Hydrogen Opens the Road to Graphene ... and Graphane

A thin flake plain carbon, just one atom thick, became world famous last year. The discovery of the super material graphene gave Andre Geim and Konstantin Novoselov the Nobel Prize in Physics 2010. Graphene has a wide range of unusual and highly interesting properties. As a conductor of electricity it performs as well as copper. As a conductor of heat it outperforms all other known materials.

There are possibilities to achieve strong variations of the graphene properties for instance by making graphene in a form of belts with various width, so called nanoribbons. Nanoribbons were prepared for the first time two years ago. A method to produce them is to start from carbon nanotubes and to use oxygen treatment to unzip into nanoribbons. However, this method leaves oxygen atoms on the edges of nanoribbons, which is not always desirable.

In the new study the research team shows that it is also possible to unzip single-walled carbon nanotubes by using a reaction with molecular hydrogen. Nanoribbons produced by the new method will have hydrogen on the edges and this can be an advantage for some applications. Alexandr Talyzin, physicist at Umeå University in Sweden, has over the past decade been studying how hydrogen reacts with fullerenes, which are football-shaped carbon molecules.

"Treating the carbon nanotubes with hydrogen was a logical extension of our research. Our previous experience has been of great help in this work," says Alexandr Talyzin.

Nanotubes are typically closed by semi-spherical cups, essentially halves of fullerene molecules. The researchers have previously proved that fullerene molecules can be completely destroyed by very strong hydrogenation. Therefore, they expected similar results for nanotube end cups and tried to open the nanotubes by using hydrogenation. The effect was indeed confirmed and they also managed to reveal some other exciting effects.

The most interesting discovery was that some carbon nanotubes were unzipped into graphene nanoribbons as a result of prolonged hydrogen treatment. What is even more exciting -- unzipping of nanotube with hydrogen attached to the side walls could possibly lead to synthesis of hydrogenated graphene: graphane. So far, graphane was attempted to be synthesized mostly by reaction of hydrogen with graphene. This appeared to be very difficult, especially if the graphene is supported on some substrate and only one side is available for the reaction. However, hydrogen reacts much easier with the curved surface of carbon nanotubes.

"Our new idea is to use hydrogenated nanotubes and unzip them into graphane nanoribbons. So far, only the first step towards graphane nanoribbon synthesis is done and a lot more work is required to make our approach effective," explains Alexandr Talyzin."Combined experience and expertise from several groups at different universities, was a key to success."

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Silver Cycle: New Evidence for Natural Synthesis of Silver Nanoparticles

A team of researchers from the Florida Institute of Technology (FIT), the State University of New York (SUNY), Buffalo, and the National Institute of Standards and Technology (NIST) reports that, given a source of silver ions, naturally occurring humic acid will synthesize stable silver nanoparticles.

"Our colleague, Virender Sharma, had read an article in which they were using wine to form nanoparticles. He thought that, based on the similar chemistry, we should be able to produce silver nanoparticles with humic acids," explains FIT chemist Mary Sohn."First we formed them by traditional methods and then we tried one of our river sediment humic acids. We were really excited that we could see the characteristic yellow color of the nanoparticles." Samples were sent to Sarbajit Banerjee at SUNY Buffalo and Robert MacCuspie at NIST for detailed analyses to confirm the presence of silver nanoparticles.

"Humic acid" is a complex mixture of many organic acids that are formed during the decay of dead organic matter. Although the exact composition varies from place to place and season to season, humic acid is ubiquitous in the environment. Metallic nanoparticles, MacCuspie explains, have characteristic colors that are a direct consequence of their size. (The effect is called"surface plasmon resonance" and is caused by surface electrons across the nanoparticle oscillating in concert.) Silver nanoparticles appear a yellowish brown.

The team mixed silver ions with humic acid from a variety of sources at different temperatures and concentrations and found that acids from river water or sediments would form detectable silver nanoparticles at room temperature in as little as two to four days. Moreover, MacCuspie says, the humic acid appears to stabilize the nanoparticles by coating them and preventing the nanoparticles from clumping together into a larger mass of silver."We believe it's actually a similar process to how nanoparticles are synthesized in the laboratory," he says, except that the lab process typically uses citric acid at elevated temperatures.

"This caught us by surprise because a lot of our work is focused on how silver nanoparticles may dissolve when they're released into the environment and release silver ions," MacCuspie says. Many biologists believe the toxicity of silver nanoparticles, the reason for their use as an antibacterial or antifungal agent, is due to their high surface area that makes them an efficient source of silver ions, he says, but"this creates the idea that there may be some sort of natural cycle returning some of the ions to nanoparticles." It also helps explain the discovery, over the past few years, of silver nanoparticles in locations like old mining regions that are not likely to have been exposed to human-made nanoparticles, but would have significant concentrations of silver ions.

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Sharpening the Nanofocus

"We have demonstrated resonant antenna-enhanced single-particle hydrogen sensing in the visible region and presented a fabrication approach to the positioning of a single palladium nanoparticle in the nanofocus of a gold nanoantenna," says Paul Alivisatos, Berkeley Lab's director and the leader of this research."Our concept provides a general blueprint for amplifying plasmonic sensing signals at the single-particle level and should pave the road for the optical observation of chemical reactions and catalytic activities in nanoreactors, and for local biosensing."

Alivisatos, who is also the Larry and Diane Bock Professor of Nanotechnology at the University of California, Berkeley, is the corresponding author of a paper in the journalNature Materialsdescribing this research. The paper is titled"Nanoantenna-enhanced gas sensing in a single tailored nanofocus." Co-authoring the paper with Alivisatos were Laura Na Liu, Ming Tang, Mario Hentschel and Harald Giessen.

One of the hottest new fields in technology today is plasmonics -- the confinement of electromagnetic waves in dimensions smaller than half-the-wavelength of the incident photons in free space. Typically this is done at the interface between metallic nanostructures, usually gold, and a dielectric, usually air. The confinement of the electromagnetic waves in these metallic nanostructures generates electronic surface waves called"plasmons." A matching of the oscillation frequency between plasmons and the incident electromagnetic waves gives rise to a phenomenon known as localized surface plasmon resonance (LSPR), which can concentrate the electromagnetic field into a volume less than a few hundred cubic nanometers. Any object brought into this locally confined field -- referred to as the nanofocus -- will influence the LSPR in a manner that can be detected via dark-field microscopy.

"Nanofocusing has immediate implications for plasmonic sensing," says Laura Na Liu, lead author of theNature Materialspaper who was at the time the work was done a member of Alivisatos' research group but is now with Rice University."Metallic nanostructures with sharp corners and edges that form a pointed tip are especially favorable for plasmonic sensing because the field strengths of the electromagnetic waves are so strongly enhanced over such an extremely small sensing volume."

Plasmonic sensing is especially promising for the detection of flammable gases such as hydrogen, where the use of sensors that require electrical measurements pose safety issues because of the potential threat from sparking. Hydrogen, for example, can ignite or explode in concentrations of only four-percent. Palladium was seen as a prime candidate for the plasmonic sensing of hydrogen because it readily and rapidly absorbs hydrogen that alters its electrical and dielectric properties. However, the LSPRs of palladium nanoparticles yield broad spectral profiles that make detecting changes extremely difficult.

"In our resonant antenna-enhanced scheme, we use double electron-beam lithography in combination with a double lift-off procedure to precisely position a single palladium nanoparticle in the nanofocus of a gold nanoantenna," Liu says."The strongly enhanced gold-particle plasmon near-fields can sense the change in the dielectric function of the proximal palladium nanoparticle as it absorbs or releases hydrogen. Light scattered by the system is collected by a dark-field microscope with attached spectrometer and the LSPR change is read out in real time."

Alivisatos, Liu and their co-authors found that the antenna enhancement effect could be controlled by changing the distance between the palladium nanoparticle and the gold antenna, and by changing the shape of the antenna.

