Tech

Deformations and defects in structures of photoelectric technologies shown to improve their efficiency

University of Warwick physicists demonstrate that strain gradient can prevent recombination of photo-excited carriers in solar energy conversion

Increasingly important as devices become miniaturised

Solar cells and light sensing technologies could be made more efficient by taking advantage of an unusual property due to deformations and defects in their structures.

Researchers from the University of Warwick's Department of Physics have found that the strain gradient (i.e. inhomogenous strain) in the solar cells, through physical force or induced during the fabrication process, can prevent photo-excited carriers from recombining, leading to an enhanced solar energy conversion efficiency. The results of their experiments have been published in Nature Communications.

The team of scientists used an epitaxial thin film of BiFeO3 grown on LaAlO3 substrate to determine the impact of inhomogenous deformation on the film's ability to convert light into electricity by examining how its strain gradient affects its ability to separate photo-excited carriers.

Most commercial solar cells are formed of two layers creating at their boundary a junction between two kinds of semiconductors, p-type with positive charge carriers (electron vacancies) and n-type with negative charge carriers (electrons). When light is absorbed, the junction of the two semiconductors sustains an internal field splitting the photo-excited carriers in opposite directions, generating a current and voltage across the junction. Without such junctions the energy cannot be harvested and the photo-excited carriers will simply quickly recombine eliminating any electrical charge.

They found that the strain gradient can help prevent recombination by separating the light-excited electron-holes, enhancing the conversion efficiency of the solar cells. The BiFeO3/LaAlO3 film also exhibited some interesting photoelectric effects, such as persistent photoconductivity (improved electrical conductivity). It has potential applications in UV light sensors, actuators and transducers.

Dr Mingmin Yang from the University of Warwick said: "This work demonstrated the critical role of the strain gradient in mediating local photoelectric properties, which is largely overlooked previously. By engineering photoelectric technologies to take advantage of strain gradient, we may potentially increase the conversion efficiency of solar cells and enhance the sensitivity of light sensors.

"Another factor to consider is the grain boundaries in polycrystalline solar cells. Generally, defects accumulate at the grain boundaries, which would induce photo-carrier recombination, limiting the efficiency. However, in some polycrystalline solar cells, such as CdTe solar cells, the grain boundaries would promote the collection of photo-carriers, where the giant strain gradient might play an important role. Therefore, we need to pay attention to the local strain gradient when we study the structure-properties relations in solar cells and light sensor materials."

Previously, the effect of this strain on efficiency was thought to be negligible. With the increasing miniaturisation of technologies, the effect of strain gradient becomes magnified at smaller sizes. So in reducing the size of a device using one of these films, the magnitude of strain gradient increases dramatically.

Dr Yang adds: "The strain gradient induced effect, such as flexo-photovoltaic effect, ionic migration, etc, would be increasingly important at low dimensions."

By combining thin organic layers with thick layers of hybrid perovskite, researchers at Kyushu University in Japan have developed micrometer-thick organic light-emitting diodes that could improve the affordability and viewing angles of high-performance displays and televisions in the near future.

Organic light-emitting diodes (OLEDs) use layers of organic molecules to efficiently convert electricity into light. The molecules, though great emitters, are generally poor electrical conductors, so the name of the game has been thin--as in 100 nm, or about 1/500 the thickness of a human hair. Only by using such thin layers can electricity easily reach where emission occurs in the middle of devices.

While extremely thin layers benefit from needing only a small amount of material, the use of such thin films complicates the reliable fabrication of millions of pixels since extremely small defects can cause device failure. Furthermore, light reflecting between the front and back of the thin layers often results in interactions--called cavity effects--that slightly distort the emission color at large viewing angles.

Thus, the challenge has been to make the devices thicker while avoiding the drawbacks of organics. To do this, researchers at Kyushu University turned to an alternative class of materials called perovskites, which are defined by their distinct crystal structure.

"Although perovskites have recently attracted a huge amount of attention as light-absorbing layers in solar cells, some perovskites are actually transparent while also being highly conductive," says Toshinori Matsushima, associate professor of the International Institute for Carbon-Neutral Energy Research at Kyushu University and lead researcher on the Nature paper announcing the new results.

"In addition, perovskites based on a blend of organic and inorganic components can be processed from low-cost starting materials using the same fabrication processes as for organics, making perovskites and organics a perfect match."

In their devices, the researchers sandwiched an emitting layer of molecules typically used in OLEDs between perovskite layers with a total thickness of 2,000 nm. The resulting devices have active layers that are 10-times thicker than typical OLEDs--though still a fraction of the width of a human hair.

The thick devices exhibited efficiencies that were similar to those in reference thin OLEDs while also having the same color from every viewing angle. On the other hand, OLEDs based on thick organic layers did not emit any light at similar operating voltages.

"These results overturn 30 years of thinking that OLEDs are limited to thin films and open new paths for low-cost, reliable, and uniform fabrication of OLED-based displays and lighting," says Prof. Chihaya Adachi, director of Kyushu University's Center for Organic Photonics and Electronics Research.

While researchers have also been attempting to use perovskites directly as light emitters, the lifetimes of the devices have been short so far. By keeping the emission process in the organic materials and using perovskites just for transporting electricity, the Kyushu team achieved similar lifetimes for both thick devices and reference OLEDs.

"Based on this work, perovskites will be seen in a new light as versatile, high-performance materials for supporting roles in not only OLEDs but also other organic electronic devices, such as lasers, memory devices, and sensors," predicts Adachi.

