Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T36: Focus Session: Graphene Growth, Characterization, and Devices: Structure, Interfaces and Transfer |
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Sponsoring Units: DMP Chair: Joost Wintterlin, Ludwig-Maximilians-Universitaet Muenchen Room: C142 |
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T36.00001: Rotated graphene bilayers : from independent layers to electronic localization Invited Speaker: Graphene outstanding electronic properties rely on its pristine honeycomb lattice. Interaction with the environment - substrate, other C layers- and how it affects graphene properties will form the guiding line of the talk. While some interactions might degrade graphene properties, others can open new and interesting possibilities. We focus first on graphene on SiC. The atomic and electronic structures of the interface and of the first C-layers will be discussed on the basis of ab initio calculations (VASP) and STM experiments. At variance with the Si face, the interaction with the substrate is weak on the C face so that the first C-layer already presents graphene properties. We propose a model for the interface which explains the observed rotational disorder. We then discuss the effect of a rotation between two graphene layers to show how it can lead to an effective decoupling of these layers and a linear graphene like dispersion. To tackle very small rotation angles, we developed a tight binding scheme based on ab initio calculations. Three regimes can be defined as a function of the rotation angle. In the first one (teta$>$15$^{\circ}$) the two layers are decoupled and behave like independent graphene layers. In the second one (1$<$teta$<$15$^{\circ}$) the Dirac velocity of the bilayer is renormalised with respect to the velocity of a monolayer while in the last one (teta$\sim$1$^{\circ}$ or smaller) the velocity drops to zero which results in localisation. Theses three regimes will be discussed in the light of analytical developments. Experimental evidence or discrepancies with these three regimes will be given. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T36.00002: Atomic Scale Transport in Graphene on Stepped SiC(0001) Surfaces Shuaihua Ji, James B. Hannon, Ruud M. Tromp, Arthur W. Ellis, Mark C. Reuter, Frances M. Ross Thermal decomposition of SiC is a promising route to wafer-scale epitaxial graphene. However, the initial SiC surface contains steps, and graphene formation induces additional steps. Here we consider how these steps affect current transport in graphene. 1-2ML graphene was grown by annealing SiC above 1300$^{\circ}$C in disilane. Low energy electron microscopy was used to determine graphene thickness, and transport through 1ML thick regions was measured by scanning tunneling potentiometry. In this technique a bias is applied between two fixed probes while a third, scanning probe measures the local electrochemical potential as well as topography. This allows us to determine the resistivity of the graphene sheet on terraces and across substrate steps. Single steps with 0.5nm height show very weak scattering. However, multiple steps of height 1.0 and 1.5nm scatter strongly, exhibiting a potential drop equivalent to $\sim $80nm and 120nm respectively of terrace graphene. Thus, step bunching is important, and steps separated by less than a few hundred nm can dominate transport through a graphene sheet. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T36.00003: Twisting and Interlayer Coupling of Few Layer Graphene Minghu Pan, Xiaoting Jia, Vincent Meunier, Mildred S. Dresselhaus, Jing Kong Few layer graphene (FLG) can be synthesized by chemical vapor deposition methods. Considering a graphene bilayer with a small angle rotation between the layers---a stacking defect was observed by high resolution scanning tunneling microscopy. Low-energy Van Hove singularities in twisted graphene layers are identified as two sharp peaks in the density of states by low temperature scanning tunneling spectroscopy. Electronic instabilities at the crossing of the Fermi energy with a Van Hove singularity in the density of states often lead to new phases of matter such as charge/spin density waves. We here observe the coexistence of a charge density wave (CDW) phase and a normal phase on the top graphene layer. By analyzing the Moir\'{e} pattern in a normal region, a twisting between the two layers by a relative large angle about 3.9$^{\circ}$ is identified. This implies that the interlayer coupling for twisted layers is playing a role in the formation of different electronic phases in FLG. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T36.00004: Electronic structure of twisted bilayer graphene with doping and under electric fields Lede Xian, Salvador Barraza-Lopez, Mei-Yin Chou Rotational stacking faults of graphene layers in epitaxial graphene are believed to electronically decouple adjacent layers, thus single-layer graphene-like behavior can be observed. In addition, the layers close to the SiC substrate are known to be electron doped. Using density functional theory and a pi-electron, highly tuned tight-binding model, we study the modifications of the band structure in rotational stack-faulted bilayer graphene induced by doping and by external electric fields. In particular, the interlayer coupling, the magnitude of the Fermi velocity, and the possible impact on charge transport will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T36.00005: Imaging the first few layers of Multilayer Epitaxial Graphene grown on SiC ($000\overline 1 )$ Jeremy Hicks, M. Sprinkle, B. Zhang, A. Tejeda, A. Taleb-Ibrahimi, P. Le F\'{e}vre, F. Bertran, W.A. de Heer, E.H. Conrad Multilayer Epitaxial Graphene (MEG) grown on the C-terminated ($000\overline 1 )$ face of SiC has been shown to behave as a series of nearly independent graphene sheets, distinguishing it from few-layer graphite. We present photoemission data from MEG films of 10 {\AA} or less, finding that the first few graphene layers on top of SiC are easily visible and are n-doped in a similar fashion to graphene grown on the Si-terminated face. Combined with the characteristic diversity of rotations in MEG films, we have obtained numerous different combinations of cone doping and rotation angles, allowing us to explore a variety of phenomena associated with the graphene-SiC interface interaction. We find that, unlike similarly-doped graphene grown on the Si-terminated face, there exists no large mismatch between the conduction and valence bands. Other potential effects of the substrate are discussed, as well as efforts in modifying the graphene-SiC interface. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T36.00006: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T36.00007: Epitaxial graphene on SiC(0001): More than just honeycombs L. Li, Y. Qi, R.H. Rhim, G.F. Sun, M. Weinert Combing scanning tunneling microscopy using transition-metal (Fe, Cr)-coated W tips and first-principles calculations, we show that the interface of epitaxial graphene/SiC(0001) is a warped graphene layer with periodic inclusions of hexagon-pentagon-heptagon (H$_{5,6,7})$ defects [1]. These defects break the six-fold honeycomb symmetry, thereby inducing a gap and two states below E$_{F}$ near the Dirac point. Furthermore, we show that the next graphene layer assumes the perfect honeycomb lattice, but its interaction with the warped interfacial layer modifies the linear dispersion about the Dirac point, leading to parabolic dispersion and an apparent gap of $\sim $0.25 eV. These results explain recent angle-resolved photoemission and carbon core-level shift data, and resolve the long-standing issue of the interfacial structure of epitaxial graphene on SiC(0001). \\[4pt] [1] Qi et al., Phys. Rev. Lett. \textbf{105}, 085502 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T36.00008: Interfacial Structures of Graphene on 4H- SiC Substrates: SCED-LCAO Molecular Dynamics Ming Yu, Sean Fancher, Joseph H. Butera, C.S. Jayanthi, S.Y. Wu The purpose of this work is to obtain a microscopic understanding of the interface between the graphene and Si-terminated as well as C-terminated 4H-SiC substrates by studying several cases of nearly commensurate overplayed structures. Relative energies of these different structures are calculated using the SCED-LCAO method [PRB \textbf{74}, 15540; PHYSE \textbf{42},1] to gain insight into the role played by the lattice mismatch in releasing the strain and thus lowering the energy of the system. Further insight into the interfacial properties is obtained by analyzing the local strain in terms local atomic and bonding arrangements [PRB \textbf{59}, 7745] which will be correlated to the lattice mismatch. Our results will be compared with current experimental [PRL \textbf{100}, 176802; PRB \textbf{77}, 155303; J. Phys: Condens Matter \textbf{21}, 134016; PRB \textbf{78}, 205424; J. Phys: Condens Matter \textbf{20}, 323202] and theoretical [PRL \textbf{99}, 076802; PRL \textbf{99}, 126805; PRB \textbf{77}, 235412; PRL \textbf{100}, 176802] results. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T36.00009: Epitaxial graphene on SiC(0001): It takes a Si jump G.F. Sun, Y. Liu, S.H. Rhim, J.F. Jia, Q.K. Xue, M. Weinert, L. Li Using scanning tunneling microscopy with transition metal (Fe, Cr)-coated W tips and first-principles calculations, we have recently shown that interface of epitaxial graphene/SiC(0001) is a warped graphene layer with periodic inclusions of hexagon-pentagon-heptagon (H$_{5,6,7})$ defects that break the six-fold honeycomb symmetry [1]. Here we show that this unique structure facilitate a novel pathway for the disposal of Si during growth: the diffusion of Si vertically through the warped interfacial layer via a series of configurations that involve the dissociation and formation of C-C and Si-C bonds within the pentagon and heptagon of the H$_{5,6,7}$ complex. The calculated energy barrier for this diffusion path is 4.7 eV. These results and their implications on the self-limiting growth of epitaxial graphene on SiC(0001) will be presented at the meeting. \\[4pt] [1] Qi et al., Phys. Rev. Lett. \textbf{105}, 085502 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T36.00010: Inhomogeneous strain fields in epitaxial graphene Diedrich A. Schmidt, Taisuke Ohta, Laura B. Biedermann, Thomas E. Beechem, Stephen W. Howell, Gary L. Kellogg We report a large, inhomogeneous in-plane compressive strain (up to 0.5{\%}) and its local variation at micrometer length scales in single layer graphene films on silicon carbide (SiC) (0001). The strain, due to the difference in lattice constants and thermal expansion coefficients of graphene and SiC substrate, is probed using Raman scattering and low energy electron diffraction. We show that both the growth mechanism and the relaxation along the mismatched symmetry of the graphene and underlying substrate can affect the exact amount of local strain. The large compressive strain implies that monolayer graphene is tightly grafted to the underlying interface layer and SiC substrate; otherwise it would delaminate to relieve the strain. The magnitudes of the structural strain and its local variation are significant and need to be taken into account for electronics applications of the graphene-SiC(0001) system. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T36.00011: Direct Printing of Graphene onto Plastic Substrates. Daniel Hines, Evgeniya Lock, Scott Walton, Mira Baraket, Matthew Laskoski, Shawn Mulvaney, Paul Sheehan, Woo Lee, Jeremy Robinson Graphene films have been synthesized on metal foils using CVD growth and have the potential to be compatible with roll-to-roll printing. To be usable in electronic devices, these films need to be removed from the metallic substrate. Currently this is accomplished by spin coating a polymer film over the graphene and chemically etching away the metal substrate. We have developed a direct printing method that allows graphene films to be printed off the metal substrate onto a polymer substrate. This printing process does not generate chemical waste, is compatible with roll-to-toll processing and renders the metal foil reusable. Adhesion of the graphene film to the polymer substrate is established by attaching perfluorophenylazides (PFPA) azide linker molecules to a plasma activated polymer surface. The transfer printing was performed by placing the PFPA treated polymer surface in contact with a graphene covered Cu foil and heating under pressure. Graphene films successfully printed onto a polystyrene substrate have been characterized by Raman spectroscopy and electrical measurements revealed the presence of Gr on the polymer surface. Details of the printing process along with characteristics of the graphene film after printing will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T36.00012: Improved methods of transfer of graphene from growth substrate to other surfaces and devices Yujie Ren, Huifeng Li, Weiwei Cai, Shanshan Chen, Richard Piner, Rodney Ruoff Transfer of graphene films from the growth substrate to other surfaces has turned out to be one of the important challenges to creating graphene based devices. In this talk we will review new techniques which we are developing to meet these challenges. In addition to describing the best technique we have to date, we will show data demonstrating the effectiveness of these techniques. Our analysis includes scanning micro-Raman spectroscopy, electronic measurements with FET devices created with our techniques and other microscopic techniques. FET measurements indicate a strong influence of transfer technique on the doping of the device. [Preview Abstract] |
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