An engineered directional nanofilm mimics nature's curious feats

October 29, 2010

Enlarge

Insects like this water strider inspire a new nanotech coating. (credit: shutterstock.com)

Although superhydrophobic self-cleaning surfaces are an active area of research, this development marks an engineering breakthrough in the ability to control the directionality of liquid transport. Using an array of poly(p-xylylene) nanorods synthesized by a bottom-up vapor-phase technique, the researchers were able to pin water droplets in one direction with enormous adhesive forces proportional to the number of nanorods and the surface tension, while releasing droplets in the opposite direction.

The differential between the pin and release force is 80 micronewtons, over ten times the values reported in other engineered surfaces with ratchet-like features, and the first such surface to be engineered at the nanoscale. Recently, the authors also demonstrated directional adhesion and friction of these surfaces, similar to the way a gecko can climb a wall (J. Applied Physics, 2010). Gecko’s feet contain approximately 4 million hairs per square millimeter, whereas polymer nanorods can be deposited at 40 million rods per square millimeter.

The nanofilm produced by this technique, called oblique angle deposition, provides a microscale smooth surface for the transport of small water droplets without pumps or optical waves and with minimal deformation for self-powered microfluidic devices for medicine and for microassembly.

In work sponsored by the U.S. Navy, the nanofilm is envisioned for use as a coating that would reduce drag on the hull of vessels and retard fouling. Potential industrial and energy related uses are as directional syringes and fluid diodes, pump-free digital fluidic devices, increased efficiency of thermal cooling for microchips, coatings for tires, and even in energy production from rain drops.

The lead on the Penn State team, Melik Demirel, associate professor of engineering science and mechanics and corresponding author on the report, believes that the current laboratory based vapor phase technique, which although relatively simple still requires a vacuum, can be replaced by a liquid phase technique, which would allow for scaling the production of their material to industry size. “The major impact of our method is that for the first time we can create a controlled directional surface at the nanoscale,” Demirel concludes.

Microreactor speeds nanotech particle production by 500 times

November 1, 2010 Engineers at Oregon State University have discovered a new method to speed the production rate of nanoparticles by 500 times, an advance that could play an important role in making nanotechnology products more commercially practical.

The approach uses an arrayed microchannel reactor and a "laminated architecture" in which many sheets, each with thousands of microchannels in them, are stacked in parallel to provide a high volume of production and excellent control of the processes involved.

Applications could be possible in improved sensors, medical imaging, electronics, and even solar energy or biomedical uses when the same strategy is applied to abundant materials such as copper, zinc or tin.

A patent has been applied for, university officials say. The work, just published in the journal Nanotechnology, was done in the research group of Brian Paul, a professor in the OSU School of Mechanical, Industrial and Manufacturing Engineering.

"A number of new and important types of nanoparticles have been developed with microtechnology approaches, which often use very small microfluidic devices," said Chih-hung Chang, a professor in the OSU School of Chemical, Biological and Environmental Engineering, and principal investigator on the study.

"It had been thought that commercial production might be as simple as just grouping hundreds of these small devices together," Chang said. "But with all the supporting equipment you need, things like pumps and temperature controls, it really wasn't that easy. Scaling things up to commercial volumes can be quite challenging."

The new approach created by a research team of five engineers at OSU used a microreactor with the new architecture that produced "undecagold nanoclusters" hundreds of times faster than conventional "batch synthesis" processes that might have been used.

"In part because it's faster and more efficient, this process is also more environmentally sensitive, using fewer solvents and less energy," Chang said. "This could be very significant in helping to commercialize nanotech products, where you need high volumes, high quality and low costs."

This research, Chang said, created nanoparticles based on gold, but the same concept should be applicable to other materials as well. By lowering the cost of production, even the gold nanoclusters may find applications, he said, because the cost of the gold needed to make them is actually just a tiny fraction of the overall cost of the finished product.

Nanoparticles are extraordinarily tiny groups of atoms and compounds that, because of their extremely small size and large surface areas, can have unusual characteristics that make them valuable for many industrial, electronic, medical or energy applications.

