Field effects on the electronic and spin properties of undoped and doped graphene nanodots

We report a spin-polarized density-functional theory study of electric-field effects on the electronic and spin properties of graphene nanodots. Both undoped and graphene nanodots doped with nitrogen and boron are considered. In the presence of nonlocal exchange-correlation interactions, undoped graphene nanodots are found to be half-semiconductors when a weak electric field is applied across the zigzag edge. At high electric fields these graphene nanodots become nonmagnetic semiconductors. When the electric field is applied across the armchair edge, these graphene nanodots maintain an antiferromagnetic ground state with the energy gap strongly dependent on the magnitude of the electric field. For graphene nanodots doped with nitrogen or boron we find that energetically the most favorable state among all possible configurations is the one in which the dopant replaces the carbon atom at the center of the zigzag edge. The substitutional dopant atom at the zigzag edge leads to a spin-polarized half-semiconducting state in which the spin degeneracy is broken. The spin-dependent energy gaps can be tuned within a wide range by applying electric fields. In addition, we find the half-semiconducting state under doping occurs even when the electric field is very strong. This indicates that edge doping can significantly widen the operating range of applied electric fields for spintronic applications because undoped graphene nanodots become spinless semiconductors under certain applied electric fields.