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An ultimate one-dimensional electronic channel in hexagonal boron nitride

Posted By Graphene Council, Wednesday, March 11, 2020
In the field of 2D electronics, the norm used to be that graphene is the main protagonist and hexagonal boron nitride (hBN) is its insulating passive support. Researchers of the Center for Multidimensional Carbon Materials (CMCM) within the Institute for Basic Science (IBS, South Korea) made a discovery that might change the role of hBN. They have reported that stacking of ultrathin sheets of hBN in a particular way creates a conducting boundary with zero bandgap. In other words, the same material could block the flow of electrons, as a good insulator, and also conduct electricity in a specific location. Published in the journal Science Advances, this result is expected to raise interest in hBN by giving it a more active part in 2D electronics.

Similarly to graphene, hBN is a 2D material with high chemical, mechanical and thermal stability. hBN sheets resemble a chicken wire, and are made of hexagonal rings of alternating boron and nitrogen atoms, strongly bound together. However, unlike graphene, hBN is an insulator with a large bandgap of more than five electronVolts, which limits its applications.

“In contrast to the wide spectrum of proposed applications for graphene, hexagonal boron nitride is often regarded as an inert material, largely confined as substrate or electron barrier for 2D material-based devices. When we began this research, we were convinced that reducing the bandgap of hBN could give to this material the versatility of graphene,” says the first author, PARK Hyo Ju.

Several attempts to lower the bandgap of hBN have been mostly ineffective because of its strong covalent boron-nitrogen bonds and chemical inertness. IBS researchers in collaboration with colleagues of Ulsan National Institute of Science and Technology (UNIST), Sejong University, Korea, and Nanyang Technological University, Singapore, managed to produce a particular stacking boundary of a few hBN layers having a bandgap of zero electronVolts.

Depending on how the hBN sheets are piled up, the material can assume different configurations. For example, in the so-called AA′ arrangement, the atoms in one layer are aligned directly on the top of atoms in another layer, but successive layers are rotated such that boron is located on nitrogen and nitrogen on boron atoms. In another type of layout, known as AB, half of the atoms of one layer lie directly over the center of the hexagonal rings of the lower sheet, and the other atoms overlap with the atoms underneath.

For the first time, the team has reported atomically sharp AA′/AB stacking boundaries formed in few-layer hBN grown by chemical vapor deposition. Characterized by a line of oblong hexagonal rings, this specific boundary has zero bandgap. To confirm this result, the research performed several simulations and tests via transmission electron microscopy, density functional theory calculations, and ab initio molecular dynamics simulations.

“An atomic conducting channel expands the application range of boron nitride infinitely, and opens new possibilities for all-hBN or all 2D nanoelectronic devices,” points out the corresponding author LEE Zonghoon.

Tags:  2D materials  Graphene  hexagonal boron nitride  Institute for Basic Science  PARK Hyo Ju 

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Researchers at TU Delft develop first model to guide large-scale production of ultrathin graphene

Posted By Graphene Council, Wednesday, March 11, 2020

Graphene is well-known for its remarkable electronic, mechanical and thermal properties, but industrial production of high-quality graphene is very challenging. A research team at Delft University of Technology has now developed a mathematical model that can be used to guide the large-scale production of these ultrathin layers of carbon. The findings were published this week in The Journal of Chemical Physics.

“Our model is the first to give a detailed view of what happens at the micro and nanoscale when graphene is produced from plain graphite using energetic fluid mixing,” says Dr. Lorenzo Botto, researcher at the department of Process & Energy at TU Delft. “The model will help the design of large-scale production processes, paving the way for graphene to be incorporated in commercial applications from energy storage devices to biomedicine”.

Graphite and graphene

Graphene can be made from graphite, which is a crystalline form of pure carbon, widely used for example in pencils and lubricants. The layers that make up graphite are called graphene and consists of carbon atoms arranged in a hexagonal structure. These extremely thin carbon layers possess remarkable electrical, mechanical, optical and thermal properties.

