Single molecule sensitivity in SERS: importance of junction of adjacent Ag nanoparticles

An enormous SERS ( Surface Enhanced Raman Scattering) signal from dye and adenine on hot or blinking silver nanoparticles is extinguished by duration of measurement possibly due to thermal diffusion or desorption of adsorbed molecules. Simultaneously, the elastic scattering peak at ca. 630 nm disappears. The Three-dimensional Finite Difference Time Domain (FDTD-3D) method manifests this scattering peak as originating from enhanced coupling of localized surface plasmon (LSP) on adjacent Ag particles through absorption of adspecies located at their junction. Distinct emission peaks were observed at 550 - 600 nm and 600 - 750 nm. Based on the spectral variations for different surface coverage and for different dye species, the shorter and longer wavelength peaks were attributed to excited electrons on metal, and from fluorescence of molecules, respectively. Furthermore, we found the shorter wavelength peak shows invariant Stokes shift irrespective of excitation wavelengths, indicating it arises from inelastic scattering of excited electron by surface roughness or by adsorbed molecules.