Researchers demystify glasses by studying crystals

But what most people don't know is that"glass transitions,"where changes in structure of a substance accompanying temperature change get"frozen in,"can show up during cooling of most any material,liquidsthrough metals. This produces"glassy states,"of that material– exotic states that can be unfrozen and refrozen by merely changing the temperature a little up and down around the transition temperature.

"For liquids,"says C. Austen Angell, an Arizona State University Regents professor of chemistry and biochemistry and a leading explorer in this domain,"it's fairly simple, glasses form when crystals don't."Beyond this"banality,"as Angell calls it, things get more complicated.

Angell has done considerable work in this realm, the best known of which has been on the classification of glass-forming liquids between extreme types–"strong"liquids, like the silica glass that optical fibers are made from, and"fragile"liquids which glassblowers stay away from because, during cooling, they set up solid too quickly for them to work with.

Now another piece of the puzzle is being reported on what, exactly, glasses are. The report uses the unusual behavior of a non-liquid substance to help unlock the secrets. It is a metallic alloy consisting of equal parts of cobalt and iron.

In a paper in the Nov. 28, 2010, issue ofNature Physics, Angell and his colleagues– Shuai Wei, Isabella Gallino and Ralf Busch, all of Saarland University, Germany– describe the behavior of iron-cobalt (Fe₅₀Co₅₀) superlattice material as it cools down from its randomly ordered high temperature state. The paper,"Glass transition with decreasing correlation length during cooling of Fe50Co50 superlattice and strong liquids,"builds on work from 1943 by Kaya and Sato in Japan who measured the heat capacity of the bi-metallic alloy.

Heat capacity is the amount of energy it takes to heat a sample by one degree Kelvin. Albert Einstein thought heat capacity was a material's most revealing property.

The iron-cobalt alloy heat capacity showed two features– a sharp spike at 1000 K (1340 F) called a lambda transition (which is quite common inmetalalloys as the two types of atoms order themselves onto two individual interpenetrating lattices)– and near 750K (890 F) another feature which is very unusual for a metallic crystal, a glass-like transition, where the state of order gets"frozen in"during cooling.

Most glassy forms of matter experience a gradual increase in heat capacity as they are heated until this special transition point is reached. At this point (called the glass temperature) the materials suddenly jump to a new, higher heat capacity zone, often 100 percent higher, and change from a solid material to a very viscous liquid.

What the new measurements in theNature Physicsarticle show is that the disordering of the superlattice has the kinetic characteristic of strong liquids. But because the alloy lambda transition is well understood, researchers know that a property called the"correlation length"is decreasing as the temperature decreases from the lambda spike towards the (glass) transition temperature. This is the opposite behavior from what has been thought to be characteristic of liquids as they cool towards their transition temperature.

"We now argue that strong and fragile extremes are not really extremes, so much as they are opposites,"Angell said.

"On a molecular level, we now think that in the strong liquids the organization of molecules in space is getting shorter-range as the glassy state is approached, while in the fragile liquids, that organization length is indeed getting longer as people have already proposed,"he explained."This shows that static correlation length changes do not, by themselves, account for the liquid turning solid at the glass transition."

To understand the paradox Angell turned to the substance water. Water is famous for having strikingly anomalous properties in its super cooled state. As 228 K (-49 F) is approached, water'sheat capacityis racing up like that of iron-cobalt alloy at its critical point. Scientists now believe that water would show the same sort of spike if it didn't crystallize first. When water is quenched at a million degrees per second it doesn't crystallize and a glass that acts like a low temperature replica of silica results, i.e. a strong liquid. Angell sees water's behavior as a sort of Rosetta stone.

"The Rosetta stone has two faces, the same statement on each but in a different language,"he said."In my case, water speaks the language of fragile liquids on its upper face (at temperatures more than 228 K, or -49 F), and the language of strong liquids on its lower face (temperatures less than 228 K)."

Previous work by Angell (with Robin Speedy in 1976) pinpointed the 228 K temperature as a"divergence temperature"of a special mathematical law, called a power law, typical of critical systems, which described the physical properties of super cooled water.

"So now we see strong liquids and fragile liquids as occupying opposite flanks of some generalized 'order-disorder' transition,"Angell explained. Angell and his colleague Dmitry Matyushov in ASU's physics department, plan to describe this generalized transition in more detail in the future.

Meantime there are practical benefits to be had. Angell points out that ifoptical fiberglasses, being silica or silica-like, have shorter range organization at lower temperatures, then fibers that have been annealed at lower temperatures than their fiber-drawing temperature (more than 2000 K) should be less scattering of light, hence better for communications purposes. Thus this new information can mean better performing materials in the future.

"Patent literature suggests that the fiber optics scientists already learned the benefits of annealing (a heat treatment that alters the microstructure of a material causing changes in properties such as strength, hardness and ductility). Now we would know exactly why this is so, and we could actually design that property into the material forming process,"Angell said.

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The water dance

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Theoretical chemist James Skinner, at the University of Wisconsin in Madison, has been researching water for over a decade. According to Skinner, understanding the dance ofwater moleculesis key to understanding how all of life works.

"We now appreciate that it is virtually impossible to grasp almost anything at the molecular level about biology, without understanding how water molecules interact withbiomoleculeswithin cells,"said Skinner, who is supported by a grant from the National Science Foundation (NSF) Division of Chemistry."This includes fundamental processes like protein folding,photosynthesisand the biology of vision."

But how do scientists search out the secrets of water's choreography? Skinner, along with three other scientists--Krzysztof Szalewicz, professor of physics and astronomy at the University of Delaware; Martin Gruebele and his research team at the University of Illinois; and Geraldine Richmond and her team at the University of Oregon--use different approaches to get at water's unique movements and interactions.

Making molecular movies

Skinner and his research group at the University of Wisconsin are analyzing and interpreting the results of experiments that use infrared light to probe the motions of molecules inliquid water.

The vibrations between oxygen and hydrogen atoms within a molecule absorbinfrared light. By varying the frequency of the light and measuring how much is absorbed at each frequency, the researchers record a graph called an"absorption spectrum."More modern techniques use ultrashort pulses of infraredlaser light.

