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Properties of graphene change due to water and oxygen

Posted By Graphene Council, The Graphene Council, Friday, December 6, 2019
We often find that food becomes rotten when we leave it outside for long and fruits turn brown after they are peeled or cut. Such phenomena can be easily seen in our daily life and they illustrate the oxidation-reduction reaction. The fundamental principle controlling physical properties of two-dimensional materials noted as next generation materials like graphene is found to be redox reactions.

The research team consisted of Professor Sunmin Ryu, Kwanghee Park, and Haneul Kang, affiliated with Department of Chemistry, POSTECH, discovered that the doping of two-dimensional materials with influx of charges from outside in the air is by an electrochemical reaction driven by the redox couples of water and oxygen molecules. Using real-time photoluminescence imaging, they observed the electrochemical redox reaction between tungsten disulfide and oxygen/water in the air. According to their study¸ the redox reaction can control the physical properties of two-dimensional materials which can be applied to bendable imaging element, high-speed transistor, next generation battery, ultralight material and other two-dimensional semiconductor applications.

Two-dimensional materials like graphene and tungsten disulfide are in the form of a single or few layers of atoms in nanometer size. They are thin and easily bended but hard. Because of these properties, they are used in semiconductors, display, solar battery and more and, they are called as a dream material. However, since all atoms exist on the surface of a material, it is limited to the ambient environment such as temperature and humidity which often causes them to modify or transform. Before the research team announced on the result of their study, it has been unknown why such phenomenon happens and has been difficult to commercialize, being unable to control material properties.

The research team used real-time photoluminescence imaging of tungsten disulfide and Raman spectroscopy of graphene. They demonstrated molecular diffusion through the two-dimensional nanoscopic space between two-dimensional materials and hydrophilic substrates. They also discovered that there was enough amount of water to mediate the redox reactions in the space. Furthermore, they proved that charge doping in the acid such as hydrochloric acid is also dictated by dissolved oxygen and hydrogen-ion concentration (pH) in the same way.

What they have accomplished in this research is the fundamental principle needed to govern electrical, magnetic, and optical properties of two-dimensional or other low-dimensional materials. It is anticipated that this method can be applied to improve pretreatment which is needed to prevent two-dimensional materials from being modified by surroundings and aftertreatment technology such as encapsulation for flexible and stretchable displays.

Professor Sunmin Ryu said, "Using the real-time photoluminescence, we were able to demonstrate that the electrochemical reaction driven by the redox couples of oxygen and water molecules in the air is the key and proved the fundamental principle for governing properties of materials. This reaction is applied to not only two-dimensional materials but also other low-dimensional materials such as quantum dot and nanowires. So, our findings will be an important steppingstone to development of nano technology based on low-dimensional materials."

Tags:  2D materials  Battery  Graphene  Haneul Kang  Kwanghee Park  POSTECH  semiconductor  Sunmin Ryu  transistor 

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Graphene Flagship partners up European academia and industry to make lighter composites for planes and cars

Posted By Graphene Council, The Graphene Council, Friday, December 6, 2019

The Graphene Flagship brought together top European researchers and companies to discuss the most disruptive ways graphene could enhance composites used in the aerospace, automotive and energy industries. The multidisciplinary team involved researchers from academic institutions, business enterprises such as Graphene Flagship Partners Nanesa and Avanzare, and large transportation end-user industries, such as Graphene Flagship Partners Airbus and Fiat. 

They showed that integrating graphene and related materials (GRMs) into fibre-reinforced composites (FRCs) has great potential to improve weight and strength, and helps to overcome the bottlenecks limiting the applications of these composites in planes, cars, wind turbines and more. Nowadays, the transportation industry is responsible for nearly one-third of global energy demand, and it is the major source of pollution and greenhouse gas emissions in urban areas. Graphene Flagship scientists are therefore continually trying to develop new materials to lower fuel usage and CO2 emissions, helping to mitigate environmental damage and climate change.

Graphene-integrated composites are an example of lighter materials with great potential for use in vehicle frameworks. They are constructed by introducing graphene sheets, a few billionths of a metre thick, into hierarchical fibre composites as a nano-additives. Hierarchical fibre composites are a type of composite material in which components of different sizes are combined in a controlled way to significantly improve the mechanical properties. They typically consist of micro- or mesoscopic carbon fibres, a few millionths of a metre thick, attached to a polymer matrix, and they are already used as building materials to make vehicles of all shapes and sizes.

Graphene's high aspect ratio, high flexibility and mechanical strength enable it to enhance the strength of weak points in these composites, such as at the interface between two different components. Its tunable surface chemistry also means that interactions with the carbon fibre and polymer matrix can be adjusted as needed. The fibre, polymer matrix and graphene layers all work together to distribute mechanical stress, resulting in a material with improved strength and other beneficial properties.

There are many challenges to consider. For instance, planes experience temperature changes between 20 °C and -40 °C every time they take off and land, with huge differences in pressure and humidity. Graphene-integrated composites therefore need to withstand water condensing and even freezing inside the fuselage. They also need to endure lightning strikes, which happen several times per month, so the conductive properties of graphene must be harnessed to create an electrically conductive framework that resists electromagnetic impulses. In cars, new structural materials must be able to withstand crash tests and be lightweight enough to ensure fuel efficiency. Graphene Flagship researchers are also investigating conductive materials to replace circuitry in car dashboards.

