EVERYTHING should look different by now. We should be surrounded by swarms of high-speed computers, each no bigger than a speck of dust. We should be living in houses that snap back into shape after an earthquake or a hurricane. We should be boarding elevators that can carry us into space. If you believed just half of what has been written about carbon nanotubes over the past decade, you might now be feeling a bit disappointed with the impact they have had on today’s world.
But you shouldn’t be. There’s a good chance you now own some nanotubes. Most American cars, not to mention a good number of European ones, contain them. If you’ve bought any electronics recently, its components may well have come to you in nanotube-laden packaging. And it won’t be long before you can go camping, gaze up at the stars and thank nanotubes for the electrical power that heated your supper. The revolution has happened – you just didn’t notice.
There’s no doubting the potential of nanotubes. They might look like a bit of rolled-up, microscopic chicken wire, but this honeycomb lattice of carbon atoms is the stuff of engineers’ dreams. For instance, their electrical properties mean that nanotubes can be made into metals or semiconductors, depending on how you roll up the sheet of carbon atoms. Roll the carbon the way you roll a cigarette, with the edges touching along their length, and you have a nanotube that acts like a tiny metal wire conducting electricity. Wind the tube askew, like a paper straw, and you have a miniature semiconductor that could replace silicon transistors, the building blocks of chips.
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What’s more, nanotubes conduct electricity better than copper, making them a contender for replacing the delicate wires that connect components together inside computer chips. Not only that, but they can carry heat far more efficiently than diamond, one of the best heat conductors around. So if you give processor chips a nanotube coating, you could pack billions of them together into a tiny space with little risk of them burning up.
Perhaps even more impressive are the mechanical properties of these lightweight structures. Nanotubes are over 50 times stronger than steel wire and only a quarter as dense. No matter how hard you squeeze a nanotube, it will bend and buckle without breaking, springing back into shape as soon as you let it go. So who can blame analysts for predicting the emergence of crash-proof cars, nanotube ropes for lassoing space junk, and bulletproof vests lighter than a silk camisole?
Indeed, the strangest thing about the nanotube story is not the hype but the history. Read textbooks, newspapers, magazines, even academic journals, and you’d think they were a recent invention. But nanotubes may already have been around for more than a century. A US patent granted in 1889 to two British men reveals how to make them using marsh gas – better known these days as methane. The method is essentially the same as that used in industrial processes today, and produced “hair-like carbon filaments” for electric lighting. According to the patent, as well as having useful electrical properties, these filaments “may be bent and twisted into various shapes and will spring back to their original form on being released”. In the 1960s and 1970s a couple of research groups – at the National Carbon company in Parma, Ohio, and the University of Canterbury in Christchurch, New Zealand, respectively – also made and characterised carbon nanotubes.
The hype began much later: in 1991, after Sumio Iijima and his colleagues created nanotubes at the research laboratory of the electronics multinational NEC in Tsukuba, Japan. Iijima’s “discovery” came just a few years after the surprise finding of buckyballs – a new molecular structure for carbon – and, perhaps more importantly, the publication of Eric Drexler’s book Engines of Creation. This raised the idea that nanotechnology, making machines on the nanoscale, could provide a solution to virtually any problem you might dream up. By the time Iijima made his announcement, nanotechnology was filtering into academic and government circles as something worth thinking about. Nanotubes were just what we had been waiting for: a material to transform the world.
One organisation unimpressed by the 1991 hype was Hyperion Catalysis, a firm based in Cambridge, Massachusetts. Hyperion has been perfecting ways to produce nanotubes by the tonne since 1983. Today, 60 per cent of cars on American roads have fuel lines containing Hyperion’s carbon nanotubes. Their high conductivity dissipates any electric charge that might build up and generate a dangerous spark as the fuel flows past the nylon walls of the fuel line. If you own a Renault Clio or Mégane, next time you polish it you’ll also be buffing some of Hyperion’s nanotubes. These are used to make the plastic wing panels so conductive that they can be earthed while the car is sprayed with paint droplets charged up to 20,000 volts. The droplets seek ground instead of floating away, making spray-painting more efficient and less polluting.
