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3D printable 2D materials based inks show promise to improve energy storage devices

Posted By Graphene Council, The Graphene Council, Sunday, August 11, 2019
Updated: Sunday, August 4, 2019
For the first time, a team of researchers, from the School of Materials and the National Graphene Institute at The the University of Manchester have formulated inks using the 2D material MXene, to produce 3D printed interdigitated electrodes.

As published in Advanced Materials, these inks have been used to 3D print electrodes that can be used in energy storages devices such as supercapacitors.

MXene, a ‘clay-like’ two-dimensional material composed of early transition metals (such as titanium) and carbon atoms, was first developed by Drexel University. However, unlike most clays, MXene shows high electrical conductivity upon drying and is hydrophilic, allowing them to be easily dispersed in aqueous suspensions and inks.

Graphene was the world’s first two-dimensional material, more conductive than copper, many more times stronger than steel, flexible, transparent and one million times thinner than the diameter of a human hair.

Since its isolation, graphene has opened the doors for the exploration of other two-dimensional materials, each with a range of different properties. However, in order to make use of these unique properties, 2D materials need to be efficiently integrated into devices and structures. The manufacturing approach and materials formulations are essential to realise this.

Dr Suelen Barg who led the team said: “We demonstrate that large MXene flakes spanning a few atoms thick, and water can be independently used to formulate inks with very specific viscoelastic behaviour for printing. These inks can be directly 3D printed into freestanding architectures over 20 layers tall. Due to the excellent electrical conductivity of MXene, we can employ our inks to directly 3D print current collector-free supercapacitors. The unique rheological properties combined with the sustainability of the approach open many opportunities to explore, especially in energy storage and applications requiring the functional properties of 2D MXene in customized 3D architectures.”

Wenji and Jae, PhD students at the Nano3D Lab at the University, said: “Additive manufacturing offers one possible method of building customised, multi-materials energy devices, demonstrating the capability to capture MXene’s potential for usage in energy applications. We hope this research will open avenues to fully unlock the potential of MXene for use in this field.”

The unique rheological properties combined with the sustainability of the approach open many opportunities to explore, especially in energy storage and applications requiring the functional properties of 2D MXene in customized 3D architectures. Dr Suelen Barg, School of Materials

The performance and application of these devices increasingly rely on the development and scalable manufacturing of innovative materials in order to enhance their performance.

Supercapacitors are devices that are able to produce massive amounts of power while using much less energy than conventional devices. There has been much work carried out on the use of 2D materials in these types of devices due to their excellent conductivity as well as having the potential to reduce the weight of the device.

Potential uses for these devices are for the automotive industry, such as in electric cars as well as for mobile phones and other electronics.

Tags:  2D materials  3D Printing  Drexel University  Graphene  Suelen Barg  Supercapacito  University of Manchester 

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A novel graphene-matrix-assisted stabilization method will help unique 2D materials become a part of quantum computers

Posted By Graphene Council, The Graphene Council, Sunday, August 11, 2019
Updated: Monday, August 5, 2019
The family of 2D materials was recently joined by a new class, the monolayers of oxides and carbides of transition metals, which have been the subject of extensive theoretical and experimental research. These new materials are of great interest to scientists due to their unusual rectangular atomic structure and chemical and physical properties. 

Scientists are particularly interested in a unique 2D rectangular copper oxide cell, which does not exist in crystalline (3D) form, as opposed to most 2D materials, whether well known or discovered recently, which have a lattice similar to that of their crystalline (3D) counterparts. The main hindrance for practical use of monolayers is their low stability.

A group of scientists from MISiS, the Institute of Biochemical Physics of RAS (IBCP), Skoltech, and the National Institute for Materials Science in Japan (NIMS) discovered 2D copper oxide materials with an unusual crystal structure inside a two-layer graphene matrix using experimental methods.

