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Engineers at the University of California, Riverside, have reported advances in so-called “spintronic” devices that will help lead to a new technology for computing and data storage. They have developed methods to detect signals from spintronic components made of low-cost metals and silicon, which overcomes a major barrier to wide application of spintronics. Previously such devices depended on complex structures that used rare and expensive metals such as platinum. The researchers were led by Sandeep Kumar, an assistant professor of mechanical engineering.

UCR researchers have developed methods to detect signals from spintronic components made of low-cost metals and silicon. Credit: UC Riverside

UCR researchers have developed methods to detect signals from spintronic components made of low-cost metals and silicon. Credit: UC Riverside

Spintronic devices promise to solve major problems in today’s electronic computers, in that the computers use massive amounts of electricity and generate heat that requires expending even more energy for cooling. By contrast, spintronic devices generate little heat and use relatively minuscule amounts of electricity. Spintronic computers would require no energy to maintain data in memory. They would also start instantly and have the potential to be far more powerful than today’s computers.

While electronics depends on the charge of electrons to generate the binary ones or zeroes of computer data, spintronics depends on the property of electrons called spin. Spintronic materials register binary data via the “up” or “down” spin orientation of electrons–like the north and south of bar magnets–in the materials. A major barrier to development of spintronics devices is generating and detecting the infinitesimal electric spin signals in spintronic materials.

In one paper published in the January issue of the scientific journal Applied Physics Letters, Kumar and colleagues reported an efficient technique of detecting the spin currents in a simple two-layer sandwich of silicon and a nickel-iron alloy called Permalloy. All three of the components are both inexpensive and abundant and could provide the basis for commercial spintronic devices. They also operate at room temperature. The layers were created with the widely used electronics manufacturing processes called sputtering. Co-authors of the paper were graduate students Ravindra Bhardwaj and Paul Lou.

In their experiments, the researchers heated one side of the Permalloy-silicon bi-layer sandwich to create a temperature gradient, which generated an electrical voltage in the bi-layer. The voltage was due to phenomenon known as the spin-Seebeck effect. The engineers found that they could detect the resulting “spin current” in the bi-layer due to another phenomenon known as the “inverse spin-Hall effect.”

The researchers said their findings will have application to efficient magnetic switching in computer memories, and “these scientific breakthroughs may give impetus” to development of such devices. More broadly, they concluded, “These results bring the ubiquitous Si (silicon) to forefront of spintronics research and will lay the foundation of energy efficient Si spintronics and Si spin caloritronics devices.”

In two other scientific papers, the researchers demonstrated that they could generate a key property for spintronics materials, called antiferromagnetism, in silicon. The achievement opens an important pathway to commercial spintronics, said the researchers, given that silicon is inexpensive and can be manufactured using a mature technology with a long history of application in electronics.

Ferromagnetism is the property of magnetic materials in which the magnetic poles of the atoms are aligned in the same direction. In contrast, antiferromagnetism is a property in which the neighboring atoms are magnetically oriented in opposite directions. These “magnetic moments” are due to the spin of electrons in the atoms, and is central to the application of the materials in spintronics.

In the two papers, Kumar and Lou reported detecting antiferromagnetism in the two types of silicon–called n-type and p-type–used in transistors and other electronic components. N-type semiconductor silicon is “doped” with substances that cause it to have an abundance of negatively-charged electrons; and p-type silicon is doped to have a large concentration of positively charged “holes.” Combining the two types enables switching of current in such devices as transistors used in computer memories and other electronics.

In the paper in the Journal of Magnetism and Magnetic Materials, Lou and Kumar reported detecting the spin-Hall effect and antiferromagnetism in n-silicon. Their experiments used a multilayer thin film comprising palladium, nickel-iron Permalloy, manganese oxide and n-silicon.

And in the second paper, in the scientific journal physica status solidi, they reported detecting in p-silicon spin-driven antiferromagnetism and a transition of silicon between metal and insulator properties. Those experiments used a thin film similar to those with the n-silicon.

The researchers wrote in the latter paper that “The observed emergent antiferromagnetic behavior may lay the foundation of Si (silicon) spintronics and may change every field involving Si thin films. These experiments also present potential electric control of magnetic behavior using simple semiconductor electronics physics. The observed large change in resistance and doping dependence of phase transformation encourages the development of antiferromagnetic and phase change spintronics devices.”

