Transition metal complexes: mixed works better

A team at BESSY II has investigated how various iron-complex compounds process energy from incident light. They were able to show why certain compounds have the potential to convert light into electrical energy. 

The results are important for the development of organic solar cells. The study has now been published in the journal PCCP, and its illustration selected for the cover.
Transition-metal complexes – that is a cumbersome word for a class of molecules with important properties: An element from the group of transition metals sits in the centre. The outer electrons of the transition-metal atom are located in cloverleaf-like extended d-orbitals that can be easily influenced by external excitation. Some transition-metal complexes act as catalysts to accelerate certain chemical reactions, and others can even convert sunlight into electricity. The well-known dye solar cell developed by Michael Graetzel (EPFL) in the 1990s is based on a ruthenium complex.

Why not Iron?
However, it has not yet been possible to replace the rare and expensive transition metal ruthenium with a less expensive element, such as iron. This is astonishing, because the same number of electrons is found on extended outer d-orbitals of iron. However, excitation with light from the visible region does not release long-lived charge carriers in most of the iron complex compounds investigated so far.

>Read more on the Bessy II at HZB website

Image: The illustration shows a molecule with an iron atom at its centre, bound to 4 CN groups and a bipyridine molecule. The highest occupied iron orbital is shown as a green-red cloud. As soon as a cyan group is present, the outer iron orbitals are observed to delocalize so that electrons are also densely present around the nitrogen atoms.
Credit: T. Splettstoesser/HZB

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.

Insulator metal transition at the nanoscale

An international team of researchers has been able to probe the insulator-conductor phase transition of materials at the nanoscale resolution. This is one of the first results of MaReS endstation of BOREAS beamline.

Controlling the flow of electrons within circuits is how electronic devices work. This is achieved through the appropriate choice of materials. Metals allow electrons to flow freely and insulators prevent conduction. In general, the electrical properties of a material are determined when the material is fabricated and cannot be changed afterwards without changing the material. However, there are materials that can exhibit metal or insulator behaviour depending on their temperature. Being able switch their properties, these materials could lead to a new generation of electronic devices.

Vanadium Dioxide (VO2) is one such material. It can switch from an insulating phase to a metallic phase just above room temperature, a feature exploited already for sensors. However, the reason why the properties of this material change so dramatically has been a matter of scientific debate for over 50 years.

One of the challenges in understanding why and how this switch occurs is due to a process called phase separation. The insulator-metal phase transition is similar to the ice to liquid transition in water. When ice melts, both liquid and solid water can coexist in separate regions. Similarly, in VO2, insulating and metallic regions of the material can be coexisting at the same time during the transition. But unlike water, where the different regions are often large enough to see with the naked eye, in VOthis separation occurs on the nanoscale and it is thus challenging to observe. As a result, it has been hard to know if the true properties of each phase, or the mixture of both phases, are being measured.

>Read more on the ALBA Synchrotron website

Image: (extract, original here) reconstructed holograms at the vanadium and oxygen edges (518, 529, and 530.5 eV) used to encode the intensities of the three color channels of an RGB (red, green, blue) image. At 330 K, an increase in intensity of the green channel, which probes the metallic rutile phase (R) through the d∥ state, is observed in small regions. As the sample is heated further, it becomes increasingly clear that the blue channel, which probes a intermediate insulating M2 phase, also changes but in different regions. At 334 K, three distinct regions can be observed corresponding to the insulating monoclinic M1, M2, and metallic R phases. As the temperature increases, the R phase dominates. The circular field of view is 2 μm in diameter. (taken from Vidas et al, Nanoletters, 2018).

Tungsten accumulation in bone raises health concerns

McGill University scientists have identified exposure to tungsten as problematic after they determined how and where high levels of the metal accumulate and remain in bone.

“Our research provides further evidence against the long-standing perception that tungsten is inert and non-toxic,” said Cassidy VanderSchee, a PhD student and a member of a McGill research group headed by chemistry professor Scott Bohle.

Tungsten is a hard metal with a high melting point and, when combined with other metals and used as an alloy, it’s also very flexible.

Because of these properties and under the assumption that tungsten is non-toxic, it has been tested for use in medical implants, including arterial stents and hip replacements, in radiation shields to protect tissue during radiation therapy, and in some drugs. Tungsten is found in ammunition as well as in tools used for machining and cutting other metals.

Tungsten also occurs naturally in groundwater where deposits of the mineral are found. Exposure to high levels of tungsten in drinking water in Fallon, Nevada, was investigated for a possible link with childhood leukemia in the early 2000s. This investigation lead scientists to question the long-held belief that exposure to tungsten is safe and prompted the Centers for Disease Control and Prevention in the U.S. to nominate tungsten for toxicology and carcinogenesis studies.

>Read more on the Canadian Light Source website

Image: Cassidy VenderSchee

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

Stressing over new materials

Titanium is a workhorse metal of the modern age. Alloyed with small amounts of aluminum and vanadium, it is used in aircraft, premium sports equipment, race cars, space craft, high-end bicycles, and medical devices because of its light weight, ability to withstand extreme temperatures, and excellent corrosion resistance. But titanium is also expensive. Metallurgists would love to understand exactly what makes it so strong so that they could design other materials with similarly desirable properties out of more common, less expensive elements. Now, researchers utilizing the U.S. Department of Energy’s Advanced Photon Source (APS) have used high-intensity x-rays to show how titanium alloy responds to stress in its (until now) hidden interior. Eventually, the researchers believe they will be able to predict how strong a titanium part such as an aircraft engine will be, just by knowing how the crystals are arranged inside of it. And materials scientists may be able to use such a computational model to swap in atoms from different metals to see how their crystalline structures compare to that of titanium.

>Read more on the Advanced Photon Source website

Figure: (extract) (A) A computational model of crystals inside a block of titanium, (B) includes effects noticed during the experiment to place permanent deformations (the darkened areas,) [not visible here, entire picture here]  while (C) models permanent deformations without incorporating the diversity of load seen in the experiment.

How metal 3-D 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.

“With 3-D printing, you can make parts with very complex geometries that are not accessible for casting like regular metal parts,” says SLAC staff scientist Johanna Nelson Weker, who is leading the project. “Theoretically, it can be a quick turnaround – simply design, send, print from a remote location. But we’re not there yet. We still need to figure out all of the parameters involved in making solid, strong parts.”

>Read more on the Stanford Synchrotron Radiation Lightsource website

Image: 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