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  • richardmitnick 7:31 am on July 15, 2016 Permalink | Reply
    Tags: , , Graphene Studies   

    From EPFL: “Graphene could revolutionize the internet of things” 

    EPFL bloc

    École Polytechnique Fédérale de Lausanne EPFL

    08.07.16
    Laure-Anne Pessina

    1
    © Thinkstock

    Wireless communications come in many forms – such as mobile phones using 4G or 5G connectivity, GPS devices, and computers connected via Bluetooth to portable sensors – and operate in different frequency bands. To work across multiple platforms, connected objects have to be compatible with a whole range of frequencies without being weighed down by excessive hardware.

    Most portable, wireless systems currently come equipped with reconfigurable circuits that can adjust the antenna to transmit and receive data in the various frequency bands. The only problem is that the technologies currently available like MEMS and MOS, using silicon or metal, do not work well at high frequencies. And that’s where data can travel much faster.

    EPFL researchers have come up with a tunable graphene-based solution that enables circuits to operate at both low and high frequencies with unprecedented efficiency. Their work has been published in Nanoletters.

    The new graphene-based solution, which was developed in the Nanoelectronic Devices Laboratory, is designed to replace tunable capacitors, which can be found in all wireless devices. The new device “tunes” the circuits to different frequencies so that they can operate across a wide range of frequency bands. It also meets other needs that neither MEMS nor MOS capacitors can: good performance at high frequency, miniaturization and the ability to be tuned using low energy.

    The EPFL researchers overcame these obstacles with a graphene-based capacitor that is compatible with traditional circuits. The device consumes very little energy and, above 2.1 GHz, easily outperforms its competitors and has a miniaturized design. “The surface area of a conventional MEMS system would have to be a thousand times greater to get the capacitance value,” said Clara Moldovan.

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    © 2016 EPFL

    How does it work?

    The researchers’ breakthrough is based on a clever sandwich structure that takes graphene’s unique characteristics into account. “When graphene was discovered more than 10 years ago, it caused a real stir,” said Moldovan. “It was considered a miracle material: it is a very good electrical and thermal conductor and it is flexible, lightweight, transparent and sturdy. But researchers discovered that it was difficult to integrate into electronic systems because its atomic thickness gives it high effective resistance.”

    The sandwich-shaped structure takes advantage of the fact that a two-dimensional gas of electrons in a quantum well can behave like a quantum capacitance. This is because it follows the Pauli Exclusion Principle, according to which a certain amount of energy is needed to fill a quantum well with electrons. Quantum capacitance can be easily measured in a single-atom layer of graphene, and the key advantage is that it is tunable by varying the charge density in graphene with a very low voltage.

    “It’s by applying voltage that we can ‘tune’ our capacitors to a given frequency, just like tuning a radio to get different stations,” said Moldovan, the lead author of the article.

    Many advantages

    The EPFL researchers’ device, which is only several hundred micrometers (around 0.05 cm) long and wide, can be stiff or flexible, is easily miniaturized, and uses very little energy. Potential applications are numerous. In addition to improving the flow of data between connected devices, it could extend battery life and lead to ever more compact devices. In its flexible state, it could be easily used in sensors placed in clothes or directly on the human body. “Our results confirm that graphene could truly revolutionize the future of wireless communications,” said Moldovan.

    The end technology will be a hybrid in which graphene will be paired with advanced silicon technologies. “Some have claimed that graphene will one day replace silicon technology,” said Adrian Ionescu, the head of the Nanolab. “But in reality, graphene is most effective in the realm of electronics when it is combined with functional silicon blocks.”

    See the full article here .

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    EPFL campus

    EPFL is Europe’s most cosmopolitan technical university with students, professors and staff from over 120 nations. A dynamic environment, open to Switzerland and the world, EPFL is centered on its three missions: teaching, research and technology transfer. EPFL works together with an extensive network of partners including other universities and institutes of technology, developing and emerging countries, secondary schools and colleges, industry and economy, political circles and the general public, to bring about real impact for society.

     
  • richardmitnick 4:49 am on July 14, 2016 Permalink | Reply
    Tags: , Graphene Studies, ,   

    From New Scientist: “Graphene sheets open like a flower’s petals when poked” 

    NewScientist

    New Scientist

    13 July 2016
    Conor Gearin

    1
    Graphene holds promise in nanotechnology. Domhnall Malone

    Give graphene a diamond and you’ll get a flower in return. Researchers have found that poking a sheet of atom-thick graphene with a diamond tool causes tiny ribbons to peel away from the surface, like flower petals opening.

    “I don’t think anyone ever expected it,” says Graham Cross at Trinity College Dublin, Ireland, whose team made the discovery. Graphene sheets, which are made of a single layer of carbon atoms, are both superstrong and highly flexible. Other researchers have folded graphene into origami shapes using chemical reactions and tiny tools, but no one knew that a little prompting could cause graphene to tear and fold itself on its own.