In further studies, Kumar and his colleagues are developing technology to switch spin currents on and off in the materials, with the ultimate goal of creating a spin transistor. They are also working to generate larger, higher-voltage spintronic chips. The result of their work could be extremely low-power, compact transmitters and sensors, as well as energy-efficient data storage and computer memories, said Kumar.

A crystal method


January 31, 2018

From Mother Nature to our must-have devices, we’re surrounded by crystals. Those courtesy of the former, such as ice and snow, can form spontaneously and symmetrically. But the silicon-based or gallium nitride crystals found in LEDs and other electronics require a bit of coaxing to attain their ideal shapes and alignments.

At UC Santa Barbara, researchers have now unlocked another piece of the theoretical puzzle that governs the growth of crystals — a development that may save time and energy in the many processes that require crystal formation.

“The way most industrial processes are designed today is by doing an exhaustively large number of experiments to find out how crystals grow and at what rate they grow under different conditions,” said UCSB chemical engineer Michael Doherty, an author of a paper that appears in the Proceedings of the National Academy of Sciences. Snowflakes, for instance, form differently as they fall, depending on variable conditions such as temperature and humidity, hence the widely held belief that no two are alike. After determining the optimal conditions for the growth of the crystal of choice, Doherty added equipment must be designed and calibrated to provide a consistent growing environment.

However, by pooling decades of expertise, Doherty, along with UCSB colleague Baron Peters and former graduate student Mark Joswiak (now at Dow Chemical) have developed a computational method to help predict growth rates for ionic crystals under different circumstances. Using a relatively simple crystal — sodium chloride (NaCl, more familiarly known as table salt) — in water, the researchers laid the groundwork for the analysis of more complex crystals.

Ionic crystals may appear to the naked eye — and even under some magnification — to consist of perfectly smooth and even faces. But look more closely and you’ll often find they actually contain surface features that influence their ability to grow, and the larger shapes that they take.

“There are dislocations and around the dislocations there are spirals, and around the spirals there are edges, and around the edges there are kinks,” Peters said, “and every level requires a theory to describe the number of those features and the rates at which they change.” At the smallest scale, ions in solution cannot readily attach to the growing crystal because water molecules that solvate (interact with) the ions are not readily dislodged, he said. With so many processes occurring at so many scales, it’s easy to see how difficult it can be to predict a crystal’s growth.

“The largest challenge was applying the various techniques and methods to a new problem — examining ion attachment and detachment at surface kink sites, where there is a lack of symmetry coupled with strong ion-water interactions,” Joswiak said. “However, as we encountered problems and found solutions, we gained additional insight on the processes, the role of water molecules and differences between sodium and chloride ions.”

Among their insights: Ion size matters. The researchers found that due to its size, the larger chloride ion (Cl-) prevents water from accessing kink sites during detachment, limiting the overall rate of sodium chloride dissolution in water.

“You have to find a special coordinate system that can reveal those special solvent rearrangements that create an opening for the ion to slip through the solvent cage and lock onto the kink site,” Peters said. “We demonstrated that at least for sodium chloride we can finally give a concrete answer.”

This proof-of-concept development is the result of the Doherty Group’s expertise with crystallization processes coupled with the Peters Group’s expertise in “rare events” — relatively infrequent and short-lived but highly significant phenomena (such as reactions) that fundamentally change the state of the system. Using a method called transition path sampling, the researchers were able to understand the events leading up to the transition state. The strategy and mechanistic insights from the work on sodium chloride provides a blueprint for predicting growth rates in materials synthesis, pharmaceuticals and biomineralization.

Boston Semi Equipment (BSE), a global semiconductor test handler manufacturer and provider of test automation technical services, today announced that it has started shipping units of its new strip load/unload module to a top 10 semiconductor manufacturer. The automation modules handle magazines containing strips holding semiconductor devices. The freestanding modules dock to strip-processing equipment via a SMEMA-compliant interface. Operators set up and control the modules using a color touch-screen monitor.

