A DFT study of the adsorption and surface enhanced Raman spectroscopy of pyridine on Au₂₀, Ag_20, and bimetallic...

This work presents a theoretical detailed analysis of the surface-enhanced Raman spectroscopy (SERS) of the pyridine - Au₂₀, pyridine - Ag_20, and pyridine - Ag₈Au₁₂ 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 Au₂₀ cubic structure, where 10² 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.