Paradigm
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NT/ New route to build materials out of tiny particles

Nanotechnology & nanomaterials biweekly vol.25, 3rd June — 17th June

TL;DR

  • Researcher Laura Rossi and her group at TU Delft have found a new way to build synthetic materials out of tiny glass particles — so-called colloids. Together with their colleagues from Queen’s University and the University of Amsterdam, they showed that they can simply use the shape of these colloids to make interesting building blocks for new materials, regardless of other properties of the colloidal particles.
  • In new research from the U.S. Department of Energy’s (DOE) Argonne National Laboratory, researchers have created extremely thin membranes of pure diamond. In a few locations in the crystal structure of the membrane, however, the team substituted carbon atoms with other atoms, notably nitrogen.
  • Researchers have now designed a micro-sized artificial cilial system using platinum-based components that can control the movement of fluids at such a scale. The technology could someday enable low-cost, portable diagnostic devices for testing blood samples, manipulating cells, or assisting in microfabrication processes.
  • Scientists have reported a nano-sized neuromorphic memory device that emulates neurons and synapses simultaneously in a unit cell, another step toward completing the goal of neuromorphic computing designed to rigorously mimic the human brain with semiconductor devices.
  • A new type of carbon allotrope, holey graphyne, has semiconductor properties and is applicable in various fields such as photoelectronics, sensors, and water purification.
  • Researchers determined exactly what happens as crystal grains in metals form during an extreme deformation process, at the tiniest scales, down to a few nanometers across. The findings could lead to better, more consistent properties in metals, such as hardness and toughness.
  • Scientists have built a mirror out of one of the strongest materials on the planet: diamond. By etching nanostructures onto the surface of a thin sheet of diamond, the research team built a highly reflective mirror that withstood, without damage, experiments with a 10-kilowatt Navy laser. In the future, the researchers envision these mirrors being used for defense applications, semiconductor manufacturing, industrial manufacturing, and deep-space communications.
  • Magnetic nanostructures are promising tools for medical applications. Incorporated into biological structures, they can be steered via external magnetic fields inside the body to release drugs or to destroy cancer cells. However, until now, only average information on the magnetic properties of those nanoparticles could be obtained, thus limiting their successful implementations in therapies. Now a team has conceived and tested a new method to assess the characteristic parameters of every single magnetic nanoparticle.
  • A research team has demonstrated an ultrathin silicon nanowire that conducts heat 150% more efficiently than conventional materials used in advanced chip technologies. The device could enable smaller, faster, energy-efficient microelectronics.
  • Summertime is almost here, a time when many people try to beat the heat. But running air conditioners constantly can be expensive and wasteful. Now, researchers have designed a lightweight foam made from wood-based cellulose nanocrystals that reflects sunlight, emits absorbed heat, and is thermally insulating. They suggest that the material could reduce buildings’ cooling energy needs by more than a third.
  • And more!

Nanotech Market

Nanotechnology deals with the ability to see, understand, measure, predict, produce or control matter at the nanoscale (below 100 nanometers). The realm of nanotechnology lies between 0.1 and 100 nanometers, wherein a nanometer is defined as one-thousandth of a micron. As a versatile technology with widespread applications in a wide range of end-use sectors, nanotechnology is currently facing a mixed bag of challenges and opportunities as the COVID-19 pandemic continues to spread across the globe. With the world fighting its biggest public health crisis in history, nanotechnology healthcare applications are storming into the spotlight led by the focus on nano intervention in terms of designing effective ways to identify, diagnose, treat and eliminate the spread of COVID-19 infections. Their role as nanocarriers has the potential to design risk-free and effective immunization strategies. In the post-COVID-19 period, the use of nanotechnology solutions in the production of a multitude of devices & products will continue to grow.

Amid the COVID-19 crisis, the global market for Nanotechnology estimated at US$42.2 Billion in the year 2020, is projected to reach a revised size of US$70.7 Billion by 2026, growing at a CAGR of 9.2% over the analysis period. Nanocomposites, one of the segments analyzed in the report, is projected to record a 8.7% CAGR and reach US$35.4 Billion by the end of the analysis period. After a thorough analysis of the business implications of the pandemic and its induced economic crisis, growth in the Nanomaterials segment is readjusted to a revised 10.1% CAGR for the next 7-year period.

Latest News & Research

Shape and interaction decoupling for colloidal preassembly

by Lucia Baldauf et al in Science Advances

Researcher Laura Rossi and her group at TU Delft have found a new way to build synthetic materials out of tiny glass particles — so-called colloids. Together with their colleagues from Queen’s University and the University of Amsterdam, they showed that they can simply use the shape of these colloids to make interesting building blocks for new materials, regardless of other properties of the colloidal particles. “This is striking, because it opens up a completely new way to think about materials design,” Rossi says.

Colloids are tiny particles, a few nanometers to a few microns in size. They consist of a collection of molecules and can have different properties depending on the material they are made of.

“Under certain circumstances colloids can behave like atoms and molecules, but their interactions are less strong,” Rossi explains. “That makes them promising building blocks for new materials, for example for interactive materials that can adapt their properties to their environment.”

If left alone, the cube-shaped colloids from this study, which are made from glass, will assemble themselves into simple structures like distorted cubic and hexagonal lattices. But instead of going immediately from the building block to the final structure, the researchers took small groups of colloids and combined them into bigger building blocks. When they assembled these clusters of colloids, they ended up with a different final structure with different material properties than the self-assembled structure.