'Express lanes' for ions: By aligning carbon nanotubes in electrodes, researchers boost performance

October 8, 2010 by Morgan Bettex

Enlarge

By aligning carbon nanotubes inside polymer composites, Wardle and his colleagues designed electrodes that allow ions to travel more quickly between the tiny cylinders. Image: Joe Klimek, Lazerpro Digital Media Group

Actuators are devices that convert electrical energy into mechanical energy, such as the battery-powered device inside a cell phone that causes the phone to vibrate. When this process is reversed -- when a device converts mechanical energy into electrical energy -- the device is called an energy harvester, and that electrical energy is often stored for future use. An example would be a device inside a pacemaker that converts mechanical energy created by the motion of a pair of breathing lungs into electrical energy that can be used to charge the pacemaker’s batteries.

Both devices typically contain electromechanical materials, such as electroactive polymers, that are made of chainlike molecules that change in size or shape when stimulated by an electric field. But their efficiency and speed depend on how quickly ions, or electrically charged particles, can move between electrodes, or the conductors that electric current passes through, to change the polymer’s size or shape. The faster ions can move between electrodes, the more ionic conductivity those electrodes will have and the more responsive the material will be to the electric field. Although these polymers usually contain nanoparticles that are randomly dispersed throughout the material to make it conductive, this slows ions by forcing them to travel in zigzag paths around the tiny particles.

Recently, an MIT researcher collaborated with a team of electrical engineers from Pennsylvania State University to devise a new way for ions to travel more quickly between electrodes than they do in traditional polymers. Brian L. Wardle, associate professor of aeronautics and astronautics, and his colleagues designed electrodes containing aligned carbon nanotubes — tiny, hollow cylinders made of carbon atoms — to be used in an electroactive polymer. As they report in a paper to be published Oct. 8 in Advanced Functional Materials, this alignment created “express lanes” that enabled the ions to travel more quickly between electrodes. Specifically, the researchers estimate that the ionic conductivity of these electrodes is about an order of magnitude greater than that of electrodes in polymers that contain randomly dispersed nanoparticles.

Wardle and his colleagues, including Qiming M. Zhang, a professor of electrical engineering at Penn State, and lead author Sheng Liu, one of Zhang’s graduate students, demonstrated that the aligned carbon-nanotube electrodes can enhance ion performance in an actuator, meaning they could be optimized for applications like artificial muscles and robots.

The researchers state that the devices could be used as energy harvesters through a reverse-conversion process. There is tremendous interest in developing energy harvesters for large-scale applications, such as to create electrical energy from the motion of wind or ocean waves, Wardle says. The devices could also be used to power vast networks of microscopic sensors in hard-to-reach areas like underground pipes.

Composite creation

The researchers’ goal was to design a composite that could function as a superior electrode. By heating natural gas and exposing it to a metal catalyst, Wardle and several of his graduate students grew the electrically conductive carbon nanotubes and poured a polymer mixed in a solvent over them. Once the solvent evaporated, it left behind a solid, ion-porous composite containing both polymer and carbon nanotubes. The researchers then used this composite to create a structure comprised of one layer of pure polymer (to act as an insulator) containing both positive and negative ions sandwiched between two layers of the composite made of both polymer and carbon nanotubes (to act as electrodes).

To test the structure’s actuator abilities, the researchers applied a low-voltage electric field. This voltage caused ions to flow from one electrode layer to the other, which resulted in one side of the structure containing more ions. This ion imbalance generated enough pressure to cause the entire structure to bend, thereby creating mechanical energy. The experiment also revealed that the composite electrode design helped to minimize electrical resistance.

The researchers believe the same device could be used as an energy harvester if it is stressed mechanically, such as through compression. That’s because compression would make the ions move differently, which would cause an unbalanced electrical charge. This, in turn, would create a voltage difference and produce a flow of electricity.

Optimizing design

Yoseph Bar-Cohen, a senior research scientist at NASA’s Jet Propulsion Laboratory, says that the study demonstrates an improvement of ionic polymers. But he is curious about the response of the device over longer periods of time, noting that the current study was limited to only a 10-minute experiment.

