  Tip enhanced Raman scattering (TERS) amplifies the intensity of vibrational
Raman scattering by employing the tip of a probe interacting, in ultra close
proximity, with a surface. Although a general understanding of the TERS process
is still to be fully elucidated, scanning tunneling microscopy (STM) feedback
is often applied with success in TERS to keep a noble metal probe in intimate
proximity with a noble metal substrate. Since such STM TERS is a common
modality, the possible implications of plasmonic fields that may be induced by
the tunneling process are investigated and reported. In addition, TERS of a 2D
resonant molecular system, a MoS2 bilayer crystal and a 2D non-resonant, lipid
molecular bilayer is compared. Data with multiple excitation wavelengths and
surfaces for the resonant system in the near- (TERS) and far-field regimes are
reported. An interpretation based on weak coupling interactions within the
framework of conventional resonance Raman scattering can explain the observed
TERS enhancements. The non-resonant molecular lipid system, on the other hand,
requires strong coupling for a full understanding of the reported observations.
