The noise about graphene

October 15, 2010 by Aditi Risbud

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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

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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."

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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)

Single-crystal films could advance solar cells (w/ Video)

October 8, 2010 By Bill Steele

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Amorphous silicon, deposited on a porous template fills the empty spaces. Laser heating melts the deposit and the top few microns of the silicon substrate. In a few nanoseconds the melted silicon recrystallizes. The substrate acts as a seed crystal for the material above, causing it to crystallize with the same alignment. This makes it easier for electric charges to flow, making possible more efficient solar cells and batteries. Image: Wiesner lab

The "holy grail" for such applications has been to create on a silicon base, or substrate, a film with a 3-D structure at the nanoscale, with the crystal lattice of the film aligned in the same direction (epitaxially) as in the substrate. Doing so is the culmination of years of research by Uli Wiesner, professor of materials science and engineering, into using polymer chemistry to create nanoscale self-assembling structures.

He and his colleagues report the breakthrough in the Oct. 8 issue of the journal Science. They used the new method to create a film with a raised texture, made up of tiny pillars just a few nanometers across. "Just the ability to make a single-crystal nanostructure has a lot of promise," Wiesner said. "We combine that with the ability of organic polymer materials to self-assemble at the nanoscale into various structures that can be templated into the crystalline material."

Wiesner's research group previously used self-assembly techniques to create Gräetzel solar cells, which use an organic dye sandwiched between two conductors. Arranging the conductors in a complex 3-D pattern creates more surface area to collect light and allows more efficient charge transport, Wiesner said.

Performance improves the most when the conducting materials are single crystals, Wiesner said. Most techniques for creating such films produce polycrystalline material -- a collection of "grains" or small crystals bunched together at random -- and grain boundaries retard the movement of electric charges, he explained.

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Uli Wienser explains his research. Video by Bill Steele

Wiesner's method uses block co-polymers to create porous templates into which a new material can flow and crystallize. A polymer consists of organic molecules that link into long chains to form a solid. A block co-polymer is made by joining two different molecules at their ends. When they chain together and are mixed with metal oxides, one forms a nanoscale pattern of repeating geometric shapes, while the other fills the space in between. Burning the polymer away leaves a porous metal oxide nanostructure that can act as a template.

Wiesner's team created a template with hexagonal pores on a silicon single-crystal substrate and deposited films of amorphous silicon or nickel silicide over it. In collaboration with Mike Thompson, associate professor of materials science and engineering, they then heated the silicon surface with very short (nanosecond) laser pulses. This melts the newly deposited layer and the top few microns (millionths of a meter) of the silicon substrate. After only a few tens of nanoseconds the molten silicon recrystallizes with the single crystal silicon substrate acting as a seed crystal to trigger crystallization in the deposited material above it, causing that crystal to line up epitaxially with the seed.

The template is dissolved away, leaving an array of hexagonal pillars about 30 nm across. The team has made porous nanostructured films up to 100 nm thick with other complex shapes. In previous work Wiesner created lattices of cylinders, planes, spheres and complex "gyroids" by varying the composition of co-polymers.

Other materials could be deposited, the researchers said. The goal here, they said, was to demonstrate the formation of film with the same material as the substrate (officially known as homoepitaxy) and with a different material (heteroepitaxy).

In a further proof-of-concept experiment, the researchers showed that the structured thin film could be arranged in micron-scale patterns, as might be necessary in designing an electronic circuit, by laying a mask over the surface before applying laser heating.

"We have essentially gotten to the holy grail," Wiesner said. "It is not only a nanostructured single crystal, but it has an epitaxial relation to the substrate. There is no better control."

Provided by Cornell University (news : web)

Watching nanosheets and molecules transform under pressure could lead to stronger materials

October 20, 2010 By Lauren Gold

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Wang and colleagues used small angle X-ray diffraction (SAXRD) and wide-angle X-ray diffraction (WAXRD) to observe changes in the molecular structure of wurtzite crystal under pressure.

(PhysOrg.com) -- When it comes to tests of strength, graphite -- actually layered sheets of carbon atoms -- fares badly. Subject it to ultra-high pressure, though, and graphite becomes diamond, the hardest substance known, and a uniquely useful material in a variety of applications.

But while diamonds may be forever, most materials that transform under high pressure revert to their original structure when the pressure is lifted -- losing any useful properties they may have gained when the squeeze was on.

Now, by understanding the process behind the transformation itself, from both experimental and theoretical perspectives, researchers have taken a potential step toward creating a new class of exceptionally strong, durable materials that maintain their high-pressure properties -- including strength and superconductivity -- in everyday low-pressure environments.

The research, led by Zhongwu Wang, staff scientist at the Cornell High Energy Synchrotron Source (CHESS) and including Roald Hoffmann, the 1981 chemistry Nobel laureate and Frank H.T. Rhodes Professor of Humane Letters Emeritus, appears in the Oct. 12, issue of the Proceedings of the National Academy of Sciences.

Additional scientists at CHESS, a group in Korea and a postdoctoral associate in the Hoffmann group, Xiao-dong Wen, also contributed.

Researchers frequently use X-ray diffraction, a technique in which X-rays are projected at a structure and captured on film after they pass through or bounce off its surfaces, to determine the static structures of atoms and molecules. But until now, the transformation and interaction between two structures happened in a metaphorical black box, said Wang.

To open the box, researchers focused on wurtzite, a cadmium-selenium crystal in which atoms are arranged in a diamondlike structure and molecules are bonded on the surface. When thin sheets of wurtzite are squeezed under 10.7 gigapascals of pressure, or 107,000 times the pressure on the Earth's surface, their atomic structure transforms into a rock salt-like structure

Subjecting a macro-sized crystal to high pressure can cause it to break (small defects in the crystal structure magnify, causing the structure, and the transformation process, to become irregular) -- so the group's Korean collaborators instead prepared wurtzite nanosheets, which are just 1.4 nanometers thick and defect-free.

As pressure was applied, Wang and colleagues integrated two X-ray diffraction techniques (small- and large-angle X-ray diffraction) to characterize changes in the crystal's surface shape and interior atomic structure, as well as the structural change of surface-bonded molecules.

They first discovered that the nanosheets required three times the pressure to undergo the transformation as the same material in larger crystal form.

They also tested the material's yield strength (the stress level at which it begins to deform), hardness (resistance to scratching or abrading) and elasticity (ability to return to its original form) during the transformation. Understanding how those properties change as the molecules interact could help researchers design stronger, tougher materials, Wang said.

And adding a bonding molecule called a soft ligand to the surface of the high-pressure nanosheets, the researchers observed the effect of that bonding to the nanosheets' internal structure, transformation pressure, and spacing.

Meanwhile, as Wang and colleagues performed the experiments at CHESS, Wen and Hoffmann worked on the corresponding theory behind the transformation interaction.

"Both the experiment and the simulation agree well," Wang said. "Now we know how the atoms move. We understand the intermediate procedure."

The next step is to test ways of blocking the reverse transformation from rock salt back to wurtzite, creating a material that maintains rock salt's unique properties under ambient pressure.

And Wang's experimental process could hold promise for understanding the transformation pathway for other compounds as well.

"It can apply to all other materials," Wang said. "Just follow our way of measurement."

Provided by Cornell University (news : web)

Structure of plastic solar cells impedes their efficiency, researchers find

October 5, 2010 (PhysOrg.com) -- A team of researchers from North Carolina State University and the U.K. has found that the low rate of energy conversion in all-polymer solar-cell technology is caused by the structure of the solar cells themselves. They hope that their findings will lead to the creation of more efficient solar cells.

Polymeric solar cells are made of thin layers of interpenetrating structures from two different conducting plastics and are increasingly popular because they are both potentially cheaper to make than those currently in use and can be “painted” or printed onto a variety of surfaces, including flexible films made from the same material as most soda bottles. However, these solar cells aren’t yet cost-effective to make because they only have a power conversion rate of about three percent, as opposed to the 15 to 20 percent rate in existing solar technology.

“Solar cells have to be simultaneously thick enough to absorb photons from the sun, but have structures small enough for that captured energy – known as an exciton – to be able to travel to the site of charge separation and conversion into the electricity that we use,” says Dr. Harald Ade, professor of physics and one of the authors of a paper describing the research. “The solar cells capture the photons, but the exciton has too far to travel, the interface between the two different plastics used is too rough for efficient charge separation, and its energy gets lost.”

The researchers’ results appear online in Advanced Functional Materials and Nano Letters.

