Category Archives: Process Materials

Soft materials are great at damping energy — that’s why rubber tires are so good at absorbing the shock of bumps and potholes. But if researchers are going to build autonomous soft systems, like soft robots, they’ll need a way to transmit energy through soft materials.

Now, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), in collaboration with colleagues at the California Institute of Technology, have developed a way to send mechanical signals through soft materials.

The research is described in the Proceedings of the National Academy of Sciences.

“Soft autonomous systems have received a lot of attention because, just like the human body or other biological systems, they can be adaptive and perform delicate movements. However, the highly dissipative nature of soft materials limits or altogether prevents certain functions,” said Jordan Raney, postdoctoral fellow at SEAS and first author of the paper. “By storing energy in the architecture itself we can make up for the energy losses due to dissipation, allowing the propagation of mechanical signals across long distances.”

The system uses the centuries-old concept of bistable beams — structures stable in two distinct state — to store and release elastic energy along the path of a wave. The system consists of a chain of bistable elastomeric beams connected by elastomeric linear springs. When those beams are deformed, they snap and store energy in the form of elastic deformation. As the signal moves down the elastomer, it snaps the beams back into place, releasing the stored energy and sending the signal downstream like a line of dominos. The bistable system prevents the signal from dissipating downstream.

“This design solves two fundamental problems in transmitting information through materials,” said Katia Bertoldi, the John L. Loeb Associate Professor of the Natural Sciences at SEAS and senior author of the paper.  “It not only overcomes dissipation, but it also eliminates dispersive effects, so that the signal propagates without distortion.  As such, we maintain signal strength and clarity from start to end.”

The beam geometry requires precise fabrication techniques. If the angle or thickness of one beam is off by one degree or millimeter, the whole system fails.

The team used advanced 3D printing techniques to fabricate the system.

“We’re developing new materials and printing methods that enable the fabrication of soft materials with programmable bistable elements,” said Jennifer A. Lewis, the Hansjorg Wyss Professor of Biologically Inspired Engineering and coauthor of the paper.

The team designed and printed a soft logic gate using this system. The gate, which looks like a tuning fork, can be controlled to act as either as an AND or as an OR gate.

“It’s amazing what you can do using simple beams — a building block that’s been around hundreds of years,” said Bertoldi. “You can do new stuff with a very old, well studied and very simple component.”

This research was supported by the National Science Foundation and the Harvard University Materials Research Science and Engineering Center (MRSEC).

Scientists at the Energy Department’s National Renewable Energy Laboratory (NREL), in collaboration with researchers at Shanghai Jiao Tong University (SJTU), devised a method to improve perovskite solar cells, making them more efficient and reliable with higher reproducibility.

The research, funded by the U.S. Department of Energy SunShot Initiative, involved hybrid halide perovskite solar cells and revealed treating them with a specific solution of methyl ammonium bromide (MABr) would repair defects, improving efficiency. The scientists converted a low-quality perovskite film with pinholes and small grains into a high-quality film without pinholes and with large grains. Doing so boosted the efficiency of the perovskite film in converting sunlight to 19 percent.

The efficiency of perovskites in converting sunlight into electricity has jumped from slightly less than 4 percent in 2009, when the first tests were done, to more than 22 percent today. However, the efficiency can fluctuate according to the skills of the researchers making perovskites at different laboratories, to somewhere between 15 percent and 20 percent.

Perovskite films are typically grown using a solution of precursor chemicals that form the crystals, which are then exposed to a second anti-solvent that removes the precursor solvent. The fast-crystallization process is almost an art. NREL researchers found that, because of the narrow time window for properly adding the anti-solvent, it is easy to miss that window and perovskite crystals with defects could form. Defects, like noncontinuous crystals and nonuniform crystals with relatively small crystallite sizes and pinholes, can significantly reduce the effectiveness of a perovskite cell.

The scientists from NREL and SJTU came up with a better method, using what’s called the Ostwald ripening process. The process involves small crystals dissolving and then redepositing onto larger crystals. The researchers were able to induce the Ostwald ripening process by treating the perovskite with a MABr solution. The amount of the solution proved key, as the ideal was proven to be about 2 milligrams per milliliter.

