Ab-initio Dynamics of the sp²/sp³ Structural Evolution in Amorphous Carbon (a-C) and (a-CN)

Amorphous carbon (a-C) has attracted considerable interest due to its desirable properties and to understand its complex structure that varies with density and number of impurities. Here we show using ab initio molecular dynamics simulations that the sp² and sp³ content of amorphous carbon, which defines its local structure, evolves over 10s of ps at 3000 K. At lower densities of 2.7 and 2.3 gcc a-C transforms into a layered graphitic structure with a large amount of sp² bonding >90 %. At higher densities of 3.7 and 3.3 gcc the sp³ content steadily increases over time. The simulations show that the structure of a-C is dependent not only on pressure and temperature but also time, i.e. the kinetics associated with its structural evolution. Amorphous carbon with 20.3 % nitrogen were found to increase the specific volume compared to pure carbon and to slightly hinder the kinetics for the transformation to a layered graphitic structure. Overall, the simulations provide insight into the structural dynamics of a-C and phase transformation processes of carbon.