Charged cluster model in the low pressure synthesis of diamond

Gas activation makes possible the synthesis of diamond without codeposition of graphite in the chemical vapor deposition (CVD) diamond process, though its exact role is not clear. Charged carbon nuclei clusters, which are expected to form in the gas phase, are suggested to be responsible for the formation of diamond. When the carbon cluster is sufficiently small, the capillary pressure built up inside the cluster can be high enough to make diamond more stable than graphite. The number of carbon atoms in the cluster that reverses the stability between diamond and graphite increases sensitively with increasing surface energy ratio of graphite to diamond. The gas activation process produces charges such as electrons and ions, which are energetically strong heterogeneous nucleation sites for the supersaturated carbon vapor, leading to the formation of charged nuclei clusters. Once the carbon clusters are charged, the surface energy of diamond can be reduced by electrocapillarity while that of graphite cannot because diamond is dielectric and graphite is conducting. The evolution of graphitic soot and diamond on the iron and the silicon substrates, respectively, can be approached based on the charged cluster model. The charged cluster model is further supported by the result that the insulating quartz block beneath the iron substrate enhances the initiation of diamond on soot.