Osaka, Japan - A research team led by Osaka University demonstrated how information encoded in the circular polarization of a laser beam can be translated into the spin state of an electron in a quantum dot, each being a quantum bit and a quantum computer candidate. The achievement represents a major step towards a "quantum internet," in which future computers can rapidly and securely send and receive quantum information.

Quantum computers have the potential to vastly outperform current systems because they work in a fundamentally different way. Instead of processing discrete ones and zeros, quantum information, whether stored in electron spins or transmitted by laser photons, can be in a superposition of multiple states simultaneously. Moreover, the states of two or more objects can become entangled, so that the status of one cannot be completely described without this other. Handling entangled states allow quantum computers to evaluate many possibilities simultaneously, as well as transmit information from place to place immune from eavesdropping.

However, these entangled states can be very fragile, lasting only microseconds before losing coherence. To realize the goal of a quantum internet, over which coherent light signals can relay quantum information, these signals must be able to interact with electron spins inside distant computers.

Researchers led by Osaka University used laser light to send quantum information to a quantum dot by altering the spin state of a single electron trapped there. While electrons don't spin in the usual sense, they do have angular momentum, which can be flipped when absorbing circularly polarized laser light.

"Importantly, this action allowed us to read the state of the electron after applying the laser light to confirm that it was in the correct spin state," says first author Takafumi Fujita. "Our readout method used the Pauli exclusion principle, which prohibits two electrons from occupying the exact same state. On the tiny quantum dot, there is only enough space for the electron to pass the so-called Pauli spin blockade if it has the correct spin."

Quantum information transfer has already been used for cryptographic purposes. "The transfer of superposition states or entangled states allows for completely secure quantum key distribution," senior author Akira Oiwa says. "This is because any attempt to intercept the signal automatically destroys the superposition, making it impossible to listen in without being detected."

The rapid optical manipulation of individual spins is a promising method for producing a quantum nano-scale general computing platform. An exciting possibility is that future computers may be able to leverage this method for many other applications, including optimization and chemical simulations.

Does the treatment success of a certain procedure depend on the number of cases in a hospital or on how often the doctors working there have already performed this procedure? In Germany, this is the subject of 8 commissions on minimum volumes awarded by the Federal Joint Committee (G-BA) to the Institute for Quality and Efficiency in Health Care (IQWiG). The IQWiG report is now available for the first indication investigated, stem cell transplantation. According to the findings, studies based on registry data show that there is indeed a correlation between treatment results and the frequency of the service provided (stem cell transplantation). This applies in particular to the survival chances of patients: The more frequently the transplantation team performs the procedure, the higher the chances of survival. In contrast, no studies could be found investigating the healthcare consequences of setting a specific minimum number of cases (25 in Germany).

Minimum Volume Regulation reformed in 2017

For about 40 years, experts have been discussing whether treatment results are better if a medical procedure is performed more frequently. In 2003, the G-BA set minimum volumes for certain plannable inpatient services for the first time: Since then, hospitals may only provide and invoice these services if they reach the set annual minimum volumes.

Since 2016, for setting minimum volumes only evidence is required "indicating a probable correlation". In 2017, the G-BA adjusted its Minimum Volume Regulation accordingly and has now commissioned IQWiG to examine the evidence in 8 indications.

Data from registries analysed

In accordance with the G-BA's commission, IQWiG searched for studies that would either allow robust conclusions to be drawn as to how the volume and quality of services provided for (autologous or allogeneic) stem cell transplantation were related, or could answer the question as to how a binding number of cases per hospital and year affected treatment success.

For the first question, IQWiG was able to include 4 observational studies (cohort studies) in the assessment, each of which had analysed patient data from international clinical registries on stem cell transplantation for malignant haematological diseases. No evidence was available for the second question, which therefore currently remains unanswered.

Only one study was highly informative

The number of patients included in the 4 studies ranged from 684 to 107,904. However, precisely the 2 largest studies were poorly informative, partly because it was unclear according to which criteria patient data had been considered in the analysis. The most informative study analysed data from 4285 patients with acute or chronic leukaemia (Loberiza 2005).

It was not possible to statistically pool the data of all studies, partly because the characteristics of the participants (e.g. age, sex, underlying disease) were too different or unknown.

Patients survived longer with a higher volume of services

Regarding mortality, the high-quality Loberiza study showed that patients survived longer after stem cell transplantation if doctors had already performed this procedure more frequently. Two studies also supported this result in terms of the volume of services provided by the hospital. However, from a methodological point of view, both of these studies were less informative.

For other outcomes, the correlations in these overall poorly informative studies were considerably weaker or even non-existent. No data at all in the 4 studies considered were available on rejection reactions in the event of a foreign donor (allogeneic stem cell transplantation) or quality of life.

It remains to be seen whether the choice of minimum volume in Germany is correct

IQWiG project manager Eva Höfer explains the results of the report: "In terms of mortality, we see a positive correlation between the volume of stem cell transplantations and treatment success. However, due to a lack of usable data, it is not possible to assess the effects of setting specific minimum volumes, for example, on patient mortality after stem cell transplantation. It therefore remains to be seen whether a case number of 25 cases per hospital location and year guarantees an optimal chance of survival for patients."

Process of report production

In July 2018, the Federal Joint Committee (G-BA) commissioned IQWiG to prepare the report in an accelerated procedure as a so-called rapid report. Interim products were therefore not published or made available for a hearing. This rapid report was sent to the contracting agency, the G-BA, in June 2019.

Studies of how global change is impacting marine organisms have long focused on physiological effects--for example an oyster's decreased ability to build or maintain a strong shell in an ocean that is becoming more acidic due to excess levels of carbon dioxide.

