Category: Physics

by Zhang Nannan, Chinese Academy of Sciences

In a study published in Science Advances, Chinese researchers have demonstrated the dynamics of liquid entrainment on solid surfaces with millimeter structures.

In some industrial processes, when transferring viscous liquid from a confined space, common tools such as pipettes and spoons are limited. The pipette is limited by high viscosity and low vacuum level. It also easily generates annoying bubbles during suction or pressure release. The confined space limits the spoon’s ability to pick up liquid from below.

For honey bees and glossophagine bats, viscous nectar is entrained and ingested as their long, hairy tongues dip into and out of the nectar. Common intuition would suggest that viscosity is undesirable in general fluid transfer processes. In this example, however, higher viscosity ensures higher, not lower, entrainment as the “hairs” on the tongue, measured in millimeters or submillimeters, further promote entrainment.

Inspired by this phenomenon, researchers from the Technical Institute of Physics and Chemistry (TIPC) of the Chinese Academy of Sciences, and the Suzhou Institute for Advanced Research of the University of Science and Technology of China integrated two key processes of liquid entrainment applied in industry, i.e., “liquid uptake” and “liquid drainage” after uptake, and found that millimeter structures moderately improve the overall entrainment process.

The essence of the entrainment process is a solid architecture passing through the interface of two fluids. For common liquids and typical industrial settings, millimeter scale is where the force such as gravity, the capillary effect, and the viscous force have the same magnitude. The viscous and capillary effects provide the driving force for entrainment, while gravity acts as the resistance. These forces control the fluid behavior and modify the classical Landau-Levich-Derjaguin theory of entrainment. Therefore, millimetric structures can accommodate liquid in larger quantities and with less deviation.

Meanwhile, liquid drainage from a millimeter-structured surface is distinct from easy drainage from an unstructured surface or the firmly trapped state of a micro-structured surface. In an uptake-drainage process, the amount of uptake is increased, which lowers friction and promotes dripping. This occurs even though higher viscosity tends to increase friction and inhibit dripping. These two opposite effects reach a good balance at the millimeter scale. The capillary force further reduces the relative influence of the viscous force. As a result, millimeter structures achieve relatively uniform droplet dripping time across a range of viscosities.

Enhanced uptake and uniform drainage combine for unique superiority in liquid transfer. The researchers built a modularized apparatus with a programmable multi-axis displacement table holding a detachable array of millimeter-structured rods. The apparatus cyclically transferred viscous liquid, much like a conveyor belt in an industrial plant. The liquid was drawn from narrow centrifuge tubes and dripped down at a convenient frequency for one to operate in practical applications. Because of the uniform draining effect, this apparatus and its settings could be adapted to liquids of varying viscosity.

As a brief demonstration, photosensitive resin was then transferred. The resin droplets were quickly cured under ultraviolet light in the short time interval between two transfer cycles. This demonstration showed that batch fabrication, packaging, or modification of lenses, chips, or other similar devices could be achieved through this process.

Uneven surfaces of millimeter morphology are ubiquitous: They include toys being painted, nasopharyngeal swabs for virus testing, tires running through a puddle, and intestinal villi absorbing nutrients, among other things. Relative motion often occurs between these surfaces and the liquid droplet, stream, bath, or water body. For this reason, an understanding of liquid entrainment is required to better control processes involving such uneven surfaces.

More information: Ziyang Cheng et al, Viscous-capillary entrainment on bioinspired millimetric structure for sustained liquid transfer, Science Advances (2023). DOI: 10.1126/sciadv.adi5990

Journal information: Science Advances 

Provided by Chinese Academy of Sciences

by Chinese Academy of Sciences

Polarimetric imaging can uncover features invisible to human eyes and conventional imaging sensors, and it is becoming an ever more essential technique in modern society. Conventional polarimetric imaging systems require complex optical components and moving parts, making system miniaturization difficult.

Recent development in optical metasurfaces and metamaterials show promising progress toward much more compact, flexible, and robust solutions for polarization detection than conventional techniques. However, current metasurface based polarimetric imaging devices suffer from issues in scalability, narrow bandwidth, low accuracy, and small field of view. So far, the demonstration of chip-integrated metasurface-based Full-Stokes polarimetric imaging sensors for visible wavelengths remains elusive.

In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Yu Yao from Arizona State University, School of Electrical, Computer and Energy Engineering, and co-workers have developed chip-integrated metasurface-based Full-Stokes polarimetric imaging sensors for visible wavelengths inspired by the mantis shrimp eye.

