ABSTRACT

This article reviews some spectroscopic techniques which allow to characterize in situ silver and gold substrates used in surface enhanced Raman scattering (SERS) experiments. The investigated substrates are colloids and roughened electrodes. In order to understand more deeply the SERS mechanisms in connection with the morphology of the substrates, we developed some models for simulating the experimental spectra and approaching the typical dimension of the localized electromagnetic surface resonances or plasmon polaritons. The first technique investigated is the low frequency Raman spectroscopy for the mechanical vibrations of the colloidal particles or surface protrusions. Using a model which describes these roughness as small (hemi)spheroids, we suggest that the objects coming into resonance are of a few nanometers size. Ultraviolet-visible spectroscopy is also a useful technique since the strong extinction bands exhibited by silver and gold colloids are assigned to plasmon polaritons. Thus, the simulation of these spectra enables to approach the size and shape of the various clusters appearing in silver and gold aggregated colloids. The objects found in major part are prolate spheroids of a few tenth nanometers semi-major axis. We observed that strong SERS spectra of organic molecules, like benzoic acid, can be easily obtained onto silver islands chemically deposited on aluminum, copper and zinc plates. For such substrates, differential reflectivity spectroscopy (DRS) is a suitable technique to observe in situ the surface plasmon resonances arising from silver islands. A simple model of reflectivity then enables us to suggest that increasing the amount of deposited silver leads to a growing of the silver islands rather than producing a greater number of islands. The islands size was found to be a few hundred nanometers in agreement with results obtained from atomic force microscopy (AFM) experiments. The comparison between experimental and calculated data, obtained from various spectroscopic techniques, demonstrates that SERS effect can be observed for roughness size ranging over at least three orders of magnitude. This latter result emphasizes the important role played by the fractal nature of the various substrates on SERS intensity.