Researchers and end-users come together
Graphene Flagship partners at Queen Mary University and the National Graphene Institute, UK, FORTH-Hellas, Greece, CNR, Italy, and Chalmers University of Technology, Sweden, collaborated with researchers at the University of Turin, the University of Trento and KET-LAB, Italy, and the University of Patras, Greece, to provide perspectives from the research community. They worked with scientists at Graphene Flagship partner companies Nanesa, Italy, and Avanzare, Spain, to review the technological viability of graphene-incorporated FRCs.

Francesco Bertocchi, co-author of the paper and President of Nanesa, believes that graphene-incorporated FRCs are indeed feasible for vehicle design, and has created new composites with many essential properties for the transportation industries. "Thanks to the Graphene Flagship, Nanesa has worked in close synergy with many partners to create many different prototypes. These include properties such as flame retardancy, water vapor absorption barrier, high electrical and thermal conductivity, EMI shielding. We also integrated thermo-resistive systems for de-icing and anti-icing ," he says.

Graphene Flagship Partners Airbus and Fiat-Chrysler Automobiles, world leading aerospace and automotive industries, evaluated the impact of graphene-incorporated FRCs on the aerospace and automotive industries and assessed their commercial viability.

Tamara Blanco-Varela, co-author and materials & processes engineer at Airbus, explains that Airbus is working hard to make these materials viable for use in new aircraft models. "We all know that the aeronautical sector is very challenging for the introduction of new materials or technologies. Airbus is committed to making graphene-related materials fly as soon as possible, and a step-by-step approach is being set up," she says. By selecting 'quick-win' applications with immediate benefits to the aerospace industry, she anticipates that graphene-integrated FRCs will reach the market soon. "One example is using these materials for anti- and de-icing purposes in aeroplanes, for which Airbus will be leading activities targeting commercial exploitation of this technology. We are hoping for it to reach a high maturity level, with a target readiness level between five and six, in the next few years."

Brunetto Martorana, co-author and researcher at Graphene Flagship partner Fiat-Chrysler Automobiles, adds: "The interesting structural properties of graphene have opened an interesting window for designing novel light composites." He explains that new lightweight composite materials do not necessarily need to be lower in strength and introduce safety issues. "New approaches must be found to enhance the 'crashworthiness' of composites – and graphene composites may be able to fill that role," he continues. Fiat-Chrysler Automobiles have now committed to the commercialization of new composite materials, and will be leading a new initiative to bring this technology to market."

An uplifting outlook
"The Graphene Flagship provides a stable, clear, long-lasting partnership for different partners to work together. They all started their collaboration as part of our Composites Work Package", comments Vincenzo Palermo, Graphene Flagship Vice-Director and lead author of the paper. "The Graphene Flagship pushes all partners to have frequent interactions, with regular meetings – like in this case, partners who begun working on graphene with different motivations have come together to address common challenges," he says.

Costas Galiotis, the Graphene Flagship's Composites Work Package leader, expresses that this collaboration has been highly valuable. "This a comprehensive review of the work undertaken in the Graphene Flagship, and elsewhere, to confirm that the addition of GRMs provides benefits to many applications in the aerospace, automotive, energy and leisure industries."

Galiotis expresses particular interest in the review's analysis of the best ways to process GRMs into composites, the effect of this on the overall composite performance, and the challenges scientists face in the search for high performance composites. "Overall, I think this is a timely review article for the composites field, which should be read with interest by all parties involved with composite development and usage," he concludes.

 

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, comments: "This paper shows the leadership of large corporations and small enterprises, all partners of the Graphene Flagship, in taking graphene composites to the market in the next few years. This yet again shows the steady progress of the Graphene Flagship along its technology and innovation roadmap."

Tags:  Aerospace  Airbus  Andrea C. Ferrari  Automotive  Avanzare  Brunetto Martorana  composites  Costas Galiotis  Fiat-Chrysler  Francesco Bertocchi  Graphene  Graphene Flagship  Nanesa  Tamara Blanco-Varela  Vincenzo Palermo 

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Using electronics to solve common biological problems

Posted By Graphene Council, The Graphene Council, Wednesday, December 4, 2019
Researchers from multiple disciplines are working together at KAUST to develop bioelectronics that can detect diseases, treat cancers and track marine animals; they may even discover the next generation of computing systems.

Cancer-killing magnets

Jurgen Kosel is an electrical engineer who loves to play with magnets. His research group has developed a technique to fabricate unique magnetic iron-oxide nanowires that can kill cancer cells1.

“Certain kinds of iron-based magnetic nanoparticles were approved many years ago by the U.S. Food and Drug Administration for use inside the human body. They are regularly used as contrast agents in magnetic resonance imaging and as nutritional supplements for people with iron deficiency,” says Kosel.

The magnetic nanoparticles currently in use are spherical in shape. Kosel and his team developed wire-shaped magnetic nanoparticles that can be rotated like a compass needle, creating a pore in cancer cell membranes that induces natural cell death. These cancer-killing nanowires can be made even more effective when coated with an anti-cancer drug or heated with a laser. They are "eaten" by cancer cells, and once released inside, they can wreak havoc.

