First-principles study of metal adatom adsorption on graphene

The adsorption of 12 different metal adatoms on graphene is studied using first-principles density-functional theory with the generalized gradient approximation. The adsorption energy, geometry, density of states (DOS), dipole moment, and work function of each adatom-graphene system are calculated. For the adatoms studied from groups I--III of the Periodic Table, the results are consistent with ionic bonding, and the adsorption is characterized by minimal change in the graphene electronic states and large charge transfer. For transition, noble, and group IV metals, the calculations are consistent with covalent bonding, and the adsorption is characterized by strong hybridization between adatom and graphene electronic states. For ionically bonded adatoms, the charge transfer is calculated quantitatively using two methods, one based on the DOS and the other based on the real-space-charge density. A variation in dipole moments and work-function shifts across the different adatoms is observed. In particular, the work-function shift shows a general correlation with the induced interfacial dipole of the adatom-graphene system and the ionization potential of the isolated atom.