Seeing the light: Scientists bring plasmonic nanofields into focus

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In typical plasmonic devices, electromagnetic waves crowd into tiny metal structures, concentrating energy into nanoscale dimensions. Due to coupling of electronics and photonics in these metal nanostructures, plasmonic devices could be harnessed for high-speed data transmission or ultrafast detector arrays. However, studying plasmonic fields in nanoscale devices presents a real roadblock for scientists, as examining these structures inherently alters their behavior.

“Whether you use a laser or alightbulb, the wavelength of light is still too large to study plasmonic fields in nanostructures. What’s more, most tools used to study plasmonic fields will alter the field distribution—the very behavior we hope to understand,” says Jim Schuck,  a staff scientist with Lawrence Berkeley National Laboratory (Berkeley Lab) who works in the Imaging and Manipulation of Nanostructures Facility at the Molecular Foundry.

Light microscopy plays a fundamental role in a scientist’s repertoire: the technique is easy to use and doesn’t inflict damage to a carefully crafted electronic circuit or delicate biological specimen. However, a typical nanoscale object of interest—such as a strand of DNA or a quantum dot—is well below the wavelength of visible light in size, which means the ability to distinguish one such object from another when they are closely spaced is lost.  Scientists are now challenging this limit using‘localization’ techniques, which count the number of photons emanating from an object to help determine its position.

In previous work, Schuck and colleagues at the Molecular Foundry, a U.S. Department of Energy (DOE) Nanoscale Science Research Centers,  engineered bowtie-shaped plasmonic devices designed to capture, filter and steer light at the nanoscale. These nano-color sorter devices served as antennae to focus and sort light in tiny spaces to a desired set of colors or energies—crucial for filters and other detectors.

In this latest advance, Schuck and his Berkeley Lab team used their innovative imaging concept to visualize plasmonic fields from these devices with nanoscale resolution. By imaging fluorescence from gold within the bowtie and maximizing the number of photons collected from their bowtie devices, the team was able to glean the position of plasmonic modes—oscillations of charge that result in optical resonance—just a few nanometers apart.

“We wondered whether there was a way to use light already present in our bowties—localized photons—to probe these fields and serve as a reporter,” says Schuck.“Our technique is also sensitive to imperfections in the system, such as tiny structural flaws or size effects, suggesting we could use this technique to measure the performance of plasmonic devices in both research and development settings.”

In parallel with Schuck’s experimental findings, Jeff Neaton, Director of the Molecular Foundry’s Theory of Nanostructured Materials Facility and Alex McLeod, an undergraduate student working at the Foundry, developed a web-based toolkit, designed to calculate images of plasmonic devices with open-source software developed at Massachusetts Institute of Technology. For this study, the researchers simulated adjusting the structure of a double bowtie antenna by a few nanometers to study how changing the size and symmetry of a plasmonic antenna affects its optical properties.

“By shifting their structure by just a few nanometers, we can focus light at different positions inside the bowtie with remarkable certainty and predictability,” said McLeod.“This work demonstrates that these nanoscale optical antennae resonate with light just as our simulations predict.”

Useful for researchers studying plasmonic and photonic structures, this toolkit will be available for download on nanoHUB, a computational resource for nanoscience and technology created through the National Science Foundation’s Network for Computational Nanotechnology.

“This work really exemplifies the very best of what the Molecular Foundry is about,” said Neaton, who is also Acting Deputy Director of Berkeley Lab’s Materials Sciences Division.“Three separate Foundry facilities—Imaging, Nanofabrication and Theory—collaborated on a significant advance in our understanding of how visible light can be localized, manipulated, and imaged at the nanoscale.”

A paper reporting this research titled,“Non-perturbative visualization of nanoscale plasmonic field distributions via photon localization microscopy,” appears inPhysical Review Lettersand is available to subscribers online. Co-authoring the paper with Schuck, McLeod and Neaton were Alexander Weber-Bargioni, Zhaoyu Zhang, Scott Dhuey, Bruce Harteneck and Stefano Cabrini.

Portions of this work at the Molecular Foundry were supported by DOE’s Office of Science.  Support for this work was also provided by the National Science Foundation through the Network for Computational Nanotechnology.

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Smallest magnetic field sensor in the world developed

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Further development of modern information technology requires computer capacities of increased efficiency at reasonable costs. In the past, integration density of the relevant electronic components was increased constantly. In continuation of this strategy, future components will have to reach the size of individual molecules. Researchers from the KIT Center for Functional Nanostructures (CFN) and IPCMS have now come closer to reaching this target.

For the first time, a team of scientists from KIT and the Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS) have now succeeded in combining the concepts of spin electronics and molecular electronics in a single component consisting of a single molecule. Components based on this principle have a special potential, as they allow for the production of very small and highly efficient magnetic field sensors for read heads in hard disks or for non-volatile memories in order to further increase reading speed and data density.

Use of organicmoleculesaselectronic componentsis being investigated extensively at the moment. Miniaturization is associated with the problem of the information being encoded with the help of the charge of the electron (current on or off). However, this requires a relatively high amount of energy. In spin electronics, the information is encoded in the intrinsic rotation of the electron, the spin. The advantage is that the spin is maintained even when switching off current supply, which means that the component can store information without any energy consumption.

The German-French research team has now combined these concepts. The organic molecule H2-phthalocyanin that is also used as blue dye in ball pens exhibits a strong dependence of its resistance, if it is trapped between spin-polarized, i.e. magnetic electrodes. This effect was first observed in purely metal contacts by Albert Fert and Peter Grünberg. It is referred to as giant magnetoresistance and was acknowledged by the Nobel Prize for Physics in 2007.

The giant magnetoresistance effect on single molecules was demonstrated at KIT within the framework of a combined experimental and theoretical project of CFN and a German-French graduate school in cooperation with the IPCMS, Strasbourg. The results of the scientists are now presented in the renowned journalNature Nanotechnology.

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Nanowire research at Stevens makes cover of Applied Physics Letters

An article by Stevens Institute of Technology researchers featured as the cover page of<i>Applied Physics Letters</i>Volume 98, Issue 7 represents a step forward in techniques for the arrangement of nanowires.

Professors Dr. Chang-Hwan Choi and Dr. Eui-Hyeok (EH) Yang, and graduate students Wei Xu, Rajesh Leeladhar, and Yao-Tsan Tsai, focuses on nanowires, structures that are mere nanometers in diameter but have enormous potential in nanotechnology to create tiny circuits that would make possible nanoelectronics, nanophotonics, and nanobiotechnology. Such devices could forever change the way we harness energy, communicate, and treat disease.

"This highly promising research can lead to the development of reliable nano-actuators which in turn stand to benefit fields and applications as diverse as biomaterials, nano robots,artificial muscles, and high frequency nano antenna applications and is an affirmation of the cutting edge research that is taking place in the Micro/Nano Devices Laboratory,"says Dr. Constantin Chassapis, Deputy Dean of the Charles V. Schaefer, Jr. School of Engineering and Science and Department Director of Mechanical Engineering.

The precise arrangement of nanowires on a large scale is crucial for any practical application. However, many current techniques for the controllable arrangement of nanowires suffer limitations.

The article, entitled,"Evaporative self-assembly of nanowires on superhydrophobic surfaces of nanotip latching surfaces,"reports a technique that is highly effective in assembling nanowires. A colloid droplet of nanowires (i.e., nanowires dispersed in a water droplet) is placed on a nano-engineered superhydrophobic surface. As the droplet evaporates, two forces cause the nanowires to self-assemble on the specially-designed surface: hydrodynamic forces inside the droplet and capillary forces of the receding contact line of the droplet. Simple and convenient, the new self-assembly technique offers a high yield rate, improving the controlled arrangement ofnanowireswhich may be used in nanodevices.

