Silicon Can Work For New-Age Spintronics Applications

Source:

http://www.sciencedaily.com/releases/2007/10/071026162320.htm

ScienceDaily (Oct. 31, 2007) — In a rapid follow-up to their achievement as the first to demonstrate how an electron's spin can be electrically injected, controlled and detected in silicon, electrical engineers from the University of Delaware and Cambridge NanoTech now show that this quantum property can be transported a marathon distance in the world of microelectronics-- through an entire silicon wafer.

The finding confirms that silicon--the workhorse material of present-day electronics--now can be harnessed up for new-age spintronics applications.
The results mark another major steppingstone in the pioneering field of spintronics, which aims to use the intrinsic “spin” property of electrons versus solely their electrical charge for the cheaper, faster, lower-power processing and storage of data than present-day electronics can offer.
The research team included Ian Appelbaum, UD assistant professor of electrical and computer engineering, and his doctoral student, Biqin Huang, and Douwe Monsma, of Cambridge NanoTech in Cambridge, Mass. Huang was the lead author of the article.*
“Our new result is significant because it means that silicon can now be used to perform many spin manipulations both within the space of thousands of devices and within the time of thousands of logic operations, paving the way for silicon-based spintronics circuits,” Appelbaum said.
In Appelbaum's lab at UD, the team fabricated a device that injected high-energy, “hot” electrons from a ferromagnet into the silicon wafer. Another hot-electron structure (made by bonding two silicon wafers together with a thin-film ferromagnet) detected the electrons on the other side.
“Electron spin has a direction, like 'up' or 'down,' ” Appelbaum said. “In silicon, there are normally equal numbers of spin-up and -down electrons. The goal of spintronics is to use currents with most of the electron spins oriented, or polarized, in the same direction.”
In another recent paper published in the Aug. 13 issue of Applied Physics Letters, the team showed how to attain very high spin polarization, achieving more than 37 percent, and then demonstrated operation as the first semiconductor spin field-effect transistor.
“One hundred percent polarization means that all injected electrons are either spin-up or spin-down,” Huang explained. “High polarization will be necessary for practical applications.”
“In the future, spintronics may bring a great change to daily life,” Huang added.
“We're taking the first steps at the beginning of a new road,” Appelbaum said. “Before our initial work on spin transport in silicon, we didn't even know where the road was,” he said with a smile. “There's a lot of fundamental work to be done, which we hope will bring us closer to a new age of electronics.”
*The article was published in the Oct. 26 issue of the American Physical Society's journal Physical Review Letters.
Adapted from materials provided by University of Delaware.

Fausto Intilla

www.oloscience.com

Star Trek Gadget? 'Tractor Beam' For Cells Developed

Source:

