Saturday, May 31, 2008

Nanowires may boost solar cell efficiency, UC San Diego engineers say

nanowire-polymer hybrid device

Caption: Schematic of the nanowire-polymer hybrid device created by UC San Diego engineers and described in the journal NanoLetters. (Top to bottom): top yellow layer is the gold (Au) electrode that attracts the holes; blue gradient is the polymer material (P3HT) that absorbs the sunlight; the yellow wires are the InP nanowires that grow directly on the green metal substrate made of indium tin oxide (ITO). Credit: UC San Diego. Usage Restrictions: mandatory photo credit: UC San Diego.
Experimental solar cells spiked with nanowires show promise as efficient thin-film solar cells of future.

University of California, San Diego electrical engineers have created experimental solar cells spiked with nanowires that could lead to highly efficient thin-film solar cells of the future.

Indium phosphide (InP) nanowires can serve as electron superhighways that carry electrons kicked loose by photons of light directly to the device’s electron-attracting electrode – and this scenario could boost thin-film solar cell efficiency, according to research recently published in NanoLetters.
The new design increases the number of electrons that make it from the light-absorbing polymer to an electrode. By reducing electron-hole recombination, the UC San Diego engineers have demonstrated a way to increases the efficiency with which sunlight can be converted to electricity in thin-film photovoltaics.

Including nanowires in the experimental solar cell increased the “forward bias current” – which is a measure of electrical current – by six to seven orders of magnitude as compared to their polymer-only control device, the engineers found.

The online journal NanoLetters published this new work on polymer/nanowire hybrid photovoltaics in February 2008.

image of n-type InP nanowire growth on indium tin oxide

Caption: Scanning electron microscope (SEM) image of n-type InP nanowire growth on indium tin oxide (ITO) taken at a 45 degree tilt with scale bar of 500 nanometers. Credit: UC San Diego. Usage Restrictions: mandatory photo credit: UC San Diego.
“If you provide electrons with a defined pathway to the electrode, you can reduce some of the inefficiencies that currently plague thin-film solar cells made from polymer mixtures. More efficient transport of electrons and holes – collectively known as carriers – is critical for creating more efficient solar cells,” said Clint Novotny the first author of the NanoLetters paper, and a recent electrical engineering Ph.D. from UC San Diego’s Jacobs School of Engineering. Novotny is now working on solar technologies at BAE Systems.
Simplified Nanowire Growth

The engineers devised a way to grow nanowires directly on the electrode. This advance allowed them to create the electron superhighways that deliver electrons from the polymer-nanowire interface directly to an electrode.

“If nanowires are going to be used massively in photovoltaic devices, then the growth mechanism of nanowires on arbitrary metallic surfaces is an issue of great importance,” said co-author Paul Yu, a professor of electrical engineering at UC San Diego’s Jacobs School of Engineering. “We contributed one approach to growing nanowires directly on metal.”

The UCSD electrical engineers grew their InP nanowires on the metal electrode –indium tin oxide (ITO) – and then covered the nanowire-electrode platform in the organic polymer, P3HT, also known as poly(3-hexylthiophene). The researchers say they were the first group to publish work demonstrating growth of nanowires directly on metal electrodes without using specially prepared substrates such as gold nanodrops.

“Just a layer of metal can work. In this paper we used ITO, but you can use other metals, including aluminum,” said Paul Yu.

Growing nanowires directly on untreated electrodes is an important step toward the goal of growing nanowires on cheap metal substrates that could serve as foundations for next-generation photovoltaics that conform to the curved surfaces like rooftops, cars or other supporting structures, the engineers say.

“By growing nanowires directly on an untreated electrode surface, you can start thinking about incorporating millions or billions of nanowires in a single device. I think this is where the field is eventually going to end up,” said Novotny. “But I think we are at least a decade away from this becoming a mainstream technology.”

Polymer Solar Cells and Nanowires Meet

As in more traditional organic polymer thin-film solar cells, the polymer material in the experimental system absorbs photons of light. To convert this energy to electricity, each photon-absorbing electron must split apart from its hole companion at the interface of the polymer and the nanowire – a region known as the p-n junction.

Once the electron and hole split, the electron travels down the nanowire – the electron superhighway – and merges seamlessly with the electron-capturing electrode. This rapid shuttling of electrons from the p-n junction to the electrode could serve to make future photovoltaic devices made with polymers more efficient.

“In effect, we used nanowires to extend an electrode into the polymer material,” said co-author Edward Yu, a professor of electrical engineering at UCSD’s Jacobs School of Engineering.

While the electrons travel down the nanowires in one direction, the holes travel along the nanowires in the opposite direction – until the nanowire dead ends. At this point, the holes are forced to travel through a thin polymer layer before reaching their electrode.

Today’s thin-film polymer photovoltaics do not provide freed electrons with a direct path from the p-n junction to the electrode – a situation which increases recombination between holes and electrons and reduces efficiency in converting sunlight to electricity. In many of today’s polymer photovoltaics, interfaces between two different polymers serve as the p-n junction. Some experimental photovoltaic designs do include nanowires or carbon nanotubes, but these wires and tubes are not electrically connected to an electrode. Thus, they do not minimize electron-hole recombination by providing electrons with a direct path from the p-n junction to the electrode the way the new UCSD design does.

Before these kinds of electron superhighways can be incorporated into photovoltaic devices, a series of technical hurdles must be addressed – including the issue of polymer degradation. “The polymers degrade quickly when exposed to air. Researchers around the world are working to improve the properties of organic polymers,” said Paul Yu.

As it was a proof-of-concept project, the UCSD engineers did not measure how efficiently the device converted sunlight to electricity. This explains, in part, why the authors refer to the device in their NanoLetters paper as a “photodiode” rather than a “photovoltaic.”

Having a more efficient method for getting electrons to their electrode means that researchers can make thin-film polymer solar cells that are a little bit thicker, and this could increase the amount of sunlight that the devices absorb. ###

Paper title: “InP Nanowire/Polymer Hybrid Photodiode,” by Clint J. Novotny, Edward T. Yu, and Paul K. Y. Yu from the Department of Electrical and Computer Engineering, University of California, San Diego. Published on the NanoLetters Web site on 02/12/2008

This project is one of the ways UCSD’s Jacobs School of Engineering is addressing the National Academy of Engineering Grand Challenge of “Make Solar Energy Economical.” Learn more about how UCSD is addressing the NAE Grand Challenges.

Contact: Daniel Kane dbkane@ucsd.edu 858-534-3262 University of California - San Diego

Friday, May 30, 2008

Federal government taps NC State experts to explain nanotech risks

Dr. David M. Berube
David M. Berube Professor, PCOST Coordinator Ph.D. New York University 1990 EMAIL

Dr. David M. Berube is Professor of Communication Studies and NanoSTS, Communication Coordinator for International Council on Nanotechnology USC NanoCenter, and Director of Nano-Ethics. He is on the advisory committee of the U.S. Environmental Protection Agency, Office of Pollution Prevention and Toxics, and on the advisory committee of the International Council on Nanotechnology. He is the author of Nano-Hype: The Truth Behind the Nanotechnology Buzz, Non-Policy Debating, the innovative Amazon download Nanotechnology politics.(FORUM): An article from: Issues in Science and Technology, and of the blog NanoHype: Nanotechnology Implications and Interactions. He is the recipient of a National Science Foundation (NSF) grant (2003-2007) to study the ethical implications of nanotechnology and is associated as a site manager, CoPI, or mentor on two others.

BIOGRAPHY CREDIT: Lifeboat Foundation Bios: Dr. David M. Berube
The arm of the federal government responsible for coordinating nanotechnology research and regulations across the country has called on experts from North Carolina State University to craft a white paper that will lay out how government and industry officials should communicate potential risks associated with nanotechnology to the media and the public. NC State communication expert Dr. David Berube has been negotiating this project for nearly 18 months.

