An enhanced sensitivity of alkanethiolate self-assembled monolayers to electron irradiation through the incorporation of a sulfide entity into the alkyl chains

A possibility to influence the response of self-assembled monolayers (SAM) of alkanethiolates (AT) to low-energy electron irradiation through the inco rporation of specific molecular groups into the alkyl chains has been studi ed by using 11-(hexylmercapto)undecane-1-thiol (HMUT, CH3(CH2)(5)S(CH2)(11) SH) SAM on gold substrate as a model system. In situ near-edge X-ray absor ption fine structure spectroscopy and X-ray photoelectron spectroscopy (XPS ) were applied as experimental tools. HMUT was found to form a dense, well- ordered self-assembled monolayer on Au with a coverage close to that of AT SAMs, a thickness of 19.8 +/- 0.5 Angstrom, and an average molecular tilt a ngle of 40 degrees +/- 2 degrees. Features related to the thiolate and sulf ide species could be easily distinguished in the S 2p XP spectra. Electron irradiation of the HMUT film gives rise to the same effects previously obse rved for AT SAMs such as disordering, partial dehydrogenation with C=C doub le bonds formation, desorption of the film fragments, reduction of the thio late moieties, and the appearance of a new sulfur species. At the same time the extent of irradiation-induced desorption from the HMUT film is found t o be noticeably larger (by approximate to 35%) than in AT SAMs, which is at tributed to a higher sensitivity of the C-S bond to electron irradiation as compared to a C-C one. Some other differences with respect to AT SAMs such as sulfide-derived formation of C=S double bonds and a slightly reduced ex tent of irradiation-induced damage at the Au-alkanethiolate interface were also observed. The close resemblance of the binding energies of alkyl sulfi de and the irradiation-induced sulfur species in AT SAMs implies an alterna tive assignment for the latter entities along with the commonly approved di sulfide formation model, namely an incorporation of sulfur into the alkyl m atrix via bonding to irradiation-induced carbon radicals in the adjacent al iphatic chains.