‘We’re excited about our progress and envision these materials as key components for emerging soft technologies’
Want a smartphone that stretches, takes damage, and still doesn’t miss a call?
A team of Virginia Tech researchers from the Department of Mechanical Engineering and the Macromolecules Innovation Institute has created a new type of soft electronics, paving the way for devices that are self-healing, reconfigurable, and recyclable. These skin-like circuits are soft and stretchy, sustain numerous damage events under load without losing electrical conductivity, and can be recycled to generate new circuits at the end of a product’s life.
Current consumer devices, such as phones and laptops, contain rigid materials that use soldered wires running throughout. The soft circuit developed by Bartlett’s team replaces these inflexible materials with soft electronic composites and tiny, electricity-conducting liquid metal droplets. These soft electronics are part of a rapidly emerging field of technology that gives gadgets a level of durability that would have been impossible just a few years ago.
The liquid metal droplets are initially dispersed in an elastomer, a type of rubbery polymer, as electrically insulated, discrete drops.
“To make circuits, we introduced a scalable approach through embossing, which allows us to rapidly create tunable circuits by selectively connecting droplets,” postdoctoral researcher and first author Ravi Tutika said. “We can then locally break the droplets apart to remake circuits and can even completely dissolve the circuits to break all the connections to recycle the materials, and then start back at the beginning.”
The circuits are soft and flexible, like skin, continuing to work even under extreme damage. If a hole is punched in these circuits, the metal droplets can still transfer power. Instead of cutting the connection completely as in the case of a traditional wire, the droplets make new connections around the hole to continue passing electricity.
The circuits will also stretch without losing their electrical connection, as the team pulled the device to over 10 times its original length without failure during the research.
At the end of a product’s life, the metal droplets and the rubbery materials can be reprocessed and returned to a liquid solution, effectively making them recyclable. From that point, they can be remade to start a new life, an approach that offers a pathway to sustainable electronics.
While a stretchy smartphone has not yet been made, rapid development in the field also holds promise for wearable electronics and soft robotics. These emerging technologies require soft, robust circuitry to make the leap into consumer applications.
“We’re excited about our progress and envision these materials as key components for emerging soft technologies,” Bartlett said. “This work gets closer to creating soft circuitry that could survive in a variety of real-world applications.”
Featured image: Current passes through a self-healing circuit. Photo by Alex Parrish.
Reference: Tutika, R., Haque, A.B.M.T. & Bartlett, M.D. Self-healing liquid metal composite for reconfigurable and recyclable soft electronics. Commun Mater 2, 64 (2021). https://doi.org/10.1038/s43246-021-00169-4
A new approach to studying conjugated polymers made it possible for an Army-funded research team to measure, for the first time, the individual molecules’ mechanical and kinetic properties during polymerization reaction. The insights gained could lead to more flexible and robust soft electronic materials, such as health monitors and soft robotics.
Conjugated polymers are essentially clusters of molecules strung along a backbone that can conduct electrons and absorb light. This makes them a perfect fit for creating soft optoelectronics, such as wearable electronic devices; however, as flexible as they are, these polymers are difficult to study in bulk because they aggregate and fall out from solution.
“Conjugated polymers are a fascinating class of materials due to their inherent optical and electronic properties which are dictated by their polymer structure,” said Dr. Dawanne Poree, program manager, U.S. Army Combat Capabilities Development Command, known as DEVCOM, Army Research Laboratory. “These materials are highly relevant to a number of applications of interest to Army and DoD including portable electronics, wearable devices, sensors, and optical communication systems. To date, unfortunately, it has been difficult to develop conjugated polymers for targeted applications due to a lack of viable tools to study and correlate their structure-property relationships.”
With Army funding, researchers at Cornell University employed an approach they pioneered on other synthetic polymers, called magnetic tweezers, that allowed them to stretch and twist individual molecules of the conjugated polymer polyacetylene. The research was published in the journal Chem.
“Through the use of novel single-molecule manipulation and imaging approaches, this work provided the first observations of single-chain behaviors in conjugated polymers which lays the foundation for the rational design and processing of these materials to enable widespread application,” Poree said.
