Edges and corners increase efficiency of catalytic converters

X-rays reveal oxide islands on noble metal nanoparticles

Catalytic converters for cleaning exhaust emissions are more efficient when they use nanoparticles with many edges. This is one of the findings of a study carried out at DESY’s X-ray source PETRA III. A team of scientists from the DESY NanoLab watched live as noxious carbon monoxide (CO) was converted into common carbon dioxide (CO2) on the surface of noble metal nanoparticles like those used in catalytic converters of cars. The scientists are presenting their findings in the journal Physical Review Letters. Their results suggest that having a large number of edges increases the efficiency of catalytic reactions, as the different facets of the nanoparticles are often covered by growing islands of a nano oxide, finally rendering these facets inactive. At the edges, the oxide islands cannot connect, leaving active sites for the catalytic reaction and an efficient oxygen supply.
Catalytic converters usually use nanoparticles because these have a far greater surface area for a given amount of the material, on which the catalytic reaction can take place. For the study presented here, the scientists at DESY’s NanoLab grew platinum-rhodium nanoparticles on a substrate in such a way that virtually all the particles were aligned in the same direction and had the same shape of truncated octahedrons (octahedrons resemble double pyramids). The scientists then studied the catalytic properties of this sample under the typical working conditions of an automotive catalytic converter, with different gaseous compositions in a reaction chamber that was exposed to intense X-rays from PETRA III on the P09 beamline.

>Read more on the PETRA III at DESY website

Image: With increasing oxygen (red) concentration, an oxide sandwich forms on the surface of the metallic nanoparticles, inhibiting the desired reaction of carbon monoxide to carbon dioxide. At the edges, however, the oxide sandwich brakes up, leaving free active sites for catalysis. The more edges the nanoparticles posses, the more efficient will the catalytic converter work.
Credit: DESY, Lucid Berlin

Tailoring the surface chemical reactivity of transition‐metal dichalcogenide PtTe2 crystals

Recently, the PtX2 (X=S, Se, Te) class of transition-metal dichalcogenides has emerged as one of the most promising among layered materials “beyond graphene” for the presence of high room-temperature electron mobility and, moreover, bulk type-II Dirac fermions, arising from a tilted Dirac cone.
Information on the ambient stability of PtTe2 is a crucial step in order to evaluate the feasibility of its exploitation in technology. Moreover, the possibility to tune surface chemical reactivity by appropriate surface modification is an essential step for its employment for diverse applications, especially in catalysis.
By means of experiments with several surface-science spectroscopies and density functional theory, an international team of researchers from Italy, Republic of Korea, and Taiwan (coordinated by Graphene Labs of Istituto Italiano di Tecnologia) has investigated the reactivity of the PtTe2 surface toward most common ambient gases (oxygen and water), under the framework of the European Graphene Flagship-Core1 project.
To assess the surface chemical reactivity of PtTe2, X-ray photoelectron spectroscopy (XPS) carried out at the APE-HE beamline has been combined with high-resolution electron energy loss (HREELS) experiments and with density functional theory.
From the analysis of Te 3d core-level spectra in XPS and from the featureless vibrational spectrum in HREELS, it has been demonstrated that as-cleaved defect-free PtTe2 surface is inert toward most common ambient gases (oxygen and water).
In the evaluation of the ambient stability of PtTe2, the possible influence of Te vacancies on surface chemical reactivity deserves particular attention. As a matter of fact, Te vacancies may appear on non-stoichiometric samples during the growth process. To check the influence of Te vacancies on ambient stability of PtTe2, Te vacancies have been intentionally introduced in stoichiometric PtTe2 samples by Ar-ion sputtering. After exposing to O2 the PtTe2 surface defected by ion sputtering, with a Pt:Te ratio of 39:61, spectral features related to Te(IV) species appear, arising from the formation of Te=O bonds in a tellurium-oxide phase. The Te(IV) components are the most intense lines in the Te 3d XPS spectra for the case of air-exposed defected samples (see Figure 1). Concerning reactivity to water, it adsorbs molecularly even at room temperature on defected PtTe2. These findings also imply that the presence of Te vacancies is able to jeopardize the ambient stability of uncapped PtTe2-based devices, with a subsequent necessity to reduce the amount of Te vacancies for a successful technological exploitation of PtTe2.