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3D nanoparticle in atomic resolution

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For the first time, scientists from Empa and ETH Zurich have, in collaboration with a Dutch team, managed to measure the atomic structure of individual nanoparticles. The technique, recently published in<i>Nature</i>, could help better understand the properties of nanoparticles in future.

In chemical terms,nanoparticleshave different properties from their«big brothers and sisters»: they have a large surface area in relation to their tiny mass and at the same time a small number of atoms. This can produce quantum effects that lead to altered material properties. Ceramics made of nanomaterials can suddenly become bendy, for instance, or a gold nugget is gold-coloured while a nanosliver of it is reddish.

The chemical and physical properties of nanoparticles are determined by their exact three-dimensional morphology, atomic structure and especially their surface composition. In a study initiated by ETH Zurich scientist Marta Rossell and Empa researcher Rolf Erni, the 3D structure of individual nanoparticles has now successfully been determined on the atomic level. The new technique could help improve our understanding of the characteristic of nanoparticles, including their reactivity and toxicity.

For their electron-microscopic study, which was published recently in the journalNature, Rossell and Erni prepared silver nanoparticles in an aluminium matrix. The matrix makes it easier to tilt the nanoparticles under the electron beam in different crystallographic orientations whilst protecting the particles from damage by the electron beam. The basic prerequisite for the study was a special electron microscope that reaches a maximum resolution of less than 50 picometres. By way of comparison: the diameter of an atom measures about oneÅngström, i.e. 100 picometres.

To protect the sample further, the electron microscope was set up in such a way as to also yield images at anatomic resolutionwith a lower accelerating voltage, namely 80 kilovolts. Normally, this kind of microscope– of which there are only a few in the world– works at 200– 300 kilovolts. The two scientists used a microscope at the Lawrence Berkeley National Laboratory in California for their experiments. The experimental data was complemented with additional electron-microscopic measurements carried out at Empa.

On the basis of these microscopic images, Sandra Van Aert from the University of Antwerp created models that sharpened the images and enabled them to be quantified: the refined images made it possible to count the individual silver atoms along different crystallographic directions.

For the three-dimensional reconstruction of the atomic arrangement in the nanoparticle, Rossell and Erni eventually enlisted the help of the tomography specialist Joost Batenburg from Amsterdam, who used the data to tomographically reconstruct theatomic structureof the nanoparticle based on a special mathematical algorithm. Only two images were sufficient to reconstruct the nanoparticle, which consists of 784 atoms."Up until now, only the rough outlines of nanoparticles could be illustrated using many images from different perspectives,"says Marta Rossell. Atomic structures, on the other hand, could only be simulated on the computer without an experimental basis.

"Applications for the method, such as characterising doped nanoparticles, are now on the cards,"says Rolf Erni. For instance, the method could one day be used to determine which atom configurations become active on the surface of the nanoparticles if they have a toxic or catalytic effect. Rossell stresses that in principle the study can be applied to any type of nanoparticle. The prerequisite, however, is experimental data like that obtained in the study.

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Armchair nanoribbons made into spintronic device

In a development that may revolutionize handheld electronics, flat-panel displays, touch panels, electronic ink, and solar cells, as well as drastically reduce their manufacturing costs, physicists in Iran have created a spintronic device based on"armchair"graphene nanoribbons. Spintronic devices are being pursued by the semiconductor and electronics industries because they promise to be smaller, more versatile, and much faster than today's electronics.

As described in the American Institute of Physics journalApplied Physics Letters, nanoribbons such as these could one day replaceindium tin oxide-- an expensive material for which researchers have been searching for suitable substitutes.

Nanoribbons are carbon nanotubes that have been"unzipped"using a room-temperature chemical process to produce ultrathin, flat ribbons of straight-edged sheets of graphene. Finite, narrow strips of graphene are cut out from a two-dimensional sheet of graphene to create the nanoribbons. And depending on how the ribbon is cut out, it results in either an"armchair"or a zigzag edge. An armchair ribbon can be thought of as essentially an unrolled zigzag nanotube.

"We proposed an electronic spin-filter device using nonmagnetic materials. Our system, which is an all-carbon device, passes only one type of spin current,"says Alireza Saffarzadeh, an associate professor in the Department of Physics at Payame Noor University. This property is due to the finite-size effect and geometry of the zigzag-edge graphene nanoribbons, Saffarzadeh explains.

"By applying a gate voltage, the type of spin current can be switched from spin-up to spin-down or vice versa,"Saffarzadeh says."For this reason, the system acts as a spin switch. And these properties are useful in spintronic applications, such as magneticrandom access memory."

Saffarzadeh and colleague Roohala Farghadan, a Ph.D. student in Tarbiat Modares University's Department of Physics, found thatgraphenenanoribbons are good candidates for electronic and spintronic devices due to high carrier mobility, long spin-relaxation times and lengths, and spin-filtering abilities.

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Manipulating molecules for a new breed of electronics

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(PhysOrg.com) -- In research appearing in today’s issue of the journal<i>Nature Nanotechnology</i>, Nongjian“NJ” Tao, a researcher at the Biodesign Institute at Arizona State University, has demonstrated a clever way of controlling electrical conductance of a single molecule, by exploiting the molecule’s mechanical properties.

Such control may eventually play a role in the design of ultra-tiny electrical gadgets, created to perform myriad useful tasks, from biological and chemical sensing to improving telecommunications and computer memory.

Tao leads a research team used to dealing with the challenges entailed in creating electrical devices of this size, where quirky effects of the quantum world often dominate device behavior. As Tao explains, one such issue is defining and controlling the electricalconductanceof a single molecule, attached to a pair of gold electrodes.

”Somemoleculeshave unusual electromechanical properties, which are unlike silicon-based materials. A molecule can also recognize other molecules via specific interactions.” These unique properties can offer tremendous functional flexibility to designers of nanoscale devices.

In the current research, Tao examines the electromechanical properties of single molecules sandwiched between conducting electrodes. When a voltage is applied, a resulting flow of current can be measured. A particular type of molecule, known as pentaphenylene, was used and its electrical conductance examined.

Tao’s group was able to vary the conductance by as much as an order of magnitude, simply by changing the orientation of the molecule with respect to the electrode surfaces. Specifically, the molecule’s tilt angle was altered, with conductance rising as the distance separating the electrodes decreased, and reaching a maximum when the molecule was poised between the electrodes at 90 degrees.

The reason for the dramatic fluctuation in conductance has to do with the so-called pi orbitals of the electrons making up the molecules, and their interaction with electron orbitals in the attached electrodes. As Tao notes, pi orbitals may be thought of as electron clouds, protruding perpendicularly from either side of the plane of the molecule. When the tilt angle of a molecule trapped between two electrodes is altered, these pi orbitals can come in contact and blend with electron orbitals contained in the gold electrode—a process known as lateral coupling. This lateral coupling of orbitals has the effect of increasing conductance.

Atoms of a molecule (gray) are shown, with their accompanying pi orbitals (red). As the distance between electrodes is decreased, the pi orbitals can interact with the electron orbitals contained in the gold electrodes—a process known as lateral coupling. This effect increases electrical conductance through the molecule.

In the case of the pentaphenylene molecule, the lateral coupling effect was pronounced, with conductance levels increasing up to 10 times as the lateral coupling of orbitals came into greater play. In contrast, the tetraphenyl molecule used as a control for the experiments did not exhibit lateral coupling and conductance values remained constant, regardless of the tilt angle applied to the molecule. Tao says that molecules can now be designed to either exploit or minimize lateral coupling effects of orbitals, thereby permitting the fine-tuning of conductance properties, based on an application’s specific requirements.

A further self-check on the conductance results was carried out using a modulation method. Here, the molecule’s position was jiggled in 3 spatial directions and the conductance values observed. Only when these rapid perturbations specifically changed the tilt angle of the molecule relative to the electrode were conductance values altered, indicating that lateral coupling of electron orbitals was indeed responsible for the effect. Tao also suggests that this modulation technique may be broadly applied as a new method for evaluating conductance changes in molecular-scale systems.

