Unlocking the secrets of metal-insulator transitions

X-ray photon correlation spectroscopy at NSLS-II’s CSX beamline used to understand electrical conductivity transitions in magnetite.

By using an x-ray technique available at the National Synchrotron Light Source II (NSLS-II), scientists found that the metal-insulator transition in the correlated material magnetite is a two-step process. The researchers from the University of California Davis published their paper in the journal Physical Review Letters. NSLS-II, a U.S. Department of Energy (DOE) Office of Science user facility located at Brookhaven National Laboratory, has unique features that allow the technique to be applied with stability and control over long periods of time.
“Correlated materials have interesting electronic, magnetic, and structural properties, and we try to understand how those properties change when their temperature is changed or under the application of light pulses, or an electric field” said Roopali Kukreja, a UC Davis professor and the lead author of the paper. One such property is electrical conductivity, which determines whether a material is metallic or an insulator.

If a material is a good conductor of electricity, it is usually metallic, and if it is not, it is then known as an insulator. In the case of magnetite, temperature can change whether the material is a conductor or insulator. For the published study, the researchers’ goal was to see how the magnetite changed from insulator to metallic at the atomic level as it got hotter.

>Read more on the NSLS-II at Brookhaven National Laboratory website

Image: Professor Roopali Kukreja from the University of California in Davis and the CSX team Wen Hu, Claudio Mazzoli, and Andi Barbour prepare the beamline for the next set of experiments.

Expanding the infrared nanospectroscopy window

The ability to investigate heterogeneous materials at nanometer scales and far-infrared energies will benefit a wide range of fields, from condensed matter physics to biology.

Scientific studies require tools that match the natural length and energy scales of the phenomena under investigation. For many questions in biology, quantum materials, and electronics, this means nanometer spatial resolution combined with far-infrared energies. For example, scientists might want to study collective electron oscillations in quantum materials for optoelectronic circuits, or the characteristic vibration modes of protein molecules in biological systems.

A recently developed infrared technique—synchrotron infrared nanospectroscopy (SINS)—combines broadband synchrotron light with atomic-force microscopes to enable infrared imaging and spectroscopy at the nanoscale. However, the technique could only be used in a narrow range of the electromagnetic spectrum that excluded far-infrared wavelengths, due to a scarcity of suitable light sources and detectors for that range. In this work, researchers extended SINS to far-infrared wavelengths, opening up a whole new experimental regime.

> Read more on the Advanced Lightsource at Berkeley Lab website

Image: Left: Nanoscale images of SiO2 hole array, obtained using atomic-force microscopy (AFM, top) and synchrotron infrared nanospectroscopy (SINS, bottom), demonstrating SINS contrast between patterned SiO2 and underlying Si substrate with ~30 nm spatial resolution (inset). Scale bar = 200 nm. Right: SINS broadband spectroscopic data for SiO2, taken along dotted line in images at left, showing amplitude (top) and phase (bottom) information from asymmetric  Si–O stretching (1200 cm–1) and bending (460 cm–1) modes. The lower-energy bending mode had previously been inaccessible with this technique.

Defense spending bill extends Air Force research partnership

For the past 10 years, the U.S. Air Force has funded research on high-performance materials at the Cornell High Energy Synchrotron Source (CHESS).

The partnership has resulted in numerous advances, including a greater understanding of metal fatigue and analysis of the best metals for aircraft.
This partnership was extended with $8 million in funding to CHESS as part of the fiscal year 2019 defense appropriations bill, a $674.4 billion package that President Donald Trump signed into law Oct. 1. The bill passed both the U.S. Senate – supported by New York Sens. Charles Schumer, who is Senate minority leader, and Kirsten Gillibrand – and the U.S. House of Representatives late last month.

