Tag: perovskite

Robust supercrystals for the LEDs of the future

Robust supercrystals for the LEDs of the future

Nanometre-sized crystals of perovskite offer great potential for applications in the field of light-emitting diodes, solar cells and optical switching elements. A particularly interesting arrangement occurs when a large number of these nanocrystals join to form a larger structure – a supercrystal. Researchers at the University of Tübingen have now found a novel way of achieving this. They resorted to a clever technique to produce perovskite supercrystals that are particularly stable and therefore useful. Some important analyses were carried out at DESY: Using its X-ray source PETRA III, the team managed to determine the precise structure of the supercrystals. They are now presenting their findings in the journal ACS Nano.

Perovskites, named after the Russian mineralogist Lev Perovski, are a class of crystalline materials with a characteristic lattice structure. Lead halide perovskites are particularly promising, as they can be produced relatively easily by chemical methods and have remarkable optoelectronic properties – which is why these materials are the subject of intensive research. The first practical applications now appear to be within reach: Experts are working on high-performance solar cells based on perovskites, as well as on a new generation of highly efficient LEDs and laser chips.

Perovskite nanocrystals are a particularly exciting area of research. “Their optoelectronic properties depend heavily on their size – which is typical of quantum behaviour,” explains Jonas Hiller from the University of Tübingen, one of the authors of the study. “Because of this, their properties can be specifically customised. The energy of the light they absorb or emit varies depending on their composition and their size.”

Under certain conditions, these perovskite nanocrystals can form larger structures, creating a supercrystal. “This is a crystal made up of crystals,” Hiller explains. “You can compare it to a Rubik’s Cube which is made up of several smaller cubes.” The exciting thing is that while the individual nanocrystals retain their desired quantum properties, they can be handled as a macroscopic unit and thus deployed in practical applications.

Forming a supercrystal in two phases

Until now, supercrystals like this have been created by allowing a solvent containing the perovskite to slowly evaporate. The resulting structures form very gradually on the substrate. “However, the supercrystals are produced at random sites around the substrate,” explains the project manager Ivan Zaluzhnyy from Tübingen. “Also, the individual nanocrystals are surrounded by a protective layer of organic molecules which makes the entire supercrystal very soft.” As a result, they break very easily when you try to move them around mechanically. This poses a real obstacle for applications in which the positioning of the materials is crucial, such as between two electrodes in an electrical component.

To solve this problem, the team opted for an alternative approach: two-phase diffusion. A solution containing the nanocrystals is layered on top of a second liquid: acetonitrile. This acts as an anti-solvent for the perovskite crystals. As it slowly penetrates the solution containing the nanocrystals, it gradually reduces their solubility. “This results in crystal growth beginning at the boundary surface between the two phases,” explains Jonas Hiller. The acetonitrile displaces the organic molecules coating the crystals, resulting in a firmer, more stable structure.

In order to examine the structure of these supercrystals more closely, the team used the narrow X-ray beam at GINIX, an instrument installed at the PETRA III beamline P10. “The beam diameter of just 300 nanometres makes it possible to examine different regions within a supercrystal with high precision,” explains DESY physicist Wojciech Roseker. And Jonas Hiller adds: “The extremely high quality of the diffraction data was a key element of this study. It enabled us to analyse the structure of the supercrystals in great detail.”

The team found that the supercrystals produced, typically had an area of 10 by 10 square micrometres but were significantly thicker than the comparatively flat structures that could be achieved using the old method. Their height was more than five micrometres which improves their stability. This makes the supercrystals robust enough to be gripped with micromanipulators and moved to other locations – a first for perovskite structures.

Read more on DESY website

Image: Like a crystal rubik’s cube: The research team has found a way to create ordered perovskite supercrystals.