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Exciting atoms on the move: Fine-tuned laser light activates oxygen atoms to escape the surface

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(PhysOrg.com) -- A new way to accelerate and remove oxygen atoms from thin films of calcium oxide has been discovered by a team of scientists from Pacific Northwest National Laboratory, University College of London, and Tohoku University. By choosing the appropriate laser wavelength, they found a way to remove oxygen at many times the speed of sound. The atoms escape from the surface of nanostructured films of calcium oxide.

This research has implications for research and development in photochemistry, catalysis, and microelectronics. It was chosen as the cover ofTheJournal of Physical Chemistry Cfor January 27, 2011.

Focusing on new and old questions, basic research helps us understand the pure principles of science and provides the building blocks for solving big scientific problems. In this new research, a team of materials and theoretical physicists collaborated to answer questions about gaining greater control over thin film structures. Thin films are important for research in photochemistry, catalysis, and microelectronics, and have applications in industries such as pharmaceuticals and energy technologies.

This project involves desorption of neutraloxygen atomsfrom a thin film of calcium oxide (CaO). Desorption is the process of removing atoms or other particles from asurface. By shining an ultraviolet-laser on the high-surface-area material, neutral oxygen atoms are desorbed at several times the speed of sound.

Researchers used a technique developed at PNNL called reactive ballistic deposition to grow a nanostructured film of CaO. Usinglaser pulsesto excite the film, researchers applied time-of-flight techniques to measure the kinetic energy and yield of desorbed oxygen atoms. By selecting the laser wavelength, they were able to produce highly energetic atoms from the film surface. The electronic excitation, known as exciton, is initially formed in the material's bulk. Excitons are mobile and can transfer electronic energy to the surface of the film. Once on the surface, some of the exciton energy is channeled into desorption of neutral oxygen atoms at high speed.

This research shows that it is possible to manipulate particular molecular structures on a material surface by tuning thelaser wavelength. This discovery could be useful for manipulating surface structures at the atomic level. Modifyingthin filmson a very fine scale may allow scientists better control over thin film structures.

Researchers continue to work on developing and exploring other mechanisms for thermal desorption processes from oxide materials.

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Researchers produce world's first programmable nanoprocessor

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Engineers and scientists collaborating at Harvard University and the MITRE Corporation have developed and demonstrated the world's first programmable nanoprocessor.

The groundbreaking prototype computer system, described in a paper appearing today in the journalNature, represents a significant step forward in the complexity of computer circuits that can be assembled from synthesized nanometer-scale components.

It also represents an advance because these ultra-tiny nanocircuits can be programmed electronically to perform a number of basic arithmetic and logical functions.

"This work represents a quantum jump forward in the complexity and function of circuits built from the bottom up, and thus demonstrates that this bottom-up paradigm, which is distinct from the way commercial circuits are built today, can yield nanoprocessors and other integrated systems of the future,"says principal investigator Charles M. Lieber, who holds a joint appointment at Harvard's Department of Chemistry and Chemical Biology and School of Engineering and Applied Sciences.

The work was enabled by advances in the design and synthesis of nanowire building blocks. These nanowire components now demonstrate the reproducibility needed to build functionalelectronic circuits, and also do so at a size and material complexity difficult to achieve by traditional top-down approaches.

Moreover, the tiled architecture is fully scalable, allowing the assembly of much larger and ever more functional nanoprocessors.

"For the past 10 to 15 years, researchers working with nanowires, carbon nanotubes, and othernanostructureshave struggled to build all but the most basic circuits, in large part due to variations in properties of individual nanostructures,"says Lieber, the Mark Hyman Professor of Chemistry."We have shown that this limitation can now be overcome and are excited about prospects of exploiting the bottom-up paradigm of biology in building future electronics."

An additional feature of the advance is that the circuits in the nanoprocessor operate using very little power, even allowing for their miniscule size, because their component nanowires contain transistor switches that are"nonvolatile."

This means that unlike transistors in conventional microcomputer circuits, once the nanowire transistors are programmed, they do not require any additional expenditure of electrical power for maintaining memory.

"Because of their very small size and very low power requirements, these new nanoprocessor circuits are building blocks that can control and enable an entirely new class of much smaller, lighter weight electronic sensors and consumer electronics,"says co-author Shamik Das, the lead engineer in MITRE's Nanosystems Group.

"This new nanoprocessor represents a major milestone toward realizing the vision of a nanocomputer that was first articulated more than 50 years ago by physicist Richard Feynman,"says James Ellenbogen, a chief scientist at MITRE.

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Carbon nanotube transistors could lead to inexpensive, flexible electronics

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(PhysOrg.com) -- Recently, researchers have been developing carbon nanotube-based thin-film transistors (TFTs) in the hopes of creating high-performance, flexible, transparent devices, such as e-paper and RFID tags. However, one of the biggest challenges holding back the transistors’ performance is a trade-off between the properties of metallic and semiconducting nanotubes that make up the transistors. In a new study, researchers have developed a new way of fabricating nanotube networks that partly overcomes this problem, and show that the nanotube networks could be used to make transistors as well as flexible integrated circuits (ICs).

The researchers, Dong-ming Sun from Nagoya University in Nagoya, Japan, and coauthors from there and Aalto University in Finland, have published their study on the fabrication of high-performance TFTs and ICs on flexible, transparent substrates in a recent issue ofNature Nanotechnology.

“We have shown that, without consideration of carbon nanotube chirality, the as-grown carbon nanotubes can be used to fabricate high-performance TFTs and ICs, leading to a simple and fast technique for low-cost, flexible electronics,” coauthor Yutaka Ohno of Nagoya University toldPhysOrg.com.“Lightweight and flexible devices such as mobile phones and electronic paper are gaining attention for their roles in achieving a smarter and green ubiquitous information society. It is important to manufacture such devices at extremely low cost in replacing conventional paper-based media such as newspapers and magazines. Our work can provide such technology.”

As the researchers explained in their study, nanotube networks contain both metallic and semiconducting nanotubes. While a greater amount of metallic nanotubes increases the transistor’s charge-carrier mobility, it also decreases the on/off ratio.

Since both of these characteristics are important for overall transistor performance, the researchers in the new study found a way to optimize both characteristics by fabricating a nanotube network with certain unique properties. For instance, the network’s morphology consists of straight, relatively long (10 micrometers) nanotubes (30% of which are metallic) compared to other nanotube networks. The new network also uses more Y-junctions than X-junctions between nanotubes. Since Y-junctions have a larger junction area than X-junctions, they also have lower junction resistance.

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The carbon nanotube film with X- and Y-junctions. Image copyright: Dong-ming Sun, et al.©2011 Macmillan Publishers Limited.

Using this nanotube network, the researchers fabricated TFTs that simultaneously demonstrate a high charge-carrier mobility and on/off ratio, offering significantly better performance than previous nanotube-based transistors. The researchers explained that the high mobility is due to the nanotube network’s unique morphology, while the high on/off ratio can be attributed to the lower density of metallic nanotubes, which can be controlled during the fabrication process.

After building the transistors, the researchers fabricated an IC capable of sequential logic– the first such circuit based oncarbon nanotubetransistorsto date. In sequential logic circuits, the output depends on both the present input as well as the history of the input, so that these circuits have storage or memory functions.

The researchers predict that, by scaling up the fabrication process and using improved printing techniques, these nanotube-based TFTs could lead to the development of large-scale, inexpensive, and flexible electronics.

“Our near-future plan is to demonstrate roll-to-roll fabrication of CNT-based TFT arrays and ICs,” Ohno said.“To do so, we need to replace all the lithographic techniques by high-throughput printing techniques. For commercialization, we have to improve the uniformity of TFT characteristics more, but we are aiming at commercializing within five years.”

