Bulk diffusion induced structural modifications of carbon-transition metal nanocomposite films

The influence of transition metal (TM=V,Co,Cu) type on the bulk diffusion induced structural changes in carbon:TM nanocomposite films is investigated. The TMs have been incorporated into the carbon matrix via ion beam co-sputtering, and subsequently the films have been vacuum annealed in the temperature range of 300-700°C. The structure of both the dispersed metal rich and the carbon matrix phases has been determined by a combination of elastic recoil detection analysis, X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The as-grown films consist of carbidic (V and Co) and metallic (Cu) nanoparticles dispersed in the carbon matrix. Thermal annealing induces surface segregation of Co and Cu starting at >=500°C, preceded by carbide-metal transformation of Co-carbide nanoparticles at ~300°C. No considerable morphological changes occur in C:V films. In contrast to the surface diffusion dominated regime where all the metals enhance the six-fold ring clustering of C, in the bulk diffusion controlled regime only Co acts as a catalyst for the carbon graphitization. The results are consistent with the metal-mediated crystallization mechanism in the C:Co films. The results are discussed on the basis of the metal-carbide phase stability, carbon solubility in metals or their carbides and interface species.