Researchers at The University of Texas at Austin's Cockrell School of Engineering have created the first transistors made of silicene, the world's thinnest silicon material. Their research holds the promise of building dramatically faster, smaller and more efficient computer chips. Made of a one-atom-thick layer of silicon atoms, silicene has outstanding electrical properties but has until now proved difficult to produce and work with. Deji Akinwande, an assistant professor in the Cockrell School's Department of Electrical and Computer Engineering, and his team, including lead researcher Li Tao, solved one of the major challenges surrounding silicene by demonstrating that it can be made into transistors --semiconductor devices used to amplify and switch electronic signals and electrical power. The first-of-their-kind devices developed by Akinwande and his teamrely on the thinnest of any semiconductor material, a long-standing dream of the chip industry, and could pave the way for future generations of faster, energy-efficient computer chips. Their work was published this week in the journal ("Silicene field-effect transistors operating at room temperature"). Until a few years ago, human-made silicene was a purely theoretical material. Looking at carbon-based graphene, another atom-thick material with promise for chip development, researchers speculated that silicon atoms could be structured in a broadly similar way. Akinwande, who also works on graphene transistors, sees value in silicene's relationship to silicon, which chipmakers already know how to work with. "Apart from introducing a new player in the playground of 2-D materials, silicene, with its close chemical affinity to silicon, suggests an opportunity in the road map of the semiconductor industry," Akinwande said. "The major breakthrough here is the efficient low-temperature manufacturing and fabrication of silicene devices for the first time." Despite its promise for commercial adaptation, silicene has proved extremely difficult to create and work with because of its complexity and instability when exposed to air. To work around these issues, Akinwande teamed with Alessandro Molle at the Institute for Microelectronics and Microsystems in Agrate Brianza, Italy, to develop a new method for fabricating the silicene that reduces its exposure to air. To start, the researchers let a hot vapor of silicon atoms condense onto a crystalline block of silver in a vacuum chamber. They then formed a silicene sheet on a thin layer of silver and added a nanometer-thick layer of alumina on top. Because of these protective layers, the team could safely peel it of its base and transfer it silver-side-up to an oxidized-silicon substrate. They were then able to gently scrape some of the silver to leave behind two islands of metal as electrodes, with a strip of silicene between them. In the near-term, Akinwande will continue to investigate new structures and methods for creating silicene, which may lead to low-energy, high-speed digital computer chips.
One-atom-thin silicon transistors hold promise for super-fast computing
read more "One-atom-thin silicon transistors hold promise for super-fast computing"
Alcohol-free zone - an effective process for coating noble nanoparticles
A straightforward and effective process for coating silver, gold and platinum nanoparticles with functionalized silica shells at room temperature has been developed by A*STAR ("Aqueous route to facile, efficient and functional silica coating of metal nanoparticles at room temperature"). Crucially, unlike conventional methods for producing silica-coated metal nanoparticles, this process is based on water and does not employ alcohol, making it both cost-effective and environmentally friendly. 
Scanning electron microscopy image showing silica-coated silver nanoparticles produced by a simple and effective alcohol-free process (inset shows high-magnification image). (© Royal Society of Chemistry) Silica-coated noble metal nanoparticles have attracted great interest because they can be used as catalysts as well as in calorimetric and optical applications. They are typically produced using silane precursors, but these are generally insoluble in water. Consequently, alcohol has to be added to water to facilitate the hydrolysis of these precursors, increasing the cost of production and making the process less green. Now, a team led by Ming-Yong Han and Shah Kwok Wei at the A*STAR Institute of Materials Research and Engineering has devised an alcohol-free method for producing silica-coated noble metal nanoparticles. To do this, the team took a commonly used precursor, tetramethoxysilane (Si(OCH3)4), and substituted a polar group (mercaptopropyl) for a methoxy group (O–CH3), which resulted in a water-soluble precursor. Then, to enable this precursor to bind directly with the metal nanoparticle surfaces, they functionalized it with a thiol group (–SH). This process has many advantages. It is straightforward to implement, efficient, universal and easily scalable. Furthermore, since the thickness of the silica shell increases with coating time, shell thickness can be readily controlled up to several tens of nanometers. By slightly modifying the process, Han and colleagues could also produce nanoparticles that have a high activity for an extremely sensitive spectroscopic technique known as surface-enhanced Raman scattering (SERS) and are promising for highly sensitive detection in analytical and biological applications. SERS is based on the hugely enhanced Raman signal generated when a Raman-active compound is adsorbed on a metal surface. The researchers prepared the fluorescence-free SERS-active nanoparticles by sandwiching Raman-active molecules between the noble metal nanoparticle and the silica shell. “The simplicity of the silica coating process means it has great potential for coating and protecting the surfaces of various kinds of metal nanoparticles,” explains Han. “Furthermore, the resulting highly negatively charged and SERS-active metal nanoparticles with thiol-functionalized silica shells and surface-protective features are very promising for various applications involving aqueous solutions.” In particular, Han notes, this water-based route to facile, efficient and functional silica coating of metal nanoparticles at room temperature could be extended to coat metal oxide nanoparticles for green building applications.
