Measuring The Immeasurable: Bond Strength Of Materials Linked To Heat Transfer

ScienceDaily (Apr. 13, 2009) — The speed at which heat moves between two materials touching each other is a potent indicator of how strongly they are bonded to each other, according to a new study by researchers at Rensselaer Polytechnic Institute.

Researchers at Rensselaer Polytechnic Institute have discovered there is a strong correlation between the speed at which heat moves between two touching materials and how strongly those materials are bonded together. The study shows that this flow of heat from one material to another can be dramatically altered by "painting" a thin atomic layer between materials. Changing the interface fundamentally alters the way the materials interact. (Credit: Rensselaer/Rahul Godawat)

Additionally, the study shows that this flow of heat from one material to another, in this case one solid and one liquid, can be dramatically altered by "painting" a thin atomic layer between materials. Changing the interface fundamentally changes the way the materials interact.

"If you have a nanoparticle that is inside a liquid solution, you can't just 'peel away' the liquid to measure how strongly it is bonded to the surrounding molecules," said Pawel Keblinski, professor in Rensselaer's Department of Materials Science and Engineering, who co-led the study. "Instead, we show that you can measure the strength of these bonds simply by measuring the rate of heat flow from the nanoparticle to the surrounding liquid."

"Interfaces are an exciting new frontier for doing fundamental studies of this type. If you peek into complex biological systems – a cell, for example – they contain a high density of interfaces, between different proteins or between protein and water," said Shekhar Garde, the Elaine and Jack S. Parker Professor and head of Rensselaer's Department of Chemical and Biological Engineering, who co-led the study with Keblinski. "Our approach possibly provides another handle to quantify how proteins talk to each other or with the surrounding water."

Results of the study, titled "How wetting and adhesion affect thermal conductance of a range of hydrophobic to hydrophilic aqueous solutions," were published April 13 in Physical Review Letters.

Keblinski and Garde used extensive molecular dynamics simulations to measure the heat flow between a variety of solid surfaces and water. They simulated a broad range of surface chemistries and showed that thermal conductance, or how fast heat is transferred between a liquid and a solid, is directly proportional to how strongly the liquid adhered to the solid.

"In the case of a mercury thermometer, thermal expansion correlates directly with temperature," Keblinski said. "What we have done, in a sense, is create a new thermometer to measure the interfacial bonding properties between liquids and solids."

"We can use this new technique to characterize systems that are very difficult or impossible to characterize by other means," Garde said.

This fundamental discovery, which helps to better understand how water sticks to or flows past a surface, has implications for many different heat transfer applications and processes including boiling and condensation. Of particular interest is how this discovery can benefit new systems for cooling and displacing heat from computer chips, a critical issue currently facing the semiconductor industry, Garde said.

More generally, the authors said the study sheds new light on the behavior of water at various solid interfaces, which has direct implications ranging from the binding of proteins and other molecules to surfaces, to biological self-assembly in interfacial environments.

Co-authors of the paper include materials science and engineering graduate student Natalia Shenogina, along with chemical and biological engineering graduate student Rahul Godawat.

Financial support for this project was provided by the U.S. National Science Foundation Nanoscale Science and Engineering Center Grant, in addition to support from U.S. Air Force Office of Scientific Research Multidisciplinary University Research Initiative.

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Adapted from materials provided by Rensselaer Polytechnic Institute.

Peering Into Nanowires To Measure Dopant Properties

ScienceDaily (Apr. 13, 2009) — Semiconductor nanowires — tiny wires with a diameter as small as a few billionths of a meter — hold promise for devices of the future, both in technology like light-emitting diodes and in new versions of transistors and circuits for next generation of electronics.

Atom-by-atom mapping of a germanium nanowire by atom probe tomography. Left: 3D reconstruction of an individual Ge nanowire with each green sphere representing an individual Ge atom. The dimensions are 50x50x100 nm3. The region enclosed by the red box is displayed at upper right, with single atomic planes visible in the center of the image. The grey spheres are phosphorous dopant atoms used to control the conductivity. (The dimensions are 5x25x15 nm3). The region enclosed by the blue box is displayed in the lower right, revealing an inhomogeneous distribution of phosphorous atoms. (The dimensions are 50x50x10 nm3). The 'shell' of enhanced doping results from surface reactions during growth of the nanowire. (Credit: Image courtesy of Northwestern University)

But in order to utilize the novel properties of nanowires, their composition must be precisely controlled, and researchers must better understand just exactly how the composition is determined by the synthesis conditions.

