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A 3D camera for safer autonomy and advanced biomedical imaging

Posted By Graphene Council, Thursday, February 6, 2020
Researchers at the University of Michigan have proven the viability of a 3D camera that can provide high quality three-dimensional imaging while determining how far away objects are from the lens. This information is critical for 3D biological imaging, robotics, and autonomous driving.

Instead of using opaque photodetectors traditionally used in cameras, the proposed camera uses a stack of transparent photodetectors made from graphene to simultaneously capture and focus in on objects that are different distances from the camera lens.

The system works because of the unique traits of graphene, which is only one atomic layer thick and absorbs only about 2.3% of the light. A pair of graphene layers can be used to construct a photodetector that can efficiently detect light, even though less than 5% of the light is absorbed. When placed on a transparent substrate, instead of a silicon chip for example, the detectors can be stacked, with each one in a different focal plane.

As described by Prof. Ted Norris: “When you have a camera, you have to have a focusing adjustment on your lens so that when you’re focusing on a particular object like a person’s face, the rays of light that are coming from that person’s face are focused onto that single plane on your detector chip. Items in front or behind the object are out of focus.

But if it were possible to stack different detector arrays each in different focal planes, then they could each image accurately a different place in the object space simultaneously. What’s more, if you can detect multiple focal planes of data all at the same time, you can use algorithms to reconstruct the object in three dimensions. That is called a light field image. We have demonstrated how to use transparent focal stacks to do light field image and image reconstruction.”

In addition to basic object identification, the current paper shows how their device can detect how far away something is – making it suitable for applications in autonomous driving and robotics. It is also ideal for biological imaging in cases where it is important to image three-dimensional volume.

For its ultimate success, the project required complementary expertise in three areas. Prof. Zhaohui Zhong’s team developed the graphene devices; Norris’ group worked on the design features of the optical instrument and demonstrated the devices in the lab; and Prof. Jeff Fessler’s group, which developed the image reconstruction algorithm.

Fessler echoed the other faculty in stating the group of nine researchers consisting of faculty, postdocs and students “coalesced as a great team, all learning from each other and contributing different aspects of the final paper.”

Inspiration for the camera came from previous research of Zhong and Norris on highly sensitive graphene photo detectors, published in Nature Nanotechnology in 2014.

The current transparent graphene sensors fabricated so far are too low-resolution to depict images, but the initial experiments showed that the lens focused light from a different distance on each of the two sensors. Work is continuing on the project.

Tags:  biological imaging  Graphene  Medical  Robotics  Sensors  Ted Norris  University of Michigan 

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Origami-inspired robots that could fit in a cell

Posted By Graphene Council, Thursday, February 6, 2020

Imagine robots that can move, sense and respond to stimuli, but that are smaller than a hair’s width. This is the project that Cornell professor and biophysicist Itai Cohen, who gave a talk on Wednesday, January 29 as a part of Duke’s Physics Colloquium, has been working on with and his team. His project is inspired by the microscopic robots in Paul McEuen’s book Spiral. Building robots at such a small scale involves a lot more innovation than simply shrinking all of the parts of a normal robot. At low Reynolds number, fluids are viscous instead of inertial, Van der Waals forces come into play, as well as other factors that affect how the robot can move and function. 

To resolve this issue, Cohen and his team decided to build and pattern their micro robots in 2D. Then, inspired by origami, a computer would print the 2D pattern of a robot that can fold itself into a 3D structure. Because paper origami is scale invariant, mechanisms built at one scale will work at another, so the idea is to build robot patterns than can be printed and then walk off of the page or out of a petri dish.

However, as Cohen said in his talk last Wednesday, “an origami artist is only as good as their origami paper.” And to build robots at a microscopic scale, one would need some pretty thin paper. Cohen’s team uses graphene, a single sheet of which is only one atom thick. Atomic layer deposition films also behave very similarly to paper, and can be cut up, stretch locally and adopt a 3D shape. Some key steps to making sure the robot self-folds include making elements that bend, and putting additional stiff pads that localize bends in the pattern of the robot. This is what allows them to produce what they call “graphene bimorphs.”