“Cornell University is deeply grateful to Leader Schumer and Senator Gillibrand for securing $8 million in additional funding for CHESS,” Cornell President Martha E. Pollack said in a statement. “Maintaining our scientific infrastructure is essential if the U.S. is to keep its competitive advantage in research and development. Over the years, taxpayers have invested more than $1 billion in CHESS, an investment that’s paid off many times over in new discoveries, breakthrough technologies, [science, technology, engineering, math] education and workforce development.”

Image: Matthew Miller, right, associate director of the Cornell High Energy Synchrotron Source (CHESS), watches graduate student Mark Obstalecki prepare a sample for analysis in the F2 hutch at CHESS.

Finding unusual performance in unconventional battery materials

Even as our electronic devices become ever more sophisticated and versatile, battery technology remains a stubborn bottleneck, preventing the full realization of promising applications such as electric vehicles and power-grid solar energy storage.  Among the limitations of current materials are poor ionic and electron transport qualities. While strategies exist to improve these properties, and hence reduce charging times and enhance storage capacity, they are often expensive, difficult to implement on a large scale, and of only limited effectiveness.  An alternative solution is the search for new materials with the desired atomic structures and characteristics.  This is the strategy of a group of researchers who, utilizing ultra-bright x-rays from the U.S. Department of Energy’s Advanced Photon Source (APS), identified and characterized two niobium tungsten oxide materials that demonstrate much faster charging rates and power output than conventional lithium electrodes.  Their work appeared in the journal Nature.

Currently, the usual approach for wringing extra capacity and performance from lithium-ion batteries involves the creation of electrode materials with nanoscale structures, which reduces the diffusion distances for lithium ions.  However, this also tends to increase the practical volume of the material and can introduce unwanted additional chemical reactions. Further, when graphite electrodes are pushed to achieve high charging rates, irregular dendrites of lithium can form and grow, leading to short circuits, overheating, and even fires.  Measures to prevent these dendrites generally cause a decrease in energy density.  These issues seriously limit the use of graphite electrodes for high-rate applications.

>Read more on the Advanced Photon Source website

Image: Artist’s impression of rapidly flowing lithium through the niobium tungsten oxide structure. This is a detail of the image, please see here for the entire art work.
Credit: Ella Maru Studio

The coolest high-energy synchrotron experiment

A French team of researchers has created and tested a cryostat where scientists can carry out the coldest experiments in the high-energy range in a synchrotron.

A photocopy of a drawing lies on the table of the control cabin of beamline ID12. It shows a cryostat and its heart: a spring-like metal tube and other components. Next to the drawing, lots of scribbles in different colours and on different dates, proof that this creation has been many years in the making. Steps away from the table, the real thing makes its appearance in the experimental hutch. Its majestic presence gives the beamline a new touch. It is the Très Basses Temperatures for miliKelvin (TBT-mK) cryostat.
Philippe Sainctavit, from the Institut de minéralogie, de physique des matériaux et de cosmochimie, together with Jean-Paul Kappler and Loïc Joly, from the Institut de Physique et Chimie des Matériaux de Strasbourg and synchrotron SOLEIL, are the fathers of this invention. “We started working on this project 20 years ago, and this is the third version of the machine”, explains Kappler. The team has installed the machine on ID12 for their experiments in magnetism. “Because this is quite a particular piece of equipment, we needed a very strong understanding with the beamline staff. Thanks to the fact that we were all in the same wavelength, the installation, which lasted 5 weeks spread throughout the year, went very smoothly. We could not have done this without the strong collaboration with the ID12 staff, namely Andrei Rogalev, Fabrice Wilhelm and Pascal Voisin”, explains Sainctavit.

>Read more on the European Synchrotron website

New NSLS-II beamline illuminates electronic structures

MIT scientists conduct the first experiment at NSLS-II’s Soft Inelastic X-ray Scattering beamline.

