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Graphene . . . a new super material 300 times stronger than steel Above: a computer-generated atomic image of graphene showing the extensive 2-dimensional honeycomb-like structure. The shape of the structure is often compared with chicken wire. Image courtesy Sébastien Sauvage, CNRS. Graphene has been billed as a new super-strong, super-thin exotic material with a vast range of exciting applications in electronics, materials science and so on. But what is it? We asked Dr David Maddison to investigate and report. G RAPHENE IS the thinnest possible material. It is actually a single 2-dimensional atomic layer of pure carbon – only one atom thick! Image of graphene through an aperture showing a single layer (central stripe) and two layers (right side) with no graphene on the left for comparison. Each layer absorbs 2.3% of incident light. Image courtesy University of Manchester. 22  Silicon Chip It has ultra-high tensile strength (300 times stronger than A36 structural steel at 130 gigapascals versus 400 megapascals), extremely low electrical resistance (almost like superconductivity under certain circumstances but at room temperature), very high stiffness whilst being able to be stretched by one fifth of its length before breaking, better thermal conductivity than copper and it is so impermeable that not even helium atoms can pass through it. Graphene is as an “allotrope” of carbon, like diamond and graphite, because it is made of the same pure carbon element but it has a different atomic structure. In recent years, many other allotropes of carbon have also been discovered and these include buckminsterfullerene (“bucky balls”), amorphous and glassy carbon, lonsdaleite (hexagonal diamond, different from common diamond) and carbon nanotubes. It is quite likely that others will also be discovered in the future. Most of these new allotropes have unique and very useful properties. Graphene can be thought of as a single atomic layer of the more familiar 3-dimensional graphite. In fact, this is an excellent example of an ordinary common material, carbon, being turned into an exotic and valuable material (much like silicon from silica or beach sand). Incredibly light Since graphene is a sheet only one atomic layer thick it is incredibly light. A single square metre of material weighs only 0.77 milligrams while an area the size of a tennis court would weigh just over 200 milligrams or one fifth of a gram. By contrast, if you used the same paper as used for printing this magazine (about 60 gsm), a tennis court-sized sheet would weigh about 16 kilograms. Despite its thinness, graphene can actually be seen with the naked eye because it is not completely transparsiliconchip.com.au ent; it absorbs around 2.3% of the light that falls on it. This makes it visible enough to see. Not only can graphene be seen with the naked eye, it is so strong that it can be picked up as a sheet. Indeed its strength is so high it is thought to represent the theoretical upper limit of how strong materials can possibly be. A sheet of multiple layers of graphene just 0.1mm thick (about the thickness of plastic cling wrap) would theoretically require a force of 2000kg to puncture it with a sharp implement. Much work is now under way to make large sheets of the material so that it can be used in practical applications where such high strength is required. For large-scale applications such as this, it is likely that the graphene would be fabricated as a composite material much like carbon-fibre composites. In addition to being extremely strong, graphene is also extremely stiff. A flake of graphene just 10 microns long (one hundredth of a millimetre, which is considered long for such a thin material) and one atomic layer thick will support itself without bending over when placed on edge vertically. This is equivalent to a 100-metre long sheet of paper supporting itself if it were placed on edge. Being only one atomic layer thin, you might reasonably expect graphene to be totally invisible but what opacity it does have is related to the fact that the electrons in graphene behave as if they have no mass, unlike electrons in normal materials. In fact, the opacity of graphene allows the direct measure of one of the most fundamental physical constants of the universe, known as the “fine structure constant”, which would normally require a very complicated apparatus to measure (the speed of light is another example of a fundamental constant). Miniature supercapacitors printed on substrate using the LightScribe method. Photo courtesy University of California, Los Angeles (UCLA). The 2.3% absorption of light by graphene equates very simply to the number π times α, where α is the fine structure constant. That it can be measured so simply using graphene is considered quite remarkable by researchers in the field. The European Union consider graphene to be so important that they have established a Future Emerging Technologies flagship with research funding of one billion Euro over 10 years to commercialise graphene technologies – see http://www.grapheneflagship.eu/GF/index.php Graphene is an extremely good electrical conductor as electrons can travel virtually without impediment within its structure. It is also an extremely good thermal conductor; better than Left: the atomic structure of graphite. Each ball represents a carbon atom and the lines represent the bonds between each atom. Note the layered, 3-dimensional structure which in reality extends indefinitely in accordance with the size of the graphite crystal. There is no strong bonding between the layers, only weak bonds and this accounts for graphite’s “slipperiness” and its use as a lubricant among other applications. Graphene is made of a single such layer. Image courtesy David Darling. siliconchip.com.au copper. Indeed, one of its proposed applications is for cooling of semiconductor devices. Bizarre properties Some graphene properties even verge on the bizarre. It is impermeable to nearly all materials, including helium