Algorithm for robot teams handles moving obstacles

Robotic consensus

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MIT researchers will present a new, decentralized planning algorithm for teams of robots that factors in not only stationary obstacles, but moving obstacles, as well. The algorithm still comes with strong mathematical guarantees that the robots will avoid collisions.

Credit: Illustration: Christine Daniloff/MIT

Planning algorithms for teams of robots fall into two categories: centralized algorithms, in which a single computer makes decisions for the whole team, and decentralized algorithms, in which each robot makes its own decisions based on local observations.

With centralized algorithms, if the central computer goes offline, the whole system falls apart. Decentralized algorithms handle erratic communication better, but they're harder to design, because each robot is essentially guessing what the others will do. Most research on decentralized algorithms has focused on making collective decision-making more reliable and has deferred the problem of avoiding obstacles in the robots' environment.

At the International Conference on Robotics and Automation in May, MIT researchers will present a new, decentralized planning algorithm for teams of robots that factors in not only stationary obstacles, but also moving obstacles. The algorithm also requires significantly less communications bandwidth than existing decentralized algorithms, but preserves strong mathematical guarantees that the robots will avoid collisions.

In simulations involving squadrons of minihelicopters, the decentralized algorithm came up with the same flight plans that a centralized version did. The drones generally preserved an approximation of their preferred formation, a square at a fixed altitude -- although to accommodate obstacles the square rotated and the distances between drones contracted. Occasionally, however, the drones would fly single file or assume a formation in which pairs of them flew at different altitudes.

"It's a really exciting result because it combines so many challenging goals," says Daniela Rus, the Andrew and Erna Viterbi Professor in MIT's Department of Electrical Engineering and Computer Science and director of the Computer Science and Artificial Intelligence Laboratory, whose group developed the new algorithm. "Your group of robots has a local goal, which is to stay in formation, and a global goal, which is where they want to go or the trajectory along which you want them to move. And you allow them to operate in a world with static obstacles but also unexpected dynamic obstacles, and you have a guarantee that they are going to retain their local and global objectives. They will have to make some deviations, but those deviations are minimal."

Rus is joined on the paper by first author Javier Alonso-Mora, a postdoc in Rus' group; Mac Schwager, an assistant professor of aeronautics and astronautics at Stanford University who worked with Rus as an MIT PhD student in mechanical engineering; and Eduardo Montijano, a professor at Centro Universitario de la Defensa in Zaragoza, Spain.

Trading regions

In a typical decentralized group planning algorithm, each robot might broadcast its observations of the environment to its teammates, and all the robots would then execute the same planning algorithm, presumably on the basis of the same information.

But Rus, Alonso-Mora, and their colleagues found a way to reduce both the computational and communication burdens imposed by consensual planning. The essential idea is that each robot, on the basis of its own observations, maps out an obstacle-free region in its immediate environment and passes that map only to its nearest neighbors. When a robot receives a map from a neighbor, it calculates the intersection of that map with its own and passes that on.

This keeps down both the size of the robots' communications -- describing the intersection of 100 maps requires no more data than describing the intersection of two -- and their number, because each robot communicates only with its neighbors. Nonetheless, each robot ends up with a map that reflects all of the obstacles detected by all the team members.

Four dimensions

The maps have not three dimensions, however, but four -- the fourth being time. This is how the algorithm accounts for moving obstacles. The four-dimensional map describes how a three-dimensional map would have to change to accommodate the obstacle's change of location, over a span of a few seconds. But it does so in a mathematically compact manner.

The algorithm does assume that moving obstacles have constant velocity, which will not always be the case in the real world. But each robot updates its map several times a second, a short enough span of time that the velocity of an accelerating object is unlikely to change dramatically.

On the basis of its latest map, each robot calculates the trajectory that will maximize both its local goal -- staying in formation -- and its global goal.

The researchers are also testing a version of their algorithm on wheeled robots whose goal is to collectively carry an object across a room where human beings are also moving around, as a simulation of an environment in which humans and robots work together.

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The above post is reprinted from materials provided by Massachusetts Institute of Technology. The original item was written by Larry Hardesty.Note: Materials may be edited for content and length.

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Massachusetts Institute of Technology. "Algorithm for robot teams handles moving obstacles: Robotic consensus."ScienceDaily.ScienceDaily,21April2016. 

<www.sciencedaily.com/releases/2016/04/160421113135.htm>.

At last: Non-toxic and cheap thin-film solar cells for 'zero-energy' buildings

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Dr Xiaojing Hao of UNSW's Australian Centre for Advanced Photovoltaics holding the new CZTS solar cells.

