First Measurement Of The Ability Of A Very Long Molecular Wire To Conduct Electric Current

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ScienceDaily (Mar. 15, 2009) — For the first time, researchers from CNRS, the Free University of Berlin and Humboldt University (Berlin) have measured the ability of a single, very long molecular wire to carry electric current. Until now, there were only statistical measurements on a collection of wires a few nanometers long. Now, thanks to an ingenious experiment using a scanning tunneling microscope, the researchers have characterized individual polymer chains of known length, up to 20 nanometers long.

They confirm what is predicted by theory: the ability to conduct electric current decreases exponentially with the length of the wire.
Tomorrow's electronic circuits will be made up of individual molecules, connected to each other by means of 'molecular electric wires' (themselves consisting of a single, long molecule). But first, researchers need to understand how electric current flows through this type of wire. On the macroscopic scale, the ability to carry current, called conductance, varies linearly as a function of the length and cross-sectional area of the wire. On the scale of a molecule, this rule is no longer valid.
Consequently, it is necessary to measure the electric current that flows through a single molecular wire connected to two electrodes and determine how it varies as a function of the length of the wire. Until now, all the experimental studies focused on very short wires (a few nanometers long) or were based exclusively on statistical measurements.
At the Free University of Berlin, in collaboration with CNRS's Center for Materials Elaboration and Structural Studies (Centre d’élaboration de matériaux et d’études structurales) (CEMES) in Toulouse and with Humboldt University (Berlin), researchers carried out an ingenious experiment to measure the conductance of a single molecule with a perfectly defined length. First they placed small molecules on a gold surface, which they bonded chemically to each other by means of a surface polymerisation reaction, which brought about the formation of long molecular chains.
They then selected one of the chains by making images of the surface with a scanning tunneling microscope and chemically bonded one end of the chain to the microscope's metal tip, which thus made up one of the two electrodes, while the other end of the polymer remained in position on the gold surface, making up the second electrode. By moving the tip away from the surface, the researchers gradually lifted up the chain, thus forming a molecular electric wire that became longer as the tip moved away from the surface. The scanning tunneling microscope was used both to measure the length of the selected molecular electric wire (since the resolution of the image is on the atomic scale it can, for example, be used to count the number of monomers) and to measure the current flowing through it. For the first time, the charge transport through a single polymer chain was measured for different lengths (up to 20 nanometers) between the two electrical contacts.
The results are in agreement with theoretical predictions: the current decreases exponentially with the length of the molecular electric wire. After the success of this innovative experiment, it is now up to chemists to come up with more conductive molecules that can be used to develop molecular wires able to carry current over even greater distances.
Journal reference:
Lafferentz et al. Conductance of a Single Conjugated Polymer as a Continuous Function of Its Length. Science, 2009; 323 (5918): 1193 DOI: 10.1126/science.1168255
Adapted from materials provided by CNRS (Délégation Paris Michel-Ange), via AlphaGalileo.

Chemists Find Secret To Increasing Luminescence Efficiency Of Carbon Nanotubes

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ScienceDaily (Mar. 14, 2009) — Chemists at the University of Connecticut have found a way to greatly increase the luminescence efficiency of single-walled carbon nanotubes, a discovery that could have significant applications in medical imaging and other areas.

