Tag Archives: letter-materials-tech

A newly discovered collective rattling effect in a type of crystalline semiconductor blocks most heat transfer while preserving high electrical conductivity – a rare pairing that scientists say could reduce heat buildup in electronic devices and turbine engines, among other possible applications.

A team led by scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) discovered these exotic traits in a class of materials known as halide perovskites, which are also considered promising candidates for next-generation solar panels, nanoscale lasers, electronic cooling, and electronic displays.

Rattling structures of halide perovskites: cesium tin iodide (left) and cesium lead iodide (right). Credit: UC Berkeley

Rattling structures of halide perovskites: cesium tin iodide (left) and cesium lead iodide (right). Credit: UC Berkeley

These interrelated thermal and electrical (or “thermoelectric”) properties were found in nanoscale wires of cesium tin iodide (CsSnI3). The material was observed to have one of the lowest levels of heat conductivity among materials with a continuous crystalline structure.

This so-called single-crystal material can also be more easily produced in large quantities than typical thermoelectric materials, such as silicon-germanium, researchers said.

“Its properties originate from the crystal structure itself. It’s an atomic sort of phenomenon,” said Woochul Lee, a postdoctoral researcher at Berkeley Lab who was the lead author of the study, published the week of July 31 in the Proceedings of the National Academy of Sciencesjournal. These are the first published results relating to the thermoelectric performance of this single crystal material.

Researchers earlier thought that the material’s thermal properties were the product of “caged” atoms rattling around within the material’s crystalline structure, as had been observed in some other materials. Such rattling can serve to disrupt heat transfer in a material.

“We initially thought it was atoms of cesium, a heavy element, moving around in the material,” said Peidong Yang, a senior faculty scientist at Berkeley Lab’s Materials Sciences Division who led the study.

Jeffrey Grossman, a researcher at the Massachusetts Institute of Technology, then performed some theory work and computerized simulations that helped to explain what the team had observed. Researchers also used Berkeley Lab’s Molecular Foundry, which specializes in nanoscale research, in the study.

“We believe there is essentially a rattling mechanism, not just with the cesium. It’s the overall structure that’s rattling; it’s a collective rattling,” Yang said. “The rattling mechanism is associated with the crystal structure itself,” and is not the product of a collection of tiny crystal cages. “It is group atomic motion,” he added.

Within the material’s crystal structure, the distance between atoms is shrinking and growing in a collective way that prevents heat from easily flowing through.

But because the material is composed of an orderly, single-crystal structure, electrical current can still flow through it despite this collective rattling. Picture its electrical conductivity is like a submarine traveling smoothly in calm underwater currents, while its thermal conductivity is like a sailboat tossed about in heavy seas at the surface.

Yang said two major applications for thermoelectric materials are in cooling, and in converting heat into electrical current. For this particular cesium tin iodide material, cooling applications such as a coating to help cool electronic camera sensors may be easier to achieve than heat-to-electrical conversion, he said.

A challenge is that the material is highly reactive to air and water, so it requires a protective coating or encapsulation to function in a device.

Cesium tin iodide was first discovered as a semiconductor material decades ago, and only in recent years has it been rediscovered for its other unique traits, Yang said. “It turns out to be an amazing gold mine of physical properties,” he noted.

To measure the thermal conductivity of the material, researchers bridged two islands of an anchoring material with a cesium tin iodide nanowire. The nanowire was connected at either end to micro-islands that functioned as both a heater and a thermometer. Researchers heated one of the islands and precisely measured how the nanowire transported heat to the other island.

They also performed scanning electron microscopy to precisely measure the dimensions of the nanowire. They used these dimensions to provide an exacting measure of the material’s thermal conductivity. The team repeated the experiment with several different nanowire materials and multiple nanowire samples to compare thermoelectric properties and verify the thermal conductivity measurements.

“A next step is to alloy this (cesium tin iodide) material,” Lee said. “This may improve the thermoelectric properties.”

Also, just as computer chip manufacturers implant a succession of elements into silicon wafers to improve their electronic properties – a process known as “doping” – scientists hope to use similar techniques to more fully exploit the thermoelectric traits of this semiconductor material. This is relatively unexplored territory for this class of materials, Yang said.

A key step in unlocking the potential for greener, faster, smaller electronic circuitry was taken recently by a group of researchers led by UAlberta physicist Robert Wolkow.

