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A bright burst of light has welded these nanowires together.(Photo: Stanford School of Engineering)

The ability to weld nano-sized wires with just a blast of light could lead to advances in electronics and solar applications.  One area of intensive research at the nanoscale is the creation of electrically conductive meshes made of metal nanowires. Promising exceptional electrical throughput, low cost, and easy processing, engineers foresee a day when such meshes are common in new generations of touch screens, video displays, light-emitting diodes, and thin-film solar cells.  Standing in the way, however, is a major engineering hurdle: In processing, these delicate meshes must be heated or pressed to unite the crisscross pattern of nanowires that form the mesh, and are thereby damaged. However, engineers at Stanford University have demonstrated a promising nanowire welding technique that harnesses plasmonics to fuse wires with a simple blast of light. At the heart of the technique is the physics of plasmonics, the interaction of light and metal in which the light flows across the surface of the metal in waves, like water on the beach. “When two nanowires lay criss-crossed, we know that light will generate plasmon waves at the place where the two nanowires meet, creating a hot spot. The beauty is that the hot spots exist only when the nanowires touch, not after they have fused. The welding stops itself. It’s self-limiting,” explains senior Mark Brongersma, associate professor of materials science and engineering at Stanford and an expert in plasmonics. “The rest of the wires and, just as importantly, the underlying material are unaffected,” notes Michael McGehee, also an associate professor of materials science and engineering. “This ability to heat with precision greatly increases the control, speed, and energy efficiency of nanoscale welding.”

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Synthesis and characterization of gold nanowires. TEM images showing the assembly of AuNPs on the SWNTs (after 30 min, left) and their welding into AuNWs (after 120 min, right).(Image Source: University of Pittsburgh)

Researchers at theUniversity of Pittsburgh have coaxed gold into nanowires as a way of creating an inexpensive material for detecting poisonous gases found in natural gas. Along with colleagues at the National Energy Technology Laboratory (NETL), Alexander Star, associate professor of chemistry in Pitt's Kenneth P. Dietrich School of Arts and Sciences and principal investigator of the research project, developed a self-assembly method that uses scaffolds (a structure used to hold up or support another material) to grow gold nanowires. “The most common methods to sense gases require bulky and expensive equipment,” says Star. “Chip-based sensors that rely on nanomaterials for detection would be less expensive and more portable as workers could wear them to monitor poisonous gases, such as hydrogen sulfide.” Star and his research team determined gold nanomaterials would be ideal for detecting hydrogen sulfide owing to gold’s high affinity for sulfur and unique physical properties of nanomaterials. They experimented with carbon nanotubes and graphene—an atomic-scale chicken wire made of carbon atoms—and used computer modeling, X-ray diffraction, and transmission electron microscopy to study the self-assembly process. They also tested the resulting materials’ responses to hydrogen sulfide. “To produce the gold nanowires, we suspended nanotubes in water with gold-containing chloroauric acid,” says Star. “As we stirred and heated the mixture, the gold reduced and formed nanoparticles on the outer walls of the tubes. The result was a highly conductive jumble of gold nanowires and carbon nanotubes.” To test the nanowires’ ability to detect hydrogen sulfide, Star and his colleagues cast a film of the composite material onto a chip patterned with gold electrodes. The team could detect gas at levels as low as 5ppb (parts per billion)—a detection level comparable to that of existing sensing techniques. Additionally, they could detect the hydrogen sulfide in complex mixtures of gases simulating natural gas.

