THE EFFECT OF UNDERPOTENTIALLY DEPOSITED LEAD THIN FILMS ON SURFACE ENHANCED RAMAN SCATTERING AT SILVER ELECTRODES.

Abstract

This dissertation details the effect of underpotentially deposited (UPD) Pb on the surface enhanced Raman scattering (SERS) ability of roughened polycrystalline Ag electrodes. The deposition of monolayer and submonolayer amounts of Pb results in a quenching of the SERS response for pyridine and Cl⁻ adsorbed at Ag electrodes. Various factors which may contribute to the loss of SERS intensity are investigated. The most significant factors include changes in surface roughness features brought about by Pb UPD, changes in surface electronic properties of Pb-modified Ag and changes in a chemical contribution to surface enhancement. Possible changes in surface roughness properties of the Ag electrode due to Pb deposition are examined using scanning electron microscopy (SEM) and SERS reversibility studies. SEMs of roughened Ag electrodes before and after Pb monolayer deposition show no significant change in the morphology of the larger roughness features. However, the deposition and stripping of 60 - 70% of a Pb monolayer results in a loss of ca. 50% of the original SERS intensity for both adsorbate bands. This irreversible loss of SERS intensity is attributed to the destruction of atomic scale roughness (ASR). These results suggest that ca. 50% of the observed SERS response arises from a mechanism involving ASR. In addition, the destruction of ASR is shown to be largely responsible for the quenching of SERS at higher Pb coverages. The morphology of the SERS quenching profiles at lower Pb coverages for pyridine and Cl⁻ varies as a function of excitation wavelength. Experimental quenching profiles are compared with theoretical quenching profiles based on an electromagnetic contribution to SERS. Theoretical quenching profiles are calculated using a model for electromagnetic enhancement at a overlayer-covered ellipsoids proposed by Murray. The experimental results for both adsorbates are in agreement with the theoretical predictions for laser excitation in the blue. Experimental results in the green and red wavelength regions are best explained in terms of photoassisted charge-transfer mechanisms for surface enhancement.