University of Toronto Engineering researchers have discovered a dose threshold that greatly increases the delivery of cancer-fighting drugs into a tumor. Determining this threshold provides a potentially universal method for gauging nanoparticle dosage and could help advance a new generation of cancer therapy, imaging and diagnostics ….
Filter ‘paper’ made from titanium oxide nanowires is capable of trapping pathogens and destroying them with light. This discovery by an EPFL laboratory could be put to use in personal protective equipment, as well as in ventilation and air conditioning systems. As part of attempts to curtail the COVID-19 pandemic …
NanoCommerce Sdn Bhd (NCSB), a subsidiary of NanoMalaysia Bhd, signed a joint venture with Pulsar UAV Sdn Bhd (PUSB) to commercialize an increased range hydrogen-powered drone known as the High Endurance Fuel Cell Powered Unmanned Aerial Vehicle (On-board H2 Generation). The partnership will give NanoCommerce a 20 per cent stake in Pulsar UAV, a Malaysian company that builds its unmanned aerial vehicles (UAV) or drones from scratch ….
All Levi Conlow’s dad wanted was an electric bike that didn’t cause sticker shock. So when he approached his son and his son’s best friend Robby Deziel with the proposal that they put their heads together to make obtaining his e-bike dream come true, the new college graduates started thinking. “My dad was just entering that phase of his life when he wanted an e-bike for himself and my mom. Their friends had e-bikes,” Conlow said. “He was frustrated. He couldn’t find one for less than $2,000 or $3,000” ….
Hyperion, a California-based company, has unveiled a hydrogen-powered supercar the company hopes will change the way people view hydrogen fuel cell technology. The Hyperion XP-1 will be able to drive for up to 1,000 miles on one tank of compressed hydrogen gas and its electric motors will generate more than 1,000 horsepower, according to the company. The all-wheel-drive car can go from zero to 60 miles per hour in a little over two seconds, the company said.
Hydrogen fuel cell cars are electric cars that use hydrogen to generate power inside the car rather than using batteries to store energy. The XP-1 doesn’t combust hydrogen but uses it in fuel cells that combine hydrogen with oxygen from the air in a process that creates water, the vehicle’s only emission, and a stream of electricity to power the car.
University of Toronto Engineering researchers have discovered a dose threshold that greatly increases the delivery of cancer-fighting drugs into a tumor.
Determining this threshold provides a potentially universal method for gauging nanoparticle dosage and could help advance a new generation of cancer therapy, imaging and diagnostics.
“It’s a very simple solution, adjusting the dosage, but the results are very powerful,” says MD/Ph.D. candidate Ben Ouyang, who led the research under the supervision of Professor Warren Chan.
Their findings were published today in Nature Materials, providing solutions to a drug-delivery problem previously raised by Chan and researchers four years ago in Nature Reviews Materials.
Nanotechnology carriers are used to deliver drugs to cancer sites, which in turn can help a patient’s response to treatment and reduce adverse side effects, such as hair loss and vomiting. However, in practice, few injected particles reach the tumor site.
In the Nature Reviews Materials paper, the team surveyed literature from the past decade and found that on median, only 0.7 percent of the chemotherapeutic nanoparticles make it into a targeted tumor.
“The promise of emerging therapeutics is dependent upon our ability to deliver them to the target site,” explains Chan. “We have discovered a new principle of enhancing the delivery process. This could be important for nanotechnology, genome editors, immunotherapy, and other technologies.”
Chan’s team saw the liver, which filters the blood, as the biggest barrier to nanoparticle drug delivery. They hypothesized that the liver would have an uptake rate threshold—in other words, once the organ becomes saturated with nanoparticles, it wouldn’t be able to keep up with higher doses. Their solution was to manipulate the dose to overwhelm the organ’s filtering Kupffer cells, which line the liver channels.
The researchers discovered that injecting a baseline of 1 trillion nanoparticles in mice, in vivo, was enough to overwhelm the cells so that they couldn’t take up particles quick enough to keep up with the increased doses. The result is a 12 percent delivery efficiency to the tumor.
“There’s still lots of work to do to increase the 12 percent but it’s a big step from 0.7,” says Ouyang. The researchers also extensively tested whether overwhelming Kupffer cells led to any risk of toxicity in the liver, heart or blood.
“We tested gold, silica, and liposomes,” says Ouyang. “In all of our studies, no matter how high we pushed the dosage, we never saw any signs of toxicity.”