The research team found a way to delete and replace out-of-place atoms that had been preventing new revolutionary circuitry designs from working. This unleashes a new kind of silicon chips for used in common electronic products, such as our phones and computers.

“For the first time, we can unleash the powerful properties inherent to the atomic scale,” explained Wolkow, noting that printing errors on silicon chips are inevitable when working at the atomic scale. “We were making things that were close to perfect but not quite there. Now that we have the ability to make corrections, we can ensure perfect patterns, and that makes the circuits work. It is this new ability to edit at the atom scale that makes all the difference.”

Think of a typing mistake and the ability to go back and white it out and type it again perfectly. Now imagine that the white out is actually single hydrogen atoms, allowing a level of precision previously unattainable.

“We can precisely erase any errors and reprint that atom in the correct place. It’s not even a compromise like white out where you either have a gooey layer or indentation. It’s actually perfect,” said Wolkow, who worked with fellow scientists from the University of Alberta, the National Research Council, and Quantum Silicon Inc.

Scientists have seen many hints that atomic circuitry was within reach. However, the necessary precision was previously possible only for simple materials that had to be maintained at ultra-low temperatures, impractical for everyday applications demanded in computers and personal digital devices. Wolkow and his team have discovered methods and material to ensure stability at room temperature, challenges that he and other scientists the world over have been working for decades on overcoming.

Wolkow’s graduate students Roshan Achal and Taleana Huff together with postdoc Moe Rashidi showed they can overcome these obstacles with a modified approach to the same silicon chips that are used in today’s circuitry. While they had previously improved accuracy of atomic silicon printing, errors in the form of misplaced atoms always occurred at the one percent level. Though the placement errors were small–about one third of a nanometer–they nevertheless large enough to upset circuit operation.

The students created a reliable procedure for picking up single hydrogen atoms with their atomically sharp probe and replacing one or more hydrogen atoms to perfectly erase atomic misprints.

With their new discovery, many remaining challenges to ultra-low power atomic circuitry have also been erased. Wolkow, Achal, and Huff’s discovery has been captured in the academic paper “Atomic Whiteout,” appearing in the scientific journal ACS Nano.

 

The ‘wonder material’ graphene has many interesting characteristics, and researchers around the world are looking for new ways to utilise them. Graphene itself does not have the characteristics needed to switch electrical currents on and off and smart solutions must be found for this particular problem. “We can make graphene structures with atomic precision. By selecting certain precursor substances (molecules), we can code the structure of the electrical circuit with extreme accuracy,” explains Peter Liljeroth from Aalto University, who conceived the research project together with Ingmar Swart from Utrecht University.

Seamless integration

The electronic properties of graphene can be controlled by synthesizing it into very narrow strips (graphene nanoribbons). Previous research has shown that the ribbon’s electronic characteristics are dependent on its atomic width. A ribbon that is five atoms wide behaves similarly to a metallic wire with extremely good conduction characteristics, but adding two atoms makes the ribbon a semiconductor. “We are now able to seamlessly integrate five atom-wide ribbons with seven atom-wide ribbons. That gives you a metal-semiconductor junction, which is a basic building block of electronic components,” according to Ingmar Swart.

Chemistry on a surface

The researchers produced their electronic graphene structures through a chemical reaction. They evaporated the precursor molecules onto a gold crystal, where they react in a very controlled way to yield new chemical compounds. “This is a different method from that currently used to produce electrical nanostructures, such as those on computer chips. For graphene, it is so important that the structure is precise at the atomic level and it is likely that the chemical route is the only effective method,” Ingmar Swart concludes.

Electronic characteristics

The researchers used advanced microscopic techniques to also determine the electronic and transport characteristics of the resulting structures. It was possible to measure electrical current through a graphene nanoribbon device with an exactly known atomic structure. “This is the first time where we can create e.g. a tunnel barrier and really know its exact atomic structure. Simultaneous measurement of electrical current through the device allows us to compare theory and experiment on a very quantitative level,” says Peter Liljeroth.

An international team of researchers has found a way to determine whether a crystal is a topological insulator — and to predict crystal structures and chemical compositions in which new ones can arise. The results, published July 20 in the journal Nature, show that topological insulators are much more common in nature than currently believed.

