Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session H21: Focus Session: Graphene: Nanoribbons |
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Sponsoring Units: DMP Chair: Nancy Sandler, Ohio University Room: Portland Ballroom 251 |
Tuesday, March 16, 2010 8:00AM - 8:12AM |
H21.00001: Electronic properties of graphene antidot lattices Vladimir M. Stojanovic, Mihajlo Vanevic, Nenad Vukmirovic, Markus Kindermann We study graphene antidot lattices -- superlattices made by perforating voids in a graphene sheet. We show that, due to their bipartite structure, such lattices display zero-energy flat bands . We also find quasi-flat bands at low energies resulting from the presence of lattice-scale defects in the system and argue that the ensuing localized electron states compete with the states induced by the superlattice-scale defects that have been proposed as hosts for electron-spin qubits. For representative antidot lattices, we predict the real-space electron density profiles due to both flat and quasi-flat bands. We also investigate the effect of phonons in antidot lattices using a model that accounts for the phonon-modulation of the hopping integrals. Based on the adopted model, we quantify the nature of charge carriers by computing the conduction-band quasiparticle weight due to the electron-phonon coupling. We find a strong phonon-induced renormalization, which provides an indication of polaronic behavior and points to the necessity of taking into account the inelastic degrees of freedom in future studies of graphene antidot lattices. [Preview Abstract] |
Tuesday, March 16, 2010 8:12AM - 8:24AM |
H21.00002: Formation of Atomic Carbon Chains from Graphene Nanoribbons Renato Pontes, Edwin Hobi Jr., Adalberto Fazzio, Ant\^onio J.R. da Silva The formation of one-dimensional carbon chains from graphene nanoribbons is investigated using spin-polarized ab initio dynamics. We show under what conditions it is possible to obtain a linear atomic chain via pulling of the graphene nanoribbons. The presence of dimers composed of two-coordinated carbon atoms at the edge of the ribbons is necessary for the formation of the linear chains, otherwise there is simply the full rupture of the structure. For a situation where a linear carbon chain is formed, when a force of 8 nN is reached there is a rupture of the chain.The presence of Stone-Wales defects close to these dimers may lead to the formation of longer chains. The local atomic configuration of the suspended atoms indicates the formation of single and triple bonds, which is a characteristic of polyynes. [Preview Abstract] |
Tuesday, March 16, 2010 8:24AM - 9:00AM |
H21.00003: Transport in Graphene Nanostructures Invited Speaker: Experimentalists have started to investigate effects of mesoscopic physics well known from experiments on GaAs, such as the quantum Hall effect, conductance quantization, Aharonov-Bohm oscillations and the Coulomb blockade effect, in graphene. It is the goal of these investigations to find ways to confine charge carriers in this material system with its gapless band structure, to exploit the field effect for electrostatic control of electronic properties on the nanoscale, and to explore the feasibility of tailored graphene quantum circuits for future electronics. We will give an overview over our experiments carried out on narrow graphene constrictions, nanoribbons and graphene quantum dot devices at low temperatures (100mK -- 2K). These nanostructure systems are fabricated by mechanical exfoliation of graphite followed by electron beam lithography and dry etching technique. The fabricated graphene nanodevices are equipped with a number of graphene in-plane gates for local electrostatic control. Our measurements demonstrate the interplay of lithographic confinement, where electron-electron interactions manifest in the appearance of the Coulomb blockade effect, and disorder, even in the simplest single constriction devices. Despite the significant complexity of the physics in single graphene constrictions and nanoribbons, well controllable quantum dot devices exhibiting Coulomb blockade physics can be fabricated. We performed measurements of excited states, inelastic co-tunneling processes and the level spectra close to the electron hole crossover in graphene quantum dots. Finally, we realized on-chip time-resolved charge detection techniques in our experiments. [Preview Abstract] |
Tuesday, March 16, 2010 9:00AM - 9:12AM |
