Researchers succeed in light-controlled molecule switching

Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the University of Konstanz are working on storing and processing information on the level of single molecules to create the smallest possible components that will combine autonomously to form a circuit. As recently reported in the academic journal ("Light-Induced Switching of Tunable Single-Molecule Junctions"), the researchers can switch on the current flow through a single molecule for the first time with the help of light. Dr. Artur Erbe, physicist at the HZDR, is convinced that in the future molecular electronics will open the door for novel and increasingly smaller – while also more energy efficient - components or sensors: “Single molecules are currently the smallest imaginable components capable of being integrated into a processor.” Scientists have yet to succeed in tailoring a molecule so that it can conduct an electrical current and that this current can be selectively turned on and off like an electrical switch. molecule Light on – molecule on. For the first time a light beam switches a single molecule to closed state (red atoms). At the ends of the diarylethene molecule gold electrodes are attached. This way, the molecule functions as an electrical switch. (Image: HZDR/Pfefferkorn ) This requires a molecule in which an otherwise strong bond between individual atoms dissolves in one location – and forms again precisely when energy is pumped into the structure. Dr. Jannic Wolf, chemist at the University of Konstanz, discovered through complex experiments that a particular diarylethene compound is an eligible candidate. The advantages of this molecule, approximately three nanometres in size, are that it rotates very little when a point in its structure opens and it possesses two nanowires that can be used as contacts. The diarylethene is an insulator when open and becomes a conductor when closed. It thus exhibits a different physical behaviour, a behaviour that the scientists from Konstanz and Dresden were able to demonstrate with certainty in numerous reproducible measurements for the first time in a single molecule. A computer from a test-tube A special feature of these molecular electronics is that they take place in a fluid within a test-tube, where the molecules are contacted within the solution. In order to ascertain what effects the solution conditions have on the switching process, it was therefore necessary to systematically test various solvents. The diarylethene needs to be attached at the end of the nanowires to electrodes so that the current can flow. “We developed a nanotechnology at the HZDR that relies on extremely thin tips made of very few gold atoms. We stretch the switchable diarylethene compound between them,” explains Dr. Erbe. When a beam of light then hits the molecule, it switches from its open to its closed state, resulting in a flowing current. “For the first time ever we could switch on a single contacted molecule and prove that this precise molecule becomes a conductor on which we have used the light beam," says Dr. Erbe, pleased with the results. "We have also characterized the molecular switching mechanism in extremely high detail, which is why I believe that we have succeeded in making an important step toward a genuine molecular electronic component.” Switching off, however, does not yet work with the contacted diarylethene, but the physicist is confident: “Our colleagues from the HZDR theory group are computing how precisely the molecule must rotate so that the current is interrupted. Together with the chemists from Konstanz, we will be able to accordingly implement the design and synthesis for the molecule.” However, a great deal of patience is required because it’s a matter of basic research. The diarylethene molecule contact using electron-beam lithography and the subsequent measurements alone lasted three long years. Approximately ten years ago, a working group at the University of Groningen in the Netherlands had already managed to construct a switch that could interrupt the current. The off-switch also worked only in one direction, but what couldn't be proven at the time with certainty was that the change in conductivity was bound to a single molecule. Nano-electronics in Dresden One area of research focus in Dresden is what is known as self-organization. “DNA molecules are, for instance, able to arrange themselves into structures without any outside assistance. If we succeed in constructing logical switches from self-organizing molecules, then computers of the future will come from test-tubes," Dr. Erbe prophesizes. The enormous advantages of this new technology are obvious: billion-euro manufacturing plants that are necessary for manufacturing today’s microelectronics could be a thing of the past. The advantages lie not only in production but also in operating the new molecular components, as they both will require very little energy.
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UNL wins $9.6 million NSF grant for nanotechnology research center

