The Great Atomic Twist: How Tiny Magnets Learned to Do the Moiré Tango
An artist's impression of giant magnetic swirls dancing across atomic layers.
Imagine, if you will, the world’s thinnest sandwich. We aren't talking about a deli sub or even a fancy tea sandwich. We are talking about layers of material so thin that if you tried to pick one up, you’d be grabbing a single layer of atoms. Specifically, we’re looking at Chromium Triiodide, a material that physicists treat like a magnetic superstar. Normally, these layers sit quietly, doing their magnetic chores, but scientists recently decided to give them a bit of a spin—literally. By stacking two of these atomic sheets and twisting them relative to one another, they’ve unlocked a secret disco of magnetic patterns that no one saw coming.
This "twist" creates something called a moiré pattern. You’ve probably seen this effect in real life without knowing its fancy name. Have you ever looked through two screen doors overlapping and seen those weird, shifting wavy lines? That’s moiré! At the atomic level, when you twist two crystalline grids, you create a brand-new "super-grid" that is much larger than the original atoms. It’s like taking two fine-mesh sieves and rotating them until a giant, beautiful kaleidoscope pattern emerges. In the world of physics, this usually means the electrons have a new playground to run around in, changing how the material conducts electricity or holds onto data.
But here’s where the story gets weird and wonderful. In most materials, the magnetic patterns are expected to stay inside the lines of that moiré grid. It’s like a coloring book where the magnets are the crayons, and they usually stay within the borders of the squares. However, in Chromium Triiodide, the magnets decided they didn't care much for the rules. Instead of tiny, predictable patterns, the researchers spotted giant magnetic "tornadoes" called skyrmions. These aren't your average, run-of-the-mill skyrmions; they are absolute behemoths, stretching across hundreds of nanometers. To an atom, that’s like a human seeing a storm the size of a continent!
The real "aha!" moment came when the scientists realized that bigger isn't always better, but there is definitely a "Goldilocks" zone for twisting. You might assume that the more you twist the layers, the bigger or smaller the patterns would get in a straight line. Nope! The universe loves a good plot twist. The size of these magnetic giants actually peaks at a very specific angle. It’s like tuning a radio: if you turn the knob too far left or too far right, you get static. But when you hit that magical sweet spot, the signal—or in this case, the giant magnetic texture—comes through loud and clear. This "angle-controlled magnetism" is a total game-changer for how we think about designing materials from the ground up.
Why should we care about giant magnetic swirls on a tiny piece of atomic paper? Because these skyrmions are "topological." That’s a fancy way of saying they are incredibly stable. Think of them like a knot in a string; you can’t just shake the string and expect the knot to disappear. You have to physically untie it. In the world of computers and gadgets, stability is the holy grail. If we can use these magnetic knots to store information, we could create devices that don't lose their memory if they get bumped or lose power. Even better, moving these textures around takes very little energy, which means your future smartphone might stay charged for weeks instead of hours.
We are entering an era of "geometric electronics," where we don't necessarily need to change the chemistry of a material to make it do something new. We just have to be really good at origami. By folding, stacking, and twisting these atomic sheets, we are essentially building tiny machines out of pure geometry. It’s like playing with the world’s most advanced set of Legos, where the bricks are atoms and the instructions are written in the laws of quantum physics. This playful manipulation of Chromium Triiodide is just the beginning of a journey toward ultra-efficient, low-power spintronics—devices that use the "spin" of electrons rather than just their charge.
So, the next time you see a pattern shifting on a screen or a set of curtains, think of the tiny magnetic dancers in Chromium Triiodide. They are performing a complex, twisted ballet that could one day power the world. Who knew that just a little bit of a twist could lead to a giant leap in technology? The future of magnetism isn't just about sticking things to your fridge; it's about the beautiful, swirling, oversized patterns created when we let the atoms dance to their own rhythm.
As researchers continue to poke and prod at these two-dimensional wonderlands, we can expect more surprises. Each degree of rotation opens up a new world of possibilities, proving that sometimes, looking at things from a slightly different angle is all it takes to see something magnificent. The giant skyrmions of the atomic world are ready for their close-up, and they’re bringing a whole new meaning to the phrase "putting a spin on things."
