Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session W17: Focus Session: Graphene Devices: Function, Fabrication, and Characterization: Graphene Surface Characterization |
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Sponsoring Units: DMP Chair: Doron Naveh, Bar-Ilan University Room: 102AB |
Thursday, March 5, 2015 2:30PM - 2:42PM |
W17.00001: High Resolution Imaging of Graphene on SiC by Contact Resonance AFM: Experiment and Theory Qing Tu, Bjoern Lange, Joao Marcelo Lopes, Stefan Zauscher, Volker Blum Contact resonance atomic force microscopy (CR-AFM)[1] is a powerful tool for mapping differences of the mechanical properties of layered, 2D materials. The deconvolution of stiffness contributions arising from the different layers is, however, difficult. Here, density functional theory (DFT) calculations can help interpret experimental results. Few-layer graphene (FLG; mono-, bi-, or trilayer thickness) on silicon carbide (SiC) exhibits very clean and distinct surfaces and yields high-contrast CR-AFM images. To interpret the contributions from surface areas with different layer thickness and structure we use DFT to calculate atomic displacements for forces acting on FLG on SiC using the $\sqrt{3} \times \sqrt{3}$ and the $6\sqrt{3} \times 6\sqrt{3}$ structure models. Based on these displacements we calculate an effective modulus using a simple spring model. The resulting moduli can then be compared with those extracted from experimental CR-AFM measurements of FLG on SiC.\\[4pt] [1] Yamanaka, K and Nakano, S, Applied Physics A: Materials Science {\&} Processing, S313--S317, (1998) [Preview Abstract] |
Thursday, March 5, 2015 2:42PM - 2:54PM |
W17.00002: Charging Ring Spectroscopy and Defect Identification in Graphene/Boron Nitride Through Scanning Tunneling Microscopy Juwon Lee, Dillon Wong, Jairo Valesco, Long Ju, Salman Kahn, Hsinzon Tsai, Chad Germany, Takashi Taniguchi, Kenji Watanabe, Alex Zettl, Feng Wang, Michael Crommie Tip-induced ionization of defects in semiconductors and surface adatoms is known to cause ring-like structures in scanning tunneling spectroscopy (STS). We report the observation and investigation of charging ring structures in bulk insulating hexagonal boron nitride (BN) capped by a monolayer of graphene. These rings provide quantitative information on the energy levels of the ionizable BN defects, providing insight into their chemical identities. This new technique suggests exciting possibilities for quantitative spectroscopic studies of defects in other insulating systems. [Preview Abstract] |
Thursday, March 5, 2015 2:54PM - 3:06PM |
W17.00003: Scanning tunneling microscopy studies of graphene and hydrogenated graphene on Cu(111) Shawna M. Hollen, Grady Gambrel, Steven Tjung, Nancy M. Santagata, Ezekiel Johnston-Halperin, Jay A. Gupta Because of the innate sensitivity of 2D material surfaces, it is increasingly important to understand and characterize surface functionalization and interactions with environmental elements, such as substrate, metallic contacts, and adatoms. We developed a method for reproducible, epitaxial growth of pristine graphene islands on Cu(111) in UHV and use scanning tunneling microscopy and spectroscopy (STM) to study the interaction of these graphene islands with the Cu substrate. Tunneling spectroscopy measurements of the electronic surface states over the graphene islands indicate a lower local work function, decreased coupling to bulk Cu states, and a decreased electron effective mass. Additionally, we developed a novel field electron dissociation technique to form hydrogen-terminated graphene at low temperatures and in UHV. This method produced what may be the first STM images of crystalline hydrogenated graphene. The pristine graphene island is then recovered by scanning at a high tip-sample bias. The hydrogenation and its reversibility suggest writing lateral 2D devices using the STM tip. Toward this end, we are developing the capability to repeat the hydrogenation on working graphene devices. [Preview Abstract] |
Thursday, March 5, 2015 3:06PM - 3:18PM |
W17.00004: ABSTRACT WITHDRAWN |
Thursday, March 5, 2015 3:18PM - 3:30PM |
W17.00005: Probing Graphene by Low-Energy Electrons under Non-normal Incidence Johannes Jobst, Jaap Kautz, Daniel Geelen, Rudolf M. Tromp, Sense Jan van der Molen Low-energy electron microscopy (LEEM) is a powerful surface analysis tool for investigating samples in real and reciprocal space. Moreover, spectroscopic information can be obtained by measuring LEEM-IV, i.e., the energy dependence of the reflected electron intensity. Here, we focus on the study of monolayer and bilayer graphene grown on silicon carbide. Its layered character gives rise to minima in the LEEM-IV, which are used to unambiguously determine the layer thickness as the number of minima is equal to the number of conducting graphene layers. In a typical LEEM experiment it is crucial to align the sample such that the electrons impinge perpendicular on the surface in order to guarantee ideal imaging conditions. In this study we, however, present a systematic analysis of the effect of beam tilt on the LEEM-IV. We find pronounced changes in shape depending on the tilt angle with respect to crystallographic axes. These changes can be related to the band structure of few-layer graphene. [Preview Abstract] |