In order for the solar cell to be most efficient, Ade says, the layer that absorbs the photons should be around 150-200 nanometers thick (a nanometer is thousands of times smaller than the width of a human hair). The resulting exciton, however, should only have to travel a distance of 10 nanometers before charge separation. The way that polymeric solar cells are currently structured impedes this process.

Ade continues, “In the all-polymer system investigated, the minimum distance that the exciton must travel is 80 nanometers, the size of the structures formed inside the thin film. Additionally, the way devices are currently manufactured, the interface between the structures isn’t sharply defined, which means that the excitons, or charges, get trapped. New fabrication methods that provide smaller structures and sharper interfaces need to be found.”

Ade and his team plan to look at different types of polymer-based solar cells to see if their low efficiencies are due to this same structural problem. They hope that their data will lead chemists and manufacturers to explore different ways of putting these cells together to increase efficiency.

“Now that we know why the existing technology doesn’t work as well as it could, our next steps will be in looking at physical and chemical processes that will correct for those problems. Once we get a baseline of efficiency, we can redirect research and manufacturing efforts.”

Provided by North Carolina State University (news : web)

New graphene fabrication method uses silicon carbide template

October 5, 2010 by John Toon

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Graphene transistors. Georgia Tech researchers have fabricated an array of 10,000 top-gated graphene transistors, believed to be the largest graphene device density reported so far.

The technique involves etching patterns into the silicon carbide surfaces on which epitaxial graphene is grown. The patterns serve as templates directing the growth of graphene structures, allowing the formation of nanoribbons of specific widths without the use of e-beams or other destructive cutting techniques. Graphene nanoribbons produced with these templates have smooth edges that avoid electron-scattering problems.

"Using this approach, we can make very narrow ribbons of interconnected graphene without the rough edges," said Walt de Heer, a professor in the Georgia Tech School of Physics. "Anything that can be done to make small structures without having to cut them is going to be useful to the development of graphene electronics because if the edges are too rough, electrons passing through the ribbons scatter against the edges and reduce the desirable properties of graphene."

The new technique has been used to fabricate an array of 10,000 top-gated graphene transistors on a 0.24 square centimeter chip – believed to be the largest density of graphene devices reported so far.

The research was reported Oct. 3 in the advance online edition of the journal Nature Nanotechnology. The work was supported by the National Science Foundation, the W.M. Keck Foundation and the Nanoelectronics Research Initiative Institute for Nanoelectronics Discovery and Exploration (INDEX).

In creating their graphene nanostructures, De Heer and his research team first use conventional microelectronics techniques to etch tiny "steps" – or contours – into a silicon carbide wafer. They then heat the contoured wafer to approximately 1,500 degrees Celsius, which initiates melting that polishes any rough edges left by the etching process.

They then use established techniques for growing graphene from silicon carbide by driving off the silicon atoms from the surface. Instead of producing a consistent layer of graphene one atom thick across the surface of the wafer, however, the researchers limit the heating time so that graphene grows only on the edges of the contours.

To do this, they take advantage of the fact that graphene grows more rapidly on certain facets of the silicon carbide crystal than on others. The width of the resulting nanoribbons is proportional to the depth of the contour, providing a mechanism for precisely controlling the nanoribbons. To form complex graphene structures, multiple etching steps can be carried out to create a complex template, de Heer explained.

"By using the silicon carbide to provide the template, we can grow graphene in exactly the sizes and shapes that we want," he said. "Cutting steps of various depths allows us to create graphene structures that are interconnected in the way we want them to be.

In nanometer-scale graphene ribbons, quantum confinement makes the material behave as a semiconductor suitable for creation of electronic devices. But in ribbons a micron or more wide, the material acts as a conductor. Controlling the depth of the silicon carbide template allows the researchers to create these different structures simultaneously, using the same growth process.

"The same material can be either a conductor or a semiconductor depending on its shape," noted de Heer, who is also a faculty member in Georgia Tech's National Science Foundation-supported Materials Research Science and Engineering Center (MRSEC). "One of the major advantages of graphene electronics is to make the device leads and the semiconducting ribbons from the same material. That's important to avoid electrical resistance that builds up at junctions between different materials."

After formation of the nanoribbons – which can be as narrow as 40 nanometers – the researchers apply a dielectric material and metal gate to construct field-effect transistors. While successful fabrication of high-quality transistors demonstrates graphene's viability as an electronic material, de Heer sees them as only the first step in what could be done with the material.

"When we manage to make devices well on the nanoscale, we can then move on to make much smaller and finer structures that will go beyond conventional transistors to open up the possibility for more sophisticated devices that use electrons more like light than particles," he said. "If we can factor quantum mechanical features into electronics, that is going to open up a lot of new possibilities."

De Heer and his research team are now working to create smaller structures, and to integrate the graphene devices with silicon. The researchers are also working to improve the field-effect transistors with thinner dielectric materials.

Ultimately, graphene may be the basis for a generation of high-performance devices that will take advantage of the material's unique properties in applications where the higher cost can be justified. Silicon will continue to be used in applications that don't require such high performance, de Heer said.

"This is another step showing that our method of working with epitaxial graphene on silicon carbide is the right approach and the one that will probably be used for making graphene electronics," he added. "This is a significant new step toward electronics manufacturing with graphene."

Provided by Georgia Institute of Technology

Smaller is better in the viscous zone

October 21, 2010

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These are nanotubes. Credit: Jei Liu

Being the right size and existing in the limbo between a solid and a liquid state appear to be the secrets to improving the efficiency of chemical catalysts that can create better nanoparticles or more efficient energy sources.

When matter is in this transitional state, a catalyst can achieve its utmost potential with the right combination of catalyst particle size and temperature, according to a pair of Duke University researchers. A catalyst is an agent or chemical that facilitates a chemical reaction. It is estimated that more than 90 percent of chemical processes used by industry involve catalysts at some point.

This finding could have broad implications in almost every catalyst-based reaction, according to an engineer and a chemist at Duke who reported their findings on line in the American Chemical Society's journal ACS-Nano. The team found that the surface-to-volume ratio of the catalyst particle – its size -- is more important than generally appreciated.

"We found that the smaller size of a catalyst will lead to a faster reaction than if the bulk, or larger, version of the same catalyst is used," said Stefano Curtarolo, associate professor in the Department of Mechanical Engineering and Materials Sciences.

"This is in addition to the usual excess of surface in the nanoparticles," said Curtarolo, who came up with the theoretical basis of the findings three years ago and saw them confirmed by a series of intricate experiments conducted by Jie Liu, Duke professor of chemistry.

"This opens up a whole new area of study, since the thermo-kinetic state of the catalyst has not before been considered an important factor," Curtarolo said. "It is on the face of it paradoxical. It's like saying if a car uses less gas (a smaller particle), it will go faster and further."

Their series of experiments were conducted using carbon nanotubes, and the scientists believe that same principles they described in the paper apply to all catalyst-driven processes.

Liu proved Curtarolo's hypothesis by developing a novel method for measuring not only the lengths of growing carbon nanotubes, but also their diameters. Nanotubes are microscopic "mesh-like" tubular structures that are used in hundreds of products, such as textiles, solar cells, transistors, pollution filters and body armor.

"Normally, nanotubes grow from a flat surface in an unorganized manner and look like a plate of spaghetti, so it is impossible to measure any individual tube," Liu said. "We were able to grow them in individual parallel strands, which permitted us to measure the rate of growth as well as the length of growth."

By growing these nanotubes using different catalyst particle sizes and at different temperatures, Liu was able to determine the "sweet spot" at which the nanotubes grew the fastest and longest. As it turned out, this happened when the particle was in its viscous state, and that smaller was better than larger, exactly as predicted before.

These measurements provided the experimental underpinning of Curtarolo's hypothesis that given a particular temperature, smaller nanoparticles are more effective and efficient per unit area than larger catalysts of the same type when they reside in that dimension between solid and liquid.

"Typically, in this field the experimental results come first, and the explanation comes later," Liu said. "In this case, which is unusual, we took the hypothesis and were able to develop a method to prove it correct in the laboratory."

Multi-component nano-structures with tunable optical properties

 BNL scientists used DNA linkers with three binding sites (black ?strings?) to connect gold nanoparticles (orange and red spheres) and fluorescent dye molecules (blue spheres) tagged with complementary DNA sequences. These units are self-assembled to form a body-center cubic lattice with nanoparticles at the corners and in the center, and fluorescent dye molecules in between.