For example, a single layer of graphene is about 100 times stronger than the strongest steel of the same thickness. It conducts heat and electricity extremely efficiently and is nearly transparent. Graphene is also intrinsically very cheap, if scalable methods to produce it in large quantities can be found. Graphene has attracted much attention of the past decade as a candidate material for applications in a variety fields such as electronics, energy generation and storage, and biomedicine. In the near future we may replace the copper wiring in our houses with graphene cables, and develop all-carbon batteries that use graphene as the main building block. However, the fabrication of high quality graphene at industrial scale and affordable price remains a challenge. A new theoretical and computational model developed at TU Delft addresses this challenge.

Production of graphene

One of the most promising techniques to produce graphene from graphite is so-called liquid-phase exfoliation. In this technique, graphite is sheared in a liquid environment until layers of graphene detach from the bulk material. The liquid causes the graphene layers to detach gently, which is important to obtain high-quality graphene.

The process has already been successful in the production of graphene on laboratory scale, and, on a trial-and-error basis, on larger scales. It has the potential to be used on industrial scales, to produce tons of material. However, in order to increase the scale of graphene production, we need to know the process parameters that make the exfoliation work efficiently without damaging the graphene sheets.

Model

A research team at TU Delft led by Dr. Lorenzo Botto has now developed the first rigorously derived and validated mathematical model to determine those parameters. This model can be embedded in large-scale industrial process optimisation software or used by practitioners to choose processing parameters.

“The exfoliation process is difficult to model,” explains Botto. “The adhesion between graphene layers is not easy to quantify and the fluid dynamical forces exerted by the liquid on the graphite depend sensitively on surface properties and geometry.” Team members Catherine Kamal and Simon Gravelle developed and tested the model against molecular dynamics simulations, and proved that that the model can be very accurate. Key to the success of the model is the inclusion of hydronamic slip of the liquid pushing against the graphite surface, and of the fluid forces on the graphene edges.

Botto: “The model forms the basis for better control of the technique at any scale. We hope it will pave the way to the large-scale production of graphene for all kinds of useful applications.”

Principal investigator
Dr. Botto obtained his Ph.D. in Fluid Mechanics at Johns Hopkins University (USA), and has worked in the USA, UK and Switzerland. He recently moved to TU Delft. The mission of his research is to use fluid mechanics knowledge to support the large-scale, sustainable production of materials that can solve important societal challenges, from sustainable energy production to environmental remediation. His work on graphene exfoliation is funded by a 1.5M€ Starting Grant (Grant agreement ID: 715475) from the European Research Council (ERC). Read more about the project.

Botto: “Fluid forces can be used to produce and process graphene at the scale required by market applications. However, to reach market readiness we need control over quality and processes. By uncovering underlying fluid mechanical principles, I aim for a profound impact on our ability to produce two-dimensional carbon nanomaterials on large scales.

Tags:  Delft University of Technology  Graphene  Lorenzo Botto  nanomaterials 

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Goodyear to Introduce Its Graphene-enhanced Bicycle Tires to Market

Posted By Graphene Council, Monday, March 9, 2020
Goodyear’s return to making bicycle tyres, after a 44-year hiatus, has been so successful that the company has launched two brand new tyres, Eagle F1 and Eagle F1 Supersport utilising graphene technology and weighing just 180g for a 23mm model.

The company’s previous range-topper was the Eagle All-Season Tubeless road tyre which we found to be a very competent all-rounder with good grip and durability for year-round riding. The new Eagle F1 aims to be more ambitious and really tap into the company's racing heritage, and has the likes of the Continental GP5000, Schwalbe Pro One and Vittoria Corsa Speed in its sights.

The new Eagle F1 is an “ultra-high-performance all-round road tyre” and the Eagle F1 Supersport, which is even lighter, is aimed at the upper echelons of competition and will be suited to road racing, time trial and triathlon where speed trumps all other requirements.

The Eagle F1 boasts numerous technologies aimed at providing this desired high-performance. It has a 120TPI casing with R:Shield Protection, an anti-puncture belt under the casing. Over the top is wrapped the company’s own Dynamic:GSR rubber compound which is claimed to provide “improved grip, reducing rolling resistance and longer wear”.