http://www.sciencedaily.com/releases/2007/10/071030095128.htm

ScienceDaily (Oct. 31, 2007) — In a feat that seems like something out of a microscopic version of Star Trek, MIT researchers have found a way to use a “tractor beam” of light to pick up, hold, and move around individual cells and other objects on the surface of a microchip.
The new technology could become an important tool for both biological research and materials research, say Matthew J. Lang and David C. Appleyard, whose work is being published in the journal Lab on a Chip.
The idea of using light beams as tweezers to manipulate cells and tiny objects has been around for at least 30 years. But the MIT researchers have found a way to combine this powerful tool for moving, controlling and measuring objects with the highly versatile world of microchip design and manufacturing.
Optical tweezers, as the technology is known, represent “one of the world's smallest microtools,” says Lang. “Now, we're applying it to building [things] on a chip.”
Says Appleyard, “We've shown that you could merge everything people are doing with optical trapping with all the exciting things you can do on a silicon wafer…There could be lots of uses at the biology-and-electronics interface.”
For example, he said, many people are studying how neurons communicate by depositing them on microchips where electrical circuits etched into the chips monitor their electrical behavior. “They randomly put cells down on a surface, and hope one lands on [or near] a [sensor] so its activity can be measured. With [our technology], you can put the cell right down next to the sensors.” Not only can motions be precisely controlled with the device, but it can also provide very precise measurements of a cell's position.
Optical tweezers use the tiny force of a beam of light from a laser to push around and control tiny objects, from cells to plastic beads. They usually work on a glass surface mounted inside a microscope so that the effects can be observed.
But silicon chips are opaque to light, so applying this technique to them not an obvious move, the researchers say, since the optical tweezers use light beams that have to travel through the material to reach the working surface. The key to making it work in a chip is that silicon is transparent to infrared wavelengths of light - which can be easily produced by lasers, and used instead of the visible light beams.
To develop the system, Lang and Appleyard weren't sure what thickness and surface texture of wafers, the thin silicon slices used to manufacture microchips, would work best, and the devices are expensive and usually available only in quantity. “Being at MIT, where there is such a strength in microfabrication, I was able to get wafers that had been thrown out,” Appleyard says. “I posted signs saying, 'I'm looking for your broken wafers'.”
After testing different samples to determine which worked best, they were able to order a set that were just right for the work. They then tested the system with a variety of cells and tiny beads, including some that were large by the standards of optical tweezer work. They were able to manipulate a square with a hollow center that was 20 micrometers, or millionths of a meter, across - allowing them to demonstrate that even larger objects could be moved and rotated. Other test objects had dimensions of only a few nanometers, or billionths of a meter. Virtually all living cells come in sizes that fall within that nanometer-to-micrometers range and are thus subject to being manipulated by the system.
As a demonstration of the system's versatility, Appleyard says, they set it up to collect and hold 16 tiny living E. coli cells at once on a microchip, forming them into the letters MIT.
Lang is an assistant professor in the Department of Biological Engineering and the Department of Mechanical Engineering. Appleyard is a graduate student in Biological Engineering.
The work was supported by the Biotechnology Training Program of the National Institutes of Health, the W.M. Keck Foundation, and MIT's Lincoln Laboratory.
Adapted from materials provided by Massachusetts Institute Of Technology.

Fausto Intilla
www.oloscience.com

Nanowire Device Fabrication Moves Into High Gear

Source:

http://www.sciencedaily.com/releases/2007/10/071026162149.htm

ScienceDaily (Oct. 30, 2007) — In the growing catalog of nanoscale technologies, nanowires--tiny rows of conductor or semiconductor atoms--have attracted a great deal of interest for their potential to build unique atomic-scale electronics. But before you can buy some at your local Nano Depot, manufacturers will need efficient, reliable methods to build them in quantity. Researchers at the National Institute of Standards and Technology (NIST) believe they have one solution--a technique that allows them to selectively grow nanowires on sapphire wafers in specific positions and orientations accurately enough to attach contacts and layer other circuit elements, all with conventional lithography techniques.

Despite their name, nanowires are more than just electrical connectors. Researchers have used nanowires to create transistors like those used in memory devices and prototype sensors for gases or biomolecules. However working with objects only tens of nanometers wide is challenging. A common approach in the lab is to grow nanowires like blades of grass on a suitable substrate, mow them off and mix them in a fluid to transfer them to a test surface, using some method to give them a preferred orientation.
When the carrier fluid dries, the nanowires are left behind like tumbled jackstraws. Using scanning probe microscopy or similar tools, researchers hunt around for a convenient, isolated nanowire to work on, or place electrical contacts without knowing the exact positions of the nanowires. It's not a technique suitable for mass production.
Building on earlier work to grow nanowires horizontally on the surface of wafers (see "Gold Nano Anchors Put Nanowires in Their Place), NIST researchers used conventional semiconductor manufacturing techniques to deposit small amounts of gold in precise locations on a sapphire wafer. In a high-temperature process, the gold deposits bead up into nanodroplets that act as nucleation points for crystals of zinc oxide, a semiconductor.
A slight mismatch in the crystal structures of zinc oxide and sapphire induces the semiconductor to grow as a narrow nanowire in one particular direction across the wafer. Because the starting points and the growth direction are both well known, it is relatively straightforward to add electrical contacts and other features with additional lithography steps.
As proof of concept, the NIST researchers have used this procedure to create more than 600 nanowire-based transistors, a circuit element commonly used in digital memory chips, in a single process. In the prototype process, they report, the nanowires typical grew in small bunches of up to eight wires at a time, but finer control over the size of the initial gold deposits should make it possible to select the number of wires in each position. The technique, they say, should allow industrial-scale production of nanowire-based devices.
Reference: B. Nikoobakht. Toward industrial-scale fabrication of nanowire-based devices. Chem. Mater., ASAP Article 10.1021/cm071798p S0897-4756(07)01798-X. Web Release Date: October 9, 2007.
Adapted from materials provided by National Institute of Standards and Technology.

