New approaches in the characterization of surface‐enhanced Raman scattering‐active substrates

Abstract

This paper develops new approaches to characterize the morphology of silver and gold colloids used as substrates underlying surface-enhanced Raman scattering (SERS). Since both electromagnetic and chemical SERS theories predict that the Raman enhancement depends strongly on surface roughness features, a better understanding of the SERS spectra and an optimization of the enhancement factor require the knowledge of the size and shape distributions of the particles and clusters of the metal sols. For this purpose, we have investigated two in situ techniques. The first deals with the mechanical vibration band (also called acoustic mode) of the metallic particles of the colloidal sols; this band can be observed at very low Raman wavenumber shifts (approximately 10 cm-1) provide it is enhanced by surface plasmon resonance excitation. The experiments on these acoustic modes were carried out on silver and gold colloids, at several laser excitation wavelengths and in the presence of various polarizable molecules (pyridine, acridine, benzoic acid) or halide ions. Since the wavenumber and the shape of these acoustic bands are related to the morphology of the colloidal particles, a previously developed model was used which permits the calculation of the acoustic mode profiles, to approach the size and shape distributions of the roughness features giving rise to the SERS enhancement. Since extinction, scattering and absorption bands of metal colloids arise from surface plasmon resonance excitation, UV–visible spectroscopy is also a powerful tool for analyzing the optical properties of colloids. From the observed extinction profiles of silver and gold colloids, aggregated with the same probe molecules as those used in the low Raman wavenumber scattering experiments, attempts were made to obtain the size and/or the shape distributions of the various aggregates. The comparison between the distributions deduced from both of the above-mentioned methods along with those obtained by transmission electron microscopy (TEM) suggests that the particles or clusters at the origin of the SERS effect are sized over several orders of magnitude. It is therefore concluded that the SERS effect arises mainly from the resonance of plasmon eigenmodes of small protrusions coupled over regions smaller than the visible wavelength. © 1998 John Wiley & Sons, Ltd.