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  • richardmitnick 12:03 pm on May 28, 2015 Permalink | Reply
    Tags: , Graphene Studies,   

    From SLAC: “Spiraling Laser Pulses Could Change the Nature of Graphene” 


    SLAC Lab

    May 27, 2015

    1
    This illustration depicts the structure of graphene, which consists of a single layer of carbon atoms arranged in a honeycomb pattern. A new simulation suggests that spiraling pulses of polarized laser light could change graphene’s nature, turning it from a metal to an insulator. Led by researchers at SLAC and Stanford, the study paves the way for experiments that create and control new states of matter with this specialized form of light. (AlexanderAlUS via Wikimedia Commons)

    A new study predicts that researchers could use spiraling pulses of laser light to change the nature of graphene, turning it from a metal into an insulator and giving it other peculiar properties that might be used to encode information.

    The results, published May 11 in Nature Communications, pave the way for experiments that create and control new states of matter with this specialized form of light, with potential applications in computing and other areas.

    “It’s as if we’re taking a piece of clay and turning it into gold, and when the laser pulse goes away the gold goes back to clay,” said Thomas Devereaux, a professor at the Department of Energy’s SLAC National Accelerator Laboratory and director of the Stanford Institute for Materials and Energy Sciences (SIMES), a joint SLAC/Stanford institute.

    “But in this case,“ he said, “our simulations show that we could theoretically change the electronic properties of the graphene, flipping it back and forth from a metallic state, where electrons flow freely, to an insulating state. In digital terms this is like flipping between zero and one, on and off, yes and no; it can be used to encode information in a computer memory, for instance. What makes this cool and interesting is that you could make electronic switches with light instead of electrons.”

    Devereaux led the study with Michael Sentef, who began the work as a postdoctoral researcher at SLAC and is now at the Max Planck Institute for the Structure and Dynamics of Matter in Germany.

    Tweaking a wonder material

    Graphene is a pure form of carbon just one atom thick, with its atoms arranged in a honeycomb pattern. Celebrated as a wonder material since its discovery 12 years ago, it’s flexible, nearly transparent, a superb conductor of heat and electricity and one of the strongest materials known. But despite many attempts, scientists have not found a way to turn it into a semiconductor – the material at the heart of microelectronics.

    An earlier study demonstrated that it might be possible to take a step in that direction by hitting a material with circularly polarized light – light that spirals either clockwise or counterclockwise as it travels, a quality that can also be described as right- or left-handedness. This would create a “band gap,” a range of energies that electrons cannot occupy, which is one of the hallmarks of a semiconductor.

    In the SIMES study, theorists used the DOE’s National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory to perform large-scale simulations of an experiment in which graphene is hit with circularly polarized pulses a few millionths of a billionth of a second long.

    Getting as close to real as possible

    “Previous studies were based on analytical calculations and on idealized situations,” said Martin Claassen, a Stanford graduate student in Devereaux’s group who made key contributions to the study. “This one tried to simulate what happens in as close to real experimental conditions as you can get, right down to the shape of the laser pulses. Doing such a simulation can tell you what types of experiments are feasible and identify regions where you might find the most interesting changes in those experiments.”

    The simulations show that the handedness of the laser light would interact with a slight handedness in the graphene, which is not entirely uniform. This interaction leads to interesting and unexpected properties, said SLAC staff scientist and study co-author Brian Moritz. Not only does it produce a band gap, but it also induces a quantum state in which the graphene has a so-called “Chern number” of either one or zero, which results from a phenomenon known as Berry curvature and offers another on/off state that scientists might be able to exploit.

    Insights go beyond graphene

    While this study does not immediately open ways to make electronic devices, it does give researchers fundamental insights that advance the science in that direction. The results are also relevant to materials called dichalcogenides (pronounced dye-cal-CAW-gin-eyeds), which are also two-dimensional sheets of atoms arranged in a honeycomb structure.

    Dichalcogenides are the focus of intense research at SIMES and around the world because of their potential for creating “valleytronic” devices. In valleytronics, electrons move through a two-dimensional semiconductor as a wave with two energy valleys whose characteristics can be used to encode information. Possible applications include light detectors, low-energy computer logic and data storage chips and quantum computing. In addition to the work on graphene, members of the research team have also been simulating experiments involving the interaction of light with dichalcogenides.

    “Ultimately,” Moritz said, “we’re trying to understand how interaction with light can alter a material’s character and properties to create something that’s both new and interesting from a technological point of view.”

    In addition to SLAC, Stanford, SIMES and the Max Planck Institute for the Structure and Dynamics of Matter, other members of the research team were from Berkeley Lab, the University of Tokyo and Georgetown University. The work was funded by the DOE Office of Science.

    See the full article here.

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    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.
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  • richardmitnick 7:16 am on May 8, 2015 Permalink | Reply
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    From MIT: “Plugging up leaky graphene” 


    MIT News

    May 8, 2015
    Jennifer Chu

    1
    In a two-step process, engineers have successfully sealed leaks in graphene. First, the team fabricated graphene on a copper surface (top left) — a process that can create intrinsic defects in graphene, shown as cracks on the surface. After lifting the graphene and depositing it on a porous surface (top right), the transfer creates further holes and tears. In a first step (bottom left), the team used atomic layer deposition to deposit hafnium (in gray) to seal intrinsic cracks, then plugged the remaining holes (bottom left) with nylon (in red), via interfacial polymerization.
    Courtesy of the researchers.

    For faster, longer-lasting water filters, some scientists are looking to graphene —thin, strong sheets of carbon — to serve as ultrathin membranes, filtering out contaminants to quickly purify high volumes of water.