    The find was an accident, discovered when the researchers were conducting an experiment to measure the friction of graphene by piercing it. Once their diamond tip punctured the sheet, they found that the energy from ambient heat was enough to cause the ribbons to keep tearing and unfolding into a tapered strip in less than a minute.

    “What for me is extraordinary is that it tears,” says Annalisa Fasolino at Radboud University in Nijmegen, the Netherlands. The atoms in a graphene sheet are bonded tightly, but there is only a weak attraction between sheets when stacked, she says. However, the researchers showed that after a layer started to tear, the weak attraction between the bottom of an unfolding ribbon and the sheet below was enough for the sheet’s internal bonds to keep ripping.

    Nanoscale control

    By changing the initial width of the tear, the researchers could control the length of the resulting ribbons, which tended to grow five times their initial width.

    “We’ve done crumpling, wrinkling and tearing of thin sheets, even one-atom-thin sheets like graphene, but not with such exquisite control,” says Scott Bunch at Boston University.

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    Graphene can be torn with a diamond tip, causing it to then self-fold – a property that could be useful in electronics. James Annett

    Graphene’s peculiar self-folding ability could be a big help in making better electronics, says Cross. By setting off ribbon formation in careful patterns, the sheets could be folded to make tiny sensors and even transistors. Such devices could allow for nanoscale electronics and fast-processing computers. “It would take a bit of work to do that, but it might be able to,” he says.

    Engineers could also prepare the sheets to tear at specific temperatures, says Cross. This could help in the food industry – for example, a graphene-based sensor on packaging could break a circuit if an item’s temperature rose above a safe level.

    Itai Cohen at Cornell University in Ithaca, New York, does not think graphene will replace silicon in electronic chips, but says graphene sheets could be used to manufacture very small robots. “Folding atomically thin sheets like graphene is a way of packing many, many features into a small 3D volume,” he says.

    Journal reference: Nature, DOI: 10:1038/nature18304

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  • richardmitnick 12:53 pm on May 16, 2016 Permalink | Reply
    Tags: , Graphene Studies, The quest to harvest light for electronics,   

    From U Washington: “UW researchers unleash graphene ‘tiger’ for more efficient optoelectronics” 

    U Washington

    University of Washington

    May 13, 2016
    James Urton

    1
    Image of one of the graphene-based devices Xu and colleagues worked with.Lei Wang

    In the quest to harvest light for electronics, the focal point is the moment when photons — light particles — encounter electrons, those negatively-charged subatomic particles that form the basis of our modern electronic lives. If conditions are right when electrons and photons meet, an exchange of energy can occur. Maximizing that transfer of energy is the key to making efficient light-captured energetics possible.

    “This is the ideal, but finding high efficiency is very difficult,” said University of Washington physics doctoral student Sanfeng Wu. “Researchers have been looking for materials that will let them do this — one way is to make each absorbed photon transfer all of its energy to many electrons, instead of just one electron in traditional devices.”

    In traditional light-harvesting methods, energy from one photon only excites one electron or none depending on the absorber’s energy gap, transferring just a small portion of light energy into electricity. The remaining energy is lost as heat. But in a paper* released May 13 in Science Advances, Wu, UW associate professor Xiaodong Xu and colleagues at four other institutions describe one promising approach to coax photons into stimulating multiple electrons. Their method exploits some surprising quantum-level interactions to give one photon multiple potential electron partners. Wu and Xu, who has appointments in the UW’s Department of Materials Science & Engineering and the Department of Physics, made this surprising discovery using graphene.

    “Graphene is a substance with many exciting properties,” said Wu, the paper’s lead author. “For our purposes, it shows a very efficient interaction with light.”

    Graphene is a two-dimensional hexagonal lattice of carbon atoms bonded to one another, and electrons are able to move easily within graphene. The researchers took a single layer of graphene — just one sheet of carbon atoms thick — and sandwiched it between two thin layers of a material called boron-nitride.

    “Boron-nitride has a lattice structure that is very similar to graphene, but has very different chemical properties,” said Wu. “Electrons do not flow easily within boron-nitride; it essentially acts as an insulator.”

    Xu and Wu discovered that when the graphene layer’s lattice is aligned with the layers of boron-nitride, a type of “superlattice” is created with properties allowing efficient optoelectronics that researchers had sought. These properties rely on quantum mechanics, the occasionally baffling rules that govern interactions between all known particles of matter. Wu and Xu detected unique quantum regions within the superlattice known as Van Hove singularities.

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    The Moiré superlattice they created by aligning graphene and boron-nitride. Sanfeng Wu

    “These are regions of huge electron density of states, and they were not accessed in either the graphene or boron-nitride alone,” said Wu. “We only created these high electron density regions in an accessible way when both layers were aligned together.”