Graphene has long been touted as the future and so far only a handful of companies have incorporated it into their products. In the tyre world, Vittoria has become well-known for adding the wonder material to its tyres for several years, and Goodyear has followed suit with its new rubber compound.

Goodyear has developed a proprietary compound enhanced with graphene and “next-generation amorphous (non-crystalline) spherical Silica” to create what it labels Dynamic:GSR. The result of all these fancy words is a rubber that is able to deliver the holy grail of low rolling resistance, improved grip in the dry and wet and long-term durability.

There are also directional tapered grooves and a smooth centre section which it claims to provide improved braking and cornering grip. The Supersport does away with these grooves to save weight.

The Eagle F1 comes in five width options from 23 to 32mm, while the Eagle F1 Supersport comes in three widths from 23 to 28mm.

Claimed weight for the Eagle F1 in 25mm width is 210g. The 28mm tyres we have here weigh 234g on our scales. If you want the lightest, a 23mm wide Eagle F1 Supersport comes in at just 180g.

The Eagle F1 Supersport has a construction designed to optimise the rolling resistance and reduce weight but at the expense of puncture prevention and durability. The anti-puncture belt is narrower and the tread thickness has been reduced, measures which bring a 25mm tyre weight down to a claimed 190g.

If you want some numbers, the new compound offers a 10.1% rolling efficiency improvement over its previous Eagle F1 tyre, +8 increase in grip in wet and dry conditions and 7.2% better wear rate.

To produce the new tyre, Goodyear has partnered with Rubber Kinetics as the official licensed partner of The Goodyear Tire & Rubber Company. "Established with the singular purpose of bringing the Goodyear bicycle tire project to life; independence allows Rubber Kinetics to be solely dedicated to establishing Goodyear as a leading brand within the global premium bicycle tire segment," the company says.

Currently the new Eagle F1 and F1 Supersport are only available as clincher tube-type tyres, but a tubeless tyre is in the pipeline for a launch later this year. We’re pretty excited to see how that performs when it comes out as the road tubeless market is really hotting up right now, and another high-end offering shows how importantly tyre companies are taking tubeless.

The new tyres will cost from £45 and be in shops in February. We’ve got a pair on test now so watch out for a review in the near future.

Tags:  Goodyear  Graphene  Tires 

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Proposed optical terahertz graphene transistor

Posted By Graphene Council, Monday, March 9, 2020
Researchers at the Center for Theoretical Physics of Complex Systems (PCS), within the Institute for Basic Science (IBS, South Korea) have proposed a transistor made of graphene and a two-dimensional superconductor that amplifies terahertz (THz) signals.

This research was conducted in collaboration with colleagues from the Micro/Nano Fabrication Laboratory Microsystem and Terahertz Research Center (China), the A. V. Rzhanov Institute of Semiconductor Physics (Russia), and Loughborough University (UK) and was published in Physical Review Letters ("Optical Transistor for Amplification of Radiation in a Broadband Terahertz Domain").

The growing interest in the THz frequency range can be easily explained by its various potential applications. This region of the electromagnetic spectrum, between radio waves and infrared light, is suited for extremely high-resolution images, non-invasive tumor detection, biosecurity, telecommunications, and encryption-decryption procedures, among others.

However, practically, finding a powerful source of rays in this frequency range is so challenging, that researchers commonly refer to this problem as the “Terahertz gap.”

In this work, the researchers proposed a novel strategy to amplify THz radiation from weak and non-uniform signals, which are common in, for instance, biological samples.

The device consists of a graphene sheet positioned in the vicinity of a two-dimensional superconductor and is connected to a power source, which provides enough energy to excite the electrons of the superconductor.

The THz signal amplification is explained by the collective oscillatory behavior of electrons in both of the two materials plus the quantum capacity of graphene.

“This work demonstrates the application-oriented perspectives of systems characterized purely by quantum effects. Light-matter interaction in these hybrid systems not only represent fundamental interest, but it can become a basis for future devices, such as terahertz logic gates, which are currently in high demand,” explains Ivan Savenko, the leader of the Light-Matter Interaction in Nanostructures (LUMIN) team at PCS IBS.