which is capable of even diffusing through glass (the ease at which helium diffuses through mat­ erials is why helium party balloons deflate so rapidly). However, one material known to be capable of passing through graphene is water. This is analogous to a chain link fence not allowing a tennis ball to pass through but allowing a basketball to do so. This property has even been proposed for use in a method to purify alcoholic spirits at room temperature without having to heat them as in a normal distillation process (graphene oxide is the material used in that application) and in water purification applications. How to make graphene In principle, it is easy to make small amounts of graphene. At the time graphene was characterised in 2004, it was made by bringing a piece of adhesive tape in contact with a piece of graphite and then examining what had been removed with an optical September 2013  23 A piece of graphite and a sticky tape dispenser was all it took to make graphene in 2004. Note the piece of tape with the graphite residue. Some of this residue will be in the form of graphene. Image source: Dr David Maddison. microscope or with powerful electron and scanning tunnelling microscopes. A video of this process can be seen at http://physicsworld.com/cws/article/ multimedia/47356 The adhesive tape method has now been superseded by another simple fabrication method. In 2012, a group at University of California, Los Angeles (UCLA) made graphene using the LightScribe feature of a consumergrade DVD drive. A water-based solution of graphite oxide is coated onto a plastic substrate and inserted into the DVD drive and then struck with a laser from the drive. The oxygen atoms are removed from the graphite oxide coating by the heat of the laser and graphene is the result. Incidentally, this method is also used to make graphene by some very enthusiastic amateur scientists who have posted some YouTube videos on the process and even made some devices. It is also possible that you have even inadvertently made graphene yourself. Although a fairly new material in terms of its discovery, naming and characterisation, graphene commonly results as a byproduct of everyday processes. For example, every time you write with a graphite pencil it is likely you will make some tiny pieces of it. It can also be created during combustion processes, along with other forms of carbon. There are many other methods of prroducing graphene. One method is by chemical vapour deposition (CVD) onto various materials such as silicon This diagram shows the atomic structure of a solar cell made with one layer of molybdenum di­sulphide on top and one layer of graphene below. Source: MIT News Service. 24  Silicon Chip carbide or metals such as copper to make so-called epitaxial layers. It is a similar process to that used to make various semiconductor devices from silicon. Such graphene layers may either be left in place or transferred elsewhere. This method is regarded by some as the likely route to mass production. Other methods can produce powdery material in a chamber which is then collected. Historical precedents Like many “new” concepts and processes, there are often historically relevant events that lead to a major discovery and graphene is no exception. As early as 1840, C. Schafheutl exfoliated graphite and may have made graphene. In 1859, Sir Benjamin Collins Brodie was aware that graphite oxide that had had its oxygen removed to make pure graphite yielded extremely thin crystals, noting: “These crystals, when examined with the microscope, are perfectly transparent, and exhibit beautiful colours by the agency of polarized light”. You can read his original scientific paper at http://www.jstor.org/ stable/view/108699 There were also other studies on graphite oxide after that and in 1947 Professor Philip Wallace undertook extensive theoretical studies of the 2-dimensional form of graphite (which was not yet called graphene) in order to better understand the properties of 3-dimensional graphite. There were also many studies on carbon thin films, including graphite of just a few layers thick, and electron microscope images of such material were produced as early as 1948. Hanns-Peter Boehm and co-workers were the first to specifically identify single-layer thick graphene sheets in 1961 and they published the results in 1962. However, they never thought of this as a discovery but merely an extension of much earlier work. The characterisation of graphene resulted in the award of the 2010 Nobel prize in physics to Andre Geim and Konstantin Novoselov of the University of Manchester. The citation was “for ground-breaking experiments regarding the 2-dimensional material graphene”. Applications and devices Because of its array of unique properties, many different applications siliconchip.com.au have been proposed for graphene. A number of prototype devices have been made, some of which are now discussed. The LightScribe fabrication method has been used by UCLA to fabricate supercapacitors. Supercapacitors are different from normal capacitors in that they have much higher capacitance per unit volume and have much higher energy density. Because of this, they are being considered as battery replacements in personal electronic devices and electric vehicles (and for cars with the start/stop feature). They are better than normal electrochemical batteries because they can be charged and discharged much more rapidly. Graphene supercapacitors For supercapacitors, graphene is important for (a) enabling the creation of electrodes with a much greater surface area and thus electron storage capacity and (b) enabling an increase of the specific energy density of the supercapacitor. In one example, a graphene-based supercapacitor had a specific energy density of around 86Wh/kg at room temperature and 136Wh/kg at 80°C. This compares favourably with lith­ ium ion batteries with a typical energy density of 100-125Wh/kg but with the added advantage that they can be charged in seconds or minutes while batteries take many hours. In addition, supercapacitors