The team used this threshold principle to improve the effectiveness of a clinically used and chemotherapy-loaded nanoparticle called Caelyx. Their strategy shrank tumors 60 percent more when compared to Caelyx on its own at a set dose of the chemotherapy drug, doxorubicin.
Because the researchers’ solution is a simple one, they hope to see the threshold having positive implications in even current nanoparticle-dosing conventions for human clinical trials. They calculate that the human threshold would be about 1.5 quadrillion nanoparticles.
“There’s a simplicity to this method and reveals that we don’t have to redesign the nanoparticles to improve delivery,” says Chan. “This could overcome a major delivery problem.”
Quantum dots have revolutionized the field of optoelectronics due to their atom-like electronic structure. However, the prospect of colloidal quantum-dot lasers has long been deemed impractical due to the high energies required to induce optical gain. But recent work published in Nature and led by Ted Sargent of the University of Toronto shows that the lasing threshold in cadmium selenide (CdSe)/cadmium sulphide (CdS) core-shell colloidal quantum dots can be lowered by squashing the CdSe core via a clever ligand exchange process.
To instigate optical gain in a semiconductor laser, the difference between the lowest electron level and the highest hole level must be wider than the band gap so that the light emitted when they recombine can stimulate emission in neighbouring nanocrystals. Colloidal quantum dots (CQDs) then, should make ideal candidates for lasing applications, as their atom-like electronic structure means that the electron and hole energy levels are easier to separate.
In practice, however, the energies required to trigger optical gain in CQDs are so high that they can heat up to the point of burning. While electrons tend to occupy one energy state upon excitation, the hole that they leave behind in the valence band can populate one of eight closely spaced states. This degeneracy pushes the hole Fermi level into the band gap and increases the amount of energy required to instigate optical gain.
To overcome this issue, the researchers took advantage of the fact that CdS imposes a strain on CdSe due to a slight lattice mismatch of 3.9%. By growing an asymmetric CdS shell around a “squashed” oblate CdSe core, they were able to induce a biaxial strain that affected the heavy and light holes of the valence band to different extents, thus lifting the degeneracy.
To produce these asymmetric CQDs the group invented a technique called facet-selective epitaxy, making use of ligands that interact differently with the surfaces of CdSe. One of these ligands, trioctylphosphine sulphide, or TOPS, binds weakly to the (0001) facet of CdSe and not at all to the (0001), while octanethiol interacts similarly with all CdSe surfaces. Therefore, by growing CdS on the (0001) facet with TOPS and then replacing with octanethiol to stimulate epitaxial growth, oblate-shaped CQDs could be made throughout the entire particle ensemble with remarkable uniformity.
The resulting lasers had an unprecedentedly high performance, exhibiting a low lasing threshold of 6.4–8.4 kW cm–2, a seven-fold reduction compared with previous attempts. They also emitted light over a narrow energy range of just 36 meV. Both of these properties can be attributed to the enhanced splitting of the valence band levels that arises due to the oblate CQD shape.
The international team of researchers has certainly proved that continuous-wave CQD lasers are possible, yet there are still some obstacles to be overcome before they are seen on the market. Most importantly, the next step will be exciting the CQDs via electrical rather than optical means, as in standard commercial lasers. Nevertheless, facet-selective epitaxy opens up a whole host of other CQD materials for lasing applications and beyond.
A University of Toronto Engineering innovation could make printing solar cells as easy and inexpensive as printing a newspaper.
Dr. Hairen Tan and his team have cleared a critical manufacturing hurdle in the development of a relatively new class of solar devices called perovskite solar cells. This alternative solar technology could lead to low-cost, printable solar panels capable of turning nearly any surface into a power generator.
“Economies of scale have greatly reduced the cost of silicon manufacturing,” said Professor Ted Sargent, an expert in emerging solar technologies and the Canada Research Chair in Nanotechnology. “Perovskite solar cells can enable us to use techniques already established in the printing industry to produce solar cells at very low cost. Potentially, perovskites and silicon cells can be married to improve efficiency further, but only with advances in low-temperature processes.”
The new perovskite solar cells have achieved an efficiency of 20.1 per cent and can be manufactured at low temperatures, which reduces the cost and expands the number of possible applications. (Image: Kevin Soobrian)
Today, virtually all commercial solar cells are made from thin slices of crystalline silicon which must be processed to a very high purity. It’s an energy-intensive process, requiring temperatures higher than 1,000 degrees Celsius and large amounts of hazardous solvents.
