Nano-paper filter can remove viruses

The illustration shows the nanofibers in white and the virus in green. Photograph: Björn Syse

Researchers at the Division of Nanotechnology and Functional Materials, Uppsala University have developed a paper filter, which can remove virus particles with an efficiency matching that of the best industrial virus filters. The paper filter consists of 100 percent high purity cellulose nanofibers, directly derived from nature.

The research was carried out in collaboration with virologists from the Swedish University of Agricultural Sciences/Swedish National Veterinary Institute and is published in the Advanced Healthcare Materials journal.

Virus particles are very peculiar objects- tiny (about thousand times thinner than a human hair) yet mighty. Viruses can only replicate in living cells but once the cells become infected the viruses can turn out to be extremely pathogenic. Viruses can actively cause diseases on their own or even transform healthy cells to malignant tumors.

‘Viral contamination of biotechnological products is a serious challenge for production of therapeutic proteins and vaccines. Because of the small size, virus removal is a non-trivial task, and, therefore, inexpensive and robust virus removal filters are highly demanded’, says Albert Mihranyan, Associate Professor at the Division of Nanotechnology and Functional Materials, Uppsala University, who heads the study.

Cellulose is one of the most common materials to produce various types of filters because it is inexpensive, disposable, inert and non-toxic. It is also mechanically strong, hydrophyllic, stable in a wide range of pH, and can withstand sterilization e.g. by autoclaving. Normal filter paper, used for chemistry, has too large pores to remove viruses.

The undergraduate student Linus Wågberg, Professor Maria Strømme, and Associate Professor Albert Mihranyan at the Division of Nanotechnology and Functional Materials, Uppsala University, in collaboration with virologists Dr. Giorgi Metreveli, Eva Emmoth, and Professor Sándor Belák from the Swedish University of Agricultural Sciences (SLU)/Swedish National Veterinary Institute (SVA), report a design of a paper filter which is capable of removing virus particles with the efficiency matching that of the best industrial virus filters. The reported paper filter, which is manufactured according to the traditional paper making processes, consists of 100 percent high purity cellulose nanofibers directly derived from nature.

The discovery is a result of a decade long research on the properties of high surface area nanocellulose materials, which eventually enabled the scientists to tailor the pore size distribution of their paper precisely in the range desirable for virus filtration.

Previously described virus removal paper filters relied heavily on interception of viruses via electrostatic interactions, which are sensitive to pH and salt concentrations, whereas the virus removal filters made from synthetic polymers and which rely on size-exclusion are produced through tedious multistep phase-inversion processing involving hazardous solvents and rigorous pore annealing processing.

Incidentally, it was the Swedish chemist J.J. Berzelius (1779-1848), one of the most famous alumni of Uppsala University, who was the first one to use the pure wet-laid-all-rag paper for separation of precipitates in chemical analysis. In a way, the virus removal nano-paper filter developed by the Uppsala scientists is the modern day analogue of the widely popular Swedish Filter Paper developed by Berzelius nearly two centuries ago.

Source: http://www.uu.se/en/media/news/article/?id=3317&area=2,10,16&typ=artikel&na=&lang=en#sthash.rKkmbmdN.dpuf

Never say never in the nano-world

Artistic impressions of the nanoparticle in a laser trap.
(Image credits: Iñaki Gonzalez and Jan Gieseler)

Objects with sizes in the nanometer range, such as the molecular building blocks of living cells or nanotechnological devices, are continuously exposed to random collisions with surrounding molecules. In such fluctuating environments the fundamental laws of thermodynamics that govern our macroscopic world need to be rewritten. 

An international team of researchers from Barcelona, Zurich and Vienna found that a nanoparticle trapped with laser light temporarily violates the famous second law of thermodynamics, something that is impossible on human time and length scale. They report about their results in the latest issue of the prestigious scientific journal Nature Nanotechnology.

Surprises at the nanoscale

Watching a movie played in reverse often makes us laugh because unexpected and mysterious things seem to happen: glass shards lying on the floor slowly start to move towards each other, magically assemble and suddenly an intact glass jumps on the table where it gently gets to a halt. Or snow starts to from a water puddle in the sun, steadily growing until an entire snowman appears as if molded by an invisible hand. When we see such scenes, we immediately realize that according to our everyday experience something is out of the ordinary. Indeed, there are many processes in nature that can never be reversed. The physical law that captures this behavior is the celebrated second law of thermodynamics, which posits that the entropy of a system – a measure for the disorder of a system – never decreases spontaneously, thus favoring disorder (high entropy) over order (low entropy).

However, when we zoom into the microscopic world of atoms and molecules, this law softens up and looses its absolute strictness. Indeed, at the nanoscale the second law can be fleetingly violated. On rare occasions, one may observe events that never happen on the macroscopic scale such as, for example heat transfer from cold to hot which is unheard of in our daily lives. Although on average the second law of thermodynamics remains valid even in nanoscale systems, scientists are intrigued by these rare events and are investigating the meaning of irreversibility at the nanoscale.