"Data from these experiments contain information about local molecular environments,"Skinner said."But this information is often hard to extract. We use first principles calculations, molecular dynamics simulations, statistical mechanics, and basically any theoretical approach that will enable us to further our understanding."

One method in particular, molecular dynamics simulation, allows the researchers to"see"water molecules in action.

Like designing the computer graphics for a movie, they start with an initial configuration of the molecules and then advance it forward in time, frame by frame. To do this, the team created a model of the potential energy for a large collection of water molecules in terms of interactions between groups of two or three molecules.

"We can show the trajectories (in time) of all the atoms in a system,"Skinner said."The simulations--and hence the movies--always involve approximations (for example, using classical mechanics instead of quantum mechanics for motion of the atomic nuclei)."Even so, the simulations have proved to be quite accurate.

Up next for Skinner's team, is a study of the ways biomolecules such as peptides, proteins, nucleic acids, and membrane lipids, dissolve in water."Experiments, coupled with new theoretical and computational techniques, will surely shed new light on the critical problem of water dynamics around biomolecules, and its effect on how they function,"he said.

In addition to boosting biomedical research, Skinner believes that understanding the dance of water is also crucial to climate science."To model the chemical and physical processes in the atmosphere one needs to understand reactivity of aerosol particles, which is often controlled by water dynamics at the surface of the aerosols,"he said.

First principles findings

Krzysztof Szalewicz , professor of physics and astronomy at the University of Delaware, uses theab initio, or"first principles,"approach to researching water molecules in motion.

With support from NSF's Quantum Calculations Program in the Division of Chemistry, Szalewicz and his team start with what is known about each type of atom within the molecule--in this case hydrogen and oxygen. Then they plug that information into the Schrödinger wave equation.

"By solving Schrödinger's equation, we can predict the properties of lighter atoms and molecules almost exactly,"Szalewicz said. Physicist Erwin Schrödinger developed his Nobel Prize-winning equation in 1926 as a way to explain the wave-like nature of particles at the nanometer (billionth of a meter) scale. His equation is central to the theory of quantum mechanics.

Once Szalewicz's team solved the equation for individual water molecules, they used the results to create a model of two molecules interacting. Then they increased the number of molecules to three. By combining several of these models, the team was able to simulate a large number of molecules interacting in bulk liquid water.

"We can model water very accurately and we did systems as large as 40 atoms fairly accurately,"Szalewicz said."However, that is about the limit at the present time."

Following the first principles method, Szalewicz and his team used no data from lab experiments to develop their model."So-called empirical approaches use experimental data to adjust their predictions,"he said."Thus, if experimental data are wrong, the predictions will be wrong."

But in this case, the model's predictions were gratifying."The agreement of our predictions with experiment for water was excellent,"he said.

"If you look at the whole range of properties, our predictions are better than any published ones, even including results obtained by empirical approaches,"Szalewicz said."Because liquid water is a rather complicated system despite the simplicity of water molecule itself, this was a demanding test for our approach."

Movers and shapers

Meanwhile, Martin Gruebele and his research team at the University of Illinois, sponsored by NSF's Division of Molecular and Cellular Biosciences, follow a different method. The team uses pulses of terahertz (THz) radiation, which falls between infrared and microwave radiation on the electromagnetic spectrum, to directly measure the movements of water molecules around proteins. Because water molecules consist of a single oxygen atom and twohydrogen atoms, they are dwarfed by the complex, folded protein molecules, made up of hundreds or thousands of individual atoms.

"Proteins influence the dynamics of up to thousands of water molecules surrounding them out to a distance comparable to the size of the protein,"Gruebele said.

These affected molecules form a thick"solvation shell"around the unwieldy protein. In the process, the two-way interaction between water and protein causes hydrogen bonds and other weak bonds in the molecules to change and rearrange.

Good vibrations

To measure just how far this influence stretches, Gruebele's group flashes laser light pulses in the THz range through a mixed sample of protein and water. When this radiation hits a molecule, if its wavelength matches the natural vibration frequency of the atomic bonds that join the molecule together, it absorbs that energy. This extra energy causes the bonds between atoms to vibrate with regular, repetitive motions.

"Pulsed THz light oscillates about once every picosecond (one-trillionth of a second),"Gruebele said. And since water molecules and proteins both vibrate on a similar time scale, they can absorb light with wavelengths in that range.

In order to"see"what is taking place inside the molecules, the researchers used a spectroscope to measure the specific wavelengths absorbed.But in the process, they discovered something totally unexpected.

"We knew that dry protein powder absorbs less THz light than water does,"Gruebele explained."So we expected that adding protein powder to water would decrease the amount of light absorbed."But instead it increased, as long as protein concentrations remained low.

"We concluded that this happens because the solvation shell actually absorbs more light than bulk water, which more than makes up for the smaller absorption of light by the protein,"Gruebele said.

"As you add more protein molecules, the solvation shells around each one begin to overlap and the absorption stops increasing,"he added."This actually allows you to measure the size of the shells: We found they are almost three nanometers (three billionths of a meter) in diameter. This is small in human terms, but gigantic in terms of the size of a water molecule."

In addition to this THz absorption spectrum method, Gruebele's team used three other measurement techniques to illuminate different aspects of the protein folding process.

"The combination of these four techniques shows that water adopts a"folded"structure and dynamics very early during the protein folding process,"he explained."Thus, water is an early driver and an integral part of that process."

In their next phase of research, Gruebele and his team will look at how water molecules mediate the binding of two biologically active molecules, such as two proteins, or a drug and a protein.

"Eventually, this would give us a much better estimate of how strongly drugs bind and why they are specific,"he said."Such information would save a great deal of live animal testing, and reduce the cost of pharmaceutical screening in the search for drugs."

Surface surprises

Another NSF-funded research effort, headed by Geraldine Richmond and her team at the University of Oregon, is probing the interactions between surface water molecules and atmospheric gases.

"I've always been fascinated by water--the way it flows, how it controls our body temperature and the temperature on this planet, how things can float on it,"said Richmond.

