Chemist Designs New Polymer Structures for Use as 'Plastic Electronics'

Those tricks improve the properties of certain organic polymers that mimic the properties of traditional inorganic semiconductors and could make the polymers very useful in organic solar cells, light-emitting diodes and thin-film transistors.

Conductive polymers date back to the late 1970s when researchers Alan Heeger, Alan MacDiarmid and Hideki Shirakawa discovered that plastics, with certain arrangements of atoms, can conduct electricity. The three were awarded the 2000 Nobel Prize in Chemistry for the discovery.

Jeffries-EL, an Iowa State assistant professor of chemistry, is working with a post-doctoral researcher and nine doctoral students to move the field forward by studying the relationship between polymer structures and the electronic, physical and optical properties of the materials. They're also looking for ways to synthesize the polymers without the use of harsh acids and temperatures by making them soluble in organic solvents.

The building blocks of their work are a variety of benzobisazoles, molecules well suited for electrical applications because they efficiently transport electrons, are stable at high temperatures and can absorb photons.

And if the polymers are lacking in any of those properties, Jeffries-EL and her research group can do some chemical restructuring.

"With these polymers, if you don't have the properties you need, you can go back and change the wheel," Jeffries-EL said."You can change the chemical synthesis and produce what's missing."

That, she said, doesn't work with silicon and other inorganic materials for semiconductors:"Silicon is silicon. Elements are constant."

The National Science Foundation is supporting Jeffries-EL's polymer research with a five-year,$486,250 Faculty Early Career Development grant. She also has support from the Iowa Power Fund (a state program that supports energy innovation and independence) to apply organic semiconductor technology to solar cells.

The research group is seeing some results, including peer-reviewed papers over the past two years inPhysical Chemistry Chemical Physics, Macromolecules, the Journal of Polymer Science Part A: Polymer Chemistry,and theJournal of Organic Chemistry.

"This research is really about fundamental science," Jeffries-EL said."We're studying the relationships between structure and material properties. Once we have a polymer with a certain set of properties, what can we do?"

She and her research group are turning to the molecules for answers.

"In order to realize the full potential of these materials, they must be engineered at the molecular level, allowing for optimization of materials properties, leading to enhanced performance in a variety of applications," Jeffries-EL wrote in a research summary."As an organic chemist, my approach to materials begins with small molecules."

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Two Graphene Layers May Be Better Than One

Graphene, a single layer of carbon atoms, is prized for its remarkable properties, not the least of which is the way it conducts electrons at high speed. However, the lack of what physicists call a band gap -- an energetic threshold that makes it possible to turn a transistor on and off -- makes graphene ill-suited for digital electronic applications.

Researchers have known that bilayer graphene, consisting of two stacked graphene layers, acts more like a semiconductor when immersed in an electric field.

According to NIST researcher Nikolai Zhitenev, the band gap may also form on its own due to variations in the sheets' electrical potential caused by interactions among the graphene electrons or with the substrate (usually a nonconducting, or insulating material) that the graphene is placed upon.

NIST fellow Joseph Stroscio says that their measurements indicate that interactions with the disordered insulating substrate material causes pools of electrons and electron holes (basically, the absence of electrons) to form in the graphene layers. Both electron and hole"pools" are deeper on the bottom layer because it is closer to the substrate. This difference in"pool" depths, or charge density, between the layers creates the random pattern of alternating charges and the spatially varying band gap.

Manipulating the purity of the substrate could give researchers a way to finely control graphene's band gap and may eventually lead to the fabrication of graphene-based transistors that can be turned on and off like a semiconductor.

Still, as shown in the group's previous work, while these substrate interactions open the door to graphene's use as a practical electronic material, they lower the window on speed. Electrons do not move as well through substrate-mounted bilayer graphene; however, this may likely be compensated for by engineering the graphene/substrate interactions.

Stroscio's team plans to explore further the role that substrates may play in the creation and control of band gaps in graphene by using different substrate materials. If the substrate interactions can be reduced far enough, says Stroscio, the exotic quantum properties of bilayer graphene may be harnessed to create a new quantum field effect transistor.

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Exploring the Superconducting Transition in Ultra Thin Films

"Understanding exactly what happens when a normally insulating copper-oxide material transitions from the insulating to the superconducting state is one of the great mysteries of modern physics," said Brookhaven physicist Ivan Bozovic, lead author on the study.

One way to explore the transition is to apply an external electric field to increase or decrease the level of"doping" -- that is, the concentration of mobile electrons in the material -- and see how this affects the ability of the material to carry current. But to do this in copper-oxide (cuprate) superconductors, one needs extremely thin films of perfectly uniform composition -- and electric fields measuring more than 10 billion volts per meter. (For comparison, the electric field directly under a power transmission line is 10 thousand volts per meter.)

Bozovic's group has employed a technique called molecular beam epitaxy (MBE) to uniquely create such perfect superconducting thin films one atomic layer at a time, with precise control of each layer's thickness. Recently, they've shown that in such MBE-created films even a single cuprate layer can exhibit undiminished high-temperature superconductivity.

Now, they've applied the same technique to build ultrathin superconducting field effect devices that allow them to achieve the charge separation, and thus electric field strength, for these critical studies.

These devices are similar to the field-effect transistors (FETs) that are the basis of all modern electronics, in which a semiconducting material transports electrical current from the"source" electrode on one end of the device to a"drain" electrode on the other end. FETs are controlled by a third electrode, called a"gate," positioned above the source-drain channel -- separated by a thin insulator -- which switches the device on or off when a particular gate voltage is applied to it.

But because no known insulator could withstand the high fields required to induce superconductivity in the cuprates, the standard FET scheme doesn't work for high-temperature superconductor FETs. Instead, the scientists used electrolytes, liquids that conduct electricity, to separate the charges.

