Electronic structure of Sp2 trap states in amorphous carbon

Amorphous carbon, a-C : H, contains an unusually high number of defect states, typically 10(19)/cm(3). These states play a major role in the process of field electron emission from a-C : H since they inhibit (by trapping) the transport of electrons from the cathode to the amorphous carbon film/vacuum interface. In this paper we examine the electronic structure of two of these defects, the pentagon-heptagon pair and a broken carbon double bond (by the addition of one hydrogen at one of the two carbon atoms). These defects are expected to be found in a graphitic environment in a-C : H. To obtain their electronic structure, we construct large enough graphitic clusters, embed the defects at their center and then use typical semiempirical quantum chemical methods such as the PM3 method - which is a parameterized Hartree-Fock method - to calculate the energy levels in the gap and the binding energies. Both defects produce energy levels just above the highest occupied molecular orbital (HOMO) of the graphite levels, but the pentagon-heptagon pair has a much higher binding energy, making it more stable. From the difference in binding energy, we conclude that the pentagon-heptagon pair is far more stable. Copyright (C) 2007 John Wiley & Sons, Ltd.