Spontaneous supercrystal discovered in switching metal-insulator

A Cornell-led research team has discovered a previously unobserved supercrystal formation in a metal-insulating material, potentially unlocking new ways to engineer materials and devices with tunable electronic properties. They demonstrated that the atomic structure in the thin-film Mott insulator Ca2RuO4, which can switch between being a metal and an insulator due to quantum effects, forms an anisotropic, organized pattern with multiple spatial periods at temperatures between 200 and 250 degrees Kelvin. Oleg Gorobtsov, postdoctoral fellow and lead author of the study, highlighted this as a prime example of complexity arising from simplicity.

“Usually, to create a supercrystal, you have to artificially engineer multiple layers of different materials on top of each other. Here is an example of how a relatively simple system demonstrates a very complex hierarchy of phase domains with different length scales.”

The discovery is the result of an analysis technique developed by the research group of Andrej Singer, associate professor of materials science and engineering and senior author of the study. In 2023, Singer and others demonstrated how a combination of high-powered X-rays, phase-retrieval algorithms and machine learning can provide a real-space visualization of materials at the nanoscale.

This technique revealed a new type of strain-induced nanopattern that spontaneously forms in Ca2RuO4 during cooling to cryogenic temperatures. By zooming out, the latest research showed that the 10-nanometer structure was embedded in a larger supercrystal.

“By leveraging the synergy between state-of-the-art synthesis and characterization of structural and electronic properties, we were able to show that the orientation of this larger structure dictates the electronic properties,” Singer said. “This is essentially a switch that can control how electricity flows through, offering potential advancements in energy-efficient electronics.”

The versatile control capabilities of Mott insulators make them ideal materials for various applications, including memory elements and optical switches. Switchable structures such as the supercrystal state in Ca2RuO4 could offer a powerful means of influencing the energy balance between competing ground states, according to Gorobtsov.

“We don’t just get the switch between conductivity and non-conductivity, but we also get this switch in the preferential direction of the current,” Gorobtsov said, “which gives us a new lever to control technologically-relevant properties, potentially not just in this material, but in others as well.”

For more information: Advanced Materials

Image: Depiction of X-ray nanodiffraction assisted by machine learning, electron microscopy, and local resistivity measurements shedding light on a novel supercrystal state forming spontaneously in a thin film Mott insulator during metal-insulator transition. Credit: Oleg Gorobtsov

New 3D reconstruction method aids analysis of property-defining defects

An international research collaboration, including a group from Cornell Engineering, has used a new X-ray-based reconstruction technique to observe topological defects in a nanoscale self-assembly-based cubic network structure of a polymer-metal composite material over a large sample volume for the first time. This technique and the new insights gained could be applied to studying other mesoscale structures with similar defects, which underpin many physical phenomena and can lead to new or enhanced material properties in both natural and synthetic self-assembled materials. Ulrich Wiesner, the Spencer T. Olin Professor in the Department of Materials Science and Engineering, emphasized that this new polymer, structure, and technique allow for unprecedented sample volumes to be reconstructed, enabling detailed examination of defect structures and their frequency.

The question regarding the importance of defects in BCP SA-generated materials has always been elusive, Wiesner said, in part because technologies necessary to measure large-enough sample volumes – with correspondingly larger defect structures – have been slow to develop.

The new technology – hard X-ray ptychography, which was conducted at the Swiss Light Source (SLS), at the Paul Scherrer Institute in Switzerland – is an advanced form of tomography that can penetrate deeper into a material than is possible with beams in electron microscopes. This technique allowed the researchers to reconstruct a very large sample volume of a BCP SA-derived polymer-metal composite material.

“If you have a smaller defect such as a line or a point defect, when you perturb the system, often you can ‘correct’ the defect structure,” Wiesner said. “In contrast, topological defects are so large, they are very stable against external perturbations.”

Once the triblock terpolymer was synthesized, researchers in the group of Ulli Steiner at the Adolphe Merkle Institute in Fribourg, Switzerland, a long-time collaborator of Wiesner, generated thin films from it and replaced one of the terpolymer blocks with gold, so the material could withstand repeated exposure to the intense coherent X-ray beams at SLS.

Imaging and image reconstruction at the SLS finally revealed a co-continuous network known as a single-diamond structure, with topological defects that the researchers expect would have substantial effects on mechanical and other properties. Importantly, the defects most closely resemble topological textures found in nematic liquid crystals and in Hydra single-celled organisms, suggesting that self-assembly can be used as a model process to investigate the role of topology in nature.

Wiesner said this collaborative research could pave the way for future studies in an area that his lab has already explored: block copolymer-directed superconductors.