“From a chemistry point of view, we always focus on how we can produce a certain type of colloid,” Rossi says. “In this study, we’ve shifted our focus to: how can we use the colloids that are already available to make interesting building blocks?”

According to Rossi and her collaborator Greg van Anders, one of the ultimate goal of their research community is to design complex colloidal structures on demand.

“What we found here is very important, because for possible applications, we need to have procedures that can be scaled up which is something that will be hard to achieve with most currently available approaches. The basic ability to pre-assemble identical pieces from different building blocks, and have them make the same structure, or to take the same building block and pre-assemble different pieces that make different structures, are really the basic ‘chess moves’ for engineering complex structures,” adds van Anders.

Although Rossi studies the fundamental aspects rather than the application of materials design, she can envision eventual applications for this specific work:

“We found that the density of the structure that we prepared was much lower than the density of the structure you would obtain by using the starting building blocks. So you can think about strong but lightweight materials for transportation.”

After Rossi’s team built clusters of colloids in the lab, they relied on the team of Greg van Anders from Queen’s University to build the final structure out of pre-assembled clusters with a computer simulation.

“With these kinds of projects, it’s great to be able to team up with others who can run simulations, not only to understand what’s happening in depth, but also to test how big the chance of a successful lab experiment will be,” Rossi explains. “And in this case, we got very convincing results that we were understanding the design process well and that the resulting material can be useful.”

The next step will be to actually build the final structure made from the groups of colloids in the lab.

“After seeing these results, I’m confident that it can be done,” says Rossi. “It would be great to have a physical version of this material and hold it in my hand.”

Tunable and Transferable Diamond Membranes for Integrated Quantum Technologies

by Xinghan Guo et al in Nano Letters

The brilliant blue of the Hope Diamond is caused by small impurities in its crystal structure. Similar diamond impurities are also giving hope to scientists looking to create materials that can be used for quantum computing and quantum sensing. In new research from the U.S. Department of Energy’s (DOE) Argonne National Laboratory, researchers have created extremely thin membranes of pure diamond. In a few locations in the crystal structure of the membrane, however, the team substituted carbon atoms with other atoms, notably nitrogen. These defects connect to neighboring atomic vacancies — regions where an atom is missing — creating unusual quantum systems known as “color centers.” Such color centers are sites for storing and processing quantum information.

Equipped with a way to cheaply and easily create diamond membranes that have robust color centers, scientists at Argonne hope to build a kind of assembly line for generating large numbers of these membranes for quantum experiments around the world.

The ability to grow the membranes could be the ticket to enhancing collaboration between different laboratories devoted to quantum information science, said University of Chicago graduate student Xinghan Guo, lead author of the study.

“Essentially, we hope this will eventually give us the ability to become a one-stop shop for quantum sensing materials,” Guo said.

“The defects in the diamond are interesting to us because they can be exploited for quantum application,” said Nazar Delegan, scientist in Argonne’s Materials Science division and the Pritzker School of Molecular Engineering at the University of Chicago and a collaborator with Q-NEXT. “Making these membranes allows us to integrate these defects with other systems and enables new experimental configurations.”

Diamond is mechanically hard, chemically stable and generally expensive — in other words, it is kind of a scientific nightmare, notoriously difficult to fabricate and integrate. At the same time, diamond’s particular structure makes it a great host for color centers that can store quantum information for a long time, Guo said.

“Conventional diamond as a substrate is super hard to work with,” he said. “Our membranes are thinner and more accessible for a wide range of experiments.”

The new diamond material fashioned by the researchers offers greater crystal and surface quality, enabling greater control over the coherence of the color centers.

“You can peel the membrane off and put it on a wide range of substrates, even put it on a silicon wafer. It’s a cheap, flexible and easy way of working with color centers without having to work directly with conventional diamond,” Guo said.

“Because we’re able to control and maintain the quantum properties in individual defects within these very thin materials, it makes this platform promising as basis for a quantum technologies,” Delegan said.

Cilia metasurfaces for electronically programmable microfluidic manipulation

by Wei Wang, Qingkun Liu, Ivan Tanasijevic, Michael F. Reynolds, Alejandro J. Cortese, Marc Z. Miskin, Michael C. Cao, David A. Muller, Alyosha C. Molnar, Eric Lauga, Paul L. McEuen, Itai Cohen in Nature

Cilia are the body’s diligent ushers. These microscopic hairs, which move fluid by rhythmic beating, are responsible for pushing cerebrospinal fluid in your brain, clearing the phlegm and dirt from your lungs, and keeping other organs and tissues clean.

A technical marvel, cilia have proved difficult to reproduce in engineering applications, especially at the microscale.

Cornell researchers have now designed a micro-sized artificial cilial system using platinum-based components that can control the movement of fluids at such a scale. The technology could someday enable low-cost, portable diagnostic devices for testing blood samples, manipulating cells or assisting in microfabrication processes.

The group’s paper, “Cilia Metasurfaces for Electronically Programmable Microfluidic Manipulation,” published May 25 in Nature. The lead author is doctoral student Wei Wang.

“There are lots of ways to make artificial cilia that respond to light, magnetic or electrostatic forces,” Wang said. “But we are the first to use our new nano actuator to demonstrate artificial cilia that are individually controlled.”

