Angularly Resolved Tip-Enhanced Raman Spectroscopy

Despite intensive research in tip-enhanced Raman spectroscopy (TERS), the angular distribution of Raman scattering in the TERS gap remains experimentally unreported leaving its relevance to the TERS signal formation to be seldomly discussed. Here, we investigate the angular distribution of the tip-enhanced Raman signal in the Fourier plane using a model system composed of flat-lying cobalt (II) hexadecafluoro-phthalocyanine (CoPcF₁₆) molecules physically adsorbed on a smooth gold surface. Both in-plane and out-of-plane vibrational modes are observed, where the out-of-plane Raman modes at about 678 and 740 cm^-1 have different angular intensity distributions than those of in-plane Raman modes at 1309 and 1373 cm^-1. We interpret the angular spectrum of the TERS signal considering the molecular vibrational modes computed with density functional theory (DFT) for the free and gold-deposited molecule, and the directed Raman scattering by the gap-mode predicted by finite-difference time-domain (FDTD) simulations. We contend that the TERS gap directs the Raman vibrational modes differently, leading to distinct angularly distributed Raman scattering intensities. These findings emphasize the nonnegligible role of the TERS detection scheme in understanding spectral features, such as the relative peak intensity ratio variations for studying molecular orientations, or for monitoring chemical reactions.