Conformational heterogeneity of molecules physisorbed on a gold surface at room temperature

TL;DR

This study demonstrates hyperspectral tip-enhanced Raman spectroscopy of single molecules at room temperature, revealing conformational heterogeneity through quantitative spectral analysis enabled by a protective capping layer.

Contribution

It introduces a robust method for single-molecule TERS imaging at room temperature, overcoming challenges of molecular dynamics and spectral diffusion.

Findings

Detected Raman peak variations up to 7.5 cm$^{-1}$ in single molecules.

Revealed conformational heterogeneity at the single-molecule level.

Established a freeze-frame approach using a capping layer to suppress spectral diffusion.

Abstract

A quantitative single-molecule tip-enhanced Raman spectroscopy (TERS) study at room temperature remained a challenge due to the rapid structural dynamics of molecules exposed to air. Here, we demonstrate the hyperspectral TERS imaging of single or a few brilliant cresyl blue (BCB) molecules at room temperature, along with quantitative spectral analyses. Robust chemical imaging is enabled by the freeze-frame approach using a thin Al$_{2}$O$_{3}$ capping layer, which suppresses spectral diffusions and inhibits chemical reactions and contaminations in air. For the molecules resolved spatially in the TERS image, a clear Raman peak variation up to 7.5 cm$^{-1}$ is observed, which cannot be found in molecular ensembles. From density functional theory-based quantitative analyses of the varied TERS peaks, we reveal the conformational heterogeneity at the single-molecule level. This work provides a facile way to investigate the single-molecule properties in interacting media, expanding the scope of single-molecule vibrational spectroscopy studies.