Quasi-Analytical Approach for Modeling of Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering has become a powerful analytical tool for the characterization of molecules adsorbed on metal surfaces. The lack of reliable computational methods to accurately assign the complicated Raman spectra has hampered its practical applications. We propose here a quasi-analytical method that allows for the effective evaluation of Raman tensors in periodic systems based on density functional perturbation theory and the finite-difference approach. Its applicability has been validated by simulating Raman spectra of 4,4'-bipyridine (4,4'-bpy) in various conditions. The calculated Raman spectra of isolated 4,4'-bpy as well as its adsorption on flat gold surfaces nicely reproduce their experimental counterparts. The same method has also been successfully applied to a more complicated system, namely 4,4'-bpy inside gold nano junctions. By comparing with the in situ experimental spectra, four interfacial configurations are identified, which are further verified by the good agreement between the simulated charge transfer properties and the experimental measurements. These results indicate that the proposed low-cost quasi-analytical method can provide accurate interpretation for the experimentally measured surface-enhanced Raman spectra and unambiguously determine the structures of the molecules on metal surfaces.