6P on Cu(110)-(2x1)O

JOHANNES KEPLER UNIVERSITY LINZ
Institute of Experimental Physics
Atomic Physics and Surface Science
http://www.exphys.jku.at/aop/
Epitaxial crystallization of rod-like molecules:
the first monolayer and beyond
Martin Oehzelt

Motivation/Outlook
Structure from the monolayer up to thick layers
- UHV is necessary for defined and reproducible studies
- the substrate has a profound influence
on the organic film:
geometric structure (molecular orientation)
electronic structure
device properties (band alignment)
film morphology
- the structure in the first monolayer is the template
for the growth of thicker films

Overview
sexiphenyl (<strong>6P</strong>) first blue LEDs
sexithiophene (6T) OFETs
pentacene (5A) OFETs
useful model molecules (clean and of
technological relevance)
can be evaporated in UHV
molecules: evaporated in UHV
substrates: single crystals
Cu(110), Cu(110)-(2x1)O, Ag(110), …
in-situ: STM, GI-XRD, PEEM, LEED, ARUPS,
XPS, NEXAFS, RDS, …
ex-situ: XRD, AFM, FM

3D-Structure
- low symmetry monoclinic
or triclinic
- layered herring-bone
structure

<strong>6P</strong> on Cu(110)
reminiscent of a „smectic“ order with
typical distances of 10.2 Å x 30-33 Å
FT
ARUPS
ARUPS:
„backdonation“ repulsive intermolecular
interaction
loosely packed monolayer
large molecular footprint!
<strong>6P</strong> covers copper at ∼ 3-4 Å
molecules are lying flat on the surface
STM at RT:
low order between molecular rows

<strong>6P</strong> on Cu(110)-(2x1)O: surface structure
LT-STM at ~ 10K
- oriented in [001]
direction
- molecules are
densely packed
~ 5 x 54 Å
rectangular surface
unit cell with
two molecules
VdW dimensions <strong>6P</strong>:
~ 28.7 x 5.9 Å

<strong>6P</strong> on Cu(110)-(2x1)O: bulk structure
Intensity a.u.
XRD
300 <strong>6P</strong> (20-3)
250
200
150 Cu(220) HH
100
50
- bulk crystallites have a
(20-3) orientation
0
0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2
Scattering vector s

<strong>6P</strong> on Cu(110)-(2x1)O: bulk structure
XRD – pole figure
- bulk crystallites have a
(20-3) orientation
- all molecules are
oriented in Cu[001]
direction
Cu[001]

<strong>6P</strong> on Cu(110)-(2x1)O
STM: <strong>6P</strong> overlayer: 5.1 Å x 54.1 Å
XRD: <strong>6P</strong> (20-3) bulk: 5.66 Å x 54.6 Å
mismatch: -8.4% x -1.1% !!
tilting to release stress

<strong>6P</strong> on Cu(110)-(2x1)O
NEXAFS
surface structure
<strong>6P</strong> (20-3)

6T on Cu(110)-(2x1)O
STM at RT
- molecules oriented
in [001] direction
- molecules are densely
packed
~ 5 x 27 Å
rectangular unit cell
not present
in the bulk structure

6T on Cu(110)-(2x1)O: bulk structure
XRD – pole figure
- bulk crystallites have a
(010) orientation
- all molecules are oriented
in [001] direction
highly ordered
GIXRD
6T(100)
6T(010)

Details of the structure on Cu(110)-(2x1)O
NEXAFS data show
again that the
molecules can tilt
to release stress

Heterostructures
<strong>6P</strong> on 6T
6T on <strong>6P</strong>
<strong>6P</strong>(20-3) on
6T(010) on
Cu(110)-(2x1)O
6T(010) on
<strong>6P</strong> (20-3) on
Cu(110)-(2x1)O

Heterostructures
6T on <strong>6P</strong>
Heterostructures of rod-like molecules orient their long
molecular axes parallel to each other

Summary
rod-like molecules can be oriented uniaxially through out
the whole surface on substrate like Cu(110)-(2x1)O
molecules have a certain degree of freedom to
adjust to the surface and release stress
highly oriented molecular films are suitable substrates
for organic heterostructures
with PEEM the growth of organic thin films can be
studied in-situ and in real-time

Acknowledgements
Johannes Kepler University Linz:
Peter Zeppenfeld
Lidong Sun
Günther Weidlinger
Thorsten Wagner
Michael Hohage
Karl-Franzens University Graz:
Michael G. Ramsey
Georg Koller
Stephen Berkebile
Alexander Fleming
University of Technology Graz:
Roland Resel
Markus Koini
Thomas Haber
Free University Berlin:
Leonhard Grill