On July 15, 2018, the Soft Inelastic X-ray Scattering (SIX) beamline at the National Synchrotron Light Source II (NSLS-II)—a U.S. Department of Energy (DOE) Office of Science User Facility at DOE’s Brookhaven National Laboratory—welcomed its first visiting researchers. SIX is an experimental station designed to measure the electronic properties of solid materials using ultrabright x-rays. The materials can be as small as a few microns—one millionth of a meter.
The first researchers to take advantage of the world-class capabilities at SIX were Jonathan Pelliciari and Zhihai Zhu, two scientists from the Massachusetts Institute of Technology (MIT). The pair used SIX to study a chromate sample, a fascinating material with novel applications in magnetism, batteries, and catalysis. Little was known about the electronic structure of the chromate sample the MIT team studied at SIX, and their research is aimed at unlocking the properties of this material. To do so, they needed the atomic sensitivity and energy resolution of the SIX beamline.

>Read more on the NSLS-II at Brookhaven National Laboratoy website

Picture: The sample chamber of the Soft Inelastic X-ray Scattering (SIX) beamline at NSLS-II allows scientists to mount their materials on a special holder that can be turned and moved into the beam of bright x-rays.

Nano-opto-electronics with Soapstone

Research shows potential of combining mineral with graphene for the design of new devices.

The development of electronic devices in the nanometric scale depends on the search for materials that have appropriate characteristics, and that are also efficient and inexpensive. This is the case of graphene, a material formed by a single layer of carbon atoms obtained from graphite. Graphene is a conductor with excellent optical and electrical properties that can be easily altered by the incidence of electric fields or light.

In addition, several other interesting structural, electronic and optical properties can be obtained by combining graphene with other materials. These new properties arise due to changes in the electronic structure in the interface of different materials when they are brought into contact. In this scenario, the search for new materials and ways of combining them becomes a natural trend.

>Read more on the Brazilian Synchrotron Light Laboratory (LNLS) website

Image: DOI: 10.1021/acsphotonics.7b01017

Boosting the efficiency of silicon solar cells

The efficiency of a solar cell is one of its most important parameters.

It indicates what percentage of the solar energy radiated into the cell is converted into electrical energy. The theoretical limit for silicon solar cells is 29.3 percent due to physical material properties. In the journal Materials Horizons, researchers from Helmholtz-Zentrum Berlin (HZB) and international colleagues describe how this limit can be abolished. The trick: they incorporate layers of organic molecules into the solar cell. These layers utilise a quantum mechanical process known as singlet exciton fission to split certain energetic light (green and blue photons) in such a way that the electrical current of the solar cell can double in that energy range.

The principle of a solar cell is simple: per incident light particle (photon) a pair of charge carriers (exciton) consisting of a negative and a positive charge carrier (electron and hole) is generated. These two opposite charges can move freely in the semiconductor. When they reach the charge-selective electrical contacts, one only allows positive charges to pass through, the other only negative charges. A direct electrical current is therefore generated, which can be used by an external consumer.

>Read more on the BESSY II at Helmholtz-Zentrum Berlin website

Picture: Darstellung des Prinzips einer Silizium-Multiplikatorsolarzelle mit organischen Kristallen
Credit: M. Künsting/HZB

A shape-induced orientation phase within 3D nanocrystal solids

Designing nanocrystal (NC) materials aims at obtaining superlattices that mimic the atomic structure of crystalline solids. In such atomic systems, spatially anisotropic orbitals determine the crystalline lattice type. Similarly, in NC systems the building block anisotropy defines the order of the final solid: here, the NC shape governs the final superlattice structure. Yet, in contrast to atomic systems, NC shape-anisotropy induces not only positional, but also orientational order, ranging from full rotational disorder to a stable, fixed alignment of all NCs. This orientational relation is of special interest, as it determines to what extent atomically coherent connections between NCs are possible, thereby enabling complete wave function delocalization within the NC solid.
In addition to predicting the final NC orientation and position structure, the realization of NC materials demands a controllable fabrication process such that the designed order can be produced. Generally, such highly ordered NC superstructures are achieved through solvent evaporation induced self‐assembly on hard substrates. For applications where the 2D nature of this substrates process is limiting, nonsolvent into solvent diffusion, a technique commonly used to grow single crystals of dissolved molecules, is an attractive option. However, the precise influence of self-assembly parameters on the final superlattice outcome remains unknown. In this work, the researchers posed two closely related questions regarding the design of novel free-standing NC materials: (i) how can the NC self-assembly process be controlled to yield highly ordered free-standing supercrystals and (ii) what is the detailed positional and orientational order within the NC solid? A multidisciplinary team of collaborators, including the Austrian Small Angle X-ray Scattering (SAXS) beamline at Elettra, approached this challenge by a combined experimental and computational strategy.

