Elastic scattering and emission correlated with single‐molecule SERS

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Abstract
With respect to the origin of single‐molecule sensitivity in surface‐enhanced Raman scattering (SERS), elastic scattering and emission spectra were studied for Ag particles adsorbed with dye or adenine molecules. The scattering peak observed at 600–650 nm was extinguished during the inactivation process of the enormous SERS signal, whereas localized surface plasmon (LSP) peaks located at 520 and 730 nm did not change significantly. It was found using a numerical simulation that the additional scattering peak arises from increased electromagnetic coupling between the LSP of adjacent Ag particles through dye molecules located at their junction. In addition, two distinct emission peaks were observed at 550–600 and 600–750 nm only for hot Ag particles with adsorbates. Based on the observations for different surface coverages and on different dye species, these bands were attributed to enhanced radiative decay of LSP and fluorescence of molecules, respectively. Interestingly, adenine molecules on the Ag nanoparticles show similar fluorescence peaks at 600–650 nm, suggesting a charge‐transfer state upon adsorption. Copyright © 2005 John Wiley & Sons, Ltd.