Kosel has been working closely with cell biologist Jasmeen Merzaban, and more recently, with organic chemist Niveen Khashab to "functionalize" the surfaces of his magnetic nanowires to ensure the body’s immune system does not treat them as foreign. They are also working on preventing the wires from sticking together and on targeting cancer cells more specifically by coating them with antibodies that recognize specific antigens on their cell membranes.

Kosel has also worked with electrical engineer Muhammad Hussain to use magnets for improving the safety of cardiac catheters. They have developed a flexible magnetic sensor that is sensitive enough to detect the Earth’s magnetic field. When these sensors are placed on the tip of a cardiac catheter, for example, clinicians can detect its orientation inside blood vessels. This enables them to direct it where it is needed in order to insert a stent, for example, to relieve blockage in a heart artery. This reduces the need for prolonged doses of X-rays and contrast dyes during procedures like coronary angioplasty.

Disease detection

“Over the past 50 years, the 500-billion-dollar semiconductor industry has mainly focused on two applications: computing and communications,” says KAUST electrical engineer, Khaled Salama. “But this technology holds a lot of promise for other areas, including medical research, as people are living longer and needing more care. We need a paradigm shift to leverage some of the technologies we’ve developed for use in this area.”

Salama has developed a sensor that can detect "C-reactive proteins," a biomarker of cardiovascular disease2. He’s done this by functionalizing electrodes with nanomaterials and gold nanoparticles to improve their sensitivity. The electrodes give a signal that is proportional to the amount of C-reactive protein in a blood sample. His group developed a unique process that 3D prints the microfluidic channels that deliver samples to the sensor for biological detection.

Elsewhere at KAUST, Sahika Inal is developing a device that can make life easier for diabetics.

Inal comes from a textile manufacturing background, but her studies on the electrical properties of polymers, which are biocompatible, have led her down the route of bioelectronics. 

Her team has developed inkjet-printed, disposable, polymer-based sensors that can measure glucose levels in saliva3. “We inkjet-print conducting polymers. The biological ink contains the enzymes used for glucose sensing, an encapsulation layer that protects the enzymes, a layer that only allows glucose penetration and an insulating layer to protect the electronics,” she explains. “And then you have a paper-based sensor within a few minutes!”

Inal is also developing other biochemical sensors that can generate their own energy from compounds already present in the body to power implantable devices, such as cardiac pacemakers.

“To conduct impactful bioelectronics work, I need to be in an environment where there are biologists, the people who can give me feedback on what I develop,” says Inal.

Bio-inspired computers and animal tracking
Bioelectronics not only encompass electronic devices designed to solve biological problems, they are also electronic solutions inspired by biology.

Khaled Salama is interested in a relatively new type of bio-inspired device called a "memristor"4. These are electrical components inspired by the neural networks and synapses of the brain. Researchers hope they will lead to the next generation of computing systems and that they will be better equipped to very rapidly process huge amounts of data. Salama has developed an approach that improves their computational efficiency while reducing power consumption in these typically energy-intensive devices.

Sensing data in harsh marine environments can be particularly challenging, says Kosel. Researchers have often resorted to electronic tags placed on large marine animals to track their movements. They also use electronic sensors to conduct flow, salinity, pressure and temperature measurements in the sea. Smaller, lighter, less power-hungry tags are needed to resist corrosion, and withstand biofouling, a bacterial crust that forms on almost anything that stays in the sea for too long.

Kosel’s solution was to develop graphene sensors fabricated with a single-step laser-printing technique for marine applications. These laser-induced graphene sensors are resistant to corrosion and can survive high temperatures. They are very light and flexible, making them suitable for attaching to smaller marine animals. They also developed a technique5 that involves conducting high-frequency measurements that allow them to withstand the effects of an accumulating biofouling layer.

The group have started a conference, which will be held annually at KAUST. Last year, among the many esteemed attendees was George Malliaras, a Prince Philip Professor of Technology at the University of Cambridge. Malliaras praised the university for its world-class instrumentation, access to excellent collaborations within the campus and mechanisms to collaborate with people abroad. He says, "Taken together, these attributes have made KAUST very successful at addressing some of the most important problems that humanity faces today." 

Tags:  Bioelectronics  George Malliaras  Graphene  Healthcare  Jasmeen Merzaban  Jurgen Kosel  Khaled Salama  King Abdullah University of Science and Technology  Muhammad Hussain  nanoparticles  Niveen Khashab  Sahika Inal  semiconductor  University of Cambridge 

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Smog-eating graphene composite reduces atmospheric pollution

Posted By Graphene Council, The Graphene Council, Wednesday, December 4, 2019
Graphene Flagship partners the University of Bologna, Politecnico di Milano, CNR, NEST, Italcementi HeidelbergCement Group, the Israel Institute of Technology, Eindhoven University of Technology, and the University of Cambridge have developed a graphene-titania photocatalyst that degrades up to 70% more atmospheric nitrogen oxides (NOx) than standard titania nanoparticles in tests on real pollutants.

Atmospheric pollution is a growing problem, particularly in urban areas and in less developed countries. According to the World Health Organization, one out of every nine deaths can be attributed to diseases caused by air pollution. Organic pollutants, such as nitrogen oxides and volatile compounds, are the main cause of this, and they are mostly emitted by vehicle exhausts and industry.

