Is graphene to replace silicon?

Photo ParisTech Review / Editors / September 30th, 2013

Forming the only known foil capable of being folded as many times as necessary without breaking, a graphene sheet is a million times thinner than a human hair, 200 times more resistant to breakage than steel (its tensile force is in excess of 130 gigapascals), more conductive than copper, perfectly transparent, and totally flexible. Furthermore, graphene is impermeable to all gases. Typically, it takes a new material thirty to forty years to be integrated into consumer products. And this particular one, in the opinion of its Mancunian discoverers, could one day revolutionize electronics, energy, aerospace and biotechnology. Already it is being used in photovoltaic cells, electric vehicle batteries, and data centers processors.

Physically, the pattern of a graphene sheet is that of a honeycomb lattice. If sheets are stacked, you get graphite, which is the gray charcoal matter that makes up our pencil leads. In just one millimeter of graphite, there are three million sheets of graphene stacked… As a matter of fact, this amazing material made it to the Hall of Fame of legendary research in the most uncanny manner, in 2004, when Andre Geim and Konstantin Novoselov, two professors at the University of Manchester – Nobel prizes in 2010 – isolated a graphene layer from a pencil lead… by simply using a roll of adhesive tape to extract ever thinner graphite layers one by one – until they formed but one single layer of atoms.

Multiple uses
Of all the materials consisting of a single layer of atoms, graphene seems to be the most promising one. An MIT team has modeled the use of these materials in photovoltaic cells. The PV industry needs a radical technological revolution because its economic model, which is largely based on government grants, is floundering. By stacking a layer of carbon atoms – graphene – with a layer of molybdenum disulfide (MoS2), one obtains a solar cell whose performance is admittedly poor – but 1% – but that is infinitely small: but one nanometer thick, i.e. one millionth of a millimeter thick. In total, it therefore generates 30 times more power per volume unit than the thinnest solar cells known (made either of gallium arsenide, silicon, or indium selenide) which are one micron thick, and whose performance near 30 %.

Modelling of a graphene sheet
couchedegraph

Researchers have calculated that by stacking six layers – three graphene layers and three layers of molybdenum disulfide – performance could theoretically reach 10%… for a thickness of only 3 nanometers. Unprecedented energy efficiency! Should such a minute graphene solar cell get to be manufactured on an industrial scale, it would beat all records in terms of power density. It is nevertheless true that at this stage, such a revolutionary cell remains purely theoretical: it has not even been tested in the laboratory.

Graphene is also capable of conferring considerable strength to ordinary materials. The Korean Advanced Institute of Science and Technology has just demonstrated that by stacking copper layers with graphene layers, the material obtained is 500 times stronger. Even though graphene only amounts to 0.00004 % of the material’s weight, it increases its overall strength by a factor of several hundred.

Graphene is also keenly anticipated to finally advance research on the battery of the future. In theory it would be possible to charge a smartphone in less than ten minutes. A graphene battery powering an electric car would be able to bestow a real autonomy to the vehicle, and would at long last make it a true mass consumption product. With graphene, which is highly conductive, a battery charges much faster – for only half the weight.

With a leading edge on the subject, a team from UCLA, led by Professor Richard Kaner, has developed an electrochemical capacitor consisting of a network of graphene micro-capacitors (watch video presentation). This “ultracapacitor” boasts performance which is incommensurate with the most efficient lithium-ion battery: it is 100 to 1000 times more powerful and three to four times denser. What we are witnessing here is a true technological breakthrough that paves the way for future electric vehicles equipped with ultra-reliable capacitors instead of expensive and heavy batteries whose performance is irregular. In particular, it would do away with the performance degradation that inevitably takes place over time, and with the eventual premature wear that batteries experience in case of prolonged non-use of vehicles. Electric vehicles could then directly compete with the heat engine, for a fraction of the cost of transport – of the order of just one euro for 100 km…

The UCLA team also successfully overcame a major challenge: the actual production of graphene. How did they pull it off? Richard Kaner and colleagues used quite the ordinary laser – that of a Lightscribe optical drive, ordinarily used to burn DVDs. After covering the DVD with a film of graphite oxide, the laser “bombards” it and produces a graphene electrode, called LSG (Laser Scribed Graphene). One day, researchers hope, it will be possible to “print” graphene on rotaries just like newspapers… But there’s still a long way to go. And, of course, both the adhesive tape technique and the DVD laser one are obviously unable to provide large quantities. However, it is undeniable that progress has been made. While in 2004, Geim and Novoselov had produced a graphene “piece” too small to be visible to the naked eye, Samsung Electronics, in 2013, managed to showcase a sheet a full 76 centimeters in diameter.

Graphene vs. silicon
In the world of electronics, where silicon has reigned unchallenged for decades, does graphene have a destiny? It could certainly perform as a coolant. Johan Liu, a professor at the Chalmers Institute of Technology in Sweden, explains it: “In a computer, the hottest spots – microprocessors for the most – reach temperatures that range between 55 and 115°C (160 to 240°F). By applying a layer of graphene, we have lowered the average temperature by 13°C (55°F).” Suffice to say that a 10°C working temperature can halve the life of electronic equipment and that half of the energy consumed by a data center is attributable to cooling to understand that this is a tremendous gain in energy efficiency. The future may thus bring chips, microprocessors and transistors that operate at impressive speeds, and yet that do not heat up. We’re one step closer to the mythical “cold computer!”