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Tuning the collective properties of artificial nanoparticle supercrystals

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Precise ordering in two-dimensional (2-D) and three-dimensional (3-D) superlattices formed by the self-assembly of individual nanocrystals (NCs) allows for control of the magnetic, optical, and electronic coupling between the individual NCs. This control can lead to useful collective properties such as vibrational coherence, reversible metal-to-insulator transitions, enhanced conductivity, spin-dependent electron transport, enhanced ferro- and ferrimagnetism, tunable magnetotransport, and efficient charge transport. These properties have many potential applications in solar cells, field-effect transistors, light-emitting devices, photodetectors, and photoconductors.

Due to precise positioning of the NCs within a 3-D superlattice, such systems are frequently referred to as“supercrystals” (SCs) in analogy to crystals built of atoms. But unlike the atomic crystals, SCs offer the flexibility of tuning the interparticle distance due to presence of the“soft” shell of organic ligands that can be used to control collective properties in such structures. Structural stability and compressibility are fundamental characteristics of any 3-D system.

A team of researchers from the Argonne National Laboratory Center for Nanoscale Materials, the X-ray Science Division at the Argonne Advanced Photon Source (APS), University of Chicago’s GeoSoilEnviroCARS, which operates Sector 13 at the APS, and Northwestern University have reported on the first combined quasi-hydrostatic, high-pressure, small-angle x-ray scattering (SAXS) and micro x-ray diffraction (XRD) studies on individual faceted, 3-D supercrystals self-assembled from colloidal 7.0-nm PbS nanocrystals. Combining the SAXS and XRD techniques allowed for precise evaluation of the interparticle spacing during the pressure cycling since the volume change of the individual NCs was taken into account. Neon was used as a pressure transmitting media to avoid the possibility of the leaching of organic ligands from the surface of the NCs and losing the structural integrity of the SCs due to sintering. Diamond anvil cell (DAC) SAXS experiments in the pressure range from ambient to 12.5 GPa, performed at X-ray Science Division x-ray beamline 12-ID-C at the APS, revealed nearly perfect structural stability of the SCs, with fcc organization of the NCs. The XRD experiments, which were carried out at GeoSoilEnviroCARS x-ray beamline 13-ID-D at the APS, demonstrated that NCs have strong preferential orientation of individual NCs in SCs up to ~55 GPa that is preserved during pressure cycling.

The mechanical properties of the individual NCs, their SCs, and the ligand matrix were analyzed using the equation of states derived from the compression data produced by SAXS and XRD. Ambient pressure bulk modulus of the SCs was calculated to be ~5 GPa during compression and ~14.5 GPa during the release cycle, respectively. NCs were found to undergo first-order phase transition above 8 GPa, and the transformation proceeds through a single nucleation event (within a pressure range of 8.1-9.2 GPa) during the first transition, and heterogeneous nucleation during the second transformation from the intermediate phase (that is not yet identified) to CsCl structure. A bulk modulus for the ligand matrix of ~2.2-2.95 GPa is an order of magnitude greater than that observed from nanoindentation study.

The high structural stability of the SCs and the ability to tune the interparticle spacing seem to offer the promise of further manipulation of the collective properties of self-organized artificial solids including the structures that consisted of NCs transformed at high pressures into a different phase. Combining high-pressure XRD and SAXS provides unique opportunities to obtain direct information about the mechanical properties of individual building blocks and their hierarchical architectures.

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Graphene and 'spintronics' combo looks promising

A team of physicists has taken a big step toward the development of useful graphene spintronic devices. The physicists, from the City University of Hong Kong and the University of Science and Technology of China, present their findings in the American Institute of Physics'<i>Applied Physics Letters</i>.

Graphene, a two-dimensional crystalline form of carbon, is being touted as a sort of"Holy Grail"of materials. It boasts properties such as a breaking strength 200 times greater than steel and, of great interest to the semiconductor anddata storageindustries,electric currentsthat can blaze through it 100 times faster than in silicon.

Spintronic devices are being hotly pursued because they promise to be smaller, more versatile, and much faster than today's electronics."Spin"is a quantum mechanical property that arises when a particle's intrinsic rotational momentum creates a tiny magnetic field. And spin has a direction, either"up"or"down."The direction can encode data in the 0s and 1s of the binary system, with the key here being that spin-based data storage doesn't disappear when the electric current stops.

"There is strong research interest in spintronic devices that process information using electron spins, because these novel devices offer better performance than traditionalelectronic devicesand will likely replace them one day,"says Kwok Sum Chan, professor of physics at the City University of Hong Kong"Grapheneis an important material for spintronic devices because itselectron spincan maintain its direction for a long time and, as a result, information stored isn't easily lost."

It is, however, difficult to generate a spin current in graphene, which would be a key part of carrying information in a graphene spintronic device. Chan and colleagues came up with a method to do just that. It involves using spin splitting in monolayer graphene generated by ferromagnetic proximity effect and adiabatic (a process that is slow compared to the speed of the electrons in the device) quantum pumping. They can control the degree of polarization of the spin current by varying the Fermi energy (the level in the distribution of electron energies in a solid at which a quantum state is equally likely to be occupied or empty), which they say is very important for meeting various application requirements.

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Researchers control conduction, surface states in topological insulator nanoribbons

In recent years, topological insulators have become one of the hottest topics in physics. These new materials act as both insulators and conductors, with their interior preventing the flow of electrical currents while their edges or surfaces allow the movement of a charge.

Perhaps most importantly, the surfaces of topological insulators enable the transport of spin-polarizedelectronswhile preventing the"scattering"typically associated with power consumption, in which electrons deviate from their trajectory, resulting in dissipation.

Because of such characteristics, these materials hold great potential for use in future transistors,memory devicesandmagnetic sensorsthat are highly energy efficient and require less power.

In a study published today inNature Nanotechnology, researchers from UCLA's Henry Samueli School of Engineering and Applied Science and from the materials division of Australia's University of Queensland show the promise of surface-conduction channels in topological insulator nanoribbons made of bismuth telluride and demonstrate that surface states in these nanoribbons are"tunable"— able to be turned on and off depending on the position of the Fermi level.

"Our finding enables a variety of opportunities in building potential new-generation, low-dissipation nanoelectronic and spintronic devices, from magnetic sensing to storage,"said Kang L. Wang, the Raytheon Professor of Electrical Engineering at UCLA Engineering, whose team carried out the research.

Bismuth telluride is well known as a thermoelectric material and has also been predicted to be a three-dimensional topological insulator with robust and unique surface states. Recent experiments with bismuth telluride bulk materials have also suggested two-dimensional conduction channels originating from the surface states. But it has been a great challenge to modify surface conduction, because of dominant bulk contribution due to impurities and thermal excitations in such small–band-gap semiconductors.

The development of topological insulator nanoribbons has helped. With their large surface-to-volume ratios, these nanoribbons significantly enhance surface conditions and enable surface manipulation by external means.

Wang and his team used thin bismuth telluride nanoribbons as conducting channels in field-effect transistor structures. These rely on an electric field to control the Fermi level and hence the conductivity of a channel. The researchers were able to demonstrate for the first time the possibility of controlling surface states in topological insulator nanostructures.

"We have demonstrated a clear surface conduction by partially removing the bulk conduction using an external electric field,"said Faxian Xiu, a UCLA staff research associate and lead author of the study."By properly tuning the gate voltage, very high surface conduction was achieved, up to 51 percent, which represents the highest values in topological insulators."

"This research is very exciting because of the possibility to build nanodevices with a novel operating principle,"said Wang, who is also associate director of the California NanoSystems Institute (CNSI) at UCLA."Very similar to the development of graphene, the topological insulators could be made into high-speed transistors and ultra–high-sensitivity sensors."

The new findings shed light on the controllability of the surface spin states in topological insulatornanoribbonsand demonstrate significant progress toward high surface electric conditions for practical device applications. The next step for Wang's team is to produce high-speed devices based on their discovery.