They first designed metasurface-based, high optical performance microscale polarization filters including broadband linear polarization filters and dual-color (green and red) chiral metasurfaces. Based on these metasurface polarization filter designs, they fabricated a microscale polarization filters array (MPFA) composed of 75.2K filters, i.e., 75.2K pixels.

They then chip-integrated the MPFA onto the imaging sensor and calibrated the sensor polarization detection with the instrument matrix calibration method. With calibration, they achieved high polarization detection accuracy: Averaged polarization measurement error less than 2% for red (630nm to 670nm) and green color (480nm to 520nm).

Moreover, they found the polarimetric imaging sensor can maintain an error of less than 5% up to ±20° oblique incidence for red color and ±5° for green color. Finally, they demonstrated the full Stokes polarimetric imaging in real-life objects invisible to the traditional imaging sensor at red and green color with total operation bandwidth of 80nm.

From the polarization images of objects, they found polarization information carried by these objects are color-dependent, revealing the advantage of dual-wavelength operation. These scientists summarize the operational principle of their polarimetric imaging sensor:

“The metasurface polarimetric imager adopted the spatial division measurement approach to obtain full Stokes polarimetric images at one snapshot. Besides, the design of circular polarizer is inspired by the eye of mantis shrimp, who can see the polarization difference of light.”

“We engineered artificial optical birefringence of Si metasurfaces working similar to a quarter waveplate with dual operation wavelengths range and stack it onto a double layered Al nanogratings with large linear polarization extinction ratio. We also found circular polarization extinction ratio of our chiral metasurface changes slowly to the oblique incidence angle, which is why our sensor can operate at a broad field of view with high detection accuracy.”

“Overall, the metasurface polarimetric imaging sensor we developed is featured with high accuracy, large field of view, high speed (single-shot measurement), superior mechanical stability, ultra-compact footprint, fabrication scalability and CMOS compatibility.” First author Jiawe Zuo added.

“Our demonstration proved a viable path to implement chip-integrated full-Stokes polarimetric imaging sensors at visible wavelengths based on metasurface device concepts, which could be widely applied in various real-world applications, such as autonomous vision, industrial inspection, space exploration, and biomedical imaging,” the scientists said.

More information: Jiawei Zuo et al, Chip-integrated metasurface full-Stokes polarimetric imaging sensor, Light: Science & Applications (2023). DOI: 10.1038/s41377-023-01260-w

Journal information: Light: Science & Applications 

Provided by Chinese Academy of Sciences 

by Science China Press

Scattering in media has unique inherent randomness. Fiber Rayleigh scattering is one of the typical scattering effects. The exploration and understanding of the characteristics of fiber Rayleigh scattering holds significant research value for optical fiber sensing, random fiber lasers, etc.

However, in practical noisy environments, accurately extracting the fiber Rayleigh scattering is a challenging issue. If deep learning models can express the general characteristics of fiber Rayleigh scattering and be trained by numerical simulation datasets only, it will have significant scientific and practical value.

The general characteristics of fiber Rayleigh scattering based on a deep learning model were successfully extracted for the first time by a research group from UESTC. Published in Science China Information Sciences, the research utilized a purely numerical simulation dataset to train a self-built deep learning model, to extract the general characteristics of fiber Rayleigh scattering.

The experimental verification was then carried out by a widely-used distributed acoustic sensing system known as the phase-sensitive optical time domain reflectometry (Φ-OTDR). The specific innovations and significance are as follows:

1. By training with a numerical simulation dataset, the deep learning model successfully captures the general characteristics of fiber Rayleigh scattering, and accurately recovers the broadband fiber Rayleigh scattering based on narrowband sensing signal in the experiment. This model has strong generalization ability, and will not overly rely on the data generated under specific experimental conditions.

Therefore, it is possible to use numerical simulation to construct large-scale and diverse training datasets for Φ-OTDR systems and train multi-functional artificial intelligence based on complex models.

2. With the fiber Rayleigh scattering obtained by the deep learning model, the optimal probe pulse can be estimated beforehand, and then the time-domain responses of the sensing signal can be optimized, achieving adaptive waveform modulation in the Φ-OTDR system. This opens up new ideas for other research directions based on Rayleigh scattering optical systems, such as random fiber laser, adaptive optics with wavefront correction, etc.