Cohen and his team are looking to use microscopic robots in making artificial cilia, which are small leg-like protrusions in cells. Cilia can be sensory or used for locomotion. In the brain, there are cavities where neurotransmitters are redirected based on cilial beatings, so if one can control the individual beating of cilia, they can control where neurotransmitters are directed. This could potentially have biomedical implications for detecting and resolving neurological disorders. 

Right now, Cohen and his lab have microscopic robots made of graphene, which have photovoltaics attached to their legs. When a light shines on the photovoltaic receptor, it activates the robot’s arm movement, and it can wave hello. The advantage of using photovoltaics is that to control the robot, scientists can shine light instead of supplying voltage through a probe—the robot doesn’t need any tethers. During his presentation, Cohen showed the audience a video of his “Brobot,” a robot that flexes its arms when a light shines on it. His team has also successfully made microscopic robots with front and back legs that can walk off a petri dish. Their dimensions are 70 microns long, 40 microns wide and two microns thick. 

Cohen wants to think critically about what problems are important to use technology to solve; he wants make projects that can predict the behavior of people in crowds, predict the direction people will go in response to political issues, and help resolve water crises. Cohen’s research has the potential to find solutions for a wide variety of current issues. Using science fiction and origami as the inspiration for his projects reminds us that the ideas we dream of can become tangible realities.

Tags:  2D materials  Cornell University  Graphene  graphene bimorphs  Itai Cohen 

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New graphene amplifier has been able to unlock hidden frequencies in the electromagnetic spectrum

Posted By Graphene Council, Thursday, February 6, 2020
Researchers have created a unique device which will unlock the elusive terahertz wavelengths and make revolutionary new technologies possible.

Terahertz waves (THz) sit between microwaves and infrared in the light frequency spectrum, but due to their low-energy scientists have been unable to harness their potential. The conundrum is known in scientific circles as the terahertz gap.

Being able to detect and amplify THz waves (T-rays) would open up a new era of medical, communications, satellite, cosmological and other technologies.

One of the biggest applications would be as a safe, non-destructive alternative to X-rays. However, until now, the wavelengths – which range between 3mm and 30μm – have proved impossible to utilise due to relatively weak signals from all existing sources.

A team of physicists has created a new type of optical transistor – a working THz amplifier – using graphene and a high-temperature superconductor.

The physics behind the simple amplifier replies on the properties of graphene, which is transparent and is not sensitive to light and whose electrons have no mass. It is made up of two layers of graphene and a superconductor, which trap the graphene massless electrons between them, like a sandwich.

The device is then connected to a power source. When the THz radiation hits the graphene outer layer, the trapped particles inside attach themselves to the outgoing waves giving them more power and energy than they arrived with – amplifying them.

Professor Fedor Kusmartsev, of Loughborough’s Department of Physics, said: “The device has a very simple structure, consisting of two layers of graphene and superconductor, forming a sandwich. “As the THz light falls on the sandwich it is reflected, like a mirror. “The main point is that there will be more light reflected than fell on the device.

“It works because external energy is supplied by a battery or by light that hits the surface from other higher frequencies in the electromagnetic spectrum.

“The THz photons are transformed by the graphene into massless electrons, which, in turn, are transformed back into reflected, energised, THz photons.

“Due to such a transformation the THz photons take energy from the graphene – or from the battery – and the weak THz signals are amplified.”

The breakthrough – made by researchers from Loughborough University, in the UK; the Center for Theoretical Physics of Complex Systems, in Korea; the Micro/Nano Fabrication Laboratory Microsystem and THz Research Center, in China and the AV Rzhanov Institute of Semiconductor Physics, in Russia – has been published in Physical Review Letters, in the journal, American Physical Society (APS).

The team is continuing to develop the device and hopes to have prototypes ready for testing soon. Prof Kusmartsev said they hope to have a working amplifier ready for commercialisation in about a year. He added that such a device would vastly improve current technology and allow scientists to reveal more about the human brain.

“The Universe is full of terahertz radiation and signals, in fact, all biological organisms both absorb and emit it. “I expect, that with such an amplifier available we will be able to discover many mysteries of nature, for example, how chemical reactions and biological processes are going on or how our brain operates and how we think.

“The terahertz range is the last frequency of radiation to be adopted by humankind. “Microwaves, infrared, visible, X-rays and other bandwidths are vital for countless scientific and technological advancements.