Scanning electron microscopy image showing silica-coated silver nanoparticles produced by a simple and effective alcohol-free process (inset shows high-magnification image). (© Royal Society of Chemistry) Silica-coated noble metal nanoparticles have attracted great interest because they can be used as catalysts as well as in calorimetric and optical applications. They are typically produced using silane precursors, but these are generally insoluble in water. Consequently, alcohol has to be added to water to facilitate the hydrolysis of these precursors, increasing the cost of production and making the process less green. Now, a team led by Ming-Yong Han and Shah Kwok Wei at the A*STAR Institute of Materials Research and Engineering has devised an alcohol-free method for producing silica-coated noble metal nanoparticles. To do this, the team took a commonly used precursor, tetramethoxysilane (Si(OCH3)4), and substituted a polar group (mercaptopropyl) for a methoxy group (O–CH3), which resulted in a water-soluble precursor. Then, to enable this precursor to bind directly with the metal nanoparticle surfaces, they functionalized it with a thiol group (–SH). This process has many advantages. It is straightforward to implement, efficient, universal and easily scalable. Furthermore, since the thickness of the silica shell increases with coating time, shell thickness can be readily controlled up to several tens of nanometers. By slightly modifying the process, Han and colleagues could also produce nanoparticles that have a high activity for an extremely sensitive spectroscopic technique known as surface-enhanced Raman scattering (SERS) and are promising for highly sensitive detection in analytical and biological applications. SERS is based on the hugely enhanced Raman signal generated when a Raman-active compound is adsorbed on a metal surface. The researchers prepared the fluorescence-free SERS-active nanoparticles by sandwiching Raman-active molecules between the noble metal nanoparticle and the silica shell. “The simplicity of the silica coating process means it has great potential for coating and protecting the surfaces of various kinds of metal nanoparticles,” explains Han. “Furthermore, the resulting highly negatively charged and SERS-active metal nanoparticles with thiol-functionalized silica shells and surface-protective features are very promising for various applications involving aqueous solutions.” In particular, Han notes, this water-based route to facile, efficient and functional silica coating of metal nanoparticles at room temperature could be extended to coat metal oxide nanoparticles for green building applications.Giving infections the brush off
A ‘one-step’ coating that blocks protein growth and kills surface-bound bacteria on silicone may significantly reduce infections from medical devices such as catheters, finds a study led by A*STAR Institute of Bioengineering and Nanotechnology researchers. Yi Yan Yang and international co-workers accomplished this with a synthetic technique that combines biomimetic surface adhesion and antimicrobial capabilities into a brush-like polymer film ("Brush-Like Polycarbonates Containing Dopamine, Cations, and PEG Providing a Broad-Spectrum, Antibacterial, and Antifouling Surface via One-Step Coating"). 
