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Paragraf and NPL Demonstrate that Paragraf’s Graphene Hall Effect Sensors are ready for High-radiation Applications in Space and Beyond

Posted By Graphene Council, 21 hours ago
Paragraf, the leader in graphene-based transformative electronic sensors and devices, has demonstrated the ability of its graphene Hall Effect sensors to withstand high levels of radiation. The discovery, based on testing from the National Physical Laboratory (NPL), proves that ‘unpackaged’ Hall Effect sensors can be used in high-radiation environments such as space. The project was funded by Innovate UK, the UK’s innovation agency.

Used to measure the magnitude of magnetic fields, Hall Effect sensors are a critical electronic component in a variety of applications, from proximity sensing and speed detection through to current sensing. However, historically, their deployment in high-radiation environments such as satellites and nuclear power plants has faced significant challenges. This is because conventional sensors made from silicon and other semiconductor materials react adversely to neutron radiation, unless they are encapsulated in radiation-hardened packaging. This entails a more complex, lengthy, and costly manufacturing process and may require the sensor to be replaced over time if, for example, the packaging is damaged.

By contrast, tests conducted by NPL have shown that following exposure to a neutron dose of 241 mSv/hour – which is about 30,000 times the expected typical neutron dose rate in the International Space Station – Paragraf graphene Hall Effect sensors are not affected by this level of radiation. This is the first time that a commercially available, graphene-based electronic device has proved impervious to neutron irradiation.

In situations where power and weight savings are as critical as radiation tolerance, for example on satellites and other space vehicles, Paragraf Hall Effect sensors really come into their own – requiring only pW’s of power and weighing only fractions of a gram.

Ivor Guiney, co-founder of Paragraf, commented: “NPL’s findings have the potential to be a game changer when it comes to high-performance satellites and other critical high-radiation applications such as nuclear decommissioning. Owing to the exceptional mechanical strength and high transparency of graphene, our Hall Effect sensor can be used reliably in high-radiation applications without requiring packaging. This is key to improving reliability and durability while reducing manufacturing costs and time to market.”

The ability of graphene Hall Effect sensors to perform under high-radiation conditions will pave the way for the deployment of a broader range of electronics in harsh environments. Thanks to Paragraf’s scalable manufacturing process for large-area graphene deposition, it may soon be possible to produce other radiation-resistant graphene-based electronic devices. This will help ensure that all critical electronics, beyond sensors, are reliable and durable even in harsh environments.

Héctor Corte-Leon at NPL added: “Our first set of findings is very promising, and we are now expecting more positive outcomes over the next few months. Testing graphene-based electronics is key to demonstrating whether they can be used in harsh environments where, traditionally, their deployment has been limited.”

Graphene Hall Effect sensors from Paragraf are now set to undergo further radiation testing (alpha, beta and gamma radiation) as well as high-frequency testing. This is expected to open-up new opportunities across critical applications such as current sensing. The project, funded by Innovate UK, the UK’s innovation agency, started in October 2019, and is due to run until the end of 2020.

Tags:  Electronics  Graphene  Héctor Corte-Leon  Innovate UK  Ivor Guiney  National Physical Laboratory  Paragraf  Sensors 

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WHAT HAPPENS BETWEEN THE SHEETS? ‘FLOATING’ GRAPHENE ON A BED OF CALCIUM ATOMS

Posted By Graphene Council, Saturday, September 19, 2020
Adding calcium to graphene creates an extremely-promising superconductor, but where does the calcium go?

Adding calcium to a composite graphene-substrate structure creates a high transition-temperature (Tc) superconductor.

In a new study, an Australian-led team has for the first time confirmed what actually happens to those calcium atoms: surprising everyone, the calcium goes underneath both the upper graphene sheet and a lower ‘buffer’ sheet, ‘floating’ the graphene on a bed of calcium atoms.

Superconducting calcium-injected graphene holds great promise for energy-efficient electronics and transparent electronics.

STUDYING CALCIUM-DOPED GRAPHENE: THROWING OFF THE DUVET

Graphene’s properties can be fine-tuned by injection of another material (a process known as ‘intercalation’) either underneath the graphene, or between two graphene sheets.