    Graphene’s unique properties make it a potentially ideal membrane for water filtration or desalination. But there’s been one main drawback to its wider use: Making membranes in one-atom-thick layers of graphene is a meticulous process that can tear the thin material — creating defects through which contaminants can leak.

    Now engineers at MIT, Oak Ridge National Laboratory, and King Fahd University of Petroleum and Minerals (KFUPM) have devised a process to repair these leaks, filling cracks and plugging holes using a combination of chemical deposition and polymerization techniques. The team then used a process it developed previously to create tiny, uniform pores in the material, small enough to allow only water to pass through.

    Combining these two techniques, the researchers were able to engineer a relatively large defect-free graphene membrane — about the size of a penny. The membrane’s size is significant: To be exploited as a filtration membrane, graphene would have to be manufactured at a scale of centimeters, or larger.

    In experiments, the researchers pumped water through a graphene membrane treated with both defect-sealing and pore-producing processes, and found that water flowed through at rates comparable to current desalination membranes. The graphene was able to filter out most large-molecule contaminants, such as magnesium sulfate and dextran.

    Rohit Karnik, an associate professor of mechanical engineering at MIT, says the group’s results, published in the journal Nano Letters, represent the first success in plugging graphene’s leaks.

    “We’ve been able to seal defects, at least on the lab scale, to realize molecular filtration across a macroscopic area of graphene, which has not been possible before,” Karnik says. “If we have better process control, maybe in the future we don’t even need defect sealing. But I think it’s very unlikely that we’ll ever have perfect graphene — there will always be some need to control leakages. These two [techniques] are examples which enable filtration.”

    Sean O’Hern, a former graduate research assistant at MIT, is the paper’s first author. Other contributors include MIT graduate student Doojoon Jang, former graduate student Suman Bose, and Professor Jing Kong.

    A delicate transfer

    “The current types of membranes that can produce freshwater from saltwater are fairly thick, on the order of 200 nanometers,” O’Hern says. “The benefit of a graphene membrane is, instead of being hundreds of nanometers thick, we’re on the order of three angstroms — 600 times thinner than existing membranes. This enables you to have a higher flow rate over the same area.”

    O’Hern and Karnik have been investigating graphene’s potential as a filtration membrane for the past several years. In 2009, the group began fabricating membranes from graphene grown on copper — a metal that supports the growth of graphene across relatively large areas. However, copper is impermeable, requiring the group to transfer the graphene to a porous substrate following fabrication.

    However, O’Hern noticed that this transfer process would create tears in graphene. What’s more, he observed intrinsic defects created during the growth process, resulting perhaps from impurities in the original material.

    Plugging graphene’s leaks

    To plug graphene’s leaks, the team came up with a technique to first tackle the smaller intrinsic defects, then the larger transfer-induced defects. For the intrinsic defects, the researchers used a process called “atomic layer deposition,” placing the graphene membrane in a vacuum chamber, then pulsing in a hafnium-containing chemical that does not normally interact with graphene. However, if the chemical comes in contact with a small opening in graphene, it will tend to stick to that opening, attracted by the area’s higher surface energy.

    The team applied several rounds of atomic layer deposition, finding that the deposited hafnium oxide successfully filled in graphene’s nanometer-scale intrinsic defects. However, O’Hern realized that using the same process to fill in much larger holes and tears — on the order of hundreds of nanometers — would require too much time.

    Instead, he and his colleagues came up with a second technique to fill in larger defects, using a process called “interfacial polymerization” that is often employed in membrane synthesis. After they filled in graphene’s intrinsic defects, the researchers submerged the membrane at the interface of two solutions: a water bath and an organic solvent that, like oil, does not mix with water.

    In the two solutions, the researchers dissolved two different molecules that can react to form nylon. Once O’Hern placed the graphene membrane at the interface of the two solutions, he observed that nylon plugs formed only in tears and holes — regions where the two molecules could come in contact because of tears in the otherwise impermeable graphene — effectively sealing the remaining defects.

    Using a technique they developed last year, the researchers then etched tiny, uniform holes in graphene — small enough to let water molecules through, but not larger contaminants. In experiments, the group tested the membrane with water containing several different molecules, including salt, and found that the membrane rejected up to 90 percent of larger molecules. However, it let salt through at a faster rate than water.

    The preliminary tests suggest that graphene may be a viable alternative to existing filtration membranes, although Karnik says techniques to seal its defects and control its permeability will need further improvements.

    “Water desalination and nanofiltration are big applications where, if things work out and this technology withstands the different demands of real-world tests, it would have a large impact,” Karnik says. “But one could also imagine applications for fine chemical- or biological-sample processing, where these membranes could be useful. And this is the first report of a centimeter-scale graphene membrane that does any kind of molecular filtration. That’s exciting.”

    De-en Jiang, an assistant professor of chemistry at the University of California at Riverside, sees the defect-sealing technique as “a great advance toward making graphene filtration a reality.”

    “The two-step technique is very smart: sealing the defects while preserving the desired pores for filtration,” says Jiang, who did not contribute to the research. “This would make the scale-up much easier. One can produce a large graphene membrane first, not worrying about the defects, which can be sealed later.”

    This research was supported in part by the Center for Clean Water and Clean Energy at MIT and KFUPM, the U.S. Department of Energy, and the National Science Foundation.

    See the full article here.

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  • richardmitnick 9:21 am on April 22, 2015 Permalink | Reply
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    From UCSD: “‘Holey’ graphene for energy storage” 

    UC San Diego bloc

    UC San Diego

    April 21, 2015
    Liezel Labios

    1
    Rajaram Narayanan, a nanoengineering graduate student at UC San Diego Jacobs School of Engineering and lead author of the Nano Letters paper.

