Plasmonic device converts light into electricity

By Lisa Zyga

Surface plasmons on the top electrode in the MIM device can increase the current from the top electrode so that it is greater than the current from the bottom electrode, generating a positive net current. Image credit: Wang and Melosh. ©2011 American Chemical Society

While the most common device for converting light into electricity may be photovoltaic (PV) solar cells, a variety of other devices can perform the same light-to-electricity conversion, such as solar-thermal collectors and rectennas. In a new study, engineers have designed a new device that can convert light of infrared (IR) and visible wavelengths into direct current by using surface plasmon excitations in a simple metal-insulator-metal (MIM) device.

The researchers, Fuming Wang and Nicholas A. Melosh of Stanford University, have published their study on the new device in a recent issue of Nano Letters. “The greatest significance thus far is to show an alternative method to rectennas and PV devices for IR and visible light conversion,” Melosh told PhysOrg.com. “The conversion efficiencies aren't amazingly high compared to a PV in visible, so it’s not going to replace PVs, but it could be used for energy scavenging later on.” The new device’s MIM architecture is similar to that of a rectenna. However, whereas rectennas operate with long-wavelength light such as microwaves and radio waves, the new device operates with a broad spectrum of infrared to visible wavelengths. When the MIM device is illuminated, incoming photons are absorbed by the top and bottom metal electrodes. Upon absorption, each photon excites an electron in the metal into a higher energy state so that it becomes a “hot electron.” About half of the hot electrons travel toward the metal-insulator interface, where they may be collected by the other electrode. However, photon absorption in the upper and lower electrodes generates currents with opposite signs, so a net DC current is achieved only if the absorption is larger at one electrode than the other.

Electron transmission in MIM devices (a) with and (b) without surface plasmon excitations. (c) The measured photocurrent in a device with surface plasmons (black line) is higher than in a device without them (red line). Image credit: Wang and Melosh. ©2011 American Chemical Society

This ability to maximize current from one electrode while minimizing it from the other is one of the biggest challenges for MIM devices. To do this, researchers can change the thicknesses of the electrodes. However, there is a tradeoff, since in a thicker electrode, more photons are absorbed but fewer electrons reach the interface due to increased scattering. Wang and Melosh’s solution is to use a prism to excite surface plamons (SPs) on the metal surface of the electrodes when under illumination. The SPs, which are small electron oscillations, can create a higher concentration of hot electrons in one electrode by efficiently coupling to light. The SP coupling efficiency depends on several factors, such as the thickness of the electrode, the type of metal used, and the wavelength of incoming light.

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Plasmonic device converts light into electricity
More information: Fuming Wang and Nicholas A. Melosh. “Plasmonic Energy Collection through Hot Carrier Extraction.” Nano Letters, DOI: 10.1021/nl203196z

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NIST Demonstrates First Quantum 'Entanglement' of Ions Using Microwaves

Laura Ost @ NIST.GOV

Physicists at the National Institute of Standards and Technology (NIST) have, for the first time, linked the quantum properties of two separated ions (electrically charged atoms) by manipulating them with microwaves instead of the usual laser beams. The feat raises the possibility of replacing today's complex, room-sized quantum computing "laser parks" with miniaturized, commercial microwave technology similar to that used in smart phones.

Gold ion trap on aluminum nitride backing. In NIST microwave quantum computing experiments, two ions hover above the middle of the square gold trap, which measures 7.4 millimeters on a side. Scientists manipulate and entangle the ions using microwaves fed into wires on the trap from the three thick electrodes at the lower right. Credit: Y. Colombe/NIST View hi-resolution bottom image

Microwaves have been used in past experiments to manipulate single ions, but the NIST group is the first to position microwaves sources close enough to the ions—just 30 micrometers away—and create the conditions enabling entanglement, a quantum phenomenon expected to be crucial for transporting information and correcting errors in quantum computers.

Described in the August 11, 2011, issue of Nature,* the experiments integrate wiring for microwave sources directly on a chip-sized ion trap and use a desktop-scale table of lasers, mirrors and lenses that is only about one-tenth of the size previously required. Low-power ultraviolet lasers still are needed to cool the ions and observe experimental results but might eventually be made as small as those in portable DVD players. Compared to complex, expensive laser sources, microwave components could be expanded and upgraded more easily to build practical systems of thousands of ions for quantum computing and simulations.

