Mathis on Graphene? Any hints?

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Post by Cr6 on Mon Jul 23, 2018 10:57 pm

Also... a quantum style explanation:
https://en.wikipedia.org/wiki/Van_Hove_singularity

A Van Hove singularity is a singularity (non-smooth point) in the density of states (DOS) of a crystalline solid. The wavevectors at which Van Hove singularities occur are often referred to as critical points of the Brillouin zone. For three-dimensional crystals, they take the form of kinks (where the density of states is not differentiable). The most common application of the Van Hove singularity concept comes in the analysis of optical absorption spectra. The occurrence of such singularities was first analyzed by the Belgian physicist Léon Van Hove in 1953 for the case of phonon densities of states.[1]

Experimental observation

The optical absorption spectrum of a solid is most straightforwardly calculated from the electronic band structure using Fermi's Golden Rule where the relevant matrix element to be evaluated is the dipole operator A → ⋅ p → {\displaystyle {\vec {A}}\cdot {\vec {p}}} {\vec {A}}\cdot {\vec {p}} where A → {\displaystyle {\vec {A}}} {\vec {A}} is the vector potential and p → {\displaystyle {\vec {p}}} {\vec {p}} is the momentum operator. The density of states which appears in the Fermi's Golden Rule expression is then the joint density of states, which is the number of electronic states in the conduction and valence bands that are separated by a given photon energy. The optical absorption is then essentially the product of the dipole operator matrix element (also known as the oscillator strength) and the JDOS.

The divergences in the two- and one-dimensional DOS might be expected to be a mathematical formality, but in fact they are readily observable. Highly anisotropic solids like graphite (quasi-2D) and Bechgaard salts (quasi-1D) show anomalies in spectroscopic measurements that are attributable to the Van Hove singularities. Van Hove singularities play a significant role in understanding optical intensities in single-walled carbon nanotubes (SWNTs) which are also quasi-1D systems. The Dirac point in graphene is a Van-Hove singularity that can be seen directly as a peak in electrical resistance, when the graphene is charge-neutral. Twisted graphene layers also show pronounced Van-Hove singularities in the DOS due to the interlayer coupling.[6]

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Post by Jared Magneson on Mon Jul 23, 2018 11:59 pm

I feel like the theory is really solid, Cr6. I just have a harder time visualizing it than I should. As a matter of potentials it makes sense. I really hope I can attack it visually at some point, or of course Nevyn's method is preferred. But it seems like a pretty simple variance, shifting the input energy from electric to magnetic?

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Post by Cr6 on Tue Jul 24, 2018 11:54 pm

Here's more observed "weirdness" that Mathis at least explains by destroying the Quantum Hall Effect:
http://milesmathis.com/hall.pdf
http://milesmathis.com/stark.pdf

Columbia researchers observe exotic quantum particle in bilayer graphene

Scientists from Columbia University have reportedly proven a 30-year-old theory called "the even-denominator fractional quantum Hall state" and established bilayer graphene as a promising platform that could lead to quantum computation.

Columbia team observes exotic quantum particle in graphene image

The team observed an intensely studied anomaly in condensed matter physics—the even-denominator fractional quantum Hall (FQH) state—via transport measurement in bilayer graphene. “Observing the 5/2 state in any system is a remarkable scientific opportunity, since it encompasses some of the most perplexing concepts in modern condensed matter physics, such as emergence, quasi-particle formation, quantization, and even superconductivity,” the team says. “Our observation that, in bilayer graphene, the 5/2 state survives to much higher temperatures than previously thought possible not only allows us to study this phenomenon in new ways, but also shifts our view of the FQH state from being largely a scientific curiosity to now having great potential for real-world applications, particularly in quantum computing.”

First discovered in the 1980s in gallium arsenide (GaAs) heterostructures, the 5/2 fractional quantum hall state remains the singular exception to the otherwise strict rule that says fractional quantum hall states can only exist with odd denominators. Soon after the discovery, theoretical work suggested that this state could represent an exotic type of superconductor, notable in part for the possibility that such a phase could enable a fundamentally new approach to quantum computation. However, confirmation of these theories has remained elusive, largely due to the fragile nature of the state; in GaAs it is observable only in the highest quality samples and even then appearing only at milikelvin temperaures (as much as 10,000 times colder than the freezing point of water).

The Columbia team has observed this same state in bilayer graphene and appearing at much higher temperatures - reaching several Kelvin. “While it’s still 100 times colder than the freezing point of water, seeing the even-denominator state at these temperatures opens the door to a whole new suite of experimental tools that previously were unthinkable,” says the team. “After several decades of effort by researchers all over the world, we may finally be close to solving the mystery of the 5/2.”

“We needed a new platform,” say the researchers. “With the successful isolation of graphene, these atomically thin layers of carbon atoms emerged as a promising platform for the study of electrons in 2D in general. One of the keys is that electrons in graphene interact even more strongly than in conventional 2D electron systems, theoretically making effects such as the even-denominator state even more robust. But while there have been predictions that bilayer graphene could host the long-sought even-denominator states, at higher temperatures than seen before, these predictions have not been realized due mostly the difficulty of making graphene clean enough.”

The Columbia team managed to improve the quality of graphene devices, creating ultra-clean devices entirely from atomically flat 2D materials: bilayer graphene for the conducting channel, hexagonal boron nitride as a protective insulator, and graphite used for electrical connections and as a conductive gate to change the charge carrier density in the channel. A crucial component of the research was having access to the high magnetic fields tools available at the National High Magnetic Field Laboratory in Tallahassee, Fla., a nationally funded user facility with which Hone and Dean have had extensive collaborations. They studied the electrical conduction through their devices under magnetic fields up to 34 Tesla, and achieved clear observation of the even-denominator states.

“By tilting the sample with respect to the magnetic field, we were able to provide new confirmation that this FQH state has many of the properties predicted by theory, such as being spin-polarized,” says the paper’s lead author. “We also discovered that in bilayer graphene, this state can be manipulated in ways that are not possible in conventional materials.”

https://www.graphene-info.com/columbia-researchers-observe-exotic-quantum-particle-bilayer-graphene

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Post by Jared Magneson on Wed Jul 25, 2018 1:18 am

“After several decades of effort by researchers all over the world, we may finally be close to solving the mystery of the 5/2.”

I think we have confirmation of Miles' theory though with that last paragraph. Tilt was the key.

I still remain beyond skeptical when these people tell us that it's "atomically thin", though. They barely know what an atom is, and have no way to tell if it's one atom thin or ten. Their scales and radii are off by a great deal, if Miles' analysis is sound. The Bohr radius itself isn't what it was supposed to be.

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Post by Cr6 on Fri Jul 27, 2018 1:22 am

I agree. Looks like they are putting the publication cart before their theoretical horse.  

----------

This article was invited by Rolf J Haug.

Abstract

We review the electronic properties of bilayer graphene, beginning with a description of the tight-binding model of bilayer graphene and the derivation of the effective Hamiltonian describing massive chiral quasiparticles in two parabolic bands at low energies. We take into account five tight-binding parameters of the Slonczewski–Weiss–McClure model of bulk graphite plus intra- and interlayer asymmetry between atomic sites which induce band gaps in the low-energy spectrum. The Hartree model of screening and band-gap opening due to interlayer asymmetry in the presence of external gates is presented. The tight-binding model is used to describe optical and transport properties including the integer quantum Hall effect, and we also discuss orbital magnetism, phonons and the influence of strain on electronic properties. We conclude with an overview of electronic interaction effects.
http://iopscience.iop.org/article/10.1088/0034-4885/76/5/056503/pdf

-----------

Tunable moiré bands and strong correlations in small-twist-angle bilayer graphene

Kyounghwan Kim, Ashley DaSilva, Shengqiang Huang, Babak Fallahazad, Stefano Larentis, Takashi Taniguchi, Kenji Watanabe, Brian J. LeRoy, Allan H. MacDonald, and Emanuel Tutuc
PNAS March 28, 2017. 114 (13) 3364-3369; published ahead of print March 14, 2017.

https://doi.org/10.1073/pnas.1620140114

Significance

Accurately controlled, very long wavelength moiré patterns are realized in small-twist-angle bilayer graphene, and studied using electron transport and scanning probe microscopy. We observe gaps in electron transport at anomalous densities equal to ±8 electrons per moiré crystal unit cell, at variance with electronic structure theory, and the emergence of a Hofstadter butterfly in the energy spectrum in perpendicular magnetic fields. These findings open up an avenue to create artificial crystals by manipulating the relative angle between individual layers in a heterostructure.

Abstract

According to electronic structure theory, bilayer graphene is expected to have anomalous electronic properties when it has long-period moiré patterns produced by small misalignments between its individual layer honeycomb lattices. We have realized bilayer graphene moiré crystals with accurately controlled twist angles smaller than 1° and studied their properties using scanning probe microscopy and electron transport. We observe conductivity minima at charge neutrality, satellite gaps that appear at anomalous carrier densities for twist angles smaller than 1°, and tunneling densities-of-states that are strongly dependent on carrier density. These features are robust up to large transverse electric fields. In perpendicular magnetic fields, we observe the emergence of a Hofstadter butterfly in the energy spectrum, with fourfold degenerate Landau levels, and broken symmetry quantum Hall states at filling factors ±1, 2, 3. These observations demonstrate that at small twist angles, the electronic properties of bilayer graphene moiré crystals are strongly altered by electron–electron interactions.

   moiré crystalgraphenetwisted bilayermoiré bandHofstadter butterfly

Moiré patterns form when nearly identical two-dimensional (2D) crystals are overlaid with a small relative twist angle (1⇓⇓–4). The electronic properties of moiré crystals depend sensitively on the ratio of the interlayer hybridization strength, which is independent of twist angle, to the band energy shifts produced by momentum space rotation (5⇓⇓⇓⇓⇓⇓–12). In bilayer graphene, this ratio is small when twist angles exceed about 2° (10, 13), allowing moiré crystal electronic structure to be easily understood using perturbation theory (5). At smaller twist angles, electronic properties become increasingly complex. Theory (14, 15) has predicted that extremely flat bands appear at a series of magic angles, the largest of which is close to 1°. Flat bands in 2D electron systems, for example the Landau level bands that appear in the presence of external magnetic fields, allow for physical properties that are dominated by electron–electron interactions, and have been friendly territory for the discovery of fundamentally new states of matter. Here we report transport and scanning probe microscopy (SPM) studies of bilayer graphene moiré crystals with carefully controlled small-twist angles (STA), below 1°. We find that conductivity minima emerge in transport at neutrality, and at anomalous satellite densities that correspond to ±8 additional electrons in the moiré crystal unit cell, and that the conductivity minimum at neutrality is not weakened by a transverse electric field applied between the layers. Our observations can be explained only by strong electronic correlations.