    2
    Zigzag and armchair defects in graphene.

    Engineers at the University of California, San Diego have discovered a method to increase the amount of electric charge that can be stored in graphene, a two-dimensional form of carbon. The research, published recently online in the journal Nano Letters, may provide a better understanding of how to improve the energy storage ability of capacitors for potential applications in cars, wind turbines, and solar power.

    Capacitors charge and discharge very fast, and are more useful for quick large bursts of energy, such as in camera flashes and power plants. Their ability to rapidly charge and discharge is an advantage over the long charge time of batteries. However, the problem with capacitors is that they store less energy than batteries.

    How can the energy storage of a capacitor be improved? One approach by researchers in the lab of mechanical engineering professor Prabhakar Bandaru at the Jacobs School of Engineering at UC San Diego was to introduce more charge into a capacitor electrode using graphene as a model material for their tests. The principle is that increased charge leads to increased capacitance, which translates to increased energy storage.

    How it’s made

    Making a perfect carbon nanotube structure ― one without defects, which are holes corresponding to missing carbon atoms ― is next to impossible. Rather than avoiding defects, the researchers in Bandaru’s lab figured out a practical way to use them instead.

    “I was motivated from the point of view that charged defects may be useful for energy storage,” said Bandaru.

    The team used a method called argon-ion based plasma processing, in which graphene samples are bombarded with positively-charged argon ions. During this process, carbon atoms are knocked out of the graphene layers and leave behind holes containing positive charges ― these are the charged defects. Exposing the graphene samples to argon plasma increased the capacitance of the materials three-fold.

    “It was exciting to show that we can introduce extra capacitance by introducing charged defects, and that we could control what kind of charged defect we could introduce into a material,” said Rajaram Narayanan, a graduate student in professor Bandaru’s research group and first author of the study.

    Using Raman spectroscopy and electrochemical measurements, the team was able to characterize the types of defects that argon plasma processing introduced into the graphene lattices. The results revealed the formation of extended defects known as “armchair” and “zigzag” defects, which are named based on the configurations of the missing carbon atoms.

    Additionally, electrochemical studies helped the team discover a new length scale that measures the distance between charges. “This new length scale will be important for electrical applications, since it can provide a basis for how small we can make electrical devices,” said Bandaru.

    Journal reference:

    R. Narayanan, H. Yamada, M. Karakaya, R. Podila, A. M. Rao, and P. R. Bandaru. Modulation of the Electrostatic and Quantum Capacitances of Few Layered Graphenes through Plasma Processing. Nano Letters 2015. DOI: 10.1021/acs.nanolett.5b00055

    This work was supported by a grant from the National Science Foundation.

    See the full article here.

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    UC San Diego Campus

    The University of California, San Diego (also referred to as UC San Diego or UCSD), is a public research university located in the La Jolla area of San Diego, California, in the United States.[12] The university occupies 2,141 acres (866 ha) near the coast of the Pacific Ocean with the main campus resting on approximately 1,152 acres (466 ha).[13] Established in 1960 near the pre-existing Scripps Institution of Oceanography, UC San Diego is the seventh oldest of the 10 University of California campuses and offers over 200 undergraduate and graduate degree programs, enrolling about 22,700 undergraduate and 6,300 graduate students. UC San Diego is one of America’s Public Ivy universities, which recognizes top public research universities in the United States. UC San Diego was ranked 8th among public universities and 37th among all universities in the United States, and rated the 18th Top World University by U.S. News & World Report ‘s 2015 rankings.

     
  • richardmitnick 2:45 pm on March 16, 2015 Permalink | Reply
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    From Rice: “Symmetry matters in graphene growth” 

    Rice U bloc

    Rice University

    March 16, 2015
    Mike Williams

    Rice researchers find subtle interactions with substrate may lead to better control

    What lies beneath growing islands of graphene is important to its properties, according to a new study led by Rice University.

    Scientists at Rice analyzed patterns of graphene – a single-atom-thick sheet of carbon – grown in a furnace via chemical vapor deposition. They discovered that the geometric relationship between graphene and the substrate, the underlying material on which carbon assembles atom by atom, determines how the island shapes emerge. The study led by Rice theoretical physicist Boris Yakobson and postdoctoral researcher Vasilii Artyukhov shows how the crystalline arrangement of atoms in substrates commonly used in graphene growth, such as nickel or copper, controls how islands form. The results appear this week in Physical Review Letters.

    1
    Graphene islands formed in two distinctly different shapes on separate grains of copper (colored in blue and red) grown simultaneously because the substrates’ atomic lattices have different orientations, according to Rice University researchers. Click on the image for a larger version. Image by Yufeng Hao/coloring by Vasilii Artyukhov

    “Experiments that show graphene’s amazing electronic properties are typically done on mechanically exfoliated graphene,” Artyukhov said. “That limits you in terms of the flake size, and it’s expensive if you need a lot of material. So everybody’s trying to come up with a better way to grow it from gases like methane (the source of carbon atoms) using different substrate metals. The problem is, the resulting crystals look different from substrate to substrate, even though it’s all graphene.”

    Yakobson said researchers often see odd-shaped graphene islands grown by chemical vapor deposition, “and we have all wondered why. In general, this is very surprising, because in graphene, the six sides should be identical.” Triangles and other shapes, he said, are examples of symmetry breaking; systems that would otherwise produce regular shapes “break” and produce less regular ones.

    Graphene forms in a chemical vapor deposition furnace when carbon atoms floating in the hot fog settle on the metallic substrate. The atoms link up in characteristic six-sided rings, but as an island grows, its overall shape can take various forms, from hexagons to elongated hexagons to more random structures, even triangles. The researchers found a strong correlation between the ultimate shape of the island and the arrangement of atoms in the exposed surface of the substrate, which can be triangular, square, rectangular or otherwise.