The project, led by the paper’s senior author, Itai Cohen, professor of physics in the College of Arts and Sciences, builds off a platinum-based, electrically-powered actuator — the part of the device that moves — his group previously created to enable microscopic robots to walk. The mechanics of those bending bot legs is similar, but the cilia system’s function and applications are different, and quite flexible.

“What we’re showing here,” Cohen said, “is that once you can individually address these cilia, you can manipulate the flows in any way you want. You can create multiple separate trajectories, you can create circular flow, you can create transport, or flows that split up into two paths and then recombine. You can get flow lines in three dimensions. Anything is possible.”

“It’s been very hard to use existing platforms to create cilia that are small, work in water, are electrically addressable and can be integrated with interesting electronics,” Cohen said. “This system solves these problems. And with this kind of platform, we’re hoping to develop the next wave of microfluid manipulation devices.”

A typical device consists of a chip that contains 16 square units with 8 cilia arrays per unit, and 8 cilia per array, with each cilium about 50 micrometers long, resulting in a “carpet” of about a thousand artificial cilia. As the voltage on each cilium oscillates, its surface oxidizes and reduces periodically, which makes the cilium bend back and forth, allowing it to pump fluid at tens of microns per second. Different arrays can be activated independently, therefore creating an endless combination of flow patterns mimicing the flexibility observed in their biological counterparts.

As a bonus, the team created a cilia device that is equipped with a complementary metal-oxide-semiconductor (CMOS) clock circuit — essentially an electronic “brain” that allows the cilia to operate without being tethered to a conventional computer system. That opens the door to developing a host of low-cost diagnostic tests that could be performed in the field.

“You can imagine in the future, people taking this tiny centimeter-by-centimeter device, putting a drop of blood on it and conducting all the assays,” Cohen said. “You wouldn’t have to have a fancy pump, you wouldn’t have to have any equipment, you would just literally put it under sunlight and it would work. It could cost on the order of $1 to $10.”

Simultaneous emulation of synaptic and intrinsic plasticity using a memristive synapse

by Sang Hyun Sung, Tae Jin Kim, Hyera Shin, Tae Hong Im, Keon Jae Lee in Nature Communications

Researchers have reported a nano-sized neuromorphic memory device that emulates neurons and synapses simultaneously in a unit cell, another step toward completing the goal of neuromorphic computing designed to rigorously mimic the human brain with semiconductor devices.

Neuromorphic computing aims to realize artificial intelligence (AI) by mimicking the mechanisms of neurons and synapses that make up the human brain. Inspired by the cognitive functions of the human brain that current computers cannot provide, neuromorphic devices have been widely investigated. However, current Complementary Metal-Oxide Semiconductor (CMOS)-based neuromorphic circuits simply connect artificial neurons and synapses without synergistic interactions, and the concomitant implementation of neurons and synapses still remains a challenge. To address these issues, a research team led by Professor Keon Jae Lee from the Department of Materials Science and Engineering implemented the biological working mechanisms of humans by introducing the neuron-synapse interactions in a single memory cell, rather than the conventional approach of electrically connecting artificial neuronal and synaptic devices.

Structure of TS-PCM composed of volatile TS and nonvolatile PCM layers. a Schematic diagram of TS-PCM composed of volatile TS and nonvolatile PCM layer. The phase transition of the top PCM layer is regulated by the Ag filament formation in the bottom TS layer. b Optical microscope image of fabricated TS-PCM cell. Scale bar, 10 μm. c SEM image of TS-PCM showing the nanopatterned electrodes via EBL process. Scale bar, 5 μm. d Illustration of biological synapse with emission neurotransmitters (upper panel). A representative example of STDP is shown in the lower panel. e Schematic diagram of PCM with the phase transition process (upper panel). The lower panel demonstrates the emulation of STDP by TS-PCM. f Illustration of a biological neuron that integrates input signals into an AP spike (upper panel). The lower panel shows the neuronal intrinsic plasticity of neuron. g Schematic diagram of TS device presenting the formation and rupture of Ag filament (upper panel). The lower panel demonstrates the emulation of intrinsic plasticity by TS-PCM and a parallel capacitor.

Similar to commercial graphics cards, the artificial synaptic devices previously studied often used to accelerate parallel computations, which shows clear differences from the operational mechanisms of the human brain. The research team implemented the synergistic interactions between neurons and synapses in the neuromorphic memory device, emulating the mechanisms of the biological neural network. In addition, the developed neuromorphic device can replace complex CMOS neuron circuits with a single device, providing high scalability and cost efficiency.

The human brain consists of a complex network of 100 billion neurons and 100 trillion synapses. The functions and structures of neurons and synapses can flexibly change according to the external stimuli, adapting to the surrounding environment. The research team developed a neuromorphic device in which short-term and long-term memories coexist using volatile and non-volatile memory devices that mimic the characteristics of neurons and synapses, respectively. A threshold switch device is used as volatile memory and phase-change memory is used as a non-volatile device. Two thin-film devices are integrated without intermediate electrodes, implementing the functional adaptability of neurons and synapses in the neuromorphic memory.

