TMCnet News
Graphene Marches Down a Long Road to Commercialization [Chemical Engineering Progress](Chemical Engineering Progress Via Acquire Media NewsEdge) With so much graphene research underway, it can be hard to keep track of the progress, as well as to evaluate the commercial viability of such developments. Here's a look at what scientists and engineers are doing and saying about graphene. Graphene has elicited a frenzy of activity in research labs around the world, all aiming to exploit one or many of the extraordinary properties that have earned graphene the nickname of wonder material (among other superlatives). Its superior strength, stiffness, electrical and thermal conductivity, transparency, and flexibility could revolutionize a wide range of applications, as well as enable previously unattainable ones. These souped-up products - from rollable electronics, to universal photodetectors, to conductive clothing - remain in the laboratory (or in the minds of scientists). Over the next 5-10 years, some products will debut on store shelves, while others will not come to be for several decades. Both technical and logistical challenges must be overcome for this to happen. "Every now and then new materials pop up, but I don't think there has really been anything that has the same wide range of promise as graphene," says Jari Kinaret, a professor of applied physics at Chalmers Univ. of Technology, in Sweden, and director of the Graphene Future Emerging Technology Flagship, which the European Union is funding with euro 1 billion (US$1.3 billion). "Some people compare graphene to the emergence of plastics in the last century. Plastics themselves were not the perfect material for any particular application, but they were so versatile that they completely revolutionized many areas in the 1 900s," Kinaret says. "In the same way, it has been argued that because of its versatility, graphene may have an impact that may be similar, perhaps not as vast as plastics, but in terms of breadth, comparable," he continues. Graphene was isolated relatively recently, in 2004 at the Univ. of Manchester by Andre Geim and Konstantin Novoselov. Long before that, though, its existence was known by way of graphite, which consists of hexagonal carbon sheets (i.e., graphene) stacked on top of one another. But until 2004, no one had ever isolated enough of the material to study it. Geim and Novoselov developed what they call the "Scotch Tape" method, in which a piece of ordinary office tape is used to peel off layers of graphene from a chunk of graphite. Many attribute the explosion of activity in graphene to the simplicity of the tape technique. With a method in hand to make small flakes of graphene, scientists have been able to quantify many of its properties - confirming expectations, but also introducing new challenges and unknowns. For example, while the tape method produces graphene with impressive material properties, it is not suitable for large-scale manufacturing, and scalable methods (e.g., chemical vapor deposition) have been unable to produce graphene with such high performance. In addition, much of the reported characterizations of graphene are based on isolated samples. Since graphene is a thin surface rather than a bulk material, its properties will be altered by the films and components with which it is integrated. Graphene basics Graphene is a one-atom-thick layer of carbon atoms arranged in hexagons (Figure 1 ) and connected by sigma and pi covalent bonds. It is not a bulk material, but instead a twosided surface. This so-called wonder material is the first two-dimensional atomic crystal discovered. There are many other 2D materials that have been isolated since, including monolayers of boron nitride (BN), molybdenum disulfide (MoS2), and niobium diselenide (NbSe2). Scientists at the Massachusetts Institute of Technology, for example, have fabricated several digital electronic devices with the 2D form of MoS2 (Figure 2), including an inverter and a ring oscillator, which consists of 12 interconnected transistors. Graphene is very thin, and thus extremely transparent. At the same time, the packing of the carbon atoms makes the material very dense, and thus not even a helium atom can move through a layer of graphene. Its peculiar electronic structure gives rise to high electrical and thermal conductivity. Another interesting property of graphene is its strong interaction with light. A single layer of graphene can absorb 2.3% of impinging light, which is large for an atomically thin material. Also, the strength of this interaction relative to the wavelength of light can be tuned via chemical and electrostatic doping. Other carbon nanomaterials, in particular carbon nanotubes (CNTs), exhibit some of the exceptional properties of graphene. However, they do not come in the planar form of graphene i.e., a flat 2D geometry), which is beneficial for processing. "One of the advantages that graphene has over carbon nanotubes is that it is a planar material," Kinaret says. "Planar technology is at the very core of today's semiconductor technology. So, from that point of view, it's rather engineer-friendly, because you can use many of the technologies that have already been developed," he says. Processing woes Most research reported in scientific journals still uses the tape method to make graphene. While this method is important for the study of graphene, it cannot be used to produce commercial quantities. By far, the largest factor limiting the use of graphene in commercial applications is its synthesis, says Michael Strano, a professor of chemical engineering at MIT. "We need ways to make large-scale, high-quality graphene," Strano