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.”
Consistent with the adaptive management process described in this strategy, the NEHI Working Group has made significant progress through the use of various evaluation tools to understand the current status of nanotechnology-related EHS (nanoEHS) research and the Federal nanoEHS research investment.
Most notably, the participating agencies reported to the NEHI Working Group examples of ongoing, completed, and anticipated EHS research (from FY 2009 through FY 2012) relevant to implementation of the 2011 NNI EHS Research Strategy.
These examples, described in this document, demonstrate the breadth of activities in all six core research areas of the 2011 NNI EHS Research Strategy: Nanomaterial Measurement Infrastructure, Human Exposure Assessment, Human Health, Environment, Risk Assessment and Risk Management Methods, and Informatics and Modeling. Overall, coordination and implementation of the 2011 NNI EHS Strategy across the NEHI agencies has enabled:
Development of comprehensive measurement tools that consider the full life cycles of engineered nanomaterials (ENMs) in various media.
Collection of exposure assessment data and resources to inform workplace exposure control strategies for key classes of ENMs.
Enhanced understanding of the modes of interaction between ENMs and physiological systems relevant to human biology.
Improved assessment of transport and transformations of ENMs in various environmental media, biological systems, and over full life cycles.
Development of principles for establishing robust risk assessment and risk management practices for ENMs and nanotechnology-enabled products that incorporate ENMs, as well as approaches for identifying, characterizing, and communicating risks to all stakeholders.
Coordination of efforts to enhance data quality, modeling, and simulation capabilities for nanotechnology, towards building a collaborative nanoinformatics infrastructure.
Extensive collaboration and coordination among the NEHI agencies as well as with international organizations is evident by the numerous research examples and by other activities such as co-sponsored workshops and interagency agreements described in this review document. These examples and activities are a small subset of the extensive research efforts at the NEHI agencies. This document addresses the NEHI Working Group’s broader efforts in coordination, implementation, and social outreach in nanoEHS, as identified in the 2011 NNI EHS Research Strategy. As the NNI agencies sustain a robust budget for EHS research, Federal agencies will continue to invest in tools and share information essential to assess and manage potential risks of current and anticipated ENMs and nanotechnology-enabled products throughout their life cycles. The agencies will also continue to engage with the stakeholder community to establish a broad EHS knowledge base in support of regulatory decision making and responsible development of nanotechnology.
Such technologies could help with the country’s energy, environmental and economic security by creating new industries and jobs as well as by reducing the pollution associated with energy production and use today. More succinctly, “ARPA–E turns things that are plausible into things that are possible,” proclaimed Acting Director Cheryl Martin at the 2014 summit.
Out of 37 projects that received initial ARPA–E funding, Sun Catalytix, a company founded by Nocera, was the poster child—or rather video favorite—featured in a U.S. Department of Energy (DoE) clip talking up the potential of transformational change. “Almost all the solar energy is stored in water-splitting,” intoned Nocera, a Massachusetts Institute of Technology chemist, at the inaugural ARPA–E summit. “Shift happens.”
The artificial leaf proved to be possible but implausible, however. It won’t be splitting water using sunlight on a mass scale anytime soon, its hydrogen dreams blown away by a gale of cheap natural gas that can also be easily converted to the lightest element.
So Sun Catalytix has set the artificial leaf aside and shifted focus to flow batteries, rechargeable fuel cells that use liquid chemistry to store electricity. A better flow battery might not shift the fundamental fuel of the American dream but it could help utilities cope with the vagaries of wind and solar power—and is more likely to become a salable product in the near future.
Five years in, ARPA–E’s priorities have shifted, too, for the same reason. The cheap natural gas freed from shale by horizontal drilling and hydraulic fracturing (or fracking) has helped kill off bleeding-edge programs like Electrofuels, a bid to use microbes to turn cheap electricity into liquid fuels, and ushered in programs like REMOTE, a bid to use microbes to turn cheap natural gas into liquid fuels. Even at the first summit in 2010, so full of alternative energy promise, this gassy revolution was becoming apparent.
Methane Opportunities for Vehicular Energy, or MOVE program cars that run on natural gas or better batteries. Is enabling the energy predominance of another fossil fuel the kind of transformation E is failing?
The measure of success ARPA–E points to follow-on funding from other entities (whether corporate, government or venture capital) as an early measure of its success. So far, the agency has invested more than $900 million in 362 different research projects. Of those projects, 22 have garnered an additional $625 million from capitalists of one type or another; it is a group that includes Sun Catalytix.
ARPA–E funding has also allowed 24 projects to form spin-off companies whereas 16 projects have found a new funding source from other government agencies, including the DoE, which runs ARPA–E, and the Department of Defense.