“BSE’s custom engineering group works with semiconductor companies to provide them the exact automation solutions they require,” said Kevin Brennan, vice president of marketing for BSE. “Our multidisciplined team started with our customer’s specification for the strip automation module, and handled the project from concept through to manufacturing of final units. With our global service organization, we can support these modules anywhere in the world.”

BSE’s custom engineering group helps companies accelerate their internal product development activities. Working with BSE, companies can implement cost savings and productivity improvement solutions sooner, helping to grow their market share and improve profits.

 

Conventional electronics rely on controlling electric charge. Recently, researchers have been exploring the potential for a new technology, called spintronics, that relies on detecting and controlling a particle’s spin. This technology could lead to new types of more efficient and powerful devices.

In a paper published in Applied Physics Letters, from AIP Publishing, researchers measured how strongly a charge carrier’s spin interacts with a magnetic field in diamond. This crucial property shows diamond as a promising material for spintronic devices.

Diamond is attractive because it would be easier to process and fabricate into spintronic devices than typical semiconductor materials, said Golrokh Akhgar, a physicist at La Trobe University in Australia. Conventional quantum devices are based on multiple thin layers of semiconductors, which require an elaborate fabrication process in an ultrahigh vacuum.

“Diamond is normally an extremely good insulator,” Akhgar said. But, when exposed to hydrogen plasma, the diamond incorporates hydrogen atoms into its surface. When a hydrogenated diamond is introduced to moist air, it becomes electrically conductive because a thin layer of water forms on its surface, pulling electrons from the diamond. The missing electrons at the diamond surface behave like positively charged particles, called holes, making the surface conductive.

Researchers found that these holes have many of the right properties for spintronics. The most important property is a relativistic effect called spin-orbit coupling, where the spin of a charge carrier interacts with its orbital motion. A strong coupling enables researchers to control the particle’s spin with an electric field.

In previous work, the researchers measured how strongly a hole’s spin-orbit coupling could be engineered with an electric field. They also showed that an external electric field could tune the strength of the coupling.

In recent experiments, the researchers measured how strongly a hole’s spin interacts with a magnetic field. For this measurement, the researchers applied constant magnetic fields of different strengths parallel to the diamond surface at temperatures below 4 Kelvin. They also simultaneously applied a steadily varying perpendicular field. By monitoring how the electrical resistance of the diamond changed, they determined the g-factor. This quantity could help researchers control spin in future devices using a magnetic field.

“The coupling strength of carrier spins to electric and magnetic fields lies at the heart of spintronics,” Akhgar said. “We now have the two crucial parameters for the manipulation of spins in the conductive surface layer of diamond by either electric or magnetic fields.”

Additionally, diamond is transparent, so it can be incorporated into optical devices that operate with visible or ultraviolet light. Nitrogen-vacancy diamonds — which contain nitrogen atoms paired with missing carbon atoms in its crystal structure — show promise as a quantum bit, or qubit, the basis for quantum information technology. Being able to manipulate spin and use it as a qubit could lead to yet more devices with untapped potential, Akhgar said.

The research team that announced the first optical rectenna in 2015 is now reporting a two-fold efficiency improvement in the devices — and a switch to air-stable diode materials. The improvements could allow the rectennas – which convert electromagnetic fields at optical frequencies directly to electrical current – to operate low-power devices such as temperature sensors.

Ultimately, the researchers believe their device design – a combination of a carbon nanotube antenna and diode rectifier – could compete with conventional photovoltaic technologies for producing electricity from sunlight and other sources. The same technology used in the rectennas could also directly convert thermal energy to electricity.

Georgia Tech researchers have developed a new higher efficiency rectenna design. Here, the device’s ability to convert blue light to electricity is tested. (Credit: Christopher Moore, Georgia Tech)

Georgia Tech researchers have developed a new higher efficiency rectenna design. Here, the device’s ability to convert blue light to electricity is tested. (Credit: Christopher Moore, Georgia Tech)

“This work takes a significant leap forward in both fundamental understanding and practical efficiency for the optical rectenna device,” said Baratunde Cola, an associate professor in the George W. Woodruff School of Mechanical Engineering at the Georgia Institute of Technology. “It opens up this technology to many more researchers who can join forces with us to advance the optical rectenna technology to help power a range of applications, including space flight.”