'Zero-energy' buildings -- which generate as much power as they consume -- are now much closer after a team at Australia's University of New South Wales achieved the world's highest efficiency using flexible solar cells that are non-toxic and cheap to make.

Until now, the promise of 'zero-energy' buildings been held back by two hurdles: the cost of the thin-film solar cells (used in façades, roofs and windows), and the fact they're made from scarce, and highly toxic, materials.

That's about to change: the UNSW team, led by Dr Xiaojing Hao of the Australian Centre for Advanced Photovoltaics at the UNSW School of Photovoltaic and Renewable Energy Engineering, have achieved the world's highest efficiency rating for a full-sized thin-film solar cell using a competing thin-film technology, known as CZTS.

NREL, the USA's National Renewable Energy Laboratory, confirmed this world leading 7.6% efficiency in a 1cm2 area CZTS cell this month.

Unlike its thin-film competitors, CZTS cells are made from abundant materials: copper, zinc, tin and sulphur.

And CZTS has none of the toxicity problems of its two thin-film rivals, known as CdTe (cadmium-telluride) and CIGS (copper-indium-gallium-selenide). Cadmium and selenium are toxic at even tiny doses, while tellurium and indium are extremely rare.

"This is the first step on CZTS's road to beyond 20% efficiency, and marks a milestone in its journey from the lab to commercial product," said Hao, named one of UNSW's 20 rising stars last year. "There is still a lot of work needed to catch up with CdTe and CIGS, in both efficiency and cell size, but we are well on the way."

"In addition to its elements being more commonplace and environmentally benign, we're interested in these higher bandgap CZTS cells for two reasons," said Professor Martin Green, a mentor of Dr Hao and a global pioneer of photovoltaic research stretching back 40 years.

"They can be deposited directly onto materials as thin layers that are 50 times thinner than a human hair, so there's no need to manufacture silicon 'wafer' cells and interconnect them separately," he added. "They also respond better than silicon to blue wavelengths of light, and can be stacked as a thin-film on top of silicon cells to ultimately improve the overall performance."

By being able to deposit CZTS solar cells on various surfaces, Hao's team believe this puts them firmly on the road to making thin-film photovoltaic cells that can be rigid or flexible, and durable and cheap enough to be widely integrated into buildings to generate electricity from the sunlight that strikes structures such as glazing, façades, roof tiles and windows.

However, because CZTS is cheaper -- and easier to bring from lab to commercialisation than other thin-film solar cells, given already available commercialised manufacturing method -- applications are likely even sooner. UNSW is collaborating with a number of large companies keen to develop applications well before it reaches 20% efficiency -- probably, Hao says, within the next few years.

"I'm quietly confident we can overcome the technical challenges to further boosting the efficiency of CZTS cells, because there are a lot of tricks we've learned over the past 30 years in boosting CdTe and CIGS and even silicon cells, but which haven't been applied to CZTS," said Hao.

Currently, thin-film photovoltaic cells like CdTe are used mainly in large solar power farms, as the cadmium toxicity makes them unsuitable for residential systems, while CIGS cells is more commonly used in Japan on rooftops.

First Solar, a US$5 billion behemoth that specialises in large-scale photovoltaic systems, relies entirely on CdTe; while CIGS is the preferred technology of China's Hanergy, the world's largest thin-film solar power company.

Thin-film technologies such as CdTe and CIGS are also attractive because they are physically flexible, which increases the number of potential applications, such as curved surfaces, roofing membranes, or transparent and translucent structures like windows and skylights.

But their toxicity has made the construction industry -- mindful of its history with asbestos -- wary of using them. Scarcity of the elements also renders them unattractive, as price spikes are likely as demand rises. Despite this, the global market for so-called Building-Integrated Photovoltaics (BIPV) is already valued at US$1.6 billion.

Hao believes CZTS's cheapness, benign environmental profile and abundant elements may be the trigger that finally brings architects and builders onboard to using thin-film solar panels more widely in buildings.

Until now, most architects have used conventional solar panels made from crystalline silicon. While these are even cheaper than CZTS cells, they don't offer the same flexibility for curved surfaces and other awkward geometries needed to easily integrate into building designs.

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Story Source:

The above post is reprinted from materials provided by University of New South Wales. The original item was written by Wilson da Silva. Note: Materials may be edited for content and length.

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University of New South Wales. "At last: Non-toxic and cheap thin-film solar cells for 'zero-energy' buildings: World's highest efficiency rating achieved for CZTS thin-film solar cells." ScienceDaily. ScienceDaily, 28 April 2016. <www.sciencedaily.com/releases/2016/04/160428103023.htm>.

Fish-eyed lens cuts through the dark

The ball-shaped, fingertip-sized artificial eye uses thousands of mirrors and a domed shape (seen in image D) to concentrate scant light.

