Graphene: Bend and flex for mobile phones

In the fourth of a five part series on the future of mobile phones, Roland Pease explores how the "wonder material" graphene could transform phones forever.

Mobile phones that fold, razor thin handsets powered by flexible batteries or see-through solar panels built directly into a colourful screen. These visions of our mobile future may seem a world away from our rigid, fragile and power-hungry smartphones today.

But they could all soon become reality thanks to the “wonder material” known as graphene. Believe the hype and these single-atom-thick sheets of carbon could soon replace just about every material and component used in modern day smartphones, making it lighter, faster and with more bells and whistles than ever before.

 If these claims seem extraordinary, then so too is the material which could make them possible. Graphene is made of a single element, carbon, arranged in a flat, unchanging crystal pattern that looks like chicken wire. Although it may sound rare and complex it is simply very thin layers of graphite – the same as found in a common pencil. In fact it’s now realised that almost every stroke of a pencil leaves fragments of graphene in the shining grey trace on the paper.

It was discovered in 2004 and could be the latest addition to a long line of material advances that have made our mobile phones possible. Think about the silicon slivers into which millions of electronic components can be etched; the lithium-ion batteries that pack a day or more’s worth of charge, and the low-energy light emitting diodes that can screen video in vivid colours.

What makes graphene remarkable is its ability to take on any of these roles. And what is more, it can conduct electricity better than copper, has strength greater than steel and also shows extraordinary elasticity. So great is its potential that in 2010 its discoverers Andre Geim and Konstantin Novoselov were awarded the Nobel prize for Physics.

Not only have thousands of scientific papers been published describing graphene’s many aspects; over 7,000 patents have been issued, many on technologies that could end up in mobile phones. No wonder electronics giant Samsung has invested huge sums into developing graphene as a material for screens and electronics; Nokia is backing a billion-euro project to exploit the carbon material and IBM has started a formidable research effort.

Rolling forward

So far, the smartphone screen is where most attention has been paid publically.

The industry currently depends on a compound called indium-tin-oxide. And it’s the little-known metal indium that’s the problem. Demand has grown massively with the development of flat screens with global production quadrupling in the two decades to 2008. Since then production has levelled off, but the price for the metal peaked at $720 for a kilogramme in 2011, and the industry is concerned about the long-term security of supply.

To further compound the problem indium-tin-oxide is a brittle material, making it difficult to work with.

Enter graphene.

The material is cheap, see-through and critically is electrically conductive. That makes it ideal for the flat-screen displays used on smart phones that need electricity to power the optical elements, and to respond to the user touch.

Samsung and Nokia [SF: avoiding repetition of a few paras above] have already shown off concepts for “bendy” phones – such as the newly unveiled Samsung “YOUM” screen or the Nokia morph concept  -with shapes unlike anything seen today. Although less extreme versions of some of these concepts may first be brought to market using more conventional technology, Nokia researchers believe that “graphene may well be one of the crucial elements of making Morph reality”.

Although these kinds of concepts excite the imagination and show the possibilities of graphene, durability is where the greatest interest lies. What the industry really wants is a touch screen a user can stab thousands of times a day without fear of the electronics ever giving out. Again, that’s what makes graphene so attractive.

Of course it has to be cost effective, too. As recently as 2009, it was only possible to manufacture samples of graphene that were a few centimetres across. But in 2010, Samsung-backed researchers at Sungkyunkwan University in South Korea showed it was possible to create roll of metres of the material, and demonstrated it on touch-sensitive tablet screens. And since then Sony has built a machine that can create rolls of the material 100m long – claimed to be the largest sheets in the world.

Samsung have also shown they can integrate onto the graphene sheets organic light-emitting diodes– the light sources used in most smartphones.  What’s more the OLEDs wired up this way are more efficient and brighter than those based on indium-tin-oxide, so they’ll help batteries last longer.

Print job

The same kind of science that lets graphene-based devices turn electricity into light allows the reverse to happen. As a result, there’s interest in whether graphene sensors could replace the CCD cameras in your phone. And, perhaps more significantly, there’s interest in whether graphene would make an efficient solar cell. Last year, a team at the University of Florida showed they could achieve 8.6% energy conversion using graphene-based cells – far behind current commercial silicon photovoltaics (which achieve around 20%), but already a factor of four better than earlier than previous attempts. And, as many point out, silicon has been under development for decades, not just a few years.

Part of the reason why researchers are so excited about graphene’s potential in highly efficient solar cells was recently revealed by research done at the European Institute of Photonic Science (ICFO). Working with researchers from the US, Spain and Germany, the team was able to show that the material is able to convert a single light particle into cascades of electrons helping more light energy to be turned into current.

"In most materials, one absorbed photon generates one electron, but in the case of graphene, we have seen that one absorbed photon is able to produce many excited electrons, and therefore generate larger electrical signals," explained Frank Koppens , group leader at ICFO at the time.

Of course, having generated your electricity, you want to capture it.

And new research from the Nokia Research Labs in Cambridge, UK, shows that graphene can help there, too.  A team at the labs recently describe the graphene-based rechargeable battery no thicker than a human hair (50 microns) – ideal for building into the case of a future, flexible phone.

