Aurelia Butler

Researchers at the University of Bath in the UK have found a way to make ‘single-crystal flake’ devices that are so thin and free of defects, they have the potential to outperform components used today in quantum computer circuits.
The study is published this month in the journal Nano Letters.
The team from the university’s Department of Physics made its discovery while exploring the junction between two layers of the superconductor niobium diselenide (NbSe?) after these layers had been cleaved apart, twisted about 30 degrees with respect to one another, then stamped back together. In cleaving, twisting and recombining the two layers, the researchers were able to build a Superconducting Quantum Interferometer Device (SQUID) — an extremely sensitive sensor used to measure incredibly tiny magnetic fields.
SQUIDs have a wide range of important applications in areas that include healthcare (as seen in cardiology and magnetoencephalography — a test that maps brain function) and mineral exploration.
SQUIDS are also the building blocks of today’s commercial quantum computers — machines that perform certain computational tasks much more rapidly than classical computers. Quantum computing is still in its infancy but in the next decade, it is likely to transform the problem-solving capacity of companies and organisations across many sectors — for instance by fast-tracking the discovery of new drugs and materials.
“Due to their atomically perfect surfaces, which are almost entirely free of defects, we see potential for our crystalline flakes to play a significant role in building quantum computers of the future,” said Professor Simon Bending, who carried out the research together with his PhD student Liam Farrar. “Also, SQUIDs are ideal for studies in biology — for instance, they are now being used to trace the path of magnetically-labelled drugs through the intestine — so we’re very excited to see how our devices could be developed in this field too.”
As Professor Bending is quick to point out, however, his work on SQUIDs made using NbSe? flakes is very much at the start of its journey. “This is a completely new and unexplored approach to making SQUIDs and a lot of research will still have to done before these applications become a reality,” he said.
Extremely thin single crystals
The flakes from which the Bath superconductors are fabricated are extremely thin single crystals (10,000 times thinner than a human hair) that bend easily, which also makes them suitable for incorporation into flexible electronics, as used in computer keyboards, optical displays, solar cells and various automotive components.
Because the bonds between layers of NbSe? are so weak, cleaved flakes — with their perfectly flat, defect-free surfaces — create atomically sharp interfaces when pushed back together again. This makes them excellent candidates for the components used in quantum computing.
While this is not the first time NbSe₂ layers have been stamped together to create a weak superconducting link, this is the first demonstration of quantum interference between two such junctions patterned in a pair of twisted flakes. This quantum interference has allowed the researchers to modulate the maximum supercurrent that can flow through their SQUIDs by applying a small magnetic field, creating an extremely sensitive field sensor. They were also able to show that the properties of their devices could be systematically tuned by varying the twist angle between the two flakes.
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The world is heading towards a trillion-sensor economy where billions of devices using multiple sensors will be connected under the umbrella of Internet-of-things. An important part of this economy is constituted of light/photo sensors, which are tiny semiconductor-based electronic components that detect light and convert them to electrical signals. Light sensors can be found everywhere around us, from household electronic gadgets and health-care equipment to optical communication systems and automobiles.
Over the years, there has been marked progress in research on photosensors. Scientists have endeavored to develop sensors that can detect a high dynamic range of lights and are easy to manufacture and energy efficient. Most light sensors used in cost-effective consumer products are energy efficient but are susceptible to noise — unwanted light information — in the external environment, which adversely affects their performance. To tackle this issue, products have been designed using light-to-frequency conversion circuits (LFCs), which show better signal to noise ratio. However, most LFCs are made of silicon-based photodetectors that can limit the range of light detection. Also, use of LFCs leads to chip area wastage, which becomes a problem when designing multi-functional electronic circuits for compact devices.
Now a team of researchers from Incheon National University, South Korea, led by Prof. Sung Hun Jin, has demonstrated a highly efficient system of photodetectors that can overcome the limitations of conventional LFCs. In their study, which was made available online on 10 June 2021 and subsequently published in volume 17, issue 26 of the journal Small, they report developing complementary photosensitive inverters with p-type single walled carbon nanotubes (SWNT) and n-type amorphous indium-gallium-zinc-oxide (a-IGZO/SWNT) thin film transistors. Prof Jin explains “Our photodetector applies a different approach with regard to the light-to-frequency conversion. We have used components that are light dependent and not voltage dependent, unlike conventional LFCs.”
The new design architecture allowed the team to design LFC with superior chip area efficiency and compact form factor, making it suitable for use in flexible electronic devices. Experiments conducted using the photosensor system indicated excellent optical properties, including high tunability and responsiveness over a broad range of light. The LFC also showed possibility of large area scalability and easy integration into state-of-the-art silicon wafer-based chips.
The LFC system developed in this study can be used to build optical sensor systems that have high-level signal integrity, as well as excellent signal processing and transmitting abilities. These promising properties make it a strong contender for application in future Internet-of-Things sensor scenarios. “LFCs based on low dimensional semiconductors will become one of the core components in the trillion sensors area. Our LFC scheme will find application in medical SpO2 detection, auto-lighting in agriculture, or in advanced displays for virtual and augmented reality” concludes Prof Jin.