This injection of foreign atoms or molecules alters the electronic properties of the graphene by either increasing its conductance, decreasing interactions with the substrate, or both.

Injecting calcium into graphite creates a composite material (calcium-intercalated graphite, CaC6) with a relatively ‘high’ superconducting transition temperature (Tc). In this case, the calcium atoms ultimately reside between graphene sheets.

Injecting calcium into graphene on a silicon-carbide substrate also creates a high-Tc superconductor, and we always thought we knew where the calcium went in this case too…

Graphene on silicon-carbide has two layers of carbon atoms: one graphene layer on top of another ‘buffer layer’: a carbon layer (graphene-like in structure) that forms between the graphene and the silicon-carbide substrate during synthesis, and is non-conducting due to being partially bonded to the substrate surface.

“Imagine the silicon carbide is like a mattress with a fitted sheet (the buffer layer bonded to it) and a flat sheet (the graphene),” explains lead author Jimmy Kotsakidis.

Conventional wisdom held that calcium should inject between the two carbon layers (between two sheets), similar to injection between the graphene layers in graphite. Surprisingly, the Monash University-led team found that when injected, the calcium atoms’ final destination location instead lies between buffer layer and the underlying silicon-carbide substrate (between the fitted sheet and the mattress!).

“It was quite a surprise to us when we realised that the calcium was bonding to the silicon surface of the substrate, it really went against what we thought would happen”, explains Kotsakidis.

Upon injection, the calcium breaks the bonds between the buffer layer and substrate surface, thus, causing the buffer layer to ‘float’ above the substrate, creating a new, quasi-freestanding bilayer graphene structure (Ca-QFSBLG).

This result was unanticipated, with extensive previous studies not considering calcium intercalation underneath the buffer layer. The study thus resolves long-standing confusion and controversy regarding the position of the intercalated calcium.

X-ray photoelectron spectroscopy (XPS) measurements at the Australian Synchrotron were able to pinpoint the location of the calcium near to the silicon carbide surface

Results were also supported by low-energy electron diffraction (LEED), and scanning tunnelling microscopy (STM) measurements, and by modelling using density functional theory (DFT).

AND MAGNESIUM TOO…
With this information at hand, the Australian team also decided to investigate if magnesium–which is notoriously difficult to inject into the graphite structure –could be inserted (intercalated) into graphene on a silicon-carbide substrate.

To the researchers’ surprise, magnesium behaved remarkably similarly to calcium, and also injected between the graphene and substrate, again ‘floating’ the graphene.

Both magnesium- and calcium-intercalated graphene n-type doped the graphene, and resulted in a low workfunction graphene, an attractive aspect when using graphene as a conducting electrical contact for other materials.

But unlike calcium, magnesium-intercalated graphene remained stable in ambient atmosphere for at least 6 hours, overcoming a major technical hurdle for alkali and alkaline earth intercalated graphene.

“The fact that Mg-QFSBLG is a low workfunction material and n-type dopes the graphene while remaining quite stable in ambient atmosphere is a huge step in the right direction for implementing these novel intercalated materials in technological applications,” explains co-author Prof Michael Fuhrer.

“Magnesium-intercalated graphene could be a stepping stone towards discovery of other similarly stable intercalants.”

Tags:  Electronics  Graphene  Graphite  Jimmy Kotsakidis  Monash University  superconductor 

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Team’s Flexible Micro LEDs May Reshape Future of Wearable Technology

Posted By Graphene Council, Monday, August 31, 2020
University of Texas at Dallas researchers and their international colleagues have developed a method to create micro LEDs that can be folded, twisted, cut and stuck to different surfaces.

The research, published online in June in the journal Science Advances, helps pave the way for the next generation of flexible, wearable technology.

Used in products ranging from brake lights to billboards, LEDs are ideal components for backlighting and displays in electronic devices because they are lightweight, thin, energy efficient and visible in different types of lighting. Micro LEDs, which can be as small as 2 micrometers and bundled to be any size, provide higher resolution than other LEDs. Their size makes them a good fit for small devices such as smart watches, but they can be bundled to work in flat-screen TVs and other larger displays. LEDs of all sizes, however, are brittle and typically can only be used on flat surfaces.

