A Frequency Domain Existence Proof of Single-Molecule Surface-Enhanced Raman Spectroscopy

The existence of single-molecule surface-enhanced Raman spectroscopy (SMSERS) is proven by employing a frequency-domain approach. This is demonstrated using two isotopologues of Rhodamine 6G that offer unique vibrational signatures. When an average of one molecule was adsorbed per silver nanoparticle, only one isotopologue was typically observed under dry N2 environment. Additionally, the distribution of vibrational frequencies hidden under the ensemble average is revealed by examining the single-molecule spectra. Correlation with transmission electron microscopy reveals that SMSERS active aggregates are composed of multiple randomly sized and shaped nanoparticles. At higher coverage and in a humid environment, adsorbate interchange was detected. Using 2D cross correlation, vibrational modes from different isotopologues were anti-correlated, indicating that the dynamic behavior was from multiple molecules competing for a single hot spot. This allows hot-spot diffusion to be directly observed without analyzing the peak intensity fluctuations.