“It has properties which would greatly improve vast areas of science such as imaging, spectroscopy, tomography, medical diagnosis, health monitoring, environmental control and chemical and biological identification.

“The device we have developed will allow scientists and engineers to harness the illusive bandwidth and create the next generation of medical equipment, detection hardware and wireless communication technology.”

Tags:  amplifier  Fedor Kusmartsev  Graphene  Healthcare  Loughborough University  Medical 

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Successful 60 tonne pilot flotation program supports Talga’s anode market development

Posted By Graphene Council, Friday, January 31, 2020

Battery anode and graphene additives provider Talga Resources Ltd is pleased to announce the successful completion of a 60 tonne pilot-scale processing program producing graphite concentrate - the feedstock for Talga’s planned European battery anode refinery and ongoing customer development programs.

The pilot processing program employed continuous test conditions for numerous key processing stages including crushing, grinding, flotation and concentration using advanced, industrial scale equipment at a Scandinavian toll milling and testing facility. The program achieved the Company’s targeted range of operational and product performance, in line with PFS assumptions, and demonstrated suitability of the process flowsheet for planned commercial production.

Talga Managing Director, Mr Mark Thompson: “This successful increase in processing scale is a positive milestone in progressing our plans for an integrated graphite mine and anode refinery in Sweden. The pilot scale program confirmed some key equipment requirements and production parameters, further improving our in-house processing knowledge and capability for future operations.”

In addition to further validating the first step of Talga’s processing flowsheet for its battery anode production, the pilot program generated information and samples for final detailed engineering design for ongoing DFS work and customer programs.

The process development and refinements of pilot-scale testing highlight the effectiveness of Talga’s preferred production process which uses large-scale European developed quality industrial equipment. Talga’s patent pending purification processes will be used in the downstream refining of the Vittangi graphite concentrate into the Company’s Talnode®-C anode product for use by Lithium-ion battery makers.

Background to scale-up program
Graphite anode is an advanced non-metal product that requires extensive physical validation by cell or battery manufacturers at increasing volumes prior to commercialisation. This is unlike most battery metals (such as lithium, copper or cobalt) that can be sold on a purity basis with little testing.

The addition of several automotive electric vehicle (EV) customers to Talga’s commercial register has driven the requirement to provide more advanced samples for testing, increased industry quality standards (high ISO standards and Six Sigma type quality operations) and most importantly, the need for samples to be sourced from larger production scale equipment.

The Company is currently investing in upscaled equipment and other components of the Talnode-C production supply chain at its European operations to meet ongoing customer qualification programs. Talga is also reviewing its options to more rapidly service the particular demands of automotive customers.

Pilot ore test program details
The pilot ore processing program was based around Talga’s unique and effective production flowsheet developed over several years from a comprehensive range of pilot and laboratory metallurgical programs.

These development programs have translated into a successful Scandinavian production schedule incorporating a 300 kg/hr pilot processing circuit, operating 24 hours per day continuously over 7 days at the facility, with a focus on developing and optimising the ore processing flowsheet.

The pilot operation included commissioning and process optimisation phases run over 15 days and testing the following key unit operations - crushing, milling, roughing, regrinding, cleaning, tailings dewatering and concentrate filtration.

Key areas of the process were tested under locked-cycle conditions simulating commercial plant operation with recycle of process streams allowing further investigation and optimisation of the process conditions, circuit configurations and operational practice.

Vital data was obtained, and targeted recoveries and product quality achieved while exploring a range of primary grinding settings, float cells residence times, regrinding energy and equipment configuration, tailings dewatering and concentrate filtration.

The results and samples from the pilot plant production will form the basis of core information in developing the graphite ore tolling process for near term ramp-up of customer sample production and guide the choice of process plant equipment for the full scale battery anode operation.

Representative samples were also taken during the pilot for further analysis and vendor trial work to support engineering and process validation activities as required.

The next stage of the anode scale-up program is currently underway, incorporating the refining of the concentrate into active anode for distribution to customers via several of Talga’s European process partner sites and at Talga’s German pilot processing facility.

