Graphene on the Ir(111) surface: from van der Waals to strong bonding

We calculated the properties of a graphene monolayer on the Ir(111) surface, using the model in which the periodicities of the two structures are assumed equal, instead of the observed slight mismatch which leads to a large superperiodic unit cell. We used the density functional theory approach supplemented with the recently developed van der Waals-density function (vdW-DF) non-local correlation functional. The latter is essential for treating the vdW interaction, which is crucial for the adsorption distances and energies of the rather weakly bound graphene. When additional iridium atoms are put on top of graphene, the electronic structure of C atoms acquires the sp(3) character and strong bonds with the iridium atoms are formed. We discuss the validity of the approximations used and their relevance to other graphene-metal systems.