A computational approach for electrochemical SERS.Key role of the surface charges and synergy between EM and CT mechanisms.

Abstract

We present a computational approach for electrochemical Surface-Enhanced Raman Scattering (EC-SERS). The surface excess of charge induced by the electrode potential (Vel) was introduced by applying an external electric eld to a set of clusters [Agn]q with (n,q) of (19, 1) or (20,0) on which a molecule adsorbs. Using DFT/TD-DFT calculations, these metal-molecule complexes were classi ed by the adsorbate partial charge, and the main Vel-dependent properties were simultaneously studied with the aid of vibronic resonance Raman computations. Namely, changes on the vibrational wavenumbers, relative intensities and enhancement factors for all SERS mechanisms: chemical or nonresonant, and resonance Raman with bright states of the adsorbate, charge-transfer states and plasmon-like excitations on the metal cluster. We selected two molecules to test our model, pyridine, for which Vel has a remarkable e ect, and 9,10-bis((E)-2-(pyridin-4-yl)vinyl)anthracene, which is almost insensitive to the applied bias. The results nicely reproduced most of the experimental observations, while the limitations of our approach were critically evaluated. We detected that accounting explicitly for the surface charges is key for EC-SERS models and that the highest calculated enhancement factors, up to 107-108, are obtained by interstate coupling of bright local excitations of the metal cluster and