"And the surface of water to me is the most fascinating,"she said."It plays such an important role in our environment and our bodies."Richmond and her group are supported by an award from NSF's Electrochemistry and Surface Chemistry Program.

"We want to understand how environmentally important gases such as sulfur dioxide (SO₂) and carbon dioxide (CO₂) interact with water surfaces,"Richmond said."We know a lot about how such gases behave once they are in water, but not much at all about what happens when they first make contact."

Richmond's team was surprised to find that SO₂tends to hang out at the surface before submerging, whereas CO₂dives right in.

"This is the first time that anyone has ever measured, with this level of molecular detail, a gas-surface complex at the surface of liquid water,"she said."We are now investigating a whole series of important environmental gases, ions and solutes at the water surface."

The group is also looking at the differences between surface chemistry and chemistry in bulk water."What we do is to try to understand the underlying principles behind the chemistry that occurs in the atmosphere,"Richmond said.

"For example, nitric acid accumulates in aerosols and clouds,"she explained."Since a lot of the reactions that happen in the atmosphere occur on the surface of aerosol particles that contain these strong acids, we wanted to understand if nitric acid continues to act as a strong acid when it sits at the surface of the aerosol."

Because aerosol interactions are so complex, the scientists simplified their study to focus on how individual molecules of nitric acid behave at the surface of a solution of nitric acid in water.

"What we find is quite remarkable,"Richmond said."When nitric acid sits at the surface of water, it acts as a weak acid -- not a strong one. This comes from the fact that at the surface, it is surrounded by fewer water molecules. Consequently it doesn't dissociate, or get pulled apart, and therefore is far less reactive there than inside the bulk liquid."

Mixing oil and water

Richmond's teamw is also fascinated by interactions between oil and water."Normally people think that oil and water don't mix; but there is a weak attraction between them,"she said."That leads to some interesting consequences."

Richmond describes the human body as"one big oil/water interface, with water continually flowing past oily membranes,"for example ions transporting through cell walls.

"We find at this oil-water interface, water is highly oriented and creates conditions that can facilitate some of the most important chemistry in our bodies,"Richmond said.

"Related to this we try then to understand how the unique properties of the junction between water and oil influences the adsorption of surfactants such as soaps, dispersants and polymers."

Why is this important?"Everything from mayonnaise to oil-spill dispersants relies on surfactants such as these adsorbing at the interface and in some cases, working to keep the oil and water separated,"she explained.

"Our laboratory work provides unique insights into the molecular properties of the interface but this information is somewhat two-dimensional,"Richmond said."There are missing pieces."

To fill in the three-dimensional picture, the scientists count on computer simulations. But there is a downside."Often you don't know how realistic the results are and whether the models you are using in your simulations are correct,"she pointed out.

"However, in our case we can check their accuracy by seeing how close the computer results match our experimental results,"Richmond added."Then we dig into the calculations to see what kind of molecular interactions gave us that match. So they really work hand-in-hand to give us a robust three dimensional picture of what is happening at the surface ofwater."

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Progress toward terabit-rate high-density recording

In theJournal of Applied Physics, researchers at Sun Yat-Sen University in China have demonstrated a way to record on ferromagnetic films using a laser-assisted ultrafast magnetization reversal dynamics.

The technique uses so-called time-resolved polar Kerr spectroscopy combined with an alternating magnetic field strong enough to re-initialize the magnetization state of gadolinium-iron-cobalt (GdFeCo)thin films. Tianshu Lai and colleagues showed that the magnetization reversal could occur in a sub-nanosecond time scale, which implies that next- generation magneto-optical storage devices can not only realize higher recording densities but also ultrafast data writing of up to a gigahertz. Such speed is at least thirty times faster than that of present hard disks in computers.

Laser-assistedmagnetic recordingwas demonstrated on a sub-picosecond time scale under a saturated external magnetic field."We found that the rate of magnetization reversal is proportional to the external magnetic field,"says Lai,"and the genuine thermo-magnetic recording should happen within several tens to hundreds of picoseconds when we apply a smallermagnetic fieldthan the coercivity of the recording films."

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Lead-free piezoelectric materials of the future

Piezoelectric materials really took off after the 1950s, with the development of a superior man-made piezoelectric ceramic crystal: lead zirconate titanate, or PZT (the initials of its chemical symbols). Over the past 60 years, PZT has been essential for myriad high-tech applications: from inkjet printers to digital camera shutters, ultrasonic imagers, fuel injector actuators, and igniters for gas barbecue grills.

Despite this success, many scientists now want to replace PZT with some as yet undiscovered lead-free material that would be more environmentally benign and that would enable new piezoelectric applications in biological settings. To date, however, no suitable successors have been found. Candidates are typically too feeble in theirpiezoelectric effectand/or physical durability.

A Swiss scientist, Dragan Damjanovic, thinks researchers should be looking more broadly. He says nearly all of today's efforts are focused on materials whose ions and electrons -- the ultimate source of the piezoelectric effect -- behave in a particular manner, called polarization rotation. Histheoretical calculationshave shown that another, overlooked behavior - polarization extension, present in other classes of materials -- can also generate an enhanced piezoelectric effect.

An article by Damjanovic in the journalApplied Physics Letters, which is published by the American Institute of Physics, details his ideas and supporting evidence. In particular, he proposes a particular type of phase diagram that he believes will lead to improved, lead-freepiezoelectric materials.

"What I have done is at odds with the dominant thinking,"Damjanovic admits."But I offer a different approach to an important problem."

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Meta-flex: Your new brand for invisibility clothing

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Flexible smart materials that can manipulate light to shield objects from view have been much-theorised but now researchers in Scotland have made a practical breakthrough that brings the possibility of an invisibility cardigan– or any other item of invisibility clothing - one step closer.

Two challenges to the creation of smart flexible materials that can cloak from visible light are making meta-atoms small enough to interact with visible light, and the fabrication ofmetamaterialsthat can be detached from the hard surfaces they are developed on to be used in more flexible constructs.

Research published today, Thursday 4 November 2010, inNew Journal of Physics, details how Meta-flex, a new material designed by researchers from the University of St Andrews, overcomes both of these challenges.

