October 6, 2010
The Many Faces of Carbon
Yesterday the Royal Swedish Academy of Sciences announced that this year’s Nobel Prize in Physics will go to Andre Geim and Konstantin Novoselov “for groundbreaking experiments regarding the two-dimensional material graphene.” Graphene is one of many allotropes, or forms, of the element carbon. Carbon is special because it has so many different allotropes (the main ones are highlighted below), many of which have special characteristics:
Diamond is just one way in which carbon atoms can be organized (via wikimedia commons)
Diamond: Carbon atoms line up and bond together in a tight lattice to create this extremely hard, transparent natural substance. The diamonds people dig up today were formed 100 miles or more beneath the Earth’s surface where the carbon was subjected to tremendous heat and pressure more than a billion years ago. Manufacturers can now also grow diamonds in a lab to create jewels or for industrial purposes.
Graphite: The soft lead in a pencil is really graphite, a flaky, flexible hexagonal lattice of carbon atoms. Unlike diamond, graphite is soft and conducts electricity.
Graphene: This thin sheet of carbon is just one atom thick. Geim and Novoselov created the first sheet of graphene by using a simple piece of adhesive tape to lift up a flake of carbon off of graphite. Graphene is a good conductor of electricity, and scientists think it might be useful in technologies such as touch screens and solar cells. (An interesting side note: Geim is the first person ever to win both a Nobel Prize and an Ig Nobel. He was awarded the Ig Nobel in 2000 for levitating a frog with magnets.)
Buckminster-fullerenes: These hollow carbon molecules, whose discoverers were awarded the 1996 Nobel Prize in Chemistry, get their name from their resemblence to the geodesic structures of Bucky Fuller. The molecules come in sphere shapes—called buckyballs—and can also be fashioned into carbon nanotubes, which are 100 times stronger than steel but one-sixth the weight.
Carbon nanofoam: This foam, made entirely of carbon atoms, is one of the least dense substances in the world. Carbon in this form acts as a semiconductor and is magnetic.
Not all carbon, however, joins together into complex crystalline structures with special properties. The jumbled up form is called amorphous carbon.
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In 1980, I published an article describing a model of one biological structure – the crypt of intestinal epithelium, completely similar to the structure later found in graphene tubes (Pyshnov, M. B., Topological Solution for Cell Proliferation in Intestinal Crypt, J. theor. Biol., 1980, v.87, 189-200). In 2005, Sergei Fedorov and myself published a computer simulation of topological transformations occuring in the model of the crypt (http://www.cell-division-program.com/index.php). Apparently, these works remain largely unknown to physicists, with the exception of one reference to them (www.mpipks-dresden.mpg.de/~coqusy06/SLIDES/vozmediano.pdf), where the structure of the crypt is called “a living curiosity”. Only from this reference I learned about graphene.
Publications describing graphene tubes are appearing, repeating the discovery of topological properties found in the model of the crypt. Some of them are adding a topological closure to the graphene tube at one end. Such closure is completely similar to the bottom part of the crypt model. I am not sure, however, that the interdependency between the structure of this closure and the structure of the cylindrical part of the graphene tube is understood in the degree it was shown in my 1980 paper and subsequently in the computer model of 2005.
The crypt model also includes the complex process of replacing dying cells with the new cells appearing by cell division, while graphene is a structure not capable of anything like multiplication of the elements of its structure, the atoms. However, when the formation of defects in the structure of graphene occurs, the explanation of structural transformations found in the crypt model can probably be helpful. To conclude, I find the striking similarities very delightful, and I hope that one day biological structures will receive at least as much attention of the researchers.