Understanding the chemical enhancement mechanism of 2D substrate enhanced Raman Spectroscopy (2D-SERS).

Surface-enhanced Raman spectroscopy (SERS) is a well established field which utilizes the enhancement of Raman signals for molecules on metal substrates, resulting in applications of this phenomena for detection and identification of trace concentrations of molecules. In recent years, two-dimensional (2D) materials like graphene, h-BN and MoS₂ are being used as substrates for SERS, giving rise to rapidly growing field of 2D-substrate enhanced Raman spectroscopy (2D-SERS). In conventional SERS, the enhancement factor is dominated by an electromagnetic enhancement mechanism, as compared to the smaller chemical enhancement effect. In 2D-SERS however, this chemical enhancement effect (which stems from electron-phonon coupling within the molecule and substrate) is thought to play a dominating role. Yet, the detailed understanding of the chemical enhancement effect in 2D-SERS is still lacking. Using first principles calculations, we study the chemical enhancement mechanism using typical probe molecules such as pyridine and pthalocyanine on 2D substrates, highlighting the role of electron-phonon coupling and charge-transfer excitons in the chemical enhancement mechanism of 2D-SERS.