Although cloaks designed to shield objects from both Terahertz and Near Infrared waves have already been designed, a flexible material designed to cloak objects from visible light poses a greater challenge because of visible light's smaller wavelength and the need to make the metamaterial's constituent part– meta-atoms– small enough to interact withvisible light.

These tiny meta-atoms have been designed but they have only traditionally been realized on flat, hard surfaces, making them rigid constructs impractical for use in clothing or other possible applications that would benefit from flexibility, such as super lenses.

The research team, led by EPSRC Career Acceleration Fellow Dr Andrea Di Falco, has developed an elaborate technique which frees the meta-atoms from the hard surface ('substrate') they are constructed on. The researchers predict that stacking them together can create an independent, flexible material, which can be adopted for use in a wide range of applications.

Di Falco says,"Metamaterials give us the ultimate handle on manipulating the behaviour of light. The impact of our new material Meta-flex is ubiquitous. It could be possible to use Meta-flex for creating smart fabrics and, in the paper, we show how easy it is to place Meta-flex on disposable contact lenses, showing how flexible superlenses could be used for visual prostheses."

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Researchers image atomic structural changes that control properties of sapphires

Peering through an electron microscope down to a level where a human hair would seem as wide as a washer and dryer set, they were able to quantify deviations from the regular columns of aluminum andoxygen atoms- the stuff of perfect sapphire crystals. The work is published in the journalScienceFriday, Nov. 26.

These structural changes are called dislocations and include very small rearrangements of some of the aluminum atoms from their normal surroundings of six oxygen atoms to a layout of four surrounding oxygen atoms.

While the changes in structure are minute, they deliver a punch.

In the orderly world of crystals, dislocations can control electrical, chemical andmagnetic propertiesas well as strength and durability. And, the information and imaging technique used in the study can be applied to all crystalline solids, from microchips to thermal protection systems that shield jet engines from extreme heat.

"We imagined this might have been the possible change in structure a year or so ago and now we're able to see how the atoms are moving with respect to one another,"said Arthur Heuer, Distinguished University Professor and Kyocera Professor of Ceramics in the department of materials science and engineering at the Case School of Engineering."The important thing is we were able to image it withatomic resolution."

Peter Lagerlöf, an associate professor of materials science and engineering at Case Western Reserve, noted that"understanding the structure of the dislocations is important because it allows increased understanding of material properties."

Heuer traveled to Julich, Germany, where he worked with Chunlin Jia at the Institute of Solid State Research and Ernst Ruska-Centre for Electron Microscopy. There, using an ultra high magnification transmissionelectron microscope, the scientists employed negative spherical aberration imaging to a section of synthetic sapphire to see dislocation cores.

This is the first time the technique was applied at subangstrom resolution to structural defects in ceramics.

The scientists were able to distinguish columns of oxygen from columns of aluminum in synthetic sapphire, used to make substrates for specialty advanced computer chips (because of sapphire's good thermal conductivity and electrical resistivity), and grocery store scanners and expensive watch faces (because of sapphire's superior scratch-resistance compared to glass).

Dislocation cores terminate with aluminum atoms and electrical neutrality is maintained as the cores occupy only half of the aluminum sites. A complex mix of six-fold and four-fold coordinated aluminum polyhedra are found in the dislocation cores.

Jacques Castaing, a materials scientist at Laboratorie Physique des Materiaux, CNRS Bellevue, F 92195 Meudon Cedex, France, was not involved in the experiment but with Heuer and Lagerlöf, last year published a theory that the atomic structure would change this way.

Castaing said that being able to see the dislocations,"for the basic knowledge of materials, is very important. These dislocations are everywhere."

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Enhanced efficiency when determining band gap in solids

Chan, a post-doc at Argonne National Lab (formerly at MIT), points out that the theory is lacking when it comes to band gaps."Theband gapproblem is a well known one. However, using current methods, there is quite a bit of inaccuracy when it comes to calculating band gaps."Band gaps in solids are important, especially if researchers want to identify the best materials for a variety of functions. The maximum efficiency of a solar cell, for example, is determined by the band gap of the material."The inability to predict the band gap is holding back research inphotovoltaics, as well as insemiconductorsand thermoelectrics,"Chan points out.

However, Chan thinks that a solution might have been found. Working with professor Gerbrand Ceder at MIT, it appears that a modified application of density functional theory– with a special generalization for solids– might hold the key to more accurate predictions of band gaps. The work is described inPhysical Review Letters:"Efficient Band Gap Prediction for Solids."

“In the past, researchers have broken down individual electrons using a sort of itemized list of individual states. This has allowed for calculations revealing different properties, including band gaps. Unfortunately, the accuracy of the band gap predictions has been off,” Chan says. She points out that scientists know that silicon has a band gap of about 1.2 eV, but when current methods are employed to calculate the band gap, the answer is 0.7 eV.“You can see how that’s a problem if you are trying to gauge the suitability of a material for specific purposes,” she continues.

To get a more accurate prediction of band gaps, Chan and Ceder created a method that involves altering the use of density functional theory so that an itemized list of individualized states is not the only consideration.“We also recognize that there are a number of interactions between electrons. So we look at the total energy, which includes these interactions,” Chan explains.

Not only do Chan and Ceder make use of the total energy, but they also demonstrate that the band gap can be viewed as a property of the ground state.“This changes the way we view the band gap, seeing that it is a part of the ground state,” Chan says.

Moving forward, Chan hopes that this technique can be used to identify the band gaps of different materials with more accuracy. This could prove useful in identifying the best options when creating future technology.“This work is part of the Materials Genome project started by Professor Ceder, with a goal of predicting properties of known compounds and using the knowledge to design new ones,” Chan says.“Part of that is understanding the band gap and being able to quickly determine the band gaps of various materials.”

“Our method is relatively inexpensive, and could be useful when learning the properties of new materials,” Chan continues.“If someone came up with a new kind of material, predicting the band gap is not a question easily answered in the past. Hopefully our work will pave the way for easier answers in the future.”