Professor Keon Jae Lee explained, “Neurons and synapses interact with each other to establish cognitive functions such as memory and learning, so simulating both is an essential element for brain-inspired artificial intelligence. The developed neuromorphic memory device also mimics the retraining effect that allows quick learning of the forgotten information by implementing a positive feedback effect between neurons and synapses.”

Nanotwinning-assisted dynamic recrystallization at high strains and strain rates

by Ahmed A. Tiamiyu, Edward L. Pang, Xi Chen, James M. LeBeau, Keith A. Nelson, Christopher A. Schuh in Nature Materials

Forming metal into the shapes needed for various purposes can be done in many ways, including casting, machining, rolling, and forging. These processes affect the sizes and shapes of the tiny crystalline grains that make up the bulk metal, whether it be steel, aluminum or other widely used metals and alloys.

Now researchers at MIT have been able to study exactly what happens as these crystal grains form during an extreme deformation process, at the tiniest scales, down to a few nanometers across. The new findings could lead to improved ways of processing to produce better, more consistent properties such as hardness and toughness.

The new findings, made possible by a detailed analysis of images from a suite of powerful imaging systems, are reported today in the journal Nature Materials, in a paper by former MIT postdoc Ahmed Tiamiyu (now assistant professor at the University of Calgary); MIT professors Christopher Schuh, Keith Nelson, and James LeBeau; former student Edward Pang; and current student Xi Chen.

“In the process of making a metal, you are endowing it with a certain structure, and that structure will dictate its properties in service,” Schuh says. In general, the smaller the grain size, the stronger the resulting metal. Striving to improve strength and toughness by making the grain sizes smaller “has been an overarching theme in all of metallurgy, in all metals, for the past 80 years,” he says.

Metallurgists have long applied a variety of empirically developed methods for reducing the sizes of the grains in a piece of solid metal, generally by imparting various kinds of strain through deforming it in one way or another. But it’s not easy to make these grains smaller.

The primary method is called recrystallization, in which the metal is deformed and heated. This creates many small defects throughout the piece, which are “highly disordered and all over the place,” says Schuh, who is the Danae and Vasilis Salapatas Professor of Metallurgy.

When the metal is deformed and heated, then all those defects can spontaneously form the nuclei of new crystals.

“You go from this messy soup of defects to freshly new nucleated crystals. And because they’re freshly nucleated, they start very small,” leading to a structure with much smaller grains, Schuh explains.

What’s unique about the new work, he says, is determining how this process takes place at very high speed and the smallest scales. Whereas typical metal-forming processes like forging or sheet rolling, may be quite fast, this new analysis looks at processes that are “several orders of magnitude faster,” Schuh says.

“We use a laser to launch metal particles at supersonic speeds. To say it happens in the blink of an eye would be an incredible understatement, because you could do thousands of these in the blink of an eye,” says Schuh.

Such a high-speed process is not just a laboratory curiosity, he says.

“There are industrial processes where things do happen at that speed.” These include high-speed machining; high-energy milling of metal powder; and a method called cold spray, for forming coatings. In their experiments, “we’ve tried to understand that recrystallization process under those very extreme rates, and because the rates are so high, no one has really been able to dig in there and look systematically at that process before,” he says.

Using a laser-based system to shoot 10-micrometer particles at a surface, Tiamiyu, who carried out the experiments, “could shoot these particles one at a time, and really measure how fast they are going and how hard they hit,” Schuh says. Shooting the particles at ever-faster speeds, he would then cut them open to see how the grain structure evolved, down to the nanometer scale, using a variety of sophisticated microscopy techniques at the MIT.nano facility, in collaboration with microscopy specialists.

The result was the discovery of what Schuh says is a “novel pathway” by which grains were forming down to the nanometer scale. The new pathway, which they call nano-twinning assisted recrystallization, is a variation of a known phenomenon in metals called twinning, a particular kind of defect in which part of the crystalline structure flips its orientation. It’s a “mirror symmetry flip, and you end up getting these stripey patterns where the metal flips its orientation and flips back again, like a herringbone pattern,” he says. The team found that the higher the rate of these impacts, the more this process took place, leading to ever smaller grains as those nanoscale “twins” broke up into new crystal grains.

In the experiments they did using copper, the process of bombarding the surface with these tiny particles at high speed could increase the metal’s strength about tenfold.

“This is not a small change in properties,” Schuh says, and that result is not surprising since it’s an extension of the known effect of hardening that comes from the hammer blows of ordinary forging. “This is sort of a hyper-forging type of phenomenon that we’re talking about.”

In the experiments, they were able to apply a wide range of imaging and measurements to the exact same particles and impact sites, Schuh says:

“So, we end up getting a multimodal view. We get different lenses on the same exact region and material, and when you put all that together, you have just a richness of quantitative detail about what’s going on that a single technique alone wouldn’t provide.”

Because the new findings provide guidance about the degree of deformation needed, how fast that deformation takes place, and the temperatures to use for maximum effect for any given specific metals or processing methods, they can be directly applied right away to real-world metals production, Tiamiyu says. The graphs they produced from the experimental work should be generally applicable.

“They’re not just hypothetical lines,” Tiamiyu says. For any given metals or alloys, “if you’re trying to determine if nanograins will form, if you have the parameters, just slot it in there” into the formulas they developed, and the results should show what kind of grain structure can be expected from given rates of impact and given temperatures.