More recently, researchers have begun to investigate how different facets of global change can disrupt animal behavior.

Now, a study led by Dr. Emily Rivest of William & Mary's Virginia Institute of Marine Science synthesizes the results of these pioneering behavioral studies--revealing both broad patterns and intriguing outliers--and provides a conceptual framework to help guide future research in this emerging field.

"Climate change will significantly impact marine organisms by altering sensory pathways," says Rivest, an assistant professor at VIMS. "This will have consequences for ecological and evolutionary interactions, including mating, predation, and habitat selection."

As an example, she cites a 2016 study led by co-author Brittany Jellison of the University of California, Davis, which showed that ocean acidification makes tidepool snails more prone to sea-star predation by interfering with their ability to process and respond to chemical cues that starfish leave in the water. An ongoing, VIMS-led study in the Chesapeake Bay is also looking at how ocean acidification might affect a predator-prey interaction--that between blue crabs and clams.

The current study, based on a review of 120 pertinent journal articles, appears in the latest issue of Frontiers in Marine Science. In addition to Rivest and Jellison, it was co-authored by Gabriel Ng, Erin Satterthwaite, Susan Williams, and Brian Gaylord of UC Davis and Hannah Bradley of James Madison University. The study was funded by the National Science Foundation.

Sensory pathways

One of the study's goals was to clarify how human pressures on the environment might impact each step in the sensory pathways between marine organisms.

"Our review," says Rivest, "emphasizes that behavior is the outcome of a sensory pathway that includes the production of information, transmission of that information through the environment, reception of the information by an organism, and then a response--what the organism decides to do with the information." The review also distinguishes between information produced incidentally--what scientists call a cue--and information that an organism produces intentionally--a signal.

"Knowing where, and in how many places, climate change breaks that pathway," she adds, "will help us anticipate how it might affect broader ecological processes, like food-web and population dynamics."

Adds Satterthwaite, "If we can better illuminate where climate change stressors impact the sensory pathway, then we can develop more targeted, effective management and conservation efforts."

The team's review turned up examples of global-change impacts at all steps in the sensory pathway. For example:

Studies with weakfish and croaker, popular recreational fish species in the Chesapeake Bay, found that warmer waters and increased turbidity alter production of the sounds that males of both species use to attract females, thus potentially affecting breeding success.

Studies with cormorants reveal that increased turbidity due to nutrient pollution and more frequent cloudbursts can degrade light transmission, possibly lowering feeding success among these highly visual avian predators.

A 2013 study showed that raising cobia larvae in waters with elevated carbon dioxide levels affects their otoliths, which likely alters their ability to detect sound. Otoliths are calcified bodies in the inner ear that vertebrates use to sense gravity and movement.

The team chose as their global-change impacts those listed by the Intergovernmental Panel on Climate Change in its most recent report: rising temperatures, ocean acidification, enhanced low-oxygen zones, salinity changes, heightened UV-B radiation, increased turbidity, altered hydrodynamics, greater stratification, and excess nutrients. The IPCC is the U.N. body tasked with guiding the international response to the climate-change crisis.

Think globally, act locally

The team's findings have implications on both the global and local scale.

"First," says Rivest, "we found there are some generalizable mechanisms. If a global change stressor affects the production of a visual cue or signal, it's likely to affect the production of acoustic and chemical cues and signals as well, and vice versa." Indeed, of the five environmental factors that affected production--rising temperatures, ocean acidification, low oxygen, salinity changes, and increased turbidity--four did so for multiple senses. Ocean acidification was found to affect the production of visual, olfactory, and auditory cues and signals.

"We think the common denominator could be physiological stress," says Rivest. "The change in temperature, salinity, or ocean acidification stresses the organism and may impair its ability to produce a cue or signal."

But when it comes to transmission, the team's review found that global change stressors tend to affect just one mode--acoustic, visual, or olfactory. For instance, Rivest cites a study that determined the decline of a Florida coral reef was substantially affected by increased nitrogen from land-based runoff.

"That's one of the benefits I see of doing work like this is," says Rivest. "If you can better understand the relationship between pathways and more local stressors such as nutrient pollution, then you have a more tractable way to make a difference. Rather than trying to affect the behavior of the whole world--to lower carbon emissions--you could work to manage local causes of pollution such as land clearing and stormwater runoff. These local solutions may actually be more effective in some places, and they are not currently getting as much attention."

The ability to combine many functions into a single microchip is a significant advance in the quest to perfect the tiny, self-powered sensors that will expand the Internet of things. KAUST researchers have managed to combine sensing, energy-harvesting, current-rectifying and energy-storage functions into a single microchip.

"Previously, researchers had to use bulky rectifiers that converted intermittent harvested electrical energy into steady direct current for storage in electrochemical microsupercapacitors," says Mrinal K. Hota, research scientist at KAUST and lead author of the study.

Hota explains that the key to integrating everything into a single chip was the development of ruthenium oxide (RuO2) as the common electrode material connecting all devices in the microcircuits. The team envisages a broad range of applications from monitoring personal health indications directly from the human body to environmental and industrial sensing.

"Our achievement simplifies device fabrication and realizes significant miniaturization of self-powered sensor devices," says project leader Husam Alshareef.

The ruthenium-oxide contacts are laid onto a glass or silicon substrate to connect sensing, energy-harvesting and current-rectifying electronics with one or more electrochemical microsupercapacitors that store the electrical energy. This creates a tiny system that can operate without any battery power. Instead it uses available body movement or machinery vibrations as the reliable and continual source of energy.

