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

Area of Science:

• Physical Chemistry
• Spectroscopy
• Nanotechnology

Background:

• Terahertz (THz) spectroscopy probes low-frequency molecular and electronic excitations.
• Surface-enhanced Raman spectroscopy (SERS) typically operates at higher frequencies.
• Direct THz probing of heterogeneous ensembles is limited compared to plasmonically confined light.

Purpose of the Study:

• To extend SERS into the terahertz spectral domain for studying molecular vibrations.
• To investigate the potential for analyzing single molecules using THz SERS.
• To assign and classify THz vibrations of aromatic thiols within plasmonic nanocavities.

Main Methods:

• Utilizing self-assembled molecular monolayers of aromatic thiols within single-particle plasmonic nanocavities.
• Applying advanced data processing to remove background noise and inelastic contributions from spectra.
• Employing environment-dependent density-functional-theory (DFT) simulations for spectral assignment and analysis.

Main Results:

• Successfully identified low-wavenumber spectral peaks below 200 cm^-1 for aromatic thiols using THz SERS.
• Assigned THz vibrations and identified intermolecular coupling and surface effects via DFT simulations.
• Observed significantly narrower THz SERS spectra from individual molecules in picocavities, reducing inhomogeneous broadening.

Conclusions:

• THz SERS provides a powerful method for studying low-frequency molecular excitations at the single-molecule level.
• DFT simulations are crucial for interpreting THz SERS spectra and understanding metal-molecule interactions.
• Picocavities enhance spectral resolution in THz SERS, enabling the study of intrinsic vibrational properties.