Tag Archives: letter-materials-tech

A powdery mix of metal nanocrystals wrapped in single-layer sheets of carbon atoms, developed at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), shows promise for safely storing hydrogen for use with fuel cells for passenger vehicles and other uses. And now, a new study provides insight into the atomic details of the crystals’ ultrathin coating and how it serves as selective shielding while enhancing their performance in hydrogen storage.

The study, led by Berkeley Lab researchers, drew upon a range of Lab expertise and capabilities to synthesize and coat the magnesium crystals, which measure only 3-4 nanometers (billionths of a meter) across; study their nanoscale chemical composition with X-rays; and develop computer simulations and supporting theories to better understand how the crystals and their carbon coating function together.

The science team’s findings could help researchers understand how similar coatings could also enhance the performance and stability of other materials that show promise for hydrogen storage applications. The research project is one of several efforts within a multi-lab R&D effort known as the Hydrogen Materials — Advanced Research Consortium (HyMARC) established as part of the Energy Materials Network by the U.S. Department of Energy’s Fuel Cell Technologies Office in the Office of Energy Efficiency and Renewable Energy.

Reduced graphene oxide (or rGO), which resembles the more famous graphene (an extended sheet of carbon, only one atom thick, arrayed in a honeycomb pattern), has nanoscale holes that permit hydrogen to pass through while keeping larger molecules at bay.

This carbon wrapping was intended to prevent the magnesium — which is used as a hydrogen storage material — from reacting with its environment, including oxygen, water vapor and carbon dioxide. Such exposures could produce a thick coating of oxidation that would prevent the incoming hydrogen from accessing the magnesium surfaces.

But the latest study suggests that an atomically thin layer of oxidation did form on the crystals during their preparation. And, even more surprisingly, this oxide layer doesn’t seem to degrade the material’s performance.

“Previously, we thought the material was very well-protected,” said Liwen Wan, a postdoctoral researcher at Berkeley Lab’s Molecular Foundry, a DOE Nanoscale Science Research Center, who served as the study’s lead author. The study was published in the Nano Letters journal. “From our detailed analysis, we saw some evidence of oxidation.”

Wan added, “Most people would suspect that the oxide layer is bad news for hydrogen storage, which it turns out may not be true in this case. Without this oxide layer, the reduced graphene oxide would have a fairly weak interaction with the magnesium, but with the oxide layer the carbon-magnesium binding seems to be stronger.

“That’s a benefit that ultimately enhances the protection provided by the carbon coating,” she noted. “There doesn’t seem to be any downside.”

David Prendergast, director of the Molecular Foundry’s Theory Facility and a participant in the study, noted that the current generation of hydrogen-fueled vehicles power their fuel cell engines using compressed hydrogen gas. “This requires bulky, heavy cylindrical tanks that limit the driving efficiency of such cars,” he said, and the nanocrystals offer one possibility for eliminating these bulky tanks by storing hydrogen within other materials.

The study also helped to show that the thin oxide layer doesn’t necessarily hinder the rate at which this material can take up hydrogen, which is important when you need to refuel quickly. This finding was also unexpected based on the conventional understanding of the blocking role oxidation typically plays in these hydrogen-storage materials.

That means the wrapped nanocrystals, in a fuel storage and supply context, would chemically absorb pumped-in hydrogen gas at a much higher density than possible in a compressed hydrogen gas fuel tank at the same pressures.

The models that Wan developed to explain the experimental data suggest that the oxidation layer that forms around the crystals is atomically thin and is stable over time, suggesting that the oxidation does not progress.

The analysis was based, in part, around experiments performed at Berkeley Lab’s Advanced Light Source (ALS), an X-ray source called a synchrotron that was earlier used to explore how the nanocrystals interact with hydrogen gas in real time.

Wan said that a key to the study was interpreting the ALS X-ray data by simulating X-ray measurements for hypothetical atomic models of the oxidized layer, and then selecting those models that best fit the data. “From that we know what the material actually looks like,” she said.

While many simulations are based around very pure materials with clean surfaces, Wan said, in this case the simulations were intended to be more representative of the real-world imperfections of the nanocrystals.

A next step, in both experiments and simulations, is to use materials that are more ideal for real-world hydrogen storage applications, Wan said, such as complex metal hydrides (hydrogen-metal compounds) that would also be wrapped in a protective sheet of graphene.

