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.

We report the gas-phase photochlorination of methyl-terminated alkyl monolayers on silicon. We argue that this process proceeds by a free-radical mechanism, as is the case for homogeneous photochlorination. This is an example of what should be a family of convenient methods for the incorporation of various functional groups into simple alkyl monolayers by chlorine-radical activation. Monolayers prepared from 1-octadecene on Si(111) were exposed to Cl2 with illumination at 350 nm. We observe that a fraction of the carbon atoms on the surface become singly chlorinated and a smaller fraction become doubly chlorinated, as measured by the chemically shifted components of the C 1s X-ray photoelectron spectrum. The elemental composition of the resulting monolayers, film thickness, and contact angles are reported as a function of exposure. Under conditions that chlorinate only a fraction of the carbon atoms, IR spectroscopy reveals complete chlorination of the methyl groups, demonstrating a strong steric preference for chlorination at the ends of the alkyl chains. In the absence of illumination, a much slower dark reaction is observed.