New Research Advances Understanding of Lead Selenide Nanowires

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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Flexible Trailing Edge Flap for Blades to Make Wind Power Cheaper

Now a three-year project co-funded by EUDP (the Danish Programme for Energy Technology Development and Demonstration), with three industry partners, is launched and is to develop the promising technology forward to a robust and durable trailing edge which can be tested on a full-scale blade.

The fierce gusts and turbulence, such as wind turbines are exposed to constantly, contribute significantly to the cost of producing electricity from wind turbines. The turbines must be designed to resist these influences throughout their lifespan of at least 20 years since repairs are costly, especially when the turbines are located far out at sea and are more than 100 meters high. Therefore, researchers and industry are aimed at finding technical solutions that can alleviate the loads on the turbines.

"We have already had a good start of the project with our first project meeting in early March. The composition of project partners is well suited in order to solve the challenges in the project" says Research Specialist and Project Manager Helge Aagaard Madsen from Risø DTU.

With a grant of DKK 9,9 million from EUDP and an own appropriation from Risø DTU, DTU Electrical Engineering and three industrial partners on DKK 3 million the exciting technology can now take one step closer to being a commercial prototype that are to be tested on a full-scale blade. Risø DTU coordinates the project and the industrial partners are AVN Energy A/S, Rehau A/S and Dansk Gummi Industri A/S.

Robust, reliable and durable! The buzz words for the project are to develop a technology that is: robust, reliable and durable. The specific solution that has been under development at Risø since 2006, supported by funds from Region Zealand, is a flexible trailing edge of rubber or plastic. Movement of the trailing edge is achieved by elastic deformations caused by fiber reinforced cavities that run through the rear and can be pressurized with air or hydraulics. It is these controlled movements that counteract the forces from the fierce wind gusts.

"The technology has already been tested under laboratory conditions and in a wind tunnel with promising results. Now the task is to have a prototype produced by the end of project that is ready for testing on a full-scale turbine"explains Research Specialist Helge Aagaard Madsen and continues:

"We want to develop and produce prototypes in 2m-long rubber or plastic in the project, depending on what's most robust and give the best result."

The three industrial partners in the project each contribute specific knowledge in key areas. Eg AVN is already experts in the hydraulic systems that are currently used for turbine pitch systems. Since AVN develops, manufactures and sells these systems for different wind turbine manufacturers they can contribute with a unique understanding of how the new flaps systems can operate together with the pitch system.

"The pitch system is what rotate the blades today so that they are positioned optimal towards the wind, but it costs both loads and energy to turn a 15-ton rotor blade as compared to what it will 'cost' for our small local movements with a flexible blade trailing edge that perhaps only has a weight of 1% of the blade's total weight,"explains Helge Aagaard Madsen

The other two project partners is Rehau, that among other supplies plastic parts for the car industry and Dansk Gummi Industri which manufactures molded rubber and polyurethane to the industry. Rehau will contribute to develop the new materials that the trailing edge can be manufactured from, and the Dansk Gummi Industri will work on the production side of the trailing edge also called CRTEF (Controllable Rubber Trailing Edge Flap).

No mechanical parts The flexible trailing edge is entirely without mechanical parts and we hope completely to avoid metal parts. And this part is important. Helge Aagaard Madsen explains:

"It is important that the technologies we develop now are virtually maintenance free. It is of no use to add another component on the turbine that needs a lot of maintenance and can break. This is also why it is very important that we have a good collaboration with the industry from this early stage. In this way we can ensure that the product matches what the industry needs and wants. Both when it comes to the production and the application side."

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Micro Aircraft Improves Avionic Systems and Sensors

A particularly important feature of the novel design is its modular construction. This allows the scientists to install a wide variety of systems to be tested under flight conditions. This also applies to components of the electric propulsion unit, since the scientists intend to use IMPULLS to investigate possible implementations of electric and hybrid propulsion systems in aircraft.

