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New study reveals unexpected softness of bilayer graphene

Posted By Graphene Council, Saturday, May 2, 2020
In the study, published in the journal Physical Review B, the researchers showed that bilayer graphene, consisting of two layers of graphene, was noticeably softer than both two-dimensional (2D) graphene and three-dimensional (3D) graphite along the stacking direction.

This surprising result differs from previous research which showed that 2D graphene, a flat single layer of carbon atoms arranged in a honeycomb structure had many of the same mechanical properties as 3D graphite, which is a naturally occurring form of carbon made up from a very weak stack of many layers of graphene.

Measuring stiffness
Graphene is a 2D material, but has 3D properties such as its stiffness in the ‘out-of-plane’ direction, perpendicular to the plane of the graphene sheets.

The behaviour of π electrons within multilayer graphene determine its out-of-plane stiffness. In this study, the researchers found that when bilayer graphene is compressed out-of-plane, some π electrons are ‘squeezed’ through the graphene planes, which are impenetrable to small molecules such as water. This response makes the material softer and much easier to compress.   

Dr Yiwei Sun, lead author of the study from Queen Mary University of London, said: “Our previous study showed that 2D graphene and 3D graphite have many of the same mechanical properties, so we were surprised to see that bilayer graphene is much softer than both of these materials. We think that the softness of bilayer graphene results from the ‘squeezing’ of pi-electronic orbitals through the graphene layers. For example, if the bread on a burger is replaced by a bagel it is even easier to compress because the contents can be squeezed out of the bagel hole.”

Realising potential
Often hailed as a 'wonder material', graphene has the highest known thermal and electrical conductivity and is stronger than steel, as well as being light, flexible and transparent. 

It was discovered in 2004 by peeling off graphene flakes from bulk graphite (used in pencil leads and lubricants) using sticky tape.

Stacking the graphene flakes one on top of the other provides more possibilities as the material’s extraordinary properties are determined by interactions between its stacked layers. Its unique characteristics can also be fine-tuned for various applications by stacking other 2D materials, such as boron nitride and molybdenum disulphide, to graphene.

This study provides insight into the complex interactions between graphene bilayers and enables quantification of its properties, which is critical for exploring future applications of the material in devices such as vertical transistors and pressure sensors.

Tags:  2D materials  boron nitride  Graphene  Queen Mary University of London  transistor  Yiwei Sun 

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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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Carbon Nanotubes & Quantum Dots: Army Thinks VERY Small

Posted By Graphene Council, Thursday, January 9, 2020
While the rest of the Army works on new hypersonic missiles, robotic mini-tanks, and ultra-high-speed helicopters, the Army Research Office is diving deep into the submicroscopic world of nanotechnology and quantum mechanics.

The military is intensely interested in the potential to improve the costs and capabilities of its electronics, which in modern warfare are as vital to survival as guns and armor. But as with the Internet, radar, and other originally military technologies, there are civilian applications as well.

Carbon Nanotubes

One Army Research Office project is looking to replace traditional silicon-based semiconductors with more efficient carbon nanotubes, program manager Joe Qiu told me. The new technology is particularly useful at the very high frequencies (30-plus gigahertz) and very short wavelengths (millimeter wave) that the telecommunications industry wants to use for 5G networks – including on military bases – and for whatever replaces 5G.

“The initial deployment of 5G, they will be lower than six gigahertz, but there are plans…to improve frequencies to 28 GHz and higher,” Qiu said. “It’s not just 5G — it’s beyond 5G.”

How soon could the private sector reap the benefits of ARO-funded research?

“Commercial use of carbon nanotube-based integrated circuits? Maybe five years,” he said, then added with a laugh: “That’s an estimate. Don’t hold me to that!”

This kind of research can take a long time to bear fruit, Qiu cautioned. Army funding actually helped get the ball rolling on carbon nanotubes for electronics 10 years ago, he said, and it’s taken that long to work out the kinks.