“Finding that a rectangular-lattice copper-oxide monolayer can be stable under given conditions is as important as showing how the binding of copper oxide and a graphene nanopore and formation of a common boundary can lead to the creation of a small, stable 2D copper oxide cluster with a rectangular lattice. In contrast to the monolayer, the small copper oxide cluster’s stability is driven to a large extent by the edge effects (boundaries) that lead to its distortion and, subsequently, destruction of the flat 2D structure. Moreover, we demonstrated that binding bilayered graphene with pure copper, which never exists in the form of a flat cluster, makes the 2D metal layer more stable,” says Skoltech Senior Research Scientist Alexander Kvashnin.

The preferability of the copper oxide rectangular lattice forming in a bigraphene nanopore was confirmed by the calculations performed using the USPEX evolutionary algorithm developed by Professor at Skoltech and MIPT, Artem Oganov. The studies of the physical properties of the stable 2D materials indicate that they are good candidates for spintronics applications.

Tags:  2D materials  Alexander Kvashnin  Artem Oganov  Graphene  MIPT  Skoltech 

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New research highlights similarities in the insulating states of twisted bilayer graphene and cuprates

Posted By Graphene Council, The Graphene Council, Sunday, August 11, 2019
Updated: Monday, August 5, 2019
In recent decades, enormous research efforts have been expended on the exploration and explanation of high-temperature (high-Tc) superconductors, a class of materials exhibiting zero resistance at particularly high temperatures.

Now a team of scientists from the United States, Germany and Japan explain in Nature ("Maximized electron interactions at the magic angle in twisted bilayer graphene") how the electronic structure in twisted bilayer graphene influences the emergence of the insulating state in these systems, which is the precursor to superconductivity in high-Tc materials.

Finding a material which superconducts at room temperature would lead to a technological revolution, alleviate the energy crisis (as nowadays most energy is lost on the way from generation to usage) and boost computing performance to an entirely new level. However, despite the progress made in understanding these systems, a full theoretical description is still elusive, leaving our search for room temperature superconductivity mainly serendipitous.

In a major scientific breakthrough in 2018, twisted bilayer graphene (TBLG) was shown to exhibit phases of matter akin to those of a certain class of high-Tc superconducting materials – the so-called high-Tc cuprates. This represents a novel inroad via a much cleaner and more controllable experimental setup.

The scientists from the Max Planck Institute for the Structure and Dynamics of Matter (MPSD), Freie Universität Berlin (both in Germany), Columbia University (USA) and the National Institute for Materials Science in Japan focused on the insulating state of TBLG.

This material is made up of two atomically thin layers of graphene, stacked at a very slight angle to each other. In this structure, the insulating state precedes the high- Tc superconducting phase. Hence, a better understanding of this phase and what leads up to it is crucial for the control of TBLG.

The scientists used scanning tunneling microscopy and spectroscopy (STM / STS) to investigate the samples. With this microscopic technique, electrically conducting surfaces can be examined atom by atom. Using the pioneering “tear and stack” method, they placed two atomically thin layers of graphene on top of one another and rotated them slightly. Then, the team directly mapped the material’s atomic-scale structural and electronic properties near the ‘magic angle’ of around 1.1°.

The findings, which have just been published in Nature, cast new light on the factors influencing the emergence of superconductivity in TBLG. The team observed that the insulating state, which precedes the superconducting state, appears at a particular level of filling the system with electrons. It enables scientists to estimate the strength and the nature of the interactions between electrons in these systems - a crucial step towards their description.

In particular, the results show that two distinct van Hove singularities (vHs) in the local density of states appear close to the magic angle which have a doping dependent separation of 40-57 meV. This demonstrates clearly for the first time that the vHs separation is significantly larger than previously thought. Furthermore, the team clearly demonstrates that the vHs splits into two peaks when the system is doped near half Moiréband filling. This doping-dependent splitting is explained by a correlation-induced gap, which means that in TBLG, electron-induced interaction plays a prominent role.

The team found that the ratio of the Coulomb interaction to the bandwidth of each individual vHs is more crucial to the magic angle than the vHs seperation. This suggests that the neighboring superconducting state is driven by a Cooper-like pairing mechanism based on electron-electron interactions. In addition, the STS results indicate some level of electronic nematicity (spontaneous breaking of the rotational symmetry of the underlying lattice), much like what is observed in cuprates near the superconducting state.

