Monday, April 30, 2007

Why Nanowires Make Great Photodetectors

Zinc oxide (ZnO) nanowires grown in the Deli Wang lab at UCSD.
Zinc oxide (ZnO) nanowires grown in the Deli Wang lab at UCSD.
San Diego, CA, -- The geometry of semiconducting nanowires makes them uniquely suited for light detection, according to a new UC San Diego study that highlights the possibility of nanowire light detectors with single-photon sensitivity.

Nanowires are crystalline fibers about one thousandth the width of a human hair, and their inherent properties are expected to enable new photodetector architectures for sensing, imaging,
memory storage, intrachip optical communications and other nanoscale applications, according to a new study in an upcoming issue of the journal Nano Letters. The UCSD engineers illustrate why the large surface areas, small volumes and short lengths of nanowires make them extremely sensitive photodetectors – much more sensitive than larger photodetectors made from the same materials.
A single ZnO nanowire held down by metal contacts. The nanowire segments between contacts serve as semiconductor nanowire photodetectors.
A single ZnO nanowire held down by metal contacts. The nanowire segments between contacts serve as semiconductor nanowire photodetectors.
“These results are encouraging and suggest a bright future for nanowire photodetectors, including single-photon detectors, built from nanowire structures,” said Deli Wang, an electrical and computer engineering (ECE) professor from the UCSD Jacobs School of Engineering and corresponding author on the Nano Letters paper.
Cesare Soci, one of two primary authors on the Nano Letters paper and a postdoctoral researcher in the Deli Wang lab at the Jacobs School. Soci is standing next to equipment used to grow nanowires.
Cesare Soci, one of two primary authors
on the Nano Letters paper and a postdoctoral researcher in the Deli Wang lab at the Jacobs School. Soci is standing next to equipment used to grow
For a nanowire to serve as a photodetector, photons of light with sufficient energy must hit the nanowire in such a way that electrons are split from their positively charged holes. Electrons must remain free from their holes long enough to zip along the nanowire and generate electric current under an applied electric field – a sure sign that light has been detected.
Drawing of a single nanowire photodetector. When light strikes the nanowire in such a way that an electron and hole in the semiconducting material split apart, the electron runs along the wire and increases the wire's current and light is detected.
Drawing of a single nanowire photodetector. When light strikes the nanowire in such a way that an electron and hole in the semiconducting material split apart, the electron runs along the wire and increases the wire's current and light is detected.
The new research demonstrates that the geometry of nanowires – with so much surface area compared to volume – makes them inherently good at trapping holes. Dangling bonds on vast nanowire surfaces trap holes – and when holes are trapped, the time it takes electrons and holes to recombine increases. Delaying the reunion of an electron and its hole increases the number of times that electron travels down the nanowire, which in turn triggers an increase in current and results in “internal photoconductive gain.”
Schematic  of the trapping and photoconduction mechanism in ZnO nanowires. At the top of each box are
Schematic of the trapping and photoconduction mechanism in ZnO nanowires. At the top of each box are 'energy band diagrams' ('b' represents the situation in darkness and 'c' under UV illumination). In ZnO nanowires (as compared to some other semiconducting nanowires), the lifetime of the unpaired electrons is further inreased by oxygen molecules desorption from the surface when holes neutralize the oxygen ions.
“Different kinds of nanowires detect different wavelengths of light. You could make a red-green-blue photodetector on the nanoscale by combining the right three kinds of nanowires,” said Cesare Soci, one of two primary authors on the Nano Letters paper and a postdoctoral researcher in the Deli Wang lab at the Jacobs School. The other primary author is Arthur Zhang, a graduate student in the lab of Yu-Hwa Lo, an electrical engineering professor at the Jacobs School.
This work supports recent theoretical work from Peter Asbeck’s High Speed Device Group, also at the Jacobs School.

“Our theoretical work showed that light-induced conductivity in nanowires can be increased by more than 10 times over similar bulk structures under the same illumination level. The work from Deli Wang’s lab has confirmed some of our calculations and provides further support for the idea that nanowires will be increasingly incorporated into photodetection and photovoltaic applications,” said Asbeck.

In the new work, short pulses of ultraviolet light (hundreds of femtoseconds wide) were detected on time scales in the nanosecond range. Moreover, using electronic measurement of photocurrent, the engineers reported internal photoconductive gain (G) as high as 10^8 – one of the highest ever reported.

"Although nanowire detectors offer both high speed and high gain, the most important figure of merit for the device is the signal-to-noise ratio or the sensitivity,” explained Yu-Hwa Lo, an author on the Nano Letters paper and the director of NANO3, the clean nanofabrication facility at Calit2's UCSD campus.

“Because of the unique geometry of nanowires, the active volume that produces dark current, a source of noise, is only one thousandth that of a normal size photodetector. This enables nanowire detectors to achieve very high sensitivity, provided that light can be efficiently coupled into the nanowires. Several methods have been proposed to achieve light coupling efficiency, such as placing the nanowires in an optical resonant cavity. In theory, a nanowire detector can achieve single photon sensitivity, which is the ultimate sensitivity for any photodetector,” said Lo.

The engineers also show that molecular oxygen absorbed at the surface of zinc oxide (ZnO) nanowires capture free electrons present in n-type ZnO nanowires and make them especially good at keeping holes and electrons apart. The oxygen mechanism the authors outline explains much of the enhanced sensitivity reported in ZnO nanowire photodetectors.

The engineers fabricated and characterized UV photodetectors made from ZnO nanowires with diameters of 150 to 300 nanometers and lengths ranging from 10 to 15 micrometers. The researchers studied the photoconductivity of zinc oxide nanowires over a broad time range and under both air and vacuum.

Analytical studies performed by Peter Asbeck and ECE graduate student Lingquan Wang and published in the proceedings of IEEE-NANO 2006 support the mechanism outlined in the Nano Letters paper.

According to Wang, this work also highlights how moving to the nanoscale can sometimes throw intuitions out the window.

“The surface trap states that help to make nanowires such sensitive light detectors are the very same surface features that engineers desperately avoid when manufacturing semiconductors for computer transistors, where they hamper performance,” Wang said. ###

Paper Reference:
ZnO Nanowire UV Photodetectors with High Internal Gain
C. Soci, A. Zhang, B. Xiang, S. A. Dayeh, D. P. R. Aplin, J. Park, X. Y. Bao, Y. H. Lo, and D. Wang

Department of Electrical and Computer Engineering, Jacobs School of Engineering, UniVersity of California, San Diego, 9500 Gilman DriVe, La Jolla, California 92093-0407, Nano Letters 7(4), 1003-1009 (2007), DOI: 10.1021/nl070111x

Funders: Office of Naval Research (ONR-Nanoelectronics), the National Science Foundation (NSF), Sharp Labs of America.

