Density-functional theory study of the microstructure, electronic structure, and optical properties of amorphous carbon

The electronic structures of amorphous carbon at densities of 2.0, 2.3, 2.6, 2.9, and 3.2 g cm3 have been calculated using density-functional theory with different simulation parameters. The carbon networks were generated by liquid-quench method using Car-Parrinello molecular dynamics. The s p3 fraction and the radial distribution function are in good agreement with experimental results. At a density of 3.2 g cm3, the highest s p3, content of the simulated carbon networks reaches 89%, which is the upper limit s p3 content of the ta-C films deposited by the filtered cathodic vacuum one technique. The optical band gap increases with density (or s p3 fraction) of the networks. The defect density of states at Fermi level can significantly reduce the optical band gap. The small rings with geometries very different from ideal s p3 bonding cause the strong distortion of σ bond angle, which decrease the splitting of σ- σ*. The distortion of π and σ bond angle can decrease the π- π* splitting. Such structural distortions can increase the states near Fermi level and reduce the optical gap.