>Read more on the Elettra Sincrotrone Trieste website

Image: Self‐assembly of 3D colloidal supercrystals built from faceted 20 nm Bi nanocrystals is studied by mens of in-situ synchrotron X‐ray scattering, combined with Monte Carlo simulations. 

X-rays uncover a hidden property that leads to failure in a lithium-ion battery material

Experiments at SLAC and Berkeley Lab uproot long-held assumptions and will inform future battery design.

Over the past three decades, lithium-ion batteries, rechargeable batteries that move lithium ions back and forth to charge and discharge, have enabled smaller devices that juice up faster and last longer.
Now, X-ray experiments at the Department of Energy’s SLAC National Accelerator Laboratory and Lawrence Berkeley National Laboratory have revealed that the pathways lithium ions take through a common battery material are more complex than previously thought. The results correct more than two decades worth of assumptions about the material and will help improve battery design, potentially leading to a new generation of lithium-ion batteries.

An international team of researchers, led by William Chueh, a faculty scientist at SLAC’s Stanford Institute for Materials & Energy Sciences and a Stanford materials science professor, published these findings today in Nature Materials.
“Before, it was kind of like a black box,” said Martin Bazant, a professor at the Massachusetts Institute of Technology and another leader of the study. “You could see that the material worked pretty well and certain additives seemed to help, but you couldn’t tell exactly where the lithium ions go in every step of the process. You could only try to develop a theory and work backwards from measurements. With new instruments and measurement techniques, we’re starting to have a more rigorous scientific understanding of how these things actually work.”

>Read more on the SLAC website

Image: When lithium ions flow into the battery’s solid electrode – illustrated here in hexagonal slices – the lithium can rearrange itself, causing the ions to clump together into hot spots that end up shortening the battery lifetime.
Credit: Stanford University/3Dgraphic

Ultralow-fluence for phase-change process

Ultrafast active materials with tunable properties are currently investigated for producing successful memory and data-processing devices. Among others, Phase-Change Materials (PCMs) are eligible for this purpose. They can reversibly switch between a high-conductive crystalline state (SET) and a low-conductive amorphous state (RESET), defining a binary code. The transformation is triggered by an electrical or optical pulse of different intensity and time duration. 3D Ge-Sb-Te based alloys, of different stoichiometry, are already employed in DVDs or Blu-Ray Disks, but they are expected to function also in non-volatile memories and RAM. The challenge is to demonstrate that the scalability to 2D, 1D up to 0D of the GST alloys improves the phase-change process in terms of lower power threshold and faster switching time. Nowadays, GST thin films and nanoparticles have been synthetized and have beenshown to function with competitive results.
A team of researchers from the University of Trieste and the MagneDyn beamline at Fermi demonstrated the optical switch from crystalline to amorphous state of Ge2Sb2Te5nanoparticles (GST NPs) with size <10 nm, produced via magnetron sputtering by collaborators from the University of Groeningen. Details were reported in the journal Nanoscale.
This work aims at showing the very low power limit of an optical pulse needed to amorphize crystalline Ge2Sb2Te5 nanoparticles. Particles of 7.8 nm and 10.4 nm diameter size were deposited on Mica and capped with ~200nm of PMMA. Researchers made use of a table-top Ti:Sapphire regenerative amplified system-available at the IDontKerr (IDK) laboratory (MagneDyn beamline support laboratory) to produce pump laser pulses at 400 nm, of ~100 fs and with a repetition rate from 1kHz to single shot.