As they develop these electrodes, Wardle and his collaborators are trying to determine an optimal design. Now that they have demonstrated how effective carbon nanotubes are for electrode efficiency, they are exploring certain details that might enable optimal performance, such as the spacing between the tiny tubes.
This story is republished courtesy of MIT News (http://web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.

Nanoscopic particles resist full encapsulation, simulations show

October 11, 2010 By Neal Singer

Enlarge

The protective capabilities of a chain-link molecular coating are lessened when, instead of attaching to a flat surface to form an unbroken wall (fig. 1), the radius of the nanoparticle is so small that the extreme surface curvatuere opens spaces between the protective molecules (fig. 2). (Drawing courtesy Matt Lane, Sandia National Labs)

Relying on that concept, fabricators of spherical nanoparticles have similarly dunked their wares in protective coatings in the belief such encapsulations would prevent clumping and unwanted chemical interactions with solvents.

Unfortunately, reactions in the nanoworld are not logical extensions of the macroworld, Sandia National Laboratories researchers Matthew Lane and Gary Grest have found.

In a cover article this past summer in Physical Review Letters, the researchers use molecular dynamics simulations to show that simple coatings are incapable of fully covering each spherical nanoparticle in a set.

Instead, because the diameter of a particle may be smaller than the thickness of the coating protecting it, the curvature of the particle surface as it rapidly drops away from its attached coating provokes the formation of a series of louvres rather than a solid protective wall (see illustration).

“We’ve known for some time now that nanoparticles are special, and that ‘small is different,’” Lane said. “What we’ve shown is that this general rule for nanotechnology applies to how we coat particles, too.”

Carlos Gutierrez, manager of Sandia’s Surfaces and Interface Sciences Department, said, “It’s well-known that aggregation of nanoparticles in suspension is presently an obstacle to their commercial and industrial use. The simulations show that even coatings fully and uniformly applied to spherical nanoparticles are significantly distorted at the water-vapor interface.”

Said Grest, “You don’t want aggregation because you want the particles to stay distributed throughout the product to achieve uniformity. If you have particles of, say, micron-size, you have to coat or electrically charge them so the particles don’t stick together. But when particles get small and the coatings become comparable in size to the particles, the shapes they form are asymmetric rather than spherical. Spherical particles keep their distance; asymmetric particles may stick to each other.”

The simulation’s finding isn’t necessarily a bad thing, for this reason: Though each particle is coated asymmetrically, the asymmetry is consistent for any given set. Said another way, all coated nanoscopic sets are asymmetric in their own way.

A predictable, identical variation occurring in every member of a nanoset could open doors to new applications.

“What we’ve done here is to put up a large ‘dead end’ sign to prevent researchers from wasting time going down the wrong path,” Lane said. “Increasing surface density of the coating or its molecular chain length isn’t going to improve patchy coatings, as it would for larger particles. But there are numerous other possible paths to new outcomes when you can control the shape of the aggregation.”

The noise about graphene

October 15, 2010 by Aditi Risbud

Enlarge

This image of a single suspended sheet of graphene taken with the TEAM 0.5, at Berkeley Lab?s National Center for Electron Microscopy shows individual carbon atoms (yellow) on the honeycomb lattice.

(PhysOrg.com) -- In last week’s announcement of the Nobel Prize in Physics, the Royal Swedish Academy of Sciences lauded graphene’s "exceptional properties that originate from the remarkable world of quantum physics." If it weren’t hot enough before, this atomically thin sheet of carbon is now officially in the global spotlight.

The promise of graphene lies in the simplicity of its structure—a ‘chicken wire’ lattice of carbon atoms just one layer thick. This sheet confines electrons in one dimension, forcing them to race across a plane. Such quantum confinement results in stellar electronic, mechanical and optical properties far beyond what silicon and other traditional semiconductor materials offer. What’s more, if graphene’s electrons were restricted in two dimensions, like in a nanoribbon, it could greatly benefit logic switching devices—the basis for computation units in today’s computer chips.