Previous efforts to address the solubility of conjugated polymers have often relied upon chemical derivatization, in which the structures are modified with functional groups of atoms. However, that approach can affect the polymer’s innate properties.
“The conjugated polymer is really a prototype,” said Dr. Peng Chen, the Peter J.W. Debye professor of chemistry and chemical biology at Cornell. “You always modify it to tailor it for applications. We are hoping everything we measured – the fundamental properties of synthesis kinetics, the mechanical property – become benchmark numbers for people to think about other polymers of the same category.”
In 2017, Chen’s group was the first to use the magnetic tweezers measurement technique to study living polymerization, visualizing it at the single-molecule level. The technique had already been used in the biophysics field for studying DNA and proteins, but no one had successfully extended it to the realm of synthetic polymers.
The process works by affixing one end of a polymer strand to a glass coverslip and the other end to a tiny magnetic particle. The researchers then use a magnetic field to manipulate the conjugated polymer, stretching or twisting it, and measuring the response of a single polymer chain that grows.
The amounts are so small, they stay soluble in solution, the way bulk amounts normally would not.
The team measured how long chains of conjugated polymers, which consist of hundreds of thousands of monomer units, grow in real time. They discovered these polymers add a new monomer per second, a much faster growth than their nonconjugated analogs.
“We found that while growing in real time, this polymer forms conformational entanglements,” Chen said. “All polymers we have studied form conformational entanglements, but for this conjugated polymer this conformational entanglement is looser, allowing it to grow faster.”
By pulling and stretching individual conjugated polymers, so-called force extension measurements, the researchers were able to assess their rigidity and better understand how they can bend in different directions while remaining conjugated and retaining electron conductivity.
They also discovered the polymers displayed diverse mechanical behaviors from one individual chain to the next–behaviors that had been predicted by theory but never observed experimentally.
The findings highlight both the uniqueness of conjugated polymers for a range of applications as well as the strength of using a single-molecule manipulation and imaging technique on synthetic materials.
“Now we have a new way to study how these conjugated polymers are made chemically and what is the fundamental mechanical property of this type of material,” Chen said. “We can study how these fundamental properties change when you start tailoring them for application purposes. Maybe you can make it more mechanically flexible and make the polymer longer, or adjust the synthesis condition to either synthesize the polymer in a faster or slower way.”
Featured image: A new approach to studying conjugated polymers made it possible for an Army-funded research team to measure, for the first time, the individual molecules’ mechanical and kinetic properties during polymerization reaction. The insights gained could lead to more flexible and robust soft electronic materials, such as health monitors and soft robotics. (Udit Chakraborty, Cornell University)
Novel crystalline form of silicon could potentially be used to create next-generation electronic and energy devices
A team led by Carnegie’s Thomas Shiell and Timothy Strobel developed a new method for synthesizing a novel crystalline form of silicon with a hexagonal structure that could potentially be used to create next-generation electronic and energy devices with enhanced properties that exceed those of the “normal” cubic form of silicon used today.
Silicon plays an outsized role in human life. It is the second most abundant element in the Earth’s crust. When mixed with other elements, it is essential for many construction and infrastructure projects. And in pure elemental form, it is crucial enough to computing that the longstanding technological hub of the U.S.–California’s Silicon Valley–was nicknamed in honor of it.
Like all elements, silicon can take different crystalline forms, called allotropes, in the same way that soft graphite and super-hard diamond are both forms of carbon. The form of silicon most commonly used in electronic devices, including computers and solar panels, has the same structure as diamond. Despite its ubiquity, this form of silicon is not actually fully optimized for next-generation applications, including high-performance transistors and some photovoltaic devices.
While many different silicon allotropes with enhanced physical properties are theoretically possible, only a handful exist in practice given the lack of known synthetic pathways that are currently accessible.
Strobel’s lab had previously developed a revolutionary new form of silicon, called Si24, which has an open framework composed of a series of one-dimensional channels. In this new work, Shiell and Strobel led a team that used Si24 as the starting point in a multi-stage synthesis pathway that resulted in highly oriented crystals in a form called 4H-silicon, named for its four repeating layers in a hexagonal structure.
