Density functional theory (DFT) study on the effects of Li+ doping on electronic states of graphene

The electronic structures of a lithium ion (Li+) doped-graphene at the ground and low-lying excited states have been investigated by means of density functional theory (DFT) method. A graphene composed of 19 benzene rings was used as a model of graphene, while the edge carbon atom was terminated by hydrogen atom (expressed by C54H18). The geometry optimization showed that the Li+ ion binds to a hexagonal site where six carbon atoms interact equivalently to the Li+ ion. When the Li+ ion interacts with the graphene surface, the electronic configuration of the Li+ ion is changed from (1s)2(2s)0 to (1s)2(2s)0.01(2p)0(3p)0.02, suggesting that the sp-hybridization of lithium ion is important in the adsorption to the graphene surface. The band gap of graphene is slightly red-shifted by the doping of Li+ ion due to the interaction with the sp-hybrid orbital. The effects of Li+ on both the ground and excited electronic states of graphene were discussed on the basis of theoretical results.