The research was reported January 26 in the journal Advanced Electronic Materials. The work has been supported by the U.S. Army Research Office under the Young Investigator Program, and by the National Science Foundation.

Optical rectennas operate by coupling the light’s electromagnetic field to an antenna, in this case an array of multiwall carbon nanotubes whose ends have been opened. The electromagnetic field creates an oscillation in the antenna, producing an alternating flow of electrons. When the electron flow reaches a peak at one end of the antenna, the diode closes, trapping the electrons, then re-opens to capture the next oscillation, creating a current flow.

The switching must occur at terahertz frequencies to match the light. The junction between the antenna and diode must provide minimal resistance to electrons flowing through it while open, yet prevent leakage while closed.

“The name of the game is maximizing the number of electrons that get excited in the carbon nanotube, and then having a switch that is fast enough to capture them at their peak,” Cola explained. “The faster you switch, the more electrons you can catch on one side of the oscillation.”

To provide a low work function – ease of electron flow – the researchers initially used calcium as the metal in their oxide insulator – metal diode junction. But calcium breaks down rapidly in air, meaning the device had to be encapsulated during operation – and fabricated in a glovebox. That made the optical rectenna both impractical for most applications and difficult to fabricate.

So Cola, NSF Graduate Research Fellow Erik Anderson and Research Engineer Thomas Bougher replaced the calcium with aluminum and tried a variety of oxide materials on the carbon nanotubes before settling on a bilayer material composed of alumina (Al2O3) and hafnium dioxide (HfO2). The combination coating for the carbon nanotube junction, created through an atomic deposition process, provides the quantum mechanical electron tunneling properties required by engineering the oxide electronic properties instead of the metals, which allows air stable metals with higher work functions than calcium to be used.

Rectennas fabricated with the new combination have remained functional for as long as a year. Other metal oxides could also be used, Cola said.

The researchers also engineered the slope of the hill down which the electrons fall in the tunneling process. That also helped increase the efficiency, and allows the use of a variety of oxide materials. The new design also increased the asymmetry of the diodes, which boosted efficiency.

“By working with the oxide electron affinity, we were able to increase the asymmetry by more than ten-fold, making this diode design more attractive,” said Cola. “That’s really where we got the efficiency gain in this new version of the device.”

Optical rectennas could theoretically compete with photovoltaic materials for converting sunlight into electricity. PV materials operate using a different principle, in which photons knock electrons from the atoms of certain materials. The electrons are collected into electrical current.

In September 2015 in the journal Nature Nanotechnology, Cola and Bougher reported the first optical rectenna – a device that had been proposed theoretically for more than 40 years, but never demonstrated.

The early version reported in the journal produced power at microvolt levels. The rectenna now produces power in the millivolt range and conversion efficiency has gone from 10-5 to 10-3 – still very low, but a significant gain.

“Though there still is room for significant improvement, this puts the voltage in the range where you could see optical rectennas operating low-power sensors,” Cola said. “There are a lot of device geometry steps you could take to do something useful with the optical rectenna today in voltage-driven devices that don’t require significant current.”

Cola believes the rectennas could be useful for powering internet of things devices, especially if they can be used to produce electricity from scavenged thermal energy. For converting heat to electricity, the principle is the same as for light – capturing oscillations in a field with the broadband carbon nanotube antenna.

“People have been excited about thermoelectric generators, but there are many limitations on getting a system that works effectively,” he said. “We believe that the rectenna technology will be the best approach for harvesting heat economically.”

In future work, the research team hopes to optimize the antenna operation, and improve their theoretical understanding of how the rectenna works, allowing further optimization. One day, Cola hopes the devices will help accelerate space travel, producing power for electric thrusters that will boost spacecraft.

“Our end game is to see carbon nanotube optical rectennas working on Mars and in the spacecraft that takes us to Mars,” he said.

This work was supported by the Army Research Office under the Young Investigator Program agreement W911NF-13-1-0491 and the National Science Foundation Graduate Research Fellowship program under grant DGE-1650044. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the sponsoring organizations.