"It's conceivable a modest-sized quantum computer could eventually look like a smart phone combined with a laser pointer-like device, while sophisticated machines might have an overall footprint comparable to a regular desktop PC," says NIST physicist Dietrich Leibfried, a co-author of the new paper.

Quantum computers would harness the unusual rules of quantum physics to solve certain problems—such as breaking today's most widely used data encryption codes—that are currently intractable even with supercomputers. A nearer-term goal is to design quantum simulations of important scientific problems, to explore quantum mysteries such as high-temperature superconductivity, the disappearance of electrical resistance in certain materials when sufficiently chilled.

Ions are a leading candidate for use as quantum bits (qubits) to hold information in a quantum computer. Although other promising candidates for qubits—notably superconducting circuits, or "artificial atoms"—are manipulated on chips with microwaves, ion qubits are at a more advanced stage experimentally in that more ions can be controlled with better accuracy and less loss of information.

The use of microwaves reduces errors introduced by instabilities in laser beam pointing and power as well as laser-induced spontaneous emissions by the ions. However, microwave operations need to be improved to enable practical quantum computations or simulations. The NIST researchers achieved entanglement 76 percent of the time, well above the minimum threshold of 50 percent defining the onset of quantum properties but not yet competitive with the best laser-controlled operations at 99.3 percent.

The research was supported by the Intelligence Advanced Research Projects Activity, Office of Naval Research, Defense Advanced Research Projects Agency, National Security Agency and Sandia National Laboratories.

For more details, see the NIST  news announcement "NIST Physicists 'Entangle' Two Atoms Using Microwaves for the First Time" at www.nist.gov/pml/div688/microwave-quantum-081011.cfm.

 

New combination of nanoparticles and graphene results in a more durable catalytic material for fuel cells

Mary Beckman, PNNL [Source]

Bracing catalyst in material makes fuel cell component work better and last longer

A nanoparticle of indium tin oxide (green and red) braces platinum nanoparticles (blue) on the surface of graphene (black honeycomb) to make a hardier, more chemically active fuel cell material. A new combination of nanoparticles and graphene results in a more durable catalytic material for fuel cells, according to work published today online at the Journal of the American Chemical Society. The catalytic material is not only hardier but more chemically active as well. The researchers are confident the results will help improve fuel cell design.

"Fuel cells are an important area of energy technology, but cost and durability are big challenges," said chemist Jun Liu. "The unique structure of this material provides much needed stability, good electrical conductivity and other desired properties."

Liu and his colleagues at the Department of Energy's Pacific Northwest National Laboratory, Princeton University in Princeton, N.J., and Washington State University in Pullman, Wash., combined graphene, a one-atom-thick honeycomb of carbon with handy electrical and structural properties, with metal oxide nanoparticles to stabilize a fuel cell catalyst and make it better available to do its job.

"This material has great potential to make fuel cells cheaper and last longer," said catalytic chemist Yong Wang, who has a joint appointment with PNNL and WSU. "The work may also provide lessons for improving the performance of other carbon-based catalysts for a broad range of industrial applications."

Muscle Metal Oxide

Fuel cells work by chemically breaking down oxygen and hydrogen gases to create an electrical current, producing water and heat in the process. The centerpiece of the fuel cell is the chemical catalyst — usually a metal such as platinum — sitting on a support that is often made of carbon. A good supporting material spreads the platinum evenly over its surface to maximize the surface area with which it can attack gas molecules. It is also electrically conductive.

Fuel cell developers most commonly use black carbon — think pencil lead — but platinum atoms tend to clump on such carbon. In addition, water can degrade the carbon away. Another support option is metal oxides — think rust — but what metal oxides make up for in stability and catalyst dispersion, they lose in conductivity and ease of synthesis. Other researchers have begun to explore metal oxides in conjunction with carbon materials to get the best of both worlds.

