True or false? It’s possible to make a sheet of carbon that is a single atom thick. If you’d asked that question before 2004, most scientists would have laughed you out of the room. It seemed as fanciful as the story of Flatland – a two-dimensional world vividly imagined by satirist Edwin Abbott in 1884.
2004 was the year maverick scientist Andre Geim and his student Kostya Novoselov introduced graphene to the world. Two-dimensional carbon is not only possible, it is promising to usher in a new industrial age. Two hundred times stronger than steel, a thousand times more conductive than copper, tougher than diamond, flexible, stretchable and see-through as well – its list of properties sounds even more remarkable than what is portrayed in Flatland. Many are forecasting a coming graphene age that will succeed that of steam, steel and silicon. Unbreakable, foldable touchscreens are just the beginning.
But perhaps graphene’s most extraordinary quality is that after a decade of intensive investigation it continues to startle the world. Last November two seemingly contradictory properties were added to the list. A paper in Science revealed that graphene was twice as bullet-proof as Kevlar. And in the same week a Nature paper showed the impenetrable barrier was actually porous to hydrogen ions, a trick that might be exploited to draw hydrogen, a potential fuel, right out of the air, says Geim.
Geim has come up with crazier ideas. In truth, graphene is the child of many crazy scientists – modern-day alchemists who spent decades bending and twisting carbon to create weird and wonderful new forms. It turns out this element doesn’t only supply the backbone of life’s chemistry; it forms the foundation of much of the non-living world as well.
Thirty years ago textbooks listed four varieties of carbon. Three were naturally occurring: diamond – the hardest material then known; graphite – better known as pencil lead; and finally amorphous carbon or soot. The fourth was man-made carbon fibre. Originally generated by Edison as a leftover from burnt out filaments for his electric light bulb, these are hard rods of pure carbon and are used as fillers to increase the strength of plastics.
Then in 1985, English chemist Harry Kroto was inspired by the composition of stars. Examining their spectra (chemical signatures detected in the emitted light spectrum), he noticed some carried an unusual form of carbon. He guessed they had the structure of a long rod. Enthralled by what carbon could do given the chance, he collaborated with American researchers who used a furnace of pure carbon gas to mimic the reactions taking place in the atmospheres of carbon-rich red giant stars. As the carbon cloud cooled it condensed to form a variety of molecules. One was made of exactly 60 carbon atoms. It turned out to be a sphere of pentagonal and hexagonal panels, like a soccer ball. It also proved to be extremely stable. In a scaled-up version, it resembled the geodesic domes created by visionary American architect Buckminster Fuller. In homage, Kroto and his colleagues named the molecule buckminsterfullerene. “Buckyballs” and similar structures known as “fullerenes” became buzzwords as materials scientists joined the new carbon craze.
Graphene is the child of many crazy scientists.
Japanese scientist Sumio Iijima was one of them. In 1991 he was synthesising fullerenes by sparking an electric current across two carbon electrodes, when he noticed a hard deposit growing on their sides. He found it was composed of long, thin tubes of carbon. Carbon nanotubes turned out to be the strongest known material. Calculations showed that, if they could be scaled up and bundled together, they would be strong enough to fulfil futurist author Arthur C. Clarke’s dream of a “space-elevator”– a cord thousands of kilometres long that could tether an orbiting satellite to the Earth. It turned out carbon nanotubes had already earned a place in history. A 2006 paper in Nature reported these structures in the steel of a Damascan sword forged in 17th century Syria. The Syrians, it appears, had inadvertently discovered how to forge carbon nanotubes.
Like buckyballs, the walls of the nanotubes were a single carbon atom thick. So how could they be so strong? Attempts by scientists to unravel this question finally led to graphene. They guessed the secret of the nanotube’s strength lay in the way the carbon atoms were bonded to each other in a hexagonal chicken wire-like structure. They had seen this structure before: in graphite. Your pencil lead is made of chicken-wired carbon sheets stacked on top of one another like cards in a pack. The weak bonding between the sheets means the layers can easily slide over one another or slough off on to paper. But the bonds within an individual sheet would have to be very strong, the scientists reasoned. The bonds had never been tested. To do so required isolating a single atomic layer – which no one believed was possible.
No one except for Andre Geim, a Russian émigré physicist at the University of Manchester with a weird sense of humour and a reputation for thinking outside the box. He had won an igNobel Prize in physics for levitating a frog in a magnetic field, and had also once co-authored a paper with his pet hamster. Geim describes his research strategy as the “Lego doctrine” – he improvises with whatever equipment is at hand. In 2004 he and his student, fellow émigré Kostya Novoselov, employed that doctrine in their quest for a single atom-thick sheet of graphite. The material might be outlandishly strong for its size, and have extraordinary properties as an electrical conductor, possibly even dethroning metals from their perch. In a now legendary tale, the duo used sticky tape to strip layers from a block of graphite. After repeatedly shaving the layer using strips of fresh tape, they were astonished to find their tape lifted away a single layer – graphene.