At present, Hyperion is the only company that churns out tens of tonnes of “multiwall” nanotubes every year. These consist of between 10 and 12 nested cylinders of carbon, each 10 micrometres long, and cost as little as $2 per gram. Hyperion only sells them incorporated into plastics, but there are plenty of other firms, such as Carbon Nanotechnologies in Houston and Sun Nanotech in Nanchang, China, that sell plain nanotubes by the gram.
Car manufacturers aren’t the only ones who find the conducting qualities of nanotubes useful. They are also prized by the electronics industry. Nanotubes are now incorporated into the carry cases and trays used to transport chips and hard drives. Carry boxes made from nanotube-spiked plastics carry away any charge before it builds up, and their super-smooth surface ensures that tiny tracks aren’t sloughed off the chips whenever they are removed from the packaging.
While the electronics industry is content to use nanotubes to wrap its chips, it would be even more pleased if nanotubes could make the chips in the first place. Every 18 months or so, engineers have been doubling the number of transistors – electrical switches composed of carefully arranged layers of semiconductors and conducting electrodes – they can cram onto processor chips. For this trend to continue, they will have to carve out ever-tinier transistors from silicon. But within a few years, they will reach the point where the transistors are so small that electrons will be able to tunnel through insulating layers between the components and render the chips useless. To shrink transistors further a radical approach will be needed.
Nanotube transistors connected by nanotube wires might be the answer. But because that would rely on expensive single-walled nanotubes, making this idea a reality is proving hard to do at a reasonable price. A gram of single-wall nanotubes costs $750, around 70 times as much as a gram of gold. The high price is due to the cost of removing any impurities formed during production. The principal means of making nanotubes is by zapping carbon with a laser or by blowing hydrocarbon vapour over a hot metal catalyst (see Diagram). The end result is that most nanotubes tend to be mixed up with impurities such as soot and metal particles. Although you can remove the metal and carbon gunk by washing the nanotubes in acid, it’s expensive and you risk damaging the honeycomb structure and thus the desirable electrical and mechanical properties. IBM is leading the way in getting round these obstacles, but nanotube processors are still a long way off (see “Micro chips”).
Using the mechanical properties of nanotubes is proving just as problematic. The idea of a tether reaching into space is looking rather far-fetched: the longest nanotube ever made is only 20 centimetres long. And, for the moment at least, embedding nanotubes in materials such as concrete is having the opposite effect to that intended. Instead of adding strength they create weak spots (see “Tangled up”).
So, for the moment at least, the nanotube revolution remains focused on plastics spiked with cheap multiwall nanotubes. But don’t scoff: it’s still revolutionary. For starters, nanotubes can make plastics conduct better than copper. David Carroll and his colleagues at Clemson University in South Carolina have been adding nanotubes to plastics that already conduct, such as polyaniline (PANI), to boost their performance. “On its own PANI isn’t quite conducting enough to replace copper wires,” explains Carroll. “With the addition of nanotubes you could potentially replace all the heavy copper in an aircraft with lightweight plastic wires.” Such weight savings would quickly lead to lower fuel consumption.
But some of the most exciting prospects for applications come from Carroll’s work on piezoelectric plastics, materials that produce a voltage when you press or heat them. Last year his group discovered that a polymer widely used in ultrasound sensors, polyvinylidene fluoride (PVDF), becomes three times as sensitive to pressure when nanotubes are sprinkled in. And it doesn’t take much to see an improvement: just one nanotube for every 8000 strands of PVDF is enough.
These improvements come about because nanotubes keep the polymer in a stable piezoelectric state. Rolling, pulling, pressing and the other processes used to turn plastics into products normally destroy the molecular structure that makes PVDF a piezoelectric plastic. But Carroll has discovered that the addition of a few nanotubes is enough to hold the piezoelectric arrangement together through thick and thin.
Carroll has grand plans for piezoelectric plastics. “The chemical industry already makes huge vats of PVDF,” he says. “It’s just one small step to make nylon fibres.” He envisages weaving them into ships’ sails so that they generate electricity as they stiffen in a buffeting wind, enough even to power heating and lighting on board. And, in the future, giant piezopolymer sails might generate electricity for homes.
