π bonding versus electronic-defect generation: An examination of band-gap properties in amorphous carbon

The electronic properties of amorphous carbon structures with varying microscopic mass densities, ranging from 2.0 to 3.52 g/cm³, are analyzed. Using a semiempirical density-functional approach the model structures were generated by molecular dynamics performing a simulated cooling of liquid carbon clusters, containing 128 atoms, within periodically arranged cubic supercells. By investigation of properties related to chemical bonding we find a strong control of the band gap by the balance between {π} bonding and electronic defect generation. As important sources that influence the {π}-π* band gap the distance distribution between non-{σ} bonded hybrid orbitals as well as the topological properties of clustering sp² (or sp) units are discussed. A number of local densities of states are calculated at representative atoms to validate the classification of {π} clusters and electronic defect states used.