Fausto Intilla

www.oloscience.com

Three First-ever Atomic Nuclei Created; New Super-heavy Aluminum Isotopes May Exist

Source:

http://www.sciencedaily.com/releases/2007/10/071024130508.htm

ScienceDaily (Oct. 27, 2007) — Researchers at Michigan State University's National Superconducting Cyclotron Laboratory, NSCL, have created three never-before-observed isotopes of magnesium and aluminum. The results not only stake out new territory on the nuclear landscape, but also suggest that variants of everyday elements might exist that are heavier than current scientific models predict.

"It's been a longstanding project since the beginning of nuclear science to establish what isotopes can exist in nature," said Dave Morrissey, University Distinguished Professor of chemistry and one of the paper's authors. "This result suggests that the limit of stability of matter may be further out than previously expected; really, it shows how much mystery remains about atomic nuclei."
Particles that comprise atomic nuclei, protons and neutrons, are held together by the nuclear force. One of the four fundamental forces that collectively describe the interactions of all matter in the cosmos, the nuclear force, has been the subject of scientific inquiry since the 1930s.
Despite much progress in nuclear physics during the subsequent decades, understanding of how the nuclear force and other effects play out inside nuclei is far from complete. For example, even today scientists aren't sure exactly what combinations of protons and neutrons can make up most atomic nuclei.
One way experimental nuclear physicists explore this issue is by using accelerator facilities to create reactions that, in effect, kluge together piles of protons. An element is defined by its number of protons. For example, hydrogen has one proton; helium, two protons; oxygen eight protons, uranium, 92 protons. Whenever physicists establish a new proton limit, they invariably garner attention for conjuring new elements. In October 2006, a team of Russian and American scientists generated worldwide headlines for creating an element with 118 protons, the most protons ever recorded in a single nucleus.
Another way to probe nuclear stability is to see how many neutrons can be loaded onto nuclei of more quotidian elements, which is the focus of much of the work at NSCL. Elements can exist as different isotopes, which contain the same number of protons but different numbers of neutrons. As an example, the most abundant stable isotope of carbon has six protons and six neutrons. However, trace amounts of carbon-13 and carbon-14 -- with seven and eight neutrons respectively -- also can be found on Earth.
The neutron-limit, referred to as the neutron-dripline, is a basic property of matter. Yet remarkably, despite more than a half-century of inquiry, scientists know the dripline location only for the eight lightest elements, hydrogen to oxygen. So one very basic question -- what's the heaviest isotope of a given element that can exist" -- remains unanswered for all but eight of the hundred or so elements on the Periodic Table.
In an experiment that ran earlier this year at NSCL, researchers successfully created and detected three new super-heavy isotopes of magnesium and aluminum: magnesium-40, with 12 protons and 28 neutrons; aluminum-42, 13 protons and 29 neutrons; and aluminum-43, 13 protons and 30 neutrons. If the everyday version of aluminum were a 160-pound adult, aluminum-43 would be a muscular, 255-pound heavyweight.
"Evidence of particle stability for magnesium-40 obtained at NSCL is a major step in the field of rare isotope physics," said Hiro Sakurai, chief scientist at RIKEN in Japan, who was not involved in the research. The RIKEN research institute in Saitama, Japan, is home to the world's most powerful accelerator facility for creating radioisotope beams.
The fleeting appearance of these three nuclear newcomers is significant for several scientific and technical reasons.
