Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session T25: Focus Session: Graphene XI: Scanning Probes I |
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Sponsoring Units: DMP Chair: Eva Andrei, Rutgers University Room: 327 |
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T25.00001: Scanning Kelvin Probe Study of Electric Field Effect Tuning of Graphene Work function Young-Jun Yu, Yue Zhao, Kwang S. Kim, Philip Kim We present the experimental work on the wok function variation of mono and bi-layer graphene device measured by scanning Kelvin probe microscopy (SKPM). Using the electric field effect (EFE), the work function of graphene can be adjusted as the gate voltage tunes the Fermi level across the charge neutrality point. Mono and bi-layer graphene samples are deposited on a silicon oxide covered silicon substrate by a mechanical exfoliation method and electrical contacts are fabricated by electron beam lithography. The underlying silicon substrate is used as a back gate to tune the carrier concentration of the graphene. After subtracting off the large back ground signal originating from the electrostatic environment, we obtain the work function of graphene samples modulated by the gate voltage. The change of work function can be ascribed by the Fermi level shift due to the EFE induced carrier doping and well quantified by the electronic band structure of mono and bi-layer graphene. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T25.00002: STM Studies of Graphene Films Prepared by Sonication-Assisted Dispersion Elena Stolyarova, Kwang Taeg Rim, George Flynn We present STM (Scanning Tunneling Microscopy) studies of thin graphene films prepared by a spraying technique via direct~exfoliation of graphite in organic solvent (dimethylformamide). Our results show that these films are a patchwork of an unperturbed hexagonal graphene network mixed with heavily functionalized areas. The area of graphene-like spots does not exceed 10nm$^{2}$. Further, we discuss the influence of chemical reduction of these films on their atomic structure. [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T25.00003: Moir\'{e} pattern for graphene on a graphite surface and its scanning tunneling spectroscopy Adina Luican, Guohong Li, Eva Andrei We report low temperature high magnetic field scanning tunneling microscopy and spectroscopy on a graphene layer rotated with respect to the orientation of a graphite substrate, which produces distinct Moir\'{e} patterns. Scanning tunneling spectroscopy in the rotated area reveals that the tunneling conductivity~has two pronounced peaks flanking the Dirac point. Comparison to a theoretical model [1] shows that the two peaks are the signature of rotated layers and their separation reflects the angle of rotation. We will discuss spatial variations of the tunneling spectra within the Moir\'{e} pattern and their dependence on magnetic field. [1] J\textit{. M. B. Lopes~dos~Santos, N. M. R. Peres, and A. H. Castro~Neto, Phys. Rev. Lett. 99, 256802 (2007)} [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T25.00004: Low temperature scanning probe imaging of electron transport in graphene nanostructures Jesse Berezovsky, Robert Westervelt Experiments on the transport properties of graphene over the last several years have revealed numerous unusual and fascinating results. These studies typically rely on lithographically patterned contacts and gates that can obscure effects arising due to spatially varying properties. Using a biased scanning probe tip, we can create a local gate or scattering potential and observe the resulting change in the transport properties of a graphene structure. Simulations show that this technique can image the fluctuating potential in a graphene sheet with a spatial resolution of tens of nanometers. By patterning the graphene into a nanowire or nanoconstriction, the local potential of the tip may be used to probe the dependence of the confinement-induced energy gap on the local atomic structure of the edges. These types of measurements provide a means for directly imaging the flow and behavior of electrons in graphene devices. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T25.00005: Imaging local potential and conductance variation around a tip gate in a graphene device with electrostatic force and scanning gate microscopes J.S. Chae, S. Jung, N.B. Zhitenev, J.A. Stroscio, Y. Kuk We fabricated graphene devices on a SiO$_{2}$ layer with 4-6 metallic contacts and a Si back gate. These devices revealed the well-known source-drain current versus gate-bias dependence with slight variation of the Dirac point upon gas adsorption on the graphene surfaces. They were inserted into an ultrahigh vacuum low temperature atomic force microscope (AFM), with which electrostatic force microscopy and scanning gate microscopy could be performed. The potential drops around the source and the drain contacts were carefully measured to estimate the barrier heights between the metallic contacts and the graphene. Using an AFM cantilever as a local gate, we measured the variation of source-drain current. This result showed some local variation, suggesting existence of electron and hole puddles. We also measured electron or hole scattering around defect states with nanometer resolution. These scanning probe microscopy results are compared with those performed by macroscopic transport measurement. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T25.00006: STM signatures for magnetic impurities in graphene Bruno Uchoa, L. Yang, S.-W. Tsai, N. M. R. Peres, A. H. Castro Neto Graphene is a two dimensional allotrope of carbon, whose elementary excitations are massless Dirac fermions which propagate ballistically in the submicron scale. With the adsorption of adatoms, such as transition metals, or simple molecules, the formation of local magnetic moments in graphene can be controlled by the application of a gate voltage [1], making graphene a potential candidate for spintronics. Unlike semiconductors, where the location of the magnetic impurities is random, in graphene the adatoms can be positioned by a scanning tunneling microscope (STM), allowing the construction of magnetic lattices. In this seminar, I will discuss the STM signatures for local magnetic impurities in graphene and promissing applications for spintronic devices. [1]B. Uchoa et al., PRL 101 026805 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T25.00007: Magnetic Oscillations in Scanning Tunneling Spectroscopy of Epitaxial Graphene on SiC Kevin D. Kubista, David L. Miller, Gregory M. Rutter, Ming Ruan, Walt A. de Heer, Phillip N. First, 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 perpendicular to the graphene plane. While changing the magnetic field we observe Shubnikov de Haas-like magnetic oscillations in the tunneling conductance, dI/dV. The peak positions of these tunneling magnetic oscillations (TMO) vary periodically with inverse magnetic field, indicating they sample a constant cross-section of the graphene k-space. This new magnetic oscillation method can map extended parts of the electronic band structure of graphene as we vary the tunneling energy in the dI/dV measurement. This is in contrast to traditional magnetic oscillations which typically only probe the Fermi level. This work was supported in part by NSF, NRI-INDEX, and the W. M. Keck Foundation. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T25.00008: Phonons, plasmons and impurities in Graphene probed with STM Victor Brar, Yunabo Zhang, Melissa Panlasigui, Caglar Girit, Alex Zettl, Michael Crommie It has been shown that the electronic structure of graphene is altered by interactions with plasmons, phonons and impurities. We probe such interactions at the atomic scale using scanning tunneling spectroscopy measurements on gated graphene flakes at 4.2K in an UHV environment. Our measurements show that collective excitations can be observed in the graphene tunneling spectra. By varying the voltage on the backgate of our devices, we are able to measure the charge density dependence of these features. We further probe atomic scale variations in the LDOS of graphene caused by impurities on the surface. We analyze our results in terms of graphene 2D electronic structure. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T25.00009: Kondo Effect for Massless Dirac Fermions in Graphene L. S. Mattos, C. R. Moon, P. B. Van Stockum, J. C. Randel, H. C. Manoharan, M. W. Sprinkle, C. Berger, W. A. de Heer, K. Sengupta, A. V. Balatsky We experimentally probe the addition of the spin degree of freedom to the local physics of graphene. This real spin degree of freedom is expected to become intertwined in various coherent scattering processes involving pseudospin and chirality intrinsic to single monolayers of graphene. A compendium of theoretical work on the interaction of localized spins with Dirac electrons predicts that low-temperature screening of localized magnetic moments by nodal quasiparticles gives rise to an unconventional Kondo effect, in which a critical exchange coupling can be controlled by charge carrier doping. We report the observation of this elusive Kondo ground state of Dirac particles, using scanning tunneling microscopy of adsorbed magnetic atoms on epitaxial graphene at low temperature. Tunneling spectroscopy and quasiparticle interference maps reveal chiral, linearly dispersing carriers that form sharp bimodal resonances around individual impurities, an effect traceable to Kondo screening of spins centered on two different graphene lattice sites. Using Fourier-transform scanning tunneling spectroscopy and concomitant measurements in a high magnetic field, we deduce the origin of these many-body ground states. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T25.00010: Atomic-scale scanning tunneling microscopy and spectroscopy studies of nanometer-sized graphene on the Si(111)-7x7 surface. Justin Koepke, Joseph Lyding We have used ultrahigh vacuum scanning tunneling microscopy to perform atomic-level studies of graphene on the Si(111)-7x7 surface. We used a dry contact transfer technique (DCT) developed by Albrecht and Lyding [1] to deposit mechanically exfoliated graphene in-situ [2] onto atomically clean Si(111)-7x7 surfaces. The DCT method deposits single, double, and thicker layers of atomically clean graphene. We observe varying degrees of transparency of the graphene monolayers and bilayers on the Si(111)-7x7 surface, where the substrate atomic structure is clearly seen through the graphene. We believe that the electronic structure of a graphene monolayer on the Si(111)-7x7 surface leads to the transparency of monolayers and bilayers, similar to the findings of Rutter, et al [3]. Room-temperature scanning tunneling spectroscopy (STS) measurements of the graphene monolayers and bilayers on the Si(111)-7x7 surface show predominantly metallic behavior. [1] P.M. Albrecht and J.W. Lyding, Appl. Phys. Lett. 83, 5029 (2003) [2] K.A. Ritter and J.W. Lyding, Nanotechnology 19, 015704 (2008) [3] G.M. Rutter, et al, Phys. Rev. B 76, 235416 (2007) [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T25.00011: Atomic-scale studies of nanometer-sized graphene on III-V semiconductors using scanning tunneling microscopy. Kevin He, Justin Koepke, Joseph Lyding We utilize the Dry Contact Transfer (DCT) method [1] to deposit nanometer-sized, monolayer graphene flakes, \textit{in situ}, onto cleaved GaAs (110) and InAs (110) surfaces. The flakes were characterized using a homebuilt, room temperature, ultrahigh-vacuum scanning tunneling microscope. We report on the apparent electronic semi-transparency of the monolayer graphene flakes, such that the underlying III-V semiconductor lattice is revealed in our topographic images. This transparency is strongly dependent on the applied sample bias, similar to results seen on SiC (1000) for large sheets of graphene grown via thermal desorption [2]. \\[3pt] [1] P.M. Albrecht and J.W. Lyding, APL 83, 5029 (2003). \\[0pt] [2] G.M. Rutter et al, Phys. Rev. B 76, 235416 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T25.00012: Epitaxial Graphene on Co(0001) Probed by STM Measurements and First Principles Calculations Deborah Prezzi, Daejin Eom, Kwang T. Rim, Hui Zhou, Michael Lefenfeld, Colin Nuckolls, Mark Hybertsen, Tony Heinz, George Flynn Structural and electronic properties of finite-sized graphene patches on Co(0001) have been investigated through a combined experimental and theoretical characterization. The analysis of low-temperature scanning tunneling microscopy images establishes an atomically uniform epitaxial configuration of graphene on the Co surface in which a C atom is a-top the interface Co atom, in agreement with total energy calculations based on a density-functional theory (DFT) approach. Scanning tunneling spectroscopy measurements show that the electronic properties of the interface are significantly different from both the clean Co surface and isolated graphene, suggesting a strong electronic coupling at the interface. DFT calculations provide a detailed analysis of the spectrocopic features in terms of spin and site contributions and reveal the coupling between graphene p and Co d states. [Preview Abstract] |
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