Self-assembled monolayers of two aromatic disulfides and a diselenide on polycrystalline silver films: An investigation by SERS and XPS

Self-assembled monolayers of diphenyldisulfide (DDS), naphthalenedisulfide (NDS), and diphenyldiselenide (DDSe) on polycrystalline silver films have b een investigated by surface enhanced Raman spectroscopy (SERS) and X-ray ph otoelectron spectroscopy (XPS), DDS adsorbs on Ag through a homolytic cleav age of the S-S bond and resultant thiolate at the surface decomposes upon p rolonged exposure to air. The geometry of the molecule is such that the ben zene ring is almost horizontal to the surface. The Raman spectrum has been assigned in the light of ab-initio molecular orbital calculations. In DDSe, the Se-Se bond is retained upon adsorption and the molecule sticks up. In contrast, NDS is highly reactive on the microscopically rough surface so th at a stable monolayer could not be prepared. A temperature dependent Raman study of the DDS monolayer shows the absence of any reorientation at the su rface as one would expect from the adsorption geometry, XPS study complemen ts the SERS data and shows the presence of Ag2S on an NDS exposed surface. (C) 1999 Academic Press.