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Post by Cr6 on Sun Aug 19, 2018 3:05 am

Surprise Graphene Discovery Could Unlock Secrets of Superconductivity

Physicists make misaligned sheets of the carbon material conduct electricity without resistance

By Elizabeth Gibney, Nature magazine on March 7, 2018

(More at link.... https://www.scientificamerican.com/article/surprise-graphene-discovery-could-unlock-secrets-of-superconductivity/ )
...
Physicists now report that arranging two layers of atom-thick graphene so that the pattern of their carbon atoms is offset by an angle of 1.1º makes the material a superconductor. And although the system still needed to be cooled to 1.7 degrees above absolute zero, the results suggest that it may conduct electricity much like known high-temperature superconductors — and that has physicists excited. The findings are published in two Nature papers1,2 on 5 March.

If confirmed, this discovery could be “very important” to the understanding of high-temperature superconductivity, says Elena Bascones, a physicist at the Institute of Materials Science of Madrid. “We can expect a frenzy of experimental activity over the next few months to fill in the missing parts of the picture,” says Robert Laughlin, a physicist and Nobel laureate at Stanford University in California.

Superconductors come broadly in two types: conventional, in which the activity can be explained by the mainstream theory of superconductivity, and unconventional, where it can’t. The latest studies suggest that graphene’s superconducting behaviour is unconventional — and has parallels with activity seen in other unconventional superconductors called cuprates. These complex copper oxides have been known to conduct electricity at up to 133 degrees above absolute zero. And although physicists have focused on cuprates for three decades in their search for room-temperature superconductors, the underlying mechanism has baffled them.

In contrast to cuprates, the stacked graphene system is relatively simple and the material is well-understood. “The stunning implication is that cuprate superconductivity was something simple all along. It was just hard to calculate properly,” says Laughlin.
Magic trick

Graphene already has impressive properties: its sheets, made of single layers of carbon atoms arranged in hexagons, are stronger than steel and conduct electricity better than copper. It has shown superconductivity before3, but it occurred when in contact with other materials, and the behaviour could be explained by conventional superconductivity.

Physicist Pablo Jarillo-Herrero at the Massachusetts Institute of Technology (MIT) in Cambridge and his team weren’t looking for superconductivity when they set up their experiment. Instead, they were exploring how the orientation dubbed the magic angle might affect graphene. Theorists have predicted that offsetting the atoms between layers of 2D materials at this particular angle might induce the electrons that zip through the sheets to interact in interesting ways — although they didn’t know exactly how.

The team immediately saw unexpected behaviour in their two-sheet set-up. First, measurements of graphene’s conductivity and the density of the particles that carry charge inside it suggested that the construction had become a Mott insulator2 — a material that has all the ingredients to conduct electrons, but in which interactions between the particles stop them from flowing. Next, the researchers applied a small electric field to feed just a few extra charge carriers into the system, and it became a superconductor1. The finding held up in experiment after experiment, says Jarillo-Herrero. “We have produced all of this in different devices and measured it with collaborators. This is something in which we’re very confident,” he says.

The existence of an insulating state so close to superconductivity is a hallmark of unconventional superconductors such as cuprates. When the researchers plotted phase diagrams that charted the material’s electron density against its temperature, they saw patterns very similar to those seen for cuprates. That provides further evidence that the materials may share a superconducting mechanism, says Jarillo-Herrero.

Finally, although graphene shows superconductivity at a very low temperature, it does so with just one-ten-thousandth of the electron density of conventional superconductors that gain the ability at the same temperature. In conventional superconductors, the phenomenon is thought to arise when vibrations allow electrons to form pairs, which stabilize their path and allow them to flow without resistance. But with so few available electrons in graphene, the fact that they can somehow pair up suggests that the interaction at play in this system should be much stronger than what happens in conventional superconductors.
Conductivity confusion

Physicists disagree wildly on how electrons might interact in unconventional superconductors. “One of the bottlenecks of high-temperature superconductivity has been the fact that we don’t understand, even now, what’s really gluing the electrons into pairs,” says Robinson.

But graphene-based devices will be easier to study than cuprates, which makes them useful platforms for exploring superconductivity, says Bascones. For example, to explore the root of superconductivity in cuprates, physicists often need to subject the materials to extreme magnetic fields. And ‘tuning’ them to explore their different behaviours means growing and studying reams of different samples; with graphene, physicists can achieve the same results by simply tweaking an electric field.

Kamran Behnia, a physicist at the Higher Institute of Industrial Physics and Chemistry in Paris, is not yet convinced that the MIT team can definitively claim to have seen the Mott insulator state, although he says the findings do suggest that graphene is a superconductor, and potentially an unusual one.

Physicists cannot yet state with certainty that the superconducting mechanism in the two materials is the same. And Laughlin adds that it is not yet clear that all the behaviour seen in cuprates is happening in graphene. “But enough of the behaviours are present in these new experiments to give cause for cautious celebration,” he says.

Physicists have been “stumbling around in the dark for 30 years” trying to understand cuprates, says Laughlin. “Many of us think that a light just switched on.”

This article is reproduced with permission and was first published on March 5, 2018.

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Post by Cr6 on Sun Aug 19, 2018 3:11 am

Posted: Jul 18, 2012
Graphene has the ability to mend itself

(more at link: https://www.nanowerk.com/spotlight/spotid=25983.php )
(Nanowerk Spotlight) Although graphene in itself has been dubbed the 'magic' material, if it is to be used for practical applications it has to integrated with the other components of possible devices. For instance, to exploit its amazing electron conduction properties, you still need to connect it to the rest of the circuit with contacts, which are typically made out of metal.

Understanding how metals interact – chemically and structurally – with graphene is therefore quite important and researchers have published a number of studies on the subject (see for instance this recent paper in Nano Letters: "Metal-Graphene Interaction Studied via Atomic Resolution Scanning Transmission Electron Microscopy").

In a quite unexpected discovery resulting from these observations, researchers have now found that graphene undergoes a self-repairing process to close holes that are caused by metal atoms. Reporting their findings in the July 5, 2012 online edition of Nano Letters ("Graphene Reknits Its Holes"), a team from The University of Manchester and SuperSTEM Laboratory, both in the UK, has shown that nanoscale holes (perhaps a 100 atoms missing or so), etched under an electron beam at room temperature in single-layer graphene sheets as a result of their interaction with metal impurities, heal spontaneously by filling up with either nonhexagon, graphene-like, or perfect hexagon 2D structures. In the process, loose carbon atoms will migrate over the surface of graphene spontaneously and will attach to the edges of the hole, filling it up quite quickly. The scientists were actually able to capture this mechanism in a series of images showing almost atom-by-atom how this hole filling process takes place.
"One of our earlier findings ("Direct Experimental Evidence of Metal-Mediated Etching of Suspended Graphene") was that in specific conditions, metal atoms – except for gold – seem to mediate a remarkable etching process: they help create holes in graphene," Quentin Ramasse, Scientific Director at SuperSTEM, tells Nanowerk. "In simple terms, put some metal near the graphene sheet, add some energy, most likely some oxygen as well, and the metals atoms will catalyze a bond breaking reaction. The carbon-carbon bonds break, a hole forms, more metal atoms are attracted to that hole and help breaking more bonds, and the hole keeps getting bigger until the reservoir of metal atoms is exhausted."

(keep in mind Miles' diagrams:
http://milesmathis.com/graphene.pdf
and
http://milesmathis.com/desig.pdf )

hole filling process in suspended graphene

Atomic resolution Z-contrast images illustrating the hole filling process in suspended graphene.
(a) A hole created at the border of the hydrocarbon contamination starts to 'mend' with C polygons.
(b) Complete reconstruction with incorporation of 5-7 rings and two 5-8 rings, and
(c) redistribution of defects in the “mended” region, by 5-7 rings.

Images (d-f) are processed versions of (a-c). A maximum entropy deconvolution algorithm was used and the contrast was optimized to visualize the carbon atoms. The carbon atom positions are highlighted by light green dots and polygons numbered according to the number of atoms in the rings. (Reprinted with permission from American Chemical Society)

Ramasse points out that this process does not take place without the metal atoms being present: "Our instruments allow us to observe graphene atom by atom for extremely long periods of time without any damage to the material."

The team – which included Recep Zan, Ursel Bangert and Konstantin S. Novoselov, who shared a Nobel prize as graphene's co-discoverer – were studying this phenomenon when they realized that some of the holes that had been created through the process described above were mending themselves, filling up with new carbon atoms that most likely came from a nearby 'reservoir' of carbon – essentially a patch of carbon-based contamination sitting not too far away from the hole.

"The fact that the hole was repairing itself is remarkable enough" says Ramasse. "Having said that, we know that holes/edges in graphene are not energetically favorable and loose carbon atoms can diffuse very fast on the surface of graphene, so it is not totally unexpected: if there is no more reason for the hole to be enlarged, in other words, if the etching process has stopped, then the material will try to compensate for this unstable hole that has been created and an easy way to do so is to fill it up."