Tags:  A. V. Rzhanov Institute of Semiconductor Physics  Center for Theoretical Physics of Complex Systems  Graphene  Institute for Basic Science  Ivan Savenko  Loughborough University  Micro/Nano Fabrication Laboratory Microsystem  Physical Review Letters  Terahertz Research Center  transistor 

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Comprehensive review of heterogeneously integrated 2D materials

Posted By Graphene Council, Monday, March 9, 2020
In a paper published in NANO, a group of researchers from Sungkyunkwan University, South Korea provide a comprehensive review of heterogeneously integrated two dimensional (2D) materials from an extensive library of atomic 2D materials with selectable material properties to open up fascinating possibilities for the design of functional novel devices.

Since the discovery of Graphene by Andre Geim and Konstantin Novoselov, 2D materials, e.g., graphene, black phosphorous (BP), transition metal dichalcogenides (TMDCs), and hexagonal boron nitride (h-BN) have attracted extensive attention due to their broad physical properties and wide range of applications to electronic and optoelectronic devices. Research on these 2D materials has matured to the point where an extensive library of atomically thin 2D materials with selectable material properties has been created and continues to grow.

By combining or stacking these 2D materials, it is possible to construct 2D heterostructures, which are built by directly stacking individual monolayers comprising different materials. Each monolayer within a 2D heterostructure is highly stable, due to strong covalent bonds between the atoms within that monolayer. However, the forces between the monolayers that keep said monolayers stacked one above the other to form the 2D heterostructure happen to be relatively weak van der Waals interactions. Due to this, each of the monolayers retains its intrinsic properties. Moreover, unlike in conventional semiconductor heterostructures where component material selection is restricted to those with similar lattice structures, the lattice mismatch requirements of stacked heterostructures can be relaxed due to the weakness of the van der Waal's forces. This means that one can combine insulating, semiconducting, or metallic 2D materials to form a single 2D heterostructure despite their different lattice structures.

When a monolayer is stacked in combination with other monolayers made out of different materials, a variety of new heterostructures with atomically thin 2D heterojunctions can be created. Heterostructures made from a particular combination of materials will have a certain set of physical characteristics depending on which materials they are made from. The unusual physical characteristics of 2D heterostructures make them suitable for use in a wide range of applications.

In this review, various 2D heterostructures are discussed along with an explanation of novel electronic and optoelectronic properties, advanced synthesis technical developments, as well as new functional applications available. It provides an understanding of the current research trends in 2D materials, so as to explore future possibilities for nanomaterial research.

Tags:  2D materials  Andre Geim  Graphene  Konstantin Novoselov  Sungkyunkwan University 

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Coherent phonon dynamics realized in spatially separated mechanical resonators

Posted By Graphene Council, Monday, March 9, 2020
The CAS Key Laboratory of Quantum Information makes a significant progress in nanomechanical resonators. A group led by Prof. GUO Guo-Ping, SONG Xiang-Xiang, DENG Guang-Wei (now at UESTC) in collaboration with Prof. TIAN Lin from University of California, Merced, and Origin Quantum Company Limited realized coherent phonon manipulations within spatially separated mechanical resonators. The research results were published online on March, 2nd, in Proceedings of the National Academy of Sciences of the United States of America.

With the rapid development of nanotechnology, devices like surface acoustic wave resonators and nanomechanical resonators are found to be suitable for generation, storage, and manipulation of few or even single phonon, which can be further applied in both classical and quantum information process. The realization of the various applications requires coherent manipulation between different phonon modes. Coherent manipulations within neighbouring phonon modes have been reported previously, while controllable coherent information transfer between spatially separated phonon modes, remains technically challenging. Focusing on this goal, the researchers designed a novel device based on their previous achievements (Nano Lett.16, 5456 (2016)?Nano Lett.17, 915 (2017); Nat. Commun. 9, 383 (2018)). Taking advantages of the extraordinary electronic and mechanical properties of graphene, they realized tunable strong coupling between non-neighbouring phonon modes, mediated by the center phonon mode. By improving sample structure design and measurement technique, the coupling strengths and quality factors are enhanced by one and two orders of magnitude, respectively, comparing to their previous work. The cooperativity reaches 107, which is several orders of magnitude higher than other works. With combined properties of high tunablitiy, large coupling strength, and excellent coherence, the researchers demonstrated electrically tunable Rabi oscillations and Ramsey interferences between non-neighbouring phonon modes in this system.