"The ideal scenario is to achieve 100 percentsurfaceconduction with a complete insulating state in the bulk,"Xiu said."Based on the current work, we are targeting high-performancetransistorswithpower consumptionthat is much less than the conventional complementary metal-oxide semiconductors (CMOS) technology used typically in today's electronics."

Study collaborators Jin Zou, a professor of materials engineering at the University of Queensland; Yong Wang, a Queensland International Fellow; and Zou's team at the division of materials at the University of Queensland contributed significantly to this work. A portion of the research was also done in Alexandros Shailos' lab at UCLA.

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Researchers use nanoscale transistors to study single-molecule interactions

An interdisciplinary team from Columbia University that includes electrical engineers from Columbia's Engineering School, together with researchers from the University's departments of Physics and Chemistry, has figured out a way to study single-molecule interactions on very short time scales using nanoscale transistors. In a paper to be published online January 23 in<i>Nature Nanotechnology</i>, they show how, for the first time, transistors can be used to detect the binding of the two halves of the DNA double helix with the DNA tethered to the transistor sensor. The transistors directly detect and amplify the charge of these single biomolecules.

Prior to this work, scientists have largely used fluorescence techniques to look at interactions at the level of single molecules. These studies have yielded fundamental understanding of folding, assembly, dynamics, and function of proteins and othercellular machinery. But these techniques require that the target molecules being studied be labeled with fluorescent reporter molecules, and the bandwidths for detection are limited by the time required to collect the very small number of photons emitted by these reporters.

The Columbia researchers, including Professor of Electrical Engineering Ken Shepard, Professor of Chemistry Colin Nuckolls, and graduate students Sebastian Sorgenfrei and Chien-Yang Chiu, realized thattransistors, like those used in modernintegrated circuits, have reached the same nanoscale dimensions as single molecules."So this raised the interesting question,"said Sorgenfrei, the lead author on the study,"as to whether these very small transistors could be used to study individual molecules."

They have discovered that the answer is"yes."The transistors employed in this study are fashioned from carbon nanotubes, which are cylindrical tubes made entirely ofcarbon atoms. While these are still emerging devices for electronics applications, they are exquisitely sensitive because the biomolecule can be directly tethered to the carbon nanotube wall creating enough sensitivity to detect a single DNA molecule.

The Columbia team expects this new technique to be a powerful tool for looking at single molecule interactions and is looking at instrumentation applications that currently rely almost exclusively on fluorescence such as protein assays and DNA sequencing. They also plan to study interactions at time scales several orders of magnitude greater than current techniques based on fluorescence.

"The area of single molecule research is an important one and pushes the envelope on our sensing systems,"commented Ken Shepard, Professor of Electrical Engineering at Columbia Engineering."There is a huge potential for modern nanoelectronics to play an important role in this field. Our work, which has been a terrific collaboration between groups from Electrical Engineering, Chemistry, and Physics, is a great example of how nanoelectronics and biotechnology can be combined to produce new, exciting results."

Shepard hopes that this research, which was funded primarily by the National Science Foundation and the National Institutes of Health, will lead to exciting new applications for nanoscale electronic circuits.

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In Brief: Ultrafast transparency in a plasmonic nanorod

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Users from the University of North Florida and King's College London collaborated with Argonne scientists in the Nanophotonics Group to show that closely spaced plasmonic gold nanorods produce an ultrafast transmission change when illuminated with a low-energy optical pulse.

The ultrafast switching behavior is due to strong coupling between the nanorodplasmons, which are collective free-electron responses of metals that are driven by the incident light.

The key discovery is that the closely spaced nanorod material exhibits nonlocality of the optical response, which has an unusually strong nonlinear dependence on incident light intensity.

These materials belong to a new class of“metamaterials”– those with optical properties and responses that do not occur naturally.

Electromagnetic modeling by Univversity of Massachusetts collaborators confirms the nonlocal response of the plasmonic metamaterial.

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Novel device sheds light on the beauty of science

The wonder of science often comes from the endless possibilities opened up by each successive discovery and the unexpected findings that result. Scientists at the University of Bristol now have a new tool that will yield yet more and unprecedented levels of information– and crucially, without disturbing the natural, physical state of the object under scrutiny.

The past few months have seen physicists at Bristol’s Interface Analysis Centre vying for time on the dualbeam instrument, which as centre Director Dr. Tom Scott says,“unlocks the key to a whole new world”.

It has so far produced hundreds of images that are as beautiful as they are revelatory, and those at the IAC are keen to see what more the dualbeam can do, working with colleagues from across the University to delve into all matter, from diamonds to insect ears.

The dualbeam looks at surface structures with a resolution of less than a nanometre– the equivalent of ten millionths of the thickness of a human hair.  The resolution of the images produced is just one nanometre, which is beyond miniscule, given that it takes 1,000 nanometres to make one micron, and 1,000 microns constitute a single millimetre. 

The dualbeam is so called because it operates using two systems– a focused ion beam (FIB) and a high spec field emission scanning electron microscope (SEM).  It operates using gallium ions derived from a liquid metal ion source that are directed at the surface in a tightly controlled beam in which individual atoms are travelling at speeds  of up to one million miles an hour. The ion beam can be precisely controlled to remove material from tightly defined areas– essentially performing micro and even nano-surgery on almost any material.

A nano-wire made using ion beam milling for gas sensing applications. It also happens to look like a small-scale version of the Clifton suspension bridge.

Unlike other techniques used for dissecting materials, the dualbeam can extract information and capture images without causing any detectable damage except over a tiny area.  It can also deposit materials such as gold and platinum, known for their conductivity, on to the surface structure, providing insights into the composition and behaviour of materials.

For physicists looking for quantum wells, biologists looking at the structure of membranes in the ears of tree crickets, and engineers keen to understand the nanostructure of exotic alloys, the dualbeam seems to hold the key to success. 

“It makes things possible which were previously considered impossible, it’s at the heart of what makes science beautiful,” says Dr. Scott. “It can do things in such a precisely defined way to such a high degree of accuracy that it really is incredible.  In fact, it’s difficult to comprehend just how small a scale this thing works on.”

Some of the project proposals under consideration that would make use of the dualbeam include an examination of the ears of Indian tree crickets, where the dualbeam could be used to slice and view in three dimensions reconstructions of cricket ears.  The findings could ultimately inform medical advancements in hearing devices for humans.

Another involves examining the materials used to build nuclear power stations.  The rate at which they age, and the outputs produced as they do so, is of serious concern.  A closer examination of the microstructure of stainless steels, and the processes by which they accommodate strain when affected by thermal cycling in power stations, would yield significant information about potential failure risks that could subsequently be safeguarded against in the design of the next generation of power stations. 

The dualbeam could also be used in quantum cryptography, to devise ways of transmitting messages in a way that is resistant to attempts to tap into the source, using emitters constructed from a single photonic light source so small and so intricately encoded as to be virtually undetectable.

In biochemistry, researchers are looking at making actuators -“gold sandwiches” with a polymer filling which could swim through the bloodstream, collecting information that could be used to inform medical approaches to human disease.

Dissecting and reconstructing structures in three dimensions can take a matter of minutes or hours, depending on the volume of the material under scrutiny.  The dualbeam also has an automation capability which allows researchers to program it to carry out operational tasks, freeing them up to continue with something else. Dr. Scott compares it to a multi-faceted kitchen aid:“This machine basically does all the slicing and dicing, leaving you to concentrate on making a really fantastic meal.”

Dr. Scott is keen to seek out other collaborations that will test the boundaries of every discipline and put materials and this new tool through its paces: “The dualbeam instrument is a clear example of the University’s commitment to groundbreaking developments in research. If we are going to be the leaders in the UK and internationally in terms of research we need to be pushing the boundaries of what is technically possible, and this new piece of equipment will certainly enable us to do that.”