“Interest in hexagonal silicon dates back to the 1960s, because of the possibility of tunable electronic properties, which could enhance performance beyond the cubic form” Strobel explained.
Hexagonal forms of silicon have been synthesized previously, but only through the deposition of thin films or as nanocrystals that coexist with disordered material. The newly demonstrated Si24 pathway produces the first high-quality, bulk crystals that serve as the basis for future research activities.
Using the advanced computing tool called PALLAS, which was previously developed by members of the team to predict structural transition pathways–like how water becomes steam when heated or ice when frozen–the group was able to understand the transition mechanism from Si24 to 4H-Si, and the structural relationship that allows the preservation of highly oriented product crystals.
“In addition to expanding our fundamental control over the synthesis of novel structures, the discovery of bulk 4H-silicon crystals opens the door to exciting future research prospects for tuning the optical and electronic properties through strain engineering and elemental substitution,” Shiell said. “We could potentially use this method to create seed crystals to grow large volumes of the 4H structure with properties that potentially exceed those of diamond silicon.”
Carnegie’s Li Zhu was also a member of the research team, along with Brenton Cook and Dougal McCulloch of RMIT University and Jodie Bradby of The Australian National University.
This work was supported by the National Science Foundation, Division of Materials Research.
Portions of this work were performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA’s Office of Experimental Sciences. The Advanced Photon Source is a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory.
Reference: Thomas B. Shiell, Li Zhu, Brenton A. Cook, Jodie E. Bradby, Dougal G. McCulloch, and Timothy A. Strobel, “Bulk Crystalline 4H-Silicon through a Metastable Allotropic Transition”, Phys. Rev. Lett. 126, 215701 – Published 25 May 2021. DOI: https://doi.org/10.1103/PhysRevLett.126.215701
A paper by Kazan Federal University appeared in IET Microwaves, Antennas & Propagation.
Problems for eigenmodes of a two-layered dielectric microcavity have become widespread thanks to the research of A.I. Nosich, E.I. Smotrova, S.V. Boriskina and others since the beginning of the 21st century. The KFU team first tackled this topic in 2014; undergraduates started working under the guidance of Evgeny Karchevsky, Professor of the Department of Applied Mathematics of the Institute of Computational Mathematics and Information Technology.
In this paper, the researchers discuss a model of a 2D active microcavity with a piercing hole and the possibility of a compromise between high directionality of radiation and low threshold gain. The analysis performed is based on the lasing eigenvalue problem (LEP) formalism. This LEP is a boundary value problem for the system of Maxwell equations with boundary and radiation conditions, adapted to study the threshold modes of open resonators with active regions. In LEP, each eigenvalue is a pair of two real numbers: the emission frequency and the threshold gain in the active region. This combination fits perfectly with the experimental results, which show that each mode has its own defined threshold, and that it is directly related to the field diagram and the location of the active region. When analyzing cavities of complex shape, we use the analytical regularization method in the form of a set of Müller boundary integral equations and reduce LEP to a nonlinear eigenvalue problem for a set of Fredholm integral equations of the second kind. To find a solution, a fast and accurate Galerkin method designed for this task is used. This method makes it possible to study symmetric and asymmetric modes separately, at the threshold of radiation that is not damped in time. The numerical results show that the directionality of the radiation of the operating modes in a given frequency range, together with their threshold gain values, is controlled by the size and location of the air hole in the resonator. In the developed code to study solutions of symmetric and asymmetric modes, the sine and cosine functions in the Galerkin scheme are used instead of exponentials. This makes the code resistant to jumps between mode families when numerically searching for eigenvalues and allows much smaller matrix equations to be used for computations with the order of machine precision. These modifications form the basis of this work. In addition, since the boundaries of the cavity and the hole are circles, this approach allows one to obtain explicit formulas for the matrix elements instead of double integrals. Thanks to all this, the algorithm is extremely fast and accurate. This makes it possible to perform an elementary optimization of the microlaser geometry, which ensures high directionality of the mode radiation while maintaining a low threshold value of the gain in the active region. This code is a promising engineering tool for microring lasers.