says. "In the future, you'll be able to pull out of a box a roll that looks like Saran Wrap, but it will be graphene," he says. "Coating a surface with graphene will be done the same way you 'coat' the surface of a bowl of leftovers." The most promising scalable techniques currently being explored to prepare graphene include chemical vapor deposition (CVD), epitaxial growth on silicon carbide, and exfoliation. Each has advantages and disadvantages, depending on the application of interest. Chemical vapor deposition. In CVD, a metal substrate such as copper is exposed to a carbon source (e.g., methane) at high temperatures, which causes the carbon atoms to stick to the metal surface. Once a carbon atom takes up residence on a particular metal plot, it nudges out other lingering carbon atoms, leaving behind one-atom-thick layers of carbon. As the temperature is reduced, the carbon atoms crystallize into graphene. CVD is scalable and produces a layer of graphene that can be transferred from copper to another substrate of interest. Although this method has been used to produce large films of graphene, several obstacles must be overcome for it to be commercially viable. The first is the large number of defects formed during CVD, which diminishes the quality of the film. These defects arise as a result of the random nucleation that takes place during CVD - crystallization starts at various spots across the substrate and each point of nucleation establishes its own orientation. Scientists at the Univ. of Oxford have figured out how the crystal orientation of the copper substrate influences the orientation of graphene - a feat that will likely aid in the development of a CVD process to produce defect-free graphene (CEP, Mar. 2013, pp. 5-6). They showed that by specifying the crystallographic orientation of the copper substrate on which the graphene is grown, they can control the orientations, edge geometries, and grain boundaries of the graphene domains. Other issues with CVD include its high cost and the difficulty associated with transferring the delicate graphene film from the copper substrate to a different substrate of interest. Epitaxial growth from SiC In this process, silicon carbide is heated to high temperatures (about 1,500°C) under vacuum to sublimate the silicon and leave behind layers of graphene. The process is scalable, but has several drawbacks. It is expensive due to the high cost of SiC wafers, and producing the necessary high temperatures consumes large amounts of energy. Graphite exfoliation. Liquid-phase exfoliation can be used to produce dispersions of graphene. In this technique, graphite is exposed to a solvent (with a surface tension that favors increasing the graphene-solvent interfacial surface area) followed by sonication. While this method can yield large quantities of material, it produces graphene in flake form (rather than a film), and therefore will be relegated to such applications as paints and inks. Wide-ranging applications Each of graphene's properties will be exploited in some way in a wide range of applications. Other advanced materials exhibit one or two superior properties, and therefore find a home in one or two industries or markets. Graphene, on the other hand, displays many superior properties and thus is being targeted at many industries and markets, including electronics, photonics, energy storage, and sensors. "Graphene is a material that I usually describe as being multi-potent," says Kinaret of Chalmers Univ. of Technology. "It has a number of very interesting properties. It's the best conductor of heat that we know. It's a single layer thick, so it's extremely flexible. It's 98% transparent, yet it conducts electricity very well. And it's very strong, between 100 and 300 times stronger than steel for the same weight," he says. "By combining these different properties, you can generate advantages in a number of different areas. Therefore, we can't say that there is a single product or single application that will be the key outcome." Electronics, photonics, and others Many electronic applications are being developed for graphene. In their recent review article published in the journal Nature, Novoselov, et al. offer a timeline for the introduction of graphene-based electronics (Figure 3). Initial electronic applications of graphene will take advantage of its transparency and conductivity, says James Tour, a professor of mechanical engineering and materials science at Rice Univ. "The first entry point for graphene in electronics will be in displays," he says. Transparent conductors. Graphene's ultrathin profile (i.e., atomic thickness) makes it extremely permeable to light. This, combined with its ultrahigh electron mobility (i.e., the speed at which electrons can move through the material) make it an excellent choice as a see-through conductive film. These conductors are found in touch-screen displays, electronic paper, and organic light-emitting diodes (OLEDs). Indium tin oxide (ITO) is the most widely used conductive film for electronic displays. However, ITO has several drawbacks. It is expensive due to the high cost of indium as well as the high-cost methods currently used to produce ITO films. In addition, ITO films are brittle. So far, the performance of CVDproduced graphene is slightly inferior to that of ITO. However, graphene 's performance has improved every year since its discovery, and thus will likely exceed that of ITO in the future, scientists say. Also, ITO lacks the thin, flexible profile of graphene, which is a must for many next-generation electronic devices. To improve its conductivity, researchers at Exeter Univ. sandwiched molecules of ferric chloride between