NC State’s Dr. Brenton Faber, who is also associated with the project, says the goal of the white paper is to advise the government on how it can “accurately communicate the risks and opportunities presented by nanotechnology.” Faber explains that “people want to know if nanotechnology is something they should worry about, and it is important for the government to be able to explain any potential risks to the public in a manner that they can understand – because what is the point of developing these technologies if people don’t trust them?”

Berube notes that the white paper could also be used for years to come to inform how the government, industry and researchers convey information about the risks of any new technologies. “The last time a white paper on risk communication was done was in 1989,” Berube says, adding “there is little doubt this could craft the direction of risk communication for some time. This is quite an honor and a challenge.”

The paper is due July 31 and will be followed up with a one-day workshop in Washington, D.C. This workshop and others planned will feature the report’s authors, who will advise government officials on some of the better ways to communicate accurately with the media and the public.

The National Nanotechnology Coordinating Office (NNCO) coordinates activities relating to the National Nanotechnology Initiative. Dr. Vivian Ota Wang, the communication director of the NNCO, who also serves on the National Science and Technology Council of the Executive Office of the President, selected Berube to author the white paper. Drs. Faber from NC State and Dietram Scheufele from the University of Wisconsin, as members of NC State’s new Public Communication of Science and Technology project, agreed to assist.
Funding for the white paper will support one graduate student and one doctoral student, who will work on the project at NC State this summer.

Nanotechnology is generally defined as technology that uses substances having a size of 100 nanometers or less (thousands of times thinner than a human hair), and is expected to have widespread uses in medicine, consumer products and industrial processes.

Faber is a professor of English at NC State. His research focuses on communications related to science and technology.

Berube is the author of Nanohype: Beyond the Nanotechnology Buzz, a contributing author to the anthology Nanotechnology & Society: Current and Emerging Ethical Issues and teaches graduate courses in risk communication and rhetoric in science and technology at NC State. ###

Contact: Matt Shipman matt_shipman@ncsu.edu 919-515-6386 North Carolina State University

Thursday, May 29, 2008

Scientists demonstrate method for integrating nanowire devices directly onto silicon

Structure of the Nanowire Device

Caption: The basic structure of the nanowire devices is based on a sandwich geometry in which a nanowire (n-type zinc oxide) is placed between the substrate (heavily doped p-type silicon) and a top metallic contact, using spin-on glass as an insulating spacer layer to prevent the metal contact from shorting to the substrate (as shown in (a) and (b)).

This allows for uniform injection of current along the length of the nanowire. A finished wafer using the team's method is shown in (c), with a typical device shown in (d). Note that a stray nanowire intercepts the device on the upper part of (d). The oval feature surrounding the stray nanowire is due to the varying thickness of the spin-on glass film. When a voltage is applied to this device, it emits ultraviolet light (as shown in image (e) obtained with a CCD camera) with a peak wavelength of ~380 nm.

Credit: Courtesy of the lab of Federico Capasso, Harvard School of Engineering and Applied Sciences. Usage Restrictions: None
Fabrication technique could yield low-cost, scalable nanowire photonic and electronic circuits

Cambridge, Mass. --- Applied scientists at Harvard University in collaboration with researchers from the German universities of Jena, Gottingen, and Bremen, have developed a new technique for fabricating nanowire photonic and electronic integrated circuits that may one day be suitable for high-volume commercial production.

Spearheaded by graduate student Mariano Zimmler and Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering, both of Harvard's School of Engineering and Applied Sciences (SEAS), and Prof. Carsten Ronning of the University of Jena, the findings will be published in Nano Letters. The researchers have filed for U.S. patents covering their invention.

While semiconductor nanowires---rods with an approximate diameter of one-thousandth the width of a human hair---can be easily synthesized in large quantities using inexpensive chemical methods, reliable and controlled strategies for assembling them into functional circuits have posed a major challenge.
By incorporating spin-on glass technology, used in Silicon integrated circuits manufacturing, and photolithography, transferring a circuit pattern onto a substrate with light, the team demonstrated a reproducible, high-volume, and low-cost fabrication method for integrating nanowire devices directly onto silicon.

"Because our fabrication technique is independent of the geometrical arrangement of the nanowires on the substrate, we envision further combining the process with one of the several methods already developed for the controlled placement and alignment of nanowires over large areas," said Capasso. "We believe the marriage of these processes will soon provide the necessary control to enable integrated nanowire photonic circuits in a standard manufacturing setting."
Federico Capasso and Mariano Zimmler of the Harvard School of Engineering and Applied Sciences

Caption: Federico Capasso and Mariano Zimmler of the Harvard School of Engineering and Applied Sciences. Credit: Eliza Grinnell, Harvard School of Engineering and Applied Sciences. Usage Restrictions: None.
The structure of the team's nanowire devices is based on a sandwich geometry: a nanowire is placed between the highly conductive substrate, which functions as a common bottom contact, and a top metallic contact, using spin-on glass as a spacer layer to prevent the metal contact from shorting to the substrate. As a result current can be uniformly injected along the length of the nanowires. These devices can then function as light-emitting diodes, with the color of light determined by the type of semiconductor nanowire used.

To demonstrate the potential scalability of their technique, the team fabricated hundreds of nanoscale ultraviolet light-emitting diodes by using zinc oxide nanowires on a silicon wafer. More broadly, because nanowires can be made of materials commonly used in electronics and photonics, they hold great promise for integrating efficient light emitters, from ultraviolet to infrared, with silicon technology. The team plans to further refine their novel method with an aim towards electrically contacting nanowires over entire wafers.
"Such an advance could lead to the development of a completely new class of integrated circuits, such as large arrays of ultra-small nanoscale lasers that could be designed as high-density optical interconnects or be used for on-chip chemical sensing," said Ronning. ###

The team's co-authors are postdoctoral fellow Wei Yi and Venkatesh Narayanamurti, John A. and Elizabeth S. Armstrong Professor and dean, both of Harvard's School of Engineering and Applied Sciences; graduate student Daniel Stichtenoth, University of G�ttingen; and postdoctoral fellow Tobias Voss, University of Bremen.

The research was supported by the National Science Foundation (NSF) and the German Research Foundation. The authors also acknowledge the support of two Harvard-based centers, the National Science Foundation Nanoscale Science and Engineering Center (NSEC) and the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Infrastructure Network (NNIN).

Contact: Michael Patrick Rutter mrutter@seas.harvard.edu 617-496-3815 Harvard University

Wednesday, May 28, 2008

New technique measures ultrashort laser pulses at focus

Georgia Tech physics professor Rick Trebino and graduate student Pam Bowlan

Caption: Georgia Tech physics professor Rick Trebino and graduate student Pam Bowlan pose with SEA TADPOLE, a device that allows non-laser scientists to easily measure complicated ultrashort pulses. Credit: Georgia Tech Photo: Gary Meek. Usage Restrictions: None.
Lasers that emit ultrashort pulses of light are used for numerous applications including micromachining, microscopy, laser eye surgery, spectroscopy and controlling chemical reactions. But the quality of the results is limited by distortions caused by lenses and other optical components that are part of the experimental instrumentation.

To better understand the distortions, researchers at the Georgia Institute of Technology developed the first device to directly measure complex ultrashort light pulses in space and time at and near the focus.
Measuring the pulse at the focus is important because that’s where the beam is most intense and where researchers typically utilize it. Knowing how the light is distorted allows researchers to correct for the aberrations by changing a lens or using a pulse shaper or compressor to manipulate the pulse into the desired form.

“Researchers have always measured the pulse immediately as it exited the laser, so they didn’t realize the extent to which the pulse became distorted by the time it reached the focus after traveling through the optics and lenses in the system,” said Rick Trebino, a professor in the Georgia Institute of Technology’s School of Physics and Georgia Research Alliance Eminent Scholar in Ultrafast Optical Physics.
Georgia Tech physics professor Rick Trebino

Caption: Georgia Tech physics professor Rick Trebino makes slight adjustments to the device he developed with graduate student Pam Bowlan that directly measures complex ultrashort light pulses in space and time at and near the focus. Credit: Georgia Tech Photo: Gary Meek. Usage Restrictions: None.
The device was described in a presentation at the Conference on Lasers and Electro-Optics on May 8. This research was funded by the National Science Foundation and published in the August 2007 issue of the journal Optics Express.

