Direct observation of sulfur dimers in alkanethiol self-assembled monolayers on Au(111)

High-resolution electron-energy-loss spectra of octadecanethiol self-assembled monolayers (SAM’s) on Au thin films have been obtained after annealing the sample to various temperatures. Annealing to 375 K results in the appearance of the S-S stretch at 530 ${\mathrm{cm}}^{\mathrm{-}1}$, a direct observation of sulfur dimers for alkanethiol SAM’s adsorbed on Au. The appearance of dimers following annealing is explained by the presence of an activation barrier to the formation of gauche defects at the S-C bond.