Surface-enhanced Raman scattering and electron spectroscopic studies of copper and silver surfaces.

Abstract

The overall goal of this research is to investigate the enhancement mechanisms associated with the surface enhanced Raman scattering (SERS) phenomenon. The approach taken in these investigations is to directly compare the SERS abilities of Cu and Ag surfaces using both SERS and electron spectroscopy. One set of experiments has been directed toward studying the contribution of the charge transfer (CT) mechanism to the SERS of pyridine adsorbed on Cu and Ag electrodes. These studies involve modifying the electrodes by depositing submonolayer amounts of Pb which serves to quench the SERS. By using a variety of excitation energies, the resulting quenching profiles track the CT process. Additionally, other electrochemical/SERS experiments have been pursued in order to probe the contribution of the CT mechanism. The second set of experiments involved measuring the optical properties of the Pb-modified Cu and Ag surfaces using electron energy loss spectroscopy in the reflection mode (REELS). Based on electromagnetic (EM) theory, the enhancement is, to a rough approximation, indirectly proportional to ε₂, the imaginary part of the dielectric constant. Thus, the goal of these studies is to determine the changes in ε₂ as a function of Pb coverage in order to determine the contribution of the EM mechanism to the SERS quenching profile. It was necessary to develop a method to determine optical constants from REELS data. A number of pure metals (Al, Cu, Ag, Au, Ti, V, Fe, Co, and Ni) were chosen to test the method. In all cases, our results compare well to the literature values with a relative standard deviation of 20% or less. Having established the method, the next step was to apply it to the Pb-modified Cu and Ag surfaces. Due to instrumental limitations, only semi-quantitative optical constants could be determined. From these values, it was found that the relative changes in the ε₂ values were larger for submonolayer coverages of Pb on Ag compared to Cu. The EM enhancements determined from these results did not predict the SERS-quenching behavior suggesting that other mechanisms must be considered.