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When size matters: Yttrium oxide breaking down under pressure

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An experimental team working at the U.S. Department of Energy’s Advanced Photon Source (APS) at Argonne National Laboratory has discovered that Y₂O₃in nanometer-sized particles undergoes a definite phase transition under pressure that results in characteristics quite different from bulk Y₂O₃, a finding with important implications for the use of yttrium oxide as a nanomaterial. Similar transitions have also been observed in silicon dioxide and other bulk materials, but the exact mechanism of the amorphization process has remained elusive.

Led by the HPSynC (the joint consortium of Geophysical Laboratory at the Carnegie Institution of Washington and the APS) scientist Lin Wang, a research team from Carnegie Institution of Washington, Argonne, Stanford University, Jilin University, Xiangtan University, Stanford University, and the SLAC National Accelerator Laboratory examined yttrium oxide samples with grain sizes ranging from 5 nm to 1μm, utilizing a variety of in situ high-pressure techniques, including x-ray diffraction, Raman spectroscopy, and high-energy pair distribution analysis at the HP-CAT 16-IDB and 16-BMD beamlines at the APS. After transmission electron microscopy revealed the same crystal structure in all of the samples under ambient conditions, the team subjected them to high pressures using diamond anvil cells with transmitting media of He and silicone oil.

The 16-nm, particle-sized material demonstrates increased stability compared to bulk Y₂O₃, with the cubic phase persisting up to 24.8 GPa (approximately 12.8 GPa greater than in the bulk material). As pressure is increased to above 24.8 GPa, disorder begins to appear, eventually yielding to a completely amorphous state above 30 GPa. The larger-grained 21-nm material, however, behaves considerably differently, maintaining its cubic structure up to 14 GPa, then undergoing a transition to the hexagonal phase up to 32.7 GPa. Meanwhile, micron-sized material transforms from cubic to the hexagonal phase at 12 GPa, which persists to the highest pressures measured. This demonstrates that yttrium oxide nanoparticles have a critical size above which they behave like the bulk Y₂O₃, but become amorphized below that size. Raman spectroscopy confirms the amorphized state of the samples subjected to high pressure.

To further characterize this observed dependence of particle size on structural stability under pressure, high-energy pair distribution function (PDF) measurements were performed on 16-nm-sized samples. Using this technique,“We can identify differences of atom-to-atom linkage,” said co-author Wenge Yang of the Carnegie Institution. Because the distance between each pair of atoms in the structure can be determined by the position of peaks in the PDF data, shifts in those peaks represent changes in the atomic bonding length, while a complete disappearance of peaks indicates breakdown of the original linkage. Under increasing pressures, the connectivity of the YO6 octohedra in the cubic Y₂O₃phase begins to break down, while the links between the edges shared by neighboring octahedra remain intact. Finally, at ~29 GPa and beyond, long-range ordering disappears and amorphization is completely dominant.

“This is the first time we’ve seen this size-dependent amorphization mechanism with the PDF measurement,” Yang said. The observed size effect explains the greater stability under pressure: While grain sizes of 21 nm and above transform from a cubic to hexagonal phase underpressure, the smaller-grained Y₂O₃maintains the cubic phase for a longer period before amorphization sets in.

The research team plans to continue their work studying both Y₂O₃and other materials, examining how various nanomaterial and bulk compounds with differing particle sizes behave underextreme conditions. Their findings will have direct implications regarding the manufacture and versatility ofnanomaterialsfor various applications. As the current work demonstrates, the high-energy PDF measurement technique adds an important new tool to the experimental toolbox.“We’ve opened up a new way to look at the disordering mechanism of nanomaterials,” said Yang.

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How do your crystals grow?

Research reported in TheJournal of Chemical Physicsuses fluorescence correlation spectroscopy (FCS) to investigate the processes at the surface of a growing crystal. By focusing a laser on the crystal surface and measuring the resulting fluorescence, FCS can resolve dimensions as small as a single wavelength of the light.

"Another advantage of fluorescence is that it provides a high signal-to-noise ratio,"says author Shinpei Tanaka of Hiroshima University in Japan."We are able to measure very dilute solutions at the crystal interface."

The researchers found that when single tetragonal crystals of egg-white lysozyme formed, there was no concentration gradient between the solution and the crystal surface. However, in formation of clumps of needle-like branched crystals, called spherulites, the observed concentration at the surface was several times higher than that of the bulk solution. The authors attributed the difference to aggregates of loosely bound molecules near the interface.

Characterization of the dynamics near the crystal by FCS may provide direction for improving the crystallization process -- currently as much an art as a science, based on trial and error -- because the spherulites are not usable for structural characterizations.

"Although we knew something was different between the two crystal forms, the degree of concentration of the molecules in spherulites compared to that of the homogeneous state around tetragonal single crystals was surprising,"says Tanaka.

The analytical result could lead to improvements in isolation of goodcrystalsofbiomolecules. For example, the results suggest that local heating by a laser could be used to control local concentrations and avoid spherulite formation.

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Forcing mismatched elements together could yield better solar cells

Highly mismatched alloys, which are still in the experimental stages of development, are combinations of elements that won't naturally mix together using conventional crystal growth techniques. Professor Rachel Goldman compares them to some extent to homogenized milk, in which the high-fat cream and low-fat milk that would naturally separate are forced to mix together at high pressure.

New mixing methods such as"molecular beam epitaxy"are allowing researchers to combine disparate elements. The results, Goldman says, are more dramatic than smooth milk.

"Highly mismatched alloys have very unusual properties,"Goldman said."You can add just a sprinkle of one element and drastically change the electrical andoptical propertiesof the alloy."

Goldman is a professor in the departments of Materials Science and Engineering, and Physics. Her team included other U-M physicists and engineers as well as researchers from Tyndall National Institute in Ireland.

Solar cells convert energy from the sun into electricity by absorbing light. However, different materials absorb light at different wavelengths. The most efficient solar cells are made of multiple materials that together can capture a greater portion of theelectromagnetic radiationin sunlight. The bestsolar cellstoday are still missing a material that can make use of a portion of the sun'sinfrared light.