First, when is comes to magnesium, the results indicate that the dripline extends at least as far as, and possibly beyond, magnesium-40. The isotope wasn't detected in several dripline-focused experiments conducted around the world since 1997 and the research community had begun to suspect that it was beyond the bounds of stability. Though it's difficult to compare across disciplines, physicists' success in detecting three magnesium-40 isotopes in the course of an 11-day experiment is roughly similar to the achievement of biologists who finally snap an image of an elusive and thought-to-be-extinct animal after years of traipsing through the jungle.
"The discovery of the hitherto unknown heaviest magnesium and aluminum isotopes at NSCL is a milestone in rare isotope research and is a great accomplishment for the worldwide scientific community exploring unstable nuclei close to the so-called neutron dripline," said Horst Stocker, director of Gesellschaft fur Schwerionenforschung, GSI, who was not involved in the research. Darmstadt, Germany-based GSI is one of the world's top accelerator facilities for producing heavy-ion beams for research.
Second, aside from being a similarly interesting outlier, aluminum-42 carries added importance since it is a near-dripline nucleus with an odd number of neutrons. Isotopes of lighter elements that toe the edge of existence generally have even numbers of neutrons due to the fact that neutrons naturally pair up inside nuclei. With an even number of neutrons, the nuclei in effect have a tidy, complete set of such pairs that collectively form a sort of energetic scaffolding that increases stability.
According to one of the leading theoretical models, aluminum-42 shouldn't exist. That it does suggests that the dripline may in fact tilt in the direction of more novel, neutron-rich isotopes, an implication that will help to extend nuclear theory and point the way to future experiments.
The NSCL result "alters the landscape of known nuclei, it alters our understanding of the forces that bind nuclei into stable objects, and it has important implications for future attempts with next-generation facilities to map the evolution of nuclear structure and existence into the most weakly bound nuclei," said Rick Casten, D. Allan Bromley Professor of Physics at Yale University, also not involved in the research.
The experimental technique itself also is noteworthy. Creating and measuring rare isotopes is always needle-in-a-haystack work that requires researchers to hunt for a few desired nuclei from a swarm of fast-moving and mostly known and therefore less interesting particles. But in this experiment, NSCL researchers achieved a hundred- to thousand-fold boost in their ability to filter out what can be thought of as junk. They did so by essentially jury-rigging the facility to filter the beam twice. The result was an ability to detect and measure isotopes so rare that they represent less than one in every million billion particles that passed by the detectors.
The dual filtering process, more properly known as two-stage separation, is a fixture in most new and planned facilities for rare isotope beam research, including the proposed upgrade of NSCL. This experiment marks one of the first uses of two-stage separation in the world and the first time the technique has been tried at NSCL, which typically filters and purifies particles only once in its A1900 separator.
NSCL detectors returned just one blip of data consistent with the existence of aluminum-43. This generally isn't enough to count as a discovery, according to the conventions of nuclear science. However, more than 20 instances of its immediate neighbor, aluminum-42, were observed. Because of this relative abundance and the fact that, due to pairing, the 30 neutrons in aluminum-43 should prove more stable than the 29 neutrons in aluminum-42, the solitary signature of aluminum-43 etched in the data logs carries more than usual amount of credibility.
"Experiments such as these are paving the way into the new era of nuclear structure studies that technological developments are opening to investigation for the first time ever," said Yale's Casten.
The findings appear in the October 25 issue of the journal Nature.
Adapted from materials provided by Michigan State University.

