Grain boundaries and grain size distributions in nanocrystalline diamond films derived from fullerene precursors

Diamond films produced by traditional CH4/H-2 methodologies usually display a columnar growth of micron-sized crystallites. Such films have been extensively characterized by a wide variety of techniques, and their microstructures have been studied in detail. By contrast, nanocrystalline diamond films have been much less investigated, perhaps because they are often referred to in the literature as ''poor quality'' diamond. The recent development of film growth from C-60/Ar mixtures may begin to change this situation in that this methodology results in very pure diamond, as demonstrated by the work discussed here and in earlier papers [1-3]. Diamond films grown using C-60 as a carbon source have been shown to be nanocrystalline with average grain sizes of 15 nm and standard deviations of 13 nm. The measured grain size distribution for two separate films, each based on measurements of over 400 grains, were found to be very similar and were well-approximated by a gamma distribution. Unlike typical CVD grown diamond films, these nanocrystalline films do not exhibit columnar growth. From the measured grain size distributions, it is estimated that 2% of the carbon atoms are located in the grain boundaries. The structure of the carbon in the grain boundaries is not known, but the films survive extended wear tests and hold together when the substrate is removed, indicating that the grains are strongly bound. The grain boundary carbon may give rise to additional features in the Raman spectrum and result in absorption and scattering of light in the films. We also expect that the grain boundary carbon may affect film properties, such as electrical and thermal conductivity.