Goldman's team made samples ofgallium arsenidenitride, a highly mismatched alloy that is spiked with nitrogen, which can tap into that underutilizedinfrared radiation.

The researchers used molecular beam epitaxy to coax the nitrogen to mix with their other elements. Molecular beam epitaxy involves vaporizing pure samples of the mismatched elements and combining them in a vacuum.

Next, the researchers measured the alloy's ability to convert heat into electricity. They wanted to determine whether its 10 parts per million of nitrogen were distributed as individual atoms or as clusters. They found that in some cases, the nitrogen atoms had grouped together, contrary to what the prevailing"band anti-crossing"model predicted.

"We've shown experimentally that the band anti-crossing model is too simple to explain the electronic properties of highly mismatchedalloys,"Goldman said."It does not quantitatively explain several of their extraordinary optical and electronic properties. Atomic clusters have a significant impact on the electronic properties of alloy films."

If researchers can learn to control the formation of these clusters, they could build materials that are more efficient at converting light and heat into electricity, Goldman said.

"The availability of higher efficiency thermoelectrics would make it more practical to generate electricity from waste heat such as that produced in power plants and car engines,"Goldman said.

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One way sound: A new material will allow for improved ultrasound imaging

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Making sound waves go in one direction only has been more difficult because of the way sound waves move through a material. Sound consists of push-pull disturbances that first compress atoms ormolecules, whether they are in a solid or even a gas like air, and then let them decompress. The disturbance acts along the direction the sound is moving.

But now for the first time, a rectifier for sound has been created at last, by scientists at Nanjing University in China. Their acoustic rectifier works on sound waves that are moving through water and was created by using two different parts.

The first part is a special material -- a liquid filled withmicroscopic bubbles-- in which sound waves entering at a particular frequency (pulsations per second) leave with a frequency twice as great. The second part consists of a sandwich-like layering of alternating thin slabs of glass and slabs of water which acts like a filter that only allows the sound waves with the doubled frequency to pass through the material but not the original sound waves.

Now, turn the rectifier around and sound waves with the original frequency do not flow through the filter at all -- so no sound gets through. In effect, the combination of the doubling part plus the filter part acts like a one-way gate for sound waves.

So why are acoustic rectifiers potentially useful? After all, when a band plays music, a listener wants the sound to be able to come and go since many musical effects depend on simultaneously hearing both the initial wave of sound as well as the reflected sound waves.

Music is one thing, but imaging is another.

In sonogram imaging -- the method often used for imaging a fetus -- sound waves are sent into the body. Reflected waves bounce back to the scanning instrument and the surrounding sensors, forming the image displayed on-screen. However, some of the reflecting waves interfere with the ingoing waves, lowering the brightness and the resolution of the image. Keeping waves from coming back toward the ultrasound source would help to keep the image strong and sharp.

"This design offers design possibilities for manipulating acoustic energy to make one-way mirrors for protecting ultrasound sources from being disturbed by backtracking sound waves,"said Jianchun Cheng, one of the Nanjing researchers."It can also serve as a sonic barrier for blocking environmental noise in a single direction."

Cheng and his colleaguespublished details of their acoustic rectifierin the Oct. 24 issue of the journalNature Materials.

Xiang Zhang, an expert on manipulating sound waves who was not involved with the Nanjing work, said that their work is"a significant step in demonstrating an acoustic diode concept by using a bubble suspension. Given more engineering optimization, such a one-way acoustic mirror may find some interesting applications in more effective medical ultrasound therapy and high resolution imaging by rejecting the back-scattered waves."

Zhang, a distinguished scientist and engineer at the University of California at Berkeley,published a paperof his own in the Nov. 8 journalNature Physicsabout how to improve the sharpness of acoustic imaging using special"metamaterials"bored through with a honeycomb pattern of empty spaces. Normally the sharpness of an image is no better than about the wavelength of the wave being used to form the image. But in Zhang's acoustic device, which he developed along with Francisco Garcia-Vidal at the Universidad Autonoma de Madrid in Spain, the image sharpness is about 50 times finer than the size of the incomingsound waves.

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New equation could advance research in solar cell materials

Without the Shockley equation, the computers of today would not be possible.

Developed in 1949 by William Shockley, the inventor of the transistor, the Shockley equation describes the relationship between electric current and voltage in inorganic semiconductors such assilicon.

The new equation describes the relationship of current to voltage at the junctions of organic semiconductors—carbon-rich compounds that don't necessarily come from a biological source, but resemble them. Organic semiconductors present special challenges for researchers because they are more disordered than their inorganic counterparts. But they could enable advanced solar cells, thin and intense OLED (organic light-emittingdiode) displays, and high-efficiency lighting.

"The field oforganic semiconductorresearch is still in its infancy. We're not making complicated circuits with them yet, but in order to do that someday, we need to know the precise relationship of current and voltage. Our new equation gives us fundamental insights into how charge moves in this class of materials. From my perspective, it's a very significant advance,"said Steve Forrest, the William Gould Dow Collegiate Professor of Electrical Engineering and U-M vice president for research.

Forrest and his doctoral students, Noel Giebink (now at Argonne National Laboratories) and Brian Lassiter, in the U-M Department of Electrical Engineering and Computer Science, contributed to this research. Two papers on the work are published in the current edition ofPhysical Review B.

About six years ago, researchers in Forrest's lab realized that they could use Shockley's equation to describe the current/voltage relationship in their organic solar cells to a degree.

"It fit nicely if you didn't look too hard,"Forrest said.

Their findings were published, and from that time on, many physicists and engineers used the Shockley equation for organic semiconductors even though it didn't describe the physics perfectly. The new equation does.

Forrest says it will allow researchers to better describe and predict the properties of the different organic semiconductors they're working with. And in that way, they'll be able to more efficiently choose which material best suits the needs of the device they're working on.

"People have been investigating organic semiconductors for 70 or 80 years, but we're just entering the world of applications,"Forrest said."This work will help advance the field forward."

The papers are titled,"The Ideal Diode Equation for Organic Heterojunctions. I. Derivation and Application,"and"The Ideal Diode Equation for Organic Heterojunctions. II. The Role of Polaron Pair Recombination."

