Picocavity Plasmon-Enhanced Vibrational Spectroscopy of Single-Molecule Chemical Processes of 4,4′-Dimercaptoazobenzene

Raman scattering measurement technology based on plasmon enhancement mechanism can improve the Raman scattering cross-sections of molecules by several orders of magnitude. Accordingly, they have a wide range of application prospects for trace substance identification and tracking. Many previous studies have reported identification of single-molecules when the enhancement factor reaches similar to 10(10). Measuring physical and chemical behaviors at the single-molecule level is expected to provide richer and more accurate results compared to measuring the statistical behavior of an ensemble of many molecules. This study uses a radially polarized laser to excite a picocavity hotspot in a longitudinally polarized plasmonic antenna. We achieve an accurate observation of the chemical processes of individual molecules using their vibrational spectra. In particular, we observe that 4-nitrobenzenethiol dimerized into 4,4'-dimercaptoazobenzene and that a single-molecule of the latter briefly switched from trans- to cis-conformation; it then desorbs from the gold atom and reverts to trans-conformation. This study preliminarily shows the power of picocavity plasmon-enhanced vibrational spectroscopy in investigating single-molecule chemical kinetics.