Bulletin of the American Physical Society
76th Annual Meeting of the Southeastern Section of APS
Volume 54, Number 16
Wednesday–Saturday, November 11–14, 2009; Atlanta, Georgia
Session DC: Condensed Matter I |
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Chair: Ramesh Mani, Georgia State University Room: Paris |
Thursday, November 12, 2009 1:30PM - 1:42PM |
DC.00001: Tunneling Magneto-Conductance Oscillations in Epitaxial Graphene Kevin D. Kubista, David L. Miller, Ming Ruan, Walt A. de Heer, Phillip N. First, Gregory M. Rutter, Joseph A. Stroscio Scanning tunneling microscopy (STM) and spectroscopy (STS) at a temperature of 4 K are used to study the electronic properties of epitaxial graphene on SiC in a magnetic field applied perpendicular to the graphene plane. While changing the magnetic field we observe oscillations in the tunneling conductance, dI/dV (tunneling magneto-conductance oscillations, or TMCO). These are similar to Shubnikov-de Haas oscillations of magnetoresistance, but differ in important aspects. Magnetic field scans acquired at multiple tunneling injection energies allow us to create a density-of-states contour map as a function of both in energy and magnetic field. The data are well-described by the monolayer-graphene density of states, with small additional features that may indicate coherent effects. This work was supported in part by NSF, NRI-INDEX, and the W. M. Keck Foundation. [Preview Abstract] |
Thursday, November 12, 2009 1:42PM - 1:54PM |
DC.00002: Mapping Quantized Cyclotron Orbits of Dirac Particles in Graphene David Miller, Kevin Kubista, Ming Ruan, Walt de Heer, Phil First, Gregory Rutter, Joseph Stroscio Monolayer graphene has unique electronic properties stemming from a low-energy band structure that is linear near the charge neutrality point (Dirac point). In a magnetic field, cyclotron orbits of electrons and holes are quantized into ``Landau levels'' (LL), with energies $E_n$ that vary proportional to $\sqrt{nB}$, where $n$ is the LL quantum number. The Dirac physics of this system provides an interesting opportunity for experiments in scanning tunneling microscopy (STM) and spectroscopy (STS). We present results from STM/STS measurements of the local density of states (LDOS) of graphene grown epitaxially on SiC (000-1). The sample was studied at 4.3K in ultra-high vacuum under an applied perpendicular magnetic field $\leq$ 8T. Using STS maps, we observed the LDOS over a 100 nm$^2$ area at energies within $\pm$200meV of the Dirac point. We find transitions from localized to extended states as the LDOS energy progresses across Landau levels. The zero-energy Landau level--unique to graphene--is observed to shift and split in areas where localized states occur. Results from Landau levels of different quantum numbers are also presented for comparison. Work supported in part by NSF, NRI-INDEX, and the W. M. Keck Foundation. [Preview Abstract] |
Thursday, November 12, 2009 1:54PM - 2:06PM |
DC.00003: Transport Properties and Surface Morphology Correlated Studies on Graphene Formed by Si Desorption of 6H-SiC William Roach, Douglas Beringer, Jonathan Skuza, Cesar Clavero, Rosa Alejandra Lukaszew Interest in graphene, a single layer of carbon atoms arranged in a hexagonal lattice, has increased in recent years due to exciting characteristics such as its predicted high mobility [1]. However, developing a method to produce graphene that is easily integrated into existing fabrication processes has proved difficult thus far. One promising method is high temperature annealing of 6H-SiC such that Si desorption occurs [2], although this method leads to graphene that exhibits lower mobility than predicted [3]. Thus, we have investigated the relationship between different growth conditions (i.e. annealing time and temperature), the resulting surface morphology and the transport properties of graphene films produced using this method. Raman spectroscopy, atomic force microscopy, and Van der Pauw Hall mobility measurements have been used to correlate the surface morphology to transport properties of graphene formed on SiC. Understanding the effect of growth conditions on the resulting transport properties will help optimize the fabrication of graphene for use in the next generation of electronic devices and other applications. [1] Novoselov \textit{et al.}, Science \textbf{306}, 666 (2004). [2] C. Berger \textit{et al.}, J. Phys. Chem. B \textbf{108}, 19912 (2004). [3] G. Gu \textit{et al.}, Appl. Phys. Lett. \textbf{90}, 253507 (2007). [Preview Abstract] |
