Specific statistical features of surface enhanced Raman scattering (SERS) spectra of graphite

A surface enhanced Raman scattering (SERS) study was carried out on highly oriented pyrolitic graphite (HOPG). In a typical SERS experiment the investigated material is in the liquid phase. For studying a solid sample, such as graphite, we put colloidal silver droplets on its surface, and let them dry. The dried colloid drops provide a strongly inhomogeneous lateral distribution of silver clusters. The pattern can be described as domains with different enhancement levels and hot spots. The increased SERS intensity was accompanied by increased spectral resolution, but with some unusual fluctuations. The SERS process on graphite seems to be an unstable one. The fluctuations suggest that the graphite Raman spectrum is inhomogeneously broadened whereby some components are selected and enhanced more strongly than the rest of the spectrum. These enhanced spectral bands are selected by some specific interaction between one of the metal clusters and the carbon surface structure. It is hard to determine at present whether these spectral components are constant, attached to certain frequencies, corresponding to definite structural units; or are distributed continuously in frequency. The physical process behind the temporal fluctuations is probably the structural variability in the silver clusters under intense illumination.