Tags:  Battery  Graphene  Li-ion batteries  Mark Thompson  Talga Resources 

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First Graphene - Quarterly Activities Report

Posted By Graphene Council, Friday, January 31, 2020

First Graphene has continued to make substantial progress in its objective of commercialising the PureGRAPH® range of graphene products during the December quarter. PureGRAPH® has provided a range of improved performance characteristics in a number of products to which it is added.

The Company continues to maintain a strong working capital position, which will drive the growth of First Graphene with increased production efficiencies and higher manufacturing throughput, market development with new customers and novel graphene applications and global supply capabilities.

Steel Blue and First Graphene showcased safety boots at the Polymers in Footwear event in Berlin In November 2019 FGR and Steel Blue gave a joint presentation at the above exhibition and showcased PureGRAPH® enhanced safety boots. First Graphene Ltd and Steel Blue explained how PureGRAPH® had enabled the development of a range of unique boot component technologies and user benefits which have the potential to revolutionise the safety boot market.

The prototype boots have been manufactured using First Graphene’s PureGRAPH® graphene additives. Unlike competing formulations, this is available in high production volumes with non-aggregated, uniform-sized graphene; this ensures it disperses evenly in thermoplastic polyurethane (TPU) masterbatches.  The prototype boots incorporate PureGRAPH®-infused TPU soles and polyurethane foam innersoles and had undergone extensive laboratory testing in accredited laboratories.

Tests had been conducted at Viclab Pty Ltd, one of Australia’s leading NATA accredited and independent mechanical testing services. 

The prototype boots complied with the following test procedures;
Impact resistance
Upper to outsole bond
Interlayer bond strength
Slip resistance on ceramic tile floor with NaLS and on steel floor with glycerine
Sole crack resistance
Tear strength
Abrasion (TPU)
Tensile
Hydrolysis
Fuel oil resistance
Chemical exposure (NaOH)

Steel Blue have continued to test different applications and processes, with ongoing measuring of outcomes by Viclab Pty Ltd. The collaboration led to the successful manufacturing of prototypes, and these are currently being wear trialled in a variety of working environments.

The new boot from Steel Blue incorporates innovative technology with new design solutions to provide significant improvements to safety footwear protection and comfort.

Positive Interim Results from Mining Industry Field Trials
In December, FGR was pleased to announce an update on the Armour-GRAPH™ bucket liner provided by newGen to a major Pilbara iron ore producer.

newGen had provided an Armour-GRAPH™ bucket liner to a major iron ore  producer for trial which contained PureGRAPH®20. The bucket had been in use for in excess of 12 weeks at the time it was inspected by the client for assessment of wear, with very pleasing results. The next assessment is due to be undertaken in January.

Preparations were underway to trial PureGRAPH® enhanced materials in equipment used by a second iron ore producer.

Continued Growth in Customer Engagement
Customer growth continued apace during the December quarter, with engagements in the UK, Europe, Asia and Australia markets. The Company’s focus continues to be in the development of elastomers and plastics and the composites industries as it is these markets which have the potential to generate the largest volume of sales.

Activities at the GEIC, Manchester,UK
The Company continues to make effective use of it’s presence as a Tier 1 partner at the Graphene Engineering and Innovation Centre, where the facilities are routinely used to host customer visits and the capabilities deployed to prepare PureGRAPH® dispersions for a range of customer evaluations; from TPU elastomers to coating precursors. The UK team have developed a strong relationship with the academic departments with active relationships with the Chemistry, Civil Engineering, and Composites & Textiles departments.

2019 – A Year in Review

The 2019 calendar year has seen FGR make considerable progress as the world’s leading graphene company. The Company has continued to automate its  production process, maintain its high-quality standards and develop its in-house product development and testing platform.

Customer engagement has accelerated as a result of the marketing efforts and the release of the Company’s B2B focussed website.

There has been extensive field testing by customers of the PureGRAPH® range of products, all of which have been successful and resulted in ongoing collaboration. That this process has not manifested itself in more extensive sales is the one area of disappointment. However, as previously stated by Warwick “The larger the organisation the longer the time frame that would typically extend beyond 12 months. Offsetting this though is the prospect of larger size sales.” The Board of Directors and Senior Management team see 2020 as the year in which FGR will achieve significant sales growth, having built a very solid base during 2019.