The researchers’ new micro LEDs aim to fill a demand for bendable, wearable electronics.

“The biggest benefit of this research is that we have created a detachable LED that can be attached to almost anything,” said Dr. Moon Kim, Louis Beecherl Jr. Distinguished Professor of materials science and engineering at UT Dallas and a corresponding author of the study. “You can transfer it onto your clothing or even rubber — that was the main idea. It can survive even if you wrinkle it. If you cut it, you can use half of the LED.”

Researchers in the Erik Jonsson School of Engineering and Computer Science and the School of Natural Sciences and Mathematics helped develop the flexible LED through a technique called remote epitaxy, which involves growing a thin layer of LED crystals on the surface of a sapphire crystal wafer, or substrate.

Typically, the LED would remain on the wafer. To make it detachable, researchers added a nonstick layer to the substrate, which acts similarly to the way parchment paper protects a baking sheet and allows for the easy removal of cookies, for instance. The added layer, made of a one-atom-thick sheet of carbon called graphene, prevents the new layer of LED crystals from sticking to the wafer.

“The biggest benefit of this research is that we have created a detachable LED that can be attached to almost anything. You can transfer it onto your clothing or even rubber — that was the main idea. It can survive even if you wrinkle it. If you cut it, you can use half of the LED.”

Dr. Moon Kim, Louis Beecherl Jr. Distinguished Professor of materials science and engineering at UT Dallas

“The graphene does not form chemical bonds with the LED material, so it adds a layer that allows us to peel the LEDs from the wafer and stick them to any surface,” said Kim, who oversaw the physical analysis of the LEDs using an atomic resolution scanning/transmission electron microscope at UT Dallas’ Nano Characterization Facility.

Colleagues in South Korea carried out laboratory tests of LEDs by adhering them to curved surfaces, as well as to materials that were subsequently twisted, bent and crumpled. In another demonstration, they adhered an LED to the legs of a Lego minifigure with different leg positions.

Bending and cutting do not affect the quality or electronic properties of the LED, Kim said.

The bendy LEDs have a variety of possible uses, including flexible lighting, clothing and wearable biomedical devices. From a manufacturing perspective, the fabrication technique offers another advantage: Because the LED can be removed without breaking the underlying wafer substrate, the wafer can be used repeatedly.

“You can use one substrate many times, and it will have the same functionality,” Kim said.

In ongoing studies, the researchers also are applying the fabrication technique to other types of materials.

“It’s very exciting; this method is not limited to one type of material,” Kim said. “It’s open to all kinds of materials.”

Other UT Dallas researchers involved in the study included Dr. Anvar Zakhidov, professor of physics; and Qingxiao Wang and Sunah Kwon, doctoral students and research assistants in materials science and engineering.

Other authors of the study were affiliated with Los Alamos National Laboratory, as well as organizations in South Korea, including Sejong University, Ewha Womans University, Korea Electronics Technology Institute, CoAsia Corp., Pohang University of Science and Technology, and Korea University. The research was funded in part by the National Research Foundation of Korea, the Korea Institute for Advanced Technology and the U.S. Department of Energy.

Tags:  Electronics  Graphene  LED  Los Alamos National Laboratory  Moon Kim  University of Texas at Dallas 

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Stack and twist: physicists accelerate the hunt for revolutionary new materials

Posted By Graphene Council, Friday, August 14, 2020
Scientists at the University of Bath have taken an important step towards understanding the interaction between layers of atomically thin materials arranged in stacks. They hope their research will speed up the discovery of new, artificial materials, leading to the design of electronic components that are far tinier and more efficient than anything known today.

Smaller is always better in the world of electronic circuitry, but there’s a limit to how far you can shrink a silicon component without it overheating and falling apart, and we’re close to reaching it. The researchers are investigating a group of atomically thin materials that can be assembled into stacks. The properties of any final material depend both on the choice of raw materials and on the angle at which one layer is arranged on top of another.