Dielectric microcavities have been the objects of intensive research in photonics and nano-optics for over 30 years. Laser radiation arising from the propagation of whispering gallery modes along the circumference of the microdisk resonators is not unidirectional. Due to a change in the structure of the laser (microcavity), it is possible to achieve unidirectional radiation with high directivity and low thresholds of its generation, which, in combination with its small size, the possibility of single-frequency generation and temperature stability, provides a wide range of applications. As an example, the authors can offer a quantum dot microcavity, which can be used to implement optical data transmission inside or between integrated circuits.
This topic can be expanded by considering and analyzing microcavities of complex structures. For example, a microcavity with several piercing holes arranged in a certain order (photonic crystal microcavity), or a microcavity with a quantum dot. Such complications will make it possible to generalize the results or to discover new dependencies.
Trade‐off between threshold gain and directionality of emission for modes of two‐dimensional eccentric microring lasers analysed using lasing eigenvalue problem
Reference: Repina, A.I., Oktyabrskaya, A.O., Spiridonov, A.O., Ketov, I.V. and Karchevskii, E.M. (2021), Trade-off between threshold gain and directionality of emission for modes of two-dimensional eccentric microring lasers analysed using lasing eigenvalue problem. IET Microw. Antennas Propag. https://doi.org/10.1049/mia2.12103
With the rise of the digital age, the amount of WiFi sources to transmit information wirelessly between devices has grown exponentially. This results in the widespread use of the 2.4GHz radio frequency that WiFi uses, with excess signals available to be tapped for alternative uses.
To harness this under-utilised source of energy, a research team from the National University of Singapore (NUS) and Japan’s Tohoku University (TU) has developed a technology that uses tiny smart devices known as spin-torque oscillators (STOs) to harvest and convert wireless radio frequencies into energy to power small electronics. In their study, the researchers had successfully harvested energy using WiFi-band signals to power a light-emitting diode (LED) wirelessly, and without using any battery.
“We are surrounded by WiFi signals, but when we are not using them to access the Internet, they are inactive, and this is a huge waste. Our latest result is a step towards turning readily-available 2.4GHz radio waves into a green source of energy, hence reducing the need for batteries to power electronics that we use regularly. In this way, small electric gadgets and sensors can be powered wirelessly by using radio frequency waves as part of the Internet of Things. With the advent of smart homes and cities, our work could give rise to energy-efficient applications in communication, computing, and neuromorphic systems,” said Professor Yang Hyunsoo from the NUS Department of Electrical and Computer Engineering, who spearheaded the project.
The research was carried out in collaboration with the research team of Professor Guo Yong Xin, who is also from the NUS Department of Electrical and Computer Engineering, as well as Professor Shunsuke Fukami and his team from TU. The results were published in Nature Communications on 18 May 2021.
Converting WiFi signals into usable energy
Spin-torque oscillators are a class of emerging devices that generate microwaves, and have applications in wireless communication systems. However, the application of STOs is hindered due to a low output power and broad linewidth.
While mutual synchronisation of multiple STOs is a way to overcome this problem, current schemes, such as short-range magnetic coupling between multiple STOs, have spatial restrictions. On the other hand, long-range electrical synchronisation using vortex oscillators is limited in frequency responses of only a few hundred MHz. It also requires dedicated current sources for the individual STOs, which can complicate the overall on-chip implementation.
To overcome the spatial and low frequency limitations, the research team came up with an array in which eight STOs are connected in series. Using this array, the 2.4 GHz electromagnetic radio waves that WiFi uses was converted into a direct voltage signal, which was then transmitted to a capacitor to light up a 1.6-volt LED. When the capacitor was charged for five seconds, it was able to light up the same LED for one minute after the wireless power was switched off.
In their study, the researchers also highlighted the importance of electrical topology for designing on-chip STO systems, and compared the series design with the parallel one. They found that the parallel configuration is more useful for wireless transmission due to better time-domain stability, spectral noise behaviour, and control over impedance mismatch. On the other hand, series connections have an advantage for energy harvesting due to the additive effect of the diode-voltage from STOs.