Tags:  First Graphene  Graphene  newGen  polymers  Steel Blue  Viclab Pty Ltd 

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Two new FLAG-ERA projects in Aachen

Posted By Graphene Council, Friday, January 31, 2020

The Aachen Graphene & 2D Materials Center  has won two projects on basic research and innovation on graphene in the last FLAG-ERA Joint Transnational Call.

FLAG-ERA is a network of national and regional funding organizations in Europe that supports the two first FET Flagship projects of the European Commission: the Graphene Flagship and the Human Brain Project. On November 2018, FLAG-ERA announced its third Joint Transnational Call (FLAG-ERA JTC 2019), with an initial budget of 20 M€. This type of call presents a number of peculiarities. First, it funds only topics where synergies with the two Flagships are expected. Second, it funds only projects that involve partners form three or more different countries participating to the FLAG-ERA net. Third, while all projects are evaluated “centrally” by an independent evaluation panel, those recommended for funding are funded by the individual funding agencies − meaning that each partner of the project is funded by its national funding agency.

“It might seem a complicated way of financing research”, says Prof. Max Lemme from the chair of Electronic Devices at RWTH Aachen University, “but graphene is a topic that profits enormously from this kind of transnational collaborations.” Lemme is partner of the project 2D-NEMS, together with Prof. Christoph Stampfer − also at RWTH − and with colleagues from the Royal Institute of Technology in Sweden and from Graphenea Semiconductor in Spain.

The goal of the project is to explore the potential of heterostructures formed by graphene and other two-dimensional materials for realizing ultra small and ultra sensitive sensors, such as accelerometers. “We want to understand which combination of 2D-materials works better for a certain type of sensors and why”, says Lemme. “And, most importantly, we want to realize prototypes that are not only good for high-impact publications, but that can be of real interest for industry.”

Christoph Stampfer, head of II Institute of Physics A, is also involved in the FLAG-ERA project TATTOOS, together with colleagues from UC Louvain in Belgium and CNRS in Paris.  TATTOOS is a more exploratory project, dedicated to some of the most fascinating properties of bilayer graphene.

As the name says, bilayer graphene is a material formed by two layers of graphene. One of the big scientific surprises of 2018 was that for certain “magic angles” between the two layers  the system can exhibit superconductivity or other exotic properties. “In TATTOOS we’ll use a technique developed by our CNRS colleague, which should allow to rotate dynamically the angle between the layers with the tip of an atomic force microscope.”, explains Stampfer. “It’s a crazy idea! Typically, changing the angle requires making a new sample. If they hadn’t already demonstrated this approach on a similar system, I would not believe it can work. I’m really excited to see what new physics we can explore in this way.”

Lemme and Stampfer are both members of the Aachen Graphene and 2D Materials Center. “The fact that the Center is participating in two of the nine projects funded in the sub-call “Graphene – Basic Research and Innovation”, is a good example of the relevance of the research done here in Aachen”, says Stampfer, who is also the spokesperson of the Center.

Tags:  2D materials  Christoph Stampfer  Graphene  Graphene Flagship  Graphenea  Max Lemme  RWTH Aachen University  Semiconductor 

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What a pair! Coupled quantum dots may offer a new way to store quantum information

Posted By Graphene Council, Friday, January 31, 2020
Researchers at the National Institute of Standards and Technology (NIST) and their colleagues have for the first time created and imaged a novel pair of quantum dots -- tiny islands of confined electric charge that act like interacting artificial atoms. Such "coupled" quantum dots could serve as a robust quantum bit, or qubit, the fundamental unit of information for a quantum computer. Moreover, the patterns of electric charge in the island can't be fully explained by current models of quantum physics, offering an opportunity to investigate rich new physical phenomena in materials.

Unlike a classical computer, which relies on binary bits that have just one of two fixed values -- "1" or "0" -- to store memory, a quantum computer would store and process information in qubits, which can simultaneously take on a multitude of values. Therefore, they could perform much larger, more complex operations than classical bits and have the potential to revolutionize computing.

