strong>on</strong> this AFM system can be found inthe manuals <str<strong>on</strong>g>of</str<strong>on</strong>g> the manufacturer [112, 113].A.1 Setting up the measurementThe first step is to put the scan head <strong>on</strong>to the AFM sample stage <str<strong>on</strong>g>and</str<strong>on</strong>g> to c<strong>on</strong>nect the scan headwith the c<strong>on</strong>troller via the scan head cable (see Fig. 10.1). Furthermore it is recommended toc<strong>on</strong>nect the scan head with the sample holder via the ground cable. After having set up theAFM system properly, the sample is put <strong>on</strong> the sample holder.132
10 AppendixFig. 10.1: Scan head (1) <strong>on</strong> the AFM sample stage. 2: Scan head cable, 3: Ground cable, 4: Levelingscrews, 5: Sample holder.After switching <strong>on</strong> the c<strong>on</strong>troller unit, the s<str<strong>on</strong>g>of</str<strong>on</strong>g>tware is started by clicking the desktop symbolshown in Fig. 10.2.Fig. 10.2: Desktop symbol <str<strong>on</strong>g>of</str<strong>on</strong>g> the nanosurf s<str<strong>on</strong>g>of</str<strong>on</strong>g>tware.Before approaching the tip to the sample, the vibrati<strong>on</strong> isolati<strong>on</strong> table has to be switched <strong>on</strong><str<strong>on</strong>g>and</str<strong>on</strong>g> by pressing E the isolati<strong>on</strong> is activated. During the isolati<strong>on</strong> process <strong>on</strong>e can m<strong>on</strong>itor thedecrease <str<strong>on</strong>g>of</str<strong>on</strong>g> the vibrati<strong>on</strong>s in the observing mode by pressing the down butt<strong>on</strong> four times.After the finishing <str<strong>on</strong>g>of</str<strong>on</strong>g> the isolati<strong>on</strong> process the red light will flash <str<strong>on</strong>g>and</str<strong>on</strong>g> now the tip can beapproached to the sample in three different ways (see also Fig. 10.3):1. manual coarse approach via the leveling screws <strong>on</strong> the scan head.2. manual coarse approach via the s<str<strong>on</strong>g>of</str<strong>on</strong>g>tware (Advance).3. automatic fine approach via the s<str<strong>on</strong>g>of</str<strong>on</strong>g>tware (Approach).133
10 AppendixFig. 10.3: Positi<strong>on</strong>ing window. Retract <str<strong>on</strong>g>and</str<strong>on</strong>g> Advance can be used for a coarse increase or decrease <str<strong>on</strong>g>of</str<strong>on</strong>g>the distance between the sample <str<strong>on</strong>g>and</str<strong>on</strong>g> the tip. Approach <str<strong>on</strong>g>and</str<strong>on</strong>g> Withdraw start the automatic fineadjustment <str<strong>on</strong>g>of</str<strong>on</strong>g> the tip positi<strong>on</strong>.When using the manual approach (either via the leveling screws or by pressing Advance) it isrecommended to keep a certain distance <str<strong>on</strong>g>of</str<strong>on</strong>g>between the cantilever <str<strong>on</strong>g>and</str<strong>on</strong>g> the samplein order to avoid any physical c<strong>on</strong>tact, which could damage the tip. If desired, this distancecan be m<strong>on</strong>itored with the magnifier glass, which is located <strong>on</strong> top <str<strong>on</strong>g>of</str<strong>on</strong>g> the scan head. In thefinal positi<strong>on</strong> the scan head should be as horiz<strong>on</strong>tal as possible. After finishing the manualcoarse approach, the automatic fine approach can be triggered independently from the sampletipdistance, but the smaller this distance, the faster the scan head reaches its measuringpositi<strong>on</strong>. After finishing a measurement or if <strong>on</strong>e desires to investigate another part <str<strong>on</strong>g>of</str<strong>on</strong>g> thesample, the tip has first to be automatically withdrawn via the s<str<strong>on</strong>g>of</str<strong>on</strong>g>tware (Withdraw) <str<strong>on</strong>g>and</str<strong>on</strong>g>afterwards manually via the s<str<strong>on</strong>g>of</str<strong>on</strong>g>tware (Retract) or the leveling screws.Right after starting the automatic approach, the s<str<strong>on</strong>g>of</str<strong>on</strong>g>tware checks the c<strong>on</strong>diti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the tip bymeasuring the res<strong>on</strong>ance curve which c<strong>on</strong>tains a measurement <str<strong>on</strong>g>of</str<strong>on</strong>g> the resulting cantilevervibrati<strong>on</strong> as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the excitati<strong>on</strong> frequency. When the search is successful, theres<strong>on</strong>ance curve c<strong>on</strong>tains a single peak at the free res<strong>on</strong>ance frequency <str<strong>on</strong>g>of</str<strong>on</strong>g> the cantilever.Afterwards a sec<strong>on</strong>d, fine-tuning search is performed in a frequency range around theres<strong>on</strong>ance frequency detected in the coarse search, as shown in Fig. 10.4.Fig. 10.4: Cantilever res<strong>on</strong>ance curve. Left: Coarse search; Right: Fine-tuning search.If the search <str<strong>on</strong>g>of</str<strong>on</strong>g> the res<strong>on</strong>ance frequency has not been successful, very <str<strong>on</strong>g>of</str<strong>on</strong>g>ten a restart <str<strong>on</strong>g>of</str<strong>on</strong>g> thes<str<strong>on</strong>g>of</str<strong>on</strong>g>tware can solve this problem. If this is not the case, the tip has to be changed (instructi<strong>on</strong>sin ref. [112], p. 31).134
10 AppendixA.2 ImagingThe imaging window allows setting up the basic image properties <str<strong>on</strong>g>and</str<strong>on</strong>g> is almost selfexplanatory(see Fig. 10.5). The rotati<strong>on</strong> determines the directi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the scan <str<strong>on</strong>g>and</str<strong>on</strong>g> thus whichslope has to be altered in order to correct the tilting <str<strong>on</strong>g>of</str<strong>on</strong>g> the sample with respect to the scanhead:• If the rotati<strong>on</strong> is set to 0° the X-slope has to be corrected.• If the rotati<strong>on</strong> is set to 90° the Y-slope has to be corrected.Another very important butt<strong>on</strong> is the photo-butt<strong>on</strong> <strong>on</strong> the right upper corner <str<strong>on</strong>g>of</str<strong>on</strong>g> Fig. 10.5. Ifthis butt<strong>on</strong> has been activated, the s<str<strong>on</strong>g>of</str<strong>on</strong>g>tware will automatically save the image as so<strong>on</strong> as it hasbeen finished. The recorded image is now displayed in a new window with the name image x(x being a sequence number) <str<strong>on</strong>g>and</str<strong>on</strong>g> can be saved in two different ways: Either by File - Save orFile - Save as, which saves the whole data file bel<strong>on</strong>ging to this image in the st<str<strong>on</strong>g>and</str<strong>on</strong>g>ard .nidformat <str<strong>on</strong>g>and</str<strong>on</strong>g> allows an <str<strong>on</strong>g>of</str<strong>on</strong>g>f-line processing <str<strong>on</strong>g>of</str<strong>on</strong>g> the image data any time. The other possibility is toexport (File – export) either the current chart (activated chart, blue frame) or the currentdocument (all charts) as an image (e.g. .jpg or .bmp file) without any data informati<strong>on</strong>.135
10 AppendixFig. 10.5: Imaging window. Here the image size (typically), the time per line (typically) <str<strong>on</strong>g>and</str<strong>on</strong>g> the resoluti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the images (points / line, st<str<strong>on</strong>g>and</str<strong>on</strong>g>ard: ) can be set up. The rotati<strong>on</strong>determines the scan directi<strong>on</strong>. Please make sure to scan forward <str<strong>on</strong>g>and</str<strong>on</strong>g> backward (framed in red) to allowfor the dual line scans which are needed to get informati<strong>on</strong> <strong>on</strong> the quality <str<strong>on</strong>g>of</str<strong>on</strong>g> the used parameters.Overscan determines the amount by which the scan range <str<strong>on</strong>g>of</str<strong>on</strong>g> each line is made larger than themeasurement range, relative to the image width (st<str<strong>on</strong>g>and</str<strong>on</strong>g>ard: 5%). Thus edge effects, caused by thereversal <str<strong>on</strong>g>of</str<strong>on</strong>g> the scanning moti<strong>on</strong>, are not displayed in the measurement. The saving <str<strong>on</strong>g>and</str<strong>on</strong>g> export opti<strong>on</strong>scan be found under File (upper left corner). The photo butt<strong>on</strong> is located in the right upper corner.There are several parameters which can be adjusted to improve the images. The mostimportant parameters for the image improvement are the loop gains (P-gain, I-gain) <str<strong>on</strong>g>and</str<strong>on</strong>g>the free vibrati<strong>on</strong> amplitude, which can be found in the Z-c<strong>on</strong>troller panel <str<strong>on</strong>g>and</str<strong>on</strong>g> in theoperating mode panel, respectively. The parameters which are now displayed as the defaultparameters had been saved before because they worked very well for most p6P <str<strong>on</strong>g>and</str<strong>on</strong>g> HATCN<str<strong>on</strong>g>film</str<strong>on</strong>g>s.Starting with the Z-c<strong>on</strong>troller panel (Fig. 10.6, right), the set point denotes the relativecantilever vibrati<strong>on</strong> amplitude (after the fine approach) with respect to its free vibrati<strong>on</strong>amplitude. For example, when a set point <str<strong>on</strong>g>of</str<strong>on</strong>g> 70% is used, the c<strong>on</strong>troller will move the tipcloser to the sample until the vibrati<strong>on</strong> amplitude has decreased to 70% <str<strong>on</strong>g>of</str<strong>on</strong>g> the vibrati<strong>on</strong>amplitude far away from the sample. Thus the set point determines the distance between thetip <str<strong>on</strong>g>and</str<strong>on</strong>g> the sample. In order to preserve the tip it is recommended to use a set point between60% <str<strong>on</strong>g>and</str<strong>on</strong>g> 70%.136
