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The nature of the charge carriers in 2D few-layer graphites (FLGs) has been recently questioned by transport measurements [K. S. Novoselov, Science 306, 666 (2004)10.1126/science.1102896] and a strong ambipolar electric field effect has been revealed. Our density functional calculations demonstrate that the electronic band dispersion near the Fermi level, and consequently the nature of the charge carriers, is highly sensitive to the number of layers and the stacking geometry. We show that the experimentally observed ambipolar transport is only possible for an FLG with a Bernal-like stacking pattern, whereas simple-carrier or semiconducting behavior is predicted for other geometries.

We examine how the misorientation of a few stacked graphene layers affects the electronic structure of carbon nanosystems. We present ab initio calculations on bi-and trilayer systems to demonstrate that the massless Fermion behavior typical of single layered graphene is also found in incommensurate multilayered graphitic systems. We also investigate the consequences of this property on experimental fingerprints, such as Raman spectroscopy and scanning tunneling microscopy (STM). Our simulations reveal that STM images of turbostratic few layer graphite are sensitive to the layer arrangement. We also predict that resonant raman signal of graphitic samples are more sensitive to the orientation of the layers than to their number.PACS numbers: 81.05. Uw, 71.15.Mb, 73.90.+f The electronic properties of 2D graphene and 3D graphite have been extensively studied for more than 50 years 1 . It is fascinating that the extraordinary properties of carbon nanotubes 2 have been deduced from those of 2D graphene many years before macroscopic samples of very thin Few Layer Graphite (FLG) could be obtained in the laboratory 3 . A particular interest has been recently given to Single Layer Graphene (SLG) because of their massless Fermion behavior, the √ B dependence of the Landau levels 4,5 , and the observation of abnormal Quantum Hall Effect (QHE), even at room temperature 6 .In that context, a precise investigation of the layer to layer interaction on the existence of massless Fermion carriers is of paramount importance. For 3D graphite, the most stable Bernal phase (AB stacking) as well as the rhombohedral (ABC stacking) have been proven to show complex electron and hole bands near the Fermi level rather than linear, massless fermion ones, due to interlayer interaction 7 . We have recently shown that regular (AB or ABC) stackings also break the linear character of the dispersion of electronic bands for FLG with 2 to 4 layers and that ambipolar electronic conduction could be related to AB stacked FLG 8 .In this Letter, we present an electronic structure analysis of misoriented (turbostratic) 2 and 3 layer FLG. A recent surface X-rays analysis of multilayer graphene grown on SiC shows that misorientation is plausible 9 . We show here that the linear dispersion of SLG is preserved in turbostratic multilayer systems despite the presence of adjacent layers. It follows that massless Fermion carriers are predicted for disoriented multilayer systems. These findings raise the question of the interpretation of the experimental observations performed on FLG samples and challenge the direct relation between SLGs and Dirac massless Fermions. More generally, the electronic properties (and consequently the optical, vibrational, and transport properties) of a given FLG film is found to be controlled mainly by the (mis)orientation of the successive layers rather than the number of layers. In the present work , we discuss also the implications of this possible misorientation on experimental signatures, in particular on STM and Raman fingerpri...

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Nitrogen-doped epitaxial graphene grown on SiC(000?1) was prepared by exposing the surface to an atomic nitrogen flux. Using Scanning Tunneling Microscopy (STM) and Spectroscopy (STS), supported by Density Functional Theory (DFT) calculations, the simple substitution of carbon by nitrogen atoms has been identified as the most common doping configuration. High-resolution images reveal a reduction of local charge density on top of the nitrogen atoms, indicating a charge transfer to the neighboring carbon atoms. For the first time, local STS spectra clearly evidenced the energy levels associated with the chemical doping by nitrogen, localized in the conduction band. Various other nitrogen-related defects have been observed. The bias dependence of their topographic signatures demonstrates the presence of structural configurations more complex than substitution as well as hole-doping.Comment: 5 pages, accepted in PR