This work is the first experimental realization of tunable coherent phonon dynamics between non-neighbouring phonon modes. It shows new possibilities towards information storage and processing using phonon modes in nanomechanical resonators, and hybrid devices based on nano-phononics. Reviewers highly evaluated this work: "These results clearly go beyond what has been achieved thus far on the coherent manipulation of resonators in the classical regime." Taking advantages of the cooling technologies, this work also shed lights on coherent manipulations of phonons in the quantum regime and development of phonon-based novel quantum devices.

Tags:  DENG Guang-Wei  Graphene  GUO Guo-Ping  Merced  National Academy of Sciences  Origin Quantum Company Limited  SONG Xiang-Xiang  TIAN Lin  University of California 

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Using molecules to draw on quantum materials

Posted By Graphene Council, Monday, March 9, 2020
Over millennia, civilizations progressed through the Stone, Bronze, and Iron Ages. Now the time has come for quantum materials to change the way we live, thanks in part to research conducted at the Institut National de la Recherche Scientifique (INRS) and McGill University.

Professor Emanuele Orgiu, a researcher at INRS and a specialist in quantum materials. These materials are only a few atoms thick, but have remarkable optical, magnetic, and electrical properties. Professor Orgiu’s research focuses on creating patterns on the surface of quantum materials in order to alter their properties.

“The shape of the drawings helps determine the properties imparted upon the surface,” he explains.

His work has potential applications for (opto)electronic devices such as transistors and photosensors, but also for biosensing devices.

The quantum materials expert has just taken a big step forward by synthesizing macrocycles—large circular molecules—on a graphite surface, a material consisting of a stack of graphene sheets. Individual sheets, which are just one atom thick, are considered quantum materials.

“Think of macrocycles as tiny Lego blocks. It’s impossible to build a ring in solution, an homogeneous mixture in which the blocks are diluted. But you can do it if you put them on a table,” said Professor Orgiu, lead author on a new study, the results of which were published online on February 18 in the journal ACS Nano.

In short, the postdoctoral researcher in Orgiu’s group, Chaoying Fu, who is the first author of the study, has found a way to use macrocycles to draw molecular patterns on a material’s surface.

“The macrocycles are deposited on the surface in solution and only the molecules are left once the liquid has evaporated. We can predict how they will fit together, but the alignment happens naturally through the interactions with neighbouring molecules and the surface,” Professor Orgiu explains.

The study was conducted in collaboration with Dmitrii F. Perepichka, a professor in McGill’s Department of Chemistry, whose expertise helped understand how certain molecules could arrange themselves on the surface of graphite.

“This is a great example of the power of a multidisciplinary approach where we combined organic synthesis and surface science. The level of control we achieved over the shape and the structure of synthesized molecules is quite remarkable,” says Perepichka.

Orgiu said the shape and size of macrocycles made them the ideal candidate to draw on the graphite’s surface.

”The advantage of these molecules is the large pores in their structure. We may eventually be able to use our macrocycles as a frame and decorate the pores with biomolecules that would promote biosensing properties of the surface. This is certainly one of our next steps for future projects.”

Tags:  Dmitrii F. Perepichka  Emanuele Orgiu  Graphene  Graphite  Institut National de la Recherche Scientifique  McGill University  quantum materials 

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Graphene: A Talented 2D Material Gets a New Gig

Posted By Graphene Council, Monday, March 9, 2020
Ever since graphene’s discovery in 2004, scientists have looked for ways to put this talented, atomically thin 2D material to work. Thinner than a single strand of DNA yet 200 times stronger than steel, graphene is an excellent conductor of electricity and heat, and it can conform to any number of shapes, from an ultrathin 2D sheet, to an electronic circuit.