Electrons orbit the center of a single quantum dot similar to the way they orbit atoms. The charged particles can only occupy specific permitted energy levels. At each energy level, an electron can occupy a range of possible positions in the dot, tracing out an orbit whose shape is determined by the rules of quantum theory. A pair of coupled quantum dots can share an electron between them, forming a qubit.

To fabricate the quantum dots, the NIST-led team, which included researchers from the University of Maryland NanoCenter and the National Institute for Materials Science in Japan, used the ultrasharp tip of a scanning tunneling microscope (STM) as if it were a stylus of an Etch A Sketch. Hovering the tip above an ultracold sheet of graphene (a single layer of carbon atoms arranged in a honeycomb pattern), the researchers briefly increased the voltage of the tip.

The electric field generated by the voltage pulse penetrated through the graphene into an underlying layer of boron nitride, where it stripped electrons from atomic impurities in the layer and created a pileup of electric charge. The pileup corralled freely floating electrons in the graphene, confining them to a tiny energy well.

But when the team applied a magnetic field of 4 to 8 tesla (about 400 to 800 times the strength of a small bar magnet), it dramatically altered the shape and distribution of the orbits that the electrons could occupy. Rather than a single well, the electrons now resided within two sets of concentric, closely spaced rings within the original well separated by a small empty shell. The two sets of rings for the electrons now behaved as if they were weakly coupled quantum dots.

This is the first time that researchers have probed the interior of a coupled quantum dot system so deeply, imaging the distribution of electrons with atomic resolution (see illustration), noted NIST co-author Daniel Walkup. To take high-resolution images and spectra of the system, the team took advantage of a special relationship between the size of a quantum dot and the spacing of the energy levels occupied by the orbiting electrons: The smaller the dot, the greater the spacing, and the easier it is to distinguish between adjacent energy levels.

In a previous quantum dot study using graphene, the team applied a smaller magnetic field and found a structure of rings, resembling a wedding cake, centered on a single quantum dot, which is the origin of the concentric quantum dot rings. By using the STM tip to construct dots about half the diameter (100 nanometers) of dots that they had previously studied, the researchers succeeded in revealing the full structure of the coupled system.

The team, which included Walkup, Fereshte Ghahari, Christopher Gutiérrez and Joseph Stroscio at NIST and the Maryland NanoCenter, describes its findings today in Physical Review B.

The way in which the electrons are shared between the two coupled dots can't be explained by accepted models of quantum dot physics, said Walkup. This puzzle may be important to solve if coupled quantum dots are eventually to be used as qubits in quantum computing, Stroscio noted.

Tags:  boron nitride  Daniel Walkup  Graphene  National Institute of Standards and Technology  quantum dots 

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How tiny misalignments in encapsulated graphene lead to a strong modification of its electronic properties

Posted By Graphene Council, Friday, January 31, 2020
Researchers at the University of Antwerp explain how higher order supermoiré periodic modulations due to the encapsulation of graphene between hexagonal boron nitride affect the electronic and structural properties of graphene, as revealed in three recent independent experiments.

High quality graphene samples are of high importance for obtaining and exploiting its theoretically described properties. Utilizing an adequate substrate reduces the corrugation and improves otherwise disorder limited properties of graphene.

Hexagonal boron nitride (hBN) is a particularly good choice, since it preserves perfectly the graphene structure, while providing a flat insulating surface. Still, this applies only if the two monolayers are misaligned. Otherwise, the van der Waals interaction induces structural relaxation on the scale of the moiré pattern formed between the two layers and modifies the electronic properties due to the periodic moiré perturbation.

Similar arguments apply if graphene is encapsulated and closely aligned to two hBN layers. In this case the effect is enhanced since both layers are expected to contribute. Furthermore, close alignment, on the order of 0.5 degrees, between the layers is responsible for the appearance of a new form of periodic supermoiré modulation, which alters graphene on a larger spatial scale, but smaller energy scale.

Recent experimental observation of such effects are a consequence of significant improvements in the experimental manipulation techniques, and among others, the possibility to rotate individual layers with high precision (Wang et al. 2019a; Wang et al. 2019b; Finney et al. 2019 – see references at the end of this article).

In their recent paper published in Nano Letters ("Double moiré with a twist: supermoiré in encapsulated graphene"), Anđelković et al. reveal under which condition the supermoiré effect appears, and how it alters the structural and electronic properties of graphene.

