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Theoretically, two layers are better than one for solar-cell efficiency

Solar cells have come a long way, but inexpensive, thin film solar cells are still far behind more expensive, crystalline solar cells in efficiency. Now, a team of researchers suggests that using two thin films of different materials may be the way to go to create affordable, thin film cells with about 34% efficiency.

“Ten years ago I knew very little about solar cells, but it became clear to me they were very important,” said Akhlesh Lakhtakia, Evan Pugh University Professor and Charles Godfrey Binder Professor of Engineering Science and Mechanics, Penn State.

Investigating the field, he found that researchers approached solar cells from two sides, the optical side — looking on how the sun’s light is collected — and the electrical side — looking at how the collected sunlight is converted into electricity. Optical researchers strive to optimize light capture, while electrical researchers strive to optimize conversion to electricity, both sides simplifying the other.

“I decided to create a model in which both electrical and optical aspects will be treated equally,” said Lakhtakia. “We needed to increase actual efficiency, because if the efficiency of a cell is less than 30% it isn’t going to make a difference.” The researchers report their results in a recent issue of Applied Physics Letters.

Lakhtakia is a theoretician. He does not make thin films in a laboratory, but creates mathematical models to test the possibilities of configurations and materials so that others can test the results. The problem, he said, was that the mathematical structure of optimizing the optical and the electrical are very different.

Solar cells appear to be simple devices, he explained. A clear top layer allows sunlight to fall on an energy conversion layer. The material chosen to convert the energy, absorbs the light and produces streams of negatively charged electrons and positively charged holes moving in opposite directions. The differently charged particles get transferred to a top contact layer and a bottom contact layer that channel the electricity out of the cell for use. The amount of energy a cell can produce depends on the amount of sunlight collected and the ability of the conversion layer. Different materials react to and convert different wavelengths of light.

“I realized that to increase efficiency we had to absorb more light,” said Lakhtakia. “To do that we had to make the absorbent layer nonhomogeneous in a special way.”

That special way was to use two different absorbent materials in two different thin films. The researchers chose commercially available CIGS — copper indium gallium diselenide — and CZTSSe — copper zinc tin sulfur selenide — for the layers. By itself, CIGS’s efficiency is about 20% and CZTSSe’s is about 11%.

These two materials work in a solar cell because the structure of both materials is the same. They have roughly the same lattice structure, so they can be grown one on top of the other, and they absorb different frequencies of the spectrum so they should increase efficiency, according to Lakhtakia.

“It was amazing,” said Lakhtakia. “Together they produced a solar cell with 34% efficiency. This creates a new solar cell architecture — layer upon layer. Others who can actually make solar cells can find other formulations of layers and perhaps do better.”

According to the researchers, the next step is to create these experimentally and see what the options are to get the final, best answers.

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Materials provided by Penn State. Original written by A’ndrea Elyse Messer. Note: Content may be edited for style and length.

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Surface deep: Light-responsive top layer of plastic film induces movement

Azobenzene-containing plastic film is a peculiar material; its surface can change shape when exposed to light, making it a valuable component in modern technologies/devices like TV screens and solar cells. Scientists now show that only a thin, topmost layer of the light-dependent azobenzene-containing plastic film needs to be light-sensitive, rather than the entire film, opening up new ways to potentially reduce production costs and revolutionize its use.

So far, it had been widely accepted that the light-sensitive nature of this material extends throughout the whole film, but scientists did not understand what was causing the shape-shifting movement. A group of scientists led by Dr Takahiro Seki of Nagoya University, Japan, set out to figure out exactly how this happens; they have published their findings in the journal Scientific Reports.

They cite a well-studied phenomenon called Marangoni flow as their inspiration: owing to this phenomenon, differences in “surface tension” (the property by which the particles in the outermost layer of liquids are always attracted inwards, creating a boundary for the liquid) cause many soft, plastic films to move in a peculiar pattern. The most famous example of this phenomenon is the formation of “wine legs” or droplets of liquid evaporating and streaking down the surfaces of wine glasses.