Notes: Thanks to a collaboration with Alan Heeger’s lab at UC Santa Barbara where Cesare Soci and Deli Wang did some of their PhD work, the engineers performed electronic measurement of the photocurrent during very short times using a technique pioneered for the study of conjugated polymers. The authors are also grateful for their collaboration with Ed Yu’s lab at UCSD’s Jacobs School and the staff of Calit2’s Nano3 Facility for maintenance of the nanofabrication environment.

Contact: Daniel Kane 858-534-3262 University of California - San Diego, April 23, 2007

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Sunday, April 29, 2007

New Materials for Making “Spintronic” Devices

New Materials for Making “Spintronic” Devices. Pushing the development of electronics beyond the limits of electric charge

L to R: Alexei Tsvelik, Dmitri Kharzeev, Igor Zaliznyak, Brookhaven National LaboratoryUPTON, NY - An interdisciplinary group of scientists at the U.S. Department of Energy’s Brookhaven National Laboratory has devised methods to make a new class of electronic devices based on a property of electrons known as “spin,” rather than merely their electric charge.
High Resolution Image. This approach, dubbed spintronics, could open the way to increasing dramatically the productivity of electronic devices operating at the nanoscale — on the order of billionths of a meter. The Brookhaven scientists have filed a U.S. provisional patent application for their invention, which is now available for licensing.

“This development can open the way for the use of spintronics in practical room temperature devices, an exciting prospect,” said DOE Under Secretary for Science Raymond L. Orbach. “The interplay between outstanding facilities and laboratory researchers is a root cause for this achievement, and a direct consequence of the collaborative transformational research that takes place in our DOE laboratories.”

In the field of electronics, devices based on manipulating electronic charges have been rapidly shrinking and, therefore, getting more efficient, ever since they were first developed in the middle of the last century. “But progress in miniaturization and increasing efficiency is approaching a fundamental technological limit imposed by the atomic structure of matter,” said physicist Igor Zaliznyak, lead author on the Brookhaven Lab patent application. Once you’ve made circuits that approach the size of a few atoms or a single atom, you simply cannot make them any smaller.

To move beyond this limit, Zaliznyak’s team has been exploring ways to take advantage of an electron’s “quantum spin” in addition to its electric charge.

You can think of spin as somewhat analogous to the spin of a toy top, where the axis of rotation can point in any direction. But unlike a top, which can be slowed down, the “spinning” electron’s rotation is a quantum property — that is, a set amount that cannot change. By aligning the spins of multiple electrons so they all point the same way — known as polarization — scientists aim to create a current of spins in addition to a current of charges.

The Brookhaven group uses magnetism to manipulate spin in graphene, a material consisting of flat sheets of carbon atoms arranged in a hexagonal pattern. They’ve proposed ways to make materials consisting of layers of graphene mated to magnetic and nonmagnetic layers.

These “graphene-magnet multilayers” (GMMs) are expected to retain their properties at room temperature, an important practical requirement for spintronic devices. By properly arranging the magnetization of the magnetic layer(s), they can be used to create a full spectrum of spintronic devices, including (re-)writable microchips, transistors, logic gates, and more. Using magnetism for spin manipulation also opens exciting possibilities for creating active, re-writable and re-configurable devices whose function changes depending on the magnetization pattern written on the magnetic medium.

“Graphene is quite unique,” Zaliznyak says, “in that an ideally balanced sheet is neither a conductor nor an insulator. Related to this is the fact that electrons in graphene behave in such a way that their mass effectively vanishes!” In other words, he explains, they move without inertia, like rays of light or particles accelerated to relativistic speeds — that is, close to the speed of light.

Such relativistic particles are studied at Brookhaven at the Relativistic Heavy Ion Collider (RHIC), a nuclear physics facility where scientists are trying to understand the fundamental properties and forces of matter. RHIC theoretical physicist Dmitri Kharzeev and condensed matter physicist Alexei Tsvelik have collaborated with Zaliznyak to gain a better understanding of the physics of magnetized graphene.

“Unifying the pool of knowledge and ideas of two fields is a great advantage for building the theoretical foundation for future devices,” Zaliznyak said. The patent application filed by the Brookhaven scientists, which puts graphene-magnet multilayers to work, leverages the large amount of scientific knowledge accumulated in both fields into developing a novel technology. Plus, the opportunity to study relativistic particles in two dimensions — on flat sheets of graphene — was an unexpected and useful arena for Brookhaven’s nuclear physicists trying to understand the properties of the matter produced at RHIC.

The patent application covers the methods for making the graphene-magnet multilayers, methods of using the GMMs, methods of magnetizing the GMMs, methods for measuring spintronic “current” in GMMs, and the spintronic devices made from GMMs.

This work was funded by the Office of Basic Energy Sciences and the Office of Nuclear Physics, both within the U.S. Department of Energy’s Office of Science. For licensing information, please contact: Kimberley Elcess, Principal Licensing Specialist, Brookhaven National Laboratory, (631) 344-4151,

Note to local editors: Igor Zaliznyak is a resident of Port Jefferson, New York.

Contact: Karen McNulty Walsh 631-344-8350 DOE/Brookhaven National Laboratory, Number: 07-49 BNL Media & Communications Office, April 25, 2007

Water flows like molasses on the nanoscale

Water flows like molasses on the nanoscale

Caption: Georgia Tech physicists have discovered that water behaves differently when its compressed in nano-sized channel. In these small spaces water behaves much like a solid, exhibiting high viscosity and organizing itself into layers. Credit: Georgia Tech, Usage Restrictions: With credit.A Georgia Tech research team has discovered that water exhibits very different properties when it is confined to channels less than two nanometers wide – behaving much like a viscous fluid with a viscosity approaching that of molasses. Determining the properties of water on the nanoscale may prove important for biological and pharmaceutical research as well as nanotechnology.
The research appears in the March 15 issue of the journal Physical Review B.

In its bulk liquid form, water is a disordered medium that flows very readily. When most substances are compressed into a solid, their density increases. But water is different; when it becomes ice, it becomes less dense. For this reason, many scientists reasoned that when water is compressed (as it is in a nanometer-sized channel), it should maintain its liquid properties and shouldn't exhibit properties that are akin to a solid. Several earlier studies came to that very conclusion – that water confined in a nano-space behaves just like water does in the macro world. Consequently, a number of scientists considered the case to be closed.

But when Georgia Tech experimental physicist Elisa Riedo and her team directly measured the force of pure water in a nanometer-sized channel, they found evidence suggesting that water was organized into layers. Riedo conducted these measurements by recording the force placed on a silicon tip of an atomic force microscope as it compressed water. The water was confined in a nanoscale thin film on top of a solid surface.