It is difficult to measure ultrashort pulses because they typically last between a few femtoseconds and a picosecond, which are 10-15 and 10-12 of a second, and faster than the response time of the fastest electronics.

“The light comes out as a train of extremely short bursts. The laser crams all of the energy of a continuous laser into a few femtoseconds, which creates really intense laser pulses,” said Pam Bowlan, a graduate student supported by the Technological Innovation: Generating Economic Results (TI:GER) program.

To achieve the highest possible intensity of the laser, the pulse must be as small as possible in space and as short as possible in time. However, focused pulses nearly always have distortions in time that vary significantly from point to point in space due to lens aberrations in focusing optics.
To address those issues, the new device, called SEA TADPOLE (Spatial Encoded Arrangement for Temporal Analysis by Dispersing a Pair of Light E-fields), allows researchers to measure complicated ultrashort pulses simultaneously in space and time as they go through the focus.

“A lot of chemists and biologists use ultrafast lasers, so it was important that our device be easy to use because non-laser scientists don’t want to spend all day measuring their laser pulses,” noted Bowlan.

The research team – which also included former graduate students Pablo Gabolde and Selcuk Akturk – used the concept of interferometry to measure a pulse in space and time. Two pulses, one reference and one unknown, were sent through optical fibers. The fibers were mounted on a scanning stage so that the pulses could be measured at many locations around the focus.

The pulses were crossed and an interference pattern was recorded for each color of the pulse at each location with a digital camera. The patterns were used to determine the shape of the unknown pulse in space and time and to create movies showing how the intensity and color of the pulse changed in space and time as it focused.

“Because the laser pulses enter SEA TADPOLE through optical fibers, which only collect a very small portion of the light, the device naturally measures pulses with high spatial resolution and can measure them at a focus spot size smaller than a micron,” explained Bowlan. To further improve the spatial resolution of the device, the research team began to use specialized fibers, called near-field scanning optical microscopy fibers, which can resolve features smaller than the wavelength of the light.

The researchers tested the device by measuring ultrashort pulses focused by various lenses, since each lens can cause different complex distortions. To validate the measurements, Bowlan performed simulations of pulses propagating through the experimental lenses. Results showed that a common plano-convex lens displayed chromatic and spherical aberrations, whereas more expensive aspheric and doublet lenses exhibited mostly chromatic aberrations.

Spherical aberrations occur when the light that strikes the edges of the lens gets focused to a different point than the light that strikes the center, creating a larger, inhomogeneous focused spot size. Chromatic aberrations occur because the many colors in the laser travel at different speeds and do not stay together in space and time as the pulse passes through glass components in the experimental setup, such as lenses. As a result, each color arrives at the focus at a different time, creating a rainbow of colors in the electric field images.

Aberrations can drastically increase the pulse length, which decreases the laser intensity. A lower intensity forces researchers to increase the power of the laser, increasing the possibility of damaging the sample. Aberrations can also yield odd pulse and beam shapes at the focus, which complicate the interpretation of the experiment or application.

“Our system tells researchers what types of aberrations are present in instrumentation, which then allows them to test different lenses in the instrumentation setup or use a pulse shaper to create the desired pulse at the focus that’s free of distortions,” added Bowlan. ###

Contact: Abby Vogel avogel@gatech.edu 404-385-3364 Georgia Institute of Technology Research News

Tuesday, May 27, 2008

UC San Diego researchers target tumors with tiny 'nanoworms'

Segmented nanoworms

Segmented “nanoworms” composed of magnetic iron oxide and coated with a polymer are able to find and attach to tumors. Credit: Ji-Ho Park, UCSD.

Scientists at UC San Diego, UC Santa Barbara and MIT have developed nanometer-sized “nanoworms” that can cruise through the bloodstream without significant interference from the body’s immune defense system and—like tiny anti-cancer missiles—home in on tumors.

Their discovery, detailed in this week’s issue of the journal Advanced Materials, is reminiscent of the 1966 science fiction movie, the Fantastic Voyage, in which a submarine is shrunken to microscopic dimensions, then injected into the bloodstream to remove a blood clot from a diplomat’s brain.


Using nanoworms, doctors should eventually be able to target and reveal the location of developing tumors that are too small to detect by conventional methods. Carrying payloads targeted to specific features on tumors, these microscopic vehicles could also one day provide the means to more effectively deliver toxic anti-cancer drugs to these tumors in high concentrations without negatively impacting other parts of the body.

“Most nanoparticles are recognized by the body's protective mechanisms, which capture and remove them from the bloodstream within a few minutes,” said Michael Sailor, a professor of chemistry and biochemistry at UC San Diego who headed the research team. “The reason these worms work so well is due to a combination of their shape and to a polymer coating on their surfaces that allows the nanoworms to evade these natural elimination processes. As a result, our nanoworms can circulate in the body of a mouse for many hours.”

“When attached to drugs, these nanoworms could offer physicians the ability to increase the efficacy of drugs by allowing them to deliver them directly to the tumors,” said Sangeeta Bhatia, a physician, bioengineer and a professor of Health Sciences and Technology at MIT who was part of the team. “They could decrease the side effects of toxic anti-cancer drugs by limiting their exposure of normal tissues and provide a better diagnosis of tumors and abnormal lymph nodes.”

The scientists constructed their nanoworms from spherical iron oxide nanoparticles that join together, like segments of an earthworm, to produce tiny gummy worm-like structures about 30 nanometers long—or about 3 million times smaller than an earthworm. Their iron-oxide composition allows the nanoworms to show up brightly in diagnostic devices, specifically the MRI, or magnetic resonance imaging, machines that are used to find tumors.

“The iron oxide used in the nanoworms has a property of superparamagnetism, which makes them show up very brightly in MRI,” said Sailor. “The magnetism of the individual iron oxide segments, typically eight per nanoworm, combine to provide a much larger signal than can be observed if the segments are separated. This translates to a better ability to see smaller tumors, hopefully enabling physicians to make their diagnosis of cancer at earlier stages of development.”

In addition to the polymer coating, which is derived from the biopolymer dextran, the scientists coated their nanoworms with a tumor-specific targeting molecule, a peptide called F3, developed in the laboratory of Erkki Ruoslahti, a cell biologist and professor at the Burnham Institute for Medical Research at UC Santa Barbara. This peptide allows the nanoworms to target and home in on tumors.

“Because of its elongated shape, the nanoworm can carry many F3 molecules that can simultaneously bind to the tumor surface,” said Sailor. “And this cooperative effect significantly improves the ability of the nanoworm to attach to a tumor.”

The scientists were able to verify in their experiments that their nanoworms homed in on tumor sites by injecting them into the bloodstream of mice with tumors and following the aggregation of the nanoworms on the tumors. They found that the nanoworms, unlike the spherical nanoparticles of similar size that were shuttled out of the blood by the immune system, remained in the bloodstream for hours.

“This is an important property because the longer these nanoworms can stay in the bloodstream, the more chances they have to hit their targets, the tumors,” said Ji-Ho Park, a UC San Diego graduate student in materials science and engineering working in Sailor’s laboratory.

Park was the motivating force behind the discovery when he found by accident that the gummy worm aggregates of nanoparticles stayed for hours in the bloodstream despite their relatively large size.

While it’s not clear yet to the researchers why, Park notes that “the nanoworm’s flexibly moving, one dimensional structure may be one the reasons for its long life in the bloodstream.”

The researchers are now working on developing ways to attach drugs to the nanoworms and chemically treating their exteriors with specific chemical “zip codes,” that will allow them to be delivered to specific tumors, organs and other sites in the body.

