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These images of a mouse's blood vessels show the difference in resolution between traditional near-infrared fluorescence imaging (top) and Stanford's new NIR-II technique (bottom). (Image Credit: Stanford University)

Stanford University scientists have developed a fluorescence imaging technique that allows them to view the pulsing blood vessels of living animals with unprecedented clarity. Compared with conventional imaging techniques, the increase in sharpness is akin to wiping fog off your glasses. The technique, called near infrared-II imaging, or NIR-II, involves first injecting water-soluble carbon nanotubes into the living subject's bloodstream. The researchers then shine a laser (its light is in the near-infrared range, a wavelength of about 0.8 micron) over the subject; in this case, a mouse. The light causes the specially designed nanotubes to fluoresce at a longer wavelength of 1-1.4 microns, which is then detected to determine the blood vessels' structure. That the nanotubes fluoresce at substantially longer wavelengths than conventional imaging techniques is critical in achieving the stunningly clear images of the tiny blood vessels: longer wavelength light scatters less, and thus creates sharper images of the vessels. Another benefit of detecting such long wavelength light is that the detector registers less background noise since the body does not does not produce autofluorescence in this wavelength range. In addition to providing fine details, the technique – developed by Stanford scientists Hongjie Dai, professor of chemistry; John Cooke, professor of cardiovascular medicine; and Ngan Huang, acting assistant professor of cardiothoracic surgery – has a fast image acquisition rate, allowing researchers to measure blood flow in near real time. The ability to obtain both blood flow information and blood vessel clarity was not previously possible, and will be particularly useful in studying animal models of arterial disease, such as how blood flow is affected by the arterial blockages and constrictions that cause, among other things, strokes and heart attacks.

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Seven-atom rings (in red) at the transition from graphene to nanotube make a new hybrid material from Rice University a seamless conductor. The hybrid may be the best electrode interface material possible for many energy storage and electronics applications. (Image Credit: Tour Group/Rice University)

A seamless graphene/nanotube hybrid created at Rice University may be the best electrode interface material possible for many energy storage and electronics applications. Led by Rice chemist James Tour, researchers have successfully grown forests of carbon nanotubes that rise quickly from sheets of graphene to astounding lengths of up to 120 microns. A house on an average plot with the same aspect ratio would rise into space. That translates into a massive amount of surface area, the key factor in making things like energy-storing supercapacitors. The Rice hybrid combines two-dimensional graphene, which is a sheet of carbon one atom thick, and nanotubes into a seamless three-dimensional structure. The bonds between them are covalent, which means adjacent carbon atoms share electrons in a highly stable configuration. The nanotubes aren’t merely sitting on the graphene sheet; they become a part of it. “Many people have tried to attach nanotubes to a metal electrode and it’s never gone very well because they get a little electronic barrier right at the interface,” Tour said. “By growing graphene on metal (in this case copper) and then growing nanotubes from the graphene, the electrical contact between the nanotubes and the metal electrode is ohmic. That means electrons see no difference, because it’s all one seamless material. "This gives us, effectively, a very high surface area of more than 2,000 square meters per gram of material. It’s a huge number,” said Tour. Tour said proof of the material’s hybrid nature lies in the seven-membered rings at the transition from graphene to nanotube, a structure predicted by theory for such a material and now confirmed through electron microscope images with subnanometer resolution.

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The graspers were coated with magnetic particles, which could allow researchers to retrieve the devices in the field by using a magnet. (Photo Credit: Manuel Ochoa, Purdue University)

Scotch tape, a versatile household staple and a mainstay of holiday gift-wrapping, may have a new scientific application as a shape-changing "smart material." Researchers used a laser to form slender half-centimeter-long fingers out of the tape. When exposed to water, the four wispy fingers morph into a tiny robotic claw that captures water droplets. The innovation could be used to collect water samples for environmental testing, said Babak Ziaie, a Purdue University professor of electrical and computer engineering and biomedical engineering. The Scotch tape - made from a cellulose-acetate sheet and an adhesive - is uniquely suited for the purpose. "It can be micromachined into different shapes and works as an inexpensive smart material that interacts with its environment to perform specific functions," he said. Doctoral student Manuel Ochoa came up with the idea. While using tape to collect pollen, he noticed that it curled when exposed to humidity. The cellulose-acetate absorbs water, but the adhesive film repels water. A laser was used to machine the tape to a tenth of its original thickness, enhancing this curling action. The researchers coated the graspers with magnetic nanoparticles so that they could be collected with a magnet. "Say you were sampling for certain bacteria in water," Ziaie said. "You could drop a bunch of these and then come the next day and collect them.”