Nanowires are synthesized from elements that form bulk semiconductors, whose electrical properties are in turn controlled by adding minute amounts of impurities called dopants. The amount of dopant determines the conductivity of the nanowire.

But because nanowires are so small — with diameters ranging from 3 to 100 nanometers — researchers have never been able to see just exactly how much of the dopant gets into the nanowire during synthesis. Now, using a technique called atom probe tomography, Lincoln Lauhon, assistant professor of materials science and engineering at Northwestern University’s McCormick School of Engineering and Applied Science, has provided an atomic-level view of the composition of a nanowire. By precisely measuring the amount of dopant in a nanowire, researchers can finally understand the synthesis process on a quantitative level and better predict the electronic properties of nanowire devices.

The results were published online March 29 in the journal Nature Nanotechnology.

“We simply mapped where all the atoms were in a single nanowire, and from the map we determined where the dopant atoms were,” he says. “The more dopant atoms you have, the higher the conductivity.”

Previously, researchers could not measure the amount of dopant and had to judge the success of the synthesis based on indirect measurements of the conductivity of nanowire devices. That meant that variations in device performance were not readily explained.

“If we can understand the origin of the electrical properties of nanowires, and if we can rationally control the conductivity, then we can specify how a nanowire will perform in any type of device,” he says. “This fundamental scientific understanding establishes a basis for engineering.”

Lauhon and his group performed the research at Northwestern’s Center for Atom Probe Tomography, which uses a Local Electrode Atom ProbeTM microscope to dissect single nanowires and identify their constituents. This instrumentation software allows 3-D images of the nanowire to be generated, so Lauhon could see from all angles just how the dopant atoms were distributed within the nanowire.

In addition to measuring the dopant in the nanowire, Lauhon’s colleague, Peter Voorhees, Frank C. Engelhart Professor of Materials Science and Engineering at Northwestern, created a model that relates the nanowire doping level to the conditions during the nanowire synthesis. The researchers performed the experiment using germanium wires and phosphorous dopants — and they will soon publish results using silicon — but the model provides guidance for nanowires made from other elements, as well.

“This model uses insight from Lincoln’s experiment to show what might happen in other systems,” Voorhees says. “If nanowires are going to be used in device applications, this model will provide guidance as to the conditions that will enable us to add these elements and control the doping concentrations.”

Both professors will continue working on this research to broaden the model.

“We would like to establish the general principles for doping semiconductor nanowires,” Lauhon says.

In addition to Lauhon and Voorhees, the other authors are Daniel E. Perea, Eric R. Hemesath, Edwin J. Schwalbach, and Jessica L. Lensch-Falk, all from Northwestern.

The research was supported by the Office of Naval Research and the National Science Foundation.

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Journal reference:

1. Perea et al. Direct measurement of dopant distribution in an individual vapour–liquid–solid nanowire. Nature Nanotechnology, 2009; DOI: 10.1038/nnano.2009.51

Adapted from materials provided by Northwestern University.

Nano Changes Rise To Macro Importance In A Key Electronics Material

ScienceDaily (Apr. 14, 2009) — By combining the results of a number of powerful techniques for studying material structure at the nanoscale, a team of researchers from the National Institute of Standards and Technology (NIST), working with colleagues in other federal labs and abroad, believe they have settled a long-standing debate over the source of the unique electronic properties of a material with potentially great importance for wireless communications.

The new study of silver niobate not only opens the door to engineering improved electronic components for smaller, higher performance wireless devices, but also serves as an example of understanding how subtle nanoscale features of a material can give rise to major changes in its physical properties.