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NRL scientists elevate warfighter readiness against invisible threats

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In asymmetric warfare, early detection and identification of trace level chemical and biological agents and explosive compounds is critical to rapid reaction, response, and survivability. While there are many methods currently being used that can detect these threats, none allow for the unique fingerprinting of threat agents at trace levels.

A research team, led by Drs. Joshua Caldwell and Orest Glembocki, scientists at the U.S. Naval Research Laboratory, Electronic Science and Technology Division, has overcome this limitation with surface enhanced Raman scattering (SERS) using optically stimulated plasmon oscillations in nanostructured substrates.

Shown to provide enhancements of the Raman signal, large-area gold (Au) coated silicon (Si) nanopillar arrays are over 100 million times more sensitive than Raman scattering sensing alone, while maintaining a very uniform response with less than 30 percent variability across the sensor area.

"These arrays are over an order-of-magnitude more sensitive than the best reported SERS sensors in the literature and the current state-of-the-art large-area commercial SERS sensors,"said Caldwell."These arrays can be a key component of fully integrated, autonomously operating chemical sensors that detect, identify and report the presence of a threat at trace levels of exposure."

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Scientists Orest Glembocki (holding a DeltaNu ExamineR hand-held Raman spectrometer) and Joshua Caldwell, display one of the silicon wafers they fabricated for the SERS sensing using ion etching and e-beam lithography. Several square arrays of gold-coated silicon nanopillar were fabricated on the wafer to perform surface-enhanced Raman scattering testing. Credit: U.S. Naval Research Laboratory/Jamie Hartman

Raman devices uselaser lightto excite molecular vibrations, which in turn causes a shift in the energy of the scattered laser photons, up or down, creating a unique visual pattern. In the case of trace amounts of molecules in gases or liquids, detection through ordinary Raman scattering is virtually impossible. However, the Raman signal can be enhanced via the SERS effect usingmetal nanoparticles.

Despite surface-enhanced Raman scattering being first observed in the late 1970s, efforts to provide reproducible SERS-basedchemical sensorshas been hindered by the inability to make large-area devices with a uniform SERS response. The ability to reproducibly pattern nanometer-sized particles in periodic arrays has finally allowed this requirement to be met.

"While many tools are currently available to detect trace amounts of chemical warfare and biological agents and explosive compounds, a device using SERS can be used to identify these minute quantities of the chemicals of interest by providing a 'fingerprint' of the material, which all but eliminates the prevalence of false alarms,"says Glembocki.

SERS offers several potential advantages over other spectroscopic techniques because of its measurement speed, high sensitivity, portability, and simple maneuverability. SERS can additionally be used to enhance existing Raman technologies, such as the hand held and standoff units that are already in use in field applications.

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Nanowires exhibit giant piezoelectricity

Gallium nitride (GaN) and zinc oxide (ZnO) are among the most technologically relevant semiconducting materials. Gallium nitride is ubiquitous today in optoelectronic elements such as blue lasers (hence the blue-ray disc) and light-emitting-diodes (LEDs); zinc oxide also finds many uses in optoelectronics and sensors.

In the past few years, though, nanostructures made of these materials have shown a plethora of potential functionalities, ranging from single-nanowire lasers and LEDs to more complex devices such as resonators and, more recently, nanogenerators that convertmechanical energyfrom the environment (body movements, for example) to power electronic devices. The latter application relies on the fact that GaN and ZnO are also piezoelectric materials, meaning that they produce electric charges as they are deformed.

In a paper published online in the journalNano Letters, Horacio Espinosa, the James N. and Nancy J. Farley Professor in Manufacturing and Entrepreneurship at the McCormick School of Engineering and Applied Science at Northwestern University, and Ravi Agrawal, a graduate student in Espinosa's lab, reported that piezoelectricity in GaN and ZnO nanowires is in fact enhanced by as much as two orders of magnitude as the diameter of the nanowires decrease.

"This finding is very exciting because it suggests that constructing nanogenerators, sensors and other devices from smaller nanowires will greatly improve their output and sensitivity,"Espinosa said.

"We used a computational method called Density Functional Theory (DFT) to model GaN and ZnO nanowires of diameters ranging from 0.6nanometersto 2.4 nanometers,"Agrawal said. Thecomputational methodis able to predict the electronic distribution of the nanowires as they are deformed and, therefore, allows calculating their piezoelectric coefficients.

The researchers' results show that the piezoelectric coefficient in 2.4 nanometer-diameter nanowires is about 20 times larger and about 100 times larger for ZnO and GaN nanowires, respectively, when compared to the coefficient of the materials at the macroscale. This confirms previous computational findings on ZnO nanostructures that showed a similar increase in piezoelectric properties. However, calculations forpiezoelectricityof GaN nanowires as a function of size were carried out in this work for the first time, and the results are clearly more promising as GaN shows a more prominent increase.

"Our calculations reveal that the increase in piezoelectric coefficient is a result of the redistribution of electrons in the nanowire surface, which leads to an increase in the strain-dependent polarization with respect to the bulk materials,"Espinosa said.

The findings by Espinosa and Agrawal may have important implications for the field ofenergy harvestingas well as for fundamental science. For energy harvesting, where piezoelectric elements are used to convert mechanical to electrical energy in order to power electronic devices, these results point to an advantage in reducing the size of the piezoelectric elements down to the nanometer scale. Energy harvesting devices built from small-diameter nanowires should in principle be able to produce more electrical energy from the same amount of mechanical energy than their bulk counterparts.

In terms of fundamental science, these results further previous conclusions that matter at the nanoscale has different properties. It is clear now that by tailoring the size of nanostructures, their mechanical, electrical and thermal properties can be tuned as well.

"Our focus remains on understanding the fundamental principles governing the behavior of nanostructures as a function of their size,"Espinosa and Agrawal say."One of the most important issues that needs to be addressed is to obtain experimental confirmation of these results, and establish up to what size the giant piezoelectric effects remain significant."

Espinosa and Agrawal hope their work will spur new interest in the electromechanical properties of nanostructures, both from theoretical and experimental standpoints, in order to clear the path for the design and optimization of future nanoscale devices.

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Atomically thin 'switch' makes for smarter electronic devices in the future

(PhysOrg.com) -- A new transistor made from graphene - the world's thinnest material - has been developed by a research team at the University of Southampton.

The new transistor achieves a record high-switching performance which will make our futureelectronic devices- such as PDAs and computers - even more functional and high-performance.

In a paper published inElectronics Letters, Dr Zakaria Moktadir of the Nano research group at the University describes how his research into graphene, a material made from a single atomic layer of carbon, arranged in a two-dimensional honeycomb structure, led to the development of graphenefield effect transistors(GFETs) with a unique channel structure atnanoscale.

According to Dr Moktadir, in the context of electronics, graphene could potentially replace or at least be used side by side with silicon integrations.

"Silicon CMOS downscaling is reaching its limits and we need to find a suitable alternative,"he says.

"Other researchers had looked at graphene as a possibility, but found that one of the drawbacks was that graphene's intrinsic physical properties make it difficult to turn off the current flow."

Dr Moktadir discovered that by introducing geometrical singularities (such as sharp bends and corners) in bilayer graphene nanowires, the current could be turned off efficiently.

According to Professor Hiroshi Mizuta, Head of the Nano group, this engineering approach has achieved an on/off switching ratio 1,000 times higher than previous attempts.

"Enormous effort has been made across the world to pinch off the channel of GFETs electrostatically, but the existing approaches require either the channel width to be much narrower than 10 nanometres or a very high voltage to be applied vertically across bilayer graphene layers,"he says.

"This hasn't achieved an on/off ratio which is high enough, and is not viable for practical use."

Dr Moktadir developed this transistor using the new heliumion beammicroscope and a focused gallium ion beam system in the Southampton Nanofabrication Centre, which has some of the best nanofabrication facilities in the world.

"This is a breakthrough in the ongoing quest to develop advanced transistors as we progress beyond our currentCMOStechnology,"says Professor Harvey Rutt, Head of Electronics and Computer Science.

