Quantum State Absorptions Coupled To Resonance Raman Spectroscopy Could Result In A General Explanation of TERS

TL;DR

This paper investigates the mechanisms behind tip-enhanced Raman scattering (TERS), exploring plasmonic effects, resonance conditions, and coupling interactions through experimental comparisons of resonant and non-resonant molecular systems.

Contribution

It provides a unified explanation of TERS enhancements based on quantum state absorptions coupled with resonance Raman scattering, considering both resonant and non-resonant systems.

Findings

Weak coupling interactions explain TERS enhancements in resonant systems.

Strong coupling is necessary to understand non-resonant lipid bilayer TERS data.

Plasmonic fields induced by tunneling may influence TERS signals.

Abstract

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.