Silver niobate is a ceramic dielectric, a class of materials used to make capacitors, filters and other basic components of wireless communications equipment and other high-frequency electronic devices. A useful dielectric needs to have a large dielectric constant—roughly, a measure of the material’s ability to hold an electric charge—that is stable in the operating temperature range. The material also should have low dielectric losses—which means that it does not waste energy as heat and preserves much of its intended signal strength. In the important gigahertz range of the radio spectrum—used for a wide variety of wireless applications—silver niobate-based ceramics are the only materials known that combine a high, temperature-stable dielectric constant with sufficiently low dielectric losses.

It’s been known for some time that silver niobate’s unique dielectric properties are temperature dependent—the dielectric constant peaks in a broad range near room temperature in these ceramics, which makes them suitable for practical applications. Earlier studies were unable to identify the structural basis of the unusual dielectric response because no accompanying changes in the overall crystal structure could be observed. “The crystal symmetry doesn’t seem to change at those temperatures,” explains NIST materials scientist Igor Levin, “but that’s because people were using standard techniques that tell you the average structure. The important changes happen at the nanoscale and are lost in averages.”

Only in recent years, says Levin, have the specialized instruments and analytic techniques been available to probe nanoscale structural changes in crystals. Even so, he says, “these subtle deviations from the average are so small that any single measurement gives only partial information on the structure. You need to combine several complementary techniques that look at different angles of the problem.” Working at different facilities* the team combined results from several high-resolution probes using X-rays, neutrons and electrons—tools that are sensitive to both the local and average crystal structure— to understand silver niobate’s dielectric properties. The results revealed an intricate interplay between the oxygen atoms, arranged in an octahedral pattern that defines the compound’s crystal structure, and the niobium atoms at the centers of the octahedra.

At high temperatures, the niobium atoms are slightly displaced, but their average position remains in the center—so the shift isn’t seen in averaging measurements. As the compound cools, the oxygen atoms cooperate by moving a little, causing the octahedral structure to rotate slightly. This movement generates strain which “locks” the niobium atoms into off-centered positions—but not completely. The resulting partial disorder of the niobium atoms gives rise to the dielectric properties. The results, the researchers say, point to potential avenues for engineering similar properties in other compounds.

The work was supported in part by the U.S. Department of Energy and the U.K. Science and Technology Facilities Council.

* The study required measurements at the Advanced Photon Source at Argonne National Laboratory, the Lujan Neutron Center at Los Alamos National Laboratory and the ISIS Pulsed Neutron and Muon Source at Rutherford Appleton Laboratory (United Kingdom). In addition to NIST, researchers from Argonne, Los Alamos, ISIS and the University of Sheffield contributed to the paper.

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Journal reference:

1. I. Levin, V. Krayzman, J.C. Woicik, J. Karapetrova, T. Proffen, M.G. Tucker and I.M. Reaney. Structural changes underlying the diffuse dielectric response in AgNbO3. Phys. Rev. B, 79, 104113, online March 26, 2009

Adapted from materials provided by National Institute of Standards and Technology.

New Security And Medical Sensor Devices Made Possible By Fundamental Physics Development In Metallic Nanostructures

ScienceDaily (Apr. 13, 2009) — Scientists have designed tiny new sensor structures that could be used in novel security devices to detect poisons and explosives, or in highly sensitive medical sensors, according to research published tomorrow (8 April) in Nano Letters.

An image of the metallic ring and disk. The scale bar shows 200 nanometres. (Credit: Image courtesy of Imperial College London)

The new ‘nanosensors’, which are based on a fundamental science discovery in UK, Belgian and US research groups, could be tailor-made to instantly detect the presence of particular molecules, for example poisons or explosives in transport screening situations, or proteins in patients’ blood samples, with high sensitivity.

The researchers were led by Imperial College London physicists funded by the Engineering and Physical Sciences Research Council. The team showed that by putting together two specific ‘nanostructures’ made of gold or silver, they can make an early prototype device which, once optimised, should exhibit a highly sensitive ability to detect particular chemicals in the immediate surroundings.