Kabiller Prize recipients Joseph DeSimone (third from left) and Warren Chan (far right) were honored by Northwestern's International Institute for Nanotechnology (IIN) at a private dinner Sept. 29. Also pictured are Northwestern trustee and alumnus David G. Kabiller (far left), whose gift established the prizes; Dr. Eric Neilson (second from left), vice president for medical affairs and Lewis Landsberg Dean at Northwestern University Feinberg School of Medicine; and Chad Mirkin (second from right), IIN director and the George B. Rathmann Professor of Chemistry. "These awards were established not only to recognize the people who are designing the technologies that will drive innovation in nanomedicine, but also to educate and shine a light on the great promises of nanomedicine," said Kabiller, co-founder of AQR Capital Management, a global investment management firm in Greenwich, Connecticut. The Kabiller Prize is among the largest monetary awards in the U.S. for outstanding achievement in the field of nanotechnology and its application to medicine and biology. "The world needs more people like David Kabiller," said Chad A. Mirkin, IIN director and the George B. Rathmann Professor of Chemistry in Northwestern's Weinberg College of Arts and Sciences. "He is dedicated to making a difference and to improving the world through advances in science." DeSimone's innovative research applying nanotechnology to medicine captures the vision of the Kabiller Prize. "Joe is a Renaissance scientist, who has made some of the most important advances in the field of nanomedicine," Mirkin said. One of those advances is PRINT (Particle Replication in Non-wetting Templates) technology, invented by DeSimone in 2005. The technology enables the fabrication of precisely defined, shape-specific nanoparticles for advances in disease treatment and prevention. Nanoparticles made with PRINT technology are being used to develop new cancer treatments, inhalable therapeutics for treating pulmonary diseases, such as cystic fibrosis and asthma, and next-generation vaccines for malaria, pneumonia and dengue. "I'm thrilled and humbled to be recognized with the inaugural Kabiller Prize by such a world-class institution as Northwestern's International Institute for Nanotechnology," DeSimone said. "The PRINT technology invented in my laboratory continues to be developed for many different applications to improve human health, and my students are leading that charge. This recognition is really a testament to their brilliant efforts." DeSimone is the Chancellor's Eminent Professor of Chemistry at the University of North Carolina at Chapel Hill (UNC-Chapel Hill). He also is the William R. Kenan Jr. Distinguished Professor of Chemical Engineering at North Carolina State University and of chemistry at UNC-Chapel Hill. DeSimone founded a startup company based on PRINT called Liquidia Technologies that is building on the promise of vaccine clinical trial results. The company already has spun out two more companies to use PRINT to improve human health, one in ophthalmology and one in oral health. "The invention of PRINT technology and its application toward improvements in human health will shape the field of nanomedicine for decades to come and improve the quality of life for many," said Dr. Eric Neilson, vice president for medical affairs and Lewis Landsberg Dean at Northwestern University Feinberg School of Medicine. The International Institute for Nanotechnology also announced that Warren Chan, a professor at the Institute of Biomaterials and Biomedical Engineering at the University of Toronto, is the recipient of the inaugural Kabiller Young Investigator Award. The award recognizes young researchers who have made a recent groundbreaking discovery with the potential to make a lasting impact in the same arena. Chan and his research group have developed an infectious disease diagnostic device for point-of-care use that can differentiate symptoms. A diagnosis occurs when a patient pricks his or her finger, the sample is amplified, and a disease is detected using a smartphone app. (More than one disease can be detected.) Results for patients infected with HIV and hepatitis B are available in less than one hour at 90 percent accuracy, and the diagnostic device costs less than $100. The device currently is being commercialized and could change the way diseases are diagnosed and tracked globally. "I am very honored to receive the Kabiller Young Investigator Award in Nanoscience and Nanomedicine, and I hope this recognition helps to inspire other young people in the field of nanotechnology," Chan said. DeSimone and Chan were celebrated at a private dinner last night in Chicago. The two will be publicly recognized and present their research Oct. 1 at the 2015 IIN Symposium, which will include talks from other prestigious speakers, including 2014 Nobel Prize in Chemistry winner William E. Moerner.
