Epitaxial graphene: A new electronic material for the 21st century
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troduction: A brief history of graphene Graphene has a long history. In the 1800s, graphite was known as a layered material that could be exfoliated by various methods. The famous chemist Acheson1 had developed exfoliation methods (that he called deflocculation) to produce colloidal suspensions of small graphitic flakes that he called “dags.” These colloidal graphite suspensions have been extensively used in the electronics industry as a conducting paint to produce conducting surfaces in vacuum tubes. In 1907, the material was described as follows: “The graphite is in what Mr. Acheson calls the ‘deflocculated’ condition, a condition of fineness beyond that attainable by mechanical means, a condition approaching the molecular slate.”1 Indeed, these suspensions probably contained graphene flakes as Acheson suspected. He then found that coating metals with deflocculated graphite prevented corrosion. But Acheson also discovered another material. He produced silicon carbide (SiC) that he called carborundum.1 It was first used as an abrasive. He then found that upon heating SiC, silicon evaporated, leaving behind very pure graphite. Exfoliated graphite and graphite on SiC would become extremely important a century later for their electronic properties. It was later found that graphene layers form on SiC.
It is not well known that SiC (a wide bandgap semiconductor) was already incorporated in electronics since the early 1900s, when SiC crystals were used as diode detectors for radio receivers.2 In fact, the first light-emitting diodes were produced using SiC at that time.3 But the quality and size of the crystals left a lot to be desired. High-quality SiC crystals are now grown using the Lely process.4 In 1859, using nitric acid and potassium chromate, Brodie5 produced “graphitic oxide” consisting of graphene sheets with epoxide groups bridging adjacent carbon atoms. Immersing this material in water caused the individual layers to separate. In 1962, Boehm6 recognized that graphite oxide monolayers could be reduced to graphene using hydrazine. He was able to measure the thicknesses of these graphene layers using a combination of surface measurements. Boehm thereby was the first to produce and to measure freely suspended graphene flakes (Figure 1). In 1982, Boehm officially coined the term “graphene,” which he saw as a macromolecule (the “ene” ending refers to hydrogen at the edges, as for example in benzene) (Figure 1).7 In 1975, van Bommel et al.8 studied the graphitic layers that are formed when hexagonal SiC is heated in vacuum (Figure 1). They confirmed Acheson’s observation that the material
Walt A. de Heer, Georgia Institute of Technology, Atlanta, GA 30332, USA; [email protected] DOI: 10.1557/mrs.2011.158
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MRS BULLETIN • VOLUME 36 • AUGUST 2011 • www.mrs.org/bulletin
© 2011 Materials Research Society
EPITAXIAL GRAPHENE: A NEW ELECTRONIC MATERIAL FOR THE 21ST CENTURY
in magnetic fields and demonstrated the unusual magnetic field dependence of the Landau levels. Others confirmed and expanded these calculations. In
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