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Mathis on Graphene? Any hints?

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Nevyn
Cr6
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Post by Cr6 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 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 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 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 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 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 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 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 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 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 3 Empty The Effective mass in graphene

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


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Post by Cr6 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 3 Nl-2011-00587h_0003


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Post by Cr6 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 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 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 3 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 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 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 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 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 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 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 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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Post by Cr6 Sun Jun 16, 2019 9:33 pm

Graphene nanoribbons

Graphene nanoribbons (GNRs, also called nano-graphene ribbons or nano-graphite ribbons) are strips of graphene with width less than 50 nm. Graphene ribbons were introduced as a theoretical model by Mitsutaka Fujita and coauthors to examine the edge and nanoscale size effect in graphene.[2][3][4]

Mathis on Graphene?  Any hints?  - Page 3 330px-B-doped_graphene_nanoribons

Nanotomy


Large quantities of width-controlled GNRs can be produced via graphite nanotomy,[5] where applying a sharp diamond knife on graphite produces graphite nanoblocks, which can then be exfoliated to produce GNRs. GNRs can also be produced by "unzipping" or axially cutting nanotubes.[6] In one such method multi-walled carbon nanotubes were unzipped in solution by action of potassium permanganate and sulfuric acid.[7] In another method GNRs were produced by plasma etching of nanotubes partly embedded in a polymer film.[8] More recently, graphene nanoribbons were grown onto silicon carbide (SiC) substrates using ion implantation followed by vacuum or laser annealing.[9][10][11] The latter technique allows any pattern to be written on SiC substrates with 5 nm precision.[12]

Epitaxy

GNRs were grown on the edges of three-dimensional structures etched into silicon carbide wafers. When the wafers are heated to approximately 1,000 °C (1,270 K; 1,830 °F), silicon is preferentially driven off along the edges, forming nanoribbons whose structure is determined by the pattern of the three-dimensional surface. The ribbons had perfectly smooth edges, annealed by the fabrication process. Electron mobility measurements surpassing one million correspond to a sheet resistance of one ohm per square— two orders of magnitude lower than in two-dimensional graphene.[13]

https://en.wikipedia.org/wiki/Graphene_nanoribbon

Silicene Transistors

The U.S. Army Research Laboratory has been supporting research on silicene since 2014. The stated goals for research efforts were to analyze atomic scale materials, such as silicene, for properties and functionalities beyond existing materials, like graphene.[40] In 2015, Deji Akinwande, led researchers at the University of Texas, Austin in conjunction with Alessandro Molle's group at CNR, Italy, and collaboration with U.S. Army Research Laboratory and developed a method to stabilize silicene in the air and reported a functional silicene field effect transistor device. An operational transistor’s material must have bandgaps, and functions more effectively if it possesses a high mobility of electrons. A bandgap is an area between the valence and conduction bands in a material where no electrons exist. Although graphene has a high mobility of electrons, the process of forming a bandgap in the material reduces many of its other electric potentials.[41]

https://en.wikipedia.org/wiki/Silicene
https://en.wikipedia.org/wiki/2D_silica
https://en.wikipedia.org/wiki/Borophene
https://en.wikipedia.org/wiki/Germanene
https://en.wikipedia.org/wiki/Stanene
https://en.wikipedia.org/wiki/Plumbene

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Post by Cr6 Fri Sep 06, 2019 1:01 am

"P-wave states"...hmm could that just be the alignment of Mathis' charge field?

----------

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.
....
(more at link)

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

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Post by Cr6 Fri Sep 06, 2019 1:02 am

Keep in mind these papers:
1/13/18, What Rutherford really Proved about the Electron. Also more information about the positron.
http://milesmathis.com/electron.pdf

1/5/18, Graphene. Where we see all the mainstream explanations are wrong. Why? Because they are based on electron bonding theory, which is a fudge from the first word.
http://milesmathis.com/graphene.pdf


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

Unconventional superconductivity in magic-angle graphene superlattices

Yuan Cao, Valla Fatemi, Shiang Fang, Kenji Watanabe, Takashi Taniguchi, Efthimios Kaxiras & Pablo Jarillo-Herrero

Nature volume 556, pages 43–50 (05 April 2018) | Download Citation

Abstract


The behaviour of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theoretical understanding has motivated the development of experimental techniques for studying such behaviour, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional superconductivity—which cannot be explained by weak electron–phonon interactions—in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°—the first ‘magic’ angle—the electronic band structure of this ‘twisted bilayer graphene’ exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature of up to 1.7 kelvin. The temperature–carrier-density phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to superconductivity. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting critical temperature of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier density of about 1011 per square centimetre), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-critical-temperature superconductors and quantum spin liquids.

https://www.nature.com/articles/nature26160

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


Insulator or superconductor? Physicists find graphene is both

When rotated at a "magic angle," graphene sheets can form an insulator or a superconductor.


Jennifer Chu | MIT News Office
March 5, 2018
Press Inquiries


It’s hard to believe that a single material can be described by as many superlatives as graphene can. Since its discovery in 2004, scientists have found that the lacy, honeycomb-like sheet of carbon atoms — essentially the most microscopic shaving of pencil lead you can imagine — is not just the thinnest material known in the world, but also incredibly light and flexible, hundreds of times stronger than steel, and more electrically conductive than copper.

Now physicists at MIT and Harvard University have found the wonder material can exhibit even more curious electronic properties. In two papers published today in Nature, the team reports it can tune graphene to behave at two electrical extremes: as an insulator, in which electrons are completely blocked from flowing; and as a superconductor, in which electrical current can stream through without resistance.

Researchers in the past, including this team, have been able to synthesize graphene superconductors by placing the material in contact with other superconducting metals — an arrangement that allows graphene to inherit some superconducting behaviors. This time around, the team found a way to make graphene superconduct on its own, demonstrating that superconductivity can be an intrinsic quality in the purely carbon-based material.

The physicists accomplished this by creating a “superlattice” of two graphene sheets stacked together — not precisely on top of each other, but rotated ever so slightly, at a “magic angle” of 1.1 degrees. As a result, the overlaying, hexagonal honeycomb pattern is offset slightly, creating a precise moiré configuration that is predicted to induce strange, “strongly correlated interactions” between the electrons in the graphene sheets. In any other stacked configuration, graphene prefers to remain distinct, interacting very little, electronically or otherwise, with its neighboring layers.

The team, led by Pablo Jarillo-Herrero, an associate professor of physics at MIT, found that when rotated at the magic angle, the two sheets of graphene exhibit nonconducting behavior, similar to an exotic class of materials known as Mott insulators. When the researchers then applied voltage, adding small amounts of electrons to the graphene superlattice, they found that, at a certain level, the electrons broke out of the initial insulating state and flowed without resistance, as if through a superconductor.

“We can now use graphene as a new platform for investigating unconventional superconductivity,” Jarillo-Herrero says. “One can also imagine making a superconducting transistor out of graphene, which you can switch on and off, from superconducting to insulating. That opens many possibilities for quantum devices.”

A large-scale interpretation of the moiré patterns formed when one graphene lattice is slightly rotated at a “magic angle,” with respect to a second graphene lattice.