"Unlike a battery, electrochemical microsupercapacitors can last for hundreds of thousands of cycles rather than just a few thousand," Hota points out. They can also deliver a significantly higher power output from a given volume.

A key to creating electrode material suitable for connecting all devices was to make optimal ruthenium-dioxide surfaces with controlled roughness, defects and conductivity. These features enabled the team to use RuO2 for both electronics and electrochemical microsupercapacitors.

Another crucial innovation was to use a gel that, after application, solidifies into the supercapacitors' electrolyte. This is a material that transports electric charge in the form of ions. The solidified gel was chosen to avoid any damage to rectifiers and thin-film transistors.

The researchers now plan to work to optimize the RuO2 electrodes further and explore linking many different types of sensors into their chips. They also want to investigate adding wireless communication into the device. This would allow biosensors and environmental sensors to send data remotely to any wireless receivers, including mobile phones and personal computers.

In the year 2026, at rush hour, your self-driving car abruptly shuts down right where it blocks traffic. You climb out to see gridlock down every street in view, then a news alert on your watch tells you that hackers have paralyzed all Manhattan traffic by randomly stranding internet-connected cars.

Flashback to July 2019, the dawn of autonomous vehicles and other connected cars, and physicists at the Georgia Institute of Technology and Multiscale Systems, Inc. have applied physics in a new study to simulate what it would take for future hackers to wreak exactly this widespread havoc by randomly stranding these cars. The researchers want to expand the current discussion on automotive cybersecurity, which mainly focuses on hacks that could crash one car or run over one pedestrian, to include potential mass mayhem.

They warn that even with increasingly tighter cyber defenses, the amount of data breached has soared in the past four years, but objects becoming hackable can convert the rising cyber threat into a potential physical menace.

"Unlike most of the data breaches we hear about, hacked cars have physical consequences," said Peter Yunker, who co-led the study and is an assistant professor in Georgia Tech's School of Physics.

It may not be that hard for state, terroristic, or mischievous actors to commandeer parts of the internet of things, including cars.

"With cars, one of the worrying things is that currently there is effectively one central computing system, and a lot runs through it. You don't necessarily have separate systems to run your car and run your satellite radio. If you can get into one, you may be able to get into the other," said Jesse Silverberg of Multiscale Systems, Inc., who co-led the study with Yunker.

Freezing traffic solid

In simulations of hacking internet-connected cars, the researchers froze traffic in Manhattan nearly solid, and it would not even take that to wreak havoc. Here are their results, and the numbers are conservative for reasons mentioned below.

"Randomly stalling 20 percent of cars during rush hour would mean total traffic freeze. At 20 percent, the city has been broken up into small islands, where you may be able to inch around a few blocks, but no one would be able to move across town," said David Yanni, a graduate research assistant in Junker's lab.

Not all cars on the road would have to be connected, just enough for hackers to stall 20 percent of all cars on the road. For example, if 40 percent of all cars on the road were connected, hacking half would suffice.

Hacking 10 percent of all cars at rush hour would debilitate traffic enough to prevent emergency vehicles from expediently cutting through traffic that is inching along citywide. The same thing would happen with a 20 percent hack during intermediate daytime traffic.

The researchers' results appear in the journal Physical Review E on July 20, 2019. The study is not embargoed.

It could take less

For the city to be safe, hacking damage would have to be below that. In other cities, things could be worse.

"Manhattan has a nice grid, and that makes traffic more efficient. Looking at cities without large grids like Atlanta, Boston, or Los Angeles, and we think hackers could do worse harm because a grid makes you more robust with redundancies to get to the same places down many different routes," Yunker said.

The researchers left out factors that would likely worsen hacking damage, thus a real-world hack may require stalling even fewer cars to shut down Manhattan.

"I want to emphasize that we only considered static situations - if roads are blocked or not blocked. In many cases, blocked roads spill over traffic into other roads, which we also did not include. If we were to factor in these other things, the number of cars you'd have to stall would likely drop down significantly," Yunker said.

The researchers also did not factor in ensuing public panic nor car occupants becoming pedestrians that would further block streets or cause accidents. Nor did they consider hacks that would target cars at locations that maximize trouble.

They also stress that they are not cybersecurity experts, nor are they saying anything about the likelihood of someone carrying out such a hack. They simply want to give security experts a calculable idea of the scale of a hack that would shut a city down.

The researchers do have some general ideas of how to reduce the potential damage.

"Split up the digital network influencing the cars to make it impossible to access too many cars through one network," said lead author Skanka Vivek, a postdoctoral researcher in Yunker's lab. "If you could also make sure that cars next to each other can't be hacked at the same time that would decrease the risk of them blocking off traffic together."

Traffic jams as physics

Yunker researches in soft matter physics, which looks at how constituent parts - in this case, connected cars - act as one whole physical phenomenon. The research team analyzed the movements of cars on streets with varying numbers of lanes, including how they get around stalled vehicles and found they could apply a physics approach to what they observed.

"Whether traffic is halted or not can be explained by classic percolation theory used in many different fields of physics and mathematics," Yunker said.

Percolation theory is often used in materials science to determine if a desirable quality like a specific rigidity will spread throughout a material to make the final product uniformly stable. In this case, stalled cars spread to make formerly flowing streets rigid and stuck.

The shut streets would be only those in which hacked cars have cut off all lanes or in which they have become hindrances that other cars can't maneuver around and do not include streets where hacked cars still allow traffic flow.

