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Nanomaterials for next-generation ultrafast lithium storage
Nanowerk Spotlight, January 21, 2011
Nanomaterials are key in developing high-performance energy storage technologies for mobile phone batteries and notebooks providing long battery times. Lithium-ion batteries are not only used in mobile gadgets but are currently being intensively developed worldwide for use in electric vehicles. As lithium-ion batteries are charged, large amounts of lithium ions are held in the anode. When the battery is used, these ions migrate to the cathode, sending electrons through the circuit. Traditional anode materials like graphite have a fairly low storage capacity and release rate, so finding alternatives is key to making batteries that last longer and produce more power.
Titanium dioxide is regarded as one of the ideal candidates for high-rate anode materials, owing not only to its structural characteristics and special surface activity, but also to its low cost, safety, and relatively low environmental impact.
Researchers in Singapore have developed a facile system to fabricate sandwich-like carbon-supported stacked titanium dioxide nanosheets, in which carbon pillars create open channels for fast lithium ion diffusion and the ultrathin framework renders the storage of lithium almost exclusively on the surface. This work provides a new route to design the electrode materials for quick-charging lithium ion batteries.
TEM images of the as-synthesized layered titanate nanosheets (left) and the carbon-supported stacked titanium dioxide nanosheets obtained after annealing at 350 °C for 2 hours (right). (Source: Xuewei Liu, Nanyang Technological University)
"The lack of open channels in bulk titanium dioxide is the main drawback that restricts its capacity and rate capability for reversible lithium insertion and extraction," Xuewei Liu, an assistant professor at Nanyang Technological University (NTU), explains to Nanowerk. "A reduction in the effective size and construction of open channels in the material are the main strategies currently employed to increase the rate performance. The capacity of ultrafine TiO2 nanocrystals and nanotubes, for example, is significantly enhanced at lower rates. However, their capacity and cycle life deteriorate dramatically at higher rates."
Liu notes that, in this respect, significant efforts have recently been made on the fabrication of anatase TiO2 nanosheets with exposed highly reactive (001) facets. These TiO2 nanosheets are shown to be an excellent host structure for lithium insertion and extraction due to the presence of exposed (001) facets and short path along the [001] direction for lithium ion diffusion.
"However, high-pressure systems including flammable solvent and corrosive additives (bases or acids) were often employed in solvothermal methods" says Liu. "There is still potential danger in the high-temperature and high-pressure systems when low boiling-point inflammable solvents are used. In addition, the nanosheets with exposed smooth facets easily overlap with each other because of the Ostwald ripening process, which leads to a decrease of surface area. To conquer these obstacles, we designed our simple ionic-liquid system."
Reporting their findings in a recent issue of Advanced Materials ("Sandwich-Like, Stacked Ultrathin Titanate Nanosheets for Ultrafast Lithium Storage"), Liu's group, working with Lou Xiong Wen's team at NTU's College of Engineering, synthesized sandwich-like carbon-supported stacked TiO2 nanosheets (CTNSs) on a large scale by a facile and clean route.
Comparison with existing methods and materials (1 C = 170 mA g-1) (Source: Xuewei Liu, Nanyang Technological University)
Owing to the unique textural features, i.e., stacked nanosheets with open channels, the storage of lithium in these CTNSs mainly takes place on surface, which is completely different from the insertion of lithium into bulk anatase TiO2 materials.
"This pseudocapacitive insertion and extraction renders remarkable improvement in high-rate charging and discharging" says Liu. "Moreover, the in situ derived carbon can significantly stabilize these stacked TiO2 nanosheets, leading to an excellent cycle life. We thus anticipate the promising use of this material in high-power lithium-ion batteries."
Liu summarizes the advantages of the new anode material: 1) Clean synthetic systems: Low pressure and safe; 2) The ionic liquid system is low-cost and facile to be synthesized; 3) High yield: the product could be synthesized in large scale and easy to mass-production; 4) Unique structure with superior performance: The sandwich structure provides open channels that are very suitable for fast lithium storage. The table above shows that the material has excellent capacities at low rates and high rates. The full charging time at 30 °C is only ~1.3 minutes.
Since the development of ultrafast-charging and high-rate electrode materials with low-cost is still a big challenge for next-generation lithium ion batteries, the NTU team will continue to focus their research on the development of novel and low-cost electrode materials for next-generation lithium ion batteries.
