We show by angle-resolved photoemission spectroscopy that a tunable gap in quasi-free-standing monolayer graphene on Au can be induced by hydrogenation. The size of the gap can be controlled via hydrogen loading and reaches approximately 1.0 eV for a hydrogen coverage of 8%. The local rehybridization from sp(2) to sp(3) in the chemical bonding is observed by X-ray photoelectron spectroscopy and X-ray absorption and allows for a determination of the amount of chemisorbed hydrogen. The hydrogen induced gap formation is completely reversible by annealing without damaging the graphene. Calculations of the hydrogen loading dependent core level binding energies and the spectral function of graphene are in excellent agreement with photoemission experiments. Hydrogenation of graphene gives access to tunable electronic and optical properties and thereby provides a model system to study hydrogen storage in carbon materials.

A new polymer with C4H stoichiometry based on graphene is synthesized in situ using template‐induced polymerization of self‐organizing hydrogen adsorbates on graphene. The polymerization is observed “live” on the surface of graphene by photoemission spectroscopy. Photoemission spectroscopy allows for an accurate determination of the carbon/hydrogen stoichiometry, an aspect that is extremely important for understanding functionalized graphene.

In their first-principles calculations of the electronic band structure of graphene under uniaxial strain, Gui, Li, and Zhong [Phys. Rev. B 78, 075435 (2008)] have found opening of band gaps at the Fermi level. This finding is in conflict with the tight-binding description of graphene which is closed gap for small strains. In this Comment, we present first-principles calculations which refute the claim that strain opens band gaps in graphene. 73.61.Wp, 72.80.Rj Gui et al. 1 have used first-principles calculations to investigate the effect of planar strain on the electronic band structure of graphene, and have found opening of band gaps at the Fermi level resulting from arbitrarily small uniaxial strains, applied parallel or perpendicular to the C-C bonds. However, tight-binding (TB) model on the honeycomb lattice with different nearest-neighbor hoppings in the three directions has been rigorously shown to be closed gap as long as the hoppings satisfy the triangle inequality. 2,3 The closed-gap TB model, which contains zero modes, has been further developed recently to include the effects of magnetic fields 4,5 and corrugations in graphene. 6 The discrepancy between the firstprinciples calculations of Ref. 1 and TB model has already been discussed 7 but the reason has not been identified. One suggested explanation 7 is that this band gap opening is an artifact of density-functional theory (DFT) calculations. However, the possibility that the nearest-neighbor TB model is an incomplete description has not been ruled out. 7,8 We remind that the DFT methods essentially solve singleparticle Schrödinger equations (Kohn-Sham equations) for effective potentials based on the underlying lattice, and the TB model solves the same problem in a simplified approximation. Therefore, it seems unlikely that qualitative differences exist between DFT and TB band structures. We also see indications of possible error in Ref. 1. First, Figs. 3(c) and 5(c) of Ref. 1 show a peculiar peak whose underlying cause is not explained. Second, an energy gap is incorrectly ascribed to the TB band structure which is then plotted in Fig. 4 of Ref. 1 with large symbols that hide the important band crossing.In this Comment, we check directly the first-principles calculations of Ref. 1 by one of the available DFT codes. We used the QUANTUM-ESPRESSO 9 package based on the pseudopotential plane-wave method. We obtained the pseudopotential C.pw91-van ak.UPF also from Ref. 9 and used a kinetic-energy cutoff of 40 Ry, a Monkhorst-Pack k-point mesh of 21 × 21 × 1, and a vacuum separation of 20.5Å along the c axis. We chose these parameters as close as possible to those of Ref. 1 for a more meaningful comparison. 10 First, we determined the equilibrium lattice constant of graphene in the absence of strain. We found a value of a = 2.464Å, defined in Fig. 1(a), which is not significantly different from the 2.4669Å found in Ref. 1. We then made calculations on graphene under uniaxial strain for two special cases, for which Ref. 1 has found maximum values...

The adsorption of an alkali-metal submonolayer on graphene occupying every third hexagon of the honeycomb lattice in a commensurate ( √ 3 × √ 3)R30 • arrangement induces an energy gap in the spectrum of graphene. To exemplify this type of band gap, we present ab initio density functional theory calculations of the electronic band structure of C6Li. An examination of the lattice geometry of the compound system shows the possibility that the nearest-neighbor hopping amplitudes have alternating values constructed in a Kekulé-type structure. The band structure of the textured tight-binding model is calculated and shown to reproduce the expected band gap as well as other characteristic degeneracy removals in the spectrum of graphene induced by lithium adsorption. More generally we also deduce the possibility of energy gap opening in periodic metal on graphene compounds CxM if x is a multiple of 3.

Many propositions have been already put forth for the practical use of N-graphene in various devices, such as batteries, sensors, ultracapacitors, and next generation electronics. However, the chemistry of nitrogen imperfections in this material still remains an enigma. Here we demonstrate a method to handle N-impurities in graphene, which allows efficient conversion of pyridinic N to graphitic N and therefore precise tuning of the charge carrier concentration. By applying photoemission spectroscopy and density functional calculations, we show that the electron doping effect of graphitic N is strongly suppressed by pyridinic N. As the latter is converted into the graphitic configuration, the efficiency of doping rises up to half of electron charge per N atom.