In contrast, perovskite solar cells depend on a layer of tiny crystals — each about 1,000 times smaller than the width of a human hair — made of low-cost, light-sensitive materials. Because the perovskite raw materials can be mixed into a liquid to form a kind of ‘solar ink’, they could be printed onto glass, plastic or other materials using a simple inkjet printing process.
But, until now, there’s been a catch: in order to generate electricity, electrons excited by solar energy must be extracted from the crystals so they can flow through a circuit. That extraction happens in a special layer called the electron selective layer, or ESL. The difficulty of manufacturing a good ESL has been one of the key challenges holding back the development of perovskite solar cell devices.
“The most effective materials for making ESLs start as a powder and have to be baked at high temperatures, above 500 degrees Celsius,” said Tan. “You can’t put that on top of a sheet of flexible plastic or on a fully fabricated silicon cell — it will just melt.”
Tan and his colleagues developed a new chemical reaction than enables them to grow an ESL made of nanoparticles in solution, directly on top of the electrode. While heat is still required, the process always stays below 150 degrees C, much lower than the melting point of many plastics.
“This is the best ever reported for low-temperature processing techniques,” said Tan. He adds that perovskite solar cells using the older, high-temperature method are only marginally better at 22.1 per cent, and even the best silicon solar cells can only reach 26.3 per cent.
Another advantage is stability. Many perovskite solar cells experience a severe drop in performance after only a few hours, but Tan’s cells retained more than 90 per cent of their efficiency even after 500 hours of use. “I think our new technique paves the way toward solving this problem,” said Tan, who undertook this work as part of a Rubicon Fellowship.
“The Toronto team’s computational studies beautifully explain the role of the newly developed electron-selective layer. The work illustrates the rapidly-advancing contribution that computational materials science is making towards rational, next-generation energy devices,” said Professor Alan Aspuru-Guzik, an expert on computational materials science in the Department of Chemistry and Chemical Biology at Harvard University, who was not involved in the work.
“To augment the best silicon solar cells, next-generation thin-film technologies need to be process-compatible with a finished cell. This entails modest processing temperatures such as those in the Toronto group’s advance reported in Science,” said Professor Luping Yu of the University of Chicago’s Department of Chemistry. Yu is an expert on solution-processed solar cells and was not involved in the work.
Keeping cool during the manufacturing process opens up a world of possibilities for applications of perovskite solar cells, from smartphone covers that provide charging capabilities to solar-active tinted windows that offset building energy use. In the nearer term, Tan’s technology could be used in tandem with conventional solar cells.
“With our low-temperature process, we could coat our perovskite cells directly on top of silicon without damaging the underlying material,” said Tan. “If a hybrid perovskite-silicon cell can push the efficiency up to 30 per cent or higher, it makes solar power a much better economic proposition.”
Gains are derived from nanowires and light trapping for better energy conversion.
Sajeev John is a University Professor at the University of Toronto and Government of Canada Research Chair holder. He received his bachelor’s degree in physics in 1979 from the Massachusetts Institute of Technology and his PhD in physics at Harvard in 1984. His PhD work introduced the theory of classical wave localization and in particular the localization of light in three-dimensional strongly scattering dielectrics.
His groundbreaking work in the field of light localization that enables light to be controlled at the microscopic level has earned him an international reputation. He is a pioneering theoretician in photonic band gap (PBG) materials. This new class of optical materials presents exciting possibilities in the fields of physics, chemistry, engineering and medicine. PBG materials could eventually be used for optical communications/information processing, clinical medicine, lighting and solar energy harvesting.
John has received numerous awards, including the King Faisal International Prize in Science (2001), the IEEE Nanotechnology Pioneer Award (2008), and the Killam Prize in Natural Sciences (2014) from the Canada Council for the Arts. He is a Fellow of the American Physical Society, OSA, the Royal Society of Canada, and a member of the Max-Planck Society of Germany.
Chemotherapy isn’t supposed to make your hair fall out—it’s supposed to kill cancer cells. A new molecular delivery system created at U of T could help ensure that chemotherapy drugs get to their target while minimizing collateral damage.
Many cancer drugs target fast-growing cells. Injected into a patient, they swirl around in the bloodstream acting on fast-growing cells wherever they find them. That includes tumours, but unfortunately also hair follicles, the lining of your digestive system, and your skin.