Combining the best features of a lobster and an African fish, University of Wisconsin-Madison engineers have created an artificial eye that can see in the dark. And their fishy false eyes could help search-and-rescue robots or surgical scopes make dim surroundings seem bright as day.

Their biologically inspired approach, published March 14, 2016 in theProceedings of the National Academy of Sciences, stands apart from other methods in its ability to improve the sensitivity of the imaging system through the lenses rather than the sensor component.

Amateur photographers attempting to capture the moon with their cellphone cameras are familiar with the limitations of low-light imaging. The long exposure time required for nighttime shots causes minor shakes to produce extremely blurry images. Yet, fuzzy photos aren't merely an annoyance. Bomb-diffusing robots, laparoscopic surgeons and planet-seeking telescopes all need to resolve fine details through almost utter darkness.

"These days, we rely more and more on visual information. Any technology that can improve or enhance image-taking has great potential," says Hongrui Jiang, professor of electrical and computer and biomedical engineering at UW-Madison and the corresponding author on the study.

Most attempts to improve night vision tweak the "retinas" of artificial eyes -- such as changing the materials or electronics of a digital camera's sensor -- so they respond more strongly to incoming packets of light.

However, rather than interfering with efforts to boost sensitivity at the back end, Jiang's group set out to increase intensity of incoming light through the front end, the optics that focus the light on the sensor. They found inspiration for the strategy from two aquatic animals that evolved different strategies to survive and see in murky waters.

Elephantnosed fishes resemble river-dwelling Cyrano de Bergerac impersonators. Looking between their prominent proboscises reveals two strikingly unusual eyes, with retinas composed of thousands of tiny crystal cups instead of the smooth surfaces common to most animals. These miniature vessels collect and intensify red light, which helps the fish discern its predators.

"We were thinking: 'Why don't we apply this idea? Can we enhance the intensity to concentrate the light?'" says Jiang, whose research is supported by the National Institutes of Health and UW-Madison.

The group emulated the fish's crystal cups by engineering thousands of miniscule parabolic mirrors, each as tall as a grain of pollen. Jiang's team then shaped arrays of the light-collecting structures across the surface of a uniform hemispherical dome. The arrangement, inspired by the superposition compound eyes of lobsters, concentrates incoming light to individual spots, further increasing intensity.

"We showed fourfold improvement in sensitivity," says Jiang. "That makes the difference between a totally dark image you can't see and an actually meaningful image."

In this case, the devices picked up a picture of UW-Madison's Bucky Badger mascot through what seemed like pitch-black darkness. The device could easily be incorporated into existing systems to visualize a variety of vistas under low light.

"It's independent of the imaging technology," says Jiang. "We're not trying to compromise among different factors. Any type of imager can use this."

Although superposition compound eyes are exquisitely sensitive, they typically suffer from less sharp vision. Increased intensity costs clarity when lots of light gets compressed down to individual pixels. To recover lost resolution, Jiang's group captured numerous raw images and processed the set with an algorithm to produce crisp, clear pictures.

The engineers in Jiang's lab -- including Hewei Liu, the postdoctoral scholar who fabricated the lenses, and Yinggang Huang, who processed the super-resolution images -- are working to refine the manufacturing process to further increase the sensitivity of the devices. With perfect precision, Jiang predicts that the artificial eyes could improve by at least an order of magnitude.

"It has always been very hard to make artificial superposition compound eyes because the curvature and alignment need to be absolutely perfect." says Jiang. "Even the slightest misalignment can throw off the entire system."

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The above post is reprinted from materials provided by University of Wisconsin-Madison. The original item was written by Sam Million-Weaver.Note: Materials may be edited for content and length.

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University of Wisconsin-Madison. "Fish-eyed lens cuts through the dark." ScienceDaily. ScienceDaily, 15 April 2016. <www.sciencedaily.com/releases/2016/04/160415125932.htm>.

Investigating world’s oldest human footprints with software designed to decode crime scenes

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The Laetoli tracks were discovered by Mary Leakey in 1976 and are thought to be around 3.6 million years old. There are two parallel trackways on the site, where two ancient hominins walked across the surface. One of these trackways was obscured when a third person followed the same path.

Researchers at Bournemouth University have developed a new software technique to uncover 'lost' tracks, hidden in plain sight at the world's oldest human footprint site in Laetoli (Tanzania). The software has revealed new information about the shape of the tracks and has found hints of a previously undiscovered fourth track-maker at the site.

The software was developed as part of a Natural Environments Research Council (NERC) Innovation Project awarded to Professor Matthew Bennett and Dr Marcin Budka in 2015 for forensic footprint analysis. They have been developing techniques to enable modern footwear evidence to be captured in three-dimensions and analysed digitally to improve crime scene practice.

