Unexpected, star-spangled find may lead to advanced electronics

For several years, a team of researchers at The University of Texas at Dallas has investigated various materials in search of those whose electrical properties might make them suitable for small, energy-efficient transistors to power next-generation electronic devices. They recently found one such material, but it was nothing anyone expected. In an article published online March 10 in the journal Advanced Materials, Dr. Moon Kim and his colleagues describe a material that, when heated to about 450 degrees Celsius, transforms from an atomically thin, two-dimensional sheet into an array of one-dimensional nanowires, each just a few atoms wide. An image caught in mid-transformation looks like a tiny United States flag, and with false colors added, is arguably the world's smallest image of Old Glory, Kim said. "The phase transition we observed, this new structure, was not predicted by theory," said Kim, the Louis Beecherl Jr. Distinguished Professor of materials science and engineering at UT Dallas. Because the nanowires are semiconductors, they might be used as switching devices, just as silicon is used in today's transistors to turn electric current on and off in electronic devices. "These nanowires are about 10 times smaller than the smallest silicon wires, and, if used in future technology, would result in powerful energy-efficient devices," Kim said. The lead authors of the study are Hui Zhu and Qingxiao Wang, graduate students in materials science and engineering in the Erik Jonsson School of Engineering and Computer Science. Just a Phase? When certain materials are subjected to changes in external conditions, such as temperature or pressure, they can undergo a phase transition. A familiar example is when liquid water is cooled to form a solid (ice), or heated to form a gas (steam). For many materials, however, a phase transition means something a little different. As external temperature and pressure change, these materials' atoms rearrange and redistribute to make a material with a different structure and composition. These changes can affect the new material's properties, such as how electrons move through it. For scientists interested in new applications for materials, understanding such transitions is paramount. In most cases, a type of graphic called a phase diagram helps researchers predict structural and property changes in a material when it undergoes a phase transition. But nothing predicted what Kim's team observed as it conducted experiments on a material called molybdenum ditelluride. Nanoflags and Nanoflowers Using a transmission electron microscope, the researchers started with atomically thin, two-dimensional sheets of molybdenum ditelluride, a material made up of one layer of molybdenum atoms and two layers of tellurium atoms. The material belongs to a class called transition metal dichalcogenides (TMDs), which show promise in replacing silicon in transistors. "We wanted to understand the thermal stability of this particular material," Kim said. "We thought it was a good candidate for next-generation nanoelectronics. Out of curiosity, we set out to see whether it would be stable above room temperature." When they increased the temperature to above 450 degrees Celsius, two things happened. "First, we saw a new pattern begin to emerge that was aesthetically pleasing to the eye," Kim said. Across the surface of the sample, the repeating rows, or stripes, of molybdenum ditelluride layers began to transform into shapes that looked like tiny six-pointed stars, or flowers with six petals. The material was transitioning into hexa-molybdenum hexa-telluride, a one-dimensional wire-like structure. The cross section of the new material is a structure consisting of six central atoms of molybdenum surrounded by six atoms of tellurium. As the phase transition progressed, part of the sample was still "stripes" and part had become "stars." The team thought the pattern looked like a United States flag. They made a false-color version with a blue field behind the stars and half of the stripes colored red, to make a "nanoflag." Not in the textbooks "Then, when we examined the material more closely, we found that the transition we were seeing from 'stripes' to 'stars' was not in any of the phase diagrams," Kim said. "Normally, when you heat up particular materials, you expect to see a different kind of material emerge as predicted by a phase diagram. But in this case, something unusual happened -- it formed a whole new phase." Each individual nanowire is a semiconductor, which means that electric current moving through the wire can be switched on and off, Kim said. When many of the individual nanowires are grouped together in bulk they behave more like a metal, which easily conducts current. "We would want to use the nanowires one at a time because we are pushing the size of a transistor as small as possible," Kim said. "Currently, the smallest transistor size is about 10 times larger than our nanowire. Each of ours is smaller than 1 nanometer in diameter, which is essentially an atomic-scale wire. "Before we can put this discovery to use and make an actual device, we have many more studies to do, including determining how to separate out the individual nanowires, and overcoming technical challenges to manufacturing and mass production," Kim said. "But this is a start." Story Source: Materials provided by University of Texas at Dallas. Note: Content may be edited for style and length.