A simple one-step immersion of a catheter surface with brush-like polycarbonates containing pendent adhesive dopamine, antifouling poly(ethylene) glycol and antibacterial cations effectively prevents the fouling of bacteria, protein and platelets with no toxicity. (© WILEY-VCH Verlag) Implanting foreign materials into biological environments inevitably leads to accumulation, or ‘fouling’, of surfaces with pathogenic biomolecules. To block this film growth, researchers from the Institute of Bioengineering and Nanotechnology are experimenting with coatings known as ‘polymer brushes’ — arrays of macromolecular chains that bind to surfaces and modify properties such as bioadhesion. One popular coating is called poly(ethylene) glycol (PEG), a soft, water-soluble material often used in drug delivery. Chemists can easily modify the structure of PEG for grafting onto device surfaces, where it provides a strong physical barrier against biomolecule adsorption. However, as PEG has no antimicrobial abilities, with time bacteria can overcome the PEG films and grow onto the surface. Yang and co-workers are working to improve polymer brushes using biodegradable materials called aliphatic polycarbonates. Recently, they developed a ‘living’ ring-opening polymerization that can attach antimicrobial molecular units and PEG chains to the polycarbonate backbone with high precision. Experiments revealed that catheters coated with this polycarbonate–PEG film eradicated Staphylococcus bacteria and had excellent blood compatibility. However, it failed to prevent fouling from one of the most dangerous pathogens in hospitals — bacteria. Now, the researchers have developed a polymer brush with broad-spectrum antibacterial capabilities and a simplified coating process. They added three key components to the polycarbonate backbone: effective antimicrobial cations, PEG chains for antifouling, and dopamine groups that stick to silicone rubber in a manner similar to adhesive proteins found in mussel shells. Linking the three components into a single polymer, however, required chemical ingenuity in the form of co-interacting, metal-free catalysts. “The amounts of co-catalysts we used were crucial,” says Yang. “PEG is a large molecule, and we had to add the catalysts in a stepwise manner to ensure a complete reaction.” The polymer’s dopamine groups allowed attachment to catheter surfaces through a single-step dipping procedure. Imaging tests showed that the film killed both and bacteria upon contact, and prevented these pathogens from establishing fouling layers (see image). Yang notes that these findings, in combination with the coating’s stability under simulated blood flow, indicate this approach’s potential for preventing infection in intravascular catheters.
A simple one-step immersion of a catheter surface with brush-like polycarbonates containing pendent adhesive dopamine, antifouling poly(ethylene) glycol and antibacterial cations effectively prevents the fouling of bacteria, protein and platelets with no toxicity. (© WILEY-VCH Verlag) Implanting foreign materials into biological environments inevitably leads to accumulation, or ‘fouling’, of surfaces with pathogenic biomolecules. To block this film growth, researchers from the Institute of Bioengineering and Nanotechnology are experimenting with coatings known as ‘polymer brushes’ — arrays of macromolecular chains that bind to surfaces and modify properties such as bioadhesion. One popular coating is called poly(ethylene) glycol (PEG), a soft, water-soluble material often used in drug delivery. Chemists can easily modify the structure of PEG for grafting onto device surfaces, where it provides a strong physical barrier against biomolecule adsorption. However, as PEG has no antimicrobial abilities, with time bacteria can overcome the PEG films and grow onto the surface. Yang and co-workers are working to improve polymer brushes using biodegradable materials called aliphatic polycarbonates. Recently, they developed a ‘living’ ring-opening polymerization that can attach antimicrobial molecular units and PEG chains to the polycarbonate backbone with high precision. Experiments revealed that catheters coated with this polycarbonate–PEG film eradicated Staphylococcus bacteria and had excellent blood compatibility. However, it failed to prevent fouling from one of the most dangerous pathogens in hospitals — bacteria. Now, the researchers have developed a polymer brush with broad-spectrum antibacterial capabilities and a simplified coating process. They added three key components to the polycarbonate backbone: effective antimicrobial cations, PEG chains for antifouling, and dopamine groups that stick to silicone rubber in a manner similar to adhesive proteins found in mussel shells. Linking the three components into a single polymer, however, required chemical ingenuity in the form of co-interacting, metal-free catalysts. “The amounts of co-catalysts we used were crucial,” says Yang. “PEG is a large molecule, and we had to add the catalysts in a stepwise manner to ensure a complete reaction.” The polymer’s dopamine groups allowed attachment to catheter surfaces through a single-step dipping procedure. Imaging tests showed that the film killed both and bacteria upon contact, and prevented these pathogens from establishing fouling layers (see image). Yang notes that these findings, in combination with the coating’s stability under simulated blood flow, indicate this approach’s potential for preventing infection in intravascular catheters.Penta-graphene, a new structural variant of carbon, discovered
Researchers at Virginia Commonwealth University and universities in China and Japan have discovered a new structural variant of carbon called "penta-graphene" - a very thin sheet of pure carbon that has a unique structure inspired by a pentagonal pattern of tiles found paving the streets of Cairo. The newly discovered material, called penta-graphene, is a single layer of carbon pentagons that resembles the Cairo tiling, and that appears to be dynamically, thermally and mechanically stable. 
