Single-Molecule Surface-Enhanced Raman Scattering Spectrum of Non-Resonant Aromatic Amine Showing Raman Forbidden Bands

TL;DR

This study reports the first single-molecule SERS spectrum of 4-aminibenzenethiol showing forbidden bands, attributed to charge-transfer effects, revealing breakdown of Raman selection rules at hotspots.

Contribution

It provides the first experimental single-molecule SERS spectrum of 4-ABT with forbidden bands and offers classical assignments explaining spectral differences due to charge-transfer effects.

Findings

Detection of Raman forbidden modes in single-molecule SERS

Differences from previous spectra due to charge-transfer effects

Breakdown of Raman selection rules at SERS hotspots

Abstract

We present the experimentally obtained single-molecule (SM) surface-enhanced Raman scattering (SERS) spectrum of 4-aminibenzenethiol (4-ABT), also known as para-aminothiophenol (PATP). Measured at a 4-ABT concentration of 8 * 10^-10 M, the spectra show Raman forbidden modes. The SM-SERS spectrum of 4-ABT obtained using a non-resonant visible laser is different from the previously reported SERS spectra of 4-ABT, and could not be reconstructed using quantum mechanical calculations. Careful classical assignments (not based on quantum-mechanical calculations) are reported, and indicate that differences in the reported spectra of 4-ABT are mainly due to the appearance of Raman forbidden bands. The presence of Raman forbidden bands can be explained by the charge-transfer (CT) effect of 4-ABT adsorbed on the silver nanostructures, indicating a breakdown of Raman selection rules at the SERS hotspot.