Thursday, March 5, 2015 3:30PM - 3:42PM |
W17.00006: Resonant Modes in Circular Graphene \textit{pn} Junctions Created by STM Probes Jonathan Wyrick, Yue Zhao, Fabian Natterer, Joaquin Rodriguez Nieva, Cyprian Lewandowski, Kenji Watanabe, Takashi Taniguchi, Leonid Levitov, Nikolai Zhitenev, Joseph Stroscio Electronic states in graphene (and similar 2D materials) are susceptible to interactions with local probes; characterization of these interactions is necessary for predicting the behavior of all graphene devices incorporating local probes and interpreting the results of related experiments. Here we report the creation of resonant modes in circular \textit{pn} junctions on graphene. Such junctions, induced by a tip, are ubiquitous in STM experiments, create states akin to whispering-gallery modes, and are quasi-localized within the \textit{pn} junction ring. These modes yield 2 types of resonances in the tunneling spectrum: first as oscillations in the otherwise linear graphene dispersion, and second when they are pulled across the Fermi-level due to tip gating. Tunable with tip height/radius, and tip work-function, the oscillations act as fingerprints of an induced \textit{pn} junction. We compare experimental results to theory for confined states in circular geometries, characterizing energy and spatial characteristics of these modes. [Preview Abstract] |
Thursday, March 5, 2015 3:42PM - 4:18PM |
W17.00007: Stacking defects and transport in bilayer graphene Invited Speaker: Francisco Guinea Pristine bilayer graphene behaves in some instances as an insulator with a transport gap of a few meV. Intriguingly, however, some samples of similar mobility exhibit good metallic properties, with a minimal conductivity of the order of $2 e^2 / \hbar$. Here we propose an explanation for this dichotomy, which is unrelated to electron interactions and based on the reversible formation of boundaries between stacking domains (``solitons''). We argue, using a numerical analysis, that the hallmark features of the previously inferred insulating state can be explained by scattering on boundaries between domains with different stacking order (AB and BA). We furthermore present experimental evidence, reinforcing our interpretation, of reversible switching between a metallic and an insulating regime in suspended bilayers when subjected to thermal cycling or high current annealing. [Preview Abstract] |
Thursday, March 5, 2015 4:18PM - 4:30PM |
W17.00008: Examination of Humidity Effects on Measured Thickness and Interfacial Phenomena of Exfoliated Graphene on SiO$_{2}$ via AC-AFM Katherine Jinkins, Jorge Camacho, Lee Farina, Yan Wu Tapping (AC) mode Atomic Force Microscopy (AFM) is commonly used to determine the thickness of graphene samples. However, AFM measurements have been shown to be sensitive to environmental conditions such as adsorbed water, in turn dependent on relative humidity (RH). In the present study, AC-AFM is used to measure the thickness and loss tangent of exfoliated graphene on silicon dioxide (SiO$_{2})$ as RH is increased from 10{\%} to 80{\%}. We show that the measured thickness of graphene is dependent on RH. Loss tangent is an AFM imaging technique that interprets the phase information as a relationship between the stored and dissipated energy in the tip-sample interaction. This study demonstrates the loss tangent of the graphene and oxide regions are both affected by humidity, with generally higher loss tangent for graphene than SiO$_{2}$. As RH increases, we observe the loss tangent of both materials approaches the same value. We hypothesize that there is a layer of water trapped between the graphene and SiO$_{2}$ substrate to explain this observation. Using this interpretation, the loss tangent images also indicate movement and change in this trapped water layer as RH increases, which impacts the measured thickness of graphene using AC-AFM. [Preview Abstract] |
Thursday, March 5, 2015 4:30PM - 4:42PM |