Experiments at Purdue's Birck Nanotechnology Center were conducted by Ochoa, doctoral student Girish Chitnis and Ziaie. The grippers close underwater within minutes and can sample one-tenth of a milliliter of liquid. "Although brittle when dry, the material becomes flexible when immersed in water and is restored to its original shape upon drying, a crucial requirement for an actuator material because you can use it over and over," Ziaie said. "Various microstructures can be carved out of the tape by using laser machining. This fabrication method offers the capabilities of rapid prototyping and batch processing without the need for complex clean-room processes."

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Flexible circuit fabricated in the Kagan lab. (Photo Credit University of Pennsylvania: David Kim and Yuming Lai)

Electronic circuits are typically integrated in rigid silicon wafers, but flexibility opens up a wide range of applications. In a world where electronics are becoming more pervasive, flexibility is a highly desirable trait, but finding materials with the right mix of performance and manufacturing cost remains a challenge. Now a team of researchers from the University of Pennsylvaniahas shown that nanoscale particles, or nanocrystals, of the semiconductor cadmium selenide can be "printed" or "coated" on flexible plastics to form high-performance electronics. Besides speed, another advantage cadmium selenide nanocrystals have over amorphous silicon is the temperature at which they are deposited. Whereas amorphous silicon uses a process that operates at several hundred degrees, cadmium selenide nanocrystals can be deposited at room temperature and annealed at mild temperatures, opening up the possibility of using more flexible plastic foundations.

Another innovation that allowed the researchers to use flexible plastic was their choice of ligands, the chemical chains that extend from the nanocrystals’ surfaces and helps facilitate conductivity as they are packed together into a film. Because the nanocrystals are dispersed in an ink-like liquid, multiple types of deposition techniques can be used to make circuits. In their study, the researchers used spincoating, where centrifugal force pulls a thin layer of the solution over a surface, but the nanocrystals could be applied through dipping, spraying or ink-jet printing as well. On a flexible plastic sheet a bottom layer of electrodes was patterned using a shadow mask — essentially a stencil — to mark off one level of the circuit. The researchers then used the stencil to define small regions of conducting gold to make the electrical connections to upper levels that would form the circuit. An insulating aluminum oxide layer was introduced and a 30-nanometer layer of nanocrystals was coated from solution. Finally, electrodes on the top level were deposited through shadow masks to ultimately form the circuits.

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UT Dallas researchers have made artificial muscles from carbon nanotube yarns that have been infiltrated with paraffin wax and twisted until coils form along their length. The diameter of this coiled yarn is about twice the width of a human hair. (Source UT Dallas)

New artificial muscles made from nanotech yarns and infused with paraffin wax can lift more than 100,000 times their own weight and generate 85 times more mechanical power than the same size natural muscle, according to scientists at The University of Texas at Dallas and their international team from Australia, China, South Korea, Canada and Brazil. The artificial muscles are yarns constructed from carbon nanotubes, which are seamless, hollow cylinders made from the same type of graphite layers found in the core of ordinary pencils. Individual nanotubes can be 10,000 times smaller than the diameter of a human hair, yet pound-for-pound, can be 100 times stronger than steel.The new artificial muscles are made by infiltrating a volume-changing “guest,” such as the paraffin wax used for candles, into twisted yarn made of carbon nanotubes. Heating the wax-filled yarn, either electrically or using a flash of light, causes the wax to expand, the yarn volume to increase, and the yarn length to contract. The combination of yarn volume increase with yarn length decrease results from the helical structure produced by twisting the yarn.  A child’s finger cuff toy, which is designed to trap a person’s fingers in both ends of a helically woven cylinder, has an analogous action. To escape, one must push the fingers together, which contracts the tube’s length and expands its volume and diameter. In the video below, Dr. Ray Baughman of UT Dallas’s NanoTech Institute explains carbon nanotube yarns.

