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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