Ever-Sharp Urchin Teeth May Yield Tools That Never Need Honing
ScienceDaily (Dec. 26, 2010)
To survive in a tumultuous environment, sea urchins literally eat through stone, using their teeth to carve out nooks where the spiny creatures hide from predators and protect themselves from the crashing surf on the rocky shores and tide pools where they live.
Sea urchin teeth are pictured in situ. New research by Pupa Gilbert, a physics professor at the University of Wisconsin-Madison, and her colleagues reveals how the sea urchin's teeth are always sharp, despite constant grinding and scraping to create the nooks that protect the marine animal from predators and crashing waves. (Credit: Photo courtesy of Pupa Gilbert)
The rock-boring behavior is astonishing, scientists agree, but what is truly remarkable is that, despite constant grinding and scraping on stone, urchin teeth never, ever get dull. The secret of their ever-sharp qualities has puzzled scientists for decades, but now a new report by scientists from the University of Wisconsin-Madison and their colleagues has peeled back the toothy mystery.
Writing in the journal Advanced Functional Materials, a team led by UW-Madison professor of physics Pupa Gilbert describes the self-sharpening mechanism used by the California purple sea urchin to keep a razor-sharp edge on its choppers.
The urchin's self-sharpening trick, notes Gilbert, is something that could be mimicked by humans to make tools that never need honing.
"The sea urchin tooth is complicated in its design. It is one of the very few structures in nature that self-sharpen," says Gilbert, explaining that the sea urchin tooth, which is always growing, is a biomineral mosaic composed of calcite crystals with two forms -- plates and fibers -- arranged crosswise and cemented together with super-hard calcite nanocement. Between the crystals are layers of organic materials that are not as sturdy as the calcite crystals.
"The organic layers are the weak links in the chain," Gilbert explains. "There are breaking points at predetermined locations built into the teeth. It is a concept similar to perforated paper in the sense that the material breaks at these predetermined weak spots."
The crystalline nature of sea urchin dentition is, on the surface, different from other crystals found in nature. It lacks the obvious facets characteristic of familiar crystals, but at the very deepest levels the properties of crystals are evident in the orderly arrangement of the atoms that make up the biomineral mosaic teeth of the sea urchin.
To delve into the fundamental nature of the crystals that form sea urchin teeth, Gilbert and her colleagues used a variety of techniques from the materials scientist's toolbox. These include microscopy methods that depend on X-rays to illuminate how nanocrystals are arranged in teeth to make the sea urchins capable of grinding rock. Gilbert and her colleagues used these techniques to deduce how the crystals are organized and melded into a tough and durable biomineral.
Knowing the secret of the ever-sharp sea urchin tooth, says Gilbert, could one day have practical applications for human toolmakers. "Now that we know how it works, the knowledge could be used to develop methods to fabricate tools that could actually sharpen themselves with use," notes Gilbert. "The mechanism used by the urchin is the key. By shaping the object appropriately and using the same strategy the urchin employs, a tool with a self-sharpening edge could, in theory, be created."
The new research was supported by grants from the U.S. Department of Energy and the National Science Foundation. In addition to Gilbert, researchers from the University of California, Berkeley; Argonne National Laboratory; the Weizmann Institute of Science; and the Lawrence Berkeley National Laboratory contributed to the report.
New Nanowires Act Like LEDs
September 30, 2010 in Energy Efficiency, General Technology
In one of those happy research accidents that lead to new breakthroughs, scientists at the National Institute of Standards and Technology (NIST) have developed a new kind of nanowire that produces light, much like the glow produced by LED technology. The team was actually fine tuning a new method for manufacturing ultra thin or nanoscale wires, hoping to create a process that yields a product with uniform traits, when their experiment began to light up.
The Trouble with Nanowires
Typically, nanowires are grown on a base material or substrate. The process involves depositing molecules such as zinc oxide in the form of a gas. The nanowires then “grow” vertically, like bristles on a brush. The problem is, they grow so densely that it is difficult to pick out the ones with better characteristics. Also, since the wires only touch the substrate at one end, their properties are not uniformly distributed.
A New Method of Growing Nanowires That Glow
The NIST team came up with a solution, which is to grow the nanowires horizontally. They converted gold into nanoparticles by superheating it, then manipulated zinc oxide nanocrystals into pushing the gold particles along the substrate, forming nanowires. Because the wire touches the substrate at all points, its characteristcs are more uniformly influenced than in the vertical growth method. When the researchers increased the size of the gold particle, the wires grew a fin-like nanowall which allowed electrons to flow, giving off a light similar to that of an LED.
