Enhanced Raman scattering with one monolayer of silver

Adatoms, clusters, and complexes have been associated with the active sites that enable the short-range mechanism of surface-enhanced Raman scattering (SERS). To clarify this relationship we have studied the interaction between pyridine and a monolayer (ML) of Ag on a Pt substrate as a function of applied potential and solution composition. Our electromagnetic calculations show only small optical fields exist at the 1-ML Ag--Pt substrate surface which are incapable of supporting the electromagnetic enhancement of SERS. This confirms our earlier report that enhancement for this system arises from an electronic resonant Raman process in the surface complex. The growth of the Ag ML was performed in a thin-layer solution-exchange electrochemical cell. The cell allowed removal of the Ag⁺ solution and introduction of a pyridine and Cl^- solution while maintaining a constant applied potential. Results reported here show that no SERS exists for pyridine adsorbed on the pre-prepared Ag ML. Electrochemical oxidation and reduction of the ML in the presence of pyridine and Cl^- is necessary before a signal is observed. This process creates Cl^--stabilized, electron-deficient sites onto which pyridine is bound. These Ag⁺ sites on the Pt substrate are analogous to the Ag₄{}⁺$ pyramidal clusters which have been discovered as SERS-active sites on bulk Ag substrates.