Proponents argue that graphene will make it possible to manufacture cell phones so thin that they can be integrated into paper or tissue. Due to its simple structure, it will be possible to craft transparent screens affixed to walls, windows or even glasses. In short, electronic devices won’t be “manufactured” in plants anymore. Having become ultrathin, they will simply be printed out. One can already picture electronic paper and roll-up communication devices! Incidentally, researchers at Northwestern University in Chicago have developed a highly conductive graphene ink that will make such communication tools possible. One significant challenge remains, though – maintaining ink conductivity after printing.

Among the innumerable proposed applications for graphene are desalination filters. To date, the process of desalination is one very expensive operation – still unaffordable in many countries – due to the amount of energy required. Lockheed Martin is developing a graphene filter, the perforene, which could revolutionize reverse osmosis desalination . Perforene is about 500 times thinner than the best filters available on the market – its membrane being even thinner than the atoms it filters! As for the energy and pressure required filter the salt, they are about a hundred times lower.

While graphene is the most famous of 2D materials to have been discovered, it is not the only one anymore. A dozen of them are being studied worldwide. They demonstrate complementary properties that, by combining with the graphene, will add further functionality. Boron nitride, for example, is also just one atom thick but unlike graphene, it is an insulator – and the most effective ever. As for molybdenum disulphide, three atoms thick, it forms a semi-conductor which is much lighter and sturdier than silicon. In Manchester, Konstantin Novoselov’s lab has combined the highly conductive graphene with dichalcogenide, a transition metal that absorbs sunlight and converts it into electricity. The combination could lead to exterior paints capable of producing the electricity needed to operate the household equipment within a building.

When cutting edge becomes literal, expect a backlash
As might be expected, the graphene-mania, as well as the funding it attracts, have caused a backlash in the scientific community. Some point out that the new carbon atom frameworks have often turned out to disappointing. Fullerenes, for instance, much touted in the 1980s, and carbon nanotubes (rolled up graphite sheets), which had prompted great excitement in the 1990s, have found no real commercial application so far.

It is true that graphene does not yet have all the qualities to revolutionize electronics. For the time being, industry giants will have to do with silicon as graphene is still hindered by certain shortcomings: for example, the output frequency of graphene devices is sometimes disappointing. It is not a semi-conductor, nor is its conduction band good enough to allow it to perform as a stand-alone transistor, the basic building block of electronics. In such an iconic market, graphene would therefore have to settle for narrower niches, such as high frequency electronics components.

Professor Novoselov himself acknowledges it: the craze has gone too far. From his point of view, commonly used materials should only be replaced whenever the characteristics of the new material can lead to applications competitive enough to justify the cost and the inconvenience of such a changeover. In short, the future of graphene depends on the development of applications designed specifically around it. Now this raises a problem of industrial culture. Mark Goerbig, professor at the Ecole Polytechnique Engineering School in Paris, explains it: “It will take a generation to train engineers who are comfortable with graphene.” What is more, according to this researcher, silicon is not done achieving electronic efficiency gains in terms of Moore’s Law (the doubling of the power of transistors every 18 months), even though graphene is certainly capable of outperforming silicon at some point in the future, but it is still uncertain when that might happen.

The economic equation is a tough one. In 2013, it costs $ 800 to produce one gram of graphene, which means the golden years are far from over for silicon and its derivatives. Especially so considering that on top of the difficulty of manufacturing, there are handling hazards to cope with. When a material is but one atom thick, any action on it is likely to modify its very structure. Furthermore, no one knows the consequences, in terms of wild fusion, in the case of a juxtaposition of several different nanomaterials. And in particular, any addition of solvent threatens the very conductivity of graphene.

As a result, graphene is raising the same concerns as other nanomaterials. According to studies conducted by Brown University in Rhode Island, the sharp edges of graphene may pierce body cells, therefore allowing the substance to get into the human cell and to disrupt its normal functioning. Researchers suggest there are very real nano-toxicity hazards: fragments may penetrate the cells up to a depth of 10 microns. Issues might then arise should we manage, for example, to create graphene-based artificial retinas. Backlash for the eyelash…

As for the strength of the material, which is its iconic property, it happens to also be questionable. Researchers at Rice University in Texas have shown that on the edges of a graphene “sheet”, the hexagonal structure of the material degenerates into pentagons and heptagons, which are considerably less robust. There is, according to them, a risk that the slightest imperfection in a sheet creates long rips that spread, just like a crack in a windshield does.

What of the geopolitics of graphene? Even though the initial breakthrough was achieved in Manchester, UK, Europe is lagging behind. The inventory of patents made by Cambridge IP, a British consultancy specialized in technological strategy, established that by Q4 2012, there were 2,204 patent publications on graphene in China, 1754 in the United States, 1160 in South Korea – and only 54 in the UK. So, the Europeans stepped up. In January 2013, the European Commission launched the Graphene project with a massive budget: a billion dollars over ten years. Its stated objective: to develop industrial applications for graphene, and for the wider family of two-dimensional materials. The project is led by a consortium of 74 academic and industrial partners from 17 countries. It brings together 126 research groups (including five French laboratories), that will work on eleven projects: materials, health & environment, basic research on graphene, two-dimensional materials, high-frequency electronics, optoelectronics, spintronics, sensors, flexible electronics, energy applications, nano-composite materials, and production technologies.

What should we retain from all this excitement? First, the challenge this diruption represents in basic research: it’s been less than ten years since this material was first isolated, and everything remains to be done to master its physics and to explore the immense possibilities it opens up in many a field. Second, even if serious questions remain about the potential dangers that could derive from mass production (think of asbestos), it now seems self-evident that in vital sectors such as photovoltaics, computer components or electric batteries, graphene can lead to technological breakthroughs that would bring about radically game-changing results. The next stage? Moving to industrial production – a case to follow closely.

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