Researchers have identified a mechanism that triggers shape-memory phenomena in organic crystals used in plastic electronics. Shape-shifting structural materials are made with metal alloys, but the new generation of economical printable plastic electronics is poised to benefit from this phenomenon, too. Shape-memory materials science and plastic electronics technology, when merged, could open the door to advancements in low-power electronics, medical electronics devices and multifunctional shape-memory materials.

The findings are published in the journal Nature Communications and confirm the shape-memory phenomenon in two organic semiconductors materials.

Illinois chemistry and biomolecular engineering professor Ying Diao, right, and graduate student Hyunjoong Chung are part of a team that has identified a mechanism that triggers shape-memory in organic crystals used in plastic electronics. Credit: L. Brian Stauffer

Illinois chemistry and biomolecular engineering professor Ying Diao, right, and graduate student Hyunjoong Chung are part of a team that has identified a mechanism that triggers shape-memory in organic crystals used in plastic electronics. Credit: L. Brian Stauffer

Devices like the expandable stents that open and unblock clogged human blood vessels use shape-memory technology. Heat, light and electrical signals, or mechanic forces pass information through the devices telling them to expand, contract, bend and morph back into their original form – and can do so repeatedly, like a snake constricting to swallow its dinner. This effect works well with metals, but remains elusive in synthetic organic materials because of the complexity of the molecules used to create them.

“The shape-memory phenomenon is common in nature, but we are not really sure about nature’s design rules at the molecular level,” said professor of chemical and biomolecular engineering and co-author of the study, Ying Diao. “Nature uses organic compounds that are very different from the metal alloys used in shape-memory materials on the market today,” Diao said. “In naturally occurring shape-memory materials, the molecules transform cooperatively, meaning that they all move together during shape change. Otherwise, these materials would shatter and the shape change would not be reversible and ultrafast.”

The discovery of the shape-memory mechanism in synthetic organic material was quite serendipitous, Diao said. The team accidentally created large organic crystals and was curious to find out how they would transform given heat.

“We looked at the single crystals under a microscope and found that the transformation process is dramatically different than we expected,” said graduate student and co-author Hyunjoong Chung. “We saw concerted movement of a whole layer of molecules sweeping through the crystal that seem to drive the shape-memory effect – something that is rarely observed in organic crystals and is therefore largely unexplored.”

This unexpected observation led the team to want to explore the merger between shape-memory materials science and the field of organic electronics, the researchers said. “Today’s electronics are dependent on transistors to switch on and off, which is a very energy-intensive process,” Diao said. “If we can use the shape-memory effect in plastic semiconductors to modulate electronic properties in a cooperative manner, it would require very low energy input, potentially contributing to advancements in low-power and more efficient electronics.”

The team is currently using heat to demonstrate the shape-memory effect, but are experimenting with light waves, electrical fields and mechanical force for future demonstrations. They are also exploring the molecular origin of the shape-memory mechanism by tweaking the molecular structure of their materials. “We have already found that changing just one atom in a molecule can significantly alter the phenomenon,” Chung said.

The researchers are very excited about the molecular cooperativity aspect discovered with this research and its potential application to the recent Nobel Prize-winning concept of molecular machines, Diao said. “These molecules can change conformation cooperatively at the molecular level, and the small molecular structure change is amplified over millions of molecules to actuate large motion at the macroscopic scale.”

For the first time an international research group has revealed the core mechanism that limits the indium (In) content in indium gallium nitride ((In, Ga)N) thin films – the key material for blue light emitting diodes (LED). Increasing the In content in InGaN quantum wells is the common approach to shift the emission of III-Nitride based LEDs towards the green and, in particular, red part of the optical spectrum, necessary for the modern RGB devices. The new findings answer the long-standing research question: why does this classical approach fail, when we try to obtain efficient InGaN-based green and red LEDs?

This is a scanning transmission electron microscopy image of the atomic ordering in (In, Ga)N monolayer: single atomic column, containing only indium (In) atoms (shown by higher intensity on the image), followed by two, containing only gallium (Ga) atoms. Credit: IKZ Berlin

This is a scanning transmission electron microscopy image of the atomic ordering in (In, Ga)N monolayer: single atomic column, containing only indium (In) atoms (shown by higher intensity on the image), followed by two, containing only gallium (Ga) atoms. Credit: IKZ Berlin

Despite the progress in the field of green LEDs and lasers, the researchers could not overcome the limit of 30% of indium content in the films. The reason for that was unclear up to now: is it a problem of finding the right growth conditions or rather a fundamental effect that cannot be overcome? Now, an international team from Germany, Poland and China has shed new light on this question and revealed the mechanism responsible for that limitation.