Credit: University of Tübingen, figure from original publication

Finetuning perovskites for new applications in solar cells, LEDs and semiconductors

Finetuning perovskites for new applications in solar cells, LEDs and semiconductors

Perovskite is a rising star in the field of materials science. The mineral is a cheaper, more efficient alternative to existing photovoltaic materials like silicon, a semiconductor used in solar cells. Now, new research has shown that applying pressure to the material can alter and fine-tune its structures — and thus properties — for a variety of applications.

Using the Canadian Light Source (CLS) at the University of Saskatchewan, a team of researchers observed in real time what happened when they “squeezed” a special type of perovskite between two diamonds. 2D hybrid perovskite is made up of alternating organic and inorganic layers. It’s the interaction between these layers, says Dr. Yang Song, professor of chemistry at Western University, that determines how the material absorbs, emits, or controls light.

The research team found that applying pressure significantly increased the material’s photoluminescence, making it brighter, which Song says hints at potential applications in LED lighting. The team also observed a continuous change in its colour from green to yellow to red. “So you can tune the colour.” Being able to observe changes to the material as they happen using ultrabright synchrotron light was critical to their research, said Song.

One of the biggest changes in the material came when the researchers applied a very large amount of pressure to the perovskite: It started glowing differently, signaling that its ability to handle light had improved. They also found the material squished more in one direction than others and that its internal structure became less twisted. Most similar materials become more twisted when they’re squeezed. The findings of the research, which also involved the Advanced Photon Source (APS)  at Argonne National Laboratory in Chicago, were published recently in the journal Advanced Optical Materials.

Read more on CLS website

Perovskite phase symmetry influence the cobalt modifier position

Perovskite phase symmetry influence the cobalt modifier position

A research group from the AGH University of Krakow, specializing in material chemistry, in collaboration with the SOLARIS Center, has published findings on the impact of phase symmetry in the CaTiO3 – SrTiO3 perovskite system on the incorporation of cobalt into the perovskite structure. These cobalt-modified materials are promising for applications in energy conversion technologies and environmental catalysis. The findings, published in Materials Chemistry and Physics, revealed distinct behaviors between calcium-rich (Ca-rich) and strontium-rich (Sr-rich) perovskite materials. The study also examined the impact of non-stoichiometry on both, the position occupied by cobalt in the structure and its oxidation state. A comprehensive understanding of the structural changes in the system was achieved through an innovative approach combining X-ray absorption spectroscopy (XAS) analysis, conducted at the ASTRA beamline, with results from temperature-programmed reduction (TPR) studies.

Ca-rich materials, characterized by orthorhombic symmetry, more effectively incorporate cobalt into the perovskite structure but also tend to form a secondary phase – CoTiO3. In contrast, Sr-rich materials with tetragonal symmetry predominantly lead to the formation of cobalt oxides, particularly Co3O4. The  slight deviations from stoichiometry (a deficiency of atoms in the Ca/Sr sublattice) intensify these effects: in Ca-rich materials, they increase the proportion of the CoTiO3 phase in the system, while in Sr-rich materials, they result in a higher content of cobalt oxides. Additionally, XAS and TPR results revealed that Ca-rich materials contain more cobalt in the +II oxidation state, while Sr-rich materials contain more cobalt in the +III oxidation state.

These findings open new possibilities for designing materials and optimizing their properties for potential applications in catalysis and electrochemical devices. At the same time, they significantly enhance the understanding of solid-state chemistry, particularly the chemistry of materials with a perovskite-type structure.

Read more on SOLARIS website

Making solar cells more weatherproof

Making solar cells more weatherproof

Dr. Tim Kelly calls it “the magic of science” – when what you think is going to happen doesn’t, but what you learn in the process promises to inform advances in a new type of solar cell. Solar cells, which convert sunlight into electricity, are increasingly being used to power everything from buildings and electric cars, to watches and toys.

In recent experiments at the Canadian Light Source (CLS) at the University of Saskatchewan (USask), Kelly, professor of chemistry at USask, and his team were trying to figure out why solar cells made with lead halide perovskite, rather than silicon, were failing prematurely. Perovskite, he explained, is a new semiconductor material that requires much less energy to produce than silicon, giving it an environmental advantage.