"It will have major implications for next generation computer, communication and electronic systems. Introducing geometrical singularities into the graphene channel is a new concept which achieves superior performance while keeping the GFET structure simple and therefore commercially exploitable."

Having created the transistor, Dr Moktadir is now undertaking further research to understand the mechanism which causes the current to stop flowing in the channel, testing its reliability and performance under various noise and temperature conditions.

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Creating a pure spin current in graphene

(PhysOrg.com) -- Graphene is a material that has the potential for a number of future applications. Scientists are interested in using graphene for quantum computing and also as a replacement for electronics. However, in order to realize these graphene applications, a solid understanding of how spin current works in graphene is important.

One of the goals is to achieve pure spin current in graphene.“Pure spin current is a spin current with zero charge current, which means electrons with different spins travel toward opposite directions,” K S Chan tellsPhysOrg.comvia email. Chan is a professor at the City University of Hong Kong. Working with Zijing Lin, a professor at the University of Science and Technology of China in Hefei, and, Qingtian Zhang, a student of the CityU-USTC joint Ph.D. program, Chan studied adiabatic pumping in graphene as a way to generate spin current. Their work is published inApplied Physics Letters:“Spin current generation by adiabatic pumping in monolayer graphene.”

“Spin current is an important tool of studying spins in graphene,” Chan explains.“With spin current, you can create polarization in a particular region, and you can study the behavior of the spin in that particular region.” Chan points out that spin current is important in the development of a graphene quantum computer. Additionally, he points out that graphene is the material of choice for spintronics, which some hope will be able to replace electronics.

“Spintronic devices are believed to be faster and consume less power than electronic devices,” Chan continues. Understanding how spin works in graphene could be an important part of making a breakthrough in spintronics. Chan and his colleagues use a method called adiabatic quantum pumping to generate spin current for study.

Chan describes the technique:“{Adiabatic pumping} is a quantum phenomenon in which a DC current is generated without a DC voltage. Two AC voltages are applied to the graphene and a DC charge current can be generated through adiabatic quantum pumping. Adiabatic means the rates of change of the voltages are very slow in comparison with the speed at which the electrons travel through the graphene structure.”

On top of that, the team created asymmetry between electrons with different spin using the ferromagnetic proximity effect.“A ferromagnetic thin film is deposited on graphene. Electrons with different spins under the ferromagnetic layer will have different energies and therefore respond differently to adiabatic pumping,” Chan says. As a result of these different responses, pure spin current is generated, with different spins traveling in opposite directions.“What is so special about the present method is that a pure spin current can be generated at some Fermi energy without an external magnetic field, which is important for making nanosized devices.”

Fundamentally, the work done by Chan and his colleagues show that it is possible to generate pure spin current in graphene without magnetic field. This could lead to more practical applications inquantum computingand perhaps, later, spintronics. The next step, though, is to learn how spin current can be detected.

“Spin current is difficult to detect,” Chan explains.“It’s not like the charge current which can be easily measured by a voltmeter.” He admits that there are other important issues that need to be studied regarding spin in graphene, but Chan points this out:“To develop graphene spintronic devices, we need to know how to measure the spin current ingraphene.”

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Physics team reports gains in thermoelectric performance for bulk semiconductor material

(PhysOrg.com) -- Researchers from Boston College, MIT, Clemson and Virginia have used nanotechnology to achieve a 60-90 percent increase in the thermoelectric figure of merit of p-type half-Heusler, a common bulk semiconductor compound, the team reported in the American Chemical Society journal<i>Nano Letters</i>.

The dramatic increase in the figure of merit, used to measure a material's relative thermoelectric performance, could pave the way for a new generation of products– from car exhaust systems and power plants to solar power technology– that that runs cleaner, according to co-author Yan Xiao, a researcher in the Department of Physics at Boston College.

The team registered improvement in half-Heusler, which has been under study for its thermal stability, mechanical sturdiness, non-toxicity and low cost. However, the application of half-Heusler has been limited because of its poor thermoelectric performance: it previously registered a peak figure of merit of approximately 0.5 at 700 degrees Celsius for bulk ingots.

Xiao, working with BC Professor of Physics Zhifeng Ren and MIT's Soderberg Professor of Power Engineering Gang Chen, said the team produced an increase in the figure of merit value of p-type half-Heusler to 0.8 at 700 degrees Celsius. Moreover, the groups' material preparation methods proved to save time and expense compared with conventional methods.

"This method is low cost and can be scaled for mass production,"Ren said."This represents an exciting opportunity to improve the performance of thermoelectric materials in a cost-effective manner."

The researchers obtained their results by first forming alloyed ingots using arc melting technique and then creating nanoscale powders by ball milling the ingots and finally obtaining dense bulk by hot pressing. Transport property measurements together with microstructure studies on the nanostructured samples, in comparison with that of bulk ingots, showed that the thermoelectric performance improves largely because of lowthermal conductivityproduced by enhanced phonon scattering at grain boundaries and defects in the material. The material was also found to have a high Seebeck coefficient, a measure of thermoelectric power.

Researchers in the BC and MIT labs are still trying to prevent grain growth during press, which accounts for the still large thermal conductivity of half-Heusler.

"Even lower thermal conductivity and improved thermoelectric performance can be expected when average grain sizes are made smaller than 100 nm,"said Ren, who was joined on the team by fellow Boston College researchers Giri Joshi, Weishu Liu, Yucheng Lan and Hui Wang, MIT's Sangyeop Lee, Virginia's Rogers Professor of Physics Joe Poons and J.W. Simonson and Clemson Professor of Physics Terry M. Tritt.

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New material enables 'information sorting' at the speed of light

(PhysOrg.com) -- An international team of scientists led by King’s College London has taken a step closer towards developing optical components for super-fast computers and high-speed internet services of the future. This has the potential to revolutionise data processing speeds by transmitting information via light beams rather than electric currents.

The researchers are studying the science of‘nanoplasmonic devices’ whose key components are tiny nanoscale metal structures, more than 1000 times smaller than the size of a human hair, that guide and direct light.

Information is routinely sorted and directed in different directions to allow computing, internet connections or telephone conversations to take place. At present, however, computers process information by encoding it in electric signals.

It would be much faster to process and transmit information in the form of light instead of electric signals, but until now, it has been difficult for the light beams to be‘changed’, that is to interact with other beams of light, while travelling through a material, and this has held up progress.

The scientists have solved this by designing a new artificial material, which allows light beams to interact efficiently and change intensity, therefore allowing information to be sorted by beams of light at very high speeds. The structure of the tailor-made material is similar to a stack of nanoscale rods, along which light can travel and, most importantly, interact.

Professor Anatoly Zayats, in the Department of Physics at King’s, explains:‘If we were able to control a flow of light in the same way as we control a flow of electrons in computer chips, a new generation of data processing machines could be built, which would be capable of dealing with huge amounts of information much faster than modern computers.

‘The new material we have developed, often called‘metamaterial’, could be incorporated into existing electronic chips to improve their performance, or used to build completely new all-optical chips and therefore revolutionise data processing speeds.

‘While there are many challenges to overcome, we would anticipate that in the future this faster technology could be in our PCs, mobile phones, aeroplanes and cars, for example.’

Other members of the team involved in this latest research include Argonne National Laboratory in the USA; University of North Florida; University of Massachusetts at Lowell; and Queen’s University of Belfast in the UK.

The research is published in the journalNature Nanotechnology.

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New inexpensive way to grow silicon microwires for sensors, batteries and solar cells

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Microwires made of silicon -- tiny wires with a thickness comparable to a human hair— have a wide range of possible uses, including the production of solar cells that can harvest much more sunlight for a given amount of material than a conventional solar cell made from a thin wafer of silicon crystal. Now researchers from MIT and Penn State have found a way of producing such wires in quantity in a highly controlled way that could be scaled up to an industrial-scale process, potentially leading to practical commercial applications.