Dr Marcin Mucha-Kruczynski who led the research from the Department of Physics, said: “We’ve found a way to determine how strongly atoms in different layers of a stack are coupled to each other, and we’ve demonstrated the application of our idea to a structure made of graphene layers.”

The Bath research, published in Nature Communications, is based on earlier work into graphene – a crystal characterised by thin sheets of carbon atoms arranged in a honeycomb design. In 2018, scientists at the Massachusetts Institute of Technology (MIT) found that when two layers of graphene are stacked and then twisted relative to each other by the ‘magic’ angle of 1.1°, they produce a material with superconductive properties. This was the first time scientists had created a super-conducting material made purely from carbon. However, these properties disappeared with the smallest change of angle between the two layers of graphene.

Since the MIT discovery, scientists around the world have been attempting to apply this ‘stacking and twisting’ phenomenon to other ultra-thin materials, placing together two or more atomically different structures in the hope of forming entirely new materials with special qualities.

“In nature, you can’t find materials where each atomic layer is different,” said Dr Mucha-Kruczynski. “What’s more, two materials can normally only be put together in one specific fashion because chemical bonds need to form between layers. But for materials like graphene, only the chemical bonds between atoms on the same plane are strong. The forces between planes – known as van der Waals interactions – are weak, and this allows for layers of material to be twisted with respect to each other.”

The challenge for scientists now is to make the process of discovering new, layered materials as efficient as possible. By finding a formula that allows them to predict the outcome when two or more materials are stacked, they will be able to streamline their research enormously.

It is in this area that Dr Mucha-Kruczynski and his collaborators at the University of Oxford, Peking University and ELETTRA Synchrotron in Italy expect to make a difference.

“The number of combinations of materials and the number of angles at which they can be twisted is too large to try out in the lab, so what we can predict is important,” said Dr Mucha-Kruczynski.

The researchers have shown that the interaction between two layers can be determined by studying a three-layer structure where two layers are assembled as you might find in nature, while the third is twisted. They used angle-resolved photoemission spectroscopy – a process in which powerful light ejects electrons from the sample so that the energy and momentum from the electrons can be measured, thus providing insight into properties of the material – to determine how strongly two carbon atoms at a given distance from each other are coupled. They have also demonstrated that their result can be used to predict properties of other stacks made of the same layers, even if the twists between layers are different.

The list of known atomically thin materials like graphene is growing all the time. It already includes dozens of entries displaying a vast range of properties, from insulation to superconductivity, transparency to optical activity, brittleness to flexibility. The latest discovery provides a method for experimentally determining the interaction between layers of any of these materials. This is essential for predicting the properties of more complicated stacks and for the efficient design of new devices.

Dr Mucha-Kruczynski believes it could be 10 years before new stacked and twisted materials find a practical, everyday application. “It took a decade for graphene to move from the laboratory to something useful in the usual sense, so with a hint of optimism, I expect a similar timeline to apply to new materials,” he said.

Building on the results of his latest study, Dr Mucha-Kruczynski and his team are now focusing on twisted stacks made from layers of transition metal dichalcogenides (a large group of materials featuring two very different types of atoms – a metal and a chalcogen, such as sulphur). Some of these stacks have shown fascinating electronic behaviour which the scientists are not yet able to explain.

“Because we’re dealing with two radically different materials, studying these stacks is complicated,” explained Dr Mucha-Kruczynski. “However, we're hopeful that in time we'll be able to predict the properties of various stacks, and design new multifunctional materials.”

Tags:  Electronics  Graphene  Marcin Mucha-Kruczynski  Massachusetts Institute of Technology  University of Bath 

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Discovery of massless electrons in phase-change materials provides next step for future electronics

Posted By Graphene Council, Friday, August 14, 2020
Researchers have found electrons that behave as if they have no mass, called Dirac electrons, in a compound used in rewritable discs, such as CDs and DVDs. The discovery of "massless" electrons in this phase-change material could lead to faster electronic devices.

The international team published their results on July 6 in ACS Nano, a journal of the American Chemical Society.