But it was a puzzle: “What makes them unstable? Why were these cells failing?”

Thinking the problem lay in the perovskite formulation, Kelly used a synchrotron technique called x-ray diffraction to visualize the 3D structure of the atoms in the material in real time.

In the experiments, Kelly and his team found cell performance started to decline with the introduction of humidity.

“We thought humidity would degrade the perovskite … because it does tend to pick up moisture more rapidly.” However, because they were able to watch – at a microscopic scale — the failure process as it unfolded, they could see that the moisture causes ions in the perovskite to become more mobile, to migrate to the electrode and to corrode it, “and now your device is no longer operative.”

“Like most of your electronics, it turns out getting these things wet is not a good idea.”

Kelly’s research identified possible solutions to address the issue of premature cell failure, including using corrosion-resistant materials for the electrodes, buffer layers to prevent the mobile ions from reaching the electrodes, or fully encapsulating the cell to keep out any moisture.

“There’s a lot of promise to the material (perovskite),” said Kelly, so solving the moisture issue could lead to high-performing and reliable solar cells.

The CLS was key to solving the cell-failure question, he said.

Read more on CLS website

Improvement of Efficiency and Stability of Lead-Free Perovskite Solar Cells

Improvement of Efficiency and Stability of Lead-Free Perovskite Solar Cells

Research Background and Objectives

Organic-inorganic halide perovskite solar cells have significant potential in solar energy development because of their long diffusion length, high light absorption coefficient, and excellent charge mobility. Due to these characteristics, the power conversion efficiency (PCE) of perovskite solar cells has rapidly increased from 3.8% to 26%. However, using lead (Pb) poses environmental and health risks, limiting commercialization. Therefore, active studies are being conducted on lead-free perovskite materials that maintain high efficiency while using less harmful substances.  

Alternative materials such as tin (Sn), germanium (Ge), antimony (Sb), bismuth (Bi), and copper (Cu) have been proposed. Among them, tin is considered a promising candidate to replace lead due to its high charge mobility, low exciton binding energy, and suitable bandgap. However, tin-based perovskites suffer instability and low efficiency (below 15%) caused by oxidation and strong self-doping. This study aims to improve structural stability and PCE by introducing additives to overcome these limitations. 

Experimental Methods and Procedures

In this study, we introduced various additives to improve the performance of tin-based perovskite solar cells, aiming to enhance grain growth and charge carrier mobility. The additives used in the experiment were bromides and various organic amine compounds, which were added to the precursor solution in small amounts. These additives were selected to help the vertical orientation of tin-based perovskite films and to increase grain size for charge recombination reduction and conductivity enhancement.  

Solar cell thin films were fabricated through spin coating and annealing, and solvent evaporation and crystallization were processed without anti-solvent treatment. Subsequently, we analyzed electrical characteristics to evaluate the efficiency and stability of the films with additive introduction and conducted the X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses in parallel to determine the crystal structure and defect states. 

Analysis Methods

To comprehensively analyze the effects of additives on tin-based perovskites, we applied synchrotron radiation analysis. In particular, small-angle X-ray scattering (SAXS) was used to investigate the effects of introducing additives on grain growth and structural orientation within the film. In addition, we observed the surface and cross-section of the film with an electron microscope to identify microstructural changes caused by introducing additives. Furthermore, to evaluate electrical characteristics, we measured open-circuit voltage (V_OC), short-circuit current density (J_SC), fill factor (FF), and PCE.  

Read more on PAL website

Extending the longevity of perovskite solar cells for cheaper solar energy

Extending the longevity of perovskite solar cells for cheaper solar energy

Study reveals the secret to treating the ‘Achilles’ heel’ of alternatives to silicon solar panels for the photovoltaics industry

Diamond’s Nanoprobe beamline I14 and the electron Physical Science Imaging Centre (ePSIC) were used by a multidisciplinary team of researchers to gain new insight into the perovskite materials that hold so much potential in the field of optoelectronics. Focusing on structural changes that can lead to degradation, the Diamond instruments were part of a suite that enabled the group to observe the nanoscale properties of thin films of perovskite materials and how they change over time under solar illumination. The research, recently published in Nature, could significantly accelerate the development of long-lasting, commercially available perovskite photovoltaics.  