“By going to complex metal hydrides, you get intrinsically higher hydrogen storage capacity and our goal is to enable hydrogen uptake and release at reasonable temperatures and pressures,” Wan said.

Some of these complex metal hydride materials are fairly time-consuming to simulate, and the research team plans to use the supercomputers at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC) for this work.

“Now that we have a good understanding of magnesium nanocrystals, we know that we can transfer this capability to look at other materials to speed up the discovery process,” Wan said.

For the first time, physicists have successfully imaged spiral magnetic ordering in a multiferroic material. These materials are considered highly promising candidates for future data storage media. The researchers were able to prove their findings using unique quantum sensors that were developed at Basel University and that can analyze electromagnetic fields on the nanometer scale. The results – obtained by scientists from the University of Basel’s Department of Physics, the Swiss Nanoscience Institute, the University of Montpellier and several laboratories from University Paris-Saclay – were recently published in the journal Nature.

Multiferroics are materials that simultaneously react to electric and magnetic fields. These two properties are rarely found together, and their combined effect makes it possible to change the magnetic ordering of materials using electric fields.

This offers particular potential for novel data storage devices: multiferroic materials can be used to create nanoscale magnetic storage media that can be deciphered and modified using electric fields.

Magnetic media of this kind would consume very little power and operate at very high speeds. They could also be used in spintronics – a new form of electronics that uses electrons’ spin as well as electrical charge.

Spiral magnetic ordering

Bismuth ferrite is a multiferroic material that exhibits electric and magnetic properties even at room temperature. While its electrical properties have been studied in depth, there was no suitable method for representing magnetic ordering on the nanometer scale until now.

The group led by Georg-H.-Endress Professor Patrick Maletinsky, from the Swiss Nanoscience Institute and the University of Basel’s Department of Physics, has developed quantum sensors based on diamonds with nitrogen vacancy centers. This allowed them, in collaboration with colleagues at the University of Montpellier and the University Paris-Saclay in France, to depict and study the magnetic ordering of a thin bismuth ferrite film for the first time, as they report in Nature.

Knowing how the electron spins behave and how the magnetic field is ordered is of crucial importance for the future application of multiferroic materials as data storage.

The scientists were able to show that bismuth ferrite exhibits spiral magnetic ordering, with two superimposed electron spins (shown in red and blue in the image) adopting opposing orientations and rotating in space, whereas it was previously assumed that this rotation took place within a plane. According to the researchers, the quantum sensors now show that a slight tilt in these opposing spins leads to spatial rotation with a slight twist.

“Our diamond quantum sensors allow not only qualitative but also quantitative analysis. This meant we were able to obtain a detailed picture of the spin configuration in multiferroics for the first time,” explains Patrick Maletinsky. “We are confident that this will pave the way for advances in research into these promising materials.”

Vacancies with special properties

The quantum sensors they used consist of two tiny monocrystalline diamonds, whose crystal lattices have a vacancy and a nitrogen atom in two neighboring positions. These nitrogen vacancy centers contain orbiting electrons whose spins respond very sensitively to external electric and magnetic fields, allowing the fields to be imaged at a resolution of just a few nanometers.

Scientists at the University of Montpellier took the magnetic measurements using the quantum sensors produced in Basel. The samples were supplied by experts from the CNRS/Thales laboratory at University Paris-Saclay, who are leading lights in the field of bismuth ferrite research.

Quantum sensors for the market

The quantum sensors used in the research are suitable for studying a wide range of materials, as they provide precisely detailed qualitative and quantitative data both at room temperature and at temperatures close to absolute zero.

In order to make them available to other research groups, Patrick Maletinsky founded the start-up Qnami in 2016 in collaboration with Dr. Mathieu Munsch. Qnami produces the diamond sensors and provides application advice to its customers from research and industry.

Researchers have shown that defects in the molecular structure of perovskites – a material which could revolutionise the solar cell industry – can be “healed” by exposing it to light and just the right amount of humidity.

The international team of researchers demonstrated in 2016 that defects in the crystalline structure of perovskites could be healed by exposing them to light, but the effects were temporary.

Now, an expanded team, from Cambridge, MIT, Oxford, Bath and Delft, have shown that these defects can be permanently healed, which could further accelerate the development of cheap, high-performance perovskite-based solar cells that rival the efficiency of silicon. Their results are reported in the inaugural edition of the journal Joule, published by Cell Press.