can deliver energy much faster than batteries. UCLA have developed a method using conventional lithography, deposition and etching processes along with a sacrificial substrate to mass-produce high-frequency graphene transistors. Switching frequencies up to 427GHz were achieved. And at the University of Manchester, individual graphene transistors have been developed with switching frequencies of up to 1.5THz (1500GHz). The applications of such devices include computation, communications, high-speed chemical sensors and, if such a transistor could be made to work at 3THz (sub-millimetre waves), detection and production of the lowfrequency side of the far-infrared radiation band. An antenna made of graphene strips 10-100nm wide and one micron long that would transmit in the terahertz frequency range has been proposed by a team at the Georgia Institute of siliconchip.com.au An illustration depicting the mass-production of graphene transistors. Image courtesy University of California, Los Angeles (UCLA). Technology. Such an antenna could transfer data at one terabit per second at distance of one metre and up to 100 Terabits per second at range of one centimetre. Nanyang Technological University in Singapore has developed a graphene -based image sensor for cameras which is 1000 times more light sensitive than current state-of-the-art sensors and sensitive over a broad spectrum of light. It also uses 10 times less energy than present sensors. Graphene tennis racket The first commercial application of graphene as an engineering material is by HEAD who have developed a tennis racket made from graphene-reinforced epoxy. Few details of its exact construction are known, however. Bulk graphene suitable for engineering applications is still extremely expensive although some grades can be obtained for around US$200 per kilogram. Samsung have recently been awarded a patent for flexible touch screens utilising graphene for various elements. Graphene can potentially replace indium tin oxide, which is expensive and inflexible, as a transparent electrode material. It has also been used as an electrode material in flexible organic light emitting diode (OLED) displays. Researchers at Rice University have recently reported the fabrication of electrodes for lithium batteries made from a mixture of tin oxide and graphene nano-ribbons. The nanoribbons are made by splitting carbon nanotubes and opening them. This new electrode material may solve a significant problem of lithium batteries which is that the lithium causes electrodes to degrade over time. September 2013  25 A flexible touch screen developed at the University of Manchester in the UK. Image courtesy University of Manchester. the portable device for more electronics or battery capacity. Graphene has been proposed as an electrode material on solar cells. Existing solar cells typically use indiumtin-oxide (ITO) as a transparent conductor but this material is expensive and brittle. Graphene has a high level of transparency and is also flexible. It could be used as an electrode on either conventional silicon solar cells or flexible organic or thin-film solar cells. In fact, such an application was recently demonstrated at the Massachusetts Institute of Technology (MIT) and the performance was found to be equivalent to ITO. A further development at MIT is the production of solar cells just two atomic layers thick. One layer is composed of graphene and the other layer is molybdenum disulphide. The cell is about 1-2% efficient; poor compared to conventional cells with efficiencies of 15-20% but greater efficiencies might be possible by stacking multiple layers together. While this concept has been successfully demonstrated, manufacturing such devices is a major challenge. Integrated circuit A photo of IBM’s graphene integrated circuit. The enlargement at top shows the graphene transistor component. Image courtesy IBM. Apple recently received a patent for graphene as a heat dissipation material. Graphite or graphite paste is used as a heat dissipation material in some mobile electronic devices. Apple proposes to replace the graphite with graphene which could be made much thinner than graphite because it is a much better thermal conductor. Apple says that replacing a 30 micron (0.03mm) thick layer of graphite with graphene will free space inside 26  Silicon Chip In 2011, IBM researchers fabricated the world’s first integrated circuit using graphene as a component. The circuit worked as a broadband frequency mixer and could operate up to frequencies of 10GHz and at temperatures to 125°C. Graphene can also be used to make conductive inks and coatings as well as act as a filler in plastics to make them more conductive. A space elevator is a proposed space transportation system consisting of an extremely strong cable or ribbon (known as a “tether”) which is attached to Earth in the region of the equator at one end and with a counterweight at the other end beyond geostationary orbit (35,800km altitude). A climbing vehicle would crawl along the ribbon to transport materials into space. Nano-engineered carbon-based materials such as graphene and carbon nanotubes are among the few ultrastrong materials that may be suitable for such an application. Hyperbole Like all new materials and technologies, graphene has been subject to its fair share of hype. The different stages A HEAD Graphene Speed Pro 18/20 Racket made by HTM Sport GmbH, Austria. of expectations and reality for new technologies is best illustrated by the Gartner Hype-Cycle (see Wikipedia). Many announcements related to graphene have been based more on hype than reality. There is no doubt, however, that graphene is an important new material. It is the first bulk 2-dimensional material. It has many possibilities but there are also many challenges to overcome before its use becomes widespread in electronics and other areas. Having said that, as mentioned above, there is now one commercial product on the market, the HEAD tennis racquet. Undoubtedly, many more SC will follow. siliconchip.com.au