(PhysOrg.com) -- Scientists at the U.S. Department of Energy's Brookhaven National Laboratory report the first successful assembly of 3-D multi-component nanoscale structures with tunable optical properties that incorporate light-absorbing and -emitting particles. This work, using synthetic DNA as a programmable component to link the nanoparticles, demonstrates the versatility of DNA-based nanotechnology for the fabrication of functional classes of materials, particularly optical ones, with possible applications in solar-energy conversion devices, sensors, and nanoscale circuits. The research was published online September 29, 2010, in the journal Nano Letters.

?For the first time we have demonstrated a strategy for the assembly of 3-D, well-defined, optically active structures using DNA encoded components of different types,? said lead author Oleg Gang of Brookhaven?s Center for Functional Nanomaterials (CFN). Like earlier work by Gang and his colleagues, this technique makes use of the high specificity of binding between complementary strands of DNA to link particles together in a precise way.

In the current study, the DNA linker molecules had three binding sites. The two ends of the strands were designed to bind to complementary strands on ?plasmonic? gold nanoparticles ? particles in which a particular wavelength of light induces a collective oscillation of the conductive electrons, leading to strong absorption of light at that wavelength. The internal part of each DNA linker was coded to recognize a complementary strand chemically bound to a fluorescent dye molecule. This setup resulted in the self-assembly of 3-D body centered cubic crystalline structures with gold nanoparticles located at each corner of the cube and in the center, with dye molecules at defined positions in between.

The scientists also demonstrated that the assembled structures can be dynamically tuned by altering the salt concentration of the solution in which they are formed. Changes in salinity alter the length of the negatively charged DNA molecules, leading to reversible contraction and expansion of the whole lattice by about 30 percent in length.

?It has long been understood that the distance between metal nanoparticles and paired dye molecules can affect the optical properties of the latter,? said Matthew Sfeir, coauthor and an optical scientist at the CFN. In this experiment, the expansion and contraction of the crystal lattice triggered by the changes in salt concentration allowed for a dramatic modulation of an optical response: a three-fold increase in the emission rate of the fluorescent molecules was observed.

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The resulting 3-D structures could be tuned by adjusting the salt concentration. As salt concentration increased, the crystals contracted by about 30 percent, decreasing the distance (D) between the particles. This contraction in interparticle distance had a dramatic effect on the fluorescence of the dye molecules, making them cycle photons faster, as indicated by the color scale at the left of the crystal images (see image below), which ranges from nearly 2 nanoseconds per cycle for the free dye (A), to about 0.7 nanoseconds per cycle in larger lattices (C), to just above 0.3 nanoseconds per cycle for the contracted crystals (E).

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These results were determined using a combination of small angle x-ray scattering at Brookhaven?s National Synchrotron Light Source (NSLS) and time-resolved fluorescent methods at the CFN. ?This combination of synchrotron-based structural methods and time-resolved optical imaging techniques provided invaluable direct insight into the relationship between the structure and fluorescent properties of these light emitting arrays,? Gang said.

?Our study tackles important questions about the self-assembly of systems from components of multiple types. Such systems potentially allow for the modulation of properties of individual components, and might lead to the emergence of new behavior due to collective effects. This assembly approach can be applied to explore such collective behavior of three-dimensional nano-optical arrays ? for example, the influence of the plasmonic lattice on quantum dots.

?An understanding of these interactions would be relevant for developing novel optical materials for photovoltaic, photocatalysis, computing, and light-emitting applications. We now have an approach to make these structures and further study these effects.?

Push-Button Logic on the Nanoscale

 (PhysOrg.com) -- Circuits that can perform logic operations at the push of a button are a dime-a-dozen these days, but a breakthrough by researchers in the USA has meant they can be smaller and simpler than ever before. Using a single material as both the button and the circuit for the first time, scientists at the Georgia Institute of Technology have created tiny logic circuits that can be used as the basis of nanometer-scale robotics and processors.

Professor Zhong Lin (ZL) Wang, who leads the research, explains how the peculiar properties of zinc oxide have made this work possible. ?Zinc oxide is unique because of its coupled piezoelectric and semiconductor properties.? The piezoelectric effect occurs when a strain on a material, caused by pushing on it for example, reversibly changes the crystal structure in one direction enough to make an electric field.

The mechanical motion induces a voltage from one side of the material to the other. Semiconductors have the ability to conduct electricity, or not, depending on some external factor. In zinc oxide, these two characteristics combine and the transport of electric current is influenced by the piezoelectric effect, meaning that changes in strain result in changes in the material?s ability to conduct electricity. This is what is known as the piezotronic effect.

By having the zinc oxide in the form of a nanowire, (diameter 300 nanometers; length 400 micrometers), and bonded with metals at each end, Wang has effectively produced a tiny transistor, which is gated (open or shut, with electricity either flowing or not) by the strain applied to the nanowire.

In results published in Advanced Materials this week, Wang and his colleagues show how by combining an appropriate number of these transistors in various arrangements, systems can be made that can process the basic logic functions of NAND, NOR, and XOR, as well as act as multiplexers (MUX) and demultiplexors (DEMUX).

Until now, logic processors have relied on the use of CMOS technology, using two Complementary components, a Metal Oxide and a Semiconductor, such as silicon. In CMOS processors, an electric signal is required to operate the gate. If a mechanical stimulus is required, yet a further component must be added to the system. By contrast, Wang claims his work represents a ?brand new approach toward logic operation that performs mechanic-electrical coupled and controlled actions in one structure unit using a single material (which is zinc oxide)?This is the very first demonstration of mechanical action-induced electronic operation with the introduction of a new driving mechanism in comparison to existing silicon-based logic operations. This is also the first demonstration of its kind using nanowires.?

Working in the nanoscale presents its own challenges, and the most difficult parts of this work were synthesizing high-quality nanowires and manipulating them on the substrate so they would work in a synchronized way. But Wang is now confident they have achieved a good control over the process, and the results testify that this is the case.

The logic circuits are not as fast as those currently in use and based on CMOS, but Wang does not see this as a problem. In fact, he sees the applications of the two technologies as being complementary. ?The strain-gated logic devices are designed to interface with the ambient environment, which is associated with low-frequency mechanical actions, and the aim and targeting applications are different from those of conventional silicon devices which aim at speed.? Envisaged applications include nanorobotics, transducers, micromachines, human-computer interfacing, and microfluidics (where tiny channels carry various liquids, usually to be mixed for reaction tightly controlled ways).

The group intends to join the new strain-gated transducers to sensors and energy-drawing components they have previously prepared also from zinc oxide nanowires to make ?self-sustainable, all-nanowire-based, multifunctional self-powered autonomous intelligent nanoscale systems.? It seems we won?t even need to push a button anymore.

Selenium makes more efficient solar cells

 

This is a sunset over the Pacific Ocean as seen from Highway 1 south of Monterey, Calif. LBNL's Marie Mayer, who took the photo, calls sunlight and water "two sustainable resources to power our world." Credit: Credit: Marie Mayer

Call it the anti-sunscreen. That's more or less the description of what many solar energy researchers would like to find -- light-catching substances that could be added to photovoltaic materials in order to convert more of the sun's energy into carbon-free electricity.

Research reported in the journal Applied Physics Letters, published by the American Institute of Physics (AIP), describes how solar power could potentially be harvested by using oxide materials that contain the element selenium. A team at the Lawrence Berkeley National Laboratory in Berkeley, California, embedded selenium in zinc oxide, a relatively inexpensive material that could be promising for solar power conversion if it could make more efficient use of the sun's energy. The team found that even a relatively small amount of selenium, just 9 percent of the mostly zinc-oxide base, dramatically boosted the material's efficiency in absorbing light.

"Researchers are exploring ways to make solar cells both less expensive and more efficient; this result potentially addresses both of those needs," says author Marie Mayer, a fourth-year University of California, Berkeley doctoral student based out of LBNL's Solar Materials Energy Research Group, which is working on novel materials for sustainable clean-energy sources.

Mayer says that photoelectrochemical water splitting, using energy from the sun to cleave water into hydrogen and oxygen gases, could potentially be the most exciting future application for her work. Harnessing this reaction is key to the eventual production of zero-emission hydrogen powered vehicles, which hypothetically will run only on water and sunlight. Like most researchers, Mayer isn't predicting hydrogen cars on the roads in any meaningful numbers soon. Still, the great thing about solar power, she says, is that "if you can dream it, someone is trying to research it."

Cheaper, better solar cell is full of holes

As the tiny holes deepen, they make the silvery-gray silicon appear darker and darker until it becomes almost pure black and able to absorb nearly all colors of light the sun throws at it.