Constructing two-dimensional holey graphyne with unusual annulative π-extension

by Xinghui Liu, Soo Min Cho, Shiru Lin, Zhongfang Chen, Wooseon Choi, Young-Min Kim, Eunbhin Yun, Eun Hee Baek, Do Hyun Ryu, Hyoyoung Lee in Matter

Diamond and graphite are two naturally occurring carbon allotropes that we have known for thousands of years. They are elemental carbons that are arranged in a manner so that they consist of sp3 and sp2 hybridized carbon atoms, respectively. More recently, the discovery of various other carbon allotrope materials, such as graphene, fullerene, carbon nanotube, graphyne, and graphdiyne, has been revolutionalizing modern nanomaterials science. In particular, graphene research has made significant advances in modern chemistry and physics because of its fascinating properties.

Graphene has been touted as a wonder material that can potentially revolutionize the semiconductor industry, owing to its exceptional electron mobility properties. Despite the hype, it appears our civilization is still far from transitioning from the silicon age to the graphene age. The main challenge of using graphene in electronics is the zero-bandgap electronic structure of graphene. This makes it impossible to switch off graphene-based transistors, which limits their application in the semiconductor industry. While it is possible to overcome this limitation by doping or functionalizing the graphene, there is also much interest in the search for new types of 2D carbon allotropes that have exceptional semiconducting properties, such as a proper energy bandgap and high mobility.

Recently, researchers discovered that it is possible to endow many characteristics suitable for a semiconductor to graphene or graphene oxides by creating many holes in its structure. This new type of material is called “holey graphene.” Compared to graphene, γ-graphyne, or graphdiyne, holey graphene not only has the ideal 2D semiconducting properties but also has nonlinear sp bonding and a special π-conjugated structure, which offers promising applications in optoelectronic, energy harvesting, gas separation, catalysis, water remediation, sensor, and energy-related fields.

So far, holey graphene has been produced in laboratories by first synthesizing graphene, then subjecting the graphene to physical, chemical, or hydrothermal treatment to puncture many holes in the structure. However, such a top-down approach for production has its limitations because the size and distribution of the ‘holes’ are uneven and difficult to control.

Led by Associate Director LEE Hyoyoung, researchers from the Center for Integrated Nanostructure Physics (CINAP) within the Institute for Basic Science, South Korea, developed a bottom-up approach for creating such material. For the first time, the group devised a method to construct topologically 2D carbon material atom by atom.

This new two-dimensional single-crystalline material was dubbed “holey-graphyne” (HGY) by the group. HGY consists of alternately linked between benzene rings and C≡C bonds, composed of a pattern of six-vertex and highly strained eight-vertex rings and an equal percentage of sp2 and sp hybridized carbon atoms.

“We were inspired by an intriguing molecule, dibenzocyclooctadiyne, which was first synthesized by Sondheimer and co-workers in 1974. In dibenzocyclooctadiyne, two aromatic benzene rings are connected by two bent acetylenic linkages, resulting in a highly strained eight-membered ring. This exciting molecule inspired us to design and synthesize the new carbon allotrope, version of the material, namely holey-graphyne,” said Associate Director Lee.

The research group successfully produced the ultra-thin single-crystalline HGY using 1,3,5-tribromo-2,4,6-triethynylbenzene as the base material. The single atomic layer thin HGY was then synthesized between the interface of two solvent-system consisting of water and dichloromethane. The new HGY displayed a direct bandgap of about 1.1 eV and excellent calculated-carrier mobility, making it suitable as a semiconductor material.

This new discovery not only demonstrates the first synthesis of the ultrathin single crystalline HGY but also introduces a new concept for the design and synthesis of such a new type of 2D carbon allotrope. It is hoped that the future application of HGY in the semiconductor industry will pave the wave for a new generation of electronics beyond the silicon age.

Diamond mirrors for high-power continuous-wave lasers

by Haig A. Atikian, Neil Sinclair, Pawel Latawiec, Xiao Xiong, Srujan Meesala, Scarlett Gauthier, Daniel Wintz, Joseph Randi, David Bernot, Sage DeFrances, Jeffrey Thomas, Michael Roman, Sean Durrant, Federico Capasso, Marko Lončar in Nature Communications

Just about every car, train and plane that’s been built since 1970 has been manufactured using high-power lasers that shoot a continuous beam of light. These lasers are strong enough to cut steel, precise enough to perform surgery, and powerful enough to carry messages into deep space. They are so powerful, in fact, that it’s difficult to engineer resilient and long-lasting components that can control the powerful beams the lasers emit.

Today, most mirrors used to direct the beam in high-power continuous wave (CW) lasers are made by layering thin coatings of materials with different optical properties. But if there is even one, tiny defect in any of the layers, the powerful laser beam will burn through, causing the whole device to fail.

If you could make a mirror out of a single material, it would significantly reduce the likelihood of defects and increase the lifespan of the laser. But what material would be strong enough?

Now, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have built a mirror out of one of the strongest materials on the planet: diamond. By etching nanostructures onto the surface of a thin sheet of diamond, the research team built a highly reflective mirror that withstood, without damage, experiments with a 10-kilowatt Navy laser.

“Our one-material mirror approach eliminates the thermal stress issues that are detrimental to conventional mirrors, formed by multi-material stacks, when they are irradiated with large optical powers,” said Marko Loncar, the Tiantsai Lin Professor of Electrical Engineering at SEAS and senior author of the paper. “This approach has potential to improve or create new applications of high-power lasers.”