They decided to test whether ultraviolet light triggered changes in the surface tension of azobenzene plastic film, and whether those changes resulted in the film moving. They chose to first cover azobenzene film with a very thin top layer that was light-sensitive, then exposed this film to UV radiation. Next, they did the same with film that was covered in a top layer unresponsive to light. To their excitement, the scientists found surface structural changes in the film with a light-sensitive top layer, but not in the film with a “light-insensitive” top layer. “This is the first time anyone has demonstrated that only the light responsiveness of a very thin ‘nanometer’ level layer is needed for azobenzene-containing film to alter its surface morphology under UV,” said Dr Seki.

An important observation of this study is that the movement of the material isn’t dependent on “light polarization,” or the direction in which light waves travel. If it were, that would suggest that there is another force on the molecular level affecting the whole film. Instead, Dr Seki concludes that it is probably the changes in chemical structure at the surface induced by the UV radiation that changes surface tension, inducing movement to the top of the film.

Describing the wider ramifications of their results, Dr Seki states: “We are only at the cusp of developing this discovery onto an industrial scale, but you can imagine how needing only a very small amount of light-sensitive material can reduce costs. Many optical devices like photocopiers, printers, and monitors depend on the light-based surface change in azobenzene polymer film. Based on our findings, azobenzene film can also act as an “actuator” (that part in a device that moves other parts) in nanomachinery.”

These newly discovered properties have vast implications, from improving the economics of production and lowering material prices, to advancing the field of nanotechnology itself.

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Physicists use oscillations of atoms to control a phase transition

The goal of “Femtochemistry” is to film and control chemical reactions with short flashes of light. Using consecutive laser pulses, atomic bonds can be excited precisely and broken as desired. So far, this has been demonstrated for selected molecules. Researchers at the University of Göttingen and the Max Planck Institute for Biophysical Chemistry have now succeeded in transferring this principle to a solid, controlling its crystal structure on the surface. The results have been published in the journal Nature.

The team, led by Jan Gerrit Horstmann and Professor Claus Ropers, evaporated an extremely thin layer of indium onto a silicon crystal and then cooled the crystal down to -220 degrees Celsius. While the indium atoms form conductive metal chains on the surface at room temperature, they spontaneously rearrange themselves into electrically insulating hexagons at such low temperatures. This process is known as the transition between two phases — the metallic and the insulating — and can be switched by laser pulses. In their experiments, the researchers then illuminated the cold surface with two short laser pulses and immediately afterwards observed the arrangement of the indium atoms using an electron beam. They found that the rhythm of the laser pulses has a considerable influence on how efficiently the surface can be switched to the metallic state.

This effect can be explained by oscillations of the atoms on the surface, as first author Jan Gerrit Horstmann explains: “In order to get from one state to the other, the atoms have to move in different directions and in doing so overcome a sort of hill, similar to a roller coaster ride. A single laser pulse is not enough for this, however, and the atoms merely swing back and forth. But like a rocking motion, a second pulse at the right time can give just enough energy to the system to make the transition possible.” In their experiments the physicists observed several oscillations of the atoms, which influence the conversion in very different ways.

Their findings not only contribute to the fundamental understanding of rapid structural changes, but also open up new perspectives for surface physics. “Our results show new strategies to control the conversion of light energy at the atomic scale,” says Ropers from the Faculty of Physics at the University of Göttingen, who is also a Director at the Max Planck Institute for Biophysical Chemistry. “The targeted control of the movements of atoms in solids using laser pulse sequences could also make it possible to create previously unobtainable structures with completely new physical and chemical properties.”

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Flexible material shows potential for use in fabrics to heat, cool

A film made of tiny carbon nanotubes (CNT) may be a key material in developing clothing that can heat or cool the wearer on demand. A new North Carolina State University study finds that the CNT film has a combination of thermal, electrical and physical properties that make it an appealing candidate for next-generation smart fabrics.