Now, Berkeley Labs materials scientist Yuegang Zhang and colleagues at University of California, Los Angeles are moving toward more efficient devices by studying the ‘noise’ in such graphene nanoribbons—one-dimensional strips of graphene with nanometer-scale widths.

“Atomically-thin graphene nanoribbons have provided an excellent platform for us to reveal the strong correlation between conductance fluctuation and the quantized electronic structures of quasi-one-dimensional systems,” says Zhang, a staff scientist in the Inorganic Nanostructures Facility at the Molecular Foundry. “This method should have much broader use to understand quantum transport phenomena in other nanoelectronic or molecular devices.”

Zhang and colleagues previously reported ways of fabricating films of graphene (http://www.physorg … 9954890.html) and revealing low-frequency signal-to-noise ratios for graphene devices on a silica substrate (http://www.physorg.com/news200314797.html). 

In the current study, the team made graphene nanoribbons using a nanowire mask-based fabrication technique. By measuring the conductance fluctuation, or ‘noise’ of electrons in graphene nanoribbons, the researchers directly probed the effect of quantum confinement in these structures. Their findings map the electronic band structure of these graphene nanoribbons using a robust electrical probing method. This method can be further applied to a wide array of nanoscale materials, including graphene-based electronic devices.

“It amazes us to observe such a clear correlation between the noise and the band structure of these graphene nanomaterials,” says lead author Guangyu Xu, a physicist at University of California, Los Angeles. “This work adds strong support to the quasi-one-dimensional subband formation in graphene nanoribbons, in which our method turns out to be much more robust than conductance measurement.”

A paper reporting this research titled, “Enhanced conductance fluctuation by quantum confinement effect in graphene nanoribbons,” appears in Nano Letters and is available to subscribers online . Co-authoring the paper with Zhang and Xu were Carlos Torres, Jr., Emil Song, Jianshi Tang, Jingwei Bai, Xiangfeng Duan and Kang L. Wang.

Portions of this work at the Molecular Foundry were supported by DOE’s Office of Science.

Provided by Lawrence Berkeley National Laboratory (news : web)

Silicon strategy shows promise for batteries

October 13, 2010

Enlarge

Microscopic pores dot a silicon wafer prepared for use in a lithium-ion battery. Silicon has great potential to increase the storage capacity of batteries, and the pores help it expand and contract as lithium is stored and released. (Credit: Biswal Lab/Rice University)

A team of Rice University and Lockheed Martin scientists has discovered a way to use simple silicon to radically increase the capacity of lithium-ion batteries.

Sibani Lisa Biswal, an assistant professor of chemical and biomolecular engineering, revealed how she, colleague Michael Wong, a professor of chemical and biomolecular engineering and of chemistry, and Steven Sinsabaugh, a Lockheed Martin Fellow, are enhancing the inherent ability of silicon to absorb lithium ions.

Their work was introduced today at Rice's Buckyball Discovery Conference, part of a yearlong celebration of the 25th anniversary of the Nobel Prize-winning discovery of the buckminsterfullerene, or carbon 60, molecule. (PhysOrg.com is an official media sponsor of the event). It could become a key component for electric car batteries and large-capacity energy storage, they said.

"The anode, or negative, side of today's batteries is made of graphite, which works. It's everywhere," Wong said. "But it's maxed out. You can't stuff any more lithium into graphite than we already have."

Silicon has the highest theoretical capacity of any material for storing lithium, but there's a serious drawback to its use. "It can sop up a lot of lithium, about 10 times more than carbon, which seems fantastic," Wong said. "But after a couple of cycles of swelling and shrinking, it's going to crack."

Enlarge

A side view of microscopic pores in silicon. (Credit: Biswal Lab/Rice University)

Other labs have tried to solve the problem with carpets of silicon nanowires that absorb lithium like a mop soaks up water, but the Rice team took a different tack.

With Mahduri Thakur, a post-doctoral researcher in Rice's Chemical and Biomolecular Engineering Department, and Mark Isaacson of Lockheed Martin, Biswal, Wong and Sinsabaugh found that putting micron-sized pores into the surface of a silicon wafer gives the material sufficient room to expand. While common lithium-ion batteries hold about 300 milliamp hours per gram of carbon-based anode material, they determined the treated silicon could theoretically store more than 10 times that amount.

