Electronic States of Defect Sites of Graphene Model Compounds: A DFT and Direct Molecular Orbital-Molecular Dynamics Study

Electronic states of normal graphene, the defective graphene (one carbon atom is removed from the normal graphene), the defective graphene anion (defective graphene plus an excess electron), and the defective graphene cation (defective graphene plus one hole) have been investigated by means of density functional theory (DFT) and direct molecular orbital-molecular dynamics (MO-MD) methods in order to elucidate the effect of vacancy defect on the electronic states of graphene. The HOMO and LUMO of normal graphene were widely delocalized as pi-conjugated orbitals over the graphene surface in the normal graphene. On the other hand, the excess electron in defective graphene anion was localized in the defect site, indicating that the excess electron on the graphene circuit is,efficiently trapped and stabilized by the vacancy defect site of graphene. The direct MO-MD calculations showed that the trapped electron in the defect site is stable at low temperature. Around room temperature (300 K), the structural change of the graphene backbone was found and the vacancy defect was reconstructed by thermal activation. The excess electron escaped from the defect site of the reconstructed graphene, while the spin density delocalized the graphene.