Near-Field Raman Enhancement of Single Molecules and Point Scatterers

Tip-enhanced Raman spectroscopy (TERS) is an emerging tool to characterize low-dimensional materials requiring high spatial resolution beyond the light diffraction limit. The most extreme example of a spatially localized scatterer is that of a zero-dimensional system, e.g., a single molecule, a nanoparticle, or an individual defect within a crystal. Here, using an analytical model for near-field Raman scattering from point-like structures in the presence of a plasmonic metal probe, we investigate the TERS patterns of representative single molecules with arbitrary orientation, for a range of Raman tensor symmetries. The results can be used to determine the orientation of single molecules as well as to differentiate molecules with diverse Raman tensors or, conversely, Raman modes with different Raman tensors from the same molecule. To demonstrate that the model is of general applicability to any spatially localized Raman modes, we further apply it to TERS imaging of individual point defects in graphene, unveiling the dependence of field enhancement and spatial resolution on the Raman mode symmetry, incident field polarization, and angle of the near-field probe.