Glowing Nanowires vs. LEDs
The NIST researchers envision uses for the new light-emitting nanowires in chip-sized “laboratories” and other miniature devices for specialized purposes. Though large-scale applications are a possibility, that seems pretty remote at the present, leaving LEDs in the lead for now in terms of providing an energy saving solution to lighting needs. LEDs are already lending themselves to large scale applications in street lighting and parking garages, for example, and new research is yielding more powerful LEDs, as well as new technologies for boosting the efficiency and lifespan of LED fixtures.
The perfect nanocube: Precise control of size, shape, and composition
(Nanowerk News) With growing interest in using nanoparticles for everything from antibacterial socks to medical imaging to electronic devices, the need to understand the environmental, health and safety risks of these particles also grows. Researchers at the National Institute of Standards and Technology (NIST) have developed a simple process for producing nanocrystals that will enable studies of certain physical and chemical properties that affect how nanoparticles interact with the world around them.
Because nanoparticles behave differently from bulk samples of the same material, new tests to understand how they affect biological systems must be developed. Toxicologists determine the hazards posed by nanoparticles by introducing them to a biological system and monitoring the effects, but they currently lack a set of control particles whose size, shape and composition have been carefully produced and characterized.
In a recent paper published in Angewandte Chemie ("Monodisperse gold-copper bimetallic nanocubes: facile one-step synthesis with controllable size and composition"), NIST scientists describe a one-step process that allows them to control the size, shape and composition of gold-copper alloy nanocrystals to create perfect-edged nanocubes as small as 3.4 nanometers—just half the thickness of a cell wall and on the same size range as DNA.
The researchers combined and heated gold and copper precursors with other chemicals to produce highly crystalline, homogeneous, perfect nanocubes with abundant yield. To study the formation process, they removed samples at 1 hour, 1.5 hours, 5 hours, and 24 hours and found that just five hours were needed to produce perfectly cubic nanoparticles of uniform size. By adjusting the ratios of the chemicals in the original solution and the reaction time, they were able to precisely control the size, shape and composition of the nanocubes. This process is unique in allowing control of the ratio of copper to gold atoms within the nanocube to either 3:1 or 1:3.
"It's a simple process, and to the best of our knowledge is the first to use synthetic chemistry, or 'bottom up' technology, to produce gold-containing nanocubes below 5 nanometers. Anything less than 10 nanometers has been extremely challenging due to the mobile behavior of the gold atoms," says NIST physicist Angela R. Hight Walker, who wrote the paper with Yonglin Liu, a guest researcher at NIST.
The NIST-developed process for creating such nanocubes will allow toxicologists to systematically alter one of the nanocubes' characteristics and observe how the change affects the biological response, if at all.
This synthesis and the resulting high-quality nanocubes may have other applications in areas such as solar energy, says Liu. "Typically, we cannot make big batches of high-quality samples for testing; now we can."
The perfect-edged nanocubes are unique from other nanocubes in the literature, says Hight Walker. The sharp edges, as opposed to truncated or rounded edges, will enable different, more reactive chemistry that could be beneficial in applications such as catalysis—in which the nanocubes would be used to initiate or enhance a chemical reaction.
The process has received a provisional patent.
Cheap nano crystals promise huge storage boost
3D memory chips built from silicon
Rice University announced today that scientists there have created the first two-terminal memory chips that use only silicon, extending the limits of miniaturisation subject to Moore's Law. The new technology places multiple layers of memory capacity on the same chip, creating what is referred to as a 3D memory architecture.
According to a Rice University spokesman, the new memory technology will improve scalability by an order of magnitude compared to NAND flash technology available today. "The fact that they can do this in 3D makes makes it highly scalable," he said. "We've got memory that's made out of dirt-cheap material and it works."
In 2008, researchers at the university showed how electrical currents could repeatedly break and reconnect 10-nanometer strips of graphite, which could potentially boost flash memory capacity by many times. The Rice researchers said then that the new technology could withstand radiation and temperatures of 200 degrees Celsius that would cause solid-state disk memory to disintegrate.
At the time, the research team acknowledged that they weren't sure why their discovery worked so well. With the latest finding, the research team, including professors James Tour, Douglas Natelson and Lin Zhong, proved the circuit doesn't need the carbon to function, only silicon. During the project, Jun Yao, a graduate student in Tour's lab, was able to confirm the hypothesis when he sandwiched an insulating layer of silicon oxide between semiconducting sheets of polycrystalline silicon that served as the top and bottom electrodes, Rice said.