“With the Ostwald ripening process, different-sized nanocrystals formed with different film qualities could then grow into pinhole-free perovskite films with similar large crystal sizes,” the researchers noted in the article. “Thus, this new chemical approach enhances processing tolerance to the initial perovskite film quality and improves the reproducibility of device fabrication.”

The improved film quality made the cells more stable. The perovskite cells treated with MABr were shown to be more efficient than those without the treatment. Untreated cells had an efficiency of about 14 percent to 17 percent, while cells treated with the MABr solution had an efficiency of more than 19 percent.

To continue advancing, next-generation electronic devices must fully exploit the nanoscale, where materials span just billionths of a meter. But balancing complexity, precision, and manufacturing scalability on such fantastically small scales is inevitably difficult. Fortunately, some nanomaterials can be coaxed into snapping themselves into desired formations-a process called self-assembly.

Scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have just developed a way to direct the self-assembly of multiple molecular patterns within a single material, producing new nanoscale architectures. The results were published in the journal Nature Communications.

“This is a significant conceptual leap in self-assembly,” said Brookhaven Lab physicist Aaron Stein, lead author on the study. “In the past, we were limited to a single emergent pattern, but this technique breaks that barrier with relative ease. This is significant for basic research, certainly, but it could also change the way we design and manufacture electronics.”

Microchips, for example, use meticulously patterned templates to produce the nanoscale structures that process and store information. Through self-assembly, however, these structures can spontaneously form without that exhaustive preliminary patterning. And now, self-assembly can generate multiple distinct patterns-greatly increasing the complexity of nanostructures that can be formed in a single step.

“This technique fits quite easily into existing microchip fabrication workflows,” said study coauthor Kevin Yager, also a Brookhaven physicist. “It’s exciting to make a fundamental discovery that could one day find its way into our computers.”

The experimental work was conducted entirely at Brookhaven Lab’s Center for Functional Nanomaterials (CFN), a DOE Office of Science User Facility, leveraging in-house expertise and instrumentation.

Cooking up organized complexity

The collaboration used block copolymers-chains of two distinct molecules linked together-because of their intrinsic ability to self-assemble.

“As powerful as self-assembly is, we suspected that guiding the process would enhance it to create truly ‘responsive’ self-assembly,” said study coauthor Greg Doerk of Brookhaven. “That’s exactly where we pushed it.”

To guide self-assembly, scientists create precise but simple substrate templates. Using a method called electron beam lithography-Stein’s specialty-they etch patterns thousands of times thinner than a human hair on the template surface. They then add a solution containing a set of block copolymers onto the template, spin the substrate to create a thin coating, and “bake” it all in an oven to kick the molecules into formation. Thermal energy drives interaction between the block copolymers and the template, setting the final configuration-in this instance, parallel lines or dots in a grid.

“In conventional self-assembly, the final nanostructures follow the template’s guiding lines, but are of a single pattern type,” Stein said. “But that all just changed.”

Lines and dots, living together

The collaboration had previously discovered that mixing together different block copolymers allowed multiple, co-existing line and dot nanostructures to form.

“We had discovered an exciting phenomenon, but couldn’t select which morphology would emerge,” Yager said. But then the team found that tweaking the substrate changed the structures that emerged. By simply adjusting the spacing and thickness of the lithographic line patterns-easy to fabricate using modern tools-the self-assembling blocks can be locally converted into ultra-thin lines, or high-density arrays of nano-dots.

“We realized that combining our self-assembling materials with nanofabricated guides gave us that elusive control. And, of course, these new geometries are achieved on an incredibly small scale,” said Yager.

“In essence,” said Stein, “we’ve created ‘smart’ templates for nanomaterial self-assembly. How far we can push the technique remains to be seen, but it opens some very promising pathways.”

Gwen Wright, another CFN coauthor, added, “Many nano-fabrication labs should be able to do this tomorrow with their in-house tools-the trick was discovering it was even possible.”

The scientists plan to increase the sophistication of the process, using more complex materials in order to move toward more device-like architectures.

“The ongoing and open collaboration within the CFN made this possible,” said Charles Black, director of the CFN. “We had experts in self-assembly, electron beam lithography, and even electron microscopy to characterize the materials, all under one roof, all pushing the limits of nanoscience.”

