Revealing Interaction of Organic Adsorbates with Semiconductor Surfaces Using Chemically Enhanced Raman

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.