Atomic Flaws Create Surprising, High-Efficiency UV LED Materials

Subtle surface defects increase UV light emission in greener, more cost-effective LED and catalyst materials

Light-emitting diodes (LEDs) traditionally demand atomic perfection to optimize efficiency. On the nanoscale, where structures span just billionths of a meter, defects should be avoided at all costs—until now.

A team of scientists from the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and Stony Brook University has discovered that subtle imperfections can dramatically increase the efficiency and ultraviolet (UV) light output of certain LED materials.

“The results are surprising and completely counterintuitive,” said Brookhaven Lab scientist Mingzhao Liu, the senior author on the study. “These almost imperceptible flaws, which turned out to be missing oxygen in the surface of zinc oxide nanowires, actually enhance performance. This revelation may inspire new nanomaterial designs far beyond LEDs that would otherwise have been reflexively dismissed.”

>Read more on the NSLS-II website

Image: The research team, front to back and left to right: Danhua Yan, Mingzhao Liu, Klaus Attenkoffer, Jiajie Cen, Dario Stacciola, Wenrui Zhang, Jerzy Sadowski, Eli Stavitski.

 

Liquid crystal molecules form nano rings

Quantised self-assembly enables design of materials with novel properties

At DESY’s X-ray source PETRA III, scientists have investigated an intriguing form of self-assembly in liquid crystals: When the liquid crystals are filled into cylindrical nanopores and heated, their molecules form ordered rings as they cool – a condition that otherwise does not naturally occur in the material. This behavior allows nanomaterials with new optical and electrical properties, as the team led by Patrick Huber from Hamburg University of Technology (TUHH) reports in the journal Physical Review Letters.

The scientists had studied a special form of liquid crystals that are composed of disc-shaped molecules called discotic liquid crystals. In these materials, the disk molecules can form high, electrically conductive pillars by themselves, stacking up like coins. The researchers filled discotic liquid crystals in nanopores in a silicate glass. The cylindrical pores had a diameter of only 17 nanometers (millionths of a millimeter) and a depth of 0.36 millimeters.

There, the liquid crystals were heated to around 100 degrees Celsius and then cooled slowly. The initially disorganised disk molecules formed concentric rings arranged like round curved columns. Starting from the edge of the pore, one ring after the other gradually formed with decreasing temperature until at about 70 degrees Celsius the entire cross section of the pore was filled with concentric rings. Upon reheating, the rings gradually disappeared again.

>Read more on the PETRA III at Desy website

Image: Stepwise self-organisation of the cooling liquid crystals. (Extract, see the entire image here)
Credit: A. Zantop/M. Mazza/K. Sentker/P. Huber, Max-Planck Institut für Dynamik und Selbstorganisation/Technische Universität Hamburg; Quantized Self-Assembly of Discotic Rings in a Liquid Crystal Confined in Nanopores, Physical Review Letters, 2018; CC BY 4.

 

40-year controversy in solid-state physics resolved

An international team at BESSY II headed by Prof. Oliver Rader has shown that the puzzling properties of samarium hexaboride do not stem from the material being a topological insulator, as it had been proposed to be.

Theoretical and initial experimental work had previously indicated that this material, which becomes a Kondo insulator at very low temperatures, also possessed the properties of a topological insulator. The team has now published a compelling alternative explanation in Nature Communications, however.

Samarium hexaboride is a dark solid with metallic properties at room temperature. It hosts Samarium, an element having several electrons confined to localized f orbitals in which they interact strongly with one another. The lower the temperature, the more apparent these interactions become. SmB6 becomes what is known as a Kondo insulator, named after Jun Kondo who was first able to explain this quantum effect.

In spite of Kondo-Effect: some conductivity remains

About forty years ago, physicists observed that SmB6 still retained remnant conductivity at temperatures below 4 kelvin, the cause of which had remained unclear until today. After the discovery of the topological-insulator class of materials around 12 years ago, hypotheses grew insistent that SmB6 could be a topological insulator as well as being Kondo insulator, which might explain the conductivity anomaly at a very fundamental level, since this causes particular conductive states at the surface. Initial experiments actually pointed toward this.