 

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Stanford chemical engineering Professor Zhenan Bao (Photo: Linda A. Cicero / Stanford News Service)

A team of Stanford University chemists and engineers has created the first synthetic material that is both sensitive to touch and capable of healing itself quickly and repeatedly at room temperature. The advance could lead to smarter prosthetics or resilient personal electronics that repair themselves.Not only is our skin sensitive – sending the brain precise information about pressure and temperature – but it also heals efficiently to preserve a protective barrier against the world. Combining these two features in a single synthetic material presented an exciting challenge for Stanford University chemical engineering Professor Zhenan Bao and her team. Now, they have succeeded in making the first material that can both sense subtle pressure and heal itself when torn or cut. The researchers succeeded by combining two ingredients to get what Bao calls "the best of both worlds" – the self-healing ability of a plastic polymer and the conductivity of a metal.

The next step was to see how well the material could restore both its mechanical strength and its electrical conductivity after damage. The researchers took a thin strip of the material and cut it in half with a scalpel. After gently pressing the pieces together for a few seconds, the researchers found the material gained back 75 percent of its original strength and electrical conductivity. The material was restored close to 100 percent in about 30 minutes. What's more, the same sample could be cut repeatedly in the same place. After 50 cuts and repairs, a sample withstood bending and stretching just like the original.

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The Bao group's all-carbon solar cell consists of a photoactive layer, which absorbs sunlight, sandwiched between two electrodes.(Image Credit: Stanford University)

Stanford University scientists have built the first solar cell made entirely of carbon, a promising alternative to the expensive materials used in photovoltaic devices.  "Carbon has the potential to deliver high performance at a low cost," said study senior author Zhenan Bao, a professor of chemical engineering at Stanford.  "To the best of our knowledge, this is the first demonstration of a working solar cell that has all of the components made of carbon. This study builds on previous work done in our lab." Unlike rigid silicon solar panels that adorn many rooftops, Stanford's thin film prototype is made of carbon materials that can be coated from solution. "Perhaps in the future we can look at alternative markets where flexible carbon solar cells are coated on the surface of buildings, on windows or on cars to generate electricity," Bao said. The coating technique also has the potential to reduce manufacturing costs. The Bao group's experimental solar cell consists of a photoactive layer, which absorbs sunlight, sandwiched between two electrodes.  In a typical thin film solar cell, the electrodes are made of conductive metals and indium tin oxide (ITO). "Materials like indium are scarce and becoming more expensive as the demand for solar cells, touchscreen panels and other electronic devices grows," Bao said.  "Carbon, on the other hand, is low cost and Earth-abundant." For the study, Bao and her colleagues replaced the silver and ITO used in conventional electrodes with graphene – sheets of carbon that are one atom thick –and single-walled carbon nanotubes that are 10,000 times narrower than a human hair. "Carbon nanotubes have extraordinary electrical conductivity and light-absorption properties," Bao said.  For the active layer, the scientists used material made of carbon nanotubes and "buckyballs" – soccer ball-shaped carbon molecules just one nanometer in diameter.  The research team recently filed a patent for the entire device.

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Yang's group has a new way of combining the structural color and superhydrophobicity found in butterfly wings. (Credit: AFM, University of Pennsylvania)

The colors of a butterfly’s wings are unusually bright and beautiful and are the result of an unusual trait; the way they reflect light is fundamentally different from how color works most of the time. A team of researchers at the University of Pennsylvania has found a way to generate this kind of “structural color” that has the added benefit of another trait of butterfly wings: super-hydrophobicity, or the ability to strongly repel water. The research was led by Shu Yang, associate professor in the Department of Materials Science and Engineering at the University of Pennsylvania’s School of Engineering and Applied Science. When water lands on a hydrophobic surface, its roughness reduces the effective contact area between water and a solid area where it can adhere, resulting in an increase of water contact angle and water droplet mobility on such surface. Yang’s method begins with a non-conventional photolithography technique, holographic lithography, where a laser creates a cross-linked 3D network from a material called a photoresist. The photoresist material in the regions that are not exposed to the laser light are later removed by a solvent, leaving the “holes” in the 3D lattice that provides structural color. Instead of using nanoparticles or plasma etching, Yang’s team was able to add the desired nano-roughness to the structures by simply changing solvents after washing away the photoresist. The trick was to use a poor solvent; the better a solvent is, the more it tries to maximize the contact with the material. Bad solvents have the opposite effect, which the team used to its advantage at the end of the photolithography step. Both superhydrophobicity and structural color are in high demand for a variety of applications. Materials with structural color could be used in as light-based analogs of semiconductors, for example, for light guiding, lasing and sensing. As they repel liquids, superhydrophobic coatings are self-cleaning and waterproof. Since optical devices are highly dependent on their degree of light transmission, the ability to maintain the device surface’s dryness and cleanliness will minimize the energy consumption and negative environmental impact without the use of intensive labors and chemicals. Yang has recently received a grant to develop such coatings for solar panels.

