Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session W22: Graphene Nanoribbons |
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Sponsoring Units: DMP DCMP Chair: Gary Pennington, University of Maryland Room: Portland Ballroom 252 |
Thursday, March 18, 2010 11:15AM - 11:27AM |
W22.00001: ABSTRACT WITHDRAWN |
Thursday, March 18, 2010 11:27AM - 11:39AM |
W22.00002: Graphene Nanoribbons with Crystallographically-Orientated Edges Javier Sanchez-Yamagishi, Ken Van Tilburg, Vitor Manfrinato, Leonardo Campos, Karl Bergerren, Pablo Jarillo-Herrero When graphene is confined to a nanoribbon a transport gap is opened which allows for field effect transistor operation. Such graphene nanoribbon FETs have been demonstrated, but are thought to be dominated by edge disorder and hence operate far from the the intrinsic regime. We present graphene devices with nanoribbons defined by crystallographically-orientated edges. The devices are formed by Ni nanoparticles which cut graphene along crystallographic directions, unlike the randomly orientated edges produced by standard plasma etching processes. We present TEM and AFM characterizations of the cutting process, as well as electronic measurements of the produced nanostructures. [Preview Abstract] |
Thursday, March 18, 2010 11:39AM - 11:51AM |
W22.00003: Spin stiffness of graphene and zigzag graphene nanoribbons Jun-Won Rhim, Kyungsun Moon We theoretically study the spin stiffness of graphene and graphene nanoribbon based on the Hubbard-type Hamiltonian. Using the Hartree-Fock method with the inclusion of the adiabatic spin twist, we have obtained the effective energy functional and investigated the magnetic excitations of the two-dimensional graphene and zigzag graphene nanoribbon (ZGNR). We have analyzed the spin stiffness of the system with varying temperature and the strength of on-site Coulomb repulsion. For ZGNR, we have also studied the effect of the lateral electric field on the spin stiffness. As the field increases, the spin stiffness decreases and reaches less than the half of the zero-field value. However, we remarkably notice that there exists a critical value of the electric field above which the stiffness starts to increase showing a cusp-like behavior. This critical point is found to coincide exactly with the metal-insulator transition point of ZGNR. [Preview Abstract] |
Thursday, March 18, 2010 11:51AM - 12:03PM |
W22.00004: Edge state engineering on graphene nano ribbons via local potential field Sungjong Woo, Young-Woo Son Charge accumulation on the edge of doped graphene nanoribbons (GNR) has been studied by Silvestrov {\it et.al.} We have investigated the electronic band structures and transport properties of doped GNRs especially with zigzag edge structure. For a doped GNR, the shift of Dirac point from the Fermi level is almost constant except the ribbon edge where charge is seriously accumulated. From our analysis, it is found that the energy of well-known localized edge states of zigzag GNRs follows the shifted Dirac point on the edge, bending the corresponding single flat band line. Based on this, we have further uncovered that one can achieve conductance enhancement exactly by the amount of a single conducting channel by applying transversely localized external potential field on one of the two edges so that the type of doping on that edge changes from electron(hole)-doping to hole(electron)-doping. More importantly, such conductance enhancement has turned out to be quite independent of the edge structure except the case of perfect armchair structure. The effect of a local potential in the presence of electron hole puddle on a GNR will also be discussed. [Preview Abstract] |
Thursday, March 18, 2010 12:03PM - 12:15PM |
W22.00005: ABSTRACT WITHDRAWN |
Thursday, March 18, 2010 12:15PM - 12:27PM |
W22.00006: Magnetic Coupling Between Transition Metal Chains Via Graphene Nanoribbons S. Vincent Ong, R. Robles, S.N. Khanna Graphene nanoribbons have generated much interest due to their unique electronic and magnetic properties. Current theoretical research has suggested that nanoribbons may have possible applications in spintronics devices. Dangling bonds at the zigzag ribbon edges have often been studied by saturation with hydrogen. We have carried out first principles theoretical studies on zigzag graphene nanoribbons of varying widths doped with 3d-elements. Our results indicate an unconventional magnetic coupling between the chains mediated via the carbon lattice. The stability of the system, magnetic ground state, and transport properties will be presented. [Preview Abstract] |
Thursday, March 18, 2010 12:27PM - 12:39PM |