Other ways of making such wires are already known, and prototypes of solar cells made from them have been produced by several researchers. But these methods have serious limitations, says Tonio Buonassisi, MIT professor of mechanical engineering and a co-author of a paper on the new work that was recentlypublished onlinein the journalSmall, and will soon appear in the print edition. Most require several extra manufacturing steps, provide little control over the exact sizes and spacing of the wires, and only work on flat surfaces. By contrast, the new process is simple yet allows precise control over the wire dimensions and spacing, and could theoretically be done on any kind of curved, 3-D surface.

Microwires are thought to be capable of reaching efficiencies close to those of conventional solar cells in converting sunlight to electricity, but because the wires are so tiny they would do so using only a small fraction of the amount of expensivesiliconneeded for the conventional cells, thus potentially achieving major reductions in cost.

In addition to microwires’ potential use in solar cells, other researchers have proposed ways such microscopic wires could be used to build new kinds of transistors and integrated circuits, as well as electrodes for advanced batteries and certain kinds of environmental monitoring devices. For any of these ideas to be practical, however, there must be an efficient, scalable manufacturing method.

The new method involves heating and intentionally contaminating the surface of a silicon wafer with copper, which diffuses into the silicon. Then, when the silicon slowly cools, the copper diffuses out to form droplets on the surface. Then, when it is placed in an atmosphere of silicon tetrachloride gas, silicon microwires begin to grow outward wherever there is a copper droplet on the surface. Silicon in the gas dissolves into these copper droplets, and then after reaching a sufficient concentration begins to precipitate out at the bottom of the droplet, onto the silicon surface below. This buildup of silicon gradually elongates to form microwires each only about 10 to 20 micrometers (millionths of a meter) across, growing up from the surface. The whole process can be carried out repeatedly on an industrial manufacturing scale, Buonassisi says, or even could potentially be adapted to a continuous process.

The spacing of the wires is controlled by textures created on the surface— tiny dimples can form centers for the copper droplets— but the size of the wires is controlled by the temperatures used for the diffusion stage of the process. Thus, unlike in other production methods, the size and spacing of the wires can be controlled independently of each other, Buonassisi says.

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This new technique for growing microwires can produce strands that are very long in relation to their diameter. The rounded“cap” at the wire’s top is a droplet of molten copper, while the wire itself is pure silicon. Image courtesy of Tonio Buonassisi

The work done so far is just a proof of principle, he says, and more work remains to be done to find the best combinations of temperature profiles, copper concentrations and surface patterning for various applications, since the process allows for orders-of-magnitude differences in the size of the wires. For example, it remains to be determined what thickness and spacing of wires produces the most efficient solar cells. But this work demonstrates a potential for a kind of solar cell based on such wires that could significantly lower costs, both by allowing the use of lower grades of silicon (that is, less-highly refined), since the process of wire growth helps to purify the material, and by using much smaller amounts of it, since the tiny wires are made up of just a tiny fraction of the amount needed for conventional silicon crystal wafers.“This is still in a very early stage,” Buonassisi says, because in deciding on a configuration for such a solar cell“there are so many things to optimize.”

Michael Kelzenberg, a postdoctoral scholar at the California Institute of Technology who has spent the last five years doing research on silicon microwires, says that while others have used the copper-droplet technique for growing microwires,“What's really new here is the method of producing those liquid metal droplets.” While others have had to place the droplets of molten copper on the silicon plate, requiring extra processing steps,“Buonassisi and his colleagues have shown that metal can be diffused into the growth substrate beforehand, and through careful heating and cooling, the metal droplets will actually form on their own— with the correct position and size.”

Kelzenberg adds that his research group has recently demonstrated that silicon microwire solar cells can equal the efficiency of today’s typical commercialsolar cells.“I think the greatest challenge remaining is to show that this technique is more cost-effective or otherwise beneficial than other catalyst metal production methods,” he says. But overall, he says, some version of silicon microwire technology“has the potential to enable dramatic cost reductions” of solar panels.

The paper was co-authored by Vidya Ganapati’10, doctoral student David Fenning, postdoctoral fellow Mariana Bertoni, and research specialist Alexandria Fecych, all in MIT’s Department of Mechanical Engineering, and postdoctoral researcher Chito Kendrick and Professor Joan Redwing of Pennsylvania State University. The work was supported by the U.S. Department of Energy, the Chesonis Family Foundation and the National Science Foundation.

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This story is republished courtesy of MIT News (http://web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.

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DNA: Building block for smaller, smarter electronics?

(PhysOrg.com) -- Using a concept called DNA origami, Arizona State University researchers are trying to pave the way to produce the next generations of electronics products.

They’re pursuing advances innanotechnologythat have the potential to enable creation of smaller components for consumer and industrial electronics such as iPods, iPads and similar devices.

Manufacturers want to make the devices smaller and“smarter.” The problem is that this requires making the internal electrical parts of such devices at an even smaller nanometer scale, while also increasing the ability of the components to perform an array of computing, communication and multimedia functions.

Making these components smaller would become vastly more expensive using the current method of manufacturing microelectronic components such as the central processing units (CPUs) of all computers.

ASU’s Hongbin Yu and Hao Yan are teaming up to develop the basis of a new manufacturing method that would keep costs down.

Yu is an assistant professor in the School of Electrical, Computer, and Energy Engineering, one of ASU’s Ira A. Fulton Schools of Engineering. Yan is a professor in the Department of Chemistry and Biochemistry in ASU’s College of Liberal Arts and Sciences.

Details of their progress have recently been reported inNano Letters, a leading nanoscience and technology journal published by the American Chemical Society. The news has also beenfeatured on Chemistry World, a science and technology news website of the Royal Society of Chemistry, the leading European organization for advancing chemical sciences.

Yu explains that he and Yan are exploring“how to use top-down lithography combined with modified bottom-up self-assembling nanostructures to guide the placement of nanostructures on silicon wafer surface.”

Top-down lithography is a process by which electrical circuit elements on a silicon wafer are constructed by cutting and etching, in a way similar to how sculptures are made. This is how today’s computer chips are manufactured.

Bottom-up self-assembly is a process in which molecules and/or nanoscale materials are self-assembled into desired structures using chemical bonds or various similar interactions.

Yu and Yan have discovered a way to useDNAto effectively combine top-down lithography with chemical bonding involving bottom-up self-assembly.

This involves a“DNA origami” design technique similar to the traditional Japanese art or technique of folding paper into decorative or representational forms. It allows DNA strands to be folded into something resembling a pegboard on which different molecules can be attached.

Enabling various molecules to attach to the DNA produces smaller nanostructure configurations– thus opening the way to construction of smaller electronic device components.

In the past it has proven difficult to combine top-down lithography with bottom-up self-assembly because the DNA nanostructures required to make it happen would bind indiscriminately to the silicon platform (called a substrate)– the material on which an electronic circuit is fabricated.

“There have been few successful demonstrations of how to put these bottom-up assembled nanostructures on the surface of the substrate where you want them to be,” Yu explains,“because you cannot just run these devices, you need to know where to connect what.”

To solve the problem, Yu’s research team prefabricated a gold“nano-island” at specific locations on a silicon substrate, then applied the DNA origami that has specific chemical ends that will bond only to the gold island and not the silicon wafer. This allows the DNA nanotubes to attach only to the islands.

The work demonstrates that it’s possible that a DNA double helix can be used to build one-dimensional and two-dimensional structures to enable the manufacture of smaller electronic memory devices– at a cost that would be far less than current manufacturing methods.

More progress is needed, Yu says.

“With this demonstration we were able to build patterns on surface that consist of only one-dimensional DNA nanotubes, but our research shows it is possible to produce two-dimensional and even more sophisticated structures that are essential building blocks for nanoscale electronic circuits,” Yu says.“So this is just the beginning of many fascinating possibilities to be realized.”

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