Thursday, November 12, 2009 2:06PM - 2:18PM |
DC.00004: Controlled growth of monolayer graphene on silicon carbide in argon atmosphere David Torrance, David Miller, Holly Tinkey, Evan Green, Madeleine Phillips, Phillip First Controlled thermal decomposition of silicon carbide is so far the most effective method for growing graphene epitaxially and at the wafer scale. In this work we study the graphenization of SiC(0001) and SiC($000\bar{1}$) as a function of ambient argon pressure and temperature in a custom-built ultrahigh vacuum (UHV) induction furnace. In-situ characterization by both Auger electron spectroscopy and low-energy electron diffraction (LEED) was used to determine the pressure-temperature ``phase boundary'' for the formation of monolayer graphene. Sample quality was further assessed ex-situ using a variety of techniques such Raman spectroscopy and scanning tunneling microscopy. The effect of the inert argon overpressure was modeled numerically with a simple kinetic growth theory. [Preview Abstract] |
Thursday, November 12, 2009 2:18PM - 2:30PM |
DC.00005: Study of the resistance induced by metal contacts in graphene by first-principles methods Salvador Barraza-Lopez, Mei-Yin Chou The detailed knowledge of the resistance induced by metals contacting graphene has been the subject of thorough experimental and theoretical efforts, as these studies conform a necessary stage before graphene could become a viable material for electronic applications. Furthermore, it is desirable to determine optimal interfaces to bring this resistance to its minimum possible values. Theoretical modeling of transport through graphene with real metallic leads is in its first stages, and some assumptions made in the models so far presented lack sound justification or validation from first-principles studies. With a combination of density-functional theory and large-scale non-equilibrium Green's function methods, a thorough study of the conductance/resistance induced by aluminum contacts on suspended graphene is presented, as a function of the graphene's width as well as the magnitude of the suspended length. Beyond an electron-hole asymmetry in the conductance features, we have been able to confirm a conduction gap for widths smaller than 10 nm (in accordance with experimental observation), and to observe the evolution of a prominent peak in the conductance that evolves as a function of the length between the metallic contacts. The insight acquired from simulation can be employed to construct a minimal tight-binding model to predict the conductance through graphene attached to metal leads. [Preview Abstract] |
Thursday, November 12, 2009 2:30PM - 2:42PM |
DC.00006: Impact of Gradient in Dynamics at the Free Surface on the Physical Aging of Polystyrene Films and Its Connection to the Glass Transition Temperature Reductions Justin Pye, Kate Rohald, Elizabeth Baker, Connie Roth The glass transition and physical aging in nanoconfined polymer films have been heavily studied. Recently there have been an increasing number of studies pointing towards a gradient in mobility emanating from the free surface in these films. Using a new streamlined ellipsometry technique, we have measured the temperature dependence of the physical aging rate $\beta $ for both bulk (2430 nm) and thin (29 nm) polystyrene (PS) films supported on silicon. We find that the thinner films have reduced physical aging rates at all temperatures that are inconsistent with a simple shift in the temperature dependence of $\beta $ corresponding to the shift in $T_{g}$ observed in these films. The reduced $\beta $ values measured at all physical aging temperatures are consistent with a gradient in dynamics originating from the free surface of the film. Our data is well fit by a simple two-layer model that has been previously employed to explain the $T_{g}$ reductions in PS thin films, suggesting that the enhanced dynamics present at the free surface are responsible for both effects. The surface layer thickness of this two-layer model, which increases with decreasing temperature, characterizes the depth to which the enhanced mobility at the free surface propagates into the film. [Preview Abstract] |
Thursday, November 12, 2009 2:42PM - 2:54PM |