In this setup, when an external voltage is applied, the electrolyte's positively charged ions travel to the negative electrode and the negatively charged ions travel to the positive electrode. But when the ions reach the electrodes, they abruptly stop, as though they've hit a brick wall. The electrode"walls" carry an equal amount of opposite charge, and the electric field between these two oppositely charged layers can exceed the 10 billion volts per meter goal.

The result is a field effect device in which the critical temperature of a prototype high-temperature superconductor compound (lanthanum-strontium-copper-oxide) can be tuned by as much as 30 degrees Kelvin, which is about 80 percent of its maximal value -- almost ten times more than the previous record.

The scientists have now used this enhanced device to study some of the basic physics of high-temperature superconductivity.

One key finding: As the density of mobile charge carriers is increased, their cuprate film transitions from insulating to superconducting behavior when the film sheet resistance reaches 6.45 kilo-ohm. This is exactly equal to the Planck quantum constant divided by twice the electron charge squared -- h/(2e)². Both the Planck constant and electron charge are"atomic" units -- the minimum possible quantum of action and of electric charge, respectively, established after the advent of quantum mechanics early in the last century.

"It is striking to see a signature of such clearly quantum-mechanical behavior in a macroscopic sample (up to millimeter scale) and at a relatively high temperature," Bozovic said. Most people associate quantum mechanics with characteristic behavior of atoms and molecules.

This result also carries another surprising message. While it has been known for many years that electrons are paired in the superconducting state, the findings imply that they also form pairs (although localized and immobile) in the insulating state, unlike in any other known material. That sets the scientists on a more focused search for what gets these immobilized pairs moving when the transition to superconductivity occurs.

Superconducting FETs might also have direct practical applications. Semiconductor-based FETs are power-hungry, particularly when packed very densely to increase their speed. In contrast, superconductors operate with no resistance or energy loss. Here, the atomically thin layer construction is in fact advantageous -- it enhances the ability to control superconductivity using an external electric field.

"This is just the beginning," Bozovic said."We still have so much to learn about high-temperature superconductors. But as we continue to explore these mysteries, we are also striving to make ultrafast and power-saving superconducting electronics a reality."

This research was funded by the DOE Office of Science and the Swiss National Science Foundation.

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Functioning Synapse Created Using Carbon Nanotubes: Devices Might Be Used in Brain Prostheses or Synthetic Brains

The team, which was led by Professor Alice Parker and Professor Chongwu Zhou in the USC Viterbi School of Engineering Ming Hsieh Department of Electrical Engineering, used an interdisciplinary approach combining circuit design with nanotechnology to address the complex problem of capturing brain function.

In a paper published in the proceedings of the IEEE/NIH 2011 Life Science Systems and Applications Workshop in April 2011, the Viterbi team detailed how they were able to use carbon nanotubes to create a synapse.

Carbon nanotubes are molecular carbon structures that are extremely small, with a diameter a million times smaller than a pencil point. These nanotubes can be used in electronic circuits, acting as metallic conductors or semiconductors.

"This is a necessary first step in the process," said Parker, who began the looking at the possibility of developing a synthetic brain in 2006."We wanted to answer the question: Can you build a circuit that would act like a neuron? The next step is even more complex. How can we build structures out of these circuits that mimic the function of the brain, which has 100 billion neurons and 10,000 synapses per neuron?"

Parker emphasized that the actual development of a synthetic brain, or even a functional brain area is decades away, and she said the next hurdle for the research centers on reproducing brain plasticity in the circuits.

The human brain continually produces new neurons, makes new connections and adapts throughout life, and creating this process through analog circuits will be a monumental task, according to Parker.

She believes the ongoing research of understanding the process of human intelligence could have long-term implications for everything from developing prosthetic nanotechnology that would heal traumatic brain injuries to developing intelligent, safe cars that would protect drivers in bold new ways.

For Jonathan Joshi, a USC Viterbi Ph.D. student who is a co-author of the paper, the interdisciplinary approach to the problem was key to the initial progress. Joshi said that working with Zhou and his group of nanotechnology researchers provided the ideal dynamic of circuit technology and nanotechnology.

"The interdisciplinary approach is the only approach that will lead to a solution. We need more than one type of engineer working on this solution," said Joshi."We should constantly be in search of new technologies to solve this problem."

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Super-Small Transistor Created: Artificial Atom Powered by Single Electrons

The researchers report inNature Nanotechnologythat the transistor's central component -- an island only 1.5 nanometers in diameter -- operates with the addition of only one or two electrons. That capability would make the transistor important to a range of computational applications, from ultradense memories to quantum processors, powerful devices that promise to solve problems so complex that all of the world's computers working together for billions of years could not crack them.

In addition, the tiny central island could be used as an artificial atom for developing new classes of artificial electronic materials, such as exotic superconductors with properties not found in natural materials, explained lead researcher Jeremy Levy, a professor of physics and astronomy in Pitt's School of Arts and Sciences. Levy worked with lead author and Pitt physics and astronomy graduate student Guanglei Cheng, as well as with Pitt physics and astronomy researchers Feng Bi, Daniela Bogorin,and Cheng Cen. The Pitt researchers worked with a team from the University of Wisconsin at Madison led by materials science and engineering professor Chang-Beom Eom, including research associates Chung Wun Bark, Jae-Wan Park, and Chad Folkman. Also part of the team were Gilberto Medeiros-Ribeiro, of HP Labs, and Pablo F. Siles, a doctoral student at the State University of Campinas in Brazil.

Levy and his colleagues named their device SketchSET, or sketch-based single-electron transistor, after a technique developed in Levy's lab in 2008 that works like a microscopic Etch A Sketch^TM, the drawing toy that inspired the idea. Using the sharp conducting probe of an atomic force microscope, Levy can create such electronic devices as wires and transistors of nanometer dimensions at the interface of a crystal of strontium titanate and a 1.2 nanometer thick layer of lanthanum aluminate. The electronic devices can then be erased and the interface used anew.