Topological materials, which hold promise for a wide range of technological applications due to their exotic electronic properties, have attracted a great deal of theoretical and experimental interest over the past decade, culminating in the 2016 Nobel Prize in physics. The materials’ electronic properties include the ability of current to flow without resistance and to respond in unconventional ways to electric and magnetic fields.

Until now, however, the discovery of new topological materials occurred mainly by trial and error. The new approach described this week allows researchers to identify a large series of potential new topological insulators. The research represents a fundamental advance in the physics of topological materials and changes the way topological properties are understood.

The team included: at Princeton University, Barry Bradlyn and Jennifer Cano, both associate research scholars at the Princeton Center for Theoretical Science, Zhijun Wang, a postdoctoral research associate, and B. Andrei Bernevig, professor of physics; professors Luis Elcoro and Mois Aroyo at the University of the Basque Country in Bilbao; assistant professor Maia Garcia Vergniory of University of the Basque Country and Donostia International Physics Center (DIPC) in Spain; and Claudia Felser, professor at the Max Planck Institute for Chemical Physics of Solids in Germany.

“Our approach allows for a much easier way to find topological materials, avoiding the need for detailed calculations,” Felser said. “For some special lattices, we can say that, regardless of whether a material is an insulator or a metal, something topological will be going on,” Bradlyn added.

Until now, of the roughly 200,000 materials catalogued in materials databases, only around a few hundred are known to host topological behavior, according to the researchers. “This raised the question for the team: Are topological materials really that scarce, or does this merely reflect an incomplete understanding of solids?” Cano said.

To find out, the researchers turned to the nearly century-old band theory of solids, considered one of the early landmark achievements of quantum mechanics. Pioneered by Swiss-born physicist Felix Bloch and others, the theory describes the electrons in crystals as residing in specific energy levels known as bands. If all the states in a group of bands are filled with electrons, then the electrons cannot move and the material is an insulator. If some of the states are unoccupied, then electrons can move from atom to atom and the material is capable of conducting an electrical current.

Because of the symmetry properties of crystals, however, the quantum states of electrons in solids have special properties. These states can be described as a set of interconnected bands characterized by their momentum, energy and shape. The connections between these bands, which on a graph resemble tangled spaghetti strands, give rise to topological behaviors such as those of electrons that can travel on surfaces or edges without resistance.

The team used a systematic search to identify many previously undiscovered families of candidate topological materials. The approach combined tools from such disparate fields as chemistry, mathematics, physics and materials science.

First, the team characterized all the possible electronic band structures arising from electronic orbitals at all the possible atomic positions for all possible crystal patterns, or symmetry groups, that exist in nature, with the exception of magnetic crystals. To search for topological bands, the team first found a way to enumerate all allowed non-topological bands, with the understanding that anything left out of the list must be topological. Using tools from group theory, the team organized into classes all the possible non-topological band structures that can arise in nature.

Next, by employing a branch of mathematics known as graph theory — the same approach used by search engines to determine links between websites — the team determined the allowed connectivity patterns for all of the band structures. The bands can either separate or connect together. The mathematical tools determine all the possible band structures in nature — both topological and non-topological. But having already enumerated the non-topological ones, the team was able to show which band structures are topological.

By looking at the symmetry and connectivity properties of different crystals, the team identified several crystal structures that, by virtue of their band connectivity, must host topological bands. The team has made all of the data about non-topological bands and band connectivity available to the public through the Bilbao Crystallographic Server. “Using these tools, along with our results, researchers from around the world can quickly determine if a material of interest can potentially be topological,” Elcoro said.

The research shows that symmetry, topology, chemistry and physics all have a fundamental role to play in our understanding of materials, Bernevig said. “The new theory embeds two previously missing ingredients, band topology and orbital hybridization, into Bloch’s theory and provides a prescriptive path for the discovery and characterization of metals and insulators with topological properties.”

David Vanderbilt, a professor of physics and astronomy at Rutgers University who was not involved in the study, called the work remarkable. “Most of us thought it would be many years before the topological possibilities could be catalogued exhaustively in this enormous space of crystal classes,” Vanderbilt said. “This is why the work of Bradlyn and co-workers comes as such a surprise. They have developed a remarkable set of principles and algorithms that allow them to construct this catalogue at a single stroke. Moreover, they have combined their theoretical approach with materials database search methods to make concrete predictions of a wealth of new topological insulator materials.”

