Hole-rich semiconductors like gallium arsenide could harness the quantum effect called "electron spin" for a new era of spintronic devices which store information on magnetic atoms inserted into a semiconductor's crystalline lattice. But the current technique of random doping of magnetic atoms makes adding spintronics capabilities a hit-or-miss process. Now researchers claim to have perfected a method of brewing exactly the right molecular arrangement. Using a scanning tunneling microscope to substitute magnetic (manganese) atoms for individual gallium atoms, Princeton University researchers were able to experiment with different crystalline lattice architectures to optimize spintronic capabilities. The researchers confirmed the optimal lattice architecture for a new spintonic material: gallium manganese arsenide. The researchers claim this is the first time atomic-level manipulations were used to verify theoretical predictions about the optimal atomic arrangement in a semiconductor. Moreover, the arrangement was achieved one atom at a time in a crystalline lattice. GaAs is a candidate for next-generation spintronic devices because of its very high electron mobility compared to silicon. By incorporating magnetic atoms into a gallium manganese arsenide semiconductor, the team said it hopes to separately control spin and charge to enable highly energetic spintronic devices.
Friday, July 28, 2006
Wednesday, July 26, 2006
Researchers gathered this week to extend the use of quantum effects in semiconductors. Quantum effects result from the confinement of electrons, or holes, by restricting their free movement (perpendicular to the direction of crystal growth for quantum dots), thereby enabling their quantum effects to dominate. At the International Conference on the Physics of Semiconductors in Vienna, Austria, University of New South Wales (Sydney, Austrailia) claimed a world's first for quantum effects: successful fabrication of quantum wires from gallium arscenide. Dubbed "Hole Quantum Wires," the researchers reported on different aspects of their discovery. Other researchers discussed controlling spin in quantum dots, including those formed in graphene sheets and nanotube transport of holes with quantum spin to "q-bit" calculations of a quantum "Hall effect." Ballistic transport in quantum wires, bound electron-holes (excitons) in semiconductor quantum dots and optical control of spin polarization were also hot topics. New methods of handling nitrides, Bose condensates and quantum-effect optical devices such as quantum-cascade lasers and single-photon lasers are also emerging, researchers said.
Posted by R. Colin Johnson at 6:10 PM
A recent Shuttle experiment could yield biosensors that harness living cells to detect harmful chemicals or biotoxins. Microbes encapsulated in biosensors by a nanoscale self-assembly method were genetically engineered to glow fluoresecent green when sensing specific toxins. After exposure to radiation and the cold vacuum of space, the biosensor prototype will return to Earth from the International Space Station on the next Shuttle flight for additional testing. If the biosensor continues to function, Sandia National Laboratories said it will develop rugged sensor technology that could be used for battlefield reconnaissance. Sandia earlier reported that it could direct the self-assembly of nanocyrstals into thin films. By controlling nanocystal structure so they self-organize to encapsulate the living cells, the researchers were able to seal them in a controlled environment that for use as biosensors. If the cells survive aboard the space station and continue functioning as biosensors when returned to Earth, then the reseachers expect to develop biosensor applications. For instance, the Defense Department is looking for a tiny biosensor carried by insects onto the battlefield. Unmanned aircraft could remotely detect any fluoresecent green generated by biological weapons or other biohazards.
Monday, July 24, 2006
The future of semiconductors is not chips: Instead of fabricating circuits on chips and soldering them to printed-circuit boards, Canadian researchers propose painting transparent "solution processed" circuits directly onto a device's surface. Such semiconductor circuits--from emitters for large-area displays to detectors for spray-on solar cells--could drastically lower the cost of electronic devices, the group says. The first beneficiaries could be night vision goggles for the military that would be 10 times more sensitive, yet less expensive, than today's models. But that is just the beginning, according to the team at the University of Toronto, because now spray-on circuits no longer have to sacrifice performance to attain low cost. Besides being inexpensive to mass-produce, the photodetector material could post a tenfold sensitivity increase for military night vision systems, which image in infrared, as well as for biomedical imaging systems that use infrared to see through skin.