Footprints reveal much about the individuals who made them; their body mass, height and their walking speed. "Footprints contain information about the way our ancestors moved," explains Professor Bennett. "The tracks at Laetoli are the oldest in the world and show a line of footprints from our early ancestors, preserved in volcanic ash. They provide a fascinating insight into how early humans walked. The techniques we have been developing for use at modern crime scenes can also reveal something new about these ancient track sites."

The Laetoli tracks were discovered by Mary Leakey in 1976 and are thought to be around 3.6 million years old. There are two parallel trackways on the site, where two ancient hominins walked across the surface. One of these trackways was obscured when a third person followed the same path. The merged trackway has largely been ignored by scientists over the last 40 years and the fierce debate about the walking style of the track-makers has predominately focused on the undisturbed trackway.

By using the software developed through the NERC Innovation Project Professor Bennett and his colleagues have been able to decouple the tracks of this merged trail and reveal for the first time the shape of the tracks left by this mysterious third track-maker. There is also an intriguing hint of a fourth track-maker at the site.

"We're really pleased that we can use our techniques to capture new data from these extremely old footprints," says Dr Marcin Budka who developed the software used in the study.

"It means that we have effectively doubled the information that the palaeo-anthropological community has available for study of these hominin track-makers," continues Dr Reynolds one of the co-authors of the study.

"As well as making new discoveries about our early ancestors, we can apply this science to help modern society combat crime. By digitising tracks at a crime scene we can preserve, share and study this evidence more easily," says Sarita Morse who helped conceive the original analysis.

For more information, please see the following video:https://www.youtube.com/watch?v=Rl8odSqoDZc

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The above post is reprinted from materials provided by Bournemouth University. Note: Materials may be edited for content and length.

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Journal Reference:

1. M. R. Bennett, S. C. Reynolds, S. A. Morse, M. Budka. Laetoli’s lost tracks: 3D generated mean shape and missing footprints. Scientific Reports, 2016; 6: 21916 DOI: 10.1038/srep21916

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Bournemouth University. "Investigating world’s oldest human footprints with software designed to decode crime scenes." ScienceDaily. ScienceDaily, 26 April 2016. <www.sciencedaily.com/releases/2016/04/160426092130.htm>.

The light stuff: A brand-new way to produce electron spin currents

Physicists have demonstrated using non-polarized light to produce a spin voltage in a metal

A schematic of the CSU team's device that demonstrates using light to create a spin current. A spin voltage drives spin-up and spin-down electrons to move in opposite directions, resulting in a pure spin current across a platinum layer.

Credit: Image courtesy of Colorado State University

With apologies to Isaac Asimov, the most exciting phase to hear in science isn't "Eureka," but "That's funny..."

A "that's funny" moment in a Colorado State University physics lab has led to a fundamental discovery that could play a key role in next-generation microelectronics.

Publishing in Nature Physics April 25, the scientists, led by Professor of Physics Mingzhong Wu in CSU's College of Natural Sciences, are the first to demonstrate using non-polarized light to produce in a metal what's called a spin voltage -- a unit of power produced from the quantum spinning of an individual electron. Controlling electron spins for use in memory and logic applications is a relatively new field called spin electronics, or spintronics, and the subject of the 2007 Nobel Prize in Physics.

Wu and his group's passion is to find new, better ways to control electron spins, the physics of which isn't completely understood. Spintronics exploits the notion that electron spins can be manipulated and used to process and store information, with a fraction of the power needed in ubiquitous, conventional electronics.

Consider that the iPhone and every electronic device out there is built upon centuries of science around charge current -- the physics of positive or negative charges flowing through a device. The perennial problem is the enormous power consumption of charge-current devices, and the electrical resistance that causes power loss in the form of heat -- which is why your laptop keeps overheating.

It's these power and heat barriers that are holding smaller, more powerful electronics back. And it's why science is turning to spintronics, because it offers a completely new way of making a device work. To utilize power from an electron spin, there's no charge current necessary. All that's needed is a magnetic field or a magnetic material, which can orient the spins "up" or "down." The up and down spins are the analogue to positive and negative charges.

What the CSU scientists have found is a brand-new method for creating spin currents. Existing methods include using a charge current, microwaves or a heat source. But for the first time, the CSU team demonstrates using light -- or in the quantum world, photons -- to generate their spin currents.

Other scientists have done similar things, but they used a special kind of polarized light. Here, the CSU scientists used unpolarized, plain light -- "a halogen bulb purchased at Ace Hardware," said graduate student David Ellsworth who is the first author on the paper. They demonstrated a "pure" spin current -- involving no charge movement whatsoever. It was an unprecedented feat.