10 AppendixFig. 10.6: Left: Operating mode panel; The mounted cantilever is a n<strong>on</strong>-c<strong>on</strong>tact cantilever (NCLR)<str<strong>on</strong>g>and</str<strong>on</strong>g> the AFM is run in the dynamic force mode. Here the free vibrati<strong>on</strong> amplitude can be optimized.Right: Z-c<strong>on</strong>troller panel. Here the set point <str<strong>on</strong>g>and</str<strong>on</strong>g> the loop gains can be optimized. The tip voltage isnot used in this mode.The P-gain (proporti<strong>on</strong>al) <str<strong>on</strong>g>and</str<strong>on</strong>g> the I-gain (integral) determine the speed <str<strong>on</strong>g>of</str<strong>on</strong>g> the c<strong>on</strong>troller. If thegain is too low, the c<strong>on</strong>troller will not follow the surface fast enough. If the gain is too high,the c<strong>on</strong>troller will overshoot <str<strong>on</strong>g>and</str<strong>on</strong>g> may start to oscillate. Thus, the image will c<strong>on</strong>tain manymeasurement artifacts. Usually the P-gain should be higher than the I-gain by a factor <str<strong>on</strong>g>of</str<strong>on</strong>g> 10.Typical values for the P-gain range from 20000 up to 30000. Unfortunately, there is a limit <str<strong>on</strong>g>of</str<strong>on</strong>g>32767 for the P-gain.The free vibrati<strong>on</strong> amplitude can be found in the operating mode panel (Fig. 10.6, left) <str<strong>on</strong>g>and</str<strong>on</strong>g>denotes the desired reference amplitude <str<strong>on</strong>g>of</str<strong>on</strong>g> the cantilever vibrati<strong>on</strong>. The cantilever vibrates atthis amplitude when it is far away from the sample. The excitati<strong>on</strong> strength is adjusted so thatthis vibrati<strong>on</strong> amplitude is reached. Typical values for the free vibrati<strong>on</strong> amplitude range fromup to, sometimes <strong>on</strong> very rough surfaces it is necessary to raise theamplitude to or . In general, the quality <str<strong>on</strong>g>of</str<strong>on</strong>g> the obtained images can besignificantly improved by altering the free vibrati<strong>on</strong> amplitude <str<strong>on</strong>g>and</str<strong>on</strong>g> this parameter is apartfrom the loop gains the most important <strong>on</strong>e. A good way to check the used parameters is tohave a look at the dual line scan during the measurement: The two lines (forward & backwardscan) should not differ too much from each other. NCLR <str<strong>on</strong>g>and</str<strong>on</strong>g> dynamic force indicate that an<strong>on</strong>-c<strong>on</strong>tact cantilever is mounted <str<strong>on</strong>g>and</str<strong>on</strong>g> that the AFM is run in the n<strong>on</strong>-c<strong>on</strong>tact (tapping) mode.137
10 AppendixA.3 Image processingOnce the image data has been recorded <str<strong>on</strong>g>and</str<strong>on</strong>g> saved, it is always possible to create new imagesdisplaying this data (Fig. 10.7, 8). The data c<strong>on</strong>tained in the .nid files (st<str<strong>on</strong>g>and</str<strong>on</strong>g>ard AFM files)can be displayed as (see Fig. 10.7, 10)• single line scan (forward)• dual line scan (forward & backward, <strong>on</strong>ly possible if activated in the imaging window)• topography map• shaded map• 3D viewFig. 10.7: Image processing bar; 1: Measure length; 2: Measure distance; 3: Measure Angle; 4: Createcross-secti<strong>on</strong>; 5: Cut out area; 6: Glitch filter; 7: Noise filter; 8: Create new chart; 9: Delete chart; 10:Select chart type; 11: Select data filter; 12: Select signal; 13: Show chart properties dialog;The topography image is the st<str<strong>on</strong>g>and</str<strong>on</strong>g>ard image type, which displays the surface corrugati<strong>on</strong>according to a color scale. On the right-h<str<strong>on</strong>g>and</str<strong>on</strong>g> side <str<strong>on</strong>g>of</str<strong>on</strong>g> the topography map there is a small blackarrow which points to the line which is depicted in the current line scan. Of uttermostimportance during the measurement is to m<strong>on</strong>itor the dual line scan, which gives informati<strong>on</strong><strong>on</strong> the reliability <str<strong>on</strong>g>of</str<strong>on</strong>g> the obtained images <str<strong>on</strong>g>and</str<strong>on</strong>g> helps to optimize the imaging parameters. Theshaded image mode is a hybrid between the st<str<strong>on</strong>g>and</str<strong>on</strong>g>ard topography map <str<strong>on</strong>g>and</str<strong>on</strong>g> its derivative. Thismode is <str<strong>on</strong>g>of</str<strong>on</strong>g>ten used because small surface structures can be seen more clearly that way than inthe st<str<strong>on</strong>g>and</str<strong>on</strong>g>ard topography mode. Eventually it is also possible to let the s<str<strong>on</strong>g>of</str<strong>on</strong>g>tware draw a 3Dimage. If desired, the scales <str<strong>on</strong>g>of</str<strong>on</strong>g> all the images can be set manually in the chart propertiesdialog (Fig. 10.7, 13), which is advisable if there is an artifact in the image which distorts thescale. The data filter panel (Fig. 10.7, 11) describes the way the data is processed before it isdisplayed.• Raw Data: The data is not processed.• Mean Fit: The mean value <str<strong>on</strong>g>of</str<strong>on</strong>g> each line <str<strong>on</strong>g>of</str<strong>on</strong>g> data points is calculated <str<strong>on</strong>g>and</str<strong>on</strong>g> then subtracted.138
10 Appendix• Line Fit: The best fit line for each line <str<strong>on</strong>g>of</str<strong>on</strong>g> data points is calculated <str<strong>on</strong>g>and</str<strong>on</strong>g> then subtracted.• Derived Data: The difference between two successive data points (derivative) iscalculated.• Parabola Fit: The best fit parabola for each line <str<strong>on</strong>g>of</str<strong>on</strong>g> data points is calculated <str<strong>on</strong>g>and</str<strong>on</strong>g> thensubtracted• Polynomial Fit: The best fit fourth order polynomial for each line <str<strong>on</strong>g>of</str<strong>on</strong>g> data points iscalculated <str<strong>on</strong>g>and</str<strong>on</strong>g> then subtracted.Typically, the mean fit <str<strong>on</strong>g>and</str<strong>on</strong>g> the line fit yield the best images.Further, the images can be evaluated <str<strong>on</strong>g>and</str<strong>on</strong>g> analyzed with the help <str<strong>on</strong>g>of</str<strong>on</strong>g> the tools bar (Fig. 10.7, 1-7). Tools can be used in all charts, both during measurement <str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g>f-line. Before using a tool<strong>on</strong> a chart, the chart must be activated by clicking <strong>on</strong>to it (blue frame appears). Am<strong>on</strong>g thetools there are also filters which can be used to reduce periodical artifacts (glitch filter, noisefilter).The most important tool is the cross secti<strong>on</strong> tool (Fig. 10.7, 4), which creates a line scan al<strong>on</strong>ga user-defined line in an activated chart. In this way structures in the sample can be analyzedin their height <str<strong>on</strong>g>and</str<strong>on</strong>g> length by the corresp<strong>on</strong>ding tools. Please note that the line al<strong>on</strong>g which thecross secti<strong>on</strong> has been created is indicated in the corresp<strong>on</strong>ding image <str<strong>on</strong>g>and</str<strong>on</strong>g> needs to be savedby exporting the chart as an image (e.g. as .jpg or .bmp) immediately. If the file is saved as an.nid file, this informati<strong>on</strong> will be lost.139