Perovskite materials offer a cheaper alternative to silicon for producing solar cells and also show great potential for other optoelectronic applications, such as energy efficient LEDs and X-ray detectors.

The metal halide salts are abundant and much cheaper to process than crystalline silicon. They can be prepared in a liquid ink that is simply printed to produce a thin film of the material.

While the overall energy output of perovskite solar cells can often meet or – in the case of multi-layered, so-called ‘tandem’ devices – exceed that achievable with traditional silicon photovoltaics, the limited longevity of the devices is a key barrier to their commercial viability.

A typical silicon solar panel, like those you might see on the roof of a house, typically lasts about 20-25 years without significant performance losses.

Because perovskite devices are much cheaper to produce, they may not need to have as long a lifetime as their silicon counterparts at least to enter some markets – but to fulfil their ultimate potential in realising widespread decarbonisation, cells will need to operate for at least a decade or more. Researchers and manufacturers have yet to develop a device with similar stability to silicon cells.

Now, researchers at the Department of Chemical Engineering and Biotechnology (CEB) and Cavendish Laboratory at the University of Cambridge, together with the Okinawa Institute of Science and Technology (OIST) in Japan, have discovered that the defects that limit perovskite efficiency are also responsible for structural changes in the material that lead to degradation.

Read more on the Diamond website

Image: A typical silicon solar panel, like those you might see on the roof of a house, typically lasts about 20-25 years without significant performance losses

New angle for perovskite research

New angle for perovskite research

Perovskite materials offer the potential for cheaper optoelectronic devices such as solar cells. Of these, the formamidinium (FA)-based FAPbI3 crystal is one of the most promising – it has a bandgap close to ideal and is very thermally stable. However, photoactive cubic (α)-FAPbI3 perovskite phase is highly unstable and quickly transforms into the non-perovskite yellow phase at room temperature in ambient atmosphere, which affects the performance of photovoltaic devices. Alloying of FA-based perovskite with caesium, methylammonium (MA) cations or a combination of both can keep the perovskite in its more efficient phase at lower temperatures. However, this can give patchy results, leading to power losses.

In work recently published in Science, researchers from the University of Cambridge Department of Chemical Engineering and Biotechnology (CEB) and the Cavendish Laboratory investigated the crystal structure of the alloyed perovskite materials to understand why adding cations improved their performance. Their results show that cation alloying induces a minor octahedral tilt that keeps the perovskite material in its highly efficient phase, and is a step towards commercial production of stable and efficient perovskite-based solar cells. 

A small distortion makes a big difference

Formamidinium (FA)-based perovskites have much better thermal stability than the methylammonium (MA)-based absorber layers commonly used in early perovskite-based solar cells. FAPbI3 is a particularly promising material, but its photoactive phase is only stable at high temperatures (above 150ºC) in inert atmosphere. It transitions to a hexagonal phase with poor optoelectronic performance at lower temperatures.  

It has been shown empirically that alloying FAPbI3 with methylammonium (MA) cations or caesium (or both) improves stability. However, although this approach led to record efficiencies, the mechanism underlying it was not fully understood. It also produces uneven materials with patches of instability that lead to performance losses. 

Co-lead author Tiarnan Doherty was a PhD student at the Cavendish Laboratory and is now an Oppenheimer Fellow in CEB. He says:

We wanted to investigate the atomic structure of the alloyed perovskite materials, but they’re very sensitive to damage. So we brought the samples to ePSIC for high-resolution electron microscopy with a low electron dose. We also used nano X-ray diffraction on beamline I14. That beamline has very sensitive detectors, which allowed us to achieve our results using low X-ray exposures.