The newly discovered material, called penta-graphene, is a single layer of carbon pentagons that resembles the Cairo tiling, and that appears to be dynamically, thermally and mechanically stable. (Image: Virginia Commonwealth University) "The three last important forms of carbon that have been discovered were fullerene, the nanotube and graphene. Each one of them has unique structure. Penta-graphene will belong in that category," said the paper's senior author, Puru Jena, Ph.D., distinguished professor in the Department of Physics in VCU's College of Humanities and Sciences. The researchers' paper, "Penta-graphene: A new carbon allotrope", will appear in the journal Proceedings of the National Academy of Sciences, and is based on research that was launched at Peking University and VCU. Qian Wang, Ph.D., a professor at Peking University and an adjunct professor at VCU, was dining in a restaurant in Beijing with her husband when she noticed artwork on the wall depicting pentagon tiles from the streets of Cairo. "I told my husband, "Come, see! This is a pattern composed only of pentagons,'" she said. "I took a picture and sent it to one of my students, and said, 'I think we can make this. It might be stable. But you must check it carefully.' He did, and it turned out that this structure is so beautiful yet also very simple." Most forms of carbon are made of hexagonal building blocks, sometimes interspersed with pentagons. Penta-graphene would be a unique two-dimensional carbon allotrope composed exclusively of pentagons. Along with Jena and Wang, the paper's authors include Shunhong Zhang, Ph.D candidate, from Peking University; Jian Zhou, Ph.D., a postdoctoral researcher at VCU; Xiaoshuang Chen, Ph.D., from the Chinese Academy of Science in Shanghai; and Yoshiyuki Kawazoe, Ph.D., from Tohoku University in Sendai, Japan. The researchers simulated the synthesis of penta-graphene using computer modelling. The results suggest that the material might outperform graphene in certain applications, as it would be mechanically stable, possess very high strength, and be capable of withstanding temperatures of up to 1,000 degrees Kelvin. "You know the saying, diamonds are forever? That's because it takes a lot of energy to convert diamond back into graphite," Jena said. "This will be similar." Penta-graphene has several interesting and unusual properties, Jena said. For example, penta-graphene is a semiconductor, whereas graphene is a conductor of electricity. "When you take graphene and roll it up, you make what is called a carbon nanotube which can be metallic or semiconducting," Jena said. "Penta-graphene, when you roll it up, will also make a nanotube, but it is always semiconducting." The way the material stretches is also highly unusual, the researchers said. "If you stretch graphene, it will expand along the direction it is stretched, but contract along the perpendicular direction." Wang said. "However, if you stretch penta-graphene, it will expand in both directions." The material's mechanical strength, derived from a rare property known as Negative Poisson's Ratio, may hold especially interesting applications for technology, the researchers said. Penta-graphene's properties suggest that it may have applications in electronics, biomedicine, nanotechnology and more. The next step, Jena said, is for scientists to synthesize penta-graphene. "Once you make it, it [will be] very stable. So the question becomes, how do you make it? In this paper, we have some ideas. Right now, the project is theoretical. It's based on computer modelling, but we believe in this prediction quite strongly. And once you make it, it will open up an entirely new branch of carbon science. Two-dimensional carbon made completely of pentagons has never been known."
The newly discovered material, called penta-graphene, is a single layer of carbon pentagons that resembles the Cairo tiling, and that appears to be dynamically, thermally and mechanically stable. (Image: Virginia Commonwealth University) "The three last important forms of carbon that have been discovered were fullerene, the nanotube and graphene. Each one of them has unique structure. Penta-graphene will belong in that category," said the paper's senior author, Puru Jena, Ph.D., distinguished professor in the Department of Physics in VCU's College of Humanities and Sciences. The researchers' paper, "Penta-graphene: A new carbon allotrope", will appear in the journal Proceedings of the National Academy of Sciences, and is based on research that was launched at Peking University and VCU. Qian Wang, Ph.D., a professor at Peking University and an adjunct professor at VCU, was dining in a restaurant in Beijing with her husband when she noticed artwork on the wall depicting pentagon tiles from the streets of Cairo. "I told my husband, "Come, see! This is a pattern composed only of pentagons,'" she said. "I took a picture and sent it to one of my students, and said, 'I think we can make this. It might be stable. But you must check it carefully.' He did, and it turned out that this structure is so beautiful yet also very simple." Most forms of carbon are made of hexagonal building blocks, sometimes interspersed with pentagons. Penta-graphene would be a unique two-dimensional carbon allotrope composed exclusively of pentagons. Along with Jena and Wang, the paper's authors include Shunhong Zhang, Ph.D candidate, from Peking University; Jian Zhou, Ph.D., a postdoctoral researcher at VCU; Xiaoshuang Chen, Ph.D., from the Chinese Academy of Science in Shanghai; and Yoshiyuki Kawazoe, Ph.D., from Tohoku University in Sendai, Japan. The