The newest Airbus and Boeing passenger jets flying today are made primarily from advanced composite materials such as carbon fiber reinforced plastic — extremely light, durable materials that reduce the overall weight of the plane by as much as 20 percent compared to aluminum-bodied planes. Such lightweight airframes translate directly to fuel savings, which is a major point in advanced composites’ favor.

But composite materials are also surprisingly vulnerable: While aluminum can withstand relatively large impacts before cracking, the many layers in composites can break apart due to relatively small impacts — a drawback that is considered the material’s Achilles’ heel.

Now MIT aerospace engineers have found a way to bond composite layers in such a way that the resulting material is substantially stronger and more resistant to damage than other advanced composites. Their results are published this week in the journal Composites Science and Technology.

The researchers fastened the layers of composite materials together using carbon nanotubes — atom-thin rolls of carbon that, despite their microscopic stature, are incredibly strong. They embedded tiny “forests” of carbon nanotubes within a glue-like polymer matrix, then pressed the matrix between layers of carbon fiber composites. The nanotubes, resembling tiny, vertically-aligned stitches, worked themselves within the crevices of each composite layer, serving as a scaffold to hold the layers together.

In experiments to test the material’s strength, the team found that, compared with existing composite materials, the stitched composites were 30 percent stronger, withstanding greater forces before breaking apart.

Roberto Guzman, who led the work as an MIT postdoc in the Department of Aeronautics and Astronautics (AeroAstro), says the improvement may lead to stronger, lighter airplane parts — particularly those that require nails or bolts, which can crack conventional composites.

“More work needs to be done, but we are really positive that this will lead to stronger, lighter planes,” says Guzman, who is now a researcher at the IMDEA Materials Institute, in Spain. “That means a lot of fuel saved, which is great for the environment and for our pockets.”

The study’s co-authors include AeroAstro professor Brian Wardle and researchers from the Swedish aerospace and defense company Saab AB.

“Size matters”

Today’s composite materials are composed of layers, or plies, of horizontal carbon fibers, held together by a polymer glue, which Wardle describes as “a very, very weak, problematic area.” Attempts to strengthen this glue region include Z-pinning and 3-D weaving — methods that involve pinning or weaving bundles of carbon fibers through composite layers, similar to pushing nails through plywood, or thread through fabric.

“A stitch or nail is thousands of times bigger than carbon fibers,” Wardle says. “So when you drive them through the composite, you break thousands of carbon fibers and damage the composite.”

Carbon nanotubes, by contrast, are about 10 nanometers in diameter — nearly a million times smaller than the carbon fibers.

“Size matters, because we’re able to put these nanotubes in without disturbing the larger carbon fibers, and that’s what maintains the composite’s strength,” Wardle says. “What helps us enhance strength is that carbon nanotubes have 1,000 times more surface area than carbon fibers, which lets them bond better with the polymer matrix.”

Stacking up the competition

Guzman and Wardle came up with a technique to integrate a scaffold of carbon nanotubes within the polymer glue. They first grew a forest of vertically-aligned carbon nanotubes, following a procedure that Wardle’s group previously developed. They then transferred the forest onto a sticky, uncured composite layer and repeated the process to generate a stack of 16 composite plies — a typical composite laminate makeup — with carbon nanotubes glued between each layer.

To test the material’s strength, the team performed a tension-bearing test — a standard test used to size aerospace parts — where the researchers put a bolt through a hole in the composite, then ripped it out. While existing composites typically break under such tension, the team found the stitched composites were stronger, able to withstand 30 percent more force before cracking.

The researchers also performed an open-hole compression test, applying force to squeeze the bolt hole shut. In that case, the stitched composite withstood 14 percent more force before breaking, compared to existing composites.

“The strength enhancements suggest this material will be more resistant to any type of damaging events or features,” Wardle says. “And since the majority of the newest planes are more than 50 percent composite by weight, improving these state-of-the art composites has very positive implications for aircraft structural performance.”

A team of scientists led by the Department of Energy’s Oak Ridge National Laboratory has developed a novel way to produce two-dimensional nanosheets by separating bulk materials with nontoxic liquid nitrogen. The environmentally friendly process generates a 20-fold increase in surface area per sheet, which could expand the nanomaterials’ commercial applications.