two layers of graphene. The resulting film, which they have named GraphExeter, outperforms ITO in terms of both conductivity and transparency. Even with its inferior electrical properties compared with ITO, graphene has the best durability of any transparent conductor available today. As such, graphene could appear in touch-screen displays over the next few years. High-frequency transistors. Highfrequency transistors - a type of analog transistor used in the generation, amplification, and transmission of electromagnetic waves e.g., cellphones, TV, radar, etc.) - is another application being explored for graphene. In this arena, graphene would compete with compound semiconductors, which are composed of elements from Group III and Group V of the periodic table. The International Technology Roadmap for Semiconductors (ITRS) suggests that existing materials will not be able to meet the requirements of future radio-frequency (RF) transistors after 2021 . A measure of the maximum frequency at which a material can operate is its cut-off frequency (fT). Theoretically, the graphene cut-off frequency can be very high, in the terahertz (THz) range. Experimentally, graphene transistors with cut-off frequencies of up to 0.4 THz have been achieved by Avouris' team at IBM. However, in practice, the cutoff frequency is not the only factor that determines the performance of an RF transistor. The structure of the device and the operating conditions determine the frequency of operation, and particularly the gain performance of the device. Avouris points out that today's graphene devices show high fT but moderate gains due to the lack of current saturation in the device. Research is focused on optimizing the conditions to achieve the desired current saturation. According to Novoselov, et al, the first graphene-based RF transistors should appear in prototype form in 2021. Photonics. Graphene has exceptional optical properties. "Graphene absorbs light over the entire spectrum, from the UV to the far-IR and terahertz frequencies," Avouris says. "And, the excitations that are produced are very short lived," he says. "So, in principle, graphene could be a universal photodetector, because any kind of light can be detected," he explains. "The disadvantage is that the absorption, although very strong for a monolayer, is relatively weak with respect to, say, a bulk crystal." For a photodetector, the magnitude of graphene 's absorption will need to be improved. Some progress has been made recently, with Avouris and his colleagues reporting last year that they had increased the absorption in the terahertz range while maintaining transparency in the visible region. An area of interest in photonics is filling what is referred to as the terahertz gap - the terahertz region of the electromagnetic spectrum between the microwave and the infrared frequencies that has yet to be harnessed by photonic devices. Terahertz-frequency radiation can penetrate insulators, such as clothing, packaging, and envelopes, without causing damage the way ionizing radiation (e.g., X-rays) does. Photodetectors that operate in this region would be useful in security and medical imaging applications. "Graphene offers one possibility to bridge the terahertz gap," Avouris says. Other applications. Graphene could also find a place in paints and inks, batteries and supercapacitors, sensors, OLEDs, and next-generation drug delivery. Some of these applications will take advantage of graphene's properties, while others call for material modifications to impart new properties. For example, graphene is an "almost-ideal" membrane for gas separation. It is thin, which maximizes flux and minimizes the energy needed to push molecules through it, and mechanically robust, which prevents fracture. The missing property is porosity. Engineers at the Univ. of Colorado at Boulder have used ultraviolet-lightinduced etching to add nano-sized pores to graphene (Figure 4). To make the porous graphene membranes, the engineers suspended flakes of graphene over a piece of silicon oxide containing etched 5^m-dia. wells (microcavities). Next, they pressurized the system with H2 and exposed the suspended membrane to UV light. Then they evaluated the membrane's permeability and selectivity. The selectivity for N2 in a mixture of N2 and C02 was 7,000 (i.e., the flowrate of 2 through the membrane was 7,000 times larger than the flowrate of C02). In a mixture of N2 and H2, the selectivity of H2 was 1 0,000. Graphene offers some hope for battery and supercapacitor technologies, as well. For example, researchers at the Lawrence Berkeley National Laboratory (LBNL) have developed an anode for lithium-ion batteries made from graphene sheets embedded with tiny pillars of tin (CEP, Sept. 201 1, p. 14). The anode consists of two layers of tin nanopillars sandwiched between three layers of graphene. The energy capacity of the tin-graphene anode is double that of graphite, which is currently used in Li-ion batteries. What to expect from graphene The development of graphene is still relatively new. It was not until 2004 that this material could be made in large enough quantities to even be studied. Over the past few years, small and large research projects have popped up around the world to move graphene forward. However, there is a long road ahead before graphene is found in mainstream products. "There are different kinds of challenges," says Kinaret of Chalmers Univ. of Technology. "The biggest is what I call organizational challenge. If you want to create a new technology, which is what we are doing, you need to address different parts of the value chain at the same time," he explains. A company developing graphene-based