Nanoparticles in laser traps

Recently, a team of physicists of the University of Vienna, the Institute of Photonic Sciences in Barcelona and the Swiss Federal Institute of Technology in Zürich succeeded in accurately predicting the likelihood of events transiently violating the second law of thermodynamics. They immediately put the mathematical fluctuation theorem they derived to the test using a tiny glass sphere with a diameter of less than 100 nm levitated in a trap of laser light. Their experimental set-up allowed the research team to capture the nano-sphere and hold it in place, and, furthermore, to measure its position in all three spatial directions with exquisite precision. In the trap, the nano-sphere rattles around due to collisions with surrounding gas molecules. 

By a clever manipulation of the laser trap the scientists cooled the nano-sphere below the temperature of the surrounding gas and, thereby, put it into a non-equilibrium state. They then turned off the cooling and watched the particle relaxing to the higher temperature through energy transfer from the gas molecules. The researchers observed that the tiny glass sphere sometimes, although rarely, does not behave as one would expect according to the second law: the nano-sphere effectively releases heat to the hotter surroundings rather than absorbing the heat. The theory derived by the researchers to analyze the experiment confirms the emerging picture on the limitations of the second law on the nanoscale.

Nanomachines out of equilibrium

The experimental and theoretical framework presented by the international research team in the renowned scientific journal Nature Nanotechnology has a wide range of applications. Objects with sizes in the nanometer range, such as the molecular building blocks of living cells or nanotechnological devices, are continuously exposed to a random buffeting due to the thermal motion of the molecules around them. As miniaturization proceeds to smaller and smaller scales nanomachines will experience increasingly random conditions. Further studies will be carried out to illuminate the fundamental physics of nanoscale systems out of equilibrium. The planned research will be fundamental to help us understand how nanomachines perform under these fluctuating conditions.

Original publication in Nature Nanotechnology

Dynamic Relaxation of a Levitated Nanoparticle from a Non-Equilibrium Steady State. Jan Gieseler, Romain Quidant, Christoph Dellago, and Lukas Novotny. Nature Nanotechnology AOP, February 28, 2014. DOI: 10.1038/NNANO.2014.40

Source: http://medienportal.univie.ac.at//presse/aktuelle-pressemeldungen/detailansicht/artikel/never-say-never-in-the-nano-world/

Bioceramic armor: tough as nails, yet clear enough to read through

A Transmission Electron Microscope (TEM) image of the region surrounding an indentation the researchers made in a piece of shell from Placuna placenta. The image shows the localization of damage to the area immediately surrounding the stress. Image: Ling Li

The shells of a sea creature, the mollusk Placuna placenta, are not only exceptionally tough, but also clear enough to read through. Now, researchers at MIT have analyzed these shells to determine exactly why they are so resistant to penetration and damage — even though they are 99 percent calcite, a weak, brittle mineral.

The shells’ unique properties emerge from a specialized nanostructure that allows optical clarity, as well as efficient energy dissipation and the ability to localize deformation, the researchers found. The results are published this week in the journal Nature Materials, in a paper co-authored by MIT graduate student Ling Li and professor Christine Ortiz.

Ortiz, the Morris Cohen Professor of Materials Science and Engineering (and MIT’s dean for graduate education), has long analyzed the complex structures and properties of biological materials as possible models for new, even better synthetic analogs.

Engineered ceramic-based armor, while designed to resist penetration, often lacks the ability to withstand multiple blows, due to large-scale deformation and fracture that can compromise its structural integrity, Ortiz says. In transparent armor systems, such deformation can also obscure visibility.

Creatures that have evolved natural exoskeletons — many of them ceramic-based — have developed ingenious designs that can withstand multiple penetrating attacks from predators. The shells of a few species, such as Placuna placenta, are also optically clear.

To test exactly how the shells — which combine calcite with about 1 percent organic material — respond to penetration, the researchers subjected samples to indentation tests, using a sharp diamond tip in an experimental setup that could measure loads precisely. They then used high-resolution analysis methods, such as electron microscopy and diffraction, to examine the resulting damage.

The material initially isolates damage through an atomic-level process called “twinning” within the individual ceramic building blocks: A crystal breaks up into a pair of mirror-image regions that share a common boundary, rather like a butterfly's wings. This twinning process occurs all around the stressed region, helping to form a kind of boundary that keeps the damage from spreading outward.

The MIT researchers found that twinning then activates “a series of additional energy-dissipation mechanisms … which preserve the mechanical and optical integrity of the surrounding material,” Li says. This produces a material that is 10 times more efficient in dissipating energy than the pure mineral, Li adds.

The properties of this natural armor make it a promising template for the development of bio-inspired synthetic materials for both commercial and military applications — such as eye and face protection for soldiers, windows and windshields, and blast shields, Ortiz says.

Huajian Gao, a professor of engineering at Brown University who was not involved in this research, calls it “an excellent and elegant piece of work.” He says it “successfully demonstrates the effectiveness of nanoscale deformation twins in energy dissipation in bioceramics, and should be able to inspire and guide the development of manmade ceramic materials.” He adds, “As a first-of-its-kind [demonstration of] the effectiveness of deformation twins in natural materials, this work should have huge practical impact.”