>Read more on the Elettra website

Figure: XPS spectra of Te-3d core levels acquired for: defected PtTe2 (green curve), the same surface exposed to 106 L of O2 (black curve) and air-exposed defected PtTe2 (yellow curve). The photon energy is 745 eV. 

Scientists have a new way to gauge the growth of nanowires

In a new study, researchers from the U.S. Department of Energy’s Argonne and Brookhaven National Laboratories observed the formation of two kinds of defects in individual nanowires, which are smaller in diameter than a human hair.

These nanowires, made of indium gallium arsenide, could be useful for a wide range of applications in a field scientists have termed optoelectronics, which encompasses devices that work by converting light energy into electrical impulses. Fiber optic relays are a good example.

The effectiveness of these devices, however, can be affected by tiny defects in their components. These defects, which can change both the optical and electronic properties of these materials, interest scientists who seek to tailor them to boost the functionality of future optoelectronics, including materials that will be able to manipulate quantum information.

>Read more on the NSLS-II website and the Advanced Photon Source website

Image: Argonne and Brookhaven researchers observed two kinds of defects forming in individual nanowires, depicted here. These nanowires are smaller in diameter than a human hair.
Credit: Megan Hill/Northwestern University

Twin Orbit operation successfully tested at BESSY II

The first “Twin Orbit User Test week” at BESSY II in February 2018 was a big success and can be considered as an important step towards real user operation.

Physicists at Helmholtz-Zentrum Berlin have been able to store two separate electron beams in one storage ring. The twin orbit operation mode can serve users with different needs of the time structure of the photon pulses simultaneously and offers elegant options regarding the future project BESSY VSR.

The Twin Orbit operation mode makes use of non-linear beam dynamics and provides two stable well separated orbits for storing two electron beams in one storage ring. The bunch fill patterns of both orbits can be chosen, to some extent, independently, which allows for fulfilling normally incompatible user needs, simultaneously. For example, one orbit can be used to store a homogenous multi bunch fill to deliver high average brilliance for photon hungry experiments, whereas only one single bunch is stored on the other orbit for timing experiments, providing a much lower pulse repetition rate.

>Read more on the Bessy II at HZB website

Image: A synchrotron source point image of a bending magnet of the Twin Orbit modus. The second orbit closes after three revolution and is winding around the standard orbit at the center. (full image here)
Credit: HZB

New technique simplifies creation of nanoparticle ‘magic-sized clusters’

One of the cool things about nanoparticles is also what makes them so difficult to work with: the fact that their properties are dependent on their size.

A critical challenge in translating nanomaterials from the laboratory into commercial applications, such as lighting or optical memory storage, is making a batch of nanoparticles all the same size. Two Cornell research groups have joined forces to lay out a solution for this issue.

Researchers in the labs of Richard Robinson and Tobias Hanrath – using X-ray analysis at the Cornell High Energy Synchrotron Source (CHESS) – have developed a new nanosynthetic pathway to achieve ultra-pure and highly stable groups of same-sized particles – known as “magic-sized clusters.”

Their paper, “Mesophase Formation Stabilizes High-Purity Magic-Sized Clusters,” published online Jan. 27 in the Journal of the American Chemical Society, and will be on a cover of the March 14 print edition. Lead authors are Curtis Williamson, doctoral student in both the Robinson and Hanrath groups, and Douglas Nevers, doctoral student in the Hanrath Group. Lena Kourkoutis, assistant professor of applied and engineering physics, also contributed.

>Read more on the Cornell Hight Energy Synchrotron Source (CHESS) website

Image: Schematic of the magic-sized clusters hexagonal mesophase. The mesophase (left) is an assembly of nanofibers (center), which are composed of magic-sized clusters (right).
Credit: Richard Robinson

Marianne Liebi winner of Swedish L’Oréal-Unesco For Women in Science 2018

L’Oréal-Unesco For Women in Science Prize is awarded in Sweden for the third time. The purpose of the prize is to pay attention to and reward young women who have shown great potential in science, while offering positive female role-models. Researchers Marianne Liebi, Chalmers, and Ruth Pöttgen, Lund University, get L’Oréal-Unesco For Women in Science Award, supported by Sweden’s young academy 2018.

