Scanning-probe Raman spectroscopy with single-molecule sensitivity

Single-molecule vibrational Raman spectroscopy of malachite green adsorbed on planar metal surfaces is achieved by means of optical local-field enhancement provided by a scanning nanoscopic metallic tip. The single-molecule signature is evident from spectral diffusion and a discretization of Raman peak intensities. The optical tip-sample coupling gives rise to a localization of the response down to a sub-10 nm length scale and a Raman enhancement up to similar to 5x10(9). This combines vibrational spectroscopy with high resolution scanning-probe microscopy for ultrasensitive in situ analysis of individual molecules.