Investigation of carbon aggregates (onions) formed on copper under the conditions of diamond chemical vapour deposition

We report further observations of graphitic carbon onions and polyhedra tha t are formed together with graphitic planes under the conditions of the che mical vapour deposition of diamond as previously described by Constant et a l. (1997, Surf: Sci., 387, 28). The occurrence of such carbon onions is qui te specific to the copper substrate. They were observed with various pre-tr eatments of the substrate. It is suggested that both the nucleation and the growth of such carbon onions occur on the copper surface rather than in th e gas phase. It is believed that, instead of high-energy electron or ion be ams as described elsewhere by Banhart (1997, J. appl. Phys., 81, 3440), the formation of carbon onions is due to the high reactivity of the atomic hyd rogen present in the gas phase. These hydrogen radicals induce a curvature of the graphitic planes by preferential reactivity on the corner and edge s ites. A dense phase in the core of few carbon onions was sometimes seen. We then measured an interplanar distance down to 0.24 nm, much lower than the distance between two graphitic sheets (0.34 nm). It is found that this dar k phase is enriched with tungsten originating from the heated filaments.