### Session P36: Focus Session: Graphene: Growth, Characterization, and Devices: Electronic Structure

 Wednesday, March 23, 2011 8:00AM - 8:12AM P36.00001: Electronic properties of the Graphene/SiC (000$\overline 1$) interface: a First Principles study Thushari Jayasekera , Shu Xu , K.W. Kim , Marco Buongiorno-Nardelli In this talk, we will discuss the electronic properties of epitaxial graphene on the SiC (000$\overline 1$) surface (C-terminated face) using Density Functional Theory. In our calculations we focus on mono- and bi-layer graphene with AA, AB and turbostratic stacking sequences. Of the three, the turbostratic is the most observed during growth on SiC (000$\overline 1$). However, no theoretical investigations are available to understand the effect of the substrate on this growth sequence. We will investigate the energetics of different stackings and explain their electronic properties. We will also discuss the role of the interfaces in the stabilization of the individual stacking sequences and indicate possible routes for chemical functionallizations at the heterojunction to facilitate the tuning of the electronic and transport properties of these systems. Wednesday, March 23, 2011 8:12AM - 8:24AM P36.00002: Local surface potential variations and charge puddling in graphene on SiC(0001) A.E. Curtin , W.G. Cullen , M.S. Fuhrer , R.L. Myers-Ward , L.O. Nyakiti , V.D. Wheeler , D.K. Gaskill We performed Kelvin probe microscopy in ultra-high vacuum on epitaxial graphene grown on SiC(0001). In agreement with previous work, we see discrete surface potentials corresponding to interface layer and monolayer regions separated by steps of $\sim$100 mV. We used the step width to determine the spatial resolution of the probe to be approximately 20 nm. Within a monolayer area we see smaller fluctuations in surface potential of only a few mV. The data set limits on the scale of possible electron/hole puddles in monolayer graphene on SiC(0001). Wednesday, March 23, 2011 8:24AM - 8:36AM P36.00003: Electric Field Effects on Electronic Structures of Epitaxial Graphene on SiC Hyungjun Lee , Seungchul Kim , Jisoon Ihm , Young-Woo Son , Hyoung Joon Choi We report first-principles calculations of atomic and electronic structures of epitaxial single-layer graphene on Si-faced SiC(0001) surface under homogeneous transverse electric fields. We find that atomic positions are insensitive to applied electric fields, but the electronic band structures of the graphene layer are shifted in energy, depending strongly on the applied electric fields, while those of the buffer layer are almost unchanged. This effect finally results in field-induced closing of the energy gap at the Dirac energy point and recovery of the conic feature of the low-energy band structures of free-standing graphene, which are verified and analyzed further with a tight-binding model. The recovery of conical dispersion of the single-layer graphene and ambipolar field-effect behavior makes epitaxial single-layer graphene one of the promising alternatives to current state-of-the-art transistors for radiofrequency applications. This work was supported by the NRF of Korea (Grant No. 2009-0081204). Computational resources have been provided by KISTI Supercomputing Center (KSC-2008-S02-0004). Wednesday, March 23, 2011 8:36AM - 9:12AM P36.00004: Plasmarons in Quasi-freestanding Epitaxial Graphene Invited Speaker: Aaron Bostwick Graphene is a remarkable new electronic material with many unique properties. To realize its promise, it is essential to understand how its charge carriers interact. By measuring the spectral function of charge carriers in quasi-free-standing graphene, we show that at finite doping, the well-known linear Dirac spectrum does not provide a full description of the charge-carrying excitations. We find that there also exist composite plasmaron'' particles, consisting of holes coupled to density oscillatons of the graphene electron gas. The Dirac crossing point is resolved into three crossings: the first between pure charge bands, the second between pure plasmaron bands, and the third a ring-shaped crossing between charge and plasmaron bands. Wednesday, March 23, 2011 9:12AM - 9:24AM P36.00005: Effective screening and the plasmaron bands in Graphene A. Walter , K.J. Jeon , A. Bostwick , L. Moreschini , Y.S. Kim , Y.J. Chang , F. Speck , M. Ostler , T. Seyllar , K. Horn , E. Rotenberg In the following we investigate the plasmaron bands in the presence of differing effective screening, by changing the interface layer between graphene and a SiC substrate. ARPES data is presented and the deviation of the band structure from the Dirac cone picture is attributed to electron, hole, plasmon interactions. Comparison to G$_{0}W$ -RPA theory is used to determine the effective dielectric constant of the