Graphene

  Graphene is a single planar sheet of sp²-bonded carbon atoms. Graphenes are the 2-D counterparts of 3-D graphite. They are aromatic.

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Description

Perfect graphenes consist exclusively of hexagonal cells; pentagonal and heptagonal cells constitute defects. If an isolated pentagonal cell is present, then the plane warps into a cone shape; insertion of 12 pentagons would create a fullerene. Likewise, insertion of an isolated heptagon causes the sheet to become saddle-shaped. Controlled addition of pentagons and heptagons would allow a wide variety of complex shapes to be made, for instance carbon NanoBuds.

Single walled carbon nanotubes may be considered to be graphene cylinders; some have a hemispherical graphene cap (that includes 6 pentagons) at each end. Graphenes have also attracted the interest of technologists who see them as a way of constructing ballistic transistors. In March 2006, Georgia Tech researchers announced that they had successfully built an all-graphene planar field-effect transistor and a quantum interference device.^[2]

The IUPAC compendium of technology states: "previously, descriptions such as graphite layers, carbon layers, or carbon sheets have been used for the term graphene…it is not correct to use for a single layer a term which includes the term graphite, which would imply a three-dimensional structure. The term graphene should be used only when the reactions, structural relations or other properties of individual layers are discussed". In this regard, graphene has been referred to as an infinite alternant (only six-member carbon ring) polycyclic aromatic hydrocarbon. The onset of graphene properties, as compared to those of a polycyclic aromatic hydrocarbon are not known. PAHs of 60, 78, and 120 carbon atoms have UV absorbance spectra that show a discreet PAH electronic structure, but a PAH of 222 carbon atoms has Raman bands similar to those in graphite..

Writing in Science,^[3] physicist Konstantin Novoselov and coworkers from the University of Manchester and the Institute of Microelectronics Technology and High Purity Materials at Chernogolovka state:

Graphene is the name given to a single layer of carbon atoms densely packed into a benzene-ring structure, and is widely used to describe properties of many carbon-based materials, including graphite, large fullerenes, nanotubes, etc. (e.g., carbon nanotubes are usually thought of as graphene sheets rolled up into nanometer-sized cylinders). Planar graphene itself has been presumed not to exist in the free state, being unstable with respect to the formation of curved structures such as soot, fullerenes, and nanotubes.

The researchers went on to construct graphenes by mechanical exfoliation (repeated peeling) of small "mesas" of highly oriented pyrolytic graphite; their motivation was to study the electrical properties of graphene. Mobilities of up to 10⁴ cm²V⁻¹s⁻¹ were reported; this value was almost independent of temperature. In addition, graphene has been shown to exhibit quantum Hall effect properties.

Similar work is ongoing at many universities and the results obtained by the Novoselov group in their PNAS paper "Two-dimensional atomic crystals" have been confirmed by several groups' work.^[4] For an example of a sample on the order of a monolayer, see figure 1.

Although theory and experiment suggest that perfect two-dimensional structures cannot exist in the free state, single-atom thick graphite has been produced. These are possible due to intrinsic microscopic roughening on the scale of 1 nm.^[5]

Chemical modification

Soluble fragments of graphene can be prepared in the laboratory^[6] through chemical modification of graphite. First, microcrystalline graphite is treated with a strongly acidic mixture of sulfuric acid and nitric acid. A series of steps involving oxidation and exfoliation result in small graphene plates with carboxyl groups at their edges. These are converted to acid chloride groups by treatment with thionyl chloride; next, they are converted to the corresponding graphene amide via treatment with octadecylamine. The resulting material (circular graphene layers of 5.3 angstrom thickness) is soluble in tetrahydrofuran, tetrachloromethane, and dichloroethane.

Properties

Electron transport

Electron transport in condensed matter physics is governed by the Schrodinger equation, due to its non-relativistic nature. But graphene is unusual in this respect. Electrons effectively obey a massless relativistic Dirac equation with a different coefficient (~10⁶ m/s, comparable to the fermi velocity) in the place of speed of light. ^[7]

Graphene oxide

By oxidising and chemically processing graphene, and then floating them in water, the graphene flakes form a single sheet and bond very powerfully. These sheets have a measured tensile modulus of 32 GPa.^[8]

References

1. ^ The image source is [1] (slide 17 image b) The height on the image is different. It is equal to 0.549 nm. See also more pictures of graphene here.
2. ^ Carbon-Based Electronics: Researchers Develop Foundation for Circuitry and Devices Based on Graphite March 14, 2006 gtresearchnews.gatech.edu Link
3. ^ Novoselov, K.S. et al. "Electric Field Effect in Atomically Thin Carbon Films", Science, Vol 306 (5696), p. 666-669, 2004 doi:10.1126/science.1102896
4. ^ Novoselov, K.S. et al. "Two-dimensional atomic crystals", PNAS, Vol 102 (30), p. 10451-10453, January 26, 2005 doi:10.1073/pnas.0502848102
5. ^ Nature 446, 60-63 (1 March 2007)doi:10.1038/nature05545
6. ^ Solution Properties of Graphite and Graphene Sandip Niyogi, Elena Bekyarova, Mikhail E. Itkis, Jared L. McWilliams, Mark A. Hamon, and Robert C. Haddon J. Am. Chem. Soc.; 2006; 128(24) pp 7720 - 7721; (Communication) doi:10.1021/ja060680r
7. ^ Two-Dimensional Gas of Massless Dirac Fermions in Graphene, Novoselov, K.S. et al, cond-mat/0509330, 2005
8. ^ [2]

See also

• Aromaticity
• Fullerenes
• Polycyclic aromatic hydrocarbons
• Carbon nanotubes
• Graphene nanoribbons
• Graphene Oxide Paper