"Since water usually has a low viscosity, the force you would expect to feel as you compress it should be very small," said Riedo, assistant professor in Georgia Tech's School of Physics. "But when we did the experiment, we found that when the distance between the tip and the surface is about one nanometer, we feel a repulsive force by the water that is much stronger than what we would expect."

As the tip compresses the water even more, the repulsive force oscillates, indicating that the water molecules are forming layers. As the tip continues to increase its pressure on a layer, the layer collapses and the water flows out horizontally.

"In effect, the confined water film behaves effectively like a solid in the vertical direction by forming layers parallel to the confining tip and surface, while maintaining it's liquidity in the horizontal direction where it can flow out – resembling some phases of liquid crystals," said Uzi Landman, director of the Center for Computational Materials Science, Regents' and Institute professor, and Callaway Chair of Physics at Georgia Tech.

A theoretical physicist, Landman conducted the first-ever computer simulations of these forces for tip-confined water films and found good correspondence between his team's theoretical predictions and the experiments.

So why did Riedo and Landman's results differ from their peers? According to Landman, most previous studies on confined water were limited by technology at the time and could not directly measure the behavior in the last two nanometers. Instead they had to measure other properties and infer the forces acting in films of one nanometer thickness or less.

"If you want force, it is preferable to measure it," he said. "This is the first experiment to directly measure the force and it's the first simulation done of these forces. The fact that we have direct measurements married with theoretical results is rather conclusive."

Riedo and Landman conducted their experiments in several different environments. They found that the layering effect was more pronounced when water was placed on top of hydrophilic surfaces that allow water to wet the solid surface, such as glass. When the water was confined by hydrophobic surfaces where water tends to bead up, like graphite, the effect was still present, but less pronounced.

At the same time, Riedo's team was measuring the vertical force exerted on the tip by the confined water film, they also measured the film viscosity by measuring the lateral force. They found that when water was placed on a hydrophilic surface, the viscosity began to increase dramatically as the thickness of the confined film reached the 1.5 nanometer range. As they continued to compress the water and measure the lateral forces, the viscosity increased by a factor of 1,000 to 10,000.

On hydrophobic surfaces, they did not see such an increase in viscosity. The results of the molecular dynamics simulations support these findings, showing a dramatically decreased mobility for sub-nanometer thick water films under hydrophilic confinement.

"Water is a wonderful lubricant," said Riedo, "but it flows too easily for many applications. At the one nanometer scale, water is a viscous fluid and could be a much better lubricant."

Understanding the properties of water at this scale could also be important for biological and pharmaceutical research, especially in understanding processes that depend on hydrated ionic transport through nanoscale channels and pores.

Riedo and Landman's next steps are to introduce impurities in the water to study how that affects its properties. ###

Tai-De Li, Robert Szoszkiewicz and Jianping Gao also contributed to this research, which was supported by the National Science Foundation, the American Chemical Society Petroleum Foundation, the Air Force Office of Scientific Research and the U.S. Department of Energy.

Contact: David Terraso 404-385-2966 Georgia Institute of Technology

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Saturday, April 28, 2007

UD receives $1.9 million for new spintronics center

UD physicist John Xiao (seated) is the director of the new UD Center for Spintronics and Biodetection, and Edmund Nowak is one of the center’s principal investigators. Photo by Kathy Atkinson.UD receives $1.9 million for new spintronics center - 4:47 p.m., April 20, 2007--The University of Delaware has been awarded $1.9 million from the U.S. Department of Energy (DOE) to establish the new Center for Spintronics and Biodetection.

Spin electronics, or “spintronics,” is an emerging science that focuses on harnessing the “spin,” or magnetic properties of electrons, to encode and process data.
The high-tech field is expected to significantly broaden the electronics industry by fostering the development of much smaller, faster, energy-saving devices, from medical diagnostic equipment to environmental sensors that can detect nano-sized particles much tinier than human cells.

UD's research grant is part of $7.5 million grant awarded to universities in four states--Delaware, Kentucky, Maine, and New Hampshire--by DOE's Experimental Program to Stimulate Competitive Research (EPSCoR), which supports scientific research in states that historically have received less federal funding for such studies. Academic institutions in 26 states and territories were eligible for the grants.

Delaware's state EPSCoR office, located at UD's Delaware Biotechnology Institute, helped coordinate the University's winning proposal. A major collaborator on the UD project is Argonne National Laboratory, one of DOE's largest research centers, located near Chicago.

“These partnerships with national labs are very important,” David McCarren, co-director of the state EPSCoR office, said. “They allow Delaware researchers access to the best instrumentation available and put them at the cutting edge of their fields, working with scientists at those sites.”

John Xiao, professor of physics and astronomy in UD's College of Arts and Sciences, will direct the new center and serve as one of its principal investigators.
The other principal investigators include Edmund Nowak, associate professor, and Branislav Nikolic and Yi Ji, assistant professors, all in the UD physics and astronomy department; James Kolodzey, the Charles Black Evans Professor of Electrical and Computer Engineering at UD; and Souheng Sun, associate professor of chemistry and engineering at Brown University in Providence, R.I.Scientists in the emerging field of spintronics are working to harness the spin or magnetic currents from electrons to encode and process data. This figures shows a spin battery device. A microwave induces magnetic resonance in a magnetic layer (blue), which will generate a 'spin current,' in which equal amounts of spin-up and spin-down electrons flow in opposing directions. The bottom magnetic layer (green) converts the spin current to a voltage for detection.
“This is a really new field,” Xiao said. “Electrons have well-known properties, such as electrical charge, which is what the whole field of electronics is based on. Yet spintronics is taking advantage of the fact that electrons also rotate around an axis, just as the Earth does,” he noted. “This spin creates the electron's magnetic properties, just as the Earth has a magnetic North Pole and South Pole.”

Electrons can spin in one of two directions--either down or up--and carry vast amounts of data on the magnetic spin currents they generate while using much less energy than present-day electronic devices, according to Xiao.

“Our new center will be at the forefront of research in how to generate spin current, detect it, and use it,” Xiao said.

The ultimate goal of the center, Xiao noted, is to develop a biosensor, patterned much like a DNA chip, which can detect the tiny magnetic field generated by a single nano-sized particle that can be used to label various biomolecules.

“There could be numerous applications for such highly sensitive sensors,” Xiao noted, “from increasing the early diagnosis of cancer, diabetes and other diseases in patients, to advancing the detection of harmful viruses in antiterrorism programs.”

The electronic sensor device will be built in collaboration with scientists at the Center for Nanoscale Materials at Argonne National Laboratory.

“Magnetism is a very old field that as a child I thought was like 'magic,'” Xiao said, smiling. “Yet about every 10 years, new physics come into the field. Currently, we're really into this nanoscale research, which we couldn't do before.”

Xiao was lured like a magnet into spintronics research when he was in graduate school at Johns Hopkins University. With UD's new center now established, Xiao said he sees lots of opportunities for UD researchers to become leaders in the field and involve students, including UD graduate and undergraduate students and Delaware high-school students, in the center's activities.