Last year, a team of researchers led by Feng Wang, a faculty scientist in Berkeley Lab’s Materials Sciences Division and a professor of physics at UC Berkeley, developed a multitasking graphene device that switches from a superconductor that efficiently conducts electricity, to an insulator that resists the flow of electric current, and back again to a superconductor.

Now, as reported in Nature today, the researchers have tapped into their graphene system’s talent for juggling not just two properties, but three: superconducting, insulating, and a type of magnetism called ferromagnetism. The multitasking device could make possible new physics experiments, such as research in the pursuit of an electric circuit for faster, next-generation electronics like quantum computing technologies.

“So far, materials simultaneously showing superconducting, insulating, and magnetic properties have been very rare. And most people believed that it would be difficult to induce magnetism in graphene, because it’s typically not magnetic. Our graphene system is the first to combine all three properties in a single sample,” said Guorui Chen, a postdoctoral researcher in Wang’s Ultrafast Nano-Optics Group at UC Berkeley, and the study’s lead author.

Using electricity to turn on graphene’s hidden potential

Graphene has a lot of potential in the world of electronics. Its atomically thin structure, combined with its robust electronic and thermal conductivity, “could offer a unique advantage in the development of next-generation electronics and memory storage devices,” said Chen, who also worked as a postdoctoral researcher in Berkeley Lab’s Materials Sciences Division at the time of the study.

The problem is that the magnetic materials used in electronics today are made of ferromagnetic metals, such as iron or cobalt alloys. Ferromagnetic materials, like the common bar magnet, have a north and a south pole. When ferromagnetic materials are used to store data on a computer’s hard disk, these poles point either up or down, representing zeros and ones – called bits.

Graphene, however, is not made of a magnetic metal – it’s made of carbon.

So the scientists came up with a creative workaround.

They engineered an ultrathin device, just 1 nanometer in thickness, featuring three layers of atomically thin graphene. When sandwiched between 2D layers of boron nitride, the graphene layers – described as trilayer graphene in the study – form a repeating pattern called a moiré superlattice.

By applying electrical voltages through the graphene device’s gates, the force from the electricity prodded electrons in the device to circle in the same direction, like tiny cars racing around a track. This generated a forceful momentum that transformed the graphene device into a ferromagnetic system.

More measurements revealed an astonishing new set of properties: The graphene system’s interior had not only become magnetic but also insulating; and despite the magnetism, its outer edges morphed into channels of electronic current that move without resistance. Such properties characterize a rare class of insulators known as Chern insulators, the researchers said.

Even more surprising, calculations by co-author Ya-Hui Zhang of the Massachusetts Institute of Technology revealed that the graphene device has not just one, but two conductive edges, making it the first observed “high-order Chern insulator,” a consequence of the strong electron-electron interactions in the trilayer graphene.

Scientists have been in hot pursuit of Chern insulators in a field of research known as topology, which investigates exotic states of matter. Chern insulators offer potential new ways to manipulate information in a quantum computer, where data is stored in quantum bits, or qubits. A qubit can represent a one, a zero, or a state in which it is both a one and a zero at the same time.

“Our discovery demonstrates that graphene is an ideal platform for studying different physics, ranging from single-particle physics, to superconductivity, and now topological physics to study quantum phases of matter in 2D materials,” Chen said. “It’s exciting that we can now explore new physics in a tiny device just 1 millionth of a millimeter thick.”

The researchers hope to conduct more experiments with their graphene device to have a better understanding of how the Chern insulator/magnet emerged, and the mechanics behind its unusual properties.

Researchers from Berkeley Lab; UC Berkeley; Stanford University; SLAC National Accelerator Laboratory; Massachusetts Institute of Technology; China’s Shanghai Jiao Tong University, Collaborative Innovation Center of Advanced Microstructures, and Fudan University; and Japan’s National Institute for Materials Science participated in the work.

This work was supported by the Center for Novel Pathways to Quantum Coherence in Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.