"What was more remarkable" he continues, "was the fact that the hole did not necessarily fill up with perfect graphene lattice, but with C atoms somewhat randomly bonded to other carbon atoms, not in the usual honeycomb 6-fold pattern but in 5-, 6-, 7- or even 8-member rings without any obvious medium- or long-range order. In other words, what we observed is a 2-dimensional 'quasi'-amorphous structure."
The balance between the etching and filling mechanisms may be the difference between a working device and a proof of concept without any real application. Therefore, it may be somewhat re-assuring that if holes are created by putting metals and graphene in close proximity, this system has a tendency to self repair. However, the team's observations are very much on the fundamental side of things: they were looking at a model system, not a real metal contact in a graphene-based device.
From a more abstract point of view, the observation of two-dimensional amorphous structures is fascinating. "For obvious reasons, it is extremely hard to say anything about amorphous materials on the atomic level" says Ramasse. "There is no order, no repeat units, so how does one go about describing how atoms bond with one-another? Seeing a 2-dimensional version of it as we did, means that we can actually study atom by atom those quasi-random structures, and get a lot of insights about how these materials might look in the full 3 dimensions."

The foundation of all this recent nanoscale work on graphene and other materials is the fact that, thanks to recent technological advances, scientists now have tools to observe materials one atom at a time, including sensitive ones such as graphene which is only one atom thick.

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Post by Cr6 on Sun Aug 19, 2018 3:17 am

Scientists predict green energy revolution after incredible new graphene discoveries

(more at link: https://www.independent.co.uk/news/science/scientists-predict-green-energy-revolution-after-incredible-new-graphene-discoveries-9885425.html )

Recently discovered wonder-material could have major new applications
11/27/2014

Steve Connor

A recently discovered form of carbon graphite – the material in pencil lead – has turned out to have a completely unexpected property which could revolutionise the development of green energy and electric cars.

Researchers have discovered that graphene allows positively charged hydrogen atoms or protons to pass through it despite being completely impermeable to all other gases, including hydrogen itself.

The implications of the discovery are immense as it could dramatically increase the efficiency of fuel cells, which generate electricity directly from hydrogen, the scientists said.

Professor Sir Andrei Geim received the Nobel Prize in Physics in 2010 (Getty)

The breakthrough raises the prospect of extracting hydrogen fuel from air and burning it as a carbon-free source of energy in a fuel cell to produce electricity and water with no damaging waste products.

“In the atmosphere there is a certain amount of hydrogen and this hydrogen will end up on the other side [of graphene] in a reservoir. Then you can use this hydrogen-collected reservoir to burn it in the same fuel cell and make electricity,” said Professor Sir Andrei Geim of Manchester Univeristy.

Ever since its discovery 10 years ago, graphene has astonished scientists. It is the thinnest known material, a million times thinner than human hair, yet more than 200 times stronger than steel, as well as being the world’s best conductor of electricity.

A computer generated illustration of graphene cells (Corbis)

Until now, being permeable to protons was not considered a practical possibility, but an international team of scientists led by Sir Andre, who shares the 2010 Nobel Prize for his work on graphene, has shown that the one-atom thick crystal acts like a chemical filter. It allows the free passage of protons but forms an impenetrable barrier to other atoms and molecules.

“There have been three or four scientific papers before about the theoretical predictions for how easy or how hard it would be for a proton to go through graphene and these calculations give numbers that take billions and billions of years for a proton to go through this same membrane,” Sir Andrei said.

“It’s just so dense an electronic field it just doesn’t let anything through. But it’s a question of numbers, no more than that. This makes a difference between billions of years and a reasonable time for permeation. There is no magic,” he said.

The study, published in the journal Nature, shows that graphene and a similar single-atom-thick material called boron nitride allowed the build-up of protons on one side of a membrane, yet prevented anything else from crossing over into a collecting chamber.

.........
Graphene revolution: Fuel breakthrough could rival splitting the atom

Editorial
@IndyVoices
Wed
(more at link: https://www.independent.co.uk/voices/editorials/graphene-revolution-fuel-breakthrough-could-rival-splitting-the-atom-9886039.html )

They have both since won Nobel Prizes and been given knighthoods. Now one of them, Sir Andre Geim, has led a team that has uncovered another amazing property of this wonder material, which is effectively a new arrangement of carbon in the form of a one-atom-thick crystal layer. As well as being incredibly strong – more than 200 times as strong as steel – and extremely light, graphene conducts electricity extremely well and has a host of potential uses in electronics and in the sphere of new materials for such high-tech industries as aerospace and car manufacturing.

Now it appears that Sir Andre has found another potential use based on graphene’s ability to form a semi-permeable membrane that is porous to positively-charged hydrogen atoms, but to nothing much else. This could prove to be the deal breaker that transforms the hydrogen fuel-cell business, which has been somewhat stalled by the technical limitations.

Even more intriguing is the possibility that graphene may be used to “harvest” hydrogen from the air, providing a new source of carbon-free fuel. Combined with fuel-cell technology, the breakthrough could prove to be as important as splitting the atom in terms of energy.

The Government, and George Osborne in particular, must therefore be congratulated in recognising the immense potential of this British discovery (albeit by two émigré Russians) by sanctioning a £61m National Graphene Institute on the Manchester University campus.


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Post by Cr6 on Sun Aug 19, 2018 3:25 am

(Note Miles on Van Der Waals forces: http://milesmathis.com/strong2.html )

Graphene and Beyond: The Astonishing Properties and Promise of 2D Materials

By Thomas Hornigold -
Aug 05, 2018 5,888

...
2D materials can seem miraculous, to the extent that experiments were even done to see if graphene could be made bulletproof. It’s not all that far-fetched—although atomically thin, graphene is very efficient at transferring momentum through its lattice, and bulletproof materials like Kevlar often work by dissipating the energy from impact across a wider area. While it took 300 layers of graphene (with gaps between each layer) to stop a specially-designed “microbullet,” scientists last year discovered that two-layer graphene can undergo a phase transition to become harder and stiffer than diamond.

Since its discovery, graphene has been joined by new 2D materials.

Stanene is atomically thin tin; stacking multiple layers of stanene could result in a phase transition to superconductivity, even though tin in bulk isn’t superconductive. As yet, the transition temperature doesn’t put bilayer stanene in the range of high-temperature superconductors, but any new manifestation of superconductivity has physicists excited.

Germanium was an element that was initially of interest due to its electronic properties. Many of the earliest transistors used germanium instead of silicon, although until recently it has been supplanted by silicon, which is easier to use in mass manufacturing.

Now, with the isolation of germanene in 2014, individual layers of germanium are among the 2D materials touted alongside graphene. While graphene’s famous hexagonal crystal structure is flat, germanene’s crystal structure is buckled; its lattice consists of two vertically separated sub-lattices. External strain or applying external electric fields to germanene can cause its bandgap to change; this owes to that double-lattice structure, but can allow germanene to be used in field effect transistors. Not to be outdone, silicon itself has a monolayer counterpart in silicene.

The early hype around graphene’s applications has been replaced by a more steady approach. We don’t have bulletproof graphene planes, trains, and automobiles yet, but graphene is slowly but surely moving towards fulfilling its potential as more research into each possible application is conducted. Graphene-based sensors are already being widely produced. Despite all the hype around replacing silicon as the basic material in electronics, some of the first commercial uses of 2D materials like graphene have been put in sports gear.

In the longer term, it seems likely that graphene and other 2D materials will find their niches. In the meantime, the experimental insight that stacking together individual layers of atomically thin materials can result in new, unexpected, and useful properties has opened up a new field of research: van der Waals heterostructures. These materials exist in a transition regime—between the bulk properties of large-scale matter that we’re familiar with, and the quantum realm on the atomic level. The result is tantalizing for theoretical physicists and technologists alike.

These heterostructures are stacks of various layers of graphene, germanene, silicene, and stanene—but also molecular monolayers. They are named after the weak van der Waals forces that attract molecules to each other. These forces are due to the shifting distributions of charge in the layers of molecules interacting with each other.

These Van der Waals forces are weaker than electrostatic forces, and tail off more rapidly with distance, but they are enough to keep these “Lego-like” structures together. Planes of atoms in hexagonal 2D arrangements can be stacked, and then the possible range of fundamental physical properties and materials to study can be multiplied exponentially. Each newly-synthesized 2D material adds more potential combinations, and already layers thirteen deep composed of four different materials can be synthesized.

Consider, for example, the quest for a high-temperature superconductor. We know that this most desirable material property is subtly linked to the structure of the crystal lattice, as in the case of the YCBO structures. Stacking 2D materials offers an exciting new way to probe these phenomena experimentally.

“What if we mimic layered superconductors by using atomic-scale Lego? Bismuth strontium calcium copper oxide superconductors (BSCCO) can be disassembled into individual atomically thin planes. Their reassembly with some intelligently guessed differences seems worth a try, especially when the mechanism of high temperature superconductivity remains unknown,” wrote Professor Geim in Nature.

For the moment, graphene remains the most likely 2D material to see near-term applications, partly due to the funding for its research and partly because it can still be produced more swiftly. The exfoliation method of gradually pulling apart layers of graphite to obtain graphene can’t be used with every 2D material, even though it produces the purest crystals.

Many of the more exotic materials must be produced by molecular beam epitaxy—painstakingly depositing individual atoms onto a surface at conditions of high vacuum and high temperature. This will limit the mass-manufacturing possibilities, or bulk uses for 2D crystals, until MBE gets cheaper—or, perhaps more likely given the high temperatures and vacuum needed for MBE, until another manufacturing technique is perfected.

Yet it seems inevitable, given the demand for ever-improved electronic components for batteries, semiconductors and transistors, and for optoelectronics like solar panels and LEDs, that we will learn how best to exploit the astonishing properties of 2D materials. This is the dream of manufacturing reaching the cutting edge of fine-tuning fundamental physical properties with a careful choice of materials. Like kids with a Lego set, the only limit to what we can build may be our imagination.

