Surface-enhanced Raman scattering from substrates with conducting or insulator overlayers: Electromagnetic model predictions and comparisons with experiment

Model electromagnetic (EM) calculations are presented of the surface-enhanc ed Raman scattering (SERS) intensities expected for gold and silver particl es coated with conducting as well as insulating dielectric films, with the objective of assessing their thickness-dependent properties and hence the a nticipated scope of such "overlayer SERS" tactics to chemically diverse int erfacial materials. Most calculations refer to Raman enhancements at the fi lm outer edge, relevant to species adsorbed on the overlayer. Spheroidal an d spherical metal particles are treated by using the electrostatic approxim ation, with a first-order electrodynamic correction for "radiation damping" , in vacuum and aqueous environments. The presence of simple insulating die lectric (such as organic) overlayers yields progressive decays in the calcu lated Raman enhancement factor, G, at the film edge with increasing thickne ss, d, throughout the visible optical region of experimental significance, even though the largest G values are necessarily obtained close to the plas mon resonance energy. These Gd dependencies are markedly milder than predic ted for bare metal particles in water or vacuum, suggesting the potential b road-based analytical utility of nanoscale (similar to 1-10 nm thick) molec ular dielectric overlayers in SERs. Furthermore, Raman enhancements increas ing with film thicknesses are typically obtained at locations close to the inner film edge, relevant to moieties imbedded in dielectric overlayers. Sh arper G-d decays are predicted at the film edge of transition-metal overlay ers, especially near the plasmon resonance; unlike dielectric insulators, t he metal overlayers progressively "quench" the optical frequency-dependent enhancement at the film edge arising from substrate plasmon resonance. Reas onable agreement is obtained in comparison with the Raman intensity-thickne ss dependence measured for chemisorbed carbon monoxide on rhodium films ele ctrodeposited onto gold. The model calculations can also account qualitativ ely for the nonmonotonic Raman intensity-thickness dependencies observed fo r phonon bands from semiconducting (cadmium chalcogenide) overlayers on gol d, attributed to film-induced redshifts in the substrate plasmon resonance. More general implications for the analytical utility of "overlayer SERS" t actics are also pointed out.