“We already have a strong research nucleus in this area at the University,” Xiao said. “We're looking forward to bringing people together, having a center where people can talk together and work together. By bringing together the strengths of different faculty, we hope to generate significant advances in spintronics science and technology.”

Article by Tracey Bryant, Contact: Tracey Bryant 302-831-8185 University of Delaware

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Electromechanical devices and high-performance membranes using ionic liquids

4 universities collaborate to synthesize new materials, nanoscale devices

Blacksburg, Va. -- The Army Research Office has awarded a potentially $7.5 million Multi-University Research Initiative (MURI) grant to scientists from Virginia Tech, the University of Pennsylvanian, Pennsylvania State University, and Drexel University to develop electromechanical devices and high-performance membranes using ionic liquids.

Caption: Virginia Tech Chemistry Professor Tim Long. Credit: Virginia Tech Photo. Usage Restrictions: with coverage about ILEAD MURI.Virginia Tech chemistry professor Tim Long and University of Pennsylvania professor of materials science and engineering Karen I. Winey are co-directors of the Ionic Liquids in Electro-Active Devices (ILEAD) MURI. Long is principal investigator.
Ionic liquids (ILs) are relatively large organic salts that offer charge and liquidity at room temperature. Some ILs are touted as safe, environmentally-friendly solvents. They are also useful in electrically conductive membranes, thermally stable at high temperatures, and do not evaporate at normal conditions. With today’s advanced ability to manipulate molecular structure and design unique molecules, ILs’ advantages are being explored for emerging applications. "The Army needs a myriad of electronic devices that take advantage of the potential synergy of these unique properties," Long said.

The team is creating synthetic ILs and evaluating their performance in sophisticated electronic devices. "Our challenge is to synthesize high performance materials with a particular device in mind. Then the device is truly created from the molecular-scale up," said Long.

The group will integrate ILs into membranes to create thin films to perform various functions, such as membranes that can transport or filter small molecules. "Applications include fuel cell membranes, where protons are transported across a membrane to create electricity. One advantage over existing fuel cell materials is that the IL will not evaporate, so future membranes will operate at higher temperatures with higher efficiency."

Another application could be stimuli-responsive materials for micro sensors and smart clothes, said Long. "The material would breathe and wick moisture away, but quickly close up in response to a chemical or biological threat. Such a suit could be used by the military, by a firefighter, or in an operating room."

Membranes can also be created that will bend, stretch, or change shape in response to a low voltage, like an artificial muscle.

And ILs can be used in coatings or as part of structures. The team will look at creating new polymeric materials that can be charged or conductive, Long said.

"ILs will serve as the building blocks for elastomers, fibers, and rigid plastics for such uses as protective gear and multilayer assemblies," Long said. "We are recharging a field that has been around for a couple of decades because now we are challenged with applications that require IL performance."

The MURI is charged to provide fundamental enabling science for future Army technologies.

Senior researchers will focus in three areas. Long and Virginia Tech chemistry professor Harry W. Gibson will work on synthesis of ILs and charged polymers. Winey and Penn State professor of materials science and engineering Ralph H. Colby will do mechanical, electrical, and morphological characterization. Yossef Elabd, professor of chemical and biological engineering at Drexel University; Virginia Tech physics professor Randy Heflin; and Qiming Zhang, distinguished professor of electrical engineering at Penn State, will research performance of actuators, electro-optical devices, and membranes. Virginia Tech and Drexel are both Army Materials Centers of Excellence. ###

Industrial collaborators include DuPont, IBM Almaden, Kraton Polymers, NexGen Aeronautics, BASF, and Discover Technologies. "The industrial collaborators will validate related commercial interests, provide cost-effective manufacturing scenarios, and facilitate technology transfer for military technologies," said Long.

The ILEAD MURI will be administered through the Macromolecules and Interfaces Institute (MII) at Virginia Tech, and both fiscal management and program administration will be provided from both MII and the Virginia Tech Institute for Critical Science and Applied Technology. Long and Winey will serve as technical co-directors of the MURI and will work jointly with the Army Research Office and multiple Army Research Lab sites to coordinate periodic technical reviews, reporting, and technical strategy. Student internships will be available at the Army labs.

Contact: Susan Trulove 540-231-5646 Virginia Tech

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Friday, April 27, 2007

The boron buckyball makes its debut

Materials scientists find stable, spherical form for boron by Mark Passwaters, Special to the Rice News. It’s bigger, it’s bolder and it’s boron.

HOUSTON, -- A new study by Rice University scientists predicts the existence and stability of another "buckyball" consisting entirely of boron atoms.

The research, which has been published online and is due to appear as an editor's selection in Physical Review Letters, was conducted bv Boris Yakobson, professor of mechanical engineering and materials science and of chemistry, and his associates Nevill Gonzalez Szwacki and Arta Sadrzadeh.

A new study by Boris Yakobson (right), professor of mechanical engineering and materials science and of chemistry,  graduate student Arta Sadrzadeh (left) and colleagues, predicts the existence and stability of another 'buckyball' consisting entirely of boron atoms. Photo Credit JEFF FITLOWThe original buckyball, a cage-shaped molecule of 60 carbon atoms, was discovered at Rice by Robert Curl, Harold Kroto and Richard Smalley in 1985. The boron buckyball is structurally similar to the original C60 fullerene, but it has an additional atom in the center of each hexagon, which significantly increases stability.
"This is the first prediction of its possible existence," Yakobson said of the boron buckyball, or B80. "This has not been observed or even conceived of before. We do hope it may lead to a significant breakthrough."

In the earliest stages of their work, the team attempted to build a "buckyball" using silicon atoms but determined that it would collapse on itself. Their search for another possible atom led them on a short trip across the periodic table.

"Boron is nearby (one atomic unit from carbon). One reason we tried it was because of proximity," Yakobson said. "Boron also has the ability to catenate, to stick together better, than other atoms, which also made it appealing."

Initial work with 60 boron atoms failed to create a hollow ball that would hold its form, so another boron atom was placed into the center of each hexagon for added stability.

Yakobson estimated that the scientific work, the consideration of the variety of boron clusters to single out the B80, took more than a year, with Szwacki initially leading the work and then Sadrzadeh gradually taking greater part in the effort.

"We thought we had the answer, essentially, after three or four months, but then we had to prove it," Yakobson said. "There are numerous possibilities, but we had to prove that this was the answer. I think we’ve made a convincing case."

Yakobson said it is too early to speculate whether the boron buckyball will prove to be equally or more useful than its Nobel Prize-winning sibling.

"It’s too early to make comparisons," he said. "All we know is that it’s a very logical, very stable structure likely to exist.

