Tuesday, May 31, 2011

Palladium nanoparticle with a gold antenna to enhance plasmonic sensing

Such highly coveted technical capabilities as the observation of single catalytic processes in nanoreactors, or the optical detection of low concentrations of biochemical agents and gases are an important step closer to fruition. Researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab), in collaboration with researchers at the University of Stuttgart in Germany, report the first experimental demonstration of antenna-enhanced gas sensing at the single particle level. By placing a palladium nanoparticle on the focusing tip of a gold nanoantenna, they were able to clearly detect changes in the palladium's optical properties upon exposure to hydrogen.

"We have demonstrated resonant antenna-enhanced single-particle hydrogen sensing in the visible region and presented a fabrication approach to the positioning of a single palladium nanoparticle in the nanofocus of a gold nanoantenna," says Paul Alivisatos, Berkeley Lab's director and the leader of this research. "Our concept provides a general blueprint for amplifying plasmonic sensing signals at the single-particle level and should pave the road for the optical observation of chemical reactions and catalytic activities in nanoreactors, and for local biosensing."

Alivisatos, who is also the Larry and Diane Bock Professor of Nanotechnology at the University of California, Berkeley, is the corresponding author of a paper in the journal Nature Materials describing this research. The paper is titled "Nanoantenna-enhanced gas sensing in a single tailored nanofocus." Co-authoring the paper with Alivisatos were Laura Na Liu, Ming Tang, Mario Hentschel and Harald Giessen.

Nanoantenna SEM Image

Caption: This is a scanning electron microscopy image showing a palladium nanoparticle with a gold antenna to enhance plasmonic sensing.

Credit: Image courtesy of Alivisatos group. Usage Restrictions: None.

Nanoantenna Plasmonic Sensing

Caption: Top figure shows hydrogen molecules (red) absorbed on a palladium nanoparticle, resulting in weak light scattering and barely detectable spectral changes. Bottom figure shows gold antenna enhancing light scattering and producing an easy to detect spectral shift.

Credit: Image courtesy of Alivisatos group. Usage Restrictions: None.
One of the hottest new fields in technology today is plasmonics – the confinement of electromagnetic waves in dimensions smaller than half-the-wavelength of the incident photons in free space. Typically this is done at the interface between metallic nanostructures, usually gold, and a dielectric, usually air. The confinement of the electromagnetic waves in these metallic nanostructures generates electronic surface waves called "plasmons." A matching of the oscillation frequency between plasmons and the incident electromagnetic waves gives rise to a phenomenon known as localized surface plasmon resonance (LSPR), which can concentrate the electromagnetic field into a volume less than a few hundred cubic nanometers. Any object brought into this locally confined field – referred to as the nanofocus - will influence the LSPR in a manner that can be detected via dark-field microscopy.

"Nanofocusing has immediate implications for plasmonic sensing," says Laura Na Liu, lead author of the Nature Materials paper who was at the time the work was done a member of Alivisatos' research group but is now with Rice University. "Metallic nanostructures with sharp corners and edges that form a pointed tip are especially favorable for plasmonic sensing because the field strengths of the electromagnetic waves are so strongly enhanced over such an extremely small sensing volume."

Plasmonic sensing is especially promising for the detection of flammable gases such as hydrogen, where the use of sensors that require electrical measurements pose safety issues because of the potential threat from sparking. Hydrogen, for example, can ignite or explode in concentrations of only four-percent. Palladium was seen as a prime candidate for the plasmonic sensing of hydrogen because it readily and rapidly absorbs hydrogen that alters its electrical and dielectric properties. However, the LSPRs of palladium nanoparticles yield broad spectral profiles that make detecting changes extremely difficult.

"In our resonant antenna-enhanced scheme, we use double electron-beam lithography in combination with a double lift-off procedure to precisely position a single palladium nanoparticle in the nanofocus of a gold nanoantenna," Liu says.

"The strongly enhanced gold-particle plasmon near-fields can sense the change in the dielectric function of the proximal palladium nanoparticle as it absorbs or releases hydrogen. Light scattered by the system is collected by a dark-field microscope with attached spectrometer and the LSPR change is read out in real time."

Alivisatos, Liu and their co-authors found that the antenna enhancement effect could be controlled by changing the distance between the palladium nanoparticle and the gold antenna, and by changing the shape of the antenna.

"By amplifying sensing signals at the single-particle level, we eliminate the statistical and average characteristics inherent to ensemble measurements," Liu says. "Moreover, our antenna-enhanced plasmonic sensing technique comprises a noninvasive scheme that is biocompatible and can be used in aqueous environments, making it applicable to a variety of physical and biochemical materials."

For example, by replacing the palladium nanoparticle with other nanocatalysts, such as ruthenium, platinum, or magnesium, Liu says their antenna-enhanced plasmonic sensing scheme can be used to monitor the presence of numerous other important gases in addition to hydrogen, including carbon dioxide and the nitrous oxides. This technique also offers a promising plasmonic sensing alternative to the fluorescent detection of catalysis, which depends upon the challenging task of finding appropriate fluorophores. Antenna-enhanced plasmonic sensing also holds potential for the observation of single chemical or biological events.

"We believe our antenna-enhanced sensing technique can serve as a bridge between plasmonics and biochemistry," Liu says. "Plasmonic sensing offers a unique tool for optically probing biochemical processes that are optically inactive in nature. In addition, since plasmonic nanostructures made from gold or silver do not bleach or blink, they allow for continuous observation, an essential capability for in-situ monitoring of biochemical behavior."

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This research was supported by the DOE Office of Science and the German ministry of research.

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 12 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy's Office of Science. For more, visit www.lbl.gov.

Contact: Lynn Yarris lcyarris@lbl.gov 510-486-5375 DOE/Lawrence Berkeley National Laboratory

Monday, May 30, 2011

UC Boulder method of producing hydrogen fuel from sunlight only approach among eight competing technologies projected to meet cost targets VIDEO

A report commissioned by the U.S. Department of Energy has concluded that a novel University of Colorado Boulder method of producing hydrogen fuel from sunlight is the only approach among eight competing technologies that is projected to meet future cost targets set by the federal agency.

The process, which is being developed by Professor Alan Weimer's research team of CU-Boulder's chemical and biological engineering department, involves an array of mirrors to concentrate the sun's rays and create temperatures as high as 2,640 degrees Fahrenheit. The process consists of two steps -- each involving reactions of a thin film of metal ferrite coating with a reactive substrate contained in a solar receiver -- to split water into its gaseous components, hydrogen and oxygen.

Currently, the lowest cost method for producing hydrogen is the steam-methane reforming of natural gas, primarily methane. In this process, significant amounts of carbon dioxide -- a powerful greenhouse gas -- are released into the atmosphere.

The DOE commissioned 76-page report was produced by TIAX, a technology processing and commercialization company headquartered in Lexington, Mass. The report authors evaluated process conditions, major capital equipment, materials and utilities usage rates, estimated equipment sizes, financial and operating assumptions.

CU's approach does not result in greenhouse gas emissions and is more cost effective than competing technologies because the water-splitting reactions occur at lower temperatures and are faster, said Weimer. In addition, less energy and fewer active materials are required, resulting in lower costs.






Weimer said the solar receiver's thin film coating on a porous active support allows heat and steam -- necessary to reactions -- to flow more easily through the device and for reactions to occur more efficiently.

"We've been able to reduce the temperature required to split water by about 250 degrees Celsius [482 degrees F] and we have eliminated what appears to be a major roadblock in terms of an unstable intermediate by using thin films and a reactive substrate," said Weimer. "It's pretty significant and it seems like there's a good shot for this to become mainstream in the southwest U.S. and other high insolation regions around the world."

Weimer refers to his water-splitting method as a "triple play." It not only uses renewable resources and produces sustainable hydrogen, but it also can purify brackish into potable water -- a byproduct that he says could address water shortage issues in the future.

Weimer is presenting his research today at the DOE Hydrogen and Fuel Cells Program and Vehicle Technologies Program Annual Merit Review and Peer Evaluation Meeting in Washington, D.C. Weimer said he hopes to garner continued research support through government and private resources.

The DOE is investigating novel approaches for solar thermochemical water splitting to produce hydrogen with the eventual goal of commercializing production. Cost targets in this analysis set hydrogen production in 2015 at 6 dollars per kilogram -- equal to 2.2 pounds -- and hydrogen delivery in 2025 at 2 to 3 dollars per kilogram. CU-Boulder's thin-film metal ferrite process is projected to meet both benchmarks.

Other technologies appearing in the analysis included reactions with hybrid-sulfur, copper chloride, sulfur-ammonia, zinc oxide, manganese oxide and cadmium oxide.