“We are now using nanoworms to construct the next generation of smart tumor-targeting nanodevices,” said Ruoslahti. We hope that these devices will improve the diagnostic imaging of cancer and allow pinpoint targeting of treatments into cancerous tumors.” ###

Other researchers involved in the development were Michael Schwartz of UC San Diego, Geoffrey von Maltzahn of MIT, and Lianglin Zhang of UC Santa Barbara. The project was funded by grants from the National Cancer Institute of the National Institutes of Health.

Comment: Michael Sailor 858-534-8188 Contact: Kim McDonald kimmcdonald@ucsd.edu 858-534-7572 University of California - San Diego

Monday, May 26, 2008

Nanotube production leaps from sooty mess in test tube to ready formed chemical microsensors

University of Warwick researchers Ioana Dumitrescu, Professor Julie Macpherson, Professor Patrick Unwin

University of Warwick researchers Ioana Dumitrescu, Professor Julie Macpherson, Professor Patrick Unwin.
Carbon nanotubes’ potential as a super material is blighted by the fact that when first made they often take the form of an unprepossessing pile of sooty black mess in the bottom of a test tube. Now researchers in the University of Warwick’s Department of Chemistry have found a way of producing carbon nanotubes in which they instantly form a highly sensitive ready made electric circuit.
The research has just been published in a paper entitled "Single-Walled Carbon Nanotube Network Ultramicroelectrodes" by University of Warwick researchers Ioana Dumitrescu, Professor Julie Macpherson, Professor Patrick Unwin, and Neil Wilson in Analytical Chemistry, 2008, 10.1021/ac702518g

The researchers used a form of chemical vapour deposition and lithography to create the ready made disc shaped single walled carbon nanotube based ultramicroelectrodes. The nanotubes deposit themselves flat on a surface in a random but relatively even manner. They also all overlap sufficiently to create a single complete metallic micro-circuit right across the final disc. What is even more impressive is that they take up less than one per cent of the surface area of the disc.

This final property makes these instant ultramicroelecrodes particular useful for the creation of ultra sensitive sensors. The low surface area of the conducting part of the disc means that they can be used to screen out background "noise" and cope with low signal to noise ratios making them up to 1000 times more sensitive than conventional ultramicroelecrodes sensors. This property also produces very fast response times allowing them to respond ten times faster than conventional ultramicroelecrodes.

As these ready made ultramicroelecrodes are carbon based they also open up a range of new possibilities for use in living systems. The biocompatibility of carbon is in stark contrast with the obvious problems that platinum and other metal based probes can pose for living tissue. The Warwick research team are already beginning to explore how their single walled carbon nanotube based ultramicroelecrodes can be used to measure levels of neurotransmitters.

The new ultramicroelecrodes also open up interesting possibilities for catalysis in fuel cells. Up till now researchers had been aware that this form of carbon nanotubes appeared to be particularly useful in the area of catalysis but there was uncertainty as to whether it was the properties of the carbon nanotubes per se that provide this benefit or whether it was due to impurities in their production. The researchers have been able to use this new method of single walled carbon nanotube assembly to prove that it is actually the properties of the carbon nanotubes themselves that are useful for catalysis. The new carbon nanotube assembly technique brings a further benefit to catalysis applications as the Warwick researchers have been able to use electrodepoistion to quickly and easily apply specific metal coatings to the ready formed single walled carbon nanotube microelectrode networks. This will be of significant benefit to anyone wanting to use single walled carbon nanotube for catalysis in fuel cell technology.

Contact: Peter Dunn p.j.dunn@warwick.ac.uk 44-024-765-23708 University of Warwick

For further information please contact:

Professor Julie Macpherson, Department of Chemistry University of Warwick Tel: +44 (0)2476 573886 J.Macpherson@warwick.ac.uk

Professor Patrick Unwin: Department of Chemistry University of Warwick, +44 (0) 2476 523264 P.R.Unwin@warwick.ac.uk

Peter Dunn, Press and Media Relations Manager, University of Warwick Tel: +44 (0)24 76 523708 or +44 (0)7767 655860 p.j.dunn@warwick.ac.uk

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Sunday, May 25, 2008

Melting defects could lead to smaller, more powerful microchips

nanometer-scale structures

Caption: These electron microscope images show before (left column) and after (right column) examples of a new technique, developed at Princeton University, for perfecting nanometer-scale structures. Credit: Stephen Chou/Nature Nanotechnology. Usage Restrictions: None.
As microchips shrink, even tiny defects in the lines, dots and other shapes etched on them become major barriers to performance. Princeton engineers have now found a way to literally melt away such defects, using a process that could dramatically improve chip quality without increasing fabrication cost.

The method, published in the May 4 issue of Nature Nanotechnology, enables more precise shaping of microchip components than what is possible with current technology. More precise component shapes could help manufacturers build smaller and better microchips, the key to more powerful computers and other devices.
"We are able to achieve a precision and improvement far beyond what was previously thought achievable," said electrical engineer Stephen Chou, the Joseph C. Elgin Professor of Engineering, who developed the method along with graduate student Qiangfei Xia. Chou's lab has previously pioneered a number of innovative chip making techniques, including a revolutionary method for making nanometer-scale patterns using imprinting.
Self-Perfecting Nanotechnology

Caption: A technique invented in the lab of Princeton engineer Stephen Chou allows for the easy correction of defects and refinement of shapes in nanostructures. The "Open" method involves using a laser to briefly melt defects, which self correct before cooling. The "Capped" method prevents the technique from rounding off the structures. The "Guided" version causes the structures to grow toward a nearby plate, causing them to become not only smoother, but taller and thinner, which are all desirable traits for creating smaller, more powerful computer chips. Credit: Stephen Chou/Nature Nanotechnology. Usage Restrictions: None.
Microchips work best when the structures fabricated on them are straight, thin and tall. Rough edges and other defects can degrade or even ruin chip performance in most applications. In integrated circuits, for instance, such flaws could cause current to leak and voltage to fluctuate. In optic devices, they could interfere with the transmission of light. In biological devices, they could impede the flow of DNA and other biomaterials.

"These chip defects pose serious roadblocks to future advances in many industries," Chou said.

To deal with this problem, researchers try to improve the process used to make the microchips.
However, Chou said such an approach works only to a point; eventually chip makers will run up against fundamental physical limits of current manufacturing techniques. In particular, the electrons and photons that are used like chisels to carve out the microscopic features on a chip always have some random behavior. This effect becomes pronounced at very small scales and limits the accuracy of component shapes.

"What we propose instead is a paradigm shift: Rather than struggle to improve fabrication methods, we could simply fix the defects after fabrication," said Chou. “And fixing the defects could be automatic -- a process of self-perfection.”

Chou's method, termed Self-Perfection by Liquefaction (SPEL), achieves this by melting the structures on a chip momentarily, and guiding the resulting flow of liquid so that it re-solidifies into the desired shapes. This is possible because natural forces acting on the molten structures, such as surface tension -- the force that allows some insects to walk on water -- smooth the structures into geometrically more accurate shapes. Lines, for instance, become straighter, and dots become rounder.

Simple melting by direct heating has previously been shown to smooth out the defects in plastic structures. This process can't be applied to a microchip, for two reasons. First, the key structures on a chip are not made of plastic, which melts at temperatures close to the boiling point of water, but from semiconductors and metals, which have much higher melting points. Heating the chip to such temperatures would melt not just the structures, but nearly everything else on the chip. Secondly, the melting process would widen the structures and round off their top and side surfaces, all of which would be detrimental to the chip.

Chou's team overcame the first obstacle by using a light pulse from so-called excimer laser, similar to those used in laser eye surgery, because it heats only a very thin surface layer of a material and causes no damage to the structures underneath. The researchers carefully designed the pulse so that it would melt only semiconductor and metal structures, and not damage other parts of the chip. The structures need to be melted for only a fraction of a millionth of a second, because molten metal and semiconductors can flow as easily as water and have high surface tension, which allows them to change shapes very quickly.