The work was supported by the National Science Foundation; the U.S. Army Research Office through the MIT Institute for Soldier Nanotechnologies; the National Security Science and Engineering Faculty Fellowships Program; and the Office of the Assistant Secretary of Defense for Research and Engineering.

Source: http://newsoffice.mit.edu/2014/tough-nails-yet-clear-enough-read-through

Rainbow-catching waveguide could revolutionize energy technologies

The image shows a “multilayered waveguide taper array.”
The different wavelengths, or colors, are absorbed
by the waveguide tapers (thimble-shaped structures)
that together form an array.

By slowing and absorbing certain wavelengths of light, engineers open new possibilities in solar power, thermal energy recycling and stealth technology.

More efficient photovoltaic cells. Improved radar and stealth technology. A new way to recycle waste heat generated by machines into energy.

All may be possible due to breakthrough photonics research at the University at Buffalo.

The work, published March 28 in the journal Scientific Reports, explores the use of a nanoscale microchip component called a “multilayered waveguide taper array” that improves the chip’s ability to trap and absorb light.

Unlike current chips, the waveguide tapers (the thimble-shaped structures pictured above) slow and ultimately absorb each frequency of light at different places vertically to catch a “rainbow” of wavelengths, or broadband light.

The paper, “Broadband absorption engineering of hyperbolic metafilm patterns,” is here: http://bit.ly/1g72Is5.

“We previously predicted the multilayered waveguide tapers would more efficiently absorb light, and now we’ve proved it with these experiments,” says lead researcher Qiaoqiang Gan, PhD, UB assistant professor of electrical engineering. “This advancement could prove invaluable for thin-film solar technology, as well as recycling waste thermal energy that is a byproduct of industry and everyday electronic devices such as smartphones and laptops.”

Each multilayered waveguide taper is made of ultrathin layers of metal, semiconductors and/or insulators. The tapers absorb light in metal dielectric layer pairs, the so-called hyperbolic metamaterial. By adjusting the thickness of the layers and other geometric parameters, the tapers can be tuned to different frequencies including visible, near-infrared, mid-infrared, terahertz and microwaves.

The structure could lead to advancements in an array of fields.

For example, there is a relatively new field of advanced computing research called on-chip optical communication. In this field, there is a phenomenon known as crosstalk, in which an optical signal transmitted on one waveguide channel creates an undesired scattering or coupling effect on another waveguide channel. The multilayered waveguide taper structure array could potentially prevent this.

It could also improve thin-film photovoltaic cells, which are a promising because they are less expensive and more flexible that traditional solar cells. The drawback, however, is that they don’t absorb as much light as traditional cells. Because the multilayered waveguide taper structure array can efficiently absorb the visible spectrum, as well as the infrared spectrum, it could potentially boost the amount of energy that thin-film solar cells generate.

The multilayered waveguide taper array could help recycle waste heat generated by power plants and other industrial processes, as well as electronic devices such as televisions, smartphones and laptop computers.

“It could be useful as an ultra compact thermal-absorption, collection and liberation device in the mid-infrared spectrum,” says Dengxin Ji, a PhD student in Gan’s lab and first author of the paper.

It could even be used as a stealth, or cloaking, material for airplanes, ships and other vehicles to avoid radar, sonar, infrared and other forms of detection. “The multilayered waveguide tapers can be scaled up to tune the absorption band to a lower frequency domain and absorb microwaves efficiently,” says Haomin Song, another PhD student in Gan’s lab and the paper’s second author.

Additional authors of the paper include Haifeng Hu, Kai Liu, Xie Zeng and Nan Zhang, all PhD candidates in UB’s Department of Electrical Engineering.

The National Science Foundation sponsored the research.

Gan is a member of UB’s electrical engineering optics and photonics research group, which includes professors Alexander N. Cartwright (also UB vice president for research and economic development), Edward Furlani and Pao-Lo Liu; associate professor Natalia Litchinitser; and assistant professor Liang Feng.

The group carries out research in nanophotonics, biophotonics, hybrid inorganic/organic materials and devices, nonlinear and fiber optics, metamaterials, nanoplasmonics, optofluidics, microelectromechanical systems (MEMS), biomedical microelectromechanical systems (BioMEMs), biosensing and quantum information processing.

Source: http://www.buffalo.edu/news/releases/2014/03/049.html#sthash.9qx1DAsE.dpuf

Record quantum entanglement of multiple dimensions

An international team of researchers, with participation from the UAB, has managed to create an entanglement of 103 dimensions with only two photons. The record had been established at 11 dimensions. The discovery could represent a great advance toward the construction of quantum computers with much higher processing speeds than current ones, and toward a better encryption of information.

The states in which elementary particles, such as photons, can be found have properties which are beyond common sense. Superpositions are produced, such as the possibility of being in two places at once, which defies intuition. In addition, when two particles are entangled a connection is generated: measuring the state of one (whether they are in one place or another, or spinning one way or another, for example) affects the state of the other particle instantly, no matter how far away from each other they are.