Weimer directs a research group of three postdoctoral research associates, 12 doctoral students and six undergraduates. He has the largest academic research group in the U.S. focused on solar thermochemical processing.

Contact: Alan Weimer alan.weimer@colorado.edu WEB: University of Colorado at Boulder

VIDEO CREDIT: toyotaesqcomm

Sunday, May 29, 2011

Cockrell School of Engineering creates new porous, three-dimensional carbon that can be used as a greatly enhanced supercapacitor VIDEO

Researchers at The University of Texas at Austin's Cockrell School of Engineering have created a new porous, three-dimensional carbon that can be used as a greatly enhanced supercapacitor, holding promise for energy storage in everything from energy grids and electric cars to consumer electronics.

The findings of the group, led by materials science and mechanical engineering Professor Rodney S. Ruoff, will be published May 12 by Science magazine in its online publication ScienceXpress.

The significance of the discovery by Ruoff's team, which included postdoctoral fellow Dr. Yanwu Zhu and graduate students Shanthi Murali and Meryl Stoller, is the potential it offers for enabling supercapacitors to deliver significantly more charge, opening the doors to many potential unprecedented uses for this type of electrical energy storage device.

Supercapacitors are known as the "sprinters" among electrical energy storage devices, able to deliver energy much faster and more efficiently than batteries, but usually holding much less electrical charge, while batteries are like marathon runners, delivering energy slowly, but steadily.


"We synthesized a new sponge-like carbon that has a surface area of up to 3,100 square meters per gram (two grams has a surface area roughly equivalent to that of a football field). It also has much higher electrical conductivity and, when further optimized, will be superb for thermal management as well," Ruoff said. "The processes used to make this porous carbon are readily scalable to industrial levels.

"After we realized that we had a new carbon with a highly novel structure that showed superb performance as an electrode, we knew that this direction of research — to create carbon materials that consist of a continuous three-dimensional porous network with single-atom-thick walls — was likely to yield the optimum electrode material for supercapacitors."

Dr. Yanwu Zhu

Dr. Yanwu Zhu and graduate students Shanthi Murali and Meryl Stoller stand with a 3-D model of the new
carbon material.
The University of Texas at Austin's Office of Technology Commercialization has filed a patent with the U.S. Patent Office on behalf of the inventors.

"Rod and his team define what we mean when we talk about innovation to address grand challenges," said Gregory L. Fenves, dean of the Cockrell School of Engineering. "This team of students, researchers and faculty has discovered a way to improve the efficiency of supercapacitor energy storage."

Ruoff's research team of about 40 people collaborated with faculty and students from The University of Texas at Dallas, scientific staff at Brookhaven National Laboratory in New York and staff members at QuantaChrome Instruments in Florida.

The process used by the university team to synthesize the carbon material involved using microwaves to exfoliate graphite oxide, followed by treatment with potassium hydroxide, which created a carbon full of tiny holes — essentially a sponge that, when combined with an electrolyte, can store a giant electrical charge.

The team at Brookhaven then analyzed the atomic structure of the carbon material at the nanoscale using very high resolution electron microscopes. Their observations confirmed Ruoff's hypothesis that the carbon was a new three-dimensional material having highly curved, single-atom-thick walls that form tiny pores.

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Contact: Maria McGivney Arrellaga maria.arrellaga@austin.utexas.edu 512-232-8060 University of Texas at Austin

Saturday, May 28, 2011

New method for making detailed X-ray images of brain cells

Researchers including members from the Niels Bohr Institute at the University of Copenhagen have developed a new method for making detailed X-ray images of brain cells. The method, called SAXS-CT, can map the myelin sheaths of nerve cells, which are important for conditions such as multiple sclerosis and Alzheimer's disease. The results have been published in the scientific journal, NeuroImage.

The myelin sheaths of nerve cells are lamellar membranes surrounding the neuronal axons. The myelin layers are important to the central nervous system as they ensure the rapid and uninterrupted communication of signals along the neuronal axons. Changes in the myelin layers are associated with a number of neurodegenerative disorders such as cerebral malaria, multiple sclerosis, and Alzheimer's disease.

The development of these diseases are still not fully understood, but are thought to be related to the damage of the myelin layers, so that messages from the brain reach the various parts of the body poorly or not at all. It is like an electric cord where the insulating material has been damaged and the current short circuits. In order to find methods to prevent or treat the diseases it is important to understand the connection between the diseases and the changes in the myelin.

Getting 3-D X-ray images

X-ray Scattering cross section

Caption: These three images show the same section of the same brain. The images have all been measured using SAXS-CT. 1: The image here shows the Small-Angle X-ray Scattering cross section measured using the SAXS-CT. It can be used as an anatomical map of the rat brain. 2: Seen here is the myelin concentration in the rat brain. It is seen that the myelin is concentrated around the corpus callosum as well as the external and internal capsule. The concentration can now be measured using SAXS-CT without cutting into the brain. 3: The image shows how the thickness of the myelin layer in the rat brain varies between 17.0 and 18.2 nm. Using SAXS-CT these variations can be measured over a large area of the brain at once. The blue color shows areas with no or a very low myelin concentration.

Credit: Torben Haugaard Jensen, et al. Niels Bohr Institute. Usage Restrictions: Credit: Torben Haugaard Jensen, et al. Niels Bohr Institute.
"We have combined two well-known medical examination methods: SAXS (Small-Angle X-ray Scattering) and CT-scanning (computed tomography scanning). Combined with a specially developed programme for data processing, we have been able to examine the variations of the myelin sheaths in a rat brain all the way down to the molecular level without surgery", explains PhD Torben Haugaard Jensen, Niels Bohr Institute at the University of Copenhagen. The method is called 'Molecular X-ray CT', because you use X-ray CT to study myelin at the molecular level.

The research has been carried out in collaboration with researchers in Switzerland, France and Germany. The experiments took place at the Paul Scherrer Institute in Switzerland, where they have a powerful X-ray source that can measure Small-Angle X-Ray Scattering, SAXS at a high resolution. Normally such experiments would give two-dimensional X-ray images that are sharp and precise, but without information on depth. But by incorporating the method from CT-scanning, where you image from different angles, the researchers have managed to get 3D X-ray images.

This has not only required the development of new X-ray methods and experiments, but has also required the development of new methods for processing data. The extremely detailed measurements of cross sections from different angles meant that there were 800,000 images to be analysed. So the researchers have also developed an image-processing programme for the SAXS-CT method. The result is that they can see all of the detailed information from SAXS in spatially resolved.

From point samples to total samples

"We can see the myelin sheaths of the neuronal axons and we can distinguish the layers which have a thickness of 17.6 nanometers", explains Torben Haugaard Jensen. "Up until now, you had to cut out a little sample in order to examine the layers in one area and get a single measuring point. With the new method we can examine 250,000 points at once without cutting into the sample. We can get a complete overview over the concentration and thickness of the myelin and this gives of the ability to determine whether the destruction of the myelin is occurring in spots or across the entire sample", he explains.

The research provides new opportunities for collaboration with doctors at Copenhagen University Hospital and the Panum Institute, who they already have close contact with. The method cannot be used to diagnose living persons. But the doctors can obtain new knowledge about the diseases, what kind of damage is taking place? – and where? They will be able to follow the development of the diseases and find out how the brain is being attacked. This knowledge could perhaps be used to develop a treatment.

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Contact: Gertie Skaarup skaarup@nbi.dk 453-532-5320 University of Copenhagen

Contact: Torben Haugaard Jensen, PhD. Niels Bohr Institute, University of Copenhagen, +45 2097-3682 torbenj@fys.ku.dk Robert Feidenhans'l, professor, Niels Bohr Institute, University of Copenhagen, +45 3532-0397, +45 2875-0397, robert@fys.ku.dk

Thursday, May 26, 2011

The practical use of visible light and zinc oxide nanorods for destroying bacterial water contamination

Exposing ZnO nanorods to visible light removes microbes, Photocatalysis for immobilizing bacteria in water using solar light on ZincOxide nanorods

The practical use of visible light and zinc oxide nanorods for destroying bacterial water contamination has been successfully demonstrated by researchers at the Asian Institute of Technology (AIT). Nanorods grown on glass substrates and activated by solar energy have been found to be effective in killing both gram positive and gram negative bacteria – a finding that has immense possibilities for affordable and environmentally friendly water purification techniques.