UAVs like IMPULLS are ideal for measuring atmospheric pollution, for aerial geo-surveying or monitoring the environment and infrastructures from above. A further field of deployment is information collection in emergencies and dangerous situations. Appropriately equipped UAVs can also be deployed in adverse weather conditions or hazardous situations that would pose an unreasonable risk to pilots.

"Thanks to advances in miniaturization and improved performance of sensor and avionics systems, we can use IMPULLS as a cornerstone for these kind of developments," says Professor Mirko Hornung, chair of the Institute of Aircraft Design. Deriving and understanding the associated business models and ranges of services are also topics that can be investigated using the IMPULLS platform.

IMPULLS has a wingspan of 5 meters and an empty weight of 20 kilograms. It is propelled by a two-kilowatt electric motor. The UAV can carry a payload of 10 kilograms and fly non-stop for up to 75 minutes. As in commercial aircraft, essential safety-relevant components are designed redundantly.

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Element Germanium Under Pressure Matches Predictions of Modern Condensed Matter Theory

New research conducted by Xiao-Jia Chen, Viktor Struzhkin, and Ho-Kwang (Dave) Mao from Geophysical Laboratory at Carnegie Institution for Science, along with collaborators from China, reveals details of the element's transitions under pressure. Their results show extraordinary agreement with the predictions of modern condensed matter theory.

Germanium (atomic number 32) is used in fiber-optic systems, specialized camera and microscope lenses, circuitry, and solar cells. Under ambient conditions it is brittle and semiconducting. But under pressure, the element should exhibit superconductivity, meaning that there is no resistance to the flow of an electric current.

The team's research, published inPhysical Review Letters, discovered that under pressure of 66 GPa (about 650,000 atmospheres), germanium undergoes a structural change from one type of solid material to another that is metallic -- meaning it conducts electricity. It then undergoes another structural change under pressure of 90 GPa (about 890,000 atmospheres). These findings matched theoretical predictions about the element's behavior under extreme pressure.

"A series of phase transitions was observed on compression of germanium that creates structures with increased density," Chen said."We found extraordinary agreement between theory and experiment for the structures, energies, and compressional behavior. Though some of this behavior had been noted earlier, the agreement between the new highly accurate experimental results and theory really was quite remarkable."

The team's results show that superconductivity in this simple element is caused by phonons, or collective vibrations in the crystal structures that germanium assumes under pressure.

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First Macro-Scale Thin-Film Solid-Oxide Fuel Cell: Strong, Nanostructured Membrane Enables Scaling for Clean-Energy Applications

While SOFCs have previously worked at the micro-scale, this is the first time any research group has overcome the structural challenges of scaling the technology up to a practical size with a proportionally higher power output.

Reported online April 3 inNature Nanotechnology, the demonstration of this fully functional SOFC indicates the potential of electrochemical fuel cells to be a viable source of clean energy.

"The breakthrough in this work is that we have demonstrated power density comparable to what you can get with tiny membranes, but with membranes that are a factor of a hundred or so larger, demonstrating that the technology is scalable," says principal investigator Shriram Ramanathan, Associate Professor of Materials Science at SEAS.

SOFCs create electrical energy via an electrochemical reaction that takes place across an ultra-thin membrane. This 100-nanometer membrane, comprising the electrolyte and electrodes, has to be thin enough to allow ions to pass through it at a relatively low temperature (which, for ceramic fuel cells, lies in the range of 300 to 500 degrees Celsius). These low temperatures allow for a quick start-up, a more compact design, and less use of rare-earth materials.

So far, however, thin films have been successfully implemented only in micro-SOFCs, where each chip in the fuel cell wafer is about 100 microns wide. For practical applications, such as use in compact power sources, SOFCs need to be about 50 times wider.

The electrochemical membranes are so thin that creating one on that scale is roughly equivalent to making a 16-foot-wide sheet of paper. Naturally, the structural issues are significant.