It was mathematically proven a decade ago that nanotubes could channel electricity much more efficiently, Qiu told me. While silicon semiconductors form a lattice that lets electrons scatter in all directions – imagine downtown traffic moving through a grid of streets – carbon nanotubes essentially act like a highway that funnels all the electrons in the desired direction. (The technical term is quantum ballistic transport). But actually producing enough nanotubes of consistent size and quality and getting them to line up right took years of further work, much of it Army funded.

Last year, under a Small Business Technology Transfer (STTR) grant from ARO, the University of South California and venture-backed startup Carbonics Inc. developed working carbon nanotube transistors. The next big step is to integrate many transistors together into an actual circuit. Then, Qiu said, you can talk about integrating many circuits together to build actual equipment.

That would be a job for other parts of the Army. “The Army Research Office, our core mission actually is investing in basic science,” Qiu emphasized. ARO is just one piece of the Army Research Laboratory, which is in turn part of Combat Capabilities Development Command (formerly RDECOM), which is in turn one of the three major components of Army Futures Command, created in 2018 to coordinate all aspects of modernization from brainstorming futuristic concepts to fielding new equipment.

At ARO, said one of Qiu’s colleagues, Joseph Myers, “we’re a bunch of program managers here who support basic research likely to lead to advances in a variety of different technologies.”

Quantum Dots

While the Chinese-born, US-trained Qiu is a physicist-turned-engineer-turned-program manager, Myers is a mathematician and head of the mathematical sciences division at ARO – a field, he jokes, notoriously disconnected from mundane reality. Qiu’s carbon nanotubes are a fraction of the size of a single human hair. Their lengths vary widely, but their thickness is typically six nanometers or less. Myers is funding research on quantum dots, miniscule crystals of semiconductor whose longest dimension is no more than six nanometers, meaning they could conceivably fit inside a nanotube.

Extremely small size allows extremely fine precision. When energized, a quantum dot will always emit a very specific wavelength (which wavelength depends on the dot’s exact size). They also emit these precise frequencies more powerfully, for a longer time, than traditional semiconductors. Some companies already sell high-end “quantum LED” TV sets that use this property to produce more vivid colors: You can even get one at Best Buy.

The downside, Myers went on, is that it’s much harder to design electronics using quantum dots. Classical models of physics start to fail as you start to enter the strange domain of quantum mechanics, where seemingly solid objects turn into fuzzy fields of energy that can pulse and jump in unpredictable ways. Unlike traditional electronics that use electrical charges to represent 1s and 0s, “the physics of what’s going on isn’t as clean as zero/one anymore,” he said. “It’s got some probability of being a zero, some probability of being a one.”

To predict those probabilities precisely, using current techniques, is arduous and slow. “We largely know the equations, but the equations are just too intractable to solve exactly,” Myers said. “If you’ve got the age of the universe… you can maybe complete one of the calculations.”

“You want to do it in less than one human lifetime,” he said. “You want to do it in a day or two, or a week or so, or maybe even a few hours.”

So how much precision can you safely give up to get your results fast enough to actually use them?

Myers funded work by Southern Methodist University professor Wei Cai, who’s figured out a streamlined modeling technique, using an old Air Force supercomputer that Myers managed to get transferred to SMU before it was scrapped. (The Pentagon has a standing High Performance Computer Modernization Reutilization Program to pass on its older machines.)

Put simply (very, very simply), Cai has figured out which parts of the traditional models tend to have such a miniscule impact on the final result – about 0.000000001 percent – that you can safely ignore them. Then you can just do the calculations that actually matter.

Cai’s technique is 750 times faster than rival approaches, Myers said proudly. In its current form, he cautioned, it is still wrong about 20 percent of the time, but Cai is working on that – he’s likely to apply for further Army funding this year – and in the meantime there are ways to double-check the results.

What kind of improved technologies could you use Cai’s model to design? Besides the QLED televisions already on sale, Myers said there’s interest from multiple parts of the Army Research Laboratory that work on everything from solar panels – a useful complement to fuel-hungry diesel generators and heavy lithium-iron batteries – to military sensors and other electronics. There’s a potential medical application in improving CT scans, as well, which is potentially life-changing not just for civilians but for survivors of skull-rattling roadside bombs.