The biggest successes include Makani Power, which makes souped up kites for wind power, and was acquired by Google after ARPA–E invested $6 million developing the technology. There’s also Ambri, which makes liquid-metal batteries for cheap energy storage on a massive scale and is now developing units capable of storing 20 kilowatt-hours for testing later this year.
The outright failures have been mostly less prominent: algae breeding for biofuels and various carbon dioxide capture technologies, along with efforts to knit together hydrocarbons from sunshine, carbon dioxide and water. But some have proved more conspicuous. ARPA–E feted a would-be breakthrough battery maker named Envia in 2012. But by 2014, while at least one of the entrepreneurs backing the company still mingled in the summit’s halls at the Gaylord National Resort & Convention Center in Maryland, Envia was mired in lawsuits and failed to deliver the energy-dense batteries it promised to General Motors.
“I don’t call them failures, I call them opportunities to learn,” argued ARPA–E’s first director, Arun Majumdar, in a 2012 interview with Scientific American about failed projects in general. “If 100 percent of these projects worked out, we’re not doing our job.”
ARPA–E is definitely doing its job then: Biofuels haven’t quite delivered on their promise, even engineering tobacco plants for oil, while electrofuels were a “crazy-ass idea,” to use a term employed by William Caesar, president of the recycling business at Waste Management, at the 2014 summit to describe some of the concepts his company has evaluated for investment. And ARPA–E’s budget has always been too small to tackle innovation in certain areas. “My real hope was to have enough of a budget to try out something different than what we are doing in the nuclear field today,” such as a prototype for a new kind of reactor, Majumdar said in a 2013 interview with Scientific American. “If you’re solving for climate change and you’re a serious person, your strategy starts with nuclear,” said David Crane, CEO of the electric utility NRG, at this year’s summit.
But ARPA–E’s budget has always been too small to encompass, for example, the hundreds of millions of dollars Crane lost during his tenure in a failed bid to build new standard nuclear reactors in Texas. An analysis of the biggest programs by year and funding shows that electrofuels drew the biggest investment (at more than $41 million) in fiscal year 2010 followed by better hardware and software for the U.S. grid to help integrate renewables in 2011.
But in fiscal year 2012 the biggest tranche of funding to a single program went to Boysen’s MOVE projects (roughly $30 million) and, in fiscal year 2013, just behind the $36 million invested in better batteries for electric cars, was the REMOTE program of projects garnering $34 million. “It could have a small environmental footprint,” argues Ramon Gonzalez, program director for Reducing Emissions using Methanotrophic Organisms for Transportation Energy (REMOTE). “We can develop something that is a bridge to renewable energy or even is renewable itself in the future.”
Natural gas hardly seems to need ARPA–E’s help to become ubiquitous. And although natural gas can help with climate change in the short term—displacing coal that emits even more pollution when burned to generate electricity—in the long run it, too, is a fossil fuel and a greenhouse gas itself. Burning methane for electricity will also one day require capturing and storing the resulting carbon dioxide in order to combat climate change.
ARPA–E has not succeeded in delivering a technological breakthrough that would allow that to happen cheaply or efficiently, despite investing more than $30 million in its Innovative Materials and Processes for Advanced Carbon Capture Technologies (IMPACCT) back in 2010. “ARPA–E needs to revisit carbon capture and storage,” said Michael Matuszewski of the National Energy Technology Laboratory at this year’s summit.
Long gameSignificant changes in energy sources—say from wood to coal or the current shift from coal to gas—take at least 50 years, judging by the record to date. “Looking at the climate risk mitigation agenda, we don’t have 50 to 60 years,” U.S. Secretary of Energy Ernest Moniz argued at the 2014 summit. “We have to cut [that time] in half,” and that will require breakthrough technologies that are cheaper, cleaner and faster to scale.
It is also exactly in times of overreliance on one energy source that funding into alternatives is not only necessary, but required. ARPA–E should continue to focus on transformational energy technologies that can be clean and cheap, even if political pressures incline the still young and potentially vulnerable agency to look for a better gas tank. After all, if ARPA–E and others succeed in finding ways to use ever-more natural gas, new shale supplies touted to last for a century at present consumption rates could be exhausted much sooner.
“Before this so-called ‘shale gale‘ came upon us, groupthink had most of us focusing on energy scarcity,” warned Alaska Sen. Lisa Murkowski (R) at the 2013 summit. “The consensus now is one of abundant energy. Don’t fall into the trap of groupthink again.” Failure is a necessary part of research at the boundaries of plausibility. As ARPA–E’s Martin said at this year’s summit: “It’s part of the process.” Many of the ideas the agency first funded were ideas that had sat unused on a shelf since the oil crisis of the 1970s.
