Graphene is graphite sloughed off in layers just atoms thick (Image: De Agostini/Getty Images)
Graphene started it all, but even as we try to exploit its distinctive properties, a host of new materials just atoms thick are already in the works
GRUBBY, grey graphite and searing, scintillating diamond – both forms of the same thing, elemental carbon. It’s a fact that probably bamboozled us at school. But for materials scientists, the lessons didn’t stop there. If they thought they knew everything there was to know about the basic element of all life we have seen so far, recent years have taught them to think again.
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Carbon, it turns out, can shape-shift in all sorts of ways, developing intriguing and unexpected personas as it does so. Football-shaped buckyballs come from outer space, but can be made with a whack of electricity in the lab. Tightly rolled nanotubes can withstand enormous pressures, and slide in and out of one another telescopically almost without friction. But top of the list is graphene: a form of carbon touted to have such whizzy properties of strength, flexibility and electrical conductivity that in the future everything from computer chips to condoms to super-light aircraft will be made of it.
The first production of graphene was announced in a paper published in October 2004 (). A decade on, how far have we come in realising its promise? Is it really the wonder material it’s cracked up to be? The picture is mixed – but perhaps those aren’t really the right questions to ask.
Graphene is essentially a two-dimensional sheet of carbon atoms arranged in hexagons, as they are in graphite. It was first isolated and characterised by and at the University of Manchester, UK. In possibly the most low-tech experiment in recent history to have been rewarded with a Nobel prize – – they did it by taking a lump of graphite and using sticky tape to rip away the thinnest of layers.
What was essentially a slimline slice of pencil lead turned out to have amazing properties. Each carbon atom in graphene is attached to three others, with each atom in a linked pair contributing an electron to make an ultra-strong bond. Because carbon atoms have four electrons available to bond, that leaves one electron per atom free to roam across the sheet. With no layers restricting their movement above and below, these electrons flow almost unimpeded, at much higher speeds than in a conventional, 3D material. The current carrying capacity of a ribbon of graphene might be as much as 100 times that of a copper wire.
Carbon-to-carbon bonds are what makes diamond so hard. Lumps of graphite have these same bonds within each of their graphene-like layers, but the layers themselves are only weakly bonded to each other, slipping over their neighbours easily and so giving this material its characteristic softness. But on its own, a graphene sheet is crazily strong for a material just one atom thick. Pressed by the diamond tip of an atomic force microscope, it has proved to be the strongest material ever measured, and also to boast extraordinary stiffness. If all that weren’t enough, graphene is highly impermeable, and absorbs infrared light while being transparent to visible light.
“The strongest material ever measured, graphene also boasts extraordinary Stiffness”
Small wonder, then, that the material’s fame has spread far and wide. The National University of Singapore established a as early as 2010. Eager to exploit a home-grown technology, in 2012 the UK government gave £50 million in funding to two research centres aimed at commercialising graphene, and . Earlier this year, £60 million was for a third, also in Manchester. The European Union has made graphene one of its 10-year . Last year, on the material were published.
But haven’t we been here before? Buckyballs – more formally known as buckminsterfullerene molecules – were a wonder form of carbon in their day, and earned their discoverers . Carbon nanotubes likewise brought great hope for stronger materials, more efficient methods of drug delivery, and even “space elevators”, structures that would provide direct access to the altitudes where some satellites are placed, or even the moon.
Beyond the hype
Those dreams have remained largely unrealised. For all the improvements they offered over more conventional materials, carbon nanotubes proved too difficult to handle and too expensive to produce for many applications, says , co-founder of G-Heat, a London-based nanotechnology company working on commercial applications of graphene. A similar fate befell buckyballs.
For of the University of Geneva in Switzerland, graphene is not a case of déjà vu, though. “There is some hype in graphene, but it is fully justified,” he says.
How so? Some graphene applications are indeed already on the market. The most advanced are those where the material’s combination of feather-light weight and peerless strength comes into play. Andy Murray, Maria Sharapova and Novak Djokovic are among the pin-ups for a range of tennis rackets from sports equipment manufacturer Head. The weight of these rackets is redistributed to the tip and the grip by using a graphene nanocomposite material for the middle section. “It does make a difference,” says of Chalmers University in Gothenburg, Sweden, who heads the EU’s graphene project.
James Baker of Manchester’s National Graphene Institute, meanwhile, is excited by how graphene might shake up the aerospace industry. “It could be disruptive,” he says. Modern aircraft wings are made from lightweight, insulating composite materials, and must incorporate copper wires to carry away electricity from any lightning strike. Should lightning hit a graphene wing, the wing itself would be the conductor, allowing the current to reach a place where it can be safely grounded.
Mind the gap
Graphene’s range of properties mean potential applications don’t end there (). Take its impermeability: for something so thin, graphene is a remarkably discriminating membrane, not even letting gases like oxygen through. That is why the Manchester institute, with funding from the Bill and Melinda Gates Foundation, that could boost efforts to prevent the spread of HIV. The first infrared-absorbing graphene photodetectors, created earlier this year at the University of Michigan in Ann Arbor, . Meanwhile, graphene’s strength, conductivity and transparency to visible light could make it the perfect material for lightweight, unscratchable touchscreens. Geim himself has a prototype made by a partner of the Manchester institute, and (see diagram).FIG-mg29920401.jpg
The outlook is not so bright on all fronts, however. One of graphene’s most touted applications is as a replacement for silicon, the material that underlies all modern computing technology. Graphene could in principle herald the dawn of an era of small, efficient and flexible devices – but this is one area where the pickings might be as thin as the material.