Professor Warren Chan (IBBME, ChemE, MSE) has spent the last decade figuring out how to deliver chemotherapy drugs into tumours—and nowhere else. Now his lab has designed a set of nanoparticles attached to strands of DNA that can change shape to gain access to diseased tissue.
“Your body is basically a series of compartments,” says Chan. “Think of it as a giant house with rooms inside. We’re trying to figure out how to get something that’s outside, into one specific room. One has to develop a map and a system that can move through the house where each path to the final room may have different restrictions such as height and width.”
One thing we know about cancer: no two tumours are identical. Early-stage breast cancer, for example, may react differently to a given treatment than pancreatic cancer, or even breast cancer at a more advanced stage. Which particles can get inside which tumours depends on multiple factors such as the particle’s size, shape and surface chemistry.
Chan and his research group have studied how these factors dictate the delivery of small molecules and nanotechnologies to tumours, and have now designed a targeted molecular delivery system that uses modular nanoparticles whose shape, size and chemistry can be altered by the presence of specific DNA sequences.
“We’re making shape-changing nanoparticles,” says Chan. “They’re a series of building blocks, kind of like a LEGO set.” The component pieces can be built into many shapes, with binding sites exposed or hidden. They are designed to respond to biological molecules by changing shape, like a key fitting into a lock.
These shape-shifters are made of minuscule chunks of metal with strands of DNA attached to them. Chan envisions that the nanoparticles will float around harmlessly in the blood stream, until a DNA strand binds to a sequence of DNA known to be a marker for cancer. When this happens, the particle changes shape, then carries out its function: it can target the cancer cells, expose a drug molecule to the cancerous cell, tag the cancerous cells with a signal molecule, or whatever task Chan’s team has designed the nanoparticle to carry out.
Their work was published this week in two key studies in the Proceedings of the National Academy of Sciences and the leading journal Science.
“We were inspired by the ability of proteins to alter their conformation—they somehow figure out how to alleviate all these delivery issues inside the body,” says Chan. “Using this idea, we thought, ‘Can we engineer a nanoparticle to function like a protein, but one that can be programmed outside the body with medical capabilities?'”
Applying nanotechnology and materials science to medicine, and particularly to targeted drug delivery, is still a relatively new concept, but one Chan sees as full of promise. The real problem is how to deliver enough of the nanoparticles directly to the cancer to produce an effective treatment.
“Here’s how we look at these problems: it’s like you’re going to Vancouver from Toronto, but no one tells you how to get there, no one gives you a map, or a plane ticket, or a car—that’s where we are in this field,” he says. “The idea of targeting drugs to tumours is like figuring out how to go to Vancouver. It’s a simple concept, but to get there isn’t simple if not enough information is provided.”
“We’ve only scratched the surface of how nanotechnology ‘delivery’ works in the body, so now we’re continuing to explore different details of why and how tumours and other organs allow or block certain things from getting in,” adds Chan.
He and his group plan to apply the delivery system they’ve designed toward personalized nanomedicine—further tailoring their particles to deliver drugs to your precise type oftumour, and nowhere else.
More information: Edward A. Sykes et al. Tailoring nanoparticle designs to target cancer based on tumor pathophysiology, Proceedings of the National Academy of Sciences(2016). DOI: 10.1073/pnas.1521265113
Funding from KAUST is helping to bring innovative solar power technology to fruition with startup company QD Solar.
Enabling researchers to take their ideas from the earliest research stages right through to the development and commercialization of a final product is a foundation goal at King Abdullah University of Science and Technology (KAUST). One route is for KAUST to provide active support to startup companies that have strong connections with the university’s research activities.
Nicola Bettio, manager of the funding scheme, says seed funding and early-stage capital are made available to startup companies through the KAUST Innovation Fund. “We invest in companies launched by our faculty, researchers and students, alongside international startups that are willing to move their operations to KAUST and Saudi Arabia,” Bettio explains. “By encouraging the transition of science into innovation and new ventures we can help to establish a fertile ecosystem for early-stage technology-based companies in Saudi Arabia.”
Efficient and cost-effective solar power generation will bring significant economic benefits and drastically reduce the carbon footprint of Saudi Arabia and the rest of the world. For the past several years, KAUST has been supporting research into new technologies using ‘colloidal quantum dots’ – a potential alternative to the established design of solar cells.