Design and simulation of a mirror in single-crystal diamond. a Graphical depiction of a diamond mirror with the “golf tee”-shaped columns arranged in a hexagonal lattice. b Typical multilayered optical coating deposited onto a substrate. c Schematic of the “golf tee” columns that comprise the diamond mirror, with all relevant dimensions labeled: angle α, radii rdisc, rmin, rsupport, and total height h. The shaded yellow region labeled n1 is of lowest refractive index (air), the red region n2 contains the top portion of the column that features optical resonances and is of highest refractive index, while the yellow region n3 is of lower refractive index and supports the top portion of the column. d Diamond mirror reflection spectrum at normal incidence for varying design angles α, with rdisc = 250 nm, rmin = 50 nm, rsupport = 250 nm, pitch 1.1 μm, and h = 3 μm. Colors indicate reflectivity. e Standing-wave pattern illustrating the reflected wavefront from a diamond mirror at a wavelength of 1064 nm. Mode is confined in the top portion of the columns due to lattice resonance. Colors indicate the electric field amplitude. Photo credit for panels (a) and (b): P. Latawiec, Harvard.

Loncar’s Laboratory for Nanoscale Optics originally developed the technique to etch nanoscale structures into diamonds for applications in quantum optics and communications.

“We thought, why not use what we developed for quantum applications and use it for something more classical,” said Haig Atikian, a former graduate student and postdoctoral fellow at SEAS and first author of the paper.

Using this technique, which uses an ion beam to etch the diamond, the researchers sculpted an array of golf-tee shaped columns on the surface on a 3-milimeter by 3-milimeter diamond sheet. The shape of the golf tees, wide on top and skinny on the bottom, makes the surface of the diamond 98.9% reflective.

“You can make reflectors that are 99.999% reflective but those have 10–20 layers, which is fine for low power laser but certainly wouldn’t be able to withstand high powers,” said Neil Sinclair, a research scientist at SEAS and co-author of the paper.

To test the mirror with a high-power laser, the team turned to collaborators at the Pennsylvania State University Applied Research Laboratory, a Department of Defense designated U.S. Navy University Affiliated Research Center.

There, in a specially designed room that is locked to prevent dangerous levels of laser light from seeping out and blinding or burning those in the adjacent room, the researchers put their mirror in front of a 10-kilowatt laser, strong enough to burn through steel.

The mirror emerged unscathed.

“The selling point with this research is that we had a 10-kilowatt laser focused down into a 750-micron spot on a 3-by-3-millimeter diamond, which is a lot of energy focused down on a very small spot, and we didn’t burn it,” said Atikian. “This is important because as laser systems become more and more power hungry, you need to come up with creative ways to make the optical components more robust.”

In the future, the researchers envision these mirrors being used for defense applications, semiconductor manufacturing, industrial manufacturing, and deep-space communications. The approach could also be used in less expensive materials, such as fused silica.

Magnetic Anisotropy of Individual Nanomagnets Embedded in Biological Systems Determined by Axi-asymmetric X-ray Transmission Microscopy

by Lourdes Marcano, Iñaki Orue, David Gandia, Lucía Gandarias, Markus Weigand, Radu Marius Abrudan, Ana García-Prieto, Alfredo García-Arribas, Alicia Muela, M. Luisa Fdez-Gubieda, Sergio Valencia in ACS Nano

Magnetic nanostructures are promising tools for medical applications. Incorporated into biological structures, they can be steered via external magnetic fields inside the body to release drugs or to destroy cancer cells. However, until now, only average information on the magnetic properties of those nanoparticles could be obtained, thus limiting their successful implementations in therapies. Now a team at HZB conceived and tested a new method to assess the characteristic parameters of every single magnetic nanoparticle.

Imagine a tiny vehicle with a nanomagnetic structure, which can be steered through the human body via external magnetic fields. Arrived at its destination, the vehicle may release a drug, or heat up cancer cells without affecting healthy tissue. Scientists of different disciplines are working on this vision to come true. A multidisciplinary research group at Universidad del País Vasco, Leioa, Spain, explores the talents of so-called magnetotactic bacteria, which have the surprising property to form magnetic iron oxide nanoparticles inside their cells. These particles, with diameters of around 50 nanometers (100 times smaller than blood cells), arrange, within the bacterium, into a chain. The Spanish team is pursuing the idea of using such “magnetic bacteria” as magnetic hyperthermia agents to treat cancer: Steered to the cancer site, the magnetic nanostructures are to be heated by external fields in order to burn the cancer cells.

Now, they have cooperated with a team of physicists led by Sergio Valencia at HZB to explore their magnetic properties in detail. The degree of success for all these applications depends sensitively on the magnetic properties of the individual nanomagnets. But since the signals originating from these super tiny magnetic structures are quite weak, it is necessary to average values over thousands of such structures in order to get meaningful data.

Until now, only these averaged values can be measured, which puts some constraints in the design of customized nanomagnet applications. But this has changed. Spanish physicist Lourdes Marcano has developed a new method during her postdoctoral stay in the team of Valencia at BESSY II:

“We can now obtain precise information on the magnetic properties of several individual nanomagnets in a simultaneous way” she says.