The nanostructures are each about 500 times smaller than the width of a human hair. One is shaped like a flat circular disk while the other looks like a doughnut with a hole in the middle. When brought together they interact with light very differently to the way they behave on their own. The scientists have observed that when they are paired up they scatter some specific colours within white light much less, leading to an increased amount of light passing through the structure undisturbed. This is distinctly different to how both structures scatter light separately. This decrease in the interaction with light is in turn affected by the composition of molecules in close proximity to the structures. The researchers hope that this effect can be harnessed to produce sensor devices.

Lead researcher on the project Professor Stefan Maier from Imperial’s Department of Physics, and an Associate of Imperial’s Institute for Security Science and Technology, said:

"Pairing up these structures has a unique effect on the way they scatter light

– an effect which could be very useful if, as our computer simulations suggest, it is extremely sensitive to changes in surrounding environment. With further testing we hope to show that it is possible to harness this property to make a highly sensitive nanosensor."

Metal nanostructures have been used as sensors before, as they interact very strongly with light due to so-called localised plasmon resonances. But this is the first time a pair with such a carefully tailored interaction with light has been created.

The device could be tailored to detect different chemicals by decorating the nanostructure surface with specific ‘molecular traps’ that bind the chosen target molecules. Once bound, the target molecules would change the colours that the device absorbs and scatters, alerting the sensor to their presence. The team’s next step is to test whether the pair of nanostructures can detect chosen substances in lab experiments.

Professor Maier concludes: "This study is a beautiful example of how concepts from different areas of physics fertilise each other – in essence our nanosensor system is a classical analogue of electromagnetically induced transparency, a famous phenomenon from quantum mechanics."

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Journal reference:

1. Verellen et al. Fano Resonances in Individual Coherent Plasmonic Nanocavities. Nano Letters, 2009; 090312170407019 DOI: 10.1021/nl9001876

Adapted from materials provided by Imperial College London.

'Powerhouses' From Living Cells -- Mitochondria -- Power New Explosives Detector

ScienceDaily (Nov. 22, 2008) — Researchers in Missouri have borrowed the technology that living cells use to produce energy to develop a tiny, self-powered sensor for rapid detection of hidden explosives. The experimental sensor, about the size of a postage stamp, represents the first of its kind to be powered by mitochondria, the microscopic "powerhouses" that provide energy to living cells, the researchers say.

Scientists have developed a stamp-sized sensor for detecting hidden explosives. The sensor is powered by mitochondria, which provide energy to living cells. (Credit: American Chemical Society)

In the new study, Shelley Minteer, Marguerite Germain, and Robert Arechederra point out that today's explosives detectors are expensive, bulky, and complex. Society needs smaller, cheaper, simpler detection devices, based on technology that perhaps could be incorporated into cell phones and portable digital music players, the researchers suggest.

The scientists describe development of an experimental sensor built from a special biofuel cell, essentially a battery-like device consisting of a thin layer of mitochondria sandwiched between a carbon-based electrode and a gas-permeable electrode. In laboratory studies using nitrobenzene as a test compound, the sensor showed a significant boost in electrical power in the presence of the substance, demonstrating the sensor's potential for detecting TNT and related explosives, the researchers say.

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Journal reference:

1. Germain et al. Nitroaromatic Actuation of Mitochondrial Bioelectrocatalysis for Self-Powered Explosive Sensors. Journal of the American Chemical Society, 2008; 130 (46): 15272 DOI: 10.1021/ja807250b

Adapted from materials provided by American Chemical Society.

Fitter Frames: Nanotubes Boost Structural Integrity Of Composites

ScienceDaily (Apr. 3, 2009) — A new research discovery at Rensselaer Polytechnic Institute could lead to tougher, more durable composite frames for aircraft, watercraft, and automobiles.

Researchers at Rensselaer have discovered a new technique for provoking unusual crazing behavior in epoxy composites. The crazing, which causes the composite to deform into a network of nanoscale pillar-like fibers that bridge together both sides of a crack and slow its growth, could lead to tougher, more durable components for aircraft and automobiles. (Credit: Image courtesy of Rensselaer Polytechnic Institute)

Epoxy composites are increasingly being incorporated into the design of new jets, planes, and other vehicles. Composite material frames are extremely lightweight, which lowers the overall weight of the vehicle and boosts fuel efficiency. The downside is that epoxy composites can be brittle, which is detrimental to its structural integrity.