At room temperature, the black silicon wafer can be made in about three minutes. At 100 degrees F, it can be made in less than a minute.

The breakthrough by NREL scientists likely will lead to lower-cost solar cells that are nonetheless more efficient than the ones used on rooftops and in solar arrays today.

R&D Magazine recently awarded the NREL team one of its R&D 100 awards for Black Silicon Nanocatalytic Wet-Chemical Etch. Called "the Oscars of Invention," the R&D 100 awards recognize the most significant scientific breakthroughs of the year.

Howard Branz, the principal investigator for the project, said his team got interested in late 2006 after he heard a talk by a scientist from the Technical University of Munich. The scientist described how his team had created black silicon by laying down a thin gold layer using a vacuum deposition technique. Quickly, NREL senior scientist Qi Wang and senior engineer Scott Ward gave it a try.

"We always ride on the shoulders of others," Branz said. "We started by replicating the Munich experiment."

Packets of Light, Golden Holes

Think of light as coming in little packets. Each packet is a photon, which potentially can be changed into an electron for solar energy. If the photon bounces off the surface of a solar cell, that's energy lost. Some of the light normally bounces off when it hits an object, but a 'black silicon' wafer will absorb all the light that hits it.

The human eye perceives the wafer as black because almost no sunlight reflects back to the retina. And that is because the trillion holes in the wafer's surface do a much better job of absorbing the wavelengths of light than a solid surface does.

It's roughly the same reason that ceiling tiles with holes in them absorb sound better than ceiling tiles without holes. Scientists by the late 19th century had already done experiments to show that what works for absorbing sound also works for absorbing light.

The team from Munich used evaporation techniques that require expensive vacuum pumps to lay down a very thin layer of gold, perhaps 10 atoms thick, Branz said. When a mixture of hydrogen peroxide and hydrofluoric acid was poured on the thin gold layer, nanoparticles of gold bored into the smooth surface of the wafer, making billions of holes.

The NREL team knew right away that the vacuum pumps and evaporative equipment needed to deposit the gold were too costly to become commercially viable.

NREL's Goal: Simplify the Process, Lower the Cost

"Our thinking was that if the goal is to make it cheaper, we want to avoid vacuum deposition completely," Branz said.

In a string of outside-the-box insights combined with some serendipity, Branz and colleagues Scott Ward, Vern Yost and Anna Duda greatly simplified that process.

Rather than laying the gold with vacuums and pumps, why not just spray it on? Ward suggested.

Rather than layering the gold and then adding the acidic mixture, why not mix it all together from the outset? Dada suggested.

In combination, those two suggestions yielded even better results.

The scientists put a suspended solution of gold nanoparticles, called colloidal gold, on the silicon surface, and let the water evaporate overnight to leave just the gold, which then etched into the wafer. The wafer turned nearly as black as with the evaporated gold.

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The process takes just three minutes at room temperature. Inside a laboratory at NREL's Solar Energy Research Facility, an acid mixture bubbles atop a silicon wafer as it etches holes and works toward turning the wafer black. Credit: Dennis Schroeder

A Lucky Accident

And then, as is often the case with important scientific breakthroughs, serendipity entered.

NREL technician and chemist Vern Yost noticed after a time that he wasn't getting such good results, and assumed it was because an old batch of colloidal nanoparticles had somehow clumped together. So he tried to separate them with aqua regia, a highly corrosive mixture of nitric acid and hydrochloric acid. Aqua regia is Latin for regal water, and refers to a liquid that can dissolve the royal metals such as silver and gold.

The aqua regia treatment got the process working better than ever, and a little investigation found that the aqua regia had reacted with the gold to form a solution of chloroauric acid.

Voila! Chloroauric acid is less expensive than colloidal gold and actually is the chemical precursor that industry uses to make colloidal gold.

Could the same black-silicon etching result be achieved by substituting the inexpensive chloroauric acid for costly colloidal gold, and then mixing it as before with hydrogen peroxide and hydrofluoric acid? Yost and Branz wondered.

Yes, it worked. "Chloroauric acid is much cheaper than colloidal gold," Branz said. "In essence, by skipping a few steps, they were able to make gold nanoparticles from the chloroauric acid at the same time as they were etching holes into the silicon with the gold they had made."

Once the concept was understood and the mix of materials solved, the actual making of a black silicon wafer became quite simple.

"You take a beaker, put a silicon wafer in, pour in the chloroauric acid, pour in the hydrofluoric acid and hydrogen peroxide, and wait," Branz said.

As little as 20 seconds later, the silvery silicon wafer turns black.

"Our method gives a blacker silicon and would replace an expensive vacuum deposition system with a single, cheap, wet etch step," Branz said.

Cheaper Process Also Makes a Better Material

They tested their black silicon and found that the much-lower-cost recipe containing chloroauric acid quickly reduced the unwanted reflection to less than 2 percent. The more costly approach using conventional silicon nitride anti-reflection layers stalled out at about 3 to 7 percent reflection. As an added bonus, black silicon prevents reflection of low-angle morning and afternoon sunlight far better than the conventional antireflection layer.

To understand why their inexpensive approach worked so well, the team brought in NREL optics expert and senior scientist Paul Stradins and NREL electron microscopists Bobby To and Kim Jones. The trio found that the black silicon squelched reflection so well because the holes were smaller in diameter than the solar wavelengths.

That's crucial, because if the holes were as big as these light wavelengths, the light rays would recognize a "sharp interface," just as they would if they encountered a stainless steel counter. Any sharp interface causes the light from the sun to reflect from the surface before it can enter the solar cell and be changed into electricity.

Another reason the sunlight never feels a sharp interface when it hits the silicon is that all those trillions of holes are bored to different depths, because of the randomness of the etch rate of each nanoparticle. Because of the variable depths of the holes, the rays very gradually move from air to silicon. The light never encounters an abrupt change from air to solid surface, so it doesn't bounce off the wafer.

But Will it Work in a Solar Cell?

Next was the formidable challenge of using the technology to make a workable solar cell.

Hao-Chi Yuan, a postdoctoral researcher, was added to the team to figure out how best to work this new kind of silicon into a solar cell, make the solar cells and determine the strengths and weaknesses of this new kind of cell. Yuan, along with Yost, Branz and NREL engineer Matthew Page worked to determine the ideal depths and diameters of the holes if the goal is to turn photons into electrons.

To keep a solar cell at or near the record 16.8 percent efficiency rate they'd achieved, they realized the holes had to adhere to the "Goldilocks" principle. The holes must be "just right": deep enough to block reflections, but not so deep that they spoil the solar cell.

Specifically, they found the best results occurred when the trillions of holes were on average about 500 nanometers or half a micron deep, and their diameters just a little bit narrower than the smallest wavelength of light. (How small? The diameter of 40 holes, added together, would be the thickness of a human hair.)

If the holes were much deeper, the solar cell would have trouble pulling all of the solar-generated electrons out. Efficiencies would be so low no one would want to put the cells on their roof.

Happily, that combination of depth and diameter can be achieved with a 3-minute wet-etch soak at room temperature.

Industry's Acutely Interested

Though they will be cheaper to manufacture, NREL's best solar cells are still a few tenths of a percent less efficient than the conventional type. But the low reflection means a jump in photovoltaic efficiency of at least 1 percentage point could be achieved. The team is still working to wrest a bit more efficiency from the black silicon cells. The solar cell world has become a game of inches, Branz said, so "even half a percentage point bump in efficiency at reduced cost would be huge."

Solar cell companies are interested in licensing the technology from NREL.

"We've had several companies come visit here to learn more about it," Chris Harris, associate director of licensing in NREL's commercialization and technology transfer division, said. "The interest is high.

"This is certainly a significant advantage in an industry where everyone is competing for market share and the cost per watt is a key selling feature," Harris added. "Black silicon provides an added benefit on top of any other improvements in efficiency a company can get."

Al Goodrich, a senior cost analyst for NREL's PV manufacturing division, found that making the black silicon wafers requires about a third less energy than adding the conventional anti-reflection layer to the finished solar cell.

The one-step process also is a lot easier on the environment.

The technology would replace a process that uses dangerous silane gas, as well as cleaning gases such as nitrogen trifluoride, which has 17,000 times more punch than carbon dioxide in contributing to global warming. A switch to the black silicon wet etch technology would mean huge reductions in greenhouse gases, and improvements in the energy payback for resulting PV devices. It also reduces the capital costs of starting a factory line by about 10 percent, because it replaces several expensive vacuum vapor tools with a simple wet bath, Goodrich said.