"By amplifying sensing signals at the single-particle level, we eliminate the statistical and average characteristics inherent to ensemble measurements," Liu says."Moreover, our antenna-enhanced plasmonic sensing technique comprises a noninvasive scheme that is biocompatible and can be used in aqueous environments, making it applicable to a variety of physical and biochemical materials."

For example, by replacing the palladium nanoparticle with other nanocatalysts, such as ruthenium, platinum, or magnesium, Liu says their antenna-enhanced plasmonic sensing scheme can be used to monitor the presence of numerous other important gases in addition to hydrogen, including carbon dioxide and the nitrous oxides. This technique also offers a promising plasmonic sensing alternative to the fluorescent detection of catalysis, which depends upon the challenging task of finding appropriate fluorophores. Antenna-enhanced plasmonic sensing also holds potential for the observation of single chemical or biological events.

"We believe our antenna-enhanced sensing technique can serve as a bridge between plasmonics and biochemistry," Liu says."Plasmonic sensing offers a unique tool for optically probing biochemical processes that are optically inactive in nature. In addition, since plasmonic nanostructures made from gold or silver do not bleach or blink, they allow for continuous observation, an essential capability forin-situmonitoring of biochemical behavior."

This research was supported by the DOE Office of Science and the German ministry of research.

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Nanopatch for the Heart, for Heart Attack Victims

When you suffer a heart attack, a part of your heart dies. Nerve cells in the heart's wall and a special class of cells that spontaneously expand and contract -- keeping the heart beating in perfect synchronicity -- are lost forever. Surgeons can't repair the affected area. It's as if when confronted with a road riddled with potholes, you abandon what's there and build a new road instead.

Needless to say, this is a grossly inefficient way to treat arguably the single most important organ in the human body. The best approach would be to figure out how to resuscitate the deadened area, and in this quest, a group of researchers at Brown University and in India may have an answer.

The scientists turned to nanotechnology. In a lab, they built a scaffold-looking structure consisting of carbon nanofibers and a government-approved polymer. Tests showed the synthetic nanopatch regenerated natural heart tissue cells­- called cardiomyocytes -- as well as neurons. In short, the tests showed that a dead region of the heart can be brought back to life.

"This whole idea is to put something where dead tissue is to help regenerate it, so that you eventually have a healthy heart," said David Stout, a graduate student in the School of Engineering at Brown and the lead author of the paper published inActa Biomaterialia.

The approach, if successful, would help millions of people. In 2009, some 785,000 Americans suffered a new heart attack linked to weakness caused by the scarred cardiac muscle from a previous heart attack, according to the American Heart Association. Just as ominously, a third of women and a fifth of men who have experienced a heart attack will have another one within six years, the researchers added, citing the American Heart Association.

What is unique about the experiments at Brown and at the India Institute of Technology Kanpur is the engineers employed carbon nanofibers, helical-shaped tubes with diameters between 60 and 200 nanometers. The carbon nanofibers work well because they are excellent conductors of electrons, performing the kind of electrical connections the heart relies upon for keeping a steady beat. The researchers stitched the nanofibers together using a poly lactic-co-glycolic acid polymer to form a mesh about 22 millimeters long and 15 microns thick and resembling"a black Band Aid," Stout said. They laid the mesh on a glass substrate to test whether cardiomyocytes would colonize the surface and grow more cells.

In tests with the 200-nanometer-diameter carbon nanofibers seeded with cardiomyocytes, five times as many heart-tissue cells colonized the surface after four hours than with a control sample consisting of the polymer only. After five days, the density of the surface was six times greater than the control sample, the researchers reported. Neuron density had also doubled after four days, they added.

The scaffold works because it is elastic and durable, and can thus expand and contract much like heart tissue, said Thomas Webster, associate professor in engineering and orthopaedics at Brown and the corresponding author on the paper. It's because of these properties and the carbon nanofibers that cardiomyocytes and neurons congregate on the scaffold and spawn new cells, in effect regenerating the area.

The scientists want to tweak the scaffold pattern to better mimic the electrical current of the heart, as well as build an in-vitro model to test how the material reacts to the heart's voltage and beat regime. They also want to make sure the cardiomyocytes that grow on the scaffolds are endowed with the same abilities as other heart-tissue cells.

Bikramjit Basu at the India Institute of Technology Kanpur contributed to the paper. The Indo-U.S. Science and Technology Forum, the Hermann Foundation, the Indian Institute of Technology, Kanpur, the government of India and California State University funded the research.

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Invisibility Cloak: Scientists Achieve Optical Invisibility in Visible Light Range of Spectrum

In invisibility cloaks, light waves are guided by the material such that they leave the invisibility cloak again as if they had never been in contact with the object to be disguised. Consequently, the object is invisible to the observer. The exotic optical properties of the camouflaging material are calculated using complex mathematical tools.

These properties result from a special structuring of the material. It has to be smaller than the wavelength of the light that is to be deflected. For example, the relatively large radio or radar waves require a material"that can be produced using nail scissors," says Wegener. At wavelengths visible to the human eye, materials have to be structured in the nanometer range.

The minute invisibility cloak produced by Fischer and Ergin is smaller than the diameter of a human hair. It makes the curvature of a metal mirror appear flat, as a result of which an object hidden underneath becomes invisible. The metamaterial placed on top of this curvature looks like a stack of wood, but consists of plastic and air. These"logs" have precisely defined thicknesses in the range of 100 nm. Light waves that are normally deflected by the curvature are influenced and guided by these logs such that the reflected light corresponds to that of a flat mirror.

"If we would succeed again in halving the log distance of the invisibility cloak, we would obtain cloaking for the complete visible light spectrum," says Fischer.

Last year, the Wegener team presented the first 3-D invisibility cloak in the journalScience. Until that time, the only invisibility cloaks existed in waveguides and were of practically two-dimensional character. When looking onto the structure from the third dimension, however, the effect disappeared. By means of an accordingly filigree structuring, the Karlsruhe invisibility cloak could be produced for wavelengths from 1500 to 2600 nm. This wavelength range is not visible to the human eye, but plays an important role in telecommunications. The breakthrough was based on the use of the direct laser writing method (DLS) developed by CFN. With the help of this method, it is possible to produce minute 3-D structures with optical properties that do not exist in nature, so-called metamaterials.

In the past year, the KIT scientists continued to improve the already extremely fine direct laser writing method. For this purpose, they used methods that have significantly increased the resolution in microscopy. With this tool, they then succeeded in refining the metamaterial by a factor of two and in producing the first 3-D invisibility cloak for non-polarized visible light in the range of 700 nm. This corresponds to the red color.

"The invisibility cloak now developed is an attractive object demonstrating the fantastic possibilities of the rather new field of transformation optics and metamaterials. The design options that opened up during the last years had not been deemed possible before," emphasizes Ergin."We expect dramatic improvements of light-based technologies, such as lenses, solar cells, microscopes, objectives, chip production, and data communication."

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Physicist Accelerates Simulations of Thin Film Growth

Jacques Amar, Ph.D., professor of physics at the University of Toledo (UT), studies the modeling and growth of materials at the atomic level. He uses Ohio Supercomputer Center (OSC) resources and Kinetic Monte Carlo (KMC) methods to simulate the molecular beam epitaxy (MBE) process, where metals are heated until they transition into a gaseous state and then reform as thin films by condensing on a wafer in single-crystal thick layers.

"One of the main advantages of MBE is the ability to control the deposition of thin films and atomic structures on the atomic scale in order to create nanostructures," explained Amar.

Thin films are used in industry to create a variety of products, such as semiconductors, optical coatings, pharmaceuticals and solar cells.