“You would expect that your macroscopic, electronic or transport properties of the superconductor will depend on defects in your materials,” he said. “That’s what I’m really excited about: Now we have a technique that allows us to visualize larger volumes of these materials and to generate defect structure – property correlations.”

For more information: Nature Nanotechnology

Image: Pictured is a two-dimensional, cut-through reconstructed sample volume showing three adjacent crystal grains (red, blue, green) separated by a 100-nanometer-thick grain boundary (yellow) together with the positions of two topological defects (plus and minus signs).

Researchers develop first voxel building blocks for 3D-printed organs

A research team at the University of Virginia School of Engineering and Applied Science, led by Assistant Professor Liheng Cai and his Ph.D. student Jinchang Zhu, has developed biomaterials with controlled mechanical properties that match various human tissues, potentially serving as the first building blocks for human-compatible organs printed on demand. Zhu highlighted this advancement as a significant leap compared to existing bioprinting technologies.

Their unique bioprinting method is called digital assembly of spherical particles. The DASP technique deposits particles of biomaterial in a supporting matrix, both of which are water-based, to build 3D structures that provide a suitable environment for the cells to grow. The assembly process is how “voxels,” the 3D version of pixels, construct 3D objects.

“Our new hydrogel particles represent the first functional voxel we have ever made,” Zhu said. “With precise control over mechanical properties, this voxel may serve as one of the basic building blocks for our future printing constructs.

“For example, with this level of control, we could print organoids, which are 3D cell-based models that function as human tissue, to study disease progression in the search for cures.”

The particles are polymer hydrogels engineered to mimic human tissue by tweaking the arrangement and chemical bonds of single-molecule monomers, which link together in chains to form networks.

Encapsuled within the particles are actual human cells.

Compared to other hydrogel bio-inks, Cai and Zhu’s are less toxic and more biocompatible for cells, they said. Their “double network” hydrogels—formed from two intertwined molecular networks—are mechanically strong, but highly tunable for mimicking the physical characteristics of human tissue.

Cai and Zhu first described their DASP technology in 2021. That work proved the concept of using biomaterial voxels as building blocks and, through lab experiments, demonstrated a DASP-printed material that functioned like a pancreas with glucose-stimulated insulin release.

But DASP 1.0 could only print brittle hydrogels with limited tunability. In their latest paper in Nature Communications, Cai and Zhu present DASP 2.0, which introduces the double-network hydrogel bio-inks formed using a “click chemistry” to rapidly cross-link, or bond, the molecular structures.

Part of what enabled this advancement was improvements to the team’s bioprinter. They designed a multichannel nozzle to mix the hydrogel components on demand. Premixing isn’t possible because the cross-linking occurs so fast, going from liquid droplets to an elastic water-swollen gel within 60 seconds.

In previous studies, the team determined that drop formation and rapid detachment from the nozzle are essential to mimic the mechanical properties—such as elasticity or stiffness—of the target human tissue.

DASP achieves this by depositing large droplets from a narrow and fast-moving nozzle into the matrix, immediately suspending them.

“We’ve now laid the foundation for voxelated bioprinting,” Cai said. “When fully realized, DASP’s applications will include artificial organ transplant, disease and tissue modeling, and screening candidates for new drugs. And it probably won’t stop there.”

For more information: Nature Communications

Image: Reminiscent of a raspberry, this voxelated hollow sphere made of a single layer of droplets was generated using digital assembly of spherical particles, or DASP, a 3D bioprinting process developed in assistant professor of materials science and engineering Liheng Cai’s lab. (Soft Biomatter Lab, UVA Engineering). Credit: University of Virginia School of Engineering and Applied Science/Liheng Cai

Spray Tips: Vibratory powder-feed devices

In addition to the powder-feeder devices most commonly used in the thermal spray industry, gravity-based devices, rotating wheel devices, and fluidized-bed systems, other systems such as vibratory powder feed devices are currently in limited use or have been used in the past.

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Recent advances in twisted bilayer graphene

Graphene, a widely studied 2D material, exhibits exceptional physical and electronic properties. Twisting two graphene monolayers at a small angle creates twisted bilayer graphene (tBLG), which forms a superlattice. The moiré pattern resulting from relative layer orientation has driven significant progress in graphene research, enhancing its optical and electrical properties, including superconductivity.

The mechanical, optical, and electronic properties of multilayer graphene structures can be tailored by altering the stacking order, interlayer spacing, and relative twisting angle (θ). Similarly, tBLG is fabricated by stacking two single-layer graphene sheets, synthesized by chemical vapor deposition (CVD), at a specific twisting angle.