Scientists have spent years combining both properties to construct networks of entangled particles in a state of superposition. This in turn allows constructing quantum computers capable of operating at unimaginable speeds, encrypting information with total security and conducting experiments in quantum mechanics which would be impossible to carry out otherwise.

Until now, in order to increase the "computing" capacity of these particle systems, scientists have mainly turned to increasing the number of entangled particles, each of them in a two-dimensional state of superposition: a qubit (the quantum equivalent to an information bit, but with values which can be 1, 0 or an overlap of both values). Using this method, scientists managed to entangle up to 14 particles, an authentic multitude given its experimental difficulty.

The research team was directed by Anton Zeilinger and Mario Krenn from the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences. It included the participation of Marcus Huber, researcher from the Group of Quantum Information and Quantum Phenomena from the UAB Department of Physics, as well as visiting researcher at the Institute of Photonic Sciences (ICFO). The team has advanced one more step towards improving entangled quantum systems.

In an article published this week in the journal Proceedings (PNAS), scientists described how they managed to achieve a quantum entanglement with a minimum of 103 dimensions with only two particles. "We have two Schrödinger cats which could be alive, dead, or in 101 other states simultaneously", Huber jokes, “plus, they are entangled in such a way that what happens to one immediately affects the other”. The results implies a record in quantum entanglements of multiple dimensions with two particles, established until now at 11 dimensions.

Instead of entangling many particles with a qubit of information each, scientists generated one single pair of entangled photons in which each could be in more than one hundred states, or in any of the superpositions of theses states; something much easier than entangling many particles. These highly complex states correspond to different modes in which photons may find themselves in, with a distribution of their characteristic phase, angular momentum and intensity for each mode.

"This high dimension quantum entanglement offers great potential for quantum information applications. In cryptography, for example, our method would allow us to maintain the security of the information in realistic situations, with noise and interference. In addition, the discovery could facilitate the experimental development of quantum computers, since this would be an easier way of obtaining high dimensions of entanglement with few particles", explains UAB researcher Marcus Huber.

Now that the results demonstrate that obtaining high dimension entanglements is accessible, scientists conclude in the article that the next step will be to search how they can experimentally control these hundreds of spatial modes of the photons in order to conduct quantum computer operations.

Source: http://www.uab.es/servlet/Satellite/latest-news/news-detail/record-quantum-entanglement-of-multiple-dimensions-1096476786473.html?noticiaid=1345668721554

Controlling electron spins by light

The picture shows the characteristic
spin texture (arrows) in a topological
insulator (bottom) and how it is
either probed by circularly polarized
light (top) or manipulated by it (middle).
Picture: Rader/Sachez-Barriga/HZB

Researchers of HZB manipulate the electron spin at the surface of topological insulators systematically by light

Topological insulators are considered a very promising material class for the development of future electronic devices. A research team at Helmholtz-Zentrum Berlin (HZB) has discovered, how light can be used to alter the physical properties of the electrons in these materials. Their results have just been published by the renowned journal "Physical Review X".

The material class of topological insulators has been discovered a few years ago and displays amazing properties: In their inside, they behave electrically insulating but at their surface they form metallic, conducting states. The electron spin, i. e., their intrinsic angular momentum, is playing a decisive role. Their sense of rotation is directly coupled to their direction of movement. This coupling leads not only to a high stability of the metallic property but also enables a particularly lossless electrical conduction. Topological insulators are, therefore, considered interesting and promising candidates for novel devices in information technology.

A particularly innovative approach is to try and influence the electron spin at the surface in such devices by light. HZB researcher Prof. Oliver Rader and his team have discovered by which means the spin at the surface of topological insulators can be altered. To this end, the researches performed experiments with light of various energies or wavelengths.

The wavelenght counts

At the synchrotron radiation source BESSY II they investigated the topological insulator bismuth selenide (Bi2Se3) using a method called "spin-resolved photoelectron spectroscopy" – and gained astonishing insights: They found an astonishing difference depending on whether the electrons at the surface of the material are excited with circularly polarized light in the vacuum ultraviolet (50-70 electron volts, eV) or in the ultraviolet spectral range (6 eV). They could demonstrate that they can measure the spin of the electrons without changing it at higher energies which are typically used at synchtrotron light sources. "When excited at 50 eV, the emitted electros display the typical spin texture of topological insulators", Dr. Jaime Sánchez-Barriga, who conducted the experiments, explains. "The electron spins are in the surface aligned on a circle, similarly to a traffic sign for roundabout." This is the ground state of the electrons in the surface of topological insulators."

When excited by low-energy circularly polarized photons (6 eV), the spin of the electrons moved completely out of the surface plane. Above all, they adopted the spin orientation imposed by the right- or left-circularly polarized light. This means that the spin can be systematically manipulated – depending on the light that is used. The scientists can also explain the entirely different behavior at different energies which they attribute to symmetry properties. "Our result delivers important insight how lossless currents could be induced in topological insulators", Oliver Rader explains. "This is important for the development of so-called optospintronic devices which could enormously enhance the speed at which information is stored and processed."