A 30-year gap

A material’s ability to conduct electricity is normally represented in terms of energy bands. A single band represents a range of energies that a material’s electrons can have. There is an energy gap between bands, and when one band is filled, an electron must embody extra energy to overcome this gap, in order to occupy the next empty band.

A material is considered an insulator if the last occupied energy band is completely filled with electrons. Electrical conductors such as metals, on the other hand, exhibit partially filled energy bands, with empty energy states which the electrons can fill to freely move.

Mott insulators, however, are a class of materials that appear from their band structure to conduct electricity, but when measured, they behave as insulators. Specifically, their energy bands are half-filled, but because of strong electrostatic interactions between electrons (such as charges of equal sign repelling each other), the material does not conduct electricity. The half-filled band essentially splits into two miniature, almost-flat bands, with electrons completely occupying one band and leaving the other empty, and hence behaving as an insulator.

“This means all the electrons are blocked, so it’s an insulator because of this strong repulsion between the electrons, so nothing can flow,” Jarillo-Herrero explains. “Why are Mott insulators important? It turns out the parent compound of most high-temperature superconductors is a Mott insulator.”

(more at link)
https://news.mit.edu/2018/graphene-insulator-superconductor-0305
----------------------------

p-wave triggered superconductivity in single-layer graphene on an electron-doped oxide superconductor


A. Di Bernardo, O. Millo, M. Barbone, H. Alpern, Y. Kalcheim, U. Sassi, A. K. Ott, D. De Fazio, D. Yoon, M. Amado, A. C. Ferrari, J. Linder & J. W. A. Robinson
Nature Communications volume 8, Article number: 14024 (2017) | Download Citation


Abstract


Electron pairing in the vast majority of superconductors follows the Bardeen–Cooper–Schrieffer theory of superconductivity, which describes the condensation of electrons into pairs with antiparallel spins in a singlet state with an s-wave symmetry. Unconventional superconductivity was predicted in single-layer graphene (SLG), with the electrons pairing with a p-wave or chiral d-wave symmetry, depending on the position of the Fermi energy with respect to the Dirac point. By placing SLG on an electron-doped (non-chiral) d-wave superconductor and performing local scanning tunnelling microscopy and spectroscopy, here we show evidence for a p-wave triggered superconducting density of states in SLG. The realization of unconventional superconductivity in SLG offers an exciting new route for the development of p-wave superconductivity using two-dimensional materials with transition temperatures above 4.2 K.


https://www.nature.com/articles/ncomms14024


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Post by Cr6 Fri Sep 06, 2019 1:12 am

In Miles' words:

-------------
1/5/18, Graphene. Where we see all the mainstream explanations are wrong. Why? Because they are based on electron bonding theory, which is a fudge from the first word.
http://milesmathis.com/graphene.pdf

We are told that electrons propagating through Graphene lose their mass, becoming quasi-particles, but
that is absurd. What really happens is that the electrons are spin-stripped by the material, becoming
photons. The reason they have to be described currently by the 2-D Dirac equation rather than the
Schrodinger equation is that the Schrodinger equation is faulty, especially regarding spin ½ particles.
Since the whole theory behind spin ½ is also faulty, we see where the mess came from. But why
would Graphene spin-strip an electron down to a real photon?
Again because the applied current is
moving opposite to the internal current of Graphene. Charge can move both directions here, but
electrons can't. The electrons they are talking about are electrons that came in with the applied current,
so they are moving with what we are calling the anticharge here. We will say they are spinning right.
But the internal charge profile of the Graphene, as created by the metal, is charge, or left spinning.
Therefore, when the introduced electron hits the field of the Graphene, it will be spun down. If its
outer spin is spun down completely, this is the same as a spin-strip. That spin is gone. An electron that
loses its outer spin becomes a photon. Therefore, under the right circumstances, we should see
Graphene “producing” X-rays. And guess what, it does. At that link to phys.org, we are told that
electrons moving through Graphene plasmons cause them to release X-rays. But the electrons aren't
releasing X-rays, they are becoming X-rays. That would be pretty easy to prove, since they now have
electron counters. All they have to do is count electrons. They will find that the X-ray production
leads to an electron reduction.
--------

Also:
https://milesmathis.forumotion.com/t259-graphene-s-crazier-than-we-thought-can-handle-1000-times-more-current-than-regular-material
https://milesmathis.forumotion.com/t292-newly-discovered-phenomenon-shows-electrons-can-move-much-faster-than-expected

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Post by Chromium6 Sun Jan 05, 2020 5:53 am

An interesting French company with a new Graphene creation process:

-----------
Mathis on Graphene?  Any hints?  - Page 3 What-is-graphene-carbon-waters.png
Our unique graphene
Why our Graphene is (so) wanted?

Per definition, graphene is a perfect, defect-free single layer of carbon, exhibiting exceptional properties. In reality, there is no large-scale production of such high-quality material.

There are different types of graphene available on the market, exhibiting different properties and therefore different advantages and disadvantages. e.g. purity, quality, and the scalability to industrial production.

Carbon Waters masters a graphene production method which sets us apart from other suppliers and brings the possibility of using “real” graphene.

Our product, “Eau de graphène”, exhibits exceptional properties. He is singled out in particular by the absence of organic solvents and surfactants. Additionally, our high-quality graphene dispersion offers versatility without the security limitations of graphene in powder form or VOCs contents.
What is graphene identity card of grap'up the graphene by carbon waters

Why our graphene is more flexible?

Carbon Waters can produce graphene directly in a liquid dispersion which allows a very broad flexibility in applications.

As our graphene can be produced in aqueous and organic solvents, it is compatible with a great variety of formulation matrices.

Moreover, this graphene form is also adapted for a lot of industrial deposition process: electrodeposition, spray, inkjet and others.

------
https://www.carbon-waters.com/surface-treatments-unit/

Our expertise on surface treatments
Graphene-enhanced surface treatments

When a protection against external environment is required, a surface treatment can be an efficient way to optimize resistance and improve the material properties.

At Carbon Waters, we have developed an expertise in surface treatment using graphene.

Taking advantage of graphene impermeability (barrier to gas and solvents) and chemical inertness, we provide solutions tailored to client’s requirements.

How to prevent corrosion with graphene surface treatment


Our deposition techniques

We can provide graphene-enhanced surface treatment adjusted to a wide range of materials: metals, plastics, polymers and composites.

Our capabilities include various deposition techniques such as electrodeposition, spray, dip-coating and inkjet.

Each technique has its own specificity and we work closely with our clients to define the most effective protocol according to their needs in order to deliver the best added-value.

https://www.carbon-waters.com/wp-content/uploads/2019/10/what-is-graphene-carbon-waters.png.webp

---

Graphene Barrier effects

Thanks to its high quality, our graphene shows very good barrier-enhancing properties.

This barrier effect provides an excellent protection against solvents (water-based and organic), extreme pH, temperature, radiations.

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Post by Chromium6 Mon Feb 10, 2020 1:34 am

https://www.extremetech.com/electronics/198563-researchers-make-graphene-magnetic-clearing-the-way-for-faster-everything

More at link..