To address the problem, researchers are continually on the hunt for new ways to remove more pollutants from the atmosphere, and photocatalysts such as titania are a great way to do this. When titania is exposed to sunlight, it degrades nitrogen oxides – which are very harmful to human health – and volatile organic compounds present at the surface, oxidising them into inert or harmless products.

Now, the Graphene Flagship team working on photocatalytic coatings, coordinated by Italcementi, HeidelbergCement Group, Italy, developed a new graphene-titania composite with significantly more powerful photodegradation properties than bare titania. "We answered the Flagship's call and decided to couple graphene to the most-used photocatalyst, titania, to boost the photocatalytic action," comments Marco Goisis, the research coordinator at Italcementi. "Photocatalysis is one of the most powerful ways we have to depollute the environment, because the process does not consume the photocatalysts. It is a reaction activated by solar light," he continues.

By performing liquid-phase exfoliation of graphite – a process that creates graphene – in the presence of titania nanoparticles, using only water and atmospheric pressure, they created a new graphene-titania nanocomposite that can be coated on the surface of materials to passively remove pollutants from the air. If the coating is applied to concrete on the street or on the walls of buildings, the harmless photodegradation products could be washed away by rain or wind, or manually cleaned off.

To measure the photodegradation effects, the team tested the new photocatalyst against NOx and recorded a sound improvement in photocatalytic degradation of nitrogen oxides compared to standard titania. They also used rhodamine B as a model for volatile organic pollutants, as its molecular structure closely resembles those of pollutants emitted by vehicles, industry and agriculture. They found that 40% more rhodamine B was degraded by the graphene-titania composite than by titania alone, in water under UV irradiation. "Coupling graphene to titania gave us excellent results in powder form – and it could be applied to different materials, of which concrete is a good example for the widespread use, helping us to achieve a healthier environment. It is low-maintenance and environmentally friendly, as it just requires the sun's energy and no other input," Goisis says. But there are challenges to be addressed before this can be used on a commercial scale. Cheaper methods to mass-produce graphene are needed. Interactions between the catalyst and the host material need to be deepened as well as studies into the long-term stability of the photocatalyst in the outdoor environment.

Ultrafast transient absorption spectroscopy measurements revealed an electron transfer process from titania to the graphene flakes, decreasing the charge recombination rate and increasing the efficiency of reactive species photoproduction – meaning more pollutant molecules could be degraded.

Xinliang Feng, Graphene Flagship Work Package Leader for Functional Foams and Coatings, explains: "Photocatalysis in a cementitious matrix, applied to buildings, could have a large effect to decrease air pollution by reducing NOx and enabling self-cleaning of the surfaces – the so-called "smog-eating" effect. Graphene could help to improve the photocatalytic behaviour of catalysts like titania and enhance the mechanical properties of cement. In this publication, Graphene Flagship partners have prepared a graphene-titania composite via a one-step procedure to widen and improve the ground-breaking invention of "smog-eating" cement. The prepared composite showed enhanced photocatalytic activity, degrading up to 40% more pollutants than pristine titania in the model study, and up to 70% more NOx with a similar procedure. Moreover, the mechanism underlying this improvement was briefly studied using ultrafast transient absorption spectroscopy."

Enrico Borgarello, Global Product Innovation Director at Italcementi, part of the HeidelbergCement Group, one of the world's largest producers of cement, comments: "Integrating graphene into titania to create a new nanocomposite was a success. The nanocomposite showed a strong improvement in the photocatalytic degradation of atmospheric NOx boosting the action of titania. This is a very significant result, and we look forward to the implementation of the photocatalytic nanocomposite for a better quality of air in the near future."

The reasons to incorporate graphene into concrete do not stop here. Italcementi is also working on another product – an electrically conductive graphene concrete composite, which was showcased at Mobile World Congress in February this year. When included as a layer in flooring, it could release heat when an electrical current is passed through it. Goisis comments: "You could heat your room, or the pavement, without using water from a tank or boiler. This opens the door to innovation for the smart cities of the future – particularly to self-sensing concrete," which could detect stress or strain in concrete structures and monitor for structural defects, providing warning signals if the structural integrity is close to failure.

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, adds: "An ever-increasing number of companies are now partners, or associate members of the Graphene Flagship, since they recognize the potential for new and improved technologies. In this work, Italcementi, leader in Italy in the field of building materials, demonstrated a clear application of graphene for the degradation of environment pollutants. This can not only have commercial benefits, but, most importantly, benefit of society by resulting in a cleaner and healthier environment"

Tags:  Andrea C. Ferrari  Eindhoven University of Technology  Enrico Borgarello  Graphene  Graphene Flagship  Healthcare  HeidelbergCement Group  Israel Institute of Technology  Italcementi  nanoparticles  University of Bologna  University of Cambridge  Xinliang Feng 

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Manchester researchers develop 2D dielectric inks suitable for print-in-place electronics

Posted By Graphene Council, The Graphene Council, Wednesday, December 4, 2019
The team at The University of Manchester have produced a two-dimensional hexagonal boron nitride ink which have been used to fabricate flexible thin-film transistors in collaboration with Duke University in North Carolina.