The breakthrough came about while the scientists were studying a different way to make spin currents, using heat from their halogen bulb, called the Spin Seebeck effect. They noticed some background data they couldn't explain.

Ever curious, they checked all possibilities and determined this seemingly light-induced spin current could be a new quantum phenomenon. They tested it by designing unique control measurements involving different magnetic insulators and metallic thin films, such as platinum. After replicating their results in the lab, they turned to theoreticians at UC Irvine and Fudan University to help them interpret the physics of what they'd discovered, and who are co-authors on the Nature Physics paper.

Wu said the discovery is too new to think about real applications; where they're at now is continuing to make breakthroughs in the understanding of spin currents. "Just like with the photovoltaic effect when it was first discovered, no one thought at first of a solar cell," Wu said. "Technologies take time before they are used in real devices. This is a fundamental, new discovery."

Said Jake Roberts, chair of the Department of Physics: "There have been tremendous technical advances in controlling light. What I see in this discovery is that now, they've linked light to spin control. Using a simple light source to produce a spin current offers new opportunities for power control and generation."

The researchers will continue exploring making spin currents with light by swapping out materials and trying different light sources. They demonstrated light control in the infrared range, Ellsworth said. Moving into the visible or UV range would likely offer more robust applications for devices.

"The framework for generating and detecting spin currents is non-trivial," Ellsworth explained. "Meanwhile, there are hundreds of years of generating charge currents and knowing how to measure them and manipulate them and characterize them. Spintronics is a new field, and devices are just now coming onto the market that utilize some small part of this."

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The above post is reprinted from materials provided by Colorado State University. Note: Materials may be edited for content and length.

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Journal Reference:

1. David Ellsworth, Lei Lu, Jin Lan, Houchen Chang, Peng Li, Zhe Wang, Jun Hu, Bryan Johnson, Yuqi Bian, Jiang Xiao, Ruqian Wu, Mingzhong Wu. Photo-spin-voltaic effect. Nature Physics, 2016; DOI:10.1038/nphys3738

Cite This Page:

Colorado State University. "The light stuff: A brand-new way to produce electron spin currents: Physicists have demonstrated using non-polarized light to produce a spin voltage in a metal." ScienceDaily. ScienceDaily, 25 April 2016. <www.sciencedaily.com/releases/2016/04/160425142352.htm>.

Battery tech with off-the-charts charging capacity

CI chemist Reginald Penner (shown) and doctoral candidate Mya Le Thai have developed a nanowire-based technology that allows lithium-ion batteries to be recharged hundreds of thousands of times.

University of California, Irvine researchers have invented nanowire-based battery material that can be recharged hundreds of thousands of times, moving us closer to a battery that would never require replacement. The breakthrough work could lead to commercial batteries with greatly lengthened lifespans for computers, smartphones, appliances, cars and spacecraft.

Scientists have long sought to use nanowires in batteries. Thousands of times thinner than a human hair, they're highly conductive and feature a large surface area for the storage and transfer of electrons. However, these filaments are extremely fragile and don't hold up well to repeated discharging and recharging, or cycling. In a typical lithium-ion battery, they expand and grow brittle, which leads to cracking.

UCI researchers have solved this problem by coating a gold nanowire in a manganese dioxide shell and encasing the assembly in an electrolyte made of a Plexiglas-like gel. The combination is reliable and resistant to failure.

The study leader, UCI doctoral candidate Mya Le Thai, cycled the testing electrode up to 200,000 times over three months without detecting any loss of capacity or power and without fracturing any nanowires. The findings were published today in the American Chemical Society's Energy Letters.

Hard work combined with serendipity paid off in this case, according to senior author Reginald Penner.

"Mya was playing around, and she coated this whole thing with a very thin gel layer and started to cycle it," said Penner, chair of UCI's chemistry department. "She discovered that just by using this gel, she could cycle it hundreds of thousands of times without losing any capacity."

"That was crazy," he added, "because these things typically die in dramatic fashion after 5,000 or 6,000 or 7,000 cycles at most."

The researchers think the goo plasticizes the metal oxide in the battery and gives it flexibility, preventing cracking.

"The coated electrode holds its shape much better, making it a more reliable option," Thai said. "This research proves that a nanowire-based battery electrode can have a long lifetime and that we can make these kinds of batteries a reality."

The study was conducted in coordination with the Nanostructures for Electrical Energy Storage Energy Frontier Research Center at the University of Maryland, with funding from the Basic Energy Sciences division of the U.S. Department of Energy.

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The above post is reprinted from materials provided by University of California - Irvine. Note: Materials may be edited for content and length.

