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Nano News & Events
Two MIT researchers have developed a thin-film material whose phase and electrical properties can be switched between metallic and semiconducting simply by applying a small voltage. The material then stays in its new configuration until switched back by another voltage. The discovery could pave the way for a new kind of “nonvolatile” computer memory chip that retains information when the power is switched off, and for energy conversion and catalytic applications. The findings, reported in the journal Nano Letters in a paper by MIT materials science graduate student Qiyang Lu and associate professor Bilge Yildiz, involve a thin-film material called a strontium cobaltite, or SrCoOx. Usually, Yildiz says, the structural phase of a material is controlled by its composition, temperature, and pressure. “Here for the first time,” she says, “we demonstrate that electrical bias can induce a phase transition in the material. And in fact we achieved this by changing the oxygen content in SrCoOx.” “It has two different structures that depend on how many oxygen atoms per unit cell it contains, and these two structures have quite different properties,” Lu explains. One of these configurations of the molecular structure is called perovskite, and the other is called brownmillerite. When more oxygen is present, it forms the tightly-enclosed, cage-like crystal structure of perovskite, whereas a lower concentration of oxygen produces the more open structure of brownmillerite. The two forms have very different chemical, electrical, magnetic, and physical properties, and Lu and Yildiz found that the material can be flipped between the two forms with the application of a very tiny amount of voltage — just 30 millivolts (0.03 volts). And, once changed, the new configuration remains stable until it is flipped back by a second application of voltage. Strontium cobaltites are just one example of a class of materials known as transition metal oxides, which is considered promising for a variety of applications including electrodes in fuel cells, membranes that allow oxygen to pass through for gas separation, and electronic devices such as memristors — a form of nonvolatile, ultrafast, and energy-efficient memory device. The ability to trigger such a phase change through the use of just a tiny voltage could open up many uses for these materials, the researchers say. Previous work with strontium cobaltites relied on changes in the oxygen concentration in the surrounding gas atmosphere to control which of the two forms the material would take, but that is inherently a much slower and more difficult process to control, Lu says. “So our idea was, don’t change the atmosphere, just apply a voltage.” “Voltage modifies the effective oxygen pressure that the material faces,” Yildiz adds. To make that possible, the researchers deposited a very thin film of the material (the brownmillerite phase) onto a substrate, for which they used yttrium-stabilized zirconia. In that setup, applying a voltage drives oxygen atoms into the material. Applying the opposite voltage has the reverse effect. To observe and demonstrate that the material did indeed go through this phase transition when the voltage was applied, the team used a technique called in-situ X-ray diffraction at MIT’s Center for Materials Science and Engineering. The basic principle of switching this material between the two phases by altering the gas pressure and temperature in the environment was developed within the last year by scientists at Oak Ridge National Laboratory. “While interesting, this is not a practical means for controlling device properties in use,” says Yildiz. With their current work, the MIT researchers have enabled the control of the phase and electrical properties of this class of materials in a practical way, by applying an electrical charge. In addition to memory devices, the material could ultimately find applications in fuel cells and electrodes for lithium ion batteries, Lu says. “Our work has fundamental contributions by introducing electrical bias as a way to control the phase of an active material, and by laying the basic scientific groundwork for such novel energy and information processing devices,” Yildiz adds. In ongoing research, the team is working to better understand the electronic properties of the material in its different structures, and to extend this approach to other oxides of interest for memory and energy applications, in collaboration with MIT professor Harry Tuller. José Santiso, the nanomaterials growth division leader at the Catalan Institute of Nanoscience and Nanotechnology in Barcelona, Spain, who was not involved in this research, calls it “a very significant contribution” to the study of this interesting class of materials, and says “it paves the way for the application of these materials both in solid state electrochemical devices for the efficient conversion of energy or oxygen storage, as well as in possible applications in a new kind of memory devices.” The work was supported by the National Science Foundation.
Two simple room-temperature and solution-based chemical processes are used to realize a new class of silver nano-network-based devices.
Nature Nanotechnology, an online manual, has published a report outlining the concept of combining nanoparticles with molecular and photodynamic therapies in order to deliver anticancer treatment, and...
A research team from the University of Amsterdam's Van’t Hoff Institute for Molecular Sciences (HIMS) has discovered a new method to achieve improved catalytic performance. In this method,...
Developing novel materials from the atoms up goes faster when some of the trial and error is eliminated. A new Rice University and Montreal Polytechnic study aims to do that for graphene and boron nitride hybrids.
Scientists from Germany and Spain have discovered a way to create a BioLED by packaging luminescent proteins in the form of rubber. This innovative device gives off a white light which is created by e...
Perovskite-based solar cells were a hot ticket in Boston this December
A team of researchers from Stanford University have constructed the first lithium-ion battery capable of turning on and off according to the temperature. The battery shuts off before overheating...
A team of scientists from MIT and Purdue University have developed an innovative technology that combines the warm look of conventional incandescent light bulbs with improved energy efficiency. This...