"Ohio's status as a worldwide manufacturing leader has led OSC to focus on the field of advanced materials as one of our areas of primary support," noted Ashok Krishnamurthy, co-interim co-executive director of the center."As a result, numerous respected physicists, chemists and engineers, such as Dr. Amar, have accessed OSC computation and storage resources to advance their vital materials science research."

Recently, Amar leveraged the center's powerful supercomputers to implement a"first-passage time approach" to speed up KMC simulations of the creation of materials just a few atoms thick.

"The KMC method has been successfully used to carry out simulations of a wide variety of dynamical processes over experimentally relevant time and length scales," Amar noted."However, in some cases, much of the simulation time can be 'wasted' on rapid, repetitive, low-barrier events."

While a variety of approaches to dealing with the inefficiencies have been suggested, Amar settled on using a first-passage-time (FPT) approach to improve KMC processing speeds. FPT, sometimes also called first-hitting-time, is a statistical model that sets a certain threshold for a process and then estimates certain factors, such as the probability that the process reaches that threshold within a certain amount time or the mean time until which the threshold is reached.

"In this approach, one avoids simulating the numerous diffusive hops of atoms, and instead replaces them with the first-passage time to make a transition from one location to another," Amar said.

In particular, Amar and colleagues from the UT department of Physics and Astronomy targeted two atomic-level events for testing the FPT approach: edge-diffusion and corner rounding. Edge-diffusion involves the"hopping" movement of surface atoms -- called adatoms -- along the edges of islands, which are formed as the material is growing. Corner rounding involves the hopping of adatoms around island corners, leading to smoother islands.

Amar compared the KMC-FPT and regular KMC simulation approaches using several different models of thin film growth: Cu/Cu(100), fcc(100) and solid-on-solid (SOS). Additionally, he employed two different methods for calculating the FPT for these events: the mean FPT (MFPT), as well as the full FPT distribution.

"Both methods provided"very good agreement" between the FPT-KMC approach and regular KMC simulations," Amar concluded."In addition, we find that our FPT approach can lead to a significant speed-up, compared to regular KMC simulations."

Amar's FPT-KMC approach accelerated simulations by a factor of approximately 63 to 100 times faster than the corresponding KMC simulations for the fcc(100) model. The SOS model was improved by a factor of 36 to 76 times faster. For the Cu/Cu(100) tests, speed-up factors of 31 to 42 and 22 to 28 times faster were achieved, respectively, for simulations using the full FPT distribution and MFPT calculations.

Amar's research was supported through multiple grants from the National Science Foundation, as well as by a grant of computer time from OSC.

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Looking Inside Nanomaterials in 3-D

Most solid materials are composed of millions of small crystals, packed together to form a fully dense solid. The orientations, shapes, sizes and relative arrangement of these crystals are important in determining many material properties.

Traditionally, it has only been possible to see the crystal structure of a material by looking at a cut surface, giving just 2D information. In recent years, x-ray methods have been developed that can be used to look inside a material and obtain a 3-D map of the crystal structure. However, these methods have a resolution limit of around 100nm (one nanometer is 100,000 times smaller than the width of a human hair).

In contrast, the newly developed technique now published in the journalScience,allows 3-D mapping of the crystal structure inside a material down to nanometer resolution, and can be carried out using a transmission electron microscope, an instrument found in many research laboratories.

Samples must be thinner than a few hundred nanometers. However, this limitation is not a problem for investigations of crystal structures inside nanomaterials, where the average crystal size is less than 100 nanometers, and such materials are investigated all over the world in a search for materials with new and better properties than the materials we use today.

For example, nanomaterials have an extremely high strength and an excellent wear resistance and applications therefore span from microelectronics to gears for large windmills. The ability to collect a 3-D picture of the crystal structure in these materials is an important step in being able to understand the origins of their special properties.

An important advantage of such 3-D methods is that they allow the changes taking place inside a material to be observed directly. For example, the mapping may be repeated before and after a heat treatment revealing how the structure changes during heating.

This new technique has a resolution 100 times better than existing non-destructive 3-D techniques and opens up new opportunities for more precise analysis of the structural parameters in nanomaterials.

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New Insight Into Viscosity and the Cytoplasm of Cancer Cells

The researchers from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) have described in a physically consistent manner the changes in viscosity as measured in various solutions and experienced by probes with size varying from a nano to a macro scale. Their findings have just been published by the journalNano Letters."We improved our earlier formulae and conclusions to successfully apply them to a larger number of systems, including the first description of the cytoplasm viscosity in cancer cells," says Prof. Robert Hołyst from the IPC PAS.

The very first scientific publication to address the viscosity of complex fluids was a paper by Albert Einstein from 1906. In the time that followed intriguing evidence has been presented with regard to the cytoplasm viscosity in cells. This experimental evidence indicated that in spite of a high cytoplasm viscosity the mobility of small proteins in the cytoplasm is high -- many orders of magnitude higher than the Stokes-Sutherland-Einstein formula would imply.

The researchers from the IPC PAS managed to describe the viscosity changes using one phenomenological formula containing coefficients of the same physical nature. The coefficients give a description for both the fluid medium (filled, for instance, with a network of long-chained polymers or clusters of molecules) and the probe (e.g., protein molecule) moving in the medium. The new formula is of universal importance and can be used for probes from a fraction of nanometer up to centimeters in size. The relationships found are generally valid for various types of fluids including solutions with elastic microscopic structure (e.g., polymer networks in various solvents) and microscopically rigid systems (e.g., composed of elongated aggregates of molecules -- micelles).

In theNano Lettersreport the researchers from the IPC PAS applied the new formula to describe the mobility of DNA fragments and other probes in mouse muscle cells (Swiss 3T3) and human cancer cells (HeLa)."We managed to show that the fluid viscosity in the cell depends actually not only on the intracellular structure but also on the size of the probe used in viscosity measurement," says Tomasz Kalwarczyk, a PhD student from the IPC PAS.

The researchers from the IPC PAS measured the so called correlation length that in the cytoplasm of the Swiss 3T3 cells was 7 nanometers (a billionth part of a meter), and in the HeLa cells -- 5 nm. The correlation length is a limiting parameter for viscosity -- the proteins smaller in size than the correlation length move freely in the cell. Another limiting parameter determined in the study was the hydrodynamic radius of the objects the fluid is made of. This is also an essential parameter, as the probes larger than the hydrodynamic radius experience macroscopic viscosity (the probes larger than the correlation length but smaller than the hydrodynamic radius experience viscosity that increases dramatically with probe size). It turned out that in the HeLa cells, the macroscopic viscosity was experienced by probes larger than 350 nm, whereas in the Swiss 3T3 cells the threshold was 120 nm only."Our research resulted in a novel method to characterise cell structure -- by measuring the viscosity of the cytoplasm," stresses Kalwarczyk.

The outcome of the research presented by the scientists from the ICP PAS will allow to estimate better the migration time of drugs introduced in cells, and will be also applied in nanotechnologies, for instance in fabrication of nanoparticles with micellar solutions. The findings of the study are also of significant importance for advanced measurement methods, such as dynamic light scattering, that allow to analyse suspensions of molecules by their sizes. If the dependence of viscosity on the size of the viscosity probe used is not taken into account, the results of the measurements can be affected by significant errors.

The research on viscosity at the IPC PAS is co-financed by the EU"European Regional Development Fund" under a TEAM grant of the Foundation for Polish Science.

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Mixing Fluids Efficiently in Confined Spaces: Let the Fingers Do the Working

Microfluidic devices were first introduced in the 1980s and for many years were best known for their use in ink-jet printers, but have since been introduced in other fields, including the chemical analysis of blood or other sera in lab-on-a-chip technologies. These devices -- usually not much larger than a stick of chewing gum -- sometimes rely on nano-sized moving components, the geometry of the grooved channels or pulsed injections to induce a mixing of the fluids. But researchers in MIT's Department of Civil and Environmental Engineering suggest that a simpler method might be equally, if not more, effective.