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Journal Reference:

1. Mya Le Thai, Girija Thesma Chandran, Rajen K. Dutta, Xiaowei Li, Reginald M. Penner. 100k Cycles and Beyond: Extraordinary Cycle Stability for MnO2Nanowires Imparted by a Gel Electrolyte. ACS Energy Letters, 2016; 57 DOI: 10.1021/acsenergylett.6b00029

Cite This Page:

University of California - Irvine. "Battery tech with off-the-charts charging capacity." ScienceDaily. ScienceDaily, 20 April 2016. <www.sciencedaily.com/releases/2016/04/160420211136.htm>.

With simple process, engineers fabricate fastest flexible silicon transistor

Using a unique method they developed, a team of UW--Madison engineers has fabricated the world's fastest silicon-based flexible transistors, shown here on a plastic substrate.

One secret to creating the world's fastest silicon-based flexible transistors: a very, very tiny knife.

Working in collaboration with colleagues around the country, University of Wisconsin-Madison engineers have pioneered a unique method that could allow manufacturers to easily and cheaply fabricate high-performance transistors with wireless capabilities on huge rolls of flexible plastic.

The researchers -- led by Zhenqiang (Jack) Ma, the Lynn H. Matthias Professor in Engineering and Vilas Distinguished Achievement Professor in electrical and computer engineering, and research scientist Jung-Hun Seo -- fabricated a transistor that operates at a record 38 gigahertz, though their simulations show it could be capable of operating at a mind-boggling 110 gigahertz. In computing, that translates to lightning-fast processor speeds.

It's also very useful in wireless applications. The transistor can transmit data or transfer power wirelessly, a capability that could unlock advances in a whole host of applications ranging from wearable electronics to sensors.

The team published details of its advance April 20 in the journal Scientific Reports.

The researchers' nanoscale fabrication method upends conventional lithographic approaches -- which use light and chemicals to pattern flexible transistors -- overcoming such limitations as light diffraction, imprecision that leads to short circuits of different contacts, and the need to fabricate the circuitry in multiple passes.

Using low-temperature processes, Ma, Seo and their colleagues patterned the circuitry on their flexible transistor -- single-crystalline silicon ultimately placed on a polyethylene terephthalate (more commonly known as PET) substrate -- drawing on a simple, low-cost process called nanoimprint lithography.

In a method called selective doping, researchers introduce impurities into materials in precise locations to enhance their properties -- in this case, electrical conductivity. But sometimes the dopant merges into areas of the material it shouldn't, causing what is known as the short channel effect. However, the UW-Madison researchers took an unconventional approach: They blanketed their single crystalline silicon with a dopant, rather than selectively doping it.

Then, they added a light-sensitive material, or photoresist layer, and used a technique called electron-beam lithography -- which uses a focused beam of electrons to create shapes as narrow as 10 nanometers wide -- on the photoresist to create a reusable mold of the nanoscale patterns they desired. They applied the mold to an ultrathin, very flexible silicon membrane to create a photoresist pattern. Then they finished with a dry-etching process -- essentially, a nanoscale knife -- that cut precise, nanometer-scale trenches in the silicon following the patterns in the mold, and added wide gates, which function as switches, atop the trenches.

With a unique, three-dimensional current-flow pattern, the high performance transistor consumes less energy and operates more efficiently. And because the researchers' method enables them to slice much narrower trenches than conventional fabrication processes can, it also could enable semiconductor manufacturers to squeeze an even greater number of transistors onto an electronic device.

Ultimately, says Ma, because the mold can be reused, the method could easily scale for use in a technology called roll-to-roll processing (think of a giant, patterned rolling pin moving across sheets of plastic the size of a tabletop), and that would allow semiconductor manufacturers to repeat their pattern and mass-fabricate many devices on a roll of flexible plastic.

"Nanoimprint lithography addresses future applications for flexible electronics," says Ma, whose work was supported by the Air Force Office of Scientific Research. "We don't want to make them the way the semiconductor industry does now. Our step, which is most critical for roll-to-roll printing, is ready."

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Story Source:

The above post is reprinted from materials provided by University of Wisconsin-Madison. Note: Materials may be edited for content and length.

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Journal Reference:

1. Jung-Hun Seo, Tao Ling, Shaoqin Gong, Weidong Zhou, Alice L. Ma, L. Jay Guo, Zhenqiang Ma. Fast Flexible Transistors with a Nanotrench Structure. Scientific Reports, 2016; 6: 24771 DOI: 10.1038/srep24771

Source link-https://www.sciencedaily.com/releases/2016/04/160420120550.htm

Cheap, efficient and flexible solar cells

Cheap, efficient and flexible solar cells: New world record for fullerene-free polymer solar cells

Polymer solar cells manufactured using low-cost roll-to-roll printing technology, demonstrated here by professors Olle Inganäs (right) and Shimelis Admassie.

