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

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


    MIT News

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

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

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

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

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

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

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

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

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

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

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

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

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

    See the full article here.

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

    KavliFoundation

    The Kavli Foundation

    09/09/2014
    No Writer Credit

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

    Graphene drums

    drum
    Graphene Drum

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

    Optomechanics

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

    Amplifier

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

    Memory

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

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

    See the full article, with video, here.

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

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

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  • richardmitnick 11:44 am on August 1, 2014 Permalink | Reply
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    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
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    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
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    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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  • richardmitnick 3:02 pm on March 27, 2014 Permalink | Reply
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    From SLAC Lab- “Science with Bling: Turning Graphite into Diamond” 

    March 27, 2014
    Manuel Gnida

    A research team led by SLAC scientists has uncovered a potential new route to produce thin diamond films for a variety of industrial applications, from cutting tools to electronic devices to electrochemical sensors.

    lead
    SLAC researchers have found a new way to transform graphite — a pure form of carbon most familiar as the lead in pencils — into a diamond-like film. (Fabricio Sousa/SLAC)

    lattice
    This illustration shows four layers of transformed graphene (single sheets of graphite, with carbon atoms represented as black spheres) on a platinum surface (blue spheres). The addition of hydrogen atoms (green spheres) to the top layer has set off a domino effect that transformed this graphite-like material into a diamond-like film. The film is stabilized by bonds between the platinum substrate and the bottom-most carbon layer. (Sarp Kaya and Frank Abild-Pedersen/SUNCAT)

    The scientists added a few layers of graphene – one-atom thick sheets of graphite – to a metal support and exposed the topmost layer to hydrogen. To their surprise, the reaction at the surface set off a domino effect that altered the structure of all the graphene layers from graphite-like to diamond-like.

    “We provide the first experimental evidence that hydrogenation can induce such a transition in graphene,” says Sarp Kaya, researcher at the SUNCAT Center for Interface Science and Catalysis and corresponding author of the recent study.

    From Pencil Lead to Diamond

    Graphite and diamond are two forms of the same chemical element, carbon. Yet, their properties could not be any more different. In graphite, carbon atoms are arranged in planar sheets that can easily glide against each other. This structure makes the material very soft and it can be used in products such as pencil lead.

    In diamond, on the other hand, the carbon atoms are strongly bonded in all directions; thus diamond is extremely hard. Besides mechanical strength, its extraordinary electrical, optical and chemical properties contribute to diamond’s great value for industrial applications.

    Scientists want to understand and control the structural transition between different carbon forms in order to selectively transform one into another. One way to turn graphite into diamond is by applying pressure. However, since graphite is the most stable form of carbon under normal conditions, it takes approximately 150,000 times the atmospheric pressure at the Earth’s surface to do so.

    Now, an alternative way that works on the nanoscale is within grasp. “Our study shows that hydrogenation of graphene could be a new route to synthesize ultrathin diamond-like films without applying pressure,” Kaya says.

    Domino Effect

    For their experiments, the researchers loaded a platinum support with up to four sheets of graphene and added hydrogen to the topmost layer. With the help of intense X-rays from SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL, Beam Line 13-2) and additional theoretical calculations performed by SUNCAT researcher Frank Abild-Pedersen, the team then determined how hydrogen impacted the layered structure.

    SLAC SSRL
    Inside SSRL

    They found that hydrogen binding initiated a domino effect, with structural changes propagating from the sample’s surface through all the carbon layers underneath, turning the initial graphite-like structure of planar carbon sheets into an arrangement of carbon atoms that resembles diamond.

    The discovery was unexpected. The original goal of the experiment was to see if adding hydrogen could alter graphene’s properties in a way that would make it useable in transistors, the fundamental building block of electronic devices. Instead, the scientists discovered that hydrogen binding resulted in the formation of chemical bonds between graphene and the platinum substrate.

    It turns out that these bonds are crucial for the domino effect. “For this process to be stable, the platinum substrate needs to bond to the carbon layer closest to it,” Kaya explains. “Platinum’s ability to form these bonds determines the overall stability of the diamond-like film.”

    Future research will explore the full potential of hydrogenated few-layer graphene for applications in the material sciences. It will be particularly interesting to determine if diamond-like films can be grown on other metal substrates, using graphene of various thicknesses.

    The research team included scientists from Stanford University, the Stanford Institute for Materials & Energy Sciences (SIMES), SUNCAT and SSRL.

    See the full article here.