As a carbon support, Liu and his colleagues thought graphene intriguing. The honeycomb lattice of graphene is porous, electrically conductive and affords a lot of room for platinum atoms to work. First, the team crystallized nanoparticles of the metal oxide known as indium tin oxide — or ITO — directly onto specially treated graphene. Then they added platinum nanoparticles to the graphene-ITO and tested the materials.

Platinumweight

The team viewed the materials under high-resolution microscopes at EMSL, DOE's Environmental Molecular Sciences Laboratory on the PNNL campus. The images showed that without ITO, platinum atoms clumped up on the graphene surface. But with ITO, the platinum spread out nicely. Those images also showed catalytic platinum wedged between the nanoparticles and the graphene surface, with the nanoparticles partially sitting on the platinum like a paperweight.

To see how stable this arrangement was, the team performed theoretical calculations of molecular interactions between the graphene, platinum and ITO. This number-crunching on EMSL's Chinook supercomputer showed that the threesome was more stable than the metal oxide alone on graphene or the catalyst alone on graphene.

But stability makes no difference if the catalyst doesn't work. In tests for how well the materials break down oxygen as they would in a fuel cell, the triple-threat packed about 40% more of a wallop than the catalyst alone on graphene or the catalyst alone on other carbon-based supports such as activated carbon.

Last, the team tested how well the new material stands up to repeated usage by artificially aging it. After aging, the tripartite material proved to be three times as durable as the lone catalyst on graphene and twice as durable as on commonly used activated carbon. Corrosion tests revealed that the triple threat was more resistant than the other materials tested as well.

The team is now incorporating the platinum-ITO-graphene material into experimental fuel cells to determine how well it works under real world conditions and how long it lasts.

---

Reference: Rong Kou, Yuyan Shao, Donghai Mei, Zimin Nie, Donghai Wang, Chongmin Wang, Vilayanur V Viswanathan, Sehkyu Park, Ilhan A. Aksay, Yuehe Lin, Yong Wang, Jun Liu, Stabilization of Electrocatalytic Metal Nanoparticles at Metal-Metal Oxide-Graphene Triple Junction Points, February 8, 2011, J. Am. Chem. Soc., DOI 10.1021/ja107719 (http://pubs.acs.org/doi/full/10.1021/ja107719u.

This work was supported by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy.

High-performance capacitor could lead to better rechargeable batteries

By Lisa Zyga @ physorg.com 

Abstract

Zeolite-templated carbon is a promising candidate as an electrode material for constructing an electric double layer capacitor with both high-power and high-energy densities, due to its three-dimensionally arrayed and mutually connected 1.2-nm nanopores. This carbon exhibits both very high gravimetric (140−190 F g⁻¹) and volumetric (75−83 F cm⁻³) capacitances in an organic electrolyte solution. Moreover, such a high capacitance can be well retained even at a very high current up to 20 A g⁻¹. This extraordinary high performance is attributed to the unique pore structure.

The unique 3D array of nanopores in zeolite-templated carbon enables it to be used as an electrode for high-performance supercapacitors that have a high capacitance and quick charge time. Image credit: Hiroyuki Itoi, et al. ©2011 American Chemical Society.

In order to develop next-generation electric vehicles, solar energy systems, and other clean energy technologies, researchers need an efficient way to store the energy. One of the key energy storage devices for these applications and others is a supercapacitor, also called an electric double-layer capacitor. In a recent study, scientists have investigated the possibility of using a material called zeolite-templated carbon for the electrode in this type of capacitor, and found that the material’s unique pore structure greatly improves the capacitor's overall performance.

To store energy, the electric double-layer capacitor is charged by ions that migrate from a bulk solution to an electrode, where they are adsorbed. Before reaching the electrode’s surface, the ions have to travel through narrow nanopores as quickly and efficiently as possible. Basically, the quicker the ions can travel down these paths, the quicker the capacitor can be charged, resulting in a high rate performance. Also, the greater the adsorbed ion density in the electrode, the greater the charge that the capacitor can store, resulting in a high volumetric capacitance.

Recently, scientists have been testing materials with pores of various sizes and structures to try to achieve both quick ion transport and high adsorption ion density. But the two requirements are somewhat contradictory, since ions can travel more quickly through larger nanopores, but large nanopores make the electrode density low and thus decrease the adsorbed ion density.

