Electromagnetic contributions to single-molecule sensitivity in surface-enhanced Raman scattering

We examine whether single molecule sensitivity in surface-enhanced Raman scattering (SERS) can be explained in the framework of classical electromagnetic theory. The influence of colloid particle shape and size, composition (Ag or Au) and interparticle separation distance on the wavelength-dependent SERS enhancement factor is reported. Our calculations indicate that the maximum enhancement factor achievable through electromagnetics is of the order 10(11). This is obtained only under special circumstances, namely at interstitial sites between particles and at locations outside sharp surface protrusions. The comparative rarity of such sites, together with the extreme spatial localization of the enhancement they provide, can qualitatively explain why only very few surface sites seem to contribute to the measured signal in single-molecule SERS experiments. Enhancement factors of the order 10(14)-10(15), which have been reported in recent experiments, are likely to involve additional enhancement mechanisms such as chemisorption induced resonance Raman effects.