By Jake Wilkinson A novel technique that uses DNA to encode information onto the surface of gold nanoparticles has been developed. This new method can be used to control the 3D structure of...
Researchers at ETH Zurich have manufactured transparent electrodes for use in touchscreens using a novel nanoprinting process. The new electrodes are some of the most transparent and conductive that have ever been developed. Read More...
A figure showing the crystal structure of strontium vanadate (orange) and calcium vanadate (blue). The red dots are oxygen atoms arranged in 8 octohedra surrounding a single strontium or calcium atom. Vanadium atoms can be seen inside each octahedron. Credit image: Lei Zhang/Penn State It is usually both interesting and useful when technology identifies multiple paths to the same goal, particularly when a new path has a major advantage, such as a much lower cost and substituting an abundant resource for a limited one. A hat tip to Kurzweil Accelerating Intelligence for reprinting this Penn State news release written by Walt Mills “Transparent metal films for smartphone, tablet and TV displays“: A new material that is both highly transparent and electrically conductive could make large screen displays, smart windows and even touch screens and solar cells more affordable and efficient, according to the Penn State materials scientists and engineers who discovered it. Indium tin oxide, the transparent conductor that is currently used for more than 90 percent of the display market, has been the dominant material for the past 60 years. However, in the last decade, the price of indium has increased dramatically. Displays and touchscreen modules have become a main cost driver in smartphones and tablets, making up close to 40 percent of the cost. While memory chips and processors get cheaper, displays get more expensive from generation to generation. Manufacturers have searched for a possible ITO replacement, but until now, nothing has matched ITO’s combination of optical transparency, electrical conductivity and ease of fabrication. A team led by Roman Engel-Herbert, assistant professor of materials science and engineering, reports … in Nature Materials [abstract] a new design strategy that approaches the problem from a different angle. The researchers use thin — 10 nanometer — films of an unusual class of materials called correlated metals in which the electrons flow like a liquid. While in most conventional metals, such as copper, gold, aluminum or silver, electrons flow like a gas, in correlated metals, such as strontium vanadate and calcium vanadate, they move like a liquid. According to the researchers, this electron flow produces high optical transparency along with high metal-like conductivity. “We are trying to make metals transparent by changing the effective mass of their electrons,” Engel-Herbert said. “We are doing this by choosing materials in which the electrostatic interaction between negatively charged electrons is very large compared to their kinetic energy. As a result of this strong electron correlation effect, electrons ‘feel’ each other and behave like a liquid rather than a gas of non-interacting particles. This electron liquid is still highly conductive, but when you shine light on it, it becomes less reflective, thus much more transparent.” To better understand how they achieved this fine balance between transparency and conductivity, Engel-Herbert and his team turned to a materials theory expert, Professor Karin Rabe of Rutgers University. “We realized that we needed her help to put a number on how ‘liquid’ this electron liquid in strontium vanadate is,” Engel-Herbert said. Rabe helped the Penn State team put together all the theoretical and mathematical puzzle pieces they needed to build transparent conductors in the form of a correlated metal. Now that they understand the essential mechanism behind their discovery, the Penn State researchers are confident they will find many other correlated metals that behave like strontium vanadate and calcium vanadate. Lei Zhang, lead author on the Nature Materials paper and a graduate student in Engel-Herbert’s group, was the first to recognize what they had discovered. “I came from Silicon Valley where I worked for two years as an engineer before I joined the group,” said Zhang. “I was aware that there were many companies trying hard to optimize those ITO materials and looking for other possible replacements, but they had been studied for many decades and there just wasn’t much room for improvement. When we made the electrical measurements on our correlated metals, I knew we had something that looked really good compared to standard ITO.” Currently indium costs around $750 per kilogram, whereas strontium vanadate and calcium vanadate are made from elements with orders of magnitude higher abundance in the earth’s crust. Vanadium sells for around $25 a kilogram, less than 5 percent of the cost of indium, while strontium is even cheaper than vanadium. “Our correlated metals work really well compared to ITO,” said Engel-Herbert. “Now, the question is how to implement these new materials into a large-scale manufacturing process From what we understand right now, there is no reason that strontium vanadate could not replace ITO in the same equipment currently used in industry.” Along with display technologies, Engel-Herbert and his group are excited about combining their new materials with a very promising type of solar cell that uses a class of materials called organic perovskites. Developed only within the last half dozen years, these materials outperform commercial silicon solar cells but require an inexpensive transparent conductor. Strontium vanadate, also a perovskite, has a compatible structure that makes this an interesting possibility for future inexpensive, high-efficiency solar cells. Engel-Herbert and Zhang have applied for a patent on their technology. The potential application to organic perovskite solar cells illustrates an interesting point: a technical development that “merely” makes an existing technology (displays) more affordable may make the difference between feasible and infeasible for a potentially even larger market (solar cells efficient, inexpensive, and robust enough to pave roads and roofs). —James Lewis, PhD
With large format interactive displays becoming more ubiquitous from large indoor and outdoor displays to electronic whiteboards the need has increased for advanced materials that are highly sensi...