Thomas Hornigold is a physics student at the University of Oxford. When he's not geeking out about the Universe, he hosts a podcast, Physical Attraction, which explains physics - one chat-up line at a time.

(more at link:  https://singularityhub.com/2018/08/05/beyond-graphene-the-promise-of-2d-materials/  )

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Post by Cr6 on Wed Aug 29, 2018 2:07 am

Twistronics’ tunes 2D material properties
23 Aug 2018 Belle Dumé
Varying the angle between crystals
Twisted electronics

(more at link: https://physicsworld.com/a/twistronics-tunes-2d-material-properties/ )

Researchers at Columbia University in the US have developed a new device structure in which they can vary the “twist” angle between layers of 2D materials (such as graphene) and study how this angle affects their electronic, optical and mechanical properties. The measurements, which are carried out on a single structure rather than multiple ones (as was the case before), could advance the emerging field of “twistronics” – a fundamentally new approach to device engineering.

“In recent years, researchers have realized that the weak coupling between different layers of 2D materials can be used to manipulate these materials in ways that are not possible with more conventional structures,” explains Cory Dean, who led this research effort together with James Hone. “One dramatic example is being able to modify their electronic properties by varying the angle between the layers.

“For instance, graphene (a 2D sheet of carbon atoms) normally does not have a band gap. It develops one, however, when placed in contact with another 2D material, hexagonal boron nitride, which has a closely matching lattice constant. The layers of graphene and boron nitride form what is called a large “Moiré superlattice”. By then twisting the layers so that they become misaligned and the angle between them becomes large, the band gap disappears.


Magic-angle graphene superlattice
Graphene moiré superlattice

“Simply varying the angle between 2D material layers thus means that graphene can be tuned from being metallic to semiconducting. Indeed, researchers at the Massachusetts Institute of Technology (MIT) recently discovered that placing two layers of graphene together, but rotated relative to one another at the ‘magic’ angle of 1.1° turns the normally metallic material into a superconductor.”

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Mathis on Graphene?  Any hints?  - Page 4 Empty The Effective mass in graphene

Post by Cr6 on Wed Aug 29, 2018 2:18 am

Effective mass of electron in monolayer graphene: Electron-phonon interaction
E Tiras, S Ardali, T Tiras, E Arslan, S Cakmakyapan, O Kazar, Jawad Hassan, Erik Janzén
and E Ozbay
http://liu.diva-portal.org/smash/get/diva2:612343/FULLTEXT01.pdf

http://www.philiphofmann.net/book_material/notes/graphene_mass2.pdf

The effective mass in graphene

Among graphene's many interesting properties, its extremely high electrical conductivity and electron mobility are particularly remarkable [1]. In fact, the room temperature conductivity of graphene is higher than that of any other known material. There are two important factors contributing to this:

The first is the high Debye temperature in graphene that suppresses phonon scattering and the second
is the very special electric structure with the linear dispersion and density of states
close to the Fermi energy, as shown in Figure 6.15. An apparent contradiction with
these properties arises when we apply our usual definition of the effective mass to
graphene. We have defined the effective mass as (see formula: )

Applying this to a linear dispersion with E(k) / k clearly results in an diverging
effective mass, something that appears to imply that it is impossible to drag the
electrons through graphene by an external field, in contrast to the experimental
observations. Matters are made even more confusing by the fact that the electrons
in graphene are often called \massless", in drastic contrast to what (1) appears
to suggest. The purpose of this note is to explain this. For a nice more in-depth
discussion see also Refs. [2, 3].

The key-issue with the apparent contradiction is our semi-classical definition of the
effective mass that implicitly assumes a parabolic band dispersion. In the following
discussion, we show that the problem can be cured using an alternative expression
for the effective mass.

-------

Graphene membrane

Researchers at Manchester University have discovered that the rate at which graphene conducts protons increases 10 fold when it is illuminated with sunlight.

Dubbed the “photo-proton” effect, the finding could lead to graphene membranes being used to produce hydrogen from artificial photosynthesis, as well as for light-induced water splitting, photo-catalysis and in photodetectors.

Graphene – a one atom-thick sheet of carbon – is already known to be an extremely good conductor of electrons, and can absorb light of all wavelengths.

But it has also recently been found to be permeable to thermal protons, the nuclei of hydrogen atoms.

To discover how light affects the behaviour of these protons, the researchers fabricated graphene membranes and decorated them on one side with platinum nanoparticles.

When they illuminated the membrane with sunlight, they found the proton conductivity increased by 10 times, according to Dr Marcelo Lozada-Hidalgo, who led the research alongside Prof Sir Andre Geim.

“This is a new effect, it can only be found in graphene, there are no other materials that can use light to produce an enhancement in proton transport,” said Lozada-Hidalgo. “Scientifically this is a new physical phenomenon, which is quite remarkable.”

What’s more, when the researchers measured the photoresponsivity of the membrane using electrical measurements and mass spectrometry, they discovered that around 5,000 hydrogen molecules were being formed in response to every light particle. Existing photovoltaic devices need thousands of photons to produce a single hydrogen molecule.

“To put this in context, people have been developing silicon photodiodes for the best part of 50 years, while we did not expect this material to be responsive to light in the first place, and found that it outperforms pretty much everything that is out there,” said Lozada-Hidalgo.

(more at link:  https://www.theengineer.co.uk/graphene-photosynthesis-membranes/  )

--------

Theory of resonant photon drag in monolayer graphene

M. V. Entin and L. I. Magarill
Institute of Semiconductor Physics, Siberian Branch of Russian Academy of Sciences, Novosibirsk 630090, Russia
http://www.quantware.ups-tlse.fr/dima/myrefs/my185.pdf


Last edited by Cr6 on Wed Aug 29, 2018 2:42 am; edited 1 time in total

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Post by Cr6 on Wed Aug 29, 2018 2:35 am


MIT may have just solved how to mass-produce graphene

by Colm Gorey

19 Apr 2018  4.67k Views
MIT may have just found how to mass produce graphene

(more at link:  https://www.siliconrepublic.com/machines/mass-produce-graphene-solved )

If graphene is to go mainstream, it needs to be mass-produced, which is where a new breakthrough from MIT comes in.

Video on Process:

...

Pinpointing what uses such a manufacturing method could have, the team said it would be ideal for desalination and biological separation in particular, but not limited to those.

To achieve the breakthrough, the team led by the director of the Laboratory for Manufacturing and Productivity at MIT, John Hart, turned to a common industrial manufacturing process for thin foils, known as the roll-to-roll approach.

This is then combined with the common graphene fabrication technique of chemical vapour deposition, whereby copper foil is fed into a heated tube before mixing with methane and hydrogen gas, creating a layer of graphene foil.

‘Like a continuous bed of pizza’

“Graphene starts forming in little islands, and then those islands grow together to form a continuous sheet,” Hart said.

“By the time it’s out of the oven, the graphene should be fully covering the foil in one layer, kind of like a continuous bed of pizza.”

The results of the experiments showed that the process could produce graphene at 5cm per minute, with its longest run lasting for almost four hours, producing 10 metres of continuous graphene.

Hart added that if it were running in a factory 24/7, it would be able to essentially create a printing press of the so-called wonder material.

The next step is to see how the team can include polymer casting, as well as other methods that are currently performed by hand, in the roll-to-roll system.

“For now, we’ve demonstrated that this process can be scaled up, and we hope this increases confidence and interest in graphene-based membrane technologies, and provides a pathway to commercialisation,” he said.

Related: materials science, MIT, graphene

Colm Gorey is a journalist with Siliconrepublic.com

editorial@siliconrepublic.com

........


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Post by Cr6 on Mon Sep 03, 2018 10:52 pm


Graphene smart membranes can control water
July 12, 2018, University of Manchester

Credit: University of Manchester

Researchers at The University of Manchester's National Graphene Institute (NGI) have achieved a long-sought-after objective of electrically controlling water flow through membranes, as reported in Nature.

This is the latest exciting membranes development benfitting from the unique properties of graphene. The new research opens up an avenue for developing smart membrane technologies and could revolutionise the field of artificial biological systems, tissue engineering and filtration.

Graphene is capable of forming a tuneable filter or even a perfect barrier when dealing with liquids and gases. New 'smart' membranes developed using an inexpensive form of graphene called graphene oxide, have been demonstrated to allow precise control of water flow by using an electrical current. The membranes can even be used to completely block water from passing through when required.

The team, led by Professor Rahul Nair, embedded conductive filaments within the electrically insulating graphene oxide membrane. An electric current passed through these nano-filaments created a large electric field which ionises the water molecules and thus controls the water transport through the graphene capillaries in the membrane.

Prof Nair said: "This new research allows us to precisely control water permeation, from ultrafast permeation to complete blocking. Our work opens up an avenue for further developing smart membrane technologies.

"Developing smart membranes that allow precise and reversible control of molecular permeation using external stimuli would be of intense interest for many areas of science; from physics and chemistry, to life-sciences."
Credit: University of Manchester

The achievement of electrical control of water flow through membranes is a step change because of its similarity to several biological process where the main stimuli are electrical signals. Controlled water transport is a key for renal water conservation, regulation of body temperature and digestion. The reported electrical control of water transport through graphene membranes therefore opens a new dimension in developing artificial biological systems and advanced nanofluidic devices for various applications.

Previously, the research group have demonstrated that graphene oxide membranes can be used as a sieve to remove salt from seawater for desalination alternatives. Last year they also showed that the membranes could remove the colour pigment from whisky without affecting its other properties.