The researchers were also able to optimize the thermal and electrical properties of the material, allowing the material to retain its desirable properties even when exposed to air for many weeks. Moreover, these properties were achieved using processes that were relatively simple and did not need excessively high temperatures.

“Many researchers are trying to develop a material that is non-toxic and inexpensive, but at the same time is efficient at heating and cooling,” said Tushar Ghosh, co-corresponding author of the study. “Carbon nanotubes, if used appropriately, are safe, and we are using a form that happens to be inexpensive, relatively speaking. So it’s potentially a more affordable thermoelectric material that could be used next to the skin.” Ghosh is the William A. Klopman Distinguished Professor of Textiles in NC State’s Wilson College of Textiles.

“We want to integrate this material into the fabric itself,” said Kony Chatterjee, first author of the study and a Ph.D. student at NC State. “Right now, the research into clothing that can regulate temperature focuses heavily on integrating rigid materials into fabrics, and commercial wearable thermoelectric devices on the market aren’t flexible either.”

To cool the wearer, Chatterjee said, CNTs have properties that would allow heat to be drawn away from the body when an external source of current is applied.

“Think of it like a film, with cooling properties on one side of it and heating on the other,” Ghosh said.

The researchers measured the material’s ability to conduct electricity, as well as its thermal conductivity, or how easily heat passes through the material.

One of the biggest findings was that the material has relatively low thermal conductivity — meaning heat would not travel back to the wearer easily after leaving the body in order to cool it. That also means that if the material were used to warm the wearer, the heat would travel with a current toward the body, and not pass back out to the atmosphere.

The researchers were able to accurately measure the material’s thermal conductivity through a collaboration with the lab of Jun Liu, an assistant professor of mechanical and aerospace engineering at NC State. The researchers used a special experimental design to more accurately measure the material’s thermal conductivity in the direction that the electric current is moving within the material.

“You have to measure each property in the same direction to give you a reasonable estimate of the material’s capabilities,” said Liu, co-corresponding author of the study. “This was not an easy task; it was very challenging, but we developed a method to measure this, especially for thin flexible films.”

The research team also measured the ability of the material to generate electricity using a difference in temperature, or thermal gradient, between two environments. Researchers said that they could take advantage of this for heating, cooling, or to power small electronics.

Liu said that while these thermoelectric properties were important, it was also key that they found a material that was also flexible, stable in air, and relatively simple to make.

“The point of this paper isn’t that we achieved the best thermoelectric performance,” Liu said. “We achieved something that can be used as a flexible, electronic, soft material that’s easy to fabricate. It’s easy to prepare this material, and easy to achieve these properties.”

Ultimately, their vision for the project is to design a smart fabric that can heat and cool the wearer, along with energy harvesting. They believe that a smart garment could help reduce energy consumption.

“Instead of heating or cooling a whole dwelling or space, you would heat or cool the personal space around the body,” Ghosh said. “If we could get the thermostat down a degree or two, that could save a tremendous amount of energy.”

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Coordinating complex behaviors between hundreds of robots

In one of the more memorable scenes from the 2002 blockbuster film Minority Report, Tom Cruise is forced to hide from a swarm of spider-like robots scouring a towering apartment complex. While most viewers are likely transfixed by the small, agile bloodhound replacements, a computer engineer might marvel instead at their elegant control system.

In a building several stories tall with numerous rooms, hundreds of obstacles and thousands of places to inspect, the several dozen robots move as one cohesive unit. They spread out in a search pattern to thoroughly check the entire building while simultaneously splitting tasks so as to not waste time doubling back on their own paths or re-checking places other robots have already visited.

Such cohesion would be difficult for human controllers to achieve, let alone for an artificial controller to compute in real-time.