And the ideas that go back on the shelf now, like the artificial leaf, provide the basic concepts—designer, metal-based molecules—for new applications, like flow batteries. The artificial leaf, for one, could benefit from ARPA–E or other research to bring down the cost of the photovoltaic cells that provide the electricity to allow the leaf’s designer molecules to do their water-splitting work. Already, cheaper photovoltaics may be ushering in an energy transition of their own, cropping up on roofs across the country from California to New Jersey.
When such renewable sources of energy become a significant source of electricity, more storage will be needed for when the sun doesn’t shine or the wind doesn’t blow—and that storage needs to be cheap and abundant. In Germany, where the wind and sun now provide roughly one quarter of all that nation’s electricity, the long-term plan is to convert any excess to gas that can then be burned in times of deficit—so-called power to gas, which is a fledgling technology at best. And why couldn’t clean hydrogen be that gas, as Nocera has suggested?
So the artificial leaf bides its time, while research continues at the Joint Center for Artificial Photosynthesis established with DoE money in California. Failure is an investment in future success. “The challenge is not that the technology doesn’t work, but the economics don’t work,” observed Waste Management’s Caesar at the 2014 ARPA–E Summit. “I don’t like to talk about dead ends. There are things that their time just hasn’t come yet.”
The lithium-ion batteries that power our laptops, smartphones and electric vehicles could have significantly higher energy density if their graphite anodes were to be replaced by lithium metal anodes. Hampering this change, however, has been the so-called dendrite problem. Over the course of several battery charge/discharge cycles, particularly when the battery is cycled at a fast rate, microscopic fibers of lithium, called “dendrites,” sprout from the surface of the lithium electrode and spread like kudzu across the electrolyte until they reach the other electrode. An electrical current passing through these dendrites can short-circuit the battery, causing it to rapidly overheat and in some instances catch fire. Efforts to solve the problem by curtailing dendrite growth have met with limited success, perhaps because they’ve just been scratching the surface of the problem.
These 3D reconstructions show how dendritic structures that can short-circuit a battery form deep within a lithium electrode, break through the surface and spread across the electrolyte.
Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have discovered that during the early stages of development, the bulk of dendrite material lies below the surface of the lithium electrode, underneath the electrode/electrolyte interface. Using X-ray microtomography at Berkeley Lab’s Advanced Light Source (ALS), a team led by Nitash Balsara, a faculty scientist with Berkeley Lab’s Materials Sciences Division, observed the seeds of dendrites forming in lithium anodes and growing out into a polymer electrolyte during cycling. It was not until the advanced stages of development that the bulk of dendrite material was in the electrolyte. Balsara and his colleagues suspect that non-conductive contaminants in the lithium anode trigger dendrite nucleation.
Nitash Balsara and Katherine Harry at ALS beamline 8.3.2 where they shed important new light on the dendrite problem in lithium batteries. (Photo by Roy Kaltschmidt)
“Contrary to conventional wisdom, it seems that preventing dendrite formation in polymer electrolytes depends on inhibiting the formation of subsurface dendritic structures in the lithium electrode,” Balsara says. “In showing that dendrites are not simple protrusions emanating from the lithium electrode surface and that subsurface non-conductive contaminants might be the source of dendritic structures, our results provide a clear prescription for the path forward to enabling the widespread use of lithium anodes.”
Balsara, who is a professor of chemical engineering at the University of California (UC) Berkeley, is the corresponding author of a paper describing this research in Nature Materials titled “Detection of subsurface structures underneath dendrites formed on cycled lithium metal electrodes.” Co-authors are Katherine Harry, Daniel Hallinan, Dilworth Parkinson and, Alastair MacDowell.
The tremendous capacity of lithium and the metal’s remarkable ability to move lithium ions and electrodes in and out of an electrode as it cycles through charge/discharge make it an ideal anode material. Until now, researchers have studied the dendrite problem using various forms of electron microscopy. This is the first study to employ microtomography using monochromatic beams of high energy or “hard” X-rays, ranging from 22 to 25 keV, at ALS beamline 8.3.2. This technique allows non-destructive three-dimensional imaging of solid objects at a resolution of approximately one micron.
“We observed crystalline contaminants in the lithium anode that appeared at the base of every dendrite as a bright speck,” says Katherine Harry, a member of Balsara’s research group and the lead author of the Nature Materials paper. “The lithium foils we used in this study contained a number of elements other than lithium with the most abundant being nitrogen. We can’t say definitively that these contaminants are responsible for dendrite nucleation but we plan to address this issue by conducting in situ X-ray microtomography.”
Balsara and his group also plan further study of the role played by the electrolyte in dendrite growth, and they have begun to investigate ways to eliminate non-conductive impurities from lithium anodes.
This research was funded by the DOE Office of Science.
