Silver-island films as substrates for enhanced Raman scattering: effect of deposition rate on intensity

The relationship between surface-enhanced resonance Raman scattering (SERRS) intensity and the rate of deposition during silver-island film preparation was examined, using zinc tetraphenylporphine (ZnTPP) as the adsorbate. The effect of the deposition rate on the optical properties of the films at specific wavelengths was also analyzed. The data show that the film extinction (the term extinction is used rather than absorption because the spectra have not been corrected for reflection or scattering losses) increases exponentially at 514 and 458 nm as the deposition rate is decreased. SERRS intensities also increase exponentially at these two excitation wavelengths with a decrease in the deposition rate. The optical density is linearly related to the SERRS intensity, and maximal enhancement is observed for films with the greatest extinction at these excitation wavelengths. In contrast, neither the extinction at 406 nm nor the SERRS scattering intensities resulting from excitation at this wavelength differ substantially. The surface-enhanced Raman scattering (SERS) intensity and the electronic spectra of 4,4'-bipyridine (BP) adsorbed onto silver films as a function deposition rate were also examined. The behavior of the nonresonantly enhanced BP is comparable to that of the resonantly enhanced ZnTPP samples. The effects of the surface morphology, as determined from transmission electron micrographs of the films at various deposition rates, on the corresponding electronic spectra and SERS/SERRS spectra are described.