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

Abstract

The low energy electron induced damage in self-assembled monolayers of dodecanethiolate, octadecanethiolate, and perdeuterated eicosanethiolate on gold and octadecanethiolate on silver has been investigated in situ by X-ray photoelectron spectroscopy and angle resolved near edge X-ray absorption fine structure spectroscopy. All investigated systems exhibit qualitatively similar behavior with respect to low energy electron irradiation. The most noticeable processes are the loss of orientational and conformational order, partial dehydrogenation with C C double bond formation, desorption of the layer fragments, reduction of the thiolate species, and the appearance of new sulfur species. The cross sections for the rates of the individual irradiation-induced processes have been determined. For the films on gold all these processes are found to evolve with similar rates, except for the formation of C C double bonds and desorption of sulfur-containing fragments. The extent 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 correlated. Whereas the irradiation-induced processes in the alkyl matrix are found to be essentially independent of the alkyl chain length and the substrate material, the extent and rate of the thiolate species reduction and new sulfur species formation are mainly determined by the strength and character of the thiolate−substrate bond. No large isotopic effect in the irradiation-induced dehydrogenation process was observed. Deuterated films are found to be only slightly less sensitive to electron irradiation as compared to their hydrogen-containing counterparts.