10 AppendixBTutorial <strong>on</strong> the use <str<strong>on</strong>g>of</str<strong>on</strong>g> the UHV Kelvin ProbeThis secti<strong>on</strong> aims to dem<strong>on</strong>strate shortly how a work functi<strong>on</strong> measurement using the built-inUHV Kelvin probe <str<strong>on</strong>g>of</str<strong>on</strong>g> KP Technology LTD is carried out. Traditi<strong>on</strong>ally, the work functi<strong>on</strong>difference between two surfaces was determined by vibrating the reference surface above thesample <str<strong>on</strong>g>and</str<strong>on</strong>g> adjusting the variable ‘backing potential’ until a zero or null output resulted.However, this mode <str<strong>on</strong>g>of</str<strong>on</strong>g> detecti<strong>on</strong> is extremely sensitive to noise since the Kelvin probe signalis diminishing with respect to the noise background. Therefore the Kelvin probe systemmanufactured by KP Technology LTD utilizes an <str<strong>on</strong>g>of</str<strong>on</strong>g>f-null detecti<strong>on</strong> where the signal ismeasured at high signal levels far from balance. For further details please refer to the manual<str<strong>on</strong>g>of</str<strong>on</strong>g> the manufacturer [134].B.1 Quick start <str<strong>on</strong>g>of</str<strong>on</strong>g> the s<str<strong>on</strong>g>of</str<strong>on</strong>g>twareIn the initial positi<strong>on</strong> the tip should be located a few away from the sample in order toavoid sample damage when the tip starts vibrating later <strong>on</strong>. After having positi<strong>on</strong>ed thesample <str<strong>on</strong>g>and</str<strong>on</strong>g> the tip properly, the first step is to start the Kelvin probe s<str<strong>on</strong>g>of</str<strong>on</strong>g>tware by clicking thedesktop symbol shown in Fig. 10.8.Fig. 10.8: Desktop symbol <str<strong>on</strong>g>of</str<strong>on</strong>g> the Kelvin probe s<str<strong>on</strong>g>of</str<strong>on</strong>g>tware.After the s<str<strong>on</strong>g>of</str<strong>on</strong>g>tware has started <strong>on</strong>e has to open a parameter file, see Fig. 10.9 <str<strong>on</strong>g>and</str<strong>on</strong>g> Fig. 10.10.For a start, I would recommend to choose the file st<str<strong>on</strong>g>and</str<strong>on</strong>g>ard.par. Experienced users can createtheir own parameter file by clicking New Parameter File in Fig. 10.9.140
10 AppendixFig. 10.9: Click File – Open Parameter File to open an existing parameter file <str<strong>on</strong>g>and</str<strong>on</strong>g> File – NewParameter File to create a new parameter file.Fig. 10.10: The Open Parameter File dialog.Once a parameter file has been opened a window similar to the <strong>on</strong>e shown in Fig. 10.11 willbe displayed. There are three tabs: <strong>on</strong>e for the Setup Probe <str<strong>on</strong>g>and</str<strong>on</strong>g> Data Acquisiti<strong>on</strong>, a sec<strong>on</strong>d tabfor the Work Functi<strong>on</strong> Measurement <str<strong>on</strong>g>and</str<strong>on</strong>g> a third for Advanced/Diagnostics.141
10 AppendixFig. 10.11: After opening the parameter file st<str<strong>on</strong>g>and</str<strong>on</strong>g>ard.par.The Setup Probe <str<strong>on</strong>g>and</str<strong>on</strong>g> Data Acquisiti<strong>on</strong> page c<strong>on</strong>tains groups for setting-up the probe, dataacquisiti<strong>on</strong>, tip translati<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> measurement opti<strong>on</strong>s. The values <str<strong>on</strong>g>of</str<strong>on</strong>g> the probe <str<strong>on</strong>g>and</str<strong>on</strong>g> dataacquisiti<strong>on</strong> parameters have turned out to work fine, but <str<strong>on</strong>g>of</str<strong>on</strong>g> course experienced users may tryto further optimize them with the help <str<strong>on</strong>g>of</str<strong>on</strong>g> the KP manual [134]. Before a measurement canbe started, the probe has to be activated by clicking Enable (top right in Fig. 10.11). Now thetip is vibrating <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>on</strong>e can start the data acquisiti<strong>on</strong> by clicking Start in the Work Functi<strong>on</strong>Measurement tab (Fig. 10.12):142
10 AppendixFig. 10.12: The measurement page. The Start-butt<strong>on</strong> in the top right corner starts the measurement.The Show group determines what is displayed in the chart: Sig is the signal from the tip, WF(automatically activated after pressing the Start-butt<strong>on</strong>) is the measured work functi<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> GD is thegradient, which gives informati<strong>on</strong> <strong>on</strong> the sample-tip distance.After starting the measurement, automatically the work functi<strong>on</strong> is displayed in the chart.Please note that the value <str<strong>on</strong>g>of</str<strong>on</strong>g> the work functi<strong>on</strong> str<strong>on</strong>gly depends <strong>on</strong> the sample-tip distance<str<strong>on</strong>g>and</str<strong>on</strong>g> has no meaning unless the tip has been moved as closely as possible towards the samplewithout touching it. It has to be pointed out here that in order to prevent damage <str<strong>on</strong>g>of</str<strong>on</strong>g> the samplethe direct c<strong>on</strong>tact with the tip must be avoided.B.2 Positi<strong>on</strong>ing the tipIn order to approach the tip to the sample, the signal <str<strong>on</strong>g>of</str<strong>on</strong>g> the tip has to be observed by clickingSig (see again Fig. 10.12). The tip is moved towards the sample by turning the wheel attachedto the UHV Kelvin probe (see Fig. 10.16). In Fig. 10.13 the evoluti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the signal is shownas a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the sample-tip distance. In the initial positi<strong>on</strong>, i.e. when the sample-tipdistance is large, the signal will probably be similar to the <strong>on</strong>e shown in Fig. 10.13a. As thesample-tip distance decreases (Fig. 10.13a-f), the amplitude <str<strong>on</strong>g>of</str<strong>on</strong>g> the signal gets higher (up tohere) <str<strong>on</strong>g>and</str<strong>on</strong>g> the curve should exhibit exactly <strong>on</strong>e minimum <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>on</strong>e maximum. During the143
10 Appendixapproach the signal must remain symmetric with respect to the line. If this is not thecase, <str<strong>on</strong>g>like</str<strong>on</strong>g> in Fig. 10.13f, the tip is already touching the sample <str<strong>on</strong>g>and</str<strong>on</strong>g> has to be withdrawnquickly. The work functi<strong>on</strong> measurement can be started when the signal looks similar to Fig.10.13d or Fig. 10.13e, indicating that the sample-tip distance is sufficiently small, but notzero.Fig. 10.13: Signal <str<strong>on</strong>g>of</str<strong>on</strong>g> the tip as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the tip-sample distance, starting from a) large distance t<str<strong>on</strong>g>of</str<strong>on</strong>g>) no distance (tip already touches the sample). When the signal looks similar to the charts shown in d)<str<strong>on</strong>g>and</str<strong>on</strong>g> e) the work functi<strong>on</strong> measurement can be started.B.3 The work functi<strong>on</strong> measurementThe work functi<strong>on</strong>, the gradient <str<strong>on</strong>g>and</str<strong>on</strong>g> the signal can be observed throughout the measurementwithout affecting it by selecting the desired mode in the Show group. By checking the signalfrom time to time it can be ascertained that the sample-tip distance stays c<strong>on</strong>stant during themeasurement, i.e. that the tip does not suddenly start touching the sample. Another way tom<strong>on</strong>itor the sample-tip distance is to observe the gradient (GD): the higher the gradient, the144
10 Appendixsmaller is the sample-tip distance. Fig. 10.14 shows the evoluti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the work functi<strong>on</strong> duringa measurement.Fig. 10.14: The result <str<strong>on</strong>g>of</str<strong>on</strong>g> a measurement after 500 points. Beneath the chart the values for the meanwork functi<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> the mean gradient are displayed.There are always slight changes in the work functi<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> the gradient during a measurement.Hence the mean work functi<strong>on</strong> is displayed beneath the work functi<strong>on</strong> chart together with themean gradient. The mean gradient becomes particularly important if the sample has to bemoved during a series <str<strong>on</strong>g>of</str<strong>on</strong>g> work functi<strong>on</strong> measurements, for instance when the work functi<strong>on</strong> isstudied as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the <str<strong>on</strong>g>film</str<strong>on</strong>g> thickness. Here the gradient helps to find again the tip-sampledistance at which the related work functi<strong>on</strong> measurements <str<strong>on</strong>g>of</str<strong>on</strong>g> the same series were taken.Since the work functi<strong>on</strong> depends <strong>on</strong> the tip-sample distance, it is <str<strong>on</strong>g>of</str<strong>on</strong>g> utmost importance that allthe measurements <str<strong>on</strong>g>of</str<strong>on</strong>g> a series, which are compared with each other later <strong>on</strong>, are taken at thesame tip-sample distance (same gradient).The reliability <str<strong>on</strong>g>of</str<strong>on</strong>g> the gained values depends <strong>on</strong> the number <str<strong>on</strong>g>of</str<strong>on</strong>g> points that have been measured.It is recommended to record at least 500 points in order to reduce the influence <str<strong>on</strong>g>of</str<strong>on</strong>g> the noise <strong>on</strong>the gained work functi<strong>on</strong> values.After the preset number <str<strong>on</strong>g>of</str<strong>on</strong>g> points has been reached, the s<str<strong>on</strong>g>of</str<strong>on</strong>g>tware <str<strong>on</strong>g>of</str<strong>on</strong>g>fers the possibility to saveit as a data (.dat) file (Fig. 10.15). These data files can be processed in ORIGIN. However,145