W17.00009: Visualization of Photo-induced Doping patterns in Graphene/Boron Nitride Heterostructures via Scanning Tunneling Microscopy Jairo Velasco Jr., Long Ju, Dillon Wong, Juwon Lee, Salman Kahn, Hsin-Zon Tsai, Chad Germany, Takashi Taniguchi, Kenji Watanabe, Alex Zettl, Feng Wang, Mike Crommie Photo-induced doping in graphene-boron nitride (G/BN) heterostructures enables flexible and repeatable writing and erasing of charge doping in graphene using optical irradiation. So far, however, this phenomenon has been explored using spatially averaging probes such as electron transport, and there have been no local studies into the underlying microscopic behavior. Here we report a combined scanning tunneling microscopy (STM) and optoelectronic measurement scheme that has been utilized to investigate the microscopic mechanisms at work in this process. We will discuss the latest experimental progress towards the visualization of light-induced charge doping patterns on G/BN heterostructures via STM. [Preview Abstract] |
Thursday, March 5, 2015 4:42PM - 4:54PM |
W17.00010: A Correlation of Raman and Single and Multiple Layer Graphene Conductivity As Detected With A Cryogenic Multiprobe AFM With On-line Raman, NSOM and Other SPM Modalities Aaron Lewis, Oleg Zinoviev, Anatoly Komissar, Eran Maayan, David Lewis It is a challenge to study 2D materials, such as Graphene, MoS$_{2}$, WeSe$_{2}$, etc. at temperatures down to 10$^{\circ}$K when considering the wide variety of physical phenomena that must be applied for a full picture of the functionality of these materials. This involves questions of structure, nanometric photoconductivity, electrical properties, thermal properties, near-field optical in the apertured {\&} scattering modes, Kelvin probe, and Raman. These phenomena are common not only to 2D materials but also to carbon nanotubes and related nanomaterials. This presentation will describe the instrumental development of such a multiprobe cryogenic system that allows for state of the art on-line optical measurements and will also include a review of the probe developments that permit such multifunctional multiprobe operation with on-line full optical access. This system has a completely free optical axis from above and below not obscured by electrical or other probes that have been developed for multiprobe operation. This permits on-line Raman and Tip Enhanced NanoRaman Scattering. With such a system we have investigated graphene and HfO2 using multiprobe electrical, Kelvin probe, NSOM and on-line Raman. The results have yielded new insights into chemical changes correlated to electrical conductivity. [Preview Abstract] |
Thursday, March 5, 2015 4:54PM - 5:06PM |
W17.00011: Electron Tunneling Spectroscopy of Single and Bilayer Graphene with Hexagonal Boron Nitride Tunneling Barrier Suyong Jung, Jaesung Park, Chanyong Hwang, Donghan Ha, Kenji Watanabe, Takashi Taniguchi, Pilkyung Moon, Young-Woo Son We have performed electron tunneling spectroscopy measurements on gated single and bilayer graphene devices with thin hexagonal boron nitride (h-BN) as a tunneling barrier. We can directly probe electronic structures of graphene devices by varying charge density and tunneling bias voltage. The evolution of bilayer energy gap identified as dI/dV dip in tunneling spectra is observed as the electric field between bottom gate and top tunneling probe varies. In addition, we can identify several spectra features which are in good agreement with the vibrational excitations; phonons of graphene and h-BN and a signature of local plasmonic excitation. Upon increasing external magnetic field, the development of Landau levels (LL) is observed as early as 0.2 T and we are able to discern individual LL as many as 20 as an index for both filled and empty states, which is unprecedented in previous tunneling spectroscopy studies on graphene devices. [Preview Abstract] |
Thursday, March 5, 2015 5:06PM - 5:18PM |
W17.00012: Thermal mirror buckling transitions in a pristine freestanding graphene membrane investigated by scanning tunneling microscopy Kevin Schoelz, Vincent Meunier, Pradeep Kumar, Mehdhi Neek-Amal, Paul Thibado, Francois Peeters Freestanding graphene membranes are not flat, but rather display an array of ripples with alternating curvature. By applying a local force using a scanning tunneling microscope tip, we can pull out these ripples, causing the graphene membrane to reversibly rise and fall. By increasing the tunneling current and exploiting the negative coefficient of thermal expansion, we can increase the strain in the graphene membrane causing an irreversible transition from this flexible state to a rigid configuration. This transition typically happens when the graphene membrane reaches 60-70\% of the total graphene height. We successfully model this transition as the transition of a spin-half Ising magnet where the ripples are modeled as Ising spins. The buckling transition can be interpreted as the transition from an antiferromagnetic state, to a ferromagnetic state. In addition, four critical exponents are measured. These results provide insight into the role of the negative thermal expansion of graphene. [Preview Abstract] |
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