Simulation of nanodiamond and nanographite formation from molten carbon in the presence of hydrogen

Hydrogen plays a significant role in the formation of nanodiamond, terminating diamond surfaces, and removing $s{p}^{2}$-bonded atoms from the surface during chemical-vapor deposition diamond growth. However, there are only few calculations that simulate nanodiamond development directly and even less that do so in a hydrogen-containing environment. Recently, nanoscale graphitic layers embedded in amorphous carbon were observed experimentally. We report here on results from a comprehensive study of nanodiamond and nanographite formation from molten carbon in the presence of hydrogen under varied conditions of external pressure and cooling rate. We find that hydrogen-free nanodiamond crystals are precipitated more readily at increased melt densities and cooling rates, whereas slower cooling rates permit formation of graphitic layers.