The method allows to measure magnetic properties of individual magnetic nanostructures, even when embedded within biological entities. Magnetic imaging at the scanning transmission X-ray microscope MAXYMUS at BESSY II with the help of theoretical simulations permits to obtain information about the so-called magnetic anisotropy of each single nanoparticle within the field of view of the microscope. The method has been proven by determining the magnetic anisotropy of magnetic nanoparticles inside a bacterium. The magnetic anisotropy is an important parameter for controlling and steering magnetic nanoparticles as it describes how a magnetic nanoparticle reacts to external magnetic fields applied at an arbitrary direction.

“Actually, magnetic imaging of magnetic nanoparticles inside a biological cell with enough spatial resolution requires the use of X-ray microscopes. Unfortunately, this is only possible at large scale research facilities, like BESSY II, providing sufficiently intense X-ray radiation. In the future, however, with the development of compact plasma X-ray sources, this method could become a standard laboratory technique,” says Sergio Valencia.

Giant Isotope Effect of Thermal Conductivity in Silicon Nanowires

by Penghong Ci, Muhua Sun, Meenakshi Upadhyaya, Houfu Song, Lei Jin, Bo Sun, Matthew R. Jones, Joel W. Ager, Zlatan Aksamija, Junqiao Wu in Physical Review Letters

Scientists have demonstrated a new material that conducts heat 150% more efficiently than conventional materials used in advanced chip technologies.

The device — an ultrathin silicon nanowire — could enable smaller, faster microelectronics with a heat-transfer-efficiency that surpasses current technologies. Electronic devices powered by microchips that efficiently dissipate heat would in turn consume less energy — an improvement that could help mitigate the consumption of energy produced by burning carbon-rich fossil fuels that have contributed to global warming.

“By overcoming silicon’s natural limitations in its capacity to conduct heat, our discovery tackles a hurdle in microchip engineering,” said Junqiao Wu, the scientist who led the Physical Review Letters study reporting the new device. Wu is a faculty scientist in the Materials Sciences Division and professor of materials science and engineering at UC Berkeley.

Our electronics are relatively affordable because silicon — the material of choice for computer chips — is cheap and abundant. But although silicon is a good conductor of electricity, it is not a good conductor of heat when it is reduced to very small sizes — and when it comes to fast computing, that presents a big problem for tiny microchips.

Within each microchip resides tens of billions of silicon transistors that direct the flow of electrons in and out of memory cells, encoding bits of data as ones and zeroes, the binary language of computers. Electrical currents run between these hard-working transistors, and these currents inevitably generate heat.

Heat naturally flows from a hot object to a cool object. But heat flow gets tricky in silicon.

In its natural form, silicon is made up of three different isotopes — forms of a chemical element containing an equal number of protons but different number of neutrons (hence different mass) in their nuclei.

About 92% of silicon consists of the isotope silicon-28, which has 14 protons and 14 neutrons; around 5% is silicon-29, weighing in at 14 protons and 15 neutrons; and just 3% is silicon-30, a relative heavyweight with 14 protons and 16 neutrons, explained co-author Joel Ager, who holds titles of senior scientist in Berkeley Lab’s Materials Sciences Division and adjunct professor of materials science and engineering at UC Berkeley.

As phonons, the waves of atomic vibration that carry heat, wind their way through silicon’s crystalline structure, their direction changes when they bump into silicon-29 or silicon-30, whose different atomic masses “confuse” the phonons, slowing them down.

“The phonons eventually get the idea and find their way to the cold end to cool the silicon material,” but this indirect path allows waste heat to build up, which in turn slows your computer down, too, Ager said.

For many decades, researchers theorized that chips made of pure silicon-28 would overcome silicon’s thermal conductivity limit, and therefore improve the processing speeds of smaller, denser microelectronics.

But purifying silicon down to a single isotope requires intense levels of energy which few facilities can supply — and even fewer specialize in manufacturing market-ready isotopes, Ager said.

Fortunately, an international project from the early 2000s enabled Ager and leading semiconductor materials expert Eugene Haller to procure silicon tetrafluoride gas — the starting material for isotopically purified silicon — from a former Soviet-era isotope manufacturing plant. (Haller founded Berkeley Lab’s DOE-funded Electronic Materials Program in 1984, and was a senior faculty scientist in Berkeley Lab’s Materials Sciences Division and a professor of materials science and mineral engineering at UC Berkeley. He died in 2018.)

This led to a series of pioneering experiments, including a 2006 study published in Nature, whereby Ager and Haller fashioned silicon-28 into single crystals, which they used to demonstrate quantum memory storing information as quantum bits or qubits, units of data stored simultaneously as a one and a zero in an electron’s spin.

Subsequently, semiconducting thin films and single crystals made with Ager’s and Haller’s silicon isotope material were shown to have a 10% higher thermal conductivity than natural silicon — an improvement, but from the computer industry’s point of view, probably not enough to justify spending a thousand times more money to build a computer from isotopically pure silicon, Ager said.

But Ager knew that the silicon isotope materials were of scientific importance beyond quantum computing. So he kept what remained in a safe place at Berkeley Lab, just in case other scientists might need it, because few people have the resources to make or even purchase isotopically pure silicon, he reasoned.

About three years ago, Wu and his graduate student Penghong Ci were trying to come up with new ways to improve the heat transfer rate in silicon chips.

