Modification of alkanethiolate monolayers by low energy electron irradiation: Dependence on the substrate material and on the length and isotopic composition of the alkyl chains

The low energy electron induced damage in self-assembled monolayers of dode canethiolate, octadecanethiolate, and perdeuterated eicosanethiolate on gol d and octadecanethiolate on silver has been investigated in situ by X-ray p hotoelectron spectroscopy and angle resolved near edge X-ray absorption fin e structure spectroscopy. All investigated systems exhibit qualitatively si milar behavior with respect to low energy electron irradiation. The most no ticeable processes are the loss of orientational and conformational order, partial dehydrogenation with C=C double bond formation, desorption of the l ayer fragments, reduction of the thiolate species, and the appearance of ne w sulfur species. The cross sections for the rates of the individual irradi ation-induced processes have been determined. For the films on gold all the se processes are found to evolve with similar rates, except for the formati on of C=C double bonds and desorption of sulfur-containing fragments. The e xtent of the latter process is noticeably smaller in the longer-chain films as compared to their shorter-chain counterparts. The response of the alkyl matrix and the S-Au interface to electron irradiation are not directly cor related. Whereas the irradiation-induced processes in the alkyl matrix are found to be essentially independent of the alkyl chain length and the subst rate material, the extent and rate of the thiolate species reduction and ne w sulfur species formation are mainly determined by the strength and charac ter of the thiolate-substrate bond. No large isotopic effect in the irradia tion-induced dehydrogenation process was observed. Deuterated films are fou nd to be only slightly less sensitive to electron irradiation as compared t o their hydrogen-containing counterparts.