Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session X37: Focus Session: Graphene Structure, Dopants, and Defects: Magnetism and Nanoribbons |
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Sponsoring Units: DMP Chair: Young-Woo Son, Korea Institute for Advanced Study Room: C146 |
Thursday, March 24, 2011 2:30PM - 2:42PM |
X37.00001: Oxygen- and Sulfur- driven Ferromagnetism in Graphitic Fragments: Ab-Initio Study Ivan Naumov, Yakov Kopelevich, Alexander Bratkovsky We study the origins of high-temperature ferromagnetic behavior in graphite by means of unbiased ab-initio calculations and compare them with our data. The experimental results show that oxygen/sulfur-induced edges of graphitic fragments (via unzipping effect) play an essential role in this phenomenon, and that the finite magnetic moment appears if edges in a graphitic ribbon are occupied asymmetrically by either oxygen or sulphur. In particular, our ab-initio calculations performed within the LDA and GGA approximations showed that in the case of pure graphene ribbon, its zig-zag edge carbon atoms carry large magnetic moment ($\sim $1 $\mu _{B}$/C). In an oxidized or sulfurized graphene, however, the magnetic moment at the edge with absorbed atoms gets considerably reduced, leading to effective ferromagnetic (more precisely, ferri-magnetic) behavior of the sample. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X37.00002: Defect Induced Resonances and Magnetic Patterns in Graphene Yi chen Chang We investergate the effects of point and line defects in monolayer graphene was investigated within the framework of the Hubbard model, using a self-consistent mean field theory. These defects are found to induce characteristic patterns into the electronic density of states and cause non-uniform distributions of magnetic moments in the vicinity of the impurity sites. Specifically, defect induced resonance bound states in the local density of states are observed at energies close to the Dirac points. The magnitudes of the frequencies of these resonance states are shown to decrease with the strength of the scattering potential, whereas their amplitudes decay algebraically with increasing distance from the defect. Furthermore, non-trivial impurity induced magnetic patterns are observed in the presence of line defects: zigzag line defects are found to introduce stronger-amplitude magnetic patterns than single line defect and armchair line defects. When the scattering strength of these topological defects is increased, the induced patterns of magnetic moments become more strongly localized. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X37.00003: Theory of Magnetic Edge States in Chiral Graphene Nanoribbons Rodrigo Capaz, Oleg Yazyev, Steven Louie Using a model Hamiltonian approach including electron Coulomb interactions, we systematically investigate the electronic structure and magnetic properties of chiral graphene nanoribbons. We show that the presence of magnetic edge states is an intrinsic feature of any smooth graphene nanoribbons with chiral edges, and discover a number of structure-property relations. Specifically, we describe how the edge-state energy gap, zone-boundary edge-state energy splitting, and magnetic moment per edge length depend on the nanoribbon width and chiral angle. The role of environmental screening effects is also studied. Our results address a recent experimental observation of signatures of magnetic ordering at smooth edges of chiral graphene nanoribbons and provide an avenue towards tuning their properties via the structural and environmental degrees of freedom. This work was supported by National Science Foundation Grant No. DMR10-1006184, the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 and the ONR MURI program. RBC acknowledges financial support from Brazilian agencies CNPq, FAPERJ and INCT-Nanomateriais de Carbono. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X37.00004: Spin-orbit interactions in graphene nanoribbons : Effects of the edge profile Jun-Won Rhim, Kyungsun Moon In graphene, it has been shown by Kane and Mele that the spin orbit coupling (SOC) connects the Dirac particles to the low- lying $p_x$ and $p_y$ orbitals so that the quantum spin Hall effect(QSHE) arises at the edges of the graphene. Their theory has drawn considerable attention as a realization of Haldane's idea of quantum Hall effect without magnetic field and as a trigger for the surging field of topological insulator. In the work, we study the band structure of the zigzag nanoribbons with the spin-orbit interaction and argue that the role of graphene edge should be considered more carefully since the realization of the QSHE is found to be largely dependent on the edge profile such as the kinds of molecules passivated. When the edge $p_x$, $p_y$ and $s$ orbitals are dangling without any passivation, the Dirac states at the edges seem to be no longer chiral for each spin species and the QSHE is not guaranteed to occur. We notice that upon the hydrogen passivation at the edges, the spin filtered chiral edge states become available. We will explain that these are due to the interaction between $\pi$-edges states and $\sigma$-edge states. The similar calculations are also performed for the armchair nanoribbons and compared with those of zigzag nanoribbons. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X37.00005: Quantum Monte Carlo Study of Edge Magnetism in Nanoribbons of Graphene Z.Y. Meng, H. Feldner, T.C. Lang, S. Wessel, A. Honecker, F. Assaad We study the electronic and magnetic properties of graphene nanoribbons, employing projective quantum Monte Carlo simulations within the Hubbard model description of electrons in graphene. We also compare our numerical results to a self-consistent mean field approximation in the weak coupling regime. Motivated by recent STM experiments about electronic resonance around atomic vacancies on multilayer graphene and graphene nanoisland, we in particular examine the local density of states throughout the sample. From this, we verify that interacting zig-zag ribbons develop an insulating ground state with a finite single particle gap from the localized edge modes observed in the non-interacting limit. In addition, we observe a drastic increase of the spin-spin correlation length along the zig-zag edge with the ribbon width. Effectively, on our finite samples ferromagnetic edges appear already for moderately wide zig-zag ribbons. This ferromagnetism is accompanied by an essentially gapless edge magnon mode, that we identify in the spin excitation spectrum. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X37.00006: First principles study of edge effects in electronic structures of graphene nanoflakes and nanoribbons Chengbo Han, Wenchang Lu, Jerry Bernholc Graphene is a promising material for future nanoelectronics. Understanding of the edge effects on the electronic structure of graphene nanoflakes and nanoribbons is important for its nanoscale applications. Using the real space multigrid method within density functional theory, we systematically simulate STM images of nanoflakes and nanoribbons with both zigzag and armchair edges. Our results explain several STM patterns seen in experiments [1], such as triangular and hexagonal lattices for different shapes of flakes. We also find that localization of edge states in zigzag flakes depends on the interior angle between two edges. Furthermore, we show that the influence of Si(001)-2x1-H substrate on the local density of states of graphene nanoflakes is not significant when the graphene layer is 0.3 nm above the substrate.\\[4pt] [1] K. A. Ritter and J. W. Lyding, Nature Materials 8, 235 (2009). [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 4:18PM |
X37.00007: Spin states in graphene quantum dots Invited Speaker: Graphene quantum dots [1,2], douple dots [3], rings [4] and nanoribbons [5] have been fabricated by electron beam lithography and dry etching. The orbital [1] properties of graphene quantum dots have been investigated in perpendicular magnetic fields and the details of the electron-hole crossover in graphene leads to a situation where electron (hole) states move down (up) in magnetic field opposite to what has been observed in standard semiconductor based quantum dots. Graphene quantum dots are thought to be good candidates for spin-based quantum information processing since spin-orbit interactions and hyperfine coupling are both expected to be weak. We investigated graphene quantum dots in the single-level transport regime in in-plane magnetic fields where orbital effects are expected to have a minor effect [6]. The g-factor is found to be $g\approx 2$ and the spin filling sequence of orbital levels can be understood in view of the strength of the exchange interaction which is independent of carrier density in graphene. \\[4pt] [1] J. Guttinger, C. Stampfer, F. Libisch, T. Frey, J. Burgdoerfer, T. Ihn, K. Ensslin, Phys. Rev. Lett. 103, 046810 (2009) \\[0pt] [2] T. Ihn, J. Guttinger, F. Molitor, S. Schnez, E. Schurtenberger, A. Jacobsen, S. Hellmuller, T. Frey, S. Droscher, C. Stampfer, and K. Ensslin, Materials Today 13, 44 (2010) \\[0pt] [3] F. Molitor, H. Knowles, S. Droscher, U. Gasser, T. Choi, P. Roulleau, J. Guttinger, A. Jacobsen, C. Stampfer, K. Ensslin and T. Ihn, Europhys. Lett. 89, 67005 (2010) \\[0pt] [4] M. Huefner, F. Molitor, A. Jacobsen, A. Pioda, C.Stampfer, K. Ensslin and T. Ihn, N. J. of Phys. 12, 043054 (2010) \\[0pt] [5] C. Stampfer, J. Guttinger, S. Hellmuller, F. Molitor, K. Ensslin, and T. Ihn, Phys. Rev. Lett. 102, 056403 (2009) \\[0pt] [6] J. Guttinger, T. Frey, C. Stampfer, T. Ihn, and K. Ensslin, Phys. Rev. Lett. 105, 116801 (2010) [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X37.00008: Complex edge effects in graphene nanoribbons due to hydrogenation Biplab Sanyal, Sumanta Bhandary, Mikhail Katsnelson, Olle Eriksson We have performed density-functional calculations as well as employed a tight-binding theory, to study the effect of hydrogenation of zigzag graphene nanoribbons (ZGNR). We show that each edge C atom bonded with 2 H atoms open up a gap and magnetism collapses for small widths of the nanoribbon. However, a re-entrant magnetism accompanied by a metallic electronic structure is observed from eight rows and thicker nanoribbons. The electronic structure and magnetic state are quite complex for this type of termination, with $sp^{3}$ bonded edge atoms being nonmagnetic whereas the nearest neighboring atoms are metallic and magnetic. We have also evaluated the phase stability of several thicknesses of ZGNR and demonstrate that $sp^{3}$ bonded edge atoms with 2 H atoms at the edge can be stabilized over 1 H atom terminated edge at high temperatures and pressures. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X37.00009: Magnetism in bulk and finite size graphene multilayers and its effect on the band gaps Bhagawan Sahu, Hongki Min, Sanjay Banerjee, Allan MacDonald In this talk, we will address the edge state magnetism and the resulting modulation of band gaps induced by quantum confinements in multilayer graphene ribbons and flakes. The magnetism arising from random point defects such as vacancies in bulk graphene layers will also be presented. The robustness of magnetism with respect to the edge disorder and the saturating agents in finite size graphene layers and with respect to the defect concentrations in bulk graphene layers will be discussed. A numerical approach based on density functional theory which uses plane-wave basis set and pseudopotentials for ion-electron interactions will be used for elucidating the complex interplay of magnetism, external electric field applied perpendicular to the layers and the resulting band gaps. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X37.00010: Exploring the Structure of Graphene Nanoribbons Using Scanning Tunneling Microscopy Yen-Chia Chen, Juanjuan Feng, Chenggang Tao, Liying Jiao, Xiaowei Zhang, Oleg Yazyev, Rodrigo Capaz, Alex Zettl, Steven Louie, Hongjie Dai, Michael Crommie The confined dimension and edges of graphene nanoribbons (GNRs) are predicted to result in novel magnetic edge states and tunable energy gaps. Such properties should be strongly dependent on GNR nanoscale structure. Here we report a scanning tunneling microscopy (STM) study of the structure of GNRs derived from unzipped carbon nanotubes that are deposited onto different substrates. These GNRs are found to have different chiralities and widths, and show some unexpected geometrical structure near the edges. We will also present new results obtained from GNRs with disordered edges. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X37.00011: STM and STS studies of CVD grown graphene nanoribbons Xiaoting Jia, Minghu Pan, Sreekar Bhaviripudi, Vincent Meunier, Jing Kong, Mildred Dresselhaus Graphene nanoribbons (GNRs) are quasi one dimensional structures which have unique transport properties, and have a potential to open a bandgap at small ribbon widths. They have been extensively studied in recent years due to their high potential for future electronics applications. We have experimentally found some GNRs in our CVD grown graphene layers. In this work, we investigated the morphology and electronic properties of the GNRs on top of a graphene layer transferred to a SiO$_{2}$ substrate by using scanning tunneling microscopy. Our results suggest that these GNRs have a surprisingly high crystallinity with one side folded. Atomic resolution images were obtained on the folded layer and the bottom layer of the GNR, which enables clear identification of the chirality for both layers. By combining with theoretical modeling we conclude that a (5,7) line defect exists at the zone of maximum curvatures to help reducing the strain energy of the folding. Low temperature spectroscopic measurements suggest that different electronic states may exist at GNR edges, when compared to the ribbon interior regions. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X37.00012: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X37.00013: Large intrinsic energy band gaps in annealed nanotube-derived graphene nanoribbons J. Haruyama, T. Shimizu, D.C. Marcano, D.V. Kosinkin, J.M. Tour, K. Hirose, K. Suenaga The usefulness of graphene for electronics is diminished by an absent energy bandgap. While graphene nanoribbons have non-zero bandgaps, lithographic fabrication methods introduce defects which decouple the bandgap from electronic properties and compromise performance [1]. Here, we present direct measurements of a large intrinsic energy bandgap of approximately 50 meV in 100 nm-width level nanoribbons fabricated by high-temperature annealing of unzipped carbon nanotubes [2]. The activation energy is seven times greater than those in [1], and is close to the width of the transport gap in the differential conductance. This similarity suggests that the activation energy is in fact the intrinsic bandgap. High-resolution TEM and Raman spectroscopy, along with an absence of hopping conductance and stochastic charging effects, suggest a low defect density. [1] M.Y. Han, P. Kim et al., PRL 104, 056801 (2010) [2] J.Haruyama, J.M.Tour, et al., Nature Nanotech. (December 2010) [Preview Abstract] |
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