Probing Local Hydrogen Impurities in Quasi-Free-Standing Graphene

We report high-resolution scanning tunneling microscopy and spectroscopy of hydrogenated, quasi-free-standing graphene. For this material, theory has predicted the appearance of a midgap state at the Fermi level, and first angle-resolved photoemission spectroscopy (ARPES) studies have provided evidence for the existence of this state in the long-range electronic structure. However, the spatial extension of H defects, their preferential adsorption patterns on graphene, or local electronic structure are experimentally still largely unexplored. Here, we investigate the shapes and local electronic structure of H impurities that go with the aforementioned midgap state observed in ARPES. Our measurements of the local density of states at hydrogenated patches of graphene reveal a hydrogen impurity state near the Fermi level whose shape depends on the tip position with respect to the center of a patch. In the low H concentration regime, we further observe predominantly single hydrogenation sites as well as extended multiple C–H sites in parallel orientation to the lattice vectors, indicating an adsorption at the same graphene sublattice. This is corroborated by ARPES measurements showing the formation of a dispersionless hydrogen impurity state which is extended over the whole Brillouin zone