Fausto Intilla

www.oloscience.com

Quantum Cascade Laser Nanoantenna Created

Source:

http://www.sciencedaily.com/releases/2007/10/071022122223.htm

ScienceDaily (Oct. 25, 2007) — In a major feat of nanotechnology engineering researchers from Harvard University have demonstrated a laser with a wide-range of potential applications in chemistry, biology and medicine. Called a quantum cascade (QC) laser nanoantenna, the device is capable of resolving the chemical composition of samples, such as the interior of a cell, with unprecedented detail.

Spearheaded by graduate students Nanfang Yu, Ertugrul Cubukcu, and Federico Capasso, Robert L. Wallace Professor of Applied Physics, all of Harvard's School of Engineering and Applied Sciences, the findings will be published as a cover feature of the October 22 issue of Applied Physics Letters. The researchers have also filed for U.S. patents covering this new class of photonic devices.
The laser's design consists of two gold rods separated by a nanometer gap (a device known as an optical antenna) built on the facet of a quantum cascade laser, which emits invisible light in the region of the spectrum where most molecules have their tell tale absorption fingerprints. The nanoantenna creates a light spot of nanometric size about fifty to hundred times smaller than the laser wavelength; the spot can be scanned across a specimen to provide chemical images of the surface with superior spatial resolution.
"There's currently a major push to develop powerful tabletop microscopes with spatial resolution much smaller than the wavelength that can provide images of materials, and in particular biological specimens, with chemical information on a nanometric scale," says Federico Capasso.
While infrared microscopes, based on the detection of molecular absorption fingerprints, are commercially available and widely used to map the chemical composition of materials, their spatial resolution is limited by the range of available light sources and optics to well above the wavelength. Likewise the so-called near field infrared microscopes, which rely on an ultra sharp metallic tip scanned across the sample surface at nanometric distances, can provide ultrahigh spatial resolution but applications are so far strongly limited by the use of bulky lasers with very limited tunability and wavelength coverage.
"By combining Quantum Cascade Lasers with optical antenna nanotechnology we have created for the first time an extremely compact device that will enable the realization of new ultrahigh spatial resolution microscopes for chemical imaging on a nanometric scale of a wide range of materials and biological specimens," says Capasso.
Quantum cascade (QC) lasers were invented and first demonstrated by Capasso and his group at Bell Labs in 1994. These compact millimeter length semiconductor lasers, which are now commercially available, are made by stacking nanometer thick layers of semiconductor materials on top of each other. By varying the thickness of the layers one can select the wavelength of the QC laser across essentially the entire infrared spectrum where molecules absorb, thus custom designing it for a specific application.
In addition by suitable design the wavelength of a particular QCL can be made widely tunable. The range of applications of QC laser based chemical sensors is very broad, including pollution monitoring, chemical sensing, medical diagnostics such as breath analysis, and homeland security.
The teams co-authors are Kenneth Crozier, Assistant Professor of Electrical Engineering, and research associates Mikhail Belkin and Laurent Diehl, all of Harvard's School of Engineering and Applied Sciences; David Bour, Scott Corzine, and Gloria Höfler, all formerly with Agilent Technologies. The research was supported by the Air Force Office of Scientific Research and the National Science Foundation. The authors also acknowledge the support of two Harvard-based centers, the Nanoscale Science and Engineering Center and the Center for Nanoscale Systems, a member of the National Nanotechnology Infrastructure Network.
Adapted from materials provided by Harvard University.

Fausto Intilla

www.oloscience.com

Nanowire Generates Its Own Electricity

Source:

http://www.sciencedaily.com/releases/2007/10/071022161425.htm

ScienceDaily (Oct. 23, 2007) — Harvard chemists have built a new wire out of photosensitive materials that is hundreds of times smaller than a human hair. The wire not only carries electricity to be used in vanishingly small circuits, but generates power as well.
Charles M. Lieber, the Mark Hyman Jr. Professor of Chemistry, and colleagues created the nanowire out of three different kinds of silicon with different electrical properties. The silicon is wrapped in layers to create the wire. When light falls on the outer material, a process begins due to the interaction of the core with the shell layers, leading to the creation of electrical charges.
The idea of creating nanoscale photovoltaics is not new, Lieber said, but prior efforts used organic compounds in combination with semiconductor nanostructures that had lower efficiency and that degraded under concentrated sunlight. Lieber’s materials have several advantages, he said. The materials are more efficient, converting 3.4 percent of the sunlight into electricity; they can withstand concentrated light without deteriorating, gaining efficiency up to about 5 percent; and they’re as cheap to make as other related nanoscale photovoltaic devices.
“The real [question] is whether there’s a new geometry that will lead to better photovoltaic technology,” Lieber said. “We worked on coaxial geometry.”
The most recent development builds on Lieber’s considerable prior work on nanoscale devices. He has developed sensors with potential bioterrorism applications that can detect a single virus or other particle, nanowire arrays that can detect signals in individual neurons, and a cracker-sized detector for cancer.
A cheap nanoscale power source broadens the potential applications of such nanoscale devices. Though the tiny photovoltaic cells can generate enough electricity to power a similarly tiny circuit, Lieber said they’re not yet efficient enough to have applications on the scale of commercial power generation.
Commercial solar cells, he said, have efficiencies around 20 percent, compared with 3.4 percent for his nano-solar cells. One avenue of future research, Lieber said, will be to explore ways to boost efficiency of the nanowire photovoltaics. If they can reach 10 to 15 percent, he said, their lower cost of production — they can be made from relatively inexpensive materials and don’t require clean rooms to produce — may make them useful in larger-scale applications.
“There’s no physical reason it couldn’t be higher,” Lieber said. “I’m pretty optimistic that we’ll be able to track down the efficiency issue.”
Until then, Lieber sees a future for the nanowire photovoltaics in niche applications, such as multiple distributed sensors or durable, flexible devices, possibly sewn into clothing or worn as a patch.
“It will have to be unique to be an economically viable application, some place where you want durability and flexibility, where if it gets destroyed, people don’t care,” Lieber said.
The work was described in the Oct. 18 issue of the journal Nature.
Adapted from materials provided by Harvard University.