Read more on the Diamond website

Image: Artist’s impression of formamidinium (FA)-based crystal

Credit: Tiarnan Doherty, University of Cambridge

World changing science

World changing science

Marion Flatken is a 3rd year PhD student working in the Department Novel Materials and Interfaces for photovoltaic solar cells led by Prof. Dr. Antonio Abate, at HZB.

In her #LightSourceSelfie, Marion describes the perovskite solar cell research she is undertaking and reflects on the opportunity light sources present to scientists.  She says,

“We are really having the chance to work in a unique environment and to use the knowledge and the facilities and the resources that we have to really change the world literally.”

Marion Flatken’s #LightSourceSelfie

Beamline filming location: HZB ASAXS-Instrument, FCM-beamline at PTB laboratory (Physikalisch-Technische Bundesanstalt), BESSYII

A properly tailored tail boosts solar-cell efficiency

A properly tailored tail boosts solar-cell efficiency

With the help of structural insights from the Advanced Light Source (ALS), researchers optimized the fit between organic and inorganic ions in a perovskite solar-cell material.

The work increased the material’s power-conversion efficiency and stability and opens up a new avenue for improving the current-carrier dynamics of a promising class of materials.

A photovoltaic rising star

To address the effects of global climate change, it’s essential that we capitalize on energy from the sun. However, although solar energy is freely available, it needs to be converted into usable electricity in a way that’s efficient, cost-effective, and commercially scalable.

Perovskites are high-performance inorganic semiconductors recognized as some of the most promising photovoltaic materials of the future. Perovskite films—thin, lightweight, and flexible—can be produced using low-cost solution-processing techniques, and their power-conversion efficiencies (PCEs) have rapidly risen to the brink of 30% in just 15 years, surpassing conventional silicon panels.

A structure with room to tinker

The most intriguing perovskite materials today are organic–inorganic hybrids. They have the general formula ABX3, in which the inorganic B and X ions form a framework of octahedral cages, and the organic A ions are located in the spaces between the cages.

Previously, it was thought that perovskite electronic performance mainly depended on the B and X electronic orbitals, and that A merely served a structural function. In this work, researchers showed that A-site organic ions with specially designed characteristics can increase charge-carrier mobility and power conversion efficiency while also improving device stability.

Read more on the ALS website

Image: Left: The basic structure of perovskite, a promising solar-cell material, has three types of sites, A (blue), B (gray), and X (purple). Right: By attaching organic tails to the interstitial “A” sites (and testing different linker lengths), researchers improved the material’s photovoltaic response.

Perfect recipe for efficient perovskite solar cells

Perfect recipe for efficient perovskite solar cells

A long-cherished dream of materials researchers is a solar cell that converts sunlight into electrical energy as efficiently as silicon, but that can be easily and inexpensively fabricated from abundant materials. Scientists at the Helmholtz-Zentrum Berlin have now come a step closer to achieving this. They have improved a process for vertically depositing a solution made from an inexpensive perovskite solute onto a moving substrate below. Not only have they discovered the crucial role played by one of the solvents used, but they have also taken a closer look at the aging and storage properties of the solution.

Solar cells made of crystalline silicon still account for the lion’s share of roof installations and solar farms. But other technologies have long since become established as well – such as those that convert sunlight into electrical energy through use of extremely thin layers of solar-cell material deposited upon a substrate. The perovskite solar cells that Prof. Eva Unger and her team at the Helmholtz-Zentrum Berlin (HZB) are researching belong to this group. “These are the best solar cells to date that can be made using a 2D ink”, the researcher explains. “And now their efficiencies are approaching those for cells made of crystalline silicon.”

Read more on the HZB website

Image: The liquid solution of perovskite precursor, solvent, and additive flows from a slit-shaped nozzle onto the glass substrate being conveyed below.

