title: Probing local hydrogen impurities in quasi-free-standing graphene.
abstract: 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.
authors.affiliation: IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany. m.scheffler@ifw-dresden.de
authors.forename: Martha
authors.initials: M
authors.lastname: Scheffler
authors.forename: Danny
authors.initials: D
authors.lastname: Haberer
authors.forename: Luca
authors.initials: L
authors.lastname: Petaccia
authors.forename: Mani
authors.initials: M
authors.lastname: Farjam
authors.forename: Ronny
authors.initials: R
authors.lastname: Schlegel
authors.forename: Danny
authors.initials: D
authors.lastname: Baumann
authors.forename: Torben
authors.initials: T
authors.lastname: Hänke
authors.forename: Alexander
authors.initials: A
authors.lastname: Grüneis
authors.forename: Martin
authors.initials: M
authors.lastname: Knupfer
authors.forename: Christian
authors.initials: C
authors.lastname: Hess
authors.forename: Bernd
authors.initials: B
authors.lastname: Büchner