    The researchers found individual atoms follow the road map set out by the substrate, as illustrated by a microscope image of two grains of copper substrate that host two distinct shapes of graphene, even though the growth conditions are identical. On one grain, the graphene islands are all nearly perfect hexagons; on the other, the hexagonal islands are elongated and aligned.

    2
    Rice University researcher Vasilii Artyukhov, left, and Professor Boris Yakobson led a study that showed islands of graphene growing in a furnace can take different shapes that depend on how their atoms align with the substrate underneath. Photo by Jeff Fitlow

    “The image shows the basic growth mechanisms are the same, but the difference in the islands is due to the subtle differences between the crystallographic surfaces of the graphene and copper,” Yakobson said.

    Because graphene’s edges are so important to its electronic properties, any step toward understanding its growth is important, he said. Whether a graphene edge ends up as a zigzag, an armchair or something in between depends on how individual atoms fall into equilibrium as they balance energies between their neighboring carbon atoms and those of the substrate.

    The atoms in metals form a specific arrangement, a crystal lattice, such as a pure copper lattice called “face-centered cubic.” But individual grains can have different surfaces in polycrystalline material like copper foils frequently used as graphene-growth substrates.

    “Depending on the way you cut a cube in half, you can end up with square, rectangular or even triangular faces,” Artyukhov said. “The surface of copper foil can have different textures in different places. Electron microscopy showed that all graphene islands growing on the same copper grain tend to have a similar shape, for instance, all perfect hexagons, or all elongated.”

    He said the islands inherit the symmetry of the grains’ surfaces and grow faster in some directions, which explains the peculiar distribution of shapes.

    When the growth process is long enough, the islands merge into larger graphene films. Where the carbon lattices don’t align with each other, the atoms seek equilibrium and form grain boundaries that control the larger sheet’s electronic properties. Researchers – and industries – desire ways to control graphene’s semiconducting properties by controlling the boundaries.

    “A good understanding of this process gives directions on how to organize the mutual orientation of islands,” Yakobson said. “So when they fuse you can, by design, create particular grain boundaries with particularly interesting properties. So this research, more than just satisfying our curiosity, is very useful.”

    He suggested the same calculations could apply to the growth of other two-dimensional materials like hexagonal boron-nitride or molybdenum disulfide and its relatives, also widely studied for their potential for electronics.

    The paper’s co-authors are Yufeng Hao, a research scientist at Columbia University, and Rodney Ruoff, director of the Center for Multidimensional Carbon Materials at the Ulsan National Institute of Science and Technology, Ulsan, South Korea.

    The U.S. Department of Energy and the Institute of Basic Science at the Ulsan National Institute of Science and Technology supported the research.

    See the full article here.

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    In his 1912 inaugural address, Rice University president Edgar Odell Lovett set forth an ambitious vision for a great research university in Houston, Texas; one dedicated to excellence across the range of human endeavor. With this bold beginning in mind, and with Rice’s centennial approaching, it is time to ask again what we aspire to in a dynamic and shrinking world in which education and the production of knowledge will play an even greater role. What shall our vision be for Rice as we prepare for its second century, and how ought we to advance over the next decade?

    This was the fundamental question posed in the Call to Conversation, a document released to the Rice community in summer 2005. The Call to Conversation asked us to reexamine many aspects of our enterprise, from our fundamental mission and aspirations to the manner in which we define and achieve excellence. It identified the pressures of a constantly changing and increasingly competitive landscape; it asked us to assess honestly Rice’s comparative strengths and weaknesses; and it called on us to define strategic priorities for the future, an effort that will be a focus of the next phase of this process.

     
  • richardmitnick 8:53 am on March 7, 2015 Permalink | Reply
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    From EPFL Lausanne: “Graphene Meets Heat Waves” 

    EPFL bloc

    Ecole Polytechnique Federale Lausanne

    06.03.15
    Laure-Anne Pessina

    1

    EPFL researchers have shed new light on the fundamental mechanisms of heat dissipation in graphene and other two-dimensional materials. They have shown that heat can propagate as a wave over very long distances. This is key information for engineering the electronics of tomorrow.

    In the race to miniaturize electronic components, researchers are challenged with a major problem: the smaller or the faster your device, the more challenging it is to cool it down. One solution to improve the cooling is to use materials with very high thermal conductivity, such as graphene, to quickly dissipate heat and thereby cool down the circuits.

    At the moment, however, potential applications are facing a fundamental problem: how does heat propagate inside these sheets of materials that are no more than a few atoms thick?

    In a study published in Nature Communications, a team of EPFL researchers has shed new light on the mechanisms of thermal conductivity in graphene and other two-dimensional materials. They have demonstrated that heat propagates in the form of a wave, just like sound in air. This was up to now a very obscure phenomenon observed in few cases at temperatures close to the absolute zero.Their simulations provide a valuable tool for researchers studying graphene, whether to cool down circuits at the nanoscale, or to replace silicon in tomorrow’s electronics.

    Quasi-Lossless Propagation

    If it has been difficult so far to understand the propagation of heat in two-dimensional materials, it is because these sheets behave in unexpected ways compared to their three-dimensional cousins. In fact, they are capable of transferring heat with extremely limited losses, even at room temperature.

    Generally, heat propagates in a material through the vibration of atoms. These vibrations are are called “phonons“, and as heat propagates though a three-dimensional material, these phonons keep colliding with each other, merging together, or splitting. All these processes can limit the conductivity of heat along the way. Only under extreme conditions, when temperature goes close to the absolute zero ( -200 0C or lower), it is possible to observe quasi-lossless heat transfer.