The researchers chose Manhattan for their simulations because a lot of data was available on that city's traffic patterns.

Stanford physicists have developed a "quantum microphone" so sensitive that it can measure individual particles of sound, called phonons.

The device, which is detailed July 24 in the journal Nature, could eventually lead to smaller, more efficient quantum computers that operate by manipulating sound rather than light.

"We expect this device to allow new types of quantum sensors, transducers and storage devices for future quantum machines," said study leader Amir Safavi-Naeini, an assistant professor of applied physics at Stanford's School of Humanities and Sciences.

Quantum of motion

First proposed by Albert Einstein in 1907, phonons are packets of vibrational energy emitted by jittery atoms. These indivisible packets, or quanta, of motion manifest as sound or heat, depending on their frequencies.

Like photons, which are the quantum carriers of light, phonons are quantized, meaning their vibrational energies are restricted to discrete values - similar to how a staircase is composed of distinct steps.

"Sound has this granularity that we don't normally experience," Safavi-Naeini said. "Sound, at the quantum level, crackles."

The energy of a mechanical system can be represented as different "Fock" states - 0, 1, 2, and so on - based on the number of phonons it generates. For example, a "1 Fock state" consist of one phonon of a particular energy, a "2 Fock state" consists of two phonons with the same energy, and so on. Higher phonon states correspond to louder sounds.

Until now, scientists have been unable to measure phonon states in engineered structures directly because the energy differences between states - in the staircase analogy, the spacing between steps - is vanishingly small. "One phonon corresponds to an energy ten trillion trillion times smaller than the energy required to keep a lightbulb on for one second," said graduate student Patricio Arrangoiz-Arriola, a co-first author of the study.

To address this issue, the Stanford team engineered the world's most sensitive microphone - one that exploits quantum principles to eavesdrop on the whispers of atoms.

In an ordinary microphone, incoming sound waves jiggle an internal membrane, and this physical displacement is converted into a measurable voltage. This approach doesn't work for detecting individual phonons because, according to the Heisenberg uncertainty principle, a quantum object's position can't be precisely known without changing it.

"If you tried to measure the number of phonons with a regular microphone, the act of measurement injects energy into the system that masks the very energy that you're trying to measure," Safavi-Naeini said.

Instead, the physicists devised a way to measure Fock states - and thus, the number of phonons - in sound waves directly. "Quantum mechanics tells us that position and momentum can't be known precisely - but it says no such thing about energy," Safavi-Naeini said. "Energy can be known with infinite precision."

Singing qubits

The quantum microphone the group developed consists of a series of supercooled nanomechanical resonators, so small that they are visible only through an electron microscope. The resonators are coupled to a superconducting circuit that contains electron pairs that move around without resistance. The circuit forms a quantum bit, or qubit, that can exist in two states at once and has a natural frequency, which can be read electronically. When the mechanical resonators vibrate like a drumhead, they generate phonons in different states.

"The resonators are formed from periodic structures that act like mirrors for sound. By introducing a defect into these artificial lattices, we can trap the phonons in the middle of the structures," Arrangoiz-Arriola said.

Like unruly inmates, the trapped phonons rattle the walls of their prisons, and these mechanical motions are conveyed to the qubit by ultra-thin wires. "The qubit's sensitivity to displacement is especially strong when the frequencies of the qubit and the resonators are nearly the same," said joint first-author Alex Wollack, also a graduate student at Stanford.

However, by detuning the system so that the qubit and the resonators vibrate at very different frequencies, the researchers weakened this mechanical connection and triggered a type of quantum interaction, known as a dispersive interaction, that directly links the qubit to the phonons.

This bond causes the frequency of the qubit to shift in proportion to the number of phonons in the resonators. By measuring the qubit's changes in tune, the researchers could determine the quantized energy levels of the vibrating resonators - effectively resolving the phonons themselves.

"Different phonon energy levels appear as distinct peaks in the qubit spectrum," Safavi-Naeini said. "These peaks correspond to Fock states of 0, 1, 2 and so on. These multiple peaks had never been seen before."

Mechanical quantum mechanical

Mastering the ability to precisely generate and detect phonons could help pave the way for new kinds of quantum devices that are able to store and retrieve information encoded as particles of sound or that can convert seamlessly between optical and mechanical signals.

Such devices could conceivably be made more compact and efficient than quantum machines that use photons, since phonons are easier to manipulate and have wavelengths that are thousands of times smaller than light particles.

"Right now, people are using photons to encode these states. We want to use phonons, which brings with it a lot of advantages," Safavi-Naeini said. "Our device is an important step toward making a 'mechanical quantum mechanical' computer."

Solar panel installations are on the rise in the U.S., with more than 2 million new installations in early 2019, the most ever recorded in a first quarter, according to a recent report by Solar Energy Industries Association and Wood Mackenzie Power & Renewables.

To meet the ever-increasing demands, low-cost and more efficient alternatives to silicon-based solar cells -- currently the most widely used technology -- are desirable. In the past decade, lead-halide perovskites have surged as the most promising class of alternative materials; however, they are unstable. They contain lead, which is toxic and poses potential health and environmental hazards such as groundwater contamination.

A team of engineers at Washington University in St. Louis has found what they believe is a more stable, less toxic semiconductor for solar applications using a novel double perovskite oxide discovered through data analytics and quantum-mechanical calculations.

Their work was published online June 11 in Chemistry of Materials.