The theoretical underpinnings for these materials, called “topological” because they are described by properties that remain intact when an object is stretched, twisted or deformed, led to the awarding of the Nobel Prize in physics in 2016 to F. Duncan M. Haldane, Princeton University’s Sherman Fairchild University Professor of Physics, J. Michael Kosterlitz of Brown University, and David J. Thouless of the University of Washington.

Chemistry and physics take different approaches to describing crystalline materials, in which atoms occur in regularly ordered patterns or symmetries. Chemists tend to focus on the atoms and their surrounding clouds of electrons, known as orbitals. Physicists tend to focus on the electrons themselves, which can carry electric current when they hop from atom to atom and are described by their momentum.

“This simple fact — that the physics of electrons is usually described in terms of momentum, while the chemistry of electrons is usually described in terms of electronic orbitals — has left material discovery in this field at the mercy of chance,” Wang said.

“We initially set out to better understand the chemistry of topological materials — to understand why some materials have to be topological,” Vergniory said.

Aroyo added, “What came out was, however, much more interesting: a way to marry chemistry, physics and mathematics that adds the last missing ingredient in a century-old theory of electronics, and in the present-day search for topological materials.”

Brewer Science Inc. today announced from SEMICON West the extension of its partnership with Arkema to develop second-generation directed self-assembly (DSA) materials using high-x (chi) block copolymers. These new materials target advanced-node wafer patterning processes, because they enable even smaller feature sizes than first-generation DSA materials. As such, they provide a cost-effective solution to achieving device nodes down to 5nm and beyond, thereby enabling the continuation of Moore’s law.

“There have been very high expectations that DSA would solve all patterning issues,” said Darron Jurajda, Business Unit Manager, Brewer Science Inc. “Like all worthwhile technologies, there are many challenges to be solved before going into production. Leveraging our earlier DSA collaboration with Arkema offers the best path for implementing the next generation of materials. Together, we look forward to unlocking DSA’s full potential in accordance with industry timelines for manufacturing.”

High-chi block copolymers will further extend DSA’s advantages, achieving feature sizes that meet the requirements for 5nm and beyond. Extending their partnership allows these companies to build on their knowledge base, giving them a head start on developing high-chi materials.

As feature sizes shrink more aggressively with each node, it has become cost prohibitive to create them using existing patterning processes, such as EUV, self-aligned double patterning and self-aligned quad patterning. This presents a challenge for foundries and integrated device manufacturers preparing to ramp to 7nm and 5nm processes. DSA provides an alternative solution to achieving fine feature patterning; can be explored for minimal investment; and is cost efficient in final production. Development of high-chi materials also expands the opportunity for implementing DSA in other applications, including photonics, membrane applications and other areas of microelectronics.

The original collaboration between the two companies combined Brewer Science’s know-how in patterning and process integration with Arkema’s leading-edge expertise in block copolymer development to develop polystyrene-polymethyl methacrylate DSA materials, which are now production-ready to manufacture sub-22nm features.

New research into the largely unstudied area of heterostructural alloys could lead to greater materials control and in turn better semiconductors, advances in nanotechnology for pharmaceuticals and improved metallic glasses for industrial applications.

Heterostructural alloys are blends of compounds made from materials that don’t share the same atom arrangement. Conventional alloys are isostructural, meaning the compounds they consist of, known as the end members, have the same crystal structure.

“Alloys are all around us,” said study co-author Janet Tate, a physicist at Oregon State University. “An example of an istostructural alloy is an LED; you have a semiconductor like aluminum gallium arsenide, dope it with a particular material and make it emit light, and change the color of the light by changing the relative concentration of aluminum and gallium.”

Structure and composition are the two means of controlling the behavior of materials, Tate said. Combining materials gives the alloy properties between those that the end members have on their own.

“If two materials have different structures, as you mix them together it’s not so clear which structure will win,” said Tate, the Dr. Russ and Dolores Gorman Faculty Scholar in the College of Science. “The two together want to take different structures, and so this is an extra way of tuning an alloy’s properties, a structural way. The transition between different crystal structures provides an additional degree of control.”

Tate and collaborators from around the world, including the National Renewable Energy Laboratory, published their findings in Science Advances.