The concoction of light with water and oxygen molecules leads to substantial defect-healing in metal halide perovskite semiconductors. Credit: Dr. Matthew T. Klug

The concoction of light with water and oxygen molecules leads to substantial defect-healing in metal halide perovskite semiconductors. Credit: Dr. Matthew T. Klug

Most solar cells on the market today are silicon-based, but since they are expensive and energy-intensive to produce, researchers have been searching for alternative materials for solar cells and other photovoltaics. Perovskites are perhaps the most promising of these alternatives: they are cheap and easy to produce, and in just a few short years of development, perovskites have become almost as efficient as silicon at converting sunlight into electricity.

Despite the potential of perovskites, some limitations have hampered their efficiency and consistency. Tiny defects in the crystalline structure of perovskites, called traps, can cause electrons to get “stuck” before their energy can be harnessed. The easier that electrons can move around in a solar cell material, the more efficient that material will be at converting photons, particles of light, into electricity.

“In perovskite solar cells and LEDs, you tend to lose a lot of efficiency through defects,” said Dr Sam Stranks, who led the research while he was a Marie Curie Fellow jointly at MIT and Cambridge. “We want to know the origins of the defects so that we can eliminate them and make perovskites more efficient.”

In a 2016 paper, Stranks and his colleagues found that when perovskites were exposed to illumination, iodide ions – atoms stripped of an electron so that they carry an electric charge – migrated away from the illuminated region, and in the process swept away most of the defects in that region along with them. However, these effects, while promising, were temporary because the ions migrated back to similar positions when the light was removed.

In the new study, the team made a perovskite-based device, printed using techniques compatible with scalable roll-to-roll processes, but before the device was completed, they exposed it to light, oxygen and humidity. Perovskites often start to degrade when exposed to humidity, but the team found that when humidity levels were between 40 and 50 percent, and the exposure was limited to 30 minutes, degradation did not occur. Once the exposure was complete, the remaining layers were deposited to finish the device.

When the light was applied, electrons bound with oxygen, forming a superoxide that could very effectively bind to electron traps and prevent these traps from hindering electrons. In the accompanying presence of water, the perovskite surface also gets converted to a protective shell. The shell coating removes traps from the surfaces but also locks in the superoxide, meaning that the performance improvements in the perovskites are now long-lived.

“It’s counter-intuitive, but applying humidity and light makes the perovskite solar cells more luminescent, a property which is extremely important if you want efficient solar cells,” said Stranks, who is now based at Cambridge’s Cavendish Laboratory. “We’ve seen an increase in luminescence efficiency from one percent to 89 percent, and we think we could get it all the way to 100 percent, which means we could have no voltage loss – but there’s still a lot of work to be done.”

The modern world relies on portable electronic devices such as smartphones, tablets, laptops, cameras or camcorders. Many of these devices are powered by lithium-ion batteries, which could be smaller, lighter, safer and more efficient if the liquid electrolytes they contain were replaced by solids. A promising candidate for a solid-state electrolyte is a new class of materials based on lithium compounds, presented by physicists from Switzerland and Poland.

Commercially available lithium-ion batteries consist of two electrodes connected by a liquid electrolyte. This electrolyte makes it difficult for engineers to reduce the size and weight of the battery, in addition, it is subjected to leakage; the lithium in the exposed electrodes then comes into contact with oxygen in the air and undergoes self-ignition. Boeing’s troubles, which for many months caused a full grounding of Dreamliner flights, are a spectacular example of the problems brought about by the use of modern lithium-ion batteries.

Laboratories have been searching for solid materials capable of replacing liquid electrolytes for years. The most popular candidates include compounds in which lithium ions are surrounded by sulphur or oxygen ions. However, in the journal Advanced Energy Materials, a Swiss-Polish team of scientists has presented a new class of ionic compounds where the charge carriers are lithium ions moving in an environment of amine (NH2) and tetrahydroborate (BH4) ions. The experimental part of the research project was carried out at Empa, the Swiss Federal Laboratories for Materials Science and Technology in Dübendorf, and at the University of Geneva (UG). The person responsible for the theoretical description of the mechanisms leading to the exceptionally high ionic conductivity of the new material was Prof. Zbigniew Lodziana from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow.