Researchers make graphene magnetic, clearing the way for faster everything

By Ryan Whitwam on
January 29, 2015 at 4:14 pm
14 Comments

Graphene has many fantastic properties that could change the course of human civilization. It’s chemically stable, highly conductive, and incredibly strong. One thing it is not, however, is magnetic. This is one of the issues cited by the likes of IBM, which has tried to dampen expectations for a future of super-efficient microprocessors built on graphene. That might not be a problem much longer, though. Scientists from the University of California, Riverside have successfully created graphene that has magnetic properties.

To make this happen, the team started with a sheet of regular (but still awesome) non-magnetic graphene. The graphene was placed on a layer of magnetic yttrium iron garnet, which actually transferred its magnetic properties to the graphene without disrupting its structure or other properties. Most magnetic substances interfere with graphene’s ability to conduct electricity, but yttrium iron garnet is also an electric insulator. That meant it was unlikely to negatively affect the graphene’s electron transport properties.

When removed and exposed to a magnetic field, the team found their treated graphene’s Hall voltage depended on the magnetic linearity of the yttrium iron garnet. This told the scientists that their graphene was magnetic all on its own and that magnetism had come from exposure to the yttrium iron garnet layer. This property should last indefinitely as it is not the result of depositing material on the graphene, but comes from the graphene itself.

yttrium iron garnet

It has been possible in the past to create magnetized graphene, but this always relied on adding additional magnetic compounds or coatings to the raw graphene — often lead or iron. This rather defeats the purpose of graphene by adding additional complications. It’s already hard enough to produce large quantities of sufficiently pure graphene without adding these additional complications. The addition of extra atoms to graphene’s single atom structure also screws up its electrical properties in the same way exposure to non-insulating magnetic materials can. What’s different this time is that the graphene is still just pure graphene.




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Post by Chromium6 Mon Feb 10, 2020 1:42 am

More at link...

https://m.phys.org/news/2017-02-layers-graphene-reveals-kind-magnet.html

Three layers of graphene reveals a new kind of magnet

February 23, 2017 , Tata Institute of Fundamental Research


A spectrum of the three layer graphene as a function of magnetic field and density of electrons. Credit: Biswajit Datta, Mandar Deshmukh

Metals have a large density of electrons and to be able to see the wave nature of electrons one has to make metallic wires that are only a few atoms wide. However, in graphene - one atom thick graphite—the density of electrons is much smaller and can be changed by making a transistor. As a result of the low density of electrons the wave nature of electrons, as described by quantum mechanics, is easier to observe in graphene.

Often in metals like copper the electron is scattered every 100 nanometers, a distance roughly 1000 times smaller than the diameter of human hair, due to impurities and imperfections. Electrons can travel much longer in graphene, upto distances of 10 micrometer, a distance roughly 10 times smaller than the diameter of human hair. This is realized by sandwiching graphene between layers of boron nitride. The layers of boron nitride have few imperfections to impede the flow of electrons in graphene.

Once electrons travel long distances, implying there are few imperfections, one notices the faint whispers of electrons "talking to each other".

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Post by Chromium6 Sun Mar 01, 2020 3:32 am

Link: https://arxiv.org/ftp/arxiv/papers/1303/1303.2391.pdf


Electrostatic Graphene Loudspeaker

Qin Zhouand A. Zettla
Center of Integrated Nanomechanical Systems, University of California at Berkeley, Berkeley, California 94720, USA

Department of Physics, University of California at Berkeley, Berkeley, California 94720, USAMaterials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

Graphene has extremely low mass density and high mechanical strength, key qualities for efficient wide-frequency-response electrostaticaudio speaker design. Low mass ensures good high frequency response, while high strength allows for relatively large free-standing diaphragms necessary for effective low frequency response. Here we report on construction and testing of a miniaturized graphene-based electrostatic audio transducer. The speaker/earphone is straightforward in design and operation and has excellent frequency response across the entire audio frequency range (20HZ –20kHz), with performance matching or surpassing commercially available audio earphones.


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Post by Chromium6 Sun Mar 01, 2020 3:35 am

How Graphene makes a faster, cooler and safer battery

Updated: Dec 4, 2019

We know many of you are curious about how graphene works and how we utilize it to bring you a faster, cooler and safer charging experience.  

https://www.realgrapheneusa.com/graphene

Why are current lithium batteries so limited?​

Heat is the number one major cause. As the device begins charging, heat is generated based on resistivity of conductor. Generated heat increases the resistivity of conductor. Since the conductor is hotter, the resistivity is higher which means the device charges even more heat. This creates a positive feedback loop that can spiral out of control and cause the battery to catch on fire.

To prevent this, batteries will regulate the speed of charging but this results in battery charging speeds slowing to a crawl.

What are the benefits of using Graphene composite?


Graphene is a near perfect conductor of electricity. This allows electricity to flow without hindrance. This dramatically slows the heating process lithium batteries face while allowing charging speeds up to 5 times as fast. This also increases the battery life by 5 times the charging cycles.

Graphene also evenly disperses heat acting as a cooling system. Graphene already generates less heat due to extremely low resistivity. But graphene also conducts heat evenly across battery to help cool the battery.


Learn more at: https://www.realgrapheneusa.com/graphene
-----------------

Lunch with Microsoft President, Brad Smith

CEO of Real Graphene USA, Samuel Gong, had lunch with Microsoft's President, Brad Smith, at the Nixon Library in Yorba Linda growing awareness for the Real Graphene brand.

The closed-doored lunch, hosted at the Nixon Library, was held to discuss several topics in upcoming technology among many industry leaders. During the lunch, Samuel Gong spoke with Brad Smith about the prospects of advancing technologies and how he viewed their integration into society. They also spoke about creating environments where innovation and new ideas can flourish during tense relations between the USA and China.

After the lunch, Brad and Samuel discussed several things such as the potential of graphene composite batteries in future devices. Real Graphene power banks can be used to charge all of Microsoft's Surface devices. However, if Real Graphene batteries could be integrated into the Surface devices themselves, it would allow for charging speeds well beyond the current capabilities.

https://www.realgrapheneusa.com/post/design-a-stunning-blog

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Post by Chromium6 Sun Mar 01, 2020 3:45 am

Ballistic miniband conduction in a graphene superlattice

   Menyoung Lee1, John R. Wallbank2, Patrick Gallagher1, Kenji Watanabe3, Takashi Taniguchi3, Vladimir I. Fal’ko2,4, David Goldhaber-Gordon1,*

See all authors and affiliations
Science  30 Sep 2016:
Vol. 353, Issue 6307, pp. 1526-1529
DOI: 10.1126/science.aaf1095

Abstract

Rational design of long-period artificial lattices yields effects unavailable in simple solids. The moiré pattern in highly aligned graphene/hexagonal boron nitride (h-BN) heterostructures is a lateral superlattice with high electron mobility and an unusual electronic dispersion whose miniband edges and saddle points can be reached by electrostatic gating. We investigated the dynamics of electrons in moiré minibands by measuring ballistic transport between adjacent local contacts in a magnetic field, known as the transverse electron focusing effect. At low temperatures, we observed caustics of skipping orbits extending over hundreds of superlattice periods, reversals of the cyclotron revolution for successive minibands, and breakdown of cyclotron motion near van Hove singularities. At high temperatures, electron-electron collisions suppress focusing. Probing such miniband conduction properties is a necessity for engineering novel transport behaviors in superlattice devices.