"Getting two fluids to mix in a very tight space is difficult because there's not much room for a disorderly flow," said Professor Ruben Juanes, the ARCO Associate Professor in Energy Studies and principal investigator on the research."But with two fluids of highly contrasting viscosity, the thinner fluid naturally creates disorder, which proves to be a marvelously efficient means of mixing."

In an analysis published online May 12 in Physical Review Letters (PRL), the researchers show that the injection of a thin or low-viscosity fluid into a much more viscous fluid (think of water spurting into molasses) will cause the two fluids to mix very quickly via a physical process known as viscous fingering. The thinner liquid, say the researchers, will form fingers as it enters the thicker liquid, and those fingers will form other fingers, and so on until the two liquids have mixed uniformly.

They also found that for maximum mixing to occur quickly, the ideal ratio of the viscosity of any two fluids depends on the speed at which the thinner liquid is injected into the thicker one.

The research team of Juanes, postdoctoral associate Luis Cueto-Felgueroso and graduate students Birendra Jha and Michael Szulczewski, made a series of controlled experiments using mixtures of water and glycerol, a colorless liquid generally about a thousand times more viscous than water. By alternating the viscosity of the liquids and the velocity of the injection flows, Jha was able to create a mathematical model of the process and use that to determine the best viscosity ratio for a particular velocity. He is lead author on the PRL paper.

"It's been known for a very long time that a low viscosity fluid will finger through the high viscosity fluid," said Juanes."What was not known is how this affects the mixing rate of the two fluids. For instance, in the petroleum industry, people have developed increasingly refined models of how quickly the low viscosity fluid will reach the production well, but know little about how it will mix once it makes contact with the oil."

Similarly, Juanes said, in microfluidics technology, the use of fluids of different viscosities has not been seriously proposed as a mixing mechanism, but the new study indicates it could work very efficiently in the miniscule channels.

"We can now say that on average, the viscosity of the fluid injected should be about 10 times lower than that of the fluid into which it is injected," said Juanes."If the contrast is greater than 10, then the injection should be done more slowly to achieve the fastest maximum mixing. Otherwise, the low viscosity fluid will create a single channel through the thicker fluid, which is not ideal."

Cueto-Felgueroso said a similar process is at work in the engraved channels of a microfluidic device and in subsurface rock containing oil."Mixing fluids at small scales or velocities is difficult because you can't rely on turbulence: it would be hard to stir milk into your coffee if you were using a microscopic cup," Cueto-Felgueroso said."With viscous fingering, you let the fluids do the job of stirring."

This work was funded by the Italian energy company, Eni, and the ARCO Chair in Energy Studies.

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Free-Standing Single-Walled Carbon Nanotube Thin Films

Thin films of SWCNTs have many unique properties such as high porosity and specific surface area, low density, high ratio of optical transmittance to sheet resistance, high thermal conductivity and chemical sensitivity, and tunable metallic and semiconducting properties.

Recently researchers from Department of Applied Physics at Aalto University (Finland) in collaboration with Canatu Ltd. (Finland) have discovered a simple and rapid method to prepare thin multifunctional single-walled carbon nanotube films without any substrate (free-standing films). Usually SWCNT films are prepared from suspensions of SWCNTs by a liquid filtration. This method typically involves several time and resource consuming and potentially detrimental liquid dispersion and purification steps ending up with dense SWCNT networks on a filter, which have a transfer issue. Moreover, preparation of free-standing films by the vacuum-filtration method is still a challenging task.

"Our method allows the preparation of SWCNT deposits both on different substrates and in the form of free-standing films during less than 15 s. This becomes possible due to the fact that the SWCNTs produced in the gas phase synthesis process gave very high purity and crystallinity, can be directly deposit from the gas to a substrate and accordingly be directly utilised without additional purification steps," says Dr. David P. Brown, CEO of the company Canatu Ltd, which commercializes the SWCNT films.

The method easily allows Canatu to alter the thickness of multifunctional free-standing SWCNT films from a sub-monolayer (when the amount of SWCNTs is insufficient to create a single continuous layer) to a few micrometers.

Collaboration with other Finnish universities

According to the researchers, the collaboration with other Finnish universities and institutions, Tampere University of Technology and Oulu University, was extremely important to investigate the unique properties and to demonstrate the multifunctionality of this unique material.

"We fabricated the state-of-the-art components for filtration of aerosol nanoparticles, transparent, flexible and highly conductive electrodes, extremely sensitive electrochemical sensors, polymer free saturable laser absorbers, gas heaters, thermo acoustic loudspeakers, and gas flow meters," says professor Esko I. Kauppinen, the leader of the research group.

SWCNT films− wide range of applications

"However, the wide range of applications is not limited to those reported in our paper. The superior mechanical and electrical properties of these films suggest potential uses in a broad range of other devices. As a filter, SWCNT films could be used for filtration of bacteria and viruses. The possibility to heat SWCNT films can be utilized for water or air sterilization. Additionally, since SWCNTs contain iron particles embedded inside them, one could exploit their magnetic properties. Their high strength coupled with high electrical conductivity could be employed in novel energy generators, electromagnetic interference shielding, flexible radio frequency identification tags, touch sensors, flat panel displays and static-charge dissipators. The ultrahigh surface area coupled with high electrical conductivity could be used in advanced solar cells and super capacitors," Dr. Albert G. Nasibulin, the leader of this project and the first author of the article related to the discovery of multifunctional free-standing SWCNTs concludes.

The results has been recently published in the journalACS Nano.

Even though graphene has recently attracted much attention from the research community, some properties of SWCNTs, such as porosity, mechanical strength, and fine-tunability of optical and electrical properties, provide many applications where the flat, single-layered carbon structure cannot compete with its tubular 'brother'.

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Carbon, Carbon Everywhere, but Not from the Big Bang

More than 50 years ago, an astronomer named Fred Hoyle deduced that when three helium nuclei -- or alpha particles -- cometogether inside the core of a star, they have difficulty combining to form carbon-12, the stuff we're made of. So he predicted a new state of carbon-12, one with an energy tuned just right to make the formation of carbon possible in stars. This new state is now known as the Hoyle state. Later experimentation demonstrated that the theory was correct, but no one had ever been able to reproduce the Hoyle state from scratch, starting from the known interactions of protons and neutrons. If the Hoyle state didn't show up in those calculations, then the calculations must be incorrect or incomplete.

NC State physicist Dean Lee, along with German colleagues Evgeny Epelbaum, Hermann Krebs, and Ulf-G. Meissner, had previously developed a new method for describing all the possible ways that protons and neutrons can bind with one another inside nuclei. This"effective field theory" is formulated on a complex numerical lattice that allows the researchers to run simulations that show how particles interact. When the researchers put six protons and six neutrons on the lattice, the Hoyle state appeared together with other observed states of carbon-12, proving the theory correct from first principles.

"We've had simple models of the Hoyle state usingthree alpha particles for a long time, but the first principles calculations weren't giving anything close," Lee says."Our method places the particles into a simulation with certain space and time parameters, then allows them to do what they want to do. Within those simulations, the Hoyle state shows up."

Their research appears in the May 13 issue ofPhysical Review Letters.

Lee adds,"This work is valuable because it gives us a much better idea of the kind of 'fine-tuning' nature has to do in order to produce carbon in stars."

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Pairing Quantum Dots With Fullerenes for Nanoscale Photovoltaics

"This is the first demonstration of a hybrid inorganic/organic, dimeric (two-particle) material that acts as an electron donor-bridge-acceptor system for converting light to electrical current," said Brookhaven physical chemist Mircea Cotlet, lead author of a paper describing the dimers and their assembly method inAngewandte Chemie.