With an ideal design for solar panels the paint made from quantum dots is highly flexible.
Colloidal quantum dots are tiny semiconductor particles with the capacity to harvest energy from both the visible spectrum and the previously-untapped near-infrared portion of the sun’s light. Edward Sargent, a senior researcher at the University of Toronto, has been working on the development and application of colloidal quantum dots for the past ten years, and helped found QD Solar, a Canadian startup company aiming to commercialize the new technology.
“When KAUST was founded back in 2009, there was a very strong emphasis on advancing solar technology that led to the creation of KAUST’s Solar and Photovoltaics Engineering Research Center (SPERC),” explains Sargent. “The university employed top professors in the field who have been a key asset during the development of the new quantum dot technique I have been working on. I was fortunate enough to be awarded a KAUST Investigators grant around that time, enabling me to pursue my research in collaboration with these top scientists.”
In 2009, Sargent’s team at the University of Toronto received a grant from KAUST to advance their research into colloidal quantum dots specifically for solar power applications. Since then, the team’s advances have evolved in leaps and bounds. “Our latest design incorporates the dots into a solution, or ‘paint’, which can be applied to flexible surfaces very easily and cheaply,” Sargent explains. “We have also vastly improved the power conversion efficiency of the dots over the past six years, thanks in part to the funding from KAUST.”
Sargent and his team, in collaboration with KAUST researchers, are now investigating ways of combining the dots with existing silicon-based solar cells to create new hybrid structure solar panels. This will allow them to harness a far greater fraction of the sun’s energy than ever before, improving the panels’ efficiency and capacity to generate more power.
Marc Vermeersch, the SPERC managing director, points out that current solar cell technologies are reaching their physical limits. He explains that a key advantage of creating QD Solar’s hybrid system is that existing solar panel producers will be able to modify their manufacturing practices relatively easily to incorporate the new design, rather than starting from scratch. Manufacturers will be able to achieve significant economic and efficiency improvements using QD Solar’s quantum dot-based solar products, Bettio adds.
KAUST anticipates welcoming a QD Solar presence to Saudi Arabia in the near future, as Bettio explains: “QD Solar is working with the KAUST Innovation Fund to raise capital to establish a significant development facility in our Research & Technology Park. Ultimately, this may lead to the establishment of a Saudi player in the photovoltaic industry and the creation of a hybrid solar panel manufacturing facility in the Kingdom.”
Researchers at the University of Toronto have unveiled a cheap, fast spray-on solar cell process that could enable the creation of solar arrays using the most modest of manufacturing methods. Even better, with the cells ‘printed’ onto flexible material, they could turn anything from airplane wings to your patio furniture into a solar power plant.
The research was led by University of Toronto researcher Illan Kramer, who calls the system sprayLD. Much as with many previous spray-on solar designs, spray LD utilizes colloidal quantum dots (CQD). CQD are tiny, light-sensitive dots—invisible to the naked eye—that can act as an absorbing photovoltaic material. The technology has long held promise, but the process of incorporating light-sensitive CQD’s onto surfaces has been expensive, slow and laborious.
What Kramer and his colleagues have done is to invent an entirely new, cheap, efficient technique for creating solar cells using CQD. They based the technique on a newspaper-printing process, and explain “SprayLD blasts a liquid containing CQDs directly onto flexible surfaces, such as film or plastic… by applying ink onto a roll of paper. This roll-to-roll coating method makes incorporating solar cells into existing manufacturing processes much simpler.”
Watch the Video:
As for their prototype, the team admits that it may look “more like junkyard wars than high-tech,” but that’s because it was made entirely from affordable, readily available parts; U of T explains “[Kramer] sourced a spray nozzle used in steel mills to cool steel with a fine mist of water, and a few regular air brushes from an art store.”
Which perhaps makes it all the more surprising that this technique does not appear to cause any major drop offs in efficiency. However, efficiency is still a major consideration with spray-on solar. The sprayLD system converts about 8.1% of sunlight to power, as opposed to 15-20% for standard rooftop solar arrays. But, speaking to Co.Exist, Kramer explained that while he is working on boosting sprayLD’s efficiency “we think of ourselves as operating in a slightly different paradigm, where we don’t have to be quite as efficient because we’re so much less expensive.”
And in the long run, Kramer explains: “My dream is that one day you’ll have two technicians with Ghostbusters backpacks come to your house and spray your roof!”