Professor Nikhil Koratkar, of Rensselaer’s Department of Mechanical, Aerospace, and Nuclear Engineering, has demonstrated that incorporating chemically treated carbon nanotubes into an epoxy composite can significantly improve the overall toughness, fatigue resistance, and durability of a composite frame.

When subjected to repetitive stress, a composite frame infused with treated nanotubes exhibited a five-fold reduction in crack growth rate as compared to a frame infused with untreated nanotubes, and a 20-fold reduction when compared to a composite frame made without nanotubes.

This newfound toughness and crack resistance is due to the treated nanotubes, which enhance the molecular mobility of the epoxy at the interface where the two materials touch. When stressed, this enhanced mobility enables the epoxy to craze – or result in the formation of a network of pillar-like fibers that bridge together both sides of the crack and slow its growth.

“This crazing behavior, and the bridging fibers it produces, dramatically slows the growth rate of a crack,” Koratkar said. “In order for the crack to grow, those fibers have to first stretch, deform plastically, and then break. It takes a lot of energy to stretch and break those fibers, energy that would have otherwise gone toward enlarging the crack.”

Results of the study were just published in the journal Small.

Epoxy composites infused with carbon nanotubes are known to be more resistant to cracks than pure epoxy composites, as the nanotubes stitch, or bridge, the two sides of the crack together. Infusing an epoxy with carbon nanotubes that have been functionalized, or treated, with the chemical group amidoamine, however, results in a completely different bridging phenomenon.

At the interface of the functionalized nanotubes and the epoxy, the epoxy starts to craze, which is a highly unusual behavior for this particular type of composite, Koratkar said. The epoxy deforms, becomes more fluid, and creates connective fibers up to 10 microns in length and with a diameter between 100 nanometers and 1,000 nanometers.

“We didn’t expect this at all. Crazing is common in certain types of thermoplastic polymers, but very unusual in the type of epoxy composite we used,” Koratkar said. “In addition to improved fatigue resistance and toughness, the treated nanotubes also enhanced the stiffness, hardness, and strength of the epoxy composite, which is very important for structural applications.”

Koratkar said the aircraft, boat, and automobile industries are increasingly looking to composites as a building material to make vehicle frames and components lighter. His research group plans to further investigate crazing behavior in epoxy composites, in order to better understand why the chemical treatment of nanotubes initiates crazing.

Co-authors of the paper include Rensselaer Associate Professor Catalin Picu, of the Department of Mechanical, Aerospace, and Nuclear Engineering; Rensselaer doctoral students Wei Zhang and Iti Srivastava; and Yue-Feng Zhu, professor in the Department of Mechanical Engineering at Tsinghua University in China.

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Adapted from materials provided by Rensselaer Polytechnic Institute.

ScienceDaily: Your source for the latest research news and science breakthroughs -- updated daily Science News Share Blog Cite Print Email Bo

ScienceDaily (Oct. 19, 2007) — Researchers in California report development of the world's first working radio system that receives radio waves wirelessly and converts them to sound signals through a nano-sized detector made of carbon nanotubes.

The "carbon nanotube radio" device is thousands of times smaller than the diameter of a human hair. The development marks an important step in the evolution of nano-electronics and could lead to the production of the world's smallest radio, the scientists say.

Peter Burke and Chris Rutherglen developed a carbon nanotube "demodulator" that is capable of translating AM radio waves into sound. In a laboratory demonstration, the researchers incorporated the detector into a complete radio system and used it to successfully transmit classical music wirelessly from an iPod to a speaker several feet away from the music player.

Although other researchers have developed nano-sized radio wave detectors in the past, the current study marks the first time that a nano-sized detector has been demonstrated in an actual working radio system, the scientists say. The study demonstrates the feasibility of making other radio components at the nanoscale in the future and may eventually lead to a "truly integrated nanoscale wireless communications system," they say. Such a device could have numerous industrial, commercial, medical and other applications.

Their findings appeared online October 17 and are scheduled for publication in the Nov. 14 print edition of ACS' Nano Letters.

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Adapted from materials provided by American Chemical Society, via EurekAlert!, a service of AAAS.