The zeolite-templated carbon consists of nanopores that are 1.2 nm in diameter (smaller than most electrode materials) and that have a very ordered structure (whereas other pores can be disordered and random). The nanopores’ small size makes the adsorbed ion density high, while the ordered structure – described as a diamond-like framework – allows the ions to quickly pass through the nanopores. In a previous study, the researchers showed that zeolite-templated carbon with nanopores smaller than 1.2 nm cannot enable fast ion transport, suggesting that this size may provide the optimal balance between high rate performance and high volumetric capacitance.

In tests, the zeolite-templated carbon’s properties exceeded those of other materials, demonstrating its potential to be used as an electrode for high-performance electric double-layer capacitors.

More information: Hiroyuki Itoi, et al. “Three-Dimensionally Arrayed and Mutually Connected 1.2-nm Nanopores for High-Performance Electric Double Layer Capacitor.” Journal of the American Chemical Society. DOI:10.1021/ja108315p

 

Metals Like Plastics: Meet the Supermaterial That Could Change Gadgets

ScienceDaily  — Imagine a material that's stronger than steel, but just as versatile as plastic, able to take on a seemingly endless variety of forms. For decades, materials scientists have been trying to come up with just such an ideal substance, one that could be molded into complex shapes with the same ease and low expense as plastic but without sacrificing the strength and durability of metal.

Jan Schroers and his team have developed novel metal alloys that can be blow molded into virtually any shape. (Credit: Image courtesy of Yale University)

Now a team led by Jan Schroers, a materials scientist at Yale University, has shown that some recently developed bulk metallic glasses (BMGs)-metal alloys that have randomly arranged atoms as opposed to the orderly, crystalline structure found in ordinary metals-can be blow molded like plastics into complex shapes that can't be achieved using regular metal, yet without sacrificing the strength or durability that metal affords. Their findings are described online in the current issue of the journal Materials Today.

"These alloys look like ordinary metal but can be blow molded just as cheaply and as easily as plastic," Schroers said. So far the team has created a number of complex shapes-including seamless metallic bottles, watch cases, miniature resonators and biomedical implants-that can be molded in less than a minute and are twice as strong as typical steel.

The materials cost about the same as high-end steel, Schroers said, but can be processed as cheaply as plastic. The alloys are made up of different metals, including zirconium, nickel, titanium and copper.

The team blow molded the alloys at low temperatures and low pressures, where the bulk metallic glass softens dramatically and flows as easily as plastic but without crystallizing like regular metal. It's the low temperatures and low pressures that allowed the team to shape the BMGs with unprecedented ease, versatility and precision, Schroers said. In order to carefully control and maintain the ideal temperature for blow molding, the team shaped the BMGs in a vacuum or in fluid.

"The trick is to avoid friction typically present in other forming techniques," Schroers said. "Blow molding completely eliminates friction, allowing us to create any number of complicated shapes, down to the nanoscale."

Schroers and his team are already using their new processing technique to fabricate miniature resonators for microelectromechanical systems (MEMS)-tiny mechanical devices powered by electricity-as well as gyroscopes and other resonator applications.

In addition, by blow molding the BMGs, the team was able to combine three separate steps in traditional metal processing (shaping, joining and finishing) into one, allowing them to carry out previously cumbersome, time- and energy-intensive processing in less than a minute.

"This could enable a whole new paradigm for shaping metals," Schroers said. "The superior properties of BMGs relative to plastics and typical metals, combined with the ease, economy and precision of blow molding, have the potential to impact society just as much as the development of synthetic plastics and their associated processing methods have in the last century."

Other authors of the paper include Thomas M. Hodges and Golden Kumar (Yale University); Hari Raman and A.J. Barnes (SuperformUSA); and Quoc Pham and Theodore A. Waniuk (Liquidmetal Technologies).

Journal Reference:

1. Jan Schroers, Thomas M. Hodges, Golden Kumar, Hari Raman, Anthony J. Barnes, Quoc Pham, Theodore A. Waniuk. Thermoplastic blow molding of metals.Materials Today, 2011; 14 (1-2): 14 DOI: 10.1016/S1369-7021(11)70018-9