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  • richardmitnick 11:17 am on September 24, 2015 Permalink | Reply
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    From Michigan: “Layered graphene beats the heat” 

    U Michigan bloc

    University of Michigan

    Kate McAlpine


    An international team of researchers, led by faculty at the University of Michigan, have found that a layered form of graphene can expel heat efficiently, which is an important feature for its potential applications in building small and powerful electronics.

    Graphene is an atomic-scale honeycomb lattice made of carbon atoms.

    “As you make devices smaller and smaller, you increase the amount of heat generated in a limited space, so you need to find ways to extract the heat,” said Momchil Mihnev, a doctoral student in electrical engineering and computer science at the University of Michigan and first author on the paper in Nature Communications.

    Graphene, a one-atom-thick sheet of carbon, has been hailed as the future of electronics because of its outstanding electrical conductivity. But a number of challenges stand between its promise and its adoption in commercial devices.

    One difficulty is simply producing the stuff. Arguably, the most reliable method is baking silicon carbide, a compound of silicon and carbon atoms, so that the silicon evaporates out, leaving behind a layered graphene structure. This type of graphene is most promising for electronic devices.

    But unlike heat in a 3D chunk of silicon, heat building up in the electrons of graphene was not expected to travel well between layers, since graphene layers tend not to interact with each other very strongly. Meanwhile, the chunk of silicon can conduct heat in any direction.

    Now, researchers led by Theodore Norris, the Gérard A. Mourou Collegiate Professor of Electrical Engineering and Computer Science, have shown the electrons can transmit heat efficiently between layers, enabling it to rapidly dissipate out of the 3D graphene. Mihnev explained that although the electrons in different layers can’t mechanically run into one another, they can interact through their electrical charges.

    The negative charges repel one another, giving the electrons an effective size that extends between the layers. When electrons collide in this way, the hotter of the two transfers energy to the colder. The heat is channeled into the graphene layers closest to the silicon carbide base because these layers had borrowed extra electrons from the silicon carbide. From there, the heat moves into the silicon carbide.

    “The reliability and performance of future graphene-based devices will depend critically on the availability of efficient mechanisms for the dissipation of excess generated heat,” Mihnev said.

    Norris’s team studied graphene samples produced by the group of Walter de Heer, a Regent Professor and professor of physics at the Georgia Institute of Technology in Atlanta. They shot a short and intense burst of laser light into the sample, causing the electrons to heat up to about 5,000°F. Then, they used a weak pulse of radiation that falls between far infrared and microwave on the electromagnetic spectrum to check the temperature of the electrons in the graphene.

    John Tolsma, co-first-author with Mihnev and a doctoral student in physics, and Allan MacDonald, the Sid W. Richardson Foundation Regents Chair Professor of Physics, both at The University of Texas at Austin, worked out a detailed new theory to explain the effect.

    “We believe that this cooling mechanism is not limited to multilayer graphene samples but is likely to be important in many other new, layered nanomaterials under active development by the scientific community,” said Norris.

    The paper on this work is called Electronic cooling via interlayer Coulomb coupling in multilayer epitaxial graphene. The study was supported in part by the National Science Foundation, the Welch foundation and the Department of Energy.

    See the full article here .

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    U MIchigan Campus

    The University of Michigan (U-M, UM, UMich, or U of M), frequently referred to simply as Michigan, is a public research university located in Ann Arbor, Michigan, United States. Originally, founded in 1817 in Detroit as the Catholepistemiad, or University of Michigania, 20 years before the Michigan Territory officially became a state, the University of Michigan is the state’s oldest university. The university moved to Ann Arbor in 1837 onto 40 acres (16 ha) of what is now known as Central Campus. Since its establishment in Ann Arbor, the university campus has expanded to include more than 584 major buildings with a combined area of more than 34 million gross square feet (781 acres or 3.16 km²), and has two satellite campuses located in Flint and Dearborn. The University was one of the founding members of the Association of American Universities.

    Considered one of the foremost research universities in the United States,[7] the university has very high research activity and its comprehensive graduate program offers doctoral degrees in the humanities, social sciences, and STEM fields (Science, Technology, Engineering and Mathematics) as well as professional degrees in business, medicine, law, pharmacy, nursing, social work and dentistry. Michigan’s body of living alumni (as of 2012) comprises more than 500,000. Besides academic life, Michigan’s athletic teams compete in Division I of the NCAA and are collectively known as the Wolverines. They are members of the Big Ten Conference.

  • richardmitnick 1:30 pm on September 13, 2015 Permalink | Reply
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    From UBC: “First superconducting graphene created by UBC researchers” 

    U British Columbia bloc

    University of British Columbia

    September 8, 2015
    No Writer Credit

    Researchers add lithium to graphene to create superconductivity. Credit: Andrea Damascelli.

    Graphene, the ultra-thin, ultra-strong material made from a single layer of carbon atoms, just got a little more extreme. UBC physicists have been able to create the first ever superconducting graphene sample by coating it with lithium atoms.

    Although superconductivity has already been observed in intercalated bulk graphite—three-dimensional crystals layered with alkali metal atoms, based on the graphite used in pencils—inducing superconductivity in single-layer graphene has until now eluded scientists.

    Andrea Damascelli

    “This first experimental realization of superconductivity in graphene promises to usher us in a new era of graphene electronics and nanoscale quantum devices,” says Andrea Damascelli, director of UBC’s Quantum Matter Institute and leading scientist of the Proceedings of the National Academy of Sciences study outlining the discovery.

