Investigation of hydrocarbon-plasma-generated carbon films by electron-energy-loss spectroscopy

The bonding in hard, hydrogenated, amorphous carbon films (a-C:H) prepared by plasma decomposition of benzene was investigated by high-resolution electron-energy-loss spectroscopy. In all asgrown films we find that one-third of the carbon atoms are trigonally bonded and two-thirds of the carbon atoms are tetrahedrally bonded. For the trigonally bonded part, the dielectric properties reveal a transformation from polymeric state for samples prepared with low ion energy to a more graphitic state for films prepared with higher ion energy. Upon annealing, the number of carbon atoms in sp² configuration increases. The graphitization of the films is observed in two steps at 200^∘{}C and at 400^∘{}C. A closing of the optical gap and a delocalization of the π electrons is observed between 400^∘{}C and 600^∘{}C.