With this research, the team has taken a crucial step towards demonstrating the equivalence of the physics of high-Tc cuprates and those of TBLG materials. The insights gained via TBLG in this study will thus further the understanding of high-temperature superconductivity in cuprates and lead to a better analysis of the detailed workings of these fascinating systems.

The team’s work on the nature of the superconducting and insulating states seen in transport will allow researchers to benchmark theories and hopefully ultimately understand TBLG as a stepping stone towards a more complete description of the high-Tc cuprates. In the future, this may pave the way towards a more systematic approach of increasing superconducting temperatures in these and similar systems.

Tags:  Columbia University  Freie Universität Berlin  Graphene  Max Planck Institute for the Structure and Dynamic 

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XG Sciences and Niagara Bottling Partner to Drive Graphene Enhanced PET Innovations in The Food & Beverage Packaging Industry

Posted By Terrance Barkan, Thursday, August 8, 2019

XG Sciences, Inc. (XGS), a market leader in designing advanced materials using xGnP® graphene nanoplatelets, announced that it has entered into an Intellectual Property License, Joint Development and Commercialization Agreement with Niagara Bottling, LLC, a market leader in  beverage packaging innovation and one of the largest beverage companies in the U.S. 

 

The Agreement provides XG Sciences with an exclusive license to Niagara’s patents and proprietary know-how related to the use of graphene nanoplatelets in PET in certain bottle applications. Under the Agreement, Niagara will assist XGS with field engineering support to install products into the manufacturing lines for new customers – greatly reducing the manufacturer’s time to market. This Agreement gives XGS access to a considerable IP portfolio relating to optimized dispersions of graphene nanoplatelets in PET and allows XGS to sell XGPET™ masterbatch pellets to global packaging companies within the next 6 to 12 months.  

 

The partnership will bring numerous advancements to the beverage bottle and packaging industry. When used in packaging production the advanced material, sold under the brand XGPET™, demonstrates improved physical strength, advanced product designs, processing benefits as well as potential reductions in the use of PET for given bottle designs.

 

“We are excited about the opportunities that partnering with an industry leader like Niagara Bottling will bring to the packaging industry. XGPET becomes the next innovative graphene enhanced material within our portfolio of high-performance composites, intended to solve major industry challenges, enable new products designs and accelerate a push towards more sustainable products,” said Bamidele Ali, Chief Commercial Officer, XG Sciences.

 

“For years we have used our expertise to innovate for Niagara Bottling’s customers. In this partnership with XG Sciences we are now advancing those innovations to the broader packaging industry,” said Jay Hanan, Ph.D., Chief Scientist, Niagara Bottling. “We are excited to further enable our industry to utilize graphene to create more efficiently produced and user-friendly packaging.”

 

Visit www.xgsciences.com/xgpet for details on the technical advantages of XGPET.

 

About XG Sciences, Inc.

XG Sciences, formed in 2006, specializes in utilizing graphene nanoplatelets to formulate advanced materials that amplify the performance of products across numerous industries. High-performance materials have been shipped to over 1,000 organizations in 47 countries. Test results have shown enhancements in manufacturing processability and improvements in mechanical, thermal, electrical, and barrier properties for many base materials.

 

For more information please visit www.xgsciences.com.

 

About Niagara Bottling, LLC

Family owned since 1963, Niagara Bottling is a leading beverage supplier in the U.S. producing a variety of beverages including bottled water, teas, sports drinks, vitamin water and sparkling water. Headquartered in Ontario, CA, Niagara operates bottling facilities throughout the U.S. and Mexico and works closely with some of the largest retailer stores throughout the country. With over 55 years of experience in advanced bottling technology, Niagara is committed to driving product innovation and environmental sustainability efforts in PET manufacturing.

 

For more information, visit www.niagarawater.com

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Graphene IP Portfolio Made Available

Posted By Dexter Johnson, IEEE Spectrum, Tuesday, August 6, 2019
Updated: Thursday, August 1, 2019


Seattle, WA-based Allied Inventors (AI) is a $600M fund that has invested in early-stage technologies to help address industrial challenges. AI manages over 5,000 intellectual property assets in technology areas such as graphene, medical platforms, energy storage, and semiconductors. 

