Hunting for the Active Sites of Surface-Enhanced Raman Scattering:  A New Strategy Based on Single Silver Particles

Abstract

Three major findings are reported here. First, a new technique based on lithographic modification with a scanning tunneling microscope (STM) has been developed for fabricating single Ag particles on atomically flat Au(111) surfaces. Second, surface-enhanced Raman scattering (SERS) from one isolated Ag particle has been observed and quantified for the first time. The enhancement factor due to one Ag particle of about 1 μm diameter is on the order of 10 4 . This factor includes contributions from all forms of chemical enhancement, which are smaller than 10 2 . Finally, strong evidence suggests the importance of clusters containing closely-spaced particles. Extra enhancement due to clusters of randomly distributed Ag particles is around 10 2 . We believe that the extra enhancement is spatially localized near the gap sites between two particles in proximity. These gap sites form an important class of SERS-active sites.