A wonder material makes your smartphone screen work. But with the world's stocks running out fast, the hunt is on for something new to keep us in touch.
A tap and a flick, and a new world is at your fingertips. Email, social networks, the digital version of New Scientist: surfing the web has never been easier thanks to the touchscreen technology built into the latest smart mobile devices. Proud owners need little excuse to demonstrate their new darling's superior, sexy features. Touch is fast, touch is fun—touch is the future.
Yet touch could soon be history, if we are not careful. Today's mobile touchscreen gadgets, along with all liquid crystal displays, rely on the unusual properties of a single material - a metallic crossbreed whose sources could be exhausted within the decade. It is not just our displays that are under threat. Solar cells and low-power LEDs, both central planks of a low-carbon energy strategy, could feel the squeeze too. No surprise, then, that companies and laboratories across the world are scrambling to find a replacement.
If this is all news to you, chances are you have never heard of the material causing all the fuss. A mixture of two metallic oxides called indium tin oxide (ITO), it is the material electronic engineers love to hate. Its principal component, indium, is a by-product of lead and zinc mining; it is difficult to come by and expensive. Once through the factory gates, ITO's brittleness and inflexibility make it a pain to work with.
And yet it has qualities that make us forgive its defects. Specifically, it is a rare example of a material that is both electrically conducting and optically transparent, which means it does not absorb photons of light.
Absorption occurs when a photon's energy matches that needed to knock an electron into an excited state. In a metallic conductor, where there is a free-flowing "sea" of electrons with many different energy states, ths almost always happens. Accordingly, almost all metals are highly absorbing—and entirely opaque.
Not so ITO. It is transparent like glass, but also conducts—not as much as most metals, to be sure, but enough. That makes it ubiquitous in modern electronic devices that manipulate light. In flatscreen televisions, each display pixel is switched on and off by a pair of transparent ITO electrodes. In thin-film solar cells, the light-absorbing layer needs an electrode front and back to form a circuit and so convert sunlight to electricity.
Touchscreens are just the latest innovation to depend on ITO. Some old touchscreens do without it, for example using infrared LEDs ranged around the screen to fire beams that are blocked by a touch. But this bulky, power-hungry set-up is ill-suited to a small device. The first mobile touchscreen gadgets came equipped with a stylus and two layers of ITO separated by a slight gap. Tapping this "analogue resistive" screen with the stylus brought the two layers into contact, allowing a current to pass that the device detected.
The sexy new handset in your pocket exploits the fact that your finger is conductive to do away even with the stylus. Touching the screen changes its capacitance at that location, a change picked up by a single layer of ITO. That innovation was the real breakthrough, says Lawrence Gasman of analysts NanoMarkets in Glen Allen, Virginia. "Multi-touch really changes the smartphone environment, almost like a mouse did for computing," he says. "Without it to expand the text, you'd probably go blind trying to read the web on such a small screen."
But how much longer can we count on the material behind that wonder? No one is quite sure how much or little indium there is left, says Thomas Graedel of Yale University, who heads the United Nations Environment Programme's working group on global metal flows. In part, that is because it is only a mining by-product and not all mines go to the trouble of recovering it. The US Geological Survey estimates that known reserves of indium worldwide amount to some 16,000 tonnes, overwhelmingly in China. Dividing that by the rate at which we are currently using the stuff suggests those reserves will be exhausted by 2020.
New sources of indium are almost certain to be found, but they are unlikely to satisfy the skyrocketing demand for ITO. This year, according to Gasman's figures, the touchscreen market alone is worth $US1.47 billion, and will balloon to $US2.5 billion by 2017. Even if the exact extent of indium supplies is hazy, ITO is set to become increasingly rare, and so increasingly expensive. This bald economic fact - and the fact that China is already curbing exports - is driving companies to search for alternative, indium-free touchscreen technologies.
Barring a fundamental shift in technology (see "Inside job"), the obvious place to start looking is among chemically similar materials. One pretender is zinc oxide, which is readily available for a fraction of ITO's cost. It is not as conductive, transparent or physically resilient as ITO, however. That is problematic, especially given that conductivity determines the responsiveness of the screen, and ITO's conductivity is already about as low as it can be and still be useful. "A little more or less makes a huge difference," says Gasman. "All that these replacements are is cheap."
Perhaps the answer is not to cut out indium altogether, but make what we have go further. Tobin Marks and his colleagues at Northwestern University in Evanston, Illinois, have developed a material based on cadmium oxide with just a sprinkling of indium that is just as transparent as ITO and three to four times as conductive. The material is prone to corrosion, so needs to be sealed under a thin layer of ITO, but ends up being just 20 per cent indium compared with 90 per cent for ITO (Thin Solid Films, vol 518, p 3694).