The compound, GeSb2Te4, is a phase-change material, meaning its atomic structure shifts from amorphous to crystalline under heat. Each structure has individual properties and is reversible, making the compound an ideal material to use in electronic devices where information can be written and rewritten several times.

"Phase-change materials have attracted a great deal of attention owing to the sharp contrast in optical and electrical properties between their two phases," said paper author Akio Kimura, professor in the Department of Physical Sciences in the Graduate School of Science and the Graduate School of Advanced Science and Engineering at Hiroshima University. "The electronic structure in the amorphous phase has already been addressed, but the experimental study of the electronic structure in the crystalline phase had not yet been investigated."

The researchers found that the crystalline phase of GeSb2Te4 has Dirac electrons, meaning it behaves similarly to graphene, a conducting material that consists of a single layer of carbon atoms. They also found that the surface of the crystalline structure shares characteristics with a topological insulator, where the internal structure remains static while the surface conducts electrical activity.

"The amorphous phase shows a semiconducting behavior with a large electrical resistivity while the crystalline phase behaves like a metallic with a much lower electrical resistivity," said Munisa Nurmamat, paper author and assistant professor in the Department of Physical Sciences in the Graduate School of Science and the Graduate School of Advanced Science and Engineering at Hiroshima University. "The crystalline phase of GeSb2Te4 can be viewed as a 3D analogue of graphene."

Graphene is already considered by researchers to be a high-speed conducting material, according to Nurmamat and Kimura, but its inherently low on- and off-current ratio limits how it is applied in electronic devices. As a 3D version of graphene, GeSb2Te4 combines speed with flexibility to engineer the next generation of electrical switching devices.

Tags:  Akio Kimura  Electronics  Graphene  Hiroshima University 

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Nanosys and Shoei Chemical Announce Quantum Dot Supply Agreement to Serve Rapidly Growing Quantum Dot Display Market

Posted By Graphene Council, Monday, August 10, 2020
Nanosys and Shoei Chemical today announced that the companies have entered into a Quantum Dot materials supply and service agreement. Under the agreement, Shoei Chemical will manufacture Nanosys’ proprietary Quantum Dot materials exclusively for Nanosys.

Shoei Chemical is now the sole manufacturing partner for capacity expansion of Quantum Dots and heavy metal free Quantum Dots. The symbiotic relationship enables Nanosys to more than double production volumes in the near term, and lower costs by producing closer to its customers in Asia. This new partnership ensures uninterrupted supply of Quantum Dot materials, due to geographic separation of manufacturing locations, which has become a requirement for end customers who increasingly rely on this technology for their products. In 2020 there will be more than 120 unique display products in mass production using quantum dot materials.

“Shoei Chemical is a world-leading supplier of nanoscale electronic materials for the electronics market. We’ve worked closely to produce our proprietary Quantum Dot materials to the highest of manufacturing and quality control standards,” said Jason Hartlove, CEO and President of Nanosys. “Over 95% of our business is generated from our customers supply chains which are outside the U.S. We look forward to working together with Shoei to meet the high- volume needs of the growing Quantum Dot display industry with the lowest cost, highest- performance Quantum Dot materials.”

Nanosys, the leading supplier of Quantum Dot materials to the rapidly growing Quantum Dot display market, will continue to mass produce Quantum Dots at the company’s Silicon Valley headquarters. According to DSCC’s Annual Quantum Dot Display Technology & Market Outlook report, Quantum Dot display panel shipments are expected to grow from 10.6 million units in 2020 to 28 million units by 2024, a 27.5% compound annual growth rate.

"Shoei Chemical is committed to producing Nanosys’ innovative, high-performance Quantum Dot materials to meet growing demand in the display market," said Shuichiro Asada, CEO and President at Shoei Chemical. "Shoei has invested in the capabilities of our people and facilities, and we are very well positioned to meet the future needs of this exciting and evolving industry.

We look forward to delivering these innovative products while maintaining our cost- effectiveness and high-quality manufacturing standards."

