posted on 2022-12-28, 18:40authored byYi Cao, Yanting Feng, Chenyu Li, Wenwen Li, Yuqing Cheng, Jindou Ru, Lingyan Meng, Mengtao Sun
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.