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From the classroom to the lab and back again

Posted By Graphene Council, The 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, The 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, The 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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