10 Appendix<str<strong>on</strong>g>of</str<strong>on</strong>g>ten it is sufficient to write down the mean WF <str<strong>on</strong>g>and</str<strong>on</strong>g> the mean GD together with thecorresp<strong>on</strong>ding st<str<strong>on</strong>g>and</str<strong>on</strong>g>ard deviati<strong>on</strong>s.Fig. 10.15: If desired, the measurement can be saved as a data file (.dat) when it is finished.B.4 Hints & tipsIt has already been pointed out that the work functi<strong>on</strong> depends <strong>on</strong> the sample-tip distance,which can be m<strong>on</strong>itored by means <str<strong>on</strong>g>of</str<strong>on</strong>g> the gradient. In general the work functi<strong>on</strong> measurementwill be ‘better’ (low-noise, reliable, stable) the higher the gradient is. Due to the macroscopicsize <str<strong>on</strong>g>of</str<strong>on</strong>g> the tip a high gradient is <strong>on</strong>ly possible if the tip <str<strong>on</strong>g>and</str<strong>on</strong>g> the sample are totally parallel toeach other, which has to be taken care <str<strong>on</strong>g>of</str<strong>on</strong>g> already when attaching the sample to the sampleholder in air: avoid any tilting <str<strong>on</strong>g>of</str<strong>on</strong>g> the sample.From time to time the signal <str<strong>on</strong>g>of</str<strong>on</strong>g> the tip stays c<strong>on</strong>stant at <str<strong>on</strong>g>and</str<strong>on</strong>g> does not change up<strong>on</strong> anyacti<strong>on</strong>. In that case an electr<strong>on</strong>ic comp<strong>on</strong>ent (AD 549 JH, operati<strong>on</strong>al amplifier) inside theUHV tip amplifier (c<strong>on</strong>nects the digital c<strong>on</strong>trol unit to the Kelvin probe) has broken down <str<strong>on</strong>g>and</str<strong>on</strong>g>must be replaced (see Fig. 10.16).146
10 AppendixFig. 10.16: Left: Photograph <str<strong>on</strong>g>of</str<strong>on</strong>g> the UHV Kelvin probe. 1: Positi<strong>on</strong>ing wheel to approach or withdrawthe tip, 2: UHV tip amplifier. Right: Detail <str<strong>on</strong>g>and</str<strong>on</strong>g> interior view <str<strong>on</strong>g>of</str<strong>on</strong>g> the UHV tip amplifier. The red arrowpoints to the electr<strong>on</strong>ic comp<strong>on</strong>ent that must be replaced if the signal remains c<strong>on</strong>stant at .Due to the sensibility <str<strong>on</strong>g>of</str<strong>on</strong>g> this electr<strong>on</strong>ic comp<strong>on</strong>ent to voltage swings <str<strong>on</strong>g>of</str<strong>on</strong>g> some <str<strong>on</strong>g>of</str<strong>on</strong>g> the otherattachments to the UHV chamber (especially XPS <str<strong>on</strong>g>and</str<strong>on</strong>g> turbo pump cooling system) it isrecommended to unplug the UHV tip amplifier if the Kelvin probe is not used for a l<strong>on</strong>gertime.147
10 AppendixCList <str<strong>on</strong>g>of</str<strong>on</strong>g> publicati<strong>on</strong>s <str<strong>on</strong>g>and</str<strong>on</strong>g> c<strong>on</strong>ference c<strong>on</strong>tributi<strong>on</strong>sC.1 Publicati<strong>on</strong>s in internati<strong>on</strong>al scientific journals1. Characterizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> step-edge barriers in <str<strong>on</strong>g>organic</str<strong>on</strong>g> thin-<str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>.G. Hlawacek, P. Puschnig, P. Frank, A. Winkler, C. Ambrosch-Draxl, C. Teichert,Science 321 (2008) 108.2. A study <strong>on</strong> the formati<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> thermal stability <str<strong>on</strong>g>of</str<strong>on</strong>g> 11-MUA SAMs <strong>on</strong> Au(111)/mica <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>on</strong>polycrystalline gold foils.J. Stettner, P. Frank, T. Grießer, G. Trimmel, R. Schennach, E. Gilli, A. Winkler,Langmuir 25 (2009) 1427.3. Characterizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> 11-MUA SAM formati<strong>on</strong> <strong>on</strong> gold surfaces.J. Stettner, P. Frank, T. Grießer, G. Trimmel, R. Schennach, R. Resel, A. Winkler,Springer proceedings in physics 129 (2009) 101.4. Layer <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> desorpti<strong>on</strong> kinetics <str<strong>on</strong>g>of</str<strong>on</strong>g> a <str<strong>on</strong>g>disc</str<strong>on</strong>g>oid molecular acceptor <strong>on</strong> Au (111).P. Frank, N. Koch, M. Koini, R. Rieger, K. Müllen, R. Resel, A. Winkler,Chemical physics letters 473 (2009) 321.5. Origin <str<strong>on</strong>g>of</str<strong>on</strong>g> the low-energy emissi<strong>on</strong> b<str<strong>on</strong>g>and</str<strong>on</strong>g> in epitaxially grown para-sexiphenylnanocrystallites.A. Kadashchuk, S. Schols, P. Heremans, Y. Skryshevski, Y. Piryatinski, I. Beinik, C.Teichert, G. Hern<str<strong>on</strong>g>and</str<strong>on</strong>g>ez-Sosa, H. Sitter, A. Andreev, P. Frank, A.Winkler,The journal <str<strong>on</strong>g>of</str<strong>on</strong>g> chemical physics 130 (2009) 084901.6. SAM <str<strong>on</strong>g>of</str<strong>on</strong>g> 11-MUA grown <strong>on</strong> polycrystalline Au foils by physical vapor depositi<strong>on</strong> in UHV.P. Frank, F. Nußbacher, J. Stettner, A. Winkler,Springer proceedings in physics 129 (2009) 107.148
10 Appendix7. Spectroscopy <str<strong>on</strong>g>of</str<strong>on</strong>g> defects in epitaxially grown para-sexiphenyl nanostructures.A. Kadashchuk, Y. Skryshevski, I. Beynik, C. Teichert, G. Hern<str<strong>on</strong>g>and</str<strong>on</strong>g>ez-Sosa, H. Sitter, A.Andreev, P. Frank, A. Winkler,Springer proceedings in physics 129 (2009) 121.8. Search for a wetting layer in the <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> para-hexaphenyl <strong>on</strong> KCl(001).P. Frank, G. Hern<str<strong>on</strong>g>and</str<strong>on</strong>g>ez-Sosa, H. Sitter, A. Winkler,<str<strong>on</strong>g>Thin</str<strong>on</strong>g> solid <str<strong>on</strong>g>film</str<strong>on</strong>g>s 516 (2008) 2939.9. Tailoring the <str<strong>on</strong>g>film</str<strong>on</strong>g> morphology <str<strong>on</strong>g>of</str<strong>on</strong>g> para-hexaphenyl <strong>on</strong> KCl(001) by surface temperature.P. Frank, A. Winkler,Applied physics / A 90 (2008) 717.10. Characterizing Chemically Reactive <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Layers: Surface Reacti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> [2-[4-(Chlorosulf<strong>on</strong>yl)phenyl]ethyl]trichlorosilane with Amm<strong>on</strong>ia.P. Pacher, A. Lex, V. Proschek, O. Werzer, P. Frank, S. Temmel, W. Kern, R. Resel, A.Winkler, C. Slugovc, R. Schennach, G. Trimmel, E. Zojer,The journal <str<strong>on</strong>g>of</str<strong>on</strong>g> physical chemistry C 111 (2007) 12407.11. Influence <str<strong>on</strong>g>of</str<strong>on</strong>g> surface temperature <str<strong>on</strong>g>and</str<strong>on</strong>g> surface modificati<strong>on</strong>s <strong>on</strong> the initial layer <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g>para-hexaphenyl <strong>on</strong> mica(001).P. Frank, G. Hlawacek, O. Lengyel, A. Satka, C. Teichert, R. Resel, A. Winkler,Surface science 601 (2007) 2152.12. The influence <str<strong>on</strong>g>of</str<strong>on</strong>g> substrate temperature <strong>on</strong> the structure <str<strong>on</strong>g>and</str<strong>on</strong>g> morphology <str<strong>on</strong>g>of</str<strong>on</strong>g> sexiphenyl thin<str<strong>on</strong>g>film</str<strong>on</strong>g>s <strong>on</strong> Au(111).S. Müllegger, G. Hlawacek, T. Haber, P. Frank, C. Teichert, R. Resel, A. Winkler,Applied physics / A 87 (2007) 103.13. Sp<strong>on</strong>taneous rearrangement <str<strong>on</strong>g>of</str<strong>on</strong>g> oligophenyl crystallites into nan<str<strong>on</strong>g>of</str<strong>on</strong>g>ibers.C. Teichert, G. Hlawacek, A. Andreev, H. Sitter, P. Frank, A. Winkler, N.S. Sariciftci,Applied physics / A 82 (2006) 665.149
10 Appendix14. Atom probe, AFM, <str<strong>on</strong>g>and</str<strong>on</strong>g> STM studies <strong>on</strong> vacuum-fired stainless steels.A. Stupnik, P. Frank, M. Leisch,Ultramicroscopy 109 (2009) 563.150