In solids, the quantum nature of electrons generates band structure, which controls conduction and optical properties. Similarly, longer-period superlattices generate minibands that disperse at a finer energy scale over a reduced Brillouin zone, enabling phenomena such as negative differential conductance and Bloch oscillations (1–3). However, fabricating long-range periodic patterns that strongly modulate the potential to form well-separated minibands without undermining the material quality and electron coherence remains challenging. Most experiments on laterally patterned semiconductor heterostructures have revealed classical commensurability effects (4–6), which do not require well-formed and separated minibands. Despite evidence for Fermi surface reconstruction in a patterned superlattice, details of Fermi surfaces were obscured by poor separation between minibands and consequent magnetic breakdown across weakly avoided crossings (7).

The arrival of high-quality graphene/h-BN van der Waals heterostructures with misalignment angle below 1° (8, 9) has drastically changed the situation. In such systems, the periodic potential for electrons in graphene is imposed by the hexagonal moiré pattern generated by the incommensurability and misalignment between the two crystals (10–12). Formation of minibands for Dirac electrons has been demonstrated by scanning tunneling (13), capacitance (14), and optical (15) spectroscopies, as well as magnetotransport (16–19). These studies have elucidated the electronic structure known as the Hofstadter butterfly, which emerges in a quantizing magnetic field (20). By contrast, a small magnetic field may be treated semiclassically. Then the connection between the miniband dispersion ε(k) and transport properties is established by the equations of motion for an electron in an out-of-plane magnetic field Embedded Image,Embedded Image (1)where e is the charge on the electron, ħ is the Planck constant divided by 2π, and the relation between carrier velocity v and momentum ħk is approximately v = vk/k (v ≈ 106 m/s) close to the Dirac point of graphene’s spectrum (10, 11, 13, 14).

The shape of the cyclotron orbit in a two-dimensional (2D) metal is a 90° rotation of the shape of the Fermi surface, and the carrier revolves along it clockwise or counterclockwise. Electron trajectories near the boundary of a metal open into skipping orbits (21), which drift in the direction determined by the effective charge of the carrier. These skipping orbits bunch along caustics (22–27), leading to the transverse electron focusing (TEF) effect (22, 23). Experimentally, TEF takes place when the magnetic field is tuned such that caustics of skipping orbits, emanating from an emitter E, end up at a collector C, located at position x = L along the boundary. Then a voltage VC is induced at C, proportional to the current IE injected into E. Figure 1B illustrates skipping orbits and caustics in a material with an isotropic Fermi surface, such as unperturbed graphene near the Dirac point, where TEF occurs for B = Bj ≡ 2jħkF/±eL (for j = 1, 2, …). An equidistant series of peaks (oscillations) appears in the focusing “spectrum”—the nonlocal magnetoresistance VC/IE(B) (Fig. 1C, lower trace), from which the Fermi momentum ħkF and the sign of effective charge ±e can be inferred. TEF was initially used to study the Fermi surfaces of bulk metals (22, 28) and was later extended to 2D systems (23), including graphene (29).
.....

The direct observation and manipulation of ballistic transport is a powerful probe of the low-energy physics of an electron system. Here, the quasiparticles propagate freely from the emitter to the collector through ballistic trajectories as long as πL/2 = 10 μm, which is 700 in dimensionless units of the superlattice period. Ballistic motion of ultracold atoms has been seen in homogeneous optical lattices as large as 100 unit cells (37), but in the solid state, the mean free path of electrons in semiconductor 1D superlattices has been limited to 10 unit cells (38). Our experiment elucidates the key features of miniband electron dynamics in a moiré superlattice and points toward further explorations of novel transport effects. For instance, the saddle-point VHS could host exotic effects caused by enhanced electron-electron interactions (19, 39), and valley-contrasting physics could be accessed by taking advantage of the severe trigonal warping of minibands (40). For technology, such a clear validation of the miniband conduction properties suggests that graphene/h-BN (and perhaps other moiré superlattices) may be a practical platform for devices based on miniband physics. Efficient photocurrent generation at the edge of a graphene superlattice in a magnetic field (41) may be caused by the skipping orbits we have observed; furthermore, THz devices such as the Bloch oscillator can benefit from the much longer scattering times in this system.

link: https://science.sciencemag.org/content/353/6307/1526.full
..................

Exceptional ballistic transport in epitaxial graphene nanoribbons

Jens Baringhaus, Ming Ruan, Frederik Edler, Antonio Tejeda, Muriel Sicot, Amina Taleb-Ibrahimi, An-Ping Li, Zhigang Jiang, Edward H. Conrad, Claire Berger, Christoph Tegenkamp & Walt A. de Heer

Nature volume 506, pages349–354(2014)Cite this article

Abstract

Graphene nanoribbons will be essential components in future graphene nanoelectronics1. However, in typical nanoribbons produced from lithographically patterned exfoliated graphene, the charge carriers travel only about ten nanometres between scattering events, resulting in minimum sheet resistances of about one kilohm per square2,3,4,5. Here we show that 40-nanometre-wide graphene nanoribbons epitaxially grown on silicon carbide6,7 are single-channel room-temperature ballistic conductors on a length scale greater than ten micrometres, which is similar to the performance of metallic carbon nanotubes. This is equivalent to sheet resistances below 1 ohm per square, surpassing theoretical predictions for perfect graphene8 by at least an order of magnitude. In neutral graphene ribbons, we show that transport is dominated by two modes. One is ballistic and temperature independent; the other is thermally activated. Transport is protected from back-scattering, possibly reflecting ground-state properties of neutral graphene. At room temperature, the resistance of both modes is found to increase abruptly at a particular length—the ballistic mode at 16 micrometres and the other at 160 nanometres. Our epitaxial graphene nanoribbons will be important not only in fundamental science, but also—because they can be readily produced in thousands—in advanced nanoelectronics, which can make use of their room-temperature ballistic transport properties.

https://www.nature.com/articles/nature12952

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Post by Chromium6 Sun Mar 01, 2020 3:54 am

Link: https://iopscience.iop.org/article/10.1088/1367-2630/aa8ec5


The focusing effect of electron flow and negative refraction in three-dimensional topological insulators

Kai-Tong Wang1, Yanxia Xing2,5, King Tai Cheung3

, Jian Wang3, Hui Pan4 and Hong-Kang Zhao1,5

Published 24 October 2017 • © 2017 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
New Journal of Physics, Volume 19, October 2017
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Abstract

We numerically study the focusing effect induced by a single p–n junction in three-dimensional topological insulators (3D TIs). It is found that, for either surface states or bulk states of 3D TIs, the corresponding electrons injected from the n/p region can be perfectly focused at the symmetric position in the p/n region. These results suggest that the focusing effect is a general phenomenon in materials which can be described by massless or massive Dirac equations. We also find that the focusing effect is robust against moderate random disorders. In the presence of external magnetic fields, the focusing effect remains good, but the position of the focus point oscillates periodically due to the finite size effect.