Congress and good-government watchdogs often wonder, with good reason, about oddball research projects that slip into the Pentagon budget with no clear connection to any military purpose. Then-undersecretary of the Army, Ryan McCarthy – now the secretary – was widely praised in 2017-2018 when he overhauled the service’s science & technology portfolio to cull low-payoff projects and focus 80 percent of investment on the service’s Big Six modernization priorities. But McCarthy was also very careful to leave 20 percent to continue basic research, unconstrained by near-term needs, to sow the seeds of real long-term breakthroughs.

Tags:  Carbon Nanotubes  Carbonics Inc  Graphene  Joe Qiu  quantum dots  Southern Methodist University  The Army Research Office  transistor  University of South California  Wei Cai 

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Properties of graphene change due to water and oxygen

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

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

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

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

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

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

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

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Graphene and layered materials boost silicon technologies

Posted By Graphene Council, Saturday, November 16, 2019
Updated: Friday, November 8, 2019
Silicon semiconductor technology has done marvels for the advancement of our society, which has benefited tremendously from its versatile use and amazing capabilities. The development of electronics, automation, computers, digital cameras and smartphones based on this material and its underpinning technology has reached skyrocket limits, downscaling the physical size of devices and wires to the nanometre regime. 

Although this technology has been growing since the late 1960s, the miniaturization of circuits seems to have reached a possible halt, since transistors can only be shrunk down to a certain size and not further beyond. Thus, there is a pressing need to complement Si CMOS technology with new materials and fulfil the future computing requirements as well as the needs for diversification of applications.

Graphene and related materials offer prospects of advances in device performance at the atomic limit.  They provide a possible solution to overcome the limitations of silicon technology, where the combination of layered materials with silicon chips promises to surpass the current technological limitations.

A team of researchers including Stijn Goossens and Frank Koppens, based at Graphene Flagship partner ICFO, and industrial leaders from Graphene Flagship partner IMEC and TSMC provided an in-depth and thorough review of opportunities, progress and challenges of integrating atomically thin materials with Si-based technology. They give insights on how and why layered materials could overcome current challenges posed by the existing technology and how they can enhance both device component function and performance, to boost the features of future technologies, in the areas of computational and non-computational applications.

For non-computational applications, they review the possible integration of these materials for future cameras, low power optical data communications and gas and bio-sensors. In particular, in image sensors and photodetectors, graphene and related materials could enable new vision in the infrared and terahertz range in addition to the visible range of the spectrum. These can serve for example in autonomous vehicles, security at airports and augmented reality.

For computational systems, and in particular in the field of transistors, they show how challenges such as doping, contact resistance and dielectrics/encapsulation can be diminished when integrating layered materials with Si technology. Layered materials could also improve memory and data storage devices with novel switching mechanisms for meta-insulator-metal structures, avoid sneak currents in memory arrays, or even push the performance gains of copper wire-based circuitry by adhering graphene to the ultrathin copper barrier materials and thus reduce resistance, scattering and self-heating.

The review provides a roadmap of layered material integration and CMOS technology, pinpointing the stage at which all challenges regarding growth, transfer, interface, doping, contacting, and design are currently standing today and what possible processes are expected to be resolved to achieve such goals of moving from a research laboratory environment to a pilot line for production of the first devices that combine both technologies. The layered materials-CMOS roadmap, as presented in this review, gives an exciting glimpse into the future, with pilot production expected to be just a few years from now.

Frank Koppens, Graphene Flagship Work Package Leader for Photonics and Optoelectronics and lead author of the study, says: "Now we have a clear industry-driven roadmap on layered material-silicon technologies and manufacturing. Complementing the established silicon technology with layered materials is key to combine the best of both worlds and enable a plethora of large volume and low-cost applications."

