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

Low-energy electron irradiation damage in alkanethiol (AT) self-assembled m onolayers (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 in situ photoelectron spectroscopy and angle resolved near edg e X-ray absorption fine structure spectroscopy. AT SAMs are found to be ver y sensitive to low-energy electron irradiation. Both the alkyl chains and t he 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 p rocesses are the loss of the orientational and conformational order, partia l dehydrogenation and desorption of the film, and the appearance of new sul fur species. The latter process can be related to the formation of disulfid e at the S/Au interface or an incorporation of the thiolate (or the corresp onding radical) into the alkyl matrix via bonding to irradiation-induced ca rbon radicals in the adjacent aliphatic chains. The most essential damage i n the AT films occurs in the early stages of irradiation. Irradiation with a dose of 1000 mu C cm(-2) (about 13 electrons per HDT chain) at the primar y electron energy of 50 eV results in almost complete breakdown of the orie ntational order in the initially well-ordered HDT film, a decrease of its t hickness by about 25%, and a destruction of approximate to 40% of the origi nal Au-thiolate bonds. The film becomes a disordered structure comprising b oth saturated and unsaturated hydrocarbons. Further irradiation of the resi dual film is accompanied by a continuous C-C bond cleavage and the desorpti on of the remaining hydrogen, which merely leads to increasing cross-linkin g and the transformation of saturated hydrocarbons into unsaturated ones th rough C-C double bond formation.