Fig. 1. (a) Schematic view of the NPoM plasmonic nanocavities, where Au nanoparticles are separated from an Au film (100 nm thick) by a self-assembled molecular monolayer. The inset illustrates the self-assembled BPhT molecules in the gap region. (b) Typical SERS spectrum of BPhT molecules in an individual plasmonic nanocavity. Arrows indicate three ever-present Raman-active vibrational modes in BPhT molecules: 1080 cm^-1 (green arrow), 1281 cm^-1 (blue arrow), and 1595 cm^-1 (red arrow), respectively. The acquisition binning time is 10 s. (c) and (d) Time series of the anti-Stokes (upper panel) and Stokes (lower panel) spectra from two typical NPoM constructs. Binning time: 10 s. (e) and (f) Time series of the integrated SERS intensity from the two typical NPoM constructs, respectively. Binning time: 10 ms. (g) Statistics of the duration of blinking events observed in 20 plasmonic nanocavities.

Fig. 2. (a) Time series of the Stokes and anti-Stokes spectra from a typical NPoM nanocavity showing long-duration blinking components. The acquisition time is 10 s. The middle column shows the time series of the integrated SERS intensity from these NPoM constructs, including signals from both Stokes and anti-Stokes. Binning time: 10 ms. (b) The spectra extracted from the time-series SERS spectra at the beginning of the acquisition and at different time points [210, 520, and 620 s as indicated by white arrows in (a)] when blinking features occur. The blue-shaded areas highlight the Raman-active peaks that are ever-present during the measurement, while the orange-shaded areas represent Raman-inactive peaks, which only appear randomly when blinking occurs. The acquisition time for each spectrum is 10 s. (c) DFT calculations of the infrared absorption (left column) and Raman cross section (right column) for a single Au-bound molecule (Au-BPhT) driven by a uniform field. The blue-shaded areas highlight both IR and Raman-active peaks.

Fig. 3. (a) Comparison of the initial moment SERS spectra of BPhT molecules with the spectra at 210 s of laser irradiation. The colored areas highlight four newly emerging vibrational modes observed in the blinking event. (b) Calculated SERS spectra for different hotspot positions relative to the molecule, which reproduce the specific vibrational modes observed in the experiment as shown in (a). The diameter of a hotspot for the simulations is 3.5 Å. (c) Schematic views of the locations of the hotspots (red dots) relative to the atomic positions in the molecule, leading to the corresponding simulations shown in (b).

Fig. 4. (a) Geometry of the NPoM structure used for simulating the optical response. (b) Near-field enhancement with and without the atomic protrusion, with the white circle highlighting the subnanometer localization of the optical field in the presence of the atomic protrusion. (c) Simulated dark-field scattering spectra for NPoM constructs with or without the atomic protrusion. (d) Dark-field scattering spectra of a representative NPoM construct, recorded before and after 15 min of laser irradiation.