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  • richardmitnick 2:45 pm on March 16, 2015 Permalink | Reply
    Tags: , Graphene Studies,   

    From Rice: “Symmetry matters in graphene growth” 

    Rice U bloc

    Rice University

    March 16, 2015
    Mike Williams

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    See the full article here.

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    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
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    From EPFL Lausanne: “Graphene Meets Heat Waves” 

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    Ecole Polytechnique Federale Lausanne

    06.03.15
    Laure-Anne Pessina

    1

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

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

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

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

    Quasi-Lossless Propagation

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

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

    A wave of quantum heat

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

    See the full article here.

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


    SLAC Lab

    February 18, 2015

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

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

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

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

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

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

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

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

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

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

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

    See the full article here.

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

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    Rice University

    February 2, 2015
    Mike Williams

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    See the full article here.

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

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

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

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

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    Berkeley Lab

    January 12, 2015
    Rachel Berkowitz 510.486.7254

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

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

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

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

    2
    Felix Fischer (on left) and Mike Crommie

    Nanoribbons past and present

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

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

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

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

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

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

    Controlling quantum properties

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

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

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

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

    New directions toward new devices

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

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

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

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

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

    See the full article here.

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  • richardmitnick 8:57 pm on October 3, 2014 Permalink | Reply
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    From MIT: “Crumpled graphene could provide an unconventional energy storage” 


    MIT News

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

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

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

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

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

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

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

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

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

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

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

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

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

    See the full article here.

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  • richardmitnick 8:30 pm on September 9, 2014 Permalink | Reply
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    From Kavli: “Tiny Graphene Drum Could Form Future Quantum Memory” 

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    The Kavli Foundation

    09/09/2014
    No Writer Credit

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

    Graphene drums

    drum
    Graphene Drum

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

    Optomechanics

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

    Amplifier

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

    Memory

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

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

    See the full article, with video, here.

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

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

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  • richardmitnick 11:44 am on August 1, 2014 Permalink | Reply
    Tags: , Graphene Studies, , ,   

    From M.I.T.: “Light pulses control graphene’s electrical behavior” 


    M.I.T.

    July 31, 2014
    David L. Chandler | MIT News Office

    Graphene, an ultrathin form of carbon with exceptional electrical, optical, and mechanical properties, has become a focus of research on a variety of potential uses. Now researchers at MIT have found a way to control how the material conducts electricity by using extremely short light pulses, which could enable its use as a broadband light detector.

    graphene
    Researchers at MIT have found a way to control how graphene conducts electricity by using extremely short light pulses. In this illustration, a lattice of graphene is shown with its bonds (bars) connecting carbon atoms (balls). When the light pulse hits the atoms, electrons can accumulate or diminish in number. By controlling the concentration of electrons in a graphene sheet, researchers can change the material’s electrical conductivity.

    Illustration: Jose-Luis Olivares/MIT

    The new findings are published in the journal Physical Review Letters, in a paper by graduate student Alex Frenzel, Nuh Gedik, and three others.

    The researchers found that by controlling the concentration of electrons in a graphene sheet, they could change the way the material responds to a short but intense light pulse. If the graphene sheet starts out with low electron concentration, the pulse increases the material’s electrical conductivity. This behavior is similar to that of traditional semiconductors, such as silicon and germanium.

    But if the graphene starts out with high electron concentration, the pulse decreases its conductivity — the same way that a metal usually behaves. Therefore, by modulating graphene’s electron concentration, the researchers found that they could effectively alter graphene’s photoconductive properties from semiconductorlike to metallike.

    The finding also explains the photoresponse of graphene reported previously by different research groups, which studied graphene samples with differing concentration of electrons. “We were able to tune the number of electrons in graphene, and get either response,” Frenzel says.

    To perform this study, the team deposited graphene on top of an insulating layer with a thin metallic film beneath it; by applying a voltage between graphene and the bottom electrode, the electron concentration of graphene could be tuned. The researchers then illuminated graphene with a strong light pulse and measured the change of electrical conduction by assessing the transmission of a second, low-frequency light pulse.

    In this case, the laser performs dual functions. “We use two different light pulses: one to modify the material, and one to measure the electrical conduction,” Gedik says, adding that the pulses used to measure the conduction are much lower frequency than the pulses used to modify the material behavior. To accomplish this, the researchers developed a device that was transparent, Frenzel explains, to allow laser pulses to pass through it.

