Structure and electronic properties of amorphous carbon: from semimetallic to insulating behaviour

Abstract

Correlations between the atomic-scale structure and electronic properties in amorphous carbon and its hydrogenated analogues are analyzed. The metastable amorphous modifications with varying density 2.0–3.5 g/cm 3 and different amount of hydrogen have been generated by density-functional-based molecular dynamics applying different annealing regimes. The atomic-scale structure is characterized with special emphasis on comparing neutron scattering with simulated diffraction data. The global electronic band gap properties are related to the chemical bonding and π-cluster formation. While at low density the π−π ∗ gap closes owing to the large size of π-clusters and the residual strain on the π-system from the rigid bonding environment, the internal strain at high density of 3.0 g/cm 3 is maximally reduced by the separation of smaller π-clusters. In the latter case, the π-bonds optimally relax consistent with the opening of large π−π ∗ gaps up to 3 eV. While the internal strain again increases with further increase in the density, incorporation of hydrogen at 3.0 g/cm 3 additionally supports the removal of internal strain by enforcing two-phase separation tendencies between chemically differently bonded carbon atoms.