The prototype power pack  is a lithium ion battery, like most rechargeables used in electronics. These already use graphite to capture the charge from the lithium-ion reactions that generate the battery power. But the belief is that the ultrathin graphene layers can achieve a much closer contact with the lithium ions, to improve performance of the battery. This is achieved because a gram of graphene has a surface area of 2,600 square metres –the equivalent of about ten tennis courts – meaning there are more opportunities for a reaction to occur in the battery.  

Currently, the very thinness of the Nokia battery limits how much charge it can hold. But other researchers have developed graphene foams, that are thicker, hold more charge, but are still completely flexible.

The prototype Nokia batteries, like the Samsung screens use graphene layers grown by condensing hot carbon vapour onto thin copper foil. It’s a remarkably successful approach, but energy intensive and relatively expensive. As a result, other researchers have explored another lower-tech approach that seems to produce perfectly good material: graphene ink.

The starting material is plain old graphite that is literally shaken to pieces using ultrasound. The resulting ink is an unexceptional black. But researchers at Cambridge University, UK, have manufactured basic electronic components using this kind of graphene solution and an ordinary office ink-jet printer.

Light speed

All of these developments could play a part in the flexible future of the mobile industry. But perhaps, where graphene has caused the most excitement is within the chip industry.  

For the past 50 years, the industry has been relentlessly driven by Moore’s law – which states that the number of transistors that can be squeezed onto an area of silicon for a fixed price double roughly every 18 months. As a result, silicon firms continually design and manufacture electronic components of ever-shrinking size, the reason more than a billion components can be squeezed onto a chip just a centimetre or two across. But each generation of shrinkage has become harder, and how to keep going beyond the horizon three or four generations away has long been a worry.

The crystalline perfection of graphene combined with its high conductivity in principle seems to hold out the promise of electronics at close to the molecular scale. Also, electronic devices like diodes and transistors had already been made with graphene’s straw-like cousin, carbon nanotubes.

But in practice, graphene doesn’t offer the same degree of electronic control as silicon. You might think of it as like running a formula one race on an ice rink: with electrons able to zip through the carbon lattice at the speed of light, there is little prospect of guiding the current, turning it on and off electronically as is essential in computer chips.

But the allure is so great that a huge amount of research effort has been pushed towards future electronics, particularly at IBM and Samsung. In 2011, IBM triumphantly proclaimed the fabrication of “a wafer-scale graphene circuit … in which all the circuit components … were monolithically integrated.” In plain language they had been able to manufacture useful circuitry at a size and scale normally used in the silicon industry.

The circuit was a so-called broadband radio-frequency mixer, an essential component of TVs, phones and radio but very different from the complex logic chip found at the heart of a smartphone or laptop. The circuit is generally used to convert the high frequency signals broadcast by radio stations to a lower frequency that we can hear.  

The circuit exploited the high speed of electrons in graphene, something that may also make graphene ideally suited to handle the high frequencies of mobile phone transmissions – both in the receiver and in generating the signal to send out to the base station.

The IBM researchers have made graphene components that operate up to 150 GHz, well above the frequencies currently used by mobile phones, and in principle opening up new portions of spectrum for cell phone coverage and improving connectivity.

Although the IBM team declared their “results open up possibilities of achieving practical graphene technology with more complex functionality,” in practice, getting the kind of data processing that goes on in silicon chips requires devising transistor circuits that can turn current on and off digitally. Nobel laureate Konstantin Novoselov has argued that despite several promising leads, it could be 2025 before large-scale integration of huge numbers of graphene components will be a reality. But by then, silicon may already have maxed out.

Hype or hope?

The list of potential uses of graphene goes on – as a replacement for the flash memory of our SD cards, or to replace the metal antennae that pick up the radio signal. But one which shows just how varied the material’s properties can be is its use in the earpiece.

Conventional speakers drive sound vibrations by pumping the air. But two groups have now shown graphene can achieve the same thing using heat - much as lightning generates thunder by dramatically heating the air. The trick with graphene is that it is so thin, it takes almost no current to heat it, but equally it cools in an instant. Pumping audio-frequency currents through a sheet of graphene – going well into the ultrasound – generates sound waves without the need for any moving parts. In theory this makes devices simpler and cheaper and opens up the possibility of new applications.

The University of Texas's Ji Won Suk, who has been developing the so-called "thermophones" says "it might be fun to have a smartphone with a transparent speaker over the whole screen".

But like just about every facet of graphene, whether this neat idea ever makes it to the market will depend on many factors.

Reading the technical papers exploring its potential, you are immediately struck by the caution expressed by the scientists doing the studies. The stumbling blocks are many. And the electronics industry is ruthlessly Darwinian. We have our mobile phones because the mobile industry and the silicon industry can turn out precision-engineered components by the bucket load, and any competing technology has to be robust in the fabrication plant and in the product. And it has to be cheap. Graphene is still yet to be tested in this environment.

Nor is graphene the only new material on the block; nanowires and plastic electronics are among the competition being explored.

But, whatever the current issues with graphene, it has to be remembered that the first attempts to make electronic components out of silicon five decades back were a complete flop. History now shows that was only a temporary setback. The ability to do so much with a single material is what made silicon (through the notion of the integrated circuit) such a success in the end.

And progress with graphene has been unexpectedly rapid – the successes of the Samsung roll-to-roll production and the IBM circuit took those in the business by surprise. More surprises are surely around the corner. As Nokia researchers have cautiously said “graphene holds tremendous promise”.

Whether it will deliver on these promises, only time will tell.

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