Published in ACS Nano, the team were able to develop insulating dielectric ink that is suitable for the print-in-place fabrication process, developed at Duke University for materials such as silver nanowires and semiconducting carbon nanotubes. Extending this process to include the hexagonal boron nitride ink led to the production of functional transistor devices, using both one and two-dimensional materials.

The use of printing technologies for flexible electronics has rapidly increased in prominence due to its simplicity, low cost and compatibility with a broad range of materials and substrates.

Currently there are a wide variety of printable functional inks, often made from organic-based materials or metal oxides, however, problems such as low carrier mobility, poor air stability, and the requirement for high processing temperatures are often encountered, limiting their application, the choice of printing method and the substrate which can be printed on.

Traditionally, there has been a particular lack of printable insulating materials that are functional without the use of high processing temperatures. Previous work carried out by the team at Duke had used silicon-based insulating materials, which are typically non-printable, rigid and brittle, thereby limiting the usage of devices in flexible electronic applications.

Using the insulating hexagonal boron nitride ink, the team were able to fully fabricate the thin-film transistors with carbon nanotube channel regions via direct-write aerosol jet printing onto flexible paper and plastic substrates in a print-in-place process, with the temperature always below 80°C. This processing temperature is one of the lowest ever reported for printed carbon nanotube-based thin-film transistors, yet the devices still show good electrical performance. The print-in-place aspect also removes the time and cost associated with the typical processing and treatment steps that are required to be performed outside of the printer.

“We had previously used our hexagonal boron nitride inks to inkjet print a graphene based transistor on paper. However, high performance transistors require a semiconducting channel, so we were pleased to see that our hexagonal boron nitride inks also perform very well in printed carbon nanotubes transistors made with the aerosol printer, showing the versatility of the hexagonal boron nitride inks in printed transistors.„
Professor Cinzia Casiraghi, Professor in Nanoscience

This paper is amongst the first reports on aerosol printing of 2D materials. Advantages of using aerosol jet printing compared to inkjet printing are the ability to print inks with a wide range of viscosity and surface tensions, in addition to the ability to print on complex surfaces.

Professor Cinzia Casiraghi who led the team at Manchester said: “We had previously used our hexagonal boron nitride inks to inkjet print a graphene based transistor on paper. However, high performance transistors require a semiconducting channel, so we were pleased to see that our hexagonal boron nitride inks also perform very well in printed carbon nanotubes transistors made with the aerosol printer, showing the versatility of the hexagonal boron nitride inks in printed transistors.”

Dr Aaron Franklin from Duke University said: “Nobody thought the aerosolized ink, especially for boron nitride, would deliver the properties needed to make functional electronics without being baked for at least an hour and a half. But not only did we get it to work, we showed that baking it for two hours after printing doesn’t improve its performance. It was as good as it could get just using our fully print-in-place process.”

The team from Duke University hope to use these inks to devise a fully print-in-place technique for electronics that is gentle enough to work on delicate surfaces including human skin.

The isolation of graphene at Manchester sparked a revolution in materials science and led to the classification of a host of other similar atomically thin materials such as hexagonal boron nitride, also known as ‘white graphene’.

But far more important is the way that these various types of 2D materials can be used as building blocks to create ‘designer materials’ or heterostructures with truly novel features on demand.

Tags:  2D materials  Aaron Franklin  ACS Nano  Cinzia Casiraghi  Duke University  Graphene  nanotubes  Nitride Ink  thin-film transistors  University of Manchester 

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Grain boundaries in graphene do not affect spin transport

Posted By Graphene Council, The Graphene Council, Wednesday, December 4, 2019
Graphene is a material that has been gaining fame in recent years due to its magnificent properties. In particular, for spintronics, graphene is a valuable material because the spins of the electrons used remain unaltered for a relatively long time. However, graphene needs to be produced on a large scale in order to be used in future devices. With that respect, chemical vapour deposition (CVD) is the most promising fabrication method.

CVD involves growing graphene on a metallic substrate at high temperatures. In this process, the generation of graphene starts at different points of the substrate simultaneously. This produces different single-crystal domains of graphene separated from one another through grain boundaries, consisting of arrays of five-, seven- or even eight-member carbon rings. The final product is, thus, polycrystalline graphene.

Is polycrystalline graphene as good as single-crystal graphene for spintronics? Grain boundaries are a significant source of charge scattering, increasing the electric resistance of the material. How do they affect spin transport?

Some experiments suggest that grain boundaries do not play a major role on spin transport. In this context, Dr Aron W. Cummings, from the ICN2 Theoretical and Computational Nanoscience Group, led by ICREA Prof. Stephan Roche, together with researchers from the Université catholique de Louvain (Belgium), have used first-principles simulations to study the impact of grain boundaries on spin transport in polycrystalline graphene. The study is published in Nano Letters.

The researchers have considered two different mechanisms by which spins could lose their original orientation (spin relaxation). One accounts for the randomisation of spins within the grains due to spin-orbit coupling, the other considers the possibility of the spins to flip due to scattering in a grain boundary. However, the researchers found that the latter case did not happen. Grain boundaries do not have any adverse effect on spin transport.