“If a control problem has three or four robots that live in a world with only a handful of rooms, and if the collaborative task is specified by simple logic rules, there are state-of-the-art tools that can compute an optimal solution that satisfies the task in a reasonable amount of time,” said Michael M. Zavlanos, the Mary Milus Yoh and Harold L. Yoh, Jr. Associate Professor of Mechanical Engineering and Materials Science at Duke University.

“And if you don’t care about the best solution possible, you can solve for a few more rooms and more complex tasks in a matter of minutes, but still only a dozen robots tops,” Zavlanos said. “Any more than that, and current algorithms are unable to overcome the sheer volume of possibilities in finding a solution.”

In a new paper published online on April 29 in the International Journal of Robotics Research, Zavlanos and his recent PhD graduate student, Yiannis Kantaros, who is now a postdoctoral researcher at the University of Pennsylvania, propose a new approach to this challenge called STyLuS*, for large-Scale optimal Temporal Logic Synthesis, that can solve problems massively larger than what current algorithms can handle, with hundreds of robots, tens of thousands of rooms and highly complex tasks, in a small fraction of the time.

To understand the basis of the new approach, one must first understand linear temporal logic, which is not nearly as scary as it sounds. Suppose you wanted to program a handful of robots to collect mail from a neighborhood and deliver it to the post office every day. Linear temporal logic is a way of writing down the commands needed to complete this task.

For example, these commands might include to visit each house in sequential order, return back to the post office and then wait for someone to retrieve the collected mail before setting out again. While this might be easy to say in English, it’s more difficult to express mathematically. Linear temporal logic can do so by using its own symbols which, although might look like Klingon to the common observer, they’re extremely useful for expressing complex control problems.

“The term linear is used because points in time have a unique successor based on discrete linear model of time, and temporal refers to the use of operators such as until, next, eventually and always,” said Kantaros. “Using this mathematical formalism, we can build complex commands such as ‘visit all the houses except house two,’ ‘visit houses three and four in sequential order,’ and ‘wait until you’ve been to house one before going to house five.’ “

To find robot controllers that satisfy such complex tasks, the location of each robot is mapped into a discrete data point called a “node.” Then, from each node, there exist multiple other nodes that are a potential next step for the robot.

A traditional controller searches through each one of these nodes and the potential paths to take between them before figuring out the best way to navigate its way through. But as the number of robots and locations to visit increase, and as the logic rules to follow become more sophisticated, the solution space becomes incredibly large in a very short amount of time.

A simple problem with five robots living in a world with ten houses could contain millions of nodes, capturing all possible robot locations and behaviors towards achieving the task. This requires a lot of memory to store and processing power to search through.

To skirt around this issue, the researchers propose a new method that, rather than constructing these incredibly large graphs in their entirety, instead creates smaller approximations with a tree structure. At every step of the process, the algorithm randomly selects one node from the large graph, adds it to the tree, and rewires the existing paths between the nodes in the tree to find more direct paths from start to finish.

“This means that as the algorithm progresses, this tree that we incrementally grow gets closer and closer to the actual graph, which we never actually construct,” said Kantaros. “Since our incremental graph is much smaller, it is easy to store in memory. Moreover, since this graph is a tree, graph search, which otherwise has exponential complexity, becomes very easy because now we only need to trace the sequence of parent nodes back to the root to find the desired path.”

It had been long accepted that growing trees could not be used to search the space of possible solutions for these types of robot control problems. But in the paper, Zavlanos and Kantaros show that they can make it work by implementing two clever tricks. First, the algorithm chooses the next node to add based on information about the task at hand, which allows the tree to quickly approximate a good solution to the problem. Second, even though the algorithm grows trees, it can still detect cycles in the original graph space that capture solutions to such temporal logic tasks.

The researchers show that this method will always find an answer if there is one, and it will always eventually find the best one possible. They also show that this method can arrive at that answer exponentially fast. Working with a problem of 10 robots searching through a 50-by-50 grid space — 250 houses to pick up mail — current state-of-the-art algorithms take 30 minutes to find an optimal solution.