researchers simulated the synthesis of penta-graphene using computer modelling. The results suggest that the material might outperform graphene in certain applications, as it would be mechanically stable, possess very high strength, and be capable of withstanding temperatures of up to 1,000 degrees Kelvin. "You know the saying, diamonds are forever? That's because it takes a lot of energy to convert diamond back into graphite," Jena said. "This will be similar." Penta-graphene has several interesting and unusual properties, Jena said. For example, penta-graphene is a semiconductor, whereas graphene is a conductor of electricity. "When you take graphene and roll it up, you make what is called a carbon nanotube which can be metallic or semiconducting," Jena said. "Penta-graphene, when you roll it up, will also make a nanotube, but it is always semiconducting." The way the material stretches is also highly unusual, the researchers said. "If you stretch graphene, it will expand along the direction it is stretched, but contract along the perpendicular direction." Wang said. "However, if you stretch penta-graphene, it will expand in both directions." The material's mechanical strength, derived from a rare property known as Negative Poisson's Ratio, may hold especially interesting applications for technology, the researchers said. Penta-graphene's properties suggest that it may have applications in electronics, biomedicine, nanotechnology and more. The next step, Jena said, is for scientists to synthesize penta-graphene. "Once you make it, it [will be] very stable. So the question becomes, how do you make it? In this paper, we have some ideas. Right now, the project is theoretical. It's based on computer modelling, but we believe in this prediction quite strongly. And once you make it, it will open up an entirely new branch of carbon science. Two-dimensional carbon made completely of pentagons has never been known."Penta-graphene, a new structural variant of carbon, discovered
The unique structure of the thin sheet of pure carbon was inspired by pentagonal tile pattern found in the streets of Cairo.
Artificial blood vessels
By combining micro-imprinting and electro-spinning techniques, researchers at Shanghai University's Rapid Manufacturing Engineering Center have developed a vascular graft composed of three layers for the first time. This tri-layered composite has allowed researchers to utilize separate materials that respectively possess mechanical strength and promote new cell growth - a significant problem for existing vascular grafts that have only consisted of a single or double layer. Vascular grafts are surgically attached to an obstructed or otherwise unhealthy blood vessel to permanently redirect blood flow, such as in coronary bypass surgery. Traditional grafts work by repurposing existing vessels from the patient's own body or from a suitable donor. However, these sources are often insufficient for a patient's needs because of the limited supply in a patient's body, and may be afflicted by the same underlying conditions that necessitate the graft in the first place. Accordingly, there has been a great deal of research towards developing synthetic vessels that can mimic natural ones, allowing new cells to grow around them and then degrade away, thereby creating new vessels. "The composite vascular grafts could be better candidates for blood vessel repair," said Yuanyuan Liu, an associate professor at the Rapid Manufacturing Engineering Center. Liu's team had previously worked with bone scaffolds, which are used to repair bone defects, before turning their attention to cardiovascular disease, and thus vascular grafts. They describe their current research in the journal ("Composite vascular repair grafts via micro-imprinting and electrospinning"). 
The 3-D bio-printing equipment can prepare all kind of scaffolds for tissue defect repair. (Image: Yuanyuan Liu/Shanghai University) As a rule, surrogate scaffolds need to mimic the natural vasculature of their targeted tissue as much as possible. For blood vessel surrogates, this structural mimicry can be fabricated by electrospinning, a process which uses an electrical charge to draw liquid inputs - here a mixture of chitosan and polyvinyl alcohol - into incredibly fine fibers. Electrospinning also allows for a high surface-to-volume ratio of nanofibers, providing ample space for host cells to grow and connect. These components all naturally degrade within six months to a year, leaving behind a new, intact blood vessel. The resulting structure, however, isn't very rigid - the fly in the ointment for many previous models. To compensate for this, the researchers designed a three-layer model, in which the mixture was electrospun onto both sides of a microimprinted middle layer of poly-p-dioxanone, a biodegradable polymer commonly used in biomedical applications. The ends of this sheet were then folded and attached to make a tube-like vessel. Liu and her team then seeded the scaffold with rat fibroblast cells, which are ideal candidates because of their ease of cultivation and quick growth rate, to test the scaffold's efficacy in promoting cellular expansion and integration. The researchers found that the cells on these composite scaffolds proliferated quickly, likely due to the functional amino and hydroxyl groups introduced by the chitosan. While a good deal of work remains before the prospect of human trials, Liu and her group are optimistic about the future of their research. Their next project is to test the implants in an animal model, to observe the structure's efficacy with live vascular cells.