Quantum dots make it possible to display any color in full brilliance. (Image: Fraunhofer IAP) The landscape is breathtaking. Because it is so real, you forget for a moment that the eagle circling the sky is not outside your window, but is instead on your television. Such deceptively realistic images are not only a result of the high resolution displays available on modern devices; the colors play a role as well, and they are becoming ever brighter and richer. This is possible thanks to tiny crystals known as quantum dots (QDs), which have a thickness of merely a few atoms. These nanoparticles located in the backlight units of QD LCD displays offer a cornucopia of colors, but also they possess another extraordinary characteristic. “One big advantage of quantum dots is that their optical properties can be selectively modified by changing their size,” explains Dr. Armin Wedel of the Fraunhofer Institute for Applied Polymer Research IAP in Potsdam, Germany. “This means you no longer have to manufacture three separate materials for the colors red, green and blue; now it is possible to do the job with just one.” This saves both time and money. Over the last several years, Fraunhofer IAP researchers in Potsdam have been developing quantum dots for customers in a wide range of industry sectors. They manufacture the nanoparticles using chemical synthesis and customize them for each application. This initially results in very small particles that radiate blue light. At sizes above approximately 2 nanometers, the color changes to green. The largest of the quantum particles, at 7 nanometers in size, emit within the red spectral range. Currently, Wedel and his team are developing quantum dots for display backlighting on behalf of Dutch company NDF Special Light Products B.V. These quantum dots will improve the color rendering and color realism of the displays. Here, the crystals are manufactured for the different emission colors and embedded in plastics. These plastics are subsequently processed into films and built into the display as a conversion film. Alternative materials based on indium phosphide With this task, researchers are facing a new challenge. The EU Commission is currently considering a ban on cadmium in consumer goods by 2017, because of its damaging effect on the environment. However, it is also considered to be the ideal material for manufacturing the crystals – cadmium-based quantum dots can achieve a narrowband spectrum sharpness of just 20 to 25 nanometers. Display manufacturers around the world are now looking for suitable replacement materials with similar characteristics. Against this backdrop, Fraunhofer IAP looks to be on a promising path. “We are testing quantum dots based on indium phosphide together with NDF Special Light Products,” says Wedel. His team has already managed to achieve a spectral sharpness of 40 nanometers. At first glance, that does not seem too far away from the quality achievable with cadmium-based quantum dots, but the differences in color fidelity are still present. “We see this as a good first milestone, but we are still striving for further improvement,” says Wedel. This effort is set to pay off, as television manufacturers are not the only ones who covet these little color wonders. There is also great market potential for special applications such as medical or aeronautical equipment displays. Furthermore, quantum dots can also increase the efficiency of solar cells, or can be employed in bioanalytics. For such special cases, the optical characteristics of the quantum dots must be precisely configured to the specific application requirements. “We’re in a good position thanks to our extensive experience in manufacturing quantum dots to meet specific customer requirements,” says Wedel.
A spray-on nanocrystal solar cell array. (Image courtesy of St. Mary's College of Maryland) A major disadvantage compared to organics, however, is that inorganic materials are difficult to deposit from solution. To overcome this, Townsend synthesized materials on the nanoscale. Inorganic nanocrystals encased in an organic ligand shell are soluble in organic solvents and can be deposited from solution (i.e., spin-, dip-, spray-coat) whereas traditional inorganic materials require a high temperature vacuum chamber. The solar devices are fabricated from nanoscale particle inks of the light absorbing layers, cadmium telluride/cadmium selenide, and metallic inks above and below. This way, the entire electronic device can be built on non-conductive glass substrates using equipment you can find in your kitchen. The outstanding challenge facing the (3-5 nm) inorganic nanocrystals is that they must be annealed or heated to form larger 'bulk scale' grains (100 nm to 1 µm) in order to produce working devices. Townsend recently teamed with Navy researchers to explore this process. "When you spray on these nanocrystals, you have to heat them to make them work," explained Townsend, "but you can't just heat the crystals by themselves, you have to add a sintering agent and that, for the last 40 years, has been cadmium chloride, a toxic salt used in commercial thin-film devices. No one has tested non-toxic alternatives for nanoscale ink devices, and we wanted to explore the mechanism of the sintering process to be able to implement safer salts." In his latest study, published this year in the ("Safer salts for CdTe nanocrystal solution processed solar cells: the dual roles of ligand exchange and grain growth"), Townsend, along with Navy researchers, found that ammonium chloride is a non-toxic, inexpensive viable alternative to cadmium chloride for nanocrystal solar cells. This discovery came after testing several different salts. Devices made using ammonium chloride (which is commonly used in bread making) had comparable device characteristics to those made with cadmium chloride, and the move away from cadmium salt treatments alleviates concerns about the environmental health and safety of current processing methods. The team also discovered that the role of the salt treatment involves crucial ligand removal reactions. This is unique to inorganic nanocrystals and is not observed for bulk-scale vacuum deposition methods. "A lot of exciting work has been done on nanocrystal ligand exchange, but, for the first time, we elucidated the dual role of the salt as a ligand exchange agent and a simultaneous sintering agent. This is an important distinction for these devices, because nanocrystals are typically synthesized with a native organic ligand shell. This shell needs to be removed before heating in order to improve the electronic properties of the film," said Townsend about the discovery. Because nanomaterials are at the forefront of emerging new properties compared to their bulk counterpart, the study is important to the future of electronic device fabrication. The research comes in the wake of the Obama Administration's announcement in July to put more solar panels on low-income housing and expand access to solar power for renters, and recent pledge to get 20 percent of the U.S. total electricity from renewable sources by the year 2030. "Right now, solar technology is somewhat unattainable for the average person," said Townsend. "The dream is to make the assembly and installation process so cheap and simple that you can go to your local home improvement store and buy a kit and then spray it on your own roof. That is why we we're working on spray-on solar cells." Townsend plans for further research to increase the efficiency of the all-inorganic nanocrystal solar cells (currently reaching five percent), while building them with completely non-toxic components.