components will wait to invest in such components until a source of highquality graphene at the right price and in the required quantities is available. At the same time, such a company will want to have systems integrators in place to buy the graphene components, he continues. Production challenges also exist, as the scalable processes for making graphene (e.g., CVD and epitaxial growth from SiC) cannot produce pristine graphene. In particular, the grain boundaries in CVD-grown graphene diminish the film's properties. Recent developments have illuminated potential ways to grow defect-free graphene, but they have not yet been demonstrated. In addition, transferring graphene from the substrate on which it is grown is not a trivial process. Another issue with graphene is that, to paraphrase former U.S. Secretary of Defense Donald Rumsfeld, we do not know what we do not know. Graphene does have exceptional properties. However, these properties are the result of graphene 's peculiar structure, which also creates unknowns. Because graphene is a one-atom-thick layer, it is very transparent and flexible. This atomic thinness also means that it is sensitive to anything it contacts. How graphene will behave when integrated within devices is unknown. And, MIT scientists reported last year that a coating of graphene can impact the properties of the underlying material, depending on that material's wettability. For materials with intermediate wettability, a graphene coating does not impact the properties of the underlying material. However, for extremely hydrophobic or hydrophilic ones, a graphene layer changes the way these materials behave. In addition to developing largescale processes for high-quality graphene, many of these fundamental science questions must be answered. The future world with graphene Although the road ahead is certainly long, the end of the road could be spectacular. Materials once thought of as insulating - clothing, plastic bags, glass windows - will take on conductive properties. They will be embedded with electronics, while maintaining their original look and feel. "We normally think of plastics as insulators," Strano says. "It's going to be trivial to add a ghost-like film to them and they're going to be anti-static, they're going to conduct electricity, you're going to be able to put electronic circuits on their surface," he says. "We think of glass as an insulator. It will be trivial at some point to make a conductive glass. It will be a simple coating," Strano continues. "There will be little things and also big things. You'll be able to modulate and bend light in very interesting ways with conventional lenses, just by adding this material." DEBUNKED: GRAPHENE VS. SILICON From research papers to press releases and news reports, graphene continues to be hailed as the silicon replacement - taking over (in a much-improved way) silicon's glorious role in digital electronics. An Internet search of graphene calls up articles with titles such as "Graphene: The New Silicon," or some variation. While graphene is certainly big news, this claim turns out to be unrealistic. "Graphene is a metal. It doesn't have a bandgap," says Phaedon Avouris, a chemical physicist and IBM Fellow and manager of Nanometer Scale Science and Technology at the IBM T. J. Watson Research Center. A bandgap is the difference in energy levels of a material between its conductive and nonconductive states; it is the gap between the valence band of electrons and the conduction band. In a metal, the valence band and conduction bands overlap, allowing electrons to move freely from one band to the other, while insulators have a very large bandgap, such that it is impossible for electrons to move from the valence band to the conduction band. On the other hand, semiconductors have a narrow bandgap that allows the material to switch between metal-like and insulator-like behavior. "To have a transistor like those made with silicon or germanium or gallium arsenide, you need a bandgap," Avouris says. "It's like a faucet. You turn the faucet on and water runs, and then you turn it off and nothing comes out," he explains. "For digital electronics, you need to be able to turn off that faucet extremely well, so nothing leaks out, and to do that you need a bandgap," he continues. "You cannot do that with graphene." Other scientists and engineers agree. "It's such a tall order to replace silicon, and very unlikely," says Michael Strano, a professor of chemical engineering at MIT. "I've been very skeptical of claims in that direction," he says. "Graphene as a starting material would be a very bad choice for making the kind of logic circuits that go into a computer, what we use silicon for now." Techniques have been developed to introduce a bandgap into graphene. However, it is not easy to do, and the bandgap that is created is very small. To put this in perspective, one metric of bandgap is the current-on to current-off ratio. For digital transistors, this ratio should be above one million. Graphene exhibits a ratio of about 10-20. IBM has published results of tests in which an electric field was used to create a bandgap. This method can achieve an on-off ratio of 200 - nowhere near what would be needed to replace silicon. Graphene's lack of a bandgap prevents its use in digital transistors, but analog transistors, which are used in wireless communication devices such as amplifiers, receivers, and transmitters, are fair game. Analog transistors need not turn off completely, but instead operate over a range of voltages. In these applications, graphene could offer improvements over compound semiconductors. (c) 2013 American Institute of Chemical Engineers |