Menlo Park, Calif. —Researchers have made the first battery electrode that heals itself, opening a new and potentially commercially viable path for making the next generation of lithium ion batteries for electric cars, cell phones and other devices. The secret is a stretchy polymer that coats the electrode, binds it together and spontaneously heals tiny cracks that develop during battery operation, said the team from Stanford University and the Department of Energy’s (DOE) SLAC National Accelerator Laboratory.
This prototype lithium ion battery, made in a Stanford lab, contains a silicon electrode protected with a coating of self-healing polymer. The cables and clips in the background are part of an apparatus for testing the performance of batteries during multiple charge-discharge cycles. (Brad Plummer/SLAC)
“Self-healing is very important for the survival and long lifetimes of animals and plants,” said Chao Wang, a postdoctoral researcher at Stanford and one of two principal authors of the paper. “We want to incorporate this feature into lithium ion batteries so they will have a long lifetime as well.”
Chao developed the self-healing polymer in the lab of Stanford Professor Zhenan Bao, whose group has been working on flexible electronic skin for use in robots, sensors, prosthetic limbs and other applications. For the battery project he added tiny nanoparticles of carbon to the polymer so it would conduct electricity.
”We found that silicon electrodes lasted 10 times longer when coated with the self-healing polymer, which repaired any cracks within just a few hours,” Bao said.
“Their capacity for storing energy is in the practical range now, but we would certainly like to push that,” said Yi Cui, an associate professor at SLAC and Stanford who led the research with Bao. The electrodes worked for about 100 charge-discharge cycles without significantly losing their energy storage capacity. “That’s still quite a way from the goal of about 500 cycles for cell phones and 3,000 cycles for an electric vehicle,” Cui said, “but the promise is there, and from all our data it looks like it’s working.”
Researchers worldwide are racing to find ways to store more energy in the negative electrodes of lithium ion batteries to achieve higher performance while reducing weight. One of the most promising electrode materials is silicon; it has a high capacity for soaking up lithium ions from the battery fluid during charging and then releasing them when the battery is put to work.
But this high capacity comes at a price: Silicon electrodes swell to three times normal size and shrink back down again each time the battery charges and discharges, and the brittle material soon cracks and falls apart, degrading battery performance. This is a problem for all electrodes in high-capacity batteries, said Hui Wu, a former Stanford postdoc who is now a faculty member at Tsinghua University in Beijing, the other principal author of the paper.
To make the self-healing coating, scientists deliberately weakened some of the chemical bonds within polymers – long, chain-like molecules with many identical units. The resulting material breaks easily, but the broken ends are chemically drawn to each other and quickly link up again, mimicking the process that allows biological molecules such as DNA to assemble, rearrange and break down.
To show how flexible their self-healing polymer is, researchers coated a balloon with it and then inflated and deflated the balloon repeatedly, mimicking the swelling and shrinking of a silicon electrode during battery operation. The polymer stretches but does not crack. (Brad Plummer/SLAC)
Researchers in Cui’s lab and elsewhere have tested a number of ways to keep silicon electrodes intact and improve their performance. Some are being explored for commercial uses, but many involve exotic materials and fabrication techniques that are challenging to scale up for production.
The self-healing electrode, which is made from silicon microparticles that are widely used in the semiconductor and solar cell industries, is the first solution that seems to offer a practical road forward, Cui said. The researchers said they think this approach could work for other electrode materials as well, and they will continue to refine the technique to improve the silicon electrode’s performance and longevity.
The research team also included Zheng Chen and Matthew T. McDowell of Stanford. Cui and Bao are members of the Stanford Institute for Materials and Energy Sciences, a joint SLAC/Stanford institute. The research was funded by DOE through SLAC’s Laboratory Directed Research and Development program and by the Precourt Institute for Energy at Stanford University.
The Stanford Institute for Materials and Energy Sciences (SIMES) is a joint institute of SLAC National Accelerator Laboratory and Stanford University. SIMES studies the nature, properties and synthesis of complex and novel materials in the effort to create clean, renewable energy technologies. For more information, please visit simes.slac.stanford.edu.
DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.
Scientists collaborate to maximize energy gains from tiny nanoparticles
“We are developing a new ‘micro-reactor’ that enables us to explore many aspects of catalytic function using multiple approaches at Brookhaven’s National Synchrotron Light Source (NSLS), the soon-to-be-completed NSLS-II, and the Center for Functional Nanomaterials (CFN),” said Stach, who works at the CFN. “This approach lets us understand multiple aspects of how catalysts work so that we can tweak their design to improve their function. This work could lead to big gains in energy efficiency and cost savings for industrial processes.” – Anatoly Frenkel, Yeshiva Univerity
Sometimes big change comes from small beginnings.That’s especially true in the research of Anatoly Frenkel, a professor of physics at Yeshiva University, who is working to reinvent the way we use and produce energy by unlocking the potential of some of the world’s tiniest structures: nanoparticles.
