Surface enhanced Raman spectroscopy: Towards single molecular spectroscopy

The surface enhancement of the Raman scattering (SERS) is based on two majo r mechanisms. First, the optical resonance of the incident light with the m etallic surface leads to the excitation of localized surface plasmons and t he enhancement of the local electromagnetic field. Second, optical resonanc es of the energy levels between the adsorbate orbitals and the metal electr ons may result in a resonant charge transfer, which causes the resonance Ra man or charge transfer enhancement. For dyes at aggregated colloids, the to tal differential cross sections can reach values of about (d sigma /d Omega ) similar to 10(-16) cm(2) sr(-1), which are comparable with cross sections for the fluorescence of laser dyes in solution. In contrast, Raman spectro scopy at surfaces that do not support surface plasmon excitation (smooth su rfaces or transition metal surfaces) is based solely on chemical or resonan ce Raman enhancement, yielding only weak signals. After reviewing recent re sults on SERS, we present an approach that combines the Raman spectroscopy at smooth surfaces with the local electromagnetic field enhancement by an o ptically active Ag STM tip. The high local enhancement of the Raman scatter ing cross section in the vicinity of the tip opens promising avenues toward s single molecule Raman spectroscopy.