10 AppendixC. 2 C<strong>on</strong>ference C<strong>on</strong>tributi<strong>on</strong>s1. Tailoring the work functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> metals using a cyano-based molecular acceptor: HATCN<strong>on</strong> Au(111) <str<strong>on</strong>g>and</str<strong>on</strong>g> Ag(111).P. Frank, M. Koini, T. Djuric, N. Koch, R. Rieger, K. Müllen, R. Resel, A. Winkler,59 th ÖPG Jahrestagung, Sept. 2009, Innsbruck, Austria.2. Temperature <str<strong>on</strong>g>and</str<strong>on</strong>g> coverage dependent dewetting <str<strong>on</strong>g>of</str<strong>on</strong>g> ultra-thin <str<strong>on</strong>g>film</str<strong>on</strong>g>s <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>disc</str<strong>on</strong>g>oid <str<strong>on</strong>g>organic</str<strong>on</strong>g><str<strong>on</strong>g>molecules</str<strong>on</strong>g> <strong>on</strong> Au(111) <str<strong>on</strong>g>and</str<strong>on</strong>g> Ag(111).P. Frank, M. Koini, N. Koch, R. Rieger, K. Müllen, R. Resel, A. Winkler,Materials Research Society (MRS) Spring Meeting, April 2009, San Francisco, USA.3. Thermal Desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> SAMs <str<strong>on</strong>g>of</str<strong>on</strong>g> 11-mercaptoundecanoic acid grown <strong>on</strong> polycrystallineAu-foils by physical vapor depositi<strong>on</strong> in ultra high vacuum.P. Frank, J. Stettner, F. Nußbacher, A. Winkler,58 th ÖPG Jahrestagung, Sept. 2008, Leoben, Austria.4. AT & S Preis: Mound formati<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> level dependent Ehrlich-Schwoebel barrier in<str<strong>on</strong>g>organic</str<strong>on</strong>g> semic<strong>on</strong>ductor thin <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>.G. Hlawacek, P. Puschnig, P. Frank, A. Winkler, C. Ambrosch-Draxl, C. Teichert,58 th ÖPG Jahrestagung, Sept. 2008, Leoben, Austria.5. Morphology <str<strong>on</strong>g>and</str<strong>on</strong>g> emissi<strong>on</strong> properties <str<strong>on</strong>g>of</str<strong>on</strong>g> ordered epitaxially grown <str<strong>on</strong>g>organic</str<strong>on</strong>g>nanocrystallites.A. Kadashchuk, C. Teichert, I. Beinik, A. Andreev, G. Hern<str<strong>on</strong>g>and</str<strong>on</strong>g>ez-Sosa, H. Sitter, P.Frank, A. Winkler,58 th ÖPG Jahrestagung, Sept. 2008, Leoben, Austria.151
10 Appendix6. Atom Probe, AFM <str<strong>on</strong>g>and</str<strong>on</strong>g> STM study <strong>on</strong> vacuum fired stainless steel.A. Stupnik, P. Frank, M. Leisch,58 th ÖPG Jahrestagung, Sept. 2008, Leoben, Austria.7. The stability <str<strong>on</strong>g>of</str<strong>on</strong>g> 11-MUA SAMs grown <strong>on</strong> gold surfaces.J. Stettner, P. Frank, T. Grießer, G. Trimmel, R. Schennach, R. Resel, A. Winkler,58 th ÖPG Jahrestagung, Sept. 2008, Leoben, Austria.8. Chemical C<strong>on</strong>trol <str<strong>on</strong>g>of</str<strong>on</strong>g> the Local Charge Carrier C<strong>on</strong>centrati<strong>on</strong> in Organic <str<strong>on</strong>g>Thin</str<strong>on</strong>g> FilmTransistors.P. Pacher, H. Etschmaier, A. Lex, V. Proschek, E. Tchernychova, M. Sezen, O. Werzer, P.Frank, W. Grogger, R. Resel, R. Schennach, A. Winkler, G. Trimmel, C. Slugovc,E. Zojer,The 8 th Internati<strong>on</strong>al Symposium <strong>on</strong> Functi<strong>on</strong>al Pi-Electr<strong>on</strong> Systems, July 2008, Graz,Austria.9. A study <str<strong>on</strong>g>of</str<strong>on</strong>g> 11-MUA SAM formati<strong>on</strong> <strong>on</strong> gold.J. Stettner, P. Frank, T. Grießer, G. Trimmel, R. Schennach, R. Resel, A. Winkler,The 8 th Internati<strong>on</strong>al Symposium <strong>on</strong> Functi<strong>on</strong>al Pi-Electr<strong>on</strong> Systems. July 2008, Graz,Austria.10. Thermal Desorpti<strong>on</strong> Spectroscopy Investigati<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> SAMs <str<strong>on</strong>g>of</str<strong>on</strong>g> 11-mercaptoundecanoic acidgrown <strong>on</strong> polycrystalline Au-foils by physical vapor depositi<strong>on</strong> in ultra high vacuum.P. Frank, J. Stettner, F. Nußbacher, A. Winkler,The 8 th Internati<strong>on</strong>al Symposium <strong>on</strong> Functi<strong>on</strong>al Pi-Electr<strong>on</strong> Systems. July 2008, Graz,Austria.11. Atom Probe <str<strong>on</strong>g>and</str<strong>on</strong>g> STM Study <strong>on</strong> Vacuum fired stainless steel.A. Stupnik, P. Frank, M. Leisch,51 st Internati<strong>on</strong>al Field Emissi<strong>on</strong> Symposium, June/July 2008, Rouen, France.12. Characterisati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> 11-MUA SAM formati<strong>on</strong> <strong>on</strong> gold surfaces.J. Stettner, P. Frank, T. Grießer, G. Trimmel, R. Schennach, R. Resel, A. Winkler,152
10 AppendixEuropean Materials Research Society (EMRS) Spring Meeting, May 2008, Strasbourg,France.13. Ehrlich-Schwöbel barrier <str<strong>on</strong>g>and</str<strong>on</strong>g> Zeno effect in <str<strong>on</strong>g>organic</str<strong>on</strong>g> thin <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>.G. Hlawacek, C. Teichert, P. Puschnig, P. Frank, A. Winkler, C. Ambrosch-Draxl,European Materials Research Society (EMRS) Spring Meeting, May 2008, Strasbourg,France.14. SAMs <str<strong>on</strong>g>of</str<strong>on</strong>g> 11-MUA grown <strong>on</strong> polycrystalline Au-foils by physical vapor depositi<strong>on</strong> in UHV.P. Frank, F. Nußbacher, J. Stettner, A. Winkler,European Materials Research Society (EMRS) Spring Meeting, May 2008, Strasbourg,France.15. Tuning <str<strong>on</strong>g>of</str<strong>on</strong>g> surface properties by photoreactive self assembled m<strong>on</strong>olayers.G. Trimmel, T. Grießer, A. Lex, T. Höfler, P. Pacher, Q. Shen, G. Hlawacek, O. Werzer,A.M. Track, P. Frank, R. Schennach, G. Koller, M. Ramsey, C. Teichert, R. Resel, A.Winkler, E. Zojer, J. Adams, G.J. Leggett, W. Kern,European Materials Research Society (EMRS) Spring Meeting, May 2008, Strasbourg,France.16. Chemical C<strong>on</strong>trol <str<strong>on</strong>g>of</str<strong>on</strong>g> Charge Carrier C<strong>on</strong>centrati<strong>on</strong> in Organic <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Film Transistors:From Depleti<strong>on</strong> to Enhancement.P. Pacher, A. Lex, V. Proschek, H. Etschmaier, E. Tchernychova, M. Sezen, O. Werzer, P.Frank, W. Grogger, R. Resel, R. Schennach, A. Winkler, G. Trimmel, C. Slugovc, E.Zojer,European Materials Research Society (EMRS) Spring Meeting, May 2008, Strasbourg,France.17. Surface study <strong>on</strong> vacuum fired stainless steel by AFM <str<strong>on</strong>g>and</str<strong>on</strong>g> STM.A. Stupnik, P. Frank, M. Leisch,Symposium <strong>on</strong> Surface Science, March 2008, St. Christoph, Austria.18. Step edge barrier in <str<strong>on</strong>g>organic</str<strong>on</strong>g> thin <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>.P. Puschnig, C. Ambrosch-Draxl, G. Hlawacek, P. Frank, A. Winkler, C. Teichert,153
10 AppendixSymposium <strong>on</strong> Surface Science, March 2008, St. Christoph, Austria.19. Ehrlich-Schwöbel barrier <str<strong>on</strong>g>and</str<strong>on</strong>g> Zeno effect in <str<strong>on</strong>g>organic</str<strong>on</strong>g> thin <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g>.G. Hlawacek, C. Teichert, P. Puschnig, P. Frank, A. Winkler, C. Ambrosch-Draxl,DPG Spring Meeting, Feb. 2008, Berlin, Germany.20. Polymer <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Film Characterizati<strong>on</strong>: A Combined Approach by X-ray Reflectivity <str<strong>on</strong>g>and</str<strong>on</strong>g>Spectroscopic Ellipsometry.H.-G. Flesch, O. Werzer, P. Frank, A. Blümel, J. Jakabovic, J. Kovac, G. Jakopic, R.Resel,NFN Winterschool <strong>on</strong> Organic Electr<strong>on</strong>ics, Jan. 2008, Planneralm, Austria.21. Self assembled m<strong>on</strong>olayers: A study <str<strong>on</strong>g>of</str<strong>on</strong>g> 11-MUA <strong>on</strong> Au surfaces.P. Frank, J. Stettner, T. Grießer, G. Trimmel, R. Schennach, R. Resel, A. Winkler,NFN Winterschool <strong>on</strong> Organic Electr<strong>on</strong>ics, Jan. 2008, Planneralm, Austria.22. Investigati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> chemically <str<strong>on</strong>g>and</str<strong>on</strong>g> photochemically reactive siloxane thin layers for surfacemodificati<strong>on</strong>.A. Lex, P. Pacher, S. Temmel, Q. Shen, G. Hlawacek, O. Werzer, A.M. Track, P. Frank,V. Proschek, R. Schennach, G. Koller, M. Ramsey, C. Teichert, R. Resel, A. Winkler, E.Zojer, W. Kern, G. Trimmel,NFN Winterschool <strong>on</strong> Organic Electr<strong>on</strong>ics, Jan. 2008, Planneralm, Austria.23. Chemically <str<strong>on</strong>g>and</str<strong>on</strong>g> Photochemically Reactive Siloxane Layers for Modificati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> In<str<strong>on</strong>g>organic</str<strong>on</strong>g>Surfaces.A. Lex, P. Pacher, S. Temmel, Q. Shen, G. Hlawacek, O. Werzer, A.M. Track, P. Frank,V. Proschek, R. Schennach, G. Koller, M. Ramsey, C. Teichert, R. Resel, A. Winkler, E.Zojer, W. Kern, G. Trimmel,Materials Research Society (MRS) Fall Meeting, Nov. 2007, Bost<strong>on</strong>, USA.24. Zeno effect <str<strong>on</strong>g>and</str<strong>on</strong>g> step-edge barrier in <str<strong>on</strong>g>organic</str<strong>on</strong>g> thin <str<strong>on</strong>g>film</str<strong>on</strong>g>s.C. Teichert, G. Hlawacek, P. Frank, A. Winkler,American Vacuum Society (AVS) Internati<strong>on</strong>al Symposium & Exhibiti<strong>on</strong>, Oct. 2007,Seattle, USA.154
