Tech

CARBON is a dirty word. We burn too much of it, producing billions of tonnes of carbon dioxide that threatens to wreck our planet's climate for generations to come. Before that it was the villain of the piece in the guise of the soot that poured from factory chimneys and turned cities black. It has a lot to live down.

Now our long-time enemy could be on the brink of becoming our high-tech best friend. As we learn to shape carbon on the nanoscale - into tubes and sheets, balls and ribbons - entirely new and unexpected vistas are opening up. The carbon atoms that were forged in the furnace of the universe's stars can be woven together into materials that may help gather energy from our own star. Similar materials promise to make our electronic world run with unprecedented efficiency, and may even hold the secret to eking out precious reserves of oil.

Carbon's potential stems from the fact that it is multitalented. Collections of carbon atoms will happily assemble themselves into a multitude of structures, from diamond to graphite, but these familiar forms are just the beginning. In the past few decades we have learned about the soccer-ball-shaped spheres called buckyballs, soon followed by the microscopic rolls of chicken wire we know as carbon nanotubes. Now they have been joined by graphene, sheets of carbon that are just one atom thick.

Of these many intriguing structures, graphene is causing the biggest stir. This is partly because of its unusual combination of properties: its two-dimensional honeycomb lattice of carbon atoms combines fantastic electrical conductivity with a strength tens of times that of steel in a material that is transparent to visible light. Best of all, we have finally learned how to make it.

This last breakthrough came in 2004, when Andre Geim and Kostya Novoselov at the University of Manchester, UK, discovered they could produce graphene sheets from a fleck of graphite by simply peeling it off with a strip of sticky tape (Science, vol 306, p 666). It has been followed by a flood of improved methods, including a technique reported earlier this year by Jing Kong and her team at the Massachusetts Institute of Technology, which involves growing graphene on top of crystals of other materials and then chemically stripping the supporting crystal away (Nano Letters, vol 9, p 30). After just five years of development, making graphene is easier than anyone ever thought possible, and ramping up to industrial scale production is just a question of demand. "It doesn't even require minor breakthroughs; it's just polish and precision now," says Geim.

After Geim isolated the first few flakes, it was quickly apparent to theorists that this material should have some pretty special properties. At the time there was too little of the material available to experiment on. "Now it's very different," says Vitor Pereira of Boston University. "There are more experimental than theoretical papers... That's really exciting because it's out of experimental results that the true breakthroughs come."

The big breakthrough everyone is looking towards arises from graphene's potential for revolutionising our gadgets, as electrons travel through graphene in a particularly efficient fashion. In conventional conductors and semiconductors, such as copper and silicon, electrons collide with atoms and dissipate their energy as heat - a typical computer chip wastes 70 to 80 per cent of its electrical power in this way. That means the materials can get hot enough at times to distort or even destroy their circuitry. But graphene is different. "The electron energy is not dissipated," Pereira says. "That gives it fantastic characteristics for electronics."

It is particularly useful for high-frequency circuits - which happens to be exactly where the electronics industry is heading. Devices such as cellphones require ever higher frequencies as engineers try to cram more information onto the signal - and the higher the operating frequency, the greater the heating effect. "At the moment, graphene looks like the most promising way forward," Novoselov says.

Switched on sensors

An even bigger market for graphene might be in fabricating the photon sensors that detect the information carried in optical telecommunications fibres, Novoselov reckons. At the moment, the job is done by silicon, but its days may be numbered. In October, Phaedon Avouris's group at IBM's Thomas J. Watson Research Center in New York unveiled the first graphene photodetector (Nature Nanotechnology, DOI: 10.1038/nnano.2009.292).

Also on the drawing board are graphene-based solar cells and LCD screens. Because the material is transparent, electrodes made of graphene let more light in or out than any alternative material. That could mean efficient solar cells and display screens - in other words, increased efficiency both when generating electricity and when using it.

Most importantly, graphene can behave as a semiconductor, marshalling the flow of electronic information by switching between conducting and insulating states. This switching behaviour - the phenomenon that underpins transistors and thus the entire computing industry - relies on materials whose electrons are organised in energy states that have what is known as a "band gap". It's the controllability of silicon's band gap that has made it the semiconductor of choice. As graphene has no band gap, it looked for a long time as if there could never be a "Carbon Valley" to compete with Silicon Valley, but that changed with the discovery in 2008 of narrow graphene "nanoribbons".

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