ORNL's Huiyuan Zhu places a sample of boron nitride, or "white graphene," into a furnace as part of a novel, nontoxic gas exfoliation process to separate 2-D nano materials. Credit: ORNL

ORNL’s Huiyuan Zhu places a sample of boron nitride, or “white graphene,” into a furnace as part of a novel, nontoxic gas exfoliation process to separate 2-D nano materials. Credit: ORNL

“It’s actually a very simple procedure,” said ORNL chemist Huiyuan Zhu, who co-authored a study published in Angewandte Chemie International Edition. “We heated commercially available boron nitride in a furnace to 800 degrees Celsius to expand the material’s 2D layers. Then, we immediately dipped the material into liquid nitrogen, which penetrates through the interlayers, gasifies into nitrogen, and exfoliates, or separates, the material into ultrathin layers.”

Nanosheets of boron nitride could be used in separation and catalysis, such as transforming carbon monoxide to carbon dioxide in gasoline-powered engines. They also may act as an absorbent to mop up hazardous waste. Zhu said the team’s controlled gas exfoliation process could be used to synthesize other 2D nanomaterials such as graphene, which has potential applications in semiconductors, photovoltaics, electrodes and water purification.

Because of the versatility and commercial potential of one-atom-thick 2D nanomaterials, scientists are seeking more efficient ways to produce larger sheets. Current exfoliation procedures use harsh chemicals that produce hazardous byproducts and reduce the amount of surface area per nanosheet, Zhu said.

“In this particular case, the surface area of the boron nitride nanosheets is 278 square meters per gram, and the commercially available boron nitride material has a surface area of only 10 square meters per gram,” Zhu said. “With 20 times more surface area, boron nitride can be used as a great support for catalysis.”

Further research is planned to expand the surface area of boron nitride nanosheets and also test their feasibility in cleaning up engine exhaust and improving the efficiency of hydrogen fuel cells.

Engineers from the University of Utah and the University of Minnesota have discovered that interfacing two particular oxide-based materials makes them highly conductive, a boon for future electronics that could result in much more power-efficient laptops, electric cars and home appliances that also don’t need cumbersome power supplies.

Their findings were published this month in the scientific journal, APL Materials, from the American Institute of Physics.

The team led by University of Utah electrical and computer engineering assistant professor Berardi Sensale-Rodriguez and University of Minnesota chemical engineering and materials science assistant professor Bharat Jalan revealed that when two oxide compounds — strontium titanate (STO) and neodymium titanate (NTO) — interact with each other, the bonds between the atoms are arranged in a way that produces many free electrons, the particles that can carry electrical current. STO and NTO are by themselves known as insulators — materials like glass — that are not conductive at all.

But when they interface, the amount of electrons produced is a hundred times larger than what is possible in semiconductors. “It is also about five times more conductive than silicon [the material most used in electronics],” Sensale-Rodriguez says.

This innovation could greatly improve power transistors — devices in electronics that regulate the electrical current –by making power supplies much more efficient for items ranging from televisions and refrigerators to handheld devices, Sensale-Rodriguez says. Today, electronics manufacturers use a material called gallium nitride for transistors in power supplies and other electronics that carry large electrical currents. But that material has been explored and optimized for many years and likely cannot be made more efficient. In this discovery made by the Utah and Minnesota team, the interface between STO and NTO can be at the very least as conductive as gallium nitride and likely will be much more in the future.

“When I look at the future, I see that we can perhaps improve conductivity by an order of magnitude through optimizing of the materials growth,” Jalan says. “We are bringing the possibility of high power, low energy oxide electronics closer to reality.”

Power transistors that use this combination of materials could lead to smaller devices and appliances because their power supplies would be more energy efficient. Laptop computers, for example, could ditch the bulky external power supplies — the big black boxes attached to the power cords — in favor of smaller supplies that are instead built inside the computer. Large appliances that consume a lot of electricity such as air conditioners could be more power efficient. And because there is less power wasted (wasted electricity usually dissipates into heat), these devices will not run as hot as before, says Sensale-Rodriguez. He also believes that if more electronics use these materials for transistors, collectively it could save significant amounts of electricity for the country.

“It’s fundamentally a different road toward power electronics, and the results are very exciting” he says. “But we still need to do more research.”

