Modification of alkanethiolate monolayers on Au-substrate by low energy electron irradiation: Alkyl chains and the S/Au interface

Abstract

Low-energy electron irradiation damage in alkanethiol (AT) self-assembled monolayers (SAM) has been studied by using hexadecanethiolate [HDT: CH₃–(CH₂)₁₅–S^-] film on Au-substrate as a model system. The induced changes were monitored by insitu photoelectron spectroscopy and angle resolved near edge X-ray absorption fine structure spectroscopy. AT SAMs are found to be very sensitive to low-energy electron irradiation. Both the alkyl chains and the S/Au interface are affected simultaneously through the electron-induced dissociation of C–H, C–C, C–S, and Au–thiolate bonds. The most noticeable processes are the loss of the orientational and conformational order, partial dehydrogenation and desorption of the film, and the appearance of new sulfur species. The latter process can be related to the formation of disulfide at the S/Au interface or an incorporation of the thiolate (or the corresponding radical) into the alkyl matrix via bonding to irradiation-induced carbon radicals in the adjacent aliphatic chains. The most essential damage in the AT films occurs in the early stages of irradiation. Irradiation with a dose of 1000 µC cm^-2 (about 13 electrons per HDT chain) at the primary electron energy of 50 eV results in almost complete breakdown of the orientational order in the initially well-ordered HDT film, a decrease of its thickness by about 25%, and a destruction of ≈40% of the original Au–thiolate bonds. The film becomes a disordered structure comprising both saturated and unsaturated hydrocarbons. Further irradiation of the residual film is accompanied by a continuous C–C bond cleavage and the desorption of the remaining hydrogen, which merely leads to increasing cross-linking and the transformation of saturated hydrocarbons into unsaturated ones through C2C double bond formation.