They show, starting from a rigid hBN/graphene/hBN heterostructure, how the supermoiré appears as a simple geometrical consideration. Furthermore, they prove that relaxation effects in the three layers are expected to enhance the effects on the electronic band structure. The supermoiré induced modifications are significant: new low energy flat sub-bands and Dirac points appear, with strong effect on electronic transport properties. In most configurations the Dirac points are gapped, while flat bands are expected to enhance electron-electron correlations.

"These new twisting degrees of freedom in heterostructures are opening up new fundamental research directions in graphene, where strong electronic correlations are expected to complement the already superlative properties of graphene," said Dr. Lucian Covaci.

"The set of multi-scale numerical simulations developed by the University of Antwerp team allows for more realistic models, which will in turn allow for a more direct comparison with experimental observations," said Dr. Miša Anđelković, a co-developer of Pybinding, the tight-binding open source software that made the simulations possible.

With a new light shed on the understanding of more complex and interfering behaviour of van der Waals heterostructures it is possible to finely tune graphene’s electronic properties and reach regimes where twist induced phenomena, such as flat bands or the appearance of mini-gaps, reveal themselves more clearly.
 

Tags:  Graphene  hexagonal boron nitride  Lucian Covaci  Miša Anđelković  University of Antwerp 

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Won’t crack under pressure: stress test reveals graphene can withstand more than one billion cycles before breaking

Posted By Graphene Council, Thursday, January 30, 2020
Graphene is a paradox: it is the thinnest material known to science, yet also one of the strongest. Now, research from U of T Engineering shows that graphene is also highly resistant to fatigue — able to withstand more than a billion cycles of high stress before it breaks.

Graphene resembles a sheet of interlocking hexagonal rings, similar to the pattern you might see in bathroom flooring tiles. At each corner is a single carbon atom bonded to its three nearest neighbours. While the sheet could extend laterally over any area, it is only one atom thick.

The intrinsic strength of graphene has been measured at more than 100 gigapascals, among the highest values recorded for any material. But materials don’t always fail because the load exceeds their maximum strength. Small repetitive stresses can weaken materials by causing microscopic dislocations and fractures that slowly accumulate over time, a process known as fatigue.

“To understand fatigue, imagine bending a metal spoon,” says Professor Tobin Filleter (MIE), one of the senior authors of the study, which was recently published in Nature Materials. “The first time you bend it, it just deforms. But if you keep working it back and forth, eventually it’s going to break in two.”

The research team — consisting of Filleter, fellow U of T Engineering professors Chandra Veer Singh (MSE) and Yu Sun (MIE), their students, and collaborators at Rice University — wanted to know how graphene would stand up to repeated stresses. Their approach included both physical experiments and computer simulations.

“In our atomistic simulations, we found that cyclic loading can lead to irreversible bond reconfigurations in the graphene lattice, causing catastrophic failure on subsequent loading,” says Singh, who along with postdoctoral fellow Sankha Mukherjee (MSE) led the modelling portion of the study. “This is unusual behaviour in that while the bonds change, there are no obvious cracks or dislocations, which would usually form in metals, until the moment of failure.”

PhD candidate Teng Cui, who is co-supervised by Filleter and Sun, used the Toronto Nanofabrication Centre to build a physical device for the experiments. The design consisted of a silicon chip etched with half a million tiny holes only a few micrometres in diameter. The graphene sheet was stretched over these holes, like the head of a tiny drum.

Using an atomic force microscope, Cui then lowered a diamond-tipped probe into the hole to push on the graphene sheet, applying anywhere from 20 to 85 per cent of the force that he knew would break the material.

“We ran the cycles at a rate of 100,000 times per second,” says Cui. “Even at 70 per cent of the maximum stress, the graphene didn’t break for more than three hours, which works out to over a billion cycles. At lower stress levels, some of our trials ran for more than 17 hours.”

As with the simulations, the graphene didn’t accumulate cracks or other tell-tale signs of stress — it either broke or it didn’t.

“Unlike metals, there is no progressive damage during fatigue loading of graphene,” says Sun. “Its failure is global and catastrophic, confirming simulation results.”

