Structural trends in amorphous carbon

Amorphous carbon (a-C) structures over a wide range of densities are generated using tight-binding molecular-dynamics simulations. The a-C networks obtained by quenching low-density liquids consist of mostly threefold coordinated atoms. These a-C structures correspond to those obtained experimentally by evaporation of sputtering of graphite. However, the a-C networks generated by quenching high-density liquids are found to be dominated by tetrahedral bonding sites, which resemble the diamondlike a-C films produced by the mass-selected ion beam deposition technique. Our study shows that the shape and position of the first peak of the radial distribution function in a-C are very sensitive to the relative concentration of sp, ${\mathit{sp}}^{2}$, and ${\mathit{sp}}^{3}$ bondings. The peak position shifts towards larger distance as the percentage of the fourfold sites increases, which gives a good indication of the relative population of threefold to fourfold sites in the system.