By varying the length of the linker molecules and the size of the quantum dots, the scientists can control the rate and the magnitude of fluctuations in light-induced electron transfer at the level of the individual dimer."This control makes these dimers promising power-generating units for molecular electronics or more efficient photovoltaic solar cells," said Cotlet, who conducted this research with materials scientist Zhihua Xu at Brookhaven's Center for Functional Nanomaterials (CFN).

Scientists seeking to develop molecular electronics have been very interested in organic donor-bridge-acceptor systems because they have a wide range of charge transport mechanisms and because their charge-transfer properties can be controlled by varying their chemistry. Recently, quantum dots have been combined with electron-accepting materials such as dyes, fullerenes, and titanium oxide to produce dye-sensitized and hybrid solar cells in the hope that the light-absorbing and size-dependent emission properties of quantum dots would boost the efficiency of such devices. But so far, the power conversion rates of these systems have remained quite low.

"Efforts to understand the processes involved so as to engineer improved systems have generally looked at averaged behavior in blended or layer-by-layer structures rather than the response of individual, well-controlled hybrid donor-acceptor architectures," said Xu.

The precision fabrication method developed by the Brookhaven scientists allows them to carefully control particle size and interparticle distance so they can explore conditions for light-induced electron transfer between individual quantum dots and electron-accepting fullerenes at the single molecule level.

The entire assembly process takes place on a surface and in a stepwise fashion to limit the interactions of the components (particles), which could otherwise combine in a number of ways if assembled by solution-based methods. This surface-based assembly also achieves controlled, one-to-one nanoparticle pairing.

To identify the optimal architectural arrangement for the particles, the scientists strategically varied the size of the quantum dots -- which absorb and emit light at different frequencies according to their size -- and the length of the bridge molecules connecting the nanoparticles. For each arrangement, they measured the electron transfer rate using single molecule spectroscopy.

"This method removes ensemble averaging and reveals a system's heterogeneity -- for example fluctuating electron transfer rates -- which is something that conventional spectroscopic methods cannot always do," Cotlet said.

The scientists found that reducing quantum dot size and the length of the linker molecules led to enhancements in the electron transfer rate and suppression of electron transfer fluctuations.

"This suppression of electron transfer fluctuation in dimers with smaller quantum dot size leads to a stable charge generation rate, which can have a positive impact on the application of these dimers in molecular electronics, including potentially in miniature and large-area photovoltaics," Cotlet said.

"Studying the charge separation and recombination processes in these simplified and well-controlled dimer structures helps us to understand the more complicated photon-to-electron conversion processes in large-area solar cells, and eventually improve their photovoltaic efficiency," Xu added.

A U.S. patent application is pending on the method and the materials resulting from using the technique, and the technology is available for licensing. This work was funded by the DOE Office of Science.

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Electromechanics Also Operates at the Nanoscale

"We have been studying carbon nanotubes theoretically, in order to see how they behave when they are stimulated to behave according to the laws quantum mechanics. The results provide a completely new platform for scientists to stand on," says Gustav Sonne of the Department of Physics at the University of Gothenburg.

Every day we use a number of different microelectromechanical components for various forms of detection, to determine whether a certain process has taken place or whether a certain substance is present. These cannot be detected without instruments. One example is the detection of rapid accelerations that is used to activate the airbag in a car during an accident. What all of these components have in common is that they combine mechanical and electronic properties in order to react to external stimuli.

Gustav Sonne has taken research down to a whole new dimension -- from the micrometer scale to the nanometer scale -- and he has studied the younger brothers of these components: nanoelectromechanical systems. The studies have been based on tiny nanotubes suspended between two electrical contacts. He has subsequently calculated how small vibrations in the suspended tubes can be coupled to a current that is led through them.

"Our research has focussed mainly on how these systems, which consist of a tiny, super-light mechanical oscillator (the suspended nanotube), can be described in quantum mechanical terms, and what effects this has on the measurements we can carry out. We have been able to demonstrate a number of new mechanisms for electromechanical coupling that should be possible to observe experimentally. This, in turn, may lead to extremely exotic physical phenomena in these structures, phenomena which may be of interest for research into quantum computers, and other fields."

Interest in nanotubes is based on their outstanding properties: they are among the strongest materials known, weigh next to nothing, and have extremely high conductivity for both electric currents and heat. Carbon nanotubes can be used to manufacture composite materials that are several orders of magnitude stronger than currently available materials.

The thesis"Mesoscopic phenomena in the electromechanics of suspended nanowires" was successfully defended in the Department of Physics. Supervisor: Associate professor Leonid Gorelik.

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Toward Optical Computing in Handheld Electronics: Graphene Optical Modulators Could Lead to Ultrafast Communications

The team of researchers, led by UC Berkeley engineering professor Xiang Zhang, built a tiny optical device that uses graphene, a one-atom-thick layer of crystallized carbon, to switch light on and off. This switching ability is the fundamental characteristic of a network modulator, which controls the speed at which data packets are transmitted. The faster the data pulses are sent out, the greater the volume of information that can be sent. Graphene-based modulators could soon allow consumers to stream full-length, high-definition, 3-D movies onto a smartphone in a matter of seconds, the researchers said.

"This is the world's smallest optical modulator, and the modulator in data communications is the heart of speed control," said Zhang, who directs a National Science Foundation (NSF) Nanoscale Science and Engineering Center at UC Berkeley."Graphene enables us to make modulators that are incredibly compact and that potentially perform at speeds up to ten times faster than current technology allows. This new technology will significantly enhance our capabilities in ultrafast optical communication and computing."

In this latest work, described in the May 8 advanced online publication of the journalNature, researchers were able to tune the graphene electrically to absorb light in wavelengths used in data communication. This advance adds yet another advantage to graphene, which has gained a reputation as a wonder material since 2004 when it was first extracted from graphite, the same element in pencil lead. That achievement earned University of Manchester scientists Andre Geim and Konstantin Novoselov the Nobel Prize in Physics last year.

Zhang worked with fellow faculty member Feng Wang, an assistant professor of physics and head of the Ultrafast Nano-Optics Group at UC Berkeley. Both Zhang and Wang are faculty scientists at Lawrence Berkeley National Laboratory's Materials Science Division.

"The impact of this technology will be far-reaching," said Wang."In addition to high-speed operations, graphene-based modulators could lead to unconventional applications due to graphene's flexibility and ease in integration with different kinds of materials. Graphene can also be used to modulate new frequency ranges, such as mid-infrared light, that are widely used in molecular sensing."

Graphene is the thinnest, strongest crystalline material yet known. It can be stretched like rubber, and it has the added benefit of being an excellent conductor of heat and electricity. This last quality of graphene makes it a particularly attractive material for electronics.

"Graphene is compatible with silicon technology and is very cheap to make," said Ming Liu, post-doctoral researcher in Zhang's lab and co-lead author of the study."Researchers in Korea last year have already produced 30-inch sheets of it. Moreover, very little graphene is required for use as a modulator. The graphite in a pencil can provide enough graphene to fabricate 1 billion optical modulators."

It is the behavior of photons and electrons in graphene that first caught the attention of the UC Berkeley researchers.

The researchers found that the energy of the electrons, referred to as its Fermi level, can be easily altered depending upon the voltage applied to the material. The graphene's Fermi level in turn determines if the light is absorbed or not.

When a sufficient negative voltage is applied, electrons are drawn out of the graphene and are no longer available to absorb photons. The light is"switched on" because the graphene becomes totally transparent as the photons pass through.

Graphene is also transparent at certain positive voltages because, in that situation, the electrons become packed so tightly that they cannot absorb the photons.

The researchers found a sweet spot in the middle where there is just enough voltage applied so the electrons can prevent the photons from passing, effectively switching the light"off."

"If graphene were a hallway, and electrons were people, you could say that, when the hall is empty, there's no one around to stop the photons," said Xiaobo Yin, co-lead author of the Nature paper and a research scientist in Zhang's lab."In the other extreme, when the hall is too crowded, people can't move and are ineffective in blocking the photons. It's in between these two scenarios that the electrons are allowed to interact with and absorb the photons, and the graphene becomes opaque."