The old rules don’t necessarily apply when building electronic components out of two-dimensional materials, according to scientists at Rice University.

The Rice lab of theoretical physicist Boris Yakobson analyzed hybrids that put 2-D materials like graphene and boron nitride side by side to see what happens at the border. They found that the electronic characteristics of such “co-planar” hybrids differ from bulkier components.

Hybrids of two-dimensional materials like the graphene-molybdenum disulfide illustrated here have electronic properties that don't follow the same rules as their 3-D cousins, according to Rice University researchers. The limited direct contact between the two materials creates an electric field that greatly increases the size of the p/n junction. Credit: Henry Yu/Rice University

Hybrids of two-dimensional materials like the graphene-molybdenum disulfide illustrated here have electronic properties that don’t follow the same rules as their 3-D cousins, according to Rice University researchers. The limited direct contact between the two materials creates an electric field that greatly increases the size of the p/n junction. Credit: Henry Yu/Rice University

Their results appear this month in the American Chemical Society journal Nano Letters.

Shrinking electronics means shrinking their components. Academic labs and industries are studying how materials like graphene may enable the ultimate in thin devices by building all the necessary circuits into an atom-thick layer.

“Our work is important because semiconductor junctions are a big field,” Yakobson said. “There are books with iconic models of electronic behavior that are extremely well-developed and have become the established pillars of industry.

“But these are all for bulk-to-bulk interfaces between three-dimensional metals,” he said. “Now that people are actively working to make two-dimensional devices, especially with co-planar electronics, we realized that the rules have to be reconsidered. Many of the established models utilized in industry just don’t apply.”

The researchers led by Rice graduate student Henry Yu built computer simulations that analyze charge transfer between atom-thick materials.

“It was a logical step to test our theory on both metals and semiconductors, which have very different electronic properties,” Yu said. “This makes graphene, which is a metal — or a semimetal, to be precise — molybdenum disulfide and boron nitride, which are semiconductors, or even their hybrids ideal systems to study.

“In fact, these materials have been widely fabricated and used in the community for almost a decade, which makes analysis of them more appreciable in the field. Furthermore, both hybrids of graphene-molybdenum disulfide and graphene-boron nitride have been successfully synthesized recently, which means our study has practical meaning and can be tested in the lab now,” he said.

Yakobson said 3-D materials have a narrow region for charge transfer at the positive and negative (or p/n) junction. But the researchers found that 2-D interfaces created “a highly nonlocalized charge transfer” — and an electric field along with it — that greatly increased the junction size. That could give them an advantage in photovoltaic applications like solar cells, the researchers said.

The lab built a simulation of a hybrid of graphene and molybdenum disulfide and also considered graphene-boron nitride and graphene in which half was doped to create a p/n junction. Their calculations predicted the presence of an electric field should make 2-D Schottky (one-way) devices like transistors and diodes more tunable based on the size of the device itself.

How the atoms line up with each other is also important, Yakobson said. Graphene and boron nitride both feature hexagonal lattices, so they mesh perfectly. But molybdenum disulfide, another promising material, isn’t exactly flat, though it’s still considered 2-D.

“If the atomic structures don’t match, you get dangling bonds or defects along the borderline,” he said. “The structure has consequences for electronic behavior, especially for what is called Fermi level pinning.”

Pinning can degrade electrical performance by creating an energy barrier at the interface, Yakobson explained. “But your Schottky barrier (in which current moves in only one direction) doesn’t change as expected. This is a well-known phenomenon for semiconductors; it’s just that in two dimensions, it’s different, and in this case may favor 2-D over 3-D systems.”

Yakobson said the principles put forth by the new paper will apply to patterned hybrids of two or more 2-D patches. “You can make something special, but the basic effects are always at the interfaces. If you want to have many transistors in the same plane, it’s fine, but you still have to consider effects at the junctions.

“There’s no reason we can’t build 2-D rectifiers, transistors or memory elements,” he said. “They’ll be the same as we use routinely in devices now. But unless we develop a proper fundamental knowledge of the physics, they may fail to do what we design or plan.”

Researchers from North Carolina State University and the U.S. Army Research Office have developed a way to integrate novel functional materials onto a computer chip, allowing the creation of new smart devices and systems.