>Read more on the Elettra Sincrotrone Trieste website

Image (extract): Trasmission Electron Microscopy image of the nanoparticles sample. Ultafast single-shot optical process with fs-pulse at 400 nm. Microscope images of amorphized and amorphized/ablated areas obtained on the nanoparticles sample. Comparison of amorphization threshold fluences between thin films and nanoparticles cases.
Please see here the entire image.

A closer look of zink behaviour under extreme conditions

Researchers have explored the phase diagram of zinc under high pressure and high temperature conditions, finding evidence of a change in its structural behaviour at 10 GPa. Experiments profited from the brightness of synchrotron light at ALBA and Diamond.

These results can help to understand the processes and phenomena happening in the Earth’s interior.

The field of materials science studies the properties and processes of solids to understand and discover their performances. Synchrotron light techniques permit to analyse these materials at extreme conditions (high pressure and high temperature), getting new details and a deep knowledge of them.

Studying the melting behaviours of terrestrial elements and materials at extreme conditions, researchers can understand the phenomena taking place inside them. This information is of great value for discovering how these materials react in the inner core of Earth but also for other industrial applications. Zinc is one of the most abundant elements in Earth’s crust and is used in multiple areas such as construction, ship-building or automobile.

>Read more on the ALBA website

Figure: P-T phase diagram of zinc for P<16 GPa and T<1600K. Square data points correspond to the X-ray diffraction measurements. Solid squares are used for the low pressure hexagonal phase (hcp) and empty symbols for the high pressure hexagonal phase (hcp’). White, red and black circles are melting points from previous studies reported in the literature. The triangles are melting points obtained in the present laser-heating measurements. In the onset of the figure is shown the custom-built vacuum vessel for resistively-heated membrane-type DAC used in the experiments at the ALBA Synchrotron. 

Empowering multicomponent cathode materials for sodium ion batteries

…by exploring three-dimensional compositional heterogeneities

Energy storage devices have revolutionized the modern electronics industry by enabling the widespread application of portable electronic devices. Moreover, these storage devices also have the potential to reduce the dependence on fossil fuels by implementing electric vehicles in the market. To date, lithium ion batteries have dominated the market because of the high energy density delivered by them. However, one should look into the sustenance of such devices because Li is not one of the most abundant metals on Earth’s crust. Thus, developing an alternative to lithium ion batteries has become one of the key issues to ensure the sustainable future of energy storage devices. Sodium ion batteries provide one such alternative. Out of all the components of a battery, cathode materials play one of the key roles in determining the overall performance of such batteries. Unfortunately, sodium-ion batteries have been lagging behind their lithium ion counterpart in terms of performance. Thus, new design strategies must be undertaken in order to improve the performance of cathode materials for sodium ion batteries.

>Read more on the SSRL at SLAC website

Image (extract): Three-dimensional elemental associations of pristine Na0.9Cu0.2Fe0.28Mn0.52O2 studied through transmission x-ray tomography. a) Visualizing the surface elemental associations at different angles with different colors corresponding to different association, and b) 2D cross-sectional association maps showing the bulk elemental associations. [Energy Environ. Sci., DOI: 10.1039/C8EE00309B (2018)] See entire figure here.

Topological excitations emerge from a vibrating crystal lattice

It has long been known that the properties of materials are crucially dependent on the arrangement of the atoms that make up the material. For example, atoms that are further apart will tend to vibrate more slowly and propagate sound waves more slowly. Now, researchers from Brookhaven National Laboratory have used Sector 30 at the Advanced Photon Source (APS) to discover “topological” vibrations in iron silicide (FeSi). These topological vibration arise from a special symmetrical arrangement of the atoms in FeSi and endow the atomic vibrations with novel properties such as the potential to transmit sound waves along the edge of the materials without scattering and dissipation. Looking to the future one might envisage using these modes to transfer energy or information within technological devices.