The SketchSET -- which is the first single-electron transistor made entirely of oxide-based materials -- consists of an island formation that can house up to two electrons. The number of electrons on the island -- which can be only zero, one, or two -- results in distinct conductive properties. Wires extending from the transistor carry additional electrons across the island.

One virtue of a single-electron transistor is its extreme sensitivity to an electric charge, Levy explained. Another property of these oxide materials is ferroelectricity, which allows the transistor to act as a solid-state memory. The ferroelectric state can, in the absence of external power, control the number of electrons on the island, which in turn can be used to represent the 1 or 0 state of a memory element. A computer memory based on this property would be able to retain information even when the processor itself is powered down, Levy said. The ferroelectric state also is expected to be sensitive to small pressure changes at nanometer scales, making this device potentially useful as a nanoscale charge and force sensor.

The research inNature Nanotechnologyalso was supported in part by grants from the U.S. Defense Advanced Research Projects Agency (DARPA), the U.S. Army Research Office, the National Science Foundation, and the Fine Foundation.

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New Kid on the Plasmonic Block: Researchers Find Plasmonic Resonances in Semiconductor Nanocrystals

"We have demonstrated well-defined localized surface plasmon resonances arising from p-type carriers in vacancy-doped semiconductor quantum dots that should allow for plasmonic sensing and manipulation of solid-state processes in single nanocrystals," says Berkeley Lab director Paul Alivisatos, a nanochemistry authority who led this research."Our doped semiconductor quantum dots also open up the possibility of strongly coupling photonic and electronic properties, with implications for light harvesting, nonlinear optics, and quantum information processing."

Alivisatos is the corresponding author of a paper in the journalNature Materialstitled"Localized surface plasmon resonances arising from free carriers in doped quantum dots." Co-authoring the paper were Joseph Luther and Prashant Jain, along with Trevor Ewers.

The term"plasmonics" describes a phenomenon in which the confinement of light in dimensions smaller than the wavelength of photons in free space make it possible to match the different length-scales associated with photonics and electronics in a single nanoscale device. Scientists believe that through plasmonics it should be possible to design computer chip interconnects that are able to move much larger amounts of data much faster than today's chips. It should also be possible to create microscope lenses that can resolve nanoscale objects with visible light, a new generation of highly efficient light-emitting diodes, and supersensitive chemical and biological detectors. There is even evidence that plasmonic materials can be used to bend light around an object, thereby rendering that object invisible.

The plasmonic phenomenon was discovered in nanostructures at the interfaces between a noble metal, such as gold or silver, and a dielectric, such as air or glass. Directing an electromagnetic field at such an interface generates electronic surface waves that roll through the conduction electrons on a metal, like ripples spreading across the surface of a pond that has been plunked with a stone. Just as the energy in an electromagnetic field is carried in a quantized particle-like unit called a photon, the energy in such an electronic surface wave is carried in a quantized particle-like unit called a plasmon. The key to plasmonic properties is when the oscillation frequency between the plasmons and the incident photons matches, a phenomenon known as localized surface plasmon resonance (LSPR). Conventional scientific wisdom has held that LSPRs require a metal nanostructure , where the conduction electrons are not strongly attached to individual atoms or molecules. This has proved not to be the case as Prashant Jain, a member of the Alivisatos research group and one of the lead authors of the Nature Materials paper, explains.

"Our study represents a paradigm shift from metal nanoplasmonics as we've shown that, in principle, any nanostructure can exhibit LSPRs so long as the interface has an appreciable number of free charge carriers, either electrons or holes," Jain says."By demonstrating LSPRs in doped quantum dots, we've extended the range of candidate materials for plasmonics to include semiconductors, and we've also merged the field of plasmonic nanostructures, which exhibit tunable photonic properties, with the field of quantum dots, which exhibit tunable electronic properties."

Jain and his co-authors made their quantum dots from the semiconductor copper sulfide, a material that is known to support numerous copper-deficient stoichiometries. Initially, the copper sulfide nanocrystals were synthesized using a common hot injection method. While this yielded nanocrystals that were intrinsically self-doped with p-type charge carriers, there was no control over the amount of charge vacancies or carriers.

"We were able to overcome this limitation by using a room-temperature ion exchange method to synthesize the copper sulfide nanocrystals," Jain says."This freezes the nanocrystals into a relatively vacancy-free state, which we can then dope in a controlled manner using common chemical oxidants."

By introducing enough free electrical charge carriers via dopants and vacancies, Jain and his colleagues were able to achieve LSPRs in the near-infrared range of the electromagnetic spectrum. The extension of plasmonics to include semiconductors as well as metals offers a number of significant advantages, as Jain explains.

"Unlike a metal, the concentration of free charge carriers in a semiconductor can be actively controlled by doping, temperature, and/or phase transitions," he says."Therefore, the frequency and intensity of LSPRs in dopable quantum dots can be dynamically tuned. The LSPRs of a metal, on the other hand, once engineered through a choice of nanostructure parameters, such as shape and size, is permanently locked-in."

Jain envisions quantum dots as being integrated into a variety of future film and chip-based photonic devices that can be actively switched or controlled, and also being applied to such optical applications as in vivo imaging. In addition, the strong coupling that is possible between photonic and electronic modes in such doped quantum dots holds exciting potential for applications in solar photovoltaics and artificial photosynthesis

"In photovoltaic and artificial photosynthetic systems, light needs to be absorbed and channeled to generate energetic electrons and holes, which can then be used to make electricity or fuel," Jain says."To be efficient, it is highly desirable that such systems exhibit an enhanced interaction of light with excitons. This is what a doped quantum dot with an LSPR mode could achieve."

The potential for strongly coupled electronic and photonic modes in doped quantum dots arises from the fact that semiconductor quantum dots allow for quantized electronic excitations (excitons), while LSPRs serve to strongly localize or confine light of specific frequencies within the quantum dot. The result is an enhanced exciton-light interaction. Since the LSPR frequency can be controlled by changing the doping level, and excitons can be tuned by quantum confinement, it should be possible to engineer doped quantum dots for harvesting the richest frequencies of light in the solar spectrum.