Tags:  Electronics  Graphene  Jason Hartlove  Nanosys  Quantum Dots  Shoei Chemical  Shuichiro Asada 

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Superior TNOx/HRGO hybrid anode for lithium-ion batteries

Posted By Graphene Council, Monday, August 10, 2020
In a paper published in NANO, a team of researchers from Chengdu Development Center of Science and Technology have significantly enhanced the performance of titanium niobium oxides for lithium-ion batteries. This has applications in electric vehicles and mobile electronics.

Due to its high security and capacity, titanium niobium oxide (TNO) has gained much attention as anode material for lithium-ion batteries. Yet, its electronic conductivity is too low to have high capability at high rates. In order to improve the high-rate performance of TNO effectively, a team of researchers from Chengdu Development Center of Science and Technology, China Academy of Engineering Physics, has combined utilized crystal structure modification, particle size reduction, porous structure, and conductive-phase compositing to solve this problem. The electrochemical performance, especially high-rate performance, of the material was significantly enhanced.

Ti2Nb10O29-x/HRGO hybrid was successfully fabricated by introducing vacancies into Ti2Nb10O29 (TNO) and hybridizing TNO with holey reduced graphene oxide. The structure of TNOx/HRGO is TNOx microspheres with oxygen vacancies wrapped by gossamer-like HRGO. Electrochemical measurements confirmed that TNOx/HRGO hybrid exhibited excellent reversible capacity of 316 mAh/g, 278 mAh/g, 242 mAh/g, 225 mAh/g, and 173 mAh/g at 1 C, 5 C, 10 C, 20 C, and 40 C, respectively. After 300 cycles at 10 C, it still has a high capacity of 238 mAh/g with a high capacity retention of 98%, revealing excellent cycling stability.

The oxygen vacancies of TNOx and the high conductivity of HRGO can effectively enhance the electronic conductivity of the TNOx/HRGO hybrid, and the HRGO holes are beneficial for the transmission of lithium-ion (Li+). The synergy effect of above features improves the rate performance of the TNOx/HRGO hybrid. In addition, the existence of HRGO can buffer volume expansion during the insertion processes of Li+, which can improve cyclic stability of the TNOx/HRGO hybrid.

In this paper, combined utilization of several methods is proved to be an effective way to improve the electrochemical performance of TNO. Ti2Nb10O29-x/HRGO hybrid can be a potential anode material for lithium-ion storage with high security and high capacity, as well as excellent high-rate and cycle performance.

Tags:  Chengdu Development Center of Science and Technolo  electric vehicles  Electronics  Graphene  Li-ion Batteries 

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USING PROTONS TO TUNE INTERLAYER FORCES IN VAN-DER-WAALS MATERIALS

Posted By Graphene Council, Wednesday, July 29, 2020
A Chinese-Australian collaboration has demonstrated for the first time that interlayer coupling in a van der Waals (vdW) material can be largely modulated by a protonic gate, which inject protons to devices from an ionic solid.

The discovery opens the way to exciting new uses of vdW materials, with insertion of protons an important new technique, now available for the wider 2D materials research community.

The study was led by FLEET researchers at RMIT, in an ongoing collaboration with FLEET partner organisation High Magnetic Field Laboratory, Chinese Academy of Sciences (CAS).

TUNING INTERLAYER FORCES IN VDW MATERIALS
Van der Waals materials, of which graphite is the most famous, are made of many 2D layers held together by weak, electrostatic forces.

Individual layers of vdW materials can be isolated individually, such as the famous Scotch tape method of producing graphene, or stacked with other materials to form new structures.

“But the same weak interlayer forces that make vdW materials so easy to separate also limit these materials’ applications in future technology,” explains the study’s first author, FLEET Research Fellow Dr Guolin Zheng.

Stronger interlayer coupling in vdW materials would significantly increase potential use in high-temperature devices utilising quantum anomalous Hall effect, and in 2D multiferroics.

The new RMIT-led study demonstrated that coupling in a vdW material, Fe3GeTe2 (FGT) nanoflakes, can be largely modulated by a protonic gate.

With the increase of the protons among layers, interlayer magnetic coupling increases.

“Most strikingly, with more protons inserted in FGT nanoflakes at a higher gate voltage, we observed a rarely seen zero-field cooled exchange bias with very large values,” says co-author A/Prof Lan Wang.