Polymer solar cells can be even cheaper and more reliable thanks to a breakthrough by scientists at Linköping University and the Chinese Academy of Sciences (CAS). This work is about avoiding costly and unstable fullerenes.

Polymer solar cells have in recent years emerged as a low cost alternative to silicon solar cells. In order to obtain high efficiency, fullerenes are usually required in polymer solar cells to separate charge carriers. However, fullerenes are unstable under illumination, and form large crystals at high temperatures.

Now, a team of chemists led by Professor Jianhui Hou at the CAS set a new world record for fullerene-free polymer solar cells by developing a unique combination of a polymer called PBDB-T and a small molecule called ITIC. With this combination, the sun's energy is converted with an efficiency of 11%, a value that strikes most solar cells with fullerenes, and all without fullerenes.

Feng Gao, together with his colleagues Olle Inganäs and Deping Qian at Linköping University, have characterized the loss spectroscopy of photovoltage (Voc), a key figure for solar cells, and proposed approaches to further improving the device performance.

The two research groups are now presenting their results in the high-profile journal Advanced Materials.

-We have demonstrated that it is possible to achieve a high efficiency without using fullerene, and that such solar cells are also highly stable to heat. Because solar cells are working under constant solar radiation, good thermal stability is very important, said Feng Gao, a physicist at the Department of Physics, Chemistry and Biology, Linköping University.

The combination of high efficiency and good thermal stability suggest that polymer solar cells, which can be easily manufactured using low-cost roll-to-roll printing technology, now come a step closer to commercialization, said Feng Gao.

Source Link-https://www.sciencedaily.com/releases/2016/04/160419103847.htm

Story Source:

The above post is reprinted from materials provided by Linköping University. Note: Materials may be edited for content and length.

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Journal Reference:

1. Wenchao Zhao, Deping Qian, Shaoqing Zhang, Sunsun Li, Olle Inganäs, Feng Gao, Jianhui Hou. Fullerene-Free Polymer Solar Cells with over 11% Efficiency and Excellent Thermal Stability. Advanced Materials, 2016; DOI: 10.1002/adma.201600281

A new way to get electricity from magnetism

'Inverse spin Hall effect' works in several organic semiconductors

The upper part of this illustration shows the device, built on a small glass slide, that was used in experiments showing that so-called spin current could be converted to electric current using several different organic polymer semiconductors and a phenomenon known as the inverse spin Hall effect. The bottom illustration shows the key, sandwich-like part of the device. An external magnetic field and pulses of microwaves create spin waves in the iron magnet. When those waves hit the polymer or organic semiconductor, they create spin current, which is converted to an electrical current at the copper electrodes.

By showing that a phenomenon dubbed the "inverse spin Hall effect" works in several organic semiconductors -- including carbon-60 buckyballs -- University of Utah physicists changed magnetic "spin current" into electric current. The efficiency of this new power conversion method isn't yet known, but it might find use in future electronic devices including batteries, solar cells and computers.

"This paper is the first to demonstrate the inverse spin Hall effect in a range of organic semiconductors with unprecedented sensitivity," although a 2013 study by other researchers demonstrated it with less sensitivity in one such material, says Christoph Boehme, a senior author of the study published April 18 in the journal Nature Materials.

"The inverse spin Hall effect is a remarkable phenomenon that turns so-called spin current into an electric current. The effect is so odd that nobody really knows what this will be used for eventually, but many technical applications are conceivable, including very odd new power-conversion schemes," says Boehme, a physics professor.

His fellow senior author, distinguished professor Z. Valy Vardeny, says that by using pulses of microwaves, the inverse spin Hall effect and organic semiconductors to convert spin current into electricity, this new electromotive force generates electrical current in a way different than existing sources.

Coal, gas, hydroelectric, wind and nuclear plants all use dynamos to convert mechanical force into magnetic-field changes and then electricity. Chemical reactions power modern batteries and solar cells convert light to electrical current. Converting spin current into electrical current is another method.

Scientists already are developing such devices, such as a thermoelectric generator, using traditional inorganic semiconductors. Vardeny says organic semiconductors are promising because they are cheap, easily processed and environmentally friendly. He notes that both organic solar cells and organic LED (light-emitting diode) TV displays were developed even though silicon solar cells and nonorganic LEDs were widely used.

Vardeny and Boehme stressed that the efficiency at which organic semiconductors convert spin current to electric current remains unknown, so it is too early to predict the extent to which it might one day be used for new power conversion techniques in batteries, solar cells, computers, phones and other consumer electronics.