That has the sound of a stop-gap solution. Unfortunately, it's not that simple. First, cadmium is a highly toxic metal, requiring careful handling and disposal. Second, materials such as cadmium oxide are prone to cracking, a decidedly inconvenient property in a screen that is designed to be repeatedly prodded and poked.
ITO suffers from a similar brittleness itself. This has been less of an issue as long as the technology has been used principally in smartphones, which have a typical lifetime in our pockets of just 18 months; within such a timeframe a screen is highly unlikely to degrade to the point of becoming unusable. But as touch technology migrates to longer-lived tablet computers and e-readers, the problem is becoming more pressing. And the impending arrival of flexible, foldable—or at least rollable—displays is giving manufacturers yet another reason to look for a radically different solution to ITO.
Conducting polymers, perhaps? These long-chain organic molecules, discovered in the 1970s, act like molecular wires and beat ITO hands down when it comes to bending and flexing. But they are about as easy to manipulate as brick dust, says Yueh-Lin Loo of Princeton University. They can't be melted without changing their properties and they won't dissolve either, so making coatings of pure conducting polymer is just about impossible. Additives intended to make them soluble, so that they can be applied like ink, have had the annoying effect of wrecking their conductivity.
Until now, that is. In February this year, Loo and her colleagues found an additive that not only dissolves the polymer, but also disrupts the interactions between individual polymer chains, allowing them to "relax". That irons out kinks in the chains that hinder the flow of electrical current (Proceedings of the National Academy of Sciences, vol 107, p 5712).
It's hardly an ideal solution, though. Conducting polymers might not be brittle like the metal oxides, but they have their own degradation problems. Prone to attack by ultraviolet light and oxygen in the air, polymers are not the perfect solution for an oft-wielded touchscreen device. So is there any material that can tick all the performance boxes? Yes, says Mark Hersam, also at Northwestern University: carbon nanomaterials.
Carbon is a chemical chameleon. In some particularly black guises, it is the most light-absorbing material known. Pare it down to nanoscale structures, however, and it becomes transparent. In June this year, for example, a team led by Jong-Hyun Ahn and Byung Hee Hong of Sungkyunkwan University in Suwon, South Korea, developed a film consisting of four layers of graphene on a plastic backing. Graphene, the wonder material behind the award of this year's Nobel prize in physics, consists of sheets of graphite just a single atom thick. The graphene-plastic combination allowed 90 per cent of visible light to pass through and had a conductivity not far behind that of the highest quality commercial ITO (Nature Nanotechnology, vol 5, p 574).
Carbon nanotubes, which are essentially graphene sheets rolled up into tiny cylinders, look promising, too. They are rough, tough, transparent and increasingly available on a commercial scale. They would even work for flexible displays, says Hersam. "You can flex them, stretch them, with little to no degradation in their performance," he says.
The problem is making a conducting network out of them. Individual nanotubes are highly conductive, but the electrons racing across their surface stop dead when they get to the end of a nanotube and have to jump to the next. Hersam has a few ideas for improving contact between the tubes, for example by soldering them together with a good conductor that wouldn't affect the optical properties too much. But it is still early days. "We've been working in the area much less time than ITO has been in development for, which gives me hope that there are further improvements to be had," he says.
Others are less sanguine. Jonathan Coleman of Trinity College Dublin in Ireland researches transparent conductors in collaboration with electronics giant Hewlett-Packard. "When we started, industry thought that carbon nanotube films would be it - but no longer," he says. After trying various ideas to get around the problem of high resistance between the tubes, he and his colleagues decided that a rethink was needed. "We realised that, if instead of nanotubes you had metal nanowires, then where they touch you might get some bonding, giving electron transfer between them," he says.
Experimenting with silver nanowires, his team discovered that they could achieve transparency of 85 per cent and a conductivity only a fraction behind that of ITO (ACS Nano, vol 3, p 1767). "Optically and electrically, the silver was almost identical to high quality commercially available ITO, but totally flexible," says Coleman. Another team led by Peter Peumans at Stanford University in California achieved similar results (Nano Letters, vol 8, p 689).
Unfortunately, this bling comes at a price: silver nanowires are 10 times as expensive to produce as the already pricey top-grade ITO. Cheaper metals just don't seem to cut it, though. With copper nanowires, for example, the conductivity is good, but the transparency is low, at 60 per cent.
But even if silver's magic properties cannot be replicated with other materials, all is not lost. As production ramps up, prices will fall - and with indium only becoming more expensive, the costs will cross over at some point. "It's just a question of when," says Coleman. "Hewlett-Packard are now looking at silver nanowires as a material of choice."
So roll up, ladies and gentlemen, place your bets. Silver, carbon, zinc, cadmium, polymer... which will become the triumphant successor to dwindling ITO? None has yet shown a clear advantage, but the soaring demand for touchscreens and the breakneck rate of innovation means one must step into the breach. After all, we all want to stay in touch.
James Mitchell Crow is a freelance writer based in Melbourne, Australia
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