Gated adatoms on graphene studied with first-principles calculations

A first-principles pseudopotential density functional method for studying the gating of adatoms on graphene is presented. A variation in gate voltage is assumed to vary the number of electrons in the adatom-graphene system. The method is applied to the cases of Li and Co on graphene. The projected density of states, charge density, and local electrostatic potential are computed as a function of gate voltage. In the case of Li, the calculations show that the Li adatom can be ionized by changing the gate voltage, and that the ionization is accompanied by a sharp change in the electrotstatic potential of the adatom. In the case of Co, correlation in the 3d shell is treated using the LDA+U method, with several values of the U parameter considered. For U = 2 eV or greater, an ionization effect analogous to the case of Li is found for the Co adatom. This result is consistent with recent scanning tunneling spectroscopy experiments for Co on graphene.