Quantum dot plasmonics also hold intriguing possibilities for future quantum communication and computation devices.

"The use of single photons, in the form of quantized plasmons, would allow quantum systems to send information at nearly the speed of light, compared with the electron speed and resistance in classical systems," Jain says."Doped quantum dots by providing strongly coupled quantized excitons and LSPRs and within the same nanostructure could serve as a source of single plasmons."

Jain and others in Alivsatos' research group are now investigating the potential of doped quantum dots made from other semiconductors, such as copper selenide and germanium telluride, which also display tunable plasmonic or photonic resonances. Germanium telluride is of particular interest because it has phase change properties that are useful for memory storage devices.

"A long term goal is to generalize plasmonic phenomena to all doped quantum dots, whether heavily self-doped or extrinsically doped with relatively few impurities or vacancies," Jain says.

This research was supported by the DOE Office of Science.

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Collecting the Sun's Energy: Novel Electrode for Flexible Thin-Film Solar Cells

The scarcity of raw materials and increasing usage of rare metals is making electronic components and devices more and more costly. Such rare metals are used, for example, to make the transparent electrodes found in mobile phone touchscreen displays, liquid-crystal displays, organic LEDs and thin-film solar cells. The material of choice in these cases is indium tin oxide (ITO), a largely transparent mixed oxide. Because ITO is relatively expensive, however, it is uneconomic to use in large area applications such as solar cells.

The search for alternatives

Indium-free transparent oxides do exist, but with demand for them increasing they too are tending to become scarce. In addition, the principal disadvantages such as brittleness remain. The search for alternative coatings which are both transparent and electrically conductive is therefore intense, with materials such as conductive polymers, carbon nanotubes or graphenes coming under scrutiny. Carbon-based electrodes, however, generally show excessive surface resistance values which make them poor electrical conductors. If a metallic grid is integrated into the organic layer, it reduces not just its resistance but also its mechanical stability. If a solar cell made out of this material is bent, the electrode layers break and are no longer conductive. The challenge thus consists of manufacturing flexible yet stable conductive substrates, ideally in a cost-effective industrial rolling process.

One solution: woven electrodes

One particularly promising possibility is the use of a transparent flexible woven polymer, which Empa has developed together with the company Sefar AG in a project financially supported by the Swiss Commission for Technology and Innovation (CTI). Sefar, which specializes in precision fabrics, is able to produce the woven polymer economically and in large quantities using a roll to roll process similar to the way newspapers are printed. Metal wires woven into the material ensure that it is electrically conductive. In a second process step the material is embedded in an inert plastic layer which does not, however, completely cover the metal filaments, thus retaining its conductivity. The electrode which results is transparent, stable and yet flexible. The Empa researchers then applied a series of coatings to this new substrate to create a novel organic solar cell whose efficiency is compatible to conventional ITO-based cells. In addition, the woven electrode is significantly more stable when deformed than commercially available flexible plastic substrates to which a thin layer of conductive ITO has been applied.

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New Spin on Graphene Makes It Magnetic

The results, reported inScience, could be a potentially huge breakthrough in the field of spintronics.

Spintronics is a group of emerging technologies that exploit the intrinsic spin of the electron, in addition to its fundamental electric charge that is exploited in microelectronics.

Billions of spintronics devices such as sensors and memories are already being produced. Every hard disk drive has a magnetic sensor that uses a flow of spins, and magnetic random access memory (MRAM) chips are becoming increasingly popular.

The findings are part of a large international effort involving research groups from the US, Russia, Japan and the Netherlands.

The key feature for spintronics is to connect the electron spin to electric current as current can be manipulated by means routinely used in microelectronics.

It is believed that, in future spintronics devices and transistors, coupling between the current and spin will be direct, without using magnetic materials to inject spins as it is done at the moment.

So far, this route has only been demonstrated by using materials with so-called spin-orbit interaction, in which tiny magnetic fields created by nuclei affect the motion of electrons through a crystal. The effect is generally small which makes it difficult to use.

The researchers found a new way to interconnect spin and charge by applying a relatively weak magnetic field to graphene and found that this causes a flow of spins in the direction perpendicular to electric current, making a graphene sheet magnetised.

The effect resembles the one caused by spin-orbit interaction but is larger and can be tuned by varying the external magnetic field.

The Manchester researchers also show that graphene placed on boron nitride is an ideal material for spintronics because the induced magnetism extends over macroscopic distances from the current path without decay.

The team believes their discovery offers numerous opportunities for redesigning current spintronics devices and making new ones such as spin-based transistors.

Professor Geim said:"The holy grail of spintronics is the conversion of electricity into magnetism or vice versa.

"We offer a new mechanism, thanks to unique properties of graphene. I imagine that many venues of spintronics can benefit from this finding."

Antonio Castro Neto, a physics professor from Boston who wrote a news article for theSciencemagazine which accompanies the research paper commented:"Graphene is opening doors for many new technologies.

"Not surprisingly, the 2010 Nobel Physics prize was awarded to Andre Geim and Kostya Novoselov for their groundbreaking experiments in this material.

"Apparently not satisfied with what they have accomplished so far, Geim and his collaborators have now demonstrated another completely unexpected effect that involves quantum mechanics at ambient conditions. This discovery opens a new chapter to the short but rich history of graphene."

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First Non-Trivial Atom Circuit: Progress Toward an Atom SQUID

The newly published work was done at the Joint Quantum Institute, a NIST/UM collaboration.