(More at link: https://phys.org/news/2018-07-graphene-smart-membranes.html )

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Post by Cr6 on Mon Sep 03, 2018 10:55 pm


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Post by Cr6 on Tue Sep 11, 2018 1:29 am

Sorry guys...I just realized how muchI've been over-posting on this thread...like really over posting instead.... Shocked

But here are a few more:

https://nano-magazine.com/news/2018/1/25/photon-friendly-graphene-membranes-mimic-photosynthesis-to-produce-hydrogen

https://pubs.acs.org/doi/abs/10.1021/nl200587h

Giant Two-Photon Absorption in Bilayer Graphene
Hongzhi Yang†, Xiaobo Feng†‡, Qian Wang†, Han Huang†, Wei Chen†§, Andrew T. S. Wee†, and Wei Ji*†
Department of Physics, National University of Singapore, Singapore 117542
School of Physics and Electronic Information Technology, Yunnan Normal University, Kunming, China 650092
Department of Chemistry, National University of Singapore, Singapore 117543
Nano Lett., 2011, 11 (7), pp 2622–2627
DOI: 10.1021/nl200587h
Publication Date (Web): June 8, 2011
Copyright ©️ 2011 American Chemical Society
E-mail address: phyjiwei@nus.edu.sg.
Cite this:Nano Lett. 11, 7, 2622-2627

Abstract


We present a quantum perturbation theory on two-photon absorption (2PA) in monolayer and bilayer graphene which is Bernal-stacked. The theory shows that 2PA is significantly greater in bilayer graphene than monolayer graphene in the visible and infrared spectrum (up to 3 μm) with a resonant 2PA coefficient of up to ∼0.2 cm/W located at half of the bandgap energy, γ1 = 0.4 eV. In the visible and terahertz region, 2PA exhibits a light frequency dependence of ω–3 in bilayer graphene, while it is proportional to ω–4 for monolayer graphene at all photon energies. Within the same order of magnitude, the 2PA theory is in agreement with our Z-scan measurements on high-quality epitaxial bilayer graphene deposited on SiC substrate at light wavelength of 780 and 1100 nm.
Mathis on Graphene?  Any hints?  - Page 4 Nl-2011-00587h_0003


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Post by Cr6 on Tue Sep 11, 2018 1:33 am

This paper from Miles is a good read:
The Specific Heat Problem of Electrons another major mainstream fudge by Miles Mathis

http://milesmathis.com/fermi.pdf

As it turns out, the Fermi energy (and Fermi level) can just as easily be assigned to charge photons, and
if that assignment is made we no longer need all the magical pushes like quantum tunneling, band
structures, electron holes, ideal crystals, and so on. Assigning the Fermi energy to photons instead of
electrons immediately simplifies all solid state theory, conduction theory, and heat theory by many
orders of magnitude.

It also solves the electron problem of specific heat. If the electron isn't the field particle of either
conduction or heat, then the original expectations vanish. This also ties into the problem of heat
capacity, which I have already solved in a previous paper. See below where I gloss it again for good
measure.

http://milesmathis.com/drude.pdf

Also notice how the explanation at Hyperphysics elides from conducted electrons to valence electrons.
But conducted electrons must be free: how else are they conducted from place to place? Valence
electrons aren't free. The definition of a valence electron is “one that is associated with an atom.”
Heat can't be transferred by electrons associated with atoms, unless they are proposing the atoms are
dragged along in conduction as well. So whether or not valence electrons are responding to kT is
beside the point. The original problem concerned the fact that conducted electrons were not adding to
the heat, and that is even admitted at Hyperphysics. Where? In their statement of the original
problem:

One of the great mysteries in physics in the early part of the 20th century was why electrons didn't appear to
contribute to specific heat. How could they contribute to electrical conduction and heat conduction and not to
specific heat?
If they are contributing to electrical conduction or heat conduction, they aren't valence electrons. So
the entire Hyperphysics explanation is just misdirection. As more proof of that, we can compare the
Hyperphysics explanation to the explanation at the number two site that comes up on a search. This is
the site at Drexel University. There, it says this in the first box:

When a metal specimen is heated from absolute zero, not every conduction electron gains an energy ~k T as
expected classically.

See, they say “conduction electron.” That would be “free electron,” not “valence electron.” These
major sites can't even fudge you in the same way on the same longstanding question.
Beyond that, you can't have valence electrons in a Fermi gas, since a valence is a type of charge
interaction. When talking of Fermi models, the fermions are non-interacting, which precludes charge
interaction.
--------------
TERS Imaging of Twisted Bilayer Graphene
Graphene observed with nanometer resolution

Graphene is famous for its gapless band structure called Dirac cones. This unique band structure makes electrons in graphene behave like massless Dirac fermions, and gives graphene some special properties such as extraordinarily high mobility and ballistic transport. These extraordinary features make graphene an ideal material for nano electronics, for example, thin-film transistors, transparent and conductive composites and electrodes, flexible and printable electronics[1].

The electronic properties of these nano electronics strongly depend on the integrity of a designed graphene sheet. Any change of its intrinsic structure, such as local strain, defect or contaminants will modify the electronic properties, results a favorable feature or an unexpected defect.

Tip-enhanced Raman spectroscopy (TERS) is able to break the diffraction limit and take a Raman image with a resolution of 10 ~ a few tens of nanometers[2]- [4]. Figures below show TERS images of 2D/G ratio, 2D band, G band and D band of graphene. For comparison, an AFM image is attached at the bottom. From 2D/G ratio image, single-layer, twisted bilayer (explained below) and multilayer are distinguished as indicated by different colors. 2D band and G band images indicate the layer difference as well. At the edges and sheet ripples of graphene, D band image distinctly indicates defects and local strain respectively. Such details of edges and sheet ripples is confirmed from the AFM image.

https://www.nanophoton.net/applications/35.html
............
Synopsis: Graphene Helps Catch Light Quanta
August 24, 2017
The use of graphene in a single-photon detector makes it dramatically more sensitive to low-frequency light.
Synopsis figure
E. D. Walsh et al., Phys. Rev. Applied (2017)

For many light-based quantum applications, failing to log the arrival of even a few photons can undermine performance. Some single-photon detectors work by registering a temperature rise when they absorb one photon, but this sensitivity diminishes for small photon energies (low frequencies.) Researchers have now shown that incorporating graphene into a particular type of single-photon detector could extend the lower end of the detector’s frequency range by four decades, to include gigahertz light (radio waves).

The device, proposed by Kin Chung Fong from Raytheon BBN Technologies, Massachusetts, and colleagues, sandwiches a sheet of graphene between two layers of superconducting material to create a Josephson junction. At low temperatures, and in the absence of photons, a superconducting current flows through the device. But the heat from a single photon is sufficient to warm the graphene, which alters the Josephson junction such that no superconducting current can flow. Thus photons can be detected by monitoring the device’s current.

https://physics.aps.org/synopsis-for/10.1103/PhysRevApplied.8.024022

...
(Hmm..."hot electrons" how about Mathis' Charge Field flows?)

21 Jan 2015 | 21:00 GMT
Proven: Graphene Makes Multiple Electrons From Light
Graphene could make super high conversion-efficiency photovoltaics
By Dexter Johnson

Researchers at École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have for the first time observed and measured graphene converting a single photon into multiple electrons in a photovoltaic device.  This work should buoy hopes that graphene can serve as a material for photovoltaics with very high energy-conversion efficiencies.

The discovery builds on work conducted last year by the Barcelona-based Institute of Photonic Science (ICFO). ICFO scientists were able to indirectly show that graphene is capable of converting one photon into multiple electrons. In that research, the team excited the graphene by exposing it to photons of different energies (colors). They then used a pulse of terahertz radiation to measure the resulting hot-electron distribution. They determined that a higher photon energy (violet) resulted in higher numbers of hot electrons than a lower photon energy (infrared).

(more at link: https://spectrum.ieee.org/nanoclast/green-tech/solar/graphene-gets-another-boost-in-high-conversion-efficiency-photovoltaics )


...

Photocarrier generation from interlayer charge-transfer transitions in WS2-graphene heterostructures


   Long Yuan1,*, Ting-Fung Chung2,3,*, Agnieszka Kuc4,5,*, Yan Wan1, Yang Xu2,3, Yong P. Chen2,3,6, Thomas Heine4,5 and Libai Huang1,†

See all authors and affiliations
Science Advances  02 Feb 2018:
Vol. 4, no. 2, e1700324
DOI: 10.1126/sciadv.1700324

Abstract

Efficient interfacial carrier generation in van der Waals heterostructures is critical for their electronic and optoelectronic applications. We demonstrate broadband photocarrier generation in WS2-graphene heterostructures by imaging interlayer coupling–dependent charge generation using ultrafast transient absorption microscopy. Interlayer charge-transfer (CT) transitions and hot carrier injection from graphene allow carrier generation by excitation as low as 0.8 eV below the WS2 bandgap. The experimentally determined interlayer CT transition energies are consistent with those predicted from the first-principles band structure calculation. CT interactions also lead to additional carrier generation in the visible spectral range in the heterostructures compared to that in the single-layer WS2 alone. The lifetime of the charge-separated states is measured to be ~1 ps. These results suggest that interlayer interactions make graphene–two-dimensional semiconductor heterostructures very attractive for photovoltaic and photodetector applications because of the combined benefits of high carrier mobility and enhanced broadband photocarrier generation.

http://advances.sciencemag.org/content/4/2/e1700324.full

...
Slippery when dry | Argonne National Laboratory
Argonne scientists reaffirm the potential of graphene as a cheaper, more efficient alternative to oil for lubrication purposes.

http://www.anl.gov/articles/slippery-when-dry

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Post by Cr6 on Mon Oct 29, 2018 12:36 am


New graphene scaffold capacitors break capacitance records


Researchers at the University of California, Santa Cruz and Lawrence Livermore National Laboratory in California have developed a new fabrication technique to make capacitors enhanced with graphene. The resulting devices store a large amount of charge over a given surface area - an important metric for measuring the performance of a capacitor.

