Raman spectroscopy is an important and widely used characterization tool of different carbon containing materials, including nano- (NCD) and ultra-nanocrystalline diamond (UNCD) thin films. The spectrum of the diamond layers excited by visible light consists of up to five bands assigned to diamond peak, G and D bands of inter-granular amorphous carbon phase and to vibrations of trans-polyacetylene formed on the grain surfaces. While similar features would be expected in the Raman spectra of these materials excited using different excitation wavelengths (with some differences in the intensity ratios and positions), our near infrared excited micro-Raman spectroscopic investigations on series of NCD samples with grain sizes above 50 nm showed that a number of narrow well separable new bands appear in the spectrum. Beside the grain size the appearance of these peaks was found to be dependent on the excitation spot size too: the bands were observable when only a few grains were falling into the excitation volume.

The small width of the bands suggests that they are related to vibrations of well defined, probably molecular structural units of the films, located on the diamond crystallite surface (grain boundary). The peaks may vary from point to point on the sample surface, but the statistical analysis performed on a series of spectra measured in different points of the surface showed that there are characteristic peaks for a given sample, being present in most of the spectra.

For UNCD thin films with grain sizes below 50 nm the excitation volume should be lowered to a few nanometers in order to observe the narrow Raman peaks. This was realized by Surface-enhanced Raman spectroscopy (SERS), where colloidal gold nanoparticles (10 nm in diameter) were placed on the UNCD layer surface and the SERS spectra were recorded from the few nanometer surrounding of the Au particles.

This work was supported by Bolyai Janos Research Scholarship of the Hungarian Academy of Sciences.

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1. As_nS_m Nanoclusters in the Structure of Amorphous Materials: Ab Initio and Raman Study