Surface-Enhanced Raman Scattering from Substrates with Conducting or Insulator Overlayers: Electromagnetic Model Predictions and Comparisons with Experiment

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Abstract
Model electromagnetic (EM) calculations are presented of the surface-enhanced Raman scattering (SERS) intensities expected for gold and silver particles coated with conducting as well as insulating dielectric films, with the objective of assessing their thickness-dependent properties and hence the anticipated scope of such “overlayer SERS” tactics to chemically diverse interfacial materials. Most calculations refer to Raman enhancements at the film outer edge, relevant to species adsorbed on the overlayer. Spheroidal and spherical metal particles are treated by using the electrostatic approximation, with a first-order electrodynamic correction for “radiation damping”, in vacuum and aqueous environments. The presence of simple insulating dielectric (such as organic) overlayers yields progressive decays in the calculated Raman enhancement factor, G, at the film edge with increasing thickness, d, throughout the visible optical region of experimental significance, even though the largest G values are necessarily obtained close to the plasmon resonance energy. These G–d dependencies are markedly milder than predicted for bare metal particles in water or vacuum, suggesting the potential broad-based analytical utility of nanoscale (∼ 1–10 nm thick) molecular dielectric overlayers in SERS. Furthermore, Raman enhancements increasing with film thicknesses are typically obtained at locations close to the inner film edge, relevant to moieties imbedded in dielectric overlayers. Sharper G–d decays are predicted at the film edge of transition-metal overlayers, especially near the plasmon resonance; unlike dielectric insulators, the metal overlayers progressively “quench” the optical frequency-dependent enhancement at the film edge arising from substrate plasmon resonance. Reasonable agreement is obtained in comparison with the Raman intensity-thickness dependence measured for chemisorbed carbon monoxide on rhodium films electrodeposited onto gold. The model calculations can also account qualitatively for the nonmonotonic Raman intensity-thickness dependencies observed for phonon bands from semiconducting (cadmium chalcogenide) overlayers on gold, attributed to film-induced redshifts in the substrate plasmon resonance. More general implications for the analytical utility of “overlayer SERS” tactics are also pointed out.

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