“This is a very interesting piece of materials science that represents a somewhat uncharted area and it may be the beginning something quite important,” Tate said. “The heterostructural alloy concept had been known before, but it’s different enough that it hadn’t really been explored in a detailed phase diagram – the mapping of exactly how, at what temperature and what concentration, it goes from one structure to another.

“This paper is primarily the NERL’s theoretical work being supported by other collaborators’ experimental work,” Tate said. “Our involvement at OSU was in making one of the kinds of heterostructural alloys used in the research, the combination of tin sulfide and calcium sulfide.”

Tate and graduate student Bethany Matthews have been focusing on the semiconductor application.

“Tin sulfide is a solar cell absorber, and the addition of calcium sulfide changes the structure and therefore the electrical properties necessary for an absorber,” Tate said “Combining tin sulfide with calcium sulfide makes it more isotropic – properties being the same regardless of orientation – and that’s usually a useful thing in devices.”

In this study, thin-film synthesis confirmed the metastable phases of the alloys that had been predicted theoretically.

“Many alloys are metastable, not stable – if you gave them enough time and temperature, they’d eventually separate,” Tate said. “The way we make them, with pulsed laser deposition, we allow the unstable structure to form, then suppress the decomposition pathways that would allow them to separate; we don’t give them enough time to equilibrate.”

Metastable materials – those that are thermodynamically stable provided they are not subjected to large disturbances – are in general understudied, Tate said.

“When theorists predict properties, they tend to work with materials that are stable,” she said. “In general the stable compounds are easier to attack. The idea here with heterostructural alloys is that they give us a new handle, a new knob to turn to change and control materials’ properties.”

Zhe Fei pointed to the bright and dark vertical lines running across his computer screen. This nano-image, he explained, shows the waves associated with a half-light, half-matter quasiparticle moving inside a semiconductor.

“These are waves just like water waves,” said Fei, an Iowa State University assistant professor of physics and astronomy and an associate of the U.S. Department of Energy’s Ames Laboratory. “It’s like dropping a rock on the surface of water and seeing waves. But these waves are exciton-polaritons.”

This image shows how researchers launched and studied half-light, half-matter quasiparticles called exciton-polaritons. A laser from the top left shines on the sharp tip of a nano-imaging system aimed at a flat semiconductor. The red circles inside the semiconductor are the waves associated with the quasiparticles. Image courtesy of Zhe Fei/Iowa State University

This image shows how researchers launched and studied half-light, half-matter quasiparticles called exciton-polaritons. A laser from the top left shines on the sharp tip of a nano-imaging system aimed at a flat semiconductor. The red circles inside the semiconductor are the waves associated with the quasiparticles. Image courtesy of Zhe Fei/Iowa State University

Exciton-polaritons are a combination of light and matter. Like all quasiparticles, they’re created within a solid and have physical properties such as energy and momentum. In this study, they were launched by shining a laser on the sharp tip of a nano-imaging system aimed at a thin flake of molybdenum diselenide (MoSe2), a layered semiconductor that supports excitons.

Excitons can form when light is absorbed by a semiconductor. When excitons couple strongly with photons, they create exciton-polaritons.

It’s the first time researchers have made real-space images of exciton-polaritons. Fei said past research projects have used spectroscopic studies to record exciton-polaritons as resonance peaks or dips in optical spectra. Until recent years, most studies have only observed the quasiparticles at extremely cold temperatures – down to about -450 degrees Fahrenheit.

But Fei and his research group worked at room temperature with the scanning near-field optical microscope in his campus lab to take nano-optical images of the quasiparticles.

“We are the first to show a picture of these quasiparticles and how they propagate, interfere and emit,” Fei said.

The researchers, for example, measured a propagation length of more than 12 microns – 12 millionths of a meter – for the exciton-polaritons at room temperature.

Fei said the creation of exciton-polaritons at room temperature and their propagation characteristics are significant for developing future applications for the quasiparticles. One day they could even be used to build nanophotonic circuits to replace electronic circuits for nanoscale energy or information transfer.

Fei said nanophotonic circuits with their large bandwidth could be up to 1 million times faster than current electrical circuits.