Fathering offspring is more than just a race to the egg

The chance of a male fathering offspring may not be a simple race to the egg, but is influenced by the length of the male's sperm, say scientists from the University of Sheffield. Using a captive population of zebra finches, the researchers carried out sperm competition experiments between pairs of males, where one male consistently produced long sperm and the other male always produced short sperm. These experiments showed that more long sperm reached and fertilized the eggs compared to short sperm. The long sperm advantage was evident even when the short sperm males mated with the females first, and were effectively given a 'head start'. The findings demonstrate that in birds, in a competitive scenario, the fertilization success of a male can be influenced by the length of his sperm. The results also suggest that the final outcome of sperm competition may be partly dependent on the female bird. Dr Clair Bennison from the University's Department of Animal and Plant Sciences, said: "We know that in the zebra finch, long sperm swim faster than short sperm, so we might expect longer, faster swimming sperm to simply reach the egg first. However, this reasoning does not explain why long sperm outcompete short sperm in our study. Long sperm win at sperm competition, by fertilizing more eggs, even when short sperm are given a head-start." Scientists at the University allowed each pair of male zebra finches to mate with a female bird so that the long and short sperm from the males could compete to fertilize the female's eggs. Female birds store sperm inside their bodies for many days, and this is one way that the females themselves could influence the fertilization success of the males. It is possible that long sperm are better at reaching and and staying inside these storage areas than short sperm. Long sperm may even be 'preferred' by the female, by some unknown process. Dr Bennison, added: "Our findings are important because they demonstrate for the first time in birds, using a controlled competitive scenario, that sperm length can influence the fertilization success of a particular male. The results also add to the body of evidence suggesting that the final outcome of sperm competition may be partly dependent on the female, and that the chance of a male siring offspring may not be an outcome of a simple 'race to the egg'." Scientists believe that a better understanding of how sperm length influences fertilization success in non-human animals such as the zebra finch may point us in new directions for investigation in human fertility research. Researchers now plan to investigate if sperm storage duration in female birds varies according to the length of the male's sperm, and the possible mechanisms responsible for this.   Story Source: Materials provided by University of Sheffield. Note: Content may be edited for style and length.

Rare-earths become water-repellent only as they age

Surfaces that have been coated with rare earth oxides develop water-repelling properties only after contact with air. Even at room temperature, chemical reactions begin with hydrocarbons in the air. In the journal Scientific Reports, researchers from the University of Basel, the Swiss Nanoscience Institute and the Paul Scherrer Institute report that it is these reactions that are responsible for the hydrophobic effect. Rare earths are metals found in rare earth minerals. They are used today in, among other things, automotive catalytic converters and batteries, in the production of screens and lamps, and as a contrast agent in magnetic resonance imaging. Their broad range of applications means that there is a high demand for rare earths, and this demand is constantly increasing. Additional uses for rare earths were opened up after American researchers reported in 2013 that surfaces that have been coated with rare earth oxides become water-repellent. Scientists from the University of Basel, the Swiss Nanoscience Institute and the Paul Scherrer Institute have now worked with the company Glas Trösch to examine these hydrophobic properties more closely. Water-repellency develops only after chemical reactions The researchers coated glass pieces with rare earth oxides, nitrides and fluorides and analyzed how well they could be wetted with water. They could not detect any hydrophobic properties when the coating was freshly deposited. It was only chemical reactions with gaseous hydrocarbons found in the ambient air that increased the surfaces' roughness and reduced wetting by water. The gaseous organic compounds from the ambient air are first adsorbed by the surface and then react with the oxides to form carbonates and hydroxides until the surface is completely covered with these compounds. This process takes place even at room temperature. "We were surprised that the hydrophobic effect was caused by the surface aging," says Professor Ernst Meyer, from the Department of Physics at the University of Basel, commenting on the results of the project supported by the Commission for Technology and Innovation (CTI). These conclusions are very revealing from a scientific point of view because catalytic processes also frequently take place at room temperature, which makes it important to understand the surface's physical properties. The examined materials are, however, unsuitable for the industrial production of water-repellent glass surfaces, because the glass requires a sophisticated storage process before it shows the desired hydrophobic characteristics.   Story Source: Materials provided by University of Basel. Note: Content may be edited for style and length.

