Ab initio structural dynamics of pure and nitrogen-containing amorphous carbon

Amorphous carbon (a-C) has attracted considerable interest due to its desirable properties, which are strongly dependent on its structure, density and impurities. Using ab initio molecular dynamics simulations we show that the sp(2)/sp(3) content and underlying structural order of a-C produced via liquid quenching evolve at high temperatures and pressures on sub-nanosecond timescales. Graphite-like densities (less than or similar to 2.7 g/cc) favor the formation of layered arrangements characterized by sp(2) disordered bonding resembling recently synthesized monolayer amorphous carbon (MAC), while at diamond-like densities (greater than or similar to 3.3 g/cc) the resulting structures are dominated by disordered tetrahedral sp(3) hybridization typical of diamond-like amorphous carbon (DLC). At intermediate densities the system is a highly compressible mixture of coexisting sp(2) and sp(3) regions that continue to segregate over 10's of picoseconds. The addition of nitrogen (20.3%) (a-CN) generates major system features similar with those of a-C, but has the unexpected effect of reinforcing the thermodynamically disfavored carbon structural motifs at low and high densities, while inhibiting phase separation in the intermediate region. At the same time, no nitrogen elimination from the carbon framework is observed above similar or equal to 2.8 g/cc, suggesting that nitrogen impurities are likely to remain embedded in the carbon structures during fast temperature quenches at high pressures.