Forrest is also a professor in the departments of Physics, and Materials Science and Engineering. Others contributing to this work are affiliated with Argonne National Laboratory's Center for Nanoscale Materials and Northwestern University.

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New technology gives on-site assessments in archaeology

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Now, however, an international team of physicists, archaeologists and materials scientists has developed a process that can tell in a matter of minutes the origin of samples thousands of years old. The new device is easily portable and works by"lifting off"the spectral fingerprint of a material with infrared light.

The first material tested was the mineral calcite, commonly found in rocks such as limestone, which forms over millions years in sediments. These rocks can also contain the mineralized shells of sea creatures.Archaeological sitesmay also feature calcite that was a part of ash, plaster, or other building materials.

In the latest issue of the journalAdvanced Materials, on-line today, Stefano Curtarolo, associate professor of mechanical engineering andmaterials sciencesand physics at Duke University, and Kristin Poduska, associate professor of physics at Memorial University in Newfoundland, and their colleagues at the Weizmann Institute of Science in Israel, describe the new approach, which has already been successfully tested in archeological sites in Israel.

"The key to determining a sample's origin lay in figuring out how well thecrystal structureis organized,"Curtarolo said."Naturally occurring calcite crystals are tightly organized, while a material created by humans from calcite is usually far less organized."

However, interpreting the information obtained using traditional methods is time-consuming and tricky, since such factors as particle size and the alignment of the atoms within the crystals can send out conflicting information.

"For this reason, getting useful and reliable information about the sample usually requires careful and time-intensive sample preparation with highly specialized equipment,"Poduska said.

The researchers used infrared spectroscopy to take advantage of the fact that different molecular units absorb light differently, yielding distinct spectral peaks, or molecular fingerprints. They put a sample through a series of grindings– sometimes as many as a dozen– while taking detailed infrared spectroscopy readings after each one. By analyzing the absorption peaks at different points in the grinding process, as the particles got smaller and smaller, they could tease out the effects of size and organization.

For example, Curtarolo said, an archaeologist finds a sample and knows that it is calcite, but what cannot be determined at the site is whether it is a naturally occurring mineral, or a building material made of calcite. Plaster is made by heatinglimestone, grinding it up and mixing it with water.

"We've shown in the field that our method can quickly detect subtle differences in the organization of a crystal by decoupling the two factors that influence the spectral peaks,"Poduska said."Our method is particularly powerful because the direct measurement of particle size is not needed, and it can be used with any crystal that can be excited byinfrared light."

Last summer, a team ofarchaeologistsand scientists from the Weizmann Institute successfully tested the new approach at an ancient site in central Israel at Tel Safit, close to where David is thought to have slain Goliath.

"Whenever they found something white, they would call us over to do tests,"Poduska said."We were able to confirm whether the sample was rock or plaster, which helps us decide how to proceed at the excavation site."

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Batteries smaller than a grain of salt

These tiny energy storage devices could one day be used to power the electronics and mechanical components of tiny micro- to nano-scale devices.

Jane Chang, an engineer at the University of California, Los Angeles, is designing one component of these batteries: theelectrolytethat allows charge to flow between electrodes. She presents her results today at the AVS 57th International Symposium&Exhibition, which takes place this week at the Albuquerque Convention Center in New Mexico.

"We're trying to achieve the same power densities, the same energy densities as traditional lithium ion batteries, but we need to make the footprint much smaller,"says Chang.

To reach this goal, Chang is thinking in three dimensions in collaboration with Bruce Dunn other researchers at UCLA. She's coating well-ordered micro-pillars or nano-wires -- fabricated to maximize the surface-to-volume ratio, and thus the potential energy density -- with electrolyte, the conductive material that allows current to flow in a battery.

Using atomic layer deposition -- a slow but precise process that allows layers of material only an atom thick to be sprayed on a surface -- she has successfully applied the solid electrolytelithiumaluminosilicate to these nanomaterials.

The research is still in its early stages: other components of these 3D microbatteries, such as theelectrodes, have also been developed, but they have yet to be assembled and integrated to make a functioning battery.

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Studying transition between insulating and conducting states in complex oxides

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Some phase transitions in complex oxides, including one called‘colossal magnetoresistance’, can lead to drastic changes in conductivity and magnetism, with potential application to computing devices. However, the mechanisms behind these transitions remain imperfectly understood. Now, Masashi Kawasaki of the RIKEN Advanced Science Institute, Japan, Wako, Zhi-Xun Shen of Stanford University, USA, and their colleagues have produced unprecedented images of an important phase transition in a thin film made from the complex oxide, manganite.

Colossal transition under the microscope

The transition in question is characterized by an increase in the manganite film conductivity by a factor of 10,000 on application of a magnetic field at low temperature, and is an example of colossal magnetoresistance. The resulting transition has been previously characterized in terms of local magnetization, atomic displacement and density of available electronic states. A microscopic and spatially resolved picture of the film conductivity during the transition, however, has proven elusive. This is partly due to the difficulty of measuring large resistivity changes underextreme temperaturesand magnetic fields and to the requirement for high-quality films with atomically flat surfaces and distinct phase transition behavior.

Kawasaki, Shen and colleagues solved the first problem by constructing a microwave impedance microscope able to operate over a wide range of temperatures and magnetic fields, but with internal shielding to prevent these conditions from interfering with the measurement. Their microscope works by passing a fine tip over the surface of the manganite film while applying microwave-frequency electrical excitation. This allows it to distinguish between conducting and insulating portions of the film even if they are structurally identical. In addition, the researchers were able to grow high-quality, single-crystal, thin manganite films by using pulsed-laser deposition, allowingphase transitionsto be clearly observed.

Networking is important

This combination of an advanced microscope and high-quality thin film allowed the researchers to map the conductivity across a manganite film as it underwent the transition from low to high conductivity. The onset of the high conductivity state was accompanied by the formation of highly conducting manganite filaments aligned along two crystal axes of the underlying substrate made from the strontium titanium oxide (STO). These filaments, which consisted of a highly conducting metallic-like phase of manganite, began to form at very low magnetic fields, below the strengths required for the phase transition to complete, and grew as the magnetic field increased. At very high fields, the filaments formed an interconnected network across which charges could travel, or‘percolate’. Using a numerical model of the film as a network of random resistors, the team was able to reproduce the observed phase change behavior, confirming the network’s importance.