The unusual stacking imparts various angle-dependent properties to tBLG.

Moiré patterns are generated by graphene-graphene interactions resulting from the relative layer orientations. These highly periodic patterns are responsible for the extraordinary optical and electronic properties of tBLG. Additionally, tBLG exhibits twisting angle-dependent Dirac spectra (similar to chirality dependence in carbon nanotubes), Fermi velocity, magnetoresistance oscillations, and quantum Hall effect.

Innovative methods are being explored to fabricate tBLG with small twist angles. For example, hexagonal boron nitride is utilized to obtain graphene layers with rotationally aligned crystal axes. tBLG is also prepared by cutting, rotating, and stacking a graphene layer through femtosecond laser micromachining and precise transfer.

Other methods for preparing tBLG films include controlled hydrophilic and hydrophobic boundary folding of single-layer graphene and vertical stacking.

Easy twisting and stacking of two graphene layers can result in a uniform and ordered moiré superlattice capable of exhibiting unusual superconductivity and correlations in tBLG. However, the twist angle becomes rigid after interlayer stacking. Alternatively, mechanical elastic strain can help control the electronic structure of tBLG by regulating the lattice spacing and symmetry.

A recent study reviewed various innovations in straining tBLG by in-plane and out-of-plane modes. It included the characterizations and calculations performed to quantitatively tune the strain-engineered electronic structures.

Another study demonstrated a topological superconducting state in tBLG depending only on the moiré minibands instead of the twist angle tuning. The method involved subjecting tBLG to induced Rashba spin-orbit coupling, s-wave superconductivity, and exchange field and is valid for 1.3 to 3 degrees twist angles. This approach could be feasible for developing a tBLG-based quantum computer.

Despite several research advances and potential applications, significant challenges remain in achieving controlled twisting of two graphene layers to fabricate and characterize tBLG. Additionally, precise stacking of bilayer graphene at the first magic angle of 1.1 degrees to observe superconductivity is tedious due to the intrinsic disruptions caused by strain and angular disorder.

Modifications in the twist angle significantly alter the spatial wavefunction distribution. While this allows engineering bandgaps in tBLG for electronic applications, the localized wavefunction at certain twist angles leads to a sudden reduction in carrier mobility. This can negatively influence the tBLG-based device performance. Hence, careful considerations are required to obtain preferred electronic characteristics and carrier mobility for ideal device functioning.

The ultrathin nature of tBLG leads to localized stretching or compression due to substrate deformation or thermal stresses during fabrication. Despite the positive impact of these localized strains on the physical properties of the material, their non-uniformity hinders practical device applications of the material.

The constant improvements in the synthesis, characterization, and electronic structure determination techniques are anticipated to accelerate the tBLG-related advances and applications. This, in turn, may lead to the advancement of “twistronics,” the fusion of “twist” and “electronics,” which exploits the electronic properties of layered materials like graphene changing with the twist angle.

The strain engineering of twisted 2D materials like tBLG can further expand “twistronics” and “straintronics” into the realm of “strain-twistronics.” Consequently, novel ferroelectric and optoelectronic devices with tunable characteristics and regulated performance may become a reality in the future.

For more information: AZO Nano

Nanosized blocks spontaneously assemble in water to create tiny floating checkerboards

The scientific community has successfully engineered nanosized cubes that exhibit the remarkable ability to spontaneously form a two-dimensional checkerboard pattern when placed on the surface of water. This innovative research introduces a simple and efficient approach to creating complex nanostructures through the application of a technique known as self-assembly.

“It’s a cool way to get materials to build themselves,” said study co-senior author Andrea Tao, a professor in the Aiiso Yufeng Li Family Department of Chemical and Nano Engineering at the University of California San Diego. “You don’t have to go into a nanofabrication lab and do all these complex and precise manipulations.”

Each nanocube is composed of a silver crystal with a mixture of hydrophobic (oily) and hydrophilic (water-loving) molecules attached to the surface. When a suspension of these nanocubes is introduced to a water surface, they arrange themselves such that they touch at their corner edges. This arrangement creates an alternating pattern of solid cubes and empty spaces, resulting in a checkerboard pattern.

The self-assembly process is driven by the surface chemistry of the nanocubes. A high density of hydrophobic molecules on the surface brings the cubes together to minimize their interaction with water. Meanwhile, the long chains of hydrophilic molecules cause enough repulsion to create voids between the cubes, creating the checkerboard pattern.