    SLAC Campus
    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.
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  • richardmitnick 12:53 pm on March 20, 2014 Permalink | Reply
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    From SLAC Lab: “Scientists Discover Potential Way to Make Graphene Superconducting” 

    March 20, 2014
    Press Office Contact:
    Andy Freeberg, afreeberg@slac.stanford.edu, (650) 926-4359

    Scientist Contact:
    Shuolong Yang, syang2@stanford.edu, (650) 725-0440

    Scientists at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University have discovered a potential way to make graphene – a single layer of carbon atoms with great promise for future electronics – superconducting, a state in which it would carry electricity with 100 percent efficiency.

    graph

    Researchers used a beam of intense ultraviolet light to look deep into the electronic structure of a material made of alternating layers of graphene and calcium. While it’s been known for nearly a decade that this combined material is superconducting, the new study offers the first compelling evidence that the graphene layers are instrumental in this process, a discovery that could transform the engineering of materials for nanoscale electronic devices.

    “Our work points to a pathway to make graphene superconducting – something the scientific community has dreamed about for a long time, but failed to achieve,” said Shuolong Yang, a graduate student at the Stanford Institute of Materials and Energy Sciences (SIMES) who led the research at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL).

    ssrl
    Stanford University / SLAC professor Zhi Xun Shen with a spectrometer at Stanford Synchrotron Radiation Lightsource (SSRL) Beamline 5-4.

    The researchers saw how electrons scatter back and forth between graphene and calcium, interact with natural vibrations in the material’s atomic structure and pair up to conduct electricity without resistance. They reported their findings March 20 in Nature Communications.

    Graphite Meets Calcium

    Graphene, a single layer of carbon atoms arranged in a honeycomb pattern, is the thinnest and strongest known material and a great conductor of electricity, among other remarkable properties. Scientists hope to eventually use it to make very fast transistors, sensors and even transparent electrodes.

    The classic way to make graphene is by peeling atomically thin sheets from a block of graphite, a form of pure carbon that’s familiar as the lead in pencils. But scientists can also isolate these carbon sheets by chemically interweaving graphite with crystals of pure calcium. The result, known as calcium intercalated graphite or CaC6, consists of alternating one-atom-thick layers of graphene and calcium.

    The discovery that CaC6 is superconducting set off a wave of excitement: Did this mean graphene could add superconductivity to its list of accomplishments? But in nearly a decade of trying, researchers were unable to tell whether CaC6’s superconductivity came from the calcium layer, the graphene layer or both.

    Observing Superconducting Electrons

    For this study, samples of CaC6 were made at University College London and brought to SSRL for analysis.

    “These are extremely difficult experiments,” said Patrick Kirchmann, a staff scientist at SLAC and SIMES. But the purity of the sample combined with the high quality of the ultraviolet light beam allowed them to see deep into the material and distinguish what the electrons in each layer were doing, he said, revealing details of their behavior that had not been seen before.

    “With this technique, we can show for the first time how the electrons living on the graphene planes actually superconduct,” said SIMES graduate student Jonathan Sobota, who carried out the experiments with Yang. “The calcium layer also makes crucial contributions. Finally we think we understand the superconducting mechanism in this material.”

    Although applications of superconducting graphene are speculative and far in the future, the scientists said, they could include ultra-high frequency analog transistors, nanoscale sensors and electromechanical devices and quantum computing devices.

    The research team was supervised by Zhi-Xun Shen, a professor at SLAC and Stanford and SLAC’s advisor for science and technology, and included other researchers from SLAC, Stanford, Lawrence Berkeley National Laboratory and University College London. The work was supported by the DOE’s Office of Science, the Engineering and Physical Sciences Research Council of UK and the Stanford Graduate Fellowship program.

    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, Calif., SLAC is operated by Stanford University for the U.S. Department of Energy’s Office of Science.

    The Stanford Institute for Materials and Energy Sciences (SIMES) is a joint institute of SLAC National Accelerator Laboratory and Stanford University. SIMES studies the nature, properties and synthesis of complex and novel materials in the effort to create clean, renewable energy technologies. For more information, visit simes.slac.stanford.edu.

    SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL) is a third-generation light source producing extremely bright X-rays for basic and applied science. A DOE national user facility, SSRL attracts and supports scientists from around the world who use its state-of-the-art capabilities to make discoveries that benefit society. For more information, visit ssrl.slac.stanford.edu.

    DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.

    See the full article here.