    This all-optical method avoids the need for adding extra electrical contacts to the graphene. Gedik, the Lawrence C. and Sarah W. Biedenharn Associate Professor of Physics, says the measurement method that Frenzel implemented is a “cool technique. Normally, to measure conductivity you have to put leads on it,” he says. This approach, by contrast, “has no contact at all.”

    Additionally, the short light pulses allow the researchers to change and reveal graphene’s electrical response in only a trillionth of a second.

    In a surprising finding, the team discovered that part of the conductivity reduction at high electron concentration stems from a unique characteristic of graphene: Its electrons travel at a constant speed, similar to photons, which causes the conductivity to decrease when the electron temperature increases under the illumination of the laser pulse. “Our experiment reveals that the cause of photoconductivity in graphene is very different from that in a normal metal or semiconductor,” Frenzel says.

    The researchers say the work could aid the development of new light detectors with ultrafast response times and high sensitivity across a wide range of light frequencies, from the infrared to ultraviolet. While the material is sensitive to a broad range of frequencies, the actual percentage of light absorbed is small. Practical application of such a detector would therefore require increasing absorption efficiency, such as by using multiple layers of graphene, Gedik says.

    Isabella Gierz, a professor at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany, who was not involved in this research, says, “The work is interesting because it presents a systematic study of the doping dependence of the low-energy dynamics, which has not received much attention so far.” She says the new research “certainly helps to reconcile previous apparently contradicting results,” and adds that these findings represent “a solid experiment, analysis, and interpretation.”

    The research team also included Jing Kong, the ITT Career Development Associate Professor of Electrical Engineering at MIT, who provided the graphene samples used for the experiments; physics postdoc Chun Hung Lui; and Yong Cheol Shin, a graduate student in materials science and engineering. The work received support from the U.S. Department of Energy and the National Science Foundation.

    See the full article here.


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  • richardmitnick 7:01 am on July 4, 2014 Permalink | Reply
    Tags: , , Graphene Studies, ,   

    From The Perimeter Institute: “From Pencil Marks To Quantum Computers” 

    Perimeter Institute
    Perimeter Institute

    July 3, 2014
    Erin Bow

    Pick up a pencil. Make a mark on a piece of paper. Congratulations: you are doing cutting-edge condensed matter physics. You might even be making the first mark on the road to quantum computers, according to new Perimeter research.

    Introducing graphene

    One of the hottest materials in condensed matter research today is graphene.

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

    Graphene had an unlikely start: it began with researchers messing around with pencil marks on paper. Pencil “lead” is actually made of graphite, which is a soft crystal lattice made of nothing but carbon atoms. When pencils deposit that graphite on paper, the lattice is laid down in thin sheets. By pulling that lattice apart into thinner sheets – originally using Scotch tape – researchers discovered that they could make flakes of crystal just one atom thick.

    The name for this atom-scale chicken wire is graphene. Those folks with the Scotch tape, Andre Geim and Konstantin Novoselov, won the 2010 Nobel Prize for discovering it. “As a material, it is completely new – not only the thinnest ever but also the strongest,” wrote the Nobel committee. “As a conductor of electricity, it performs as well as copper. As a conductor of heat, it outperforms all other known materials. It is almost completely transparent, yet so dense that not even helium, the smallest gas atom, can pass through it.”

    Developing a theoretical model of graphene

    Graphene is not just a practical wonder – it’s also a wonderland for theorists. Confined to the two-dimensional surface of the graphene, the electrons behave strangely. All kinds of new phenomena can be seen, and new ideas can be tested. Testing new ideas in graphene is exactly what Perimeter researchers Zlatko Papić and Dmitry (Dima) Abanin set out to do.

    p
    Perimeter postdoctoral researcher Zlatko Papić

    abannin
    Perimeter Faculty member Dmitry Abanin

    “Dima and I started working on graphene a very long time ago,” says Papić. “We first met in 2009 at a conference in Sweden. I was a grad student and Dima was in the first year of his postdoc, I think.”

    The two young scientists got to talking about what new physics they might be able to observe in the strange new material when it is exposed to a strong magnetic field.

    “We decided we wanted to model the material,” says Papić. They’ve been working on their theoretical model of graphene, on and off, ever since. The two are now both at Perimeter Institute, where Papić is a postdoctoral researcher and Abanin is a faculty member. They are both cross-appointed with the Institute for Quantum Computing (IQC) at the University of Waterloo.