"Most studies so far either work on the use of ultraviolet light or involve a suspension of nanoparticles," revealed Prof. Joydeep Dutta, director of the Center for Excellence in Nanotechnology at AIT. The AIT research group has dispensed with both. Instead of using a suspension of nanoparticles, which have to be removed later after the water purification process, or relying on UV light, the group demonstrated a system featuring visible light and ZnO nanorods. "The key concept was to incorporate deliberate defects in ZnO nanorods by creating oxygen vacancies and interstitials, which then allows visible light absorption," he explained.

Environmentally friendly approach

Researchers at AIT's Center of Excellence in Nanotechnology

Researchers at AIT's Center of Excellence in Nanotechnology.
Such ZnO nanorods grown on glass were tested on Escherichia coli and Bacillus subtilis bacteria, which are commonly used as model microbes. In the dark, ZnO dissolves slowly releasing zinc ions, which have anti bacterial properties, as it penetrates the bacterial cell envelope thereby thwarting the growth of microbes. Under well lit conditions, the effect is doubled with both photocatalysis and zinc ions playing their part in killing the microbes.

The implications of these experiments are enormous. "Since ZnO has now been tested under solar light, instead of the traditionally used UV light, the potential for commercial applications is huge, particularly since the levels of zinc ions removed from the rods to the water are safe for human consumption," added Dutta.

The team, which also includes Dr. Oleg V Shipin, Ajaya Sapkota, Dr. Alfredo J Anceno, Mr. Sunandan Baruah and Ms. Mayuree Jaisai, is continuing its work on photocatalysis for use in water decontamination.

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Contact: Prof. Joydeep Dutta nano@ait.ac.th WEB: Asian Institute of Technology

Wednesday, May 25, 2011

Supercapacitor energy storage devices high capacity, superfast energy release, quick recharge time, and a lifetime of 10,000 charge discharge cycles

Activated graphene makes superior supercapacitors for energy storage, New material combines high storage capacity with quick energy release and unlimited recharge

UPTON, NY - Scientists at the U.S. Department of Energy's Brookhaven National Laboratory have helped to uncover the nanoscale structure of a novel form of carbon, contributing to an explanation of why this new material acts like a super-absorbent sponge when it comes to soaking up electric charge. The material, which was recently created at The University of Texas - Austin, can be incorporated into "supercapacitor" energy-storage devices with remarkably high storage capacity while retaining other attractive attributes such as superfast energy release, quick recharge time, and a lifetime of at least 10,000 charge/discharge cycles.

"Those properties make this new form of carbon particularly attractive for meeting electrical energy storage needs that also require a quick release of energy - for instance, in electric vehicles or to smooth out power availability from intermittent energy sources, such as wind and solar power," said Brookhaven materials scientist Eric Stach, a co-author on a paper describing the material published in Science on May 12, 2011.

Dong Su and Eric Stach

Dong Su and Eric Stach use a powerful electron microscope to analyze samples of activated graphene at Brookhaven’s Center for Functional Nanomaterials. Says Stach: “The CFN provides access to scientists around the world to solve cutting-edge problems in nanoscience and nanotechnology. This work is exactly what this facility was established to do.”
Supercapacitors are similar to batteries in that both store electric charge. Batteries do so through chemical reactions between metallic electrodes and a liquid electrolyte. Because these chemicals take time to react, energy is stored and released relatively slowly. But batteries can store a lot of energy and release it over a fairly long time.

Supercapacitors, on the other hand, store charge in the form of ions on the surface of the electrodes, similar to static electricity, rather than relying on chemical reactions. Charging the electrodes causes ions in the electrolyte to separate, or polarize, as well - so charge gets stored at the interface between the electrodes and the electrolyte. Pores in the electrode increase the surface area over which the electrolyte can flow and interact - increasing the amount of energy that can be stored.

But because most supercapacitors can't hold nearly as much charge as batteries, their use has been limited to applications where smaller amounts of energy are needed quickly, or where long life cycle is essential, such as in mobile electronic devices.

The new material developed by the UT-Austin researchers may change that. Supercapacitors made from it have an energy-storage capacity, or energy density, that is approaching the energy density of lead-acid batteries, while retaining the high power density - that is, rapid energy release - that is characteristic of supercapacitors.

"This new material combines the attributes of both electrical storage systems," said University of Texas team leader Rodney Ruoff. "We were rather stunned by its exceptional performance."

The UT-Austin team had set out to create a more porous form of carbon by using potassium hydroxide to restructure chemically modified graphene platelets - a form of carbon where the atoms are arrayed in tile-like rings laying flat to form single-atom-thick sheets. Such "chemical activation" has been previously used to create various forms of "activated carbon," which have pores that increase surface area and are used in filters and other applications, including supercapacitors.

But because this new form of carbon was so superior to others used in supercapacitors, the UT-Austin researchers knew they'd need to characterize its structure at the nanoscale.

Ruoff had formed a hypothesis that the material consisted of a continuous three-dimensional porous network with single-atom-thick walls, with a significant fraction being "negative curvature carbon," similar to inside-out buckyballs. He turned to Stach at Brookhaven for help with further structural characterization to verify or refute this hypothesis.

Stach and Brookhaven colleague Dong Su conducted a wide range of studies at the Lab's Center for Functional Nanomaterials (CFN, http://www.bnl.gov/cfn/), the National Synchrotron Light Source (NSLS, http://www.nsls.bnl.gov/), and at the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory, all three facilities supported by the DOE Office of Science. "At the DOE laboratories, we have the highest resolution microscopes in the world, so we really went full bore into characterizing the atomic structure," Stach said.

"Our studies revealed that Ruoff's hypothesis was in fact correct, and that the material's three-dimensional nanoscale structure consists of a network of highly curved, single-atom-thick walls forming tiny pores with widths ranging from 1 to 5 nanometers, or billionths of a meter."

The study includes detailed images of the fine pore structure and the carbon walls themselves, as well as images that show how these details fit into the big picture. "The data from NSLS were crucial to showing that our highly local characterization was representative of the overall material," Stach said.

"We're still working with Ruoff and his team to pull together a complete description of the material structure. We're also adding computational studies to help us understand how this three-dimensional network forms, so that we can potentially tailor the pore sizes to be optimal for specific applications, including capacitive storage, catalysis, and fuel cells," Stach said.

Meanwhile, the scientists say the processing techniques used to create the new form of carbon are readily scalable to industrial production. "This material - being so easily manufactured from one of the most abundant elements in the universe - will have a broad range impacts on research and technology in both energy storage and energy conversion," Ruoff said.

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The work at Brookhaven was supported by DOE's Office of Science; the UT - Austin team's research was supported by the Office of Science, the National Science Foundation, and the Advanced Technology Institute.

The Center for Functional Nanomaterials at Brookhaven National Laboratory is one of the five DOE Nanoscale Science Research Centers (NSRCs), premier national user facilities for interdisciplinary research at the nanoscale. 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 nano.energy.gov.

One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. Brookhaven is operated and managed for DOE's Office of Science by Brookhaven Science Associates, a limited-liability company founded by the Research Foundation of State University of New York on behalf of Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.

Contact: Karen McNulty Walsh kmcnulty@bnl.gov 631-344-8350 DOE/Brookhaven National Laboratory

Tuesday, May 24, 2011

Light absorbing colloidal quantum dots linked to carbon-based fullerene nanoparticles can convert light to electricity in a precisely controlled way

Surface-based assembly produces promising power-generating units for molecular electronics.

UPTON, NY — In a step toward engineering ever-smaller electronic devices, scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have assembled nanoscale pairings of particles that show promise as miniaturized power sources. Composed of light-absorbing, colloidal quantum dots linked to carbon-based fullerene nanoparticles, these tiny two-particle systems can convert light to electricity in a precisely controlled way.

“This is the first demonstration of a hybrid inorganic/organic, dimeric (two-particle) material that acts as an electron donor-bridge-acceptor system for converting light to electrical current,” said Brookhaven physical chemist Mircea Cotlet, lead author of a paper describing the dimers and their assembly method in Angewandte Chemie.

By varying the length of the linker molecules and the size of the quantum dots, the scientists can control the rate and the magnitude of fluctuations in light-induced electron transfer at the level of the individual dimer. “This control makes these dimers promising power-generating units for molecular electronics or more efficient photovoltaic solar cells,” said Cotlet, who conducted this research with materials scientist Zhihua Xu at Brookhaven’s Center for Functional Nanomaterials (CFN).

Mircea Cotlet (standing) and Zhihua Xu

Mircea Cotlet (standing) and Zhihua Xu.

