Graphene Silicon Comparison Essay

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 %.

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.

Written By Jesus de La Fuente / CEO Graphenea / j.delafuente@graphenea.com

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Graphene, the well-publicised and now famous two-dimensional carbon allotrope, is as versatile a material as any discovered on Earth. Its amazing properties as the lightest and strongest material, compared with its ability to conduct heat and electricity better than anything else, mean that it can be integrated into a huge number of applications. Initially this will mean that graphene is used to help improve the performance and efficiency of current materials and substances, but in the future it will also be developed in conjunction with other two-dimensional (2D) crystals to create some even more amazing compounds to suit an even wider range of applications. To understand the potential applications of graphene, you must first gain an understanding of the basic properties of the material.

The first time graphene was artificially produced; scientists literally took a piece of graphite and dissected it layer by layer until only 1 single layer remained. This process is known as mechanical exfoliation. This resulting monolayer of graphite (known as graphene) is only 1 atom thick and is therefore the thinnest material possible to be created without becoming unstable when being open to the elements (temperature, air, etc.). Because graphene is only 1 atom thick, it is possible to create other materials by interjecting the graphene layers with other compounds (for example, one layer of graphene, one layer of another compound, followed by another layer of graphene, and so on), effectively using graphene as atomic scaffolding from which other materials are engineered. These newly created compounds could also be superlative materials, just like graphene, but with potentially even more applications.

After the development of graphene and the discovery of its exceptional properties, not surprisingly interest in other two-dimensional crystals increased substantially. These other 2D crystals (such as Boron Nitride, Niobium Diselenide and Tantalum (IV) sulphide), can be used in combination with other 2D crystals for an almost limitless number of applications. So, as an example, if you take the compound Magnesium Diboride (MgB2), which is known as being a relatively efficient superconductor, then intersperse its alternating boron and magnesium atomic layers with individual layers of graphene, it improves its efficiency as a superconductor. Or, another example would be in the case of combining the mineral Molybdenite (MoS2), which can be used as a semiconductor, with graphene layers (graphene being a fantastic conductor of electricity) when creating NAND flash memory, to develop flash memory to be much smaller and more flexible than current technology, (as has been proven by a team of researchers at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland).

The only problem with graphene is that high-quality graphene is a great conductor that does not have a band gap (it can’t be switched off). Therefore to use graphene in the creation of future nano-electronic devices, a band gap will need to be engineered into it, which will, in turn, reduce its electron mobility to that of levels currently seen in strained silicon films. This essentially means that future research and development needs to be carried out in order for graphene to replace silicon in electrical systems in the future. However, recently a few research teams have shown that not only is this possible, it is probable, and we are looking at months, rather than years, until this is achieved at least at a basic level. Some say that these kinds of studies should be avoided, though, as it is akin to changing graphene to be something it is not.

In any case, these two examples are just the tip of the iceberg in only one field of research, whereas graphene is a material that can be utilized in numerous disciplines including, but not limited to: bioengineering, composite materials, energy technology and nanotechnology.

Biological Engineering

Bioengineering will certainly be a field in which graphene will become a vital part of in the future; though some obstacles need to be overcome before it can be used. Current estimations suggest that it will not be until 2030 when we will begin to see graphene widely used in biological applications as we still need to understand its biocompatibility (and it must undergo numerous safety, clinical and regulatory trials which, simply put, will take a very long time). However, the properties that it displays suggest that it could revolutionise this area in a number of ways. With graphene offering a large surface area, high electrical conductivity, thinness and strength, it would make a good candidate for the development of fast and efficient bioelectric sensory devices, with the ability to monitor such things as glucose levels, haemoglobin levels, cholesterol and even DNA sequencing. Eventually we may even see engineered ‘toxic’ graphene that is able to be used as an antibiotic or even anticancer treatment. Also, due to its molecular make-up and potential biocompatibility, it could be utilised in the process of tissue regeneration.

Optical Electronics

One particular area in which we will soon begin to see graphene used on a commercial scale is that in optoelectronics; specifically touchscreens, liquid crystal displays (LCD) and organic light emitting diodes (OLEDs). For a material to be able to be used in optoelectronic applications, it must be able to transmit more than 90% of light and also offer electrical conductive properties exceeding 1 x 106 Ω1m1 and therefore low electrical resistance. Graphene is an almost completely transparent material and is able to optically transmit up to 97.7% of light. It is also highly conductive, as we have previously mentioned and so it would work very well in optoelectronic applications such as LCD touchscreens for smartphones, tablet and desktop computers and televisions.