Tags:  2D materials  Berkeley Lab’s Materials Sciences Division  DNA  Feng Wang  Graphene 

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Supercomputers drive ion transport research

Posted By Graphene Council, Saturday, March 7, 2020
For a long time, nothing. Then all of a sudden, something. Wonderful things in nature can burst on the scene after long periods of dullness -- rare events such as protein folding, chemical reactions, or even the seeding of clouds. Path sampling techniques are computer algorithms that deal with the dullness in data by focusing on the part of the process in which the transition occurs.

Scientists are using XSEDE-allocated supercomputers to help understand the relatively rare event of salts in water passing through atomically-thin, nanoporous membranes. From a practical perspective, the rate of ion transport through a membrane needs to be minimized. In order to achieve this goal, however, it is necessary to obtain a statistically representative picture of individual transport events to understand the factors that control its rate. This research could not only help make progress in desalination for fresh water; it has applications in decontaminating the environment, better pharmaceuticals, and more.

Advanced path sampling techniques and molecular dynamics (MD) simulations captured the kinetics of solute transport through nanoporous membranes, according to a study published online in the Cell journal Matter, January 2020.

"The goal was to calculate the mean first passage times for solutes irrespective of their magnitude," said study co-author Amir Haji-Akbari, an assistant professor of chemical and environmental engineering at Yale University.

The team was awarded supercomputing time by XSEDE, the Extreme Science and Engineering Discovery Environment (XSEDE) funded by the National Science Foundation. The XSEDE-allocated Stampede2 system at TACC was used for the simulations in this study, in particular the Skylake nodes of Stampede2.

"XSEDE was extremely useful and indispensable to what we did," Haji-Akbari said. "That's because the underlying trajectories that are part of the forward flux sampling method are fairly expensive atomistic simulations. We definitively couldn't have finished these studies using the resources that we have locally at the Yale lab."

MD simulations were used to calculate forces in the system studied at the atomic level. The problem with MD is that even today's most powerful supercomputers can only handle number crunching at timescales of a few hundred microseconds. The semi-permeable membranes under study that rejected certain solutes or ions had mean first passage times that could be much longer than the times accessible to MD.

"We used a technique called forward flux sampling, which can be equally used with equilibrium and non-equilibrium MD. The non-equilibrium aspect is particularly important for us because, when you're thinking about driven solute or ion transport, you're dealing with a non-equilibrium process that is either pressure-driven or is driven through external electric fields," Haji-Akbari said.

One can get an idea for this by imagining salty water being pushed by pistons against a membrane skin that only squeezes water out, leaving the sodium and chloride ions behind.

Haji-Akbari and colleagues used this experimental set-up with a special membrane with a nanopore through three layers of graphene. Surprisingly, even at that small scale, solutes that are supposed to be rejected can still fit.

"Geometrically, these solutes can enter the pores and pass the membrane accordingly," Haji-Akbari said. "However, what seems to be keeping them back from doing that is the fact that, when you have a solute that is in water, for example, there usually is a strong association between that solute and what we call its solvation shell, or in the case of aqueous solutions, the hydration shell."

In this example, solvent molecules can clump together, binding to the central solute. In order for the solute to enter the membrane, it has to lose some of these chunky molecules, and losing the molecules costs energy, which amounts to a barrier for their entrance into the membrane. However, it turns out that this picture, although accurate, is not complete.

"When you have an ion that passes through a nanoporous membrane, there is another factor that pulls it back and prevents it from entering and traversing the pore," Haji-Akbari said. "We were able to identify a very interesting, previously unknown mechanism for ion transport through nanopores. That mechanistic aspect is what we call induced charge anisotropy."

To give you a simple perspective of what that is, imagine a chloride ion that enters a nanopore. Once it approaches and then enters the nanopore, it sorts the remaining ions that are in in the feed. Because of the presence of that chloride inside the pore, it will be more likely for sodium ions in the feed to be closer to the pore mouth than the chloride ions.

"That is the additional factor that pulls back the leading ion," Haji-Akbari explained. "You basically have two factors, partial dehydration, which was previously known; but also this induced charge anisotropy that as far as we know is the first time this has been identified."