Increasing the luminescence efficiency of carbon nanotubes may someday make it possible for doctors to inject patients with microscopic nanotubes to detect tumors, arterial blockages and other internal problems. Rather than relying on potentially harmful x-rays or the use of radioactive dyes, physicians could simply scan patients with an infrared light that would capture a very sharp resolution of the luminescence of the nanotubes in problem areas.
UConn's process of increasing the luminescence efficiency of single-walled carbon nanotubes will be featured in the journal Science on March 6, 2009. The research was performed in the Nanomaterials Optoelectronics Laboratory at the Institute of Materials Science at the University of Connecticut, in Storrs, CT. A patent for the process is pending.
University of Connecticut Chemist Fotios Papadimitrakopoulos describes the discovery as a major breakthrough and one of the most significant discoveries in his 10 years of working with single-walled carbon nanotubes. Assisting Papadimitrakopoulos with the research were Polymer Program graduate student Sang-Yong Ju (now a researcher at Cornell University) and William P. Kopcha, a former Chemistry undergraduate assistant in the College of Liberal Arts and Sciences who is now a first-year graduate student at UConn.
Although carbon is used in many diverse applications, scientists have long been stymied by the element's limited ability to emit light. The best scientists have been able to do with solution-suspended carbon nanotubes was to raise their luminescence efficiency to about one-half of one percent, which is extremely low compared to other materials, such as quantum dots and quantum rods.
By tightly wrapping a chemical 'sleeve' around a single-walled carbon nanotube, Papadimitrakopoulos and his research team were able to reduce exterior defects caused by chemically absorbed oxygen molecules.
This process can best be explained by imagining sliding a small tube into a slightly larger diameter tube, Papadimitrakopoulos says. In order for this to happen, all deposits or protrusions on the smaller tube have to be removed before the tube is allowed to slip into the slightly larger diameter tube. What is most fascinating with carbon nanotubes however, Papadimitrakopoulos says, is the fact that in this case the larger tube is not as rigid as the first tube (i.e. carbon nanotube) but is rather formed by a chemical "sleeve" comprised of a synthetic derivative of flavin (an analog of vitamin B2) that adsorbs and self organizes onto a conformal tube.
Papadimitrakopoulos claims that this process of self-assembly is unique in that it not only forms a new structure but also actively "cleans" the surface of the underlying nanotube. It is that active cleaning of the nanotube surface that allows the nanotube to achieve luminescence efficiency to as high as 20 percent.
"The nanotube is the smallest tube on earth and we have found a sleeve to put over it," Papadimitrakopoulos says. "This is the first time that a nanotube was found to emit with as much as 20 percent luminescence efficiency."
Papadimitrakopoulos has been working closely with the UConn Center for Science and Technology Commercialization (CSTC) in transferring his advances in research into the realm of patents, licenses and corporate partnerships. The CSTC was created several years ago as a way to help expand Connecticut's innovation-based economy and to help create new businesses and jobs around new ideas.
This is the second major nanotube discovery at UConn by Papadimitrakopoulos in the past two years. Last year, Papadimitrakopoulos and Sang-Young Ju, along with other UConn researchers, patented a way to isolate certain carbon nanotubes from others by seamlessly wrapping a form of vitamin B2 around the nanotubes. It was out of that research that Papadimitrakopoulos and Sang-Yong Ju began wrapping nanotubes with helical assemblies and probing their luminescence properties.
The more luminescent the nanotube, the brighter it appears under infrared irradiation or by electrical excitation (such as that provided by a light-emitting diode or LED). A number of important applications may be possible as a result of this research, Papadimitrakopoulos says. Carbon nanotube emissions are not only extremely sharp, but they also appear in a spectral region where minimal absorption or scattering takes place by soft tissue. Moreover, carbon nanotubes display superb photo bleaching stability and are ideally suited for near-infrared emitters, making them appropriate for applications in medicine and homeland security as bio-reporting agents and nano-sized beacons. Carbon nanotube luminescence also has important applications in nano-scaled LEDs and photo detectors, which can readily integrate with silicon-based technology. This provides an enormous repertoire for nanotube use in advanced fiber optics components, infrared light modulators, and biological sensors, where multiple applications are possible due to the nanotube's flavin-based (vitamin B2) helical wrapping.
To see a QuickTime animation of how a single-walled carbon nanotube is wrapped with the synthetic flavin derivative to increase its luminescence go to:
http://www.ims.uconn.edu/~papadim/research.htm
Journal reference:
Ju et al. Brightly Fluorescent Single-Walled Carbon Nanotubes via an Oxygen-Excluding Surfactant Organization. Science, 2009; 323 (5919): 1319 DOI: 10.1126/science.1166265
Adapted from materials provided by University of Connecticut.

New Experiments Constrain Higgs Mass

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ScienceDaily (Mar. 13, 2009) — The territory where the Higgs boson may be found continues to shrink. The latest analysis of data from the CDF and DZero collider experiments at the U.S. Department of Energy's Fermilab now excludes a significant fraction of the allowed Higgs mass range established by earlier measurements. Those experiments predict that the Higgs particle should have a mass between 114 and 185 GeV/c2. Now the CDF and DZero results carve out a section in the middle of this range and establish that it cannot have a mass in between 160 and 170 GeV/c2.