Ultracold gases, such as the Bose-Einstein condensate of sodium atoms in this experiment, are fluids that exhibit the unusual rules of the quantum world. Atomic quantum fluids show promise for constructing ultraprecise versions of sensors and other devices such as gyroscopes (which stabilize objects and aid in navigation). Superfuid helium circuits have already been used to detect rotation. Superconducting quantum interference devices (SQUIDs) use superconducting electrons in a loop to make highly sensitive measurements of magnetic fields. Researchers are striving to create an ultracold-gas version of a SQUID, which could detect rotation. Combined with ultracold atomic-gas analogs of other electronic devices and circuits, or"atomtronics" that have been envisioned, such as diodes and transistors, this work could set the stage for a new generation of ultracold-gas-based precision sensors.

To make their atom circuit, researchers created a long-lived persistent current -- a frictionless flow of particles -- in a Bose-Einstein condensate of sodium atoms held by an arrangement of lasers in a so-called optical trap that confines them to a toroidal, or donut, shape. Persistent flow -- occurring for a record-high 40 seconds in this experiment -- is a hallmark of superfluidity, the fluid analog of superconductivity.

The atom current does not circle the ring at just any velocity, but only at specified values, corresponding in this experiment to just a single quantum of angular momentum. A focused laser beam creates the circuit element -- a barrier across one side of the ring. The barrier constitutes a tunable"weak link" that can turn off the current around the loop.

Superflow stops abruptly when the strength of the barrier is sufficiently high. Like water in a pinched garden hose, the atoms speed up in the vicinity of the barrier. But when the velocity reaches a critical value, the atoms encounter resistance to flow (viscosity) and the circulation stops, as there are no external forces to sustain it.

In atomic Bose-Einstein condensates, researchers have previously created Josephson junctions, a thin barrier separating two superfluid regions, in a single atomic trap. SQUIDs require a Josephson junction in a circuit. This present work represents the implementation of a complete atom circuit, containing a superfluid ring of current and a tunable weak link barrier. This is an important step toward realizing an atomic SQUID analog.

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New Research Advances Understanding of Lead Selenide Nanowires

Now, a research team at the University of Pennsylvania's schools of Engineering and Applied Science and Arts and Sciences has shown how to control the characteristics of semiconductor nanowires made of a promising material: lead selenide.

Led by Cherie Kagan, professor in the departments of Electrical and Systems Engineering, Materials Science and Engineering and Chemistry and co-director of Pennergy, Penn's center focused on developing alternative energy technologies, the team's research was primarily conducted by David Kim, a graduate student in the Materials Science and Engineering program.

The team's work was published online in the journalACS Nanoand will be featured in the Journal's April podcast.

The key contribution of the team's work has to do with controlling the conductive properties of lead selenide nanowires in circuitry. Semiconductors come in two types,nandp, referring to the negative or positive charge they can carry. The ones that move electrons, which have a negative charge, are called"n-type." Their"p-type" counterparts don't move protons but rather the absenceof an electron -- a"hole" -- which is the equivalent of moving a positive charge.

Before they are integrated into circuitry, the semiconductor nanowire must be"wired up" into a device. Metal electrodes must be placed on both ends to allow electricity to flow in and out; however, the"wiring" may influence the observed electrical characteristics of the nanowires, whether the device appears to ben-type orp-type. Contamination, even from air, can also influence the device type. Through rigorous air-free synthesis, purification and analysis, they kept the nanowires clean, allowing them to discover the unique properties of these lead selenide nanomaterials.

Researchers designed experiments allowing them to separate the influence of the metal"wiring" on the motion of electrons and holes from that of the behavior intrinsic to the lead selenide nanowires. By controlling the exposure of the semiconductor nanowire device to oxygen or the chemical hydrazine, they were able to change the conductive properties betweenp-type andn-type. Altering the duration and concentration of the exposure, the nanowire device type could be flipped back and forth.

"If you expose the surfaces of these structures, which are unique to nanoscale materials, you can make themp-type, you can make themn-type, and you can make them somewhere in between, where it can conduct both electrons and holes," Kagan said."This is what we call 'ambipolar.'"

Devices combining onen-type and onep-type semiconductor are used in many high-tech applications, ranging from the circuits of everyday electronics, to solar cells and thermoelectrics, which can convert heat into electricity.

"Thinking about how we can build these things and take advantage of the characteristics of nanoscale materials is really what this new understanding allows," Kagan said.

Figuring out the characteristics of nanoscale materials and their behavior in device structures are the first steps in looking forward to their applications.

These lead selenide nanowires are attractive because they may be synthesized by low-cost methods in large quantities.

"Compared to the big machinery you need to make other semiconductor devices, it's significantly cheaper," Kagan said."It doesn't look much more complicated than the hoods people would recognize from when they had to take chemistry lab."

In addition to the low cost, the manufacturing process for lead selenide nanowires is relatively easy and consistent.

"You don't have to go to high temperatures to get mass quantities of these high-quality lead selenide nanowires," Kim said."The techniques we use are high yield and high purity; we can use all of them."

And because the conductive qualities of the lead selenide nanowires can be changed while they are situated in a device, they have a wider range of functionality, unlike traditional silicon semiconductors, which must first be"doped" with other elements to make them"p" or"n."

The Penn team's work is a step toward integrating these nanomaterials in a range of electronic and optoelectronic devices, such as photo sensors.

The research was conducted by Kim and Kagan, along with Materials Science and Engineering undergraduate and graduate students Tarun R. Vemulkar and Soong Ju Oh; Weon-Kyu Koh, a graduate student in Chemistry; and Christopher B. Murray, a professor in Chemistry and in Materials Science and Engineering.

This work was supported with funding from the National Science Foundation Division of Materials Research, the National Science Foundation Solar Program and the National Science Foundation Nano-Bio Interface Center.

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Scientists Develop New Technology for Stroke Rehabilitation

In a paper to be presented this week (6 April) at the Institution of Engineering and Technology (IET) Assisted Living Conference, Dr Geoff Merrett, a lecturer in electronic systems and devices, will describe the design and evaluation of three technologies which could help people who are affected by stroke to regain movement in their hand and arm.