The 3-D bio-printing equipment can prepare all kind of scaffolds for tissue defect repair. (Image: Yuanyuan Liu/Shanghai University) As a rule, surrogate scaffolds need to mimic the natural vasculature of their targeted tissue as much as possible. For blood vessel surrogates, this structural mimicry can be fabricated by electrospinning, a process which uses an electrical charge to draw liquid inputs - here a mixture of chitosan and polyvinyl alcohol - into incredibly fine fibers. Electrospinning also allows for a high surface-to-volume ratio of nanofibers, providing ample space for host cells to grow and connect. These components all naturally degrade within six months to a year, leaving behind a new, intact blood vessel. The resulting structure, however, isn't very rigid - the fly in the ointment for many previous models. To compensate for this, the researchers designed a three-layer model, in which the mixture was electrospun onto both sides of a microimprinted middle layer of poly-p-dioxanone, a biodegradable polymer commonly used in biomedical applications. The ends of this sheet were then folded and attached to make a tube-like vessel. Liu and her team then seeded the scaffold with rat fibroblast cells, which are ideal candidates because of their ease of cultivation and quick growth rate, to test the scaffold's efficacy in promoting cellular expansion and integration. The researchers found that the cells on these composite scaffolds proliferated quickly, likely due to the functional amino and hydroxyl groups introduced by the chitosan. While a good deal of work remains before the prospect of human trials, Liu and her group are optimistic about the future of their research. Their next project is to test the implants in an animal model, to observe the structure's efficacy with live vascular cells.Looking at graphene and other 2D crystals in energy conversion and storage
Posted: Feb 03, 2015 (Nanowerk News) The single-atom thick allotrope of carbon known as graphene has many potential applications, among them energy conversion and storage. Graphene and related two-dimensional crystals combine high electrical conductivity with physical flexibility and a huge surface to weight ratio. Such qualities make them suitable for storing electric charge in batteries and supercapacitors, and as catalysts in solar and fuel-cell electrodes. A number of energy applications for 2D crystals are under development worldwide, and Europe’s Graphene Flagship has invested significant resources in this area. In an article for the journal Science ("Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage"), flagship scientists, together with colleagues in the US and Korea, have reviewed the potential for graphene and related materials in the energy sector. The authors hope that their review will guide researchers in academia and industry in drawing up a strategy for energy applications and their implementation. In the Science review, the researchers – led by physicist Francesco Bonaccorso, who is based at the Graphene Labs of the Instituto Italiano di Tecnologia in Genova, and is a Royal Society Newton Fellow at the Cambridge Graphene Centre – note the substantial progress made in material preparation at the laboratory level. They also highlight the challenge of producing the materials on an industrial scale in a cost-effective manner. Surface, function and production Graphene, the best-known of the hundreds of two-dimensional crystals investigated to date, has a very high surface-to-mass ratio. With around 2,600 square metres for every gramme, graphene is all surface and no bulk, and it is this 2D nature which gives graphene its unique electrical, thermal and mechanical properties. Other two-dimensional materials, including the transition metal dichalcogenide molybdenum disulphide (MoS2), various transition metal oxides, and the MAX-phase class of 2D crystals, display properties complementary to those of graphene. In all cases, the potential for mass production and chemical functionalisation make graphene and related materials an ideal platform for energy applications. The challenge is to produce them on a large scale with properties tailored for specific purposes. There are various ways of producing graphene in bulk, but, when it comes to industrial scale manufacture of the volumes required for energy applications, liquid-phase exfoliation looks very promising. This involves exploiting the agitating effect of high-frequency sound waves to separate graphene flakes from graphite held in suspension. Electronic-grade graphene can be produced through chemical vapour deposition, but the process is less suitable for energy applications. A third possibility is the chemical synthesis of graphene flakes, but there are questions of scalability. Solar cells Barely a week goes by without reports of new developments in solar cell technology. Efficiencies are on the rise, and various nanomaterials are employed in the manufacture of photovoltaic films and electrodes. Silicon remains the most widely used photon absorber, and this established semiconductor material dominates the solar cell market. Second and third-generation photovoltaic cells, including those based on organic materials and quantum dots, have lower photocurrent efficiency than those fabricated from silicon, and there are issues with material stability and durability. Graphene and related materials are attracting serious interest in solar energy conversion, on the grounds that they could improve efficiency, reduce production costs, and deal with a number of environmental issues. In addition, 2D crystals have the advantages of transparency and flexibility, and in time they could replace indium