Yao applied a charge to the electrodes, which created a conductive pathway by stripping oxygen atoms from the silicon oxide, forming a chain of nanometer-sized silicon crystals. Once formed, the chain can be repeatedly broken and reconnected by applying a pulse of varying voltage, the University said.
"It is more than 5 times denser than 20 nanometer flash... without 3D stacking," Zhong said. "I would argue the nanowire-based solution is much more amenable to vertical stacking, which makes the technology very scalable as process technology improves. The density can be further doubled or tripled with two or three layers."
Unlike NAND flash memory, which is controlled by three terminals or wires, the new silicon memory requires two terminals, making it more viable for three-dimensional or stacked silicon arrays, multiplying a chip's capacity. But like flash memory, chips made with silicon consume virtually no power while keeping data intact.
The nanocrystal wires are as small as 5 nanometers wide. A nanometer is one billionth of a meter. "The beauty of it is its simplicity," said Tour, a professor of mechanical engineering and materials science and computer science, in a statement.
Because the layers of silicon-oxide memory are not required to hold a charge, they can also be stacked one atop another, he added. "I've been told by industry that if you're not in the 3D memory business in four years, you're not going to be in the memory business. This is perfectly suited for that," Tour said. "Manufacturers feel they can get pathways down to 10 nanometers. Flash memory is going to hit a brick wall at about 20 nanometers," Tour said. "But how do we get beyond that? Well, our technique is perfectly suited for sub-10-nanometer circuits."
PrivaTran, a tech design company, is already testing a silicon-oxide chip with 1,000 memory elements built in collaboration with Tour's lab. "We're real excited about where the data is going here," said PrivaTran CEO Glenn Mortland, who is using the technology in several projects supported by the Army Research Office, National Science Foundation, Air Force Office of Scientific Research, and the Navy Space and Naval Warfare Systems Command Small Business Innovation Research (SBIR) and Small Business Technology Transfer programs.
"Our original customer funding was geared toward more high-density memories," Mortland said. "That's where most of the paying customers see this going. I think, along the way, there will be side applications in various nonvolatile configurations."
NuPGA, a company formed last year through collaborative patents with Rice University, is also demonstrating how silicon oxide also works in reprogrammable gate arrays. NuPGA's devices will assist in the design of computer circuitry based on vertical arrays of silicon oxide embedded in "vias," the holes in integrated circuits that connect layers of circuitry.
Such rewritable gate arrays could drastically cut the cost of designing complex electronic devices, according to Tour.
===============================================================
Will the next generation memory chips be made of silicon oxide, not carbon?
There’s a reason why you can store what seems like a gazillion songs and pictures on your smart phone. The gadget has an incredible memory in its inner core.
But there’s a limit to the development of electronic memory. Computer experts worry that we might reach that limit in five years.
It turns out, a chip made from silicon dioxide can store more information than a flash drive. As a bonus, this method does not require any carbon.
Rice University chemists discovered that silicon dioxide can actually store information in the breaks in its crystal structure. Eventually, the researchers hope the nanocrystals can be used to create 3-D storage devices, which would severely cut the cost of electronics.
It wasn’t easy for graduate student Jun Yao to prove that building a silicon oxide circuit was even possible. When Yao was looking for a graphite replacement, nothing could properly charge the circuit.
However, Yao got rid of carbon and instead, used silicon oxide next to silicon terminals. The student found that the silicon oxide nanowire performed just as well as the graphite device. The silicon oxide nanowire only had two terminals, which didn’t require it to hold a charge.
“I’ve been told by industry that if you’re not in the 3-D memory business in four years, you’re not going to be in the memory business. This is perfectly suited for that,” Rice professor James Tour said in a statement.
Flash memory is expected to hit a wall when it gets down to the 20 nanometer scale.
That’s why the Rice researchers’ technique looks promising — it’s a proof of concept that it can be used to build 10 nanometer circuits.
And the best part is that the chemistry is rather simple.
A Texas based company called RivaTran is working with the lab on a number of projects. According to a statement, the projects are supported by the:
* Army Research Office
* National Science Foundation
* Air Force Office of Scientific Research
* Navy Space and Naval Warfare Systems Command
* Small Business Innovation Research (SBIR)
* Small Business Technology Transfer program
Whether or not this discovery can break the memory barrier and help computer chips continue to shrink remains to be seen.
