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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.

“As we learn to shape carbon on the nanoscale, new and unexpected vistas are opening up”

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 and 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 (). It has been followed by a flood of improved methods, including a technique reported earlier this year by 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 (). 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, Thomas J. Watson Research Center in New York unveiled the first graphene photodetector ().

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”.

When graphene is chopped into ribbons less than 10 nanometres wide, its electronic properties undergo a dramatic change. Because of the way electrons are forced to move through the narrow strips, nanoribbon graphene does have a band gap. That turns it into a semiconductor ready to challenge silicon on its home turf.

“Cut into nanoribbons, graphene is a semiconductor ready to challenge silicon on its home turf”

Manufacturing nanoribbons remains something of a problem, though progress is being made. At first it was only possible to make them by breaking up a graphene sheet. This has been done using chemicals that break some of the carbon-carbon bonds, ultrasound, or with a scanning tunnelling microscope that saws the atoms apart. With all these techniques, however, the quantities produced were minute. Then, in June this year, at MIT showed how nickel nanoparticles can be used to cut nanoribbons out of graphene sheets (). The method produced such neat cuts that it could provide a way to create graphene nanocircuits.

Also this year, , California, shredded carbon nanotubes into ribbons using a beam of argon plasma (). Because nanotubes are already produced in bulk – Mitsubishi, for example, makes tonnes per year – this is an encouraging route to nanoribbon mass production. With Dai’s method, “the possibility of making tonnes of ribbons is also here”, says Mauricio Terrones, a nanotech researcher at the Institute for Scientific and Technological Research in San Luis Potosí, Mexico.

There are, however, more subtle ways of turning graphene into a semiconductor, and they could make nanoribbons redundant long before they ever reach the production line. Pereira’s group has shown that inducing a strain in the bonds between carbon atoms generates ribbon-like semiconductor behaviour in the strained regions (). “You can generate a ribbon without having to cut the ribbon,” Pereira says. One way of doing this is to drape the graphene over a pre-shaped structure. The strain induced at each curve and fold will give the sheet a variety of electronic properties. Lay it over an etched channel, for instance, and the part of the graphene that lies along the bottom of the channel will behave as if it is an isolated nanoribbon.

This has two advantages over both silicon-based semiconductors and graphene nanoribbons. First, it is highly adaptable. In silicon, the band gap is created by “doping” a silicon crystal with various impurities, but once set it is fixed. A nanoribbon, once cut, also has fixed properties. But a slight change in the twist or the angle of pull of a graphene sheet gives it a completely different set of properties.

The second advantage is simple: the process is entirely reversible. Remove the strain and the graphene sheet returns to its normal conducting self. The implication of this discovery, Pereira says, is that it might be possible to manufacture all-graphene electronics, with every component created by carefully orchestrated strain on a region of the graphene sheet.

It remains to be seen whether such claims will stand up outside the lab. As Avouris points out, many of the experiments so far have been carried out in rather favourable circumstances – at low temperatures that keep electronic noise to a minimum, for example. “The basic physics and basic engineering has been done, and it’s very promising,” he says, “but people forget this is done in idealised conditions.” Geim agrees that it’s too early to tell whether graphene could be the material that knocks silicon off its perch and takes computing to the nanoscale. It certainly can’t be ruled out, he says.

Meanwhile, Geim and his team continue to lead graphene research in new directions. Their most recent advance has opened up a range of possibilities by creating “graphane”, a graphene sheet studded with hydrogen atoms. The carbon-hydrogen bonds act as a sink for electrons, preventing the material from conducting electricity. The interest graphane has sparked, however, is not really to do with its electronic properties. Instead, the excitement focuses on the swathe of other new molecules that might be just around the corner. The changes that the addition of hydrogen atoms induced are nothing compared with what the rest of the periodic table might do to graphene, Geim predicts. “Graphane has opened the floodgates,” he says.

His team is about to unveil several other new materials based on chemically modified graphene. Though they are still under wraps, Novoselov promises that they will be more stable, more reliable and much more useful than graphane.

Carbon chemistry is about to get a whole new lease of life, says Rodney Ruoff of the University of Texas, Austin. As graphene is nothing more than a layer of carbon atoms, modifications with other elements will radically alter its response to heat, light and other stimuli. On a more practical front, a carbon-based electronics revolution has already begun, and Geim predicts that there should be plenty of other applications waiting in the wings. “As far as graphene’s magnetic, superconducting or mechanical properties, or even its basic chemistry, is concerned, we haven’t even started,” he says.