“The nanoparticle is the smallest unit in most novel materials, and all of its properties are linked in one way or another to its structure,” said Frenkel. “If we can understand that connection, we can derive much more information about how it can be used for catalysis, energy, and other purposes.”
“This work could lead to big gains in energy efficiency and cost savings for industrial processes.”
— Eric Stach, CFN
Frenkel is collaborating with materials scientist Eric Stach and others at the U.S. Department of Energy’s Brookhaven National Laboratoryto develop new ways to study how nanoparticles behave in catalysts—the “kick-starters” of chemical reactions that convert fuels to useable forms of energy and transform raw materials to industrial products.
“We are developing a new ‘micro-reactor’ that enables us to explore many aspects of catalytic function using multiple approaches at Brookhaven’s National Synchrotron Light Source (NSLS), the soon-to-be-completed NSLS-II, and the Center for Functional Nanomaterials (CFN),” said Stach, who works at the CFN. “This approach lets us understand multiple aspects of how catalysts work so that we can tweak their design to improve their function. This work could lead to big gains in energy efficiency and cost savings for industrial processes.”
High-tech tools for science
Until now, the methods for understanding catalytic properties could only be used one at a time, with the catalyst ending up in a different state for each of the experiments. This made it difficult to compare information obtained using the different instruments. The new micro-reactor will employ multiple techniques—microscopy, spectroscopy, and diffraction—to examine different properties of catalysts simultaneously under operating conditions. By keeping particles in the same structural and dynamic state under the same reaction conditions, the micro-reactor will give scientists a much better sense of how they function.
This high-resolution transmission electron micrograph taken at the CFN reveals the arrangement of cerium oxide nanoparticles (bright angular “slashes” at the bottom of the image) supported on a titania substrate (background)‹a combination being explored as a catalyst for splitting water molecules to release hydrogen as fuel and for other energy-transformation reactions.
“These developments have resulted from the combination of unique facilities available at Brookhaven,” said Frenkel. “By working closely with Eric, we realized that there was a way to make both x-ray and electron-based methods work in a truly complementary fashion.
Each technique has strengths, Stach explained. “At the NSLS, using powerful beams of x-rays, we can tell how the entire group of nanoparticles behaves, while electron microscopy at the CFN lets us see the atomic structure of each nanoparticle. By having both of these views of the catalysts we can more clearly understand the relationship between catalyst structure and function.”
Said Frenkel, “It was very satisfying for us to conduct the first tests with the reactor at each facility and receive positive results. I am particularly grateful to Ryan Tappero, the scientist who runs NSLS beamline X27A, for his expert help with x-ray data acquisition.”
Frenkel has had an ongoing collaboration with scientists at Brookhaven. Last year, with post-doctoral research associate Qi Wang, Frenkel and Stach measured properties of nanoparticles using the x-rays produced by the NSLS as well as atomic-scale imaging with electrons at the CFN. As reported in a paper published in the Journal of the American Chemical Society earlier this year, they discovered that rather than changing completely from one state to another at a certain temperature and size, as had been previously believed, there is a transition zone between states when particles are changing forms.
“This is of significance fundamentally because until now, the structures were known to merely change from one form to another—they were never envisioned to coexist in different forms,” Frenkel said. “With our information we can explain why catalysts often don’t work as expected and how to improve them.”
Training for young scientists
Anatoly Frenkel of Yeshiva University with students from Stern College for Women at the National Synchrotron Light Source at Brookhaven National Laboratory.
The collaboration also offers opportunities for students to experience the challenges of research, giving them access to the world-class tools at Brookhaven. Frenkel’s undergraduate students at Yeshiva University’s Stern College for Women help with measurements, data analysis, and interpretation, and many have already accompanied him to Brookhaven to assist in his work using NSLS and other cutting-edge instruments.
“I’m giving them firsthand experience about what a researcher’s life is like early on as they conduct first-rate research,” said Frenkel. “This experience opens doors to any field they want to be in.”
Alyssa Lerner, a pre-engineering major who has been working with Frenkel at Brookhaven, said the research “has helped me develop skills like computational analysis and critical thinking, which are essential in any scientific field. The hands-on experimental experience has given me a better understanding of how the scientific community operates, helping me make more informed career-related choices as I continue to advance my education.”
Pairing up students and mentors to advance education and making use of complementary imaging techniques to enhance energy efficiency—just two of the positive outcomes of this successful collaboration.
“By bringing together multiple complementary techniques to illuminate the same process we’re going to understand how nanomaterials work,” Frenkel said. “Ultimately, this research will create a better way of using, storing, and converting energy.”
The CFN and NSLS facilities at Brookhaven Lab are supported by the Department of Energy’s Office of Science. The collaborative work of Frenkel and Stach is funded by the Office of Science and Brookhaven’s Laboratory Directed Research and Development program.