Dr Merrett worked with Dr Sara Demain, a lecturer in physiotherapy and Dr Cheryl Metcalf, a researcher in electronic systems and devices, to develop three 'tactile' devices which generate a realistic 'sense of touch' and sensation -- mimicking those involved in everyday activities.

Dr Demain says:"Most stroke rehabilitation systems ignore the role of sensation and they only allow people repetitive movement. Our aim is to develop technology which provides people with a sense of holding something or of feeling something, like, for example, holding a hot cup of tea, and we want to integrate this with improving motor function."

Three tactile devices were developed and tested on patients who had had a stroke and on healthy participants. The devices were: a 'vibration' tactile device, which users felt provided a good indication of touch but did not really feel as if they were holding anything; a 'motor-driven squeezer' device, which users said felt like they were holding something, a bit like catching a ball; and a 'shape memory alloy' device which has thermal properties and creates a sensation like picking up a cup of tea.

Dr Merrett adds:"We now have a number of technologies, which we can use to develop sensation. This technology can be used on its own as a stand-alone system to help with sensory rehabilitation or it could be used alongside existing health technologies such as rehabilitation robots or gaming technologies which help patient rehabilitation."

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Golden Window Electrodes Developed for Organic Solar Cells

This ultra-low thickness means that even at the current high gold price the cost of the gold needed to fabricate one square metre of this electrode is only around£4.5. It can also be readily recouped from the organic solar cell at the end of its life and since gold is already widely used to form reliable interconnects it is no stranger to the electronics industry.

Organic solar cells have long relied on Indium Tin Oxide (ITO) coated glass as the transparent electrode, although this is largely due to the absence of a suitable alternative. ITO is a complex, unstable material with a high surface roughness and tendency to crack upon bending if supported on a plastic substrate. If that wasn't bad enough one of its key components, indium, is in short supply making it relatively expensive to use.

An ultra-thin film of air-stable metal like gold would offer a viable alternative to ITO, but until now it has not proved possible to deposit a film thin enough to be transparent without being too fragile and electrically resistive to be useful.

Now research led by Dr Ross Hatton and Professor Tim Jones in the University of Warwick 's department of Chemistry has developed a rapid method for the preparation of robust, ultra-thin gold films on glass. Importantly this method can be scaled up for large area applications like solar cells and the resulting electrodes are chemically very well-defined.

Dr Hatton says"This new method of creating gold based transparent electrodes is potentially widely applicable for a variety of large area applications, particularly where stable, chemically well-defined, ultra-smooth platform electrodes are required, such as in organic optoelectronics and the emerging fields of nanoelectronics and nanophotonics."

The paper documents the team's success in creating this simple, practical and effective method of depositing the films onto glass, and also reports how the optical properties can be fine tuned by perforating the film with tiny circular holes using something as simple as polystyrene balls. The University of Warwick research team has also had some early success in depositing ultra-thin gold films directly on plastic substrates, an important step towards realising the holy grail of truly flexible solar cells. This innovation is set to be exploited by Molecular Solar Ltd, a Warwick spinout company dedicated to commercialising the discoveries of its academic founders in the area of organic solar cells.

This work was supported by the European Regional Development Fund (ERDF) / Advantage West Midlands Science City SCRA AM2 project, the Engineering and Physical Sciences Research Council (EPSRC) and the Royal Academy of Engineering.

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Replacing Batteries May Become a Thing of the Past, Thanks to 'Soft Generators'

A class of variable capacitor generators known as"dielectric elastomer generators" (DEGs) shows great potential for wearable energy harvesting. In fact, researchers at the Auckland Bioengineering Institute's Biomimetics Lab believe DEGs may enable light, soft, form-fitting, silent energy harvesters with excellent mechanical properties that match human muscle. They describe their findings in the American Institute of Physics' journalApplied Physics Letters.

"Imagine soft generators that produce energy by flexing and stretching as they ride ocean waves or sway in the breeze like a tree," says Thomas McKay, a Ph.D. candidate working on soft generator research at the Biomimetics Lab."We've developed a low-cost power generator with an unprecedented combination of softness, flexibility, and low mass. These characteristics provide an opportunity to harvest energy from environmental sources with much greater simplicity than previously possible."

Dielectric elastomers, often referred to as artificial muscles, are stretchy materials that are capable of producing energy when deformed. In the past, artificial muscle generators required bulky, rigid, and expensive external electronics.

"Our team eliminated the need for this external circuitry by integrating flexible electronics -- dielectric elastomer switches -- directly onto the artificial muscles themselves. One of the most exciting features of the generator is that it's so simple; it simply consists of rubber membranes and carbon grease mounted in a frame," McKay explains.

McKay and his colleagues at the Biomimetics Lab are working to create soft dexterous machines that comfortably interface with living creatures and nature in general. The soft generator is another step toward fully soft devices; it could potentially be unnoticeably incorporated into clothing and harvest electricity from human movement. When this happens, worrying about the battery powering your cell phone or other portable electronics dying on you will become a thing of the past. And as an added bonus, this should help keep batteries out of landfills.

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Self-Cooling Observed in Graphene Elctronics

Led by mechanical science and engineering professor William King and electrical and computer engineering professor Eric Pop, the team will publish its findings in the April 3 advance online edition of the journalNature Nanotechnology.

The speed and size of computer chips are limited by how much heat they dissipate. All electronics dissipate heat as a result of the electrons in the current colliding with the device material, a phenomenon called resistive heating. This heating outweighs other smaller thermoelectric effects that can locally cool a device. Computers with silicon chips use fans or flowing water to cool the transistors, a process that consumes much of the energy required to power a device.

Future computer chips made out of graphene -- carbon sheets 1 atom thick -- could be faster than silicon chips and operate at lower power. However, a thorough understanding of heat generation and distribution in graphene devices has eluded researchers because of the tiny dimensions involved.