"But this opens up a whole new direction, a whole new continent to explore. There should be a strong effort to find it experimentally. That may not be an easy path, but we gave them a good road map."

Following the paper's acceptance, there was a little debate with the journal's editors about whether or not the structure could be named "buckyball." Yakobson mentioned this to Curl.

"Bob (Curl) said with a chuckle that it was more of a ‘buckyball’ than his buckyball," Yakobson said. The reason being that C60 was named for famed architect Buckminster Fuller, because the buckyball looked like conjoined geodesic domes, a structure that Fuller had invented.

"When Fuller made his domes, he made them from triangles because hexagons would collapse," Yakobson said. "In B80, we fill the hexagon with one more atom, making triangles."

Yakobson said having the paper published in Physical Review Letters will help get the attention of experimentalists in the field.

"It is very helpful that this work can be seen and this is just a good instrument for it," he said. "To be able to deliver it to this broad a base of physicists and chemists is a good start." ###

The research was supported by the Robert A. Welch Foundation, the Office of Naval Research and the Department of Defense. April 23, 2007

Contact: Jade Boyd 713-348-6778 Rice University

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Thursday, April 26, 2007

Trips to the NanoFrontier PODCAST

NanoFrontiers: Visions for the Future of NanotechnologyNew podcasts, newsletter look to the future of nanotechnology. PODCAST in M4a FORMAT

WASHINGTON, D.C.—Nanotechnology's many anticipated benefits will arrive in waves of innovation, beginning with today's stain-resistant clothing and other first-generation applications and extending decades into the future, when extraordinarily advanced products, from self-repairing tissues to quantum computers, may become practical.
Given the incredible promise of the fast emerging field—and the billions in public and private investment that it has attracted — the Wilson Center's Project on Emerging Nanotechnologies (PEN) launched today a new series of NanoFrontiers newsletters and podcasts focused on progress toward exciting applications on the horizon of nanotechnology. Intended to encourage broader public understanding of nanotechnology, both are available on the PEN website at:

Prepared by freelance science writer Karen Schmidt, the first Trips to the NanoFrontier podcast features a discussion with Dr. Samuel I. Stupp, director of the Institute of BioNanotechnology in Medicine at Northwestern University, on prospective nanotechnology applications in tissue engineering. Dr. Stupp and colleagues are investigating how self-assembling nanofibers can be used to jump start repairs of damaged cells and restore functions. He also shares predictions on the long-term potential of using nanotechnology to treat specific medical conditions.

The debut issue of the monthly NanoFrontiers newsletter explores several developments in nanomedicine and examines where they might lead over the long term. Efforts focused on detecting, diagnosing, treating, and – ultimately – preventing cancer are used to illustrate nanotechnology-enabled progress in biomedicine.

The podcasts and newsletter build on the report NanoFrontiers: Visions for the Future of Nanotechnology, the outcome of a technology-forecasting meeting, sponsored by PEN, the National Institutes of Health, and the National Science Foundation. The newly issued report, written by Schmidt, also is available on the PEN website.

Future podcasts will feature discussions with experts on a variety of topics, including energy and clean water. The second issue of the NanoFrontiers newsletter will discuss how nanotechnology can be used to tackle challenges facing developing nations.

"As nanotechnology progresses, scientists and engineers are creating novel applications that have the potential to transform everything from manufacturing to medicine to energy production," said PEN Director David Rejeski. "Whether you are an expert, policymaker, or the 'average' citizen, it is becoming increasingly important to understand the prospects for nanotechnology and how the boundaries of innovation may be defined." ###

The Project on Emerging Nanotechnologies is an initiative launched by the Woodrow Wilson International Center for Scholars and The Pew Charitable Trusts in 2005. It is dedicated to helping business, government and the public anticipate and manage possible health and environmental implications of nanotechnology.

LINKS: Media interested in further information should contact Sharon McCarter at (202) 691-4016 or

Contact: Sharon McCarter 202-691-4016 Project on Emerging Nanotechnologies

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Plastic solar cell efficiency breaks record at WFU nanotechnology center

Plastic solar cell efficiency breaks record at WFU nanotechnology center

The global search for a sustainable energy supply is making significant strides at Wake Forest University as researchers at the university’s Center for Nanotechnology and Molecular Materials have announced that they have pushed the efficiency of plastic solar cells to more than 6 percent.

In a paper to be published in an upcoming issue of the journal Applied Physics Letters, Wake Forest researchers describe how they have achieved record efficiency for organic or flexible, plastic solar cells by creating “nano-filaments” within light absorbing plastic, similar to the veins in tree leaves. This allows for the use of thicker absorbing layers in the devices, which capture more of the sun’s light.

Efficient plastic solar cells are extremely desirable because they are inexpensive and light weight, especially in comparison to traditional silicon solar panels. Traditional solar panels are heavy and bulky and convert about 12 percent of the light that hits them to useful electrical power. Researchers have worked for years to create flexible, or “conformal,” organic solar cells that can be wrapped around surfaces, rolled up or even painted onto structures.

David Carroll, director of the Wake Forest nanotechnology centerThree percent was the highest efficiency ever achieved for plastic solar cells until 2005 when David Carroll, director of the Wake Forest nanotechnology center, and his research group announced they had come close to reaching 5 percent efficiency. High Resolution Image
Now, a little more than a year later, Carroll said his group has surpassed the 6 percent mark.

"Within only two years we have more than doubled the 3 percent mark,” Carroll said. “I fully expect to see higher numbers within the next two years, which may make plastic devices the photovoltaic of choice.”

In order to be considered a viable technology for commercial use, solar cells must be able to convert about 8 percent of the energy in sunlight to electricity. Wake Forest researchers hope to reach 10 percent in the next year, said Carroll, who is also associate professor of physics at Wake Forest.

Because they are flexible and easy to work with, plastic solar cells could be used as a replacement for roof tiling or home siding products or incorporated into traditional building facades. These energy harvesting devices could also be placed on automobiles. Since plastic solar cells are much lighter than the silicon solar panels structures do not have to be reinforced to support additional weight.
A large part of Carroll’s research is funded by the United States Air Force, which is interested in the potential uses of more efficient, light-weight solar cells for satellites and spacecraft. Other members of Carroll’s research team include Jiwen Liu and Manoj Namboothiry, postdoctoral associates at Wake Forest’s nanotechnology center, and Kyungkon Kim, a postdoctoral researcher at the center High Resolution ImageDr. Jiwen Liu, a researcher in the Wake Forest University Center for Nanotechnology and Molecular Materials, tests a new solar cell in the center's laboratory in Winston-Salem, NC.
, who has moved to the Materials Science & Technology Division at the Korea Institute of Science and Technology in Seoul.