Engineers devise new way to produce clean hydrogen

Duke University engineers have developed a novel method for producing clean hydrogen, which could prove essential to weaning society off of fossil fuels and their environmental implications. While hydrogen is ubiquitous in the environment, producing and collecting molecular hydrogen for transportation and industrial uses is expensive and complicated. Just as importantly, a byproduct of most current methods of producing hydrogen is carbon monoxide, which is toxic to humans and animals. The Duke engineers, using a new catalytic approach, have shown in the laboratory that they can reduce carbon monoxide levels to nearly zero in the presence of hydrogen and the harmless byproducts of carbon dioxide and water. They also demonstrated that they could produce hydrogen by reforming fuel at much lower temperatures than conventional methods, which makes it a more practical option. Catalysts are agents added to promote chemical reactions. In this case, the catalysts were nanoparticle combinations of gold and iron oxide (rust), but not in the traditional sense. Current methods depend on gold nanoparticles ability to drive the process as the sole catalyst, while the Duke researchers made both the iron oxide and the gold the focus of the catalytic process. The study appears online in the May issue of the Journal of Catalysis. "Our ultimate goal is to be able to produce hydrogen for use in fuel cells," said Titilayo "Titi" Shodiya, a graduate student working in the laboratory of senior researcher Nico Hotz, assistant professor of mechanical engineering and materials science at Duke's Pratt School of Engineering. "Everyone is interested in sustainable and non-polluting ways of producing useful energy without fossil fuels," said Shodiya, the paper's first author.   Fuel cells produce electricity through chemical reactions, most commonly involving hydrogen. Also, many industrial processes require hydrogen as a chemical reagent and vehicles are beginning to use hydrogen as a primary fuel source. "We were able through our system to consistently produce hydrogen with less than 0.002 percent (20 parts per million) of carbon monoxide," Shodiya said. The Duke researchers achieved these levels by switching the recipe for the nanoparticles used as catalysts for the reactions to oxidize carbon monoxide in hydrogen-rich gases. Traditional methods of cleaning hydrogen, which are not nearly as efficient as this new approach, also involve gold-iron oxide nanoparticles as the catalyst, the researchers said. "It had been assumed that the iron oxide nanoparticles were only 'scaffolds' holding the gold nanoparticles together, and that the gold was responsible for the chemical reactions," Sodiya said. "However, we found that increasing the surface area of the iron oxide dramatically increased the catalytic activity of the gold." One of the newest approaches to producing renewable energy is the use of biomass-derived alcohol-based sources, such as methanol. When methanol is treated with steam, or reformed, it creates a hydrogen-rich mixture that can be used in fuel cells. "The main problem with this approach is that it also produces carbon monoxide, which is not only toxic to life, but also quickly damages the catalyst on fuel cell membranes that are crucial to the functioning of a fuel cell," Hotz said. "It doesn't take much carbon monoxide to ruin these membranes." The researchers ran the reaction for more than 200 hours and found no reduction in the ability of the catalyst to reduce the amount of carbon monoxide in the hydrogen gas. "The mechanism for this is not exactly understood yet. However, while current thinking is that the size of the gold particles is key, we believe the emphasis of further research should focus on iron oxide's role in the process," Shodiya said. The Duke team's research was supported by the California Energy Commission and the Oak Ridge Associated Universities. Duke postdoctoral associates Oliver Schmidt and Wen Peng were also part of the research team.   Story Source: Materials provided by Duke University. Note: Content may be edited for style and length.

