Uncovering low-frequency vibrations in surface-enhanced Raman of organic molecules

TL;DR

This paper demonstrates the detection and analysis of low-frequency vibrational modes in organic molecules using surface-enhanced Raman spectroscopy within plasmonic nanocavities, revealing environment-dependent spectral features and molecular interactions.

Contribution

It introduces a method to distinguish low-wavenumber spectral peaks in single-molecule SERS and assigns these to specific THz vibrations through advanced simulations.

Findings

Identification of low-frequency spectral peaks below 200 cm-1 in SERS

Narrower THz spectra from individual molecules at picocavities

Environment-dependent vibrational properties observed

Abstract

Accessing the terahertz (THz) spectral domain through surface-enhanced Raman spectroscopy (SERS) is challenging and opens up the study of low-frequency molecular and electronic excitations. Compared to direct THz probing of heterogenous ensembles, the extreme plasmonic confinement of visible light to deep sub-wavelength scales allows the study of hundreds or even single molecules. We show that self-assembled molecular monolayers of a set of simple aromatic thiols confined inside single-particle plasmonic nanocavities can be distinguished by their low-wavenumber spectral peaks below 200 cm-1, after removal of a bosonic inelastic contribution and an exponential background from the spectrum Developing environment-dependent density-functional-theory simulations of the metal-molecule configuration enables the assignment and classification of their THz vibrations as well as the identification of intermolecular coupling effects and of the influence of the gold surface configuration Furthermore, we show dramatically narrower THz SERS spectra from individual molecules at picocavities, which indicates the possibility to study intrinsic vibrational properties beyond inhomogeneous broadening further supporting the key role of local environment.