Specific statistical features of surface enhanced Raman scattering (SERS) spectra of graphite I. P ocsik * , M. Veres, M. F€ ule, S. T oth, M. Ko os Research Institute for Solid State Physics and Optics, P.O. Box 49, H-1525 Budapest, Hungary Available online 7 May 2004 Abstract 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. Ó 2004 Elsevier B.V. All rights reserved. PACS: 81.70.)g 1. Introduction Surface enhanced Raman scattering (SERS) is an effective tool to enhance the Raman cross section, and through this enhancement the sensitivity of Raman spectroscopy is increased by up to 14 orders of magni- tude. The phenomenon takes place in the close vicinity of electrochemically roughened surfaces of various metals (typically silver and gold), or their colloids [1–10]. The technique is mostly used to study materials in the liquid phase. In the present case, we will use SERS to study HOPG. To realize this, silver islands films can be deposited on the carbon surface or silver colloid can be dried onto it. Island films are less effective, because of the relatively larger and fewer silver clusters. Raman scattering is one of the most frequently used analysis methods to study carbonaceous material. Gra- phitic materials are usually characterized by broad two Gaussian component spectra [11]. The graphitic, or G mode (the vibration normal mode of the two-dimen- sional hexagonal graphite lattice) occurs around 1582 cm À1 , and the excitation wavelength dependent, disor- der related D mode, occurs in the 1280–1400 cm À1 range. This latter mode is excited over the 1000–250 nm laser wavelength range [12]. Amorphous carbon mate- rials are frequently characterized by the intensity ratio of these two bands [13], however spectra excited by the 1064 nm wavelength infrared light of YAG lasers shows an additional component also [14], similarly to the Ra- man spectra for the diamond nucleation and growth [15]. The action radius of the SERS effect was found to be in the range of atomic distances, by covering the roughened silver surface, i.e., the SERS active medium, with spacer layers, and determining the enhancement factor as function of the spacer thickness [2]. Utilizing this short action radius we expect to be able to enhance characteristic vibration modes of the different local structures. This small action radius and large enhance- ment factor makes SERS able to provide spectra of single molecule-like structures, like carbon nanotubes * Corresponding author. Tel.: +36-1 392 2763; fax: +36-1 392 2215. E-mail address: pocsik@szfki.hu (I. P ocsik). 0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.03.027 Journal of Non-Crystalline Solids 338–340 (2004) 496–498 www.elsevier.com/locate/jnoncrysol