tin oxide films, which are relatively brittle. They can also act as catalysts in dye-sensitised solar cells, in place of platinum. This may result in a four orders-of-magnitude reduction in device cost. Graphene can be engineered to match the low sheet resistance and high transmittance of transparent conducting films made from established materials, and graphene-based materials have with varying degrees of success been implemented in solar energy systems, including inorganic, organic, dye-sensitised and hybrid organic/inorganic cells. This is an especially active area of research and development, with chemically-doped graphene a particular focus of attention. As well as the photo-sensitivity of solar cells, charge collection and transport are important issues to consider, and this applies both to dye-sensitised cells and organic photovoltaics. For example, graphene and related materials can be used to lessen the negative effects of charge carrier recombination. In the case of dye-sensitised cells, reduced graphene oxide is employed in combination with titanium dioxide nanoparticles, whilst graphene quantum dots are efficient electron hole transport layers in organic photovoltaics. There is a need to replace or at least reduce the amount of platinum in solar cell counter-electrodes, owing to the cost of the precious metal, and its tendency to degrade when in contact with iodide electrolytes, which reduces device efficiency. Functionalised graphene flakes and other 2D crystals have been used in place of platinum in counter-electrodes, sometimes outperforming the platinum. The efficiency of photovoltaic devices based on graphene and related materials is improving at a pace superior to those based on established materials, with the highest reported efficiency of 13% for dye-sensitised cells achieved using graphene nanoplatelets as a counter-electrode. With graphene-based perovskite solar cells we have seen an efficiency of 15.6%, and that achieved with relatively low-temperature processing, yielding a significant cost reduction. Thermoelectric devices Recovering waste heat is clearly a good thing, and this may be done with solid-state devices which generate electricity from temperature gradients. Thermoelectric devices can also convert heat produced by sunlight into electricity, and here they can enhance solar cells by capturing heat produced by photons with energies below the band gaps of the photosensitive materials employed. The effectiveness of thermoelectric devices is assessed by the fraction of absorbed heat converted into electricity, along with a figure of merit which depends on the material’s electrical and thermal conductivities. Traditional thermoelectric devices have conversion efficiencies of around 5%, and this low value limits their widespread use. In thermoelectric materials one is looking for high electrical and low thermal conductivity. On the face of it this rules out graphene, in which the electrical and thermal conductivities are both high. However, it is possible to tailor the thermal transport properties of graphene by introducing defects, edge roughness, and periodic holes into the material. This can reduce thermal conductivity by up to 100 times when compared with pristine graphene. It can also raise the figure of merit by a factor of three over that obtained with other materials. The problem here is the difficulty in scaling tailored graphene nanoribbons via chemical synthesis. Still, we can blend two-dimensional materials with carbon nanotubes to increase electrical conductivity without reducing thermoelectric sensitivity. Fuel cells A fuel cell converts chemical energy from a fuel such as hydrogen gas into electricity via a reaction with oxygen or another oxidising agent. Much of the attention devoted to fuel cells focuses on high power applications. When it comes to improving and better exploiting fuel cells, the principal difficulty is one of cost, and specifically the need to replace expensive noble metals such as platinum and gold as catalysts in the chemical reaction. Fuel cell electrodes must also be chemically stable over the long term, and for some applications physically flexible. Graphene and related materials are promising candidates for fuel cell electrodes, and also as membranes in proton exchange fuel cells. Where they cannot replace precious metal catalysts, the addition of two-dimensional materials may lower the amount of expensive catalyst materials required. For example, reduced graphene oxide modifies the properties of platinum electro-catalysts supported on it, leading to improved methanol oxidation when compared with commercial platinum/carbon black mixtures. Batteries Today’s state-of-the-art rechargeable batteries are based on lithium-based cathodes and graphite anodes, and crucial to their performance is the capacity of the anode material to store lithium ions. Graphene has a larger gravimetric capacity than graphite, and the flexible nature of the material has advantages when it comes to certain applications and environments. Graphene and related materials are also appealing as fuel cell cathodes, owing to their high electrical capacity per unit weight. Graphene-based hybrid electrodes may also be employed in rechargeable batteries to increase electron transport, capacity, discharge current and device longevity. Where graphene is not actually incorporated into battery electrodes, it may be used as a substrate for the growth of high-performance anode/cathode nanoparticles. For example, lithium-based nanorods grown on reduced graphene oxide flakes show a significantly lower degradation over a fixed number of cycles than reduced graphene oxide or graphite. Wrapping