The electronic band-gap properties of high-density, highly tetrahedrally bonded, hydrogenated amorphous carbon are related to the size and overlap distribution of small π-bonded clusters embedded within a strained rigid ${\mathit{sp}}^3$ matrix of atomic-scale models generated by semiempirical density-functional molecular dynamics. Compared to the hydrogen-free analogues of similar density and chemical composition, the residual strain in the network is reduced. As a consequence the overlap of p orbitals between undercoordinated sites is enforced in favor of forming strong π bonds, which is consistent with a further band-gap opening relative to the hydrogen-free structures.

• Received 6 May 1994

DOI:https://doi.org/10.1103/PhysRevB.50.7940