{"60372":{"#nid":"60372","#data":{"type":"news","title":"Study of Electron Orbits in Multilayer Graphene Finds Energy Gaps","body":[{"value":"\u003Cp\u003EResearchers have taken one more step toward understanding the unique and often unexpected properties of graphene, a two-dimensional carbon material that has attracted interest because of its potential applications in future generations of electronic devices.\u003C\/p\u003E\u003Cp\u003EIn the Aug. 8 advance online edition of the journal \u003Cem\u003ENature Physics\u003C\/em\u003E, researchers from the Georgia Institute of Technology and the National Institute of Standards and Technology (NIST) describe for the first time how the orbits of electrons are distributed spatially by magnetic fields applied to layers of epitaxial graphene. \u003C\/p\u003E\u003Cp\u003EThe research team also found that these electron orbits can interact with the substrate on which the graphene is grown, creating energy gaps that affect how electron waves move through the multilayer material. These energy gaps could have implications for the designers of certain graphene-based electronic devices. \u003C\/p\u003E\u003Cp\u003E\u0022The regular pattern of energy gaps in the graphene surface creates regions where electron transport is not allowed,\u0022 said Phillip N. First, a professor in the Georgia Tech School of Physics and one of the paper\u2019s co-authors. \u0022Electron waves would have to go around these regions, requiring new patterns of electron wave interference. Understanding such interference will be important for bi-layer graphene devices that have been proposed, and may be important for other lattice-matched substrates used to support graphene and graphene devices.\u0022 \u003C\/p\u003E\u003Cp\u003EIn a magnetic field, an electron moves in a circular trajectory -- known as a cyclotron orbit -- whose radius depends on the size of the magnetic field and the energy of electron. For a constant magnetic field, that\u0027s a little like rolling a marble around in a large bowl, First said. \u003C\/p\u003E\u003Cp\u003E\u0022At high energy, the marble orbits high in the bowl, while for lower energies, the orbit size is smaller and lower in the bowl,\u0022 he explained. \u0022The cyclotron orbits in graphene also depend on the electron energy and the local electron potential -- corresponding to the bowl -- but until now, the orbits hadn\u2019t been imaged directly.\u0022 \u003C\/p\u003E\u003Cp\u003EPlaced in a magnetic field, these orbits normally drift along lines of nearly constant electric potential. But when a graphene sample has small fluctuations in the potential, these \u0022drift states\u0022 can become trapped at a hill or valley in the material that has closed constant potential contours. Such trapping of charge carriers is important for the quantum Hall effect, in which precisely quantized resistance results from charge conduction solely through the orbits that skip along the edges of the material. \u003C\/p\u003E\u003Cp\u003EThe study focused on one particular electron orbit: a zero-energy orbit that is unique to graphene. Because electrons are matter waves, interference within a material affects how their energy relates to the velocity of the wave -- and reflected waves added to an incoming wave can combine to produce a slower composite wave. Electrons moving through the unique \u0022chicken-wire\u0022 arrangement of carbon-carbon bonds in the graphene interfere in a way that leaves the wave velocity the same for all energy levels. \u003C\/p\u003E\u003Cp\u003EIn addition to finding that energy states follow contours of constant electric potential, the researchers discovered specific areas on the graphene surface where the orbital energy of the electrons changes from one atom to the next. That creates an energy gap within isolated patches on the surface. \u003C\/p\u003E\u003Cp\u003E\u0022By