Plasmonic Gradient and Plexcitonic Effects in Single-Molecule Tip-Enhanced (Resonance) Raman Spectroscopy

A basic understanding of the electromagnetic processes in tip-enhanced Raman spectroscopy (TERS) is crucial for the high-sensitivity Raman signal and high-resolution optical image. Here, we first explored the physical mechanism of the plasmonic gradient effects in TERS, where the plasmon and plasmonic gradient can activate the dipole and dipole-quadrupole polarizability, respectively, which results in the simultaneous observation of Raman and infrared radiation (IR) spectra in TERS. The horizontal electric gradient dominantly contributes to the IR-active modes, while the Raman-active modes mainly resulted from the vertical electric field. Second, we revealed the physical mechanism of the plexcitonic effect in tip-enhanced resonance Raman spectroscopy (TERRS), in which the localized surface plasmon resonance strongly couples with the exciton of molecular resonance absorption in the electronic state transition. The plexciton-enhanced resonance Raman scattering is much stronger than plasmon-enhanced resonance Raman scattering in TERRS. Our results could not only reveal the plasmon gradient effect in TERS and plexciton effect in TERRS but also promote a deeper understanding on the design of TERS and TERRS instruments with high efficiency.