10 Appendix25. Ultra <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Hexaphenyl Films <strong>on</strong> KCl (001) - Is there a Wetting Layer?.P. Frank, G. Hern<str<strong>on</strong>g>and</str<strong>on</strong>g>ez-Sosa, H. Sitter, A. Winkler,57 th ÖPG Jahrestagung, Sept. 2007, Krems, Austria.26. Self assembled m<strong>on</strong>olayers <strong>on</strong> gold: A study <str<strong>on</strong>g>of</str<strong>on</strong>g> 11-mercaptoundecanoic acid (11-MUA) <strong>on</strong>gold.J. Stettner, P. Frank, A. Winkler, T. Grießer, A.M. Track, G. Trimmel, W. Kern,57 th ÖPG Jahrestagung, Sept. 2007, Krems, Austria.27. Zeno effect <str<strong>on</strong>g>and</str<strong>on</strong>g> step-edge barrier in <str<strong>on</strong>g>organic</str<strong>on</strong>g> thin <str<strong>on</strong>g>film</str<strong>on</strong>g>s.G. Hlawacek, C. Teichert, P. Frank, A. Winkler,57 th ÖPG Jahrestagung, Sept. 2007, Krems, Austria.28. Hexaphenyl <strong>on</strong> KCl (001): TDS, XPS <str<strong>on</strong>g>and</str<strong>on</strong>g> AFM measurements.P. Frank, G. Hern<str<strong>on</strong>g>and</str<strong>on</strong>g>ez-Sosa, H. Sitter, A. Winkler,NFN Meeting, Sept. 2007, Eisenerz, Austria.29. SAM formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> 11-mercapto-undecanoic acid (MUA) <strong>on</strong> gold: The influence <str<strong>on</strong>g>of</str<strong>on</strong>g>substrate structure <str<strong>on</strong>g>and</str<strong>on</strong>g> compositi<strong>on</strong>.J. Stettner, T. Grießer, P. Frank, A.M. Track, G. Trimmel, W. Kern, A. Winkler,NFN Meeting, Sept. 2007, Eisenerz, Austria.30. Amm<strong>on</strong>ia Sensing Organic Field Effect Transistor.P. Pacher, A. Lex, V. Proschek, E. Tchernychova, M. Sezen, O. Werzer, P. Frank, S.Temmel, W. Grogger, R. Resel, R. Schennach, A. Winkler, C. Slugovc, G. Trimmel, E.Zojer,EUROMAT, Sept. 2007, Nürnberg, Germany.31. Photoreactive thin layers for <str<strong>on</strong>g>organic</str<strong>on</strong>g> electr<strong>on</strong>ics.A.M. Track, A. Lex, T. Grießer, G. Koller, P. Pacher, L. Romaner, P. Frank, J. Stettner,A. Winkler, R. Schennach, G. Trimmel, W. Kern, E. Zojer, M. Ramsey,EUROMAT, Sept. 2007, Nürnberg, Germany.32. Tailoring the thin flm layer <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> 6P <strong>on</strong> mica(001).155
10 AppendixP. Frank, G. Hlawacek, T. Haber, R. Resel, C. Teichert, A. Winkler,EUROMAT, Sept. 2007, Nürnberg, Germany.33. The importance <str<strong>on</strong>g>of</str<strong>on</strong>g> a wetting layer for the initial <str<strong>on</strong>g>film</str<strong>on</strong>g> <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> p-Hexaphenyl <strong>on</strong> varioussubstrates.P. Frank, S. Müllegger, G. Hlawacek, C. Teichert, G. Hern<str<strong>on</strong>g>and</str<strong>on</strong>g>ez-Sosa, H. Sitter, A.Winkler,9 th European C<strong>on</strong>ference <strong>on</strong> Molecular Electr<strong>on</strong>ics (ECME), Sept. 2007, Metz, France.34. Tuning the Local Doping Level in Organic <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Film Transistors by a ChemcialReacti<strong>on</strong>: from Depleti<strong>on</strong> to Accumulati<strong>on</strong>.P. Pacher, A. Lex, V. Proschek, E. Tchernychova, M. Sezen, O. Werzer, P. Frank, S.Temmel, W. Grogger, W. Kern, R. Resel, R. Schennach, A. Winkler, C. Slugovc, G.Trimmel, E. Zojer,9 th European C<strong>on</strong>ference <strong>on</strong> Molecular Electr<strong>on</strong>ics (ECME), Sept. 2007, Metz, France.35. Para-sexiphenyl <str<strong>on</strong>g>growth</str<strong>on</strong>g> <strong>on</strong> modified mica(001).G. Hlawacek, C. Teichert, P. Frank, A. Winkler,Internati<strong>on</strong>al Vacuum C<strong>on</strong>gress, July 2007, Stockholm, Sweden.36. Zeno effect <str<strong>on</strong>g>and</str<strong>on</strong>g> step edge barrier in <str<strong>on</strong>g>organic</str<strong>on</strong>g> thin <str<strong>on</strong>g>film</str<strong>on</strong>g>s.G. Hlawacek, P. Frank, C. Teichert, A. Winkler,Gord<strong>on</strong> C<strong>on</strong>ference, June 2007, South Hadley, USA.37. Organic <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Film Transistor with a Reactive Intermediate Layer for Vapour SensingApplicati<strong>on</strong>s.P. Pacher, A. Lex, V. Proschek, O. Werzer, P. Frank, S. Temmel, E. Tchernychova, M.Sezen, G. Hlawacek, Q. Shen, C. Teichert, W. Grogger, R. Resel, R. Schennach, A.Winkler, C. Slugovc, G. Trimmel, E. Zojer,106 th Bunsentagung: Heterogene Neuartige Kohlenst<str<strong>on</strong>g>of</str<strong>on</strong>g>fstrukturen, May 2007, Graz,Austria38. Amm<strong>on</strong>ia Sensing Organic <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Film Transistors.156
10 AppendixP. Pacher, A. Lex, V. Proschek, O. Werzer, P. Frank, S. Temmel, E. Tchernychova, M.Sezen, G. Hlawacek, Q. Shen, C. Teichert, W. Grogger, R. Resel, R. Schennach, A.Winkler, C. Slugovc, G. Trimmel, E. Zojer,DPG Spring Meeting, March 2007, Regensburg, Germany.39. Characterisati<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> modificati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the hexaphenyl wetting layer <strong>on</strong> mica(001).P. Frank, G. Hlawacek, C. Teichert, A. Winkler,DPG Spring Meeting, March 2007, Regensburg, Germany.40. Using <str<strong>on</strong>g>organic</str<strong>on</strong>g> thin <str<strong>on</strong>g>film</str<strong>on</strong>g> transistors with thin reactive layers for amm<strong>on</strong>ia detecti<strong>on</strong>.P. Pacher, A. Lex, V. Proschek, O. Werzer, P. Frank, S. Temmel, E. Tchernychova, M.Sezen, G. Hlawacek, Q. Shen, C. Teichert, W. Grogger, R. Resel, R. Schennach, A.Winkler, C. Slugovc, G. Trimmel, E. Zojer,Nano <str<strong>on</strong>g>and</str<strong>on</strong>g> Phot<strong>on</strong>ics Mauterndorf 2007, March 2007, Mauterndorf, Austria.41. Chemical Surface Reacti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> 2-(4-Chlorosulf<strong>on</strong>ylphenyl)ethyltrichlorosilane withAmm<strong>on</strong>ia: A Combined Experimental <str<strong>on</strong>g>and</str<strong>on</strong>g> Theoretical Study.V. Proschek, P. Pacher, A. Lex, O. Werzer, P. Frank, S. Temmel, R. Resel, R.Schennach, A. Winkler, C. Slugovc, G. Trimmel, E. Zojer,NFN Winter School <strong>on</strong> Organic Electr<strong>on</strong>ics, Jan. 2007, Planneralm, Austria.42. Fundamental aspects <str<strong>on</strong>g>of</str<strong>on</strong>g> oligo-phenylen adsorpti<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> thin <str<strong>on</strong>g>film</str<strong>on</strong>g> layer <str<strong>on</strong>g>growth</str<strong>on</strong>g> <strong>on</strong> goldsurfaces.S. Müllegger, P. Frank, I. Salzmann, T. Haber, R. Resel, G. Hlawacek, C. Teichert, K.Hänel, T. Strunskus, C. Wöll, S. Mitsche, P. Pölt, A. Winkler,NFN Winter School <strong>on</strong> Organic Electr<strong>on</strong>ics, Jan. 2007, Planneralm, Austria.43. <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Reactive Layers for Amm<strong>on</strong>ia Detecti<strong>on</strong> with Organic <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Film Transistors.P. Pacher, A. Lex, V. Proschek, O. Werzer, P. Frank, S. Temmel, E. Tchernychova, M.Sezen, G. Hlawacek, Q. Shen, C. Teichert, W. Grogger, R. Resel, R. Schennach, A.Winkler, C. Slugovc, G. Trimmel, E. Zojer,NFN Winter School <strong>on</strong> Organic Electr<strong>on</strong>ics, Jan. 2007, Planneralm, Austria.157
10 Appendix44. Tailoring <str<strong>on</strong>g>of</str<strong>on</strong>g> Ultra <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Hexaphenyl (6P) Films <strong>on</strong> Mica (001) through SurfaceModificati<strong>on</strong>s.P. Frank, G. Hlawacek, C. Teichert, A. Winkler,NFN Winter School <strong>on</strong> Organic Electr<strong>on</strong>ics, Jan. 2007, Planneralm, Austria.45. Oligo-phenylene adsorpti<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> thin <str<strong>on</strong>g>film</str<strong>on</strong>g> layer <str<strong>on</strong>g>growth</str<strong>on</strong>g> <strong>on</strong> gold surfaces.S. Müllegger, P. Frank, I. Salzmann, T. Haber, R. Resel, G. Hlawacek, C. Teichert, K.Hänel, T. Strunskus, C. Wöll, S. Mitsche, P. Pölt, A. Winkler,Wilhelm <str<strong>on</strong>g>and</str<strong>on</strong>g> Else Heraeus Seminar: Physik hochgeordneter organischer Grenzschichten,Jan. 2007, Bad H<strong>on</strong>nef, Germany.46. Adsorpti<strong>on</strong>, <str<strong>on</strong>g>growth</str<strong>on</strong>g> kinetics <str<strong>on</strong>g>and</str<strong>on</strong>g> desorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> hexaphenyl <strong>on</strong>/from mica(001).P. Frank, A. Winkler,11 th Joint Vacuum C<strong>on</strong>ference (JVC), Sept. 2006, Prague, Czech Republic.47. Tailoring <str<strong>on</strong>g>growth</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> morphology <str<strong>on</strong>g>of</str<strong>on</strong>g> 6P <strong>on</strong> mica(0001)P. Frank, A. Winkler,56 th ÖPG Jahrestagung, Sept. 2006, Graz, Austria.48. Reactive Gate Oxide Modificati<strong>on</strong> for Organic <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Film Transistors.P. Pacher, A. Lex, V. Proschek, G. Hlawacek, Q. Shen, C. Teichert, A. Satka, J. Kovac, O.Werzer, S. Temmel, P. Frank, R. Resel, R. Schennach, A. Winkler, C. Slugovc, G.Trimmel, E. Zojer,56 th ÖPG Jahrestagung, Sept. 2006, Graz, Austria.49. Gate Oxide Modificati<strong>on</strong> by Reactive <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Layers in Polymer <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Film Transistors.P. Pacher, A. Lex, V. Proschek, S. Temmel, O. Werzer, P. Frank, G. Hlawacek, Q. Shen,K. Stubenrauch, E. Zojer, R. Resel, A.Winkler, C. Teichert, C. Slugovc, G. Trimmel,8 th Austrian Polymer Meeting, Sept. 2006, Linz, Austria.50. Experimental <str<strong>on</strong>g>and</str<strong>on</strong>g> Theoretical Study <str<strong>on</strong>g>of</str<strong>on</strong>g> Chemically Sensitive Layers in Organic <str<strong>on</strong>g>Thin</str<strong>on</strong>g> FilmTransistors.P. Pacher, A. Lex, V. Proschek, S. Temmel, O. Werzer, P. Frank, K. Stubenrauch, R.Schennach, R. Resel, A.Winkler, C. Slugovc, G. Trimmel, E. Zojer,158