Photoinduced electron transfer

Left: Photoinduced electron transfer occurring in quantum dot-bridge-fullerene hererodimers and observed with single molecule microscopy. Right: Control of electron transfer (ET) rate by variation of interparticle distance (R, upper panel) and quantum dot size (D, lower panel).
Scientists seeking to develop molecular electronics have been very interested in organic donor-bridge-acceptor systems because they have a wide range of charge transport mechanisms and because their charge-transfer properties can be controlled by varying their chemistry. Recently, quantum dots have been combined with electron-accepting materials such as dyes, fullerenes, and titanium oxide to produce dye-sensitized and hybrid solar cells in the hope that the light-absorbing and size-dependent emission properties of quantum dots would boost the efficiency of such devices. But so far, the power conversion rates of these systems have remained quite low.

“Efforts to understand the processes involved so as to engineer improved systems have generally looked at averaged behavior in blended or layer-by-layer structures rather than the response of individual, well-controlled hybrid donor-acceptor architectures,” said Xu.

The precision fabrication method developed by the Brookhaven scientists allows them to carefully control particle size and interparticle distance so they can explore conditions for light-induced electron transfer between individual quantum dots and electron-accepting fullerenes at the single molecule level.

The entire assembly process takes place on a surface and in a stepwise fashion to limit the interactions of the components (particles), which could otherwise combine in a number of ways if assembled by solution-based methods. This surface-based assembly also achieves controlled, one-to-one nanoparticle pairing.

To identify the optimal architectural arrangement for the particles, the scientists strategically varied the size of the quantum dots — which absorb and emit light at different frequencies according to their size — and the length of the bridge molecules connecting the nanoparticles. For each arrangement, they measured the electron transfer rate using single molecule spectroscopy.

“This method removes ensemble averaging and reveals a system’s heterogeneity — for example fluctuating electron transfer rates — which is something that conventional spectroscopic methods cannot always do,” Cotlet said.

The scientists found that reducing quantum dot size and the length of the linker molecules led to enhancements in the electron transfer rate and suppression of electron transfer fluctuations.

“This suppression of electron transfer fluctuation in dimers with smaller quantum dot size leads to a stable charge generation rate, which can have a positive impact on the application of these dimers in molecular electronics, including potentially in miniature and large-area photovoltaics,” Cotlet said.

“Studying the charge separation and recombination processes in these simplified and well-controlled dimer structures helps us to understand the more complicated photon-to-electron conversion processes in large-area solar cells, and eventually improve their photovoltaic efficiency,” Xu added.

A U.S. patent application is pending on the method and the materials resulting from using the technique, and the technology is available for licensing. Please contact Kimberley Elcess at (631) 344-4151 for more information.

This work was funded by the DOE Office of Science.

The Center for Functional Nanomaterials at Brookhaven National Laboratory is one of the five DOE Nanoscale Science Research Centers (NSRCs), premier national user facilities for interdisciplinary research at the nanoscale. 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 nano.energy.gov.

Contact: Karen McNulty Walsh kmcnulty@bnl.gov 631-344-8350 DOE/Brookhaven National Laboratory

Monday, May 23, 2011

The acidification of the world’s oceans could have major consequences for the marine environment

The acidification of the world’s oceans could have major consequences for the marine environment. New research shows that coccoliths, which are an important part of the marine environment, dissolve when seawater acidifies. Associate Professor Tue Hassenkam and colleagues at the Nano-Science Center, University of Copenhagen, are the first to have measured how individual coccoliths react to water with different degrees of acidity.

Coccoliths are very small shells of calcium carbonate that encapsulate a number of species of alga. Algae plays an important role in the global carbon-oxygen cycle and thus in our ecosystem. Our seawater has changed because of our emissions of greenhouse gases and therefore it was interesting for Hassenkam and his colleagues to investigate how the coccoliths react to different types of water.

"We know that the world’s oceans are acidifying due to our emissions of CO2 and that is why it is interesting for us to find out how the coccoliths are reacting to it. We have studied algae from both fossils and living coccoliths, and it appears that both are protected from dissolution by a very thin layer of organic material that the algae formed, even though the seawater is extremely unsaturated relative to calcite. The protection of the organic material is lost when the pH is lowered slightly. In fact, it turns out that the shell falls completely apart when we do experiments in water with a pH value that many researchers believe will be the found in the world oceans in the year 2100 due to the CO2 levels," explains Tue Hassenkam, who is part of the NanoGeoScience research group at the Department of Chemistry, University of Copenhagen.

Atomic Force Microscope ViewProfessor of Biological Oceanography Katherine Richardson has followed research in the acidification of the oceans and climate change in general and she hopes that the results can help to bring the issue into public focus.

"These findings underscore that the acidification of the oceans is a serious problem. The acidification has enormous consequences not only for coccoliths, but also for many other marine organisms as well as the global carbon cycle," explains Katherine Richardson, professor of biological oceanography and vice dean at the Faculty of Science at the University of Copenhagen.
Nano-microscope is the key

Tue Hassenkam is a nano-specialist and has been working for several years with the AFM (Atomic Force Microscope), which is an important instrument for nano researchers, because they can see and manipulate very small samples of, for example, geological materials like coccoliths.

"Using the AFM I weighed the coccoliths before and after they have been immersed in water with different compositions. The coccoliths weigh around 500 pg (0.0000000005 g). Specifically, I have set a coccolith on tip of an AFM and immersed the tip in water and looked at and weighed the coccolith afterwards. In that way I can say something about how much and how long it takes for a coccolith to dissolve in water with different degrees of acidity. I can use these results to say something about how important the water acidity is for the marine environment," explains Tue Hassenkam, who has just had his results published in the journal PNAS.

Measurements of such small materials are unique and very precise and there is therefore great potential in using the technique on other materials. For example, Tue Hassenkam has recently measured the dissolution of salt in ash from the Icelandic volcano Eyjafjallajökull which erupted last year.

Contact: Associate Professor Tue Hassenkam tue@nano.ku.dk 452-655-2030 University of Copenhagen

Sunday, May 22, 2011

New detector so sensitive it can pick up a single molecule of an explosive such as TNT

CAMBRIDGE, Mass. — MIT researchers have created a new detector so sensitive it can pick up a single molecule of an explosive such as TNT.

To create the sensors, chemical engineers led by Michael Strano coated carbon nanotubes — hollow, one-atom-thick cylinders made of pure carbon — with protein fragments normally found in bee venom. This is the first time those proteins have been shown to react to explosives, specifically a class known as nitro-aromatic compounds that includes TNT.

If developed into commercial devices, such sensors would be far more sensitive than existing explosives detectors — commonly used at airports, for example — which use spectrometry to analyze charged particles as they move through the air.

“Ion mobility spectrometers are widely deployed because they are inexpensive and very reliable. However, this next generation of nanosensors can improve upon this by having the ultimate detection limit, [detecting] single molecules of explosives at room temperature and atmospheric pressure,” says Strano, the Charles (1951) and Hilda Roddey Career Development Associate Professor of Chemical Engineering.

coated carbon nanotubes

The system in action. Image: Ardemis Boghossian and Daniel Helle
A former graduate student in Strano’s lab, Daniel Heller (now a Damon Runyon Fellow at MIT’s David H. Koch Institute for Integrative Cancer Research), is lead author of a paper describing the technology in the Proceedings of the National Academy of Sciences. The paper appears online this week.

Strano has filed for a patent on the technology, which makes use of protein fragments called bombolitins. “Scientists have studied these peptides, but as far as we know, they’ve never been shown to have an affinity for and recognize explosive molecules in any way,” he says.

In recent years, Strano’s lab has developed carbon-nanotube sensors for a variety of molecules, including nitric oxide, hydrogen peroxide and toxic agents such as the nerve gas sarin. Such sensors take advantage of carbon nanotubes’ natural fluorescence, by coupling them to a molecule that binds to a specific target. When the target is bound, the tubes’ fluorescence brightens or dims.

The new explosives sensor works in a slightly different way. When the target binds to the bee-venom proteins coating the nanotubes, it shifts the fluorescent light’s wavelength, instead of changing its intensity. The researchers built a new type of microscope to read the signal, which can’t be seen with the naked eye. This type of sensor, the first of its kind, is easier to work with because it is not influenced by ambient light.

“For a fluorescent sensor, using the intensity of the fluorescent light to read the signal is more error-prone and noisier than measuring a wavelength,” Strano says.

Each nanotube-peptide combination reacts differently to different nitro-aromatic compounds. By using several different nanotubes coated in different bombolitins, the researchers can identify a unique “fingerprint” for each explosive they might want to detect. The nanotubes can also sense the breakdown products of such explosives.

“Compounds such as TNT decompose in the environment, creating other molecule types, and those derivatives could also be identified with this type of sensor,” Strano says. “Because molecules in the environment are constantly changing into other chemicals, we need sensor platforms that can detect the entire network and classes of chemicals, instead of just one type.”

