Latest Method to Detect and Analyze the Electronic Properties of Graphene

By Benedette Cuffari, M.Sc.Feb 15 2017

Considered to be a material that is approximately 200 times stronger than steel while also remaining one of the lightest materials on earth, graphene is a two-dimensional hexagonal allotrope of carbon, capable of a wide variety of useful, yet unusual properties. As an active conducer of both heat and electricity, graphene is a nearly transparent and highly flexible material that also has an impressively high tensile strength¹. As a result of these extensive properties, graphene has been applied to several different industries including, but not limited to, biological engineering, optical electronics, ultrafiltration, composite materials, photovoltaic cells, energy storage, and others.

Within the field of electronics, graphene has been used commercially in touchscreens, liquid crystal displays (LCD) and organic light emitting dioides¹. As an almost completely transparent material, graphene is able to transmit up to 97.7% of light optically, while also eliciting its highly conductive properties, which is an ideal feature in certain electronics. One of the most important indicators of graphene’s electric conductibility is a result of its remarkable band structure. Within graphene’s structure exists the spherical arrangement of carbon atoms in which a σ band is present between each carbon atom, accounting for the robustness of its lattice structure in graphene, as well as all other types of carbon allotropes². These σ bands each contain a filled shell, whose ability to covalently bind to neighboring carbon atoms allows for the formation of a half-filled π band, which plays an important role in the ability of graphene to elicit its strong electronic effects.

Interest in two-dimensional materials, such as graphene, has become of increasing interest in both academic and industrial settings, as these extremely thin sheets have quite different properties as compared to conventional three-dimensional materials. By manipulating different two-dimensional sheets by stacking them on top of each other, enhanced electrical, optical and thermal properties emerge³. Physicists at the Center for Theoretical Physics of Complex Systems (PCS) have collaborated with the Research Institute for Standards and Science (KRISS) in an effort to further the understanding of the electronic properties of both single and double layer graphene through their developed nanoscale device – the first in its kind.

The PCS and KRISS teams have constructed a nanodevice in which both single and bilayer graphene sheets have been sandwiched by a thick back-gate with a diameter of approximately 20 nm and a thin tunneling insulator, whose diameter measures between 1.0-1.8 nm⁴. Both the thick back-gate an thin tunneling insulator layers are comprised of hexagonal boron-nitride thin films, whose compatibility with similar two-dimensional crystals has been studied as a promising platform for the engineering of specific device functionalities, as well as for quantum measurement capabilities⁴. On top of this device, graphite, which is essentially composed of hundreds to thousands of layers of graphene is added, while the bottom layer of the device consists of one layer of silicon and one layer of silica³.

Researchers in this study were primarily interested in probing the electronic structures of both single and bilayer graphene through the application of electron tunneling microscopy (ETM). A magnetic field was applied to both the single and double layered-graphene in order to detect the change in electrical properties elicited by its addition. What was fdiscovered was that at a zero magnetic field, the tunneling spectra has a direct effect on the charge neutrality point, as well as the opening of the electric-field induced bilayer energy gap⁴.

By understanding this difference in the conductance of the double-layered graphene in the presence of an electric field perpendicular to it, as compared to the single-layered graphene model, this energy gap allows for this form of graphene more closely resemble current tunable semiconductor models. The discovery and reproducibility of this developed model by PCS and KRISS provides a foundational step for future studies to fully understand the potential of graphene’s striking electronic properties.

References

1. "Graphene Applications & Uses." Graphenea. Web. https://www.graphenea.com/pages/graphene-uses-applications#.WKOJB1eTTww.
2. Castro Neto, A. H., F. Guinea, N. M. R. Peres, K. S. Novoselov, and A. K. Geim. "The Electronic Properties of Graphene." Reviews of Modern Physics 81 (2009): 109-62. Web.
3. "New Platform to Study Graphene's Electronic Properties." ScienceDaily. ScienceDaily, 13 Feb. 2017. Web. https://www.sciencedaily.com/releases/2017/02/170213090735.htm.
4. Suyong Jung, Nojoon Myoung, Jaesung Park, Tae Young Jeong, Hakseong Kim, Kenji Watanabe, Takashi Taniguchi, Dong Han Ha, Chanyong Hwang, Hee Chul Park. “Direct Probing of the Electronic Structures of Single-Layer and Bilayer Graphene with a Hexagonal Boron Nitride Tunneling Barrier.” Nano Letters, 2017; 17 (1): 206.
5. Image Credit: shutterstock.com/highlevelspecialist

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