First Direct Observation of a Nearly Ideal Graphene Band Structure

First Direct Observation of a Nearly Ideal Graphene Band Structure M. Sprinkle,‘ D. Siegel,2 Y. Hu,' J. Hicks,' A. Tejeda,3‘4 A. Taleb—Ibrahimi,5 P. Le Fevref F. Bertran,4 s. Vizzini,6’7 H. Enriquez,6‘7 S. Chiang,6’8 P. Soukiassian,6’7 C. Berger,q9 W. A. de Heer,l A. Lanzara,2 and E. H. Conradl [The Georgia Institute of Technology, Atlanta, Georgia 30332-0430, USA 2Department of Physics, University of California, Berkeley, California 94720, USA and Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 3Institut Jean Lamom; CNRS—Universite' de Nancy~UPV-Metz, 54506 Vandoeuvre les Nancy, France 4Synehrotron SOLEIL, L’0rme des Merisiers, Saint—Aubin, 91192 Gif sar Yvette, France 5UR1 CNRS/Synchrotron SOLEIL, Saint—/lubin, 91192 Gif sur Yvette, France 6Commissariata l’Energie Atomique, SIMA, DSM-lRAM1S—-SPCSI, Saclay, 91191 Gif sur Yvette, France 7De’partement ale Physique, Universite de Paris-Sud, 91405 Orsay, France 8Department of Physics, University of California—Davis, California 95616-8677, USA 9CNRS/Institut Neel, BP166, 38042 Grenoble, France (Received 29 July 2009; published 24 November 2009) Angle—resolved photoemission and x—ray diffraction experiments show that multilayer epitaxial graphene grown on the SiC(000l) surface is a new form of carbon that is composed of effectively isolated graphene sheets. The unique rotational stacking of these ﬁlms causes adjacent graphene layers to electronically decouple leading to a set of nearly independent linearly dispersing bands (Dirac cones) at the graphene K point. Each cone corresponds to an individual macroscale graphene sheet in a multilayer stack where AB—stacked sheets can be considered as low density faults. In an ideal graphene sheet (near the Dirac point, ED) the 7r and 7rq‘ bands disperse linearly E(Ak) = hvFAk, where Up‘ is the Fermi velocity and Ak is the momentum relative to the K points of the hexagonal reciprocal unit cell [1]. The two-dimensional dispersion is isotropic and deﬁnes a cone with an apex at ED [1]. For undoped graphene the Fermi energy E; coincides with ED so that the Fermi ‘surface consists of six points [see Fig. l(a)]. This speciﬁc electronic structure of graphene is relevant for graphene based electronics for several reasons. For example, elec- tronic energies above (or below) ED of the order of ~1 eV correspond to wavelengths of order ~1 nm. Consequently, quantum conﬁnement energies in nanoscopic graphene structures will be of the order of ~l eV, which is consid- erably greater than the thermal energy at 300 K [2]. This graphene property is essential for room temperature gra- phene nanoelectronics. Epitaxial graphene (EG) grown directly on both the SiC(0001) Si face and (0001) C face has exceptional ﬁlm quality [3,4]. It is atomically ﬂat and the EG sheets are continuous over macroscopic distances (if not the entire crystal surface). In Si—face few layer EG ﬁlms, substrate’ interactions cause charge doping, signiﬁcant electron- phonon coupling, and distortions in the linear dispersion of the ﬁrst graphene layer near ED [S-7]. These are similar to the more substantial substrate induced distortions ob- served in exfoliated graphene [8,9]. Beyond the ﬁrst gra- phene layer, the graphitic AB stacking of few layer Si—face graphene causes the band structure to converge to graphite when the number of layers becomes large [6,10]. In contrast to other forms of graphene, multilayer epi- taxial graphene (MEG) grown on the C face of SiC exhibits all the transport properties of an isolated graphene sheet [1l—l7]. Moreover, Landau level spectroscopy from C-face ﬁlms has demonstrated unprecedented graphene properties including exceptionally high room temperature mobilities (g200 000 cm2/ V s) and resolved Landau lev- els in magnetic ﬁelds as low as 40 mT [17]. MEG has been shown to have a unique crystal structure. Rather than AB stacked like graphite, MEG ﬁlms have successive layers that are typically rotated by angles other than the 60° rotation of graphite. This rotational stacking has been theoretically predicted to cause the layers to electronically decouple [18—20]. Here we provide direct experimental evidence for this effect using angle-resolved photoemission spectroscopy (ARPES). In particular we show that the electronic band structure of the individual graphene layers in the MEG stack indeed are essentially unperturbed Dirac cones as expected for isolated graphene sheets. We further show that these ﬁlms have exceptionally long electron relaxation times and a remarkable absence of distortions in the Dirac cone. The measurements experimentally conﬁrm that the electronic structure of each individual sheet in MEG is essentially that of an isolated graphene sheet as predicted [18—20] and indicated in prior experiments [11- 17]. Our results clearly demonstrate that a quasiperiodic (not random) rotational stacking order is responsible for MEG’s exquisite 2D properties. The substrates used in these studies were both n—doped n = 2 X 1018 cmq2 6H and insulating 4H SiC. The gra- phene layers were grown in a closed rf induction furnace at a temperature of 1550 °C (see Ref. [3] for details). The graphene ﬁlm thicknesses ranged from 11-12 layers as