examining their distribution over the surface for different magnetic fields, we determined that the energy gap is due to a subtle interaction with the substrate, which consists of multilayer graphene grown on a silicon carbide wafer,\u0022 First explained. \u003C\/p\u003E\u003Cp\u003EIn multilayer epitaxial graphene, each layer\u0027s symmetrical sublattice is rotated slightly with respect to the next. In prior studies, researchers found that the rotations served to decouple the electronic properties of each graphene layer. \u003C\/p\u003E\u003Cp\u003E\u0022Our findings hold the first indications of a small position-dependent interaction between the layers,\u0022 said David L. Miller, the paper\u0027s first author and a graduate student in First\u0027s laboratory. \u0022This interaction occurs only when the size of a cyclotron orbit -- which shrinks as the magnetic field is increased -- becomes smaller than the size of the observed patches.\u0022 \u003C\/p\u003E\u003Cp\u003EThe origin of the position dependent interaction is believed to be the \u0022moir\u00e9 pattern\u0022 of atomic alignments between two adjacent layers of graphene. In some regions, atoms of one layer lie atop atoms of the layer below, while in other regions, none of the atoms align with the atoms in the layer below. In still other regions, half of the atoms have neighbors in the underlayer, an instance in which the symmetry of the carbon atoms is broken and the Landau level -- discrete energy level of the electrons -- splits into two different energies. \u003C\/p\u003E\u003Cp\u003EExperimentally, the researchers examined a sample of epitaxial graphene grown at Georgia Tech in the laboratory of Professor Walt de Heer, using techniques developed by his research team over the past several years. \u003C\/p\u003E\u003Cp\u003EThey used the tip of a custom-built scanning-tunneling microscope (STM) to probe the atomic-scale electronic structure of the graphene in a technique known as scanning tunneling spectroscopy. The tip was moved across the surface of a 100-square nanometer section of graphene, and spectroscopic data was acquired every 0.4 nanometers. \u003C\/p\u003E\u003Cp\u003EThe measurements were done at 4.3 degrees Kelvin to take advantage of the fact that energy resolution is proportional to the temperature. The scanning-tunneling microscope, designed and built by Joseph Stroscio at NIST\u0027s Center for Nanoscale Science and Technology, used a superconducting magnet to provide the magnetic fields needed to study the orbits. \u003C\/p\u003E\u003Cp\u003EAccording to First, the study raises a number of questions for future research, including how the energy gaps will affect electron transport properties, how the observed effects may impact proposed bi-layer graphene coherent devices -- and whether the new phenomenon can be controlled. \u003C\/p\u003E\u003Cp\u003E\u0022This study is really a stepping stone in long path to understanding the subtleties of graphene\u0027s interesting properties,\u0022 he said. \u0022This material is different from anything we have worked with before in electronics.\u0022 \u003C\/p\u003E\u003Cp\u003EIn addition to those already mentioned, the study also included Walt de Heer, Kevin D. Kubista, Ming Ruan, and Markus Kinderman from Georgia Tech and Gregory M. Rutter from NIST. The research was supported by the National Science Foundation, the Semiconductor Research Corporation and the W.M. Keck Foundation. Additional assistance was provided by Georgia Tech\u0027s Materials Research Science and Engineering Center (MRSEC). \u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EResearch News \u0026amp; Publications Office\u003Cbr \/\u003EGeorgia Institute of Technology\u003Cbr \/\u003E75 Fifth Street, N.W., Suite 314\u003Cbr \/\u003EAtlanta, Georgia 30308 USA\u003C\/strong\u003E \u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EMedia Relations Contacts\u003C\/strong\u003E: John Toon (404-894-6986)(\u003Ca href=\u0022mailto:jtoon@gatech.edu\u0022\u003Ejtoon@gatech.edu\u003C\/a\u003E) or Abby Vogel Robinson (404-385-3364)(\u003Ca href=\u0022mailto:abby@innovate.gatech.edu\u0022\u003Eabby@innovate.gatech.edu\u003C\/a\u003E). \u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EWriter\u003C\/strong\u003E: John Toon \u003C\/p\u003E","summary":null,"format":"limited_html"}],"field_subtitle":[{"value":"Findings May Have Implications for Device Designers"}],"field_summary":[{"value":"\u003Cp\u003EResearchers have taken one more step toward understanding the unique and often unexpected properties of graphene, a two-dimensional carbon material that has attracted interest because of its potential applications in future generations of electronic devices.\u003C\/p\u003E","format":"limited_html"}],"field_summary_sentence":[{"value":"Researchers take a new step to understanding graphene properties."}],"uid":"27303","created_gmt":"2010-08-09 00:00:00","changed_gmt":"2016-10-08 03:07:15","author":"John Toon","boilerplate_text":"","field_publication":"","field_article_url":"","dateline":{"date":"2010-08-09T00:00:00-04:00","iso_date":"2010-08-09T00:00:00-04:00","tz":"America\/New_York"},"extras":[],"hg_media":{"60373":{"id":"60373","type":"image","title":"Moire alignment of graphene","body":null,"created":"1449176267","gmt_created":"2015-12-03 20:57:47","changed":"1475894523","gmt_changed":"2016-10-08 02:42:03","alt":"Moire alignment of graphene","file":{"fid":"191110","name":"tpx85581.jpg","image_path":"\/sites\/default\/files\/images\/tpx85581_0.jpg","image_full_path":"http:\/\/tlwarc.hg.gatech.edu\/\/sites\/default\/files\/images\/tpx85581_0.jpg","mime":"image\/jpeg","size":953599,"path_740":"http:\/\/tlwarc.hg.gatech.edu\/sites\/default\/files\/styles\/740xx_scale\/public\/images\/tpx85581_0.jpg?itok=4b2fa4es"}},"60374":{"id":"60374","type":"image","title":"Graphene Electron Motion","body":null,"created":"1449176267","gmt_created":"2015-12-03 20:57:47","changed":"1475894523","gmt_changed":"2016-10-08 02:42:03","alt":"Graphene Electron Motion","file":{"fid":"191111","name":"tdc85581.jpg","image_path":"\/sites\/default\/files\/images\/tdc85581_0.jpg","image_full_path":"http:\/\/tlwarc.hg.gatech.edu\/\/sites\/default\/files\/images\/tdc85581_0.jpg","mime":"image\/jpeg","size":192342,"path_740":"http:\/\/tlwarc.hg.gatech.edu\/sites\/default\/files\/styles\/740xx_scale\/public\/images\/tdc85581_0.jpg?itok=TH4hzXiY"}}},"media_ids":["60373","60374"],"related_links":[{"url":"http:\/\/www.physics.gatech.edu\/","title":"Georgia Tech School of Physics"},{"url":"http:\/\/www.mrsec.gatech.edu\/","title":"Materials Research Science and Engineering Center"},{"url":"http:\/\/www.physics.gatech.edu\/people\/faculty\/pfirst.html","title":"Phillip First"}],"groups":[{"id":"1188","name":"Research Horizons"}],"categories":[{"id":"149","name":"Nanotechnology and Nanoscience"},{"id":"135","name":"Research"},{"id":"150","name":"Physics and Physical Sciences"}],"keywords":[{"id":"6884","name":"electron"},{"id":"609","name":"electronics"},{"id":"429","name":"graphene"},{"id":"10361","name":"orbits"}],"core_research_areas":[],"news_room_topics":[],"event_categories":[],"invited_audience":[],"affiliations":[],"classification":[],"areas_of_expertise":[],"news_and_recent_appearances":[],"phone":[],"contact":[{"value":"\u003Cp\u003E\u003Cstrong\u003EJohn Toon\u003C\/strong\u003E\u003Cbr \/\u003EResearch News \u0026amp; Publications Office\u003Cbr \/\u003E\u003Ca href=\u0022http:\/\/www.gatech.edu\/contact\/index.html?id=jt7\u0022\u003EContact John Toon\u003C\/a\u003E\u003Cbr \/\u003E\u003Cstrong\u003E404-894-6986\u003C\/strong\u003E\u003C\/p\u003E","format":"limited_html"}],"email":["jtoon@gatech.edu"],"slides":[],"orientation":[],"userdata":""}}}