In their work the scientists tried to push the indium content to the limit by growing single atomic layers of InN on GaN. However, independent on growth conditions, indium concentrations have never exceeded 25% – 30% – a clear sign of a fundamentally limiting mechanism. The researchers used advanced characterization methods, such as atomic resolution transmission electron microscope (TEM) and in-situ reflection high-energy electron diffraction (RHEED), and discovered that, as soon as the indium content reaches around 25 %, the atoms within the (In, Ga)N monolayer arrange in a regular pattern – single atomic column of In alternates with two atomic columns of Ga atoms. Comprehensive theoretical calculations revealed that the atomic ordering is induced by a particular surface reconstruction: indium atoms are bonded with four neighboring atoms, instead of expected three. This creates stronger bonds between indium and nitrogen atoms, which, on one hand, allows to use higher temperatures during the growth and provides material with better quality. On the other hand, the ordering sets the limit of the In content of 25%, which cannot be overcome under realistic growth conditions.

“Apparently, a technological bottleneck hampers all the attempts to shift the emission from the green towards the yellow and the red regions of the spectra. Therefore, new original pathways are urgently required to overcome these fundamental limitations,” states Dr. Tobias Schulz, scientist at the Leibniz-Institut fuer Kristallzuechtung; “for example, growth of InGaN films on high quality InGaN pseudo-substrates that would reduce the strain in the growing layer.”

However, the discovery of ordering may help to overcome well known limitations of the InGaN material system: localization of charge carriers due to fluctuations in the chemical composition of the alloy. Growing stable ordered (In, Ga)N alloys with the fixed composition at high temperatures could thus improve the optical properties of devices.

One of the big challenges in computer architecture is integrating storage, memory and processing in one unit. This would make computers faster and more energy efficient. University of Groningen physicists have taken a big step towards this goal by combining a niobium doped strontium titanate (SrTiO3) semiconductor with ferromagnetic cobalt. At the interface, this creates a spin-memristor with storage abilities, paving the way for neuromorphic computing architectures. The results were published on 22 January in Scientific Reports.

The device developed by the physicists combines the memristor effect of semiconductors with a spin-based phenomenon called tunnelling anisotropic magnetoresistance (TAMR) and works at room temperature. The SrTiO3 semiconductor has a non-volatile variable resistance when interfaced with cobalt: an electric field can be used to change it from low to high resistance and back. This is known as the electroresistance effect.

Tunability

Furthermore, when a magnetic field was applied across the same interface, in and out of the plane of the cobalt, this showed a tunablity of the TAMR spin voltage by 1.2 mV. This coexistence of both a large change in the value of TAMR and electroresistance across the same device at room temperature has not previously been demonstrated in other material systems.

‘This means we can store additional information in a non-volatile way in the memristor, thus creating a very simple and elegant integrated spin-memristor device that operates at room temperature’, explains Professor of Spintronics of Functional Materials Tamalika Banerjee. She works at the Zernike Institute for Advanced Materials at the University of Groningen. So far, attempts to combine spin-based storage, memory and computing have been hampered by a complex architecture in addition to other factors.

Brain

The key to the success of the Banerjee group device is the interface between cobalt and the semiconductor. ‘We have shown that a one-nanometre thick insulating layer of aluminium oxide makes the TAMR effect disappear’, says Banerjee. It took quite some work to engineer the interface. They did so by adjusting the niobium doping of the semiconductor and thus the potential landscape at the interface. The same coexistence can’t be realized with silicon as a semiconductor: ‘You need the heavy atoms in SrTiO3 for the spin orbit coupling at the interface that is responsible for the large TAMR effect at room temperature.’