However, not everyone is convinced silicon oxide circuits are the way to go. The New York Times reports that I.B.M and Intel are looking at phase-change memory.
In 2011 a global commodities exchange will go live, linking up buyers and investors with suppliers of nano-engineered materials, to help unlock the commercial potential of this far-reaching field of technology.
Trading exchange for nanomaterials to launch in 2011
The Integrated Nano-Science and Commodity Exchange (INSCX) will provide a 24-hour trading platform for nanomaterials, akin to exchanges for commodities such as oil, metals and grains.
The exchange could help suppliers of nanomaterials - for example carbon nanotubes, quantum dots and nanoceramics - find buyers in the form of companies developing products that exploit the properties of various advanced materials. Developers work across applications as diverse as LCD and OLED displays, new types of solar cells, optical films, conductive inks, medical diagnostic devices, barrier packaging and engine lubricants.
Buyers and investors would benefit from being able to buy nanomaterials, on an open and transparent platform, which have been subject to the AssuredNano Safety, Health and Environment (SHE) accreditation scheme. The exchange will provide them with the option to buy from more than one source.
INSCX's CEO is Charles McGovern, a qualified stockbroker and professional risk trader across securities and commodity markets for US and international investment houses. McGovern announced the exchange initiative at this year's NanoMaterials2010 conference on 8-10 June, in London. NanoCentral, which founded the event as part of its strategy to aid the commercial application of nanomaterials, supports the proposed platform.
Nanomaterials maturing
Presentations at the event came from the likes of David Baxter, the lead researcher in emerging risks for the global insurer Lloyds, Cientifica's Tim Harper, who has also founded a specialist VC firm, and Matthias Hölling, who manages technology transfer activity within ETH Zurich. The conference tackled some of the key challenges and risks facing suppliers of nanomaterials and developers of products exploiting them.
Discussions and presentations focused on alternative financing sources, with instructive examples of failed nanotechnology start-ups; an emphasis on developing products and solutions based on nanomaterials to meet real societal needs; demands for the long-term, addressing how to equip start-ups with the right management team that can take new technology from the realms of science and guide it towards commercial opportunities; media perceptions of nano and other emerging technologies; and managing risk from an insurer's perspective.
Revolutionary New Solution for Semiconductor, Nano Materials
Schematic of hybrid core-shell growth process
University of Maryland researchers have created a completely new way to produce high quality semiconductor materials critical for advanced microelectronics and nanotechnology. Published in the March 26 issue of Science, their research is a fundamental step forward in nanomaterials science that could lead to significant advances in computer chips, photovoltaic cells, biomarkers and other applications, according to the authors and other experts.
"This is a major, major advance that shows it is possible to do something that was impossible to do before," said Massachusetts Institute of Technology Associate Professor Francesco Stellacci, whose own work focuses on discovery of new properties in nanoscale materials and the development of new nanofabrication schemes. "This research actually shows that it's possible at the nanoscale for two materials to happily coexist at their interface, two materials that would not coexist otherwise," explained Stellacci, who was not involved in the study.
Led by Min Ouyang, an assistant professor in the department of physics and the Maryland NanoCenter, the University of Maryland team has created a process that uses chemical thermodynamics to produce, in solution, a broad range of different combination materials, each with a shell of structurally perfect mono-crystal semiconductor around a metal core.
Ouyang and fellow researchers Jiatao Zhang, Yun Tang and Kwan Lee, say their method offers a host of benefits over the existing process, known as epitaxy, used to create single crystal semiconductors and related devices. The biggest advantage of their non-epitaxial process may be that it avoids two key constraints of epitaxy -- a limit on deposition semiconductor layer thickness and a rigid requirement for "lattice matching."
The constraints of the epitaxial method restrict the materials that can be formed with it. For example, authors Ouyang, Zhang, Tang and Lee note that attempts to use epitaxy to achieve the kind of hybrid core-shell nanostructures they demonstrate in their article have been unsuccessful.
"Our process should allow creation of materials that yield highly integrated multi-functional microelectronic components; better, more efficient materials for photovoltaic cells; and new biomarkers," said Ouyang, who noted his team is in the process of applying for a patent. "We envision for example that we can use this method to create new types of photovoltaic cells that are ten times more efficient in converting sunlight to electricity than current cells.
