Thursday, April 27, 2006

It’s flat, it’s hot, and it’s very weird

Graphene, that is. I have been talking to some fellows about this new wonder-stuff, which wowed the crowds at the American Physical Society meeting in March. Mainly to Andre Geim at Manchester, who is one of those wry chaps you feel you can inherently trust not to load you down with hype. I’m working on a feature on this for New Scientist, which will delve into the decidedly wacky physics of these single-atom-thick sheets of pure carbon. It’s not your ordinary two-dimensional semimetal (yes I know, name me another), mainly because the electrons behave as though they are travelling at close to the speed of light. So here’s an everyday material in which one can investigate Dirac’s relativistic quantum mechanics, which normally applies only in the kind of astrophysical environment you wouldn’t want to end up in by mistake. Anyway, that’s to come. By way of an hors d’oeuvre, here’s a short piece on the materials aspects of graphene which will appear in the June issue of Nature Materials :

Carbon goes flat out

Graphene has revealed itself from a direction that, in retrospect, seems opposite to what one might have expected. First came the zero-dimensional form: C60 and the other fullerenes, nanoscopically finite in every direction. Then there was the carbon nanotube, whose one-dimensional, tubular form set everyone thinking in terms of fibres and wires. It was just two years ago that the two-dimensional form, graphene itself, appeared: flat sheets of carbon one atom thick (Novoselov et al., Science 306, 666; 2004), which, when stacked in the third dimension, return us to familiar, lustrous graphite.

Now it’s tempting to wonder if the earlier focus on reduced dimensionality and curvature may have been misplaced. C60 is a fascinating molecule, but useful materials tend to be extended in at least one dimension. Carbon nanotubes can be matted into ‘bucky paper’, but without exceptional strength. Long, thin single-molecule transistors are all very well, but today’s microelectronics is inherently two-dimensional. Graphene is the master substance of all these structures, and perhaps, so far as materials and electronics are concerned, sheets were what we needed all along.

You can cut up these sheets into device-styled patterns – but that’s best done with chemistry (etching with an oxygen plasma, say), since attempts to tear single-layer graphene with a diamond tip just make it blunt. (As carbon nanotubes have shown, graphite’s reputation for weakness gives a false impression.) And graphene is a semimetal with a tunable charge-carrier density that makes it suitable for the conducting channel of transistors.

But its conductivity is more extraordinary than that. For one thing, the electron transport is ballistic, free from scattering. That recommends graphene for ultrahigh-frequency electronics, since scattering processes limit the switching speeds. More remarkably, the mobile electrons behave as Dirac fermions (Novoselov et al., Nature 438, 197; 2005), which mimic the characteristics of electrons travelling close to the speed of light.

From the perspective of applications, however, one key question is how to make the stuff. Peeling away flakes of graphite with Scotch tape, or in fact just rubbing a piece of graphite on a surface (popularly known as drawing) will produce single-layer films – but neither reliably nor abundantly. Walt de Heer of the Georgia Institute of Technology and coworkers have recently flagged up the value of a method several years old, by which silicon carbide heated in a vacuum will decompose to form graphitic films one layer at a time (Berger et al., Science Express, doi:10.1126/science.1125925).

But maybe wet chemistry will be better still. Graphite was exfoliated (separated into layers) nearly 150 years ago by oxidation, producing platelets of water-soluble oxidized graphene, which may include single sheets. But reducing them triggers aggregation via hydrophobic interactions. This can be prevented by the use of amphiphilic polymers (Stankovich et al., J. Mat. Chem. 16, 155; 2006). Anchoring bare, single graphene sheets to a surface remains a challenge – but one that may benefit, in this approach, from the wealth of experience of organic chemists.


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