To fabricate the structure, researchers applied drops of the nanocube suspension onto a petri dish containing water. The resulting checkerboard can be easily transferred to a substrate by dipping the substrate into the water and slowly withdrawing it, allowing the nanostructure to coat it.

This study stems from a collaborative effort between multiple research groups that are part of the UC San Diego Materials Research Science and Engineering Center (MRSEC). The work featured a synergistic combination of computational and experimental techniques. “We’ve built a continuous feedback loop between our computations and experiments,” said Tao. “We used computer simulations to help us design the materials at the nanoscale and predict how they will behave. We also used our experimental results in the lab to validate the simulations, fine tune them and build a better model.”

In designing the material, researchers chose silver crystal nanocubes due to the Tao lab’s expertise in their synthesis. Determining the optimal surface chemistry required extensive computational experimentation, which was led by Gaurav Arya, a professor in the Department of Mechanical Engineering and Materials Science at Duke University and co-senior author of the study.

The simulations identified the best molecules to attach to the nanocubes and predicted how the cubes would interact and assemble on the water surface. The simulations were iteratively refined using experimental data obtained by Tao’s lab. Electron microscopy performed by the lab of study co-author Alex Frañó, a professor in the Department of Physics at UC San Diego, confirmed the formation of the desired checkerboard structures.

Tao envisions applications for the nanocube checkerboard in optical sensing. “Such a nanostructure can manipulate light in interesting ways,” she explained. “The spaces between the cubes, particularly near the corner edges where the cubes connect, can act as tiny hotspots that focus or trap light. That could be useful for making new types of optical elements like nanoscale filters or waveguides.”

For more information: Nature Communications

Image: SEM image of the mesophase constructed with edge–edge connected checkerboard lattice obtained with Ag NCs post-synthetically modified with a feedstock mixture of 50 µM PEG20k-SH and 6 µM C16-SH. Scale bar = 500 nm, inset = 100 nm.

Team develops predictive tool for designing complex metal alloys that can withstand extreme environments

Many cooks appreciate the durability, rust resistance, and even heating abilities of stainless steel. However, not everyone knows the secret behind its popularity. Stainless steel contains chromium, which reacts with oxygen in the air to form a thin, protective coating that keeps the steel underneath safe.

Nowadays, scientists and engineers are developing alloys that can withstand extreme conditions for use in nuclear fusion reactors, hypersonic flights, and high-temperature jet engines. They are experimenting with combinations of multiple metals in equal proportions, known as multi-principal element alloys or medium- to high-entropy alloys, to achieve specific design goals such as strength, toughness, and corrosion resistance.

Specifically, researchers seek alloys resistant to corrosion that can happen when metals react with oxygen in the atmosphere, a process called oxidation. These alloys are typically tested in a “cook-and-look” procedure where alloy materials are exposed to high-temperature oxidation environments to see how they respond.

But now, a multidisciplinary research team led by scientists at the Department of Energy’s Pacific Northwest National Laboratory and North Carolina State University combined atomic-scale experiments with theory to create a tool to predict how such high-entropy alloys will behave under high-temperature oxidative environments.

“We are working toward developing an atomic-scale model for material degradation of these complex alloys, which then can be applied to design next-generation alloys with superior resistance to extreme environments for a wide variety of applications such as the aerospace and nuclear power industries,” said Arun Devaraj, co-principal investigator of the study and a PNNL materials scientist specializing in understanding metal degradation in extreme environments.

“The goal here is to find ways to rapidly identify medium- to high-entropy alloys with the desired properties and oxidation resistance for your chosen application.”

For their recent experiments, the research team studied the degradation of a high-entropy alloy with equal amounts of the metals cobalt, chromium, iron, nickel and manganese (CoCrFeNiMn, also called the Cantor alloy). The research team examined oxide formed on the Cantor alloy using a variety of advanced atomic-scale methods to understand how each element arranges itself in the alloy and the oxide.

They discovered that chromium and manganese tend to migrate quickly toward the surface and form stable chromium and manganese oxides. Subsequently, iron and cobalt diffuse through these oxides to form additional layers.

By adding a small amount of aluminum, they discovered that aluminum oxide can act as a barrier for other elements migrating to form the oxide, thereby reducing the overall oxidation of the aluminum-containing Cantor alloy and increasing its resistance to degradation at high temperatures.

“This work sheds light on the mechanisms of oxidation in complex alloys at the atomic scale,” said Bharat Gwalani, co-corresponding author of the study. Gwalani began the study while a scientist at PNNL and continued the research in his current role as an assistant professor of materials science and engineering at North Carolina State University. He added, “by understanding the fundamental mechanisms involved, this work gives us a deeper understanding of oxidation across all complex alloys.”