H21.00004: Gate-defined graphene double quantum dot and excited state spectroscopy Xing Lan Liu, Dorothee Hug, Lieven Vandersypen A graphene double quantum dot is a highly attractive system for quantum information processing. In particular, spin relaxation and coherence times in graphene are expected to be very long due to the absence of nuclear spin and spin-orbit interaction. We experimentally demonstrate a double quantum dot based on a graphene nano-ribbon device. Three top gates are fabricated on a graphene nano-ribbon to form a double quantum dot, where two gates on the left and right control independently the number of carriers on the left and right dot, and a middle gate is used to tune the inter-dot coupling. The inter-dot coupling changes non-monotonously when the middle gate voltage is varied, indicating that disorder influences the tunability. Transport through excited states is observed when the inter-dot coupling is switched off by the middle gate. We extract from the measurements the quantized level spacing in the double dot, and the results are comparable with expectations. The same device design principle can be applied to defining multiple quantum dots along a graphene nano-ribbon with independent gate control over barriers and charges. [Preview Abstract] |
Tuesday, March 16, 2010 9:12AM - 9:24AM |
H21.00005: Optical properties of graphene nanostructures from first-principles: from 1D to 0D Daniele Varsano, Deborah Prezzi, Alice Ruini, Elisa Molinari The possibility of patterning graphene sheets in a controllable manner to design semiconducting low-dimensional nanostructures opens exciting opportunities also in view of novel phenomena occurring under light excitation as well as nanoscale optoelectronics applications. We discuss the main characteristics of optical excitations in quasi-1D armchair graphene nanoribbons (A-GNRs) by means of ab-initio many-body calculations [1]. Our theoretical approach includes both self-energy corrections and excitonic effects through the GW-BSE formalism, providing full understanding of excited-state properties. Electron-hole interaction is found to suppress the van Hove singularities -as known for other 1D systems- and introduces strongly bound excitonic peaks. Starting from these ideal structures, we discuss the effect of width modulation on confinement and optical response [2]. Our results show that edge-modulated A-GNRs are efficient systems for the creation of carbon-based QD structures with prominent exciton localization features. [1] D. Prezzi, D. Varsano, A. Ruini, A. Marini, and E. Molinari, Phys. Rev. B 77, 041404 (2008). [2] D. Prezzi, D. Varsano, A. Ruini, and E. Molinari, to be published (2009) [Preview Abstract] |
Tuesday, March 16, 2010 9:24AM - 9:36AM |
H21.00006: Edge effects in graphene nanoislands on Co(0001) Deborah Prezzi, Daejin Eom, Andrea Ferretti, Kwang T. Rim, Hui Zhou, Michael Lefenfeld, Colin Nuckolls, Mark S. Hybertsen, Tony F. Heinz, George W. Flynn We recently demonstrated the growth of regularly shaped, nanoscale graphene islands of graphene on Co(0001) surfaces [1]. Here we combine low-temperature scanning tunneling microscopy (STM) measurements and DFT based calculations to study their edge properties. These graphene nanoislands reveal a well-ordered structure at the edges, with predominant zigzag termination. STS tunneling spectra show prominent peaks at low bias, where the edges dominate the images. DFT calculations provide insights into the relative stability of different edge configurations and passivation conditions, as driven by interactions with Co. The coupling with the substrate results also in a dramatic modification of both electronic and magnetic properties at the edges. In order to study hybridization and size effets, we transform to localized Wannier states and develop a minimal model for the effective $\pi$ states of these graphene nanostructures. [1] D. Eom et al., Nano Lett. 9, 2844 (2009). [Preview Abstract] |
Tuesday, March 16, 2010 9:36AM - 9:48AM |
H21.00007: Graphene Nanoribbons Fabricated by Helium Ion microscope D. Pickard, B. Oezyilmaz, J. Thong, K.P. Loh, V. Viswanathan, A. Zhongkai, S. Mathew, T. Kundu, C. Park, Z. Yi, X. Xu, K. Zhang, T.C. Tat, H. Wang, T. Venkatesan, G. Botton, M. Couillard Graphene, a monolayer graphitic lattice of carbon atoms has tremendous promise for a variety of applications on account of the zero mass of electrons, high mobility and the sensitivity of transport to perturbations at the interface. Patterning graphene is an obvious challenge and mesoscopic devices based on graphene require high spatial resolution patterning that will induce as little damage as possible. We use a helium ion microscope with its 0.4nm spot size beam to directly write patterns on free standing graphene films. TEM images of the patterns reveal holes as small as 4 nm and ribbons with line widths as narrow as 3 nm. The images show recovery of the graphene lattice at a distance of about a nm from the patterned edge. The linewidths of the ribbon can be varied considerably in a controllable fashion over ribbon lengths of the order of microns. . . [Preview Abstract] |
Tuesday, March 16, 2010 9:48AM - 10:00AM |
