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Graphene in High-Frequency Electronics

This two-dimensional form of carbon has properties not seen in any other substance

Keith A. Jenkins

2012-09JenkinsF1.jpgClick to Enlarge ImageThe Nobel Prize in Physics for 2010 was awarded jointly to Andre Geim and Konstantin Novoselov of the University of Manchester “for groundbreaking experiments regarding the two-dimensional material graphene.” Essentially it was awarded for the discovery in 2004 of the form of carbon known as graphene, which led to an explosion of experimental and theoretical work with the material around the world. It is remarkable not only that the Nobel Prize was given for the discovery of a material (rather than for the elucidation of some physical principle), but also that the material was already in one of the most common substances in human history, and that the prize was awarded such a short time after the discovery.

In part, the speed of this recognition is due to the amazing excitement created by Geim and Novoselov’s findings. According to Geim, several technical papers are published every day on the subject of graphene. This enthusiasm arose from the quick realization that graphene has many remarkable physical properties not seen in any other material.

What is graphene and what are its properties? It is a two-dimensional form of carbon, a single layer of carbon atoms, in which the atoms are arranged in a hexagonal chicken-wire or honeycomb configuration. It is found in nature and is simply one of the layers of the common substance graphite. Each layer of graphene stacked up to make graphite is only loosely bonded to the others. The layers adhere only by van der Waals forces, weak dipole-to-dipole attractions between adjacent molecules, rather than by stronger covalent bonds, in which the molecules share electrons. This loose bonding explains why graphite is a good lubricant. Technically, graphene was known since x-ray crystallography was able to uncover graphite’s structure in the early 1900s, but it was not isolated into individual planes of graphene until 2004. And there was little interest in graphene by itself until Geim and Novoselov were the first to detail its properties.

Graphene is one of very few two-dimensional materials with a crystalline structure. Its physical layout and electronic properties result in many remarkable characteristics. It is an excellent heat conductor. It is flexible, yet 10 times stronger than any other measured material, at equivalent thicknesses. It is a good absorber of light over a wide spectrum, yet it is effectively transparent. (This dichotomy is not a paradox: A layer of graphene absorbs 2.3 percent of the light impinging on it, which is a lot for a single atomic layer, but still allows most light to pass through.) It is fairly chemically inert, yet when appropriately treated it is a sensitive electrical detector of very small concentrations of chemicals. And it has very high electron mobility, allowing the transit of electrical charges through the material at a rate that is at least 100 times greater, in its purest state, than conventional semiconductors used in electronics, such as silicon or gallium arsenide.

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