Marianne Liebi gets the award “for the constructive use of advanced imaging methods for biomaterials with the aim of understanding the connection between molecular and mechanical properties”. Marianne Liebi uses powerful X-ray technology to study how, for example, the smallest building blocks, collagen fibrils, the bone tissue, look and are organised. The goal is to develop a mimicking, biomimetic material, where nature’s own design principles are imitated and applied to develop artificial bone and cartilage.
“It’s important to show that in research, it does not matter where you come from or who you are – what matters is passion and dedication. At best, this kind of award will not be needed in the future, it would be aimed at all young researchers. It would not matter who you were, says Marianne Liebi.

>Read more on the MAXIV Laboratory website

Photo: Researchers Ruth Pöttgen (left), Lund University, and Marianne Liebi (right), Chalmers, get L’Oréal-Unesco For Women in Science Award 2018, supported by Young Academy Sweden.
Credit: Emma Burendahl

Precise layer growth in a superlattice controls electron coupling and magnetism

Two-dimensional (2-D) crystalline films often exhibit interesting physical characteristics, such as unusual magnetic or electric properties. By layering together distinct crystalline thin films, a so-called “superlattice” is formed. Due to their close proximity, the distinct layers of a superlattice may significantly affect the properties of other layers. In this research, single 2-D layers of strontium iridium oxide were sandwiched between either one, two, or three layers of strontium titanium oxide to form three distinct superlattices. Researchers then used x-ray scattering at the U.S. Department of Energy’s Advanced Photon Source (APS) to probe the magnetic structure of each superlattice. The x-ray data revealed that the number of layers of the titanium-based material produced a dramatic difference in the magnetic behavior of the iridium-based layer. These findings are especially significant because the iridium compound is one of the perovskites, a class of materials known for their unique electric, magnetic, optical, and other properties that have proven useful in sensor and energy-related devices, and which are being intensively investigated for their application towards improved electronics and other technologies.

>Read more on the Advance Photon Source website
Image: Fig. 1. Illustration of superlattices. Panel (a) shows the Sr2IrO4 crystalline superlattice, with alternating layers of SrIrO3 and SrO. The SrIrO3 layers are perovskites, depicted as diamond-like shapes formed by six oxygen atoms; inside each diamond is a gold-colored iridium ion (cation), while pink strontium ions lay near the diamond’s ends. The SrIrO3 layers are separated by non-perovskite (inert) SrO layers, depicted as pink bars. Panel (b) shows the more-recently developed SrIrO3/SrTiO3 superlattice used for this research. Three distinct SrIrO3/SrTiO3 superlattices were created, having 1, 2, or 3 layers of inert SrTiO3 layers (colored green) separating the active SrIrO3 layers. Bold green boxes highlight the number of inert SrTiO3 layers in the three distinct lattices. While both SrIrO3 (gold diamonds) and SrTiO3 (green diamonds) are perovskites, the green-colored SrTiO3 layers buffer the active SrIrO3 layers. (The entire image is visible here)

The Molecular Scale Structure of Electrolyte-Metal Oxide Interface

Li-ion batteries (LIBs) are key components of portable electronic devices, as well as in electric vehicles, military and medical equipment, backup power supplies, and even grid storage. However, the energy storage capacity and rate capability of current LIBs is still too low to meet the increasing demand of key markets. For the latter, the properties of the electrolyte-electrode interface play a decisive role.

From a more general point of view, interfaces, or surfaces, are the outer boundary of any condensed matter. Due to the resulting symmetry breaking, the arrangement of atoms or molecules at the interface often varies significantly from that in the bulk. Studies of the molecular scale structural properties of liquids at interfaces are intriguing, as these give insights into the fundamental molecule–molecule and molecule–substrate interactions. Investigations have included layering of ionic liquids [1], layering of metallic [2] and non-metallic liquids [3], and the (potential-dependent) structure of water adsorbed on solid surfaces [4]. However, basic insights into how a non-aqueous electrolyte–salt solution organizes at a solid interface, in particular from experiments, is still missing [5]. In many technological applications, the atomic scale properties of interfaces govern the functionality of the system. A prominent example is the importance of the structure and molecular arrangement of the liquid at the functional solid–liquid interface in batteries. More specifi­cally, in LIBs, the arrangement of the electrolyte molecules directly at the electrode interface, and the electric double layer (EDL) formation are expected to govern the interfacial ion transport during charge/discharge, as well as affect the origin and properties of the solid electrolyte interphase (SEI).