His group has also added nanotubes into plastic solar cells, and found that they are 50,000 times as efficient at converting sunlight into electricity as other plastic photovoltaic devices. Researchers are keen to make solar cells from plastic because polymers are so cheap and they can be made into huge sheets. When sunlight hits the polymer, it releases electrons and positively charged holes that travel through the material to electrodes, generating a current. But, until now, plastics have made poor photovoltaics because the electrons and holes find it hard to move through the polymer. Instead they meet up and recombine to form light well before they reach the electrodes, giving polymer solar cells an efficiency of only 0.0001 per cent – for every million photons that land on the solar cell, just one produces an electric current. They also tend to have a short lifetime, each sheet only working for a couple of hours before oxygen from the atmosphere works its way into the plastic and traps the charge carriers.
But a network of nanotubes running through the polymer gives the electrons and holes a free run towards the electrodes. Carroll’s team has made solar cells that convert 5 per cent of light into electricity and live long enough to be commercially viable. “Some of the devices I made a year ago are still working,” he says.
Although the best silicon solar cells are many times more efficient, Carroll predicts strong interest in his material because it could be used to make swathes of electricity-generating solar cells. He believes the market will be driven by an unusual group: campers fed up with cooking on gas stoves who will relish the chance to plug in their electrical appliances to their wired-up tent.
OK, so it’s not an earth-shattering technological vision, but it’s no pipe dream either. Nanotubes may not be guiding the space elevator to a holiday destination above the clouds, but the fact remains that they are already worth billions of dollars worldwide. If you were about to cash in your nanotube stock, disappointed by the revolution that never came, you might want to think again.
Micro chips
It didn’t take long for researchers to realise that nanotubes might satisfy demand for ever-faster computer chips. The best way to gain speed is by shrinking the transistors that make up the chips, and so nanotube transistors are an ideal solution.
Cees Dekker’s group at the Delft University of Technology in the Netherlands built the first transistor from a nanotube in 1998. Although it worked, it was nowhere near as good as its silicon counterparts. Last year, however, Phaedon Avouris and his colleagues at IBM’s TJ Watson Research Center in New York state made nanotube transistors that outperform state-of-the-art silicon devices. They achieved this by spreading a solution of nanotubes over a silicon wafer. Then they searched for isolated semiconducting nanotubes that would make good transistors and laid gold electrodes on top of them. Next came an insulating layer, followed by the final electrode.
The only remaining problem is economic: producing semiconducting nanotubes cheaply. The IBM team has managed to make a pinch of single-wall nanotubes by passing hydrocarbon gas over a silicon carbide crystal rather than a metal catalyst. This meant the nanotubes came out clean, and there was no need for a damaging washing process. They even managed to isolate the required semiconducting nanotubes from the conducting, metallic ones by sending a huge surge of current through them. The metallic ones burn up like blown fuses, leaving the semiconducting nanotubes behind.
Nevertheless, Avouris admits, they are still a long way from the production line. It’s a low-yield process, and for the moment it remains prohibitively expensive. There’s a long, slow road to travel before the first nanotube-powered PC hits the shops.
Tangled up
Multiwall nanotubes might be cheap, but that doesn’t make space lassos and lightweight bulletproof vests any easier to develop. At present, for instance, nanotubes are way too short to reach into space. Pulickel Ajayan of Rensselaer Polytechnic Institute in New York state and colleagues currently hold the record for growing the longest nanotube strands, which can be up to 20 centimetres long. They make them in a chimney that blows hydrocarbon gas upwards over the strands, whose length is limited only by the size of the apparatus.
Kaili Jiang’s group at Tsinghua University in Beijing, China, has made a thread 30 centimetres long composed of 3000 nanotubes, each 100 micrometres long. It is created by slowly plucking one nanotube out from an array. The forces between the nanotubes are so strong that, as the first tube is pulled away, another one sticks to its end and is pulled out, and so on. Jiang reckons a continuous thread up to 10 metres long is possible.
While they may provide the route to a nanotube rope, these strong interactions are exactly the reason that applications like the nanotube bulletproof vest are so elusive. Put a hefty dose of nanotubes into concrete or plastic and they tend to clump together rather than zigzag through the material. Instead of strengthening materials, the nanotubes create weak points: not quite the result anyone was expecting.