Commenting on the significance of their results, Dr Raghav Sharma, the first author of the paper, shared, “Aside from coming up with an STO array for wireless transmission and energy harvesting, our work also demonstrated control over the synchronising state of coupled STOs using injection locking from an external radio-frequency source. These results are important for prospective applications of synchronised STOs, such as fast-speed neuromorphic computing.”
To enhance the energy harvesting ability of their technology, the researchers are looking to increase the number of STOs in the array they had designed. In addition, they are planning to test their energy harvesters for wirelessly charging other useful electronic devices and sensors.
The research team also hopes to work with industry partners to explore the development of on-chip STOs for self-sustained smart systems, which can open up possibilities for wireless charging and wireless signal detection systems.
Reference: Sharma, R., Mishra, R., Ngo, T. et al. Electrically connected spin-torque oscillators array for 2.4 GHz WiFi band transmission and energy harvesting. Nat Commun 12, 2924 (2021). https://doi.org/10.1038/s41467-021-23181-1
Quantum dots are manmade nanoparticles of semiconducting material comprising only a few thousand atoms. Because of the small number of atoms, a quantum dot’s properties lie between those of single atoms or molecules and bulk material with a huge number of atoms. By changing the nanoparticles’ size and shape, it is possible to fine-tune their electronic and optical properties – how electrons bond and move through the material, and how light is absorbed and emitted by it.
Thanks to increasingly refined control of the nanoparticles’ size and shape, the number of commercial applications has grown. Those already available include lasers, LEDs, and TVs with quantum dot technology.
However, there is a problem that can impair the efficiency of devices or appliances using this nanomaterial as an active medium. When light is absorbed by a material, the electrons are promoted to higher energy levels, and when they return to their fundamental state, each one can emit a photon back to the environment. In conventional quantum dots the electron’s return trip to its fundamental state can be disturbed by various quantum phenomena, delaying the emission of light to the exterior.
The imprisonment of electrons in this way, known as the “dark state”, retards the emission of light, in contrast with the path that lets them return quickly to the fundamental state and hence to emit light more efficiently and directly (“bright state”).
This delay can be shorter in a new class of nanomaterial made from perovskite, which is arousing considerable interest among researchers in materials science as a result (read more at: agencia.fapesp.br/32682/).
A study conducted by researchers in the Chemistry and Physics Institutes of the University of Campinas (UNICAMP) in the state of São Paulo, Brazil, in collaboration with scientists at the University of Michigan in the United States, made strides in this direction by providing novel insights into the fundamental physics of perovskite quantum dots. An article on the study is published in Science Advances.
“We used coherent spectroscopy, which enabled us to analyze separately the behavior of the electrons in each nanomaterial in an ensemble of tens of billions of nanomaterials. The study is groundbreaking insofar as it combines a relatively new class of nanomaterials – perovskite – with an entirely novel detection technique,” Lázaro Padilha Junior, principal investigator for the project on the Brazilian side, told Agência FAPESP.
“We were able to verify the energy alignment between the bright state [associated with triplets] and the dark state [associated with singlets], indicating how this alignment depends on the size of the nanomaterial. We also made discoveries regarding the interactions between these states, opening up opportunities for the use of these systems in other fields of technology, such as quantum information,” Padilha said.
“Owing to the crystal structure of perovskite, the level of bright energy divides into three, forming a triplet. This provides various paths for excitation and for the electrons to return to the fundamental state. The most striking result of the study was that by analyzing the lifetimes of each of the three bright states and the characteristics of the signal emitted by the sample we obtained evidence that the dark state is present but located at a higher energy level than two of the three bright states. This means that when light is shone on the sample the excited electrons are trapped only if they occupy the highest bright level and are then shifted to the dark state. If they occupy the lower bright levels, they return to the fundamental state more efficiently.”
To study how electrons interact with light in these materials, the group used multidimensional coherent spectroscopy (MDCS), in which a burst of ultrashort laser pulses (each lasting about 80 femtoseconds, or 80 quadrillionths of a second) is beamed at a sample of perovskite chilled to minus 269 degrees Celsius.