“We were dealing with lithium amide-borohydride, a substance previously regarded as not being too good an ionic conductor. This compound is made by milling two constituents in a ratio of 1 to 3. To date, nobody has ever tested what happens to ionic conductivity when the proportions between these constituents are changed. We were the first to do so and suddenly it turned out that by reducing the number of NH2 groups to a certain limit we could significantly improve the conductivity. It increases so much that it becomes comparable to the conductivity of liquid electrolytes!” says Prof. Lodziana.

The several dozen-fold boost in ionic conductivity of the new material – the effect of a change in the proportion of its constituents – opens up a new, unexplored direction in the search for a candidate for a solid-state electrolyte. Previously, throughout the world, the focus was almost exclusively on changes in the composition of the chemical substance. It has now become apparent that, at the stage of production of the compound, a key role can be played by the proportions themselves of the ingredients used to manufacture them.

“Our lithium amide-borohydride is a representative of a promising new class of solid-state electrolyte materials. However, it will be some time before batteries built on such compounds come into use. For example, there should be no chemical reactions between the electrolyte and the electrodes leading to their degradation. This problem is still waiting for an optimal solution”, comments Prof. Lodziana.

The research prospects are promising. The scientists from Empa, UG and IFJ PAN did not confine themselves to just characterizing the physico-chemical properties of the new material. The compound was used as an electrolyte in a typical Li4Ti5O12 half-cell. The half-cell performed well, in tests of running down and recharging 400 times it proved to be stable. Promising steps have also been taken towards resolving another important issue. The lithium amide-borohydride described in the publication exhibited excellent ionic conductivity only at about 40 °C. In the most recent experiments this has already been lowered to below room temperature.

Theoretically, however, the new material remains a challenge. Hitherto models have been constructed for substances in which the lithium ions move in an atomic environment. In the new material, ions move among light molecules that adjust their orientation to ease the lithium movement.

“In the proposed model, the excellent ionic conductivity is a consequence of the specific construction of the crystalline lattice of the tested material. This network in fact consists of two sub-lattices. It turns out that the lithium ions are present here in the elementary cells of only one sub-lattice. However, the diffusion barrier between the sub-lattices is low. Under appropriate conditions, the ions thus travel to the second, empty sub-lattice, where they can move quite freely,” explains Prof. Lodziana.

The theoretical description presented here explains only some of the observed features of the new material. The mechanisms responsible for its high conductivity are certainly more complex. Their better understanding should significantly accelerate the search for optimal compounds for a solid-state electrolyte and consequently shorten the process of commercialization of new power sources that are most likely to revolutionize portable electronics.

Silicon – the second most abundant element in the earth’s crust – shows great promise in Li-ion batteries, according to new research from the University of Eastern Finland. By replacing graphite anodes with silicon, it is possible to quadruple anode capacity.

In a climate-neutral society, renewable and emission-free sources of energy, such as wind and solar power, will become increasingly widespread. The supply of energy from these sources, however, is intermittent, and technological solutions are needed to safeguard the availability of energy also when it’s not sunny or windy. Furthermore, the transition to emission-free energy forms in transportation requires specific solutions for energy storage, and lithium-ion batteries are considered to have the best potential.

Researchers from the University of Eastern Finland introduced new technology to Li-ion batteries by replacing graphite used in anodes by silicon. The study analysed the suitability of electrochemically produced nanoporous silicon for Li-ion batteries. It is generally understood that in order for silicon to work in batteries, nanoparticles are required, and this brings its own challenges to the production, price and safety of the material. However, one of the main findings of the study was that particles sized between 10 and 20 micrometres and with the right porosity were in fact the most suitable ones to be used in batteries. The discovery is significant, as micrometre-sized particles are easier and safer to process than nanoparticles. This is also important from the viewpoint of battery material recyclability, among other things. The findings were published in Scientific Reports.

“In our research, we were able to combine the best of nano- and micro-technologies: nano-level functionality combined with micro-level processability, and all this without compromising performance,” Researcher Timo Ikonen from the University of Eastern Finland says. “Small amounts of silicon are already used in Tesla’s batteries to increase their energy density, but it’s very challenging to further increase the amount,” he continues.

Next, researchers will combine silicon with small amounts of carbon nanotubes in order to further enhance the electrical conductivity and mechanical durability of the material.