Every electrical device, from a simple lightbulb to the latest microchips, is enabled by the movement of electrical charge, or current. The nascent field of ‘spintronics’ taps into a different electronic attribute, an intrinsic quantum property known as spin, and may yield devices that operate on the basis of spin-transport.
Atom-optical lattice systems offer a clean, well-controlled way to study the manipulation and movement of spins because researchers can create particle configurations analogous to crystalline order in materials. JQI/CMTC* theorists Xiaopeng Li, Stefan Natu, and Sankar Das Sarma, in collaboration with Arun Paramekanti from the University of Toronto, have been developing a model for what happens when ultracold atomic spins are trapped in an optical lattice structure with a “double-valley” feature, where the repeating unit resembles the letter “W”. This new theory result, recently published in the multidisciplinary journal Nature Communications, opens up a novel path for generating what’s known as the spin Hall effect, an important example of spin-transport.
Behavior in double-valley lattices has previously been studied for a collection of atoms that all have the same value of spin. In this new work, theorists consider two-component atoms–here, the spin state each atom has can vary between “spin-up” and “spin-down” states.
The spontaneous chiral spin superfluid. When loaded into a hexagonal lattice, chiral spin currents are expected to develop. Here, the orange represents spin-up particles, and green spin-down. Arrows are used to guide the eye along the path of movement. Credit S. Kelley/JQI – See more at: http://jqi.umd.edu/news/restoring-order#sthash.pjtKa0oy.dpuf
Atomic spins in a lattice can be thought of as an array of tiny bar magnets. In the single-component case, the atom-magnets are all oriented the same direction in the lattice. In this case, the magnets can have a tendency to favor only one of the wells of the double-valley. In the two-component system studied here, each atom-magnet can have its’ north-pole pointed either up or down, with respect to a particular magnetic field.
Adding this kind of freedom to the model leads to some very curious behavior – the atoms spontaneously separate, with the spin-up atoms collecting into one well of the double-valley, and spin-down atoms in the other. Theorists have dubbed this new state a “spontaneous chiral spin superfluid” (for further explanation of what a superfluid is, see here).
This sort of spin-dependent organization is of great interest to researchers, who could employ it to study the spin Hall effect, analogous to the Hall effect for electrons. Normally, this effect is seen as a result of spin-orbit coupling, or the association of an atom’s spin with its motion. In fact, this has long been the approach for producing a spin Hall effect – apply a current to material with spin-orbit coupling, and the spins will gather at the edges according to their orientation.
Ian Spielman’s group at the JQI has pioneered laser-based methods for realizing both spin-orbit coupling and the spin Hall effect phenomenon in ultracold atomic gases. In contrast, for the superfluid studied here, spin-sorting is not the result of an applied field or asymmetric feature of the system, but rather emerges spontaneously. It turns out this behavior is driven by tiny, random quantum fluctuations, in a paradoxical phenomenon known as quantum order by disorder.
Quantum order by disorder
Generally speaking, the transition from disorder to order is a familiar one. Consider water condensing into ice: this is a disordered system, a liquid, transitioning to a more ordered one, a solid. This phase transformation happens because the molecules become limited in their degrees of freedom, or the ability to move in different ways. Conversely, adding noise to a system, such as heating an ice cube, generally leads to a more disordered state, a pool of water. Amazingly, noise or fluctuations in a system can sometimes drive a system into a more ordered state.
The theory team showed that this is indeed the case for certain kinds of atoms loaded into a double-valley optical lattice. While this system is a quiet, mostly non-thermal environment, noise still lurks in the form of quantum mechanical fluctuations. In this system, the spin-up and spin-down atoms can potentially be configured in four different, but energetically identical arrangements. This is known as degeneracy and can be indicative of the amount of order in a system–the more equal energy states, the more disordered a system. It turns out that these arrangements have different amounts of quantum noise and these fluctuations play a crucial role. Surprisingly, the quantum fluctuations will break up the degeneracy, thus restoring order.
What’s the upshot? In this system, the resulting lowest energy configuration–a chiral spin superfluid– is preferred independent of the type of double-valley lattice geometry, indicating a type of universal behavior. With this in mind, the theorists examine a number of lattice structures where this phenomenon might be realized. For instance, if the fluid is placed into a hexagonal lattice configuration, similar to the structure of graphene, they expect the characteristic spin currents of the spin Hall effect to emerge, as depicted in the graphic, above.