“ The outstanding performance of the Tevatron and CDF and DZero together have produced this important result,” said Dennis Kovar, Associate Director of the Office of Science for High Energy Physics at the U.S. Department of Energy. “We're looking forward to further Tevatron constraints on the Higgs mass."
The Higgs particle is a keystone in the theoretical framework known as the Standard Model of particles and their interactions. According to the Standard Model, the Higgs boson explains why some elementary particles have mass and others do not.
So far, the Higgs particle has eluded direct detection. Searches at the Large Electron Positron collider at the European laboratory CERN established that the Higgs boson must weigh more than 114 GeV/c2. Calculations of quantum effects involving the Higgs boson require its mass to be less than 185 GeV/c2.
"A cornerstone of NSF's support of particle physics is the search for the origin of mass, and this result takes us one step closer," said Physics Division Director Joe Dehmer, of the National Science Foundation.
The observation of the Higgs particle is also one of the goals of the Large Hadron Collider experiments at CERN, which plans to record its first collision data before the end of this year.
The success of probing the Higgs territory at the Tevatron has been possible thanks to the excellent performance of the accelerator and the continuing improvements that the experimenters incorporate into the analysis of the collider data.
“Fermilab’s Tevatron collider typically produces about ten million collisions per second,” said DZero co-spokesperson Darien Wood, of Northeastern University. “The Standard Model predicts how many times a year we should expect to see the Higgs boson in our detector, and how often we should see particle signals that can mimic a Higgs. By refining our analysis techniques and by collecting more and more data, the true Higgs signal, if it exists, will sooner or later emerge.”
To increase their chances of finding the Higgs boson, the CDF and DZero scientists combine the results from their separate analyses, effectively doubling the data available.
“A particle collision at the Tevatron collider can produce a Higgs boson in many different ways, and the Higgs particle can then decay into various particles,” said CDF co-spokesperson Rob Roser, of Fermilab. “Each experiment examines more and more possibilities. Combining all of them, we hope to see a first hint of the Higgs particle.”
So far, CDF and DZero each have analyzed about three inverse femtobarns of collision data---the scientific unit that scientists use to count the number of collisions. Each experiment expects to receive a total of about 10 inverse femtobarns by the end of 2010, thanks to the superb performance of the Tevatron. The collider continues to set numerous performance records, increasing the number of proton-antiproton collisions it produces.
The Higgs search result is among approximately 70 results that the CDF and DZero collaborations presented at the annual conference on Electroweak Physics and Unified Theories known as the Rencontres de Moriond, held March 7-14. In the past year, the two experiments have produced nearly 100 publications and about 50 Ph.D.s that have advanced particle physics at the energy frontier.
Adapted from materials provided by DOE/Fermi National Accelerator Laboratory.

New Platinum Catalysts For The Dehydrogenation Of Propane Discovered

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ScienceDaily (Mar. 13, 2009) — The process to turn propane into industrially necessary propylene has been expensive and environmentally unfriendly. That was until scientists at U.S. Department of Energy's Argonne National Laboratory devised a greener way to take this important step in chemical catalysis.

"Using platinum clusters, we have devised a way to catalyze propane not only in a more environmentally friendly way, but also using far less energy than previous methods," Argonne scientist Stefan Vajda said.
Alkanes are typical feedstocks for transformation to alkenes, aromatics and chemicals containing value added moieties. Dehydrogenation is a route to such transformations, but it is an endothermic process requiring significant energy input.
Oxidative dehydrogenation (ODH) of propane to propylene is a multibillion dollar industrial process. ODH of alkanes is exothermic, and thus an attractive alternative to dehydrogenation. However, current ODH catalysts have limited activity and/or poor selectivity resulting from inability to prevent complete oxidation. Two classes of catalysts are used: vanadia and platinum. The vanadia based catalysts are highly selective, but their activity is relatively low. Pt-based catalysts are more active, but their selectivity is low.
Argonne scientists showed that the size preselected Pt8-10 clusters stabilized on high-surface-area supports are 40 to 100 times more active for the oxidative dehydrogenation of propane than previously studied platinum and vanadia catalysts, while at the same time maintaining high selectivity towards formation of propylene over by-products.
This new class of catalysts may lead to energy-efficient and environmentally friendly synthesis strategies and the possible replacement of petrochemical feedstocks by abundant small alkanes.
"The oxidative dehydrogenation of alkanes is a reaction that is exothermic and thus an attractive alternative to the endothermic process of dehydrogenation of alkanes," Argonne scientist Larry Curtiss said. "The endothermic process requires a significant energy input with an increased chance of environmentally unfriendly by-products."
Using high performance computing facilities at Argonne and elsewhere, Vajda and his colleagues proved theoretically that attractive interaction between the under-coordinated platinum and propane was the cause for the higher selectivity towards propylene and its high activity.
Funding for this research was provided by the U.S. Department of Energy, Office of Science and by the Air Force Office of Scientific Research.
Journal reference:
Vajda et al. Subnanometre platinum clusters as highly active and selective catalysts for the oxidative dehydrogenation of propane. Nature Materials, 2009; 8 (3): 213 DOI: 10.1038/nmat2384
Adapted from materials provided by DOE/Argonne National Laboratory.