The new technique uses a 3D printer to construct a microscopic scaffold with porous graphene and then fills the structure with a kind of material called a pseudocapacitive gel, which is a kind of capacitor material that also behaves like a battery in some ways.

The team explains that currently, supercapacitors - capacitors that store their energy through a reaction at the electrode surface - have a “mass-loading” problem. In order to increase capacitance, intuitively one would want to load more electrode materials into the device, but this approach is limited by the total surface area of the material.

Existing supercapacitor designs tackle this problem by stacking alternating layers of electrodes and thin metal sheets. Using the new 3D-printed scaffold with porous graphene, the researchers were able to increase the mass-loading in their device more than 10 times compared to the stacking technique, resulting in a much higher areal capacitance.

Source:
insidescience
Joule
Tags:Graphene Aerogel
Supercapacitors
Graphene 3D Printing
Technical / Research
Posted: Oct 21, 2018 by Roni Peleg
https://www.graphene-info.com/new-graphene-scaffold-capacitors-break-capacitance-records
https://www.insidescience.org/news/3d-printed-graphene-scaffold-breaks-capacitor-records

------------------


Highly porous graphene used to develop high-performance supercapacitor electrodes

Researchers from Korea's Gwangju Institute of Science and Technology in Korea developed high-performance supercapacitors based on graphene. They say these capacitors can store almost as much energy as a Li-Ion battery and can charge/discharge in seconds. They also last for many tens of thousands of charging cycles.

The researchers use a highly porous graphene that has a huge internal surface area. To fabricate this material they reduced graphene oxide with hydrazine in water agitated with ultrasound. This results in a graphene powder that they then packed into a cell shaped like a cell and dried it at 140 degrees Celsius under pressure for five hour. The material was used as an electrode.

The porous material has a huge surface area - a single gram has a surface area that is bigger than a basketball court. This allows the electrode to accommodate much more electrolyte and ultimately determines the amount of charge the supercapacitor can hold. The researcher report that their supercapacitors can store over 60 Watts Hour per kg (at density of 5 Amps per gram) and has a specific capacitance of over 150 Farrads per gram. Just to compare, Li-Io batteries have an energy density of between 100 and 200 Watt hours per kilogram. The supercapacitor though can fully charge in 16 seconds.

http://www.technologyreview.com/view/521651/graphene-supercapacitors-ready-for-electric-vehicle-energy-storage-say-korean-engineers/
https://www.graphene-info.com/highly-porous-graphene-used-develop-high-performance-supercapacitor-electrodes

----------------------

Sunvault Energy and Edison Power present a 10,000 Farad graphene supercapacitor

Sunvault Energy, along with Edison Power, announced the creation of the world's largest 10,000 Farad Graphene Supercapacitor. The companies declared that this development is the most significant breakthrough in the development of Graphene Supercapacitors to date.

Sunvault's CEO says that the technology can be defined as a hybrid, bringing the power density associated with a battery together with the high impact fast charging known to capacitors. He claims that at 10,000 Farads, a Graphene Supercapacitor is powerful enough to power up a Semi Truck while being the size of a paperback novel. the companies are focused on developing their technology and shrinking the size of the unit in the near future.These graphene storage units can be connected together like storage building blocks to form a larger form of electrical storage for many markets. The higher the Farad combined with the smallest sized unit allows an optimal building block design. Some initial markets of concentration for Sunvault Energy (in conjunction with the Edison Power Company) are the smartphone market, the electric car market, electric grid stabilization and the recently popularized concept of a home off the Grid with Solar.

Sunvault expects to have solved its building block configuration design base unit within the next month, and will move into the phase of product approval and manufacturing immediately following.

In April 2015, Sunvault entered into a collaboration with Edison Power and declared its intentions to revolutionize supercapacitor technology with aims to replace Li-ion batteries.

https://www.graphene-info.com/sunvault-energy-and-edison-power-present-10000-farad-graphene-supercapacitor



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Post by Cr6 on Mon Oct 29, 2018 12:46 am

(Kind of curious to see a model based on Mathis's charge flow....)
---------
New method doubles performance of 3D printed supercapacitors

Researchers from Lawrence Livermore National Laboratory (LLNL) and UC Santa Cruz (UCSC) have designed a technique that could double the performance of 3D printed graphene-based supercapacitors. The new technique involves sandwiching lithium ion and perchlorate ion between layers of graphene in aerogel electrodes—a process which greatly improves the capacity of the electrodes while maintaining the high rate capability of the devices.

The 3D printing process used by the researchers to build the supercapacitors is a form of direct ink writing, consisting of two ion-intercalation steps before the hydrolysis of perchlorate ion intercalation compounds. According to the team “this two-step electrochemical process increases the surface area of graphene-based materials for charge storage, as well as the number of pseudo-capacitive sites that contribute additional storage capacity”.

Due to the relatively small ion-accessible surface area of the aerogel, which is caused by the aggregation and stacking of graphene sheets, its capacitance is limited. Thanks to the new research, however, that capacitance can now be increased. “This study presents a facile method to boost the capacitive performance of 3D printed graphene aerogel by exfoliating the stacked graphene layers and functionalizing their surface, without damaging structural integrity,” explained the team.

By improving the performance of the 3D printed supercapacitors, the scientists believe that the devices could be used in future custom-built electronics which require supercapacitors with unusual shapes. With 3D printing adding an unprecedented level of customization to consumer electronics, they see no reason why customers couldn’t someday be designing their own smartphones. Those customized electronic devices would then require customized supercapacitors to fit their particular shape and form, with this new 3D printed graphene aerogel devices seemingly fitting the bill very well.

https://www.graphene-info.com/new-method-doubles-performance-3d-printed-supercapacitors
http://www.3ders.org/articles/20160617-scientists-double-performance-of-3d-printed-graphene-aerogel-supercapacitors.html


......
https://en.wikipedia.org/wiki/Perchlorate
Perchlorate
A perchlorate is the name for a chemical compound containing the perchlorate ion, ClO−4.
Mathis on Graphene?  Any hints?  - Page 4 195px-Perchlorate-3D-balls
Perchlorate is used to control static electricity in food packaging. Sprayed onto containers it stops statically charged food from clinging to plastic or paper/cardboard surface.[7]

Niche uses include lithium perchlorate, which decomposes exothermically to produce oxygen, useful in oxygen "candles" on spacecraft, submarines, and in other situations where a reliable backup oxygen supply is needed.[citation needed] For example, oxygen "candles" are used in commercial aircraft during emergency situations to compensate for oxygen insufficiency.[citation needed]
Potassium perchlorate has, in the past, been used therapeutically to treat hyperthyroidism resulting from Graves' disease. It impedes the accumulation of iodide in the thyroid, which blocks thyroid hormone production.[8]

Nevyn's MBL Renderer:

https://www.nevyns-lab.com/mathis/app/mbl/mbl.html?mbl=CIO-4&align=X&atom=nucleus
...........

Quote from Mathis' "Solid Light" paper:  http://www.milesmathis.com/solidlight.pdf

Miles Mathis wrote:But when we are looking at what we call electrical conduction, we are looking at the stream from south
pole to north. This stream is linear, directionalized, and coherent. If we align the poles of adjacent
nuclei, we create longer lines of conduction.

As you can probably see already, this explains the Meissner Effect in superconductivity, where interior
magnetic lines disappear. We have never been given a simple mechanical explanation for that, but my
diagram of Copper supplies it immediately. If this Copper nucleus begins superconducting, that simply
means that all photons being recycled are going from pole to pole. None are being recycled out the
equatorial or carousel level. As we know, the magnetic field lines are always orthogonal to the
electrical field lines. Well, the electrical fields lines go with the conduction. They run south to north
here. The magnetic field lines are then orthogonal to that and in a circle, by the old right hand rule.
Well, since we have no photons being emitted out the equator in this case, we have no magnetic field
being created. Both the electrical field and magnetic field are caused by the charge field, and the
charge field is just the recycled photons. Photons that are recycled from south to north in a line create
the electrical field, and photons that are recycled through the carousel level create the magnetic field.
So if all charge is channeled south to north as through charge, nothing is left to create the magnetic
field. It disappears. This disappearance is what we call the Meissner Effect.

This tells us how the magnetic field and electrical field are related at the foundational level. Given my
theory, we should have expected the magnetic field to go to zero when the electrical field was at a
maximum, since the field creation is a zero-sum game. Since the same charge field creates both, a
maximal electrical conduction

Nevyn's MBL on Copper:
https://www.nevyns-lab.com/mathis/app/mbl/mbl.html?mbl=Cu4&align=Y&atom=nucleus

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Post by Cr6 on Wed Oct 31, 2018 11:10 pm

Looks like they are going to just stumble onto a real "Mathis' Charge Field style Cheap Energy device" with 3D printing/Graphene:
.............
3D printed graphene scaffold offers highest-ever capacitance for a supercapacitor

Oct 25, 2018 | By Thomas

Researchers at the University of California, Santa Cruz and Lawrence Livermore National Laboratory in California have invented a new fabrication technique to make capacitors. The researchers fabricated electrodes using a printable graphene aerogel to build a porous three-dimensional scaffold loaded with pseudocapacitive gel, which is a kind of capacitor material that also behaves like a battery in some ways. In laboratory tests, the novel electrodes achieved the highest areal capacitance (electric charge stored per unit of electrode surface area) ever reported for a supercapacitor.

This schematic illustration shows the fabrication of a 3D-printed graphene aerogel/manganese oxide supercapacitor electrode. (Credit: Yat Li et al., Joule, 2018)

As energy storage devices, supercapacitors have the advantages of charging very rapidly (in seconds to minutes) and retaining their storage capacity through tens of thousands of charge cycles. They are used for regenerative braking systems in electric vehicles and other applications. Compared to batteries, they hold less energy in the same amount of space, and they don't hold a charge for as long. But advances in supercapacitor technology could make them competitive with batteries in a much wider range of applications.