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Research from Rice University and the University of California at Berkeley may give science and industry a new way to manipulate graphene, the new material expected to play a role in advanced electronic, mechanical and thermal applications. When graphene – a one-atom thick sheet of carbon – rips under stress, it does so in a unique way that puzzled scientists who first observed the phenomenon. Instead of tearing randomly like a piece of paper would, it seeks the path of least resistance and creates new edges that give the material desirable qualities. Because graphene's edges determine its electrical properties, finding a way to control them will be significant, said Boris Yakobson, Rice's Karl F. Hasselmann Professor of Mechanical Engineering and Materials Science and professor of chemistry. Yakobson and Vasilii Artyukhov, a postdoctoral researcher at Rice, recreated in computer simulations the kind of ripping observed through an electron microscope by researchers at Berkeley. The California team noticed that cracks in flakes of graphene followed armchair or zigzag configurations, terms that refer to the shape of the edges created. It seemed that molecular forces were dictating how graphene handles stress. Those forces are robust. Carbon-carbon bonds are the strongest known to man. But the importance of this research, Yakobson said, lies in the nature of the edge that results from the rip. The edge of a sheet of graphene gives it particular qualities, especially in the way it handles electric current. Graphene is so conductive that current flows straight through without impediment – until it reaches the edge. What the current finds there makes a big difference, he said, in whether it stops in its tracks or flows to an electrode or another sheet of graphene.
 httpvh://www.youtube.com/watch?v=7V54YQjIJO0

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Imagine dropping your phone on the hard concrete sidewalk—but when you pick it up, you find its battery has already healed itself. A team of researchers from the University of Illinois at Urbana-Champaign (UIUC) and the U.S. Department of Energy's (DOE) Argonne National Laboratory are exploring ways to design batteries that heal themselves when damaged. "This would help electronics survive daily use—both the long-term damage caused by charging over and over again, and also the inevitable physical damage of everyday life," said Jeff Moore, a UIUC scientist on the team. Scientists think that loss of electrical conductivity is what causes a battery to fade and die. Theories abound on the specific molecular failures; perhaps chemicals build up on electrodes, or the electrodes themselves pull away. Perhaps it's simply the inevitable stress fractures in materials forced to expand and contract repeatedly as the battery is charged and used. In any case, the battery's storage capacity drops due to loss of electrical conductivity. This is what the team wants to address. The idea is to station a team of "emergency repairmen" already contained in the battery. These are tiny microspheres, each smaller than a single red blood cell, and containing liquid metal inside. Added along with the battery components, they lie dormant for most of the battery's lifetime. But if the battery is damaged, the capsules burst open and release their liquid metal into the battery. The metal fills in the gaps in the electrical circuit, connecting the broken lines, and power is restored.
 httpvh://www.youtube.com/watch?v=Kc-Vjbn9M4g

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Rice University chemists have found a way to load more than 2 million tiny gold particles, called nanorods, into a single cancer cell. The breakthrough could speed development of cancer treatments that would use nanorods like tiny heating elements to cook tumors from the inside. "The breast cancer cells that we studied were so laden with gold nanorods that their masses increased by an average of about 13 percent," said study leader Eugene Zubarev, associate professor of chemistry at Rice. "Remarkably, the cells continued to function normally, even with all of this gold inside them." Though the ultimate goal is to kill cancer, Zubarev said the strategy is to deliver nontoxic particles that become deadly only when they are activated by a laser. The nanorods, which are about the size of a small virus, can harvest and convert otherwise harmless light into heat. But because each nanorod radiates miniscule heat, many are needed to kill a cell.  Unfortunately, scientists who study gold nanorods have found it difficult to load large numbers of particles into living cells. For starters, nanorods are pure gold, which means they won't dissolve in solution unless they are combined with some kind of polymer or surfactant. The most commonly used of these is cetyltrimethylammonium bromide, or CTAB, a soapy chemical often used in hair conditioner.  CTAB is a key ingredient in the production of nanorods, so scientists have often relied upon it to make nanorods soluble in water. CTAB does this job by coating the surface of the nanorods in much the same way that soap envelopes and dissolves droplets of grease in dishwater. CTAB-encased nanorods also have a positive charge on their surfaces, which encourages cells to ingest them. Unfortunately, CTAB is also toxic, which makes it problematic for biomedical applications. In the new research, Zubarev, Rice graduate student Leonid Vigderman and former graduate student Pramit Manna, now at Applied Materials Inc., describe a method to completely replace CTAB with a closely related molecule called MTAB that has two additional atoms attached at one end. The additional atoms -- one sulfur and one hydrogen -- allow MTAB to form a permanent chemical bond with gold nanorods. In contrast, CTAB binds more weakly to nanorods and has a tendency to leak into surrounding media from time to time, which is believed to be the underlying cause of CTAB-encased nanorod toxicity. It took Zubarev, Vigderman and Manna several years to identify the optimal strategy to synthesize MTAB and substitute it for CTAB on the surface of the nanorods. In addition, they developed a purification process that can completely remove all traces of CTAB from a solution of nanorods.
 httpvh://www.youtube.com/watch?v=kMjRBXi2DIY