NREL estimates that the black silicon can reduce cell conversion costs by 4 to 8 percent, while using widely available industrial materials and equipment.

"That's big," Goodrich added. "The people who are interested in this technology recognize that that difference is valuable real estate."

Research gives insight into using graphene in electronics

September 22, 2010 (PhysOrg.com) -- New findings from the laboratory of University of Illinois researcher Joe Lyding are providing valuable insight into graphene, a single two-dimensional layer of graphite with numerous electronic and mechanical properties that make it attractive for use in electronics.

Lyding, a researcher at Illinois?s Beckman Institute, and his lab report using a dry deposition method they developed to deposit pieces of graphene on semiconducting substrates and on the electronic character of graphene at room temperature they observed using the method. The paper, by Lyding, lead author Kevin He of the Lyding lab, and their collaborators, is titled Separation-Dependent Electronic Transparency of Monolayer Graphene Membranes on III-V Semiconductor Substrates and appeared last month in the journal Nanoletters.

The researchers wrote this of graphene?s potential, especially as compared to its elemental cousin, carbon nanotubes, for use in electronics and other applications: ?It exhibits the quantum hall effect, even at room temperature, and its optical transparency is directly related to the fine structure constant. Graphene is more and more being thought of as a fairly strong and elastic membrane (with an associated potential as a material for NEMS applications). Unlike carbon nanotubes, graphene can be patterned using standard e-beam lithographic techniques, making it an attractive prospect for use in semiconductor devices.?

To reach that goal, issues associated with graphene must be overcome, and this paper gives insight into a much-needed step in that direction: understanding substrate-graphene interactions toward integration into future nanoelectronic devices. The project investigated the electronic character of the underlying substrate of graphene at room temperature and reports on ?an apparent electronic semitransparency at high bias of the nanometer-sized monolayer graphene pieces observed using an ultrahigh vacuum scanning tunneling microscope (UHV-STM) and corroborated via first-principles studies.? This semitransparency was made manifest through observation of the substrate atomic structure through the graphene.

Lyding?s research group had developed a non-chemical (dry) technique for depositing carbon nanotubes (CNTs) on a surface called Dry Contact Transfer that allowed the CNTs to maintain their electronic properties. They later applied the method to graphene and were able to deposit pristine, nanometer-sized graphene pieces in situ onto atomically flat UHV-cleaved Gallium arsenide and Indium arsenide semiconductor substrates with low amounts of extraneous contamination.

The electronic semitransparency of the graphene pieces was observed when the UHV STM probe pushed the graphene 0.05nm closer to the surface, causing its electronic structure to mix with that of the surface.

In summary, the researchers write, their results ?highlight the significance of graphene-substrate interactions and suggest that proper control of the substrate can have a major effect on the electronic properties of the graphene it supports.?

'Greening' your flat screen TV

Electronic products pollute our environment with a number of heavy metals before, during and after they're used. In the U.S. alone, an estimated 70% of heavy metals in landfill come from discarded electronics. With flat screen TVs getting bigger and cheaper every year, environmental costs continue to mount.

To counter this, a new Tel Aviv University solution applies a discovery in nanotechnology, based on self-assembled peptide nanotubes, to "green" the optics and electronics industry. Researchers Nadav Amdursky and Prof. Gil Rosenman of Tel Aviv University's Department of Electrical Engineering say their technology could make flat screen TV production green and can even make medical equipment -- like subcutaneous ultrasound devices -- more sensitive.

Inspired by a biomaterial involved in Alzheimer's disease research discovered by Prof. Ehud Gazit of the university's Department of Microbiology and Biotechnology, the scientists developed a new nano-material, applying the scientific disciplines of both biology and physics. This biological material is the basis for their new, environmentally-friendly variety of light-emitting diodes (LED) used in both consumer and medical electronics.

TV in a test tube?

Their new invention is more than a clean, green way to create light, the researchers say. It also generates a strong signal that can be used in other applications in the nano-world of motors, actuators and ultrasound.

"We are growing our own light sources," says Amdursky, a doctoral student working under Prof. Rosenman's supervision. The organic nano-lightsticks he and his supervisors have developed using organic chemistry are made from carbon, making them cheap as well as environmentally friendly.

Unlike conventional light sources, the biologically-derived light source has a nanoscale architecture, easing the integration into light-emitting devices such as LED TVs and improving the resolution of the picture as well. The Tel Aviv University team has recently written a patent to cover their "organic LED" lights.

From living rooms to hospital rooms

According to Amdursky, the light emitted by the new light sticks is not appreciably different than that which emanates from today's inorganically engineered LED lights.

"We don't need a special plant, bacterium or a big machine to grow these structures in," says Amdursky, who says the applications of the technology are wider than the widest screen television. The core technology and structures, described in Advanced Materials, Nano Letters, and ACS Nano, exhibit "piezoelectric characteristics," necessary for the development of tiny nano-ultrasound machines that could scan cells from inside the body. Piezoelectric motors or actuators are only dozens of nanometers wide, which can lead to their application in energy harvesting systems as super-capacitors -- large energy storage devices, necessary for the solar energy and wind energy business.

Robotics breakthrough: Scientists make artificial skin

The lab-tested material responds to almost the same pressures as human skin and with the same speed, they reported in the British journal Nature Materials.

Important hurdles remain but the exploit is an advance towards replacing today's clumsy robots and artificial arms with smarter, touch-sensitive upgrades, they believe.

"Humans generally know how to hold a fragile egg without breaking it," said Ali Javey, an associate professor of computer sciences at the University of California at Berkeley, who led one of the research teams.

"If we ever wanted a robot that could unload the dishes, for instance, we'd want to make sure it doesn't break the wine glasses in the process. But we'd also want the robot to grip the stock pot without dropping it."

The "e-skin" made by Javey's team comprises a matrix of nanowires made of germanium and silicon rolled onto a sticky polyimide film.

The team then laid nano-scale transistors on top, followed by a flexible, pressure-sensitive rubber. The prototype, measuring 49 square centimetres (7.6 square inches), can detect pressure ranging from 0 to 15 kilopascals, comparable to the force used for such daily activities as typing on a keyboard or holding an object.

A different approach was taken by a team led by Zhenan Bao, a Chinese-born associate professor at Stanford University in California who has gained a reputation as one of the top women chemists in the United States.

Their approach was to use a rubber film that changes thickness due to pressure, and employs capacitors, integrated into the material, to measure the difference. It cannot be stretched, though.

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This is an artist's illustration of an artificial e-skin with nanowire active matrix circuitry covering a hand. A fragile egg is held, illustrating the functionality of the e-skin device for prosthetic and robotic applications. Credit: Ali Javey and Kuniharu Takei

"Our response time is comparable with human skin, it's very, very fast, within milliseconds, or thousandths of a second," Bao told AFP. "That means in real terms that we can feel the pressure instantaneously."

The achievements are "important milestones" in artificial intelligence, commented John Boland, a nanoscientist at Trinity College Dublin, Ireland, who hailed in particular the use of low-cost processing components.

In the search to substitute the human senses with electronics, good substitutes now exist for sight and sound, but lag for smell and taste.

Touch, though, is widely acknowledged to be the biggest obstacle.

Even routine daily actions, such as brushing one's teeth, turning the pages of a newspaper or dressing a small child would easily defeat today's robots.

Bao added important caveats about the challenges ahead.

One is about improving the new sensors. They respond to constant pressure, whereas in human skin more complex sensations are possible.

This is because the pressure-sensing cells in the skin can send different frequencies of signal -- for instance, when we feel something painful or sharp, the frequency increases, alerting us to the threat.

In addition, Bao warned, "connecting the artificial skin with the human nerve system will be a very challenging task".

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This is an optical image of a fully fabricated e-skin device with nanowire active matrix circuitry. Each dark square represents a single pixel. Credit: Ali Javey and Kuniharu Takei, UC Berkeley

"Ultimately, in the very distant future, we would like to make a skin which performs really like human skin and to be able to connect it to nerve cells on the arm and thus restore sensation.

"Initially, the prototype that we envision would be more like a handheld device, or maybe a device that connects to other parts of the body that have skin sensation.

"The device would generate a pulse that would stimulate other parts of the skin, giving the kind of signal 'my (artificial) hand is touching something', for instance."

In the future, artificial skin could be studded with sensors that respond to chemicals, biological agents, temperature, humidity, radioactivity or pollutants.

"This would be especially useful in applications where we want to send robots into environments, including space, where it could be dangerous for humans to go," said Bao. "They could collect information and send it back."