    Graphene, roughly 200 times stronger than steel by weight, is a single layer of carbon atoms arranged in a honeycomb pattern. Along with studying its extreme physical properties, scientists eventually hope to make very fast transistors, semiconductors, sensors and transparent electrodes using graphene.

    “This is an amazing material,’” says Bart Ludbrook, first author on the PNAS paper and a former PhD researcher in Damascelli’s group at UBC. “Decorating monolayer graphene with a layer of lithium atoms enhances the graphene’s electron–phonon coupling to the point where superconductivity can be stabilized.”

    Given the massive scientific and technological interest, the ability to induce superconductivity in single-layer graphene promises to have significant cross-disciplinary impacts. According to financial reports, the global market for graphene reached $9 million in 2014 with most sales in the semiconductor, electronics, battery, energy, and composites industries.

    The researchers, which include colleagues at the Max Planck Institute for Solid State Research through the joint Max-Planck-UBC Centre for Quantum Materials, prepared the lithium-decorated graphene in ultra-high vacuum conditions and at ultra-low temperatures (-267 degrees Celsius or 5 Kelvin), to achieve this breakthrough.

    UBC’s Quantum Matter Institute

    UBC’s Quantum Matter Institute (QMI) is internationally recognized for its research and discoveries in quantum structures, quantum materials, and applications towards quantum devices. A recent $66.5-million investment from the Canada First Research Excellence Fund will broaden the scope of QMI’s research and support the discovery of practical applications for computing, electronics, medicine and sustainable energy technologies.

    Study: Evidence for superconductivity in Li-decorated monolayer graphene in Proceedings of the National Academy of Sciences. Tracking number: 2015-10435R

    See the full article here .

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    U British Columbia Campus

    The University of British Columbia is a global centre for research and teaching, consistently ranked among the 40 best universities in the world. Since 1915, UBC’s West Coast spirit has embraced innovation and challenged the status quo. Its entrepreneurial perspective encourages students, staff and faculty to challenge convention, lead discovery and explore new ways of learning. At UBC, bold thinking is given a place to develop into ideas that can change the world.

    • flowerpoet 3:47 pm on September 13, 2015 Permalink | Reply

      an amazing breakthrough with ever-increasing possibilities…thanks for sharing this info


  • richardmitnick 11:01 am on August 13, 2015 Permalink | Reply
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    From MIT Tech Review: “How to Make Graphene Using Supersonic Buckyballs” 

    MIT Technology Review
    M.I.T Technology Review

    August 13, 2015
    By C60 Supersonic Molecular Beam Epitaxy


    Graphene is one of the wonder materials of our age. It is some 200 times stronger than steel, it is an extraordinary conductor of heat and electricity and it is almost transparent. And yet making graphene is still tricky, particularly when it needs to sit on a substrate for applications such as electronics.

    Today, Simone Taioli at the Trento Institute for Fundamental Physics and Applications in Italy and a few pals say they’ve worked out how to do it starting with the famous football-shaped molecule buckminsterfullerene.

    Their idea is remarkably simple: bombard the substrate with buckyballs travelling at supersonic speeds. That’s fast enough to crack them open when they hit and the resulting unzipped cages then bond together to form a graphene film.

    Researchers have long thought of using buckyballs as a precursor for graphene. But the only way to get them to unzip and bind together is to heat them to temperatures in excess of around 600 °C.

    That’s not particularly effective because those high temperatures can change the properties of the substrate, in particular the amount of carbon it adsorbs. That results in irregular films with serious defects.

    The new technique gets around these problems. The team accelerates the buckyballs by releasing them into a helium or hydrogen gas which they allow to expand at supersonic speeds, carrying the carbon balls with it. That gives the buckyballs energies of around 40 keV without changing their internal dynamics (unlike ordinary heating which dramatically increases the molecular vibrations).

    These guys then aim the buckyballs at a copper sheet and watch them smash into it like flies onto a windscreen. The result is a fairly even coating of graphene-like material in a single layer.

    This material has its own idiosyncrasies. For a start, it is not made of regular hexagons, like perfect graphene. Instead it also contains pentagons which come from the original buckyball structures. That’s potentially useful because the pentagons could introduce a band gap into the material, something materials scientists have longed hoped to create in graphene.

    Although just a proof of principle at this stage, the technique looks interesting not least because it produces relatively high quality films and could also be applied to a wide range of other materials such as metals, semiconductors and insulators. And that could pave the way for a new generation of electronic devices.

    Interesting stuff!

    See the full article here.

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    The mission of MIT Technology Review is to equip its audiences with the intelligence to understand a world shaped by technology.

  • richardmitnick 12:03 pm on May 28, 2015 Permalink | Reply
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    From SLAC: “Spiraling Laser Pulses Could Change the Nature of Graphene” 

    SLAC Lab

    May 27, 2015

    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.

  • 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

    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.

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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

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

    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.

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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.

    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.

    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.

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    Rice U campus

    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
    Tags: , , Graphene Studies,   

    From EPFL Lausanne: “Graphene Meets Heat Waves” 

    EPFL bloc

    Ecole Polytechnique Federale Lausanne

    Laure-Anne Pessina


    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.

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    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
    Tags: , Graphene Studies, ,   

    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.

    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.

    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.

    Please help promote STEM in your local schools.

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    Stem Education Coalition

    SLAC Campus
    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.

  • richardmitnick 11:24 am on February 3, 2015 Permalink | Reply
    Tags: , Graphene Studies,   

    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

    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.

    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.

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Rice U campus

    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.

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