We show experimentally that multilayer graphene grown on the carbon terminated SiC(0001[over ]) surface contains rotational stacking faults related to the epitaxial condition at the graphene-SiC interface. Via first-principles calculation, we demonstrate that such faults produce an electronic structure indistinguishable from an isolated single graphene sheet in the vicinity of the Dirac point. This explains prior experimental results that showed single-layer electronic properties, even for epitaxial graphene films tens of layers thick.

Recent transport measurements on thin graphite films grown on SiC show large coherence lengths and anomalous integer quantum Hall effects expected for isolated graphene sheets. This is the case eventhough the layer-substrate epitaxy of these films implies a strong interface bond that should induce perturbations in the graphene electronic structure. Our DFT calculations confirm this strong substrate-graphite bond in the first adsorbed carbon layer that prevents any graphitic electronic properties for this layer. However, the graphitic nature of the film is recovered by the second and third absorbed layers. This effect is seen in both the (0001)and (0001) 4H SiC surfaces. We also present evidence of a charge transfer that depends on the interface geometry. It causes the graphene to be doped and gives rise to a gap opening at the Dirac point after 3 carbon layers are deposited in agreement with recent ARPES experiments (T.Ohta et al, Science 313 (2006) 951).PACS numbers: 73.20. At, 71.15.Mb The possibility of carbon nanotubes (CNT) switching devices has been pursued in the last decade because of their attractive electronic properties. Nevertheless, problems with large intrinsic resistance in contacts and the inability to control tube helicity, and thus whether or not they are metallic or semiconducting, have made large scale circuit designs problematic. The proposed solution to these problems is an all carbon nanoelectronics paradigm based on the planar 2D form of carbon, graphene. [1] Graphene consists of a single carbon plane arranged on a honeycomb lattice. From a fundamental point of view, graphene ribbons can be seen as an unrolled CNT but with different boundary conditions (finite versus cyclic). Therefore, their electronic properties should be similar. In fact this has been demonstrated in recent experiments on single and multi-graphene sheets that show the existence of Dirac Fermions, large electron coherence lengths and anomalous integer quantum Hall effect [2,3,4]. The advantage of graphene over CNTs for electronics resides in its planar 2D structure that enables circuit design with standard lithography techniques. This enables the graphene to be cut with different shapes and selected edge direction. By simply selecting the ribbon edge direction it is possible to design metallic or semiconductor graphene ribbons [5,6] (analogous to helicity in CNTs).Since single or multiple sheets must be supported on a surface for fabrication, the pressing question becomes: how does the interface between a graphene sheet and its support affect its electronic properties? In other words can the symmetry of an isolated graphene sheet be maintained in the presence of an interface? It is this question that is the focus of this paper. Specifically we have studied the system of graphite grown on both polar faces of hexagonal SiC.The graphene layers are produced by sublimating Si from either the 4H-or 6H-SiC (0001) (Si terminated) or (0001) (C terminated) surfaces at sufficiently high temperatures to graphitize the excess car...

Interaction with a substrate can modify the graphene honeycomb lattice and thus alter its outstanding properties. This could be particularly true for epitaxial graphene where the carbon layers are grown from the SiC substrate. Extensive ab initio calculations supported by Scanning Tunneling Microscopy experiments demonstrate here that the substratei n d e e di n d u c e sas t r o n gn a n o s t r u cturation of the interface carbon layer. It generates an apparent 6x6 modulation different from the interface 6 √ 3×6 √ 3R30 symmetry used for the calculation. The top carbon layer roughly follows the interface layer morphology. This creates soft 6x6 ripples in the otherwise graphene-like honeycomb lattice. The wavelength and height of the ripples are much smaller than the one found in exfoliated graphene. Their formation mechanism also differs: They are due to the weak interaction with the interface layer and not to a roughening of the plane due to the instability of a strictly two-dimensional crystal.

We present a scanning tunneling microscopy (STM) study of a gently-graphitized 6H-SiC(0001) surface in ultra high vacuum. From an analysis of atomic scale images, we identify two different kinds of terraces, which we unambiguously attribute to mono-and bilayer graphene capping a C-rich interface. At low temperature, both terraces show ( √ 3 × √ 3) quantum interferences generated by static impurities. Such interferences are a fingerprint of π-like states close to the Fermi level. We conclude that the metallic states of the first graphene layer are almost unperturbed by the underlying interface, in agreement with recent photoemission experiments (A. Bostwick et al., Nature Physics 3, 36 (2007)).

Graphene exhibits unconventional two-dimensional electronic properties resulting from the symmetry of its quasiparticles, which leads to the concepts of pseudospin and electronic chirality. Here, we report that scanning tunneling microscopy can be used to probe these unique symmetry properties at the nanometer scale. They are reflected in the quantum interference pattern resulting from elastic scattering off impurities, and they can be directly read from its fast Fourier transform. Our data, complemented by theoretical calculations, demonstrate that the pseudospin and the electronic chirality in epitaxial graphene on SiC(0001) correspond to the ones predicted for ideal graphene.

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We have analyzed by Scanning Tunnelling Microscopy (STM) thin films made of few (3-5) graphene layers grown on the C terminated face of 6H-SiC in order to identify the nature of the azimuthal disorder reported in this material. We observe superstructures which are interpreted as Moiré patterns due to a misorientation angle between consecutive layers. The presence of stacking faults is expected to lead to electronic properties reminiscent of single layer graphene even for multilayer samples. Our results indicate that this apparent electronic decoupling of the layers can show up in STM data. 73.20.-r, 68.55.-a

An efficient computational methodology is used to explore charge transport properties in chemically modified (and randomly disordered) graphene-based materials. The Hamiltonians of various complex forms of graphene are constructed using tight-binding models enriched by first-principles calculations. These atomistic models are further implemented into a real-space order-N Kubo-Greenwood approach, giving access to the main transport length scales (mean free paths, localization lengths) as a function of defect density and charge carrier energy. An extensive investigation is performed for epoxide impurities with specific discussions on both the existence of a minimum semiclassical conductivity and a crossover between weak to strong localization regime. The 2D generalization of the Thouless relationship linking transport length scales is here illustrated based on a realistic disorder model.