In earlier work, the UCSC and LLNL researchers demonstrated ultrafast supercapacitor electrodes fabricated using a 3D-printed graphene aerogel. In the new study, they used an improved graphene aerogel to build a porous scaffold which was then loaded with manganese oxide, a commonly used pseudocapacitive material.

As part of an electrochemical capacitor, pseudocapacitors store energy through a reaction at the electrode surface, providing a more battery-like performance.

"The problem for pseudocapacitors is that when you increase the thickness of the electrode, the capacitance decreases rapidly because of sluggish ion diffusion in bulk structure," said Yat Li, professor of chemistry and biochemistry at UC Santa Cruz. "So the challenge is to increase the mass loading of pseudocapacitor material without sacrificing its energy storage capacity per unit mass or volume."

Existing supercapacitor designs tackle this problem by applying a thin coating of electrode material to a thin metal sheet that serves as a current collector. Using the new 3D-printed scaffold with porous graphene, the researchers were able to increase mass loading to record levels of more than 100 milligrams of manganese oxide per square centimeter without compromising performance, compared to typical levels of around 10 milligrams per square centimeter for commercial devices.

Most importantly, the areal capacitance increased linearly with mass loading of manganese oxide and electrode thickness, while the capacitance per gram (gravimetric capacitance) remained almost unchanged. This indicates that the electrode's performance is not limited by ion diffusion even at such a high mass loading.
(more at link:   http://www.3ders.org/articles/20181025-3d-printed-graphene-scaffold-offers-highest-ever-capacitance-for-a-supercapacitor.html )

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Post by Cr6 on Mon Jan 14, 2019 2:14 am

Graphene's Superconductive Power Has Finally Been Unlocked, And It's Crazier Than We Expected
FIONA MACDONALD
20 JAN 2017

It's official: graphene has been made into a superconductor in its natural state - which means electrical current can flow through it with zero resistance.

Last year, physicists managed to do this by doping graphene with calcium atoms, but this is the first time researchers have achieved superconductivity in the material without having to alter it. And the results so far show that the material achieves an incredibly rare type of superconductivity that's even crazier and more powerful than scientists expected.

The new research is a big deal even for a material as innately impressive as graphene, seeing as superconductivity is the key to more efficient electronics, better power grids, and new medical technology.

"It has long been postulated that, under the right conditions, graphene should undergo a superconducting transition, but can't," said one of the researchers, Jason Robinson from the University of Cambridge in the UK.

Now, he says his team has managed to awaken that ability. And it appears graphene isn't just a normal superconductor - it could be conducting current with no resistance as a result of an unconfirmed and elusive type of superconductivity called p-wave state. Further research is needed to confirm this result, but it's a pretty intriguing possibility.

Already an over-achiever, graphene is a two-dimensional sheet of carbon atoms that's super flexible, harder than diamond, and stronger than steel.

But since its discovery in 2004, researchers have suspected that graphene might also have the ability to be a superconductor - which means it could shuttle electrons through it with no resistance at all.

Even materials that are good conductors are still inefficient in comparison to superconductors - for example, energy companies lose about 7 percent of their energy as heat as a result of resistance in the grid.

We already use superconducting materials to create the strong magnetic fields needed in MRI machines and maglev trains, but right now, these materials only become superconductive at temperatures of around –269 degrees Celsius (–452.2 degrees Fahrenheit), which is incredibly expensive, and not exactly practical.

If we could find a way to achieve superconductivity sustainably and at high temperatures, it would open up a the possibility of supercomputers that run without resistance and more efficient medical technology - and graphene is often thought of as a prime candidate for achieving this, given all of its other weird and wonderful properties.

Researchers succeeded in making graphene a superconductor last year by inserting calcium atoms into its lattice. And other teams have achieved a similar result by placing it on a superconducting material.

But in the new study, researchers from the University of Cambridge were able to activate the dormant potential of graphene without it being influenced by another material.

"Placing graphene on a metal can dramatically alter the properties so it is technically no longer behaving as we would expect," said one of the team, Angelo di Bernardo.

"What you see is not graphene's intrinsic superconductivity, but simply that of the underlying superconductor being passed on."

Instead, the team achieved superconductivity by coupling it with a material called praseodymium cerium copper oxide (PCCO).

That might sound similar to what happened in previous experiments - after all, they're still putting graphene on top of another material - but the difference here is that PCCO is a type of superconducting material called a cuprate, which has well-understood electronic properties.

So the team was able to clearly distinguish the superconductivity in PCCO from the superconductivity in graphene.

And what they saw was crazier than they'd expected.

Superconductivity occurs when electrons pair up and travel more efficiently through a material.

The spin alignment, or symmetry, of these electron pairs changes depending on the type of superconductivity involved. For example, in PCCO, the electrons pair up with a spin state that's antiparallel, which is called d-wave state.

But what the team saw happening in graphene was very different - they found evidence of a rare and still unverified type of superconductivity called p-wave state occurring.

"What we saw in the graphene was, in other words, a very different type of superconductivity than in PCCO," said Robinson.

"This was a really important step because it meant that we knew the superconductivity was not coming from outside it and that the PCCO was therefore only required to unleash the intrinsic superconductivity of graphene."

It remains unclear exactly what kind of superconductivity the team unleashed in graphene - but if it's confirmed that it really is the elusive p-wave form, it could prove once and for all that this type of superconductivity exists, and allow researchers to study it properly for the first time.

P-wave superconductivity was first proposed in 1994, when Japanese researchers found evidence of it occurring in a crystal material called strontium ruthenate. But the crystal is too bulky to study it well enough to achieve the type of proof scientists need to confirm that the state exists.

If it's happening in graphene, it would be a lot easier to investigate.

"If p-wave superconductivity is indeed being created in graphene, graphene could be used as a scaffold for the creation and exploration of a whole new spectrum of superconducting devices for fundamental and applied research areas," said Robinson.

"Such experiments would necessarily lead to new science through a better understanding of p-wave superconductivity, and how it behaves in different devices and settings."

(More at link: https://www.sciencealert.com/graphene-s-superconductive-power-has-finally-been-unlocked-and-it-s-crazier-than-we-expected )

The research has been published in Nature Communications.

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Post by Cr6 on Mon Jan 14, 2019 2:15 am

What happens when you feed spiders graphene? Their silk gets crazy strong
By Luke Dormehl — Posted on August 16, 2017 11:30AM PST


“You are what you eat.” Everybody knows that saying. Well, it goes for spiders, too. At the University of Trento in Italy, a team of researchers led by Professor Nicola Pugno has developed stronger spider silk by feeding spiders a diet of the 2D wonder material graphene

. The nanomaterial-laced silk is up to three times the strength and 10 times the toughness of the silk the spiders produce in the wild.

“For this study, we created a solution of water and nanomaterials, carbon nanotubes, and graphene,” Pugno told Digital Trends. “We then sprayed this solution into a box of spiders, where it would likely be ingested. When the spiders spin their webs, we saw that the silk contained the nanomaterials. When we tested this silk with a nano-tensile testing machine, we found that it was stronger and tougher than regular silk.”

The silk produced as part of the study has a fracture strength up to 5.4 gigapascals, and a toughness modulus up to 1,570 joules per gram. By comparison, regular spider silk has a failure strength of around 1.5 gigapascals, and a toughness modulus of just 150 joules per gram.

Spider silk is of interest to engineers and material scientists because of its unique properties — including strength that’s equivalent to steel, toughness that’s superior to Kevlar, and an impressive amount of flexibility. Other varied spider-silk-based projects we’ve covered recently include extreme shock-absorbing spider silk, and the use of it as a biomedical material in repairing extensive nerve injuries. In the case of Pugno’s research, it’s too early to talk specific use cases, although strengthened spider silk would likely be greatly appreciated by researchers working on a broad range of applications.

Pugno said that he was not surprised by the findings of the experiment, since previous studies have shown that diet can play a role in the properties of silk. This has been most widely studied among silkworms, most notably in a 2013 study that found silkworms fed on mulberry leaves that had been sprayed with fabric dyes went on to produce colored silk.

https://www.digitaltrends.com/cool-tech/spiders-graphene-silk/

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Post by Cr6 on Mon Jan 14, 2019 2:21 am


Scientists may have found the keystone use for graphene oxide


By Jessica Hall on April 12, 2017 at 1:03 pm
23 Comments


graphene hexagons

This looks like a graphene story at first glance, but it isn’t.

Graphene has two major problems: it’s tough to make, and we don’t really know what its keystone use is. High-quality graphene is made by vapor deposition, except that that’s extremely expensive, which is one reason why graphene doesn’t see wider use. But we’re inching toward better techniques for studying and using graphene. Recently scientists discovered a new way of using one derivative form of graphene called graphene oxide, and beyond its use in research it could see a lot of use in commercial applications. It turns out graphene oxide makes a pretty great filter for desalinating water. And it’s all because of physical chemistry.

Graphene oxide comes in sheets, like sheets of graphene. That’s about where the similarities end. Where graphene has (ideally) a perfectly regular, one-atom-thick structure of adjoining benzene rings, graphene oxide is several layers thick. Graphene oxide is also loaded with oxygen — hence the oxide — and that gives it some very different chemical properties than regular graphene, which is pure carbon.

Graphene is a great conductor, if you can make it behave. But that isn’t what graphene oxide does best. In somewhat the same way as oxidation rusts metal, making it a less effective conductor, graphene oxide isn’t necessarily what we’re after in terms of applications for the semiconductor industry. It just isn’t quite as compelling as graphene, or even reduced graphite oxide. No, graphene oxide has a different set of talents.