Therefore, spin diffusion length in polycrystalline graphene is independent of grain size and depends only on the strength of the substrate-induced spin-orbit coupling. Moreover, this is valid not only for the diffusive regime of transport, but also for the weakly localized one, in which quantum phenomena begin to prevail. This is the first quantum mechanical simulation confirming that the same expression for spin diffusion length holds in both regimes.

The research highlights the fact that single-domain graphene may not be a requirement for spintronics applications, and that polycrystalline CVD-grown graphene may work just as well. This puts the focus on other aspects to enhance in graphene production, such as the elimination of magnetic impurities.

Tags:  Aron W. Cummings  chemical vapour deposition  CVD  Graphene  Nano Letters  Nanoscience  polycrystalline  Stephan Roche  Universite catholique de Louvain 

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Graphene Nanoplatelets: a future role in pipecoating?

Posted By Graphene Council, The Graphene Council, Tuesday, December 3, 2019
Pipelines constitute a major infrastructure investment frequently carrying materials which in the event of failure can cause significant loss to the owner and serious potential for environmental damage. To fulfil their role pipelines often run long distances either underwater or underground. This physical challenge is often further complicated by the crossing of international borders introducing complex codes and standards of management. Coatings are essential to the protection of pipelines from corrosion and subsequent failure but are themselves subject to degradation by severe abrasion, hydrothermal aging and chemical degradation. These coating systems are typically considered to be passive or active. Passive systems prevent corrosion by blocking key elements of water, oxygen and salts from reaching the pipe surface. Cathodic protection systems (CP) are reactive systems designed to protect pipelines in the event of failure.

Graphene was first produced and identified in 2004 by the group of Andre Geim and Konstantin Novoselev at the University of Manchester, an event which was followed by the Nobel prize for Physics in 2010. One of the remarkable properties of graphene is its impermeability to gases. Graphene manufactured as a single monolayer is time consuming, expensive and difficult to scale. Graphene nanoplatelets (GNPs) offer a cheap and scalable alternative for use in barrier systems. Much research has been carried out on the implementation and use of graphene in coatings including those for pipelines. Direct application of GNP into epoxy has been discussed by Battocchi et al (1) who observed that low level additions of GNP offered improved barrier properties and corrosion mitigation together with improved abrasion resistance. Budd et al(2) applied GNP in laminate structures for flexible risers demonstrating the potential barrier properties of graphene in aggressive conditions. Applied Graphene Materials (AGM) GNPs are manufactured using the company’s patented proprietary “bottom up” process, yielding high specification graphene materials. AGM produce a range of GNP dispersions capable of easy addition into coating systems and have undertaken significant development activity to demonstrate their use in coating systems enabling improved in barrier performance and corrosion resistance.

Corrosion Testing

Current organic coating systems designed for protective coatings applied in harsh environments, such as bridges, are typically comprised of a number of different coating layer, each providing a different set of properties. A basic system usually consists of three layers, which may include a zinc rich primer coat offering sacrificial protection, an intermediate coat and a final topcoat for environmental protection. Typical dry film thicknesses of these coats is around 50 to 150 µm for the primer and intermediate coat and 50 µm for the top coat. Recently it has been demonstrated that GNPs, both as prepared and chemically functionalised, when incorporated into an organic coating system or host matrix, provide via a highly tortuous path which acts to impede the movement of corrosive species towards the metal surface (Okafor et al[3) ) creating a passive corrosion protection mechanism. In support of this, previous work by Choi et al (4) has also shown that very small additions of GNPs decreased water vapour transmission rates indicating a barrier type property, while some authors Aneja et al(5) also report an electrochemical activity provided by graphene within coatings. The introduction of GNPsinto the intermediate coat has recently been demonstrated by AGM(6) to increase significantly the impedance of a protective coating system as measured by EIS when studied in conjunction with Neutral Salt Spray testing (ASTM B117). The intermediate epoxy was formulated as shown below in Table 1.

Three different GNP-containing variants of the control were prepared (D1-D3) using the same initial preparation route as for the epoxy prototype base, by substituting commercially available GNPcontaining dispersion additives (formulation component 10) for epoxy in the final step (formulation component 9). The GNP dispersion additives were effectively treated as masterbatches, and were added in varying amounts according to their graphene content and the final GNP content specified in the end coating (Table 1). The dispersion used in the preparation of D1 and D3 contained a reduced graphene oxide type GNPs (A-GNP10). The dispersions used in the preparation of D2 contained GNPs of a ‘crumpled sheet’ type morphology with a relatively low density and high surface area (A-GNP35). In addition, dispersion D3 based on A-GNP10 contained an active corrosion inhibitor.

Prior to coating application, all substrates were degreased using acetone. Each first coat was applied to grit blasted mild steel CR4 grade panels (Impress North East Ltd.), of dimensions 150 x 100 x 2mm, by means of a gravity fed conventional spray gun. The over coating interval was 3 hours with all panels permitted a final curing period of 7 days at 23°C (+/-2°C). Dry film thickness of the prepared coatings were in the range of 50-60 microns for single coat samples and 150-160 microns for multi coat samples. Full details of the coating systems prepared can be seen in Table 2. All substrates were backed and edged prior to testing.