“We now have a good understanding of the material properties required in large-scale use of silicon in Li-ion batteries. However, the silicon we’ve been using is too expensive for commercial use, and that’s why we are now looking into the possibility of manufacturing a similar material from agricultural waste, for example from barley husk ash,” Professor Vesa-Pekka Lehto explains.

Two-dimensional materials are a sort of a rookie phenom in the scientific community. They are atomically thin and can exhibit radically different electronic and light-based properties than their thicker, more conventional forms, so researchers are flocking to this fledgling field to find ways to tap these exotic traits.

Applications for 2-D materials range from microchip components to superthin and flexible solar panels and display screens, among a growing list of possible uses. But because their fundamental structure is inherently tiny, they can be tricky to manufacture and measure, and to match with other materials. So while 2-D materials R&D is on the rise, there are still many unknowns about how to isolate, enhance, and manipulate their most desirable qualities.

Now, a science team at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has precisely measured some previously obscured properties of moly sulfide, a 2-D semiconducting material also known as molybdenum disulfide or MoS2. The team also revealed a powerful tuning mechanism and an interrelationship between its electronic and optical, or light-related, properties.

To best incorporate such monolayer materials into electronic devices, engineers want to know the “band gap,” which is the minimum energy level it takes to jolt electrons away from the atoms they are coupled to, so that they flow freely through the material as electric current flows through a copper wire. Supplying sufficient energy to the electrons by absorbing light, for example, converts the material into an electrically conducting state.

As reported in the Aug. 25 issue of Physical Review Letters, researchers measured the band gap for a monolayer of moly sulfide, which has proved difficult to accurately predict theoretically, and found it to be about 30 percent higher than expected based on previous experiments. They also quantified how the band gap changes with electron density – a phenomenon known as “band gap renormalization.”

“The most critical significance of this work was in finding the band gap,” said Kaiyuan Yao, a graduate student researcher at Berkeley Lab and the University of California, Berkeley, who served as the lead author of the research paper.

“That provides very important guidance to all of the optoelectronic device engineers. They need to know what the band gap is” in orderly to properly connect the 2-D material with other materials and components in a device, Yao said.

Obtaining the direct band gap measurement is challenged by the so-called “exciton effect” in 2-D materials that is produced by a strong pairing between electrons and electron “holes” ¬- vacant positions around an atom where an electron can exist. The strength of this effect can mask measurements of the band gap.

Nicholas Borys, a project scientist at Berkeley Lab’s Molecular Foundry who also participated in the study, said the study also resolves how to tune optical and electronic properties in a 2-D material.

“The real power of our technique, and an important milestone for the physics community, is to discern between these optical and electronic properties,” Borys said.

The team used several tools at the Molecular Foundry, a facility that is open to the scientific community and specializes in the creation and exploration of nanoscale materials.

The Molecular Foundry technique that researchers adapted for use in studying monolayer moly sulfide, known as photoluminescence excitation (PLE) spectroscopy, promises to bring new applications for the material within reach, such as ultrasensitive biosensors and tinier transistors, and also shows promise for similarly pinpointing and manipulating properties in other 2-D materials, researchers said.

The research team measured both the exciton and band gap signals, and then detangled these separate signals. Scientists observed how light was absorbed by electrons in the moly sulfide sample as they adjusted the density of electrons crammed into the sample by changing the electrical voltage on a layer of charged silicon that sat below the moly sulfide monolayer.

Researchers noticed a slight “bump” in their measurements that they realized was a direct measurement of the band gap, and through a slew of other experiments used their discovery to study how the band gap was readily tunable by simply adjusting the density of electrons in the material.

“The large degree of tunability really opens people’s eyes,” said P. James Schuck, who was director of the Imaging and Manipulation of Nanostructures facility at the Molecular Foundry during this study.

“And because we could see both the band gap’s edge and the excitons simultaneously, we could understand each independently and also understand the relationship between them,” said Schuck, now at Columbia University. “It turns out all of these properties are dependent on one another.”

Moly sulfide, Schuck also noted, is “extremely sensitive to its local environment,” which makes it a prime candidate for use in a range of sensors. Because it is highly sensitive to both optical and electronic effects, it could translate incoming light into electronic signals and vice versa.