The Illinois team used an atomic force microscope tip as a temperature probe to make the first nanometer-scale temperature measurements of a working graphene transistor. The measurements revealed surprising temperature phenomena at the points where the graphene transistor touches the metal connections. They found that thermoelectric cooling effects can be stronger at graphene contacts than resistive heating, actually lowering the temperature of the transistor.

"In silicon and most materials, the electronic heating is much larger than the self-cooling," King said."However, we found that in these graphene transistors, there are regions where the thermoelectric cooling can be larger than the resistive heating, which allows these devices to cool themselves. This self-cooling has not previously been seen for graphene devices."

This self-cooling effect means that graphene-based electronics could require little or no cooling, begetting an even greater energy efficiency and increasing graphene's attractiveness as a silicon replacement.

"Graphene electronics are still in their infancy; however, our measurements and simulations project that thermoelectric effects will become enhanced as graphene transistor technology and contacts improve" said Pop, who is also affiliated with the Beckman Institute for Advanced Science, and the Micro and Nanotechnology Laboratory at the U. of I.

Next, the researchers plan to use the AFM temperature probe to study heating and cooling in carbon nanotubes and other nanomaterials.

King also is affiliated with the department of materials science and engineering, the Frederick Seitz Materials Research Laboratory, the Beckman Institute, and the Micro and Nanotechnology Laboratory.

The Air Force Office of Scientific Research and the Office of Naval Research supported this work. Co-authors of the paper included graduate student Kyle Grosse, undergraduate Feifei Lian and postdoctoral researcher Myung-Ho Bae.

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A Measurement First: 'Noise Thermometry' System Measures Boltzmann Constant

The Boltzmann constant* relates energy to temperature for individual particles such as atoms. The accepted value of this constant is based mainly on a 1988 NIST measurement performed using acoustic gas thermometry, with a relative standard uncertainty of less than 2 parts per million (ppm). The technique is highly accurate but the experiment is complex and difficult to perform. To assure that the Boltzmann constant can be determined accurately around the world, scientists have been trying to develop different methods that can reproduce this value with comparable uncertainty.

The latest NIST experiment used an electronic technique called Johnson noise thermometry (JNT) to measure the Boltzmann constant with an uncertainty of 12 ppm. The results are consistent with the currently recommended value for this constant. NIST researchers aim to make additional JNT measurements with improved uncertainties of 5 ppm or less, a level of precision that would help update crucial underpinnings of science, including the definition of the Kelvin, the international unit of temperature.

The international metrology community is expected to soon fix the value of the Boltzmann constant, which would then redefine the Kelvin as part of a larger effort to link all units to fundamental constants. This approach would be the most stable and universal way to define measurement units, in contrast to traditional measurement unit standards based on physical objects or substances. The Kelvin is now defined in terms of the triple-point temperature of water (273.16 K, or about 0 degrees C and 32 degrees F), or the temperature and pressure at which water's solid, liquid and vapor forms coexist in balance. This value may vary slightly depending on chemical impurities.

The NIST JNT system measures very small electrical noise in resistors, a common electronic component, when they are cooled to the water triple point temperature. This"Johnson noise" is created by the random motion of electrons, and the signals they generate are directly proportional to temperature. The electronic devices measuring the noise power are calibrated with electrical signals synthesized by a superconducting voltage source based on fundamental principles of quantum mechanics. This unique feature enables the JNT system to match electrical power and thermal-noise power at the triple point of water, and assures that copies of the system will produce identical results. NIST researchers recently improved the apparatus to reduce the statistical uncertainty, systematic errors and electromagnetic interference. Additional improvements in the electronics are expected to further reduce measurement uncertainties.

The new measurements were made in collaboration with guest researchers from the Politecnico di Torino, Italy; the National Institute of Metrology, China; the University of Twente, The Netherlands; the National Metrology Institute of Japan, Tsukuba, Japan; and the Measurement Standards Laboratory, New Zealand.

* The currently accepted value of the Boltzmann Constant is 1.380 6504 x 10-23 joules/kelvin.

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Next-Generation Computers: Advance in Microchannel Manufacturing Opens New Industry Applications

This type of technology will be needed, researchers say, in next-generation computers, lasers, consumer electronics, automobile cooling systems, fuel processors, miniature heat pumps and more.

New industries and jobs are possible. A patent has been applied for, the findings reported in theJournal of Manufacturing Processes, and the university is seeking a partner for further commercial development.

"Even though microchannel arrays have enormous potential for more efficient heat transfer and chemical reactions, high production costs have so far held back the broad, mainstream use of the technology," said Brian Paul, a professor in the OSU School of Mechanical, Industrial and Manufacturing Engineering.

"In certain applications, this new approach has reduced material costs by 50 percent," Paul said."It could cut production bonding costs by more than 90 percent, compared to existing approaches to microchannel lamination. And the use of surface-mount adhesives is directly translatable to the electronics assembly industry, so there is less risk going to market.

"This type of manufacturing research could enable a microchannel revolution," he said.

Microchannels, the diameter of a human hair, can be patterned into the surface of a metal or plastic, and can be designed to speed up the heat exchange between fluids, or the mixing and separation of fluids during chemical reactions. The accelerated heat and mass transfer leads to smaller heat exchangers and chemical reactors and separators, such as a portable"home dialysis" system that evolved out of previous OSU research.

Cost and production issues, however, have until now constrained the wider industrial use of this technology. The new manufacturing technique developed at OSU should help change that.

"We have demonstrated the use of surface-mount adhesives to create microchannels on a wide variety of metals, including aluminum, which is very cheap," said Prawin Paulraj, an OSU doctoral candidate and lead author on the recent study."Bonding aluminum is difficult with conventional techniques."

These very thin pieces of patterned metal -- akin to aluminum foil -- can be bonded one on top of another to increase the number of microchannels in a heat exchanger, and the amount of fluid that can be processed. Creation of laminated microchannel arrays in a wide variety of materials is possible, including aluminum, copper, titanium, stainless steel and other metals.