The University of Nebraska-Lincoln has earned a $9.6 million grant from the National Science Foundation to support its Materials Research Science and Engineering Center and its nanotechnology research through 2020. Through this multidisciplinary center, UNL physicists, chemists and engineers collaborate to study nanostructures and materials that could lead to more energy-efficient electronic devices. UNL's is one of 21 NSF-funded MRSECs nationwide. UNL established its MRSEC in 2002 with a $5.4 million NSF grant. In 2008, NSF awarded UNL $8.1 million to continue the center. UNL was one of 12 universities nationwide that received grants in the latest round of competition. "With this award from NSF, we continue to be part of a prestigious group of institutions recognized for our expertise in materials research and education through the MRSEC program, which includes Columbia, Harvard, MIT, the University of Chicago, Penn State and Ohio State," said UNL Chancellor Harvey Perlman. "The achievements of our materials researchers are highly valued by U.S. and international scientific communities and greatly contribute to UNL's reputation." The center receives a new name with this latest funding -- Polarization and Spin Phenomena in Nanoferroic Structures, or P-SPINS -- to reflect its expanding research focus on nanoferroic materials, which may one day transform electronics and computing technologies. "Our MRSEC scientists are doing research at the frontiers of materials and nanoscience and although this is very basic research, it leads to advanced technologies and products that affect our everyday lives," said Prem S. Paul, UNL vice chancellor for research and economic development. "An important part of the center's work is developing collaborations with industry and national laboratories to focus on potential applications." The center's success is based on several major accomplishments in understanding the properties and performance of nanomaterials, key steps toward improving computing power and creating advanced technologies, said Evgeny Tsymbal, George Holmes University Professor of Physics and MRSEC director. These discoveries have led the center to focus on two key areas: magnetoelectric materials and functional interfaces, and polarization-enabled electronic phenomena. UNL physicist Christian Binek leads the magnetoelectric materials and functional interfaces research group. It's based on Binek's work with spintronics, which manipulates electron spin, in addition to charge, to generate power and store digital information. Traditional magnetic memory devices use an electric current to reverse the magnetic direction, which is the binary method of storing information. Binek's team discovered how to switch magnetization using voltage instead, which doesn't generate heat and thus opens the avenue to energy-efficient computing. This team now is developing voltage-powered logical and memory devices. UNL physicist Alexei Gruverman leads the polarization-enabled electronic phenomena research group. This research takes advantage of nano-thin ferroelectric oxide, a material with both positive and negative polarization directions that, like spintronics, can be read out as a binary code using less energy than current technology. The work is driven by Tsymbal's theoretical predication and Gruverman's experimental demonstration of quantum tunneling across nano-thin ferroelectrics. The phenomenon of quantum tunneling, in which particles, such as electrons, can pass through a barrier, occurs only at the quantum, or atomic, level. When voltage is applied, electrons are able to tunnel through the barrier, creating a current with resistance. By experimenting with tunnel junctions, in which an ultra-thin barrier made of ferroelectric oxide is placed between two electrodes, they have shown that reversing the polarization changes dramatically the resistance through the tunnel junction. Measuring that resistance would allow devices to read the binary polarization direction, and thus, the information it contains. Each of these nanomaterials holds promise for overcoming the limitations of traditional silicon-based electronics, which engineers say are fast approaching their functional limits. Harnessing nanomaterials would enable smaller, more powerful and less expensive computers and other electronics. Other applications include more energy-efficient solar panels and refrigeration. "Our niche, which I think is very exciting in terms of fundamental science, is very important from the point of view of applications," Tsymbal said. "It's a focused research area where we're leading the field." The center's work is highly collaborative, with researchers from diverse disciplines sharing expertise. UNL's MRSEC includes 18 UNL faculty from the departments of physics and astronomy, chemistry, electrical engineering, mechanical and materials engineering, and teaching, learning and teacher education. Two other MRSEC affiliates are at North Carolina A&T State University and the University of Wisconsin-Madison. UNL MRSEC faculty collaborate with industry, national laboratories and scientists internationally and will interact closely with UNL's Center for Nanoferroic Devices, established in 2013 and funded jointly by a consortium of industrial companies known as the Nanoelectronics Research Initiative and the National Institute for Standards and Technology, to develop device applications. The latest NSF funding also will support expansion of the center's traditionally strong education and outreach programs. UNL physicist Axel Enders will lead several ongoing and new initiatives, including those designed to encourage women and minorities into materials science research. Activities include conferences and mentorship programs with minority-serving institutions, such as the University of Puerto Rico and North Carolina A&T State University. During summer research programs, undergraduates and faculty from non-research-intensive four-year institutions, as well as high school and middle school teachers, will continue to tackle research projects alongside the center's faculty and staff.
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Engineers introduce design that mimics nature's camouflage

It can shift from red to green to violet. It can mimic patterns and designs. And it can do all of this in a flash -- literally. The same qualities that define the cuttlefish -- a sea dweller that uses its powers of dynamic camouflage to survive and communicate -- also apply to a new engineering feat that behaves much like nature's master of disguise. A team of UNL researchers has developed a structure that can begin replicating color and texture within seconds of exposure to pulses of light ("Color and Texture Morphing with Colloids on Multilayered Surfaces"). The new design responds to much lower-intensity light and at faster rates than its few predecessors, said Li Tan, associate professor of mechanical and materials engineering. Li Tan Li Tan "This is a relatively new community of research," said Tan, who co-authored a recent paper outlining the team's design. "Most of the people (in it) are inspired by the cuttlefish, whose skin changes color and texture, as well. "Changing color is relatively easy; a TV can do that. Changing texture is harder. We wanted to combine the two." To do so, the team has created a structure consisting of three layers: a base that insulates against heat, a middle that readily absorbs light, and a top made of either a liquid or solid. Paper, glass, foil, silicon and other materials have all proven suitable for the middle layer, so long as it includes a distribution of colored pixels. The middle and top layers also contain colloids: microscopic particles of soda lime, glass or copper. When a moderately intense laser strikes the middle layer, it begins warming any pixels that absorb it -- that is, those that don't share the light's color. Through the process of convection, these localized increases in temperature trigger ruptures along the surface of the top layer or volcano-like eruptions within it. In both cases, the resulting suction draws the colloids toward the heated, light-absorbing areas -- thereby reproducing the color that shines upon the surface. If the light is red, for instance, the colloids migrate to cover green pixels and leave only the red exposed. Under violet light, the particles obscure the majority of red pixels while leaving most of the green uncovered. This same photo-thermal principle allows the team to replicate or create patterns, Tan said, either by directing lights in deliberate trajectories or simply flashing them through transparent images overlaying the structure. The team has used these techniques to write words and mimic checkerboard patterns, among others. When the top layer is a solid, cooling it will obscure the word or pattern, which reappears upon reheating. According to Tan, the design's first apparent application -- camouflage -- probably won't see the light of day for a while. The more immediate potential application, Tan said, relies on the photo-thermal principle that drives the design's color and texture changes. In the same way that the technique can direct the assembly of microscopic colloids, it might also accelerate the accumulation of cells and facilitate the growth of biological tissue, he said. "Starting from small building blocks and growing them into large structures usually takes a very long time," Tan said. "In our case, it really doesn't."
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