Exploring charge-transfer effects at metal-molecule interfaces through modeling surface-enhanced Raman spectroscopy...

Understanding charge transfer (CT) at metal-molecule interfaces is central to the design of new materials for catalysis, sensing, and energy applications. Surface-enhanced Raman spectroscopy (SERS) provides a powerful technique to probe metal-molecule interfaces since the SERS spectra reflect changes in geometry, orientation, and electronic structure which are influenced by CT. While CT can either be intrinsic or excitation-driven, we will focus on how the former contributes to SERS. However, it remains difficult to model these effects since it requires electronic structure methods capable of treating large systems. In this study, we present an efficient model to study these effects by combining a simplified time-dependent density functional theory (TDDFT) approach with a Raman bond model. The first-principles Raman bond model partitions Raman intensities into bond contributions such that the chemical mechanism in SERS can be interpreted as interatomic charge-flow modulations. Using this new model, we will examine how molecular orientation and intermolecular interactions affect the interfacial CT, employing N-heterocyclic carbenes (NHCs) as a model system. Using the Raman bond model, we will characterize the degree to which the interfacial charge flow is influenced by these effects. We then apply this model to characterize how charge flow affects molecular imaging using tip-enhanced Raman scattering (TERS). Using TERS, it is possible to image single molecules with sub-nanometer resolution. However, the role of CT in this process remains to be elucidated. We will show how the Raman bond model can characterize the importance of interfacial CT in TERS imaging. Overall, our results demonstrate that the Raman bond model, when combined with efficient first-principles calculations, offers a powerful approach for interpreting SERS spectra and providing new insights into interfacial CT.