10 Appendix3rd Semianual ISOTEC Meeting, July 2006, Graz, Austria.51. Hexaphenyl <strong>on</strong> Mica: Basic Research into Film Growth <str<strong>on</strong>g>and</str<strong>on</strong>g> Film Morphology.P. Frank, O. Lengyel, G. Hlawacek, C. Teichert, R. Resel, A. Winkler,EUROFET Meeting, June 2006, Wildau, Germany.52. Reactive Dipole Layers for Chemical Sensing Applicati<strong>on</strong>s.P. Pacher, A. Lex, V. Proschek, C. Auner, S. Rentenberger, O. Werzer, P. Frank, S.Temmel, R. Resel, A. Winkler, R. Schennach, C. Slugovc, G. Trimmel, E. Zojer,European Materials Research Society (EMRS) Spring Meeting, June 2006, Nice, France.53. Morphology <str<strong>on</strong>g>of</str<strong>on</strong>g> sexiphenyl thin <str<strong>on</strong>g>film</str<strong>on</strong>g>s.G. Hlawacek, A. Andreev, C. Teichert, P. Frank, A. Winkler, R. Resel, M. Ramsey,DPG Spring Meeting, Mar. 2006, Dresden, Germany.159
10 AppendixDBibliography[1] P. Puschnig, C. Ambrosch-Draxl, Phys. Rev. B 60 (1999) 7891.[2] G. Leising, S. Tasch, F. Meghdadi, L. Athouel, G. Froyer, U. Scherf,Synth. Met. 81 (1996) 185.[3] W. Graupner, G. Grem, F. Meghdadi, C. Paar, G. Leising, U. Scherf, K. Müllen,W. Fischer, F. Stelzer, Mol. Cryst. Liq. Cryst. 256 (1994) 549.[4] F. Meghdadi, S. Tasch, B. Winkler, W. Fischer, F. Stelzer, G. Leising,Synth. Met. 85 (1997) 1441.[5] H. Yanagi, S. Okamoto, T. Mikami, Synth. Met. 91 (1997) 91.[6] G. Leising, S. Tasch, W. Graupner, H<str<strong>on</strong>g>and</str<strong>on</strong>g>book <str<strong>on</strong>g>of</str<strong>on</strong>g> C<strong>on</strong>ducting Polymers: Fundamentals<str<strong>on</strong>g>of</str<strong>on</strong>g> Electroluminescence in Paraphenylenetype C<strong>on</strong>jugated Polymers <str<strong>on</strong>g>and</str<strong>on</strong>g> Oligomers,sec<strong>on</strong>d ed., Marcel Dekker, 1997.[7] D.J. Gundlach, Y.-Y. Lin, T.N. Jacks<strong>on</strong>, D.G. Schlom,Appl. Phys. Lett. 7 (1997) 3853.[8] S. Tasch, C. Br<str<strong>on</strong>g>and</str<strong>on</strong>g>stätter, F. Meghdadi, G. Leising, G. Froyer, L. Athouel,Adv. Mater. 9 (1997) 33.[9] G. Meinhardt, W. Graupner, G. Feistritzer, R. Schröder, E.J.W. List, A. Pogantsch, G.Dicker, B. Schlicke, N. Schulte, A.D. Schluter, G. Winter, M. Hanack, U. Scherf, K.Müllen, G. Leising, Proc. SPIE 3623 (1999) 46.[10] T. Mikami, H. Yanagi, Appl. Phys. Lett. 73 (1998) 563.[11] H. Yanagi, S. Okamoto, Appl. Phys. Lett. 71 (1997) 2565.[12] R. Resel, I. Salzmann, G. Hlawacek, C. Teichert, B. Koppelhuber, B.Winter, J.K. Krenn, J. Ivanco, M.G. Ramsey, Org. Electr<strong>on</strong>. 5 (2004) 45.[13] B. Winter, J. Ivanco, F.P. Netzer, M.G. Ramsey, <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Solid Films 433 (2003) 269.[14] B. Winter, J. Ivanco, F.P. Netzer, M.G. Ramsey, I. Salzmann, R. Resel,Langmuir 20 (2004) 7512.[15] G. Hlawacek, C. Teichert, A. Andreev, H. Sitter, S. Berkebile, G. Koller, M. Ramsey,R. Resel, Phys. Status Solidi (a) 202 (2005) 2376.[16] G. Koller, S. Berkebile, J.R. Krenn, G. Tzvetkov, G. Hlawacek, O. Lengyel, F.P.Netzer, C. Teichert, R. Resel, M. Ramsey, Adv. Mater 16 (2004) 2159.[17] S. Berkebile, G. Koller, G. Hlawacek, C. Teichert, F.P. Netzer, M.G. Ramsey,160
10 AppendixSurf. Sci. 600 (2006) L313.[18] M. Oehzelt, L. Grill, S. Berkebile, G. Koller, F.P. Netzer, M. Ramsey,Chem. Phys. Chem. 8 (2007) 1707.[19] Y. Hu, K. Maschek, L.D. Sun, M. Hohage, P. Zeppenfeld, Surf. Sci. 600 (2006) 762.[20] T. Haber, S. Müllegger, A. Winkler, R. Resel, Phys. Rev. B 74 (2006) 045419.[21] S. Müllegger, A. Winkler, Surf. Sci. 600 (2006) 1290.[22] C.B. France, B.A. Parkins<strong>on</strong>, Appl. Phys. Lett. 82 (2003) 1194.[23] A. Andreev, T. Haber, D.-M. Smilgies, R. Resel, H. Sitter, N.S. Sariciftci, L. Valek, J.Cryst. Growth 275 (2005) 2037.[24] A. Andreev, G. Matt, C.J. Brabec, H. Sitter, D. Badt, H. Seyringer, N.S. Sariciftci,Adv. Mater. 12 (2000) 629.[25] F. Balzer, H.G. Rubahn, Appl. Phys. Lett. 79 (2001) 3860.[26] F. Balzer, H.G. Rubahn, Surf. Sci. 507–511 (2002) 588.[27] H. Plank, R. Resel, S. Purger, J. Keckes, A. Thierry, B. Lotz, A. Andreev, N.S.Sariciftci, H. Sitter, Phys. Rev. B 64 (2001) 235423.[28] H. Plank, R. Resel, H. Sitter, A. Andreev, N.S. Sariciftci, G. Hlawacek, C. Teichert, A.Thierry, B. Lotz, <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Solid Films 443 (2003) 108.[29] F. Balzer, J. Beermann, S.I. Bozhevolnyi, A.C. Sim<strong>on</strong>sen, H.-G. Rubahn,Nano Lett. 3 (2003) 1311.[30] S. Müllegger, O. Stranik, E. Zojer, A. Winkler, Appl. Surf. Sci. 221 (2004) 184.[31] H. Sitter, A. Andreev, C. Teichert, G. Hlawacek, T. Haber, D.-M. Smilgies, R. Resel, A.M. Ramil, N.S. Sariciftci, Phys. Status Solidi (b) 242 (9) (2005) 1877.[32] H. Plank, R. Resel, A. Andreev, N.S. Sariciftci, H. Sitter, J. Cryst.Growth 237 (2002) 2076.[33] A. Andreev, H. Sitter, C.J. Brabec, P. Hinterdorfer, G. Springholz, N.S. Sariciftci, R.Resel, H. Plank, Mater. Res. Soc. Symp. Proc. 665 (2001).[34] A. Andreev, H. Sitter, R. Resel, D.-M. Smilgies, H. Hoppe, G. Matt, N.S. Sariciftci, D.Meissner, D. Lysacek, L. Valek, Mol. Cryst. Liq. Cryst. Sci. Technol. 385 (2002) 61.[35] A. Andreev, F. Quochi, F. Cordella, A. Mura, G. B<strong>on</strong>giovanni, H. Sitter, G. Hlawacek,C. Teichert, N.S. Sariciftci, J. Appl. Phys. 99 (2006) 034305.[36] A. Andreev, H. Sitter, C.J. Brabec, P. Hinterdorfer, G. Springholz, N.S. Sariciftci,Synth. Met. 121 (2001) 1379.[37] A. Andreev, R. Resel, D.-M. Smilgies, H. Hoppe, G. Matt, H. Sitter, N.S. Sariciftci, D.Meissner, H. Plank, O. Zrzavecka, Synth. Met. 138 (2003) 59.161
10 Appendix[38] A. Andreev, H. Sitter, N.S. Sariciftci, C.J. Brabec, G. Springholz, P. Hinterdorfer, H.Plank, R. Resel, A. Thierry, B. Lotz, <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Solid Films 403–404 (2002) 444.[39] G. Hlawacek, Q. Shen, C. Teichert, R. Resel, D.-M. Smilgies, Surf.Sci. 601 (2006) 2584.[40] T. Mikami, H. Yanagi, Appl. Phys. Lett. 73 (1998) 563.[41] H. Yanagi, S. Okamoto, Appl. Phys. Lett. 71 (1997) 2565.[42] H. Yanagi, T. Morikawa, Appl. Phys. Lett. 75 (1999) 187.[43] Y. Yoshida, H. Takiguchi, T. Hanada, A. Tanigaki, E.M. Han, K. Yase,J. Cryst. Growth 198 (1999) 923.[44] D.M. Smilgies, N. Boudet, H. Yanagi, Appl. Surf. Sci. 189 (2002) 24.[45] E.J. Kinztel Jr., D.M. Smilgies, J.G. S<str<strong>on</strong>g>of</str<strong>on</strong>g>r<strong>on</strong>ik, S.A. Safr<strong>on</strong>, D.H. Van Winkle, J. Vac.Sci. Technol. A 22 (2004) 107.[46] F. Balzer, H.G. Rubahn, Surf. Sci. 548 (2004) 170.[47] T. Haber, A. Andreev, A. Thierry, H. Sitter, M. Oehzelt, R. Resel,J. Cryst. Growth 284 (2005) 209.[48] T. Haber, M. Oehzelt, R. Resel, A. Andreev, A. Thierry, H. Sitter, D.M. Smilgies, B.Schaffer, W. Grogger, J. Nanosci. Nanotechnol. 6 (2006) 1.[49] A. Andreev, A. M<strong>on</strong>taigne, G. Hlawacek, H. Sitter, C. Teichert,J. Vac. Sci. Technol. A 24 (2006) 1660.[50] W. Chen, C. Huang, X.Y. Gao, L. Wang, C.G. Zhen, D. Qi, S. Chen, H.L. Zhang,K.P.Loh, Z.K. Chen, A.T.S. Wee, J. Phys. Chem B 110 (2006) 26075.[51] G. Heimel, L. Romaner, J.-L. Brédas, E. Zojer, Surf. Sci. 600 (2006) 4548.[52] W. Osikowicz, X. Crispin, C. Tengstedt, L. Lindell, T. Kugler, W.R. Salaneck,Appl. Phys. Lett. 85 (2004) 1616.[53] L. Romaner, G. Heimel, J.-L.Brédas, A. Gerlach, F. Schreiber, R.L. Johns<strong>on</strong>, J.Zegenhagen, S. Duhm, N. Koch, E. Zojer, Phys. Rev. Lett. 99 (2007) 256801.[54] S. Duhm, A. Gerlach, I. Salzmann, B. Bröker, R.L. Johns<strong>on</strong>, F. Schreiber, N. Koch,Org. Electr<strong>on</strong>. 9 (2008) 111.[55] J. Meyer, S. Hamwi, T. Bulow, H. Johannes, T. Riedl, W. Kowalsky,Appl. Phys. Lett. 91(2007) 113506.[56] N. Koch, S. Duhm, J.P. Rabe, A. Vollmer, R.L. Johns<strong>on</strong>, Phys. Rev.Lett. 95 (2005) 237601.[57] Z.Q. Gao, B.X. Mi, G.Z. Xu, Y.Q. Wan, M.L. G<strong>on</strong>g, K.W. Cheah, C.H. Chen,Chem. Commun. 1 (2008) 117.162