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Nanoscale plasmonic antennas Nanoscale plasmonic antennas called nonamers placed on graphene have the potential to create electronic circuits by hitting them with light at particular frequencies, according to researchers at Rice University. The positively and negatively doped graphene can be prompted to form phantom circuits on demand.(Image Credit: Rice University)

Researchers are doping graphene with light in a way that could lead to the more efficient design and manufacture of electronics, as well as novel security and cryptography devices. Nanoscale plasmonic antennas called nonamers placed on graphene have the potential to create electronic circuits by hitting them with light at particular frequencies, according to the researchers. The positively and negatively doped graphene can be prompted to form phantom circuits on demand. Manufacturers chemically dope silicon to adjust its semiconducting properties. But this involves a novel concept: plasmon-induced doping of graphene, the ultrastrong, highly conductive, single-atom-thick form of carbon. That could facilitate the instant creation of circuitry – optically induced electronics – on graphene patterned with plasmonic antennas that can manipulate light and inject electrons into the material to affect its conductivity. The research incorporates both theoretical and experimental work to show the potential for making simple, graphene-based diodes and transistors on demand. The work was done by Rice University scientists Naomi Halas, Stanley C. Moore Professor in Electrical and Computer Engineering, a professor of biomedical engineering, chemistry, physics and astronomy and director of the Laboratory for Nanophotonics; and Peter Nordlander, professor of physics and astronomy and of electrical and computer engineering; physicist Frank Koppens of the Institute of Photonic Sciences in Barcelona, Spain; lead author Zheyu Fang, a postdoctoral researcher at Rice; and their colleagues. Researchers have investigated many strategies for doping graphene, including attaching organic or metallic molecules to its hexagonal lattice. Making it selectively – and reversibly – amenable to doping would be like having a graphene blackboard upon which circuitry can be written and erased at will, depending on the colors, angles or polarization of the light hitting it.

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Rice University researchers have come up with a set of calculations to predict how graphene grows in the process known as chemical vapor deposition. The graph set against an illustration of graphene growing on a nickel catalyst shows the initial energy barrier a carbon atom must overcome to join the bloom; subsequent atoms face an ever-smaller energy barrier until the process begins again for the next line.(Image Credit: Vasilii Artyukhov/Rice Universityxas)

Like tiny ships finding port in a storm, carbon atoms dock with the greater island of graphene in a predictable manner. But until recent research by scientists at Rice University, nobody had the tools to make that kind of prediction. Electric current shoots straight across a sheet of defect-free graphene with almost no resistance, a feature that makes the material highly attractive to engineers who would use it in things like touchscreens and other electronics, said Rice theoretical physicist Boris Yakobson. To examine exactly what happens at the atomic level, Yakobson and his Rice colleagues took a close look at the now-common process called chemical vapor deposition (CVD), in which a carbon source heated in a furnace is exposed to a metal catalyst to form graphene, a single-atom layer of pure carbon. Yakobson, Rice’s Karl F. Hasselmann Professor of Mechanical Engineering and Materials Science and professor of chemistry, and his colleagues calculated the energies of individual atoms as they accrete to form graphene at the “nanoreactor” dock where the carbon vapor and catalyst meet. With the help of theories long applied to crystal growth, they determined that, at equilibrium, some patterns of graphene are more likely to form than others depending on the catalyst used.

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