H21.00008: Superconducting Graphene Nanoelectronic Devices Joel Wang, Michele Zaffalon, Pablo Jarillo-Herrero Graphene, a single atom-thick sheet of graphite discovered in recent years, has attracted tremendous attention due to its exotic electronic properties. At low energy, its gapless linear band structure results in transport properties described by the Dirac equation, making it an ideal system for the study of exotic quantum phenomena and other new physics. Graphene may also exhibit many novel transport characteristics in the superconducting regime. New phenomena, such as pseudo-diffusive dynamics of ballistic electrons, the relativistic Josephson effect, and specular Andreev reflection are predicted by theoretical models combining relativistic quantum mechanics and superconductivity. We study these phenomena experimentally with superconductor-graphene-superconductor junctions. The supercurrent in graphene is induced by the superconducting contacts through proximity effect. Various superconducting materials are considered for different explorations. Preliminary tests indicate clean electrical contact with graphene and superconducting properties as expected. [Preview Abstract] |
Tuesday, March 16, 2010 10:00AM - 10:12AM |
H21.00009: Bistability and oscillatory motion of graphene nano-membranes on silicon dioxide Torge Mashoff, Marco Pratzer, Viktor Geringer, Tim Echtermeyer, Max Lemme, Marcus Liebmann, Markus Morgenstern The truly two-dimensional material graphene is an ideal candidate for nanoelectromechanics due to its large strength and high electron mobility. Here, we show that a monolayer of graphene on SiO$_2$ provides natural, ultra-small membranes of diameters down to 3\,nm within its intrinsic rippling. These membranes can be lifted either reversibly or hysteretically by the tip of a scanning tunneling microscope (STM). The clamped-membrane model including van-der-Waals and dielectric forces explains the results quantitatively. Application of an AC-voltage oscillates the nanomembrane, which might lead to a completely novel approach to controlled quantized oscillations or single atom mass detection. [Preview Abstract] |
Tuesday, March 16, 2010 10:12AM - 10:24AM |
H21.00010: Edge effects in quantum transport and quasiparticle spectra of graphene nanostructures J. Wurm, I. Adagideli, K. Richter In this work, we focus on the spectral and transport properties of graphene nanostructures. In recent work, we studied the effects of edges on the transport and spectral properties of graphene quantum dots, as well as on the conductance of graphene nanoribbons numerically[1]. Some edges can lead to effective time reversal symmetry breaking, others are effective intervalley scatterers. In this work, we develop a theory of transport that is capable of handling such effects in graphene nanostructures. We do this in two steps. First, we derive an exact expression for the Green function of a graphene flake, where each term in this expansion corresponds to the specific number of times the quasiparticle hits the edge. Second, we use the Green function to calculate: (i) the spectra for closed systems and; (ii) the conductance of open systems. In particular, we focus on the phase coherent effects, such as the weak localization correction to the conductance, and the universal conductance fluctuations. Moreover, we show how the size of these effects depends on the edges. [1] Wurm et al., Phys. Rev. Lett. 102, 056806 (2009) [Preview Abstract] |
Tuesday, March 16, 2010 10:24AM - 10:36AM |
H21.00011: First-principles study of the graphene edges: edge orientation, stability, equilibrium shape, and the effect of hydrogen passivation Chee K. Gan, David J. Srolovitz We use density functional theory to determine the equilibrium shape of graphene flakes, through the calculation of the edge orientation dependence of the edge energy and edge stress of graphene nanoribbons (see C. K. Gan and D. J. Srolovitz, arXiv:0909.4373 (2009)). The edge energy is a nearly linear function of edge orientation angle; increasing from the armchair orientation to the zigzag orientation. Reconstruction of the zigzag edge lowers its energy to less than that of the armchair edge. The edge stress for all edge orientations is compressive, however, reconstruction of the zigzag edge reduces this edge stress to near zero. Hydrogen adsorption is favorable for all edge orientations; dramatically lowering all edge energies and all edge stresses. It also removes the reconstruction of the zigzag edge. Using the new edge energy data, we determine the equilibrium shape of a graphene sheet (with unreconstructed edges) to be hexagonal with straight armchair edges in the presence and absence of hydrogen. However, zigzag edge reconstruction produces graphene flakes with a six-fold symmetry, but with rounded edges. This shape is dominated by near zigzag edges. The compressive edge stresses will lead to edge buckling (out-of-the-plane of the graphene sheet) for all edge orientations, in the absence of hydrogen. Exposing the graphene flake to hydrogen dramatically decreases the buckling amplitude. [Preview Abstract] |
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