“Right now there are no universally applicable governing models to extrapolate how a given complex, multi-principal element alloy will oxidize and degrade over time in a high-temperature oxidation environment,” said Devaraj. “This is a substantial step in that direction.”

The team’s careful analysis revealed some universal rules that can predict how the oxidation process will proceed in these complex alloys. Computational colleagues from NCSU developed a model called the Preferential Interactivity Parameter for early prediction of oxidation behavior in complex metal alloys.

Ultimately, the research team expects to expand this research to develop complex alloys with exceptional high-temperature properties, and to do so very quickly by rapid sampling and analysis. The ultimate goal is to choose a combination of elements that favor the formation of an adherent oxide, said Devaraj. “You know oxide formation will happen, but you want to have a very stable oxide that will be protective, that would not change over time, and would withstand extreme heat inside a rocket engine or nuclear reactors.”

A next step will be to introduce automated experimentation and integrate additive manufacturing methods, along with advanced artificial intelligence, to rapidly evaluate promising new alloys. That project is now getting underway at PNNL as a part of the Adaptive Tunability for Synthesis and Control via the Autonomous Learning on Edge (AT SCALE) Initiative.

“That kind of discovery loop for materials discovery will be very relevant for further expanding our knowledge of these novel alloys,” said Devaraj, who also has a joint faculty appointment at the Colorado School of Mines.

For more information: Nature Communications

Image: Researchers have developed a tool to predict how new high-entropy alloys will behave under high-temperature oxidative environments. Development of new alloys is important for the aerospace and nuclear power industries. Credit: Nathan Johnson | Pacific Northwest National Laboratory

Spray Tips: Fluidized-bed systems

Other commonly used gravity-based powder-feeding systems for feedstock delivery are fluidized-bed systems, where the powder feedstock is suspended by an upward flow of gas, creating a fluid-like state that allows for consistent and controlled delivery of the powder to the process.

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NSL Analytical Services relocates Metallurgical Testing Laboratory

NSL Analytical Services, Cleveland, Ohio, an independent analytical and metallurgical testing services company serving U.S. and global customers, announces a significant milestone in its growth trajectory with the relocation of one of its two Cleveland-area testing laboratories and the addition of high-temperature stress rupture testing to its offerings.

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Unlocking the cause of pitting corrosion in 3D-printed stainless steel

Scientists from Lawrence Livermore National Laboratory (LLNL), Livermore, Calif., delved into the mysterious world of pitting corrosion in additively manufactured (3D-printed) stainless steel 316L in seawater. Stainless steel 316L is a popular choice for marine applications due to its excellent combination of mechanical strength and corrosion resistance. This holds even more true after 3D printing, but even this resilient material isn’t immune to the scourge of pitting corrosion. The LLNL team used transmission electron microscopy and x-ray photoelectron spectroscopy to do a deep-dive microscopy study to figure out what could potentially be responsible for corrosion.

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‘Surprising’ hidden activity of semiconductor material spotted by researchers

New research suggests that materials commonly overlooked in computer chip design actually play an important role in information processing, a discovery which could lead to faster and more efficient electronics.

Using advanced imaging techniques, an international team led by Penn State researchers found that the material that a semiconductor chip device is built on, called the substrate, responds to changes in electricity much like the semiconductor on top of it.

The researchers worked with the semiconductor material, vanadium dioxide, which they said shows great potential as an electronic switch. They also studied how vanadium dioxide interacts with the substrate material titanium dioxide and said they were surprised to discover that there seems to be an active layer in the substrate that behaves similarly to the semiconductor material on top of it when the semiconductor switches between an insulator — not letting electricity flow — and a metal — letting electricity flow. The revelation that substrates can play an active role in semiconductor processes is significant for designing future materials and devices, said study lead Venkatraman Gopalan, professor of materials science and engineering and of physics at Penn State.

The potential of vanadium dioxide as a metal-to-insulator transistor is well-documented and the material is considered promising for semiconductor technology due to its low energy consumption, Gopalan said. However, the material’s properties are still not fully understood, and until now, it has usually been observed in isolation rather than while functioning in a real device.

Vanadium dioxide has strongly correlated electronic effects, meaning the repulsion between electrons interferes with the device, so cannot be ignored as is currently done in silicon-based electronics. This characteristic can result in materials with novel functionalities such as high-temperature superconductivity and enhanced magnetic properties.