“The pulses irradiate the sample at tightly controlled intervals. By modifying the intervals and detecting the light emitted by the sample as a function of the interval, we can analyze the electron-light interaction and its dynamics with high temporal precision, mapping the typical interaction times, the energy levels with which they couple, and the interactions with other particles,” Padilha said.
The MDCS technique can be used to analyze billions of nanoparticles at the same time and to distinguish between different families of nanoparticles present in the sample.
The experimental system was developed by a team led by Steven Cundiff, principal investigator for the study at the University of Michigan. Some of the measurements were made by Diogo Almeida, a former member of Cundiff’s team and now at UNICAMP’s ultrafast spectroscopy laboratory with a postdoctoral fellowship from FAPESP under Padilha’s supervision.
Quantum dots were synthesized by Luiz Gustavo Bonato, a PhD candidate at UNICAMP’s Chemistry Institute. “The care Bonato took in preparing the quantum dots and his protocol were fundamentally important, as evidenced by their quality and size, and by the properties of the nanometric material,” said Ana Flávia Nogueira, co-principal investigator for the study in Brazil. Nogueira is a professor at the Chemistry Institute (IQ-UNICAMP) and principal investigator for Research Division 1 at the Center for Innovation in New Energies (CINE), an Engineering Research Center (ERC) established by FAPESP and Shell.
“The results obtained are very important since knowledge of the optical properties of the material and how its electrons behave opens up opportunities for the development of new technologies in semiconductor optics and electronics. The incorporation of perovskite is highly likely to be the most distinctive feature of the next generation of television sets,” Nogueira said.
Featured image: Nanomaterials of perovskite dispersed in hexane and irradiated by laser. Light emission by these materials is intense thanks to resistance to surface defects (photo: Luiz Gustavo Bonato)
Cornell scientists have identified a new contender when it comes to quantum materials for computing and low-temperature electronics.
Using nitride-based materials, the researchers created a material structure that simultaneously exhibits superconductivity – in which electrical resistance vanishes completely – and the quantum Hall effect, which produces resistance with extreme precision when a magnetic field is applied.
“This is a beautiful marriage of the two things we know, at the microscale, that give electrons the most startling quantum properties,” said Debdeep Jena, the David E. Burr Professor of Engineering in the School of Electrical and Computer Engineering and Department of Materials Science and Engineering. Jena led the research, published Feb. 19 in Science Advances, with doctoral student Phillip Dang and research associate Guru Khalsa, the paper’s senior authors.
The two physical properties are rarely seen simultaneously because magnetism is like kryptonite for superconducting materials, according to Jena.
“Magnetic fields destroy superconductivity, but the quantum Hall effect only shows up in semiconductors at large magnetic fields, so you’re having to play with these two extremes,” Jena said. “Researchers in the past few years have been trying to identify materials which show both properties with mixed success.”
The research is the latest validation from the Jena-Xing Lab that nitride materials may have more to offer science than previously thought. Nitrides have traditionally been used for manufacturing LEDs and transistors for products like smartphones and home lighting, giving them a reputation as an industrial class of materials that has been overlooked for quantum computation and cryogenic electronics.
“The material itself is not as perfect as silicon, meaning it has a lot more defects,” said co-author Huili Grace Xing, the William L. Quackenbush Professor of Electrical and Computer Engineering and of Materials Science and Engineering. “But because of its robustness, this material has thrown pleasant surprises to the research community more than once despite its extremely large irregularities in structure. There may be a path forward for us to truly integrate different modalities of quantum computing – computation, memory, communication.”
Such integration could help to condense the size of quantum computers and other next-generation electronics, just as classical computers have shrunk from warehouse to pocket size.
“We’re wondering what this sort of material platform can enable because we see that it’s checking off a lot of boxes,” said Jena, who added that new physical phenomena and technological applications could emerge with further research. “It has a superconductor, a semiconductor, a filter material – it has all kinds of other components, but we haven’t put them all together. We’ve just discovered they can coexist.”
For this research, the Cornell team began engineering epitaxial nitride heterostructures – atomically thin layers of gallium nitride and niobium nitride – and searching for conditions in which magnetic fields and temperatures in the layers would retain their respective quantum Hall and superconducting properties.