Credit: © Jinzhao Li / HZB

Hybrid photoactive perovskites imaged with atomic resolution for the first-time

Hybrid photoactive perovskites imaged with atomic resolution for the first-time

A huge step towards better performing solar cells – a collaboration identified information previously invisible using Diamond’s ePSIC microscopes of Oxford University’s Departments of Materials and Physics

A new technique has been developed allowing reliable atomic-resolution images to be taken, for the first time, of hybrid photoactive perovskite thin films.- highly favourable materials for efficient photovoltaic and optoelectronic applications. These images have significant implications for improving the performance of solar cell materials and have unlocked the next level of ability to understand these technologically important materials. The breakthrough was achieved by a joint team from the University of Oxford and Diamond who have just released a new paper published in Science.

Using the ePSIC (the Electron Physical Science Imaging Centre) E02 microscope and the ARM200 microscope in at the Department of Materials, University of Oxford, the team developed a new technique which allowed them to image the hybrid photoactive perovskites thin films with atomic resolution. This gave them unprecedented insights into their atomic makeup and provided them with information that is invisible to every other technique.

Read more on the Diamond website

Image: An example of one of the images obtained using the new protocol, which illustrates several of the phenomena that the team has been able to describe for the first time, including a range of grain boundaries, extended planar defects, stacking faults, and local inclusions of non-perovskite material.

Scientists probe Earth’s deep mantle in the laboratory

Scientists probe Earth’s deep mantle in the laboratory

Extreme conditions experiments sharpen view of our planet’s interior

Simulating the conditions 2700 kilometres deep underground, scientists have studied an important transformation of the most abundant mineral on Earth, bridgmanite. The results from the Extreme Conditions Beamline at DESY’s X-ray light source PETRA III reveal how bridgmanite turns into a structure known as post-perovskite, a transformation that affects the dynamics of Earth’s lower mantle, including the spreading of seismic waves. The analysis can provide an explanation for a range of peculiar seismic observations, as the team headed by Sébastien Merkel from the Université de Lille in France report in the Journal Nature Communications.
Bridgmanite is a magnesian-iron mineral ((Mg,Fe)SiO3) with a crystal structure that is not stable under ambient conditions. It forms about 660 kilometres below the surface of the Earth, and microcrystalline grains found as inclusions in meteorites are the only samples ever recovered on the surface. “In order to study bridgmanite under the conditions of the lower mantle, we had to produce the mineral first,” explains Merkel. To do so, the scientists compressed tiny amounts of iron-magnesium-silicon-oxide in a diamond anvil cell (DAC), a device that can squeeze samples with high pressure between two small diamond anvils.

Image: The crystal structures of bridgmanite (left) and post-perovskite (right).

Credit: Université de Lille, Sébastien Merkel
>Read more on the PETRA III (DESY) website

Unleashing perovskites’ potential for solar cells

Unleashing perovskites’ potential for solar cells

Perovskites — a broad category of compounds that share a certain crystal structure — have attracted a great deal of attention as potential new solar-cell materials because of their low cost, flexibility, and relatively easy manufacturing process. But much remains unknown about the details of their structure and the effects of substituting different metals or other elements within the material. Now, researchers using the U.S. Department of Energy’s (DOE’s) Advanced Photon Source (APS) have been able to decipher a key aspect of the behavior of perovskites made with different formulations: With certain additives there is a kind of “sweet spot” where sufficient amounts will enhance performance and beyond which further amounts begin to degrade it. The findings were detailed in the journal Science.
Conventional solar cells made of silicon must be processed at temperatures above 1,400 degrees Celsius, using expensive equipment that limits their potential for production scale-up. In contrast, perovskites can be processed in a liquid solution at temperatures as low as 100 degrees, using inexpensive equipment. What’s more, perovskites can be deposited on a variety of substrates, including flexible plastics, enabling a variety of new uses that would be impossible with thicker, stiffer silicon wafers.