This work investigates the utilization of a deep learning model for extracting the characteristics of Rayleigh scattering in optical fibers. The discoveries provide a basis for developing a multi-functional artificial intelligence specifically designed for distributed optical fiber sensing and also offer valuable insights for other optical systems based on Rayleigh scattering.

More information: Yongxin Liang et al, Prediction of fiber Rayleigh scattering responses based on deep learning, Science China Information Sciences (2023). DOI: 10.1007/s11432-022-3734-0. www.sciengine.com/SCIS/doi/10. … 4b-bd8f-6fe346fa3db8

Provided by Science China Press 

by Chalmers University of Technology

Microcombs can help us discover planets outside our solar system and track new diseases in our bodies. But current microcombs are inefficient and unable to reach their full potential. Now, researchers at Chalmers University of Technology in Sweden have scored a world first with their solution to make microcombs 10 times more efficient. Their breakthrough opens the way to new discoveries in space and health care and paves the way for high-performance lasers in a range of other technologies.

The study “Surpassing the nonlinear conversion efficiency of soliton microcombs” has been published in Nature Photonics

Laser frequency combs can measure frequencies with revolutionary precision and are considered the most disruptive technological advance in the field since the birth of the laser.

Simply put, a microcomb is like a ruler made of light. The principle is based on a laser sending photons that circulate inside a small cavity, a so-called microresonator, where the light is divided into a wide range of frequencies. These frequencies are precisely positioned in relation to each other, like the markings on a ruler. Thus, a new kind of light source can be created consisting of hundreds—or even thousands—of frequencies, like lasers beaming in unison.

Since virtually all optical measurements are connected to light frequencies, the microcomb has a multitude of applications—from calibrating instruments that measure signals at light-year distances in space in the search for exoplanets, to identifying and keeping track of our health via the air we exhale.

Unprecedented efficiency

Up to now, a fundamental problem with microcombs has been that their efficiency was too weak to produce a wider technological impact on society. The conversion efficiency between the laser and the microcomb was too weak, meaning that only a fraction of the power contained in the laser beam was usable. But now, a Chalmers research team has successfully developed a method to increase the effect of the microcomb’s laser beams tenfold.

“We’ve developed a new method that breaks what was previously thought to be a fundamental limit for optical conversion efficiency. Our method increases the laser power of the soliton microcomb by ten times and raises its efficiency from around 1% to over 50%,” says Victor Torres Company, Professor of Photonics at Chalmers.

The new method uses two microresonators, as opposed to just one. They form a unique ensemble with properties that are greater than the sum of its parts. One of the resonators enables the light coming from the laser to couple with the other resonator; rather like impedance matching in electronics.

“The new microcombs have transformative potential because they make high-performance laser technology available to many more markets. For example, frequency combs could be used in lidar modules for autonomous driving, or in GPS satellites and environmental sensing drones, or in data centers to enable bandwidth-intensive AI apps,” says Torres Company.

The method opens up completely new areas of application for high-performance lasers. The technology was recently patented by the project’s researchers. They have founded Iloomina AB, a company which will launch the technology onto a wider market.

The study was conducted by researchers Óskar B. Helgason, Marcello Girardi, Zhichao Ye, Fuchuan Lei, Jochen Schröder and Victor Torres Company at the Department of Photonics at the Department of Microtechnology and Nanoscience at Chalmers University of Technology.

More information: Óskar B. Helgason et al, Surpassing the nonlinear conversion efficiency of soliton microcombs, Nature Photonics (2023). DOI: 10.1038/s41566-023-01280-3

Journal information: Nature Photonics 

Provided by Chalmers University of Technology 

by Tokyo Institute of Technology

Blue light is vital for light-emitting devices, lighting applications, as well as smartphone screens and large screen displays. However, it is challenging to develop efficient blue organic light-emitting diodes (OLEDs), owing to the high applied voltage required for their function. Conventional blue OLEDs typically require around 4 V for a luminance of 100 cd/m². This is higher than the industrial target of 3.7 V—the voltage of lithium-ion batteries commonly used in smartphones. Therefore, there is an urgent need to develop novel blue OLEDs that can operate at lower voltages.

In this regard, Associate Professor Seiichiro Izawa from Tokyo Institute of Technology and Osaka University has collaborated with researchers from University of Toyama, Shizuoka University, and the Institute for Molecular Science. The team recently presented a novel OLED device with a remarkable ultralow turn-on voltage of 1.47 V for blue emission and a peak wavelength at 462 nm (2.68 eV). Their work is published in Nature Communications.