"If you make a conventional thin membrane on that scale without a support structure, you can't do anything -- it will just break," says co-author Bo-Kuai Lai, a postdoctoral fellow at SEAS."You make the membrane in the lab, but you can't even take it out. It will just shatter."

With lead author Masaru Tsuchiya (Ph.D. '09), a former member of Ramanathan's lab who is now at SiEnergy, Ramanathan and Lai fortified the thin film membrane using a metallic grid that looks like nanoscale chicken wire.

The tiny metal honeycomb provides the critical structural element for the large membrane while also serving as a current collector. Ramanathan's team was able to manufacture membrane chips that were 5 mm wide, combining hundreds of these chips into palm-sized SOFC wafers.

While other researchers' earlier attempts at implementing the metallic grid showed structural success, Ramanathan's team is the first to demonstrate a fully functional SOFC on this scale. Their fuel cell's power density of 155 milliwatts per square centimeter (at 510 degrees Celsius) is comparable to the power density of micro-SOFCs.

When multiplied by the much larger active area of this new fuel cell, that power density translates into an output high enough for relevance to portable power.

Previous work in Ramanathan's lab has developed micro-SOFCs that are all-ceramic or that use methane as the fuel source instead of hydrogen. The researchers hope that future work on SOFCs will incorporate these technologies into the large-scale fuel cells, improving their affordability.

In the coming months, they will explore the design of novel nanostructured anodes for hydrogen-alternative fuels that are operable at these low temperatures and work to enhance the microstructural stability of the electrodes.

The research was supported in part by the National Science Foundation (NSF) and performed in part at the Harvard University Center for Nanoscale Systems, a member of the NSF-funded National Nanotechnology Infrastructure Network.

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Seeing Below the Surface: Engineers Devise a New Way to Inspect Advanced Materials Used to Build Airplanes

"With aluminum, if you hit it, there's a dent there. With a composite, oftentimes if you hit it, there's no surface damage, even though there may be internal damage," says Brian L. Wardle, associate professor of aeronautics and astronautics.

Wardle and his colleagues have devised a new way to detect that internal damage, using a simple handheld device and heat-sensitive camera. Their approach also requires engineering the composite materials to include carbon nanotubes, which generate the heat necessary for the test.

Their approach, described in the March 22 online edition of the journalNanotechnology, could allow airlines to inspect their planes much more quickly, Wardle says. This project is part of a multiyear, aerospace-industry-funded effort to improve the mechanical properties of existing advanced aerospace-grade composites. The U.S. Air Force and Navy are also interested in the technology, and Wardle is working with them to develop it for use in their aircraft and vessels.

Uncovering damage

Advanced composite materials are commonly found not only in aircraft, but also cars, bridges and wind-turbine blades, Wardle says.

One method that inspectors now use to reveal damage in advanced composite materials is infrared thermography, which detects infrared radiation emitted when the surface is heated. In an advanced composite material, any cracks or delamination (separation of the layers that form the composite material) will redirect the flow of heat. That abnormal flow pattern can be seen with a heat-sensitive (thermographic) camera.

This is effective but cumbersome because it requires large heaters to be placed next to the surface, Wardle says. With his new approach, carbon nanotubes are incorporated into the composite material. When a small electric current is applied to the surface, the nanotubes heat up, which eliminates the need for any external heat source. The inspector can see the damage with a thermographic camera or goggles.

"It's a very clever way to utilize the properties of carbon nanotubes to deliver that thermal energy, from the inside out," says Douglas Adams, associate professor of mechanical engineering at Purdue University. Adams, who was not involved in the research, notes that two fundamental challenges remain: developing a practical way to manufacture large quantities of the new material, and ensuring that the addition of nanotubes does not detract from the material's primary function of withstanding heavy loads.

The new carbon nanotube hybrid materials that Wardle is developing have so far shown better mechanical properties, such as strength and toughness, than existing advanced composites.

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Novel Nanowires Boost Fuel Cell Efficiency

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

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

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

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

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

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

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

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

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

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

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