Marco Romagnoli, Graphene Flagship Work Package Leader for Wafer-Scale System Integration, comments: "This is an interesting paper complementing a previous one focused on graphene photonics for telecommunications that completes the range of applications in which graphene can be exploited for large scale production in CMOS environments. Also interesting is the type of application, in which graphene can best exploit its characteristics, from IR/THz cameras to low-power electronic switching and memories.

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, adds: "The integration of graphene and related materials with silicon and CMOS technology is the next goal for the Flagship. For this reason, we will fund the first foundry focussed on the integration of layered materials. This work clearly spells out the vision for the transformative technology that integration will enable."

Tags:  Andrea C. Ferrari  Frank Koppens  Graphene  Graphene Flagship  ICFO  Marco Romagnoli  optoelectronics  photonics  Semiconductor  Stijn Goossens  transistor 

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From the classroom to the lab and back again

Posted By Graphene Council, Monday, October 28, 2019
Updated: Monday, October 28, 2019
Adithya Sriram has always had an interest in physics. But after learning about Penn’s Vagelos Integrated Program in Energy Research (VIPER), he decided that the chance to study both physics and chemical engineering was one he couldn’t turn down. “I wasn’t going to do engineering, but the fact that I got into this program afforded me the opportunity,” says the senior from Columbus, Ohio. 

A joint initiative between the School of Arts and Sciences and the School of Engineering and Applied Science, VIPER enables students to earn degrees from both schools while also conducting summer research projects. For Sriram, that has entailed working in the experimental nanoscale physics lab of Charlie Johnson since the summer following his freshman year, developing graphene field effect transistors to detect biological molecules such as proteins or DNA in biological samples. And to top things off, he and a group of Penn students travel to Paul Robeson High School each Friday to teach physics to 10th graders. 

Transistors are found everywhere in modern electronics, enabling the creation of small, inexpensive devices from radios to clocks to computers. Using a voltage signal, the transistor can control the flow of electrons, switching between an “on” and “off” state. 

To broaden the application of transistors to disease diagnosis, the Johnson lab focuses attention on transistors’ transitional state, between “on” and “off,” where the device is extremely sensitive and the flow of electrons can be precisely measured.

Using atomically-thin graphene as a starting point, the transistors can be customized to detect a wide range of targets based on the biological molecule that the researchers attach to the graphene. In a typical week, Sriram spends his time synthesizing new devices, testing the sensor’s response against known concentrations of biomarkers, and figuring out what changes can be made to the structure of the material to improve its sensitivity. 

The goal is to use these devices to detect disease biomarkers, biological molecules that can help diagnose diseases before symptoms appear. To make progress, Sriram spearheaded a collaboration with Kelvin Luk at the Center For Neurodegenerative Disease Research to add aptamers, small chains of DNA that work as sensors for a wide array of biological molecules, onto graphene devices. “He went and found a collaborator at Penn—only one other student has ever done that before. Understanding that there’s a connection and realizing that it’s a real opportunity are terrific things.”

Sriram also helped generate preliminary data that was part of a recently funded NIH grant with engineer David Issadore for developing a filtration process that will make it easier to measure small amounts of biological materials in complex samples like blood or cerebral spinal fluids, work that could help transforms the Johnson lab’s transistors into compact, handheld devices. “There’s still basic science that we need to look at, there’s still applied science that we’re doing, and Adithya has done a great job embracing that challenge,” says Johnson, adding that their group has only recently started to look more closely at direct applications. 

Outside of courses and lab work, Sriram and senior Alex Zhou from Yorktown, Virginia, serve as teaching assistants for an academically-based community service (ABCS) course for Penn students. With guidance from physics professors Philip Nelson and Masao Sako, Sriram and Zhou develop the curriculum for an introductory physics course that Penn students implement in hands-on and inquiry-based physics labs for high schoolers, with ABCS support provided by the Barbara and Edward Netter Center for Community Partnerships. Every Friday, the Penn students and Nelson travel to Paul Robeson High School—not far from Penn’s campus—and lead three hours of teaching and demonstrations for a group of 20 tenth graders based on the newly developed curriculum. 