The researchers also showed that the nanotubes can detect two pesticides that are nitro-aromatic compounds as well, making them potentially useful as environmental sensors. The research was funded by the Institute for Soldier Nanotechnologies at MIT.

Philip Collins, a professor of physics at the University of California at Irvine, says the new approach is a novel extension of Strano’s previous work on carbon-nanotube sensors. “It’s nice what they’ve done — combined a couple of different things that are not sensitive to explosives, and shown that the combination is sensitive,” says Collins, who was not involved in this research.

The technology has already drawn commercial and military interest, Strano says. For the sensor to become practical for widespread use, it would have to be coupled with a commercially available concentrator that would bring any molecules floating in the air in contact with the carbon nanotubes.

“It doesn’t mean that we are ready to put these onto a subway and detect explosives immediately. But it does mean that now the sensor itself is no longer the bottleneck,” Strano says. “If there’s one molecule in a sample, and if you can get it to the sensor, you can now detect and quantify it.”

Other researchers from MIT involved in the work include former postdocs Nitish Nair and Paul Barone; graduate students Jingqing Zhang, Ardemis Boghossian and Nigel Reuel; George Pratt ’10 and junior Adam Hansborough.

Written by: Anne Trafton, MIT News Office.

Contact: Caroline McCall cmccall5@mit.edu WEB:Massachusetts Institute of Technology

Unique properties of carbon nanotubes can be combined with classical physics

What limits the behaviour of a carbon nanotube? This is a question that many scientists are trying to answer. Physicists at University of Gothenburg, Sweden, have now shown that electromechanical principles are valid also at the nanometre scale. In this way, the unique properties of carbon nanotubes can be combined with classical physics – and this may prove useful in the quantum computers of the future.

"We have been studying carbon nanotubes theoretically, in order to see how they behave when they are stimulated to behave according to the laws quantum mechanics. The results provide a completely new platform for scientists to stand on", says Gustav Sonne of the Department of Physics at the University of Gothenburg.

Every day we use a number of different microelectromechanical components for various forms of detection, to determine whether a certain process has taken place or whether a certain substance is present. These cannot be detected without instruments. One example is the detection of rapid accelerations that is used to activate the airbag in a car during an accident. What all of these components have in common is that they combine mechanical and electronic properties in order to react to external stimuli.

Carbon Nanotube

Caption: A suspended carbon nanotube can be made to vibrate like a guitar string. Gustav Sonne has studied how these oscillations influence the properties of the system if a magnetic field (H) is used to couple the mechanical motion of the tube to the electric current through it.

Credit: University of Gothenburg. Usage Restrictions: None.
Gustav Sonne has taken research down to a whole new dimension – from the micrometer scale to the nanometer scale – and he has studied the younger brothers of these components: nanoelectromechanical systems. The studies have been based on tiny nanotubes suspended between two electrical contacts. He has subsequently calculated how small vibrations in the suspended tubes can be coupled to a current that is led through them.

"Our research has focussed mainly on how these systems, which consist of a tiny, super-light mechanical oscillator (the suspended nanotube), can be described in quantum mechanical terms, and what effects this has on the measurements we can carry out. We have been able to demonstrate a number of new mechanisms for electromechanical coupling that should be possible to observe experimentally. This, in turn, may lead to extremely exotic physical phenomena in these structures, phenomena which may be of interest for research into quantum computers, and other fields."

Interest in nanotubes is based on their outstanding properties: they are among the strongest materials known, weigh next to nothing, and have extremely high conductivity for both electric currents and heat. Carbon nanotubes can be used to manufacture composite materials that are several orders of magnitude stronger than currently available materials.

###

The thesis "Mesoscopic phenomena in the electromechanics of suspended nanowires" was successfully defended in the Department of Physics. Supervisor: Associate professor Leonid Gorelik.

Contact: Gustav Sonne gustav.sonne@physics.gu.se 46-703-143-094 University of Gothenburg

Friday, May 20, 2011

First precise measurements of the "edge states" of well-ordered nanoribbons.

Berkeley Lab scientists discover the edge states of graphene nanoribbons.

As far back as the 1990s, long before anyone had actually isolated graphene – a honeycomb lattice of carbon just one atom thick – theorists were predicting extraordinary properties at the edges of graphene nanoribbons. Now physicists at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), and their colleagues at the University of California at Berkeley, Stanford University, and other institutions, have made the first precise measurements of the "edge states" of well-ordered nanoribbons.

A graphene nanoribbon is a strip of graphene that may be only a few nanometers wide (a nanometer is a billionth of a meter). Theorists have envisioned that nanoribbons, depending on their width and the angle at which they are cut, would have unique electronic, magnetic, and optical features, including band gaps like those in semiconductors, which sheet graphene doesn't have.

"Until now no one has been able to test theoretical predictions regarding nanoribbon edge-states, because no one could figure out how to see the atomic-scale structure at the edge of a well-ordered graphene nanoribbon and how, at the same time, to measure its electronic properties within nanometers of the edge," says Michael Crommie of Berkeley Lab's Materials Sciences Division (MSD) and UC Berkeley's Physics Division, who led the research. "We were able to achieve this by studying specially made nanoribbons with a scanning tunneling microscope."

Graphene Nanoribbon Structure

Caption: Graphene nanoribbons are narrow sheets of carbon atoms only one layer thick. Their width, and the angles at which the edges are cut, produce a variety of electronic states, which have been studied with precision for the first time using scanning tunneling microscopy and scanning tunneling spectroscopy.

Credit: Crommie et al, Lawrence Berkeley National Laboratory. Usage Restrictions: with credit as given.

Unzipped Carbon Nanotube

Caption: By "unzipping" carbon nanotubes, regular edges with differing chiralities can be produced between the extremes of the zigzag configuration and, at a 30-degree angle to it, the armchair configuration.

Credit: Hongjie Dai, Stanford University, and Michael Crommie et al, Lawrence Berkeley National Laboratory. Usage Restrictions: with credit as given.

Scanning Nanoribbon

Caption: A scanning tunneling microscope determines the topography and orientation of the graphene nanoribbons on the atomic scale. In spectroscopy mode, it determines changes in the density of electronic states, from the nanoribbon's interior to its edge.

Credit: Crommie et al, Lawrence Berkeley National Laboratory. Usage Restrictions: with credit as given.
The team's research not only confirms theoretical predictions but opens the prospect of building quick-acting, energy-efficient nanoscale devices from graphene-nanoribbon switches, spin-valves, and detectors, based on either electron charge or electron spin. Farther down the road, graphene nanoribbon edge states open the possibility of devices with tunable giant magnetoresistance and other magnetic and optical effects.

Crommie and his colleagues have published their research in Nature Physics, available May 8, 2011 in advanced online publication.

The well-tempered nanoribbon

"Making flakes and sheets of graphene has become commonplace," Crommie says, "but until now, nanoribbons produced by different techniques have exhibited, at best, a high degree of inhomogeneity" – typically resulting in disordered ribbon structures with only short stretches of straight edges appearing at random. The essential first step in detecting nanoribbon edge states is access to uniform nanoribbons with straight edges, well-ordered on the atomic scale.

Hongjie Dai of Stanford University's Department of Chemistry and Laboratory for Advanced Materials, a member of the research team, solved this problem with a novel method of "unzipping" carbon nanotubes chemically. Graphene rolled into a cylinder makes a nanotube, and when nanotubes are unzipped in this way the slice runs straight down the length of the tube, leaving well-ordered, straight edges.

Graphene can be wrapped at almost any angle to make a nanotube. The way the nanotube is wrapped determines the pitch, or "chiral vector," of the nanoribbon edge when the tube is unzipped. A cut straight along the outer atoms of a row of hexagons produces a zigzag edge. A cut made at a 30-degree angle from a zigzag edge goes through the middle of the hexagons and yields scalloped edges, known as "armchair" edges. Between these two extremes are a variety of chiral vectors describing edges stepped on the nanoscale, in which, for example, after every few hexagons a zigzag segment is added at an angle.

These subtle differences in edge structure have been predicted to produce measurably different physical properties, which potentially could be exploited in new graphene applications. Steven Louie of UC Berkeley and Berkeley Lab's MSD was the research team's theorist; with the help of postdoc Oleg Yazyev, Louie calculated the expected outcomes, which were then tested against experiment.