STyLuS* does it in about 20 seconds.

“We have even solved problems with 200 robots that live on a 100-by-100 grid world, which is far too large for today’s algorithms to handle,” said Zavlanos. “While there currently aren’t any systems that use 200 robots to do something like deliver packages, there might be in the future. And they would need a control framework like STyLuS* to be able to deliver them while satisfying complex logic-based rules.”

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Adhesive film turns smartwatch into biochemical health monitoring system

UCLA engineers have designed a thin adhesive film that could upgrade a consumer smartwatch into a powerful health-monitoring system. The system looks for chemical indicators found in sweat to give a real-time snapshot of what’s happening inside the body. A study detailing the technology was published in the journal of Science Advances.

Smartwatches can already help keep track of how far you’ve walked, how much you’ve slept and your heart rate. Newer models even promise to monitor blood pressure. Working with a tethered smartphone or other devices, someone can use a smartwatch to keep track of those health indicators over a long period of time.

What these watches can’t do, yet, is monitor your body chemistry. For that, they need to track biomarker molecules found in body fluids that are highly specific indicators of our health, such as glucose and lactate, which tell how well your body’s metabolism is working.

To address that need, the researchers engineered a disposable, double-sided film that attaches to the underside of a smartwatch. The film can detect molecules such as metabolites and certain nutrients that are present in body sweat in very tiny amounts. They also built a custom smartwatch and an accompanying app to record data.

“The inspiration for this work came from recognizing that we already have more than 100 million smartwatches and other wearable tech sold worldwide that have powerful data-collection, computation and transmission capabilities,” said study leader Sam Emaminejad, an assistant professor of electrical and computer engineering at the UCLA Samueli School of Engineering. “Now we have come up with a solution to upgrade these wearables into health-monitoring platforms, enabling them to measure molecular-level information so that they give us a much deeper understanding of what’s happening inside our body in real time.”

The skin-touching side of the adhesive film collects and analyzes the chemical makeup of droplets of sweat. The watch-facing side turns those chemical signals into electrical ones that can be read, processed and then displayed on the smartwatch.

The co-lead authors on the paper are graduate student Yichao Zhao and postdoctoral scholar Bo Wang. Both are members of Emaminejad’s Interconnected and Integrated Bioelectronics Lab at UCLA.

“By making our sensors on a double-sided adhesive and vertically conductive film, we eliminated the need for the external connectors,” Zhao said. “In this way, not only have we made it easier to integrate sensors with consumer electronics, but we’ve also eliminated the effect of a user’s motion that can interfere with the chemical data collection.”

“By incorporating appropriate enzymatic-sensing layers in the film, we specifically targeted glucose and lactate, which indicate body metabolism levels, and nutrients such as choline,” Wang said.

While the team designed a custom smartwatch and app to work with the system, Wang said the concept could someday be applied to popular models of smartwatches.

The researchers tested the film on someone who was sedentary, someone doing office work and people engaged in vigorous activity, such as boxing, and found the system was effective in a wide variety of scenarios. They also noted that the stickiness of the film was sufficient for it to stay on the skin and on the watch without the need for a wrist strap for an entire day.

Over the past few years, Emaminejad has led research on using wearable technology to detect indicator molecules through sweat. This latest study shows a new way that such technologies could be widely adopted.

“We are particularly excited about our technology because by transforming our smartwatches and wearable tech into biomonitoring platforms, we could capture multidimensional, longitudinal and physiologically relevant datasets at an unprecedented scale, basically across hundreds of millions of people,” Emaminejad said. “This thin sensing film that works with a watch shows such a path forward.”

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Nanomaterial gives robots chameleon skin

A new film made of gold nanoparticles changes color in response to any type of movement. Its unprecedented qualities could allow robots to mimic chameleons and octopi — among other futuristic applications.

Unlike other materials that try to emulate nature’s color changers, this one can respond to any type of movement, like bending or twisting. Robots coated in it could enter spaces that might be dangerous or impossible for humans, and offer information just based on the way they look.