electrochemically-active particles within graphene or MoS2 flakes is another possibility. Supercapacitors Electrochemical capacitors store energy in an electric field set up between conducting plates separated by an insulating material. Supercapacitors, which divide into electrostatic double-layer capacitors, hybrid and pseudocapacitors, are ideal for high power applications in which the required energy density is an order of magnitude greater than is possible with lithium-ion batteries. Such applications include electric and hybrid motor vehicles, heavy lifting, load levelling and backup power for electric utilities and industrial plant. Supercapacitors are necessarily large devices, and the materials of which they are made are produced in bulk into electrodes 100 to 200 microns thick. Most double-layer capacitors have carbon electrodes impregnated with organic electrolytes. Another type of supercapacitor is based on lithium-ion hybrid cells, in which a graphite lithium-ion anode is coupled with an activated carbon cathode. In this case the energy density is around twice that of double-layer capacitors. The performance of supercapacitors is dependent on the electrode surface area accessible to the electrolyte, and this is where 2D crystals such as graphene have the advantage over graphite and activated carbon. In practice this can include graphene-based platelets with spacer materials such as carbon nanotubes, mesoporous carbon spheres, water and ionic liquids, and resins chemically activated to create a porous structure. Note that large material surface areas do not always translate into high-performance supercapacitors, especially if the material packing density is low. It is possible to increase the packing density through evaporation drying of graphene hydrogels, and by capillary compression of reduced graphene oxide. Raising the operating voltage is another way of increasing the energy storage capacity. Double-layer capacitors are marked by their rapid charge/discharge and long cycle life, and lithium-ion batteries by a high energy storage capacity. Combine the two qualities, and the result is a lithium-ion hybrid supercapacitor, albeit with performance trade-offs common to both types of device. Electrodes for double-layer capacitors made from microwave-expanded graphite oxide and lithium-ion battery electrodes comprising graphite, lithium and iron oxides have been studied, as have electrodes containing metal oxides based on ruthenium, manganese and molybdenum, with conducting polymers to increase the specific capacitance via chemical reduction and oxidation reactions. Graphene is included in these systems as a conductive support. Hydrogen production and storage Hydrogen has an energy density more than three times that of petrol, and its combustion by-product is water. The principal challenges in using hydrogen as a green fuel are to produce and store the gas. Electrolysis is the key mechanism for hydrogen gas production, and the edges of 2D crystals including MoS2 and reduced graphene oxide are active catalytic sites in the reaction. Whilst simple in outline, the detail of the hydrogen evolution reaction varies from material to material, with resistive losses a significant issue with non-metallic electrodes. Combining graphene and related materials with carbon nanotubes can enhance the reaction by improving electron transport efficiency. When it comes to storing hydrogen, carbon-based structures are attractive owing to their high gravimetric capacity, and graphene particularly so. Calculations show that the gravimetric density may in principal be up to 8% in graphene multi-layers spaced with pillar structures such as carbon nanotubes. This applies to cryogenic temperature and/or high pressure regimes. With ambient conditions the maximum theoretical density is 4%. In practice the numbers are lower, with up to 1% density at room temperature and pressure. Doping graphene with alkaline or transition metals can increase the gravimetric density to 10%. This can be achieved by functionalising graphene with metals such as palladium, which catalyse the dissociation of hydrogen molecules into ions on the graphene surface. Perspectives Graphene and related two-dimensional crystals may play a major role in future energy conversion and storage technologies, and this is an active area of research and development for Graphene Flagship partners, both academic and industrial. “The huge interest in two-dimensional crystals for energy applications comes both from their physico-chemical properties, and the possibility of producing and processing them in large quantities, in a cost-effective manner,” says Bonaccorso. “In this context, the development of functional inks based on two-dimensional crystals is the gateway for the realisation of new generation electrodes in energy storage and conversion devices.” Bonaccorso adds that the challenge ahead is to demonstrate a disruptive technology in which two-dimensional materials not only replace traditional electrodes, but more importantly enable whole new device concepts. Review co-author Andrea Ferrari, who chairs the Executive Board of the Graphene Flagship, and is director of the Cambridge Graphene Centre, offers a soberly optimistic view of the potential for graphene in this area: “Graphene and related materials have great promise in these areas, and the Graphene Flagship has identified energy applications as a key area of investment. We hope that our critical overview will guide researchers in academia and industry in identifying optimal pathways toward applications and implementation, with an eventual benefit for society as a whole.”
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