Currently the most widely used material is indium tin oxide (ITO), and the development of manufacture of ITO over the last few decades time has resulted in a material that is able to perform very well in this application. However, recent tests have shown that graphene is potentially able to match the properties of ITO, even in current (relatively under-developed) states. Also, it has recently been shown that the optical absorption of graphene can be changed by adjusting the Fermi level. While this does not sound like much of an improvement over ITO, graphene displays additional properties which can enable very clever technology to be developed in optoelectronics by replacing the ITO with graphene. The fact that high quality graphene has a very high tensile strength, and is flexible (with a bending radius of less than the required 5-10mm for rollable e-paper), makes it almost inevitable that it will soon become utilized in these aforementioned applications.

In terms of potential real-world electronic applications we can eventually expect to see such devices as graphene based e-paper with the ability to display interactive and updatable information and flexible electronic devices including portable computers and televisions.

Ultrafiltration

Another standout property of graphene is that while it allows water to pass through it, it is almost completely impervious to liquids and gases (even relatively small helium molecules). This means that graphene could be used as an ultrafiltration medium to act as a barrier between two substances. The benefit of using graphene is that it is only 1 single atom thick and can also be developed as a barrier that electronically measures strain and pressures between the 2 substances (amongst many other variables). A team of researchers at Columbia University have managed to create monolayer graphene filters with pore sizes as small as 5nm (currently, advanced nanoporous membranes have pore sizes of 30-40nm). While these pore sizes are extremely small, as graphene is so thin, pressure during ultrafiltration is reduced. Co-currently, graphene is much stronger and less brittle than aluminium oxide (currently used in sub-100nm filtration applications). What does this mean? Well, it could mean that graphene is developed to be used in water filtration systems, desalination systems and efficient and economically more viable biofuel creation.

Composite Materials

Graphene is strong, stiff and very light. Currently, aerospace engineers are incorporating carbon fibre into the production of aircraft as it is also very strong and light. However, graphene is much stronger whilst being also much lighter. Ultimately it is expected that graphene is utilized (probably integrated into plastics such as epoxy) to create a material that can replace steel in the structure of aircraft, improving fuel efficiency, range and reducing weight. Due to its electrical conductivity, it could even be used to coat aircraft surface material to prevent electrical damage resulting from lightning strikes. In this example, the same graphene coating could also be used to measure strain rate, notifying the pilot of any changes in the stress levels that the aircraft wings are under. These characteristics can also help in the development of high strength requirement applications such as body armour for military personnel and vehicles.

Photovoltaic Cells

Offering very low levels of light absorption (at around 2.7% of white light) whilst also offering high electron mobility means that graphene can be used as an alternative to silicon or ITO in the manufacture of photovoltaic cells. Silicon is currently widely used in the production of photovoltaic cells, but while silicon cells are very expensive to produce, graphene based cells are potentially much less so. When materials such as silicon turn light into electricity it produces a photon for every electron produced, meaning that a lot of potential energy is lost as heat. Recently published research has proved that when graphene absorbs a photon, it actually generates multiple electrons. Also, while silicon is able to generate electricity from certain wavelength bands of light, graphene is able to work on all wavelengths, meaning that graphene has the potential to be as efficient as, if not more efficient than silicon, ITO or (also widely used) gallium arsenide. Being flexible and thin means that graphene based photovoltaic cells could be used in clothing; to help recharge your mobile phone, or even used as retro-fitted photovoltaic window screens or curtains to help power your home.

Energy Storage

One area of research that is being very highly studied is energy storage. While all areas of electronics have been advancing over a very fast rate over the last few decades (in reference to Moore’s law which states that the number of transistors used in electronic circuitry will double every 2 years), the problem has always been storing the energy in batteries and capacitors when it is not being used. These energy storage solutions have been developing at a much slower rate. The problem is this: a battery can potentially hold a lot of energy, but it can take a long time to charge, a capacitor, on the other hand, can be charged very quickly, but can’t hold that much energy (comparatively speaking). The solution is to develop energy storage components such as either a supercapacitor or a battery that is able to provide both of these positive characteristics without compromise.

Currently, scientists are working on enhancing the capabilities of lithium ion batteries (by incorporating graphene as an anode) to offer much higher storage capacities with much better longevity and charge rate. Also, graphene is being studied and developed to be used in the manufacture of supercapacitors which are able to be charged very quickly, yet also be able to store a large amount of electricity. Graphene based micro-supercapacitors will likely be developed for use in low energy applications such as smart phones and portable computing devices and could potentially be commercially available within the next 5-10 years. Graphene-enhanced lithium ion batteries could be used in much higher energy usage applications such as electrically powered vehicles, or they can be used as lithium ion batteries are now, in smartphones, laptops and tablet PCs but at significantly lower levels of size and weight.

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