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/ cm(3) has been investigated using Car-Parrinello ab initio molecular dynami cs. The networks were generated by rapidly cooling samples of 125 atoms to form a solid amorphous structure. The important structural attributes of ea ch network were found to be in good agreement with experimental observation s. In particular, the fraction of sp(2) atoms was found to have a linear de pendence on the density of the network. The relationship between stress, an important parameter in experimental synthesis, and the structural properti es of each;network, was also investigated. A nonlinear dependence of stress on sp(3) fraction was observed. The electronic properties of each network is also explored with a widening of the band gap and a corresponding increa se in the optical gap observed with increasing density (or sp(3) fraction) of the network. The simulations also provided qualitative support for the w idely used experimental method for determining the sp(3) fraction from the carbon core loss edge in. electron-energy-loss spectra.