    When Xu and Wu directed energetic photons toward the superlattice, they discovered that those Van Hove singularities were sites where one energized photon could transfer its energy to multiple electrons that are subsequently collected by electrodes— not just one electron or none with the remaining energy lost as heat. By a conservative estimate, Xu and Wu report that within this superlattice one photon could “kick” as many as five electrons to flow as current.

    With the discovery of collecting multiple electrons upon the absorption of one photon, researchers may be able to create highly efficient devices that could harvest light with a large energy profit. Future work would need to uncover how to organize the excited electrons into electrical current for optimizing the energy-converting efficiency and remove some of the more cumbersome properties of their superlattice, such as the need for a magnetic field. But they believe this efficient process between photons and electrons represents major progress.

    “Graphene is a tiger with great potential for optoelectronics, but locked in a cage,” said Wu. “The singularities in this superlattice are a key to unlocking that cage and releasing graphene’s potential for light harvesting application.”

    Co-authors were Lei Wang, Xian Zhang, Cory Dean and James Hone at Columbia University; You Lai and Zhiqiang Li at the National High Magnetic Field Laboratory in Florida; Wen-Yu Shan and Di Xiao at Carnegie Mellon University; former UW graduate student Grant Aivazian; and Takashi Taniguchi and Kenji Watanabe at the National Institute for Materials Science in Japan. The work at the UW was funded by the National Science Foundation and the U.S. Air Force Office of Scientific Research. Xu acknowledges the support from the Boeing Distinguished Professorship and Washington’s state-funded Clean Energy Institute.

    *Science paper:
    Multiple hot-carrier collection in photo-excited graphene Moiré superlattices

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    So what defines us — the students, faculty and community members at the University of Washington? Above all, it’s our belief in possibility and our unshakable optimism. It’s a connection to others, both near and far. It’s a hunger that pushes us to tackle challenges and pursue progress. It’s the conviction that together we can create a world of good. Join us on the journey.

     
  • richardmitnick 12:50 pm on April 6, 2016 Permalink | Reply
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    From EPFL: “A graphene chip filters light to boost communications” 

    EPFL bloc

    École Polytechnique Fédérale de Lausanne EPFL

    1

    06.04.16
    Hillary Sanctuary

    A microchip that filters out unwanted radiation with the help of graphene has been developed by scientists from the EPFL and tested by researchers of the University of Geneva (UNIGE). The invention could be used in future devices to transmit wireless data ten times faster.

    EPFL and UNIGE scientists have developed a microchip using graphene that could help wireless telecommunications share data at a rate that is ten times faster than currently possible. The results are published today in Nature Communications.

    “Our graphene based microchip is an essential building block for faster wireless telecommunications in frequency bands that current mobile devices cannot access,” says EPFL scientist Michele Tamagnone.

    Graphene acts like polarized sunglasses

    Their microchip works by protecting sources of wireless data — which are essentially sources of invisible radiation — from unwanted radiation, ensuring that the data remain intact by reducing source corruption.

    They discovered that graphene can filter out radiation in much the same way as polarized glasses. The vibration of radiation has an orientation. Like polarized glasses, their graphene-based microchip makes sure that radiation that only vibrates a certain way gets through. In this way, graphene is both transparent and opaque to radiation, depending on the orientation of vibration and signal direction. The EPFL scientists and their colleagues from Geneva used this property to create a device known as an optical isolator.

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    Faster Uploads in the Terahertz Bandwidth

    Moreover, their microchip works in a frequency band that is currently empty, called the terahertz gap.

    Wireless devices work today by transmitting data in the gigahertz range or at optical frequencies. This is imposed by technological constraints, leaving the potential of the terahertz band currently unexploited for data transmission.

    But if wireless devices could use this terahertz bandwidth, your future mobile phone could potentially send or receive data tens of times faster than now, meaning better sound quality, better image quality and faster uploads.

    The graphene-based microchip brings this terahertz technology a step closer to reality. This discovery addresses an important challenge that was so far unsolved due to lacking technologies, confirming once more the extraordinary physical properties of graphene.

    This joint project between EPFL and the University of Geneva was funded by the European Graphene Flagship project and by the Swiss National Science Foundation.

    See the full article here .

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    EPFL campus

    EPFL is Europe’s most cosmopolitan technical university with students, professors and staff from over 120 nations. A dynamic environment, open to Switzerland and the world, EPFL is centered on its three missions: teaching, research and technology transfer. EPFL works together with an extensive network of partners including other universities and institutes of technology, developing and emerging countries, secondary schools and colleges, industry and economy, political circles and the general public, to bring about real impact for society.

     
  • 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

    9/24/2015
    Kate McAlpine

    1

    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.

    2
    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

    1
    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.

    2
    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

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    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

      Like

  • 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

    1

    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!

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  • 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

    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
    Tags: , Graphene Studies,   

    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.

     
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