Rohan Mishra, assistant professor of mechanical engineering & materials science in the McKelvey School of Engineering, led an interdisciplinary, international team that discovered the new semiconductor, made up of potassium, barium, tellurium, bismuth and oxygen (KBaTeBiO6). The lead-free double perovskite oxide was one of an initial 30,000 potential bismuth-based oxides. Of those 30,000, only about 25 were known compounds.

Using materials informatics and quantum mechanical calculations on one of the fastest supercomputers in the world, Arashdeep Singh Thind, a doctoral student in Mishra's lab based at Oak Ridge National Laboratory, found KBaTeBiO6 to be the most promising out of the 30,000 potential oxides.

"We found that this looked to be the most stable compound and that it could be synthesized in the lab," Mishra said. "More importantly, whereas most oxides tend to have a large band, we predicted the new compound to have a lower band gap, which is close to the halide perovskites, and to have reasonably good properties."

The band gap is the energy barrier that electrons must overcome to form free carriers that, in the context of a solar cell, can be extracted to power an electrical device or stored in a battery for later use. The energy to overcome this barrier is provided by sunlight. The most promising compounds for solar cell applications have a band gap of about 1.5 eV, or electronvolt, Mishra said.

Mishra discussed the possibility of synthesizing KBaTeBiO6 with Pratim Biswas, assistant vice chancellor, the Lucy & Stanley Lopata Professor and chair of the Department of Energy, Environmental & Chemical Engineering. Shalinee Kavadiya, then a McKelvey Engineering doctoral student and now a postdoctoral research associate at Arizona State University, got to work on perfecting the recipe.

"Shalinee spent about six months synthesizing the material," Mishra said. "Once she was able to synthesize it, as we had predicted, it was stable and had a band gap of 1.88 eV, which we also predicted."

Mishra said these are first-generation solar cells that need more fine tuning of the band gap, but it is a good first step toward nontoxic solar cells.

"This shows that we can go away from these lead-halide perovskites," Mishra said. "This opens up a really big space for designing semiconductors not just for solar cell applications but also for other semiconductor applications, such as LCD displays."

Next, the team will study the role of any defects in this new semiconductor and look to more advanced synthesis techniques, including using aerosol techniques.

Social dilemmas are ever-present in contemporary society; these are situations where individual incentives aren't aligned with group goals. One way to overcome social dilemmas is through prosocial institutions.

Explaining the origin of these institutions, from individual incentives, is problematic, because it's hard to incentivise individuals to contribute to establish such institutions.

A new computational model shows that the perception of the possibility of punishment (for example, by way of fines) is key to promote the kind of institutions that prevent cheating behaviour and sustain cooperative interactions.

Social dilemmas are ever-present in contemporary society, with people 'cheating the system' and jeopardising collective outcomes in place of their own personal gains.

Using excess water during summer restrictions, avoiding the payment of taxes and refusing to vote are all examples of cheating a public good.

One way to overcome these social dilemmas is through prosocial institutions - arrangements in which those who don't contribute to a common good get punished. From a mathematical perspective, explaining the origin of these institutions, from individual incentives, is problematic, because it's hard to incentivise individuals to contribute to establish such institutions.

However, new international research has found that one way to overcome these social dilemmas is through visible prosocial punishment - the existence of collective institutions that punish individuals who don't cooperate.

A new computational model shows that the perception of the possibility of punishment (for example, by way of fines) is key to paving the way for the institutions that prevent antisocial behaviour.

Dr Julian Garcia, Senior Lecturer in Monash University's Faculty of Information Technology, and Professor Arne Traulsen, from the Max Planck Institute for Evolutionary Biology in Germany, found that punishment in itself isn't enough to promote prosocial behaviour. In addition, the institutions that implement this punishment should be visible and advertised.

"Most modern societies put law enforcement into the hands of institutions and do not allow their citizens to punish others directly," Dr Garcia said.

"Our modelling suggests that the human instinct against taking the law into our own hands is justified. The value of the signal conferred by the presence of punishment institutions may be crucial in promoting the kind of cooperation we observe in humans.

"These findings are useful in understanding contemporary social dilemmas, and the potential of institutions to solve them."

Researchers used a mathematical and computational model to understand the incentives and effectiveness provided by prosocial punishment in light of the tendency by some individuals to 'cheat' the system.

As opposed to antisocial punishment - where good people are punished by law-breakers in some sort of vengeful way - study findings showed that prosocial punishment strategies can preserve the public good and generate cooperative behaviour.

"We show that institutions play a role in enabling society to function holistically, not only by implementing punishments to law-breakers, but also through their visibility," Dr Garcia said.

"This work is also useful in understanding how to design groups of artificial agents in which cooperation is a required emergent feature."

Bottom Line: This observational study looked at how green space is associated with mental health. Some research has suggested living near more green space may be associated with benefits. This analysis included nearly 47,000 city-dwelling adults in Australia and examined how living near different kinds of green space (including tree canopy, grass and low-lying vegetation) may be associated with risk of psychological distress, self-reported physician-diagnosed depression or anxiety, and fair to poor self-reported general health. The three outcomes were examined at baseline and  follow-up about six years later. The authors report exposure to more tree canopy was associated with a lower likelihood of psychological distress and better self-rated general health. No green space indicator was associated with depression or anxiety. Exposure to low-lying vegetation wasn't consistently associated with any outcome. Exposure to more grass was associated with a higher likelihood of reporting fair to poor general health and prevalent psychological distress. Limitations of the study include self-reported health outcomes and green space availability that may have decreased in some areas over time, which may mean the results underestimate the associations.

Authors: Thomas Astell-Burt, Ph.D., and Xiaoqi Feng, Ph.D., of the University of Wollongong, Wollongong, New South Wales, Australia.