>Read more on the Bessy II website

Image: Electrons with differing energies are emitted along various crystal axes in the interior of the sample as well as from the surface. These can be measured with the angular-resolved photoemission station (ARPES) at BESSY II. Left image shows the sample temperature at 25 K, right at only 1 K. The energy distribution of the conducting and valence band electrons can be derived from these data. The surface remains conductive at very low temperature (1 K).
Credit: Helmholtz Zentrum Berlin

Questioning the universality of the charge density wave nature…

… in electron-doped cuprates

The first superconductor materials discovered offer no electrical resistance to a current only at extremely low temperatures (less than 30 K or −243.2°C). The discovery of materials that show superconductivity at much higher temperatures (up to 138 K or −135°C) are called high-temperature superconductors (HTSC). For the last 30 years, scientists have researched cuprate materials, which contain copper-oxide planes in their structures, for their high-temperature superconducting abilities. To understand the superconducting behavior in the cuprates, researchers have looked to correlations with the charge density wave (CDW), caused by the ordered quantum field of electrons in the material. It has been assumed that the CDW in a normal (non-superconducting) state is indicative of the electron behavior at the lower temperature superconducting state. A team of scientists from SLAC, Japan, and Michigan compared the traits of superconducting and non-superconducting cuprate materials in the normal state to test if the CDW is correlated to superconductivity.

>Read more on the SSRL website

Picture: explanation in detail to read in the full scientific highlight (SSRL website)

 

 

 

SLAC scientists investigate how metal 3D printing can avoid producing flawed parts

The goal of these X-ray studies is to find ways to improve manufacturing of specialized metal parts for the aerospace, aircraft, automotive and healthcare industries.

Scientists at the Department of Energy’s SLAC National Accelerator Laboratory are using X-ray light to observe and understand how the process of making metal parts using three-dimensional (3-D) printing can leave flaws in the finished product – and discover how those flaws can be prevented. The studies aim to help manufacturers build more reliable parts on the spot – whether in a factory, on a ship or plane, or even remotely in space – and do it more efficiently, without needing to store thousands of extra parts.

The work is taking place at the lab’s Stanford Synchrotron Radiation Lightsource (SSRL) in collaboration with scientists from the DOE’s Lawrence Livermore National Laboratory and Ames Laboratory.

The 3-D printing process, also known as additive manufacturing, builds solid, three-dimensional objects from a computer model by adding material layer by layer. The use of plastics and polymers in 3-D printing has advanced rapidly, but 3-D printing with metals for industrial purposes has been more challenging to sort out.

>Read more on the SSRL website

Picture: SLAC staff scientist Johanna Nelson Weker, front, leads a study on metal 3-D printing at SLAC’s Stanford Synchrotron Radiation Lightsource with researchers Andrew Kiss and Nick Calta, back.
Credit: Dawn Harmer/SLAC

 

X-ray experiments suggest high tunability of 2-D material

Scientists at Berkeley Lab use a new platform, called MAESTRO, to see microscale details in monolayer material’s electronic structure

To see what is driving the exotic behavior in some atomically thin – or 2-D – materials, and find out what happens when they are stacked like Lego bricks in different combinations with other ultrathin materials, scientists want to observe their properties at the smallest possible scales.

Enter MAESTRO, a next-generation platform for X-ray experiments at the Advanced Light Source (ALS) at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), that is providing new microscale views of this weird 2-D world.

In a study published Jan. 22 in the journal Nature Physics, researchers zeroed in on signatures of exotic behavior of electrons in a 2-D material with microscale resolution.

The new insights gleaned from these experiments show that the properties of the 2-D semiconductor material they studied, called tungsten disulfide (WS2), may be highly tunable, with possible applications for electronics and other forms of information storage, processing, and transfer.

Those applications could include next-gen devices spawned from emerging fields of research like spintronics, excitonics and valleytronics. In these fields, researchers seek to manipulate properties like momentum and energy levels in a material’s electrons and counterpart particles to more efficiently carry and store information – analogous to the flipping of ones and zeroes in conventional computer memory.

>Read more on the ALS website

Picture: Extract of a rendering showing a “ball-and-stick” representation of the atomic structure of a 2-D single crystalline layer of tungsten disulfide (blue and yellow) on top of layers of 2-D boron nitride (silver and gold). On top of these is a representation of the structure of electronic energy levels, or valence bands, within the tungsten disulfide, and the increased splitting between the two valence bands observed using an x-ray technique at the MAESTRO beamline. The experiments suggest the effect could be due to “trions,” made up of two holes and an electron in the bands, depicted as clear and dark spheres. The background is raw data of the electronic structure of the tungsten disulfide, as measured in the experiment.
Credit: Chris Jozwiak/Berkeley Lab

 

Complex tessellations, extraordinary materials

Simple organic molecules form complex materials through self-organisation

An international team of researchers lead by the Technical University of Munich (TUM) has discovered a reaction path that produces exotic layers with semiregular structures. These kinds of materials are interesting because they frequently possess extraordinary properties. In the process, simple organic molecules are converted to larger units which form the complex, semiregular patterns. With experiments at BESSY II at Helmholtz-Zentrum Berlin this could be observed in detail.