“It would take an elephant, balanced on a pencil, to break through a sheet of graphene.”
The newly-minted 2-D material did not disappoint. Graphene was extraordinarily conductive – electrons could skate unfettered across a surface of carbon atoms, instead of bouncing off them pinball-style as happens in metals. Graphene was also super strong. It resisted poking with a sharp diamond tip, meaning its chicken-wired atoms were more strongly bonded than those in diamond. In a relatively swift recognition of their discovery, Geim and Novoselov were awarded the Nobel prize in physics in 2010. Graphene is the strongest, sheerest material on the planet. “Our research establishes graphene as the strongest material ever measured, some 200 times stronger than structural steel,” says James Hone, the mechanical engineer at Columbia University who was a member of the team that conducted the diamond-point strength test. “It would take an elephant, balanced on a pencil, to break through a sheet of graphene the thickness of Saran wrap,” he enthused.
Last November Jae-Hwang Lee at Rice University in Texas and colleagues decided to test graphene’s mettle against a speeding bullet. Using the sticky-tape method they fixed multi-layer graphene sheets 10 to 100 nanometres thick across a tiny metal frame to make a membrane, a bit like a micro-scale drumskin. They then fired micro-scale silica bullets into it. The team used high-speed cameras to measure the bullets’ speed before and after hitting the graphene, allowing them to calculate how much speed (and energy) was lost in the process. A lot, it turns out. When struck by a projectile travelling at 600 metres per second, Kevlar – which has been the gold standard for ballistic armour – can absorb 0.4 megajoules of energy per kilogram of material. Graphene could absorb 0.92 megajoules per kilogram. To stop the projectile completely, Lee calculated, you’d need a thickness of only 500 nanometres – that’s one hundred times thinner than a human hair. It wasn’t possible for them to test that exact thickness because the sticky tape method didn’t allow them that degree of control.
Graphene was able to perform this feat through a combination of its strength and its ability to flex and stretch – by up to 20%. The bullet’s force rapidly dissipated when it hit the graphene – like dropping a bowling ball on a trampoline – rippling through the membrane at 22 kilometres per second. And when graphene did eventually crack, the fault lines were predictable. Lee expects that by reinforcing these lines with a polymer layer he could make a composite armour even better than pure graphene. “I am expecting that commercial graphene armour vests will be possible within a decade,” says Lee.
Scientists also believed graphene would present a barrier to sub-atomic particles. Despite its gaping chicken wire structure, the electron cloud between the carbon atoms was deemed impenetrable. But once again a team led by Geim has managed to surprise the world. The team placed a single sheet of graphene between two proton-conducting materials. In theory when they turned up the voltage, no current should have flowed. But a current did flow. The explanation? Despite the electron cloud barrier, “there are, however, areas where it is very, very thin”, says Marcelo Lozada-Hidalgo, first author of a paper on the finding published in Nature. Those thinned areas allow protons to squeeze through like minnows through a fishing net where the graphene net is only one atom thick. Adding even one extra layer of graphene stops the protons squeezing through.
It’s a “surprising and interesting result”, says Zhe Liu, a materials scientist at Monash University in Melbourne, Australia. Researchers had been so convinced of graphene’s impermeability, they were planning to use it for filtration membranes in which they would drill holes to fit particular atoms or molecules and be confident nothing else would cross. If gaps in graphene allow hydrogen ions (protons) through but not neutral hydrogen atoms, that could be useful for hydrogen fuel cells, says Liu.
Fuel cells are being developed to power hydrogen cars or life support systems for space travel. Instead of burning hydrogen in a combustion engine which is relatively inefficient, they unlock its energy by directly splitting the hydrogen atom into its electron and proton components. But in order to produce useful electricity, the particles must be well segregated: electrons have to go one way round a circuit, and the protons another. One of the leading designs relies on a membrane at the gateway into the circuit – a material that acts like a nightclub bouncer, only letting the VIPs (Very Important Protons) through. The problem is that the best membrane in commercial use today, DuPont’s Nafion polymer, is not a discriminating bouncer; it sometimes allows hydrogen molecules through, which wastes precious fuel. And at tens of microns thick it’s also so bulky that it slows down the flow of protons, reducing power. Replacing the Nafion with a single graphene sheet as a sleek, highly discerning new “bouncer” could solve both problems at once.
Graphene might also be used to pull hydrogen from the air
Geim goes a step further, suggesting graphene could be used to generate hydrogen fuel directly from the air. Generating hydrogen in the first place is the biggest obstacle in the production of hydrogen fuel cells. Hydrogen is made commercially from methane when it reacts with steam at high temperatures or by splitting water into its hydrogen and oxygen components with an electric current. Neither method is ideal. The first releases carbon dioxide; the second uses significantly more energy than it produces in the form of hydrogen fuel.
