Nearly Flat Unoccupied State in Intercalated Graphene: Observation and Interpretation*

By using two-photon-photoemission, we observe a near-zero-dispersion empty state in oxygen-intercalated graphene-Ir interface, approximately 2.6 eV above the Fermi energy and near the Brillouin zone center. DFT method and band-unfolding technique are used to calculate the spectral weights for the graphene-Ir supercells with and without the intercalants. The calculations reproduce the measurements. Our analysis reveals the nature of the observed nearly flat band to be the replicates states from near the Dirac cone that have little dispersion due to trigonal warping. This interpretation is further supported by the results from angle-resolved photoemission. Our work explicitly demonstrates the persisting perturbation on the graphene even for the intercalated quasi-freestanding graphene. Our work demonstrates a pathway for tailoring the graphene electronic structure and generating nearly flat bands by using intercalation.