Schuck said the team hopes to use a suite of techniques at the Molecular Foundry to create other types of monolayer materials and samples of stacked 2-D layers, and to obtain definitive band gap measurements for these, too. “It turns out no one yet knows the band gaps for some of these other materials,” he said.

The team also has expertise in the use of a nanoscale probe to map the electronic behavior across a given sample.

Borys added, “We certainly hope this work seeds further studies on other 2-D semiconductor systems.”

The Molecular Foundry is a DOE Office of Science User Facility that provides free access to state-of-the-art equipment and multidisciplinary expertise in nanoscale science to visiting scientists.

Researchers from the Kavli Energy NanoSciences Institute at UC Berkeley and Berkeley Lab, and from Arizona State University also participated in this study, which was supported by the National Science Foundation.

While lithium-ion batteries, widely used in mobile devices from cell phones to laptops, have one of the longest lifespans of commercial batteries today, they also have been behind a number of recent meltdowns and fires due to short-circuiting in mobile devices. In hopes of preventing more of these hazardous malfunctions researchers at Drexel University have developed a recipe that can turn electrolyte solution — a key component of most batteries — into a safeguard against the chemical process that leads to battery-related disasters.

Yury Gogotsi, PhD, Distinguished University and Bach professor in the College of Engineering, and his research team from the Department of Materials Science and Engineering, recently published their work — entitled “Nanodiamonds Suppress Growth of Lithium Dendrites” — in the journal Nature Communications. In it, they describe a process by which nanodiamonds — tiny diamond particles 10,000 times smaller than the diameter of a hair — curtail the electrochemical deposition, called plating, that can lead to hazardous short-circuiting of lithium ion batteries.

As batteries are used and charged, the electrochemical reaction results in the movement of ions between the two electrodes of a battery, which is the essence of an electrical current. Over time, this repositioning of ions can create tendril-like buildups — almost like stalactites forming inside a cave. These battery buildups, called dendrites, are one of the main causes of lithium battery malfunction. As dendrites form inside the battery over time, they can reach the point where they push through the separator, a porous polymer film that prevents the positively charged part of a battery from touching the negatively charged part. When the separator is breached, a short-circuit can occur, which can also lead to a fire since the electrolyte solution in most lithium-ion batteries is highly flammable.

To avoid dendrite formation and minimize the probability of fire, current battery designs include one electrode made of graphite filled with lithium instead of pure lithium. The use of graphite as the host for lithium prevents the formation of dendrites. But lithium intercalated graphite also stores about 10 times less energy than pure lithium. The breakthrough made by Gogotsi’s team means that a great increase in energy storage is possible because dendrite formation can be eliminated in pure lithium electrodes.

“Battery safety is a key issue for this research,” Gogotsi said. “Small primary batteries in watches use lithium anodes, but they are only discharged once. When you start charging them again and again, dendrites start growing. There may be several safe cycles, but sooner or later a short-circuit will happen. We want to eliminate or, at least, minimize that possibility.”

Gogotsi and his collaborators from Tsinghua University in Beijing, and Hauzhong University of Science and Technology in Wuhan, China, focused their work on making lithium anodes more stable and lithium plating more uniform so that dendrites won’t grow.

They’re doing this by adding nanodiamonds to the electrolyte solution in a battery. Nanodiamonds have been used in the electroplating industry for some time as a way of making metal coatings more uniform. While they are much, much smaller — and cheaper — than the diamonds you’d find in a jeweler’s case, nanodiamonds still retain the regular structure and shape of their pricey progenitors. When they are deposited, they naturally slide together to form a smooth surface.

The researchers found this property to be exceedingly useful for eliminating dendrite formation. In the paper, they explain that lithium ions can easily attach to nanodiamonds, so when they are plating the electrode they do so in the same orderly manner as the nanodiamond particles to which they’re linked. They report in the paper that mixing nanodiamonds into the electrolyte solution of a lithium ion battery slows dendrite formation to nil through 100 charge-discharge cycles.

If you think about it like a game of Tetris, that pile of mismatched blocks inching perilously close to “game over” is the equivalent of a dendrite. Adding nanodiamonds to the mix is kind of like using a cheat code that slides each new block into the proper place to complete a line and prevent a menacing tower from forming.