Shaping the future: Iron nanocubes may be key in the future of NO2 sensing

While nanoparticles sound like a recent discovery, these tiny structures have been used for centuries. The famous Lycurgus cup, made by 4th century Roman artisans, features dichroic glass, with gold and silver nanoparticles sprinkled throughout, producing a green appearance when light is shining on it from the front, and a red appearance when illuminated from behind. In the centuries since the time of the ancient artisans, researchers have come a long way in understanding nanoparticles. The production of nanocubes has been of particular interest due to their potential applications as biosensors and gas sensors. Nanoparticles can be produced using either physical or chemical methods, though physical methods are advantageous due to the absence of organic contaminants commonly introduced by chemical methods. However, uniformly sized nanocubes are difficult to produce in sufficient quantities by physical methods. Researchers from the Nanoparticles by Design Unit at the Okinawa Institute of Science and Technology (OIST) Graduate University have recently discovered a new approach to overcome this problem. Their research was recently published in Advanced Functional Materials. "The cube shape is not the lowest energy structure for iron nanoparticles," explains Dr. Jerome Vernieres, first author of the publication, "thus, we couldn't rely on equilibrium thermodynamics considerations to self-assemble these nanocubes." Instead, the OIST scientists, under the guidance of Prof. Mukhles Sowwan, exploited the possibilities offered by a technique called magnetron-sputtering inert-gas condensation to create their iron nanocubes. With this method, argon gas is first heated up and turned into ionized plasma. Then, a magnet, suitably located behind a target made of the desired material, in this case, iron, controls the shape of the plasma, and ensures that argon ions bombard the target; hence the name "magnetron." As a result, iron atoms are sputtered away from the target, collide with argon atoms and with each other, and form nanoparticles. Accurate control of the plasma via controlling the magnetic field can produce uniform nanocubes. "Uniformity is key in sensing applications. We needed a way to control the size, shape, and number of the nanocubes during their production," explained Dr. Stephan Steinhauer.   To control the size and shape of these cubes, the researchers made a simple but significant observation: iron is magnetic in its own right! In other words, the researchers discovered that they could exploit the intrinsic magnetism of the target itself as an innovative way to modify the magnetic field of the magnetron. This way they managed to manipulate the plasma where the particles are grown, and thus to control the nanocube sizes during formation. "This is the first time uniform iron nanocubes have been made using a physical method that can be scaled for mass production" clarifies Vernieres. To better understand the mechanics of this process, the OIST team collaborated with researchers from the University of Helsinki to make theoretical calculations. "The work relied heavily on both experimental methods and theoretical calculations. The simulations were important for us to explain the phenomena we were observing," illuminates Dr. Panagiotis Grammatikopoulos. Once the researchers invented a way to produce these uniform iron cubes, the next step was to build an electronic device that can utilize these nanocubes for sensing applications. "We noticed that these cubes were extremely sensitive to the levels of gaseous NO2. NO2 sensing is used for a variety of different purposes, from diagnosis of asthma patients to detecting environmental pollution, so we immediately saw an application for our work," states Steinhauer. The researchers from the Nanoparticles by Design Unit, in collaboration with researchers from the Université de Toulouse, then built a prototype NO2 sensor that measured the change in electrical resistance of the iron nanocubes due to exposure to NO2 gas. Because exposure to even a very tiny amount of NO2 can produce a measurable change in electrical resistance that is considerably larger than for other atmospheric pollutants, the iron nanocube-based sensor is both extremely sensitive and specific. "These nanocubes have many potential uses. The fact that we can produce a relatively large quantity of uniform nanocubes using an increasingly common synthesis method makes this research highly promising for industrial applications," emphasized Vernieres. Story Source: Materials provided by Okinawa Institute of Science and Technology (OIST) Graduate University. Note: Content may be edited for style and length.

New method to program nanoparticle organization in polymer thin films

Controlling the organization of nanoparticles into patterns in ultrathin polymer films can be accomplished with entropy instead of chemistry, according to a discovery by Dr. Alamgir Karim, UA's Goodyear Tire and Rubber Company Professor of Polymer Engineering, and his student Dr. Ren Zhang. Polymer thin films are used in a variety of technological applications, for example paints, lubricants, and adhesives. Karim and Zhang have developed an original method -- soft-confinement pattern-induced nanoparticle segregation (SCPINS) -- to fabricate polymer nanocomposite thin films with well-controlled nanoparticle organization on a submicron scale.   This new method uniquely controls the organization of any kind of nanoparticles into patterns in those films, which may be useful for applications involving sensors, nanowire circuitry or diffraction gratings, with proper subsequent processing steps like thermal or UV sintering, that are likely required but the self-organization into directed patterns. This work, "Entropy-driven segregation of polymer-grafted nanoparticles under confinement," has been published in the February 2017 issue of Proceedings of the National Academy of Sciences (PNAS). Intuitively, entropy is associated with disorder of a system. However, for colloidal matter, it has been shown that a system can experience transitions which increase both entropy and visible order. Inspired by this observation, Karim and Zhang investigated the role of entropy in directed organization of polymer-grafted nanoparticles (PGNPs) in polymer thin films. By simply imprinting the blend films into patterned mesa-trench regions, nanoparticles are spontaneously enriched within mesas, forming patterned microdomain structures which coincide with the topographic pattern. This selective segregation of PGNPs is induced by entropic penalty due to the alteration of the grafted chain conformation when confined in ultrathin trench regions. For the first time, the desired spatial organization of nanoparticles is achieved by topographic pattern-induced entropic confinement effect, without tuning enthalpic interactions through chemistry. This facile method, SCPINS, is applicable to versatile particle compositions and pattern geometries. This work can be extended to multicomponent particle systems, which has potential applications in nanomaterial-based technologies such as nanoelectronics and plasmonics. "The process is highly efficient as it can incorporate all the nanoparticles without wastage in the remaining matrix film upon patterning -- 100% of the nanoparticles are patterned," explains Karim. "The remnant matrix can be rinsed way with no loss of expensive nanoparticles." Story Source: Materials provided by University of Akron. Note: Content may be edited for style and length.