When the manganite was switched to its insulating state, and then back to its high-conductivity state, the filaments reappeared in the same locations. According to Kawasaki, Shen and colleagues, this suggests that filament direction was not completely random, but was set by disorder and strain in the film, which result from the boundary between the film’s metallic and insulating phases, as well as from strain between the film and the underlying STO substrate. The ordered growth of filaments during the phase transition is reminiscent of the self-organized patterning of snowflakes that accompanies water’s phase change from liquid to solid.

Moving beyond silicon

The observed filament growth represents the first microscopic view of thin film conductivity during the colossal magnetoresistance phase transition, with direct implications for the science in this field. It may also be useful for future‘post-silicon’ or ‘post-CMOS’ electronic devices. “Most proposed post-CMOS devices mimic existing silicon technologies like diodes and transistors,” says Kawasaki. “The directional nature of the phase separation that we have observed may allow for a completely new method to process and transfer information.” Such a technology would exploit the fact that insulating and metallic phases of the manganite thin film involve different electronic orbitals and can be controlled. Therefore, it may be possible to effectively store information in electron orbitals, rather than in charge quantity or electron spin as is done in other devices. The research team has dubbed this new approach ‘orbitronics’.

In fact, Kawasaki notes that the potential to develop radically new electronics devices is what first sparked the RIKEN team’s interest in manganitethin films.“We were thinking about using electronic phase separation for future electronics,” he explains. “It was a fortuitous meeting with Zhi-Xun Shen at a workshop in Okinawa in 2009 that led to the present work.” Shen had been developing a novel microwave impedance microscope at Stanford and was looking for interesting high-quality films to characterize with it. When the RIKEN team presented their manganite thin film and explained its phase separation properties, Shen had what he wanted.

While the new data may have an immediate impact on the colossal magnetoresistance community, Kawasaki says the RIKEN team’s original focus on new devices has been well served: “This is a step forwards in a long-term basic research program directed towards creating an entirely new kind of electronics.”

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Physicists find fractal boundaries in crystals

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The researchers made computer-synthesized models of what such metals as aluminum andcopperlook like at the atomic level while being stretched, heated and cooled. They simulated how crystals, whoseatomsstart in a regular grid, transform as they are bent into different shapes.

Such new theories could lead to a better understanding of structural materials, from buildings to bridges, to make them less susceptible to tearing or breaking.

"We're really at the beginning stages of trying to develop a systematic theory of how materials evolve as we vary strain and temperature,"said James Sethna, Cornell professor of physics, who leads the research.

The work is published in the Sept. 1 edition (Vol. 105 Issue 10) ofPhysical Review Letters, a publication of the American Physical Society.

When a single crystal is bent, portions of the crystal shift and create defects in the lattice called dislocations. The researchers found that their crystals exhibited starkly contrasting properties depending on temperature.

When hotcrystalswere bent, the dislocations arranged into grain boundaries, which are the places where lattice planes suddenly tilt. At low temperatures, the dislocations formed self-similar, random patterns known as fractals.

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Water's interaction with platinum requires a closer look, researchers find

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“The way thatwater moleculesprefer to arrange themselves on platinum has always been largely a matter of speculation,” Sandia researcher Peter Feibelman said.

Accurate knowledge is important because the first layer ofwatermolecules at the surface of a solid determines how easily water can flow past, guides the growth of ice crystals, and plays an important role in surface chemical phenomena. The metal is, among other things, a widely used catalyst.

The Sandia group present a radically new picture by working at ice-forming temperatures (to stabilize the interaction long enough to view molecular details with a scanning tunneling microscope), and then basing computer simulations on the results.

They found that to exist in the state of least energy– the endpoint sought by all undisturbed materials— water molecules arrange themselves in pentagons and heptagons in the primary wetting layer, not hexagons as their well-known arrangement in ice crystals suggests.

The unexpected result“underlines the importance of directly characterizing the first stages of water adsorption before claiming that one understands how water interacts with solids,” the researchers write in an article published July 9 inPhysical Review Letters.

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Zooming into a patch of one-molecule-thick wetting layer reveals its unexpected molecular structure.

The Sandia team of Shu Nie, Norm Bartelt, Konrad Thürmer and Feibelman began with 1997 experimental results from the University of Göttingen in Germany. Those experiments showed that water initially grows as two-dimensional crystals on platinum, unexpectedly rotated so that they seem poorly aligned with the metal atoms.

Formation of a two-dimensional wetting arrangement has the well-understood meaning that water molecules are more attracted to platinum atoms than to each other. But why the rotation?

High-resolution scanning-tunneling microscope images of the delicate system led the Sandia researchers to attribute the rotation to a clustering of water molecules lying parallel to the metal surface in a way that allows the molecules at the center of the cluster to bind particularly strongly to the metal, becoming, in effect,“molecular anchors.”

The theoretical model proposed by the researchers added the surprising detail that the anchors’ connections to the rest of the wetting layer is through water molecules arrayed in pentagons and heptagons, not hexagons.

“We think the pentagon/heptagon arrangement allows bonds to bend down, connecting higher-lying with lower-lying molecules in a relatively strain-free way,” Feibelman said.“This also suggests, however, that 3-D islands will not grow atop the wetting layer without substantial molecular rearrangement.”

A similar scenario is likely true for water on other metal crystal surfaces, he said.“More image data and more calculations will clarify the picture, probably pretty soon.”

Whether there are lessons from the metal studies that carry over to oxides and other insulators is an interesting question,“one I am drawn to,” Feibelman said.“We hope our results will yield a picture of value, down the track, for applications, but are happy enough to have made the progress we did.”

He notes that water on platinum has become a model system over time because scientists have learned to prepare highly perfect, extremely pureplatinumsurfaces, and developed tools to study molecules deposited on them.

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