Chenggang Tao of MSD and UCB led a team of graduate students in performing scanning tunneling microscopy (STM) of the nanoribbons on a gold substrate, which resolved the positions of individual atoms in the graphene nanoribbons. The team looked at more than 150 high-quality nanoribbons with different chiralities, all of which showed an unexpected feature, a regular raised border near their edges forming a hump or bevel. Once this was established as a real edge feature – not the artifact of a folded ribbon or a flattened nanotube – the chirality and electronic properties of well-ordered nanoribbon edges could be measured with confidence, and the edge regions theoretically modeled.

Electronics at the edge

"Two-dimensional graphene sheets are remarkable in how freely electrons move through them, including the fact that there's no band gap," Crommie says.

"Nanoribbons are different: electrons can become trapped in narrow channels along the nanoribbon edges. These edge-states are one-dimensional, but the electrons on one edge can still interact with the edge electrons on the other side, which causes an energy gap to open up."

Using an STM in spectroscopy mode (STS), the team measured electronic density changes as an STM tip was moved from a nanoribbon edge inward toward its interior. Nanoribbons of different widths were examined in this way. The researchers discovered that electrons are confined to the edge of the nanoribbons, and that these nanoribbon-edge electrons exhibit a pronounced splitting in their energy levels.

"In the quantum world, electrons can be described as waves in addition to being particles," Crommie notes. He says one way to picture how different edge states arise is to imagine an electron wave that fills the length of the ribbon and diffracts off the atoms near its edge. The diffraction patterns resemble water waves coming through slits in a barrier.

For nanoribbons with an armchair edge, the diffraction pattern spans the full width of the nanoribbon; the resulting electron states are quantized in energy and extend spatially throughout the entire nanoribbon. For nanoribbons with a zigzag edge, however, the situation is different. Here diffraction from edge atoms leads to destructive interference, causing the electron states to localize near the nanoribbon edges. Their amplitude is greatly reduced in the interior.

The energy of the electron, the width of the nanoribbon, and the chirality of its edges all naturally affect the nature and strength of these nanoribbon electronic states, an indication of the many ways the electronic properties of nanoribbons can be tuned and modified.

Says Crommie, "The optimist says, 'Wow, look at all the ways we can control these states – this might allow a whole new technology!' The pessimist says, 'Uh-oh, look at all the things that can disturb a nanoribbon's behavior – how are we ever going to achieve reproducibility on the atomic scale?'"

Crommie himself declares that "meeting this challenge is a big reason for why we do research. Nanoribbons have the potential to form exciting new electronic, magnetic, and optical devices at the nanoscale. We might imagine photovoltaic applications, where absorbed light leads to useful charge separation at nanoribbon edges. We might also imagine spintronics applications, where using a side-gate geometry would allow control of the spin polarization of electrons at a nanoribbon's edge."

Although getting there won't be simple -- "The edges have to be controlled," Crommie emphasizes -- "what we've shown is that it's possible to make nanoribbons with good edges and that they do, indeed, have characteristic edge states similar to what theorists had expected. This opens a whole new area of future research involving the control and characterization of graphene edges in different nanoscale geometries."

###

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 12 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy's Office of Science. For more, visit www.lbl.gov.

Contact: Paul Preuss paul_preuss@lbl.gov 510-486-6249 DOE/Lawrence Berkeley National Laboratory

Thursday, May 19, 2011

A bold new design for thin film solar cells that requires significantly less silicon – and may boost their efficiency

College Park, Md. — A bold new design for thin film solar cells that requires significantly less silicon – and may boost their efficiency – is the result of an industry/academia collaboration between Oerlikon Solar in Switzerland and the Institute of Physics' photovoltaic group at the Academy of Sciences of the Czech Republic.

One long-term option for low-cost, high-yield industrial production of solar panels from abundant raw materials can be found in amorphous silicon solar cells and microcrystalline silicon tandem cells (a.k.a. Micromorph)—providing an energy payback within a year.

A drawback to these cells, however, is that the stable panel efficiency is less than the efficiency of presently dominate crystalline wafer-based silicon, explains Milan Vanecek, who heads the photovoltaic group at the Institute of Physics in Prague.

"To make amorphous and microcrystalline silicon cells more stable they're required to be very thin because of tight spacing between electrical contacts, and the resulting optical absorption isn't sufficient," he notes. "They're basically planar devices. Amorphous silicon has a thickness of 200 to 300 nanometers, while microcrystalline silicon is thicker than 1 micrometer."

nanostructured ZnO layer

Caption: This SEM micrograph shows the nanostructured ZnO layer, Swiss cheese design for Micromorph solar cells.

Credit: Milan Vanecek, Institute of Physics, Prague. Usage Restrictions: None.
The team's new design focuses on optically thick cells that are strongly absorbing, while the distance between the electrodes remains very tight. They describe their design in the American Institute of Physics' journal Applied Physics Letters.

"Our new 3D design of solar cells relies on the mature, robust absorber deposition technology of plasma-enhanced chemical vapor deposition, which is a technology already used for amorphous silicon-based electronics produced for liquid crystal displays. We just added a new nanostructured substrate for the deposition of the solar cell," Vanecek says.

This nanostructured substrate consists of an array of zinc oxide (ZnO) nanocolumns or, alternatively, from a "Swiss cheese" honeycomb array of micro-holes or nano-holes etched into the transparent conductive oxide layer (ZnO) (See Figure).

"This latter approach proved successful for solar cell deposition," Vanecek elaborates. "The potential of these efficiencies is estimated within the range of present multicrystalline wafer solar cells, which dominate solar cell industrial production. And the significantly lower cost of Micromorph panels, with the same panel efficiency as multicrystalline silicon panels (12 to 16 percent), could boost its industrial-scale production."

The next step is a further optimization to continue improving efficiency.

###

The article, "Nanostructured 3-dimensional thin film silicon solar cells with very high efficiency potential," by Milan Vanecek, Oleg Babchenko, Adam Purkrt, Jakub Holovsky, Neda Neykova, Ales Poruba, Zdenek Remes, Johannes Meier, and Ulrich Kroll, appears in the journal Applied Physics Letters.

Contact: Charles E. Blue cblue@aip.org 301-209-3091 American Institute of Physics

nanoflowers that will help people who've lost their sight

Forecast calls for nanoflowers to help return eyesight, University of Oregon physicist Richard Taylor is leading effort to design fractal devices to put in eyes.

EUGENE, Ore. -- University of Oregon researcher Richard Taylor is on a quest to grow flowers that will help people who've lost their sight, such as those suffering from macular degeneration, to see again.

These flowers are not roses, tulips or columbines. They will be nanoflowers seeded from nano-sized particles of metals that grow, or self assemble, in a natural process -- diffusion limited aggregation. They will be fractals that mimic and communicate efficiently with neurons.

Fractals are "a trademark building block of nature," Taylor says. Fractals are objects with irregular curves or shapes, of which any one component seen under magnification is also the same shape. In math, that property is self-similarity. Trees, clouds, rivers, galaxies, lungs and neurons are fractals, Taylor says. Today's commercial electronic chips are not fractals, he adds.

Richard Taylor, University of Oregon

Caption: Richard Taylor, physics professor and director of the University of Oregon Material Science Institute, is leading an effort to design a fractal-based retinal implant to help return vision to the blind.

Credit: Photo by Jim Barlow. Usage Restrictions: None.
Eye surgeons would implant these fractal devices within the eyes of blind patients, providing interface circuitry that would collect light captured by the retina and guide it with almost 100 percent efficiency to neurons for relay to the optic nerve to process vision.

In an article titled "Vision of beauty" for Physics World, Taylor, a physicist and director of the UO Materials Science Institute, describes his envisioned approach and how it might overcome the problems occurring with current efforts to insert photodiodes behind the eyes. Current chip technology is limited, because it doesn't allow sufficient connections with neurons.

"The wiring -- the neurons -- in the retina is fractal, but the chips are not fractal," Taylor says.

"They are just little squares of electrodes that provide too little overlap with the neurons."

Beginning this summer, Taylor's doctoral student Rick Montgomery will begin a yearlong collaboration with Simon Brown at the University of Canterbury in New Zealand to experiment with various metals to grow the fractal flowers on implantable chips.

The idea for the project emerged as Taylor was working under a Cottrell Scholar Award he received in 2003 from the Research Corporation for Science Advancement. His vision is now beginning to blossom under grants from the Office of Naval Research (ONR), the U.S. Air Force and the National Science Foundation.

Taylor's theoretical concept for fractal-based photodiodes also is the focus of a U.S. patent application filed by the UO's Office of Technology Transfer under Taylor's and Brown's names, the UO and University of Canterbury.