"I want to invoke a degree of caution," Boehme says. "This is a power conversion effect that is new and mostly unstudied."

Boehme notes that the experiments in the new study converted more spin current to electrical current than in the 2013 study, but Vardeny cautioned the effect still "would have to be scaled up many times to produce voltages equivalent to household batteries."

The new study was funded by the National Science Foundation and the University of Utah-NSF Materials Research Science and Engineering Center. Study co-authors with Vardeny and Boehme were these University of Utah physicists: research assistant professors Dali Sun and Hans Malissa, postdoctoral researchers Kipp van Schooten and Chuang Zhang, and graduate students Marzieh Kavand and Matthew Groesbeck.

From spin current to electric current

Just as atomic nuclei and the electrons that orbit them carry electrical charges, they also have another inherent property: spin, which makes them behave like tiny bar magnets that can point north or south.

Electronic devices store and transmit information using the flow of electricity in the form of electrons, which are negatively charged subatomic particles. The zeroes and ones of computer binary code are represented by the absence or presence of electrons within silicon or other nonorganic semiconductors.

Spin electronics -- spintronics -- holds promise for faster, cheaper computers, better electronics and LEDs for displays, and smaller sensors to detect everything from radiation to magnetic fields.

The inverse spin Hall effect first was demonstrated in metals in 2008, and then in nonorganic semiconductors, Vardeny says. In 2013, researchers elsewhere showed it occurred in an organic semiconductor named PEDOT:PSS when it was exposed to continuous microwaves that were relatively weak to avoid frying the semiconductor.

But Boehme and Vardeny say the electrical current generated in that study by the inverse spin Hall effect was small -- nanovoltages -- and was obscured by microwave heating of the sample and other undesired effects.

"We thought, let's build different devices so these spurious effects were eliminated or very small compared with the effect we wanted to observe," Boehme says.

In the new study, the researchers used short pulses of more powerful microwaves to utilize the inverse spin Hall effect and convert a spin current to electric current in seven organic semiconductors, mostly at room temperature.

One organic semiconductor was PEDOT:PSS -- the same material in the 2013 study. The others were three platinum-rich organic polymers, two so-called pi-conjugated polymers and the spherical carbon-60 molecule named buckminsterfullerene because it looks like a pair of geodesic domes popularized by the late architect Buckminster Fuller.

The carbon-60 proved surprisingly to be the most efficient semiconductor at converting spin waves into electrical current, Vardeny says.

How the experiments were performed

The Utah physicists take multiple steps to convert spin current to electrical current. They begin with a small glass slide, about 2.1-inches long and one-sixth-inch wide. Two electrical contacts are attached to one end of the glass slide. Thin, flat copper wires run the length of the slide, connecting the contacts at one end with a "sandwich" at the other end that includes the glass at the bottom, the organic polymer semiconductor being tested in the middle and a nickel-iron ferromagnet on top.

This device then is inserted lengthwise into a metal tube about 1-inch diameter and 3.5 inches long. A nonconducting material surrounds the device inside this tube, which then is inserted into a table-sized magnet that generates a magnetic field.

"We apply a magnetic field and leave it more or less constant," Boehme says. "Then we hook up the two contacts to a voltage meter and start measuring the voltage coming out of the device as a function of time."

With just the magnetic field, no electrical current was detected. But then the Utah physicists bombarded the organic semiconductor device with pulses of microwaves -- as powerful as those from a home microwave oven but in pulses ranging from only 100 to 5,000 nanoseconds (the latter equal to one 200,000th of a second).

"All of a sudden we saw a voltage during that pulse," Boehme says.

Vardeny says the microwave pulses generate spin waves in the device's magnet, then the waves are converted into spin current in the organic semiconductor, and then into an electric current detected as a voltage.

Compared with the 2013 study, the use of microwave pulses in the Utah experiments meant "our power is much higher but the heating is much less and the inverse spin Hall effect is about 100 times stronger," Boehme says.

In effect, the pulsed microwaves provide a way to enhance the inverse spin Hall effect so it can be used to convert power, Vardeny adds.

The new study also showed that the conversion of spin current to electric current works in organic semiconductors via "spin-orbit coupling" -- the same process found in inorganic conductors and semiconductors -- even though the phenomenon in inorganic and organic materials works in fundamentally different ways, Boehme says.

This coupling is much weaker in organic than in nonorganic semiconductors, but "the big achievement we made was to find an experimental method sensitive enough to reliably measure these very weak effects in organic semiconductors," Boehme says.

Source link-https://www.sciencedaily.com/releases/2016/04/160418120049.htm