Photoreactivity of Unsaturated Compounds with Hydrogen-Terminated Silicon(111)

Abstract

We report the photoreactivity of H−Si(111) with dioxygen and terminally unsaturated hydrocarbons. Illumination of H−Si(111) in air with ultraviolet light of wavelength of 350 nm or shorter produces oxidized silicon; longer wavelengths cause no oxidation. When H−Si(111) is immersed in unsaturated hydrocarbons (1-octene, 1-octadecene, 1-octyne, styrene, and phenylacetylene) that have been deoxygenated, illumination with a Hg lamp results in densely packed hydrocarbon films of molecular thickness. Previous work and the results presented here suggest that the H−Si bond adds across the unsaturated bond in these reactions similar to hydrosilylation reactions known for small-molecule chemistry and produce adsorbates covalently bonded to the surface. We present spectroscopic evidence showing films resulting from terminal acetylenes consist of adsorbates linked to the surface by a vinyl group. We propose that all of these reactions occur through a radical-chain mechanism initiated by the wavelength-dependent photodesorption of surface hydrogen by UV light. We also present a method in the Appendix to quantify surface adsorbate coverage using XPS and ellipsometric data.

Photochemical Attachment of Organic Monolayers onto H-Terminated Si(111): Radical Chain Propagation Observed via STM Studies

“…Furthermore, the irregular, meandering shape of the islands suggest that these structures were formed by a process analogous to the self-avoiding, near-random walk radical chain mechanism proposed for UV-lightinduced monolayer attachment. 11 Similar images were also obtained for photochemical reactions using styrene, although the islands in this case appear to be less extended. It is also clear from the images that longer irradiation times lead primarily to the nucleation of more islands rather than growth of larger islands.…”

“…This is in agreement with island nucleation being the rate-limiting step along with the self-limiting nature of the subsequent chain reaction. 10,11 The progress of the attachment reaction has also been monitored with high-resolution electron energy loss spectroscopy (HREELS). † Wageningen University.…”

“…light of < 365 nm) is required. 11,14 Since the visible-light-induced attachment employed here is initiated with photons of significantly less energy, homolytic bond cleavage can be excluded as the starting point for the reaction. Additionally, the 1-decene does not absorb any 447 nm light.…”

Wet chemical routes to the assembly of organic monolayers on silicon surfaces via the formation of Si–C bonds: surface preparation, passivation and functionalization

“…However, the observation that monolayer growth is self-limiting and comes to a halt after a finite number of adsorption events, e.g. when adjacent hydrogen atoms are no longer available for abstraction, 88,133 is a strong evidence of monolayer rather than multilayer formation. 138 Further, in contrast to layers grafted via siloxo (Si-O-Si) bridges, 81,139,140 the silicon-to-carbon bond 141 structure is relatively ordered and not prone to hydrolysis.…”

“…165,166 Initially reported by Terry et al in a study aimed to provide direct evidence of the silicon-carbon bond formation for the reaction of 1-pentene with Si(111)-H, 146 the UV-mediated hydrosilylation reaction of unsaturated hydrocarbons has since seen numerous applications. [50][51][52]113,133,[167][168][169] The process generally affords good hydrocarbon chain surface coverages, 113,133 chemically stable surfaces, 83 and allows for a straightforward light-addressable surface patterning. 51,52,168 For further details on the early body of work on the photochemical hydrosilylation, the reader should consult the comprehensive reviews of Buriak.…”