    A wave of quantum heat

    The situation is very different in two dimensional materials, as shown by researchers at EPFL. Their work demonstrates that heat can propagate without significant losses in 2D even at room temperature, thanks to the phenomenon of wave-like diffusion, called “second sound“. In that case, all phonons march together in unison over very long distances. “Our simulations, based on first-principles physics, have shown that atomically thin sheets of materials behave, even at room temperature, in the same way as three-dimensional materials at extremely low temperatures” says Andrea Cepellotti, the first author of the study. “We can show that the thermal transport is described by waves, not only in graphene but also in other materials that have not been studied yet,” explains Cepellotti. “This is an extremely valuable information for engineers, who could adapt the design of future electronic components using some of these novel two-dimensional materials properties.”

    See the full article here.

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    EPFL is Europe’s most cosmopolitan technical university. It receives students, professors and staff from over 120 nationalities. With both a Swiss and international calling, it is therefore guided by a constant wish to open up; its missions of teaching, research and partnership impact various circles: universities and engineering schools, developing and emerging countries, secondary schools and gymnasiums, industry and economy, political circles and the general public.

     
  • richardmitnick 4:23 am on February 19, 2015 Permalink | Reply
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    From SLAC: “Semiconductor Works Better when Hitched to Graphene” 


    SLAC Lab

    February 18, 2015

    Experiments at SLAC Show Potential for Graphene-based Organic Electronic Devices

    Graphene – a one-atom-thick sheet of carbon with highly desirable electrical properties, flexibility and strength – shows great promise for future electronics, advanced solar cells, protective coatings and other uses, and combining it with other materials could extend its range even further.

    Experiments at the Department of Energy’s SLAC National Accelerator Laboratory looked at the properties of materials that combine graphene with a common type of semiconducting polymer. They found that a thin film of the polymer transported electric charge even better when grown on a single layer of graphene than it does when placed on a thin layer of silicon.

    1
    A material made of semiconducting polymer placed on top of graphene conducts electric charge extremely well and may enable new electronic devices. This work was featured on the cover of the journal Advanced Functional Materials. (David Barbero)

    “Our results are among the first to measure the charge transport in these materials in the vertical direction – the direction that charge travels in organic photovoltaic devices like solar cells or in light-emitting diodes,” said David Barbero of Umeå University in Sweden, leader of the international research team that performed the experiments at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL), a DOE Office of Science User Facility. “The result was somewhat expected, because graphene and silicon have different crystalline structures and electrical properties.”

    But the team also discovered something very unexpected, he said.

    Although it was widely believed that a thinner polymer film should enable electrons to travel faster and more efficiently than a thicker film, Barbero and his team discovered that a polymer film about 50 nanometers thick conducted charge about 50 times better when deposited on graphene than the same film about 10 nanometers thick.

    2
    Studies conducted at the Stanford Synchrotron Radiation Lightsource revealed that when deposited atop graphene, a thicker polymer film (top) conducted charge significantly better than a thinner polymer film (bottom). This is likely because the orientation of the polymer crystallites within the thick film allows the formation of a continuous pathway for the charge to flow. (David Barbero)

    The team concluded that the thicker film’s structure, which consists of a mosaic of crystallites oriented at different angles, likely forms a continuous pathway of interconnected crystals. This, they theorize, allows for easier charge transport than in a regular thin film, whose thin, plate-like crystal structures are oriented parallel to the graphene layer.

    By better controlling the thickness and crystalline structure of the semiconducting film, it may be possible to design even more efficient graphene-based organic electronic devices.

    “The fields most likely to benefit from this work are probably next-generation photovoltaic devices and flexible electronic devices,” said Barbero. “Because graphene is thin, lightweight and flexible, there are a number of potential applications.”

    See the full article here.

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  • richardmitnick 11:24 am on February 3, 2015 Permalink | Reply
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    From Rice: “Winding borders may enhance graphene” 

    Rice U bloc

    Rice University

    February 2, 2015
    Mike Williams

    Rice University theory suggests ‘sinuous’ grain boundries add strength, predictable semiconducting properties

    1
    The grain boundary in a computation model at left and a simulated microscope image at center were found to be a near-perfect match for an actual grain boundary, right, depicted in a 2011 Nature paper led by scientists at Cornell. The out-of-place rings highlighted in blue were likely due to distortion caused by irradiation from the microscope’s electron beam. Courtesy of the Yakobson Group.

    Far from being a defect, a winding thread of odd rings at the border of two sheets of graphene has qualities that may prove valuable to manufacturers, according to Rice University scientists.

    Graphene, the atom-thick form of carbon, rarely appears as a perfect lattice of chicken wire-like six-atom rings. When grown via chemical vapor deposition, it usually consists of “domains,” or separately grown sheets that bloom outward from hot catalysts until they meet up.

    Where they meet, the regular rows of atoms aren’t necessarily aligned, so they have to adjust if they are to form a continuous graphene plane. That adjustment appears as a grain boundary, with irregular rows of five- and seven-atom rings that compensate for the angular disparity.

    The Rice lab of theoretical physicist Boris Yakobson had calculated that rings with seven carbon atoms can be weak spots that lessen the legendary strength of graphene. But new research at Rice shows meandering grain boundaries can, in some cases, toughen what are known as polycrystalline sheets, nearly matching the strength of pristine graphene.

    3
    Periodic grain boundaries in graphene may lend mechanical strength and semiconducting properties to the atom-thick carbon material, according to calculations by scientists at Rice University. Illustration by Zhuhua Zhang

    Conveniently, they can also create a “sizable electronic transport gap,” or band gap, according to the paper. Perfect graphene allows for the ballistic transport of electricity, but electronics require materials that can controllably stop and start the flow. These are known as semiconductors, and the ability to control semiconducting characteristics in graphene (and other two-dimensional materials) is a much-sought goal.