To overcome the second obstacle, Chou's team placed a plate on top of the melting structures to guide the flow of liquid. The plate prevents a molten structure from widening, and keeps its top flat and sides vertical, Chou said. In one experiment, it made the edges of 70 nanometer-wide chromium lines more than five times smoother. The resulting line smoothness was far more precise than what semiconductor researchers believe to be attainable with existing technology.

The conventional approach to fixing chip defects is to measure the exact shape of each defect, and provide a correction precisely tailored to it -- a slow and expensive process, Chou said. In contrast, Chou's guided melting process fixes all defects on a chip in a single quick and inexpensive step. "Regardless of the shape of each defect, it always gets fixed precisely and with no need for individual shape measurement or tailored correction," Chou said.

One of the big surprises from this work is observed when the guiding plate is placed not in direct contact with the molten structures, but at a distance above it. In this situation, the liquid material from the structures rises up and reaches the plate by itself, causing line structures to become taller and narrower -- both highly desirable outcomes from a chip design perspective.

"The authors demonstrate improved edge roughness and dramatically altered aspect ratios in nanoscale features," said Donald Tennant, director of operations at the NanoScale Science and Technology Facility at Cornell University. The techniques "may be a way forward when nanofabricators bump up against the limits of lithography and pattern transfer," he said.

Next, Chou's group plans to demonstrate this technique on large (8-inch) wafers. Several leading semiconductor manufacturers have expressed keen interest in the technique, Chou said. ###

The work was supported by the Defense Advanced Research Projects Agency and the Office of Naval Research.

Contact: Steven Schultz sschultz@princeton.edu 609-258-3617 Princeton University, Engineering School

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Saturday, May 24, 2008

Environmental fate of nanoparticles depends on properties of water carrying them

Equipment for Studying C60 Transport

Caption: Laboratory equipment used to study the transport and retention of C60 nanoparticles in water. Credit: Georgia Tech Photo: Gary Meek. Usage Restrictions: None.
Nanomaterials in the environment - The fate of carbon-based nanoparticles spilled into groundwater – and the ability of municipal filtration systems to remove the nanoparticles from drinking water – depend on subtle differences in the solution properties of the water carrying the particles, a new study has found.

In slightly salty water, for example, clusters of Carbon 60 (C60) would tend to adhere tightly to soil or filtration system particles.
But where natural organic compounds or chemical surfactants serve as stabilizers in water, the C60 fullerene particles would tend to flow as easily as the water carrying them.

“In some cases, the nanoparticles move very little and you would get complete retention in the soil,” said Kurt Pennell, a professor in the School of Civil and Environmental Engineering at the Georgia Institute of Technology. “But in different solution conditions or in the presence of a stabilizing agent, they can travel just like water. The movement of these nanoparticles is very sensitive to the solution conditions.”
Researchers Kurt Pennell (standing) and Younggang Wang

Caption: Researchers Kurt Pennell (standing) and Younggang Wang examine glass microbeads and sand used to study the transport and retention of C60 particles in water. Credit: Georgia Tech Photo: Gary Meek. Usage Restrictions: None.
Research into the transport and retention of C60 nanoparticles was reported April 11 in the online version of the American Chemical Society journal Environmental Science and Technology and will be published later in the print edition. The research was funded by the U.S. Environmental Protection Agency.

Comparatively little research has been done on what happens to nanoparticles when they are released through accidental spills – or when products containing them are discarded.
Researchers want to know more about the environmental fate of nanoparticles to avoid creating problems like those of polychlorinated biphenyls (PCBs), in which the harmful effects of the compounds were discovered only after their use became widespread.

“It will be difficult to control the waste stream, so these nanoparticles are likely to get everywhere,” said Pennell. “We want to figure out now what will happen to them and how toxic they will be in the environment.”
Researchers Kurt Pennell (standing) and Younggang Wang

Caption: Researchers Kurt Pennell (standing) and Younggang Wang examine glass columns used to measure the transport and retention of C60 particles in water. Credit: Georgia Tech Photo: Gary Meek. Usage Restrictions: None.
To study the flow and retention of the nanoparticles in simulated soil and filtration systems, Pennell’s research team filled glass columns with either glass microbeads or sand, and saturated the columns with water. They then sent a “pulse” of water containing C60 nanoparticles through the columns, followed by additional water containing no nanoparticles.

They measured the quantity of nanoparticles emerging from the columns and analyzed the sand and glass beads to observe the quantity of C60 retained there. They also extracted the contents of the columns to measure the distribution of retained nanoparticles.

“In sand, we saw a uniform distribution of the nanoparticles throughout the column, which suggests that under the circumstances we examined, there is a limited retention capability due to filtration,” Pennell explained. “Once that capacity is reached, the particles will pass through until they are retained by other grains of soil or sand.”
Traditional theories regarding the activity of such packed-bed filters suggest that particles would build up near the column entrance, with concentrations falling off thereafter. The study findings suggest that the predictions of “filter theory” will have to be modified to explain the transport of nanoparticles in soil, Pennell said.

The nanoparticles retained were tightly bound to the sand or beads and could only be removed by changing the pH of the water.

“That would be a good thing if you were trying to filter these particles from a water system and were worried about them moving into the environment,” Pennell said. “Once they go onto the soil system, it’s unlikely that they will come off as long as the conditions don’t change.”

The researchers observed that up to 77 percent of the nanoparticle mass was retained by the sand, while the glass beads retained between 8 and 49 percent. Preparation of the solutions containing C60 dramatically affected the retention; when no salt was added, the particles flowed through the columns like water.

“We want to make a mechanistic assessment of why the particles are attaching,” Pennell said. “When we look at real soils with finer particles, we will expect to see more retention.”

For municipal drinking water filtration, the sensitivity to solution characteristics means local conditions may play a key role.

“Under most conditions, you should be able to remove nanoparticles from the water,” Pennell explained. “But you will have to be careful if the nanoparticles are stabilized by a natural surfactant or humic acid. If those are present in the water, the nanoparticles could go right through.”

In a continuation of the work, Pennell and his Georgia Tech collaborators – Joseph Hughes, John Fortner and Younggang Wang – are now studying more complicated transport issues in real soils and with other types of nanoparticles. In field conditions, the nanoparticles are likely to be found with other types of carbon – and potentially with other nanostructures.

“When we study systems with real soil, we will have background interference with humics and other materials,” Pennell noted. “Ramping up the complexity will make this research a real challenge.”

Ultimately, Pennell hopes to develop information about a broad range of nanoparticles to predict how they’ll be retained and transported under a variety of conditions. Facilitating that is mathematical modeling being done by collaborators Linda Abriola and Yusong Li at Tufts University in Medford, Mass.

“We want to build up to the point that we can systematically vary properties and parameters,” Pennell explained. “Over time, we should be able to classify nanoparticles based on their properties and have a good idea of how they will behave in the environment.” ###

Technical Contact: Kurt Pennell (404-894-9365); E-mail: (kpennell@ce.gatech.edu). Contact: John Toon jtoon@gatech.edu 404-894-6986 Georgia Institute of Technology Research News

Friday, May 23, 2008

Revving up the world's fastest nanomotors

Nanomotor Racing

Green lines show results of "racing," where images a, b, c, and d represent the tracks left by various types of speeding nanomotors. The winner is "c," a "catalytic nanomotor" composed of gold and platinum nanowires supercharged with carbon nanotubes. Credit: Courtesy of the American Chemical Society. Usage Restrictions: None.
Boosting' research to develop world's fastest nanomotor

In a “major step” toward a practical energy source for powering tomorrow’s nanomachines, researchers at Arizona State University’s Biodesign Institute report the development of a new generation of tiny nanomotors that are up to 10 times more powerful than existing motors.
Just like weekend hot-rodders who tinker with their car engines in the ultimate quest for speed, a research team led by Joseph Wang, who directs the institute’s Center for Biosensors and Bioelectronics, set out to improve on the design of current nanomotors. These so-called “catalytic nanomotors” are made with gold and platinum nanowires and use hydrogen peroxide (the same chemical that bleaches hair) as a fuel for self-propulsion.