The novel functional materials are oxides, including several types of materials that, until now, could not be integrated onto silicon chips: multiferroic materials, which have both ferroelectric and ferromagnetic properties; topological insulators, which act as insulators in bulk but have conductive properties on their surface; and novel ferroelectric materials. These materials are thought to hold promise for applications including sensors, non-volatile computer memory and microelectromechanical systems, which are better known as MEMS.

“These novel oxides are normally grown on materials that are not compatible with computing devices,” says Jay Narayan, the John C. Fan Distinguished Chair Professor of Materials Science and Engineering at NC State and co-author of a paper describing the work. “We are now able to integrate these materials onto a silicon chip, allowing us to incorporate their functions into electronic devices.”

The approach developed by the researchers allows them to integrate the materials onto two platforms, both of which are compatible with silicon: a titanium nitride platform, for use with nitride-based electronics; and yttria-stabilized zirconia, for use with oxide-based electronics.

Specifically, the researchers developed a suite of thin films that serve as a buffer, connecting the silicon chip to the relevant novel materials. The exact combination of thin films varies, depending on which novel materials are being used.

For example, if using multiferroic materials, researchers use a combination of four different thin films: titanium nitride, magnesium oxide, strontium oxide and lanthanum strontium manganese oxide. But for topological insulators, they would use a combination of only two thin films: magnesium oxide and titanium nitride.

These thin film buffers align with the planes of the crystalline structure in the novel oxide materials, as well as with the planes of the underlying substrate – effectively serving as a communicating layer between the materials.

This approach, called thin film epitaxy, is based on the concept of domain-matching epitaxy, and was first proposed by Narayan in a 2003 paper.

“Integrating these novel materials onto silicon chips makes many things possible,” Narayan says. “For example, this allows us to sense or collect data; to manipulate that data; and to calculate a response – all on one compact chip. This makes for faster, more efficient, lighter devices.”

Another possible application, Narayan says, is the creation of LEDs on silicon chips, to make “smart lights.” Currently, LEDs are made using sapphire substrates, which aren’t directly compatible with computing devices.

“We’ve already patented this integration technology, and are currently looking for industry partners to license it,” Narayan says.

Quantum drag


July 20, 2016

Friction and drag are commonplace in nature. You experience these phenomena when riding in an airplane, pairing electrical wiring, or rubbing pieces of sandpaper together.

Friction and drag also exist at the quantum level, the realm of atoms and molecules invisible to the naked eye. But how these forces interact across materials and energy sources remain in doubt.

In a new study, University of Iowa theoretical physicist Michael Flatté proposes that a magnetic current flowing through a magnetic iron sheet will cause a current in a second, nearby magnetic iron sheet, even though the sheets aren’t connected. The movement is created, Flatté and his team say, when electrons whose magnetic spin is disturbed by the current on the first sheet exert a force, through electromagnetic radiation, to create magnetic spin in the second sheet.

The findings may prove beneficial in the emerging field of spintronics, which seeks to channel the energy from spin waves generated by electrons to create smaller, more energy-efficient computers and electronic devices.

“It means there are more ways to manipulate through magnetic currents than we thought, and that’s a good thing,” says Flatté, senior author and team leader on the paper published June 9 in the journal Physical Review Letters.

Flatté has been studying how currents in magnetic materials might be used to build electronic circuits at the nanoscale, where dimensions are measured in billionths of a meter, or roughly 1/50,000 the width of a human hair. Scientists knew that an electrical current introduced in a wire will drag a current in another nearby wire. Flatté’s team reasoned that the same effects may hold true for magnetic currents in magnetic layers.

In a magnetic substance, such as iron, each atom acts as a small, individual magnet. These atomic magnets tend to point in the same direction, like an array of tiny compasses fixated on a common magnetic point. But the slightest disturbance to the direction of just one of these atomic magnets throws the entire group into disarray: The collective magnetic strength in the group decreases. The smallest individual disturbance is called a magnon.

Flatté and his team report that a steady magnon current introduced into one iron magnetic layer will produce a magnon current in a second layer–in the same plane of the layer but at an angle to the introduced current. They propose that the electron spins disturbed in the layer where the current was introduced engage in a sort of “cross talk” with spins in the other layer, exerting a force that drags the spins along for the ride.