The goal of the course is to provide Penn students with high school teaching experience while giving science-minded tenth graders a chance to see physics in action. “We’re trying to give some authentic science experience,” says Nelson, adding that Sriram and Zhou have taken the lead and reinvented this community service course from scratch.

Sriram has a strong interest in science outreach, something he hopes to continue doing in the next stage of his career. It’s a challenging endeavor that Nelson says Sriram has embraced extremely well. “Adithya and Alex have had to reach back into their past, to things that confused them at first. It takes an element of empathy that goes beyond just knowing the science.”

Now busy with his final year of courses and his senior capstone project, Sriram is actively applying for graduate schools. He says that being in the VIPER program was a great opportunity to learn about a number of different topics, including biology courses and advanced graduate-level electives. “As of now I’m more interested in pursuing fundamental physics, though later I might consider trying to get into policy,” he says. “If I do, I foresee knowledge of the energy industry and chemical engineering to be useful.”

With perspectives from physics and engineering, a solid grounding in fundamental research, and his time spent teaching, Sriram is bound to make an impact regardless of where he lands. “He has a plan, and a goal, and a lot of energy,” says Nelson. “What makes my job exciting and rewarding is that I see students that fill me with hope about the future.”

Johnson adds that the Roy and Diana Vagelos endowed programs, including the Life Sciences and Management and the Molecular Life Sciences programs, have positively impacted Penn while providing students with a strong grounding in fundamental science. “Science is about phenomena, discovering new things, but solving a problem is another layer. Engineers get told that they can solve problems, and I think if you can combine that attitude with a really deep understanding of the basic science, that’s a real combo.”

Tags:  Adithya Sriram  David Issadore  Graphene  Healthcare  Penn State University  transistor 

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Scientists create fully electronic 2-dimensional spin transistors

Posted By Graphene Council, Tuesday, October 15, 2019
Updated: Tuesday, October 15, 2019

Physicists from the University of Groningen constructed a two-dimensional spin transistor, in which spin currents were generated by an electric current through graphene. A monolayer of a transition metal dichalcogenide (TMD) was placed on top of graphene to induce charge-to-spin conversion in the graphene. This experimental observation was described in the issue of the journal Nano Letters published on 11 September 2019.

Spintronics is an attractive alternative way of creating low-power electronic devices. It is not based on a charge current but on a current of electron spins. Spin is a quantum mechanical property of an electron, a magnetic moment that could be used to transfer or store information.

Heterostructure
Graphene, a 2D form of carbon, is an excellent spin transporter. However, in order to create or manipulate spins, interaction of its electrons with the atomic nuclei is needed: spin-orbit coupling. This interaction is very weak in carbon, making it difficult to generate or manipulate spin currents in graphene. However, it has been shown that spin-orbit coupling in graphene will increase when a monolayer of a material with heavier atoms (such as a TMD) is placed on top, creating a Van der Waals heterostructure.

In the Physics of Nanodevices group, led by Professor Bart van Wees at the University of Groningen, Ph.D. student Talieh Ghiasi and postdoctoral researcher Alexey Kaverzin created such a heterostructure. Using gold electrodes, they were able to send a pure charge current through the graphene and generate a spin current, referred to as the Rashba-Edelstein effect. This happens due to the interaction with the heavy atoms of the TMD monolayer (in this case, tungsten disulfide). This well-known effect was observed for the first time in graphene that was in proximity to other 2D materials.

Symmetries

'The charge current induces a spin current in the graphene, which we could measure with spin-selective ferromagnetic cobalt electrodes,' says Ghiasi. This charge-to-spin conversion makes it possible to build all-electrical spin circuits with graphene. Previously, the spins had to be injected through a ferromagnet. 'We have also shown that the efficiency of the generation of the spin accumulation can be tuned by the application of an electric field,' adds Ghiasi. This means that they have built a spin transistor in which the spin current can be switched on and off.