"In computers and electronics, the heat generated by the electrical circuit is a limiting factor in how small you can make it," Paulraj said."Microchannel process technology provides an efficient way to cool computers and consumer electronics, and make them even smaller."

The adhesives are limited in temperature to about that of boiling water. The researchers say that possible uses might include radiators to cool an automobile engine or small, very efficient heat pumps for efficient air conditioning within buildings.

This research was conducted at the Microproducts Breakthrough Institute, a user facility of the Oregon Nanoscience and Microtechnologies Institute.

University officials are now seeking a commercial partner in private industry to continue development and marketing of the technology, according to Denis Sather, a licensing associate in the OSU Office for Commercialization and Corporate Development.

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Optical Transistor Advance: Physicists Rotate Beams of Light With Semiconductor

Light waves can oscillate in different directions -- much like a string that can vibrate up and down or left and right -- depending on the direction in which it is picked. This is called the polarization of light. Physicists at the Vienna University of Technology have now, together with researchers at Würzburg University, developed a method to control and manipulate the polarization of light using ultra thin layers of semiconductor material.

For future research on light and its polarization this is an important step forward -- and this breakthrough could even open up possibilities for completely new computer technology. The experiment can be viewed as the optical version of an electronic transistor. The results of the experiment have now been published in the journalPhysical Review Letters.

Controlling light with magnetic fields

The polarization of light can change, when it passes through a material in a strong magnetic field. This phenomenon is known as the"Faraday effect.""So far, however, this effect had only been observed in materials in which it was very weak," professor Andrei Pimenov explains. He carried out the experiments at the Institute for Solid State Physics of the TU Vienna, together with his assistant Alexey Shuvaev. Using light of the right wavelength and extremely clean semiconductors, scientists in Vienna and Würzburg could achieve a Faraday effect which is orders of magnitude stronger than ever measured before.

Now light waves can be rotated into arbitrary directions -- the direction of the polarization can be tuned with an external magnetic field. Surprisingly, an ultra-thin layer of less than a thousandth of a millimeter is enough to achieve this."Such thin layers made of other materials could only change the direction of polarization by a fraction of one degree," says professor Pimenov. If the beam of light is then sent through a polarization filter, which only allows light of a particular direction of polarization to pass, the scientists can, rotating the direction appropriately, decide whether the beam should pass or not.

The key to this astonishing effect lies in the behavior of the electrons in the semiconductor. The beam of light oscillates the electrons, and the magnetic field deflects their vibrating motion. This complicated motion of the electrons in turn affects the beam of light and changes its direction of polarization.

An optical transistor

In the experiment, a layer of the semiconductor mercury telluride was irradiated with light in the infrared spectral range."The light has a frequency in the terahertz domain -- those are the frequencies, future generations of computers may operate with," professor Pimenov believes."For years, the clock rates of computers have not really increased, because a domain has been reached, in which material properties just don't play along anymore." A possible solution is to complement electronic circuits with optical elements. In a transistor, the basic element of electronics, an electric current is controlled by an external signal. In the experiment at TU Vienna, a beam of light is controlled by an external magnetic field. The two systems are very much alike."We could call our system a light-transistor," Pimenov suggests.

Before optical circuits for computers can be considered, the newly discovered effect will prove useful as a tool for further research. In optics labs, it will play an important role in research on new materials and the physics of light.

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Novel Nanowires Boost Fuel Cell Efficiency

But one reason fuel cells aren't already more widespread is their lack of endurance. Over time, the catalysts used even in today's state-of-the-art fuels cells break down, inhibiting the chemical reaction that converts fuel into electricity. In addition, current technology relies on small particles coated with the catalyst; however, the particles' limited surface area means only a fraction of the catalyst is available at any given time.

Now a team of engineers at the Yale School of Engineering& Applied Science has created a new fuel cell catalyst system using nanowires made of a novel material that boosts long-term performance by 2.4 times compared to today's technology. Their findings appear on the cover of the April issue ofACS Nano.

Yale engineers Jan Schroers and André Taylor have developed miniscule nanowires made of an innovative metal alloy known as a bulk metallic glass (BMG) that have high surface areas, thereby exposing more of the catalyst. They also maintain their activity longer than traditional fuel cell catalyst systems.

Current fuel cell technology uses carbon black, an inexpensive and electrically conductive carbon material, as a support for platinum particles. The carbon transports electricity, while the platinum is the catalyst that drives the production of electricity. The more platinum particles the fuel is exposed to, the more electricity is produced. Yet carbon black is porous, so the platinum inside the inner pores may not be exposed. Carbon black also tends to corrode over time.

"In order to produce more efficient fuel cells, you want to increase the active surface area of the catalyst, and you want your catalyst to last," Taylor said.

At 13 nanometers in scale (about 1/10,000 the width of a human hair), the BMG nanowires that Schroers and Taylor developed are about three times smaller than carbon black particles. The nanowires' long, thin shape gives them much more active surface area per mass compared to carbon black. In addition, rather than sticking platinum particles onto a support material, the Yale team incorporated the platinum into the nanowire alloy itself, ensuring that it continues to react with the fuel over time.

It's the nanowires' unique chemical composition that makes it possible to shape them into such small rods using a hot-press method, said Schroers, who has developed other BMG alloys that can also be blow molded into complicated shapes. The BMG nanowires also conduct electricity better than carbon black and carbon nanotubes, and are less expensive to process.

So far Taylor has tested their catalyst system for alcohol-based fuel cells (including those that use ethanol and methanol as fuel sources), but they say the system could be used in other types of fuel cells and could one day be used in portable electronic devices such as laptop computers and cell phones as well as in remote sensors.

"This is the introduction of a new class of materials that can be used as electrocatalysts," Taylor said."It's a real step toward making fuel cells commercially viable and, ultimately, supplementing or replacing batteries in electronic devices."

Other authors of the paper include Marcelo Carmo, Ryan C. Sekol, Shiyan Ding and Golden Kumar (all of Yale University).

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