Degradation of Polystyrene on Silver Substrates during Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) by films of polystyrene adsorbed onto silver island films was investigated. Films that were only a few tens of angstroms in thickness degraded rapidly during laser irradiation to form graphite-like species at the silver surface. However, no degradation was observed while Raman spectra of the solid polymer were obtained, indicating that the graphitization was probably induced by laser heating of the substrate and catalyzed by silver. For thin films of polystyrene, the rate of graphitization was high and was proportional to laser power. However, the degradation reaction was inhibited for thick films or for thin films overcoated with thick films of a second polymer. The Raman spectra were similar for all films thicker than approximately a hundred angstroms, even those overcoated with a thick film of a second polymer having a large Raman scattering cross section, indicating that most of the observed scattering originated from polymer molecules within a few tens of angstroms of the silver surface. It was concluded that SERS can be used to probe the molecular structure of polymer/metal interfaces without interference by scattering from the bulk of the polymer.