The Rashba-Edelstein effect is not the only effect that produces a spin current. The study shows that the Spin-Hall effect does the same, but that these spins are oriented differently. 'When we apply a magnetic field, we make the spins rotate in the field. Different symmetries of the spin signals generated by the two effects in interaction with the magnetic field help us to disentangle the contribution of each effect in one system,' explains Ghiasi. It was also the first time that both types of charge-to-spin conversion mechanisms were observed in the same system. 'This will help us to gain more fundamental insights into the nature of spin-orbit coupling in these heterostructures.'

Graphene Flagship

Apart from the fundamental insights that the study can provide, building an all-electrical 2D spin transistor (without ferromagnets) has considerable significance for spintronic applications, which is also a goal of the EU Graphene Flagship. 'This is especially true because we were able to see the effect at room temperature. The spin signal decreased with increasing temperature but was still very much present under ambient conditions.'

Tags:  2D materials  Bart van Wees  Graphene  Graphene Flagship  Talieh Ghiasi  transistor  University of Groningen 

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MIT engineers build advanced microprocessor out of carbon nanotubes

Posted By Graphene Council, Tuesday, September 3, 2019

After years of tackling numerous design and manufacturing challenges, MIT researchers have built a modern microprocessor from carbon nanotube transistors, which are widely seen as a faster, greener alternative to their traditional silicon counterparts.

The microprocessor, described today in the journal Nature, can be built using traditional silicon-chip fabrication processes, representing a major step toward making carbon nanotube microprocessors more practical.

Silicon transistors — critical microprocessor components that switch between 1 and 0 bits to carry out computations — have carried the computer industry for decades. As predicted by Moore’s Law, industry has been able to shrink down and cram more transistors onto chips every couple of years to help carry out increasingly complex computations. But experts now foresee a time when silicon transistors will stop shrinking, and become increasingly inefficient.

Making carbon nanotube field-effect transistors (CNFET) has become a major goal for building next-generation computers. Research indicates CNFETs have properties that promise around 10 times the energy efficiency and far greater speeds compared to silicon. But when fabricated at scale, the transistors often come with many defects that affect performance, so they remain impractical.

The MIT researchers have invented new techniques to dramatically limit defects and enable full functional control in fabricating CNFETs, using processes in traditional silicon chip foundries. They demonstrated a 16-bit microprocessor with more than 14,000 CNFETs that performs the same tasks as commercial microprocessors. The Nature paper describes the microprocessor design and includes more than 70 pages detailing the manufacturing methodology.

The microprocessor is based on the RISC-V open-source chip architecture that has a set of instructions that a microprocessor can execute. The researchers’ microprocessor was able to execute the full set of instructions accurately. It also executed a modified version of the classic “Hello, World!” program, printing out, “Hello, World! I am RV16XNano, made from CNTs.”

“This is by far the most advanced chip made from any emerging nanotechnology that is promising for high-performance and energy-efficient computing,” says co-author Max M. Shulaker, the Emanuel E Landsman Career Development Assistant Professor of Electrical Engineering and Computer Science (EECS) and a member of the Microsystems Technology Laboratories. “There are limits to silicon. If we want to continue to have gains in computing, carbon nanotubes represent one of the most promising ways to overcome those limits. [The paper] completely re-invents how we build chips with carbon nanotubes.”

Joining Shulaker on the paper are: first author and postdoc Gage Hills, graduate students Christian Lau, Andrew Wright, Mindy D. Bishop, Tathagata Srimani, Pritpal Kanhaiya, Rebecca Ho, and Aya Amer, all of EECS; Arvind, the Johnson Professor of Computer Science and Engineering and a researcher in the Computer Science and Artificial Intelligence Laboratory; Anantha Chandrakasan, the dean of the School of Engineering and the Vannevar Bush Professor of Electrical Engineering and Computer Science; and Samuel Fuller, Yosi Stein, and Denis Murphy, all of Analog Devices.