Fausto Intilla
www.oloscience.com

Computer Memory May Leap With Solution To Chemical Mystery

Source:

http://www.sciencedaily.com/releases/2007/10/071019123619.htm

ScienceDaily (Oct. 22, 2007) — A Florida State University researcher has helped solve a scientific mystery that stumped chemists for nearly seven decades. In so doing, his team's findings may lead to the development of more-powerful computer memories and lasers.

Naresh S. Dalal, the Dirac Professor of Chemistry and Biochemistry at FSU, recently collaborated with three colleagues, Jorge Lasave, Sergio Koval and Ricardo Migoni, all of the Universidad Nacional de Rosario in Argentina, to determine why a certain type of crystal known as ammonium dihydrogen phosphate, or ADP, behaves the way it does.
"ADP was discovered in 1938," Dalal said. "It was observed to have some unusual electrical properties that weren't fully understood -- and for nearly 70 years, scientists have been perplexed by these properties. Using the supercomputer at SCRI (FSU's Supercomputer Computations Research Institute), we were able to perform in-depth computational analyses that explained for the very first time what causes ADP to have these unusual properties."
ADP, like many crystals, exhibits an electrical phenomenon known as ferroelectricity. Ferroelectric materials are analogous to magnets in that they maintain a positively charged and a negatively charged pole below a certain temperature that is characteristic for each compound.
"Ferroelectric materials can stay in a given state of charge for a long time -- they retain their charge after the external electrical source is removed," Dalal said. "This has made ADP and other materials like it very useful for storing and transmitting data.
ADP is commonly used in computer memory devices, fiber optic technology, lasers and other electro-optic applications."
What researchers found perplexing about ADP was that it often displays a very different electrical phase -- one known as antiferroelectricity.
"With antiferroelectricity, one layer of molecules in a crystal has a plus and a minus pole, but in the next layer, the charges are reversed," Dalal said. "You see this reversal of charges, layer by layer, throughout the crystal."
Using the supercomputer at SCRI enabled Dalal and his colleagues to perform numerous highly complex calculations that couldn't be duplicated in a laboratory environment. For example, they were able to theoretically alter the angles of ADP's ammonium ions and then measure the effects on the crystal's electrical charge. That approach ultimately led to their solution to the seven-decade mystery.
"We found that the position of the ammonium ions in the compound, as well as the presence of stresses or defects in the crystal, determine whether it behaves in a ferroelectric or antiferroelectric manner," Dalal said.
The team's research is important for two main reasons, Dalal said: "First, this allows us to further understand how to design new materials with both ferroelectric and antiferroelectric properties. Doing so could open new doors for computer memory technology -- and possibly play a role in the development of quantum computers.
"Second, our research opens up new ways of testing materials," Dalal said. "Using supercomputers, we can quickly perform tests to see how materials would react under a variety of conditions. Many such tests can't even be performed in the lab."
A paper "Origin of Antiferroelectricity in NH4H2PO4 from First Principles,"describing Dalal, Lasave and Migoni's findings was published recently in Physical Review Letters.
Adapted from materials provided by Florida State University.

Fausto Intilla

www.oloscience.com