(doi:10.1001/jamanetworkopen.2019.8209)

Editor's Note: The article includes conflict of interest and funding/support disclosures. Please see the article for additional information, including other authors, author contributions and affiliations, financial disclosures, funding and support, etc.

Researchers from LSTM, working alongside colleagues from the Wellcome Sanger Institute, Cambridge and the Big Data Institute, University of Oxford, have used whole genome sequencing to understand copy-number variants (CNVs) in malaria mosquitoes and their role in insecticide resistance.

Many diseases, such as malaria, Zika and Dengue, are transmitted by mosquitoes, making control of mosquito populations a cornerstone of efforts to tackle these diseases. This is usually achieved through the use of insecticides, traditionally to great effect. Dr Eric Lucas, first author on a paper published in the journal Genome Research, explained: "Cases of malaria have been greatly reduced in the last 20 years, primarily due to improved vector control. These efforts are however threatened by the evolution of resistance to insecticides in many medically-important mosquito species. To better understand and address insecticide resistance, we need to understand the genetic mutations that cause it, but only a few mutations have so far been discovered."

In order to better understand the evolution of resistance, LSTM along with collaborators at the University of Oxford and the Wellcome Sanger Institute, are sequencing the genomes of thousands of individuals of the main malaria mosquito, Anopheles gambiae, from across Sub-Saharan Africa as part of a project called the An. gambiae 1000 Genomes project (Ag1000G).

One type of mutation that could lead to increased insecticide resistance is the gain of extra copies of genes that help to break down the insecticide in the mosquito body, yet there has been little research into such copy-number variants (CNVs) in malaria mosquitoes. For this work, the team used the Ag1000G data to look for increases in copy-number in An. gambiae and found that CNVs were much more likely to occur in genes that play a role in insecticide resistance than in the rest of the genome. "These resistance-associated CNVs were found in nearly every population in our study," continued Dr Lucas, "and over 90% of mosquitoes had increases in copy-number in some populations. Overall, in the five genetic regions known to be associated with the detoxification of insecticides in An. gambiae, we found a total of 44 different CNVs. The repeated origins of increased copy-number in the same genes, suggest that this type of mutation is relatively frequent and could provide a means of rapid evolutionary response to insecticide for the mosquitoes."

Professor Martin Donnelly, Head LSTM's Department of Vector Biology, was Senior author on the paper. He said: "This research demonstrates the importance of increases in gene copy-number in the evolution of insecticide resistance and should spur on research into understanding the exact effect of each of the CNVs, and the insecticides against which they act. Once these effects are understood, testing the presence of these mutations and tracking their spread between populations will help us predict the insecticides against which a mosquito population may still be susceptible."

Soon we will be able to replace fossil fuels with a carbon-neutral product created from solar energy, carbon dioxide and water. Researchers at Uppsala University have successfully produced microorganisms that can efficiently produce the alcohol butanol using carbon dioxide and solar energy, without needing to use solar cells.

This has been presented in a new study published in the scientific journal Energy & Environmental Science.

- We have systematically designed and created a series of modified cyanobacteria that gradually produced increasing quantities of butanol in direct processes. When the best cells are used in long-term laboratory experiments, we see levels of production that exceed levels that have been reported in existing articles. Furthermore, it is comparable with indirect processes where bacteria are fed with sugar, says Pia Lindberg, Senior Lecturer at the Department of Chemistry Ångström Laboratory, Uppsala University.

The knowledge and ability to modify cyanobacteria so they can produce a variety of chemicals from carbon dioxide and solar energy is emerging in parallel with advances in technology, synthetic biology, genetically changing them. Through a combination of technical development, systematic methods and the discovery that as more product removed from the cyanobacteria, the more butanol is formed, the study shows the way forward for realising the concept.

We already know it is possible to produce butanol using this process (proof-of-concept). What researchers have now been able to show is that it is possible to achieve significantly higher production, so high that it becomes possible to use in production. In practical terms, butanol can be used in the automotive industry as both an environmentally friendly vehicle fuel - fourth generation biofuel - and as an environmentally friendly component of rubber for tyres. In both cases, fossil fuels are replaced by a carbon-neutral product created from solar energy, carbon dioxide and water.

Even larger industries, in all trades, that currently produce high greenhouse gas emissions from carbon dioxide will be able to use the process with cyanobacteria to bind carbon dioxide and consequently significantly reduce their emissions.

- Microscopic cyanobacteria are the most efficient photosynthetic organisms on earth. In this study, we utilise their ability to efficiently capture the sun's energy and bind to carbon dioxide in the air, alongside with all the tools we have to modify cyanobacteria to produce desirable products. The results show that a direct production of carbon-neutral chemicals and fuels from solar energy will be a possibility in the future, explains Peter Lindblad, Professor at the Department of Chemistry Ångström Laboratory at Uppsala University who is leading the project.

Research at Uppsala University is part of the larger EU Photofuel project (http://www.photofuel.eu) being coordinated by vehicle manufacturer VW whose aim is to develop the next generation of techniques for sustainable manufacture of alternative fuels in the transport sector.

A joint research group including Ryo Yoshida (Professor and Director of the Data Science Center for Creative Design and Manufacturing at the Institute of Statistical Mathematics [ISM], Research Organization of Information and Systems) Junko Morikawa (Professor at the School of Materials and Chemical Technology, Tokyo Institute of Technology [Tokyo Tech]), and Yibin Xu (Group Leader of Thermal Management and Thermoelectric Materials Group, Center for Materials Research by Information Integration, Research and Services Division of Materials Data and Integrated System [MaDIS], NIMS) has demonstrated the promising application of machine learning (ML) -- a form of AI that enables computers to "learn" from given data -- for discovering innovative materials.