The panels were placed in a Neutral Salt Spray corrosion chamber, running ISO 9227 for a period of up to 1440 hours. This test method consists of a continuous salt spray mist at a temperature of 35°C. Panels were assessed at 10 day (240 hour intervals) for signs of blistering, corrosion, and corrosion creep in accordance with ISO4628. These assessments were complimented with electrochemical measurements, carried out at the same intervals. All electrochemical measurements were recorded using a Gamry 1000E potentiostat in conjunction with a Gamry ECM8 multiplexer to permit the concurrent testing of up to 8 samples per run. Each individual channel was connected to a Gamry PCT1 paint test cell, specifically designed for the electrochemical testing of coated metal substrates.

Figure 1 shows the progression of impedance modulus for the three coat system samples, measured at 0.1 Hz, over the time period during which the samples were subjected to NSS conditions. Initial impedance values (recorded at t=0) range from the orders of 108 to 1010 Ω.cm2 . The control sample, consisting of a zinc rich primer coat, a layer of commercial equivalent epoxy and polyurethane topcoat, displays the lowest overall impedance values in addition to one of the higher rates of decrease of impedance from the t=0 point. When GNPs are introduced to the intermediate layer, the impedance modulus is increased suggesting that the inclusion of GNPs is acting to increase the barrier performance properties of the system as a whole. The incorporation of A-GNP35 into D2 gave a final system uplift of 5 orders of magnitude above the control. Throughout the testing the D2 formulation showed little change in impedance, compared to the other samples. The achievement of >109 Ohm.cm2 @ 0.1Hz over a period of 1440 hours in neutral salt spray outperformed existing technology in barrier performance equating to a C5 high rating for salt spray performance according to ISO12944-1.

The choice of coating system for pipelines is typically influenced by the geographical region and is often made between thick or thin film build. Critical requirements of coatings in either case are:

• Excellent adhesion

• Low permeability

• Resistance to cathodic disbondment

• High electrical resistance

Thin build coating systems are typically based on Fusion Bonded Epoxy (FBE) either single or double layer being the preferred approach in the North American market. Alternatives might also include high build epoxy or polyurethane. Typically such thin build systems utilise an active CP system to provide additional corrosion protection. Graphene modification as shown by Battochi(1) and by AGM(6) might easily be incorporated into such epoxy or polyurethane systems through the use of AGM’s dispersions. The known electrical conductivity of Graphene might give cause for concern if the incorporation changes the insulating characteristics of the film. The GNP modification demonstrated by AGM is however substantially below the percolation threshold required for conductivity and the net impact on epoxy conductivity is considered negligible (Figure 2).

Thick build coating systems used in other parts of the world are typically 3 layer polyolefin (3LPO and might be polyethylene or polypropylene). AGM has experience in master-batching Graphene into thermoplastics and as such there is no obstacle to the introduction of GNPs into of the main body of the coating. GNP might also be introduced into the adhesive copolymer layer applied to the FBE typically used as a base for the 3LPO coating system.

Tags:  Andre Geim  Applied Graphene Materials  Coatings  Graphene  hydrothermal  Konstantin Novoselev  Nanoplatelets  Pipelines  Pipes  University of Manchester 

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Robust electrodes could pave the way to lighter electric vehicles

Posted By Graphene Council, The Graphene Council, Tuesday, December 3, 2019
One of the biggest remaining problems facing electric vehicles – whether they are road-going, waterborne or flying – is weight. Vehicles must carry their energy storage, and in the case of electric vehicles this inevitably means batteries.

No matter how many advances electrical engineers make in improving energy density, batteries remain dense and heavy components, and this is a drag on vehicle performance.

One approach to reducing the weight of electric vehicles might be to incorporate energy storage into the structure of the vehicle itself, thereby distributing the mass all over the vehicle and reducing the need for a single large battery or even eliminating it altogether.

The stumbling block to this approach is that materials that are good for energy storage and release tend to have properties that are not useful for structural applications: they are often brittle, which has obvious safety implications.

A team led by a Texas A&M University chemical engineer, Jodie Lutkenhaus, now claims to have made progress towards solving this problem using an approach inspired by brain chemistry and a trick employed by shellfish to stick themselves to rocks.

In a paper in the journal Matter, Lutkenhaus and her colleagues explain how their studies of redox active polymers for energy storage led them to investigate the properties of dopamine, most familiar as a signal-carrying molecule in the brain involved in movement, but also a very sticky substance that mimics proteins found in the material that mussels use to fasten themselves tightly to any surface underwater.

The team used dopamine to functionalise – that is, chemically bond to – graphene oxide, and then combine this material into a composite with aramid fibres, better known as Kevlar. This composite is both strong and tough, with a structure and properties similar to the famously tough natural material nacre or mother-of-pearl, and the graphene in its structure conveys both lightness and electrical properties that make it useful as an electrode.

The researchers describe using this material to form the electrodes for a super capacitor, a kind of energy storage device which can be charged and discharged very quickly.

The paper reports the highest ever multifunctional efficiency (a metric which evaluates material based on both its mechanical and electrochemical performance) for graphene-based materials.