In their experiment, the researchers layered graphene on top of a silicon waveguide to fabricate optical modulators. The researchers were able to achieve a modulation speed of 1 gigahertz, but they noted that the speed could theoretically reach as high as 500 gigahertz for a single modulator.

While components based upon optics have many advantages over those that use electricity, including the ability to carry denser packets of data more quickly, attempts to create optical interconnects that fit neatly onto a computer chip have been hampered by the relatively large amount of space required in photonics.

Light waves are less agile in tight spaces than their electrical counterparts, the researchers noted, so photon-based applications have been primarily confined to large-scale devices, such as fiber optic lines.

"Electrons can easily make an L-shaped turn because the wavelengths in which they operate are small," said Zhang."Light wavelengths are generally bigger, so they need more space to maneuver. It's like turning a long, stretch limo instead of a motorcycle around a corner. That's why optics require bulky mirrors to control their movements. Scaling down the optical device also makes it faster because the single atomic layer of graphene can significantly reduce the capacitance -- the ability to hold an electric charge -- which often hinders device speed."

Graphene-based modulators could overcome the space barrier of optical devices, the researchers said. They successfully shrunk a graphene-based optical modulator down to a relatively tiny 25 square microns, a size roughly 400 times smaller than a human hair. The footprint of a typical commercial modulator can be as large as a few square millimeters.

Even at such a small size, graphene packs a punch in bandwidth capability. Graphene can absorb a broad spectrum of light, ranging over thousands of nanometers from ultraviolet to infrared wavelengths. This allows graphene to carry more data than current state-of-the-art modulators, which operate at a bandwidth of up to 10 nanometers, the researchers said.

"Graphene-based modulators not only offer an increase in modulation speed, they can enable greater amounts of data packed into each pulse," said Zhang."Instead of broadband, we will have 'extremeband.' What we see here and going forward with graphene-based modulators are tremendous improvements, not only in consumer electronics, but in any field that is now limited by data transmission speeds, including bioinformatics and weather forecasting. We hope to see industrial applications of this new device in the next few years."

Other UC Berkeley co-authors of this paper are graduate student Erick Ulin-Avila and post-doctoral researcher Thomas Zentgraf in Zhang's lab; and visiting scholar Baisong Geng and graduate student Long Ju in Wang's lab.

This work was supported through the Center for Scalable and Integrated Nano-Manufacturing (SINAM), an NSF Nanoscale Science and Engineering Center. Funding from the Department of Energy's Basic Energy Science program at Lawrence Berkeley National Laboratory also helped support this research.

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'Swiss Cheese' Design Enables Thin Film Silicon Solar Cells With Potential for Higher Efficiencies

One long-term option for low-cost, high-yield industrial production of solar panels from abundant raw materials can be found in amorphous silicon solar cells and microcrystalline silicon tandem cells (a.k.a. Micromorph) -- providing an energy payback within a year.

A drawback to these cells, however, is that the stable panel efficiency is less than the efficiency of presently dominate crystalline wafer-based silicon, explains Milan Vanecek, who heads the photovoltaic group at the Institute of Physics in Prague.

"To make amorphous and microcrystalline silicon cells more stable they're required to be very thin because of tight spacing between electrical contacts, and the resulting optical absorption isn't sufficient," he notes."They're basically planar devices. Amorphous silicon has a thickness of 200 to 300 nanometers, while microcrystalline silicon is thicker than 1 micrometer."

The team's new design focuses on optically thick cells that are strongly absorbing, while the distance between the electrodes remains very tight. They describe their design in the American Institute of Physics' journalApplied Physics Letters.

"Our new 3D design of solar cells relies on the mature, robust absorber deposition technology of plasma-enhanced chemical vapor deposition, which is a technology already used for amorphous silicon-based electronics produced for liquid crystal displays. We just added a new nanostructured substrate for the deposition of the solar cell," Vanecek says.

This nanostructured substrate consists of an array of zinc oxide (ZnO) nanocolumns or, alternatively, from a"Swiss cheese" honeycomb array of micro-holes or nano-holes etched into the transparent conductive oxide layer (ZnO).

"This latter approach proved successful for solar cell deposition," Vanecek elaborates."The potential of these efficiencies is estimated within the range of present multicrystalline wafer solar cells, which dominate solar cell industrial production. And the significantly lower cost of Micromorph panels, with the same panel efficiency as multicrystalline silicon panels (12 to 16 percent), could boost its industrial-scale production."

The next step is a further optimization to continue improving efficiency.

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New Mineral Discovered: One of Earliest Minerals Formed in Solar System

This particular grain is known affectionately as"Cracked Egg" for its distinctive appearance. Dr. Harold C. Connolly, Jr. and student Stuart A. Sweeney Smith at the City University of New York (CUNY) and the American Museum of Natural History (AMNH) first recognized the grain to be of a very special type, known as a calcium-aluminum-rich refractory inclusion. ("Refractory" refers to the fact that these grains contain minerals that are stable at very high temperature, which attests to their likely formation as very primitive, high-temperature condensates from the solar nebula.)

Cracked Egg refractory inclusion was sent to Dr. Chi Ma at California Institute of Technology (Caltech) for very detailed nano-mineralogy investigation. Dr. Ma then sent it to Dr. Anthony Kampf, Curator of Mineral Sciences at the Natural History Museum of Los Angeles County (NHM), for X- ray diffraction study. Kampf's findings, confirmed by Ma, showed the main component of the grain was a low-pressure calcium aluminum oxide (CaAl2O4) never before found in nature. Kampf's determination of the atomic arrangement in the mineral showed it to be the same as that of a human-made component of some types of refractory (high-temperature) concrete.

What insight can we get from knowing that a common human-made component of modern concrete is found in nature only as a very rare component of a grain formed more than 4.5 billion years ago? Such investigations are essential in deciphering the origins of our solar system. The creation of the human-made compound requires temperature of at least 1,500°C (2,732°F). This, coupled with the fact that the compound forms at low pressure, is consistent with krotite forming as a refractory phase from the solar nebula. Therefore, the likelihood is that krotite is one of the first minerals formed in our solar system.

Studies of the unique Cracked Egg refractory inclusion are continuing, in an effort to learn more about the conditions under which it formed and subsequently evolved. In addition to krotite, the Cracked Egg contains at least eight other minerals, including one other mineral new to science.

TheAmerican Mineralogistpaper is authored by Chi Ma (Caltech), Anthony R. Kampf (NHM), Harold C. Connolly Jr. (CUNY and AMNH), John R. Beckett (Caltech), George R. Rossman (Caltech), Stuart A. Sweeney Smith (who was a NSF funded Research for Undergraduate (REU) student at CUNY/AMNH) and Devin L. Schrader (University of Arizona). Krotite is named for Alexander N. Krot, a cosmochemist at the University of Hawaii, in recognition of his significant contributions to the understanding of early solar system processes.

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Forecast Calls for Nanoflowers to Help Return Eyesight: Physicist Leads Effort to Design Fractal Devices to Put in Eyes

These flowers are not roses, tulips or columbines. They will be nanoflowers seeded from nano-sized particles of metals that grow, or self assemble, in a natural process -- diffusion limited aggregation. They will be fractals that mimic and communicate efficiently with neurons.

Fractals are"a trademark building block of nature," Taylor says. Fractals are objects with irregular curves or shapes, of which any one component seen under magnification is also the same shape. In math, that property is self-similarity. Trees, clouds, rivers, galaxies, lungs and neurons are fractals, Taylor says. Today's commercial electronic chips are not fractals, he adds.

Eye surgeons would implant these fractal devices within the eyes of blind patients, providing interface circuitry that would collect light captured by the retina and guide it with almost 100 percent efficiency to neurons for relay to the optic nerve to process vision.