Nanospheres made of aromatic amino acids: The most rigid organic nanostructures to date

 (PhysOrg.com) -- Organic nanostructures are key elements of nanotechnology because these building blocks can be made with tailored chemical properties. Their disadvantage has been that their mechanical properties have so far been significantly inferior to those of metallic nanostructures.

Ehud Gazit, Itay Rousso, and a team from the Tel Aviv University, the Weizmann Institute of Science and the Ben-Gurion University of the Negev (Israel) have now introduced organic nanospheres that are as rigid as metal. As the scientists report in the journal Angewandte Chemie, they are interesting components for ultrarigid biocomposite materials.

Nanoscale biological structures often exhibit unique mechanical properties; for example spider silk is 25 times as strong as steel by weight. The most rigid synthetic organic materials known to date are aramids, such as Kevlar. Their secret is a special spatial arrangement of their aromatic ring systems and the network of interactions between their planar amide bonds. The new nanospheres are based on a similar construction principle. However, unlike the large polymeric chains, they are formed in a self-organization process from very simple molecules based on aromatic dipeptides of the amino acid phenylalanine.

Using an atomic force microscope, the scientists examined the mechanical properties of their nanospheres. This device uses a nanotip (cantilever), a tiny flexible lever arm with a very fine tip at the end. When this tip is pressed against a sample, the deflection of the lever indicates whether the tip of the needle can press into the sample object and how far in it can go. A metal needle was not able to make any impression on the nanospheres; only a needle made of diamond was able to do it. The researchers used these measurements to calculate the elasticity modulus (Young?s modulus) for the nanospheres. This value is a measure of the stiffness of a material. The larger the value, the more resistance a material has to its deformation. By using a high-resolution scanning electron microscope equipped with a nanomanipulator, it was possible to directly observe the deformation of the spheres.

For the nanospheres, the team measured a remarkably high elasticity modulus (275 GPa), which is higher than many metals and similar to the values found for steel. This makes these nanostructures the stiffest organic molecules to date; they may even eclipse aramids. In addition to having outstanding mechanical properties, the nanospheres are also transparent. This makes them ideal elements for the reinforcement of ultrarigid biocomposite materials, such as reinforced plastics for implants or materials for tooth replacement, aerospace, and other applications that require inexpensive, lightweight materials with high stiffness and unusual stability.

New ultracapacitor recharges in under a millisecond

 (A) Plan SEM micrograph of coated Ni electrode. (B) SEM micrograph of a coated fiber, showing plan and shallow-angle views. Image credit: Science, DOI:10.1126/science.1194372

(PhysOrg.com) -- A new ultracapacitor or electric double-layer capacitor (DLC) design has been announced in the journal Science this week, and could pave the way for smaller and lighter portable electronics devices.

Ultracapacitors are capable of charging and discharging in only seconds and this gives them an advantage over batteries, which take much longer, and make them extremely useful in applications such as regenerative braking. However, for some applications even a few seconds is too long, and this is where a new nanoscale ultracapacitor comes in. Researchers in the US have built an ultracapacitor from nanometer-scale fins of graphene, and this design gives them a device that can charge/discharge in under 200 microseconds.

Ultracapacitors store charge in electric fields between conducting surfaces, so a larger surface area of conducting surfaces enables the device to hold more charge. A larger amount of stored charge enables ultracapacitors to work in devices needing more energy than ordinary capacitors can provide, and they can deliver the energy much faster than a battery.

A team of researchers led by John Miller, president of JME, an electrochemical capacitor company based in Shaker Heights, Ohio has been able to increase the speed of the ultracapacitor by redesigning the electrodes to give more surface area. The new electrode, developed by Ron Outlaw, a team member from the College of William and Mary, in Williamsburg, Virginia, consists of sheets of graphene sticking up vertically from a graphite base. The graphene sheets are made of carbon one atom thick, and grown by a plasma-assisted chemical vapor deposition process. The graphite base is 10 nanometers thick. Miller described the design as resembling "rows of 600-nanometer tall potato chips standing on edge."

The design allows for much faster charging and recharging than stacked graphene sheets used in earlier ultracapacitors or the pored surfaces of activated carbon ultracapacitors.

According to Miller's team, the new ultracapacitor could replace bulky capacitors in portable devices to free up more space while still smoothing out peaks and troughs in power supplies. It has been tested in a filtering circuit in an AC rectifier, a task at which other ultracapacitors fail. (AC rectifiers tend to leave a voltage ripple that the capacitor smooths out.) Other ultracapacitors fail because their porous electrodes make them act like resistors in filter circuits. The new ultracapacitor worked well in the test, which means they could replace the current capacitors, which are six times larger.

Ron Outlaw said work is continuing on increasing the capacitance and attempting to make the graphene sheets taller and more parallel with the aim of finding the perfect balance of maximum charge storage with minimum restriction of ion flow in the electrolyte. As size and weight of the ultracapacitors are reduced, they will find more applications in areas such as airlines, the military, and NASA.

Nanotechnology promises better catalytic converter

 Catalytic converters clear the air of this kind of noxious fug, at least if they're still in good condition and up to operating temperature. (WIKIMEDIA COMMONS)

(PhysOrg.com) -- Control over material properties would reduce the amount of platinum needed.

The toxic byproducts made when a car?s engine burns fuel are funneled into the catalytic converter, where chemical reactions turn them into much less toxic substances like water and carbon dioxide.

The catalyst that lowers the activation energy of these chemical reactions so that they can take place at reasonable temperatures and speeds is usually platinum, one of the world?s rarest and most precious metals. Because platinum is so expensive, automakers want to use as little as possible, and to use it as effectively as possible.

To maximize its specific surface area (surface area per unit mass) and therefore its chemical activity, manufacturers coat a ceramic support with small particles of platinum. But when the converter gets hot, the platinum aggregates, forming large clumps that cannot carry out the detoxification reactions as effectively. To compensate for the loss of efficiency, converters must contain more platinum, a scarce metal badly neede for other clean-energy applications, such as fuel cells.

A model catalytic system, described online this week of the German Chemical Society's Journal Angewantde Chemie International Edition, prevents the platinum from aggregating, so that less is needed for each converter.

The system was devised by a team of scientists including Younan Xia, PhD, the James M. McKelvey Professor of Biomedical Engineering in the School of Engineering and Applied Science at Washington University in St. Louis. The team also includes Charles T. Camvell, Phd, the Lloyd E. and Florence M. West Professor of Chemistry at the University of Washington in Seattle and Paul T. Fanson, PhD, a chemist at ToyotaToyota Motor Engineering & Manufacturing North America in Ann Arbor.

The key development is to coat platinum nanoparticles with a porous silica layer. Because of its weak interaction with the platinum, the silica coating provides an energy barrier that holds the platinum in place even at very high temperatures, preventing aggregation and maintaining catalytic activity.

The first step in making the new system is to load titanium dioxide nanofibers with platinum nanoparticles. This support makes the platinum catalyst more active by providing additional electrons for some of the detoxifying reactions. The loaded fibers are then coated with silica containing an organic pore-generating agent, which was then removed by heating to 350 degrees C to create porous sheaths.

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The new design loads platinum nanoparticles onto nanofirbers and then coats the nanoparticles with silica and an organic pore-generating compound that can be removed by gentle heating. The porous sheath allows gases to reach the platinum, but prevents the particles from aggregating. (YOUNAN XIA/WUSTL)

?It?s very tricky to make this kind of coating thin enough and porous enough so you don?t really affect the activity of the platinum catalyst. So that?s a major development,? Xia says.

Experiments then showed that the silica-coated platinum maintained its catalytic ability at much higher temperatures than uncoated platinum, which began to aggregate at temperatures as low as 350 degrees C.

Yunqian Dai, the lead author on the paper and a visiting graduate student from China, says this development ?will greatly improve the thermostability? of platinum catalysts, although it is not yet clear if whether it will lead to new catalytic converter designs.

?It looks like we can run these up to 750 degrees without any significant agglomeration,? says Xia. ?The typical temperature for a catalytic converter is about 550, so in that sense, it should be able to last for a longer time.?

Next up for Xia's tea, is to study catalytic systems with different compositions, such as aluminum oxide rather than silica sheathing.

Platinum is so expensive, Xia says, the converters are sometimes stolen and it is economic to recycle old ones to recover the precious metal. The goal of his research, however, is to use much less platinum, so cars cost less and more of the metal is available for other uses.