    SLAC Campus
    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.
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  • richardmitnick 5:08 pm on March 5, 2014 Permalink | Reply
    Tags: , Graphene Studies, , ,   

    From UC Berkeley: “Using Carbon to Control the Light” 

    UC Berkeley

    The flip of a light switch – a nano-scale light switch – may some day dramatically boost the speed of data transmission, from streaming movies to accelerating the most data-intense computation. Today, information flow in a computer is based on electrical pulses. But if an electrical signal could instead control a light switch, the “ones and zeros” that give data meaning could race through computer circuits at ten times the current speed. A ten-fold increase in speed would mean a similar spike in the volume of information that can be processed.

    fw
    Feng Wang performing optoelectronic measurements in the lab. Photo: Peg Skorpinski.

    Of course, electrical signals are used to modulate light in the optical fibers that transmit massive amounts of data around the corner and around the world. But harnessing light to boost communication between chips within a computer circuit has proved an elusive goal. At the scale of computer circuitry, materials such as silicon can’t absorb light efficiently, and devices that can perform well are too bulky to integrate into a chip.

    So excitement runs high that graphene, a material under intense study for only a decade, might do the trick. Single atom-thick carbon graphene crystals absorb all wavelengths of light, and at certain voltages, electrical pulses can turn the material’s light absorption on and off – the key to data transmission. This trait and graphene’s nano-size “footprint” make it an ideal candidate for ultra-miniature optical devices that could be installed by the thousands on a chip to control traffic flow.

    “We’re not there yet,” says Feng Wang, assistant professor of physics and a Bakar Fellow, “but graphene’s remarkable combination of electrical and optical properties, and its potential for nanofabrication hold great promise for optoelectronics.”

    Wang’s lab studies how electrical fields modulate the optical properties of a number of materials. The Bakar Fellows Program supports his efforts to develop graphene modulators for chip-to-chip communication. Because he’s manipulating photons, he can do much of the research under an optical microscope. At this relatively low magnification, a graphene layer looks like a continuous thin sheet. But under the power of a scanning tunneling microscope that can resolve individual atoms, the material’s chicken wire-like atomic configuration appears.

    Wang grew up in Nanchang in the south of China and went to college in Shanghai. He received his PhD in physics at Columbia and was a postdoc at Berkeley before joining the physics faculty. His focus on graphene’s potential to boost chip-to-chip performance in computers circuits began about six years ago. Before that, he studied carbon nanotubes, a one dimensional carbon material.

    fw2
    Feng Wang began to focus on graphene’s potential to boost chip-to-chip performance in computer circuits about six years ago. Photo: Peg Skorpinski.

    “Our lab mainly focuses on the fundamental physics of how light interacts with materials at the nano-scale, and what novel properties emerge,” Wang says. This holds a lot of fascination for me.

    “But exploring ways to exploit some of these novel behaviors in microelectronics is just as exciting. The basic research can reveal real-world applications. It’s a great combination.”

    Out on the horizon, Wang can see graphene integrated into infrared imagers and optical sensors, and possibly being used to detect telltale changes in diseased cells. Metabolism changes the pH, or acidity, of cells, and fast-metabolizing cancer cells have distinct metabolic signatures. Local pH variations, in turn, alter the optical absorption properties of graphene. This could be measured to aid in diagnosis.

    Similarly, graphene may one day aid detection of neurological disease. Neurons communicate with pulses of ions – their so-called “action potential” – and the release of ions modified the optical absorption of graphene. Such a change in graphene could potentially be used detect neuron activity.

    These applications are well down the line, Wang says, though not at all out of the question. For now, he seems fully absorbed by the physics of this truly absorbing nano-material.
    ____________________________

    The Bakar Fellows Program supports innovative research by early career faculty at UC Berkeley with a special focus on projects that hold commercial promise. For more information, see http://bakarfellows.berkeley.edu.

    See the full article here.

    Founded in the wake of the gold rush by leaders of the newly established 31st state, the University of California’s flagship campus at Berkeley has become one of the preeminent universities in the world. Its early guiding lights, charged with providing education (both “practical” and “classical”) for the state’s people, gradually established a distinguished faculty (with 22 Nobel laureates to date), a stellar research library, and more than 350 academic programs.

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  • richardmitnick 11:14 am on September 10, 2013 Permalink | Reply
    Tags: , Graphene Studies, , ,   

    From SLAC: “Stanford Scientists Use DNA to Assemble a Transistor From Graphene” 

    September 5, 2013
    Tom Abate, Stanford Engineering, tabate@stanford.edu, 650-736-2245

    DNA is the blueprint for life. Could it also become the template for making a new generation of computer chips based not on silicon, but on an experimental material known as graphene?

    That’s the theory behind a process that Stanford chemical engineering professor Zhenan Bao reveals in Nature Communications.

    Bao and her co-authors, former post-doctoral fellows Anatoliy Sokolov and Fung Ling Yap, hope to solve a problem clouding the future of electronics: consumers expect silicon chips to continue getting smaller, faster and cheaper, but engineers fear that this virtuous cycle could grind to a halt.