These devices could be used in a brain-like computer architecture. They would act like the synapses that connect the neurons. The synapse responds to an external stimulus, but this response also depends on the synapse’s memory of previous stimuli. ‘We are now considering how to create a bio-inspired computer architecture based on our discovery.’ Such a system would move away from the classical Von Neumann architecture. The big advantage is that it is expected to use less energy and thus produce less heat. ‘This will be useful for the “Internet of Things”, where connecting different devices and networks generates unsustainable amounts of heat.’

Energy efficiency

The physics of what exactly happens at the interface of cobalt and the strontium semiconductor is complicated, and more work needs to be done to understand this. Banerjee: ‘Once we understand it better, we will be able to improve the performance of the system. We are currently working on that. But it works well as it is, so we are also thinking of building a more complex system with such spin-memristors to test actual algorithms for specific cognition capabilities of the human brain.’ Banerjee’s device is relatively simple. Scaling it up to a full computing architecture is the next big step.

How to integrate these devices in a parallel computing architecture that mimics the working of the brain is a question that fascinates Banerjee. ‘Our brain is a fantastic computer, in the sense that it can process vast amounts of information in parallel with an energy efficiency that is far superior to that of a supercomputer.’ Banerjee’s team’s findings could lead to new architectures for brain-inspired computing.

Sometimes it pays to be two-dimensional. The merits of graphene, a 2D sheet of carbon atoms, are well established. In its wake have followed a host of “post-graphene materials” – structural analogues of graphene made of other elements like silicon or germanium.

Now, an international research team led by Nagoya University (Japan) involving Aix-Marseille University (France), the Max Planck Institute in Hamburg (Germany) and the University of the Basque country (Spain) has unveiled the first truly planar sample of stanene: single sheets of tin (Sn) atoms. Planar stanene is hotly tipped as an extraordinary electrical conductor for high technology.

High-resolution STM image of stanene prepared on a Ag2Sn surface alloy. The honeycomb stanene structure model is superimposed. Credit: Junji Yuhara

High-resolution STM image of stanene prepared on a Ag2Sn surface alloy. The honeycomb stanene structure model is superimposed. Credit: Junji Yuhara

Just as graphene differs from ordinary graphite, so does stanene behave very differently to humble tin in bulk form. Because of relatively strong spin-orbit interactions for electrons in heavy elements, single-layer tin is predicted to be a “topological insulator,” also known as a quantum spin Hall (QSH) insulator. Materials in this remarkable class are electrically insulating in their interiors, but have highly conductive surfaces/edges. This, in theory, makes a single-layered topological insulator an ideal wiring material for nanoelectronics. Moreover, the highly conductive channels at the edge of these materials can carry special chiral currents with spins locked with transport directions, which makes them also very appealing for spintronics applications.

In previous studies, where stanene was grown on substrates of bismuth telluride or antimony, the tin layers turned out to be highly buckled and relatively inhomogeneous. The Nagoya team instead chose silver (Ag) as their host – specifically, the Ag(111) crystal facet, whose lattice constant is slightly larger than that of the freestanding stanene, leading to the formation of flattened tin monolayer in a large area, one step closer to the scalable industrial applications.

Individual tin atoms were slowly deposited onto silver, known as epitaxial growth. Crucially, the stanene layer did not form directly on top of the silver surface. Instead, as shown by core-level spectroscopy, the first step was the formation of a surface alloy (Ag2Sn) between the two species. Then, another round of tin deposition produced a layer of pure, highly crystalline stanene atop the alloy. Tunneling microscopy shows striking images of a honeycomb lattice of tin atoms, illustrating the hexagonal structure of stanene.

The alloy guaranteed the flatness of the tin layer, as confirmed by density-functional theory calculations. Junji Yuhara, lead author of an article by the team published in 2D Materials, explains: “Stanene follows the crystalline periodicity of the Ag2Sn surface alloy. Therefore, instead of buckling as it would in isolation, the stanene layer flattens out – at the cost of a slight strain – to maximize contact with the alloy beneath.” This mutual stabilization between stanene and host not only keeps the stanene layers impeccably flat, but lets them grow to impressive sizes of around 5,000 square nanometers.