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Living or biological systems cannot be easily understood using the standard laws of physics, such as thermodynamics, as scientists would for gases, liquids or solids. Living systems are active, demonstrating fascinating properties such as adapting to their environment or repairing themselves. Exploring the questions posed by living systems using computer simulations, researchers at the University of Göttingen have now discovered a novel type of ordering effect generated and sustained by a simple mechanical deformation, specifically steady shear. The results were published in PNAS.
Understanding living systems, such as tissues formed by cells, poses a significant challenge because of their unique properties, such as adaptation, self-repair and self-propulsion. Nonetheless, they can be studied using models that treat them as just an unusual, “active” form of physical matter. This can reveal extraordinary dynamical or mechanical properties. One of the puzzles is how active materials behave under shear (the deformation produced by moving the top and bottom layers sideways in opposite directions, like sliding microscope cover plates against each other). Researchers at the Institute for Theoretical Physics, University of Göttingen explored this question and discovered a novel type of ordering effect that is generated and sustained by steady shear deformation. The researchers used a computer model of self-propelling particles where each particle is driven by a propulsion force that changes direction slowly and randomly. They found that while the flow of the particles looks similar to that in ordinary liquids, there is a hidden order revealed by looking at the force directions: these tend to point towards the nearest (top or bottom) plate, while particles with sideways forces aggregate in the middle of the system.
“We were exploring the response of a model active material under steady driving, where the system is sandwiched between two walls, one stationary and the other moving to generate shear deformation. What we saw was that at a sufficiently strong driving force, an interesting ordering effect emerges,” comments Dr Rituparno Mandal, Institute for Theoretical Physics at the University of Göttingen. “We now also understand the ordering effect using a simple analytical theory and the predictions from this theory match surprisingly well with the simulation.”
Senior author Professor Peter Sollich, also from the Institute for Theoretical Physics, Universiy of Göttingen, explains, “Often an external force or driving force destroys ordering. But here the driving by shear flow is key in providing mobility to the particles that make up the active material, and they actually need this mobility to achieve the observed order. The results will open up exciting possibilities for researchers investigating the mechanical responses of living matter.”
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A research team led by Prof. Kei-May LAU of the Department of Electronic and Computer Engineering at Hong Kong University of Science and Technology (HKUST) has recently developed a novel semiconductor deposition scheme and demonstrated high-performance photodetectors (PDs) grown on silicon-on-insulators (SOI) for silicon photonics. These III-V photodetectors are qualified candidates for high-speed data communications in silicon photonics. These results point to a practical solution for the monolithic integration of III-V active devices and Si-based passive devices on the SOI platform in the future.
The ever-growing communication traffic is pushing the conventional electronic interconnection to the limit. Silicon photonics is regarded as an enabling technology to solve this pressing issue with its high-speed and large bandwidth capability, as well as scalable and high-throughput manufacturing. High-performance PDs are crucial optical building blocks in silicon photonic integrated circuits (Si-PICs). In addition to characteristics such as high responsivity, low dark current, large bandwidth, operation over a wide wavelength band, efficient light coupling with Si waveguides and CMOS compatibility are also needed for the PDs.
III-V photodetectors have long been deployed in InP-based photonic integrated circuits (PICs) because of their superior device performance. Recently, interest on III-V PDs grown on Si started to flourish complementing the research on integrating III-V lasers on Si and the eventual goal of having high-performance III-V-photonics integrated on the Si-photonics platform. For the III-V PDs on Si realized by traditional blanket hetero-epitaxy method, the thick buffer layers used for defect reduction make it challenging for light coupling with Si-waveguides and reported 3 dB bandwidths of these PDs often fall in the range of sub-10GHz.
The HKUST team developed the lateral aspect ratio trapping (ART) method to grow III-V materials on SOI without the need of thick buffers. III-V PDs grown on SOI by this method feature an in-plane configuration with the Si-device layer, which allows easy integration of the PDs and Si-waveguides. The team designed and fabricated III-V PDs with a variety of dimensions on a monolithic InP/SOI platform, also developed by the team. The PDs feature a large 3 dB bandwidth exceeding 40 GHz, a high responsivity of 0.3 A/W at 1550 nm and 0.8 A/W at 1310 nm, a wide operation wavelength span over 400 nm, and a low dark current of 0.55 nA. The photocurrents is adjustable for various applications by varying the length of the PDs. Design of interfacing these PDs with Si-waveguides can be flexible and simple.
For the first time, the team demonstrates III-V photodetectors grown on the monolithic InP/SOI platform (paper to appear in Light: Science and Application) to fulfill the stringent criteria for PDs in silicon photonics. “This was made possible by our latest development of a monolithic InP/SOI platform with both sub-micron InP bars and large-dimension InP membranes. Our team’s combined expertise and insights into both device physics and growth mechanisms allow us to accomplish the challenging task of cross-correlated analysis of epitaxial growth, material characteristics and device performance,” says Prof. Lau.