Nanocups Brim With Potential

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ScienceDaily (Mar. 13, 2009) — Researchers at Rice University have created a metamaterial that could light the way toward high-powered optics, ultra-efficient solar cells and even cloaking devices.

Naomi Halas, an award-winning pioneer in nanophotonics, and graduate student Nikolay Mirin created a material that collects light from any direction and emits it in a single direction. The material uses very tiny, cup-shaped particles called nanocups.
In a paper in the February issue of the journal Nano Letters, co-authors Halas and Mirin explain how they isolated nanocups to create light-bending nanoparticles.
In earlier research, Mirin had been trying to make a thin gold film with nano-sized holes when it occurred to him the knocked-out bits were worth investigating. Previous work on gold nanocups gave researchers a sense of their properties, but until Mirin's revelation, nobody had found a way to lock ensembles of isolated nanocups to preserve their matching orientation.
"The truth is a lot of exciting science actually does fall in your lap by accident," said Halas, Rice's Stanley C. Moore Professor in Electrical and Computer Engineering and professor of chemistry and biomedical engineering. "The big breakthrough here was being able to lift the nanocups off of a structure and preserve their orientation. Then we could look specifically at the properties of these oriented nanostructures."
Mirin's solution involved thin layers of gold deposited from various angles onto polystyrene or latex nanoparticles that had been distributed randomly on a glass substrate. The cups that formed around the particles – and the dielectric particles themselves – were locked into an elastomer and lifted off of the substrate. "You end up with this transparent thing with structures all oriented the same way," he said.
In other words, he had a metamaterial, a substance that gets its properties from its structure and not its composition. Halas and Mirin found their new material particularly adept at capturing light from any direction and focusing it in a single direction.
Redirecting scattered light means none of it bounces off the metamaterial back into the eye of an observer. That essentially makes the material invisible. "Ideally, one should see exactly what is behind an object," said Mirin.
"The material should not only retransmit the color and brightness of what is behind, like squid or chameleons do, but also bend the light around, preserving the original phase information of the signal."
Halas said the embedded nanocups are the first true three-dimensional nanoantennas, and their light-bending properties are made possible by plasmons. Electrons inside plasmonic nanoparticles resonate with input from an outside electromagnetic source in the same way a drop of water will make ripples in a pool. The particles act the same way radio antennas do, with the ability to absorb and emit electromagnetic waves that, in this case, includes visible wavelengths.
Because nanocup ensembles can focus light in a specific direction no matter where the incident light is coming, they make pretty good candidates for, say, thermal solar power. A solar panel that doesn't have to track the sun yet focuses light into a beam that's always on target would save a lot of money on machinery.
Solar-generated power of all kinds would benefit, said Halas. "In solar cells, about 80 percent of the light passes right through the device. And there's a huge amount of interest in making cells as thin as possible for many reasons."
Halas said the thinner a cell gets, the more transparent it becomes. "So ways in which you can divert light into the active region of the device can be very useful. That's a direction that needs to be pursued," she said.
Using nanocup metamaterial to transmit optical signals between computer chips has potential, she said, and enhanced spectroscopy and superlenses are also viable possibilities.
"We'd like to implement these into some sort of useful device," said Halas of her team's next steps. "We would also like to make several variations. We're looking at the fundamental aspects of the geometry, how we can manipulate it, and how we can control it better.
"Probably the most interesting application is something we not only haven't thought of yet, but might not be able to conceive for quite some time."
Journal reference:
Nikolay A. Mirin and Naomi J. Halas. Light-Bending Nanoparticles. Nano Letters, 2009, 9 (3), pp 1255%u20131259; February 19, 2009 DOI: 10.1021/nl900208z
Adapted from materials provided by Rice University.

Metal Discovered To Become Transparent Under High Pressure

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ScienceDaily (Mar. 14, 2009) — An international team of scientists have discovered a transparent form of the element sodium (Na). The team, led by Artem Oganov, Professor of Theoretical Crystallography at Stony Brook University, and Yanming Ma, the lead author and professor of physics at Jilin University in China, was able to demonstrate that sodium defies normal physical expectations by going transparent under pressure.