    In January 2014, they published a paper in Physical Review Letters presenting new ideas about how to induce a strange but interesting state in graphene – one where it appears as if particles inside it have a fraction of an electron’s charge.

    It’s called the fractional quantum Hall effect (FQHE), and it’s head turning. Like the speed of light or Planck’s constant, the charge of the electron is a fixed point in the disorienting quantum universe.

    Every system in the universe carries whole multiples of a single electron’s charge. When the FQHE was first discovered in the 1980s, condensed matter physicists quickly worked out that the fractionally charged “particles” inside their semiconductors were actually quasiparticles – that is, emergent collective behaviours of the system that imitate particles.

    Graphene is an ideal material in which to study the FQHE. “Because it’s just one atom thick, you have direct access to the surface,” says Papić. “In semiconductors, where FQHE was first observed, the gas of electrons that create this effect are buried deep inside the material. They’re hard to access and manipulate. But with graphene you can imagine manipulating these states much more easily.”

    In the January paper, Abanin and Papić reported novel types of FQHE states that could arise in bilayer graphene – that is, in two sheets of graphene laid one on top of another – when it is placed in a strong perpendicular magnetic field. In an earlier work from 2012, they argued that applying an electric field across the surface of bilayer graphene could offer a unique experimental knob to induce transitions between FQHE states. Combining the two effects, they argued, would be an ideal way to look at special FQHE states and the transitions between them.

    Experimental tests

    Two experimental groups – one in Geneva, involving Abanin, and one at Columbia, involving both Abanin and Papić – have since put the electric field + magnetic field method to good use. The paper by the Columbia group appears in the July 4 issue of Science. A third group, led by Amir Yacoby of Harvard, is doing closely related work.

    “We often work hand-in-hand with experimentalists,” says Papić. “One of the reasons I like condensed matter is that often even the most sophisticated, cutting-edge theory stands a good chance of being quickly checked with experiment.”

    Inside both the magnetic and electric field, the electrical resistance of the graphene demonstrates the strange behaviour characteristic of the FQHE. Instead of resistance that varies in a smooth curve with voltage, resistance jumps suddenly from one level to another, and then plateaus – a kind of staircase of resistance. Each stair step is a different state of matter, defined by the complex quantum tangle of charges, spins, and other properties inside the graphene.

    “The number of states is quite rich,” says Papić. “We’re very interested in bilayer graphene because of the number of states we are detecting and because we have these mechanisms – like tuning the electric field – to study how these states are interrelated, and what happens when the material changes from one state to another.”

    For the moment, researchers are particularly interested in the stair steps whose “height” is described by a fraction with an even denominator. That’s because the quasiparticles in that state are expected to have an unusual property.

    There are two kinds of particles in our three-dimensional world: fermions (such as electrons), where two identical particles can’t occupy one state, and bosons (such as photons), where two identical particles actually want to occupy one state. In three dimensions, fermions are fermions and bosons are bosons, and never the twain shall meet.

    But a sheet of graphene doesn’t have three dimensions – it has two. It’s effectively a tiny two-dimensional universe, and in that universe, new phenomena can occur. For one thing, fermions and bosons can meet halfway – becoming anyons, which can be anywhere in between fermions and bosons. The quasiparticles in these special stair-step states are expected to be anyons.

    In particular, the researchers are hoping these quasiparticles will be non-Abelian anyons, as their theory indicates they should be. That would be exciting because non-Abelian anyons can be used in the making of qubits.

    Graphene qubits?

    Qubits are to quantum computers what bits are to ordinary computers: both a basic unit of information and the basic piece of equipment that stores that information. Because of their quantum complexity, qubits are more powerful than ordinary bits and their power grows exponentially as more of them are added. A quantum computer of only a hundred qubits can tackle certain problems beyond the reach of even the best non-quantum supercomputers. Or, it could, if someone could find a way to build stable qubits.

    The drive to make qubits is part of the reason why graphene is a hot research area in general, and why even-denominator FQHE states – with their special anyons – are sought after in particular.

    “A state with some number of these anyons can be used to represent a qubit,” says Papić. “Our theory says they should be there and the experiments seem to bear that out – certainly the even-denominator FQHE states seem to be there, at least according to the Geneva experiments.”

