Characterization of SiO2-overcoated silver-island films as substrates for surface-enhanced Raman scattering

Vapor deposition of a 45-Angstrom layer of silver and a subsequent 50-65-Angstrom layer of SiO2 onto a glass slide is shown to be a viable substrate for surface-enhanced Raman spectroscopy (SERS) and a model of the silica surface suitable for adsorption kinetic studies. The chemistry of the silica is characterized by UV absorption and contact angle measurements. The physical structure is probed by atomic force microscopy to compare the surface roughness of the silver-island film and its SiO2 overcoat. Auger electron spectroscopy and X-ray photoelectron spectroscopy are used to probe the silica layer for exposure of silver through gaps in the overcoat. Comparison of SERS of dibenzyl disulfide bound to a silver-island film versus that of a silver film protected by the SiO2 overlayer tests the continuity of the SiO2 film and its ability to block access of adsorbates to the underlying silver surface. The model silica surface is used to detect the Raman spectrum of adsorbed pyridine and its adsorption isotherm; the adsorbate vibrational frequencies are consistent with previous reports of pyridine on silica.