Wake Forest University Press Contacts: Jacob McConnico (336) 758-5237, Kevin Cox (336) 758-5237, April 18, 2007

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Wednesday, April 25, 2007

Nanotechnology's Past, Present, and Future: A Congressional Perspective VIDEO

Nanotechnology's Past, Present, and Future: A Congressional Perspective VIDEO When upstate New York Republican Congressman Sherwood “Sherry” Boehlert retired last year, the U.S. Congress lost its most passionate “cheerleader for science.”

New York Republican Congressman Sherwood “Sherry” BoehlertIn his 24 years in the House of Representatives, including the last six as chair of the House Science Committee, Boehlert engaged in numerous science policy debates and groundbreaking programs, including the establishment of America’s National Nanotechnology Initiative (NNI) in 2000. He helped forge bipartisan support for the first U.S. government funds—$422 million—dedicated to nanoscale science and engineering research.
In his last year in Congress, Boehlert chaired several hearings on nanotechnology safety, particularly on the need to create and fund a prioritized federal nanotechnology environmental, health and safety research plan.

What was it like to be present at the creation of the NNI? What are the promises and potential pitfalls of nanotechnology and nanomanufacturing, which many predict will enable “The Next Industrial Revolution”?

Robert Service, nanotechnology reporter at Science magazine, will interview former Congressman Boehlert about the beginnings of the NNI and about the future of this transformative technology.

Speakers: Launch in external player
• Sherwood Boehlert, Public Policy Scholar, Woodrow Wilson International Center for Scholars; Former Chair, House Science Committee.
• Robert F. Service, Correspondent, Science, Interviewer
• David Rejeski (moderator), Director, Project on Emerging Nanotechnologies, Moderator
web: Project on Emerging Nanotechnology, e-mail:

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Tuesday, April 24, 2007

Measuring exposure to airborne nanomaterials

Measuring exposure to airborne nanomaterials Photo by Beamie Young/NISTNew methods and tools needed to measure exposure to airborne nanomaterials

WASHINGTON, DC—New methods and tools for measuring exposure to airborne engineered nanomaterials will be required to protect the health of workers in nanotechnology-related jobs— estimated to total 10 million people by 2014—according to two occupational health experts writing in the inaugural issue of the journal Nanotoxicology.
The article, "Assessing Exposure to Airborne Nanomaterials: Current Abilities and Future Requirements," written by Andrew Maynard, chief science advisor at the Wilson Center's Project on Emerging Nanotechnologies, and Robert Aitken, director of strategic consulting at the Institute of Occupational Medicine (Edinburgh, UK), can be viewed online at

"Airborne engineered nanomaterials present complex exposure measurement challenges," Maynard said. "Conventional approaches—measuring the mass of airborne material—will not always be sufficient. This presents a challenge because studies have indicated that, on a mass-for-mass basis, certain nanometer-scale particles may be more toxic than larger particles with a similar composition. In other words, smaller particles may be more harmful than conventional thinking would lead us to believe."
Maynard and Aitken reduced the incredibly diverse set of possible engineered nanoparticles into nine distinct categories, ranging from very simple spherical particles to complex multifunctional particles. By pairing these categories with particle properties associated with potential health effects, they teased out possible monitoring approaches for each particle-property combination.
"What our analysis shows is that in the complex new 'nano world' there is no single or simple method for monitoring nanoaerosol exposures in order to assess and manage potential health effects," Aitken explained. "There are instruments that present partial solutions to the measurement challenges we face. But at the end of the day, we lack the tools and devices that are sophisticated, cost-effective and fast enough to do the job."

Maynard and Aitken conclude that current approaches of measuring the number of particles in a volume of air, surface areas, and mass concentration, will all be useful to some degree. However, further research is needed to identify which is most important for specific nanomaterials and which measurement methods are most effective.

The authors advocate developing a new "universal aerosol monitor" capable of providing detailed information on the nature of airborne engineered nanomaterials to which people are exposed. Maynard, Aitken and 12 other experts included development of such a versatile measurement tool among five grand challenges that they viewed as essential to achieving the safe handling of nanotechnology in an article that appeared in the November 16, 2006 issue of the journal Nature.

The proposed wearable sampling device would measure aerosol number, surface area, and concentration mass simultaneously and would be low cost. Today, stand-alone instruments can perform the individual types of measurements called for by Maynard and Aitken. "Bringing these technologies together into a single package within the size and cost parameters discussed does present a significant challenge," they write.

"An economical integrated device will empower small and large nanotechnology industries alike to reduce uncertainty over what their workers are exposed to, and enable them to develop safer working environments" said Maynard. "This will require targeted research into developing new methodologies and new instruments. But the rapid advancement and commercialization of nanotechnologies are leading to the need for effective—if not necessarily perfect—exposure measurement approaches and devices to be developed as soon as possible."

In 2005, nanotechnology was incorporated into $30 billion in manufactured goods—a number predicted to grow to $2.6 trillion in annual manufactured goods by 2014. Already, there are almost 400 manufacturer-identified nanotechnology-based consumer products on the market—ranging from computer chips to automobile parts and from clothing to cosmetics and dietary supplements (see:

Nanotechnology is the ability to measure, see, manipulate and manufacture things usually between 1 and 100 nanometers. A nanometer is one billionth of a meter; a human hair is roughly 100,000 nanometers wide. ###

The Project on Emerging Nanotechnologies is an initiative launched by the Woodrow Wilson International Center for Scholars and The Pew Charitable Trusts in 2005. It is dedicated to helping business, government and the public anticipate and manage possible health and environmental implications of nanotechnology. For more information about the project, log on to

Contact: Sharon McCarter 202-691-4016 Project on Emerging Nanotechnologies

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Monday, April 23, 2007

Nanoparticles can damage DNA

Description: Image of DNA Double Helix, Courtesy: National Human Genome Research Institute.Cancer Tip: Nanoparticles Can Damage DNA, Increase Cancer Risk High Resolution Image, 300 DPI.

LOS ANGELES -- Tissue studies indicate that nanoparticles, engineered materials about a billionth of a meter in size,
could damage DNA and lead to cancer, according to research presented at the 2007 Annual Meeting of the American Association for Cancer Research.

Nanoparticles are small enough to penetrate cell membranes and defenses, yet they are large enough to cause trouble by interfering with normal cell processes, researchers at the University of Massachusetts say. Such nanoparticles are currently in use in electronics, cosmetics, and chemical manufacturing, among others industries. Because of their extremely small size, they can be difficult to isolate from the larger environment, as they are much too small for removal by conventional filtering techniques.

When nanoparticles find their way into cancer cells, they can wreak havoc, according to Sara Pacheco, an undergraduate researcher at the University of Massachusetts. Yet very little is known about how they behave in the environment or how they interact with and affect humans.

"Unfortunately, only a very small portion of research on nanoparticles is focused on health and safety risks, or on threats to the environment," Pacheco said. "I am concerned because so many new nanoparticles are being developed and there is little regulation on their manufacture, use and disposal."