Gogotsi notes that his group’s discovery is just the beginning of a process that could eventually see electrolyte additives, like nanodiamonds, widely used to produce safe lithium batteries with a high energy density. Initial results already show stable charge-discharge cycling for as long as 200 hours, which is long enough for use in some industrial or military applications, but not nearly adequate for batteries used in laptops or cell phones. Researchers also need to test a large number of battery cells over a long enough period of time under various physical conditions and temperatures to ensure that dendrites will never grow.

“It’s potentially game-changing, but it is difficult to be 100 percent certain that dendrites will never grow,” Gogotsi said. “We anticipate the first use of our proposed technology will be in less critical applications — not in cell phones or car batteries. To ensure safety, additives to electrolytes, such as nanodiamonds, need to be combined with other precautions, such as using non-flammable electrolytes, safer electrode materials and stronger separators.”

 

Transition metal silicides, a distinct class of semiconducting materials that contain silicon, demonstrate superior oxidation resistance, high temperature stability and low corrosion rates, which make them promising for a variety of future developments in electronic devices. Despite their relevance to modern technology, however, fundamental aspects of the chemical bonding between their transition metal atoms and silicon remain poorly understood. One of the most important, but poorly known, properties is the strength of these chemical bonds — the thermochemical bond dissociation energy.

With funding from the National Science Foundation, a team of researchers from the University of Utah has investigated this property, and in this week’s The Journal of Chemical Physics, from AIP Publishing, they present their valuable findings for a number of specific compounds. These include precise values of the bond dissociation energies of the group four and five transition metal silicide molecules: TiSi, ZrSi, HfSi, VSi, NbSi and TaSi.

“The team measured the energy at which the diatomic silicides fall apart more quickly than they can be ionized by absorption of a second photon. This amount of energy is called the predissociation threshold. It provides an upper limit to the bond dissociation energy. However, the researchers have found that for molecules with certain electron configurations, if the molecule is cold, then the observation of a sharp predissociation threshold provides an accurate value of the thermochemical bond dissociation energy, and not simply an upper limit.”

“What I’m so pleased about with this new technique that we’ve developed is that it’s not just applicable to a small set of molecules,” said Michael Morse, one of the work’s authors. “It’s based on the fact that these small transition metal molecules have a density of electronic states that increases very rapidly as you get close to the dissociation limit, and that’s key in causing the molecule to fall apart as soon as you get above that limit […] The peculiarities of the transition metals make the method broadly applicable to that entire class of molecules, which are quite difficult to investigate by other means.”

This sharp threshold observation in a dense vibronic spectrum provides a new and highly effective means of estimating the bond dissociation energy for transition metals bonded to other p-block elements. According to the researchers, the uncertainties using this new method are much smaller than those seen with previous approaches.

Along with measuring the bond dissociation values for these molecules, the researchers were also able to use the predissociation thresholds to determine other fundamental values for certain molecules using thermochemical cycles, namely enthalpies of formation and ionization energies.

The data acquired can be used by chemists to develop more accurate computational methods regarding transition metal chemical bonding, along with bettering our understanding of these bonds.

“Quantum chemists are trying to develop new, efficient and accurate means of calculating these systems, and they’ve been quite successful with main group systems, and especially organic compounds,” Morse said. “But, the transition metals are much more difficult because there are so many more ways the electrons can be arranged. Another problem is that in the past, there hasn’t been as much highly accurate data available that can be used to compare theory and experiment. Without accurate data, it’s hard to tell how good a computational method may be.”

The research team has plans to work with other diatomic molecules containing transition metals. In fact, they already have results for the bond dissociation energies of TiC, ZrC, HfC, VC, NbC, TaC, WC, WSi, WS, WSe, and WCl that are in preparation for publication. By examining series of chemically related molecules, like these studies of the metal-carbon and tungsten-halogen molecules, the team intends to develop a broad picture of chemical bonding in the transition metal molecules.

“There’s a big advantage that comes from this sort of wide-ranging, systematic study. It allows us to develop what I like to call ‘chemical intuition’ about chemical bonds,” said Morse.

Advances in modern electronics has demanded the requisite hardware, transistors, to be smaller in each new iteration. Recent progress in nanotechnology has reduced the size of silicon transistors down to the order of 10 nanometers. However, for such small transistors, other physical effects set in, which limit their functionality. For example, the power consumption and heat production in these devices is creating significant problems for device design. Therefore, novel quantum materials and device concepts are required to develop a new generation of energy-saving information technology. The recent discoveries of topological materials — a new class of relativistic quantum materials — hold great promise for use in energy saving electronics.