The choice of materials used in this OLED significantly influences its turn-on voltage. The device utilizes NDI-HF (2,7-di(9H-fluoren-2-yl)benzo[lmn][3,8]-phenanthroline-1,3,6,8(2H,7H)-tetraone) as the acceptor, 1,2-ADN (9-(naphthalen-1-yl)-10-(naphthalen-2-yl)anthracene) as the donor, and TbPe (2,5,8,11-tetra-tert-butylperylene) as the fluorescent dopant. This OLED operates via a mechanism called upconversion (UC). Herein, holes and electrons are injected into donor (emitter) and acceptor (electron transport) layers, respectively. They recombine at the donor/acceptor (D/A) interface to form a charge transfer (CT) state.

Dr. Izawa observes, “The intermolecular interactions at the D/A interface play a significant role in CT state formation, with stronger interactions yielding superior results.”

Subsequently, the energy of the CT state is selectively transferred to the low-energy first triplet excited states of the emitter, which results in blue light emission through the formation of a high-energy first singlet excited state by triplet-triplet annihilation (TTA).

“As the energy of the CT state is much lower than the emitter’s bandgap energy, the UC mechanism with TTA significantly decreases the applied voltage required for exciting the emitter. As a result, this UC-OLED reaches a luminance of 100 cd/m², equivalent to that of a commercial display, at just 1.97 V,” explains Dr. Izawa.

In effect, this study efficiently produces a novel OLED, with blue light emission at an ultralow turn-on voltage, using a typical fluorescent emitter widely utilized in commercial displays, thus marking a significant step toward meeting the commercial requirements for blue OLEDs. It emphasizes the importance of optimizing the design of the D/A interface for controlling excitonic processes and holds promise not only for OLEDs, but also for organic photovoltaics and other organic electronic devices.

More information: Blue Organic Light-Emitting Diode with a Turn-on Voltage of 1.47 V, Nature Communications (2023). DOI: 10.1038/s41467-023-41208-7

Journal information: Nature Communications 

Provided by Tokyo Institute of Technology 

by Lehigh University

Engineering researchers at Lehigh University have discovered that sand can actually flow uphill.

The team’s findings were published today in the journal Nature Communications. A corresponding video shows what happens when torque and an attractive force is applied to each grain—the grains flow uphill, up walls, and up and down stairs.

“After using equations that describe the flow of granular materials,” says James Gilchrist, the Ruth H. and Sam Madrid Professor of Chemical and Biomolecular Engineering in Lehigh’s P.C. Rossin College of Engineering and Applied Science and one of the authors of the paper, “we were able to conclusively show that these particles were indeed moving like a granular material, except they were flowing uphill.”

The researchers say the highly unusual discovery could unlock many more lines of inquiry that could lead to a vast range of applications, from health care to material transport and agriculture.

The paper’s lead author, Dr. Samuel Wilson-Whitford, a former postdoctoral research associate in Gilchrist’s Laboratory of Particle Mixing and Self-Organization, captured the movement entirely by serendipity in the course of his research into microencapsulation. When he rotated a magnet beneath a vial of iron oxide-coated polymer particles called microrollers, the grains began to heap uphill.

Wilson-Whitford and Gilchrist began studying how the material reacted to the magnet under different conditions. When they poured the microrollers without activating them with the magnet, they flowed downhill. But when they applied torque using the magnets, each particle began to rotate, creating temporary doublets that quickly formed and broke up. The result, says Gilchrist, is cohesion that generates a negative angle of repose due to a negative coefficient of friction.

“Up until now, no one would have used these terms,” he says. “They didn’t exist. But to understand how these grains are flowing uphill, we calculated what the stresses are that cause them to move in that direction. If you have a negative angle of repose, then you must have cohesion to give a negative coefficient of friction. These granular flow equations were never derived to consider these things, but after calculating it, what came out is an apparent coefficient of friction that is negative.”

Increasing the magnetic force increases the cohesion, which gives the grains more traction and the ability to move faster. The collective motion of all those grains, and their ability to stick to each other, allows a pile of sand particles to essentially work together to do counterintuitive things—like flow up walls, and climb stairs. The team is now using a laser cutter to build tiny staircases, and is taking videos of the material ascending one side and descending the other. A single microroller couldn’t overcome the height of each step, says Gilchrist. But working together, they can.