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(Image Credit: University of Pittsburgh )

Researchers at the University of Pittsburgh have invented a new type of electronic switch that performs electronic logic functions within a single molecule.  The incorporation of such single-molecule elements could enable smaller, faster, and more energy-efficient electronics. “This new switch is superior to existing single-molecule concepts,” said Hrvoje Petek, principal investigator and professor of physics and chemistry in the Kenneth P. Dietrich School of Arts and Sciences and codirector of the Petersen Institute for NanoScience and Engineering (PINSE) at Pitt. “We are learning how to reduce electronic circuit elements to single molecules for a new generation of enhanced and more sustainable technologies.” The switch was discovered by experimenting with the rotation of a triangular cluster of three metal atoms held together by a nitrogen atom, which is enclosed entirely within a cage made up entirely of carbon atoms. Petek and his team found that the metal clusters encapsulated within a hollow carbon cage could rotate between several structures under the stimulation of electrons. This rotation changes the molecule’s ability to conduct an electric current, thereby switching among multiple logic states without changing the spherical shape of the carbon cage. Petek says this concept also protects the molecule so it can function without influence from outside chemicals. Because of their constant spherical shape, the prototype molecular switches can be integrated as atom-like building blocks the size of one nanometer (100,000 times smaller than the diameter of a human hair) into massively parallel computing architectures. The prototype was demonstrated using an Sc3N@C80 molecule sandwiched between two electrodes consisting of an atomically flat copper oxide substrate and an atomically sharp tungsten tip. By applying a voltage pulse, the equilateral triangle-shaped Sc3N could be rotated predictably among six logic states.

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AMikael Fogelström and Sergey Kubatkin are two of the Chalmers researchers investigating the supermaterial graphene. The cryostat in the picture is used to cool graphene samples to one hundredth of a degree above absolute zero.(Image Credit: Jan-Olof Yxell,Chalmers University of Technology)

Chalmers University of Technology will receive the lion's share of a new Swedish research grant of SEK 40 million for the supermaterial graphene. Following the new financing from the Knut and Alice Wallenberg Foundation, a group of some 30 Swedish graphene researchers will be formed, in a close collaboration between Chalmers and the universities of Uppsala and Linköping. The effort will form the Swedish spearhead in international graphene research – a hot topic ever since the Nobel Physics Prize in 2010. “The money will be used for everything from producing graphene to developing a variety of products, with basic research into experimental and theoretical physics along the way,” says Mikael Fogelström, the project coordinator. The graphene production process needs to be improved and made more reproducible. The researchers will develop reliable synthesis methods designed to produce high-quality graphene surfaces. Following that, the material will be investigated and processed at the nano level, ultimately to be used for specific components with far better performance than today's electronic devices.
Graphene can enable the best quantum resistance standard. This is one of many advances emerging from the active research into graphene at Chalmers. The researchers have already achieved several important breakthroughs with graphene, despite the fact that the material was first produced as recently as 2004. One example is a new standard for the quantum of resistance – a “tuning fork” for calibrating the correct resistance in electrical instruments and devices. State-of-the-art resistance standards are based on silicon or gallium arsenide. These are difficult to manufacture, and the method only works at extremely low temperatures and in large magnetic fields. A new generation of resistance standards based on graphene are at least as accurate as those in use today, while benefitting from being substantially easier to produce and use.

