[Prof. Kyoung-Duck Park] Conformational heterogeneity of molecules phy…

caption. (Left) Schematic illustration of bottom-illumination mode TERS. The encapsulated BCB molecules on the Au surface are excited by a radially polarized beam and the back-scattered TERS responses are collected. The inset illustration shows a tip-contamination process in conventional TERS without using an Al2O3 capping layer. (Right) Models of a BCB molecule on the Au surface for the DFT calculations of normal modes in different conditions exhibiting the origin of the observed peak shift of single-molecule TERS measurements.

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 Al2O3 capping layer, which suppresses spectral diffusions and inhibits chemical reactions and contamination 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.