>Read more on the Stanford Synchrotron Radiation Lightsource website

Image: (a) Fresnel-normalized XRR (symbols) of the sapphire/LiPF6:EC:DMC and corresponding model fits (lines). (b) Fit-derived electron density profiles. (c) Comparison of the XRR- and MD-derived (blue) density profiles. The MD-derived profile is smeared by the XRR-derived roughness. All curves are spaced vertically for clarity. (d) Periodicity at the solid/liquid interface vs. LiPF6 concentration. (e) Normalized correlation lengths. (f) Schematic illustration of the proposed origin induced increased layer spacing with increasing salt-concentration.

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

Kilian Peter Heeg wins ESRF Young Scientist award

Kilian Peter Heeg has been awarded the title of Young Scientist 2018 by the ESRF User Organisation in recognition of his pioneering work on light-matter interactions enabling resonant brilliance enhancement of X-ray pulses. This award is presented every year at the ESRF annual User Meeting to a scientist aged 37 or younger for outstanding work conducted at the ESRF.

Kilian Heeg is a physicist and postdoctoral researcher at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. Aged just 31, Kilian has already significantly shaped the field of X-ray quantum optics.

Kilian says: “I wanted to be a mathematician when I was a child and I was always fascinated by natural sciences. However in my final years in school I fell in love with physics and very quickly became fascinated with quantum mechanics and especially quantum optics. I feel very honoured and pleased to have been chosen as the winner of this year’s ESRF Young Scientist Award.”

>Read more on the ESRF website

Image: Kilian on ESRF’s ID18 beamline
Credit: ESRF/C. Argoud

High coherence and intensity at FERMI enables new X-ray interfacial probe

Interfaces are involved in a wide range of systems and have significant implications in many fields of scientific and technological advancement, often determining device performance or chemical reactivity. Vital examples include solar cells, protein folding, and computer chips. A class of commonly used surface science techniques are comprised of even-ordered nonlinear spectroscopies (i.e., second harmonic and sum frequency generation) which exhibit no response in centrosymmetric media due to symmetry constraints.As a result, they have been widely used at optical wavelengths to explore physical and chemical properties of interfaces, where centrosymmetry is broken. Extending this to x-ray wavelengths would effectively combine the element specificity and spectral sensitivity of x-ray spectroscopy with the rigorous interfacial/surface specificity of optical even-ordered nonlinear spectroscopies. Unfortunately, at hard x-ray energies (x-ray wavelength order of the spacing between atoms) these even-ordered nonlinear spectroscopies are effectively bulk probes, as each individual atom breaks inversion symmetry. As soft x-ray wavelengths fall in between the UV and hard x-ray regimes, there has been uncertainty regarding the interface specificity of soft x-ray second harmonic generation.

>Read more on the FERMI webpage

Figure: (extract)  Experimental Design. X-ray pulses are passed through a 2 mm iris and focused onto the graphite sample at normal incidence. The transmitted beam is then passed through a 600 nm aluminum filter and onto a spectrometer grating, spatially resolving the second harmonic signal from the fundamental. Inset: A schematic energy level diagram of the second harmonic generation process. (entire figure here)


Magnetic trick triples the power of SLAC’s X-ray laser

The new technique will allow researchers to observe ultrafast chemical processes previously undetectable at the atomic scale.

Scientists at the Department of Energy’s SLAC National Accelerator Laboratory have discovered a way to triple the amount of power generated by the world’s most powerful X-ray laser. The new technique, developed at SLAC’s Linac Coherent Light Source (LCLS), will enable researchers to observe the atomic structure of molecules and ultrafast chemical processes that were previously undetectable at the atomic scale.

The results, published in a Jan. 3 study in Physical Review Letters (PRL), will help address long-standing mysteries about photosynthesis and other fundamental chemical processes in biology, medicine and materials science, according to the researchers.

“LCLS produces the world’s most powerful X-ray pulses, which scientists use to create movies of atoms and molecules in action,” said Marc Guetg, a research associate at SLAC and lead author of the PRL study. “Our new technique triples the power of these short pulses, enabling higher contrast.”

>Read more on the LCSL website

Picture: The research team, from left: back row, Yuantao Ding, Matt Gibbs, Nora Norvell, Alex Saad, Uwe Bergmann, Zhirong Huang; front row, Marc Guetg and Timothy Maxwell.
Credit: Dawn Harmer/SLAC


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)




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


G. Ghiringhelli and L. Braicovich win 2018 Europhysics Prize of Condensed Matter

>Read more on the ESRF website