The successful realization of both field-cooled and zero-field cooled exchange bias in FGT implies the interlayer coupling can be largely modulated by gate-induced proton insertion, opening the road to many applications of vdW materials requiring strong interface coupling.

THE STUDY

Gate-Tuned Interlayer Coupling in van der Waals Ferromagnet Fe3GeTe2 Nanoflakes was published in APS Physical Review Letters July 2020 (DOI 10.1103/physrevlett.125.047202).

As well as funding from the Australian Research Council the researchers acknowledge the support of the RMIT Micro Nano Research Facility (MNRF) in the Victorian Node of the Australian National Fabrication Facility (ANFF) and the RMIT Microscopy and Microanalysis Facility (RMMF). Theoretical calculations were performed by Professor Yujun Zhao’s group at South China University of Technology.

VAN DER WAALS MATERIALS STUDY AT FLEET
VdW materials and heterostructures are keenly studied at FLEET, an Australian Research Council Centre of Excellence. The Centre for Future Low-Energy Electronics Technologies (FLEET) brings together over a hundred Australian and international experts, with the shared mission to develop a new generation of ultra-low energy electronics.

The impetus behind such work is the increasing challenge of energy used in computation, which uses 5–8% of global electricity and is doubling every decade.

FLEET’s research sits at the very boundary of what is possible in condensed-matter physics. At the nano scale, nanofabrication of functioning devices will be key to the Centre’s success. Specialised techniques needed to integrate novel atomically-thin, 2D materials into high-quality, high-performance nano-devices are coordinated within the Centre’s Enabling technology B, led by RMIT’s A/Prof Lan Wang.

Professor Mingliang Tian is vice-director of the Chinese Academy of Science’s High Magnetic Field Laboratory in Anhui province, China, a partner organisation for FLEET.  The FLEET-CAS partnership studies 2D magnetic materials, vdW ferromagnetic heterostructures and topological condensed matter systems.

Tags:  2D materials  Electronics  Graphene  Guolin Zheng  Lan Wang  RMIT  Yujun Zhao 

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Taking the guesswork out of twistronics

Posted By Graphene Council, Wednesday, July 29, 2020

The twist has been taking the field of condensed matter physics by storm. No, not the 1960s dance craze made famous by Chubby Checker— the stunning discovery that two sheets of graphene, a flat honeycomb-shaped lattice of carbon, could be stacked and twisted at so-called magic angles to exhibit vastly different properties, including superconducting behavior. 

Since 2018, when the first experimental verification was published, researchers around the world have been exploring this rapidly expanding subfield of condensed matter physics and materials science. But when there are millions of different ways to stack and twist layers of two-dimensional materials such as graphene, how do you know which way will yield interesting properties?

That’s where two recent research articles from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Department of Physics come in. First author of the publications Georgios Tritsaris, a research scholar at SEAS, with the research group of Efthimios Kaxiras, the John Hasbrouck Van Vleck Professor of Pure and Applied Physics in the Department of Physics and Director of the Institute for Applied Computational Science in SEAS, designed a computational system to screen twisted multi-layer graphene stacks for twist angles associated with potentially interesting electronic properties. 

The approach can identify nanostructures with tailored properties that could help accelerate the development and commercialization of quantum and other technologies. 

The research articles were published in 2D Materials and the Journal of Chemical Information and Modeling and were co-authored by Stephen Carr, Ziyan Zhu, Yiqi Xie, Steven Torrisi, Marios Mattheakis, Daniel Larson, and Alexander Rush from Harvard, and Jing Tang from Nanjing University. 

The research builds on the team’s expertise in materials modeling and machine learning, and its previous work in this emerging field, named twistronics. The term twistronics was first introduced by the Kaxiras Research Group in earlier theoretical studies of layered graphene. It refers to the ability to tune the electrical properties of two-dimensional materials through a rotation between successive layers. 