“This first paper just focuses on how the material flows uphill, but our next several papers will look at applications, and part of that exploration is answering the question, can these microrollers climb obstacles? And the answer is yes.”

Potential applications could be far ranging. The microrollers could be used to mix things, segregate materials, or move objects. Because these researchers have discovered a new way to think about how the particles essentially swarm and work collectively, future uses could be in microrobotics, which in turn could have applications in health care. Gilchrist recently submitted a paper exploring their use on soil as a means of delivering nutrients through a porous material.

“We’re studying these particles to death,” he says, “experimenting with different rotation rates, and different amounts of magnetic force to better understand their collective motion. I basically know the titles of the next 14 papers we’re going to publish.”

More information: Samuel R. Wilson-Whitford et al, Microrollers flow uphill as granular media, Nature Communications (2023). DOI: 10.1038/s41467-023-41327-1

Journal information: Nature Communications 

Provided by Lehigh University 

by Chinese Academy of Sciences

Non-Hermitian systems have numerous alluring optical properties at exceptional points (EPs) and have attracted extensive attention because of their great prospects in applications such as optical sensing, integrated optics, and other fields.

Recently, metasurface, a class of artificial materials that transcends natural materials through the orderly design of subwavelength structures, has become a new platform to realize complex optical EPs.

A common way to implement scattering EP in the acoustic/microwave range is regulating the loss of the gradient metasurfaces by introducing a specific loss in a unit cell. However, direct extensions of in-plane loss in a gradient metasurface from non-visible waveband to visible light remain a formidable challenge, since the adjustable in-plane loss and corresponding manufacturing process in the visible are lacking.

Furthermore, the complex and rebellious interplay between in-plane lossy structure and lightwave restricts optical efficiency. Therefore, achieving high-efficiency EP at optical non-Hermitian metasurface is still a challenging task in photonics. Notably, two-dimensional scattering systems operating at EPs in the visible are unexplored.

In a paper published in Light: Science & Applications, a team of scientists, led by Professor Cheng-Wei Qiu from Department of Electrical and Computer Engineering, National University of Singapore, Singapore and Xinbin Cheng from Institute of Precision Optical Engineering, School of Physics Science and Engineering, Tongji University, Shanghai, and co-workers have reported a universal paradigm for achieving a high-efficiency EP in the visible by leveraging interlayer loss to accurately control the interplay between the lossy structure and scattering lightwaves.

A bilayer framework is demonstrated to reflect back the incident light from the left side (|r_-1| > 0.999) and absorb the incident light from the right side (|r₊₁| < 10^-4).

“As a proof of concept, a bilayer metasurface composed of TiO₂ metagrating and Si subwavelength grating is designed: the metagrating in the upper layer achieves the directional regulation of lightwave, and the lossy subwavelength grating in the lower layer achieves an adjustable absorption.”

“When the proper absorption of Si subwavelength grating is selected, a wave vector-dependent perfect retroreflector and absorber is realized (Fig. 2a). The eigenvalues and phase change all proves that we have reached the EP (Fig. 2b and 2c).”

“We fabricated the sample (Fig. 3a) and conducted two separate tests with incident angles of 30° and −30° as shown in the purple and red boxes of Fig. 3b. When the incident light impacted from the left, most of the light was reflected by the sample to the incident path. When the incident light came from the right, there was very little retroreflection light. The fabricated sample is experimentally demonstrated to reflect and absorb incident light with efficiencies of 88% and 85%, respectively, at 532 nm. (Fig. 3c).”

“Our work paves a new avenue toward the design of versatile optical metasurface platforms involving the EP or higher-order EP, which may inspire more functional photonic devices for wave manipulation,” the scientists say.

More information: Tao He et al, Scattering exceptional point in the visible, Light: Science & Applications (2023). DOI: 10.1038/s41377-023-01282-4

Journal information: Light: Science & Applications 

Provided by Chinese Academy of Sciences 

by Chinese Academy of Sciences

As a promising candidate for the next-generation mobile platform, mixed reality (MR) such as Apple Vision Pro and Meta Quest Pro (both are passthrough virtual reality headsets) has the potential to revolutionize the way we perceive and interact with various digital information.

By providing more direct interactions with digital information, MR is one of the key enablers for metaverse, spatial computing, and digital twins that have found widespread applications in smart tourism, smart health care, smart manufacturing, and smart construction, just to name a few.

