Modification of Alkanethiolate Self-Assembled Monolayers by Atomic Hydrogen: Influence of Alkyl Chain Length

The interaction of atomic hydrogen with four alkanethiolate self-assembled monolayers (SAMs), octadecanethiolate, hexadecanethiolate, dodecanethiolate, and nonanethiolate, has been studied in situ, using X-ray photoelectron spectroscopy. Atomic hydrogen reactions with gold−sulfur bonds result in sulfur desorption and the formation of new sulfur species within the film. The loss of sulfur from each SAM exhibits first-order kinetics with a rate constant that decreases with increasing alkyl chain length. This supports the idea that sulfur desorption is controlled by the diffusion of atomic hydrogen through the alkyl chains to the film/substrate interface. In the two shorter chain SAMs, sulfur and carbon desorption is rapid and concerted. This indicates that the modification of dodecanethiolate and nonanethiolate SAMs is dominated by desorption of intact adsorbate chains and alkyl sulfur fragments formed by atomic hydrogen reactions at the film/substrate interface. For octadecanethiolate and hexadecanethiolate SAMs, however, the increased residence time of atomic hydrogen within the hydrocarbon film is responsible for the formation of a disordered hydrocarbon overlayer that prevents desorption of intact adsorbate chains or alkyl sulfide species. As a result, the rate of carbon desorption from these two SAMs is dramatically reduced and the loss of carbon and sulfur is not concerted. In the longer alkyl chain SAMs, sulfur is lost as small volatile fragments (e.g., H 2 S), while carbon desorption proceeds through chemical erosion of the hydrocarbon film by atomic hydrogen. For alkanethiolate SAMs exposed to atomic hydrogen, reactions between carbon-centered radicals and atmospheric oxygen containing species increase the surface hydrophilicity. Results from this investigation highlight the determinant role that film thickness can play in moderating the reactivity of gas-phase species with organized thin film assemblies.