>Read more on the Advanced Photon Source (APS) website

Image: Perovskite-based solar cells are flexible, lightweight, can be produced cheaply, and could someday bring down the cost of solar energy. Shown here is the type of perovskite solar cell measured at the CNM/XSD Hard X-ray Nanoprobe at the APS.
Credit: Rob Felt

Perovskites, the rising star for energy harvesting

Perovskites, the rising star for energy harvesting

Perovskites are promising candidates for photovoltaic cells, having reached an energy harvesting of more than 20% while it took silicon three decades to reach an equivalent. Scientists from all over the world are exploring these materials at the ESRF.

Photovoltaic (PV) panels exist in our society since several years now. The photovoltaic market is currently dominated by wafer-based photovoltaics or first generation PVs, namely the traditional crystalline silicon cells, which take a 90% of the market share.

Although silicon (Si) is an abundant material and the price of Si-PV has dropped in the past years, their manufacturing require costly facilities. In addition, their fabrication typically takes place in countries that rely on carbon-intensive forms of electricity generation (high carbon footprint).

But there is room for hope. There is a third generation of PV: those based on thin-film cells. These absorb light more efficiently and they currently take 10% of the market share.

>Read more on the European Synchrotron website

Image: The CEA-CNRS team on ID01. From left to right: Peter Reiss, from CEA-Grenoble/INAC, Tobias Schulli from ID01, Tao Zhou from ID01, Asma Aicha Medjahed, Stephanie Pouget (both from CEA-Grenoble/INAC) and David Djurado, from the CNRS. 
Credits: C. Argoud.

Spin and charge frozen by strain

Spin and charge frozen by strain

In the development of next-generation microelectronics, a great deal of attention has been given to the use of epitaxy (the deposition of a crystalline overlayer on a crystalline substrate) to tailor the properties of materials to suit particular applications. Correlated electron systems provide an excellent platform for the development of new microelectronic devices due to the presence of multiple competing ground states of similar energy. In some cases, strain can drive these systems between two or more such states, resulting in phase transitions and dramatic changes in the properties of the material. Often, the specific mechanism by which strain accomplishes such a feat is unknown. This was precisely the case in lanthanum cobaltite, LaCoO3, which undergoes a strain-induced transition from paramagnet to ferromagnet, until a recent study carried out at the U.S. Department of Energy’s Advanced Photon Source (APS) revealed the intriguing microscopic phenomena at work in this system. These phenomena may play a role in spin-state and magnetic-phase transitions, regardless of stimulus, in many other correlated systems.

Lanthanum cobaltite is a perovskite, which means the structure can be thought of as made up of distorted cubes with cobalt at the cube centers, oxygen at the cube faces, and lanthanum at the cube corners. The cobalt ions have a nominal 3+ valence, meaning they lose three electrons to the neighboring oxygen ions. Bulk LaCoO3 is paramagnetic (that is, having a net magnetization only in the presence of an externally applied magnetic field) above 110 Kelvin, and non-magnetic below that temperature. In its ground state, all the electrons on a given cobalt ion are paired, meaning their magnetic spins cancel each other out. These are so-called low-spin (LS) Co3+ ions, and when all of the cobalt ions are in this form, LaCoO3 is non-magnetic.

>Read more on the Advanced Photon Source website

Image: Upper left: Resonant x-ray scattering at the cobalt K-edge. Inversion of the spectra at the reflections shown indicates the presence of charge order. Upper right: X-ray diffraction reciprocal space maps at the (002) and (003) reflection indicating the high epitaxial quality of the films. The satellite peaks result from lattice modulations associated with the reduced symmetry in the film. Lower left: Schematic crystal structure of epitaxial LaCoO3 showing the arrangement of cobalt sites with different charge and spin. The circulated charge transfer from oxygen to the different cobalt sites is also shown. Lower right: Calculated total energy as a function of the difference between the in-plane Co-O bond lengths of HS and LS cobalt ions (∆rCo-O).

Precise layer growth in a superlattice controls electron coupling and magnetism

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)