Surface-enhanced and STM-tip-enhanced Raman Spectroscopy at Metal Surfaces

Abstract

The strong electromagnetic field enhancement, occurring at illuminated metal structures with sub-wavelength dimensions, is exploited to develop Raman spectroscopy with exceedingly high sensitivity and lateral resolution, possibly down to single molecule detection. After reviewing Surface Enhanced Raman Spectroscopy (SERS) on single crystalline surfaces and colloids we present recent results on the Tip Enhanced Raman Spectroscopy (TERS) approach, where a metal tip is used as an external enhancing unit. In this way the electromagnetic and chemical surface enhancement are physically separated: the former is confined to the tip, the latter to the metal-adsorbate system. So far, TERS has been reported only for a few molecules exhibiting large Raman cross sections, such as sulphur or dye molecules. Here, we present combined SERS and TERS studies for the CN- ions and TERS for Brilliant Cresyl Blue adsorbed at smooth thin gold films.