Understanding intercalation structures formed under graphene on Ir(111)

Abstract

The coverage-dependent intercalation of oxygen, CO, hydrogen, alkali metals, and halogens between graphene and Ir(111) is investigated using density functional theory with van der Waals corrections. By comparing adsorption on clean Ir to intercalation we show that the presence of the graphene layer shifts the stability of the adsorption structures towards higher coverages, with oxygen as the only exception preferring low-coverage intercalation structures. In general, we find that the preferred adsorption site of the intercalant is important for the stability of intercalation structures, where an atop adsorption site favors higher-coverage structures compared to a hollow adsorption site. Overall, the predicted stable intercalation structures are in good agreement with experimentally observed intercalation structures. We calculate doping levels of intercalated graphene and show that there is a correlation between the amount of charge transfer to or from the graphene sheet and the graphene binding energy, which is an indication for ionic bonding between the graphene sheet and the intercalants. We show that the graphene doping level can be tuned almost continuously between strong $n$-type doping for the alkali metals and strong $p$-type doping for F. Further, we calculate C $1s$ core-level shifts for intercalated graphene and show that these are correlated with the calculated doping level. The obtained values for graphene doping levels and core-level shifts are qualitatively in good agreement with available experimental values, but we find quantitative disagreements of up to 0.3 eV.