Tags:  batteries  electric vehicle  energy storage  Graphene  graphene oxide  Jodie Lutkenhaus  Journal Matter  mimics proteins  polymers  super capacitor  Texas A&M University 

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Microcavities save organic semiconductors from going dark

Posted By Graphene Council, The Graphene Council, Monday, December 2, 2019
More and more electronics manufacturers are favoring organic LED displays for smartphones, TVs and computers because they are brighter and offer a greater color range.

The organic semiconductors that drive these devices are highly flexible and easily controlled. They also have the potential to be mass produced more readily than inorganic semiconductors such as silicon, which require higher temperatures for processing.

But there is a dark side to purely organic LEDs: They can be incredibly wasteful, losing up to 75% of their energy because organic semiconductors have a tendency to enter “dark states” in which they don’t emit light. These states sometimes even lead to the devices breaking down. Researchers have been looking for ways to either harness these dark states or jettison them altogether.

A collaboration led by Andrew Musser, assistant professor of chemistry and chemical biology in the College of Arts and Sciences, and Jenny Clark of the University of Sheffield, United Kingdom, has found a way to keep these organic semiconductors from going dark. Musserused tiny sandwich structures of mirrors, called microcavities, to trap light and force it to interact with a layer of molecules, forming a new hybrid state, known as a polariton, that mixes light and matter.This approach could lead to brighter, more efficient LEDs, sensors and solar cells.

The team’s paper, “Manipulating Molecules with Strong Coupling: Harvesting Triplet Excitons in Organic Exciton Microcavities,” published in Chemical Science.

“In the LED world, people are putting huge efforts into designing these vast libraries of molecules and testing them in different device configurations to see if, by tweaking the bonds or changing energy levels, they can harvest these dark states more efficiently,” Musser said. “It’s a cumbersome, difficult battle because it’s really hard to design molecules. And you don’t necessarily know how to make them do what you want.

“So what we’ve done here is address that problem with a standard molecule, purely by putting it between these mirrors and tuning the way it interacts with light,” he said. “This suggests that, for some phenomena, we can bypass a lot of this cumbersome synthetic exploration and tune the molecules at a distance.”

Musser’s interest in polaritons began while he was studying the ways organic semiconductors can improve light-harvesting efficiency in solar cells. In that case, molecules undergo a process called singlet fission, in which they absorb one photon and split that energy into two “packets” – essentially two excited electrons – thereby doubling the photon current efficiency in the solar cell.

Musser began investigating how the reverse process can also occur, with two packets of energy combining into a single, high-energy state that can emit a high-energy photon. That led him to microcavities and the ways these simple optical structures can have a profound effect on organic material through light.

In addition to manipulating a molecule’s electronic properties for enhanced brightness, recent research has demonstrated that these structures also can be used to target specific bonds and change their chemical reactivity.

Musser said different molecules interact with light in the microcavities in different ways, and further research is needed to explore the rules that underpin their behavior.

“Right now, it serves to show that when you have these complex materials and you do something even more complicated with them – putting them between these mirrors – weird and wonderful things can happen,” Musser said.

“This work literally sheds light on dark states,” said Clark. “We’ve shown that we can use polaritons to force dark states to emit light. Apart from immediate applications for LEDs, this offers a new method for studying organic semiconductors more broadly, using previously unavailable techniques.” 

Tags:  Andrew Musser  Chemical Science  energy  Graphene  Jenny Clark  LED  Semiconductor  silicon  smartphones  solar cell  U.S. Department of Energy  University of California  University of Cambridge  University of Kentucky  University of Sheffield 

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Growing nano-tailored surfaces using micellar brushes

Posted By Graphene Council, The Graphene Council, Monday, December 2, 2019

Growing nanoscale polymer brushes on materials' surfaces overcomes a key challenge in surface chemistry, researchers report, creating a new way to fabricate a diverse array of materials that could hold advanced uses in catalysis or chemical separation applications, for example. Their approach represents a crucial step forward in the search for simple and general techniques to create functional surfaces with tailor-made chemical properties, writes Alejandro Presa Soto in a related Perspective; "Pandora's box is now open, and the limits of this approach are only restricted by the imagination and skills of the scientific community." As technology advances, the ability to create advanced materials with specific surface properties and functionalities is becoming critically significant in a wide variety of areas including chemical engineering and biomedicine.

One recently developed approach for creating functionalized surfaces makes use of polymer chains, grafted to surfaces in brush-like patches. However limited, the method allows for tailoring of the surface chemistry at the molecular level. Similar approaches using nano- or micron-scale structures hold great promise for greatly expanded functionality and applications; however, the precise fabrication of these surfaces remains a prohibitive challenge. Jiandon Cai and colleagues address this by growing nanoscale micellar brushes directly on a material's surface. Cai et al. attached small crystalline micelle-seeds on a variety of surfaces, including silicon wafers, graphene oxide nanosheets and gold nanoparticles.

Unimers are used to initiate the crystallization-driven growth of well-defined cylindrical nanostructures over the seed-coated surface. The approach allows for the precise control over the density, length and chemistry of the micellar brushes, which can further be outfitted with other functional molecules and nanoparticles to enable a variety of catalysis and antibacterial and chemical separation applications.

Tags:  Alejandro Presa Soto  biomedicine  Graphene  Jiandon Cai  micellar brushes  nanoparticles  nanoscale  polymer  silicon wafers 

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