A research team led by Fei recently reported its findings in the scientific journal Nature Photonics. The paper’s first author is Fengrui Hu, an Iowa State postdoctoral research associate in physics and astronomy. Additional co-authors are Yilong Luan, an Iowa State doctoral student in physics and astronomy; Marie Scott, a recently graduated undergraduate at the University of Washington; Jiaqiang Yan and David Mandrus of Oak Ridge National Laboratory and the University of Tennessee; and Xiaodong Xu of the University of Washington.

The researchers’ work was supported by funds from Iowa State and the Ames Laboratory to launch Fei’s research program. The W.M. Keck Foundation of Los Angeles also partially supported the nano-optical imaging for the project.

The researchers also learned that by changing the thickness of the MoSe2 semiconductor, they could manipulate the properties of the exciton-polaritons.

Fei, who has been studying quasiparticles in graphene and other 2-D materials since his graduate school days at University of California San Diego, said his earlier work opened the doors for studies of exciton-polaritons.

“We need to explore further the physics of exciton-polaritons and how these quasiparticles can be manipulated,” he said.

That could lead to new devices such as polariton transistors, Fei said. And that could one day lead to breakthroughs in photonic and quantum technologies.

A team of researchers at the Israel Institute of Technology has developed a new capacitor with a metal-insulator-semiconductor (MIS) diode structure that is tunable by illumination. The capacitor, which features embedded metal nanoparticles, is similar to a metal-insulator-metal (MIM) diode, except that the capacitance of the new device depends on illumination and exhibits a strong frequency dispersion, allowing for a high degree of tunability.

This new capacitor has the potential to enhance wireless capability for information processing, sensing and telecommunications. The researchers report their findings this week in the Journal of Applied Physics, from AIP Publishing.

“We have developed a capacitor with the unique ability to tune the capacitance by large amounts using light. Such changes are not possible in any other device,” said Gadi Eisenstein, professor and director of the Russell Berrie Nanotechnology Institute at the Technion Israel Institute of Technology in Haifa and a co-author of the paper. “The observed photo sensitivity of this MIS diode structure expands its potential in optoelectronic circuits that can be used as a light-sensitive variable capacitor in remote sensing circuits.”

MIM diodes are common elements in electronic devices, especially those utilizing radio frequency circuits. They comprise thin-film metal plate electrodes that are separated by an insulator. Like the MIM structure, the researchers’ new MIS capacitor is bias independent, meaning the constant capacitance is independent of its supply voltage. Bias-independent capacitors are important for high linearity, and therefore straightforward predictability, of circuit performance.

“We have demonstrated that our MIS structure is superior to a standard MIM diode,” said Vissarion (Beso) Mikhelashvili, senior research fellow at the Israel Institute of Technology and also a co-author of the paper. “On one hand, it has all the features of an MIM device, but the voltage independent capacitance is tunable by light, which means that the tuning functionality can be incorporated in photonic circuits.”

“The illumination causes a twofold effect,” Eisenstein said. “First, the excitation of trap states enhances the internal polarization. Second, it increases the minority carrier density (due to photo generation) and reduces the depletion region width. This change modifies the capacitance.”

The researchers created three MIS structures, fabricated on a bulk silicon substrate, based on a multilayer dielectric stack, which consisted of a thin thermal silicon dioxide film and a hafnium oxide layer. The two layers were separated by strontium fluoride (SrF2) sublayers in which ferrum (Fe, iron) or cobalt (Co) nanoparticles were embedded.

The researchers found that the fluoridation-oxidation process of the iron atoms causes the formation of a gradient in the valence state of iron ions across the active layer, which results in the generation of an electronic polarization. The polarization causes a bias-independent depletion region and hence an MIM-type characteristic.

Four additional structures were prepared for comparison: Two lacked the SrF2 sublayers and one of them was prepared without the iron film. The other two structures contained SrF2: One did not have cobalt and the second included a one-nanometer Co layer.

The comparison with other MIS capacitors that contained the metal nanoparticles with or without the SrF2 sublayers led to the unequivocal conclusion that only devices consisting of the combination of Fe and SrF2 turn the MIS structure into a photo-sensitive MIM-like structure.

An engineer with the Erik Jonsson School of Engineering and Computer Science at The University of Texas at Dallas has designed a novel computing system made solely from carbon that might one day replace the silicon transistors that power today’s electronic devices.

“The concept brings together an assortment of existing nanoscale technologies and combines them in a new way,” said Dr. Joseph S. Friedman, assistant professor of electrical and computer engineering at UT Dallas who conducted much of the research while he was a doctoral student at Northwestern University.