“Besides increasing our theoretical knowledge of arbitrarily layered graphene, an important goal was to minimize the need for time-consuming, trial-and-error experimentation since achieving a magic-angle configuration in the lab remains a painstaking endeavor,” said Tritsaris. “We wanted to develop an automated system that an experimentalist, engineer, or perhaps an algorithm, could use to quickly answer the question, is this layered configuration likely to be interesting or not.” 

To do that, the team leveraged existing knowledge about these materials. The material’s electrical properties are determined by how the energy of electrons throughout the layers varies as a function of their momentum. One indicator as to whether or not a twisted configuration will exhibit interesting electronic phenomena is whether the energy of a single electron in the presence of other electrons can be constrained to a narrow window, giving rise to nearly flat bands in the plots of electronic energy levels.

In order to look for these flat bands for a given configuration, the researchers used a supercomputer to perform accurate calculations of the allowed energy levels of electrons, combined with a computer vision algorithm commonly used in autonomous vehicles to spot flat objects such as lane dividers. The research team used the approach to quickly sort through stacks of graphene up to ten layers.

“By automating data collection and analysis and using machine learning to create informative visualizations of the entire database, we were able to search for magic-angle multi-layer graphene stacks in a resource-effective fashion,” said Tritsaris. “Our streamlined approach is also applicable to two-dimensional layered materials beyond graphene.”

Data-centric approaches for the discovery and optimization of materials are already being used in a range of fields, including in pharmaceuticals to identify new drug targets and in consumer electronics to find new organic light-emitting diodes (OLEDs) for TV screens. 

“It is not always straightforward how to best leverage data mining and machine learning for materials research, as researchers are often dealing with sparse and high-dimensional data, and solutions tend to be domain-specific. We wanted to share our findings to increase confidence in combining physics-based and data-driven models, in a way which is going to be interesting and useful to scientists and technologists in the field of two-dimensional materials," said Tritsaris.

Source: Leah Burrows, Harvard John A. Paulson School Of Engineering and Applied Sciences

Tags:  2D materials  Efthimios Kaxiras  Electronics  Georgios Tritsaris  Graphene  Harvard University 

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Graphene transistors enable selective ion sensing

Posted By Graphene Council, Friday, June 26, 2020
New research shows that graphene field effect transistors can be used to selectively detect ions in a liquid solution. The work, just published in Nature Communications, paves the way to applications such as genome sequencing, medical diagnostics, environmental monitoring, and industrial process control.

State of the art technology for detecting and resolving ions in solution relies on ion sensitive field effect transistors (ISFETs). Standard ISFETs are made of silicon, due to the ease of technological processing, however silicon ISFETs have some drawbacks that hinder their performance in real-life scenarios.

To achieve selectivity to different ionic species, ISFETs that are selective to specific ions are assembled into arrays and post-processing is used to estimate ion concentration. Since many ISFETs are packed on small areas to implement selectivity, each ISFET has to be made small, which leads to low-frequency noise that is prominent in silicon. Increasing the size of individual ISFETs leads to loss of resolution, which imposes a tradeoff that limits practical use.

The present research, reported by teams in Canada and Spain, overcomes the tradeoff by using graphene instead of silicon as the ISFET channel. Graphene has high carrier mobility even in large-area devices, which enables construction of a single large sensor for multiple ionic species. Post-processing of the transistor signal enables the measuring of concentration of K+, Na+, NH4+, NO3-, SO42-, and Cl- ions down to concentrations lower than 10-5 M in a multianalyte solution. These ions were chosen due to their prominence in agriculture runoff, hence the importance of their detection in water quality monitoring.

Practical graphene ISFET use was demonstrated by monitoring the uptake of ions by duckweed in an aquarium over a period of three weeks. The researchers tracked, with high precision and selectivity, the concentration of seven different ionic species over time after adding plant nutrients to the aquarium. This novel work demonstrates that large-area graphene ISFETs can be fabricated from wafer scale graphene by a facile method, yielding ISFETs with a high signal-to-noise-ratio and high-resolution sensing. Graphene ISFETs hence overcome poor selectivity typically associated with ISFETs made of other materials and can be applied to real-life scenarios in environmental sensing.

Tags:  Electronics  Graphene  Sensors  transistor 

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