The team also tested a related material, graphene oxide, which has small groups of atoms such as oxygen and hydrogen bonded to both the top and bottom of the sheet. Its fatigue behaviour was more like traditional materials, in that the failure was more progressive and localized. This suggests that the simple, regular structure of graphene is a major contributor to its unique properties.

“There are no other materials that have been studied under fatigue conditions that behave the way graphene does,” says Filleter. “We’re still working on some new theories to try and understand this.”

In terms of commercial applications, Filleter says that graphene-containing composites — mixtures of conventional plastic and graphene — are already being produced and used in sports equipment such as tennis rackets and skis.

In the future, such materials may begin to be used in cars or in aircraft, where the emphasis on light and strong materials is driven by the need to reduce weight, improve fuel efficiency and enhance environmental performance.

“There have been some studies to suggest that graphene-containing composites offer improved resistance to fatigue, but until now, nobody had measured the fatigue behaviour of the underlying material,” he says. “Our goal in doing this was to get at that fundamental understanding so that in the future, we’ll be able to design composites that work even better.”

Tags:  Graphene  Graphene Composites  nanofabrication  Rice University  Teng Cui  Tobin Filleter  University of Toronto Engineering 

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Method detects defects in 2D materials for future electronics, sensors

Posted By Graphene Council, Tuesday, January 28, 2020
Two-dimensional materials are atomically thin, the most well-known being graphene, a single-atom-thick layer of carbon atoms. "People have struggled to make these 2D materials without defects," said Mauricio Terrones, Verne M. Willaman Professor of Physics, Penn State. "That's the ultimate goal. We want to have a 2D material on a four-inch wafer with at least an acceptable number of defects, but you want to evaluate it in a quick way."

The researchers' -- who represent Penn State, Northeastern University, Rice University and Universidade Federal de Minas Gerais in Brazil -- solution is to use laser light combined with second harmonic generation, a phenomenon in which the frequency of the light shone on the material reflects at double the original frequency. They add dark field imaging, a technique in which extraneous light is filtered out so that defects shine through. According to the researchers, this is the first instance in which dark field imaging was used, and it provides three times the brightness of the standard bright field imaging method, making it possible to see types of defects previously invisible.

"The localization and identification of defects with the commonly used bright field second harmonic generation is limited because of interference effects between different grains of 2D materials," said Leandro Mallard, a senior author on a recent paper in Nano Letters and a professor at Universidade Federal de Minas Gerais. "In this work we have shown that by the use of dark field SHG we remove the interference effects and reveal the grain boundaries and edges of semiconducting 2D materials. Such a novel technique has good spatial resolution and can image large area samples that could be used to monitor the quality of the material produced in industrial scales."

Vincent H. Crespi, Distinguished Professor of Physics, Materials Science and Engineering, and Chemistry, Penn State, added, "Crystals are made of atoms, and so the defects within crystals -- where atoms are misplaced -- are also of atomic size.

"Usually, powerful, expensive and slow experimental probes that do microscopy using beams of electrons are needed to discern such fine details in a material," said Crespi. "Here, we use a fast and accessible optical method that pulls out just the signal that originates from the defect itself to rapidly and reliably find out how 2D materials are stitched together out of grains oriented in different ways."

Another coauthor compared the technique to finding a particular zero on a page full of zeroes. "In the dark field, all the zeroes are made invisible so that only the defective zero stands out," said Yuanxi Wang, assistant research professor at Penn State's Materials Research Institute.

The semiconductor industry wants to have the ability to check for defects on the production line, but 2D materials will likely be used in sensors before they are used in electronics, according to Terrones. Because 2D materials are flexible and can be incorporated into very small spaces, they are good candidates for multiple sensors in a smartwatch or smartphone and the myriad of other places where small, flexible sensors are required.

"The next step would be an improvement of the experimental setup to map zero dimension defects -- atomic vacancies for instance -- and also extend it to other 2D materials that host different electronic and structural properties," said lead author Bruno Carvalho, a former visiting scholar in Terrones' group.

Tags:  2D materials  Graphene  Leandro Mallard  Mauricio Terrones  Northeastern University  Penn State  Rice University  Universidade Federal de Minas Gerais in Brazil  Vincent H. Crespi  Yuanxi Wang 

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