10 Appendix[58] P.S. Szalay, J.R. Galan-Mascaros, R. Clerac, K.R. Dunbar, Synth. Met. 122 (2001) 535.[59] P.S. Szalay, J. R.Galan-Mascaros, B.L. Schottel, J. Bacsa, L.M. Perez, A.S. Ichimura,A. Chouai, K.R. Dunbar, Journal <str<strong>on</strong>g>of</str<strong>on</strong>g> Cluster Science 15 (2004) 503.[60] S. Müllegger, G. Hlawacek, T. Haber, P. Frank, C. Teichert, R. Resel, A. Winkler,Appl. Phys. A 87 (2007) 103.[61] C. Teichert, G. Hlawacek, A. Andreev, H. Sitter, P. Frank, A. Winkler, N.S. Sariciftci,Appl. Phys. A 82 (2006) 665.[62] P. Frank, G. Hlawacek, O. Lengyel, A. Satka, C. Teichert, R. Resel, A. Winkler,Surf. Sci. 601 (2007) 2152.[63] P. Frank, A. Winkler, Appl. Phys. A 90 (2008) 717.[64] P. Frank, G. Hern<str<strong>on</strong>g>and</str<strong>on</strong>g>ez-Sosa, H. Sitter, A. Winkler, <str<strong>on</strong>g>Thin</str<strong>on</strong>g> solid <str<strong>on</strong>g>film</str<strong>on</strong>g>s 516 (2008) 2939.[65] G. Hlawacek, P. Puschnig, P. Frank, A. Winkler, C. Ambrosch-Draxl, C. Teichert,Science 321 (2008) 108.[66] P. Frank, N. Koch, M. Koini, R. Rieger, K. Müllen, R. Resel, A. Winkler,Chem. Phys. Lett. 473 (2009) 321.[67] Bröker et al., in preparati<strong>on</strong>.[68] P. Frank et al., in preparati<strong>on</strong>.[69] H. Ehrenreich, F. Spaepen, Solid State Physics, Vol. 49, Academic Press Inc., SanDiego, California, 1995.[70] P.W. Atkins, Physical Chemistry, Oxford University Press, Oxford, 1998.[71] S. Müllegger, Adsorpti<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Film Growth <str<strong>on</strong>g>of</str<strong>on</strong>g> Oligo-phenylenes <strong>on</strong> goldsurfaces. PhD thesis, Institute for solid state physics, Graz University <str<strong>on</strong>g>of</str<strong>on</strong>g> Technology,Graz, Austria (2005).[72] K. Seki, U.O. Karlss<strong>on</strong>, R. Engelhardt, E.-E. Koch, W. Schmidt, Chem.Phys. 91 (1984) 459.[73] P. Puschnig, C. Ambrosch-Draxl, Phys. Rev. B 56 (1999) 7891.[74] St<str<strong>on</strong>g>and</str<strong>on</strong>g>ard reference database. NIST;http://webbook.nist.gov/cgi/cbook.cgi?ID=C4499836&Units=SI , 2008.[75] K.N. Baker, A.V. Fratini, T. Resch, H.C. Knachel, W.W. Adams, E.P. Socci, B.L.Farmer, Polymer 34 (1993) 1571.[76] Y. Delugeard, J. Desuche, J.L. Badour, Acta Cryst. B 32 (1976) 702.[77] G. Hlawacek, Molecular <str<strong>on</strong>g>growth</str<strong>on</strong>g> mechanisms in para-sexiphenyl thin <str<strong>on</strong>g>film</str<strong>on</strong>g> depositi<strong>on</strong>.PhD thesis, Institute <str<strong>on</strong>g>of</str<strong>on</strong>g> physics, University <str<strong>on</strong>g>of</str<strong>on</strong>g> Leoben, Leoben, Austria.[78] T.L. Cairns, R.A. Carb<strong>on</strong>i, D.D. C<str<strong>on</strong>g>of</str<strong>on</strong>g>fman, V.A. Engelhardt, R.E. Heckert, E.L. Little,163
10 AppendixE.G. McGeer, B. C. McKusick, W. J. Middlet<strong>on</strong>, J. Am. Chem. Soc. 79 (1957) 2341.[79] D.S. Acker, W.R. Hertler, J. Am. Chem. Soc. 84 (1962) 3370.[80] P. Frank, Adsorpti<strong>on</strong>, Schichtwachstum und Desorpti<strong>on</strong> v<strong>on</strong> p-Hexaphenyl aufGlimmer-Oberflächen. Master’s thesis, Institute for solid state physics, Graz University<str<strong>on</strong>g>of</str<strong>on</strong>g> Technology, Graz, Austria (2006).[81] T. Haber, Structural Order in Epitaxially Grown Para-Sexiphenyl <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Films <strong>on</strong>KCl(100) <str<strong>on</strong>g>and</str<strong>on</strong>g> Mica Surfaces. Master’s thesis, Institute for solid state physics, GrazUniversity <str<strong>on</strong>g>of</str<strong>on</strong>g> Technology, Graz, Austria (2004).[82] Ch. Wöll, S. Chiang, R.J. Wils<strong>on</strong>, P.H. Lippel, Phys. Rev. B 39 (1989) 7988.[83] I.V. Markov, Crystal <str<strong>on</strong>g>growth</str<strong>on</strong>g> for beginners. World Scientific, New Jersey,2 nd editi<strong>on</strong>, (2003).[84] M.A. Herman, H. Sitter. Molecular Beam Epitaxy. Vol. 7 <str<strong>on</strong>g>of</str<strong>on</strong>g> Springer Series inMaterials Science, Springer Verlag, Berlin-Heidelberg, (1989).[85] J. Venables, G. Spiller, M. Hanbücken, Rep. Prog. Phys. 47 (1984) 399.[86] J. Krug, P. Politi, T. Michely, Phys. Rev. B: C<strong>on</strong>dens. Matter Mater.Phys. 61 (2000) 14037.[87] M. Kalff, P. Smilauner, G. Comsa, T. Michely, Surf. Sci. 426 (1999) L447.[88] T. Michely, J. Krug, Isl<str<strong>on</strong>g>and</str<strong>on</strong>g>s, Mounds <str<strong>on</strong>g>and</str<strong>on</strong>g> Atoms. Vol. 42 <str<strong>on</strong>g>of</str<strong>on</strong>g> Springer Series in SurfaceScience, Springer Verlag, Berlin-Heidelberg, (2004).[89] G. Ehrlich, F. Hudda, J. Chem. Phys. 44 (1966) 1039.[90] R.L. Schwöbel, E.J. Shipsey, J. Appl. Phys. 37 (1966) 3682.[91] J. Krug, J. Stat. Phys. 87 (1997) 505.[92] P. Politi, J. Phys. I 17 (1997) 797.[93] M.P. Seah, Surf. Sci. 32 (1972) 703.[94] E. Umbach, M. Sokolowski, R. Fink, Appl. Phys. A, 63 (1996) 565.[95] O. Stranik, Adsorpti<strong>on</strong> und Schichtwachstum v<strong>on</strong> p-Quarterphenyl aufGoldoberflächen. Master’s Thesis, Institute for solid state physics, Graz University <str<strong>on</strong>g>of</str<strong>on</strong>g>Technology, Graz, Austria (2002).[96] Infic<strong>on</strong> Inc., Operating Manual, XTM/2 Depositi<strong>on</strong> M<strong>on</strong>itor. Infic<strong>on</strong> Inc.,New York (1995).[97] P.A. Redhead, Vacuum 12 (1962) 203.[98] K.J. Laidler, Chemical Kinetics. HarperCollins, New York (1987).[99] K. Christmann, Int<str<strong>on</strong>g>rod</str<strong>on</strong>g>ucti<strong>on</strong> to Surface Physical Chemistry. Springer Verlag,Berlin-Heidelberg, (1991).164
10 Appendix[100] S.L. Tait, Z. Dohnalek, C.T. Campbell, B.D. Kay, J. Chem. Phys. 122 (2005) 164708.[101] K.R. Paserba, A.J. Gellman, Phys. Rev. Lett. 86 (2001) 4338.[102] K.A. Fichthorn, R.A. Mir<strong>on</strong>, Phys. Rev. Lett. 89 (2002) 196103.[103] K.A. Fichthorn, K.E. Becker, R.A. Mir<strong>on</strong>, Cat. Today 123 (2007) 71.[104] E. Habenschaden, J. Küppers, Surf. Sci. Lett. 138 (1984) L147.[105] D. Briggs, M.P. Seah, Practical Surface Analysis by Auger <str<strong>on</strong>g>and</str<strong>on</strong>g> X-ray Photoelectr<strong>on</strong>Spectroscopy. John Wiley & S<strong>on</strong>s, Chichester/New York/Brisbane/Tor<strong>on</strong>to/Singapore (1983).[106] K. Oura, V.G. Lifshits, A.A. Saranin, A.V. Zotov, M. Katayama, Surface Science.Springer Verlag, Berlin (2003).[107] H. Bubert, H. Jenett, Surface <str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>Thin</str<strong>on</strong>g> Film Analysis: Principles, Instrumentati<strong>on</strong>,Applicati<strong>on</strong>s. Wiley-VCH, Weinheim (2002).[108] http://www.wsu.edu/~scudiero/, 2009.[109] G. Ertl, J. Küppers, Low Energy Electr<strong>on</strong>s <str<strong>on</strong>g>and</str<strong>on</strong>g> Surface Chemistry. Vol. 4 <str<strong>on</strong>g>of</str<strong>on</strong>g>M<strong>on</strong>ographs in Modern Chemistry, Verlag Chemie, Weinheim (1974).[110] D.P. Woodruff, T.A. Delchar, Modern Techniques <str<strong>on</strong>g>of</str<strong>on</strong>g> Surface Science. CambridgeUniversity Press, Cambridge (1986).[111] G. Binning, C.F. Quate, C. Gerber, Phys. Rev. Lett. 56 (1986) 930.[112] Nanosurf AG, Nanosurf easyScan 2 AFM System. Nanosurf AG, Switzerl<str<strong>on</strong>g>and</str<strong>on</strong>g> (2005).[113] Nanosurf AG, Nanosurf easyScan 2 S<str<strong>on</strong>g>of</str<strong>on</strong>g>tware Reference. Nanosurf AG,Switzerl<str<strong>on</strong>g>and</str<strong>on</strong>g> (2005).[114] http://www3.physik.uni-greifswald.de/method/afm/eafm.htm, 2009.[115] I.D. Baikie, M. Porterfield, P. Smith, P.J. Estrup, Rev. Sci. Instrum. 70 (1999) 1842.[116] I.D. Baikie, P.J. Estrup, Rev. Sci. Instrum. 69 (1998) 3902.[117] I.D. Baikie, S. Mackenzie, P.J. Estrup, J.A. Meyer, Rev. Sci. Instrum. 62 (1991) 1326.[118] I.D. Baikie, E. Venderbosch, J.A. Meyer, P.J. Estrup, Rev. Sci. Instrum. 62 (1991) 725.[119] I.D. Baikie, K.O. van der Werf, J. Broeze, A. van Silfhout,Rev. Sci. Instrum. 60 (1989) 930.[120] I.D. Baikie, U. Petermann, B. Lagel, J. Vac. Sci. Technol. A 19 (2001) 1460.[121] I.D. Baikie, U. Petermann, A. Speakman, B. Lagel, K.M. Dirscherl, P.J. Estrup,J. Appl. Phys. 88 (2000) 4371.[122] K. Müller, C.C. Chang, Surf. Sci. 14 (1969) 39.[123] K. Müller, C.C. Chang, Surf. Sci. 8 (1968) 455.[124] H. Poppa, A.G. Elliot, Surf. Sci. 24 (1971) 149.165
10 Appendix[125] F. Balzer, L. Kankate, H. Niehus, R. Frese, C. Maibohm, H.G. Rubahn,Nanotechnology 17 (2006) 984.[126] H. Glowatzki, B. Bröker, R.-P. Blum, O.T. H<str<strong>on</strong>g>of</str<strong>on</strong>g>mann, A. Vollmer, R. Rieger, K.Müllen, E. Zojer, J.P. Rabe, N. Koch, Nano Lett. 8 (2008) 3825.[127] N. Yoshimoto, T. Sato, Y. Saito, S. Ogawa, Mol. Cryst. Liq. Cryst. 425 (2004) 279.[128] R. Resel, T. Haber, O. Lengyel, H. Sitter, F. Balzer, H.G. Rubahn, in preparati<strong>on</strong>.[129] I. Elkinani, J. Villain, Solid State Commun. 87 (1993) 105.[130] G. Koller, S. Surnev, M. Ramsey, F. Netzer, Surf. Sci. 559 (2004) L187.[131] G. Witte, S. Lukas, P.S. Bagus, C. Wöll, Appl. Phys. Lett. 87 (2005) 263502.[132] T. Haber, A. Andreev, A. Thierry, H. Sitter, M. Oehzelt, R. Resel, J. Cryst. Growth 284(2005) 209.[133] O. H<str<strong>on</strong>g>of</str<strong>on</strong>g>mann, private communicati<strong>on</strong>s.[134] KP Technology LTD, UHV Kelvin Probe. KP Technology LTD, UK (2006).166