Observation of intercalation-driven zone folding in quasi-free-standing graphene energy bands

Abstract

Two-photon photoemission measurements reveal a near-zero-dispersion empty electronic state, approximately 2.6 eV above the Fermi energy and near the Brillouin zone center, induced by oxygen intercalation at the graphene-Ir(111) interface. While oxygen intercalation leads to quasi-free-standing graphene, electron diffraction shows $2\ifmmode\times\else\texttimes\fi{}2$ periodicity due to the patterned intercalant. Near the zone center, large-wave-vector zone folding, driven by this $2\ifmmode\times\else\texttimes\fi{}2$ periodicity, replicates states from near the Dirac cone that have little dispersion due to trigonal warping, explaining the nearly flat band. The zone-folding mechanism is supported by results from angle-resolved photoemission measurements and from density-functional-theory-based calculations of the unfolded energy bands. These results demonstrate zone-folding effects in graphene on a wave vector and energy scale that has largely been unexplored, and may open new opportunities to engineer the graphene electronic states.