Pacheco and her colleagues looked at how two different types of nanoparticles could cause DNA damage in the MCF-7 line of breast cancer cells.

She and her team examined the genotoxicity of silica and C60 fullerene nanoparticle suspensions using the alkaline single-cell gel electrophoresis assay (Comet assay) to quantify breaks in single and double stranded DNA. The team chose these particular nanoparticle types because they are commonly used commercially - in electronics, textiles and sporting goods - and easy to work with in the laboratory setting.

"We observed both dose-dependent and time-dependent increases in DNA damage in breast cancer cells exposed to either aqueous colloidal silica or C60 fullerenes," Pacheco said. "The DNA damage could potentially lead to mutations and ultimately increase the risk of cancer."

One problem is that, while it's clear that some nanoparticles can be more toxic than others, there's not enough data as yet to determine the most dangerous types.

"A lot is unknown about nanoparticle function, but clearly both size and composition are important," Pacheco said. "Several studies have shown that smaller particles are more likely to enter cells and cause more toxicity."

According to Pacheco, what makes matters worse is the fact that so far, aside from preventing their release, there are no known ways to prevent the harmful effects of environmental nanoparticles.

"It is important to know whether the nanoparticles are entering the cell and causing DNA damage directly or if they are acting on the membrane and inducing a cascade of events resulting in DNA damage," Pacheco said. "Once we understand the mechanisms by which nanoparticles induce their toxicity, we will be better able to prevent or mitigate their harmful effects."

In the meantime, the experimental team suggests that great caution should be taken in handling such nanoparticle suspensions and that any uncontrolled release should be avoided.

"Until we understand which types of nanoparticles are harmless and which have the potential to be harmful, I think it is prudent to limit their introduction into the environment," recommended Pacheco. # # # #

The mission of the American Association for Cancer Research is to prevent and cure cancer. Founded in 1907, AACR is the world's oldest and largest professional organization dedicated to advancing cancer research. The membership includes more than 25,000 basic, translational, and clinical researchers; health care professionals; and cancer survivors and advocates in the United States and more than 70 other countries.

AACR marshals the full spectrum of expertise from the cancer community to accelerate progress in the prevention, diagnosis and treatment of cancer through high-quality scientific and educational programs. It funds innovative, meritorious research grants. The AACR Annual Meeting attracts over 17,000 participants who share the latest discoveries and developments in the field. Special Conferences throughout the year present novel data across a wide variety of topics in cancer research, diagnosis and treatment.

AACR publishes five major peer-reviewed journals: Cancer Research; Clinical Cancer Research; Molecular Cancer Therapeutics; Molecular Cancer Research; and Cancer Epidemiology, Biomarkers & Prevention. Its most recent publication, CR, is a magazine for cancer survivors, patient advocates, their families, physicians, and scientists. It provides a forum for sharing essential, evidence-based information and perspectives on progress in cancer research, survivorship and advocacy.

Contact: Staci Vernick Goldberg 267-646-0616, American Association for Cancer Research

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Sunday, April 22, 2007

Earth Day “green nanotechnology”

Green Nanotechnology: It's Easier Than You Think. Project on Emerging NanotechnologiesGreen Nanotechnology: It's Easier Than You Think. The ability to eliminate waste and toxins from production processes early on, to create more efficient and flexible solar panels, and to remove contaminants from water is becoming an exciting reality with nanotechnology. This “green nanotechnology” involves designing nanoproducts for the environment and with the environment in mind.
Green nano is not just a niche among a few scientists or environmentalists, but is commercially viable among businesses; the investment community has recognized these green nano advances as big business and rewarded corporate innovators. A recent article, Green is Gold, advises investors: “Nowhere is the vision of technology in the service of sustainability more promising than in the field of nanotechnology,” (Forex Market, 3/15/07).

Last spring, several scientists, policymakers, lawyers, and industry representatives came together to participate in a series of dialogues on green nanotechnology held at the Woodrow Wilson Center. The American Chemical Society also held a symposium on Nanotechnology and the Environment at its annual meeting. On April 26, 2007, the Project on Emerging Nanotechnologies will release its first report on green nanotechnology, which highlights the research breakthroughs, industry perspectives, and policy options discussed at those meetings. The report, Green Nanotechnology: It’s Easier Than You Think, is written by journalist and science writer, Karen Schmidt.

Please join us for the release of this report with James Hutchison, a University of Oregon chemist who applies green chemistry principles in his production of gold nanoparticles; Barbara Karn, an Environmental Protection Agency scientist who kicked off the Project on Emerging Nanotechnologies’ Green Nano initiative while on detail at the Wilson Center; and John Carberry, an industry representative, who will discuss how it is both possible and profitable to be green with nanotechnology.

  • James E. Hutchison, Director, Oregon Nanoscience and Microtechnologies Institute’s Safer Nanomaterials and Nanomanufacturing Initiative, University of Oregon
  • Barbara Karn, Office of Research & Development, U.S. Environmental Protection Agency; Former Visiting Environmental Scientist, Project on Emerging Nanotechnologies, Woodrow Wilson International Center for Scholars
  • John Carberry, Director, Environmental Technologies, DuPont
  • Download the report: (link will be available at time and date of the event)
  • Webcast (available at time and date of event
Thursday April 26, 2007 • 10:00 – 11:00 AM • Woodrow Wilson Center• 5th Floor Conference Room. RSVP Required (No response required for webcast) Acceptances to

Contact: Sharon McCarter 202-691-4016 Project on Emerging Nanotechnologies

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Saturday, April 21, 2007

New World Record in Generation of High-Frequency Submillimeter Waves

UCLA Engineers Set New World Record in Generation of High-Frequency Submillimeter Waves

UCLA Engineering professor Frank Chang.Researchers at the UCLA Henry Samueli School of Engineering and Applied Science have achieved a new world record in high-frequency submillimeter waves. The record-setting 324-gigahertz frequency was accomplished using a voltage-controlled oscillator (Download high-quality images for this release )
in a 90-nanometer complementary metal-oxide semiconductor (CMOS) integrated circuit, a technology used in chips such as microprocessors.

The signal generator, which produces frequencies nearly 70 percent faster than other CMOS oscillators, paves the way for a new generation of submillimeter devices that could someday be used in high-resolution sensors on spacecraft, and here on Earth in a new class of highly integrated and lightweight imagers that could literally cut through fog and see through clothing fabrics. And because frequency ultimately means bandwidth, "the higher frequency increases the available bandwidth," said M.C. Frank Chang, UCLA professor of electrical engineering, who leads the research team. That greater bandwidth translates into faster communication speeds.

With traditional 90-nanometer CMOS circuit approaches, it is virtually impossible to generate usable submillimeter signals with a frequency higher than about 190 GHz. That's because conventional oscillator circuits are nonlinear systems in which increases in frequency are accompanied by a corresponding loss in gain or efficiency and an increase in noise, making them unsuitable for practical applications.