"Our method doesn't require a clean room facility and the materials don't have to be formed in a vacuum the ways those made by conventional epitaxy do," Ouyang said. "Thus it also would be much simpler and cheaper for companies to mass produce materials with our process."
Epitaxy is one of the cornerstones of contemporary semiconductor industry and nanotechnology. It has been considered the most affordable method of high quality crystal growth for many semiconductor materials including silicon-germanium, gallium nitride, gallium arsenide, indium phosphide and graphene.
A Quantum Leap
The new method also can be used to design and fabricate artificial quantum structures that help scientists understand and manipulate the basic physics of quantum information processing at the nanoscale, said Ouyang, noting that he and his team have a separate paper on the quantum science applications of this method that they expect to be published in the near future.
This work was supported by the Office of Naval Research, the National Science Foundation (NSF) and the Beckman Foundation. Facility support was from Maryland Nanocenter and its Nanoscale Imaging, Spectroscopy and Properties Laboratory, which is supported in part by the NSF as a Materials Research Science and Engineering Centers shared experiment facility.
More information: "Nonepitaxial Growth of Hybrid Core-Shell Nanostructures with Large Lattice Mismatches," Jiatao Zhang, Yun Tang, Kwan Lee, Min Ouyang*, Science, March 26, 2010. http://www.sciencemag.org/
Provided by University of Maryland
Paper supercapacitor could power future paper electronics
Photos at left and center show the paper supercapacitor printed on Xerox paper. The right photo shows the paper supercapacitor printed on newspaper. Image credit: Liangbing Hu, et al. ©2010 AIP.
All those paper transistors and paper displays that scientists have been designing can now be powered by an onboard power source, thanks to the development of a new paper supercapacitor. Designed by researchers at Stanford University, the paper supercapacitor is made by simply printing carbon nanotubes onto a treated piece of paper. The researchers hope that the integrated design could lead to the development of low-cost, disposable paper electronics.
In the paper supercapacitor, all the necessary components are integrated onto a single sheet of paper in the form of single walled carbon nanotubes (SWNTs). High-speed printing could be used to print the SWNTs directly onto a piece of paper - anything from Xerox paper to newspaper and even grocery ads will work. At first, the researchers found that the SWNTs were so small that they penetrated the paper through micron-sized pores, which would cause the device to short-circuit. To solve this problem, the researchers first coated both sides of the paper with polyvinylidene fluoride (PVDF), which blocked the pores but still allowed for electrolytes to be transported through the paper. As such, the treated paper could function as an electrolyte membrane and separator without short-circuiting.
“The key design is that SWNTs stick well on paper and do not penetrate through paper completely to avoid shorting,” Yi Cui of Stanford University told PhysOrg.com.
Once the SWNTs were printed onto the treated paper, they experienced strong bonding forces similar to those experienced when writing with a pen or pencil on paper. Even when rubbed or subjected to tape, the SWNTs remained attached to the paper. After printing SWNTs on both sides of single sheets of paper, electrolyte was loaded to form a supercapacitor. The SWNTs served as both the electrodes and current collectors in the supercapacitor, which had a capacitance of about 3 F/g. The device also showed an excellent cycling stability, with very little loss of capacitance after 2500 cycles. The researchers say the same concept could be extended to make batteries, as well.
The fully integrated supercapacitor is based on an earlier version that the researchers made, in which nanomaterials were coated separately onto different anode and cathode substrates and then assembled together with a separator. The advantage of the new integrated structure is that it allows for high-speed printing, which greatly reduces fabrication costs and brings disposable, flexible, and lightweight paper electronics closer to reality. Cui said that, in the future, the researchers plan “to use this new design for real applications.”
More information: Liangbing Hu, Hui Wu, and Yi Cui. “Printed energy storage devices by integration of electrodes and separators into single sheets of paper.” Applied Physics Letters 96, 183502 (2010).
Nanoshell structures: Self-assembly method yields materials with unique optical properties
Heptamers containing seven nanoshells have unique optical properties. Credit: Rice University
Scientists from four U.S. universities have created a way to use Rice University's light-activated nanoshells as building blocks for 2-D and 3-D structures that could find use in chemical sensors, nanolasers and bizarre light-absorbing metamaterials. Much as a child might use Lego blocks to build 3-D models of complex buildings or vehicles, the scientists are using the new chemical self-assembly method to build complex structures that can trap, store and bend light.