The Center for Functional Nanomaterials is one of five DOE Nanoscale Science Research Centers (NSRCs), national user facilities for interdisciplinary research at the nanoscale, supported by the DOE Office of Science. Together the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative. The NSRCs are located at DOE’s Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge and Sandia and Los Alamos National Laboratories. For more information about the DOE NSRCs, please visit http://science.energy.gov.
The National Synchrotron Light Source (NSLS) provides intense beams of infrared, ultraviolet, and x-ray light for basic and applied research in physics, chemistry, medicine, geophysics, and environmental and materials sciences. Supported by the Office of Basic Energy Sciences within the U.S. Department of Energy, the NSLS is one of the world’s most widely used scientific facilities. For more information, visit http://www.nsls.bnl.gov.
DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov
(Nanowerk News) Advanced plasma-based etching is a key enabler of Moore’s Law that observes that the number of transistors on integrated circuits doubles nearly every two years. It is the plasma’s ability to reproduce fine patterns on silicon that makes this scaling possible and has made plasma sources ubiquitous in microchip manufacturing.
A groundbreaking fabrication technique, based on what is called a DC-augmented capacitively coupled plasma source, affords chip makers unprecedented control of the plasma. This process enables DC-electrode borne electron beams to reach and harden the surface of the mask that is used for printing the microchip circuits. More importantly, the presence of the beam creates a population of suprathermal electrons in the plasma, producing the plasma chemistry that is necessary to protect the mask. The energy of these electrons is greater than simple thermal heating could produce—hence the name “suprathermal.” But how the beam electrons transform themselves into this suprathermal population has been a puzzle.
A plasma wave can give rise to a population of suprathermal electrons. (Credit: I.D. Kaganovich and D. Sydorenko)
Plasma Physics Laboratory in collaboration with the University of Alberta has shed light on this transformation. The simulation reveals that the initial DC-electrode borne beam generates intense plasma waves that move through the plasma like ripples in water. And it is this beam-plasma instability that leads to the generation of the crucial suprathermal electrons.
Understanding the role these instabilities play provides a first step toward still-greater control of the plasma-surface interactions, and toward further increasing the number of transistors on integrated circuits. Insights from both numerical simulations and experiments related to beam-plasma instabilities thus portend the development of new plasma sources and the increasingly advanced chips that they fabricate.
OAK RIDGE, Tenn., Oct. 28, 2013 – Gas and oil deposits in shale have no place to hide from an Oak Ridge National Laboratory technique that provides an inside look at pores and reveals structural information potentially vital to the nation’s energy needs.
The research by scientists at the Department of Energy laboratory could clear the path to the more efficient extraction of gas and oil from shale, environmentally benign and efficient energy production from coal and perhaps viable carbon dioxide sequestration technologies, according to Yuri Melnichenko, an instrument scientist at ORNL’s High Flux Isotope Reactor.
Scanning electron microscope image illustrating mineralogy and texture of unconventional gas reservoir. Note that nanoporosity is not resolvable with this image. SANS and USANS analysis is required to quantify pore size distribution and interconnectivity. (hi-res image)
Melnichenko’s broader work was emboldened by a collaboration with James Morris and Nidia Gallego, lead authors of a paper recently published in Journal of Materials Chemistry A and members of ORNL’s Materials Science and Technology Division.
Researchers were able to describe a small-angle neutron scattering technique that, combined with electron microscopy and theory, can be used to examine the function of pore sizes.
Using their technique at the General Purpose SANS instrument at the High Flux Isotope Reactor, scientists showed there is significantly higher local structural order than previously believed in nanoporous carbons. This is important because it allows scientists to develop modeling methods based on local structure of carbon atoms. Researchers also probed distribution of adsorbed gas molecules at unprecedented smaller length scales, allowing them to devise models of the pores.
“We have recently developed efficient approaches to predict the effect of pore size on adsorption,” Morris said. “However, these predictions need verification – and the recent small-angle neutron experiments are ideal for this. The experiments also beg for further calculations, so there is much to be done.”
While traditional methods provide general information about adsorption averaged over an entire sample, they do not provide insight into how pores of different sizes contribute to the total adsorption capacity of a material. Unlike absorption, a process involving the uptake of a gas or liquid in some bulk porous material, adsorption involves the adhesion of atoms, ions or molecules to a surface.
This research, in conjunction with previous work, allows scientists to analyze two-dimensional images to understand how local structures can affect the accessibility of shale pores to natural gas.
“Combined with atomic-level calculations, we demonstrated that local defects in the porous structure observed by microscopy provide stronger gas binding and facilitate its condensation into liquid in pores of optimal sub-nanometer size,” Melnichenko said. “Our method provides a reliable tool for probing properties of sub- and super-critical fluids in natural and engineered porous materials with different structural properties.