Sinsabaugh described the breakthrough as one of the first fruits of the Lockheed Martin Advanced Nanotechnology Center of Excellence at Rice (LANCER). He said the project began three years ago when he met Biswal at Rice and compared notes. "She was working on porous silicon, and I knew silicon nanostructures were being looked at for battery anodes. We put two and two together," he said.

Nanopores are simpler to create than silicon nanowires, Biswal said. The pores, a micron wide and from 10 to 50 microns long, form when positive and negative charge is applied to the sides of a silicon wafer, which is then bathed in a hydrofluoric solvent. "The hydrogen and fluoride atoms separate," she said. "The fluorine attacks one side of the silicon, forming the pores. They form vertically because of the positive and negative bias."

The treated silicon, she said, "looks like Swiss cheese."

The straightforward process makes it highly adaptable for manufacturing, she said. "We don't require some of the difficult processing steps they do -- the high vacuums and having to wash the nanotubes. Bulk etching is much simpler to process.

"The other advantage is that we've seen fairly long lifetimes. Our current batteries have 200-250 cycles, much longer than nanowire batteries," said Biswal.

They said putting pores in silicon requires a real balancing act, as the more space is dedicated to the holes, the less material is available to store lithium. And if the silicon expands to the point where the pore walls touch, the material could degrade.

The researchers are confident that cheap, plentiful silicon combined with ease of manufacture could help push their idea into the mainstream.

"We are very excited about the potential of this work," Sinsabaugh said. "This material has the potential to significantly increase the performance of lithium-ion batteries, which are used in a wide range of commercial, military and aerospace applications

Biswal and Wong plan to study the mechanism by which silicon absorbs lithium and how and why it breaks down. "Our goal is to develop a model of the strain that silicon undergoes in cycling lithium," Wong said. "Once we understand that, we'll have a much better idea of how to maximize its potential."

Provided by Rice University (news : web)

New nano techniques integrate electron gas-producing oxides with silicon

A team led by University of Wisconsin-Madison Materials Science and Engineering Professor Chang-Beom Eom has demonstrated methods to harness essentially this concept for broad applications in nanoelectronic devices, such as next-generation memory or tiny transistors. The discoveries were published Oct. 19 by the journal Nature Communications.

Eom's team has developed techniques to produce structures based on electronic oxides that can be integrated on a silicon substrate—the most common electronic device platform.

"The structures we have developed, as well as other oxide-based electronic devices, are likely to be very important in nanoelectronic applications, when integrated with silicon," Eom says.

The term "oxide" refers to a compound with oxygen as a fundamental element. Oxides include millions of compounds, each with unique properties that could be valuable in electronics and nanoelectronics.

Usually, oxide materials cannot be grown on silicon because oxides and silicon have different, incompatible crystal structures. Eom's technique combines single-crystal expitaxy, postannealing and etching to create a process that permits the oxide structure to reside on silicon—a significant accomplishment that solves a very complex challenge.

The new process allows the team to form a structure that puts three-atom-thick layers of lanthanum-aluminum-oxide in contact with strontium-titanium-oxide and then put the entire structure on top of a silicon substrate.

These two oxides are important because an "electron gas" forms at the interface of their layers, and a scanning probe microscope can make this gas layer conductive. The tip of the microscope is dragged along the surface with nanometer-scale accuracy, leaving behind a pattern of electrons that make the one-nanometer-thick gas layer. Using the tip, Eom's team can "draw" lines of these electrons and form conducting nanowires. The researchers also can "erase" those lines to take away conductivity in a region of the gas.

In order to integrate the oxides on silicon, the crystals must have a low level of defects, and researchers must have atomic control of the interface. More specifically, the top layer of strontium-titanium-oxide has to be totally pure and match up with a totally pure layer of lanthanum-oxide at the bottom of the lanthanum-aluminum-oxide; otherwise, the gas layer won't form between the oxide layers. Finally, the entire structure has been tuned to be compatible with the underlying silicon.

Provided by University of Wisconsin-Madison (news : web)