One strategy to make more efficient transistors involves using a type of nanowire called a Gate-All-Around Field Effect Transistor. In these devices, silicon nanowires are stacked to conduct electricity, and heat is generated simultaneously, Wu explained.

“And if the heat generated is not extracted out quickly, the device would stop working, akin to a fire alarm blaring in a tall building without an evacuation map,” he said.

But heat transport is even worse in silicon nanowires, because their rough surfaces — scars from chemical processing — scatter or “confuse” the phonons even more, he explained.

“And then one day we wondered, ‘What would happen if we made a nanowire from isotopically pure silicon-28?’” Wu said.

Silicon isotopes are not something one can easily buy on the open market, and word had it that Ager still had some silicon isotope crystals in storage at Berkeley Lab — not a lot, but still enough to share

“If someone has a great idea about how to use it,” Ager said. “And Junqiao’s new study was such a case.”

“We’re really fortunate that Joel happened to have the isotopically enriched silicon material ready to use for the study,” Wu said.

Using Ager’s silicon isotope materials, the Wu team tested the thermal conductivity in bulk 1-millimeter-size silicon-28 crystals versus natural silicon — and again, their experiment confirmed what Ager and his collaborators discovered years ago — that bulk silicon-28 conducts heat only 10% better than natural silicon.

Now for the nano test. Using a technique called electroless etching, Ci made natural silicon and silicon-28 nanowires just 90 nanometers (billionths of a meter) in diameter — about a thousand times thinner than a single strand of human hair.

To measure the thermal conductivity, Ci suspended each nanowire between two microheater pads outfitted with platinum electrodes and thermometers, and then applied an electrical current to the electrode to generate heat on one pad that flows to the other pad via the nanowire.

“We expected to see only an incremental benefit — something like 20% — of using isotopically pure material for nanowire heat conduction,” Wu said.

But Ci’s measurements astonished them all. The Si-28 nanowires conducted heat not 10% or even 20%, but 150% better than natural silicon nanowires with the same diameter and surface roughness.

This defied everything that they had expected to see, Wu said. A nanowire’s rough surface typically slows phonons down. So what was going on?

High-resolution TEM (transmission electron microscopy) images of the material captured by Matthew R. Jones and Muhua Sun at Rice University uncovered the first clue: a glass-like layer of silicon dioxide on the silicon-28 nanowire surface.

Computational simulation experiments at the University of Massachusetts Amherst led by Zlatan Aksamija, a leading expert on the thermal conductivity of nanowires, revealed that the absence of isotope “defects” — silicon-29 and silicon-30 — prevented phonons from escaping to the surface, where the silicon dioxide layer would drastically slow down the phonons. This in turn kept phonons on track along the direction of heat flow — and therefore less “confused” — inside the silicon-28 nanowire’s “core.” (Aksamija is currently an associate professor of materials science and engineering at the University of Utah.)

“This was really unexpected. To discover that two separate phonon-blocking mechanisms — the surface versus the isotopes, which were previously believed to be independent of each other — now work synergistically to our benefit in heat conduction is very surprising but also very gratifying,” Wu said.

“Junqiao and the team discovered a new physical phenomenon,” Ager said. “This is a real triumph for curiosity-driven science. It’s quite exciting.”

Wu said that the team next plans to take their discovery to the next step: by investigating how to “control, rather than merely measure, heat conduction in these materials.”

Dynamically Tunable All-Weather Daytime Cellulose Aerogel Radiative Supercooler for Energy-Saving Building

by Chenyang Cai, Zechang Wei, Chunxiang Ding, Bianjing Sun, Wenbo Chen, Christoph Gerhard, Evgeny Nimerovsky, Yu Fu, Kai Zhang in Nano Letters

Summertime is almost here, a time when many people try to beat the heat. But running air conditioners constantly can be expensive and wasteful. Now, researchers reporting in ACS’ Nano Letters have designed a lightweight foam made from wood-based cellulose nanocrystals that reflects sunlight, emits absorbed heat and is thermally insulating. They suggest that the material could reduce buildings’ cooling energy needs by more than a third.

Although scientists have developed cooling materials, they have disadvantages. Some materials that passively release absorbed heat let a lot of heat through to buildings under the direct, midday sun of the summer months. And other materials that reflect sunlight don’t work well in hot, humid, or cloudy weather. So, Yu Fu, Kai Zhang, and colleagues wanted to develop a robust material that could reflect sunlight, passively release heat, and keep wayward heat from passing through.

To generate a cooling material, the researchers connected cellulose nanocrystals together with a silane bridge, before freezing and freeze-drying the material under a vacuum. This process vertically aligned the nanocrystals, making a white, lightweight foam, which reflected 96% of visible light and emitted 92% of absorbed infrared radiation. When placed over an aluminum foil-lined box sitting outdoors at noon, the material kept the temperature inside the box 16 degrees F cooler than outside it. Also, the material kept the inside of the box 13 degrees F cooler when the air was humid. As the cellulose-based foam was compressed, its cooling ability decreased, revealing tunable cooling properties. The team calculated that placing the foam on the roof and exterior walls of a building could reduce its cooling energy needs by an average of 35.4%. Because the wood-based cellulose foam’s performance can be tuned depending on weather conditions, the researcher says that the technology could be applied in a wide range of environments.

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