Source: http://www.helmholtz-berlin.de/pubbin/news_seite?nid=13952&sprache=en&typoid=49880

Engineers design ‘living materials’

An artist's rendering of a bacterial cell engineered 

to produce amyloid nanofibers that incorporate 

particles such as quantum dots 

(red and green spheres) or gold nanoparticles. 

@ YAN LIANG

Hybrid materials combine bacterial cells with nonliving elements that can conduct electricity or emit light.

Inspired by natural materials such as bone — a matrix of minerals and other substances, including living cells — MIT engineers have coaxed bacterial cells to produce biofilms that can incorporate nonliving materials, such as gold nanoparticles and quantum dots.

These “living materials” combine the advantages of live cells, which respond to their environment, produce complex biological molecules, and span multiple length scales, with the benefits of nonliving materials, which add functions such as conducting electricity or emitting light.

The new materials represent a simple demonstration of the power of this approach, which could one day be used to design more complex devices such as solar cells, self-healing materials, or diagnostic sensors, says Timothy Lu, an assistant professor of electrical engineering and biological engineering. Lu is the senior author of a paper describing the living functional materials in the March 23 issue of Nature Materials.

“Our idea is to put the living and the nonliving worlds together to make hybrid materials that have living cells in them and are functional,” Lu says. “It’s an interesting way of thinking about materials synthesis, which is very different from what people do now, which is usually a top-down approach.”

The paper’s lead author is Allen Chen, an MIT-Harvard MD-PhD student. Other authors are postdocs Zhengtao Deng, Amanda Billings, Urartu Seker, and Bijan Zakeri; recent MIT graduate Michelle Lu; and graduate student Robert Citorik.

Self-assembling materials

Lu and his colleagues chose to work with the bacterium E. coli because it naturally produces biofilms that contain so-called “curli fibers” — amyloid proteins that help E. coli attach to surfaces. Each curli fiber is made from a repeating chain of identical protein subunits called CsgA, which can be modified by adding protein fragments called peptides. These peptides can capture nonliving materials such as gold nanoparticles, incorporating them into the biofilms.

By programming cells to produce different types of curli fibers under certain conditions, the researchers were able to control the biofilms’ properties and create gold nanowires, conducting biofilms, and films studded with quantum dots, or tiny crystals that exhibit quantum mechanical properties. They also engineered the cells so they could communicate with each other and change the composition of the biofilm over time.

First, the MIT team disabled the bacterial cells’ natural ability to produce CsgA, then replaced it with an engineered genetic circuit that produces CsgA but only under certain conditions — specifically, when a molecule called AHL is present. This puts control of curli fiber production in the hands of the researchers, who can adjust the amount of AHL in the cells’ environment. When AHL is present, the cells secrete CsgA, which forms curli fibers that coalesce into a biofilm, coating the surface where the bacteria are growing.

The researchers then engineered E. coli cells to produce CsgA tagged with peptides composed of clusters of the amino acid histidine, but only when a molecule called aTc is present. The two types of engineered cells can be grown together in a colony, allowing researchers to control the material composition of the biofilm by varying the amounts of AHL and aTc in the environment. If both are present, the film will contain a mix of tagged and untagged fibers. If gold nanoparticles are added to the environment, the histidine tags will grab onto them, creating rows of gold nanowires, and a network that conducts electricity. 

‘Cells that talk to each other’

The researchers also demonstrated that the cells can coordinate with each other to control the composition of the biofilm. They designed cells that produce untagged CsgA and also AHL, which then stimulates other cells to start producing histidine-tagged CsgA.

“It’s a really simple system but what happens over time is you get curli that’s increasingly labeled by gold particles. It shows that indeed you can make cells that talk to each other and they can change the composition of the material over time,” Lu says. “Ultimately, we hope to emulate how natural systems, like bone, form. No one tells bone what to do, but it generates a material in response to environmental signals.”

To add quantum dots to the curli fibers, the researchers engineered cells that produce curli fibers along with a different peptide tag, called SpyTag, which binds to quantum dots that are coated with SpyCatcher, a protein that is SpyTag’s partner. These cells can be grown along with the bacteria that produce histidine-tagged fibers, resulting in a material that contains both quantum dots and gold nanoparticles.

These hybrid materials could be worth exploring for use in energy applications such as batteries and solar cells, Lu says. The researchers are also interested in coating the biofilms with enzymes that catalyze the breakdown of cellulose, which could be useful for converting agricultural waste to biofuels. Other potential applications include diagnostic devices and scaffolds for tissue engineering.

“I think this is really fantastic work that represents a great integration of synthetic biology and materials engineering,” says Lingchong You, an associate professor of biomedical engineering at Duke University who was not part of the research team. 
The research was funded by the Office of Naval Research, the Army Research Office, the National Science Foundation, the Hertz Foundation, the Department of Defense, the National Institutes of Health, and the Presidential Early Career Award for Scientists and Engineers.