1. Introduction


In the 1960s, Veselago theoretically predicted the existence of negative refractive index materials, i.e., left-handed materials [1, 2]. After about 30 years, the first artificial left-handed material was experimentally verified [3]. In general, electromagnetic negative refraction can only be realized in artificial metamaterials with negative epsilon and μ, [4] where epsilon and μ are the electric permittivity and the magnetic permeability, respectively. When concerning the wave vector and the group velocity, the optical rays and the electron flow (the electron's de Broglie wave) are similar. Hence the negative refraction would be achieved in massless Dirac fermion materials, such as graphene. In such a material, the negative refraction is directly related to the perfect Veselago lens [5] and the Klein paradox [6].

The existence of negative refraction in massless Dirac material is natural. The electrons and holes in massless Dirac material are conjugately linked, and the chiralities (or dispersions) in the conduction band and valence band are opposite. Therefore, the potential barrier induced by a p–n junction (PNJ) in massless Dirac materials is highly transparent for the charge carriers [7]. As a result, the electron flow would be negatively refracted and symmetrically focused by the straight interface of the PNJ in the linear dispersion region [8]. Beyond the linear region, the statement on Dirac fermion fall through, but the focusing effect still exists [9]. It means the negative refraction is not limited to two-dimensional (2D) massless Dirac materials. In fact, as shown in figure 1, when electrons with momentum $({k}_{x},{k}_{\parallel })$ and velocity $({v}_{x},{v}_{\parallel })$ penetrate through the PNJ and become holes with momentum $(-{k}_{x},{k}_{\parallel })$, due to the opposite dispersion for electrons and holes, the velocity of holes becomes $({v}_{x},-{v}_{\parallel })$. Then the negative refraction is formed. As a result, the electron flow is focused by the straight interface induced by the PNJ. Here, '∥' denotes the direction along y for a 2D system or y–z plane for three-dimensional systems. So, there are two essential conditions to the focusing effect of the electron flow. One is the opposite dispersions in the conduction band and valence band, the other is the nearly transparent PNJ. In principle, besides massless Dirac fermions, [10, 11] all gapless semi-metals and topological materials [12, 13] described by the quadratic massive Dirac equation in 2D or beyond 2D, such as the 3D topological insulator (TI), are expected to have the same effect. Considering the helical resolved characteristics of the TI materials, the focusing effect in TIs can have great potential in the applications of helicity-based electronic optics [12].
....
4. Conclusion

In summary, based on the tight-binding Hamiltonian and NEGF technique, the focusing effect of electron flows in 3D TIs with a single PNJ is systematically studied. It is found that, for either surface states or bulk states of 3D TIs, the electron flow injected from the n/p region can be perfectly focused at the symmetric position in the p/n region of the PNJ. These facts suggest that the focusing effect is a general phenomenon in TIs described by massless or massive Dirac equations. However, focusing patterns are destroyed when the Fermi energy of electron flows is at the band edges, where surface and bulk states are mixed. This exception is attributed to the incompatible dispersion relations of the surface and bulk states of TIs. We also found the focusing pattern is robust against moderate random disorders, but the focusing intensity are severely reduced at strong disorders. In the presence of a weak magnetic field, the focusing effect remains well, but the position of the focus point oscillates periodically due to the finite size effect. These numerical findings are beneficial to the application of topological materials.

https://iopscience.iop.org/article/10.1088/1367-2630/aa8ec5/pdf

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Post by Chromium6 Sun Mar 01, 2020 4:06 am

History and Physics of the Klein Paradox
A Calogeracos, N Dombey
(Submitted on 24 May 1999)

The early papers by Klein, Sauter and Hund which investigate scattering off a high step potential in the context of the Dirac equation are discussed to derive the 'paradox' first obtained by Klein. The explanation of this effect in terms of electron-positron production is reassessed. It is shown that a potential well or barrier in the Dirac equation can become supercritical and emit positrons or electrons spontaneously if the potential is strong enough. If the well or barrier is wide enough, a seemingly constant current is emitted. This phenomenon is transient whereas the tunnelling first calculated by Klein is time-independent. It is shown that tunnelling without exponential suppression occurs when an electron is incident on a high barrier, even when the barrier is not high enough to radiate. Klein tunnelling is therefore a property of relativistic wave equations and is not necessarily connected to particle emission. The Coulomb potential is investigated and it is shown that a heavy nucleus of sufficiently large Z will bind positrons. Correspondingly, as Z increases the Coulomb barrier should become increasingly transparent to positrons. This is an example of Klein tunnelling. Phenomena akin to supercritical positron emission may be studied experimentally in superfluid.

https://arxiv.org/pdf/quant-ph/9905076.pdf
.....
We begin with a summary of the Dirac equation in one dimension in the presence of a potential V(x) and show how Klein’s original result for RS and TS is obtained. We go on to the papers of Sauter in 1931, who replaced Klein’s potential step with a barrier with a finite slope, and then to Hund in 1940 who realised that the Klein potential step gives rise to the production of pairs of charged particles when the potential strength is sufficiently strong. This result although not well known is a precursor of the famous results of modern quantum field theory of Schwinger [6] and Hawking [7] which show that particles are spontaneously produced in the presence of strong electric and gravitational fields. In Part II we turn to the underlying physics of the Klein paradox and show that particle production and Klein tunnelling arise naturally in the Dirac equation: when a potential well is deep enough it becomes super critical (defined as the potential strength for which the bound state energy E=−m) and positrons will be spontaneously produced. Supercriticality is well-understood [8], [9] and can occur in the Coulomb potential with finite nuclear size when thenuclear chargeZ >137. Positron production via this mechanism has been the subject of experimental investigations in heavy ion collisions for many years. We then show that if a potential well is wide enough, a steady but transient current will flow when the potential becomes supercritical. In order to analyse these processes it is necessary to introduce the concept of vacuum charge. We consider the implications of these concepts for the Coulomb potential and for other physical phenomena and we end by pointing out that Klein was unfortunate in that the example he chose to calculate was pathological. Smile

(Miles' solves this with the charge field.)

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Post by Chromium6 Tue Mar 03, 2020 9:53 pm

Mar 16, 2012

Engineered piezoelectric graphene could yield dramatic degree of control in nanotechnology


(Nanowerk News) In what became known as the 'Scotch tape technique," researchers first extracted graphene with a piece of adhesive in 2004. Graphene is a single layer of carbon atoms arranged in a honeycomb, hexagonal pattern. It looks like chicken wire.

Graphene is a wonder material. It is one-hundred-times better at conducting electricity than silicon. It is stronger than diamond. And, at just one atom thick, it is so thin as to be essentially a two-dimensional material. Such promising physics have made graphene the most studied substance of the last decade, particularly in nanotechnology. In 2010, the researchers who first isolated it shared the Nobel Prize.

Yet, while graphene is many things, it is not piezoelectric. Piezoelectricity is the property of some materials to produce electric charge when bent, squeezed or twisted. Perhaps more importantly, piezoelectricity is reversible. When an electric field is applied, piezoelectric materials change shape, yielding a remarkable level of engineering control.

lithium atoms (red) adhered to a graphene lattice that will produce electricity when bent, squeezed or twisted

This illustration shows lithium atoms (red) adhered to a graphene lattice that will produce electricity when bent, squeezed or twisted.