The project, he writes in the Physics World article, is based on "the striking similarities between the eye and the digital camera." (Physics World article is available at: http://physicsworld.com/cws/article/indepth/45840)

"The front end of both systems," he writes, "consists of an adjustable aperture within a compound lens, and advances bring these similarities closer each year." Digital cameras, he adds, are approaching the capacity to capture the 127 megapixels of the human eye, but current chip-based implants, because of their interface, are only providing about 50 pixels of resolution.

Among the challenges, Taylor says, is determining which metals can best go into body without toxicity problems. "We're right at the start of this amazing voyage," Taylor says. "The ultimate thrill for me will be to go to a blind person and say, we're developing a chip that one day will help you see again. For me, that is very different from my previous research, where I've been looking at electronics to go into computers, to actually help somebody … if I can pull that off that will be a tremendous thrill for me."

Taylor also is working under a Research Corp. grant to pursue fractal-based solar cells. ###

About the University of Oregon

The University of Oregon is among the 108 institutions chosen from 4,633 U.S. universities for top-tier designation of "Very High Research Activity" in the 2010 Carnegie Classification of Institutions of Higher Education. The UO also is one of two Pacific Northwest members of the Association of American Universities.

Contact: Jim Barlow, director of science and research communications, 541-346-3481, jebarlow@uoregon.edu

Contact: Jim Barlow jebarlow@uoregon.edu 541-346-3481 University of Oregon

Tuesday, May 17, 2011

Electrical injection, detection and precession of spin accumulation in silicon

Researchers in the Materials Science and Technology Division of the Naval Research Laboratory have recently demonstrated electrical injection, detection and precession of spin accumulation in silicon, the cornerstone material of modern device technology, at temperatures up to 225 degrees Celsius. These results provide the first demonstration that spin accumulation in Si is viable as a basis for practical devices which meet the operating temperatures specified for commercial (85°C), industrial (100°C) and military (125°C) applications. This is a key enabling step for developng devices which rely on electron spin rather than electron charge, an approach known as semiconductor spintronics that is expected to provide devices with higher performance, lower power consumption and less heat dissipation. The complete findings of this study titled, "Electrical injection and detection of spin accumulation in silicon at 500K with magnetic metal / silicon dioxide contacts" are published in the 22 March 2011 issue of Nature Communications 2:245 DOI: 10.1038/ncomms1256 (2011).

The electron possesses an internal angular momentum called the spin. The International Technology Roadmap for Semiconductors has identified the electron's spin as a new state variable that should be explored as an alternative to the electron's charge for use beyond Moore's Law, a projection named after Intel co-founder Gordon E. Moore.

Measurement geometry

Measurement geometry. In the three-terminal device measurement geometry, a current is applied to contacts 1 and 2, and a voltage is measured across contacts 2 and 3. Electrical spin injection produces spin accumulation in the transport channel under the magnetic contact 2 and a corresponding output voltage. When a magnetic field Bz is applied, the injected spins precess and dephase, and the spin accumulation decreases to zero.
U.S. Naval Research Laboratory
Moore predicted in 1965 that the number of transistors per unit area in an integrated circuit would double approximately every two years as advances in fabrication technology enabled the devices to be made smaller. Although this approach has been remarkably successful, critical device dimensions now approach atomic length scales, so that further size scaling becomes untenable. "Researchers have been forced to look beyond the simple reduction of size to develop future generations of electronic devices," states NRL senior scientist Dr. Berry Jonker. "Electrical generation, manipulation and detection of significant spin polarization in silicon at temperatures that meet commercial and military requirements are essential to validate spin as an alternative to charge for a device technology beyond Moore's Law."

Using ferromagnetic metal / silicon dioxide contacts on silicon, NRL scientists Connie Li, Olaf van 't Erve and Jonker electrically generate and detect spin accumulation and precession in the silicon transport channel at temperatures up to 225°C, and conclude that the spin information can be transported in the silicon over distances readily compatible with existing fabrication technology. They thus overcome a major obstacle in achieving control of the spin variable at temperatures required for practical applications in the most widely utilized semiconductor.

To make a semiconductor spintronic device, one needs contacts that can both generate a current of spin-polarized electrons (called a spin injector), and detect the spin polarization of the electrons (spin detector) in the semiconductor. Because the magnetic contact interface is likely to introduce additional scattering and spin relaxation mechanisms not present in the silicon bulk, the region of the semiconductor directly beneath the contact is expected to be a critical factor in the development of any future spin technology. The NRL scientists probe the spin environment directly under the magnetic metal / silicon dioxide contact using the three terminal geometry illustrated in the accompanying figure. Demonstration of spin precession and dephasing in a magnetic field transverse to the injected spin orientation, known as the Hanle effect, is conclusive evidence of spin accumulation, and enables a direct measure of the spin lifetime, a critical parameter for device operation. The NRL researchers observed Hanle precession of the electron spin accumulation in the silicon channel under the contact for biases corresponding to both spin injection and extraction, and determine the corresponding spin lifetimes.

Electronic states can form at the contact interface and introduce deleterious effects for both charge and spin transport. These undesirable states can serve as traps which prevent propagation of either charge or spin in the silicon channel. In bulk silicon, the spin lifetime is known to depend upon the carrier density, and generally decreases as the electron density increases.. "In this study we show that the spin lifetime determined from our measurements changes systematically as one changes carrier concentration of the particular silicon sample used," adds Jonker. "Our results were obtained for a number of different carrier densities and show this trend, thus making it very clear that we obtain spin injection and accumulation in the silicon itself rather than in interface defect states." The result of this research rules out spin accumulation in interface states and demonstrates spin injection, accumulation and precession in the silicon channel.

Contact: Daniel Parry daniel.parry@nrl.navy.mil 202-767-2541 Naval Research Laboratory

Monday, May 16, 2011

Merging the optics of nanoscale antennas with the electronics of semiconductors

Basic scientific curiosity paid off in unexpected ways when Rice University researchers investigating the fundamental physics of nanomaterials discovered a new technology that could dramatically improve solar energy panels.

The research is described in a new paper this week in the journal Science.

"We're merging the optics of nanoscale antennas with the electronics of semiconductors," said lead researcher Naomi Halas, Rice's Stanley C. Moore Professor in Electrical and Computer Engineering. "There's no practical way to directly detect infrared light with silicon, but we've shown that it is possible if you marry the semiconductor to a nanoantenna. We expect this technique will be used in new scientific instruments for infrared-light detection and for higher-efficiency solar cells."

More than a third of the solar energy on Earth arrives in the form of infrared light. But silicon -- the material that's used to convert sunlight into electricity in the vast majority of today's solar panels -- cannot capture infrared light's energy. Every semiconductor, including silicon, has a "bandgap" where light below a certain frequency passes directly through the material and is unable to generate an electrical current.

nanoscale antennas

By attaching nanoscale antennas to silicon semiconductors, Rice researchers showed they could harvest infrared light and turn it into electricity.
By attaching a metal nanoantenna to the silicon, where the tiny antenna is specially tuned to interact with infrared light, the Rice team showed they could extend the frequency range for electricity generation into the infrared. When infrared light hits the antenna, it creates a "plasmon," a wave of energy that sloshes through the antenna's ocean of free electrons. The study of plasmons is one of Halas' specialties, and the new paper resulted from basic research into the physics of plasmons that began in her lab years ago. It has been known that plasmons decay and give up their energy in two ways; they either emit a photon of light or they convert the light energy into heat.

The heating process begins when the plasmon transfers its energy to a single electron -- a 'hot' electron. Rice graduate student Mark Knight, lead author on the paper, together with Rice theoretical physicist Peter Nordlander, his graduate student Heidar Sobhani, and Halas set out to design an experiment to directly detect the hot electrons resulting from plasmon decay.

Patterning a metallic nanoantenna directly onto a semiconductor to create a "Schottky barrier," Knight showed that the infrared light striking the antenna would result in a hot electron that could jump the barrier, which creates an electrical current. This works for infrared light at frequencies that would otherwise pass directly through the device.

"The nanoantenna-diodes we created to detect plasmon-generated hot electrons are already pretty good at harvesting infrared light and turning it directly into electricity," Knight said. "We are eager to see whether this expansion of light-harvesting to infrared frequencies will directly result in higher-efficiency solar cells."

Contact: David Ruth druth@rice.edu713-348-6327 Rice University

Sunday, May 15, 2011

Developing nanoparticles that work as "magic bullets" — selectively targeting tumors while sparing normal, healthy tissues

The ability to use nanoparticles to deliver payloads of cancer-fighting drugs to tumors in the body could herald a fundamental change in chemotherapy treatment. But scientists are still at a relatively early stage in the implementation of this technology.

