A wonder material makes your smartphone screen work. But with the
world's stocks running out fast, the hunt is on for new stuff to
keep us in touch
by James Mitchell Crow
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.
Sexy touch
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
$1.47 billion, and will balloon to $2.5 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."
Toxic stop-gap
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.
The impending arrival of flexible, foldable displays is yet another
reason to look for a radically different touchscreen technology
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.
Inside job
Could big screens be the saviour of the smartphone? As supplies of
vital indium tin oxide (ITO) for touchscreen applications dwindle (see
main story), the expansion of the technology into bigger devices such
as tablet PCs has piqued the interest of manufacturers of liquid
crystal displays.
Touchscreens are currently made simply by sticking a touch-sensitive
ITO surface onto an LCD from a big manufacturer such as Samsung or
Sony. But these companies could build touch sensitivity into the
display itself, integrating it into each pixel, says Lawrence Gasman,
principal analyst at NanoMarkets in Glen Allen, Virginia.
One such system would simply move the indium-based technology used by
today's multi-touch smartphones to within the pixel layer. Two other
techniques are indium-free. The first of these employs a mechanical
switch behind every pixel, registering the force as the screen is
touched. But using pressure-sensing technology means doing away with
the protective glass cover that usually fronts a touchscreen device,
leaving it vulnerable to damage.
The second possibility is an optical technology that incorporates a
light-detecting element into each pixel. These light sensors turn the
screen into a kind of scanner that can detect and follow a finger as
it strokes the screen. That too has its problems: optical touch needs
significant processing power to continually analyse the screen surface
for touch inputs, and works only at about a quarter of the speed of a
traditional laptop touchpad. But that needn't be a deal-breaker. "As
processing power gets ever faster, that is a problem that will solve
itself," says Gasman.
Indeed, the first optical touch devices have already hit the market.
Last year the electronics company Sharp released a laptop in Japan
fitted with a touch-sensitive second screen where the touchpad would
usually be. The fact that the product hasn't been rolled out outside
Japan, however, makes Gasman wonder if the technology is experiencing
teething problems.
In any case, such innovations do not address the more fundamental
problem that, touch or no touch, the electrodes that supply power to
the pixels of LCD displays themselves depend on ITO. That will be
solved only by the development of new materials that mimic ITO's
intensely desirable combination of transparency and conductivity.
James Mitchell Crow is a freelance writer based in Melbourne,
Australia
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