Size dependence of structural stability in nanocrystalline diamond

We describe experiments which demonstrate that carbon atoms introduced into a fused-silica substrate by means of MeV ion implantation can, after suitable annealing, form nanocrystalline diamond. Unlike other methods of creating diamond, the coalescence of the carbon into diamond nanocrystals occurs when the samples are heated in a conventional furnace and does not require the application of high external pressures, or any pre-existing diamond template. Following a dose of 5 × 1016 C/cm2 into fused quartz and after annealing in forming gas (4% hydrogen in argon), perfect cubic diamond crystallites of 5-7 nm diameter are formed. For higher doses, the same annealing treatments produce larger crystallites which are comprised of other varieties of solid carbon phases. We conclude that diamond is the stable form of carbon provided that the crystallite size is sufficiently small (less than 7 nm) and that the nanocrystallites are appropriately surface passivated.