underlying layer and a range of values ($\varepsilon _{S} \quad \sim$219 to $\sim$11.6) is found. This investigation indicates that, in addition to the long list of unique and interesting properties, graphene is an ideal candidate for investigating the effective screening in the context of electron-hole-plasmon interactions. It is also shown that plasmaronic and electronic properties of graphene can be manipulated semi-independently, a necessity if it is to be employed in future plasmaronic'' devices. Wednesday, March 23, 2011 9:24AM - 9:36AM P36.00006: Phonon-Induced Gaps in Graphite and Graphene Observed by Angle-Resolved Photoemission Yang Liu , Longxiang Zhang , Matthew Brinkley , Guang Bian , Tom Miller , Tai-chang Chiang Graphene systems, made of sheets of carbon atomic layers, have unusual electronic structures known as Dirac cones. While strong interest in the electronic structure of these graphitic materials has driven extensive ARPES studies, prior work has mostly focused on the quasiparticle band dispersion relations associated with the Dirac cones. Largely unexplored are spectral regions far away from the quasiparticle bands, where direct emission from the quasiparticles is forbidden, but indirect emission through coupling to phonons is allowed. Our ARPES measurements of graphite and graphene layers at low temperatures reveal heretofore unreported gaps at normal emission, one at around 67 meV and another much weaker one at around 150 meV. The major gap features persist to room temperature and beyond, and diminish for increasing emission angles. We show that these gaps arise from electronic coupling to out-of-plane and in-plane vibrational modes at the K point in the surface Brillouin zone, respectively, in accordance with conservation laws and selection rules governed by quantum mechanics. Our study suggests a new approach for characterizing phonons and electron-phonon coupling in solids. Wednesday, March 23, 2011 9:36AM - 9:48AM P36.00007: Many-Body Interactions in Quasi-Freestanding Graphene David Siegel , Cheol-Hwan Park , Choongyu Hwang , Jack Deslippe , Alexei Fedorov , Steven Louie , Alessandra Lanzara The Landau-Fermi liquid picture for quasiparticles assumes that charge carriers are dressed by many-body interactions, forming the basis of any theory of solids. Whether this picture still holds for a semimetal like graphene at the neutrality point, i.e. when the chemical potential coincides with the Dirac point energy, is one of the long-standing puzzles in this field. Here we present the first direct measurements of the self-energy in graphene near the neutrality point, by using high-resolution angle-resolved photoemission spectroscopy. These exciting findings set a new benchmark in our understanding of many-body physics in graphene and a variety of novel materials with Dirac fermions. Wednesday, March 23, 2011 9:48AM - 10:00AM P36.00008: Abnormal temperature-dependent self-energy in graphene Choongyu Hwang , Daniel Garcia , David Siegel , Pu Yu , Swanee Shin , Xiaozhu Yu , Alexei Fedorov , Eugene Haller , Ramamoorthy Ramesh , Dung-Hai Lee , Alessandra Lanzara Dynamics of charge carriers are determined by their self-energy associated with many-body interactions. By using angle-resolved photoemission spectroscopy, we study the origin of abnormal temperature-dependent self-energy in graphene, and discuss the results in terms of a quantum phase transition as a function of temperature. Our findings provide another example of novel electronic properties of graphene and deeper understanding of the ground state of charge carriers in graphene. Wednesday, March 23, 2011 10:00AM - 10:12AM P36.00009: Graphene/substrate charge transfer characterized by inverse photoelectron spectroscopy Lingmei Kong , Cameron Bjelkevig , Sneha Gaddam , Mi Zhou , YoungHee Lee , GangHee Han , HaeKyung Jeong , Ning Wu , Zhengzheng Zhang , Jie Xiao , Peter Dowben , Jeffry Kelber Wave vector-resolved inverse photoelectron spectroscopy (IPES) measurements demonstrate that there is a large variation of interfacial charge transfer between graphene and various substrates. IPES measurements of CVD single layer graphene on BN(0001)/Ru(0001), Ru, Ni(poly), and Cu(poly) indicate a substrate-to-graphene charge transfer of approximately 0.07, 0.06, 0.03 e- per carbon atom respectively and a charge transfer of 0.02 e- from graphene to the MgO substrate per carbon atom. IPES and photoemission data also indicate that graphene/MgO(111) has a band gap. These data demonstrate that IPES is an effective method for precise measurement of substrate/graphene charge transfer due to the extreme surface sensitivity of IPES. Wednesday, March 23, 2011 10:12AM - 10:24AM P36.00010: Characterization of Image States in Graphene on Ir(111) Jerry I Dadap , Marko Kralj , Marin Petrovic , Kevin Knox , Nader Zaki , Rohan Bhandari , Po-Chun Yeh , Richard M. Osgood Jr. Two dimensional electron systems involving graphene and graphene/metallic interfaces are increasingly of interest in condensed matter physics. Here, we demonstrate two-photon photoemission to map the image states of highly perfect and weakly bonded graphene on an Ir(111) substrate to reveal the effects of interaction with the underlying metal substrate. We observe a monotonic decrease in the work function with increasing graphene coverage from 5.6$\pm$0.1 eV for clean Ir to 4.5$\pm$0.1 eV for full graphene ML. We observe $n$=1, 2, 3 image states with nearly free electron dispersion. Despite the minimal coupling between the graphene and Ir, the energy spacing of the image states is consistent with a single Rydberg series description, in contrast to the expected bifurcation of the image states into odd and even states for a pure graphene layer. At large $k_{\vert \vert }$, we observe a weak state deviating from the $n$=1 dipersion. We explain this effect in terms of scattering from the Ir substrate. Wednesday, March 23, 2011 10:24AM - 10:36AM P36.00011: Electronic and chemical properties of epitaxial graphene intercalated with FeCl$_{3}$ Kristin Shepperd , Feng Wang , Jeremy Hicks , Holly Tinkey , Edward Conrad Epitaxial graphene has emerged as the platform for large-scale graphene-based electronics. To fully exploit the unique properties of graphene for electronic materials, a number of materials issues need to be resolved. One important challenge is being able to control the doping of graphene without altering its band structure and disrupting the sp2 graphene bonding. One approach to accomplish this is intercalation of atomic or molecular species between individual graphene layers. We report the intercalation of multilayers of epitaxial graphene (EG) with the electron-acceptor FeCl$_{3}$. We will present results on experiments focused on the intercalation of FeCl$_{3}$ into multilayers EG grown on the C-face of SiC(000-1). Intercalation with different staging was achieved by a standard two-zone vapor transport method. The chemical and electronic properties of the EG-FeCl$_{3}$ intercalation compounds were analyzed using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). Wednesday, March 23, 2011 10:36AM - 10:48AM P36.00012: Accessing high energy sub-bands in bilayer graphene - a transport study Dmitri K. Efetov , Patrick Maher , Simas Glinskis , Philip Kim In contrast to single layer graphene sheets with its two distinct valence and conduction bands merging at the Dirac Point, multilayer graphene sheets are known to have additional sub-bands at higher energies. Whereas the low energy sub-bands in these systems are well studied, the higher energy sub-bands could so far not be accessed in a transport measurement of graphene samples sitting on typical SiO$_{2}$/Si back gates. Employing a poly(ethylene)oxide-CsClO$_{4}$ solid polymer electrolyte gate we demonstrate the filling up of the high energy sub-bands in bilayer graphene samples at carrier densities above $\sim$ 2.7x10$^{13}$ cm$^{-2}$ . The onset of these sub-bands is defined by a slight increase of the resistivity and the onset of Shubnikov de Haas (SdH) oscillations. Measurements of the magneto-resistance, the SdH oscillations and the Hall Effect enable us to deduce the carrier densities and mobilities for both, the high and low energy bands simultaneously. In addition, we find that the onset energy of these sub-bands can be tuned by varying the bilayer interlayer asymmetry. Wednesday, March 23, 2011 10:48AM - 11:00AM P36.00013: Magnetic-field induced Electron-K-Phonon Interaction in the optical response of multi-layer epitaxial graphene Gerard Martinez , Milan Orlita , Marek Potemski , Mike Sprinkle , Claire Berger , Walter de Heer , Liang Tan , Steven Louie Absolute magneto-optical transmission measurements have been performed in the far-infra-red range under magnetic fields up to 32 T and at a temperature of 1.8 K on a series of multi-layer epitaxial graphene samples. In all samples, transmission data show for the main optical transition involving the n=0 landau level a clear splitting of the transition in the field range 17-18 T corresponding to an energy of about 150 meV which coincides with that of the K zone boundary phonon of graphene. A global analysis of the data using a multi-dielectric model, to fit them with a single transition, reveals in that range of energies an additional increase of the line-width accompanied by a softening of the transition energy. The energy variation of these quantities is characteristic of the emission of phonons. Possible origins of this effect will be discussed but seems to be the consequence of electron-electron interactions between the two valleys K and K' assisted by K-phonons between these two valleys.