Reporting their findings in the open-access journal npj Computational Materials, the researchers show that their ML method, involving "transfer learning", enables the discovery of materials with desired properties even from an exceeding small data set.

The study drew on a data set of polymeric properties from PoLyInfo, the largest database of polymers in the world housed at NIMS. Despite its huge size, PoLyInfo has a limited amount of data on the heat transfer properties of polymers. To predict the heat transfer properties from the given limited data, ML models on proxy properties were pre-trained where sufficient data were available on the related tasks; these pre-trained models captured common features relevant to the target task. Re-purposing such machine-acquired features on the target task yielded outstanding prediction performance even with the exceedingly small datasets, as if highly experienced human experts can make rational inferences even for considerably less experienced tasks. The team combined this model with a specially designed ML algorithm for computational molecular design, which is called the iQSPR algorithm previously developed by Yoshida and his colleagues. Applying this technique enabled the identification of thousands of promising "virtual" polymers.

From this large pool of candidates, three polymers were selected based on their ease of synthesis and processing.

Tests confirmed that the new polymers have a high thermal conductivity of up to 0.41 Watts per meter-Kelvin (W/mK). This figure is 80 percent higher than that of typical polyimides, a group of commonly used polymers that have been mass-produced since the 1950s for applications ranging from fuel cells to cookware.

By verifying the heat transfer properties of the computationally designed polymers, the study represents a key breakthrough for fast, cost-effective, ML-supported methods for materials design. It also demonstrates the team's combined expertise in data science, organic synthesis and advanced measurement technologies.

Yoshida comments that many aspects remain to be explored, such as "training" computational systems to work with limited data by adding more suitable descriptors. "Machine learning for polymer or soft material design is a challenging but promising field as these materials have properties that differ from metals and ceramics, and are not yet fully predicted by the existing theories," he says.

The study is a starting point for the discovery of other innovative materials, as Morikawa adds: "We would like to try to create an ML-driven high-throughput computational system to design next-generation soft materials for applications going beyond the 5G era. Through our project, we aim to pursue not only the development of materials informatics but also contribute to fundamental advancement of materials science, especially in the field of phonon engineering."

A study led by KU Leuven for the first time explains how a promising type of perovskites - man-made crystals that can convert sunlight into electricity - can be stabilized. As a result, the crystals turn black, enabling them to absorb sunlight. This is necessary to be able to use them in new solar panels that are easy to make and highly efficient. The study was published in Science.

Perovskites are semiconductor materials that have many applications. They show particular promise in harvesting solar energy. Currently, most solar cells are made with silicon crystals, a relatively straightforward and effective material to process for this purpose. However, perovskite-based devices offer higher conversion efficiencies than silicon. The only problem: some of the most promising perovskites, namely caesium lead triiodide (CsPbI3), are very unstable at room temperature. Under these conditions, they have a yellow colour, as the atoms in the crystal do not form a perovskite structure. For the crystals to absorb sunlight efficiently and turn it into electricity, they should be in a black, perovskite state - and stay that way.

"Silicon forms a very strong, rigid crystal. If you press on it, it won't change its shape. On the other hand, perovskites are much softer and more malleable," explains Dr. Julian Steele of the KU Leuven Centre for Membrane Separations, Adsorption, Catalysis, and Spectroscopy for Sustainable Solutions (cMACS). "We can stabilize them under various lab conditions, but at room temperature, the black perovskite atoms really want to reshuffle, change structure, and ultimately turn the crystal yellow."

Together with an international team of scientists, Steele discovered that by binding a thin film of perovskite solar cells to a sheet of glass, the cells can obtain and maintain their desired black state. The thin film is heated to a temperature of 330 degrees Celsius, causing the perovskites to expand and adhere to the glass. After heating, the film is rapidly cooled down to room temperature. This process fixates the atoms in the crystals, restricting their movement, so that they stay in the desired black form.

"There are three pillars that determine the quality of solar cells: price, stability, and performance. Perovskites score high on performance and price, but their stability is still a major issue," says Steele. Scientists had already been observing for several years that perovskites can retain their blackness after heating, but it was as of yet unclear why. "In our study, we chose CsPbI3 because its performance is very high," Steel explains. "Additionally, it is one of the most unstable types of perovskites, which means it is sensitive to the method we describe, and should translate to other unstable perovskites."

Much of the data used in the study were collected at the European Synchrotron Radiation Facility. To understand the experimental observations on a molecular scale, colleagues at Ghent University's Center for Molecular Modeling (CMM) supported the finding with theoretical simulations of the black and yellow phases of the perovskites. The computational results were necessary to rationalize why the black phase is stabilized when fixating it as a thin film to a glass substrate.

How the bonding takes place exactly, is still a mystery, though there are hypotheses. "Normally, we would take a microscope with atomic resolution and directly have a look. However, that's impossible with perovskites, as they are hard to observe with such a high-resolution imaging instrument, since they are so soft and prone to falling apart under the relatively high energy of common probes."

"Understanding how this mechanism works will help further research to ultimately develop solar panels that use pure perovskite crystals," Steele says. "Since the entry level for processing perovskite-based solar cells is relatively low, they can be very beneficial for people in developing countries operating in a more limited infrastructure." Additionally, perovskites can be used in LEDs, photoelectric sensors, transistors, x-ray detectors and more.