Understanding the chemical enhancement mechanism of graphene-enhanced Raman Spectroscopy (GERS) – A first principles study.

Surface-enhanced Raman spectroscopy (SERS), involving the enhancement of Raman signals for molecules on metal substrates, is a well-established field that has enabled the detection of trace concentrations of molecules. In recent years, graphene was found to be a candidate substrate for SERS, giving rise to a rapidly expanding field of graphene-enhanced Raman spectroscopy (GERS), and opening doors for other two-dimensional materials to be used for SERS. In conventional SERS, the Raman signal is enhanced by a combination of the dominant electromagnetic enhancement effect, and the smaller chemical enhancement effect, which stems from electron-phonon coupling in the system. The latter chemical enhancement effect is thought to play a dominating role in GERS, thus enabling detailed studies of electron-phonon coupling effects for organic-graphene interfaces. Yet, the details of the chemical enhancement effect in GERS are not well understood. Using first principles calculations, we study the GERS chemical enhancement mechanism using typical probe molecules such as pyridine adsorbed on graphene. We uncover a novel ground state enhancement mechanism that is distinct from the typical ground state charge transfer mechanism in conventional SERS.