Conversely, the graphene will deform when an electric field is applied, opening new possibilities in nanotechnology.

Illustration: Mitchell Ong, Stanford School of Engineering

Piezoelectrics have found application in countless devices from watches, radios and ultrasound to the push-button starters on propane grills, but these uses all require relatively large, three-dimensional quantities of piezoelectric materials.

Now, in a paper published in the journal ACS Nano ("Engineered Piezoelectricity in Graphene"), two materials engineers at Stanford have described how they have engineered piezoelectrics into graphene, extending for the first time such fine physical control to the nanoscale.

https://dx.doi.org/doi:10.1021/nn204198g
Straintronics

"The physical deformations we can create are directly proportional to the electrical field applied and this represents a fundamentally new way to control electronics at the nanoscale," said Evan Reed, head of the Materials Computation and Theory Group at Stanford and senior author of the study. "This phenomenon brings new dimension to the concept of 'straintronics' for the way the electrical field strains—or deforms—the lattice of carbon, causing it to change shape in predictable ways."
"Piezoelectric graphene could provide an unparalleled degree of electrical, optical or mechanical control for applications ranging from touchscreens to nanoscale transistors," said Mitchell Ong, a post-doctoral scholar in Reed's lab and first author of the paper.

Using a sophisticated modeling application running on high-performance supercomputers, the engineers simulated the deposition of atoms on one side of a graphene lattice — a process known as doping — and measured the piezoelectric effect.
They modeled graphene doped with lithium, hydrogen, potassium and fluorine, as well as combinations of hydrogen and fluorine and lithium and fluorine on either side of the lattice. Doping just one side of the graphene, or doping both sides with different atoms, is key to the process as it breaks graphene's perfect physical symmetry, which otherwise cancels the piezoelectric effect.
The results surprised both engineers.
"We thought the piezoelectric effect would be present, but relatively small. Yet, we were able to achieve piezoelectric levels comparable to traditional three-dimensional materials," said Reed. "It was pretty significant.

More at link: https://www.nanowerk.com/news/newsid=24620.php?utm_source=feedburner&utm_medium=email&utm_campaign=Feed%3A+nanowerk%2FagWB+%28Nanowerk+Nanotechnology+News%29&utm_content=Yahoo%21+Mail

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Post by Chromium6 Wed Mar 04, 2020 1:07 am

Graphene Repairs Holes By Knitting Itself Back Together, Say Physicists

Enter Konstantin Novoselov at the University of Manchester and a few pals who have spent more than a few hours staring at graphene sheets through an electron microscope to see how it behaves.

Today, these guys say they've discovered why graphene appears so unpredictable. It turns out that if you make a hole in graphene, the material automatically knits itself back together again.

Novoselov and co made their discovery by etching tiny holes into a graphene sheet using an electron beam and watching what happens next using an electron microscope. They also added a few atoms of palladium or nickel, which catalyse the dissociation of carbon bonds and bind to the edges of the holes making them stable.

They found that the size of the holes depended on the number of metal atoms they added--more metal atoms can stabilise bigger holes.

But here's the curious thing. If they also added extra carbon atoms to the mix, these displaced the the metal atoms and reknitted the holes back together again.

Novoselov and co say the structure of the repaired area depends on the form in which the carbon is available. So when available as a hydrocarbon, the repairs tend to contain non-hexagonal defects where foreign atoms have entered the structure.

http://www.technologyreview.com/view/42 ... 2012-07-11


http://arxiv.org/abs/1207.1487

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Post by Chromium6 Sun Jun 21, 2020 3:42 pm

More unexpected properties:

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

New study reveals unexpected softness of bilayer graphene

May 1, 2020 , Queen Mary, University of London
graphene

This visualisation shows layers of graphene used for membranes. Credit: University of Manchester
In the study, published in the journal Physical Review B, the researchers showed that bilayer graphene, consisting of two layers of graphene, was noticeably softer than both two-dimensional (2-D) graphene and three-dimensional (3-D) graphite along the stacking direction.

This surprising result differs from previous research which showed that 2-D graphene, a flat single layer of carbon atoms arranged in a honeycomb structure had many of the same mechanical properties as 3-D graphite, which is a naturally occurring form of carbon made up from a very weak stack of many layers of graphene.

Measuring stiffness

Graphene is a 2-D material, but has 3-D properties such as its stiffness in the 'out-of-plane' direction, perpendicular to the plane of the graphene sheets.

The behaviour of π electrons within multilayer graphene determine its out-of-plane stiffness. In this study, the researchers found that when bilayer graphene is compressed out-of-plane, some π electrons are 'squeezed' through the graphene planes, which are impenetrable to small molecules such as water. This response makes the material softer and much easier to compress.

Dr. Yiwei Sun, lead author of the study from Queen Mary University of London, said: "Our previous study showed that 2-D graphene and 3-D graphite have many of the same mechanical properties, so we were surprised to see that bilayer graphene is much softer than both of these materials. We think that the softness of bilayer graphene results from the 'squeezing' of pi-electronic orbitals through the graphene layers. For example, if the bread on a burger is replaced by a bagel it is even easier to compress because the contents can be squeezed out of the bagel hole."

Realising potential

Often hailed as a 'wonder material', graphene has the highest known thermal and electrical conductivity and is stronger than steel, as well as being light, flexible and transparent.

It was discovered in 2004 by peeling off graphene flakes from bulk graphite (used in pencil leads and lubricants) using sticky tape.

Stacking the graphene flakes one on top of the other provides more possibilities as the material's extraordinary properties are determined by interactions between its stacked layers. Its unique characteristics can also be fine-tuned for various applications by stacking other 2-D materials, such as boron nitride and molybdenum disulphide, to graphene.

More at link:  https://phys.org/news/2020-05-news-story-reveals-unexpected-softness.html

.......

30 Mar 2020 in Research & Technology

Graphene displays unexpected permeability
In a two-step process, diatomic hydrogen flips through the monolayer’s intrinsic pores.


More at link: https://physicstoday.scitation.org/do/10.1063/PT.6.1.20200330a/full/

Christine Middleton

The angstrom-scale hexagonal voids in a sheet of graphene resemble the holes in chicken wire. But despite the material’s appearance, studies have shown that defect-free graphene is nearly impermeable. It blocks all liquids and gases; only protons have been found to pass through (see Physics Today, February 2015, page 11). Quantum calculations indicate that any atom or molecule attempting to cross faces an all but insurmountable energy barrier of a few electron volts or more.

Now Pengzhan Sun, Andre Geim (both at the University of Manchester in the UK), and their collaborators have found another exception to graphene’s impermeability: hydrogen gas.

Diagram of graphene membrane setup
Credit: Adapted from P. Z. Sun et al., Nature 579, 229, 2020

The researchers’ experimental setup, depicted in the first image, resembled that used in previous experiments to test graphene’s permeability. Micron-scale wells of trapped air were sealed with graphene membranes and surrounded by the gas being investigated. If any gas molecules got into a well, its membrane would balloon outward because of the increased pressure inside the well, and that deflection could be measured using atomic force microscopy. The new setup’s main advantage was an improved seal between the graphene and the well. Previous experiments with oxidized silicon wells had leaky seals, so Sun, Geim, and coworkers instead used graphite and hexagonal boron nitride wells. The graphene membranes formed leak-proof seals with the atomically flat ring-shaped surfaces surrounding each well.