“It is well known that at sufficiently high compression all materials must go metallic,” said Oganov. “This is seen in the metallization of hydrogen at high pressures and temperatures inside planets Jupiter and Saturn.”
However, as the researchers found, element sodium does just the opposite. A perfect white metal at atmospheric pressure, on increasing pressure sodium first turns black, then (at the pressure of 2 million atmospheres) red transparent, and eventually becomes a colorless transparent material – just like glass.
“This fundamental result is important for understanding properties of highly compressed matter, particularly within stars and giant planets,” said Oganov.
The unexpected transformation was first predicted by Professor Ma. His sophisticated calculations clearly indicated that sodium adopts unusual crystal structures and becomes an insulator at high pressures. Together with Professor Oganov, Ma was able to demonstrate that, at very high compressions, overlapping sodium atoms force their outer electrons into the “holes” between the atoms.
“In these holes electrons demonstrate an extremely localized behavior, responsible for the collapse of the metallic state,” said Ma. “These electrons behave as ‘fake atoms,’ just like in electrides - ionic compounds where the role of the anion is played by localized electrons.”
To test this unexpected prediction, Ma and Oganov contacted Mikhail Eremets, the leader of an experimental group at Max Planck Institute of Chemistry, Mainz (Germany). Eremets was initially skeptical of the Ma-Oganov predictions, yet he was undeterred by extreme technical challenges of reaching pressures of over 2 million atmospheres. Eremets and his group initiated a cohort of several extremely difficult experiments, using a tiny micrometer-size sample. Through these experiments, the group was able to confirm the predictions of a new structure and transparency of sodium.
“What fascinated us most is that the pressures at which this transformation was predicted were experimentally reachable,” Oganov said, “and that at these conditions such a remarkable change of chemistry occurs”.
The results are published in the March 12 edition of the journal Nature.
Adapted from materials provided by Stony Brook University, via Newswise.

Major Step Toward Less Energy Loss In New Electromagnetic Materials

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ScienceDaily (Mar. 13, 2009) — Researchers at Uppsala University have managed for the first time to measure magnetic properties in new materials quantitatively with the help of electron microscopy – with unparalleled precision. The secret behind the breakthrough is a successful elaboration of electron microscope technology.

The findings, published in the scientific journal Physical Review Letters, means that the energy loss entailed in all electromagnetic materials can ultimately be minimized.
Apace with the miniaturization of electronic components, new methods are needed to analyze the properties of materials down to the atomic level. In 2006 a scientific article showed that it is possible to use a transmission electron microscope to study the magnetic properties of a material, using a technique called “Electron Magnetic Circular Dichroism,” (EMCD). As different materials are combined, often in thin atomic monolayer films, exciting new magnetic properties are created.
This is an interesting research field that is used in hard drives, for example. Today scientists are primarily studying magnetic properties with the aid of an extremely expensive synchrotron light source, whereas EMCD affords a cheaper and considerably more detailed study of the magnetic properties of each layer down to one nanometer.
Until now it has only been shown that EMCD works qualitatively. The Uppsala University researchers have further elaborated the technology to enable it to measure the magnetic forces of the sample quantitatively as well.
“This means we can put a number on the magnetic strength of the sample, which is key to understanding how various materials interact,” says Klaus Leifer, professor of experimental physics at the Department of Engineering Sciences.
By combining practical experiments and theoretical calculations, the method of measuring the EMCD signal has now been optimized and the computer processing of the experimental data further developed. The article is the result of collaborative work involving researchers in materials theory (Professor Olle Eriksson), physical materials synthesis (Professor Björgvin Hjörvarsson), and experimental physics.
These findings are important for our ability to analyze the magnetic properties of a material using equipment that is standard in most electron microscopy laboratories today.
“The technology will also enhance our knowledge of the energy losses that occur in magnetic components in generators and transformers,” says Klaus Leifer.
Journal reference:
Hans Lidbaum, Ján Rusz, Andreas Liebig, Björgvin Hjörvarsson, Peter M. Oppeneer, Ernesto Coronel, Olle Eriksson, and Klaus Leifer. Quantitative Magnetic Information from Reciprocal Space Maps in Transmission Electron Microscopy. Physical Review Letters, 2009; 102 (3): 037201 DOI: 10.1103/PhysRevLett.102.037201
Adapted from materials provided by Uppsala University.