Only a few basic geometric shapes lend themselves to covering a surface without overlaps or gaps using uniformly shaped tiles: triangles, rectangles and hexagons. Considerably more and significantly more complex regular patterns are possible with two or more tile shapes. These are so-called Archimedean tessellations or tilings.

Materials can also exhibit tiling characteristics. These structures are often associated with very special properties, for example unusual electrical conductivity, special light reflectivity or extreme mechanical strength. But, producing such materials is difficult. It requires large molecular building blocks that are not compatible with traditional manufacturing processes.

 

>Read more on the Bessy II website

Image: The new building block (left, red outline) comprises two modified starting molecules connected to each other by a silver atom (blue). This leads to complex, semiregular tessellations (right, microscope image).
Credit: Klappenberger and Zhang / TUM

Scientists discover material ideal for smart photovoltaic windows

Berkeley Lab researchers make thermochromic windows with perovskite solar cell

Smart windows that are transparent when it’s dark or cool but automatically darken when the sun is too bright are increasingly popular energy-saving devices. But imagine that when the window is darkened, it simultaneously produces electricity. Such a material – a photovoltaic glass that is also reversibly thermochromic – is a green technology researchers have long worked toward, and now, scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) have demonstrated a way to make it work.

Researchers at Berkeley Lab, a Department of Energy (DOE) national lab, discovered that a form of perovskite, one of the hottest materials in solar research currently due to its high conversion efficiency, works surprisingly well as a stable and photoactive semiconductor material that can be reversibly switched between a transparent state and a non-transparent state, without degrading its electronic properties.

>Read more on the Advanced Light Source website

Image Credit: iStock

 

X-ray detector for studying characteristics of materials

Sol M. Gruner’s group, Physics, has been a leader in the development of x-ray detectors for scientific synchrotron applications, and the team’s technology is used around the world. Their detectors utilize pixelated integrated circuit silicon layers to absorb x-rays to produce electrical signals. The wide dynamic range, high sensitivity, and rapid image frame rate of the detectors enable many time-resolved x-ray experiments that have been difficult to perform until now.

The detectors are limited by the silicon layer. Low atomic number materials such as silicon become increasingly transparent to x-rays as the energy of the x-rays rises. Gruner’s group is now developing a variant of their detector that will use semiconductors comprised of high atomic weight elements to absorb the x-rays and produce the resultant electrical signals. The Detector Group, led by Antonio Miceli, at the United States Department of Energy’s Advanced Photon Source (APS) will simultaneously develop the ancillary electronics and interfacing required to produce fully functional prototypes suitable for high x-ray energy experiments at the APS and CHESS.

 

>Read more on the CHESS website

Image: Sol M. Gruner, Physics, College of Arts and Sciences
Credit: Jesse Winter

 

Control of magnetoresistance in spin valves

Molecules, due to their wide-ranging chemical functionalities that can be tailored on demand, are becoming increasingly attractive components for applications in materials science and solid-state physics. Remarkable progress has been made in the fields of molecular-based electronics and optoelectronics, with devices such as organic field-effect transistors and light emitting diodes. As for spintronics, a nascent field which aims to use the spin of the electron for information processing, molecules are proposed to be an efficient medium to host spin-polarized carriers, due to their weak spin relaxation mechanisms. While relatively long spin lifetimes are measured in molecular devices, the most promising route toward device functionalization is to use the chemical versatility of molecules to achieve a deterministic control and manipulation of the electron spin.

Spin-polarized hybrid states induced by the interaction of the first molecular monolayers on ferromagnetic substrates are expected to govern the spin polarization at the molecule–metal interface, leading to changes in the sign and magnitude of the magnetoresistance in spin-valve devices. The formation of spin-polarized hybrid states has been determined by spin-polarized spectroscopy methods and principle-proven in nanosized molecular junctions, but not yet verified and implemented in large area functional device architectures.

>Read more on the ALBA website

Image: Magnetoresistance (top) and X-ray spectroscopy (bottom) measurements, evidencing the control of the magnetoresistance sign and amplitude by engineering spin valves with NaDyClq/NiFe and NaDyClq/Co interfaces, and their corresponding interfacial molecule-metal hybridization states.

Perovskite solar cells: perfection not required!