Thursday, July 20, 2006
A University of Delaware engineer claims to have solved a power issue that has prevented wider use of digital light processors (DLP) in power-sensitive applications. DLPs normally use microelectromechanical systems (MEMS) to control the angle of micromirror arrays. However, for power-sensitive applications such as space exploration, even the microamps required to tilt micromirrors make use of DLPs prohibitive. The University of Delaware engineering professor claims to have solved the problem using micro-optomechanical systems. MOMS use lasers to actuate tiny mirror-tipped cantilevers instead of electrical current to power pnematic, piezoelectric or electrostatic actuators. An optically-active nanotube film enables MOMs to be actuated by a ultralow-power laser rather than power-draining electrical current. The researcher claims power consumption is low enough for space exploration and new applications of field-emission displays and biomedical scanners. The technique patterns a carbon nanotube thin-film using standard CMOS processing steps, resulting in arrays of optically-actuated cantilevers measuring 300 microns long by 30 microns wide by 7 microns thick. The cantilevers deflected 23 microns when their base was illuminated by a 808-nm wavelength, 170 mW semiconductor laser.
Posted by R. Colin Johnson at 6:57 AM
Monday, July 17, 2006
Researchers believe they have unlocked the mystery to what makes high-temperature superconductors tick. According to a team from Oak Ridge National Laboratory and the University of Tennessee, the reason these materials superconduct at such high temperatures may be a magnetic resonance that causes their anti-ferromagnetic lattice to oscillate opposing-spin orientations in synchronization with the opposing-spin orientations of the so-called Cooper pairs passing through the superconductor's molecular lattice. Magnetic-resonance excitation is believed to be the mechanism that generates Cooper electron pairs in high-critical-temperature superconductors. Recent experiments at the National Institute of Standards and Technology have confirmed the theory in the superconductor called praseodymium lanthanum cerium copper oxide. The Oak Ridge-UT team also reported a universal law governing all high-temperature materials--their magnetic-resonance energy is proportional to their superconductivity transition temperature. If the researchers are correct that magnetic resonance serves the same function as phonon lattice vibrations in low-temperature superconductors, then room-temperature superconductors could be on the horizon.
Thursday, July 13, 2006
The National Science Foundation will fund research on a solar heating and cooling prototype that seeks to replace conventional systems. Details of the prototype technology called the Active Building Envelope (ABE) were disclosed this week at Solar 2006 in Denver. Rensselaer Polytechnic Institute (RPI) professor Steven Van Dessel described his group's work on the ABE system. He said ABE could allow the hitching of solar panels to thermoelectric heat pumps to reduce the cost of cooling and heating. The National Science Foundation will fund Van Dessel's next project to make ABE technology economically feasible by switching to low-cost thin-film materials. If successful, thin-films could then be used for other applications such as auto glass that heats or cools vehicle interiors. Thermoelectric heat pumps become cool on one end and hot on the other when electric current passes through them. By putting one end outside a container and the other inside, the thermoelectric device can pump heat into or out of the container. ABE combines the thermoelectric element with solar panels that can cover an entire building. Combined with a storage device, it could then heat or cool a building day or night. Van Dessel's group hopes to use low-cost, thin-film materials that allow both solar cells and thermoelectric heat pumps to be integrated into windows and other surfaces to enable climate control.
