Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session P28: Focus Session: Graphene III |
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Sponsoring Units: DMP Chair: Pablo Jarillo-Herrero, Columbia University Room: Colorado Convention Center 302 |
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P28.00001: Band Gaps and Quasiparticle Energies of Graphene Nanoribbons Li Yang, Cheol Hwan Park, Young-Woo Son, Marvin L. Cohen, Steven G. Louie We present calculations of the quasiparticle energies and band gaps of graphene nanoribbons (GNRs) carried out using a first-principles many-electron Green's function approach. The self-energy of electrons is evaluated within the GW approximation. In our supercell calculation, due to the geometry of GNRs, a rectangular truncation of the Coulomb interaction is applied, which significantly improves the efficiency of the calculation. The quasiparticle results are compared and contrasted with results from previous studies that have been carried out either within the tight-binding or density functional formalism. [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P28.00002: Graphene ribbon electronics Zhihong Chen, Phaedon Avouris Graphene consists of a single layer of carbon atoms that are arranged in a hexagonal structure. This ideal two-dimensional system represents a gapless semiconductor with six intersecting points per Brillouin zone between the valence and conduction band. In principle, a semiconducting gap can be introduced when the width of the graphene sheet is made small enough and the carbon hexagons are orientated in certain directions. In this study, we have combined e-beam lithography and etching techniques to form graphene ribbons of different widths. Electrical properties of these ribbons were studied through gate dependent transport measurements at various temperatures. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P28.00003: Engineering The Energy Band Gap of Graphene Quantum Structures Melinda Han, Yuanbo Zhang, Barbaros Oezyilmaz, Philip Kim We report on experimental studies of electrical transport in patterned graphene ribbon structures where the lateral confinement of charge carriers creates an energy gap near the charge neutral Dirac point. Single graphene layers are contacted with metal electrodes and patterned into ribbons of varying widths (10 to 100nm) and orientations. Energy gaps of the ribbons are investigated using both stability diagrams obtained at low temperatures (1.7K) and temperature dependent conduction behavior. An understanding of ribbon dimension and orientation as control parameters for the electrical properties of graphene structures can be seen as a first step toward the development of graphene-based electronic devices. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:27PM |
P28.00004: Electronic, magnetic and transport properties of graphene nanoribbons Invited Speaker: The recent fabrication of a single graphite layer opens a new possibility in the area of nanoelectronics. These experimental findings motivated us to study a novel one dimensional nanomaterial $-$ a graphene nanoribbon (GNR). Based on a first-principles approach, we have established the scaling rules for electronic energy bandgaps as a function of ribbon width. Both armchair and zigzag edged GNRs, with homogeneous edges passivated with hydrogen, are shown to have bandgaps, differing from the results of simple tight-binding calculations or solutions of the Dirac's equation based on them. Our {\it ab initio} calculations show that the origin of energy gaps for GNRs with armchair shaped edges arises from both quantum confinement and the crucial effect of the edges. The variations in energy bandgap of GNRs with armchair shaped edges exhibit three distinct family behaviors. For GNRs with zigzag shaped edges, gaps appear because of a staggered sublattice potential on the hexagonal lattice due to edge magnetizations. Based on electronic structure calculations on GNRs, we present two novel phenomena in GNRs and GNR nano-constrictions. First, our calculations show that the magnetic properties of nanoribbons can be controlled by electric fields. In particular, half-metallicity is predicted in GNRs if in-plane homogeneous electric fields are applied across zigzag shaped edges of these systems. Such asymmetric electronic structure for each spin originates from the fact that the spatially separated spin polarized states with opposite spin orientations in the semiconducting GNRs are shifted oppositely in energy by the applied fields. This closes the gap associated with one spin orientation and widens the other. Second, in GNR nano-constrictions with armchair shaped edge, conductances are shown to depend on the family behavior of energy gap of GNRs forming nano-constrictions. Depending on the width of nano-constriction, the incoming electrons from GNR leads are shown to experience perfect transmissions or nearly complete reflection in a wide range of energy. This work has been collaborated with M. L. Cohen and S. G. Louie. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P28.00005: First-Principles Simulations of Armchair-Edge Graphene Nanostrips. Junwen Li, John W. Mintmire, Daniel Gunlycke, Carter T. White We have carried out a series of first-principles, local-density functional band structure calculations of finite-width graphene nanostrips with armchair edges. A simple nearest-neighbor tight-binding model predicts that the band structures of these materials should be directly related to those of zigzag single wall carbon nanotubes, with two-thirds of the structures being small gap semiconductors and one-third of the structures being zero gap systems. The band gap in the semiconducting strips would be expected to decrease monotonically with increasing strip width. In our first-principles results, we find that in addition to the zero gap systems becoming finite gap quasimetallic systems because of symmetry breaking (as in the single-walled nanotubes), we also find that the semiconducting strips split into two families with band gaps that deviate from the simple nearest-neighbor tight binding model. Within the framework of our computational results, we compare the band structures of graphene, single-walled nanotubes, graphene nanostrips, and other carbon nanostructures. This work was supported by the US Office of Naval Research and the DoD HPCMO CHSSI program, both directly and through the US Naval Research Laboratory. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P28.00006: Performance Limit and Scaling Behaviors of Carbon Nanoribbon Transistors Jing Guo, Yijian Ouyang Carbon-based nanostructures promise near ballistic transport and are being intensively explored for device applications. In this work, the performance limits of carbon nanoribbon (CNR) field-effect transistors (FETs) are assessed using a semiclassical model, and compare to those of carbon nanotube (CNT) FETs. The ballistic channel conductance and the quantum capacitance of the CNRFET are about a factor of 2 smaller than those of the CNTFET, because of the different valley degeneracy factors for CNTs and CNRs. The intrinsic speed of the CNRFET is faster due to a larger average carrier injection velocity. The gate capacitance plays an important role in determining which transistor delivers a larger on-current. The scaling behaviors of CNRFETs are studied using an atomistic quantum simulation. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P28.00007: Applications of Nanoribbon Devices Thushari Jayasekera, John W. Mintmire Modern experiments allow us to grow ultra-thin epitaxial graphene which shows two-dimensional electron gas (2DEG) behavior. Electron transport in these 2DEG systems can be further confined in lateral directions using micro-electronics lithography methods (nano-patterned epitaxial graphene, NPEG). We study the properties of the NPEG multi-terminal devices made at a crossing of a zig-zag and armchair nanoribbons, in particular, plus junction and T-junction devices. We investigate the effect of size, shape, and, chirality on the transport properties of the device. We also discuss the effect of defects in the junction region on the electron transport of the device. Our results find that the properties of nanoribbon junctions are highly sensitive to the details of the junction region, thus we can engineer different properties by changing those details of the device. This work was supported by the DoD HPCMO CHSSI program through the Naval Research Laboratory. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P28.00008: Ballistic transport in zigzag-edge graphene nanostrips Daniel Gunlycke, Hadley M. Lawler, Denis A. Areshkin, Carter T. White Graphene nanostrips (GNSs) constitute a class of materials where one of the two in-plane dimensions of graphene has a small finite width. We present results of zigzag-edge GNSs terminated with hydrogen atoms which suggest that ballistic transport may be possible over micrometer lengths. The single channel near the Fermi level appears to possess a natural resistance to back-scattering. Long-range disorder have a negligible back-scattering since the only allowed coupling requires a large crystal momentum change. We find that disorder on atomic scale and edge disorder have also little impact on the conductance in the single-channel window. Not only are the zigzag-edge GNSs resistent to static disorder, they may also offer longer electron-phonon mean-free paths which are longer than those in carbon nanotubes. Back-scattering in the conduction band requires a large transfer of crystal momentum from phonons which immediately eliminate long-wavelength acoustic phonon scattering. Therefore, it might be feasible to have single-channel ballistic transport in zigzag-edge GNSs at room temperature. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P28.00009: first-principles tight-binding study of band gaps in graphene ribbons Daniel Finkenstadt, Gary Pennington, Chris Ashman, Mike Mehl Graphene has recently received much attention for the many interesting physical properties that it exhibits, including light Dirac fermion characteristics of its charge carriers and some experimental evidence of a minimum conductivity, even as the carrier concentration goes to zero. From a practical standpoint, the potential for large carrier mobility in graphene provides an attractive alternative to silicon-based devices, e.g. for field-effect transistors. Theoretical efforts towards designing these devices are focused on determining the geometry and chemistry needed to open up a semiconducting gap in the otherwise semi-metal band structure of a perfect, infinite graphene sheet. Such effects may allow gate control of the electronic conductance as found in semiconducting carbon nanotube devices. Here we use the NRL tight-binding method, which is fit to first-principles calculated data, to study the possibility of opening a gap in graphene by varying strip-width, edge shape with and without termination, and by allowing Peierl's distortion of the edges for narrow ribbons. We compare the tight-binding results with calculations based on the density functional theory. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P28.00010: Effects of disorder in the biased graphene bilayer Johan Nilsson, Antonio Castro Neto We discuss the effects of disorder on the peculiar electronic properties of a biased graphene bilayer, which is a semiconductor that has the property that its band-gap can be controlled externally by the field effect. We focus on the low-energy region inside of and near the band-gap and have studied the properties of bound states as well as possible effects due to a finite density of impurities such as impurity band formation and band gap renormalization. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P28.00011: Edge disorder in armchair-edge graphene nanostrips Denis A. Areshkin, Daniel Gunlycke, Carter T. White Graphene nanostrips created using current lithography techniques will likely contain edge irregularities due to lack of atomic precision. We present tight-binding calculations which show that these edge irregularities have a strong effect on electron transport in armchair-edge graphene nanostrips. The edge disorder causes Anderson localization which effectively suppress the electronic conductance in samples which are longer than the mean-free path. We estimate the mean-free path via the localization length which is calculated by averaging over a large number of disordered nanostrips. We find that the localization length approximately decreases with the square of the width of the nanostrip and is of the order of tens of nanometers at the width $20$ nm. The localization length also depends on the concentration of edge defects and energy. Only nanostrips with low concentration of edge disorder reflect expected semiconducting gaps in the localization length. We also find that the Anderson localization extends over the entire $\pi$-electron energy range. With this result in mind, we predict that long and narrow armchair-edge graphene nanostrips are insulators. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P28.00012: Carrier transport in 2D graphene layers near the Dirac point Shaffique Adam, E.H. Hwang, S. Das Sarma In a recent work we studied carrier transport in gated 2D graphene monolayers theoretically in the presence of scattering by random charged impurity centers using a Boltzmann theory formalism (cond-mat/0610157). Comparing our results with available experimental data suggested that the low density saturation of conductivity arises from charged impurity induced inhomogeneity in the graphene carrier density. In the present work, we develop a model for carrier transport in a disorder-induced inhomogeneous potential and examine the consequences on conductivity. This work was partially supported by U.S. ONR. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P28.00013: Dislocation and pentagon-heptagon pair generation in vacancy-induced graphene layer Byoung Wook Jeong, Hoonkyung Lee, Gun-Do Lee, Jisoon Ihm We investigate the mechanism of the generation of long range order defects in graphene layer by tight binding molecular dynamics simulations and first-principles total energy methods. It is found that the vacancies are diffused and coalesced to make the dislocation defect with the two 5-7 pair defects when more than a certain number of vacancies are present. We examine the magic number of the vacancy which gives dislocation defects in a graphene layer. STM simulation results related to the graphite lattice with the period of $\sqrt 3 \times \sqrt 3 $ in an STM topograph will be discussed. [Preview Abstract] |
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