    In the new work, which appears in Advanced Functional Materials, Yakobson and his team led by postdoctoral researcher Zhuhua Zhang determined that at certain angles, these “sinuous” boundaries relieve stress that would otherwise weaken the sheet.

    “If stress along the boundary were alleviated, the strength of the graphene would be enhanced,” Zhang said. “But this only applies to sinuous grain boundaries as compared with straight boundaries.”

    Yakobson and his team calculate the mechanical strength of grain boundaries to determine how they influence each other: where the boundaries are inclined to bind and where they are likely to break under tensile stress. Grain boundaries could minimize the interface energy between sheets by forming pairs of rings called dislocations, where an atom shifts from one six-member ring to its neighbor to form connected five- and seven-atom units.

    Sometimes the domains’ angles dictate winding rather than straight boundaries. Zhang and his co-authors simulated these sinuous boundaries to measure their tensile strength and band-gap properties. He determined that where these small sections are periodic — that is, when their patterns repeat along the length of the boundary — their qualities apply to the entire polycrystalline sheet.

    Remarkably, one of his simulations of energetically “preferred” sinuous grain boundaries was a near-perfect match for the asymmetric boundary he spotted in a 2011 paper in the journal Nature. The scanning transmission electron microscopy image showed an atomic grain-boundary structure with a very similar arrangement of dislocations. Only one pair of rings out of the hundred in view was out of place, likely due to a distortion caused by irradiation from the microscope’s electron beam, Zhang said.

    To take advantage of the Rice lab’s predictions, scientists would have to figure out how to grow polycrystalline graphene with precise misalignment of the components. This is a tall order, Yakobson said.

    “But this — so far, hypothetically — can be achieved if graphene nucleates at the polycrystalline metal substrate with prescribed grain orientations so that the emergent carbon isles inherit the misalignment of the template underneath,” Yakobson said.

    Co-authors include graduate students Yang Yang, Fangbo Xu and Luqing Wang. Yakobson is Rice’s Karl F. Hasselmann Professor of Materials Science and NanoEngineering and a professor of chemistry.

    The Department of Energy and the U.S. Air Force Office of Scientific Research supported the research. The researchers utilized the National Science Foundation-supported DAVinCI and SUGAR supercomputer clusters administered by Rice’s Ken Kennedy Institute for Information Technology.

    See the full article here.

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    In his 1912 inaugural address, Rice University president Edgar Odell Lovett set forth an ambitious vision for a great research university in Houston, Texas; one dedicated to excellence across the range of human endeavor. With this bold beginning in mind, and with Rice’s centennial approaching, it is time to ask again what we aspire to in a dynamic and shrinking world in which education and the production of knowledge will play an even greater role. What shall our vision be for Rice as we prepare for its second century, and how ought we to advance over the next decade?

    This was the fundamental question posed in the Call to Conversation, a document released to the Rice community in summer 2005. The Call to Conversation asked us to reexamine many aspects of our enterprise, from our fundamental mission and aspirations to the manner in which we define and achieve excellence. It identified the pressures of a constantly changing and increasingly competitive landscape; it asked us to assess honestly Rice’s comparative strengths and weaknesses; and it called on us to define strategic priorities for the future, an effort that will be a focus of the next phase of this process.

     
  • richardmitnick 1:35 pm on January 12, 2015 Permalink | Reply
    Tags: , Graphene Studies,   

    From LBL: “From the Bottom Up: Manipulating Nanoribbons at the Molecular Level” 

    Berkeley Logo

    Berkeley Lab

    January 12, 2015
    Rachel Berkowitz 510.486.7254

    Narrow strips of graphene called nanoribbons exhibit extraordinary properties that make them important candidates for future nanoelectronic technologies. A barrier to exploiting them, however, is the difficulty of controlling their shape at the atomic scale, a prerequisite for many possible applications.

    Now, researchers at the US Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California, Berkeley, have developed a new precision approach for synthesizing graphene nanoribbons from pre-designed molecular building blocks. Using this process the researchers have built nanoribbons that have enhanced properties—such as position-dependent, tunable bandgaps—that are potentially very useful for next-generation electronic circuitry.

    The results appear in a paper titled Molecular bandgap engineering of bottom-up synthesized graphene nanoribbon heterojunctions, published in Nature Nanotechnology.

    “This work represents progress towards the goal of controllably assembling molecules into whatever shapes we want,” says Mike Crommie, senior scientist at Berkeley Lab, professor at UC Berkeley, and a leader of the study. “For the first time we have created a molecular nanoribbon where the width changes exactly how we designed it to.”

    2
    Felix Fischer (on left) and Mike Crommie

    Nanoribbons past and present

    Previously, scientists made nanoribbons that have a constant width throughout. “That makes for a nice wire or a simple switching element,” says Crommie, “but it does not provide a lot of functionality. We wanted to see if we could change the width within a single nanoribbon, controlling the structure inside the nanoribbon at the atomic scale to give it new behavior that is potentially useful.”

    Felix Fischer, Professor of Chemistry at UC Berkeley who jointly led the study, designed the molecular components to find out whether this would be possible. Together, Fischer and Crommie discovered that molecules of different widths can indeed be made to chemically bond such that width is modulated along the length of a single resulting nanoribbon.

    “Think of the molecules as different sized Lego blocks,” explains Fischer. Each block has a certain defined structure and when pieced together they result in a particular shape for the whole nanoribbon. “We want to see if we can understand the exotic properties that emerge when we assemble these molecular structures, and to see if we can exploit them to build new functional devices.”

    Until now, nanoribbon synthesis has mostly involved etching ribbons out of larger 2D sheets of graphene. The problem, according to Fischer, is that this lacks precision and each resulting nanoribbon has a unique, slightly random structure. Another method has been to unzip nanotubes to yield nanoribbons. This produces smoother edges than the “top-down” etching technique, but it is difficult to control because nanotubes have different widths and chiralities.