The researchers tested a few dozen wells with helium—the smallest and potentially leakiest gas—and saw no membrane deflection over 30 days. Neon, nitrogen, oxygen, argon, krypton, and xenon also showed no discernable penetration. The precise measurements set an upper bound on graphene’s permeability, which is around that of a 1-km-thick piece of glass. However, when they tested hydrogen, the researchers were surprised to see a slow but steady upward membrane deflection of a few nanometers.

Diagram of graphene with chemisorbed hydrogen
Credit: Adapted from P. Z. Sun et al., Nature 579, 229, 2020

Although the researchers couldn’t directly observe the hydrogen transport, theoretical predictions for one route agreed with the observed leak rates. Hydrogen molecules first chemisorb on the membrane. That process is facilitated by the graphene’s local curvature; numerous ripples in the sheet make it more reactive

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Post by Chromium6 Sun Jun 21, 2020 4:03 pm

Princeton team detects a cascade of electronic transitions in "magic-angle" twisted bilayer graphene

A team of researchers at Princeton has looked for the origins of the unusual behavior known as magic-angle twisted bilayer graphene, and detected signatures of a cascade of energy transitions that could help explain how superconductivity arises in this material.

"This study shows that the electrons in magic-angle graphene are in a highly correlated state even before the material becomes superconducting, "said Ali Yazdani, Professor of Physics and the leader of the team that made the discovery. "The sudden shift of energies when we add or remove an electron in this experiment provides a direct measurement of the strength of the interaction between the electrons."

This is significant because these energy jumps provide a window into the collective behaviors of electrons, such as superconductivity, that emerge in magic-angle twisted bilayer graphene, a material composed of two layers of graphene in which the top sheet is rotated by a slight angle relative to the other.

In everyday metals, electrons can move freely through the material, but collisions among electrons and from the vibration of atoms give rise to resistance and the loss of some electrical energy as heat - which is why electronic devices get warm during use.

In superconducting materials, electrons cooperate. "The electrons are kind of dancing with each other," said Biao Lian, a postdoctoral research associate in the Princeton Center for Theoretical Science and one of the co-first authors of the study. "They have to collaborate to go into such a remarkable state."

By some measures, magic-angle graphene, discovered two years ago by Pablo Jarillo-Herrero and his team at the Massachusetts Institute of Technology (MIT), is one of the strongest superconductors ever discovered. Superconductivity is relatively robust in this system even though it occurs when there are very few freely moving electrons.



The researchers set out to explore how the unique crystal structure of magic-angle graphene enables collective behaviors. Electrons not only have a negative charge, but also two other characteristics: angular momentum or "spin," and possible movements in the crystal structure known as "valley" states. Combinations of spin and valley make up the various "flavors" of electrons.

The team particularly wanted to know how these flavors affect collective behaviors, so they conducted their experiments at temperatures just slightly above the point at which the electrons become strongly interacting, which the researchers likened to the parent phase of the behaviors.

"We measured the force between the electrons in the material at higher temperatures in the hopes that understanding this force will help us understand the superconductor that it becomes at lower temperatures," said Dillon Wong, a postdoctoral research fellow in the Princeton Center for Complex Materials and a co-first author.

They used a scanning tunneling microscope, in which a conductive metal tip can add or remove an electron from magic-angle graphene and detect the resulting energy state of that electron.

Because strongly interacting electrons resist the addition of a new electron, it costs some energy to add the additional electron. The researchers can measure this energy and from it determine the strength of the interaction force.

"I'm literally putting an electron in and seeing how much energy it costs to shove this electron into the cooperative bath," said Kevin Nuckolls, a graduate student in the Department of Physics, also a co-first author.

The team found that the addition of each electron caused a jump in the amount of energy needed to add another one - which would not have been the case if the electrons were able to go into the crystal and then move freely among the atoms. The resulting cascade of energy transitions resulted from an energy jump for each of the electrons' flavors -- since electrons need to assume the lowest energy state possible while also not being of the same energy and same flavor as other electrons at the same location in the crystal.

More at link: https://www.graphene-info.com/princeton-team-detects-cascade-electronic-transitions-magic-angle-twisted

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Post by Chromium6 Fri Jun 26, 2020 12:14 am

June 22, 2020
A new symmetry-broken parent state discovered in twisted bilayer graphene

by Weizmann Institute of Science

The symmetry-breaking phase transition in magic-angle graphene. The four ‘flavours’ of Dirac electrons filling their energy levels are represented by four ‘liquids’ filling conical glasses.

Credit: Weizmann Institute of Science

In 2018 it was discovered that two layers of graphene twisted one with respect to the other by a "magic" angle show a variety of interesting quantum phases, including superconductivity, magnetism and insulating behaviors. Now, a team of researchers from the Weizmann Institute of Science led by Prof. Shahal Ilani of the Condensed Matter Physics Department, in collaboration with Prof. Pablo Jarillo-Herrero's group at MIT, have discovered that these quantum phases descend from a previously unknown high-energy "parent state" with an unusual breaking of symmetry.

Graphene is a flat crystal of carbon, just one atom thick. When two sheets of this material are placed on top of each other, misaligned at small angle, a periodic "moiré" pattern appears. This pattern provides an artificial lattice for the electrons in the material. In this twisted bilayer system the electrons come in four "flavors": spins "up" or "down," combined with two "valleys" that originate in the graphene's hexagonal lattice. As a result, each moiré site can hold up to four electrons, one of each flavor.

While researchers already knew that the system behaves as a simple insulator when all the moiré sites are completely full (four electrons per site), Jarillo-Herrero and his colleagues discovered to their surprise, in 2018, that at a specific "magic" angle, the twisted system also becomes insulating at other integer fillings (two or three electrons per moiré site). This behavior, exhibited by magic-angle twisted bilayer graphene (MATBG), cannot be explained by single particle physics, and is often described as a "correlated Mott insulator." Even more surprising was the discovery of exotic superconductivity close to these fillings. These findings led to a flurry of research activity aiming to answer the big question: what is the nature of the new exotic states discovered in MATBG and similar twisted systems?

Imaging magic-angle graphene electrons with a carbon nanotube detector

The Weizmann team set out to understand how interacting electrons behave in MATBG using a unique type of microscope that utilizes a carbon nanotube single-electron transistor, positioned at the edge of a scanning probe cantilever. This instrument can image, in real space, the electric potential produced by electrons in a material with extreme sensitivity.

"Using this tool, we could image for the first time the 'compressibility' of the electrons in this system—that is, how hard it is to squeeze additional electrons into a given point in space," explains Ilani. "Roughly speaking, the compressibility of electrons reflects the phase they are in: In an insulator, electrons are incompressible, whereas in a metal they are highly compressible."

