A DFT study of the adsorption and surface enhanced Raman spectroscopy of pyridine on Au, Ag and bimetallic AgAu clusters.

This work presents a theoretical detailed analysis of the surface-enhanced Raman spectroscopy (SERS) of the pyridine - Au, pyridine - Ag and pyridine - AgAu model systems considering different symmetries of the clusters. In addition to the well-known Td geometry of this twenty atoms metal cluster, low energy structures have been analyzed (Cs and Cb). Density functional methodology with the use of PBE, PBE0, and scalar - relativistic pseudopotentials have been employed for the electronic structure calculations of these molecule-metal complexes. The projected state density analysis has shown a different behavior that distinguishes vertex (V) pyridine position from surface (S) adsorption site on both Td and Cs geometries. Adsorption of pyridine on the V position is always energetically favored as compared to S substitution. The chemical mechanism of enhancement has been analyzed through charge-transfer in the adsorption of pyridine to the metal cluster and charge-transfer excitations between both moieties. There is a close relationship between the amount of charge transfer from the pyridine molecule to the cluster and the SERS enhancements. The highest enhancement factors were obtained precisely for the Au cubic structure, where 10 order enhancements were calculated. A back - donation of charge triggers the highest enhancements obtained for this case. The bimetallic case also shows an improvement in the enhancements as compared to Td and Cs geometries. In addition, a direct relationship between resonant charge - transfer excitations in the 500-530 nm range and SERS enhancement have been obtained. Cs and cubic clusters show higher chemical enhancements than Td structures, thus representing a promise as SERS substrates.