The chemical properties of graphene oxide make it swell up when it gets water on it. The formal term is hydration. Hydrated graphene oxide forms a molecular mesh of very regular size, which does not permit anything larger than 9 angstroms through it. It’s like a tiny, unforgiving sieve. That’s why it’s a great water filter. The tiny, regular holes in the hydrated oxide mesh are of a size that’s small enough to permit H2O molecules, but not larger compounds. Researchers call these tiny pores graphene capillaries.

The problem is that salts are smaller than 9 angstroms. So the researchers thought a while, and then mashed a layer of graphene oxide between two layers of epoxy, which left the graphene oxide nowhere to expand into but its own empty space. Envision yourself squashing a bunch of water balloons between two sheets of Plexiglas, and that’s not too far off what happens when you hydrate graphene oxide while it’s constrained. The spaces between units get smaller. And in that smushed form factor, the holes in the mesh are small enough that most things no longer fit through. Researchers from the University of Manchester achieved a graphene oxide pore size of less than 7 Å, and at that size, even hydrated ions don’t fit through. Just water. The researchers achieved 97 percent filtration of NaCl ions, and it’s safe to expect that further development would yield good results with many different solutes.

Thoroughly oxygenated graphite oxide molecule, including epoxide, hydroxyl and carboxyl groups. These substituents give graphite oxide very different properties to “regular” graphene.

https://www.extremetech.com/wp-content/uploads/2017/04/500px-Graphite_oxide.svg_.png

Predictably, this is some shiny news for water desalination. Osmotic movement across a semipermeable membrane happens without human energy input, which makes this development compelling. And it’s even better because of the microfluidics aspect of the story. Between the graphene capillaries and the wicking action of water between two narrowly separated plates, this is absolutely ripe for passive exploitation and low-power applications, which could make it really great for deployment in the roughest regions of interior, water-challenged countries. There may be semiconductor applications, but none with the same potential for ubiquitous worldwide deployment.
(more at link: )
https://www.extremetech.com/extreme/247568-scientists-may-found-keystone-use-graphene-oxide

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Post by Cr6 on Mon Feb 04, 2019 2:59 am

Hmmm... maybe it is just Miles Mathis' charge field at work do the "crinkling"?  
----------
Graphene crinkles can be used as ‘molecular zippers’
January 31, 2019   Media contact: Kevin Stacey   401-863-3766

New research shows that electrically charged crinkles in layered graphene can be useful in the directed self- assembly of molecules.

PROVIDENCE, R.I. [Brown University] — A decade ago, scientists noticed something very strange happening when buckyballs — soccer ball shaped carbon molecules — were dumped onto a certain type of multilayer graphene, a flat carbon nanomaterial. Rather than rolling around randomly like marbles on a hardwood floor, the buckyballs spontaneously assembled into single-file chains that stretched across the graphene surface.

Now, researchers from Brown University’s School of Engineering have explained how the phenomenon works, and that explanation could pave the way for a new type of controlled molecular self-assembly. In a paper published in Proceedings of the Royal Society A, the Brown team shows that tiny, electrically charged crinkles in graphene sheets can interact with molecules on the surface, arranging those molecules in electric fields along the paths of the crinkles.

“What we show is that crinkles can be used to create ‘molecular zippers’ that can hold molecules onto a graphene surface in linear arrays,” said Kyung-Suk Kim, director of the Center for Advanced Materials Research in Brown’s Institute for Molecular and Nanoscale Innovation and the study’s senior author. “This linear arrangement is something that people in physics and chemistry really want because it makes molecules much easier to manipulate and study.”

The new paper is a follow-up to earlier research by Kim’s team. In that first paper, they described how gently squeezing sheets of layered graphene from the side causes it to deform in a peculiar way. Rather than forming gently sloping wrinkles like you might find in a rug that’s been scrunched against a wall, the compressed graphene forms pointy saw-tooth crinkles across the surface. They form, Kim’s research showed, because the arrangement of electrons in the graphene lattice causes the curvature of a wrinkle to localize along a sharp line. The crinkles are also electrically polarized, with crinkle peaks carrying a strong negative charge and valleys carrying a positive charge.

Kim and his team thought the electrical charges along the crinkles might explain the strange behavior of the buckyballs, partly because the type of multilayer graphene used in the original buckyball experiments was HOPG, a type of graphene that naturally forms crinkles when it’s produced. But the team needed to show definitely that the charge created by the crinkles could interact with external molecules on the graphene’s surface. That’s what the researchers were able to do in this new paper.

Their analysis using density functional theory, a quantum mechanical model of how electrons are arranged in a material, predicted that positively charged crinkle valleys should create an electrical polarization in the otherwise electrically neutral buckyballs. That polarization should cause buckyballs to line up, each in the same orientation relative to each other and spaced around two nanometers apart.

Those theoretical predictions match closely the results of the original buckyball experiments as well as repeat experiments newly reported by Kim and his team. The close agreement between theory and experiment helps confirm that graphene crinkles can indeed be used to direct molecular self-assembly, not only with buckyballs but potentially with other molecules as well.

Kim says that this molecular zippering capability could have many potential applications, particularly in studying biomolecules like DNA and RNA. For example, if DNA molecules can be stretched out linearly, it could be sequenced more quickly and easily. Kim and his team are currently working to see if this is possible.

(More at link:  https://news.brown.edu/articles/2019/01/crinkles )

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Post by Cr6 on Mon Mar 04, 2019 12:52 am


Hall effect becomes viscous in graphene
February 28, 2019, University of Manchester

(Snip...more at link)

Graphene can come in very useful here: the carbon sheet is a highly clean material that contains only a few defects, impurities and phonons (vibrations of the crystal lattice induced by temperature) so that electron-electron interactions become the main source of scattering, which leads to a viscous electron flow.

"In previous work, our group found that electron flow in graphene can have a viscosity as high as 0.1 m2s-1, which is 100 times higher than that of honey," said Dr. Bandurin "In this first demonstration of electron hydrodynamics, we discovered very unusual phenomena like negative resistance, electron whirlpools and superballistic flow."

Even more unusual effects occur when a magnetic field is applied to graphene's electrons when they are in the viscous regime. Theorists have already extensively studied electro-magnetohydrodynamics because of its relevance for plasmas in nuclear reactors and in neutron stars, as well as for fluid mechanics in general. But, no practical experimental system in which to test those predictions (such as large negative magnetoresistance and anomalous Hall resistivity) was readily available until now.

In their latest experiments, the Manchester researchers made graphene devices with many voltage probes placed at different distances from the electrical current path. Some of them were less than one micron from each other. Geim and colleagues showed that while the Hall effect is completely normal if measured at large distances from the current path, its magnitude rapidly diminishes if probed locally, using contacts close to the current injector.

"The behaviour is radically different from the standard textbook physics" says Alexey Berdyugin, a Ph.D. student who conducted the experimental work. "We observe that if the voltage contacts are far from the current contacts, we measure the old, boring Hall effect, instead of this new 'viscous Hall effect'. But, if we place the voltage probes near the current injection points—the area in which viscosity shows up most dramatically as whirlpools in electron flow—then we find that the Hall effect diminishes.

"Qualitative changes in the electron flow caused by viscosity persist even at room temperature if graphene devices are smaller than one micron in size, says Berdyugin. "Since this size has become routine these days as far as electronic devices are concerned, the viscous effects are important when making or studying graphene devices."


Read more at: https://phys.org/news/2019-02-hall-effect-viscous-graphene.html#jCp

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Post by Cr6 on Mon Mar 18, 2019 12:00 am


Graphene and cobalt used together to create new electromagnetic devices


Researchers from IMDEA Nanociencia and other European centers have discovered that the combination of graphene with cobalt offers relevant properties in the field of magnetism. This breakthrough sets the stage for the development of new logic devices that can store large data amounts quickly and with reduced energy consumption.

One of the latest technologies for digitally encoding information is spin orbitronics, which not only exploits the charge of the electron (electronics) and its spin (spintronics), but also the interaction of the spin with its orbital motion, offering a multitude of properties.

This technology is applied in certain materials to generate magnetic configurations that are very stable but which can be controlled and moved quickly with very small electrical currents. The resulting structures are considered very promising for future spin-orbitronic devices, as they provide high processing speed and a high capacity for storing data, with low energy consumption.

The European team led by the IMDEA Nanociencia Institute has developed a methodology to prepare such a system. It consists of a device made of stacked graphene films placed on ferromagnetic cobalt, arranged in turn on a platinum layer with a certain crystallographic orientation. The details are published in Nano Letters.

The main author of the study, Paolo Perna from IMDEA Nanociencia, explains the advantages of this configuration: "On the one hand, the exceptional properties of graphene make it possible to obtain a homogeneous, flat and protected magnetic layer, which is also atomically perfect. However, what matters most are the two magnetic properties that are achieved: an improvement in the magnetic anisotropy of cobalt (its spines are preferably oriented in a certain direction), and a strong interaction called Dzyaloshinskii-Moriya, which allows the presence of chiral magnetic structures, as they do not overlap with its specular image."

These chiral magnetic structures of nanometric size are called skyrmions. They are very stable and act as carriers of binary information as they travel through graphene.


"By passing through two electrical contacts, each skyrmion produces a change in the electrical response that can be decoded into zeros and ones," explains Perna.

"In this way, in the near future, it will be possible to produce spin-orbitronic magnetic devices such as magnetic memories or sensors that are much faster and denser than current ones, and with much lower energy consumption," the researcher says.

In order to detect the properties, the authors have used combined spectroscopy and microscopy techniques, including some with light at the ALBA synchrotron near Barcelona. Researchers from the Complutense and Autonomous Universities of Madrid, together with the Néel Institute of Grenoble (France), have also participated in the study.

As the basis of the device, the authors have used oxide insulating substrates. In order to obtain high-quality graphene, metallic substrates are usually used in laboratories, but they are very expensive for the industry and, as conductors, they would not allow the electrical insulation of the device with the chip.

https://www.graphene-info.com/graphene-and-cobalt-used-together-create-new-electromagnetic-devices (more at link...)

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