Crossover in the adsorption properties of alkali metals on graphene

The adsorption of alkali metals (AMs) on single layer graphene is studied using first principles methods. We observe a common trend in the binding distance, the charge transfer, and the work function $(W)$ at certain coverage of AMs with increase in the proportion ρ (adatom/C atom) of the graphene covered by the AM. A dip in these properties occurs at ρ≈0.04 for all AMs except Li, for which it occurs at ρ≈0.08. This behavior is due to a transition of adsorbed metals from individual atoms to two-dimensional metallic sheets that exert a depolarization effect. W of graphene exhibits asymmetric dependence on ρ: a dip in the adatom layer side but saturation on the graphene side, which is in contrast to the case of bulk graphite.