The resulting all-carbon spin logic proposal, published by lead author Friedman and several collaborators in the June 5 issue of the online journal Nature Communications, is a computing system that Friedman believes could be made smaller than silicon transistors, with increased performance.

Today’s electronic devices are powered by transistors, which are tiny silicon structures that rely on negatively charged electrons moving through the silicon, forming an electric current. Transistors behave like switches, turning current on and off.

In addition to carrying a charge, electrons have another property called spin, which relates to their magnetic properties. In recent years, engineers have been investigating ways to exploit the spin characteristics of electrons to create a new class of transistors and devices called “spintronics.”

Friedman’s all-carbon, spintronic switch functions as a logic gate that relies on a basic tenet of electromagnetics: As an electric current moves through a wire, it creates a magnetic field that wraps around the wire. In addition, a magnetic field near a two-dimensional ribbon of carbon — called a graphene nanoribbon — affects the current flowing through the ribbon. In traditional, silicon-based computers, transistors cannot exploit this phenomenon. Instead, they are connected to one another by wires. The output from one transistor is connected by a wire to the input for the next transistor, and so on in a cascading fashion.

In Friedman’s spintronic circuit design, electrons moving through carbon nanotubes — essentially tiny wires composed of carbon — create a magnetic field that affects the flow of current in a nearby graphene nanoribbon, providing cascaded logic gates that are not physically connected.

Because the communication between each of the graphene nanoribbons takes place via an electromagnetic wave, instead of the physical movement of electrons, Friedman expects that communication will be much faster, with the potential for terahertz clock speeds. In addition, these carbon materials can be made smaller than silicon-based transistors, which are nearing their size limit due to silicon’s limited material properties.

“This was a great interdisciplinary collaborative team effort,” Friedman said, “combining my circuit proposal with physics analysis by Jean-Pierre Leburton and Anuj Girdhar at the University of Illinois at Urbana-Champaign; technology guidance from Ryan Gelfand at the University of Central Florida; and systems insight from Alan Sahakian, Allen Taflove, Bruce Wessels, Hooman Mohseni and Gokhan Memik at Northwestern.”

While the concept is still on the drawing board, Friedman said work toward a prototype of the all-carbon, cascaded spintronic computing system will continue in the interdisciplinary NanoSpinCompute research laboratory, which he directs at UT Dallas.

Scientists have developed a new method of characterizing graphene’s properties without applying disruptive electrical contacts, allowing them to investigate both the resistance and quantum capacitance of graphene and other two-dimensional materials. Researchers from the Swiss Nanoscience Institute and the University of Basel’s Department of Physics reported their findings in the journal Physical Review Applied.

Graphene consists of a single layer of carbon atoms. It is transparent, harder than diamond and stronger than steel, yet flexible, and a significantly better conductor of electricity than copper. Since graphene was first isolated in 2004, scientists across the world have been researching its properties and the possible applications for the ultrathin material. Other two-dimensional materials with similarly promising fields of application also exist; however, little research has been carried out into their electronic structures.

No need for electrical contacts

Electrical contacts are usually used to characterize the electronic properties of graphene and other two-dimensional materials. However, these can significantly alter the materials’ properties. Professor Christian Schönenberger’s team from the Swiss Nanoscience Institute and the University of Basel’s Department of Physics has now developed a new method of investigating these properties without applying contacts.

To do this, the scientists embedded graphene in the isolator boron nitride, placed it on a superconductor and coupled it with a microwave resonator. Both the electrical resistance and the quantum capacitance of the graphene affect the quality factor and resonance frequency of the resonator. Although these signals are very weak, they can be captured using superconducting resonators.

By comparing the microwave characteristics of resonators with and without encapsulated graphene, the scientists can determine both the electrical resistance and quantum capacitance. “These parameters are important in the determination of graphene’s exact properties and in the identification of limiting factors for its application,” explains Simon Zihlmann, a PhD student in Schönenberger’s group.

Also suitable for other two-dimensional materials

The boron nitride-encapsulated graphene served as a prototype material during the method’s development. Graphene integrated into other materials can be investigated in the same way. In addition, other two-dimensional materials can also be characterized without the use of electrical contacts; for example, the semiconductor molybdenum disulfide, which has applications in solar cells and optics.