In an article titled"Vision of beauty" forPhysics World, Taylor, a physicist and director of the UO Materials Science Institute, describes his envisioned approach and how it might overcome the problems occurring with current efforts to insert photodiodes behind the eyes. Current chip technology is limited, because it doesn't allow sufficient connections with neurons.

"The wiring -- the neurons -- in the retina is fractal, but the chips are not fractal," Taylor says."They are just little squares of electrodes that provide too little overlap with the neurons."

Beginning this summer, Taylor's doctoral student Rick Montgomery will begin a yearlong collaboration with Simon Brown at the University of Canterbury in New Zealand to experiment with various metals to grow the fractal flowers on implantable chips.

The idea for the project emerged as Taylor was working under a Cottrell Scholar Award he received in 2003 from the Research Corporation for Science Advancement. His vision is now beginning to blossom under grants from the Office of Naval Research (ONR), the U.S. Air Force and the National Science Foundation.

Taylor's theoretical concept for fractal-based photodiodes also is the focus of a U.S. patent application filed by the UO's Office of Technology Transfer under Taylor's and Brown's names, the UO and University of Canterbury.

The project, he writes in thePhysics Worldarticle, is based on"the striking similarities between the eye and the digital camera." (Physics Worldarticle is available at:http://physicsworld.com/cws/article/indepth/45840)

"The front end of both systems," he writes,"consists of an adjustable aperture within a compound lens, and advances bring these similarities closer each year." Digital cameras, he adds, are approaching the capacity to capture the 127 megapixels of the human eye, but current chip-based implants, because of their interface, are only providing about 50 pixels of resolution.

Among the challenges, Taylor says, is determining which metals can best go into body without toxicity problems."We're right at the start of this amazing voyage," Taylor says."The ultimate thrill for me will be to go to a blind person and say, we're developing a chip that one day will help you see again. For me, that is very different from my previous research, where I've been looking at electronics to go into computers, to actually help somebody… if I can pull that off that will be a tremendous thrill for me."

Taylor also is working under a Research Corp. grant to pursue fractal-based solar cells.

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New Way to Control Conductivity: Reversible Control of Electrical and Thermal Properties Could Find Uses in Storage Systems

"It's a new way of changing and controlling the properties" of materials -- in this case a class called percolated composite materials -- by controlling their temperature, says Gang Chen, MIT's Carl Richard Soderberg Professor of Power Engineering and director of the Pappalardo Micro and Nano Engineering Laboratories. Chen is the senior author of a paper describing the process that was published online on April 19 and will appear in a forthcoming issue ofNature Communications. The paper's lead authors are former MIT visiting scholars Ruiting Zheng of Beijing Normal University and Jinwei Gao of South China Normal University, along with current MIT graduate student Jianjian Wang. The research was partly supported by grants from the National Science Foundation.

The system Chen and his colleagues developed could be applied to many different materials for either thermal or electrical applications. The finding is so novel, Chen says, that the researchers hope some of their peers will respond with an immediate,"I have a use for that!"

One potential use of the new system, Chen explains, is for a fuse to protect electronic circuitry. In that application, the material would conduct electricity with little resistance under normal, room-temperature conditions. But if the circuit begins to heat up, that heat would increase the material's resistance, until at some threshold temperature it essentially blocks the flow, acting like a blown fuse. But then, instead of needing to be reset, as the circuit cools down the resistance decreases and the circuit automatically resumes its function.

Another possible application is for storing heat, such as from a solar thermal collector system, later using it to heat water or homes or to generate electricity. The system's much-improved thermal conductivity in the solid state helps it transfer heat.

Essentially, what the researchers did was suspend tiny flakes of one material in a liquid that, like water, forms crystals as it solidifies. For their initial experiments, they used flakes of graphite suspended in liquid hexadecane, but they showed the generality of their process by demonstrating the control of conductivity in other combinations of materials as well. The liquid used in this research has a melting point close to room temperature -- advantageous for operations near ambient conditions -- but the principle should be applicable for high-temperature use as well.

The process works because when the liquid freezes, the pressure of its forming crystal structure pushes the floating particles into closer contact, increasing their electrical and thermal conductance. When it melts, that pressure is relieved and the conductivity goes down. In their experiments, the researchers used a suspension that contained just 0.2 percent graphite flakes by volume. Such suspensions are remarkably stable: Particles remain suspended indefinitely in the liquid, as was shown by examining a container of the mixture three months after mixing.

By selecting different fluids and different materials suspended within that liquid, the critical temperature at which the change takes place can be adjusted at will, Chen says.

"Using phase change to control the conductivity of nanocomposites is a very clever idea," says Li Shi, a professor of mechanical engineering at the University of Texas at Austin. Shi adds that as far as he knows"this is the first report of this novel approach" to producing such a reversible system.

"I think this is a very crucial result," says Joseph Heremans, professor of physics and of mechanical and aerospace engineering at Ohio State University."Heat switches exist," but involve separate parts made of different materials, whereas"here we have a system with no macroscopic moving parts," he says."This is excellent work."

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Removable 'Cloak' for Nanoparticles Helps Them Target Tumors

Such particles could target nearly any type of tumor, and can be designed to carry virtually any type of drug, says Paula Hammond, a member of the David H. Koch Institute for Integrative Cancer Research at MIT and senior author of a paper describing the particles in the journalACS Nano.

Like most other drug-delivering nanoparticles, the new MIT particles are cloaked in a polymer layer that protects them from being degraded by the bloodstream. However, the MIT team, including lead author and postdoctoral associate Zhiyong Poon, designed this outer layer to fall off after entering the slightly more acidic environment near a tumor. That reveals another layer that is able to penetrate individual tumor cells.

In theACS Nanopaper, which went online April 23, the researchers reported that, in mice, their particles can survive in the bloodstream for up to 24 hours, accumulate at tumor sites and enter tumor cells.

A new target

The new MIT approach differs from that taken by most nanoparticle designers. Typically, researchers try to target their particles to a tumor by decorating them with molecules that bind specifically to proteins found on the surface of cancer cells. The problem with that strategy is that it's difficult to find the right target -- a molecule found on all of the cancer cells in a particular tumor, but not on healthy cells. Also, a target that works for one type of cancer might not work for another.

Hammond and her colleagues decided to take advantage of tumor acidity, which is a byproduct of its revved-up metabolism. Tumor cells grow and divide much more rapidly than normal cells, and that metabolic activity uses up a lot of oxygen, which increases acidity. As the tumor grows, the tissue becomes more and more acidic.

To build their targeted particles, the researchers used a technique called"layer-by-layer assembly." This means each layer can be tailored to perform a specific function.

When the outer layer (made of polyethylene glycol, or PEG) breaks down in the tumor's acidic environment, a positively charged middle layer is revealed. That positive charge helps to overcome another obstacle to nanoparticle drug delivery: Once the particles reach a tumor, it's difficult to get them to enter the cells. Particles with a positive charge can penetrate the negatively charged cell membrane, but such particles can't be injected into the body without a"cloak" of some kind because they would also destroy healthy tissues.

The nanoparticles' innermost layer can be a polymer that carries a cancer drug, or a quantum dot that could be used for imaging, or virtually anything else that the designer might want to deliver, says Hammond, who is the Bayer Professor of Chemical Engineering at MIT.

Layer by layer

Other researchers have tried to design nanoparticles that take advantage of tumors' acidity, but Hammond's particles are the first that have been successfully tested in living animals.

Jinming Gao, professor of oncology and pharmacology at the University of Texas Southwestern Medical Center, says it is"quite clever" to use layer-by-layer assembly to create particles with a protective layer that can be shed when the particles reach their targets."It is a nice proof of concept," says Gao, who was not part of the research team."This could serve as a general strategy to target acidic tumor microenvironment for improved drug delivery."