Solar cells thinner than wavelengths of light hold huge power potential

 

This schematic diagram of a thin film organic solar cell shows the top layer, a patterned, roughened scattering layer, in green. The organic thin film layer, shown in red, is where light is trapped and electrical current is generated. The film is sandwiched between two layers that help keep light contained within the thin film.

(PhysOrg.com) -- Ultra-thin solar cells can absorb sunlight more efficiently than the thicker, more expensive-to-make silicon cells used today, because light behaves differently at scales around a nanometer, say Stanford engineers. They calculate that by properly configuring the thicknesses of several thin layers of films, an organic polymer thin film could absorb as much as 10 times more energy from sunlight than was thought possible.

In the smooth, white, bunny-suited clean-room world of silicon wafers and solar cells, it turns out that a little roughness may go a long way, perhaps all the way to making solar power an affordable energy source, say Stanford engineers.

Their research shows that light ricocheting around inside the polymer film of a solar cell behaves differently when the film is ultra thin. A film that's nanoscale-thin and has been roughed up a bit can absorb more than 10 times the energy predicted by conventional theory.

The key to overcoming the theoretical limit lies in keeping sunlight in the grip of the solar cell long enough to squeeze the maximum amount of energy from it, using a technique called "light trapping." It's the same as if you were using hamsters running on little wheels to generate your electricity - you'd want each hamster to log as many miles as possible before it jumped off and ran away.

"The longer a photon of light is in the solar cell, the better chance the photon can get absorbed," said Shanhui Fan, associate professor of electrical engineering. The efficiency with which a given material absorbs sunlight is critically important in determining the overall efficiency of solar energy conversion. Fan is senior author of a paper describing the work published online this week by Proceedings of the National Academy of Sciences.

Light trapping has been used for several decades with silicon solar cells and is done by roughening the surface of the silicon to cause incoming light to bounce around inside the cell for a while after it penetrates, rather than reflecting right back out as it does off a mirror. But over the years, no matter how much researchers tinkered with the technique, they couldn't boost the efficiency of typical "macroscale" silicon cells beyond a certain amount.

Eventually the scientists realized that there was a physical limit related to the speed at which light travels within a given material.

But light has a dual nature, sometimes behaving as a solid particle (a photon) and other times as a wave of energy, and Fan and postdoctoral researcher Zongfu Yu decided to explore whether the conventional limit on light trapping held true in a nanoscale setting. Yu is the lead author of the PNAS paper.

"We all used to think of light as going in a straight line," Fan said. "For example, a ray of light hits a mirror, it bounces and you see another light ray. That is the typical way we think about light in the macroscopic world.

"But if you go down to the nanoscales that we are interested in, hundreds of millionths of a millimeter in scale, it turns out the wave characteristic really becomes important."

Visible light has wavelengths around 400 to 700 nanometers (billionths of a meter), but even at that small scale, Fan said, many of the structures that Yu analyzed had a theoretical limit comparable to the conventional limit proven by experiment.

"One of the surprises with this work was discovering just how robust the conventional limit is," Fan said.

It was only when Yu began investigating the behavior of light inside a material of deep subwavelength-scale - substantially smaller than the wavelength of the light - that it became evident to him that light could be confined for a longer time, increasing energy absorption beyond the conventional limit at the macroscale.

"The amount of benefit of nanoscale confinement we have shown here really is surprising," said Yu. "Overcoming the conventional limit opens a new door to designing highly efficient solar cells."

Yu determined through numerical simulations that the most effective structure for capitalizing on the benefits of nanoscale confinement was a combination of several different types of layers around an organic thin film.

He sandwiched the organic thin film between two layers of material - called "cladding" layers - that acted as confining layers once the light passed through the upper one into the thin film. Atop the upper cladding layer, he placed a patterned rough-surfaced layer designed to send the incoming light off in different directions as it entered the thin film.

By varying the parameters of the different layers, he was able to achieve a 12-fold increase in the absorption of light within the thin film, compared to the macroscale limit.

Nanoscale solar cells offer savings in material costs, as the organic polymer thin films and other materials used are less expensive than silicon and, being nanoscale, the quantities required for the cells are much smaller.

The organic materials also have the advantage of being manufactured in chemical reactions in solution, rather than needing high-temperature or vacuum processing, as is required for silicon manufacture.

"Most of the research these days is looking into many different kinds of materials for solar cells," Fan said. "Where this will have a larger impact is in some of the emerging technologies; for example, in organic cells."

"If you do it right, there is enormous potential associated with it," Fan said.

Carbon nanotubes twice as strong as once thought

Carbon nanotubes -- those tiny particles poised to revolutionize electronics, medicine, and other areas ? are much bigger in the strength department than anyone ever thought, scientists are reporting.

New studies on the strength of these submicroscopic cylinders of carbon indicate that on an ounce-for-ounce basis they are at least 117 times stronger than steel and 30 times stronger than Kevlar, the material used in bulletproof vests and other products. The findings, which could expand commercial and industrial applications of nanotube materials, appear in the monthly journal ACS Nano.

Stephen Cronin and colleagues point out that nanotubes ? barely 1/50,000th the width of a human hair ? have been renowned for exceptional strength, high electrical conductivity, and other properties. Nanotubes can stretch considerably like toffee before breaking. This makes them ideal for a variety of futuristic applications, even, if science fiction ever become reality, as cables in "space elevators" that lift objects from the Earth's surface into orbit.

To resolve uncertainties about the actual strength of nanotubes, the scientists applied immense tension to individual carbon nanotubes of different lengths and widths. They found that nanotubes could be stretched up to 14 percent of their normal length without breaking, or more than twice that of previous reports by others. The finding establishes "a new lower limit for the ultimate strength of carbon nanotubes," the article noted.

Scientists explain graphene mystery

 ORNL simulations demonstrate how loops (seen above in blue) between graphene layers can be minimized using electron irradiation (bottom).

Nanoscale simulations and theoretical research performed at the Department of Energy's Oak Ridge National Laboratory are bringing scientists closer to realizing graphene's potential in electronic applications.

A research team led by ORNL's Bobby Sumpter, Vincent Meunier and Eduardo Cruz-Silva has discovered how loops develop in graphene, an electrically conductive high-strength low-weight material that resembles an atomic-scale honeycomb.

Structural loops that sometimes form during a graphene cleaning process can render the material unsuitable for electronic applications. Overcoming these types of problems is of great interest to the electronics industry.

"Graphene is a rising star in the materials world, given its potential for use in precise electronic components like transistors or other semiconductors," said Bobby Sumpter, a staff scientist at ORNL.

The team used quantum molecular dynamics to simulate an experimental graphene cleaning process, as discussed in a paper published in Physical Review Letters. Calculations performed on ORNL supercomputers pointed the researchers to an overlooked intermediate step during processing.

Imaging with a transmission electron microscope, or TEM, subjected the graphene to electron irradiation, which ultimately prevented loop formation. The ORNL simulations showed that by injecting electrons to collect an image, the electrons were simultaneously changing the material's structure.

"Taking a picture with a TEM is not merely taking a picture," Sumpter said. "You might modify the picture at the same time that you're looking at it."

The research builds on findings discussed in a 2009 Science paper (Jia et al.), where Meunier and Sumpter helped demonstrate a process that cleans graphene edges by running a current through the material in a process known as Joule heating. Graphene is only as good as the uniformity or cleanliness of its edges, which determine how effectively the material can transmit electrons. Meunier said the ability to efficiently clean graphene edges is crucial to using the material in electronics.

"Imagine you have a fancy sports car, but then you realize it has square wheels. What good is it? That's like having jagged edges on graphene," Meunier said.

Recent experimental studies have shown that the Joule heating process can lead to undesirable loops that connect different graphene layers. The PRL paper provides an atomistic understanding of how electron irradiation from a transmission electron microscope affects the graphene cleaning process by preventing loop formation.

"We can clean the edges, and not only that, we're able to understand why we can clean them," Meunier said.

A versatile, clean and efficient way to enhance widespread application of carbon nanotubes

August 26, 2010 (PhysOrg.com) -- Researchers at Imperial College London have developed a versatile, practical and efficient method for activating sites on the surface of carbon nanotubes (CNTs) and subsequently binding a wide range of molecules to them. This new method will enable large-scale manufacture of modified CNTs.

The new method, reported this month in the journal Chemical Science, overcomes a major hurdle in the development of industrial scale applications for CNTs. It provides manufacturers with a method that, in principle, can be used to modify the surface chemistry of the underlying nanotube structure, on a large scale. Surface modification can provide new properties or enable subsequent processing steps: for example, molecules grafted to the CNTs may introduce catalytic activity or provide compatibility with particular solvents.