W22.00007: Electronic properties of graphene nanoribbons with multiple passivating edge species Maria Stournara, Vivek Shenoy, Ashwin Ramasubramaniam We suggest a novel approach of engineering the band gap of zigzag nanoribbons (ZGNRs) with multiple coexisting functional edge species, in particular, ZGNRs with varying ratios and spatial distribution of mixed oxygen and hydrogen passivation. Prior studies have already demonstrated that H-termination is responsible for semiconducting behavior of graphene, whereas O-termianation is purely metallic. Based on density functional studies, we show that ribbons with mixed O and H termination exhibit a rich variety of behavior that depends both upon the ratio of O and H atoms as well as their spatial distribution. In some instances it is even possible to double the band gap of H-terminated ZGNRs by introducing coexisting O edge atoms. We present a systematic analysis of these trends and propose a simple model to explain the observed behavior. [Preview Abstract] |
Thursday, March 18, 2010 12:39PM - 12:51PM |
W22.00008: Width-dependent dimensional crossover in zigzag graphene nanoribbons Mahdi Zarea, Nancy Sandler The exact solution of a tight-binding Hamiltonian shows that the unusual width- dependent band-structure of zigzag graphene nanoribbons (ZGNR) is caused by spinor wavefunction phase shifts. These results suggest the correct continuum description of these systems to be in terms of effective coupled-chains models. The particular anisotropic continuum limit reproduces the physics of ZGNR with high accuracy while capturing the width-dependence effect. It also reveals the underlying connection between tight-binding ZGNR models and a continuous set of two-dimensional models that include the square lattice and the $\pi$-flux model. The Majorana fermion language used in this approach shows that ZGNRs are physical realizations of various quantum spin chain (QSC) models. [Preview Abstract] |
Thursday, March 18, 2010 12:51PM - 1:03PM |
W22.00009: Unimpeded Tunneling in Graphene Nanoribbons Andrii Iurov, Oleksiy Roslyak, Godfrey Gumbs, Danhong Huang The Klein paradox is unimpeded tunneling of the purely bonded Dirac electron state across arbitrary wide gated region. Its another manifestation is perfect reflection in the graphene stacks. We studied the Klein paradox in zigzag (ZNR) and anti- zigzag (AZNR) graphene nanoribbons. For ZNR (AZNR), the number $N$ of lattice sites across the nanoribbon is even (odd). Since the ZNR and AZNR (configurations are indistinguishable in the Dirac formalism, we supplemented the model with a pseudo-parity operator whose eigenvalues correctly give the dependence on $N$ for the sublattice wavefunctions, in agreement with the tight- binding model. We have shown that the Klein tunneling in zigzag nanoribbons is determined by conservation of the pseudo-parity rather than pseudo-spin which is required in infinite graphene. Chirality is the projection of the pseudo-spin on momentum at different corners of the Brillouin zone. Perfect transmission for head-on incidence is replaced by perfect transmission at the center of the ribbon. [Preview Abstract] |
Thursday, March 18, 2010 1:03PM - 1:15PM |
W22.00010: Graphene nanoribbons without cutting graphene Matheus Paes Lima, Alexandre Reily Rocha, Ant\^onio J.R. da Silva, Adalberto Fazzio We show that the 2D periodic graphene deposited on Silicon Carbide surface with a trench mimics a grapheme nanoribbon. Our study is carried out with calculations based on Density Functional Theory. In our work, the graphene is deposited at the [0001] and the [0001 \={ }] surfaces. We investigate the influence of the charge transfer between the graphene and the substrate, the local magnetic moment, as well as the direction of the trench on the electronic properties of such systems. Our results suggest that at the [0001] surface the charge transfer is large resulting in a large change in the Fermi energy. As a consequence, the mimicked armchair graphene nanoribbons turn out to be metallic and the mimicked zigzag graphene nanoribbons are nonmagnetic. These properties are distinct from the corresponding free standing graphene nanoribbons. On the other hand, at the [0001 \={ }] surface, the charge transfer is small, and the properties of the mimicked ribbons are very similar to the free standing ones. [Preview Abstract] |
Thursday, March 18, 2010 1:15PM - 1:27PM |