But these motors are too slow and inefficient for practical use, with top speeds of about 10 micrometers per second, the researchers say. One micrometer is about 1/25,000 of an inch or almost 100 times smaller than the width of a human hair. (If one could somehow magnify the nanoworld to human scale by multiplying by a factor of 100,000 the speed would roughly equal a walking speed of 3.6 miles per hour).

Wang and colleagues supercharged their nanomotors by inserting carbon nanotubes into the platinum, thus boosting average speed to 60 micrometers per second. This was the first time that carbon nanotubes had been added to the existing gold and platinum nanowires. The tiny tubes, only a few atoms thick, help conduct electricity and heat.

"This is the first example of a powerful, man-made nanomotor," said Wang, who is an ASU professor with a joint appointment in the departments of Chemical and Material Engineering in the Ira A. Fulton School of Engineering and Chemistry and Biochemistry in the College of Liberal Arts and Science.

Spiking the hydrogen peroxide fuel with hydrazine (a type of rocket fuel) kicked up the speed still further, to 94- 200 micrometers per second (using the same multiplying factor of 100,000 the top speed would now be whirring to a moped-like speed of 43.2 miles per hour). This innovation “offers great promise for self-powered nanoscale transport and delivery systems,” Wang states.

The Biodesign team is interested in more than just bragging rights at the nanotechnology research racetrack. By packaging the nanomotors with the right cargo, Wang says the powerful nanomotors could one day deliver disease-fighting drugs inside the body to invading pathogens or tumor cells, or help clean up environmental toxins by using the toxins as fuel.

Authors on the paper include: Rawiwan Laocharoensuk, Jared Burdick, and Joseph Wang. Their study is scheduled for the May 27 issue of ACS Nano, a monthly journal. They also reported their findings in the online edition of ACS Nano "Carbon-Nanotube-Induced Acceleration of Catalytic Nanomotors." ### Adapted from materials provided by the American Chemical Society

CONTACT: Joseph Wang, Ph.D. Arizona State University Tempe, Arizona 85287 Phone: 480-727-0399 Fax: 480-727-0412 Email: joseph.wang@asu.edu

Contact: Michael Bernstein m_bernstein@acs.org 202-872-4400 American Chemical Society

Joe Caspermeyer, Media Relations Manager & Science Editor (480) 727-0369 | joseph.caspermeyer@asu.edu

Thursday, May 22, 2008

Spiraling nanotrees offer new twist on growth of nanowires

Spiraling pine tree-like nanowires

Spiraling pine tree-like nanowires created by University of Wisconsin-Madison chemistry professor Song Jin and graduate student Matthew Bierman are evidence of an entirely different way of growing the tiny wires, one that could be harnessed to make better nanowires for applications such as high performance integrated circuits, LEDs and lasers, biosensors, and solar cells. The rapid elongation of the trunks is driven by a spiral defect within them called "screw dislocation," which causes them to twist as they grow and their branches to spiral. Photo by: courtesy Song Jin Date: April 2008
Since scientists first learned to make nanowires, the tiny wires just a few millionths of a centimeter thick have taken many forms, including nanobelts, nanocoils and nanoflowers.

But when UW-Madison chemistry professor Song Jin and graduate student Matthew Bierman accidentally made some pine tree shapes one day — complete with tall trunks and branches that tapered in length as they spiraled upward — they knew they'd stumbled upon something peculiar.

“At the beginning we saw just a couple of trees, and we said, ‘What the heck is going on here?’” recalls Jin. “They were so curious.”

Writing in the May 1 edition of Science Express, Jin and his team reveal just how curious the nanotrees truly are. In fact, they’re evidence of an entirely different way of growing nanowires, one that promises to give scientists a powerful means to create new and better nanomaterials for all sorts of applications, including high-performance integrated circuits, biosensors, solar cells, LEDs and lasers.

Until now, most nanowires have been made with metal catalysts, which promote the growth of nanomaterials along one dimension to form long rods. While the branches on Jin’s trees also elongate in this way, growth of the trunks is driven by a “screw” dislocation, or defect, in their crystal structure. At the top of the trunk, the defect provides a spiral step for atoms to settle on an otherwise perfect crystal face, causing them stack together in a spiral parking ramp-type structure that quickly lengthens the tip.

Dislocations are fundamental to the growth and characteristics of all crystalline materials, but this is the first time they’ve been shown to aid the growth of one-dimensional nanostructures. Engineering these defects, says Jin, may not only allow scientists to create more elaborate nanostructures, but also to investigate the fundamental mechanical, thermal and electronic properties of dislocations in materials.

His team created its nanotrees specifically by applying a slight variation of a synthesis technique called chemical vapor deposition to the material lead sulfide. But the chemists believe the new mechanism will be applicable to many other materials, as well.
“We think these findings will motivate a lot of people to do this purposefully, to design dislocation and try to grow nanowires around it,” Jin says. “Or perhaps people who have grown a structure and were puzzled by it will read our paper and say, ‘Hey, we see something similar in our system, so maybe now we have the solution.’”

What initially puzzled Jin and his students about their pine tree structures was the long length of the trunks compared with the branches, a difference that indicated the trunks were growing much faster. The result was surprising because when complex, branching nanostructures are grown with metal catalysts, the branches are usually all of similar length because of similar growth rates, leading to boxy shapes rather than the cone-shapes of the trees.

Another oddity was the twist to the trunks, which sent the branches spiraling.

“The long and twisting trunks were telling us we had a new growth mode,” says Jin. Suspecting dislocation, the team set about refining their technique for growing the pine trees – they soon learned to produce entire forests with ease – and then confirmed the presence of dislocations with a special type of transmission electron microscopy.

Upon closer examination, the twisting trunks and spiraling branches also turned out to embody a well-known general theory about the mechanical deformation of crystalline materials caused by screw dislocations. Although this so-called “Eshelby twist” was first calculated back in 1953 and is discussed in many textbooks, Jin’s experimental results likely offer the best support yet for the theory.

“These are beautiful, truly intriguing structures, but behind them is also a really beautiful, interesting science,” says Jin. “Once you understand it, you just feel so…satisfied.” ###

The paper’s other authors are Y.K. Albert Lau, Alexander Kvit and Andrew Schmitt. The work was funded by the National Science Foundation.

by Madeline Fisher Contact: Song Jin jin@chem.wisc.edu 608-262-1562 University of Wisconsin-Madison

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Wednesday, May 21, 2008

Graphene-based gadgets may be just years away

graphene as a transparent conductive coatingResearchers at The University of Manchester have produced tiny liquid crystal devices with electrodes made from graphene – an exciting development that could lead to computer and TV displays based on this technology.
Writing in the American Chemical Society’s journal Nano Letters, Dr Kostya Novoselov and colleagues from The School of Physics and Astronomy and The School of Computer Science, report on the use of graphene as a transparent conductive coating for electro-optical devices – and show that its high transparency and low resistivity make it ideal for electrodes in liquid crystal devices.

Graphene was discovered at The University of Manchester back in 2004, by Professor Andre Geim FRS and Royal Society Research Fellow Dr Kostya Novoselov. This incredible one-atom-thick gauze of carbon atoms, which resembles chicken wire, has quickly become one of the hottest topics in physics and materials science.

“Graphene is only one atom thick, optically transparent, chemically inert, and an excellent conductor,” says Dr Novoselov, from the Manchester research team.

“These properties seem to make this material an excellent candidate for applications in various electro-optical devices that require conducting but transparent thin films. We believe graphene should improve the durability and simplify the technology of potential electronic devices that interact with light.”

Prof Geim said: “Transparent conducting films are an essential part of many gadgets including common liquid crystal displays (LCDs) for computers, TVs and mobile phones.

“The underlying technology uses thin metal-oxide films based on indium. But indium is becoming an increasingly expensive commodity and, moreover, its supply is expected to be exhausted within just 10 years.