For example, a camouflaged robot could enter tough-to-access underwater crevices. If the robot changes color, biologists could learn about the pressures facing animals that live in these environments.

Although some other color-changing materials can also respond to motion, this one can be printed and programmed to display different, complex patterns that are difficult to replicate. The UC Riverside scientists who created this nanomaterial documented their process in a Nature Communications paper published this past week.

Nanomaterials are simply materials that have been reduced to an extremely small scale — tens of nanometers in width and length, or, about the size of a virus. When materials like silver or gold become smaller, their colors will change depending on their size, shape, and the direction they face.

“In our case, we reduced gold to nano-sized rods. We knew that if we could make the rods point in a particular direction, we could control their color,” said chemistry professor Yadong Yin. “Facing one way, they might appear red. Move them 45 degrees, and they change to green.”

The problem facing the research team was how to take millions of gold nanorods floating in a liquid solution and get them all to point in the same direction to display a uniform color.

Their solution was to fuse smaller magnetic nanorods onto the larger gold ones. The two different-sized rods were encapsulated in a polymer shield, so that they would remain side by side. That way, the orientation of both rods could be controlled by magnets.

“Just like if you hold a magnet over a pile of needles, they all point in the same direction. That’s how we control the color,” Yin said.

Once the nanorods are dried into a thin film, their orientation is fixed in place and they no longer respond to magnets. “But, if the film is flexible, you can bend and rotate it, and will still see different colors as the orientation changes,” Yin said.

Other materials, like butterfly wings, are shiny and colorful at certain angles, and can also change color when viewed at other angles. However, those materials rely on precisely ordered microstructures, which are difficult and expensive to make for large areas. But this new film can be made to coat the surface of any sized object just as easily as applying spray paint on a house.

Though futuristic robots are an ultimate application of this film, it can be used in many other ways. UC Riverside chemist Zhiwei Li, the first author on this paper, explained that the film can be incorporated into checks or cash as an authentication feature. Under normal lighting, the film is gray, but when you put on sunglasses and look at it through polarized lenses, elaborate patterns can be seen. In addition, the color contrast of the film may change dramatically if you twist the film.

The applications, in fact, are only limited by the imagination. “Artists could use this technology to create fascinating paintings that are wildly different depending on the angle from which they are viewed,” Li said. “It would be wonderful to see how the science in our work could be combined with the beauty of art.”

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From dark to light in a flash: Smart film lets windows switch autonomously

Researchers have developed a new easy-to-use smart optical film technology that allows smart window devices to autonomously switch between transparent and opaque states in response to the surrounding light conditions.

The proposed 3D hybrid nanocomposite film with a highly periodic network structure has empirically demonstrated its high speed and performance, enabling the smart window to quantify and self-regulate its high-contrast optical transmittance. As a proof of concept, a mobile-app-enabled smart window device for Internet of Things (IoT) applications has been realized using the proposed smart optical film with successful expansion to the 3-by-3-inch scale. This energy-efficient and cost-effective technology holds great promise for future use in various applications that require active optical transmission modulation.

Flexible optical transmission modulation technologies for smart applications including privacy-protection windows, zero-energy buildings, and beam projection screens have been in the spotlight in recent years. Conventional technologies that used external stimuli such as electricity, heat, or light to modulate optical transmission had only limited applications due to their slow response speeds, unnecessary color switching, and low durability, stability, and safety.

The optical transmission modulation contrast achieved by controlling the light scattering interfaces on non-periodic 2D surface structures that often have low optical density such as cracks, wrinkles, and pillars is also generally low. In addition, since the light scattering interfaces are exposed and not subject to any passivation, they can be vulnerable to external damage and may lose optical transmission modulation functions. Furthermore, in-plane scattering interfaces that randomly exist on the surface make large-area modulation with uniformity difficult.

