Photoactivated fluorescence of small silver nanoclusters and their relation to Raman spectroscopy

Photoactivated fluorescence from individual silver nanoclusters ranging in size from 2--8 atoms has been demonstrated at room temperature. The optical properties of such clusters are far superior to those of fluorescence dyes with absorption cross sections ∼50 times stronger than those of even the best organic dyes. The strong oscillator strengths produced from such nanoclusters has been shown to yield comparable enhancement factors in the surface-enhanced Raman spectroscopy (SERS) process to those observed in the presence of a plasmon-supporting nanoparticle. Raman transitions are in fact so strong that antistokes scattering is also observable on a single molecule (SM) level marking the first true demonstration of SM-SERS to date. Capable of generating true scaffold specific Raman scattering on the single molecule level, the combination of fluorescence from the small nanoclusters and strong observed Raman signals in the absence of a nanoparticle strongly indicate a chemical or charge transfer SERS enhancement mechanism.