Now AI is looking to monetize its graphene IP portfolio consisting of 87 patents and pending applications through licenses or sale of the patent package. Over 91% of the patent portfolio has been granted in multiple jurisdictions including the US, China, Germany Japan, and India.

AI curated their technology portfolio by partnering with a large network of inventors from well-known universities, research institutions, and companies. In developing its graphene IP portfolio, AI sourced novel technologies relevant to producing quality large scale graphene, detecting graphene defects, and using graphene for a variety of applications.  The resulting IP portfolio consists of patents related to graphene manufacture and graphene applications like batteries, filtration, and nanoparticle composites. 

In one manufacturing process patent (US Patent 8,828,193 and 14/459,860), this technology is an electromagnetic radiation process that can operate at low temperatures and offers a way to rapidly produce graphene from graphite oxide on an industrial scale. Another patent (US Patent 15/313,855) involves the process of and system for converting carbon dioxide into graphene by focusing light beam on it.

In addition to graphene manufacturing patents, the portfolio includes technologies for making graphene-based materials. One of the patents (US Patent 9,944,774) is a simple and cost-effective process for forming graphene wrapped carbon nanotube based polymer composites. These composites can be used for strain sensing applications such as structural health monitoring.

Another patent (US Patent 9,499,410) describes a method for making metal oxide-graphene composites. The technology is based on a solvo-thermal process that can synthesize a variety of metal oxide-graphene composites. It is a simple one-step method for use in applications such as batteries and capacitors. 

“Our carefully-curated graphene portfolio has a wide range of important technologies for the manufacture and application of high quality graphene. This portfolio would be beneficial to companies in the graphene space that are interested in enhancing the value of their technology portfolio,” said Norman Ong, Business Analyst for AI. “While the preference is to monetize the entire IP portfolio, we would be open to exploring different options.” 

Ong invites any organization that is interested in the graphene IP portfolio to visit their website and to contact them directly at info@alliedinventors.com.

 

***

 

DISCLOSURE: The Graphene Council has NO INTEREST in the referenced patents and has no financial gain from the sale or license of any of the above referenced patents. This article is provided for informational purposes only and you are requested to contact the patent owners directly. 

Tags:  batteries  graphene production  Investment  sensors 

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Argonne-led center receives award for pivotal discovery in battery technology

Posted By Graphene Council, The Graphene Council, Monday, August 5, 2019
This year marks the tenth anniversary of the U.S. Department of Energy's (DOE's) Energy Frontier Research Centers (EFRCs). The DOE Office of Basic Energy Sciences launched forty-six such centers in 2009 to bring together teams of scientists to perform basic research beyond what is possible for individuals or small groups. To celebrate the ten-year milestone, DOE selected ten awardees to recognize their having made a major impact on scientific ideas, technologies and tools, and people. Hence, the award name is "Ten at Ten."

"This award is the consequence of the long-range vision, established at the very start of CEES in 2009, that a robust fundamental understanding of the electrode processes in lithium-ion batteries would have broad benefits." -- Paul Fenter, CEES director

One of the Ten at Ten Awards has gone to three researchers in the Center for Electrochemical Energy Science (CEES), a multi-organizational EFRC led by Argonne National Laboratory in partnership with Northwestern University, University of Illinois and Purdue University. The CEES mission is to explore the fundamental chemistry and materials underlying batteries and energy storage by means of state-of-the-art materials synthesis and characterization.

"This award is the consequence of the long-range vision, established at the very start of CEES in 2009, that a robust fundamental understanding of the electrode processes in lithium-ion batteries would have broad benefits," said Paul Fenter, CEES director and senior physicist in the Chemical Sciences and Engineering division. Such batteries could power electric vehicles and drones as well as provide energy storage for the grid.

