Controlling the electronic structure of graphene using surface-adsorbate interactions

We show that strong coupling between graphene and the substrate is mitigated when 0.8 monolayer of Na is adsorbed and consolidated on top graphene-on-Ni(111). Specifically, the {\pi} state is partially restored near the K-point and the energy gap between the {\pi} and {\pi}* states reduced to 1.3 eV after adsorption, as measured by angle-resolved photoemission spectroscopy. We show that this change is not caused by intercalation of Na to underneath graphene but it is caused by an electronic coupling between Na on top and graphene. We show further that graphene can be decoupled to a much higher extent when Na is intercalated to underneath graphene. After intercalation, the energy gap between the {\pi} and {\pi}* states is reduced to 0 eV and these states are identical as in freestanding and n-doped graphene. We conclude thus that two mechanisms of decoupling exist: a strong decoupling through intercalation, which is the same as one found using noble metals, and a weak decoupling caused by electronic interaction with the adsorbate on top.