Abstract
Throughout the history of science, carbon-based research has played a defining role in the development of a range of fundamental and technological fields. It was used in Avagadro's definition of the mole in the early 18th century, it provides the 'backbone' of molecules in organic compounds, and in the environmental debate currently raging in the press and international government discussions, the 'carbon footprint' has become the metric of our species' impact on our planet. Also in nanotechnology, with the discovery of various wonder materials, carbon is once again asserting its claim as king of the elements.
Until the 1980s the only known forms of carbon were diamond, graphite and amorphous carbon, as in soot or charcoal. In 1985 Robert Curl, Harold Kroto and Richard Smalley reported the existence of fullerenes, spherical structures comprising hexagonal carbon rings [1], work for which they won the Nobel Prize for Chemistry in 1996 [2]. The discovery of fullerenes was followed in 1991 by Sumio Ijima with the discovery of rolled graphite sheets, the carbon nanotube [3]. The discovery of these novel carbon nanostructures inspired researchers in a range of fields, largely as a result of the extraordinary capacity for investigations of these structures to reveal ever more intriguing properties.
One of the fascinating properties attributed to carbon nanotubes is their phenomenal strength, with a Young's modulus of single walled carbon nanotubes approaching a terapascal [4]. Ingenious methods of harnessing this strength have since been developed, including bucky paper, a term used to refer to a mat of randomly self-entangled carbon nanotubes. Steven Crannford and Markus Buehler have recently reported a novel computational technique for probing the mechanical properties of these structures and show that the Young's modulus of bucky paper can be tuned by manipulation of the carbon nanotube type and density [5].
The electrical properties of carbon nanotubes, which depend on the chirality or wrapping angle of the graphite sheet with respect to the tube axis [6], have captured the imagination of researchers working in nanoelectronics. Carbon nanotubes also revealed interesting thermal properties that could lend them to the next generation of nanoscale devices. In 2000, researchers at the California Institute of Technology published the results of molecular dynamics simulations of thermal conductivity in carbon nanotubes [7]. The thermal properties predicted from this work added further promise to the potential of carbon nanotubes in micro- and nanoelectromechanical devices. More recently, researchers from the University of Columbia have studied how to exploit the thermal properties of carbon nanotubes in nanofluids. They report enhancement of heat transfer properties of carbon-nanotube-based nanofluids using a plasma treatment to aid stable dispersion of the nanotubes in water [8]. Eric Pop in Illinois has reported on the role of electrical and thermal contact resistance in Joule breakdown of single-walled carbon nanotubes, including analysis of several published data sets [9]. The work finds universal scaling rules, whereby the breakdown scales linearly with length for carbon nanotubes above a certain length, below which the breakdown is entirely limited by contact resistance.
In 2004 another form of carbon came to the fore when researchers at the University of Manchester and the Institute of Microelectronics Technology isolated a single plane of graphite, that is, graphene, using a kind of scotch tape [10]. As with other forms of carbon, investigation of graphene has also revealed fascinating properties that lend the material to a number of applications, such as sensing. The electronic properties of graphene are highly sensitive to the adsorption of molecules such as CO, NO, NO2 and NH3, and a collaboration of researchers from Lanzhou University in China and the University of Sheffield in the UK have further reported on how this sensitivity can be enhanced by two orders of magnitude when the graphene is doped with an impurity such as boron [11].
Work on graphene has also prompted the possibility of a new field of application in electron-spin-based quantum computing. In this issue, Patrik Recher and Björn Trauzettel in Germany present an overview on the latest research on quantum dots of graphene, graphene nanoribbons and discs in single- and bilayer graphene with a view to their possible application as qubits in computing [12].
Nanodiamonds with nitrogen vacancies have been shown to behave as quantum-dot-like fluorescent nanostructures that are not susceptible to bleaching. Once certain nanotoxicological concerns have been settled, such nanodiamonds may one day be useful for imaging, and means of industrial scale fabrication have already been reported [13]. Other possible medical applications of carbon nanostructures include the use of carbon nanofibres for improved neural and orthopaedic implants [14].
Electronics, sensing, medicine, quantum computing; the utility of carbon in nanotechnology is apparently unlimited. Of course there is as much work in inorganic research advancing as rapidly and in fields just as diverse. Actual commercialisation of devices based on these carbon wonder materials is still pending, but the extraordinary properties of carbon-based materials holds an unavoidable fascination that is likely to endure and inspire further research and discoveries for some time to come.
References
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[2] The Nobel Prize in Chemistry 1996 Nobelprize.org http://nobelprize.org/nobel_prizes/chemistry/laureates/1996
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