The Ten at Ten Award recipients are two former CEES members, Harold Kung and Cary Hayner, and a current CEES member, Mark Hersam. Both Kung and Hersam are professors at Northwestern University, and Hayner is chief technical officer and co-founder of NanoGraf Corp. (formerly SiNode Systems).

"The interdisciplinary collaborative environment within CEES provides a breeding ground not only for fundamental discoveries but also for disruptive thinking that spawns new technologies," said Hersam.  "The EFRC program is a poignant example of how government investment in research ultimately fuels the innovation that underlies economic growth."

The Ten at Ten Award recognizes two new electrode technologies for next-generation lithium-ion batteries that were developed based on research that was initiated in CEES. Both technologies use "graphene," carbon layers just one atom thick, to coat the active materials within the battery electrode to create a "composite" electrode structure.  The first advance by Hayner and Kung used graphene in the battery anode, encapsulating particles of silicon. The second advance by Hersam incorporated graphene in the cathode, to encapsulate manganese-based oxides.

The resulting electrodes consist of graphene-coated active materials that have substantially improved properties, such as increased battery power, lifetime, and safety, as well as diminished likelihood of safety problems such as a violent reaction.

Another important feature of these technologies is that they enable lithium-ion batteries to function at temperatures well below the freezing point -- a capability critical for electric car owners in cold regions.

"CEES is especially proud that the award-winning research has given birth to two startups," noted Fenter. A startup company co-founded by Kung and Hayner in 2012 (NanoGraf) is commercializing the graphene-based silicon anode, while a startup company co-founded by Hersam in 2018 (Volexion) is bringing the graphene-based cathode to market.

"We owe our entire existence as a company to the research and people who are part of CEES," said NanoGraf co-founder Hayner. "The transformative discoveries made by CEES scientists has enabled us to further develop these technologies and bring them to the market to drive a cleaner, more sustainable future."

The award presentation took place on July 29 in Washington, DC. The Center for Electrochemical Energy Science is an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.

Tags:  Cary Hayner  CEES  Graphene  Harold Kung  Mark Hersam  NanoGraf  Paul Fenter 

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Synthesizing single-crystalline hexagonal graphene quantum dots

Posted By Graphene Council, The Graphene Council, Monday, August 5, 2019
A KAIST team has designed a novel strategy for synthesizing single-crystalline graphene quantum dots, which emit stable blue light. The research team confirmed that a display made of their synthesized graphene quantum dots successfully emitted blue light with stable electric pressure, reportedly resolving the long-standing challenges of blue light emission in manufactured displays. The study, led by Professor O Ok Park in the Department of Chemical and Biological Engineering.

Graphene has gained increased attention as a next-generation material for its heat and electrical conductivity as well as its transparency. However, single and multi-layered graphene have characteristics of a conductor so that it is difficult to apply into semiconductor. Only when downsized to the nanoscale, semiconductor's distinct feature of bandgap will be exhibited to emit the light in the graphene. This illuminating featuring of dot is referred to as a graphene quantum dot.

Conventionally, single-crystalline graphene has been fabricated by chemical vapor deposition (CVD) on copper or nickel thin films, or by peeling graphite physically and chemically. However, graphene made via chemical vapor deposition is mainly used for large-surface transparent electrodes. Meanwhile, graphene made by chemical and physical peeling carries uneven size defects.

The research team explained that their graphene quantum dots exhibited a very stable single-phase reaction when they mixed amine and acetic acid with an aqueous solution of glucose. Then, they synthesized single-crystalline graphene quantum dots from the self-assembly of the reaction intermediate. In the course of fabrication, the team developed a new separation method at a low-temperature precipitation, which led to successfully creating a homogeneous nucleation of graphene quantum dots via a single-phase reaction.

Professor Park and his colleagues have developed solution phase synthesis technology that allows for the creation of the desired crystal size for single nanocrystals down to 100 nano meters. It is reportedly the first synthesis of the homogeneous nucleation of graphene through a single-phase reaction.

Professor Park said, "This solution method will significantly contribute to the grafting of graphene in various fields. The application of this new graphene will expand the scope of its applications such as for flexible displays and varistors."

