Ab initio studies of amorphous carbon films

Amorphous carbon films can be prepared with a range of sp3 fractions, ranging from pure sp2 to approximately 80% sp3 (tetrahedral amorphous carbon). In this paper we study the bonding and energetics of structures with different sp3 ratios using ab-initio Car-Parrinello Molecular Dynamics (CPMD). By taking into account the elastic strain energy that is contained in the structures as formed, we can compare the energetics at the same imposed stress to investigate the effects of annealing on amorphous carbon (a-C) films. We show that in order to minimise total energy, intermediate sp3 films will either decrease their sp3 fraction and generate stress or increase their sp3 fraction and relieve stress. On the other hand, high sp3 films retain their high sp3 fraction following annealing. We also show that Wannier function analysis is useful for examining the bonding in amorphous carbon structures generated by liquid quench methods, resolving difficulties associated with the bonds drawn on a distance criterion alone. Wannier functions also reveal unstable bonding configurations, which convert atoms from sp3 bonded to sp2 bonded and vice versa. Such unstable bonding sites may be important to understanding transport properties in these networks.