    That’s still a step away from experimental proof that those even-denominator stair-step states actually contain non-Abelian anyons. More work remains, but Papić is optimistic: “It might be easier to prove in graphene than it would be in semiconductors. Everything is happening right at the surface.”

    It’s still early, but it looks as if bilayer graphene may be the magic material that allows this kind of qubit to be built. That would be a major mark on the unlikely line between pencil lead and quantum computers.

    See the full article here.

    About Perimeter

    Perimeter Institute is a leading centre for scientific research, training and educational outreach in foundational theoretical physics. Founded in 1999 in Waterloo, Ontario, Canada, its mission is to advance our understanding of the universe at the most fundamental level, stimulating the breakthroughs that could transform our future. Perimeter also trains the next generation of physicists through innovative programs, and shares the excitement and wonder of science with students, teachers and the general public.

     
  • richardmitnick 7:25 am on May 8, 2014 Permalink | Reply
    Tags: , , Graphene Studies, ,   

    From physicsworld.com: “Lattice mismatch opens up a band gap in graphene” 

    physicsworld
    physicsworld.com

    May 7, 2014
    Anna Demming

    A new way of modifying the electronic properties of graphene has been discovered by a team led by Andre Geim and Kostya Novoselov at the University of Manchester. The physicists have shown that when graphene is grown on a hexagonal substrate, a small change in its crystal structure results in a gap opening in the material’s electron energy bands. They also found that graphene grown in this way can exist in an alternative structure in which the band gap is much smaller. The result could point to an exciting new way of controlling the electronic properties of graphene-based devices.

    mismatch
    Moiré pattern in a sample of of graphene-on-hBN

    Graphene is a honeycomb lattice of carbon just one atom thick that was first isolated in 2004 by Geim and Novoselov. Graphene is blessed with a wealth of fascinating electronic properties, many of which arise from the fact that it is a semiconductor with a zero-energy gap between its valence and conduction bands. One important consequence of how the bands meet is that conduction electrons travel through graphene at extremely high speeds. This means that the material could be used to create extremely fast electronic devices.

    But there is an important snag: electronic devices such as transistors rely on the fact that semiconductors such as silicon have a non-zero band gap. Therefore, the challenge for device developers is to create a modified version of graphene that has a band gap. Several schemes have been explored – including applying an electric field, adding chemical impurities or modifying the structure of graphene – but none have proved ideal.

    Moiré superlattices

    In this latest study, the Manchester team looked at graphene grown on hexagonal boron nitride (hBN), which has a lattice that is very similar to graphene. When the two lattices are overlaid in certain ways, a moiré superlattice is created (see figure). The periodic potential associated with this superlattice causes a number of new and interesting electronic phenomena to occur in graphene, including Hofstadter’s butterfly (see: “Hofstadter’s butterfly spotted in graphene”).

    Now the team has added “the commensurate–incommensurate transition” to the list of interesting phenomenon. In the commensurate state, the distance between carbon atoms in the graphene increases by about 1.8%, so that the lattice exactly matches that of hBN. This occurs when the two lattices are more or less aligned in a moiré structure. However, if this alignment is off by as little as one degree, the structure exists in an incommensurate state in which the graphene adopts its natural atomic spacing.

    “Although it is extremely difficult to rotate a graphene sheet on a hBN substrate, we have overcome this problem by making many samples at varying angles and testing each one,” explains the Manchester Condensed Matter Physics Group.

    Solitons and strain

    The team, which also includes researchers from China, the Netherlands, Russia and Japan, mapped the locations of commensurate and incommensurate states by measuring the strain across the graphene surface. “In the commensurate state, the strain distribution becomes very abrupt,” adds Woods. “This is because there must be a network of domain walls [marked yellow in the figure above], also known as solitons in 1D, between the stretched regions [grey/blue].”

    The team then measured the electronic properties of commensurate and incommensurate samples. In the former it found a relatively large band gap, and in the latter a much smaller gap. The team believes that this could explain why previous studies of graphene-on-hBN often resulted in conflicting values for the band gap.

    In addition to clearing up the confusion surrounding the value of the band gap, Woods believes that the research has identified a new and exciting way to control and fine-tune the electronic properties of graphene devices.

    The research is described in Nature Physics.

    See the full article here.

    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
    IOP Institute of Physics


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