This research was a joint project with a team from Korea University under Professor Sang Hyuk Im from the Department of Chemical and Biological Engineering, and was supported by the National Research Foundation of Korea, the Nano-Material Technology Development Program from the Electronics and Telecommunications Research Institute (ETRI), KAIST EEWS, and the BK21+ project from the Korean government.

Tags:  CVD  Graphene  KAIST  O Ok Park  quantum dots  Sang Hyuk Im 

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New quantum phenomena helps to understand fundamental limits of graphene electronics

Posted By Graphene Council, The Graphene Council, Wednesday, July 31, 2019
Updated: Tuesday, July 30, 2019
A team of researchers from the Universities of Manchester, Nottingham and Loughborough have discovered quantum phenomena that helps to understand the fundamental limits of graphene electronics. As published in Nature Communications, the work describes how electrons in a single atomically-thin sheet of graphene scatter off the vibrating carbon atoms which make up the hexagonal crystal lattice.

By applying a magnetic field perpendicular to the plane of graphene, the current-carrying electrons are forced to move in closed circular “cyclotron” orbits. In pure graphene, the only way in which an electron can escape from this orbit is by bouncing off a “phonon” in a scattering event. These phonons are particle-like bundles of energy and momentum and are the “quanta” of the sound waves associated with the vibrating carbon atom. The phonons are generated in increasing numbers when the graphene crystal is warmed up from very low temperatures.

By passing a small electrical current through the graphene sheet, the team were able to measure precisely the amount of energy and momentum that is transferred between an electron and a phonon during a scattering event.

Their experiment revealed that two types of phonon scatter the electrons: transverse acoustic (TA) phonons in which the carbon atoms vibrate perpendicular to the direction of phonon propagation and wave motion (somewhat analogous to surface waves on water) and longitudinal acoustic (LA) phonons in which the carbon atoms vibrate back and forth along the direction of the phonon and the wave motion; (this motion is somewhat analogous to the motion of sound waves through air).

The measurements provide a very accurate measure of the speed of both types of phonons, a measurement which is otherwise difficult to make for the case of a single atomic layer. An important outcome of the experiments is the discovery that TA phonon scattering dominates over LA phonon scattering.

We were pleasantly surprised to find such prominent magnetophonon oscillations appearing in graphene. We were also puzzled why people had not seen them before, considering the extensive amount of literature on quantum transport in graphene. Laurence Eaves and Roshan Krishna Kumar, The University of Manchester

The observed phenomena, commonly referred to as “magnetophonon oscillations”, was measured in many semiconductors years before the discovery of graphene. It is one of the oldest quantum transport phenomena that has been known for more than fifty years, predating the quantum Hall effect. Whereas graphene possesses a number of novel, exotic electronic properties, this rather fundamental phenomenon has remained hidden.

Laurence Eaves & Roshan Krishna Kumar, co-authors of the work said: “We were pleasantly surprised to find such prominent magnetophonon oscillations appearing in graphene. We were also puzzled why people had not seen them before, considering the extensive amount of literature on quantum transport in graphene.”

Their appearance requires two key ingredients. First, the team had to fabricate high quality graphene transistors with large areas at the National Graphene Institute. If the device dimensions are smaller than a few micrometres the phenomena could not be observed.

Piranavan Kumaravadivel from The University of Manchester, lead author of the paper said: “At the beginning of quantum transport experiments, people used to study macroscopic, millimetre sized crystals. In most of the work on quantum transport on graphene, the studied devices are typically only a few micrometres in size. It seems that making larger graphene devices is not only important for applications but now also for fundamental studies.”

The second ingredient is temperature. Most graphene quantum transport experiments are performed at ultra-cold temperatures in-order to slow down the vibrating carbon atoms and “freeze-out” the phonons that usually break quantum coherence. Therefore, the graphene is warmed up as the phonons need to be active to cause the effect.

Mark Greenaway, from Loughborough University, who worked on the quantum theory of this effect said: “This result is extremely exciting - it opens a new route to probe the properties of phonons in two-dimensional crystals and their heterostructures. This will allow us to better understand electron-phonon interactions in these promising materials, understanding which is vital to develop them for use in new devices and applications.”