The research appears in this week's issue of the journal Science.
"We used the method to make a seven-nanoshell structure that creates a particular type of interference pattern called a Fano resonance," said study co-author Peter Nordlander, professor of physics and astronomy at Rice. "These resonances arise from peculiar light wave interference effects, and they occur only in man-made materials. Because these heptamers are self-assembled, they are relatively easy to make, so this could have significant commercial implications."
Because of the unique nature of Fano resonances, the new materials can trap light, store energy and bend light in bizarre ways that no natural material can. Nordlander said the new materials are ideally suited for making ultrasensitive biological and chemical sensors. He said they may also be useful in nanolasers and potentially in integrated photonic circuits that run off of light rather than electricity.
The research team was led by Harvard University applied physicist Federico Capasso and also included nanoshell inventor Naomi Halas, Rice's Stanley C. Moore Professor in Electrical and Computer Engineering and professor of physics, chemistry and biomedical engineering.
Nordlander, the world's leading theorist on nanoparticle plasmonics, had predicted in 2008 that a heptamer of nanoshells would produce Fano resonances. That paper spurred Capasso's efforts to fabricate the structure, Nordlander said.
The new self-assembly method developed by Capasso's team was also used to make magnetic three-nanoshell "trimers." The optical properties of these are described in the Science paper, which also discusses how the self-assembly method could be used to build even more complex 3-D structures.
Nanoshells, the building blocks that were used in the new study, are about 20 times smaller than red blood cells. In form, they resemble malted milk balls, but they are coated with gold instead of chocolate, and their center is a sphere of glass. By varying the size of the glass center and the thickness of the gold shell, Halas can create nanoshells that interact with specific wavelengths of light.
"Nanoshells were already among the most versatile of all plasmonic nanoparticles, and this new self-assembly method for complex 2-D and 3-D structures simply adds to that," said Halas, who has helped develop a number of biological applications for nanoshells, including diagnostic applications and a minimally invasive procedure for treating cancer.
Solexant's Nanocrystal Solar Technology Lands $41.5M Series C Round
“Solexant’s innovative printed nanocrystal technology platform has the potential to deliver the lowest cost per watt in the solar industry”
Berkeley Researchers Light Up White OLEDs
Biwu Ma, a staff scientist with the Molecular Foundry, was part of a research team that found a new way to process white OLEDs for solid state lighting. (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs)
Light-emitting diodes, which employ semiconductors to produce artificial light, could reduce electricity consumption and lighten the impact of greenhouse gas emissions. However, moving this technology beyond traffic signals and laser pointers to illumination for office buildings and homes -- the single largest use of electricity -- requires materials that emit bright, white light cheaply and efficiently. White light is the mix of all the colors, or wavelengths, in the visible spectrum.
Organic light-emitting diodes (OLEDs), based on organic and/or polymer semiconductor materials, are promising candidates for general lighting applications, as they can cover large-area displays or panels using low-cost processing techniques. Indeed, single-color OLED displays are already available commercially. A mix of red-, green- and blue-emitting materials can be used to generate white light, but these bands of color often interact with one another, degrading device performance and reducing color quality.
Using polymer nanoparticles to house light-emitting ‘inks’, scientists at the Molecular Foundry, a U.S. Department of Energy nanoscience center located at Berkeley Lab, and the University of California, Berkeley, have made a thin film OLED using iridium-based guest molecules to emit various colors of visible light. The polymer nanoparticle surrounding a guest light-emitter serves as a ‘do not disturb’ sign, isolating guest molecules from one another. Each guest can then emit light without pesky interactions with neighboring nanoparticles, resulting in white luminescence.
“This simple and bright approach to achieving nanoscale site isolation of phosphors opens a new door for facile processing of white OLEDs for solid state lighting,” said Biwu Ma, a staff scientist with the Molecular Foundry’s Organic Nanostructures Facility who contributed to this study. With this proof-of-concept device under their belts, Ma and his colleagues plan to vary the ratio of each color nanoparticle in the OLED to enhance efficiency and brightness. White light from OLEDs can be adjusted from cooler to warmer whites, making these materials easy to use in office or home environments. Buildings account for more than 40 percent of carbon emissions in the United States, so replacing even a fraction of conventional lighting with OLEDs could result in a significant reduction in electricity use.
A paper reporting this research titled, “Site isolation of emitters within cross-linked polymer nanoparticles for white electroluminescence,” appears in the journal Nano Letters.
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