In quantum mechanics, atomic motions in crystals are described in terms of vibrational modes called phonons. Similar to electrons moving in metals, phonons can also propagate through materials. The detailed properties of these excitations determine many of the thermal, mechanical and electronic properties of the material. In 2017, part of the current collaborative team from the Chinese Academy of Science, theoretically predicted the existence of the topological phonons in transition metal monosilicides. As shown in Fig.1, these topological phonons are formed by two Dirac-cones with different slopes and are protected by symmetry. Since the mathematical description of each Dirac-cone is intimately related to the famous Weyl-equation that was originally proposed in high-energy physics, these topological phonons are consequently called double-Weyl excitations.

>Read more on the Advanced Photon Source website

Image: (extract) Schematic view of the double-Weyl phonon dispersion. Full image here.
Credit: Brookhaven National Laboratory

High-caliber research launches NSLS-II beamline into operations

Pratt & Whitney conduct the first experiments at a new National Synchrotron Light Source II beamline.

A new experimental station (beamline) has begun operations at the National Synchrotron Light Source II (NSLS-II)—a U.S. Department of Energy (DOE) Office of Science User Facility at DOE’s Brookhaven National Laboratory. Called the Beamline for Materials Measurement (BMM), it offers scientists state-of-the-art technology for using a classic synchrotron technique: x-ray absorption spectroscopy.

“There are critical questions in all areas of science that can be solved using x-ray absorption spectroscopy, from energy sciences and catalysis to geochemistry and materials science,” said Bruce Ravel, a physicist at the National Institute of Standards and Technology (NIST), which constructed and operates BMM through a partnership with NSLS-II.

X-ray absorption spectroscopy is a research technique that was developed in the 1980s and, since then, has been at the forefront of scientific discovery.

“The reason we’ve used this technique for 40 years and the reason why NIST built the BMM beamline is because it adds a great value to the scientific community,” Ravel explained.

The first group of researchers to conduct experiments at BMM came from jet engine manufacturer Pratt & Whitney. Senior Engineer Chris Pelliccione and colleagues used BMM to study the chemistry of jet engines.

>Read more on the National Synchrotron Light Source II (NSLS-II) website

Image: Pratt & Whitney Senior Engineer Chris Pelliccione (left) with NIST’s Bruce Ravel (right) at BMM’s workstation.

Understanding reaction pathways leading to MnO2 polymorph formation

Computational driven design of materials has provided guidelines for designing novel materials with desired properties, especially for metastable materials, which may have superior functionalities than its stable counterparts [1]. However, the synthesis of these metastable materials is usually challenging. The current computational approaches are not able to predict reaction pathways passing through intermediate or metastable phases. As a consequence, the synthesis of many compounds still remains Edisonian, meaning that repeated iteration is usually required to find the reaction conditions needed for synthesizing targeted materials with desired properties. To reduce the amount of cost and effort during this discovery process, a predictive theory for directing the synthesis of materials is necessary.

In the recent article “Understanding Crystallization Pathways Leading to Manganese Oxide Polymorph Formation [2]”, researchers from SLAC, LBNL, MIT, Colorado School of Mines, and NREL combined theory and experimental approaches to develop and demonstrate a theoretical framework that guides the synthesis of intermediate/metastable phases. This ab initio-computation based framework calculates the influence of particle size and solution composition on the stability of polymorph (substances having the same composition but different crystallographic structures), and predicts the phases that will appear along the different reaction pathways.

>Read more on the SSRL at SLAC website

Image (extract): (a) Size-dependent phase diagram of MnO2 polymorphs. The three arrows mark the reaction progression from nano-size to bulk at different potassium concentrations. (b-d) The evolution of x-ray scattering pattern with time along [K+] = 0 M (b), 0.2 M (c), and 0.33M (d). The identities and the fractions of the phases are marked in the subfigure to the right. (e-f) Electron beam diffraction patterns of the δ” phase and δ’ phase harvested from [K+] = 0 M and 0.2 M, respectively. See all figures here.