Tags:  2D materials  Graphene  Laurence Eaves  Loughborough University  Mark Greenaway  Piranavan Kumaravadivel  Roshan Krishna Kumar  University of Manchester  University of Nottingham 

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Graphene in Electronic Circuits

Posted By Graphene Council, The Graphene Council, Wednesday, July 31, 2019
Updated: Tuesday, July 30, 2019
Ever since graphene was discovered in 2004, researchers around the world have been working to develop commercially scalable applications for this high-performance material.

Graphene is 100 to 300 times stronger than steel at the atomic level and has a maximum electrical current density orders of magnitude greater than that of copper, making it the strongest, thinnest and, by far, the most reliable electrically conductive material on the planet. It is, therefore, an extremely promising material for interconnects, the fundamental components that connect billions of transistors on microchips in computers and other electronic devices in the modern world.

For over two decades, interconnects have been made of copper, but that metal encounters fundamental physical limitations as electrical components that incorporate it shrink to the nanoscale. “As you reduce the dimensions of copper wires, their resistivity shoots up,” said Kaustav Banerjee, a professor in the Department of Electrical and Computer Engineering. “Resistivity is a material property that is not supposed to change, but at the nanoscale, all properties change.”

As the resistivity increases, copper wires generate more heat, reducing their current-carrying capacity. It’s a problem that poses a fundamental threat to the $500 billion semiconductor industry. Graphene has the potential to solve that and other issues. One major obstacle, though, is designing graphene micro-components that can be manufactured on-chip, on a large scale, in a commercial foundry.

“Whatever the component, be it inductors, interconnects, antennas or anything else you want to do with graphene, industry will move forward with it only if you find a way to synthesize graphene directly onto silicon wafers,” Banerjee said. He explained that all manufacturing processes related to the transistors, which are made first, are referred to as the ‘front end.’ To synthesize something at the back-end — that is, after the transistors are fabricated — you face a tight thermal budget that cannot exceed a temperature of about 500 degrees Celsius. If the silicon wafer gets too hot during the back-end processes employed to fabricate the interconnects, other elements that are already on the chip may get damaged, or some impurities may start diffusing, changing the characteristics of the transistors.

Now, after a decade-long quest to achieve graphene interconnects, Banerjee’s lab has developed a method to implement high-conductivity, nanometer-scale doped multilayer graphene (DMG) interconnects that are compatible with high-volume manufacturing of integrated circuits. A paper describing the novel process was named one of the top papers at the 2018 IEEE International Electron Devices Meeting (IEDM),  from more than 230 that were accepted for oral presentations. It also was one of only two papers included in the first annual “IEDM Highlights” section of an issue of the journal Nature Electronics.

Banerjee first proposed the idea of using doped multi-layer graphene at the 2008 IEDM conference and has been working on it ever since. In February 2017 he led the experimental realization of the idea by Chemical Vapor Deposition (CVD) of multilayer graphene at a high temperature, subsequently transferring it to a silicon chip, then patterning the multilayer graphene, followed by doping. Electrical characterization of the conductivity of DMG interconnects down to a width of 20 nanometers established the efficacy of the idea that was proposed in 2008. However, the process was not “CMOS-compatible” (the standard industrial-scale process for making integrated circuits), since the temperature of CVD processes far exceed the thermal budget of back-end processes.

To overcome this bottleneck, Banerjee’s team developed a unique pressure-assisted solid-phase diffusion method for directly synthesizing a large area of high-quality multilayer graphene on a typical dielectric substrate used in the back-end CMOS process. Solid-phase diffusion, well known in the field of metallurgy and often used to form alloys, involves applying pressure and temperature to two different materials that are in close contact so that they diffuse into each other.

Banerjee’s group employed the technique in a novel way. They began by depositing solid-phase carbon in the form of graphite powder onto a deposited layer of nickel metal of optimized thickness. Then they applied heat (300 degrees Celsius) and nominal pressure to the graphite powder to help break down the graphite. The high diffusivity of carbon in nickel allows it to pass rapidly through the metal film.

