Spectroscopic chemical characterization of atomic-scale interfaces is a challenging scientific problem. In the search for new spectroscopic capabilities, we investigate nonresonant Raman interactions that occur at the interface between organic adsorbates and inorganic surfaces. Our system is a trans-1,2-bis(4-pyridyl)ethylene molecule adsorbed to a semiconductor PbSe surface. We employ first-principles density functional methods to investigate the vibrational dynamics and Raman spectra of this absorbate−surface motif, and use an external electric bias to tune the degree of the interfacial chemical coupling. As a result, changes in the Raman spectra reveal a continuous transition between the weak and the strong coupling regimes. The strong coupling is associated with the vibration-induced charge transfer, which appears to be a damping mechanism that caps the chemical enhancement (CE) of the Raman spectra. This effect reduces the potential of CE to be used as merely an enhancing mechanism, but shows the potential of Raman scattering to probe changes in the interfacial electronic structure.

Surface enhanced Raman spectroscopy (SERS) is frequently associated with "chemical enhancement" (CE), which is an effect of the chemical coupling between reporting molecules and surfaces. While SERS technique is mainly attributed to the studies of metallic surfaces, chemical coupling must be present on semiconductor surfaces as well. Here, we examine binding of trans-1,2-two(4-pyridyl) ethylene (BPE) to various crystallographic facets of PbSe semiconductor. The calculated off-resonant Raman spectra vary significantly on different crystallographic facets of PbSe, correlating with the electronic structure of each type of semiconductor surface. We distinguish situations when the charge transfer is present and when it is not, which raises the question about what exactly should be called the "chemical enhancement". We attempt to clarify this situation by introducing the concept of the "charge-transfer" and "charge-transfer-less" chemical enhancement. We also demonstrate a transition between these two regimes, which exhibits a nonlinear behavior of the vibrational coupling and a significantly stronger contribution to the Raman intensity.