“This is a crucial step toward predicting and designing materials with enhanced gas adsorption properties.”
Together, the application of neutron scattering, electron microscopy and theory can lead to new design concepts for building novel nanoporous materials with properties tailored for the environment and energy storage-related technologies. These include capture and sequestration of man-made greenhouse gases, hydrogen storage, membrane gas separation, environmental remediation and catalysis.
Other authors of the paper, titled “Modern approaches to studying gas adsorption in nanoporous carbons,” are Cristian Contescu, Matthew Chisholm, Valentino Cooper, Lilin He, Yungok Ihm, Eugene Mamontov, Raina Olsen, Stephen Pennycook, Matthew Stone and Hongxin Zhang. The research, funded by DOE’s Office of Basic Energy Sciences, utilized the following DOE Office of Science user facilities:
The ShaRE User Facility (http://web.ornl.gov/sci/share/) makes available state-of-the-art electron beam microcharacterization facilities for collaboration with researchers from universities, industry and other government laboratories.
UT-Battelle manages ORNL for the Department of Energy’s Office of Science. DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of the time. For more information, please visit science.energy.gov.
The technique, described in a paper published online by Nature Nanotechnology on October 20, 2013 (“A general strategy for the DNA-mediated self-assembly of functional nanoparticles into heterogeneous systems”), opens many opportunities for mixing and matching particles with different magnetic, optical, or chemical properties to form new, multifunctional materials or materials with enhanced performance for a wide range of potential applications.
The approach takes advantage of the attractive pairing of complementary strands of synthetic DNA-based on the molecule that carries the genetic code in its sequence of matched bases known by the letters A, T, G, and C.
After coating the nanoparticles with a chemically standardized “construction platform” and adding extender molecules to which DNA can easily bind, the scientists attach complementary lab-designed DNA strands to the two different kinds of nanoparticles they want to link up. The natural pairing of the matching strands then “self-assembles” the particles into a three-dimensional array consisting of billions of particles. Varying the length of the DNA linkers, their surface density on particles, and other factors gives scientists the ability to control and optimize different types of newly formed materials and their properties.
DNA linkers allow different kinds of nanoparticles to self-assemble and form relatively large-scale nanocomposite arrays. This approach allows for mixing and matching components for the design of multifunctional materials.
“Our study demonstrates that DNA-driven assembly methods enable the by-design creation of large-scale ‘superlattice’ nanocomposites from a broad range of nanocomponents now available-including magnetic, catalytic, and fluorescent nanoparticles,” said Brookhaven physicist Oleg Gang, who led the research at the Lab’s Center for Functional Nanomaterials (CFN). “This advance builds on our previous work with simpler systems, where we demonstrated that pairing nanoparticles with different functions can affect the individual particles’ performance, and it offers routes for the fabrication of new materials with combined, enhanced, or even brand new functions.”
Future applications could include quantum dots whose glowing fluorescence can be controlled by an external magnetic field for new kinds of switches or sensors; gold nanoparticles that synergistically enhance the brightness of quantum dots’ fluorescent glow; or catalytic nanomaterials that absorb the “poisons” that normally degrade their performance, Gang said.
“Modern nano-synthesis methods provide scientists with diverse types of nanoparticles from a wide range of atomic elements,” said Yugang Zhang, first author of the paper. “With our approach, scientists can explore pairings of these particles in a rational way.”
Pairing up dissimilar particles presents many challenges the scientists investigated in the work leading to this paper. To understand the fundamental aspects of various newly formed materials they used a wide range of techniques, including x-ray scattering studies at Brookhaven’s National Synchrotron Light Source (NSLS) and spectroscopy and electron microcopy at the CFN.
For example, the scientists explored the effect of particle shape. “In principle, differently shaped particles don’t want to coexist in one lattice,” said Gang. “They either tend to separate into different phases like oil and water refusing to mix or form disordered structures.”
The scientists discovered that DNA not only helps the particles mix, but it can also improve order for such systems when a thicker DNA shell around the particles is used.
They also investigated how the DNA-pairing mechanism and other intrinsic physical forces, such as magnetic attraction among particles, might compete during the assembly process.
For example, magnetic particles tend to clump to form aggregates that can hinder the binding of DNA from another type of particle. “We show that shorter DNA strands are more effective at competing against magnetic attraction,” Gang said.
For the particular composite of gold and magnetic nanoparticles they created, the scientists discovered that applying an external magnetic field could “switch” the material’s phase and affect the ordering of the particles.
“This was just a demonstration that it can be done, but it could have an application-perhaps magnetic switches, or materials that might be able to change shape on demand,” said Zhang.
