Surface Enhanced Raman Spectroscopy: Towards Single Molecule Spectroscopy

Abstract

The surface enhancement of the Raman scattering (SERS) is based on two major mechanisms. First, the optical resonance of the incident light with the metallic surface leads to the excitation of localized surface plasmons and the enhancement of the local electromagnetic field. Second, optical resonances of the energy levels between the adsorbate orbitals and the metal electrons may result in a resonant charge transfer, which causes the resonance Raman or charge transfer enhancement. For dyes at aggregated colloids, the total differential cross sections can reach values of about (dσ/dω) ∼10-16 cm2 sr-1, which are comparable with cross sections for the fluorescence of laser dyes in solution. In contrast, Raman spectroscopy at surfaces that do not support surface plasmon excitation (smooth surfaces or transition metal surfaces) is based solely on chemical or resonance Raman enhancement, yielding only weak signals. After reviewing recent results on SERS, we present an approach that combines the Raman spectroscopy at smooth surfaces with the local electromagnetic field enhancement by an optically active Ag STM tip. The high local enhancement of the Raman scattering cross section in the vicinity of the tip opens promising avenues towards single molecule Raman spectroscopy.