The science team based their computational method on forward flux sampling, which is parallelizable because the computational components do not interact that strongly with one another. "High performance computing is very suitable for using these types of methods," Haji-Akbari said. "We have previously used it to study crystal nucleation. This is the first time that we're using it to study ion transport through membranes."

As supercomputers get better and better, they offer scientists tools to explore the unexplained in a more realistic way.

"We know that in real systems, the electronic cloud of any molecule or ion will be affected by its environment," Haji-Akbari said. "Those kinds of effects are usually accounted for in polarizable force fields, which are more accurate, but more expensive to simulate. Because the calculation that we conducted was already very expensive, we didn't afford to use those polarizable force fields. That's something that we would like to do at some point, especially if we have the resources to do so."

"Supercomputers are extremely useful in addressing questions that we can't address with regular computing resources. For example, we couldn't have done this calculation without a supercomputer. They're extremely valuable in accessing scales that are not accessible to either experiments, because of their lack of resolution; or simulations, because you need a large number of computer nodes and processors to be able to address that," Haji-Akbari concluded.

Tags:  Amir Haji-Akbari  computer algorithms  environmental  Graphene  Yale University 

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Biomaterial discovery enables 3D printing of tissue-like vascular structures

Posted By Graphene Council, Saturday, March 7, 2020
An international team of scientists have discovered a new material that can be 3D printed to create tissue-like vascular structures.

In a new study published today in Nature Communications, led by Professor Alvaro Mata at the University of Nottingham and Queen Mary University London, researchers have developed a way to 3D print graphene oxide with a protein which can organise into tubular structures that replicate some properties of vascular tissue.

Professor Mata said: “This work offers opportunities in biofabrication by enabling simulatenous top-down 3D bioprinting and bottom-up self-assembly of synthetic and biological components in an orderly manner from the nanoscale. Here, we are biofabricating micro-scale capillary-like fluidic structures that are compatible with cells, exhibit physiologically relevant properties, and have the capacity to withstand flow."

This could enable the recreation of vasculature in the lab and have implications in the development of safer and more efficient drugs, meaning treatments could potentially reach patients much more quickly, Professor Alvaro Mata.

Material with remarkable properties

Self-assembly is the process by which multiple components can organise into larger well-defined structures. Biological systems rely on this process to controllably assemble molecular building-blocks into complex and functional materials exhibiting remarkable properties such as the capacity to grow, replicate, and perform robust functions. 

The new biomaterial is made by the self-assembly of a protein with graphene oxide. The mechanism of assembly enables the flexible (disordered) regions of the protein to order and conform to the graphene oxide, generating a strong interaction between them. By controlling the way in which the two components are mixed, it is possible to guide their assembly at multiple size scales in the presence of cells and into complex robust structures.

The material can then be used as a 3D printing bioink to print structures with intricate geometries and resolutions down to 10 mm. The research team have demonstrated the capacity to build vascular-like structures in the presence of cells and exhibiting biologically relevant chemical and mechanical properties.

Dr. Yuanhao Wu is the lead researcher on the project, she said: “There is a great interest to develop materials and fabrication processes that emulate those from nature. However, the ability to build robust functional materials and devices through the self-assembly of molecular components has until now been limited. This research introduces a new method to integrate proteins with graphene oxide by self-assembly in a way that can be easily integrated with additive manufacturing to easily fabricate biofluidic devices that allow us replicate key parts of human tissues and organs in the lab.”

Close-up of a tubular structure made by simultaneous printing and self-assembling between graphene oxide and a protein.

Cross-section of a bioprinted tubular structure with endothelial cells (green) on and embedded within the wall.

Confocal microscopy image depicting junctions between endothelial cells (green) growing within the printed tubular structures.

Scanning electron microscopy image depicting endothelial cells growing on the surface of the printed tubular structures.

Tags:  3D Printing  Alvaro Mata  biofabricating  biomaterials  Graphene  Graphene Oxide  Queen Mary University London  University of Nottingham  Yuanhao Wu 

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