W22.00011: Polaron Formation in Graphene Nanoribbons Ivo Batistic, Avadh Saxena, Alan Bishop The tight-binding SSH model, commonly used to describe trans- and cis-polyacetylene and other conducting polymers, can also be applied to study polaron formation in the recently synthesized graphene nanoribbons. Within the model we find that the stability of a polaron is strongly dependent not only on the type of the ribbon (armchair vs. zig-zag), but also on the ribbon width. The formation of polaron implies the existence of localized energy states within the electronic band and localized lattice deformation as well as localized phonon excitations. All these properties can be calculated within the SSH model, which in turn can be used as a reference for a comparison with experimental measurements on optical, electronic and transport properties of graphene nanoribbons with differing width and type. [Preview Abstract] |
Thursday, March 18, 2010 1:27PM - 1:39PM |
W22.00012: Tunable Band Structure Effects on Ballistic Transport in Graphene Nanoribbons Oleksiy Roslyak, Godfrey Gumbs, Danhong Huang Recent advantages in the fabrication techniques of graphene nanoribbons (GNR) together with the long electron mean free path have stimulated considerable interest in their potential applications as interconnects in nano circuits. We have demonstrated that when GNRs are placed in mutually perpendicular electric and magnetic fields, there are dramatic changes in their band structure and transport properties. The electric field across the ribbon induces multiple chiral Dirac points, whereas a perpendicular magnetic field induces partially formed Landau levels accompanied by dispersive surface-bound states. Each of the fields by itself preserves the original even parity of the subband dispersion, i.e. $E_{n,k}=E_{n,-k}$, maintaining the Dirac fermion symmetry. When applied together, their combined effect is to reverse the dispersion parity to being odd with $E_{{\rm e},k}=-E_{{\rm h},-k}$ and to mix electron and hole subbands within an energy range equal to the potential drop across the ribbon. Broken Dirac symmetry suppresses the wave function localization and the Zitterbewegung effect.The B\"utikker formula for the conductance holds true for the odd $k$ symmetry. This, in turn, causes the ballistic conductance to oscillate within this region which can be used to design tunable field-effect transistors. [Preview Abstract] |
Thursday, March 18, 2010 1:39PM - 1:51PM |
W22.00013: Ballistic Thermal Conductance of a Graphene Ribbon Enrique Munoz, Jianxin Lu, Boris Yakobson Recent experiments on thermal transport in graphene suggest that the phonon mean free path may exceed 500 nm,\footnote{S. Ghosh, et al., ``Extremely high thermal conductivity of graphene: Prospects for thermal management applications in nanoelectronic circuits'' Applied Physics Letters, 2008. \textbf{92}: p. 151911.} with thermal conductivities in the range 3000 -- 5000 (W/m/K). In this scenario, it is expected that thermal transport is dominated by a ballistic rather than diffusive mechanism. We present an analytical theory to calculate the thermal conductance of a graphene ribbon in the ballistic regime. For that purpose, we analyze the vibrational modes of a continuum thin plate with isotropic elastic properties. To address the effect of nanoscale dimensions, we consider a finite width $w$ in the model. At low temperatures, our analytical theory shows a power law dependence of the thermal conductance per unit width, were the exponent $\beta $ is a function of the ribbon width, ranging from $\beta $ = 1 for thin graphene ribbons, towards $\beta $ = 1.5 in the limit of a large graphene sheet. Quantitative predictions of our theory at room temperature are in good agreement with experiments.\footnote{Ibid.} [Preview Abstract] |
Thursday, March 18, 2010 1:51PM - 2:03PM |
W22.00014: Thermal conductivity of graphene nanoribbons Guo Zhixin, Zhang Dier, Gong Xin-Gao We have investigated thermal conductivity of graphene nanoribbons (GNRs) with different edge shapes as a function of the length, width, and strain in use of the nonequilibrium molecular dynamics method. The thermal conductivity does not converge to a finite value with the increase of GNRs' length up to 60 nm, while follows a power law of K$\sim $L$^{\beta }$, indicating very high thermal conductivities and long PMFPs of GNRs. Moreover, the thermal conductivity is very sensitive to the edge shapes. It is found the zigzag GNR's thermal conductivity increases first and then decreases with the width increasing, while, the armchair GNR's thermal conductivity monotonously increases with width. A competitive mechanism is further proposed to explain such interesting phenomena. Very remarkable decrease of thermal conductivity is also obtained when a tensile/compressive uniaxial strain is applied on the GNRs [1]. \\[4pt] [1] Zhixin Guo, Dier Zhang, and Xin-Gao Gong, Appl. Phys. Lett. 95, 163103 (2009). [Preview Abstract] |
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