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We report the near-edge x-ray absorption fine structure (NEXAFS) spectrum of a single layer of graphite (graphene) obtained by micromechanical cleavage of Highly Ordered Pyrolytic Graphite (HOPG) on a SiO2 substrate. We utilized a PhotoEmission Electron Microscope (PEEM) to separately study single-double-and few-layers graphene (FLG) samples. In single-layer graphene we observe a splitting of the π * resonance and a clear signature of the predicted interlayer state. The NEXAFS data illustrate the rapid evolution of the electronic structure with the increased number of layers.The recent discovery of a single sheet of graphite [1], called graphene, has opened up a new area of condensed matter physics.Graphene proves that materials just one atom thick may exist, with exciting prospects for applications. Its unusual electronic spectrum, where charge carriers mimic massless relativistic particles [2,3], also provides an unexpected bridge between condensed matter physics and quantum electrodynamics.The method to obtain single sheets of graphite [1], called micro-mechanical cleavage, allows easy production of sample with a typical size of few tens of microns, ideal for ballistic transport and Quantum Hall effect measurements, but inappropriate for many conventional spectroscopy investigations in Ultra High Vacuum (UHV) conditions. In the absence of new and more efficient ways to make graphene, samples obtained by micro-mechanical cleavage of bulk graphite are used in a limited class of experiments, where the size and the identification of thin flakes is possible. Indeed, after the cleavage with simple adhesive tape, graphene crystallites left on the SiO 2 substrate are extremely rare and hidden amongst hundreds of thicker flakes. Conventional surface science probes of the electronic and structural properties of materials, are then ruled out, unless they are coupled to a microscope. On the other hand, single-and few-layers graphene (FLG) samples have been grown epitaxially by chemical vapour deposition of hydrocarbons on metal substrates [4] and by thermal decomposition of SiC [5]. In both cases, the hybridization of graphene with the substrate is an unavoidable complication, although graphene on SiC preserves most of the electronic properties expected for a free layer [6,7,8].In this Letter we report the near-edge x-ray absorption fine structure (NEXAFS) spectra of a free layer of graphene, and of few-layers graphene (FLG) samples, obtained by a PhotoEmission Electron Microscope (PEEM) in UHV conditions. The spectrum of graphene exhibits a new structure below the π * resonance, reflecting its peculiar density of states (DOS) above the

We present a new method to engineer the charge carrier mobility and its directional asymmetry in epitaxial graphene by using metal cluster superlattices self-assembled onto the moiré pattern formed by graphene on Ir(111). Angle-resolved photoemission spectroscopy reveals threefold symmetry in the band structure associated with strong renormalization of the electron group velocity close to the Dirac point giving rise to highly anisotropic Dirac cones. We further find that the cluster superlattice also affects the spectral-weight distribution of the carbon bands as well as the electronic gaps between graphene states. DOI: 10.1103/PhysRevLett.105.246803 PACS numbers: 73.20.Àr, 73.21.Cd, 73.22.Pr, 79.60.Ài Graphitic materials have attracted strong scientific interest because they exhibit exotic phenomena such as superconductivity or the anomalous quantum Hall effect [1]. Graphene (gr) is the building block of these materials; it is wrapped up into fullerenes, rolled up into carbon nanotubes, or stacked into 3D graphite. It presents a model system to investigate the influence of many-body interactions on the electron dynamics in these materials. In addition, the exceptional electronic mobility makes graphene a candidate material for next generation electronic devices [2]. Freestanding graphene is a zero-gap semiconductor. Because most electronic applications require a gap between valence and conduction bands, considerable effort has been spent to induce and control the opening of such a band gap [3][4][5][6].A related, and for applications equally relevant, issue is the ability to tailor the band dispersion. The speed with which information can be transmitted along a graphene layer depends on the charge carrier group velocity. Close to the K points, the bands of freestanding graphene have a linear dispersion well described by the relativistic Dirac equation for massless neutrinos. The resulting Dirac cones are trigonally warped due to the chiral nature of graphene charge carriers in the equivalent A and B carbon sublattices [7]. The ability to increase and tailor this anisotropy would open a manifold of new applications. Several theoretical studies suggest that this goal can be reached by applying an external periodic potential with nanometer period giving rise to highly anisotropic Dirac cones [8][9][10][11].Epitaxial graphene layers grown on lattice mismatched close-packed metal substrates, such as Pt (111) Here we demonstrate with angle-resolved photoemission spectroscopy (ARPES) that an Ir cluster superlattice grown on the gr=Irð111Þ-(9:32 AE 0:15 Â 9:32 AE 0:15) moiré structure [13,15] gives rise to significant group velocity and Dirac cone asymmetries. We attribute this to a much stronger periodic potential caused by the cluster superlattice than by the moiré itself. H decorated gr=Irð111Þ has been reported to give rise to band gap opening but not to the Dirac cone asymmetries reported here [6]. We thereby confirm theoretical predictions and present a new way to tailor the directionality of carrier mobilit...

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We demonstrate that the spin-orbit coupling of two-dimensional surface states can be detected locally by scanning-tunneling spectroscopy ͑STS͒. The spin splitting of the surface state induces a singularity in the local density of states which can be detected as a distinct peak in the differential conductance spectrum. From the STS spectrum we can determine the Rashba energy as a measure of the strength of the spin splitting. Its detection and imaging are demonstrated for the surface alloys Bi and Pb on Ag͑111͒, which exhibit particularly large spin-split band structures. The influence of the spin splitting on the surface-state STS spectra of closepacked noble metal surfaces is also discussed.

We investigate the effects of Na adsorption on the electronic structure of bare and Ir cluster superlattice covered epitaxial graphene on Ir(111) using angle-resolved photoemission spectroscopy and scanning tunneling microscopy. At Na saturation coverage a massive charge migration from sodium atoms to graphene raises the graphene Fermi level by about 1.4 eV relative to its neutrality point. We find that Na is adsorbed on top of the graphene layer and when coadsorbed onto an Ir cluster superlattice it results in the opening of a large bandgap of Δ Na/Ir/G = 740 meV comparable to the one of Ge and with preserved high group velocity of the charge carriers.

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We report the structural and electronic properties of an artificial graphene/Ni (111) (111), and mildly corrugated graphene on Ir(111), allows to disentangle the two key properties which lead to the observed increased interaction, namely lattice matching and electronic interaction. Although the latter determines the strength of the hybridization, we find an important influence of the local carbon configuration resulting from the lattice mismatch.