“What’s exciting is you get this response (in the layer with no introduced current), even though there’s no physical connection between the layers,” says Flatté, professor in the physics department and director of the Optical Science and Technology Center at the UI. “This is a physical reaction through electromagnetic radiation.”

How electrons in one layer communicate and dictate action to electrons in a separate layer is somewhat bizarre.

Take electricity: When an electrical current flows in one wire, a mutual friction drags current in a nearby wire. At the quantum level, the physical dynamics appear to be different. Imagine that each electron in a solid has an internal bar magnet, a tiny compass of sorts. In a magnetic material, those internal bar magnets are aligned. When heat or a current is applied to the solid, the electrons’ compasses get repositioned, creating a magnetic spin wave that ripples through the solid. In the theoretical case studied by Flatté, the disturbance to the solid excites magnons in one layer that then exert influence on the other layer, creating a spin wave in the other layer, even though it is physically separate.

“It turns out there is the same effect with spin waves,” Flatté says.

Contributing authors include Tianyu Liu with the physics and astronomy department at the UI and Giovanni Vignale at the University of Missouri, Columbia.

The U.S. National Science Foundation funded the research through grants to the Center for Emergent Materials.

With an eye to the next generation of tech gadgetry, a team of physicists at The University of Texas at Austin has had the first-ever glimpse into what happens inside an atomically thin semiconductor device. In doing so, they discovered that an essential function for computing may be possible within a space so small that it’s effectively one-dimensional.

In a paper published July 18 in the Proceedings of the National Academy of Sciences, the researchers describe seeing the detailed inner workings of a new type of transistor that is two-dimensional.

Transistors act as the building blocks for computer chips, sending the electrons on and off switches required for computer processing. Future tech innovations will require finding a way to fit more transistors on computer chips, so experts have begun exploring new semiconducting materials including one called molybdenum disulfide (MoS2). Unlike today’s silicon-based devices, transistors made from the new material allow for on-off signaling on a single flat plane.

Keji Lai, an assistant professor of physics, and a team found that with this new material, the conductive signaling happens much differently than with silicon, in a way that could promote future energy savings in devices. Think of silicon transistors as light bulbs: The whole device is either turned on or off at once. With 2-D transistors, by contrast, Lai and the team found that electric currents move in a more phased way, beginning first at the edges before appearing in the interior. Lai says this suggests the same current could be sent with less power and in an even tinier space, using a one-dimensional edge instead of the two-dimensional plane.

“In physics, edge states often carry a lot of interesting phenomenon, and here, they are the first to turn on. In the future, if we can engineer this material very carefully, then these edges can carry the full current,” Lai says. “We don’t really need the entire thing, because the interior is useless. Just having the edges running to get a current working would substantially reduce the power loss.”

Researchers have been working to get a view into what happens inside a 2-D transistor for years to better understand both the potential and the limitations of the new materials. Getting 2-D transistors ready for commercial devices, such as paper-thin computers and cellphones, is expected to take several more years. Lai says scientists need more information about what interferes with performance in devices made from the new materials.

“These transistors are perfectly two-dimensional,” Lai says. “That means they don’t have some of the defects that occur in a silicon device. On the other hand, that doesn’t mean the new material is perfect.”

Lai and his team used a microscope that he invented and that points microwaves at the 2-D device. Using a tip only 100 nanometers wide, the microwave microscope allowed the scientists to see conductivity changes inside the transistor. Besides seeing the currents’ motion, the scientists found thread-like defects in the middle of the transistors. Lai says this suggests the new material will need to be made cleaner to function optimally.

“If we could make the material clean enough, the edges will be carrying even more current, and the interior won’t have as many defects,” Lai says.

The paper’s other authors are postdoctoral researchers Di Wu and Xiao Li; research scientist Lan Luan, and graduate students Xiaoyu Wu and Zhaodong Chu, and professor Qian Niu in UT Austin’s Department of Physics; and graduate student Wei Li, former graduate student Maruthi N. Yogeesh, postdoctoral researcher Rudresh Ghosh, and associate professor Deji Akinwande of UT Austin’s Department of Electrical and Computer Engineering.

Earlier this year, both Lai and Akinwande won Presidential Early Career Awards for Scientists and Engineers, the U.S. government’s highest honor for early-stage scientists and engineers.