Although developing nanoparticles that work as "magic bullets" — selectively targeting tumors while sparing normal, healthy tissues — is still the goal, the reality is that most of these nanocarriers are removed through the liver and spleen before ever reaching their intended target. And many of the encapsulated drugs can be lost while the carriers circulate in the blood or degraded on the way to tumors.

In a study recently published in the journal ACS Nano, UCLA scientists report that by using engineered mesoporous silica nanoparticles (MSNPs) as delivery vehicles, they were able to achieve significant increases in the percentage of drug-carrying nanoparticles that reach and are retained at tumor sites.

The MSNP platform allows for the introduction of multiple and customized design features that can help optimize the delivery of chemotherapeutic drugs to a variety of cancer types, said the researchers, led by Dr. Andre Nel, a professor of medicine, pediatrics and public health and chief of the nanomedicine division in the UCLA Department of Medicine, and Jeffrey Zink, a professor in the UCLA Department of Chemistry and Biochemistry. Nel and Zink are also members of the California NanoSystems Institute at UCLA.

mesoporous silica nanoparticles

MSNPs in drug delivery: UCLA scientists demonstrate optimal design of mesoporous silica nanoparticles for improved in vivo delivery of anticancer drug doxorubicin in tumor-bearing mice.

Credit: the Nel and Zink Group.
A key challenge in enhancing drug delivery has been improving nanocarriers' access to tumors by capitalizing on features like the leakiness of abnormal tumor blood vessels, which allows nanoparticles to slip through and be retained at tumor sites. To achieve that, particles must be designed to be the ideal size, to remain in the blood stream long enough by temporarily evading the liver and spleen, and to stably bind the drug.

The dynamic design features employed by the UCLA research team include the manipulation of the size and surface properties of the nanoparticle to improve tumor biodistribution and protected delivery. The study demonstrates how, through an iterative design process, the first-generation MSNP was redesigned and optimized to deliver doxorubicin to a cancer xenograft in a mouse model.

The team demonstrated a significant increase in particle retention at the tumor site: Approximately 10 to 12 percent of all the drug-loaded particles injected intravenously reached the tumor site. This high tumor distribution is exceptionally good, compared with other polymer- and copolymer-based nanodelivery platforms for which the best passive tumor targeting is in the range of 3.5 to 10 percent of injected particles, the researchers said.

The study also demonstrated efficient drug delivery and tumor cell–killing using the redesigned and optimized MSNP system in mice.

"The amount of doxorubicin being delivered to the tumor site was considerably higher than what could be achieved by the free drug, in addition to allowing efficient delivery into the cancer cells at the tumor site," said Nel, who is also a member of UCLA's Jonsson Comprehensive Cancer Center.

Moreover, the improved drug delivery was accompanied by a significant reduction in systemic side effects such as weight loss and reduced liver and renal injury.

"This is an important demonstration of how the optimal design of the MSNP platform can achieve better drug delivery in vivo," Nel said. "This delivery platform allows effective and protective packaging of hydrophobic and charged anticancer drugs for controlled and on-demand delivery. Not only are these design features superior to induce tumor shrinkage and apoptosis compared to the free drug, but they also dramatically improve the safety profile of systemic doxorubicin delivery."

The UCLA research team also included Dr. Huan Meng and Dr. Tian Xia of the division of nanomedicine; Xue Min and Derrick Y. Tarm of the department of chemistry; and Dr. Zhaoxia Ji of the Center for Environmental Implications of Nanotechnology.

This study was funded by a U.S. Health Service grant from the National Cancer Institute.

The California NanoSystems Institute at UCLA is an integrated research facility located at UCLA and UC Santa Barbara. Its mission is to foster interdisciplinary collaborations in nanoscience and nanotechnology; to train a new generation of scientists, educators and technology leaders; to generate partnerships with industry; and to contribute to the economic development and the social well-being of California, the United States and the world. The CNSI was established in 2000 with $100 million from the state of California. An additional $850 million of support has come from federal research grants and industry funding. CNSI members are drawn from UCLA's College of Letters and Science, the David Geffen School of Medicine, the School of Dentistry, the School of Public Health and the Henry Samueli School of Engineering and Applied Science. They are engaged in measuring, modifying and manipulating atoms and molecules — the building blocks of our world. Their work is carried out in an integrated laboratory environment. This dynamic research setting has enhanced understanding of phenomena at the nanoscale and promises to produce important discoveries in health, energy, the environment and information technology.

UCLA's Jonsson Comprehensive Cancer Center has more than 240 researchers and clinicians engaged in disease research, prevention, detection, control, treatment and education. One of the nation's largest comprehensive cancer centers, the Jonsson Center is dedicated to promoting research and translating basic science into leading-edge clinical studies. In July 2010, the center was named among the top 10 cancer centers nationwide by U.S. News & World Report, a ranking it has held for 10 of the last 11 years.

Media Contacts Jennifer Marcus, 310-267-4839 marcus@cnsi.ucla.edu

Saturday, May 14, 2011

MIT chemical engineers have designed a new type of drug-delivery nanoparticle that exploits a trait shared by almost all tumors

MIT chemical engineers have designed a new type of drug-delivery nanoparticle that exploits a trait shared by almost all tumors: They are more acidic than healthy tissues.

Such particles could target nearly any type of tumor, and can be designed to carry virtually any type of drug, says Paula Hammond, a member of the David H. Koch Institute for Integrative Cancer Research at MIT and senior author of a paper describing the particles in the journal ACS Nano.

Like most other drug-delivering nanoparticles, the new MIT particles are cloaked in a polymer layer that protects them from being degraded by the bloodstream. However, the MIT team, including lead author and postdoctoral associate Zhiyong Poon, designed this outer layer to fall off after entering the slightly more acidic environment near a tumor. That reveals another layer that is able to penetrate individual tumor cells.

In the ACS Nano paper, which went online April 23, the researchers reported that, in mice, their particles can survive in the bloodstream for up to 24 hours, accumulate at tumor sites and enter tumor cells.

A new target

Paula Hammond

Paula Hammond, Bayer Professor of Chemical Engineering and a member of the David H. Koch Institute for Integrative Cancer Research at MIT. Photo: Dominick Reuter
The new MIT approach differs from that taken by most nanoparticle designers. Typically, researchers try to target their particles to a tumor by decorating them with molecules that bind specifically to proteins found on the surface of cancer cells. The problem with that strategy is that it’s difficult to find the right target — a molecule found on all of the cancer cells in a particular tumor, but not on healthy cells. Also, a target that works for one type of cancer might not work for another.

Hammond and her colleagues decided to take advantage of tumor acidity, which is a byproduct of its revved-up metabolism. Tumor cells grow and divide much more rapidly than normal cells, and that metabolic activity uses up a lot of oxygen, which increases acidity. As the tumor grows, the tissue becomes more and more acidic.

To build their targeted particles, the researchers used a technique called “layer-by-layer assembly.” This means each layer can be tailored to perform a specific function.

When the outer layer (made of polyethylene glycol, or PEG) breaks down in the tumor’s acidic environment, a positively charged middle layer is revealed. That positive charge helps to overcome another obstacle to nanoparticle drug delivery: Once the particles reach a tumor, it’s difficult to get them to enter the cells. Particles with a positive charge can penetrate the negatively charged cell membrane, but such particles can’t be injected into the body without a “cloak” of some kind because they would also destroy healthy tissues.

The nanoparticles’ innermost layer can be a polymer that carries a cancer drug, or a quantum dot that could be used for imaging, or virtually anything else that the designer might want to deliver, says Hammond, who is the Bayer Professor of Chemical Engineering at MIT.

Layer by layer

Other researchers have tried to design nanoparticles that take advantage of tumors’ acidity, but Hammond’s particles are the first that have been successfully tested in living animals.

Jinming Gao, professor of oncology and pharmacology at the University of Texas Southwestern Medical Center, says it is “quite clever” to use layer-by-layer assembly to create particles with a protective layer that can be shed when the particles reach their targets. “It is a nice proof of concept,” says Gao, who was not part of the research team. “This could serve as a general strategy to target acidic tumor microenvironment for improved drug delivery.”

The researchers are planning to further develop these particles and test their ability to deliver drugs in animals. Hammond says she expects it could take five to 10 years of development before human clinical trials could begin.

drug delivering nanoparticles

The polymer coating (light blue) is shed as the particle approaches a tumor, exposing positive charges. Those charges help the particle be absorbed through the tumor cell membrane.
Image: Stephen Morton

Hammond’s team is also working on nanoparticles that can carry multiple payloads. For example, the outer PEG layer might carry a drug or a gene that would “prime” the tumor cells to be susceptible to another drug carried in the particle’s core.

Anne Trafton, MIT News Office