To further enhance the human factors and ergonomics of MR displays, it is essential to improve the overall user experience, particularly for long-term wearing comfort. To achieve this goal, an ultracompact formfactor and a lightweight device are key goals.

In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Shin-Tson Wu from College of Optics and Photonics, University of Central Florida, U.S., and co-workers, have demonstrated an achromatic diffractive liquid-crystal (LC) optics system with an ultrathin formfactor and light weight.

Unlike refractive optics that use optical path difference to produce phase patterns, diffractive LC optics produce desired phase patterns by satisfying the half-wave condition along the thickness direction, which is typically several microns for a visible light.

These LC optics offer several advantages, such as high diffraction efficiency (nearly 100%), easy fabrication, polarization selectivity, and dynamic switching, making them promising candidates for near-eye display applications. However, the diffraction angle of LC optics depends on the wavelength, which in turn leads to a severe chromatic aberration and cannot be used for imaging purposes .

To overcome this longstanding chromatic aberration issue while maintaining the ultrathin formfactor, Prof. Wu’s team proposed an achromatic LC optics system consisting of three components taking advantage of polarization selectivity. The transmission spectrum and phase pattern of each optical element is carefully designed to control the polarization states and correct the chromatic aberrations.

Specifically, for an achromatic LC lens system to eliminate the focal shift between blue and red light, the first component would be a broadband lens that shows high efficiency in the visible spectral region, and the second element would be a half-wave plate that is designed to switch the polarization state of blue light.

The last component is a LC lens with specially designed transmission spectrum, which is effective only for the blue light and red light. The achromatic LC lens system can be achieved by simply stacking these three LC components together. Both achromatic grating and deflector systems can be constructed based on the same principle.

The concept was validated using two different types of light engines: a laser projector and an organic light-emitting diode (OLED) display panel. The image of single LC lens exhibits severe chromatic aberrations which arises from the wavelength dependent optical power of the diffractive optics. The achromatic lens system shows a significantly improved color performance; its chromatic aberrations are dramatically suppressed.

This approach can be extended to other types of diffractive optics by controlling the polarization states properly, which is expected to lead to more compact optical components. It will have widespread applications in MR-enabled smart transportation, smart cities, smart health care, and smart manufacturing.

More information: Zhenyi Luo et al, Achromatic diffractive liquid-crystal optics for virtual reality displays, Light: Science & Applications (2023). DOI: 10.1038/s41377-023-01254-8

Journal information: Light: Science & Applications 

Provided by Chinese Academy of Sciences 

by Helmholtz Association of German Research Centres

It sounds like magic: photoelectrodes could convert the greenhouse gas CO₂ back into methanol or N₂ molecules into valuable fertilizer—using only the energy of sunlight.

An HZB study has now shown that diamond materials are in principle suitable for such photoelectrodes. By combining X-ray spectroscopic techniques at BESSY II with other measurement methods, Tristan Petit’s team has succeeded for the first time in precisely tracking which processes are excited by light as well as the crucial role of the surface of the diamond materials.

At first glance, lab-grown diamond materials have little in common with their namesakes in the jewelry shop. They are often opaque, dark and look not spectacular at all. But even if their looks are unimpressive, they are promising in many different applications, for example in brain implants, quantum sensors and computers, as well as metal-free photoelectrode in photo-electrochemical energy conversion.

They are fully sustainable and made of carbon only, they degrade little in time compared to metal-based photoelectrodes and they can be industrially produced!

Diamond materials are suitable as metal-free photoelectrodes because when excited by light, they can release electrons in water and trigger chemical reactions that are difficult to initiate otherwise. A concrete example is the reduction of CO₂ to methanol which turns the greenhouse gas into a valuable fuel. It would also be exciting to use diamond materials to convert N₂ into nitrogen fertilizer NH₃, using much less energy than the Haber-Bosch process.

However, diamond electrodes oxidize in water and oxidized surfaces, it was assumed, no longer emit electrons into the water. In addition, the bandgap of diamond is in the UV range (at 5.5 eV), so visible light is unlikely to be sufficient to excite electrons. In spite of this expectation, previous studies have shown puzzling emission of electrons from visible light excitation. A new study by Dr. Tristan Petit’s group at HZB now brings new insights and gives cause for hope.