Monday, July 10, 2006
Two problems with conventional radar make it unsuitable for many applications: Anyone with a radar receiver can tell when you activate it, and it can't image objects closer than about 100 feet. Granted, radar automatically opens the door for you at the grocery store, and Stealth bombers are supposedly transparent to radar. But the grocery store radar uses a Doppler algorithm that can only sense movement, not make images, and an aircraft can only be made invisible to radar directed at it from the ground. Now Ohio State University electrical engineer Eric Walton claims to solve both problems with $100 worth of parts. Walton's "noise radar" hides its signal in wideband noise, making it undetectable by the enemy, and it can image objects right through concrete walls. By spreading low-level noise across gigahertz of radio spectrum, the noise radar signal becomes undetectable to normal radar receivers, which are designed to look for high-level signals and to filter out weak signals assumed to be noise. Spread-spectrum transmitters and receivers are widely used today, but spread-spectrum receivers cannot decode noise radar signals, because the signals are spread across gigahertz of bandwidth--simply too much territory for the receivers to cover when searching for correlations. In fact, even two of Walton's own noise radar transceivers, sitting side by side, cannot detect each other, because the signals they send out are random and unique. The only receiver that can detect the signal from a noise radar, Walton said, is the very device that sent out the signal in the first place, its unique code being the random signal itself.
Thursday, July 06, 2006
Researchers said they have moved a step closer to understanding the mechanism behind high-temperature superconductivity. The discovery of a high-temperature superconductor (bismuth strontium calcium copper oxide) by IBM in 1986 made lower-cost devices feasible. Since then, researchers have been trying to understand why these materials superconduct at such a high temperature. Their aim is to design materials that superconduct at even higher temperatures—perhaps even at room temperature. Earlier this year, IBM confirmed that high-temperature superconduction results from a condensate of Cooper pairs—two electrons bound together with opposing spins. But the mechanism responsible for condensing the Cooper pairs remains a mystery. Working at NIST's Center for Neutron Research, the team claims to have observed what may be the mechanism that binds Cooper pairs, thereby explaining high-temperature superconductivity.
Monday, July 03, 2006
A spherical-aberration corrector has enabled the transmission electron microscope at IBM's T.J. Watson Research Center (Yorktown Heights, N.Y.) to make the highest-resolution images in the world. Instead of blurry pictures of individual atoms, the researchers have obtained clear images of the individual molecular bonds among the different types of atoms in the crystalline lattice of a semiconductor surface. IBM recently installed a second-generation spherical-aberration correction system made by Nion Co. (Kirkland, Wash.) As a result, the world's highest-resolution images are now made on IBM's 120,000-electron-volt (eV) scanning-tunneling electron microscope (STEM). The researchers clearly imaged a crystalline aluminum nitride surface, showing the hexagonal "wurtzite" arrangement of atoms. The crystalline aluminum nitride layer was fabricated to experiment with storing charge in aluminum-nitride/gallium-nitride/aluminum-nitride quantum wells. The gallium nitride behaves as a semiconductor, storing as little as one charge carrier, while the aluminum-nitride sandwich insulates the quantum well from electrodes above and below it. The images clearly revealed for the first time the location and orientation of both the aluminum and the much tinier nitrogen atoms in the hexagonal wurtzite crystalline lattice pattern.
A diagnostic spark that finds defects in wiring systems as complex as those on aircraft has been developed by researchers at Sandia National Laboratories. The Pulsed Arrested Spark Discharge (PASD) enables engineers to pinpoint the location of future short circuits before they occur, by exposing weaknesses that would eventually cause the short, according to the researchers. Ordinarily, a 10,000 to 15,000-volt, 200-amp test signal would fry any electronics under test, but when it only lasts 10 nanoseconds, it can't damage wiring systems. Airplanes have miles of wiring harnesses, with many of the wires running hundreds of feet between connections. In the past, airlines have had to wait for shorts to develop before they could be diagnosed, possibly risking debilitating failures. Intermittent electrical short circuits in aging airliners often make cabin lights blink, but they have also caused fatal crashes, such as flights SwissAir 111 and TWA 800. If PASD had been used to precondition their wiring harnesses, then those aircraft could have been maintained short-free by replacing problem wires before they failed in flight. PASD pulses actually improve the performance of wiring harnesses, according to the laboratory, since they burn off any foreign matter that might be bridging a hot line to ground, thereby preconditioning the wiring system and raising its overall breakdown voltage.