In the publication, the team points out that optical lattice systems are a flexible, pristine platform for examining the effect of these tiny variations in quantum fluctuations, which are often masked in real materials. Outside of exploring novel forms of matter like the one found here, research into spin and atom manipulation has applications in emerging electronic-like technologies, such as spintronics, valleytronics and atomtronics.
*JQI (Joint Quantum Institute) and CMTC (Condensed Matter Theory Center)
Quantum Dot Photovoltaics: A New Breed of Solar Cells: Setting New Records for Efficiency
May 28, 2014
Solar-cell technology has advanced rapidly, as hundreds of groups around the world pursue more than two dozen approaches using different materials, technologies, and approaches to improve efficiency and reduce costs.
Now a team at MIT has set a new record for the most efficient quantum-dot cells—a type of solar cell that is seen as especially promising because of its inherently low cost, versatility, and light weight.
While the overall efficiency of this cell is still low compared to other types—about 9 percent of the energy of sunlight is converted to electricity—the rate of improvement of this technology is one of the most rapid seen for a solar technology. The development is described in a paper, published in the journal Nature Materials, by MIT professors Moungi Bawendi and Vladimir Bulović and graduate students Chia-Hao Chuang and Patrick Brown.
The new process is an extension of work by Bawendi, the Lester Wolfe Professor of Chemistry, to produce quantum dots with precisely controllable characteristics—and as uniform thin coatings that can be applied to other materials. These minuscule particles are very effective at turning light into electricity, and vice versa. Since the first progress toward the use of quantum dots to make solar cells, Bawendi says, “The community, in the last few years, has started to understand better how these cells operate, and what the limitations are.”
The new work represents a significant leap in overcoming those limitations, increasing the current flow in the cells and thus boosting their overall efficiency in converting sunlight into electricity.
Many approaches to creating low-cost, large-area flexible and lightweight solar cells suffer from serious limitations—such as short operating lifetimes when exposed to air, or the need for high temperatures and vacuum chambers during production.
By contrast, the new process does not require an inert atmosphere or high temperatures to grow the active device layers, and the resulting cells show no degradation after more than five months of storage in air.
Bulović, the Fariborz Maseeh Professor of Emerging Technology and associate dean for innovation in MIT’s School of Engineering, explains that thin coatings of quantum dots “allow them to do what they do as individuals—to absorb light very well—but also work as a group, to transport charges.” This allows those charges to be collected at the edge of the film, where they can be harnessed to provide an electric current.
The new work brings together developments from several fields to push the technology to unprecedented efficiency for a quantum-dot based system: The paper’s four co-authors come from MIT’s departments of physics, chemistry, materials science and engineering, and electrical engineering and computer science. The solar cell produced by the team has now been added to the National Renewable Energy Laboratories’ listing of record-high efficiencies for each kind of solar-cell technology.
The overall efficiency of the cell is still lower than for most other types of solar cells. But Bulović points out, “Silicon had six decades to get where it is today, and even silicon hasn’t reached the theoretical limit yet. You can’t hope to have an entirely new technology beat an incumbent in just four years of development.” And the new technology has important advantages, notably a manufacturing process that is far less energy-intensive than other types.
Chuang adds, “Every part of the cell, except the electrodes for now, can be deposited at room temperature, in air, out of solution. It’s really unprecedented.”
The system is so new that it also has potential as a tool for basic research. “There’s a lot to learn about why it is so stable. There’s a lot more to be done, to use it as a testbed for physics, to see why the results are sometimes better than we expect,” Bulović says.
A companion paper, written by three members of the same team along with MIT’s Jeffrey Grossman, the Carl Richard Soderberg Associate Professor of Power Engineering, and three others, appears this month in the journal ACS Nano, explaining in greater detail the science behind the strategy employed to reach this efficiency breakthrough.
The new work represents a turnaround for Bawendi, who had spent much of his career working with quantum dots. “I was somewhat of a skeptic four years ago,” he says. But his team’s research since then has clearly demonstrated quantum dots‘ potential in solar cells, he adds.
Arthur Nozik, a research professor in chemistry at the University of Colorado who was not involved in this research, says, “This result represents a significant advance for the applications of quantum-dot films and the technology of low-temperature, solution-processed, quantum-dot photovoltaic cells. … There is still a long way to go before quantum-dot solar cells are commercially viable, but this latest development is a nice step toward this ultimate goal.”