SOLVENT-INDUCED MORPHOLOGY CHANGES IN THIN SILVER FILMS

Thin Ag films supported on dielectric materials have been extremely popular as surface-enhanced Raman scattering (SERS) substrates. In an effort to characterize SERS-active films under the conditions of typical SERS experiments, we have extended our previous work to elucidate the effect of solvent exposure on thin Ag film optical properties and surface morphology. When thin Ag films are dipped in a solvent, a common method for analyte application, the optical extinction spectra blue-shift more than 20 nm with a significant reduction in optical density. Over a time period of 60 min, the optical spectra gradually shift back toward the original pre-dipped profiles, but do not return completely. While it is known that the solvent modifies the local dielectric, the effect of solvent on the Ag surface morphology has only briefly been addressed. In this study, atomic absorbance spectrometry was employed to rule out significant loss of Ag from the substrate during the dipping process. A numerical algorithm was developed for automated analysis of atomic force microscopy images. With this algorithm, for a typical 1 mu m X 1 mu m scan, approximately 2000 Ag islands can be evaluated for height, radius, and the distribution in both these values. In addition, radial distribution functions are easily generated. Based on an atomic force microscopic investigation of Ag films before and after solvent exposure, the dominant morphological change in Ag islands appears to be an overall increase in aspect ratio. This increase in aspect ratio is consistent, according to the electromagnetic theory, with observed trends in the optical spectra.