Inspired by these limitations, a KAIST research team led by Professor Seokwoo Jeon from the Department of Materials Science and Engineering and Professor Jung-Wuk Hong of the Civil and Environmental Engineering Department used proximity-field nanopatterning (PnP) technology that effectively produces highly periodic 3D hybrid nanostructures, and an atomic layer deposition (ALD) technique that allows the precise control of oxide deposition and the high-quality fabrication of semiconductor devices.

The team then successfully produced a large-scale smart optical film with a size of 3 by 3 inches in which ultrathin alumina nanoshells are inserted between the elastomers in a periodic 3D nanonetwork.

This “mechano-responsive” 3D hybrid nanocomposite film with a highly periodic network structure is the largest smart optical transmission modulation film that exists. The film has been shown to have state-of-the-art optical transmission modulation of up to 74% at visible wavelengths from 90% initial transmission to 16% in the scattering state under strain. Its durability and stability were proved by more than 10,000 tests of harsh mechanical deformation including stretching, releasing, bending, and being placed under high temperatures of up to 70°C. When this film was used, the transmittance of the smart window device was adjusted promptly and automatically within one second in response to the surrounding light conditions. Through these experiments, the underlying physics of optical scattering phenomena occurring in the heterogeneous interfaces were identified. Their findings were reported in the online edition of Advanced Science on April 26. KAIST Professor Jong-Hwa Shin’s group and Professor Young-Seok Shim at Silla University also collaborated on this project.

Donghwi Cho, a PhD candidate in materials science and engineering at KAIST and co-lead author of the study, said, “Our smart optical film technology can better control high-contrast optical transmittance by relatively simple operating principles and with low energy consumption and costs.”

“When this technology is applied by simply attaching the film to a conventional smart window glass surface without replacing the existing window system, fast switching and uniform tinting are possible while also securing durability, stability, and safety. In addition, its wide range of applications for stretchable or rollable devices such as wall-type displays for a beam projection screen will also fulfill aesthetic needs,” he added.

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Magnetic nano-vortex: Swirling boundaries

For the first time, researchers at the Paul Scherrer Institute PSI have recorded a “3D film” of magnetic processes on the nanometer scale. This reveals a variety of dynamics inside the material, including the motion of swirling boundaries between different magnetic domains. The insights were gained with a method newly developed at the Swiss Light Source SLS. It could help to make magnetic data storage devices more compact and efficient. The researchers are publishing the results of their investigations today in the journal Nature Nanotechnology.

A magnetic sticker staying attached to a refrigerator door is hardly surprising. However, when one approaches the nanometre range (where one nanometer is one millionth of a millimeter), physicists still find magnets, and their behaviour, puzzling. At the same time, the effects occurring on this small scale are highly relevant for future technologies. Now, for the first time, PSI researchers were able to record a short “film” of the three-dimensional magnetic structure inside a material with nanoscale resolution.

“Magnetism plays a role in many ways in our everyday lives; but at this very small, fundamental level, the phenomena are not yet fully understood,” explains Claire Donnelly, lead author of the study. Donnelly was a researcher at PSI at the time of the experiment and now works at the University of Cambridge in the UK.

The researchers used X-ray light from the Swiss Light Source SLS at PSI and a special tomographic method they recently developed there, which they call “time-resolved ptychographic laminography.” Their team consisted of scientists at PSI and ETH Zurich, as well as in the UK. The sample they examined consisted of a gadolinium-cobalt compound patterned into a circular disc.

More than four days for seven images

“With our method we can non-destructively scan the material and from the data reconstruct several successive 3D images of the inner magnetic structure,” says PSI researcher Manuel Guizar-Sicairos. “We can visualise the orientation of the magnetic moment at every measured point in the material and represent them as tiny magnetic compass needles.”