Researchers in the Louisiana Consortium for Neutron Scattering, or LaCNS, led by LSU Department of Physics & Astronomy Chair and Professor John F. DiTusa and Tulane University Professor Zhiqiang Mao, with collaborators at Oak Ridge National Lab, the National High Magnetic Field Laboratory, Florida State University, and the University of New Orleans, recently reported the first observation of this topological behavior in a magnet, Sr1-yMn1-zSb2 (y, z < 0.1). These results were published this week in Nature Materials(doi:10.1038/nmat4953).

“This first observation is a significant milestone in the advancement of novel quantum materials and this discovery opens the opportunity to explore its consequences. The nearly massless behavior of the charge carriers offers possibilities for novel device concepts taking advantage of the extremely low power dissipation,” DiTusa said.

The phrase “topological materials” refers to materials where the current carrying electrons act as if they have no mass similar to the properties of photons, the particles that make up light. Amazingly, these electronic states are robust and immune to defects and disorder because they are protected from scattering by symmetry. This symmetry protection results in exceedingly high charge carrier mobility, creating little to no resistance to current flow. The result is expected to be a substantial reduction in heat production and energy saving efficiencies in electronic devices.

This new magnet displays electronic charge carriers that have almost no mass. The magnetism brings with it an important symmetry breaking property – time reversal symmetry, or TRS, breaking where the ability to run time backward would no longer return the system back to its starting conditions. The combination of relativistic electron behavior, which is the cause of much reduced charge carrier mass, and TRS breaking has been predicted to cause even more unusual behavior, the much sought after magnetic Weyl semimetal phase. The material discovered by this collaboration is thought to be an excellent one to investigate for evidence of the Weyl phase and to uncover its consequences.

Flux-closure domain (FCD) structures are microscopic topological phenomena found in ferroelectric thin films that feature distinct electric polarization properties. These closed-loop domains have garnered attention among researchers studying new ferroelectric devices, ranging from data storage components and spintronic tunnel junctions to ultra-thin capacitors.

In the development of thin films for such devices, researchers have thought that contact with commonly used oxide electrodes limits FCD formation. However, a group of researchers in China has shown otherwise. The findings are reported this week as the cover article in Applied Physics Letters, from AIP Publishing.

Ferroelectric materials are typically developed and studied as thin films, sometimes as thin as only a few nanometers. As a result, researchers have begun discovering the abundant domain structures and unique physical properties that these ferroelectrics possess, such as skyrmion and FCD formation that could benefit next-generation electronic devices. Because the films are so thin, however, their interaction with electrodes is inevitable.

“The general thinking has been that oxide electrodes would destabilize flux-closure domains. However, our work has shown that this is no longer true when the top and bottom electrodes are symmetric, which physically makes sense,” said Yinlian Zhu, professor at the Institute of Metal Research at the Chinese Academy of Sciences and a co-author of the paper.

Zhu and colleagues used two types of oxide electrodes: one based on strontium ruthenate, the other based on lanthanum strontium manganite, chosen as oxide electrodes because of their similar perovskite structures, which work well in layer-by-layer film growth. They studied how these electrodes influenced FCD formation in PbTiO3 (PTO) perovskite-oxide-based thin films deposited on gadolinium scandium oxide (GSO) substrates.

The research team’s previous studies indicated that flux-closure domains can be stabilized in strained ferroelectric films in which the strain plays a critical role in the formation of flux-closure domains, such as multilayer PTO/strontium titanate systems grown on GSO-based (specifically GdScO3) substrates.

Based on their previous studies, the researchers consequently anticipated that similar phenomenon might also occur in PTO/electrode systems. They then grew PTO films sandwiched between symmetric oxide electrodes on GSO substrates using pulsed laser deposition.

They found that periodic FCD arrays can be stabilized in PTO films when the top and bottom electrodes are symmetric, while alternating current domains appear when they apply asymmetric electrodes.

“We successfully grew ferroelectric thin films with symmetric oxide electrodes in which flux-closure domains and their periodic arrays clearly do exist,” Zhu said. “Our work sheds light on understanding the nature of flux-closure domains in ferroelectrics. We expect that it will open research possibilities in the evolution of these structures under external electric fields.”