    Why has to do with how silicon chips work.

    Everything starts with the notion of the semiconductor, a type of material that can be induced to either conduct or stop the flow of electricity. Silicon has long been the most popular semiconductor material used to make chips.

    The basic working unit on a chip is the transistor. Transistors are tiny gates that switch electricity on or off, creating the zeroes and ones that run software.

    To build more powerful chips, designers have done two things at the same time: they’ve shrunk transistors in size and also swung those gates open and shut faster and faster.

    The net result of these actions has been to concentrate more electricity in a diminishing space. So far that has produced small, faster, cheaper chips. But at a certain point, heat and other forms of interference could disrupt the inner workings of silicon chips.

    “We need a material that will let us build smaller transistors that operate faster using less power,” Bao said.

    Graphene has the physical and electrical properties to become a next-generation semiconductor material – if researchers can figure out how to mass-produce it.

    graph
    To the right is a honeycomb of graphene atoms. To the left is a double strand of DNA. The white spheres represent copper ions integral to the chemical assembly process. The fire represents the heat that is an essential ingredient in the technique. (Anatoliy Sokolov)

    Graphene is a single layer of carbon atoms arranged in a honeycomb pattern. Visually it resembles chicken wire. Electrically this lattice of carbon atoms is an extremely efficient conductor.

    Bao and other researchers believe that ribbons of graphene, laid side-by-side, could create semiconductor circuits. Given the material’s tiny dimensions and favorable electrical properties, graphene nano ribbons could create very fast chips that run on very low power, she said.

    “However, as one might imagine, making something that is only one atom thick and 20 to 50 atoms wide is a significant challenge,” said co-author Sokolov.

    To handle this challenge, the Stanford team came up with the idea of using DNA as an assembly mechanism.

    Physically, DNA strands are long and thin, and exist in roughly the same dimensions as the graphene ribbons that researchers wanted to assemble.

    Chemically, DNA molecules contain carbon atoms, the material that forms graphene.

    The real trick is how Bao and her team put DNA’s physical and chemical properties to work.

    Now, it gets interesting. See the full article here.

    SLAC Campus
    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.
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  • richardmitnick 8:01 pm on August 12, 2013 Permalink | Reply
    Tags: , Graphene Studies, , , ,   

    From Berkeley Lab: “New Twist in the Graphene Story” 


    Berkeley Lab

    Berkeley Lab Researchers Discover a Tiny Twist in Bilayer Graphene That May Solve a Mystery

    August 12, 2013
    Lynn Yarris (510) 486-5375 lcyarris@lbl.gov

    “Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have discovered a unique new twist to the story of graphene, sheets of pure carbon just one atom thick, and in the process appear to have solved a mystery that has held back device development.

    Electrons can race through graphene at nearly the speed of light – 100 times faster than they move through silicon. In addition to being superthin and superfast when it comes to conducting electrons, graphene is also superstrong and superflexible, making it a potential superstar material in the electronics and photonics fields, the basis for a host of devices, starting with ultrafast transistors. One big problem, however, has been that graphene’s electron conduction can’t be completely stopped, an essential requirement for on/off devices.

    graph
    The Dirac spectrum of bilayer graphene when the two layers are exactly aligned (left) shifts with a slight interlayer twist that breaks interlayer-coupling and potential symmetry, leading to a new spectrum with surprisingly strong signatures in ARPES data. (Image courtesy of Keun Su Kim)

    Working at Berkeley Lab’s Advanced Light Source (ALS), a DOE national user facility, a research team led by ALS scientist Aaron Bostwick has discovered that in the stacking of graphene monolayers subtle misalignments arise, creating an almost imperceptible twist in the final bilayer graphene. Tiny as it is – as small as 0.1 degree – this twist can lead to surprisingly strong changes in the bilayer graphene’s electronic properties.

    ab
    Aaron Bostwick at Berkeley Lab’s Advanced Light Source led the discovery of a tiny twist in the formation of bilayer graphene that has a large impact on electronic properties. (Photo by Roy Kaltschmidt)

    ‘The introduction of the twist generates a completely new electronic structure in the bilayer graphene that produces massive and massless Dirac fermions,’ says Bostwick. ‘The massless Dirac fermion branch produced by this new structure prevents bilayer graphene from becoming fully insulating even under a very strong electric field. This explains why bilayer graphene has not lived up to theoretical predictions in actual devices that were based on perfect or untwisted bilayer graphene.’”

    See the full article here.

    A U.S. Department of Energy National Laboratory Operated by the University of California

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