Image obtained from scanning tunneling microscopy on a porous 3D-gold structure. (Image: Anaïs Ferris – LAAS) With the development of on-board electronic systems
A special invisibility cloak (right) guides sunlight past the contacts for current removal to the active surface area of the solar cell. (Image: Martin Schumann, KIT) Energy efficiency of solar panels has to be improved significantly not only for the energy turnaround, but also for enhancing economic efficiency. Modules that are presently mounted on roofs convert just one fifth of the light into electricity, which means that about 80% of the solar energy are lost. The reasons of these high losses are manifold. Up to one tenth of the surface area of solar cells, for instance, is covered by so-called contact fingers that extract the current generated. At the locations of these contact fingers, light cannot reach the active area of the solar cell and efficiency of the cell decreases. “Our model experiments have shown that the cloak layer makes the contact fingers nearly completely invisible,” doctoral student Martin Schumann of the KIT Institute of Applied Physics says, who conducted the experiments and simulations. Physicists of KIT around project head Carsten Rockstuhl, together with partners from Aachen, Freiburg, Halle, Jena, and Jülich, modified the optical invisibility cloak designed at KIT for guiding the incident light around the contact fingers of the solar cell. Normally, invisibility cloak research is aimed at making objects invisible. For this purpose, light is guided around the object to be hidden. This research project did not focus on hiding the contact fingers visually, but on the deflected light that reaches the active surface area of the solar cell thanks to the invisibility cloak and, hence, can be used. To achieve the cloaking effect, the scientists pursued two approaches. Both are based on applying a polymer coating onto the solar cell. This coating has to possess exactly calculated optical properties, i.e. an index of refraction that depends on the location or a special surface shape. The second concept is particularly promising, as it can potentially be integrated into mass production of solar cells at low costs. The surface of the cloak layer is grooved along the contact fingers. In this way, incident light is refracted away from the contact fingers and finally reaches the active surface area of the solar cell (see Figure). By means of a model experiment and detailed simulations, the researchers demonstrated that both concepts are suited for hiding the contact fingers. In the next step, it is planned to apply the cloaking layer onto a solar cell in order to determine the efficiency increase. The physicists are optimistic that efficiency will be improved by the cloak under real conditions: “When applying such a coating onto a real solar cell, optical losses via the contact fingers are supposed to be reduced and efficiency is assumed to be increased by up to 10%,” Martin Schumann says.
Formation of metallic Tantalum (Ta) filament within Ta/TaO
A look into the Oxide Cluster at Forschungszentrum Jülich in which resistive cells and other layers of material are produced and examined in an ultrahigh vacuum. (Image: Forschungszentrum Jülich) Incorporating such a carbon interlayer would make it possible to jump from one switching process to the other in VCMs. This would lead to new options for designing ReRAMs. “Depending on the application, our findings could be exploited and the effect purposely enhanced or intentionally suppressed,” says Valov. The scientists’ findings give rise to several questions. “Existing models and studies will have to be reworked and adapted on the basis of these findings,” says the Jülich scientist. Further tests will clarify how such novel components behave in practice.