On the significance of Shalaev's "hot spots' in ensemble and single-molecule SERS by adsorbates on metallic films at the percolation threshold

Recent models of plasmon localization and local electromagnetic (EM) field distribution in silver-on-glass films at the percolation threshold (Shalaev and collaborators) have successfully modelled the scanning near-field optical microscopy (SNOM) pictures. The clearly visible so-called hot spots are believed to contribute heavily to the Raman enhancement for adsorbates at the hot spots exceeding 10(11). However, when the theorem of optical reciprocity is properly taken into account, the ensemble EM enhancement factor contributing to surface-enhanced Raman scattering (SERS) in silver island films at the percolation threshold is calculated here as about 3 X 10(5), an order of magnitude lower than the overall SERS enhancement measured by Shalaev and collaborators. Given the experimental probability of 0.003 in finding a single dye molecule that is embedded within a Langmuir-Blodgett (LB) film coating a silver island film at a hot spot (observations of Aroca and collaborators), and assuming that these hot spots are of 'Shalaev-type', the corresponding EM enhancement does not exceed 10(6) to 10(7). In addition, this is in good agreement with the experiment. This enhancement is sufficient to observe the SERS from single dye molecules. Earlier enhancement numbers produced by the rule of thumb, that the enhancement is given by the fourth power of the local field enhancement, are misleading by several orders of magnitude for Stoke shifts exceeding approximately 300 cm(-1). Copyright (c) 2006 John Wiley & Sons, Ltd.