The researchers are planning to further develop these particles and test their ability to deliver drugs in animals. Hammond says she expects it could take five to 10 years of development before human clinical trials could begin.

Hammond's team is also working on nanoparticles that can carry multiple payloads. For example, the outer PEG layer might carry a drug or a gene that would"prime" the tumor cells to be susceptible to another drug carried in the particle's core.

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Improved Electrical Conductivity in Polymeric Composites

The researchers in Luxembourg, in cooperation with scientists from the Netherlands, have studied the electrical percolation of carbon nanotubes in a polymer matrix and shown the percolation threshold -- the point at which the polymer composite becomes conductive -- can be considerably lowered if small quantities of a conductive polymer latex are added. The simulations were done in Luxembourg, while the experiments took place at Eindhoven University.

"In this project, the idea is to use as little as possible carbon nanotubes and still benefit from their favourable properties," says the project leader at the University of Luxembourg, Prof. Tania Schilling,"we have discovered that, by adding a second component, we could make use of the resulting interactions to reach our goal." By mixing finely dispersed particles, so-called colloidal particles, of differing shapes and sizes in the medium, system-spanning networks form: the prerequisite for electrically conductive composites.

The recent finding of the materials scientists of the University of Luxembourg was published in the peer-reviewed, scientific journalNature Nanotechnology.This finding is a result of a cooperation of scientists at the University of Luxembourg, the Technische Universiteit Eindhoven and the Dutch Polymer Institute.

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New Material Could Improve Safety for First Responders to Chemical Hazards

In a recent issue of the journalAdvanced Materials, a team of researchers from the University of California, San Diego and Tyco Electronics describe how they made the carbon nanostructures and demonstrate their potential use as microsensors for volatile organic compounds.

First responders protect themselves from such vapors, whose composition is often unknown, by breathing through a canister filled with activated charcoal -- a gas mask. Airborne toxins stick to the carbon in the filter, trapping the dangerous materials.

As the filters become saturated, chemicals will begin to pass through. The respirator can then do more harm than good by providing an illusion of safety. But there is no easy way to determine when the filter is spent. Current safety protocols base the timing of filter changes on how long the user has worn the mask.

"The new sensors would provide a more accurate reading of how much material the carbon in the filters has actually absorbed," said team leader Michael Sailor, professor of chemistry and biochemistry and bioengineering at UC San Diego."Because these carbon nanofibers have the same chemical properties as the activated charcoal used in respirators, they have a similar ability to absorb organic pollutants."

Sailor's team assembled the nanofibers into repeating structures called photonic crystals that reflect specific wavelengths, or colors, of light. The wing scales of the Morpho butterfly, which give the insect its brilliant iridescent coloration, are natural examples of this kind of structure.

The sensors are an iridescent color too, rather than black like ordinary carbon. That color changes when the fibers absorb toxins -- a visible indication of their capacity for absorbing additional chemicals.

The agency that certifies respirators in the U.S., the National Institute of Occupational Safety and Health, has long sought such a sensor but the design requirements for a tiny, sensitive, inexpensive device that requires little power, have proved difficult to meet.

The materials that the team fabricated are very thin -- less than half the width of a human hair. Sailor's group has previously placed similar photonic sensors on the tips of optical fibers less than a millimeter across and shown that they can be inserted into respirator cartridges. And the crystals are sensitive enough to detect chemicals such as toluene at concentrations as low as one part per million.

Ting Gao, a senior researcher at the Polymers, Ceramics, and Technical Services Laboratories of Tyco Electronics in Menlo Park, California and Timothy L. Kelly, a NSERC post-doctoral fellow at UC San Diego co-authored the paper. The National Science Foundation, the Department of Homeland Security, the Natural Sciences and Engineering Research Council of Canada, and TYCO Electronics provided funding for the work.

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Electron Ping Pong in the Nano-World

The research is reported in the journalNature Physics(April 24, 2011).

When intense laser light interacts with electrons in nanoparticles that consist of many million individual atoms, these electrons can be released and strongly accelerated. Such an effect in nano-spheres of silica was recently observed by an international team of researchers in the Laboratory for Attosecond Physics (LAP) at the Max Planck Institute of Quantum Optics. The researchers report how strong electrical fields (near-fields) build up in the vicinity of the nanoparticles and release electrons. Driven by the near-fields and collective interactions of the charges resulting from ionization by the laser light, the released electrons are accelerated, such that they can by far exceed the limits in acceleration that were observed so far for single atoms. The exact movement of the electrons can be precisely controlled via the electric field of the laser light. The new insights into this light-controlled process can help to generate energetic extreme ultraviolet (XUV) radiation. The experiments and their theoretical modeling, which are described by the scientists in the journal"Nature Physics," open up new perspectives for the development of ultrafast, light-controlled nano-electronics, which could potentially operate up to one million times faster than current electronics.

Electron acceleration in a laser field is similar to a short rally in a ping pongmatch: a serve, a return and a smash securing the point. A similar scenario occurs when electrons in nanoparticles are hit by light pulses. An international team, led by three German groups including Prof. Matthias Kling from the Laboratory for Attosecond Physics (LAP) at the Max Planck Institute of Quantum Optics in Garching and the Ludwig-Maximilian University Munich, Prof. Eckart Rühl from the Free University of Berlin and Prof. Thomas Fennel from the University of Rostock, was now successful in observing the mechanisms and aftermath of such a ping pong play of electrons in nanoparticles interacting with strong laser light-fields.

The researchers illuminated silica nanoparticles with a size of around 100 nm with very intense light pulses, lasting around five femtoseconds (one femtosecond is a millionth of a billionth of a second). Such short laser pulses consist of only a few wave cycles. The nanoparticles contained around 50 million atoms each. The electrons are ionized within a fraction of a femtosecond and accelerated by the electric field of the remaining laser pulse. After travelling less than one nanometer away from the surface of the nano-spheres, some of the electrons can be returned to the surface by the laser field to the surface, where they were smashed right back (such as the ping pong ball being hit by the paddle). The resulting energy gain of the electrons can reach very high values. In the experiment electron energies of ca. 60 times the energy of a 700 nm wavelength laser photon (in the red spectral region of light) have been found.

For the first time, the researchers could observe and record the direct elastic recollision phenomenon from a nanosystem in detail. The scientists used polarized light for their experiments. With polarized light, the light waves are oscillating only along one axis and not, as with normal light, in all directions."Intense radiation pulses can deform or destroy nanoparticles. We have thus prepared the nanoparticles in a beam, such that fresh nanoparticles were used for every laser pulse. This was of paramount importance for the observation of the highly energetic electrons.," explains Eckart Rühl.

The accelerated electrons left the atoms with different directions and different energies. The flight trajectories were recorded by the scientists in a three-dimensional picture, which they used to determine the energies and emission directions of the electrons."The electrons were not only accelerated by the laser-induced near-field, which by itself was already stronger than the laser field, but also by the interactions with other electrons, which were released from the nanoparticles," describes Matthias Kling about the experiment. Finally, the positive charging of the nanoparticle-surface also plays a role. Since all contributions add up, the energy of the electrons can be very high."The process is complex, but shows that there is much to explore in the interaction of nanoparticles with strong laser fields," adds Kling.

The electron movements can also produce pulses of extreme ultraviolet light when electrons that hit the surface do not bounce back, but are absorbed releasing photons with wavelengths in the XUV. XUV light is of particular interest for biological and medical research."According to our findings, the recombination of electrons on the nanoparticles can lead to energies of the generated photons, which are up to seven-times higher than the limit that was so far observed for single atoms," explains Thomas Fennel. The evidence of collective acceleration of electrons with nanoparticles offers great potential. Matthias Kling believes that"From this may arise new, promising applications in future, light-controlled ultrafast electronics, which may work up to one million times faster than conventional electronics."

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