How much carbon flows through the nickel depends on its thickness and the number of grains it holds. “Grains” refer to the fact that deposited nickel is not a single-crystal metal, but rather a polycrystalline metal, meaning it has areas where two single-crystalline regions meet each other without being perfectly aligned. These areas are called grain boundaries, and external particles — in this case, the carbon atoms — easily diffuse through them. The carbon atoms then recombine on the other surface of the nickel closer to the dielectric substrate, forming multiple graphene layers.

Banerjee’s group is able to control the process conditions to produce graphene of optimal thickness. “For interconnect applications, we know how many layers of graphene are needed,” said Junkai Jiang, a Ph.D. candidate in Banerjee’s lab and lead author of the 2018 IEDM paper. “So we optimized the nickel thickness and other process parameters to obtain precisely the number of graphene layers we want at the dielectric surface. “Subsequently, we simply remove the nickel by etching so that what’s left is only very high-quality graphene — virtually the same quality as graphene grown by CVD at very high temperatures,” he continued. “Because our process involves relatively low temperatures that pose no threat to the other fabricated elements on the chip, including the transistors, we can make the interconnects right on top of them.”

UCSB has filed a provisional patent on the process, which overcomes the obstacles that, until now, have prevented graphene from replacing copper. Bottom line: graphene interconnects help to create faster, smaller, lighter, more flexible, more reliable and more cost-effective integrated circuits. Banerjee is currently in talks with industry partners interested in potentially licensing this CMOS-compatible graphene synthesis technology, which could pave the way for what would be the first 2D material to enter the mainstream semiconductor industry.

Tags:  2D materials  CVD  Graphene  Graphite  Junkai Jiang  Kaustav Banerjee  Semiconductor  UC Santa Barbara 

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Unconventional phenomena triggered by acoustic waves in 2D materials

Posted By Graphene Council, The Graphene Council, Tuesday, July 30, 2019
Researchers at the Center for Theoretical Physics of Complex Systems (PCS), within the Institute for Basic Science (IBS, South Korea), and colleagues have reported a novel phenomenon, called Valley Acoustoelectric Effect, which takes place in 2D materials, similar to graphene. This research is published in Physical Review Letters and brings new insights to the study of valleytronics.

In acoustoelectronics, surface acoustic waves (SAWs) are employed to generate electric currents. In this study, the team of theoretical physicists modelled the propagation of SAWs in emerging 2D materials, such as single-layer molybdenum disulfide (MoS2). SAWs drag MoS2 electrons (and holes), creating an electric current with conventional and unconventional components. The latter consists of two contributions: a warping-based current and a Hall current. The first is direction-dependent, is related to the so-called valleys -- electrons' local energy minima -- and resembles one of the mechanisms that explains photovoltaic effects of 2D materials exposed to light. The second is due to a specific effect (Berry phase) that affects the velocity of these electrons travelling as a group and resulting in intriguing phenomena, such as anomalous and quantum Hall effects.

The team analyzed the properties of the acoustoelectric current, suggesting a way to run and measure the conventional, warping, and Hall currents independently. This allows the simultaneous use of both optical and acoustic techniques to control the propagation of charge carriers in novel 2D materials, creating new logical devices.

The researchers are interested in controlling the physical properties of these ultra-thin systems, in particular those electrons that are free to move in two dimensions, but tightly confined in the third. By curbing the parameters of the electrons, in particular their momentum, spin, and valley, it will be possible to explore technologies beyond silicon electronics. For example, MoS2 has two district valleys, which could be potentially used in the future for bit storage and processing, making it an ideal material to delve into valleytronics.

"Our theory opens a way to manipulate valley transport by acoustic methods, expanding the applicability of valleytronic effects on acoustoelectronic devices," explains Ivan Savenko, leader of the Light-Matter Interaction in Nanostructures Team at PCS.

Tags:  2D materials  Center for Theoretical Physics of Complex Systems  Electronics  Graphene  Institute for Basic Science  Ivan Savenko 

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