Ab initio simulations of the structure of amorphous carbon

The structure of amorphous carbon at densities of 2.0, 2.6, 2.9, and 3.2 g/cm³ has been investigated using Car-Parrinello ab initio molecular dynamics. The networks were generated by rapidly cooling samples of 125 atoms to form a solid amorphous structure. The important structural attributes of each network were found to be in good agreement with experimental observations. In particular, the fraction of sp² atoms was found to have a linear dependence on the density of the network. The relationship between stress, an important parameter in experimental synthesis, and the structural properties of each network, was also investigated. A nonlinear dependence of stress on sp³ fraction was observed. The electronic properties of each network is also explored with a widening of the band gap and a corresponding increase in the optical gap observed with increasing density (or sp³ fraction) of the network. The simulations also provided qualitative support for the widely used experimental method for determining the sp³ fraction from the carbon core loss edge in electron-energy-loss spectra.