Surface-enhanced Raman scattering of pyrazine at the junction between two Ag20 nanoclusters

In this work we present a detailed time-dependent density functional theory (TDDFT) investigation of the absorption and Raman spectra of a pyrazine molecule located at the junction between two Ag-20 clusters. Surface-enhanced Raman scattering enhancements of the order of 106 have been found for the junction system, which are similar to enhancements of 105 found for individual silver nanoparticles. Surprisingly, the chemical enhancement is found to account for as much as 105, suggesting that this mechanism might be more important than previously believed, in particular for nanoparticle aggregates. Moreover, TDDFT calculations suggest that unlike larger nanoparticles, the junction between small Ag20 tetrahedral clusters does not provide an electromagnetic "hot spot".