Static secondary ion mass spectrometry studies of self-assembled monolayers: electron beam degradation of alkanethiols on gold

Abstract

Static secondary ion mass spectrometry (SIMS) has been used to investigate the degradation of self-assembled monolayers (SAMs) of alkanethiols subjected to bombardment by keV electrons. Because of its remarkable structural specificity, SIMS revealed significant structural modifications to the SAMs following irradiation. Both positive and negative ion spectra exhibited dramatic changes after exposure of SAMs to electron bombardment. In the positive ion spectra, peaks were observed between m/z 100 and 200 that were attributed to polycyclic aromatic ions with masses greater than the adsorbate molecule. These species are the result of interchain crosslinking initiated by electron impact. In the negative ion spectra, gold-molecular clusters disappeared after only small doses of electrons, attributed to the rapid oxidation of thiolates to disulfides. After doses as high as 3×10 ¹⁷ electrons cm ^–2 , there were still significant levels of sulfur at the surface along with graphitised carbonaceous material. It was concluded that keV electron impact leads to only slow removal of material from the SAM. These data illustrate the power of SIMS for probing surface reactions in SAMs.