The Band Gap of Graphene Is Efficiently Tuned by Monovalent Ions

Small monovalent ions are able to polarize carbonaceous nanostructures significantly. We report a systematic investigation of how monovalent and divalent ions influence valence electronic structure of graphene. Pure density functional theory is employed to compute electronic energy levels. We show that the lowest unoccupied molecular orbital (LUMO) of an alkali ion (Li+, Na+) fits between the highest occupied molecular orbital (HOMO) and LUMO of graphene, in such a way as to tune the bottom of the conduction band (i.e., band gap). In turn, Mg2+ shares its orbitals with graphene. The corresponding binding energy is ca. 4 times higher than that in the case of alkali ions. The reported insights provide inspiration for engineering electrical properties of the graphene-containing systems.