"Front Matter". In: Organosilanes in Radical Chemistry - Index of
"Front Matter". In: Organosilanes in Radical Chemistry - Index of "Front Matter". In: Organosilanes in Radical Chemistry - Index of
Radical Chemistry on Silicon Surfaces 203 41 + Br 2 Ph2 Si SiPh2 Si SiPh2 Ph 45 hν (daylight) Ph2 Si Ph2Si Si Si Ph H Ph O SiPh2 Ph2Si Si Ph Si Ph 47 Br 2, H 2O Ph2 Si Ph2Si Si Ph Si Ph Scheme 8.9 Proposed reaction mechanism for the formation of 9,10-disilaanthracene derivative 47 residing on or near the surface became significant. Although the success of silicon is mainly due to the presence of robust native oxide on silicon surfaces, much attention is being directed towards the synthesis of an organic monolayer, which can be modified upon demand for specific requirements. In this section, we deal with the chemistry of hydrogen-terminated silicon surfaces and, in particular, with radical reactions that have been found to be the most convenient methods for organic modification of silicon surface [46,47]. Indeed, the chemistry can be understood in many cases by analogy with radical reactions of organosilicon hydrides described in other chapters of this book. Structural properties of hydrogen-terminated silicon surfaces are of critical importance for their chemical behaviour. Flat (single crystal) and porous silicon surfaces are available. The Si(111) and Si(100) orientation of a single crystal are shown in Figure 8.3 together with porous silicon as examples of silicon hydrideterminated surfaces [47]. The porous silicon is terminated with SiH, SiH2 and SiH3 moieties in a variety of different local orientations and environments. These materials are reasonably stable and can be prepared and manipulated in air for tens of minutes as well as in a number of organic solvents. However, on prolonged exposure to air, single crystal silicon becomes coated with a thin, native oxide that can be removed chemically from Si(111) using 40 % aqueous NH4F or from Si(100) and porous silicon using dilute aqueous HF. Under ultrahigh vacuum conditions it is also possible to produce uniform monohydride H w Si(100) surfaces (see below). It is worth underlining that the monohydride terminal surface of H w Si(111) resembles (Me3Si) 3SiH. For example, the Si(111) surface after fluoride ion treatment exhibits a sharp peak at 2084 cm 1 with p-polarized infrared light due to Si w H stretching absorption 46 + C 60
204 Silyl Radicals in Polymers and Materials Si Si Si H Si Si H Si Si Si Si Si H Si H H Si Si Si Si Si(111) Si Si Si H Si H H Si Si Si Si Si Si H Si H H Si Si Si Si Si(100) H Si H H H H Si Si H H Si Si Si Si H H porous silicon H H Si Si Si H Si Si Si Si Si H H H Figure 8.3 Hydrogen-terminated Si(111), Si(100) and porous silicon surfaces. [46], which is comparable with the nSi w H of 2052 and 2075 cm 1 for (Me3Si) 3Si w H and (Me3Si) 2MeSi w H, respectively. Therefore, it is not surprising that several of (Me3Si) 3SiH reactions have been adopted and applied to surfaces, as described here. This section will cover aspects of monohydride terminal surface reactions that were carried out under free-radical conditions. The description will be circumscribed to the reactions with molecular oxygen and monounsaturated compounds. Mechanistic information for these reactions is scarce mainly due to the complexity of the system, and mechanistic schemes are often proposed in analogy with radical chemistry of organosilane molecules. H w Si(111) has a band gap of about 1.1 eV while the HOMO–LUMO gap in (Me3Si) 3SiH is within 8–11 eV and, therefore, has very important consequences for the reactions with nucleophilic and electrophilic species where frontier orbital inter-
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204 Silyl <strong>Radical</strong>s <strong>in</strong> Polymers and Materials<br />
Si<br />
Si<br />
Si<br />
H<br />
Si<br />
Si<br />
H<br />
Si<br />
Si<br />
Si<br />
Si<br />
Si<br />
H<br />
Si<br />
H H<br />
Si<br />
Si<br />
Si<br />
Si<br />
Si(111)<br />
Si<br />
Si<br />
Si<br />
H<br />
Si<br />
H H<br />
Si<br />
Si<br />
Si<br />
Si<br />
Si<br />
Si<br />
H<br />
Si<br />
H H<br />
Si Si Si Si<br />
Si(100)<br />
H Si<br />
H<br />
H H<br />
H<br />
Si<br />
Si<br />
H<br />
H<br />
Si<br />
Si<br />
Si Si<br />
H<br />
H<br />
porous silicon<br />
H<br />
H<br />
Si Si<br />
Si<br />
H<br />
Si<br />
Si<br />
Si<br />
Si<br />
Si<br />
H<br />
H H<br />
Figure 8.3 Hydrogen-term<strong>in</strong>ated Si(111), Si(100) and porous silicon surfaces.<br />
[46], which is comparable with the nSi w H <strong>of</strong> 2052 and 2075 cm 1 for<br />
(Me3Si) 3Si w H and (Me3Si) 2MeSi w H, respectively. Therefore, it is not surpris<strong>in</strong>g<br />
that several <strong>of</strong> (Me3Si) 3SiH reactions have been adopted and applied to<br />
surfaces, as described here.<br />
This section will cover aspects <strong>of</strong> monohydride term<strong>in</strong>al surface reactions<br />
that were carried out under free-radical conditions. The description will be<br />
circumscribed to the reactions with molecular oxygen and monounsaturated<br />
compounds. Mechanistic <strong>in</strong>formation for these reactions is scarce ma<strong>in</strong>ly due to<br />
the complexity <strong>of</strong> the system, and mechanistic schemes are <strong>of</strong>ten proposed <strong>in</strong><br />
analogy with radical chemistry <strong>of</strong> organosilane molecules. H w Si(111) has a<br />
band gap <strong>of</strong> about 1.1 eV while the HOMO–LUMO gap <strong>in</strong> (Me3Si) 3SiH is<br />
with<strong>in</strong> 8–11 eV and, therefore, has very important consequences for the reactions<br />
with nucleophilic and electrophilic species where frontier orbital <strong>in</strong>ter-
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