Tardigrades use unique protein to protect themselves from desiccation

Tardigrades, the microscopic animals also known as water bears and moss piglets, have captured the imagination of scientists for almost 250 years, thanks to their Muppet-like appearance and their ability to survive extreme environments that would destroy most other living things. One of these skills is the ability to endure being dried out for up to a decade or longer. In Molecular Cell on March 16, a team of scientists report that this knack for survival is due to a unique set of proteins they dubbed tardigrade-specific intrinsically disordered proteins (TDPs) "The big takeaway from our study is that tardigrades have evolved unique genes that allow them to survive drying out," says Thomas Boothby, the Life Sciences Research Foundation Postdoctoral Fellow at the University of North Carolina, Chapel Hill, and the study's first author. "In addition, the proteins that these genes encode can be used to protect other biological material -- like bacteria, yeast, and certain enzymes -- from desiccation." For a long time, it was assumed that a sugar called trehelose gave tardigrades the ability to tolerate desiccation. Trehelose is found in a number of other organisms that can survive being dried out, including yeast, brine shrimp, and some nematodes. But biochemical studies of tardigrades have found trehelose at low levels or not at all, and sequencing has not revealed the gene for the enzyme required to make this sugar. "The question has been, 'If tardigrades aren't relying on trehelose to survive desiccation, what do they use instead?'" Boothby says. He and his team set out to discover how they do it. The first step of the research was to look at which genes were active under various conditions: unstressed, drying out, and frozen. The researchers identified genes that were upregulated and expressed at high levels when the animals began to dry out. The proteins that these genes encode, the TDPs, are in a class of proteins called intrinsically disordered proteins (IDPs). Unlike most proteins, IDPs have no fixed three-dimensional structure.   After they found the TDP genes expressed at high levels during the drying-out period in one species of tardigrade, the team looked at two other species and found the same genes. One species, which had the genes turned on all the time, is able to survive drying out much more quickly that the others. "We think it can do this because it has so many of these proteins around already and doesn't need time to make them," Boothby says. To verify that these TDPs were what gave tardigrades their unique abilities, the researchers put the genes encoding them into yeast and bacteria, and confirmed that the TDPs protected these other organisms. Trehelose helps other organisms to survive drying out by forming glass-like solids when they dry, rather than crystals. Boothby and his colleagues found that TDFs form similar glass-like solids, and showed that when the glassiness of TDPs was disrupted, it correlated with a loss of their protective abilities. Boothby says TDPs have a number of potential uses, including protecting crops from drought and safeguarding medications that normally require cold storage. "Being able to stabilize sensitive pharmaceuticals in a dry state is very important to me personally," he says. "I grew up in Africa, where lack of refrigeration in remote areas is a huge problem. These real-world applications are one of the things that led me to study tardigrades."   Story Source: Materials provided by Cell Press. Note: Content may be edited for style and length.

First thin films of spin ice reveal cold secrets

Thin films of spin ice have been shown to demonstrate surprising properties which could help in the development of applications of magnetricity, the magnetic equivalent of electricity. Published today in Nature Communications, a team of researchers based at the London Centre for Nanotechnology (LCN), in collaboration with scientists from Oxford and Cambridge, found that, against expectations, the Third Law of Thermodynamics could be restored in thin films of the magnetic material spin ice. In the familiar world around us it is always possible to make things colder, but science has established that there is a limit to how cold an object can be -- the so-called `absolute zero' of temperature, or minus 273 degrees centigrade. At the absolute zero it is expected that the entropy of a substance, a measure of the randomness of the atoms within it, should itself be zero. The concept that absolute zero equates to zero entropy or randomness is called the Third Law of Thermodynamics. A famous exception to the Third Law is spin ice, in which atomic magnetic moments or `spins' remain random in the approach to absolute zero. This randomness gives spin ice properties that more conventional materials don't have, most notably `magnetic monopoles'. In this study, the researchers fabricated, for the first time, thin spin ice films with a thicknesses of only a few nanometres. At about half a degree above absolute zero, the normal entropy of spin ice within the film was found to disappear, showing that the Third Law is restored in these spin ice thin films. Using X-ray diffraction at the LCN, the researchers showed that the films are slightly strained by the `substrate' on which they are grown, which causes the loss of entropy. "This result shows that we can use strain to drastically alter and control the spin ice state" says Dr. Laura Bovo (UCL London Centre for Nanotechnology) the leading author of the team's paper. "It opens up new possibilities for the control and manipulation of magnetricity and magnetic monopoles in spin ice." Prof. Steve Bramwell (UCL Department of Physics and Astronomy), another of the paper's authors, added, "Restoration of the Third Law in spin ice thin films adds an unexpected twist to the story of spin ice. How the Third Law is first violated and then restored in spin ice is an interesting question of basic physics." Control of the entropy of spin ice could potentially be used in a number of applications; for example, magnetic technology in computer hard disks is often based on thin magnetic films. Story Source: Materials provided by University College London. Note: Content may be edited for style and length.
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