    A third route, discovered by Roman Fasel of Swiss Federal Laboratories for Materials Science & Technology along with his co-workers, involves placing molecules on a metal surface and chemically fusing them together to form perfectly uniform nanoribbons. Crommie and Fischer modified this last approach and have shown that if the shapes of the constituent molecules are varied then so is the shape of the resulting nanoribbon.

    “What we’ve done that is new is to show that it is possible to create atomically-precise nanoribbons with non-uniform shape by changing the shapes of the molecular building blocks,” says Crommie.

    Controlling quantum properties

    Electrons within the nanoribbons set up quantum mechanical standing-wave patterns that determine the nanoribbon’s electronic properties, such as its “bandgap”. This determines the energetics of how electrons move through a nanoribbon, including which regions they accumulate in and which regions they avoid.

    In the past, scientists spatially engineered the bandgap of micron-scale devices through doping, the addition of impurities to a material. For the smaller nanoribbons, however, it is possible to change the bandgap by modifying their width in sub-nanometer increments, a process that Crommie and Fischer have dubbed “molecular bandgap engineering.” This kind of engineering allows the researchers to tailor the quantum mechanical properties of nanoribbons so they might be flexibly used for future nanoelectronic devices.

    2
    Figure 1: Bottom-up synthesis of graphene nanoribbons from molecular building blocks 1 and 2. (a) The resulting ribbon, or heterojunction, has varied widths as a result of different width molecules 1 and 2. (b) Scanning transmission microscope image of graphene nanoribbon heterojunction, with larger-scale inset of multiple ribbons.

    To test their molecular bandgap engineering, Crommie’s group used scanning tunneling microscopy (STM), a technique that can spatially map the behavior of electrons inside a single nanoribbon. “We needed to know the atomic-scale shape of the nanoribbons, and we also needed to know how the electrons inside adapt to that shape,” says Crommie. UC Berkeley professor of physics Steven Louie and his student Ting Cao calculated the electronic structure of the nanribbons in order to correctly interpret the STM images. This “closed the loop” between nanoribbon design, fabrication, and characterization.

    New directions toward new devices

    A major question in this work is how best to build useful devices from these tiny molecular structures. While the team has shown how to fabricate width-varying nanoribbons, it has not yet incorporated them into actual electronic circuits. Crommie and Fischer hope to use this new type of nanoribbon to eventually create new device elements – such as diodes, transistors, and LEDs – that are smaller and more powerful than those in current use. Ultimately they hope to incorporate nanoribbons into complex circuits that yield better performance than today’s computer chips. To this end they are collaborating with UC Berkeley electrical engineers such as Jeffrey Bokor and Sayeef Salahuddin.

    The required spatial precision already exists: the team can modulate nanoribbon width from 0.7 nm to 1.4nm, creating junctions where narrow nanoribbons fuse seamlessly into wider ones. “Varying the width by a factor of two allows us to modulate the bandgap by more than 1eV,” says Fischer. For many applications this is sufficient for building useful devices.

    While the potential applications are exciting, Crommie points out that a central motivation for the research is the desire to answer basic scientific questions like how nanoribbons with non-uniform width actually behave. “We set out to answer an interesting question, and we answered it,” he concludes.

    The complete list of authors on the paper includes Yen-Chia Chen, Ting Cao, Chen Chen, Zahra Pedramrazi, Danny Haberer, Dimas de Oteyza, Felix Fischer, Steven Louie, and Michael Crommie.

    This research was supported by the Office of Naval Research BRC Program (molecular synthesis and characterization), and by the DOE Office of Science (instrumentation development, STM operation and simulations); and by the National Science Foundation (image analysis, theory formalism).

    See the full article here.

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  • richardmitnick 8:57 pm on October 3, 2014 Permalink | Reply
    Tags: , , Graphene Studies, ,   

    From MIT: “Crumpled graphene could provide an unconventional energy storage” 


    MIT News

    October 3, 2014
    David L. Chandler | MIT News Office

    Two-dimensional carbon “paper” can form stretchable supercapacitors to power flexible electronic devices.

    When someone crumples a sheet of paper, that usually means it’s about to be thrown away. But researchers have now found that crumpling a piece of graphene “paper” — a material formed by bonding together layers of the two-dimensional form of carbon — can actually yield new properties that could be useful for creating extremely stretchable supercapacitors to store energy for flexible electronic devices.

    temp
    To form the crumpled graphene, a sheet of polymer material is stretched in both dimensions, then graphene paper is bonded to it. When the polymer is released in one direction, the graphene forms pleats, as shown in the bottom left image, taken with a scanning electron microscope (SEM). Then, when released in the other direction, it forms a chaotic crumpled pattern (top left). At top right, an SEM image shows the material in a partially crumpled state. At bottom right, SEM image of a piece that has been crumpled and then flattened out. Image courtesy of the researchers

    The finding is reported in the journal Scientific Reports by MIT’s Xuanhe Zhao, an assistant professor of mechanical engineering and civil and environmental engineering, and four other authors. The new, flexible supercapacitors should be easy and inexpensive to fabricate, the team says.

    “Many people are exploring graphene paper: It’s a good candidate for making supercapacitors, because of its large surface area per mass,” Zhao says. Now, he says, the development of flexible electronic devices, such as wearable or implantable biomedical sensors or monitoring devices, will require flexible power-storage systems.

    Like batteries, supercapacitors can store electrical energy, but they primarily do so electrostatically, rather than chemically — meaning they can deliver their energy faster than batteries can. Now Zhao and his team have demonstrated that by crumpling a sheet of graphene paper into a chaotic mass of folds, they can make a supercapacitor that can easily be bent, folded, or stretched to as much as 800 percent of its original size. The team has made a simple supercapacitor using this method as a proof of principle.

