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We employ angle-resolved photoemission spectroscopy (ARPES) to investigatethe electronic structures of two rotational variants of epitaxial, single-layergraphene on Ir(111). As grown, the more-abundant R0 variant is nearlycharge-neutral, with strong hybridization between graphene and Ir bands nearthe Fermi level. The graphene Fermi surface and its replicas exactly coincidewith Van Hove singularities in the Ir Fermi surface. Sublattice symmetrybreaking introduces a small gap-inducing potential at the Dirac crossing, whichis revealed by n-doping the graphene using K atoms. The energy gaps betweenmain and replica bands (originating from the moir\'e interference patternbetween graphene and Ir lattices) is shown to be non-uniform along the mini-zone boundary due to hybridization with Ir bands. An electronically mediatedinteraction is proposed to account for the stability of the R0 variant. Thevariant rotated 30{\deg} in-plane, R30, is p-doped as grown and K dopingreveals no band gap at the Dirac crossing. No replica bands are found in ARPESmeasurements. Raman spectra from the R30 variant exhibit the characteristicphonon modes of graphene, while R0 spectra are featureless. These results showthat the film/substrate interaction changes from chemisorption (R0) tophysisorption (R30) with in-plane orientation. Finally, graphene-covered Ir hasa work function lower than the clean substrate but higher than graphite.

In-plane orientation effects on the electronic structure, stability and Raman scattering of monolayer graphene on Ir(111)