But free hydrogen is floating all around us at about 0.5 parts per million in the air. These trace amounts add up to more than two trillion tonnes of free fuel. Perhaps graphene might also be used to filter out the hydrogen from air?
Geim proposes using a chemical catalyst to split the airborne hydrogen into a plasma of protons and electrons. Air containing the plasma stream could then be blown through a graphene membrane. While the air molecules (mostly nitrogen and oxygen) would bounce off, a pure stream of protons would filter through. Once combined with electrons on the other side, it would provide a source of ultra-pure hydrogen. No doubt that would take an awful lot of filtering to generate any useful amount of hydrogen for a fuel cell. “It’s speculation,” Geim told Nature, “but before this paper, it would be science fiction”. His team has already used a similar approach to sift hydrogen from water where the concentration of hydrogen ions, depending on the pH, is much higher.
The fabric from flatland has wowed us with its marvels for more than a decade now. But most of these feats are performed in the lab. So when will the forecast graphene revolution be upon us?
A roadblock is the cost of manufacturing high quality sheets of the fabric – only the pure form would be able to hold an elephant on the top of a pencil. But at a market price of hundreds of dollars per gram, graphene can add the title “one of the most expensive materials known” to its list of properties.
That title may soon be shed as production methods superior to shaving with sticky tape are being developed. In 2010, scientists at Samsung laboratories created single sheets of graphene as large as TV screens by carefully growing the crystal on a surface of copper foil – this could prove to be the method of choice for making flexible touch screens. But if graphene is to be made on the scale of tonnes, the method will have to be liquid-based. The idea is to suspend flakes of graphite in a liquid and use exfoliating chemicals to slough off layers to make a suspension of graphene sheets. In 2014, in a paper published in Nature Materials, Jonathan Coleman and colleagues at Trinity College, Dublin, found that with the right detergent, a kitchen blender was strong enough to peel graphite apart to provide large quantities of cheap, high-quality graphene.
Though expensive, graphene has been getting a niche foothold in the marketplace, in high-end sports equipment such as skis, bike helmets and tennis rackets. Novak Djokovic and Maria Sharapova, for instance, use graphene racquets.
But this trickle is the calm before the storm. Governments are laying down huge investments in the race to be at the forefront of the graphene era. In the UK the $120 million National Graphene Institute is due to open this year. The Graphene Research Centre in Singapore has been built with funding of more than $100 million. Not to mention the EU’s Graphene Flagship program, where $1.2 billion will be divvied up across the 29 member states over the coming decade, supporting collaborations between academia and industry to ease graphene into commercial products.
While graphene is not yet used in consumer electronics there have already been more than 11,000 graphene patents filed worldwide, including hundreds each by technology giants such as Samsung, Apple, Sony and IBM.
What sets graphene apart is how rapidly the research is translating into real world products, says Gordon Wallace, director of the ARC Centre of Excellence for Electromaterials Science in Wollongong, Australia. “For more than two decades you had all these people working to make devices with nanomaterials, then along came graphene, stood on the shoulders of carbon nanotubes and the rest, and suddenly it’s turning up in devices within just a few years.”
“Where graphene will be really exciting is that it enables applications we cannot even imagine now. These are the ones that are going to really change society. I believe we are just at the start of that,”
Deeper into Graphene
New Materials from Flatland
In the past 10 years, dozens of materials capable of forming 2-D structures have been found. Some have properties that outperform graphene in one or two areas, but none come close to graphene’s superlative versatility.
Among the most interesting candidates is silicene. It’s made of silicon atoms arrayed in a hexagonal structure. It was first detected when researchers managed to grow it on a silver substrate in 2010. A freestanding sheet of silicene has yet to be made and so many of its properties are still unknown. But like the slabs of silicon used in computer chips, silicene can easily be “doped” to turn it into a semiconductor, giving it an edge over graphene in the creation of exotic new electronic devices. In February 2015, an international team reported in Nature Nanotechnology they had created the first working silicene transistors, which operated at the blazing speeds theory predicted.
Another recent highlight is phosphorene, a single layer of black phosphorus. It was first isolated in 2014, using the sticky tape method. In January 2015 researchers at Trinity College, Dublin, found a new method to produce phosphorene in larger quantities, using a solution-based process similar to the one used to isolate graphene with a kitchen blender. Rather than forming a chicken wire structure, the phosphorus atoms are arranged in a slightly kinked zig-zag pattern. This semiconducting material could have an array of applications in nanoscale electronics. In the lab, it’s already been used to make high-quality transistors and to increase the strength of plastics. It seems that in a scientific field eternally wowed by novelty, we can expect cries of “the new graphene” to keep echoing for some time yet.