Fighting the “bane” of CNFETs

The microprocessor builds on a previous iteration designed by Shulaker and other researchers six years ago that had only 178 CNFETs and ran on a single bit of data. Since then, Shulaker and his MIT colleagues have tackled three specific challenges in producing the devices: material defects, manufacturing defects, and functional issues. Hills did the bulk of the microprocessor design, while Lau handled most of the manufacturing.

For years, the defects intrinsic to carbon nanotubes have been a “bane of the field,” Shulaker says. Ideally, CNFETs need semiconducting properties to switch their conductivity on an off, corresponding to the bits 1 and 0. But unavoidably, a small portion of carbon nanotubes will be metallic, and will slow or stop the transistor from switching. To be robust to those failures, advanced circuits will need carbon nanotubes at around 99.999999 percent purity, which is virtually impossible to produce today.  

The researchers came up with a technique called DREAM (an acronym for “designing resiliency against metallic CNTs”), which positions metallic CNFETs in a way that they won’t disrupt computing. In doing so, they relaxed that stringent purity requirement by around four orders of magnitude — or 10,000 times — meaning they only need carbon nanotubes at about 99.99 percent purity, which is currently possible.

Designing circuits basically requires a library of different logic gates attached to transistors that can be combined to, say, create adders and multipliers — like combining letters in the alphabet to create words. The researchers realized that the metallic carbon nanotubes impacted different pairings of these gates differently. A single metallic carbon nanotube in gate A, for instance, may break the connection between A and B. But several metallic carbon nanotubes in gates B may not impact any of its connections.

In chip design, there are many ways to implement code onto a circuit. The researchers ran simulations to find all the different gate combinations that would be robust and wouldn’t be robust to any metallic carbon nanotubes. They then customized a chip-design program to automatically learn the combinations least likely to be affected by metallic carbon nanotubes. When designing a new chip, the program will only utilize the robust combinations and ignore the vulnerable combinations.

“The ‘DREAM’ pun is very much intended, because it’s the dream solution,” Shulaker says. “This allows us to buy carbon nanotubes off the shelf, drop them onto a wafer, and just build our circuit like normal, without doing anything else special.”

Exfoliating and tuning

CNFET fabrication starts with depositing carbon nanotubes in a solution onto a wafer with predesigned transistor architectures. However, some carbon nanotubes inevitably stick randomly together to form big bundles — like strands of spaghetti formed into little balls — that form big particle contamination on the chip.  

To cleanse that contamination, the researchers created RINSE (for “removal of incubated nanotubes through selective exfoliation”). The wafer gets pretreated with an agent that promotes carbon nanotube adhesion. Then, the wafer is coated with a certain polymer and dipped in a special solvent. That washes away the polymer, which only carries away the big bundles, while the single carbon nanotubes remain stuck to the wafer. The technique leads to about a 250-times reduction in particle density on the chip compared to similar methods.

Lastly, the researchers tackled common functional issues with CNFETs. Binary computing requires two types of transistors: “N” types, which turn on with a 1 bit and off with a 0 bit, and “P” types, which do the opposite. Traditionally, making the two types out of carbon nanotubes has been challenging, often yielding transistors that vary in performance. For this solution, the researchers developed a technique called MIXED (for “metal interface engineering crossed with electrostatic doping”), which precisely tunes transistors for function and optimization.

In this technique, they attach certain metals to each transistor — platinum or titanium — which allows them to fix that transistor as P or N. Then, they coat the CNFETs in an oxide compound through atomic-layer deposition, which allows them to tune the transistors’ characteristics for specific applications. Servers, for instance, often require transistors that act very fast but use up energy and power. Wearables and medical implants, on the other hand, may use slower, low-power transistors.  

The main goal is to get the chips out into the real world. To that end, the researchers have now started implementing their manufacturing techniques into a silicon chip foundry through a program by Defense Advanced Research Projects Agency, which supported the research. Although no one can say when chips made entirely from carbon nanotubes will hit the shelves, Shulaker says it could be fewer than five years. “We think it’s no longer a question of if, but when,” he says.

Tags:  Analog Devices  Carbon Nanotubes  Graphene  Max M. Shulaker  MIT  transistor 

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