Source: http://web.mit.edu/newsoffice/2014/engineers-design-living-materials.html

A mathematical equation that explains the behavior of nanofoams

The scientific team, made up of researchers from the Consejo Superior de Investigaciones Científicas (Spanish National Research Council) - CSIC, the Universidad Pontificia Comillas de Madrid- UPCO, and UC3M, reached this conclusion after producing and characterizing nanofoam formed by ion radiation on a silicon surface. This study, recently published in the journal, Physical Review Letters, describes the evolution of these nanostructures during the time of irradiation.

For this purpose, the scientists carried out an experiment that consisted in “bombardment” of a small silicon plate with energetic particles from a plasma. The objective was to observe how the surface of this crystal reacted to these different “attacks” from this type of ion radiation (ions are used: atoms of a gas that have lost an electron). “At the outset, we were studying other methods of erosion and looking for a rippled structure at the edge of our sample after applying this technique, but when we looked at its center we observed a cellular structure that got our attention because of its similarity to many other natural and artificial systems,” one of the authors of the study, Mario Castro, UPCO Professor, revealed.

Cellular structures that are more or less disordered can be found in many natural systems: from the hides of animals, such as a giraffe, to bath froth or beer foam, to microscopic fluid convection, basalt column landscapes or diverse crystalline materials. This particular order is also evident in artificial structures and even political ones, such as modern architecture or demarcation of provinces on maps.

“It is of interest to confirm that the same universal laws which regulate the cellular structures in other systems are also regulating at the nanoscale,” Rodolfo Cuerno from the UC3M Mathematics Department noted. “Furthermore,” he added “it is the first time that the evolution of a system of this kind is reproduced quite well by a single differential equation,” which also is applied to other systems. The validity of the model in this study means that the formation of certain self-organized patterns and the dynamics of the foam would be different manifestations of a same principle.

“The results of this study help us to understand how certain material systems evolve in the presence of an external agent, as in this case of ion radiation. In addition, there exists interest of a practical nature because of the importance of the technological applications of silicon as well as for the nanometric dimensions in which the phenomenon unfolds,” explained Luis Vázquez, from the Instituto de Ciencia de Materiales (Materials Science Institute) de Madrid at the CSIC.

The experimental observations have been carried out using an atomic force microscope, a machine with great precision. This type of microscope has enormous spatial resolution: it distinguishes variations in height up to a nanometer (the millionth part of a millimeter) and movements on a horizontal plane of up to 10 nanometers.

This research could have further future applications, since in general, methods are being sought to produce structures with nanometric dimensions for diverse uses, according to the scientists: for example, in order to obtain favorable conditions in certain catalytic chemical reactions, to optimize displacement of fluids in circuits on such small scale or in optoelectronics, to generate laser light if certain structures are sufficiently ordered.

Watch Video: https://www.youtube.com/watch?v=F2GHL0jhW84

Source: http://portal.uc3m.es/portal/page/portal/news_repository/general_news/A%20mathematical%20equation%20that%20explains%20the%20behavior%20of%20nanofo?_template=/SHARED/pl_noticias_detalle_pub_ingles

Researchers Grow Carbon Nanofibers Using Ambient Air, Without Toxic Ammonia

Researchers have shown they can grow vertically-aligned carbon nanofibers using ambient air, rather than ammonia gas. Click to enlarge image. (Image free for use. Credit: Anatoli Melechko.)

Researchers from North Carolina State University have demonstrated that vertically aligned carbon nanofibers (VACNFs) can be manufactured using ambient air, making the manufacturing process safer and less expensive. VACNFs hold promise for use in gene-delivery tools, sensors, batteries and other technologies.

Conventional techniques for creating VACNFs rely on the use of ammonia gas, which is toxic. And while ammonia gas is not expensive, it’s not free.

“This discovery makes VACNF manufacture safer and cheaper, because you don’t need to account for the risks and costs associated with ammonia gas,” says Dr. Anatoli Melechko, an adjunct associate professor of materials science and engineering at NC State and senior author of a paper on the work. “This also raises the possibility of growing VACNFs on a much larger scale.”

In the most common method for VACNF manufacture, a substrate coated with nickel nanoparticles is placed in a vacuum chamber and heated to 700 degrees Celsius. The chamber is then filled with ammonia gas and either acetylene or acetone gas, which contain carbon. When a voltage is applied to the substrate and a corresponding anode in the chamber, the gas is ionized. This creates plasma that directs the nanofiber growth. The nickel nanoparticles free carbon atoms, which begin forming VACNFs beneath the nickel catalyst nanoparticles. However, if too much carbon forms on the nanoparticles it can pile up and clog the passage of carbon atoms to the growing nanofibers.

Ammonia’s role in this process is to keep carbon from forming a crust on the nanoparticles, which would prevent the formation of VACNFs.

“We didn’t think we could grow VACNFs without ammonia or a hydrogen gas,” Melechko says. But he tried anyway.

Melechko’s team tried the conventional vacuum technique, using acetone gas. However, they replaced the ammonia gas with ambient air – and it worked. The size, shape and alignment of the VACNFs were consistent with the VACNFs produced using conventional techniques.

