Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session H26: Focus Session: Graphene IV: Electronic and Structural Properties |
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Sponsoring Units: DMP Chair: Stefano Curtarolo, Duke University Room: 328 |
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H26.00001: Graphene-based nanomechanical cooling device Wan-Ju Li, Daoxin Yao, Erica Carlson We propose a novel structure for nanomechanical cooling, based upon graphene. Because thermal transport occurs perpendicular to the surface of the substrate, the proposed structure can be used to facilitate thermal transport between two objects in contact. Furthermore, the strength of the Seeback coefficient may be tuned by applying pressure. We calculate the Seebeck coefficient in this geometry, as a function of applied voltage and strain. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H26.00002: Doping Single-Layer Graphene from SiO2 Substrates or Jointed Few-Layer Graphene Lain-Jong Li, Yumeng Shi Electrostatic force microscopy (EFM) measurements reveal that the surface potential of graphene either increases or decreases, depending on the initial surface potential of substrates, to a ``bulk'' value with their thickness. Graphene layers tend to screen of the substrate potential, which drives the charge exchange between graphene and the substrates. The direct consequence is the effective doping of graphene, where the substrate-dependent p- or n-doping of graphene is corroborated by Raman spectroscopy and electrical measurements for the transistor devices. Our results promise a practical method for tailoring the electronic properties of graphene for nanoelectronics. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 9:00AM |
H26.00003: Graphene-based Materials Invited Speaker: Our top-down approaches [Lu et al.] inspired physicists to obtain graphene by micromechanical exfoliation. Another approach to individual layers involves converting graphite to graphite oxide (GO) to generate aqueous colloidal suspensions of `graphene oxide'(GO') sheets. (i) Reduced GO' (RGO') sheets were embedded in polymers such as polystyrene and their dispersion/morphology studied by SEM/TEM, and the conductivity/ percolation threshold of such composites was determined; (ii) individual GO' and RGO' sheets were studied to elucidate their chemical, optical, and electrical properties, (iii) GO' and RGO' sheets were embedded in thin glass films by a sol-gel route yielding conductive/transparent films, (iii) a `paper' material of stacked GO' sheets was made and characterized, (iv) powders composed of RGO' showed exceptional promise for use in ultracapacitors, and (v) C13-labeled GO was made and the detailed chemical structure of GO was determined with SS NMR.~ --Lu,Yu,Huang,Ruoff, ``Tailoring graphite with the goal of achieving single sheets'', Nanotechnology, 10, 269-272 (1999). See also http://bucky-central.me.utexas.edu/publications.htm 139, 146, 150, 155, 160, 164, 166, 168, 169, 174, 179-182, 184 where collaborators are shown as coauthors. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H26.00004: Quantum transport and Klein tunneling in graphene heterojunctions Andrea Young, Philip Kim I will discuss the observation of quantum conductance oscillations in extremely narrow graphene heterostructures where a resonant cavity is formed between two electrostatically created bipolar junctions. From analysis of the observed interference pattern, it can be inferred that individual p-n junctions have a collimating effect on transmitted carriers, leading directly to the observation of resonant oscillations despite the largely diffusive carrier dynamics. The oscillatory part of the conductance is insensitive to scattering within individual p-n junctions, electrons which scatter in the junctions, making them a novel probe of the ballistic physics of graphene at the Dirac point and allowing an estimate of the electric field due to nonlinear screening. In a weak applied magnetic field, the oscillations undergo a phase shift characteristic of reflectionless normal transmission, or ``Klein Tunneling,'' at the individual p-n junctions. Finally, at high magnetic field, graphene heterostructures show modified Shubnikov de Haas oscillations due to the inhomogenous external potential. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H26.00005: Electronic transport properties of graphene irradiated by charged particles Jian-Hao Chen, William Cullen, Chaun Jang, Michael Fuhrer, Ellen Williams We have measured the effect of low energy charged particle irradiation (electrons, He ions, Ne ions or Ar ions) on the electronic transport properties of clean graphene devices on SiO$_{2}$. Charged particle irradiation induces additional scattering which is consistent with adding both short-ranged (i.e. point defect) and long-ranged (i.e. charged) impurities to the device. We also performed temperature-dependent conductivity of the irradiated devices from 9K to 400K in ultra high vacuum. In sharp contrast to graphene with charged impurity disorder, which remains metallic at low temperature, even a small amount of irradiation-induced disorder (one order of magnitude increase in room-temperature resistivity compared to pristine devices) produces a divergence of the resistivity and insulating behavior at low temperature for carrier densities below 4*10$^{12}$cm$^{-2}$. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H26.00006: Fractionalization in graphene-like systems Conan Weeks, Marcel Franz I will discuss the possible interaction-driven instabilities that can arise in a system of massles Dirac fermions modeled by the extended Hubbard model on the $\pi$ flux square lattice and the honeycomb lattice, and their relevance to fractionalization in 2D graphene-like systems. Through numerical studies we have shown that these instabilities can result in a number of interesting phases. In addition to the charge density wave and various stripe phases these include the exotic ``quantum anomalous Hall'' (Haldane) phase and the dimerized ``Kekul\'e'' phase. A self consistent calculation inside the Kekul\'e phase on the $\pi$ flux lattice indicates that a discretized $U(1)$ vortex can be stabilized in this region leading to a zero-energy bound state with fractional charge $e/2$. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H26.00007: Diffraction Symmetries in Epitaxially-Grown Graphene and the SiC Substrate David Siegel, Shuyun Zhou, Farid El Gabaly, Andreas Schmid, Kevin McCarty, Alessandra Lanzara Diffraction patterns in epitaxially-grown graphene and the SiC substrate are studied by observing dark field low energy electron microscopy (LEEM) and low energy electron diffraction (LEED) images. The breaking of 6-fold symmetry is mapped out as a function of the position on the sample for different crystal periodicities. Such observations provide information about the stacking and domain boundaries in epitaxially-grown graphene. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H26.00008: Dynamical conductivity of graphene Oskar Vafek Frequency dependent conductivity of Coulomb interacting massless Dirac fermions coupled to random scalar and random vector potentials is found as a function of frequency in the regime controlled by a line of fixed points. Such model provides a low energy description of a weakly rippled suspended graphene. The main finding is that at the neutrality point the a.c. conductivity is not frequency independent and may either increase or decrease with decreasing frequency, depending on the values of the disorder variances $\Delta_{\phi}$, $\Delta_ {A}$ and the Coulomb coupling $\alpha=e^2/(\epsilon v_F)$. The low frequency behavior is characterized by the values of two dimensionless parameters $\gamma=\Delta_{\phi}/\alpha^2$ and $\Delta_A$ which are RG invariants, and for small values of which the electron-hole ``puddles'' are effectively screened making the results asymptotically exact. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H26.00009: Charge transfer and electronic structure of graphene and graphite intercalation compounds T. Valla, J. Camacho, M. H. Upton, Z.-H. Pan, A. V. Fedorov, A. C. Walters, C. A. Howard, M. Ellerby In graphite intercalation compounds (GIC), layers of different chemical species (intercalants) are introduced between graphene sheets. Due to the charge transfer between the intercalant and graphene layers, intercalation allows a controlled doping of graphene sheets and a broad variation of many physical properties, including the emergence of relatively high transition temperature superconductivity in some GICs. We have studied the changes in the electronic structure of various GICs in angle-resolved photoemission spectroscopy and found that, with the doping of graphene sheets, the electronic correlations become stronger and more anisotropic. In particular, the electron phonon coupling of graphene-derived electronic states with high-frequency graphene-derived vibrations increases dramatically with the amount of charge transfer. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H26.00010: Low Temperature Atomic Layer Deposition of Thin HfO$_{2}$ Film as Top Gate Oxide in Graphene Field Effect Transistors K. Zou, D. Keefer, X. Hong, J. Zhu We explore the possibility of using HfO$_{2}$ as top gate oxide in graphene field effect transistors (FETs). Graphene flakes are obtained by mechanically exfoliating HOPG graphite on SiO$_ {2}$ (300nm)/doped Si substrates. We fabricate graphene FETs using e-beam lithography and metal electrodes deposition. A second e-beam writing is used to define the area of the HfO$_{2} $ over-layer. It is followed by an atomic layer deposition (ALD) of 30 nm HfO$_{2}$ film at low temperature without the use of an adhesion layer. This low-temperature recipe produces smooth HfO$_{2}$ films with RMS roughness of 2-3{\AA} over a 1x1 $\mu$m area. These films exhibit a dielectric constant of $\sim$12-15 and a breakdown field of $\sim$0.8 MV/cm. Carrier mobility in HfO$_{2}$-covered FETs is comparable to that of uncovered graphene. We report and discuss the influence of the HfO$_{2}$ over-layer on the transport properties of graphene. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H26.00011: Graphene nanoribbons and flakes produced from graphite in solution Gabriel Setzler, L.K.K.D Chamath, Zhixian Zhou We developed a method to fabricate graphene nanoribbons and flakes from graphite in solution without covalent chemical functionalization. Atomic force microscopy (AFM) was used to characterize individual graphene samples deposited onto Si/SiO$_{2}$ substrates. Few layer (and possibly monolayer) graphene sheets were observed, including micron size graphene flakes and nanoribbons with very high aspect ratios. Possible mechanisms of graphite exfoliation and graphene nanoribbon formation will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H26.00012: Electronic structure of graphene nanoflakes: band gap evolution with increasing size E. Martinez-Guerra, M.E. Cifuentes-Quintal, I. Rodriguez-Vargas, S. Vlaev, R. de Coss Graphene has unusual electronic properties which make it a promising material for electronic devices. Neverthless, the absence of a band gap sets limitations on its applications. Thus, it is crucial to tune the band gap of systems based on graphene. In this way, we explore to modulate the electronic properties of graphene through the size system. In particular, we studied the evolution of the electronic structure of graphene nanoflakes (C$_{6n2}$H$_{6n})$ as a function of size. The calculations were performed using the pseudopotential LCAO method with a Generalized Gradient Approximation for the exchange-correlation functional. We found that the energy gap decreases as the diameter of the nanoflakes increases. From a tight-binding analysis, we show that the energy gap reduction is due to an HOMO-LUMO bands widening. This effect results from the fact that as nanoflakes size increases the ratio between the number of internal and periferical atoms decreases. Thus, we found that controlling the size of graphene nanoflakes is a useful way for the electronic modulation. Supported from CONACyT Grant 83604. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H26.00013: Fabrication of graphene nanoribbon by local anodic oxidation lithography using atomic force microscope Satoru Masubuchi, Masashi Ono, Kenji Yoshida, Kazuhiko Hirakawa, Tomoki Machida We conducted local anodic oxidation (LAO) lithography in single-layer, bilayer, and multilayer graphene using tapping-mode atomic force microscope (AFM). We found that the width of the insulating oxidized area depends systematically on the number of graphene layers. An 800-nm-wide bar-shaped device fabricated in single-layer graphene exhibits the half-integer quantum Hall effect, which indicates that the conducting channel region of graphene is intact during LAO. We also fabricated a 55-nm-wide graphene nanoribbon (GNR). The conductance of the GNR at the charge neutrality point was suppressed at low temperature, which suggests the opening of an energy gap due to the lateral confinement of charge carriers. These results show that LAO lithography is an effective technique for the fabrication of graphene nanodevices. [Preview Abstract] |
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