The team investigated vanadium dioxide in a device rather than in isolation, applying a voltage to it to make it switch from an insulating to a conducting state. They used the Advanced Photon Source (APS) at Argonne National Laboratory, which uses powerful X-ray beams to study the behavior and structure of materials on the atomic level. When mapping the spatial and temporal response of the material to the switching event, the researchers observed unexpected changes to the structure of the material and substrate.

“What we found was that as the vanadium dioxide film changes to a metal, the whole film channel bulges, which is very surprising,” Gopalan said. “Normally it is supposed to shrink. So clearly something else was going on in the film geometry that was missed before.”

The APS X-ray penetrated through the vanadium dioxide film and into the titanium dioxide (TiO2) substrate — which is normally considered an electrically and mechanically passive material — that the thin film was grown on.

To understand these findings, the theory and simulation effort — led by Long-Qing Chen, Hamer Professor of Materials Science and Engineering, professor of engineering science and mechanics and of mathematics at Penn State — developed a theoretical framework to explain the entire process of the film and the substrate bulging instead of shrinking. When their model incorporated naturally occurring missing oxygen atoms in this material of two types, charged and uncharged, the experimental results could be satisfactorily explained.

Gopalan credited the multidisciplinary team’s combined expertise in material growth, synthesis, structure analysis and synchrotron beamline operation with the new understanding. Using a collaborative approach led by Greg Stone, a physical scientist with the U.S. Army and the lead experimental author, and Yin Chi, postdoctoral scholar at Penn State and the lead theory author, the researchers disentangled the material’s responses and observed them individually using phase field simulations, a simulation that helps scientists understand material changes over time by depicting various states of matter in a virtual setting.

The responses themselves require further investigation, researchers said, but they believe that understanding them will assist in identifying previously unknown capabilities of vanadium dioxide, including potential yet-to-be discovered phenomena in the TiO2 substrate that was considered passive before this study. The study itself unfolded over 10 years, Gopalan noted, including validating the results.

“This is what it takes to go from interesting science to a working device you can hold in the palm of your hand,” Gopalan said. “Experiments and theory are complex and require large-scale collaborative teams working closely together over an extended period of time to solve difficult problems that could have a large impact. We hope and expect that this will accelerate the progress towards a new generation of electronic devices.”

For more information: AdvancedMaterials

Image: Venkatraman Gopalan, professor of materials science and engineering and of physics, in his optical lab

Bio-inspired materials’ potential for efficient mass transfer boosted by a new twist on a century-old theory

Did you know leaves could be the key to unlocking new energy storage and catalysis improvements? The key lies within their natural vein structure and the century-old biophysical law, Murray’s Law.

An international team of researchers, led by the NanoEngineering Group at the Cambridge Graphene Centre, has developed a new materials theory based on ‘Murray’s Law’, applicable to a wide range of next-generation functional materials, with applications in everything from rechargeable batteries to high-performance gas sensors.

Murray’s Law, put forward by Cecil D. Murray in 1926, describes how natural vascular structures, such as animal blood vessels and veins in plant leaves, efficiently transport fluids with minimum energy expenditure.

“But whereas this traditional theory works for cylindrical pore structures, it often struggles for synthetic networks with diverse shapes — a bit like trying to fit a square peg into a round hole,” says first author Cambridge PhD student Binghan Zhou.

Dubbed ‘Universal Murray’s Law’, the researchers’ new theory bridges the gap between biological vessels and artificial materials and is expected to benefit energy and environmental applications.

“The original Murray’s Law was formulated by minimising the energy consumption to maintain the laminar flow in blood vessels, but it was unsuited for synthetic materials,” says Binghan Zhou.

“To broaden its applicability to synthetic materials, we expanded this Law by considering the flow resistance in hierarchical channels. Our proposed Universal Murray’s Law works for the pores of any shape and suits all common transfer types, including laminar flow, diffusion, and ionic migration.”

Ranging from daily usage to industrial production, many applications involve ion or mass transfer processes through highly porous materials — applications that could benefit from Universal Murray’s Law, say the researchers.

For instance, when charging or discharging batteries, ions physically move between the electrodes through a porous barrier. Gas sensors rely on the diffusion of gas molecules through porous materials. Chemical industries often use catalytic reactions, involving laminar flow of reactants through catalysts.

“Employing this new biophysical law could greatly reduce the flow resistance in the above processes, boosting overall efficiency,” adds Binghan Zhou.

The researchers proved their theory using graphene aerogel, a material known for its extraordinary porosity. They carefully varied the pore sizes and shapes by controlling the growth of ice crystals within the material. Their experiments showed that the microscopic channels following the newly proposed Universal Murray’s Law offer minimum resistance against fluid flow, while deviations from this Law increase the flow resistance.