In a semiconductor such as silicon, electrons have to be given a small amount of energy to free them up to conduct. This switching between on and off states can be finely controlled to process information – the function of the device at the heart of silicon electronics, the transistor. The energy difference between the two states is called the band gap, and the main problem is that graphene doesn’t have one: its electrons are essentially free to conduct all the time. “It will be very difficult to turn off a graphene transistor,” says Kinaret.
There are ways around this, which have been used to create prototype graphene switching devices, for example introducing other elements in tiny amounts at the graphene surface to stop electrons moving too freely. But there is a more fundamental obstacle in the way of all these applications: the difficulty of making graphene consistently in commercially viable quantities. Scraping the top off a pencil lead isn’t a practical way to supply large amounts of graphene to the electronics, aerospace or any other demanding industry, and researchers are still refining techniques for making large sheets that are flaw-free. “I don’t think anybody in the world can make graphene at the moment in its perfect form,” says Ian Walters of , a company with a production facility in Ammanford, UK. The most successful approach so far has been heating graphite to a vapour and depositing the carbon atoms on a surface. Last year researchers at the electronics manufacturer Sony of graphene in this way.
New dimensions
Walters’s worry is that the hype is leading some suppliers to label bags of black powder as graphene when they contain clumps of several graphene layers – in other words, graphite. Although “multilayer graphene” a few atomic layers thick does have some of graphene’s wonder properties, at the moment there is no way to guarantee what buyers are getting. “The market is being misled,” he says. Kinaret agrees. “There are a lot of people who sell something they call graphene,” he says. “There’s no standardisation.”
All this means there is much still to be done before graphene lives up to all the claims made for it. “Theoreticians say that graphene has remarkable properties,” says Walters – but in practice he remains concerned it is all hype. For , an electrical engineer from the Massachusetts Institute of Technology, it is still too early to make that call: the coming years will determine what worth graphene truly has in areas such as computing. “It’s going to be very difficult and expensive to displace silicon,” he says. “It will take decades.”
But to obsess over whether graphene will fulfil its promise is to overlook perhaps its most significant contribution: as a trailblazer for a whole family of novel two-dimensional materials. Silicon, phosphorus, germanium, tin – 2D versions of all these elements are already being hailed as the “new graphene”. In the case of the tin version, stanene, this is before it has even been made in the lab.
“Graphene’s True contribution is as a trailblazer for a whole new family of 2d materials”
Why the excitement? “No other materials are thinner, stronger, more transparent than these,” says Palacios. “They are reaching the limit of what is physically and chemically possible.” Crucially, these 2D materials often have a property that graphene lacks: a band gap. Silicene for example, the 2D version of silicon isolated in 2012, looks on paper the ideal candidate to create slimmed down versions of existing electronics.
But no silicene transistor or device yet exists – in fact, it was only in August that researchers at the Italian National Research Council in Rome , at least for 24 hours. Other compound 2D materials are further advanced. One of the most widely studied is a 2D version of molybdenum disulphide that also has a band gap. Thin, 2D crystals of the stuff can gather light at their edges and convert its energy very efficiently into electricity. And although it isn’t as strong as graphene, says Kinaret, it can be used .
In total we know of around 500 solids that could be promising starting points for 2D materials. “This is really a family of materials,” says Palacios. “Graphene just happens to be the first.”
Experience gained with graphene is paying dividends. Methods devised to manufacture it, for example, are speeding up our exploitation of the novel materials. “Graphene has inspired a new field,” says , director of the Cambridge Graphene Centre.
That has the potential to lead into a virtuous circle. Using graphene in combination with another 2D material might get around the band-gap problem: in 2012, a team including Geim and Novoselov produced a transistor of molybdenum disulphide sandwiched between two graphene layers. Similarly, if graphene’s transparency makes it unsuitable for, say, a super-efficient solar cell, then you might combine it with a 2D material that is better at absorbing light, and exploit graphene’s electronic properties just to whizz the electrons away. “There is no material that is well suited to every occasion,” says Palacios. “The solution will be a combination.” All the nascent graphene institutes have the wider study of 2D materials as a goal, with Singapore’s this year “.
Geim is certainly moving his attention away from the material he co-discovered. “Graphene is scientifically well explored, and hundreds of groups are now working on the subject. Let them continue,” he says. He’s now investigating what he calls : 2D layers of different substances weakly held together, as in graphite, by van der Waals bonds. It is terra incognita, he says, and that is where he wants to be. “I am not an industrialist or a property developer. I do what I am doing best, better than others: that is, exploring new science.”
For others, it is a different story. Graphene’s potential, whether used singly or in combination with other materials, is enough to keep any number of researchers happy for a while. Aeroplane wings? Condoms? Flexible wearable electronics? Perhaps all those things will emerge – or perhaps not. For Ferrari, the real aim in the world of 2D materials should not be to reproduce something that already exists, but to make something no one has even dreamed of yet. Palacios thinks so too. “Nobody knew about the transistor before the transistor existed,” he says.
This article appeared in print under the headline “Carbon and… on”
Article amended on 1 January 1970
When this article was first published, Melinda Gates was mis-named.
