Roles of silver nanoclusters in surface-enhanced Raman spectroscopy

The cause for the huge enhancement factors of surface-enhanced Raman spectroscopy (SERS) by the addition of small silver nanoclusters is theoretically investigated by focusing on the difference between resonance Raman activity and surface plasmon effects. First, the resonance and off-resonance Raman spectra are calculated using the incident light wavenumbers of the low-lying charge transfer excitations for the surface (S) and vertex (V) complexes of the pyridine molecule attaching to three small silver nanoclusters: Ag₅, Ag₁₀, and Ag₂₀. As a result, it is found that the incident radiation dramatically increases the resonance Raman activities with the enhancement factors up to 10¹². This indicates that the resonance Raman effects are dominant in the enhancement factors of SERS, at least when to use small silver clusters. It is also found that the resonance Raman spectra significantly depend on the adsorption sites given in S or V complexes, and on the inclusion or exclusion of the long-range correction for density functional theory, irrespective of the size of the silver clusters. The electromagnetic field enhancement effects called "surface plasmon effects" are also examined for the Ag₂₀ cluster to confirm this conclusion. Consequently, the enhancement in the electric field is roughly evaluated as less than one for the static polarizability of this small cluster. It is, therefore, concluded that the resonance Raman activity effect is dominant in the huge SERS enhancement factors for, at least, small silver nanoclusters.