“We designed a scaled-down hierarchical model for numerical simulation and found that simple shape changes following the proposed Law indeed reduce the flow resistance,” says co-author Dongfang Liang, Professor of Hydrodynamics at the Department of Engineering.

The team also demonstrated the practical value of Universal Murray’s Law by optimising a porous gas sensor. The sensor, designed in accordance with the Law, shows a significantly faster response compared to sensors following a porous hierarchy, traditionally considered to be highly efficient.

“The only difference between the two structures is a slight variation in shape, showing the power and ease of application of our proposed Law,” says Binghan Zhou.

“We have incorporated this special natural Law into synthetic materials,” adds Tawfique Hasan, Professor of Nanoengineering at the Cambridge Graphene Centre, who led the research. “This could be an important step towards theory-guided structural design of functional porous materials. We hope our work will be important for new generation porous materials and contribute to applications for a sustainable future.”

For more information: University of Cambridge

Scientists make the first single-atom-thick sheet of gold

The discovery of graphene created great interest in two-dimensional (2D) materials, but it is tricky to synthesize 2D materials comprised solely of metals. The synthesis of monolayer gold has so far been limited to free-standing several-atoms-thick layers, or monolayers confined on or inside templates.

Gold nanoparticles are of interest due to their application in electronics, catalysis, photonics, sensing and biomedicine.

For the first time, scientists from Linköping University in Sweden have successfully developed gold sheets that are only a single atom layer thick. Dubbed “goldene,” this groundbreaking material exhibits new properties that could transform several technological applications, from environmental catalysis to advanced electronics.

The creation of goldene marks a significant achievement in materials science. Typically, gold atoms naturally tend to clump together, making the formation of such thin layers challenging. Inspired by ancient techniques, the researchers applied a centuries-old Japanese method, refining it to suit modern scientific requirements.

In the process, the scientists have completely revamped some of the properties of gold. For instance, gold is a great electrical conductor, which is used in a lot of electronic hardware. Your phone and computer have gold in it for this exact reason.

But when reduced to a single sheet only one atom thick, gold (or goldene) becomes a semiconductor. Just as graphene exhibits extraordinary properties at a single-layer thickness, so does this 2D material.

“Since the discovery of graphene, 2D materials have gained interest for their extraordinary properties. Diverse 2D materials comprising non-metallic elements or covalently bonded blends have been investigated. However, the synthesis of 2D materials solely comprising metals is challenging,” said the researchers.

“Goldene is one of few elemental 2D materials comprising metals, which are produced via scalable methods. Metals, especially noble metals such as gold due to their plasmonic properties, are used in a wide range of applications such as chemical, biological, pharmaceutical, and electrical applications. Thus, goldene would find unique applications different from those of other 2D materials,” they added.

The journey to goldene began unexpectedly and had many twists and turns. The researchers were working with a special conductive ceramic called titanium silicon carbide in which silicon is embedded in very thin layers. They added some gold to the bulk material at high temperature to make it more conductive. But, to everyone’s surprise, they observed that atomic layers of gold replaced silicon within the ceramic matrix, leading to the formation of titanium gold carbide — a precursor to goldene.

This process was painstaking. It involved tweaking different concentrations of Murakami’s reagent (an alkaline potassium ferricyanide solution) over different periods. Still, even this wasn’t enough. After much trial and error, the researchers found a sweet spot for the reagent’s dilution. But they also found the secret sauce: performing the etching under complete darkness. Light triggers the formation of cyanide ions from the reagent, which attack the gold. Finally, the researchers added surfactants to preserve the resulting golden sheets from curling and coalescence.

Potassium ferricyanide can be toxic, which is why it must be handled with extreme care. However, the researchers note that their method involves diluting this reagent to less than one percent in the solution, minimizing its impact. Moreover, the cyanide ions remain confined in the reagent’s molecules and not released.

Using an electron microscope, the researchers confirmed goldene’s unique structure, characterized by having two free bonds in its two-dimensional form, thus enhancing its chemical reactivity. This makes it ideal as a catalyst for applications in carbon dioxide conversion, hydrogen production, and the creation of value-added chemicals.

Beyond the lab, the implications of goldene are vast. The material’s efficiency in catalytic applications means that less gold is needed for processes that currently depend on larger quantities of the metal. Goldene also exhibits semiconductor properties, unlike its bulk form. So, it opens up new uses for gold in technologies where traditional metals are not feasible.

For more information: Nature Synthesis

“Nanostitches” enable lighter and tougher composite materials

Interest has been growing in the development of nanostructured hybrid composite materials, where nanoparticles such as carbon nanotubes (CNTs) are used alongside microscale-fiber composite laminates.