“Forget about oil – our civilisation will first run out of indium. Scientists have an urgent task on their hands to find new types of conductive transparent films.”

The Manchester research team has now demonstrated highly transparent and highly conductive ultra-thin films that can be produced cheaply by ‘dissolving’ chunks of graphite – an abundant natural resource – into graphene and then spraying the suspension onto a glass surface.

The resulting graphene-based films can be used in LCDs and, to prove the concept, the research team have demonstrated the first liquid crystal devices with graphene electrodes.

Dr Novoselov believes that there are only a few small, incremental steps remain for this technology to reach a mass production stage. “Graphene-based LCD products could appear in shops as soon as in a few years”, he adds.

A research team from the Max Planck Institute for Polymer Research in Germany recently reported in Nano Letters how they had used graphene-based films to create transparent electrodes for solar cells (1).

But the German team used a different technology for obtaining graphene films, which involved several extra steps.

The Manchester team says the films they have developed are much simpler to produce, and they can be used not only in LCDs but also in solar cells. ###

Notes to editors

(1) Wang, X.; Zhi, L.; Mullen, K. Nano Lett. 2008, 8, 323.

A copy of the paper- ‘Graphene-Based Liquid Crystal Device’ - is available on request. A selection of images are also available to illustrate the story.

Dr Novoselov is available for comment.

For further information please contact Contact: Alex Waddington 01-612-758-387 University of Manchester

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Tuesday, May 20, 2008

Atomic force microscopy reveals liquids adjust viscosity when confined, shaken

A liquid cell containing water allows viscosity measurements

Caption: A liquid cell containing water allows viscosity measurements of the water to be collected with an atomic force microscope. A study led by Elisa Riedo, an assistant professor in the Georgia Tech School of Physics, revealed that confined liquids can respond and modify their viscosity based on environmental changes. Credit: Georgia Tech Photo: Gary Meek. Usage Restrictions: None.
Getting ketchup out of the bottle isn’t always easy. However, shaking the bottle before trying to pour allows the thick, gooey ketchup to flow more freely because it becomes more fluid when agitated. The opposite is not typically true – a liquid such as water does not become a gel when shaken.

However, new research published in the March 14 issue of the journal Physical Review Letters shows that when fluids like water and silicon oil are confined to a nanometer-sized space, they behave more like ketchup or toothpaste. Then, if these confined liquids are shaken, they become fluidic and exhibit the same structural and mechanical properties as those in thicker layers.
The study – the first to use an atomic force microscope to measure the viscosity of confined fluids – revealed that these liquids can respond and modify their viscosity based on environmental changes.
Tai-De Li (left) and Elisa Riedo, an assistant professor

Caption: As graduate student Tai-De Li (left) looks on, Elisa Riedo, an assistant professor in the Georgia Tech School of Physics, holds a liquid cell containing water that allows viscosity measurements of the water to be collected with a atomic force microscope. Credit: Georgia Tech Photo: Gary Meek. Usage Restrictions: None.
“Knowing this could be very important,” said Elisa Riedo, an assistant professor in the Georgia Tech School of Physics. “If a lubricant used in a piece of machinery becomes thick and gelatinous when squeezed between two solid surfaces, serious problems could occur. However, if the machine vibrated, the liquid could become fluidized.”

With funding from the National Science Foundation and the U.S. Department of Energy, Riedo and graduate student Tai-De Li used atomic force microscopy (AFM) to measure the behavior of thin and thick layers of liquids while they were vibrated.
A nanometer-size spherical silicon tip was used to approach a mica surface immersed in water or silicon oil, while small lateral oscillations were applied to the cantilever support.

“Some researchers have measured the force it takes to squeeze out a fluid, but we took a different approach,” explained Riedo. “We are the first group to use AFM to study the viscosity of confined fluids from direct high-resolution lateral force measurements.”
Graduate student Tai-De Li

Caption: Graduate student Tai-De Li examines the AFM scanner head used to measure the viscosity of confined fluids. A study led by Elisa Riedo, an assistant professor in the Georgia Tech School of Physics, revealed that these liquids can respond and modify their viscosity based on environmental changes. Credit: Georgia Tech Photo: Gary Meek. Usage Restrictions: None.
The normal and lateral forces acting on the tip were measured directly and simultaneously as a function of the liquid film thickness. The ratio of stress to strain under vibratory conditions, called the viscoelastic modulus, was also measured at different frequencies and strains.

Riedo and Li measured the relaxation times of two wetting liquids: water and silicone oil (octamethylcylotetrasiloxane), which is primarily used as a lubricant or hydraulic fluid, and is the main ingredient in Silly Putty®.

“The relaxation time describes how active the molecules are. A longer relaxation time means it takes longer for the molecules to rearrange themselves back into their original shape after shaking them,” said Li. “Liquids have very short relaxation times – as soon as one stops shaking a bottle of water, it reverts to its original configuration.”

Experimental results showed that the relaxation time became orders of magnitude longer in water and silicone oil when they were confined, meaning they behaved more like gels or glass.
The researchers also showed that the relaxation times depended on the shaking speed when the liquids were confined. However, in thick layers that were not confined, the molecules showed no dependence on the shaking speed and always relaxed very quickly, meaning they behaved like a “normal” liquid.

Longer relaxation times were observed when the water film was less than one nanometer thick, composed of about three molecules of water stacked on top of each other. Otherwise, its properties were the same as in a bottle of water. For silicone oil, a thickness of four nanometers was required before the properties were like those of a glassy material.

“We observed a nonlinear viscoelastic behavior remarkably similar to that widely observed in metastable complex fluids, such as gels or supercooled liquids,” noted Riedo. “Because we observed these phenomena in both water and silicone oil, we believe they are very general phenomena and may apply to all wetting liquids.”

Since the behavior of confined water observed in these experiments is similar to the behavior of supercooled water at -98.15 degrees Celsius, the researchers are currently examining whether confinement defines a lower effective temperature in the confined liquid. ###

The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Contact: Abby Vogel avogel@gatech.edu 404-385-3364 Georgia Institute of Technology Research News

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Monday, May 19, 2008

Making a good impression: Nanoimprint lithography tests at NIST

Nanoimprint Lithography

Caption: Electron micrograph shows a cross-section of a typical SOG microcircuit feature. Nanoporous regions in the interior are lighter. The process forms a dense, stronger skin about 2 nanometers thick on the outside. (Color added for clarity.) Credit: NIST. Usage Restrictions: None.
In what should be good news for integrated circuit manufacturers, recent studies by the National Institute of Standards and Technology (NIST) have helped resolve two important questions about an emerging microcircuit manufacturing technology called nanoimprint lithography—yes, it can accurately stamp delicate insulating structures on advanced microchips, and, no, it doesn’t damage them, in fact it makes them better.

An emerging manufacturing technique, nanoimprint lithography (NIL) is basically an embossing process. A stamp with a nanoscale pattern in its surface is pressed into a soft film on the surface of a semiconductor wafer.
The film is hardened, usually by heating or exposure to ultraviolet light, and the film retains the impressed pattern from the stamp. The process is astonishingly accurate. NIL has been used to create features as small as ten nanometers across with relatively complex shapes.

NIL is being eyed in particular for building the complexly patterned insulating layers sandwiched between layers of logic devices in future generations of integrated circuits. State-of-the-art semiconductors contain over a billion transistors, packed together into a footprint of silicon that is no bigger than a few square centimeters. Several miles of nanoscale copper wiring are required to connect the devices, and these wires must be separated by a highly efficient insulator. One candidate is a porous glassy material called SOG* that can be applied as a thin fluid film. When heated, SOG turns into a thin glass film laced with nanometer pores that enhance the electrical insulation. But SOG is relatively delicate, and the conventional photoresist etching process used to cut trenches for the wiring can compromise it. NIL, on the other hand, might be able to pattern SOG layers with wiring trenches and eliminate several time-consuming and expensive photolithography steps if it could pattern the film accurately and do so without destroying the delicate nanopore lacework.

