Surface enhanced Raman spectroscopy of aromatic compounds on silver nanoclusters

Surface enhanced Raman spectroscopy (SERS) has been used to characterize multilayers of three different aromatic compounds in the proximity of silver nanoclusters. SERS of mercapto benzoic acid (MBA), which adsorbs onto the silver nanoclusters through the sulfur moiety, exhibits frequency shifts in comparison to the Raman spectrum of crystalline MBA. Conversely, benzoic acid and benzophenone that adsorb through the oxygen species lack these frequency shifts, and show only a typical SERS enhancement. We employed X-ray photoelectron spectroscopy (XPS), to probe the nature of the binding between the silver and the three different aromatic compounds. Thereafter, we assigned the major Raman peaks of all three molecules to specific molecular vibrations. Overall, this enables us to determine the origin of the observed shifts in the SERS spectrum of MBA and similar molecules.