“We did this using the vacuum technique without ammonia,” Melechko says. “But it creates the theoretical possibility of growing VACNFs without a vacuum chamber. If that can be done, you would be able to create VACNFs on a much larger scale.”

Melechko also highlights the role of two high school students involved in the work: A. Kodumagulla and V. Varanasi, who are lead authors of the paper. “This discovery would not have happened if not for their approach to the problem, which was free from any preconceptions,” Melechko says. “I think they’re future materials engineers.”

The paper, “Aerosynthesis: Growth of Vertically-aligned Carbon Nanofibres with Air DC Plasma,” is published online inNanomaterials and Nanotechnology. Co-authors include former NC State Ph.D. student Dr. R.C. Pearce; NC State Ph.D. student W.C. Wu; Dr. Joseph Tracy, an associate professor of materials science and engineering at NC State; and D.K. Hensley and T.E. McKnight of Oak Ridge National Laboratory. The work was partially supported by National Science Foundation grant DMR-1056653.

Source: http://news.ncsu.edu/releases/wms-melechko-cnf-ambient-2014/

Cold Chaos

Chaotic motion and complex chemistry might lurk at nano-Kelvin temperatures 

At sub-micro-kelvin temperatures atoms or molecules move so slowly that it is better to think of them as spread-out, wavelike things a micron or more across, many times larger than any putative bond length (typically sub-nanometer in size) that would characterize bound molecules. Experiments over several years have shown that collisions and chemical reactions do take place---surprising, considering the scant energy available to the reactants---but under the sway of wavelike, and not particle-like, considerations.

A new experiment conducted at the University of Innsbruck in Austria adds a new twist to this picture. There swarms of erbium atoms were held in a special trap and cooled to a temperature of about 300 nK. Er-166 and Er-168 are boson species, which means that these atoms can clump very closely together in a single quantum state known as Bose Einstein Condensate. But because of the erbium’s large intrinsic magnetic moment, inter-atom interactions are strong. By imposing an extra magnetic field---the better to excite Feshbach resonances, which are delicate bound molecular states---the types of collisions among atoms can be controlled. The Innsbruck physicists expected that the use of Er atoms would give rise to a number of such resonance states.

It came as a great surprise to them, therefore, to observe a hundred and more resonances rather than a dozen or so. The resonances were so great in number and so densely packed that the researchers deduce that a form of quantum chaos is at work here.

Quantum chaos, as a research subject, is only a few decades old. It features systems of particles exhibiting both quantum and chaotic behavior---chaos being usually thought of as classical-physics phenomenon. A classical system is generally deemed to be chaotic if its future course is described by nonlinear equations and if predictions of future behavior are exquisitely dependent our knowledge of the initial conditions of the system. The signature of quantum chaos is somewhat different: a dense set of energy levels with a special kind of spacing between levels.

The Innsbruck experiment represents the first instance of quantum chaos observed for ultracold atomic collisions. The results are reported in Nature in a paper published online on 12 March (1). JQI scientist Paul Julienne, as an expert on particle collisions at cold temperatures, was asked to write a commentary on this appearance of cold chaos. His essay appears in the same issue of Nature (2).

MORE THAN THEY BARGAINED FOR

As the strength of the external magnetic field is varied in the Innsbruck apparatus, new collision conditions become available. At certain field values the nearly-at-rest atoms come into resonance, and form weakly bound molecules; these molecules, when struck by a third atom, form a threefold object which can no longer be held by the trap. Thus a decrease in the atomic population marks the location of the resonance energies.

The hundred resonances actually observed while varying magnetic field across a fixed range was more than the researchers had expected and far more than had ever been observed before---typically in studies of alkali elements such as cesium. The bad news was that studying atomic collisions at these temperatures suddenly got a lot more complicated.

The good news is that all these new collision alternatives might open up new avenues in low-temperature physics research. “Part of the power and the beauty of cold atom physics,” said Julienne in his News & Views essay in Nature, “is that the scattering length can be made to take on any value by tuning a magnetic field close to a Feshbach resonance. Its value controls the two-body, few-body, and many-body physics of ultracold quantum matter. Thus, controlling the field makes the system dance to our tune.”

Still more complex than erbium atoms are molecules. They are currently hard to cool to nK temperatures because of their great complexity; energy continues to lurk in various rotational and vibrational modes of molecules. But molecular cooling is advancing and soon molecular chemistry at the coldest temperatures might be doable. It would not be surprising to find a similar kind of chaos at play there.

Source: http://jqi.umd.edu/news/cold-chaos#sthash.b1us507R.dpuf

Dance of the skyrmions

Skyrmions are swirling patterns in the magnetic
orientations of atoms (arrows) that can be arranged in ordered patterns.© 2014 M. Mochizuki et al.