Experiments at BESSY II reveal why even inhomogeneous perovskite films are highly functional

Metal-organic perovskite layers for solar cells are frequently fabricated using the spin coating technique. If you follow the simplest synthesis pathway and use industry-relevant compact substrates, the perovskite layers laid down by spin coating generally exhibit numerous holes, yet attain astonishingly high levels of efficiency. The reason that these holes do not lead to significant short circuits between the front and back contact and thus high-rate charge carrier recombination has now been discovered by a HZB team headed by Dr.-Ing. Marcus Bär in cooperation with the group headed by Prof. Henry Snaith (Oxford Univ.) at BESSY II.

>Read more on the HZB website.

Topological insulator gap in graphene contacted with Pb

Up to now the proposed modifications do not allow to introduce graphene to existing electronic devices.

Graphene is the most promising two dimensional material for nanoelectronic applications featuring the relativistic-like electronic spectrum. Contact of graphene with various materials and its functionalization allows to manipulate the electronic structure, e.g. to change the conductivity type and band gap creation. The latter is of great interest due to the requirements for graphene transistor realisation. Furthermore, graphene contact with heavy/magnetic metals results in the lifting of the spin degeneracy of the Dirac cone, opening the spintronics field for its applications. However, up to now the proposed modifications do not allow to introduce graphene to existing electronic devices.

>Read more on the Elettra website.

Image: a) Sketch of the studied system, the Pb atoms presented by yellow spheres; b) ARPES image of graphene/Pb/Pt(111) in the region of K point, taken as a sum of two spectra with p-and s-polarization of light; c) schematic spin structure of the graphene states in the case of large “intrinsic” spin-orbit interaction d) ARPES mapping of the system in two orthogonal k-directions near the K point of graphene.

Brittle star shows how to make tough ceramics

Nature inspires innovation.

An international team lead by researchers at Technion – Israel Institute of Technology, together with ESRF scientists, have discovered how a brittle star can create material like tempered glass underwater. The findings are published in Science and may open new bio-inspired routes for toughening brittle ceramics in various applications.

A beautiful, brainless brittle star that lives in coral reefs has the clue to super tough glass. Hundreds of focal lenses are located on the arms of this creature, which is an echinoderm called Ophiocoma wendtii. These lenses, made of chalk, are powerful and accurate, and the deciphering of their crystalline and nanoscale structure has occupied Boaz Pokroy and his team, from the Technion-Israel Institute of Technology, for the past three years. Thanks to research done on three ESRF beamlines, ID22, ID13 and ID16B, among other laboratories, they have figured out the unique protective mechanism of highly resistant lenses.

Read more on the ESRF website

Image: The brittle star Ophiocoma wendtii shows amazing properties.
Credit: Sinhyu.

Studying Gas Mask Filters So People Can Breathe Easier

Scientists have put the x-ray spotlight on composite materials in respirators used by the military, police, and first responders. The results provide reassuring news about the effectiveness of current filters and provide fundamental information that could lead to more advanced gas masks as well as protective gear for civilian applications.

Read more on the ALS website.

Image: credit ALS

A new x-ray technique to unravel electronic properties of actinide compounds

A new research demonstrates a direct and selective way to investigate 5f electrons in actinide compounds as well as their interaction with other valence electrons

Actinides are a series of chemical elements that form the basis of nuclear fission technology, finding applications in strategic areas such as power generation, space exploration, diagnostics and medical treatments, and also in some special glass. Thorium (Th) and Uranium (U) are the most abundant actinides in the Earth’s crust.

Read more on the LNLS website.

Image: X-ray Magnetic Circular Dichroism (XMCD) measurements for UCu2Si2 and UMn2Si2 performed at temperatures of 10 K and 300 K, respectively.

Watching a Quantum Material Lose Its Stripes

Berkeley Lab study uses terahertz laser pulses to reveal ultrafast coupling of atomic-scale patterns

Stripes can be found everywhere, from zebras roaming in the wild to the latest fashion statement. In the world of microscopic physics, periodic stripe patterns can be formed by electrons within so-called quantum materials.

Scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have now disentangled the intriguing dynamics of how such atomic-scale stripes melt and form, providing fundamental insights that could be useful in the development of novel energy materials.

>Read more on the ALS website

Image: Illustration of an ultrashort laser light striking a lanthanum strontium nickel oxide crystal, triggering the melting of atomic-scale stripes. The charges (yellow) quickly become mobile while the crystal distortions react only with delay, exposing the underlying interactions.
Credit: Robert Kaindl/Berkeley Lab