Just like magnetic filings, these compass needles react to an external magnetic field and to each other, forming intricate patterns throughout the entire object. The patterns contain areas — so-called domains — in which the magnetisation points predominantly in one direction. The transitions between two such areas, i.e., the domain walls, are of particular interest to researchers: “People have proposed using them as memory bits, which could possibly be used to pack data even more tightly than when using the domains,” says Donnelly. The details of these domain walls have only recently been made visible in 3D at PSI, among other places, using state-of-the-art imaging methods.

In the present study, the researchers went one step further by mapping the motion of both the domains and the domain boundaries. “We have taken seven snapshots showing points in time that are only a quarter of a billionth of a second apart. In these we can see how a domain boundary moves back and forth.” It took the scientists a little more than four days of constant measuring to collect the data, which later yielded this sequence of seven images.

Like stroboscopic light

The observed movement of the domain boundary was repeatedly and specifically induced by the researchers themselves through an externally applied magnetic field. Their images were therefore not actually recorded within a quarter of a billionth of a second. Instead, the scientists created a time loop of the changing magnetic field and took images at different points in time within it- similar to stroboscopic light seemingly slowing down a repetitive movement.

The recording of the 3D images from inside the sample in turn draws on a basic principle from computed tomography (CT). Similar to medical CT scans, the X-rays were used to take many radioscopic images of the sample one after the other, each from a slightly different angle. From the data collected, the researchers were able to recover their 3D maps of the magnetisation using software they had developed for this purpose.

“With this method, we have not just achieved time-resolved 3D movies of the interior of an object,” says Donnelly happily. “We also have been able to map the nanoscale dynamics in a magnet. In other words, we have shown that our new technique is really relevant to the development of new technology.” And Guizar-Sicairos adds: “Our new method is also suitable for other materials and could therefore have many more useful applications in the future.”

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Why is ice so slippery?

The answer lies in a film of water that is generated by friction, one that is far thinner than expected and much more viscous than usual water through its resemblance to the “snow cones” of crushed ice we drink during the summer. This phenomenon was recently demonstrated by researchers from the CNRS and ENS-PSL, with support from the École polytechnique, in a study that appeared in Physical Review X on 2019, November 4.

The “slippery” nature of ice is generally attributed to the formation of a thin layer of liquid water generated by friction, which for instance allows an ice skater to “surf” on top of this liquid film. The properties of this thin layer of water had never been measured: its thickness remained largely unknown, while its properties, and even its very existence, were the subject of debate. What’s more, since liquid water is known to be a poor lubricant, how could this liquid film reduce friction and make ice slippery?

To solve this paradox, researchers from the Laboratoire de physique de l’ENS (CNRS/ENS-PSL /Sorbonne Université/Université de Paris), in collaboration with a team from the Laboratoire d’hydrodynamique (LadHyX,CNRS/École polytechnique), developed a device equipped with a tuning fork — similar to those used in music — that can “hear” the forces at work during ice gliding with remarkable precision. Despite the instrument’s size, which measures a few centimetres, it is sensitive enough to probe ice and analyse the properties of friction on a nanometric scale.

Thanks to their unique device, the scientists were able to clearly demonstrate for the first time that friction does indeed generate a film of liquid water. This film nevertheless offered a number of surprises: with a thickness measuring a few hundred nanometres to a micron, or one hundredth the thickness of a strand of hair, it is much thinner than theoretical estimates had suggested. Even more unexpectedly, this film is not at all “simple water,” but consists of water that is as viscous as oil, with complex viscoelastic properties. This unexpected behaviour suggests that surface ice does not completely transform into liquid water, but instead ends up in a mixed state similar to “snow cones,” a mix of ice water and crushed ice. The mystery of sliding on ice can therefore be found in the “viscous” nature of this film of water.

These results show that a thorough overhaul is needed of the theoretical descriptions that have been proposed to describe friction on ice. The unusual properties of meltwater are a key factor that has not been taken into consideration until now. This will help better understand the phenomenon of ice gliding, in winter sports for example, and will also help propose innovative solutions for increasing friction in order to avoid skidding on icy roads.

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