In a paper published last fall,** NIST materials scientists addressed the first question. Using sensitive X-ray measurements they demonstrated that NIL could be used on a functional SOG material to transfer patterns with details finer than 100 nanometers with minimal distortion due to the processing. In a new paper this month,*** they extend this work to study the effect of the embossing process on the nanopore structure in the glass. Using a combination of techniques to measure the distribution of nanopores in the insulator material, they found that the NIL embossing process actually has a beneficial effect—it increases the population of small pores, which improve performance, reduces the population of larger pores that can cause problems and creates a thin, dense protective skin across the surface of the material. All of these effects are highly attractive for minimizing short circuits in semiconductor devices.

Taken together, the two papers suggest that nanoimprint lithography can produce superior nanoporous insulator layers in advanced semiconductor devices with significantly fewer—and easier—processing steps than conventional lithography. ###

* “Spin-on organosilicate glass”

** H.W. Ro, R.L. Jones, H. Peng, D.R. Hines, H-J. Lee, E.K. Lin, A. Karim, D.Y. Yoon, D.W. Gidley and C.L. Soles. The direct patterning of nanoporous interlayer dielectric insulator films by nanoimprint lithography. Advanced Materials. 2007, 19, 2919–2924.

*** H.W. Ro, H. Peng, K.-i. Niihara, H.-J. Lee, E.K. Lin, A. Karim, D.W. Gidley, H. Jinnai, D.Y. Yoon and C.L. Soles. Self-sealing of nanoporous low dielectric constant patterns fabricated by nanoimprint lithography. Advanced Materials 2008, Early View: April 15, 2008.

Contact: Michael Baum michael.baum@nist.gov 301-975-2763 National Institute of Standards and Technology (NIST)

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Sunday, May 18, 2008

Too much technology may be killing beneficial bacteria

Zhiqiang Hu , Assistant Professor

Zhiqiang Hu , Assistant Professor, 125 Engr. Bldg. North University of Missouri - Columbia Columbia, MO 65211. Phone: (573) 884-0497 Fax: (573) 882-4784 E-mail: huzh@missouri.edu
Homepage: web.missouri.edu/huzh
MU engineer concerned about environmental impact of silver nanoparticles in wastewater treatment

COLUMBIA, Mo. –Too much of a good thing could be harmful to the environment. For years, scientists have known about silver’s ability to kill harmful bacteria and, recently, have used this knowledge to create consumer products containing silver nanoparticles. Now, a University of Missouri researcher has found that silver nanoparticles also may destroy benign bacteria that are used to remove ammonia from wastewater treatment systems. The study was funded by a grant from the National Science Foundation.

Several products containing silver nanoparticles already are on the market, including socks containing silver nanoparticles designed to inhibit odor-causing bacteria and high-tech, energy-efficient washing machines that disinfect clothes by generating the tiny particles.
The positive effects of that technology may be overshadowed by the potential negative environmental impact.

“Because of the increasing use of silver nanoparticles in consumer products, the risk that this material will be released into sewage lines, wastewater treatment facilities, and, eventually, to rivers, streams and lakes is of concern,” said Zhiqiang Hu, assistant professor of civil and environmental engineering in MU’s College of Engineering. “We found that silver nanoparticles are extremely toxic. The nanoparticles destroy the benign species of bacteria that are used for wastewater treatment. It basically halts the reproduction activity of the good bacteria.”

Hu said silver nanoparticles generate more unique chemicals, known as highly reactive oxygen species, than do larger forms of silver. These oxygen species chemicals likely inhibit bacterial growth. For example, the use of wastewater treatment “sludge” as land-application fertilizer is a common practice, according to Hu. If high levels of silver nanoparticles are present in the sludge, soil used to grow food crops may be harmed.

Hu is launching a second study to determine the levels at which the presence of silver nanoparticles become toxic. He will determine how silver nanoparticles affect wastewater treatment processes by introducing nanomaterial into wastewater and sludge. He will then measure microbial growth to determine the nanosilver levels that harm wastewater treatment and sludge digestion.

The Water Environment Research Foundation recently awarded Hu $150,000 to determine when silver nanoparticles start to impair wastewater treatment. Hu said nanoparticles in wastewater can be better managed and regulated. Work on the follow-up research should be completed by 2010. ###

The silver nanoparticle research conducted by Hu and his graduate student, Okkyoung Choi, was recently published in Water Research and Environmental Science & Technology.

Contact: Bryan E. Jones jonesbry@missouri.edu 573-882-9144 University of Missouri-Columbia

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Saturday, May 17, 2008

Micro-origami: USC folds up micrometer-scale 'voxels' for drug delivery

5-sided Voxel Pyramid

Caption: After starting the folds using magnetic forces, the structure is sealed using capillary action. Credit: USC Information Sciences Institute. Usage Restrictions: Credit USC Information Sciences Institute.
Egypt has its great pyramids; the Information Sciences Institute has its microscopic ones

Researchers at the USC Information Sciences Institute have demonstrated a way to manufacture miniscule closed containers that might be used to deliver precise micro- or even nano-quantities of drugs.

According to ISI project leader Peter Will, who is also a research professor in the Viterbi School of Engineering, the new technique, described in a paper in the Journal of Micromechanics and Microengineering, is a two-step process.
Part one is the creation of flat patterns, origami, of exactly the fold up shapes familiar to kindergarten children making paper pyramids, cubes or other solids, except that these are as small 40 micrometers (µm) on a side. (1 inch = 25,400 µm)
Sheet of Voxels to Be

Caption: Flat forms fabricated in polisilicon, ready for additional processing and subsequent folding. Credit: USC Information Sciences Institute. Usage Restrictions: Credit USC Information Sciences Institute.
Instead of paper, the USC researchers created the patterns in polysilicon sitting on top of a thin film of gold, using a well-established commercial silicon wafer process called PolyMUMPs. The next step was clearing the polysilicon off the hinge areas by etching.

When the blanks were later electrocoated with permalloy to make them magnetic, the photomask used left hinge areas uncoated, to make sure they were the places that folded.

Then the folding had to be accomplished. First the researchers bent the hinges by application of magnetic force to the permalloy.
Water pressure and capillary forces generated by submerging the tiny blanks in water, and drying them off did the final folding into shape.

The experiments spend considerable time comparing various methods of controlling the closure effects of water drying with simple flaps designed to close over each other to form "envelops," the directing water from different directions sequence the closing. Varying the time of trying could produce tighter seams.
Voxel Schematics

Caption: Voxel as initially fabricated (a), etched,(b) and ready for final folding (c). Credit: USC Information Sciences Institute. Usage Restrictions: Credit USC Information Sciences Institute.
"Our experiments show" says the paper, that "the combination of partial folding of structures by magnetic actuation and liquid closure to bring the structures to their final closed state is an extremely promising technique for mass production of large arrays of micrometer size …voxels. Furthermore, we believe that future optimization of the voxel hinge geometry and composition should allow for extensions of our work to" much smaller voxels.

The Voxel team - consisting of Will, professor of chemistry Bruce Koel (who has since gone to Lehigh University), former post-doctoral researcher Alejandro Bugacov and former grad student (now graduate) Rob Gagler folded a number of different shapes, including four- and five-sided pyramids, pentagonal 'lotus' shapes, and also simple square plates that folded over each other to make flat mini-envelopes.

Will has been pursuing the idea of creating voxels for many years, "way back to my days in HP labs, when I was working in Medical and Chemical applications." The USC team designed the chips using MEMSPRO CAD software; the actual chip fabrication was done in France.
"The experimental work was done on campus," said Will, "since ISI doesn't have a wet lab." ###

The National Science Foundation supported the research, under an exploratory research grant. The paper is "Voxels: volume-enclosing microstructures," J. Micromech. Microeng. 18 (2008) 055025.

Contact: Eric Mankin 310-448-9112 University of Southern California

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