Dr. Arsène Chemin, a postdoctoral researcher in Petit’s team, studied samples of diamond materials produced at the Fraunhofer Institute for Applied Solid State Physics in Freiburg. The samples were engineered to facilitate the CO₂ reduction reaction: doped with boron to insure good electrical conductivity and nanostructured, which gives them huge surfaces to increase the emission of charge carriers such as electrons.

Chemin used four X-ray spectroscopic methods at BESSY II to characterize the surface of the sample and the energy needed to excite specific electronic surface states. Then, he used the surface photovoltage measured in a specialized laboratory at HZB to determine which ones of these states are excited and how the charge carriers are displaced in the samples. In complement, he measured the photoemission of electrons of samples either in air or in liquid.

By combining these results, he was able for the first time to draw a comprehensive picture of the processes that take place on the surfaces of the sample after excitation by light.

“Surprisingly, we found almost no difference in the photoemission of charges in liquid, regardless of whether the samples were oxidized or not,” says Chemin. This shows that diamond materials are well suited for use in aqueous solutions. Excitation with visible light is also possible: in the case of the boron-doped samples, violet light (3.5 eV) is sufficient to excite the electrons.

“These results are a great cause for optimism,” says Chemin: “With diamond materials we have a new class of materials that can be explored and widely used.” What’s more, also the methodology of this study is interesting: The combination of these different spectroscopic methods could also lead to new breakthroughs in other photoactive semiconductor materials, the physicist points out.

The work is published in the journal Small Methods.

More information: Arsène Chemin et al, Surface‐Mediated Charge Transfer of Photogenerated Carriers in Diamond, Small Methods (2023). DOI: 10.1002/smtd.202300423

Journal information: Small Methods 

Provided by Helmholtz Association of German Research Centres 

by Tohoku University

A group of researchers have made a significant breakthrough which could revolutionize next-generation electronics by enabling non-volatility, large-scale integration, low power consumption, high speed, and high reliability in spintronic devices.

Details of their findings were published in the journal Physical Review B on August 25, 2023.

Spintronic devices, represented by magnetic random access memory (MRAM), utilize the magnetization direction of ferromagnetic materials for information storage and rely on spin current, a flow of spin angular momentum, for reading and writing data.

Conventional semiconductor electronics have faced limitations in achieving these qualities.

However, the emergence of three-terminal spintronic devices, which employ separate current paths for writing and reading information, presents a solution with reduced writing errors and increased writing speed. Nevertheless, the challenge of reducing energy consumption during information writing, specifically magnetization switching, remains a critical concern.

A promising method for mitigating energy consumption during information writing is the utilization of the spin Hall effect, where spin angular momentum (spin current) flows transversely to the electric current. The challenge lies in identifying materials that exhibit a significant spin Hall effect, a task that has been clouded by a lack of clear guidelines.

“We turned our attention to a unique compound known as cobalt-tin-sulfur (Co₃Sn₂S₂), which exhibits ferromagnetic properties at low temperatures below 177 K (-96°C) and paramagnetic behavior at room temperature,” explains Yong-Chang Lau and Takeshi Seki, both from the Institute for Materials Research (IMR), Tohoku University and co-authors of the study. “Notably, Co₃Sn₂S₂ is classified as a topological material and exhibits a remarkable anomalous Hall effect when it transitions to a ferromagnetic state due to its distinctive electronic structure.”

Lau, Seki and colleagues employed theoretical calculations to explore the electronic states of both ferromagnetic and paramagnetic Co₃Sn₂S₂, revealing that electron-doping enhances the spin Hall effect. To validate this theoretical prediction, thin films of Co₃Sn₂S₂ partially substituted with nickel (Ni) and indium (In) were synthesized. These experiments demonstrated that Co₃Sn₂S₂ exhibited the most significant anomalous Hall effect, while (Co2Ni)Sn2S2 displayed the most substantial spin Hall effect, aligning closely with the theoretical predictions.

“We uncovered the intricate correlation between the Hall effects, providing a clear path to discovering new spin Hall materials by leveraging existing literature as a guide,” adds Seki. “This will hopefully accelerate the development of ultralow-power-consumption spintronic devices, marking a pivotal step toward the future of electronics.”

More information: Yong-Chang Lau et al, Intercorrelated anomalous Hall and spin Hall effect in kagome-lattice Co3Sn2S2 -based shandite films, Physical Review B (2023). DOI: 10.1103/PhysRevB.108.064429

Journal information: Physical Review B 

Provided by Tohoku University