ORIENTATION AND CHEMISTRY OF ALKYL THIOLATES ON THE NI(100) SURFACE

As models for boundary layer films we have examined the structure and chemistry of the straight-chain thiols (CH3(CH2)(n)SH, n = 0-5) on the clean Ni(100) surface. On the clean surface the thiols decompose at low temperatures (T < 140 K) to form adsorbed alkylthiolates (CH3(CH2)(n)S-(ad)). Further decomposition at higher temperatures results in the formation and desorption of alkanes, olefins and hydrogen, and leaves sulfur adsorbed to the surface. Since no sulfur containing species desorb it is possible to determine the alkylthiolate coverage by measuring the sulfur coverage on the surface following decomposition. These measurements have been made for saturated monolayers of methylthiolate through hexylthiolate. The saturation coverage remains constant at theta = 0.28 +/- 0.04 monolayers as the number of methylene groups in the alkyl chain increases. This suggests that the thiolates in these films have self-assembled into a structure in which the alkyl chains are oriented away from the surface.