STRUCTURAL MODELS OF A-C AND A-C-H

Diamond-like forms of amorphous carbon and hydrogenated amorphous carbon tend to have significant disorder due to ion bombardment during deposition. Their electronic structure is analyzed in the case where the disorder potential has similar size to the energy gained from clustering of sp(2) sites. Disorder is found to greatly reduce the probability of clustering, and single 6-fold rings become the most probable configuration. A variety of configurations are analyzed to account for the relatively low optical band gap. Chair distortions of 6-fold rings or pairs of distorted negatively charged 5-fold and positive 7-fold rings can give gaps as low as 1 eV. It is argued that some form of sp(2) clustering is required to account for the high luminescence efficiency of wide-gap a-C:H.