Our approach is potentially a very significant step towards manufacturing carbon nanotubes with specific chemical characteristics, so-called functionalisation, at an industrial scale," said Professor Milo Shaffer, lead author of the study from the Department Chemistry at Imperial College London. "Our method is extremely practical because, in principle, it can exploit existing infrastructure and yet it remains extremely versatile; the huge range of molecules that can be bound to the CNTs makes the technology adaptable to almost any application."

"Our technique is intrinsically scalable and, for the first time, it should be feasible to functionalise CNTs on the same scale as they are produced. This change is significant as industry's current capacity to manufacture CNTs is hundreds, if not thousands, of times greater than its capacity to add complex surface chemistry. This technique should increase the availability of functionalised CNTs, enable new applications that require manufacturing in bulk, and hence enhance the growth of the market," added Professor Shaffer.

The method that Professor Shaffer and his colleagues have developed should allow CNTs to be readily tailored to potential applications such as sensor networks, filters, electrodes for electrochemical devices, advanced catalysts and to improve CNT compatibility in, for example, composite materials, solvents, and electrolytes.

The key step in the new method involves activating CNTs at high temperatures under an inert atmosphere or vacuum. The high temperature treatment drives desorption of surface oxides on the CNT surface, producing reactive radicals that can subsequently bind a wide range of functional molecules to modify CNT physico-chemical properties. The radicals can also initiate the polymerisation of monomers, so that oligomers of functional molecules are bound to CNTs. The treatment does not cause any significant damage to the CNT structure, because the surface sites that it activates are already present on nanotubes manufactured using standard industrial methods. The number of reactive sites, and hence degree of functionalisation, can be increased by additional oxidation steps.

Professor Shaffer's team has demonstrated that the functional molecules are bound to, and uniformly distributed over, the surface of the CNTs. While the molecules are bound at relatively low densities, the degree of functionalisation is sufficient to offer benefits in industrial applications. The team has already demonstrated the attachment of catalytic metal particles, enhanced solubilities, and improved wetting with polymer matrices.

Professor Shaffer said: "The heat treatment to activate CNTs is compatible with some existing production technologies and can be easily adopted to work with others. Where the functional molecules to be added are volatile, the method can be carried out in the gas phase without the need for solvents, at any stage. The absence of solvent simplifies purification of the functionalised CNTs and, as many solvents used in wet-activation methods are corrosive and toxic, this option has environmental and hazard control benefits. It also has the advantage of being less damaging, less wasteful, and less time consuming than existing methods."

More information: A versatile, solvent-free methodology for the functionalisation of carbon nanotubes. Chemical Science 2010 DOI:10.1039/C0SC00287A

Provided by Imperial College London (news : web)

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Researchers develop a way to funnel solar energy

 This filament containing about 30 million carbon nanotubes absorbs energy from the sun as photons and then re-emits photons of lower energy, creating the fluorescence seen here. The red regions indicate highest energy intensity, and green and blue are lower intensity. Image: Geraldine Paulus

(PhysOrg.com) -- Using carbon nanotubes (hollow tubes of carbon atoms), MIT chemical engineers have found a way to concentrate solar energy 100 times more than a regular photovoltaic cell. Such nanotubes could form antennas that capture and focus light energy, potentially allowing much smaller and more powerful solar arrays.

"Instead of having your whole roof be a photovoltaic cell, you could have little spots that were tiny photovoltaic cells, with antennas that would drive photons into them," says Michael Strano, the Charles and Hilda Roddey Associate Professor of Chemical Engineering and leader of the research team.

Strano and his students describe their new carbon nanotube antenna, or "solar funnel," in the Sept. 12 online edition of the journal Nature Materials. Lead authors of the paper are postdoctoral associate Jae-Hee Han and graduate student Geraldine Paulus.

Their new antennas might also be useful for any other application that requires light to be concentrated, such as night-vision goggles or telescopes.

Solar panels generate electricity by converting photons (packets of light energy) into an electric current. Strano's nanotube antenna boosts the number of photons that can be captured and transforms the light into energy that can be funneled into a solar cell.

The antenna consists of a fibrous rope about 10 micrometers (millionths of a meter) long and four micrometers thick, containing about 30 million carbon nanotubes. Strano's team built, for the first time, a fiber made of two layers of nanotubes with different electrical properties ? specifically, different bandgaps.

In any material, electrons can exist at different energy levels. When a photon strikes the surface, it excites an electron to a higher energy level, which is specific to the material. The interaction between the energized electron and the hole it leaves behind is called an exciton, and the difference in energy levels between the hole and the electron is known as the bandgap.

The inner layer of the antenna contains nanotubes with a small bandgap, and nanotubes in the outer layer have a higher bandgap. That's important because excitons like to flow from high to low energy. In this case, that means the excitons in the outer layer flow to the inner layer, where they can exist in a lower (but still excited) energy state.

Therefore, when light energy strikes the material, all of the excitons flow to the center of the fiber, where they are concentrated. Strano and his team have not yet built a photovoltaic device using the antenna, but they plan to. In such a device, the antenna would concentrate photons before the photovoltaic cell converts them to an electrical current. This could be done by constructing the antenna around a core of semiconducting material.

The interface between the semiconductor and the nanotubes would separate the electron from the hole, with electrons being collected at one electrode touching the inner semiconductor, and holes collected at an electrode touching the nanotubes. This system would then generate electric current. The efficiency of such a solar cell would depend on the materials used for the electrode, according to the researchers.

Strano's team is the first to construct nanotube fibers in which they can control the properties of different layers, an achievement made possible by recent advances in separating nanotubes with different properties.

While the cost of carbon nanotubes was once prohibitive, it has been coming down in recent years as chemical companies build up their manufacturing capacity. "At some point in the near future, carbon nanotubes will likely be sold for pennies per pound, as polymers are sold," says Strano. "With this cost, the addition to a solar cell might be negligible compared to the fabrication and raw material cost of the cell itself, just as coatings and polymer components are small parts of the cost of a photovoltaic cell."

Strano's team is now working on ways to minimize the energy lost as excitons flow through the fiber, and on ways to generate more than one exciton per photon. The nanotube bundles described in the Nature Materials paper lose about 13 percent of the energy they absorb, but the team is working on new antennas that would lose only 1 percent.

International research team develops ultrahigh-power energy storage devices

August 17, 2010 A team of researchers from the U.S. and France report the development of a micro-supercapacitor with remarkable properties. The paper will be published in the premier scientific journal Nature Nanotechnology online on August 15.

These micro-supercapacitors have the potential to power nomad electronics, wireless sensor networks, biomedical implants, active radiofrequency identification (RFID) tags and embedded microsensors, among other devices.

Supercapacitors, also called electric double layer capacitors (EDLCs) or ultracapacitors, bridge the gap between batteries, which offer high energy densities but are slow, and ?conventional? electrolytic capacitors, which are fast but have low energy densities.

The newly developed devices described in Nature Nanotechnology have powers per volume that are comparable to electrolytic capacitors, capacitances that are four orders of magnitude higher, and energies per volume that are an order of magnitude higher. They were also found to be three orders of magnitude faster than conventional supercapacitors, which are used in backup power supplies, wind power generators and other machinery. These new devices have been dubbed ?micro-supercapacitors? because they are only a few micrometers (0.000001 meters) thick.

What makes this possible? ?Supercapacitors store energy in layers of ions at high surface area electrodes,? said Dr. Yury Gogotsi, Trustee Chair Professor of materials science and engineering at Drexel University, and a co-author of the paper. ?The higher the surface area per volume of the electrode material, the better the performance of the supercapacitor.?

Vadym Mochalin, research assistant professor of materials science and engineering at Drexel and co-author, said, ?We use electrodes made of onion-like carbon, a material in which each individual particle is made up of concentric spheres of carbon atoms, similar to the layers of an onion. Each particle is 6-7 nanometers in diameter.?

This is the first time a material with very small spherical particles has been studied for this purpose. Previously investigated materials include activated carbon, nanotubes, and carbide-derived carbon (CDC).

?The surface of the onion-like carbons is fully accessible to ions, whereas with some other materials, the size or shape of the pores or of the particles themselves would slow down the charging or discharging process,? Mochalin said. ?Furthermore, we used a process to assemble the devices that did not require a polymer binder material to hold the electrodes together, which further improved the electrode conductivity and the charge/discharge rate. Therefore, our supercapacitors can deliver power in milliseconds, much faster than any battery or supercapacitor used today.?