Planar stanene has exciting prospects in electronics and computing. “The QSH effect is rather delicate, and most topological insulators only show it at low temperatures”, according to project team leader Guy Le Lay at Aix-Marseille University. “However, stanene is predicted to adopt a QSH state even at room temperature and above, especially when functionalized with other elements. In the future, we hope to see stanene partnered up with silicene in computer circuitry. That combination could drastically speed up computational efficiency, even compared with the current cutting-edge technology.”

Scientists used spiraling X-rays at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) to observe, for the first time, a property that gives handedness to swirling electric patterns – dubbed polar vortices – in a synthetically layered material.

This property, also known as chirality, potentially opens up a new way to store data by controlling the left- or right-handedness in the material’s array in much the same way magnetic materials are manipulated to store data as ones or zeros in a computer’s memory.

Researchers said the behavior also could be explored for coupling to magnetic or optical (light-based) devices, which could allow better control via electrical switching.

Chirality is present in many forms and at many scales, from the spiral-staircase design of our own DNA to the spin and drift of spiral galaxies; it can even determine whether a molecule acts as a medicine or a poison in our bodies.

A molecular compound known as d-glucose, for example, which is an essential ingredient for human life as a form of sugar, exhibits right-handedness. Its left-handed counterpart, l-glucose, though, is not useful in human biology.

“Chirality hadn’t been seen before in this electric structure,” said Elke Arenholz, a senior staff scientist at Berkeley Lab’s Advanced Light Source (ALS), which is home to the X-rays that were key to the study. The study was published online this week in the journal Proceedings of the National Academy of Sciences.

The experiments can distinguish between left-handed chirality and right-handed chirality in the samples’ vortices. “This offers new opportunities for fundamentally new science, with the potential to open up applications,” she said.

“Imagine that one could convert a right-handed form of a molecule to its left-handed form by applying an electric field, or artificially engineer a material with a particular chirality,” said Ramamoorthy Ramesh, a faculty senior scientist in Berkeley Lab’s Materials Sciences Division and associate laboratory director of the Lab’s Energy Technologies Area, who co-led the latest study.

Ramesh, who is also a professor of materials science and physics at UC Berkeley, custom-made the novel materials at UC Berkeley.

Padraic Shafer, a research scientist at the ALS and the lead author of the study, worked with Arenholz to carry out the X-ray experiments that revealed the chirality of the material.

The samples included a layer of lead titanate (PbTiO3) and a layer of strontium titanate (SrTiO3) sandwiched together in an alternating pattern to form a material known as a superlattice. The materials have also been studied for their tunable electrical properties that make them candidates for components in precise sensors and for other uses.

Neither of the two compounds show any handedness by themselves, but when they were combined into the precisely layered superlattice, they developed the swirling vortex structures that exhibited chirality.

“Chirality may have additional functionality,” Shafer said, when compared to devices that use magnetic fields to rearrange the magnetic structure of the material.

The electronic patterns in the material that were studied at the ALS were first revealed using a powerful electron microscope at Berkeley Lab’s National Center for Electron Microscopy, a part of the Lab’s Molecular Foundry, though it took a specialized X-ray technique to identify their chirality.

“The X-ray measurements had to be performed in extreme geometries that can’t be done by most experimental equipment,” Shafer said, using a technique known as resonant soft X-ray diffraction that probes periodic nanometer-scale details in their electronic structure and properties.

Spiraling forms of X-rays, known as circularly polarized X-rays, allowed researchers to measure both left-handed and right-handed chirality in the samples.

Arenholz, who is also a faculty member of the UC Berkeley Department of Materials Science & Engineering, added, “It took a lot of time to understand the results, and a lot of modeling and discussions.” Theorists at the University of Cantabria in Spain and their network of computational experts performed calculations of the vortex structures that aided in the interpretation of the X-ray data.

The same science team is pursuing studies of other types and combinations of materials to test the effects on chirality and other properties.

“There is a wide class of materials that could be substituted,” Shafer said, “and there is the hope that the layers could be replaced with even higher functionality materials.”

Researchers also plan to test whether there are new ways to control the chirality in these layered materials, such as by combining materials that have electrically switchable properties with those that exhibit magnetically switchable properties.

“Since we know so much about magnetic structures,” Arenholz said, “we could think of using this well-known connection with magnetism to implement this newly discovered property into devices.”