    The material can be crumpled and flattened up to 1,000 times, the team has demonstrated, without a significant loss of performance. “The graphene paper is pretty robust,” Zhao says, “and we can achieve very large deformations over multiple cycles.” Graphene, a structure of pure carbon just one atom thick with its carbon atoms arranged in a hexagonal array, is one of the strongest materials known.

    To make the crumpled graphene paper, a sheet of the material was placed in a mechanical device that first compressed it in one direction, creating a series of parallel folds or pleats, and then in the other direction, leading to a chaotic, rumpled surface. When stretched, the material’s folds simply smooth themselves out.

    Forming a capacitor requires two conductive layers — in this case, two sheets of crumpled graphene paper — with an insulating layer in between, which in this demonstration was made from a hydrogel material. Like the crumpled graphene, the hydrogel is highly deformable and stretchable, so the three layers remain in contact even while being flexed and pulled.

    Though this initial demonstration was specifically to make a supercapacitor, the same crumpling technique could be applied to other uses, Zhao says. For example, the crumpled graphene material might be used as one electrode in a flexible battery, or could be used to make a stretchable sensor for specific chemical or biological molecules.

    “This work is really exciting and amazing to me,” says Dan Li, a professor of materials engineering at Monash University in Australia who was not involved in this research. He says the team “provides an extremely simple but highly effective concept to make stretchable electrodes for supercapacitors by controlled crumpling of multilayered graphene films.” While other groups have made flexible supercapacitors, he says, “Making supercapacitors stretchable has been a great challenge. This paper provides a very smart way to tackle this challenge, which I believe will bring wearable energy storage devices closer.”

    The research team also included Jianfeng Zang at Huazhong University of Science and Technology and Changyang Cao, Yaying Feng, and Jie Liu at Duke University. The work was supported by the Office of Naval Research, the National Science Foundation, and the National 1000 Talents Program of China.

    See the full article here.

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  • richardmitnick 8:30 pm on September 9, 2014 Permalink | Reply
    Tags: , Graphene Studies, ,   

    From Kavli: “Tiny Graphene Drum Could Form Future Quantum Memory” 

    KavliFoundation

    The Kavli Foundation

    09/09/2014
    No Writer Credit

    Scientists from TU Delft’s Kavli Institute of Nanoscience have demonstrated that they can detect extremely small changes in position and forces on very small drums of graphene. Graphene drums have great potential to be used as sensors in devices such as mobile phones. Using their unique mechanical properties, these drums could also act as memory chips in a quantum computer. The researchers present their findings in an article in the August 24th edition of Nature Nanotechnology. The research was funded by the FOM Foundation, the EU Marie-Curie program, and NWO.

    Graphene drums

    drum
    Graphene Drum

    Graphene is famous for its special electrical properties, but research on the one-layer thin graphite was recently expanded to explore graphene as a mechanical object. Thanks to their extreme low mass, tiny sheets of graphene can be used the same was as the drumhead of a musician. In the experiment, scientists use microwave-frequency light to ‘play’ the graphene drums, to listen to its ‘nano sound’, and to explore the way graphene in these drums moves.

    Optomechanics

    Dr. Vibhor Singh and his colleagues did this by using a 2D crystal membrane as a mirror in an ‘optomechanical cavity’. “In optomechanics you use the interference pattern of light to detect tiny changes in the position of an object. In this experiment, we shot microwave photons at a tiny graphene drum. The drum acts as a mirror: by looking at the interference of the microwave photons bouncing off of the drum, we are able to sense minute changes in the position of the graphene sheet of only 17 femtometers, nearly 1/10000th of the diameter of an atom.”, Singh explains.

    Amplifier

    The microwave ‘light’ in the experiment is not only good for detecting the position of the drum, but can also push on the drum with a force. This force from light is extremely small, but the small mass of the graphene sheet and the tiny displacements they can detect mean that the scientist can use these forces to ‘beat the drum’: the scientists can shake the graphene drum with the momentum of light. Using this radiation pressure, they made an amplifier in which microwave signals, such as those in your mobile phone, are amplified by the mechanical motion of the drum.

    Memory

    The scientists also show you can use these drums as ‘memory chips’ for microwave photons, converting photons into mechanical vibrations and storing them for up to 10 milliseconds. Although that is not long by human standards, it is a long time for a computer chip. “One of the long-term goals of the project is explore 2D crystal drums to study quantum motion. If you hit a classical drum with a stick, the drumhead will start oscillating, shaking up and down. With a quantum drum, however, you can not only make the drumhead move up and then down, but also make it into a ‘quantum superposition’, in which the drum head is both moving up and moving down at the same time ”, says research group leader Dr. Gary Steele. “This ‘strange’ quantum motion is not only of scientific relevance, but also could have very practical applications in a quantum computer as a quantum ‘memory chip’”.

    In a quantum computer, the fact that quantum ‘bits’ that can be both in the state 0 and 1 at the same time allow it to potentially perform computations much faster than a classical computer like those used today. Quantum graphene drums that are ‘shaking up and down at the same time’ could be used to store quantum information in the same way as RAM chips in your computer, allowing you to store your quantum computation result and retrieve it at a later time by listening to its quantum sound.

    See the full article, with video, here.

    The Kavli Foundation, based in Oxnard, California, is dedicated to the goals of advancing science for the benefit of humanity and promoting increased public understanding and support for scientists and their work.

    The Foundation’s mission is implemented through an international program of research institutes, professorships, and symposia in the fields of astrophysics, nanoscience, neuroscience, and theoretical physics as well as prizes in the fields of astrophysics, nanoscience, and neuroscience.

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