Study of Single-Molecule Rhodamine 6G Raman Scattering on Silver Nanocrystal Aggregates |
Surface enhanced Raman scattering (SERS) fromRhodamine 6G adsorbed on small, optically resolvable aggregates of silvernanoparticles is detectable at the single-molecule level. The signal fluctuateson a second time scale, presumably due to movement of the molecule in to andout of the SERS active “hot spots”. The depolarization of the signal is found tobe independent of the unstabilitys, to vary from one spot to the next, and tobe highly dependent on the direction of the incident electric field. The “SERScontinuum” dominates the signal and is assumed to arise from electronic Ramanprocesses in the metal that are triggered by chemisorption of the molecule inthe hot spot. The source of the Raman signal is therefore highly localizedaround the hot spot. The local structure determines the depolarizationproperties and is often extremely anisotropic. Conversely, the Rayleighscattering does not depend on the molecule or the hot spot and is delocalizedover the whole aggregate, which is always found to be highly isotropic. TheRayleigh scattering therefore does not fluctuate and is always highly polarized.