This is a collaborative work with a research team led by Prof. Hon-Ki TSANG of Department of Electronic Engineering at Chinese University of Hong Kong (CUHK).
The device fabrication technology in the work was developed at HKUST’s Nanosystem Fabrication Facility (NFF) on Clear Water Bay campus. The work is supported by Research Grants Council of Hong Kong and Innovation Technology Fund of Hong Kong. This work has recently been published in Optica.
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Quantum dot light-emitting diode (QLED), which employs quantum dots as a light-emitting material, has attracted significant attention as a promising alternative for next-generation display technologies, owing to its outstanding electroluminescence properties. Since it does not require any bulky components such as backlight units, QLED displays can potentially be manufactured into an ultrathin form factor. A joint research team led by KIM Dae-Hyeong (Professor at Seoul National University) and HYEON Taeghwan (Distinguished professor at Seoul National University) from the Center for Nanoparticle Research within the Institute for Basic Science has previously unveiled a prototype QLED back in 2015. The device had a thickness of only 3 micrometers, which is only one-thirtieth of that of human hair. Due to such an extremely reduced thickness, the ultrathin QLED exhibited outstanding mechanical flexibility, which allowed it to be readily applicable in various wearable devices, such as electronic tattoos.
Recently, the team further advanced this technology and developed a foldable variant of the ultrathin QLED, inspired by the ancient art of paper folding known as origami. The IBS researchers reported three-dimensional foldable QLEDs, which can be freely transformed into various user-customized 3D structures, such as butterflies, airplanes, and pyramids. Considering the rising popularity of foldable smartphones, the advancement of foldable display technology is gaining greater importance. It is expected this technology can provide unprecedented opportunities for next-generation electronics with user-customized form factors with complex structures, as well as allowing for dynamic three-dimensional display of visual information.
The researchers endowed foldability to the conventional planar QLED via a new fabrication process that can partially etch the epoxy film deposited on the QLED surface without damaging the underlying QLED. Using a power-controllable carbon dioxide pulsed laser and the silver-aluminum alloy-based etch-stop layers, the etching depth can be precisely controlled. As the laser-etched part of the device is relatively thinner than the surrounding region, it is possible to etch out deformation lines along which the device can be folded like origami paper.
Based on the selective laser-etching technique, researchers were able to precisely control the radius of curvature down to less than 50 micrometers. Under such a small curvature radius, the fold line resembles a sharp edge with no visible curvature. By using mechanical simulation to carefully engineer the device, researchers were able to minimize the strain loaded on the light-emitting components. The entire QLED including the crease region (a fold line) was able to maintain a stable light-emitting performance even when after it was repeatedly folded 500 times. The technology was applied to fabricate 3D foldable QLEDs with various complex shapes such as butterflies, airplanes, and pyramids.
“We were able to build a 3D foldable QLED that can be freely folded just like a paper artwork,” said KIM Dae-Hyeong, the vice-director of the Center for Nanoparticle Research. He also said, “By fabricating the passively driven, 3D foldable QLED arrays composed of 64 individual pixels, we have shown the possibility of developing displays with greater complexity in the future.” HYEON Taeghwan, the director of the Center for Nanoparticle Research, states that “Through the technology reported in this research, paper-like QLEDs that can be folded into various complex structures have been successfully fabricated. Who knows when the day will come when electronic paper with a display unit can replace real paper?”
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While very little is known on the effects of immersive VR on adults, there is next to no knowledge on the impact of such systems on the sensorimotor abilities of young children.
In 2016 at EPFL’s Open House, EPFL graduate Jenifer Miehlbradt was showcasing her virtual reality setup to allow users to pilot drones using their torso. Users from the general public were invited to wear a VR headset, and movements of their torso would allow them to navigate through a series of obstacles in a virtual landscape.
“Adults had no problem using simple torso movements to fly through the virtual obstacles, but I noticed that children just couldn’t do it,” remembers Miehlbradt. “That’s when Silvestro asked me to come to his office.”
Silvestro Micera, Bertarelli Foundation Chair in Translational Neuroengineering, was Miehlbradt’s supervisor at the time. They realized that their virtual reality torso experiment may be revealing something about the way a child’s nervous system develops, and that no study in the literature had assessed the effect of virtual reality headsets on children. They embarked on a study of several years, in collaboration with the Italian Institute of Technology, involving 80 children between the ages of 6 and 10. The results are published today in Scientific Reports.
“This study confirms the potential of technology to understand motor control,” says Micera.
The development of upper body coordination
Healthy adults have no problem disconnecting their head movements from their torso for piloting, like looking elsewhere while riding a bike. This requires complex integration of multiple sensory inputs: vision, from the inner ear for balance, and proprioception, the body’s ability to sense movement, action and location. More