They eventually discovered a small window in which the properties were observed simultaneously, thanks to advances in the quality of the materials and structures produced in close collaboration with colleagues at the Naval Research Laboratory.
“The quality of the niobium-nitride superconductor was improved enough that it can survive higher magnetic fields, and simultaneously we had to improve the quality of the gallium-nitride semiconductor enough that it could exhibit the quantum Hall effect at lower magnetic fields,” Dang said. “And that’s what will really allow for potential new physics to be seen at low temperature.”
Potential applications for the material structure include more efficient electronics, such as data centers cooled to extremely low temperatures to eliminate heat waste. And the structure is the first to lay the groundwork for the use of nitride semiconductors and superconductors in topological quantum computing, in which the movement of electrons must be resilient to the material defects typically seen in nitrides.
“What we’ve shown is that the ingredients you need to make this topological phase can be in the same structure,” Khalsa said, “and I think the flexibility of the nitrides really opens up new possibilities and ways to explore topological states of matter.”
Other co-authors of the paper include David Muller, the Samuel B. Eckert Professor of Engineering in the School of Applied and Engineering Physics; and researchers from the U.S. Naval Research Laboratory, the National High Magnetic Field Laboratory, and semiconductor company Qorvo.
The research was funded by the Office of Naval Research and the National Science Foundation.
Reference: Phillip Dang, Guru Khalsa, Celesta S. Chang et al., “An all-epitaxial nitride heterostructure with concurrent quantum Hall effect and superconductivity”, Science Advances 19 Feb 2021: Vol. 7, no. 8, eabf1388 DOI: 10.1126/sciadv.abf1388
Walking quadruped is controlled and powered by pressurized air
Engineers at the University of California San Diego have created a four-legged soft robot that doesn’t need any electronics to work. The robot only needs a constant source of pressurized air for all its functions, including its controls and locomotion systems.
The team, led by Michael T. Tolley, a professor of mechanical engineering at the Jacobs School of Engineering at UC San Diego, details its findings in the Feb. 17, 2021 issue of the journal Science Robotics.
“This work represents a fundamental yet significant step towards fully-autonomous, electronics-free walking robots,” said Dylan Drotman, a Ph.D. student in Tolley’s research group and the paper’s first author.
Applications include low-cost robotics for entertainment, such as toys, and robots that can operate in environments where electronics cannot function, such as MRI machines or mine shafts. Soft robots are of particular interest because they easily adapt to their environment and operate safely near humans.
Most soft robots are powered by pressurized air and are controlled by electronic circuits. But this approach requires complex components like circuit boards, valves and pumps—often outside the robot’s body. These components, which constitute the robot’s brains and nervous system, are typically bulky and expensive. By contrast, the UC San Diego robot is controlled by a light-weight, low-cost system of pneumatic circuits, made up of tubes and soft valves, onboard the robot itself. The robot can walk on command or in response to signals it senses from the environment.
“With our approach, you could make a very complex robotic brain,” said Tolley, the study’s senior author. “Our focus here was to make the simplest air-powered nervous system needed to control walking.”
The robot’s computational power roughly mimics mammalian reflexes that are driven by a neural response from the spine rather than the brain. The team was inspired by neural circuits found in animals, called central pattern generators, made of very simple elements that can generate rhythmic patterns to control motions like walking and running.
To mimic the generators’ functions, engineers built a system of valves that act as oscillators, controlling the order in which pressurized air enters air-powered muscles in the robot’s four limbs. Researchers built an innovative component that coordinates the robot’s gait by delaying the injection of air into the robot’s legs. The robot’s gait was inspired by sideneck turtles.
The robot is also equipped with simple mechanical sensors—little soft bubbles filled with fluid placed at the end of booms protruding from the robot’s body. When the bubbles are depressed, the fluid flips a valve in the robot that causes it to reverse direction.
The Science Robotics paper builds on previous work by other research groups that developed oscillators and sensors based on pneumatic valves, and adds the components necessary to achieve high-level functions like walking.