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Stanford researchers have developed a new method of attaching nanowire electronics to the surface of virtually any object, regardless of its shape or what material it is made of. The method could be used in making everything from wearable electronics and flexible computer displays to high-efficiency solar cells and ultrasensitive biosensors. Nanowire electronics are promising building blocks for virtually every digital electronic device used today, including computers, cameras and cell phones.  The electronic circuitry is typically fabricated on a silicon chip. The circuitry adheres to the surface of the chip during fabrication and is extremely difficult to detach, so when the circuitry is incorporated into an electronic device, it remains attached to the chip.  But silicon chips are rigid and brittle, limiting the possible uses of wearable and flexible nanowire electronics. The key to the new method is coating the surface of the silicon wafer with a thin layer of nickel before fabricating the electronic circuitry. Nickel and silicon are both hydrophilic, or "water-loving," meaning when they are exposed to water after fabrication of nanowire devices is finished, the water easily penetrates between the two materials, detaching the nickel and the overlying electronics from the silicon wafer.
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httpvh://www.youtube.com/watch?v=oPSeLHYwV9g

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Suenne Kim, Nazanin Bassiri-Gharb, and Yaser Bastani have developed a way to draw nanostructures directly on plastic. (Credit: Georgia Tech)

Using a technique known as thermochemical nanolithography (TCNL), researchers at Georgia Tech have developed a new way to fabricate nanometer-scale ferroelectric structures directly on flexible plastic substrates that would be unable to withstand the processing temperatures normally required to create such nanostructures. The technique, which uses a heated atomic force microscope (AFM) tip to produce patterns, could facilitate high-density, low-cost production of complex ferroelectric structures for energy harvesting arrays, sensors and actuators in nano-electromechanical systems (NEMS) and micro-electromechanical systems (MEMS).

"We can directly create piezoelectric materials of the shape we want, where we want them, on flexible substrates for use in energy harvesting and other applications," said Nazanin Bassiri-Gharb, assistant professor in the School of Mechanical Engineering at the Georgia Institute of Technology. "This is the first time that structures like these have been directly grown with a CMOS-compatible process at such a small resolution. Not only have we been able to grow these ferroelectric structures at low substrate temperatures, but we have also been able to pattern them at very small scales." In addition to the Georgia Tech researchers, the work also involved scientists from the University of Illinois Urbana-Champaign and the University of Nebraska Lincoln. Ultimately, arrays of AFM tips under computer control could produce complete devices, providing an alternative to current fabrication techniques.

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A graphene waveguide and splitter.(Image Credit: University of Pennsylvania)

Two University of Pennsylvania engineers have proposed the possibility of two-dimensional metamaterials. These one-atom-thick metamaterials could be achieved by controlling the conductivity of sheets of graphene, which is a single layer of carbon atoms. Professor Nader Engheta and graduate student Ashkan Vakil, both of the Department of Electrical and Systems Engineering in Penn’s School of Engineering and Applied Science, have recently published their theoretical research. The study of metamaterials is an interdisciplinary field of science and engineering that has grown considerably in recent years. It is premised on the idea that materials can be designed so that their overall wave qualities rely not only upon the material they are made of but also on the pattern, shape and size of irregularities, known as “inclusions,” or “meta-molecules” that are embedded within host media. These unusual properties generally have to do with manipulating electromagnetic (EM) or acoustic waves; in this case, it is EM waves in the infrared spectrum. Changing the shape, speed and direction of these kinds of waves is a subfield of metamaterials known as “transformation optics” and may find applications in everything from telecommunications to imaging to signal processing. Engheta and Vakil’s research shows how transformation optics might now be achieved using graphene, a lattice of carbon a single atom thick.

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