Composite materials have one main vulnerability: the space between layers, which is typically filled with polymer “glue” to bond the layers together. In the event of an impact or strike, cracks can easily spread between layers and weaken the material, even though there may be no visible damage to the layers themselves. Over time, as these hidden cracks spread between layers, the composite could suddenly crumble without warning.

Now, MIT engineers have shown they can prevent cracks from spreading between composite’s layers, using an approach they developed called “nanostitching,” in which they deposit chemically grown microscopic forests of carbon nanotubes between composite layers. The tiny, densely packed fibers grip and hold the layers together, like ultrastrong Velcro, preventing the layers from peeling or shearing apart.

In experiments with an advanced composite known as thin-ply carbon fiber laminate, the team demonstrated that layers bonded with nanostitching improved the material’s resistance to cracks by up to 60 percent, compared with composites with conventional polymers. The researchers say the results help to address the main vulnerability in advanced composites.

“Just like phyllo dough flakes apart, composite layers can peel apart because this interlaminar region is the Achilles’ heel of composites,” says Brian Wardle, professor of aeronautics and astronautics at MIT. “We’re showing that nanostitching makes this normally weak region so strong and tough that a crack will not grow there. So, we could expect the next generation of aircraft to have composites held together with this nano-Velcro, to make aircraft safer and have greater longevity.”

At MIT, Wardle is director of the necstlab (pronounced “next lab”), where he and his group first developed the concept for nanostitching. The approach involves “growing” a forest of vertically aligned carbon nanotubes — hollow fibers of carbon, each so small that tens of billions of the the nanotubes can stand in an area smaller than a fingernail. To grow the nanotubes, the team used a process of chemical vapor deposition to react various catalysts in an oven, causing carbon to settle onto a surface as tiny, hair-like supports. The supports are eventually removed, leaving behind a densely packed forest of microscopic, vertical rolls of carbon.

The lab has previously shown that the nanotube forests can be grown and adhered to layers of composite material, and that this fiber-reinforced compound improves the material’s overall strength. The researchers had also seen some signs that the fibers can improve a composite’s resistance to cracks between layers.

In their new study, the engineers took a more in-depth look at the between-layer region in composites to test and quantify how nanostitching would improve the region’s resistance to cracks. In particular, the study focused on an advanced composite material known as thin-ply carbon fiber laminates.

The study’s experiments were led by Carolina Furtado, who joined the effort as part of the MIT-Portugal program in 2016, continued the project as a postdoc, and is now a professor at the University of Porto in Portugal, where her research focuses on modeling cracks and damage in advanced composites.

In her tests, Furtado used the group’s techniques of chemical vapor deposition to grow densely packed forests of vertically aligned carbon nanotubes. She also fabricated samples of thin-ply carbon fiber laminates. The resulting advanced composite was about 3 millimeters thick and comprised 60 layers, each made from stiff, horizontal fibers embedded in a polymer sheet.

She transferred and adhered the nanotube forest in between the two middle layers of the composite, then cooked the material in an autoclave to cure. To test crack resistance, the researchers placed a crack on the edge of the composite, right at the start of the region between the two middle layers.

“In fracture testing, we always start with a crack because we want to test whether and how far the crack will spread,” Furtado explains.

The researchers then placed samples of the nanotube-reinforced composite in an experimental setup to test their resilience to “delamination,” or the potential for layers to separate.

“There’s lots of ways you can get precursors to delamination, such as from impacts, like tool drop, bird strike, runway kickup in aircraft, and there could be almost no visible damage, but internally it has a delamination,” Wardle says. “Just like a human, if you’ve got a hairline fracture in a bone, it’s not good. Just because you can’t see it doesn’t mean it’s not impacting you. And damage in composites is hard to inspect.”

To examine nanostitching’s potential to prevent delamination, the team placed their samples in a setup to test three delamination modes, in which a crack could spread through the between-layer region and peel the layers apart or cause them to slide against each other, or do a combination of both. All three of these modes are the most common ways in which conventional composites can internally flake and crumble.

The tests, in which the researchers precisely measured the force required to peel or shear the composite’s layers, revealed that the nanostitched held fast, and the initial crack that the researchers made was unable to spread further between the layers. The nanostitched samples were up to 62 percent tougher and more resistant to cracks, compared with the same advanced composite material that was held together with conventional polymers.

“This is a new composite technology, turbocharged by our nanotubes,” Wardle says.

For more information: ACS Applied Materials and Interfaces