Ab initio studies of amorphous carbon films

Amorphous carbon films can be prepared with a range of sp(3) fractions, ranging from pure sp(2) to approximately 80% sp(3) (tetrahedral amorphous carbon). In this paper we study the bonding and energetics of structures with different sp(3) ratios using ab-initio Car-Parrinelto 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 sp(3) films will either decrease their sp(3) fraction and generate stress or increase their sp fraction and relieve stress. On the other hand, high sp(3) films retain their high sp(3) 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. Warmier functions also reveal unstable bonding configurations, which convert atoms from sp(3) bonded to sp(2) bonded and vice versa. Such unstable bonding sites may be important to understanding transport properties in these networks.