Resolving Single Molecules in Surface-Enhanced Raman Scattering within the Inhomogeneous Broadening of Raman Peaks

We demonstrate both the observation of either a single or a few molecules resolved within the inhomogeneous broadening of a peak in surface-enhanced raman scattering (SERS). Our results demonstrate a fundamental aspect of spectroscopy and also a possible technique to learn more about the varying interactions that single molecules can have with a given SERS substrate. Resolving more than one molecule within the inhomogeneous broadening is only possible thanks to the combination of (i) high-resolution measurements, and (ii) low temperatures (to narrow down the intrinsic homogeneous broadening as much as possible). Besides being a textbook-like example of laser spectroscopy, this result provides yet another confirmation of single molecule sensitivity in SERS. We show specific experimental examples for these effects in single molecule SERS spectra of the molecules nile blue (NB) and rhodamine 800 (RH800). The possible physical origins of the fluctuations in terms of (i) interactions with the substrate, (ii) isotopic effects, or (iii) instrumental contributions, are explained and discussed.