Chang, who also is director of UCLA Engineering's High Speed Electronics Laboratory, and researchers Daquan Huang and Tim LaRocca skirted the issues using a technological sleight of hand — and some unique analog signal processing.

The researchers first generated a voltage-controlled CMOS oscillator, or CMOS VCO, operating at a fundamental frequency of 81GHz with phase-shifted outputs at 0, 90, 180 and 270 degrees, respectively. By linearly superimposing these four (or quadruple) rectified phase-shifted outputs in real time, they ultimately generated a waveform with a resultant oscillation frequency that is four times the fundamental frequency, or 324 GHz. This new frequency generation method, in principle, has high DC-to-RF conversion efficiency (up to 8 percent) and has low phase noise, comparable to that of the constituent fundamental oscillation signal.

"When you go back to the fundamental math and physics, you find that you can do this and not pay much of a price. That's the beauty of it," Chang said. "If you use digital signal processing, you can synthesize this and synthesize that, but you pay the price for it with a loss of energy."

The measurement test of the 324-GHz signal was conducted by engineers Lorene Samoska and Andy Fung of NASA's Jet Propulsion Laboratory in Pasadena, which has facilities to test these high-frequency ranges. JPL and NASA are particularly interested in submillimeter technology because submillimeter-range wavelengths are ideal for deep-space remote sensing — there is no atmosphere in space to dampen the signals. Higher frequency signals, in turn, produce higher resolution images. "You can see better," Chang said.

Chang and Huang, in collaboration with JPL colleagues, have jointly applied for government grants to use the technology to design lightweight, low-power and highly integrated signal generators that can produce signals at frequencies up to 600 GHz. Applications for these high-frequency VCOs include imaging systems for both commercial and future space missions.

Creating 600-GHz signals requires a relatively straightforward modification of the circuit — either by increasing the fundamental frequency of the VCO or increasing the number of superimposed oscillator outputs (using eight or 16 instead of four).

"Because the algorithm has been validated, we know that we can achieve these frequencies," Chang says.

For example, if quadruple 85-GHz VCO outputs are used, the resulting output frequency would be 340 GHz. That frequency is something of a Holy Grail to the commercial aerospace industry and the military because it represents a "window" in our atmosphere where there is very little attenuation of submillimeter signals. (Essentially, they are invisible to the air.)

Normally, millimeter-range waves excite the atomic and molecular bonds in water, oxygen, carbon dioxide and other molecules in the atmosphere, and the gases absorb the waves. Signals at 340 GHz, however, "sneak through," Chang said, and can propagate long distances.

"One result is that waves of these frequencies can see through the fog, which is of interest to commercial aerospace companies," he said. Chang estimates that he and his colleagues will be able to produce the 340-GHz signals within the next six months.

Another application of the high-frequency CMOS VCOs of interest to the United States military is in submillimeter wavelength imaging.

"Because the wavelength is submillimeter, you may image through people's clothing," Chang said. "For example, it would be possible to remotely view if some civilian walking up to you has plastic explosives hidden under his coat."

CMOS technology makes future submillimeter-wave devices easily integrated with advanced microprocessors on-chip and can be very lightweight, so these sensors would be portable. "Foot soldiers could backpack them into the battle zone," Chang said.

About the UCLA Henry Samueli School of Engineering and Applied Science, Established in 1945, the UCLA Henry Samueli School of Engineering and Applied Science offers 28 academic and professional degree programs, including an interdepartmental graduate degree program in biomedical engineering. Ranked among the top 10 engineering schools at public universities nationwide, the school is home to seven multimillion-dollar interdisciplinary research centers in space exploration, wireless sensor systems, nanomanufacturing and defense technologies, which are funded by top national and professional agencies. For more information, visit -UCLA- MA154

Contact: Melissa Abraham 310-206-0540 University of California - Los Angeles

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Friday, April 20, 2007

New light on nanotubes

NIST's stretching exercises shed new light on nanotubes

Caption: Biomedical applications could exploit the natural fluorescence of the carbon nanotubes. When light is polarized along a single-walled carbon nanotube (left), this fluorescent emission is strong. Credit: NIST. Usage Restrictions: None.Stretching a carbon nanotube composite like taffy, researchers at the National Institute of Standards and Technology (NIST) and the Rochester Institute of Technology (RIT) 72 DPI Image, 150 DPI Image, 300 DPI Image
have made some of the first measurements* of how single-walled carbon nanotubes (SWNTs) both scatter and absorb polarized light, a key optical and electronic property.

SWNTs have excited materials scientists with the promise of novel materials that have exceptional mechanical, electronic, and optical properties. One fundamental issue is how light interacts with SWNTs. Is there, for example, a way to use appropriately tailored light to exert a force on SWNTs so that they can be trapped or aligned? Or can they be designed to be nanoscale tags for medical diagnostics? Semiconducting SWNTs can fluoresce in the near infrared region, an ideal characteristic for medical applications because body fluids and tissues are nearly transparent in that range.

Recent research on the optics of SWNTs has focused on the behavior of “excitons” — the pairing of a negatively charged electron with the positively charged “hole” that it leaves behind when it gets excited by a photon into a higher energy state. An important optical characteristic is how excitons in SWNTs impact the way the nanotubes absorb and scatter light.For example, how easy is it for the incident light to deform an exciton to create positive and negative poles? Theory says it should be significantly harder to do in a nearly one-dimensional nanotube than in a bulk semiconductor, where nearby electrons and holes reduce the amount of energy required.

Measuring that is difficult because the effect depends on the orientation of the nanotubes, and they’re hard to line up neatly. The NIST/RIT team solved the problem elegantly by wrapping SWNTs with DNA to keep them from clumping together, and dispersing them in a polymer. When they heated the polymer and stretched it in one direction, the nanotubes aligned like sugar crystals lining up in pulled taffy, making the optical measurements possible. The team obtained the first experimental verification of the full optical response of individual semiconducting SWNTs, finding good agreement with theory.

The stretching alignment technique is applicable to a broad range of SWNT experiments where orientation is important, particularly in optics. The work should further our current understanding of how nanotubes interact with light, with important practical applications in optical sensing and the manipulation of individual nanotubes using electromagnetic fields. ###

* J.A. Fagan, J.R. Simpson, B.J. Landi, L.J. Richter, I.Mandelbaum, V. Bajpai, D.L. Ho, R. Raffaelle, A.R. Hight Walker, B.J. Bauer and E.K. Hobbie. Dielectric response of aligned semiconducting single-wall nanotubes. Physical Review Letters. 98, 147402 (2007).

Contact: Michael Baum 301-975-2763 National Institute of Standards and Technology (NIST)

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