DNA linkers allow different kinds of nanoparticles to self-assemble and form relatively large-scale nanocomposite arrays. This approach allows for mixing and matching components for the design of multifunctional materials.
The third fundamental factor the scientists explored was how the particles were ordered in the superlattice arrays: Does one type of particle always occupy the same position relative to the other type-like boys and girls sitting in alternating seats in a movie theater-or are they interspersed more randomly?
“This is what we call a compositional order, which is important for example for quantum dots because their optical properties-e.g., their ability to glow-depend on how many gold nanoparticles are in the surrounding environment,” said Gang. “If you have compositional disorder, the optical properties would be different.” In the experiments, increasing the thickness of the soft DNA shells around the particles increased compositional disorder.
These fundamental principles give scientists a framework for designing new materials. The specific conditions required for a particular application will be dependent on the particles being used, Zhang emphasized, but the general assembly approach would be the same.
Said Gang, “We can vary the lengths of the DNA strands to change the distance between particles from about 10 nanometers to under 100 nanometers-which is important for applications because many optical, magnetic, and other properties of nanoparticles depend on the positioning at this scale. We are excited by the avenues this research opens up in terms of future directions for engineering novel classes of materials that exploit collective effects and multifunctionality.”
From solar cells to optoelectronic sensors to lasers and imaging devices, many of today’s semiconductor technologies hinge upon the absorption of light. Absorption is especially critical for nano-sized structures at the interface between two energy barriers called quantum wells, in which the movement of charge carriers is confined to two-dimensions. Now, for the first time, a simple law of light absorption for 2D semiconductors has been demonstrated.
Working with ultrathin membranes of the semiconductor indium arsenide, a team of researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) has discovered a quantum unit of photon absorption, which they have dubbed “AQ,” that should be general to all 2D semiconductors, including compound semiconductors of the III-V family that are favored for solar films and optoelectronic devices. This discovery not only provides new insight into the optical properties of 2D semiconductors and quantum wells, it should also open doors to exotic new optoelectronic and photonic technologies.
“We used free-standing indium arsenide membranes down to three nanometers in thickness as a model material system to accurately probe the absorption properties of 2D semiconductors as a function of membrane thickness and electron band structure,” says Ali Javey, a faculty scientist in Berkeley Lab’s Materials Sciences Division and a professor of electrical engineering and computer science at the University of California (UC) Berkeley. “We discovered that the magnitude of step-wise absorptance in these materials is independent of thickness and band structure details.”
Javey is one of two corresponding authors of a paper describing this research in the Proceedings of the National Academy of Sciences (PNAS). The paper is titled “Quantum of optical absorption in two-dimensional semiconductors.” Eli Yablonovitch, an electrical engineer who also holds joint appointments with Berkeley Lab and UC Berkeley, is the other corresponding author. Co-authors are Hui Fang, Hans Bechtel, Elena Plis, Michael Martin and Sanjay Krishna.
Previous work has shown that graphene, a two-dimensional sheet of carbon, has a universal value of light absorption. Javey, Yablonovitch and their colleagues have now found that a similar generalized law applies to all 2D semiconductors. This discovery was made possible by a unique process that Javey and his research group developed in which thin films of indium arsenide are transferred onto an optically transparent substrate, in this case calcium fluoride.
“This provided us with ultrathin membranes of indium arsenide, only a few unit cells in thickness, that absorb light on a substrate that absorbed no light,” Javey says. “We were then able to investigate the optical absorption properties of membranes that ranged in thickness from three to 19 nanometers as a function of band structure and thickness.”
Using the Fourier transform infrared spectroscopy (FTIR) capabilities of Beamline 1.4.3 at Berkeley Lab’s Advanced Light Source, a DOE national user facility, Javey, Yablonovitch and their co-authors measured the magnitude of light absorptance in the transition from one electronic band to the next at room temperature. They observed a discrete stepwise increase at each transition from indium arsenide membranes with an AQ value of approximately 1.7-percent per step.
“This absorption law appears to be universal for all 2D semiconductor systems,” says Yablonovitch. “Our results add to the basic understanding of electron–photon interactions under strong quantum confinement and provide a unique insight toward the use of 2D semiconductors for novel photonic and optoelectronic applications.”
Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more information, visit http://www.lbl.gov.
The DOE Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.
The Advanced Light Source is a third-generation synchrotron light source producing light in the x-ray region of the spectrum that is a billion times brighter than the sun. A DOE national user facility, the ALS attracts scientists from around the world and supports its users in doing outstanding science in a safe environment. The Advanced Light Source is a third-generation synchrotron light source producing light in the x-ray region of the spectrum that is a billion times brighter than the sun. A DOE national user facility, the ALS attracts scientists from around the world and supports its users in doing outstanding science in a safe environment. For more information, visit http://www.als.lbl.gov/.