DC.00007: Switching Behavior of Ag/Molecule/Metal Junctions Using $m$-1-Carboranethiol, 3-Chloro-1-propanethiol, and 1,1',4',1''-Terphenyl-4-thiol Kevin Andring, Lam Yu The investigation of nanoscale switch elements is an important step towards developing low-power, low-cost, high-density electronic devices. Here we present how the switching behavior of a Ag/molecule/metal junction changes with the variation of monolayers in a crossed-wire tunnel junction geometry. By creating self-assembled monolayers (SAMs) on silver and gold wires, this geometry is used to show how the switching rates, threshold voltages, and on/off ratios vary with monolayers of $m$-1-Carboranethiol, 3-Chloro-1-propanethiol, and 1,1',4',1''-Terphenyl-4-thiol. [Preview Abstract] |
Thursday, November 12, 2009 2:54PM - 3:06PM |
DC.00008: Influence of Residual Stresses from Thermal Expansion Mismatch during Quenching on the Subsequent Physical Aging of Polymer Thin Films James Davidheiser, Suk Yoon, Connie Roth Physical aging studies in the research literature over the past 15 years have observed large increases in physical aging rate with decreasing film thickness. Surprisingly these effects are observed for micron thick films, an order of magnitude or two larger than thicknesses where nanoconfinement effects on the glass transition and modulus are typically observed. We present physical aging measurements using ellipsometry on polystyrene (PS) films thermally quenched in different states suggesting that the increased physical aging rate with decreasing film thickness may be attributable to residual stresses in these films. For example, we find that 1400 nm thick PS films quenched in a free-standing state exhibit physical aging rates comparable to those for bulk PS measured by dilatometry, while 600 nm thick PS films quenched in a free-standing state exhibit a much faster aging response more comparable to those observed for supported PS films of any thickness. We are working towards correlating the residual stresses due to the thermal expansion mismatch between the film and support with the observed physical aging rates. [Preview Abstract] |
Thursday, November 12, 2009 3:06PM - 3:18PM |
DC.00009: Effect of Ultra Violet Radiation on Surface Properties: Comparison of RTV 655 and Silica--Based Aerogels MacKenzie Sinden-Redding, Firouzeh Sabri The broad spectrum of ultra violet (UV) radiation causes material property changes such as chalkiness, brittleness, color change, and ultimately complete mechanical failure. UV radiation is also known to modify the surface charge state of insulators. These effects are intensified for materials used in space exploration due to the lack of an atmosphere. In this work, we compare the radiation response and the material properties of RTV 655 (existing calibration targets material on Phoenix Mars Lander) and silica-based aerogels. The extreme light-weight and ability to color-code aerogels makes this material a candidate for the next generation of calibration targets. The radiation response of both materials will be studied using Kelvin Probe, UV-VIS spectrophotometry, and ESR techniques. [Preview Abstract] |
Thursday, November 12, 2009 3:18PM - 3:30PM |
DC.00010: Freshman Research Experience: Deposition and Characterization of thin films Travis Vaughn, Tatiana Allen, Phillip Broussard In the summer of my freshman year, I participated in research which was a collaborative effort between the University of Tennessee-Chattanooga (advisor Dr. Tatiana Allen) and Covenant College (advisor Dr. Phillip Broussard). The goal of the research is to make thin metallic and semiconductor films to observe and study phase transitions: niobium films to study superconducting properties; molybdenum-nickel layers to study ferromagnetic transition, and manganese-based perovskite oxides to study metal-insulator transition. The films are deposited by magnetron sputtering. The transport properties of the films such as resistivity (by van-der-Pauw method), Hall Effect and magnetoresistance are studied at the University of Tennessee-Chattanooga in the temperature range between 77 and 700K. Resistivity at lower temperatures is studied at the Covenant College. During the summer I learned the theoretical aspects of the deposition and characterization procedures and was able to deposit several films. In the fall semester I will continue to work on film characterization. [Preview Abstract] |
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