How it works?
The robot is equipped with three valves acting as inverters that cause a high pressure state to spread around the air-powered circuit, with a delay at each inverter.
Each of the robot’s four legs has three degrees of freedom powered by three muscles. The legs are angled downward at 45 degrees and composed of three parallel, connected pneumatic cylindrical chambers with bellows. When a chamber is pressurized, the limb bends in the opposite direction. As a result, the three chambers of each limb provide multi-axis bending required for walking. Researchers paired chambers from each leg diagonally across from one another, simplifying the control problem.
A soft valve switches the direction of rotation of the limbs between counterclockwise and clockwise. That valve acts as what’s known as a latching double pole, double throw switch—a switch with two inputs and four outputs, so each input has two corresponding outputs it’s connected to. That mechanism is a little like taking two nerves and swapping their connections in the brain.
In the future, researchers want to improve the robot’s gait so it can walk on natural terrains and uneven surfaces. This would allow the robot to navigate over a variety of obstacles. This would require a more sophisticated network of sensors and as a result a more complex pneumatic system.
The team will also look at how the technology could be used to create robots, which are in part controlled by pneumatic circuits for some functions, such as walking, while traditional electronic circuits handle higher functions.
This work is supported by the Office of Naval Research, grant numbers N00014-17-1-2062 and N00014-18-1-2277.
Reference: Dylan Drotman, Saurabh Jadhav, David Sharp, Christian Chan and Michael T. Tolley, “Electronics-free pneumatic circuits for controlling soft legged robots”, Science Robotics 17 Feb 2021: Vol. 6, Issue 51, eaay2627 DOI: 10.1126/scirobotics.aay2627
A Russian physicist and his international colleagues studied a quantum point contact (QCP) between two conductors with external oscillating fields applied to the contact. They found that, for some types of contacts, an increase in the oscillation frequency above a critical value reduced the current to zero – a promising mechanism that can help create nanoelectronics components. This research supported by the Russian Science Foundation (RSF) was published in the Physical Review B journal.
A persistent trend in the modern electronics, miniaturization has spurred demand for new nano-sized devices that boast advanced performance and leverage quantum effects with electrons behaving as particles and waves at the same time. Of particular importance is precise control of charge transport by means of external electric and magnetic fields. This can be achieved in a tiny QPC comparable in size to an atom (several angstroms) and with just a few electron wavelengths fitting in. Such contacts can be obtained experimentally by connecting two massive electrodes with a layer of two-dimensional electron gas, i.e. gas with particles freely moving in two directions only, and then applying voltage to the plates. The higher the voltage, the larger the forbidden area for the electrons and the narrower the contact.
The authors did theoretical research on two conductors connected by a QPC subjected to external oscillating fields. The charge carriers in the conductors were assumed to have different initial concentrations. At low oscillation frequencies, the current at the contact tends to equalize the concentrations. However, the scientists discovered that, for a certain type of contacts, the current drops to zero and the concentrations are never equal at frequencies above the critical value. This provides telling evidence of a non-equilibrium phase transition − a dynamic phenomenon which accounts for the fundamental difference between the system properties below and above the critical value of an external parameter, in this case, oscillation frequency.
“This striking effect is best illustrated by a simple example. Imagine two vessels filled with water and their bottoms connected by a tube. If the water levels are different, water will keep flowing from one vessel to the other until its levels are the same in both vessels. Now imagine that we shake the tube with a frequency above some critical value. Water will stop flowing and will never balance out to the same level. Of course, this does not happen to water in real life, but it does happen to electrons flowing through a quantum contact “shaken” by external electric and magnetic fields,” explains Oleg Lychkovskiy, a PhD in physics and mathematics and a senior research scientist at the Skolkovo Institute of Science and Technology (Skoltech), Moscow Institute of Physics and Technology and (MIPT) and V.A. Steklov Mathematical Institute of RAS.
This research can pave the way for new nanometer-scale electronic devices with a broad range of potential applications. Electronic devices and systems based on quantum effects are a promising avenue of research, considering that the Russian nanoelectronics and photonics market may balloon to 20 billion rubles by 2027.