Direct observation of a dispersionless impurity band in hydrogenated graphene

D. Haberer, L. Petaccia, M. Farjam, S. Taioli, S. A. Jafari, A. Nefedov, W. Zhang, L. Calliari, G. Scarduelli, B. Dora, D. V. Vyalikh, T. Pichler, Ch. Wöll, D. Alfè, S. Simonucci, M. S. Dresselhaus, M. Knupfer, B. Büchner, and A. Grüneis

Phys. Rev. B 83, 165433 – Published 22 April 2011

Abstract

We show with angle-resolved photoemission spectroscopy that a new energy band appears in the electronic structure of electron-doped hydrogenated monolayer graphene (H-graphene). Its occupation can be controlled with the hydrogen amount and allows for tuning of graphene’s doping level. Our calculations of the electronic structure of H-graphene suggest that this state is largely composed of hydrogen 1 $s$ orbitals and remains extended for low H coverages despite the random chemisorption of H. Further evidence for the existence of a hydrogen state is provided by x-ray absorption studies of undoped H-graphene which are clearly showing the emergence of an additional state in the vicinity of the $π^{*}$ resonance.

Received 21 October 2010

DOI:https://doi.org/10.1103/PhysRevB.83.165433

Authors & Affiliations

D. Haberer¹, L. Petaccia², M. Farjam³, S. Taioli^4,5, S. A. Jafari^3,6, A. Nefedov⁷, W. Zhang^7,8, L. Calliari⁴, G. Scarduelli⁴, B. Dora⁹, D. V. Vyalikh¹⁰, T. Pichler¹¹, Ch. Wöll⁷, D. Alfè^12,13,14, S. Simonucci¹⁵, M. S. Dresselhaus¹⁶, M. Knupfer¹, B. Büchner¹, and A. Grüneis^1,11

¹IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany
²Elettra Synchrotron Light Laboratory, Sincrotrone Trieste SCpA, S.S. 14 Km 163.5, 34149 Trieste, Italy
³School of Nano Science, Institute for Research in Fundamental Sciences (IPM), P.O. Box 19395-5531, Tehran, Iran
⁴Interdisciplinary Laboratory for Computational Science (LISC), FBK-CMM and University of Trento, Via Sommarive 18, I-38123 Trento, Italy
⁵Department of Physics, University of Trento, Via Sommarive 14, I-38100 Trento, Italy
⁶Department of Physics, Sharif University of Technology, Tehran 11155-9161, Iran
⁷Institut für Funktionelle Grenzflächen (IFG), Karlsruher Institut für Technologie (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany
⁸National Synchrotron Radiation Laboratory, University of Science and Technology of China, 230029 Hefei, Anhui, People’s Republic of China
⁹Department of Physics, Budapest University of Technology and Economics, Budafoki t 8, H-1111 Budapest, Hungary
¹⁰Institut für Festkörperphysik, TU Dresden, Mommsenstrasse 13, D-01069 Dresden, Germany
¹¹Faculty of Physics, University of Vienna, Strudlhofgasse 4, A-1090 Wien, Austria
¹²Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK
¹³Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹⁴London Centre for Nanotechnology, University College London, Gower Street, London WC1E 6BT, UK
¹⁵Department of Physics, University of Camerino, Via Madonna delle Carceri 9, I-62032 Camerino, Italy
¹⁶Department of Physics and Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA

Article Text (Subscription Required)

Click to Expand

Supplemental Material (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 83, Iss. 16 — 15 April 2011

Reuse & Permissions

Access Options

Article Available via CHORUS

Download Accepted Manuscript

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
NEXAFS spectra of (a) pristine graphene and (b) hydrogenated graphene (H/C $~$ 15%) for various incident angles between normal (90 $^{°}$ ) and grazing incidence (20 $^{°}$ ). The inset in (a) shows the experimental geometry and $ϕ$ , the angle of light incidence. (c) Comparison of pristine (green) and H-graphene (blue) in the region of the $π^{*}$ resonance. The additional shoulder at lower photon energy denoted by two arrows is best visible for grazing incidence. The red line indicates the C 1 $s$ energy position of unhydrogenated graphene/Au. (d) Calculated density of states for hydrogenated graphene with various H/C ratios.Reuse & Permissions
Figure 2
(a) ARPES intensities around the $K$ point of the Brillouin zone of graphene intercalated with one monolayer Au for increasing potassium doping. The last viewgraph (bottom right) denotes the photoemission intensities of graphene intercalated with potassium in between the graphene/Au interface. (b) Energy dispersion curves taken at the $K$ point for the six doping steps in (a) with the shift of $E$ $_{F}$ as indicated. For the doped graphene, the two peaks correspond to the $π$ and $π^{*}$ bands. Upon intercalation of potassium between graphene and Au, the value of the gap between $π$ and $π^{*}$ bands doubles.Reuse & Permissions
Figure 3
(a) ARPES intensities around the $K$ point of the Brillouin zone of hydrogenated $n$ -doped graphene with increasing H/C ratios as denoted in percent. The size of the Fermi surfaces is indicated by two black vertical ticks on top and an arrow for the H/C $=$ 0 and H/C = 2.1% ratios. The dots in the last panel are a guide to the eye and depict the dispersionless midgap state. The photoemission intensity scale is depicted and applies to all graphs. (b) Energy dispersion curves of the ARPES intensity at the $K$ point. (c) DFT calculation of the electronic energy bands and (d) the partial DOS of HC $_{32}$ . A rigid band shift of 1 eV was applied in (c) to account for the potassium doping. The dispersionless band 1 eV below $E$ $_{F}$ in blue corresponds to the midgap state. The inset to (d) depicts the unit cell and the atoms (H, $C_{1}$ , and $C_{2}$ ) in the graphene lattice which contribute significantly to the midgap-state DOS.Reuse & Permissions
Figure 4
Sketches of symmetry-breaking, hybridization, and charge-transfer processes in pristine and $n$ -doped H-graphene. In the case of undoped hydrogenated graphene the hydrogen-derived midgap state can be measured with absorption spectroscopies probing unoccupied states such as NEXAFS. Only if $E_{F}$ is above the energetic position of the midgap state (denoted by $σ_{H}^{*}$ ) can it be directly observed in ARPES. In this case, the midgap state likely accepts electrons from the $π^{*}$ band of graphene denoted by arrows (see the right panel).Reuse & Permissions
Figure 5
The calculated typical (black line) and average (red line) DOS for three different H/C ratios. The vanishing of the typical DOS at energies slightly higher or lower than the acceptor level (located at $E = - 1$ eV) indicates a localization of the $π$ -band edges. The typical DOS of the midgap state never reaches zero which indicates that it is resistant to localization. A relatively large width of the impurity band helps the states in the middle of the impurity band remain extended.Reuse & Permissions

Physical Review B

covering condensed matter and materials physics