Turning magnetic whirls using an electron beam

Skyrmions are ‘whirls’ in the magnetization of certain magnetic materials that show promise for future electronics and spintronics applications if they can be harnessed and manipulated. Naoto Nagaosa and colleagues from the RIKEN Center for Emergent Matter Science, in collaboration with Masahito Mochizuki from Aoyama Gakuin University and other researchers, have discovered that skyrmions can be manipulated thermally using an electron beam.

Each atom in a ferromagnetic material acts like a tiny bar magnet. Although all of the magnets usually point in the same direction, under certain conditions some can tilt away from their neighbors.

Skyrmions are whirls within this ‘sea’ of atomic bar magnets. They are usually free to drift around, but an external magnetic field can lock them into regular patterns (Fig. 1) in crystals of manganese silicide and copper oxoselenite.

While studying these materials by transmission electron microscopy, Nagaosa and his colleagues were surprised to find that the skyrmion patterns rotated continuously, completing a full revolution every few seconds. Under a more intense electron beam, they rotated faster.

In searching for a cause, the researchers quickly ruled out the minuscule magnetic field of the electron beam, along with the electric current that the beam might induce. Instead, they concluded that the heating effect of the electron beam was responsible for the skyrmion dance.

Noting that the rotation always occurred in a clockwise direction, the researchers then developed a mathematical model to describe the motion. Their model accurately simulated the observed motion, revealing that the rotation is driven solely by the thermal gradient that runs outward from the center of the sample.

The heat is carried outward by small ripples in the magnetic fabric of the material known as magnons. As the magnons flow, they bounce off the swirling skyrmions—a phenomenon referred to as the magnon Hall effect—and force them to rotate in a clockwise direction. Reversing the external magnetic field switches the rotation to an anticlockwise direction. The discovery is likened to Feynman’s ratchet, essentially a tiny engine driven by heat that was described by the physicist Richard Feynman in the 1960s.

Nagaosa says that the findings could aid the development of low-energy memory and logic devices where information is encoded by skyrmions. Previously, electric currents have been used to manipulate skyrmions in metallic magnets. Heat could now be used to drive their motion in electrically insulating magnets, which tend to have lower energy dissipation and would better preserve the high-density data held by the skyrmions.

Source: http://www.rikenresearch.riken.jp/eng/research/7739

Thermal conductance can be controlled like waves using nanostructures

A phononic crystal device fabricated in silicon nitride (SiN) 
using electron-beam lithography. Green = SiN, blue and 
red lines = aluminium and copper heaters and 
thermometers, and black areas = holes.
@ University of Jyväskylä 

Thermal conduction is a familiar everyday phenomenon. In a hot sauna, for instance, you can sit comfortably on a wooden bench that has a temperature of 100C (212F), but if you touch a metallic nail with the same temperature, you will hurt yourself. 

The difference of these two experiences is due to the fact that some materials, such as metals, conduct heat well, whereas some other materials, such as wood, do not. 

It is therefore commonly thought that thermal conductance is simply a materials parameter. 

Now, researchers at the University of Jyväskylä, Finland, led by Professor Ilari Maasilta, have demonstrated for the first time that it is possible to change the thermal conductance of a material by tuning the wave-like properties of heat flow, by orders of magnitude, using nanostructuring. The results were published on 19 March in Nature Communications. The research was funded by the Academy of Finland.

Theoretically, it has been long known that heat can be thought of as a collection of the motion of many different kinds of waves; atoms vibrate, but not randomly. These little wave packets follow the laws of quantum mechanics, which leads to that only certain ranges of wavelengths are excited, depending on the temperature. The situation is very similar in thermal radiation, where the wave packets or particles are photons (visible light also consists of photons). In the case of material vibrations, the particles are called phonons, theoretically introduced by Albert Einstein over a hundred years ago.

The wave nature of phonons has never before been used to control heat transport. Until now, thermal transport has been engineered by adding one material (e.g. nanoparticles) inside another, or by changing the roughness of the surfaces. In both cases, phonons scatter more and thus carry heat less effectively. Now, the University of Jyväskylä study shows that it is possible to change the phonon thermal conductance strongly based on the wave properties of the phonons. 

This was achieved by fabricating a nanoscale mesh structure (a so-called phononic crystal), whose period is of the same order as the wavelength of the phonons that carry heat, about one micrometre in this case. The phonon waves interact strongly with the phononic crystal structure and change their speed by almost an order of magnitude. Because the waves move much more slowly, the thermal conductance is significantly reduced. The experiment was performed at a temperature near absolute zero in order to increase the wavelength of the thermal phonons to a length scale, where fabrication using common nanofabricating tools is possible.

In the future, the demonstrated concept can possibly be used in many ways. At low temperatures, there are direct applications in the development of ultrasensitive radiation detectors, where the control of heat transport is essential. This kind of applied research is also conducted in Professor Maasilta’s group. In addition, if the demonstrated concept can be made to work at room temperature range, it might have an impact on the future development of more efficient thermoelectric devices, which can be harnessed to generate electricity from waste heat.

Source: http://www.aka.fi/en-GB/A/Academy-of-Finland/Media-services/Releases1/Thermal-conductance-can-be-controlled-like-waves-using-nanostructures/