Compressibility also reveals the "effective mass" of electrons. For example, in regular graphene the electrons are extremely "light," and thus behave like independent particles that practically ignore the presence of their fellow electrons.
In magic-angle graphene, on the other hand, electrons are believed to be extremely "heavy" and their behavior is thus dominated by interactions with other electrons ‒ a fact that many researchers attribute to the exotic phases found in this material. The Weizmann team therefore expected the compressibility to show a very simple pattern as a function of electron filling: interchanging between a highly-compressible metal with heavy electrons and incompressible Mott insulators that appear at each integer moiré lattice filling.

To their surprise, they observed a vastly different pattern. Instead of a symmetric transition from metal to insulator and back to metal, they observed a sharp, asymmetric jump in the electronic compressibility near the integer fillings.

"This means that the nature of the carriers before and after this transition is markedly different," says study lead author Uri Zondiner. "Before the transition the carriers are extremely heavy, and after it they seem to be extremely light, reminiscent of the 'Dirac electrons' that are present in graphene." (See Mathis on stripping to "photons")

The same behavior was seen to repeat near every integer filling, where heavy carriers abruptly gave way and light Dirac-like electrons re-emerged.

But how can such an abrupt change in the nature of the carriers be understood?
To address this question, the team worked together with Weizmann theorists Profs. Erez Berg, Yuval Oreg and Ady Stern, and Dr. Raquel Quiroez; as well as Prof. Felix von-Oppen of Freie Universität Berlin. They constructed a simple model, revealing that electrons fill the energy bands in MATBG in a highly unusual "Sisyphean" manner: when electrons start filling from the "Dirac point" (the point at which the valence and conduction bands just touch each other), they behave normally, being distributed equally among the four possible flavors. "However, when the filling nears that of an integer number of electrons per moiré superlattice site, a dramatic phase transition occurs," explains study lead author Asaf Rozen. "In this transition, one flavor 'grabs' all the carriers from its peers, 'resetting' them back to the charge-neutral Dirac point."

"Left with no electrons, the three remaining flavors need to start refilling again from scratch. They do so until another phase transition occurs, where this time one of the remaining three flavors grabs all the carriers from its peers, pushing them back to square one. Electrons thus need to climb a mountain like Sisyphus, being constantly pushed back to the starting point in which they revert to the behavior of light Dirac electrons," says Rozen. While this system is in a highly symmetric state at low carrier fillings, in which all the electronic flavors are equally populated, with further filling it experiences a cascade of symmetry-breaking phase transitions that repeatedly reduce its symmetry.

A 'parent state'

"What is most surprising is that the phase transitions and Dirac revivals that we discovered appear at temperatures well above the onset of the superconducting and correlated insulating states observed so far," says Ilani. "This indicates that the broken symmetry state we have seen is, in fact, the 'parent state' out of which the more fragile superconducting and correlated insulating ground states emerge."

The peculiar way in which the symmetry is broken has important implications for the nature of the insulating and superconducting states in this twisted system.

"For example, it is well known that stronger superconductivity arises when electrons are heavier. Our experiment, however, demonstrates the exact opposite: superconductivity appears in this magic-angle graphene system after a phase transition has revived the light Dirac electrons. How this happens, and what it tells us about the nature of superconductivity in this system compared to other more conventional forms of superconductivity remain interesting open questions," says Zondiner.

A similar cascade of phase transitions was reported in another paper published in the same Nature issue by Prof. Ali Yazdani and colleagues at Princeton University. "The Princeton team studied MATBG using a completely different experimental technique, based on a highly-sensitive scanning tunneling microscope, so it is very reassuring to see that complementary techniques lead to analogous observations," says Ilani.

The Weizmann and MIT researchers say they will now use their scanning nanotube single-electron-transistor platform to answer these and other basic questions about electrons in various twisted-layer systems: What is the relationship between the compressibility of electrons and their apparent transport properties? What is the nature of the correlated states that form in these systems at low temperatures? And what are the fundamental quasiparticles that make up these states?

More at link:
https://phys.org/news/2020-06-symmetry-broken-parent-state-bilayer-graphene.html

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Post by Chromium6 Fri Jun 26, 2020 12:20 am

News Release 25-Jun-2020
Science study: Chemists achieve breakthrough in the synthesis of graphene nanoribbons

Martin-Luther-Universität Halle-Wittenberg

Graphene Nanoribbons might soon be much easier to produce. An international research team led by Martin Luther University Halle-Wittenberg (MLU), the University of Tennessee and Oak Ridge National Laboratory in the U.S. has succeeded in producing this versatile material for the first time directly on the surface of semiconductors. Until now, this was only possible on metal surfaces. The new approach also enables scientists to customise the properties of the nanoribbons. Storage technology is one of the potential applications of the material. The research team reports on its results in the upcoming issue of Science.

For years, graphene has been regarded as the material of the future. In simple terms, it is a two-dimensional carbon surface that resembles a honeycomb. This special structure gives the material distinctive properties: for example, it is extremely stable and ultra-light. There is a particular interest in graphene nanoribbons as they are a semiconductor material that could be used, for instance, in the electrical and computer industry. "This is why many research groups around the world are focusing their efforts on graphene nanoribbons," explains chemist Professor Konstantin Amsharov at MLU. These ribbons, which are only nanometres in size, are made up of just a few carbon atoms wide. Their properties are determined by their shape and width. When graphene research was just beginning, the bands were produced by cutting up larger sections. "This process was very complicated and imprecise," says Amsharov.

He and colleagues from Germany, the U.S. and Poland, have now succeeded in simplifying the production of the coveted nanoribbons. The team produces the material by joining together individual atoms, which enables the properties to be customised. The researchers have succeeded for the first time in producing the ribbons on the surface of titanium oxide, a non-metallic material. "Until now, the ribbons were mainly synthesised on gold surfaces. This is not only comparatively expensive, but also impractical," explains Amsharov. The problem with this approach is that gold conducts electricity. This would directly negate the properties of the graphene nanoribbons, which is why this method has only been used in basic research. However, the gold was needed as a catalyst to produce the nanoribbons in the first place. In addition, the nanoribbons had to be transferred from the gold surface to another surface - a very tricky undertaking. The new approach discovered by Amsharov and his colleagues solves this set of problems.

More at link: https://www.eurekalert.org/pub_releases/2020-06/mh-sc062320.php

............


Graphene Nanomesh: New Nanotechnology ‘Brick’ for Modern Micromachines
TOPICS:2D MaterialsGrapheneNanotechnology

By Japan Advanced Institute of Science and Technology May 7, 2020

Researchers at Japan advanced institute of science and technology (JAIST) have successfully fabrication the suspended graphene nanomesh in a large area by the helium ion beam microscopy. 6nm diameter nanopores were pattern on the 1.2 um long and 500 nm wide suspended graphene uniformly. By systematically controlling the pitch (nanopore’s center to nanopore’s center) from 15 nm to 50 nm, a series of stable graphene nanomesh devices were achieved. This provides a practical way to investigate the intrinsic properties of graphene nanomesh towards the application for gas sensing, phonon engineering, and quantum technology.

Graphene, with its excellent electrical, thermal and optical properties, is promising for many applications in the next decade. It is also a potential candidate instead of silicon to build the next generation of electrical circuits. However, without a bandgap, it is not straightforward to use graphene as field-effect transistors (FETs). Researchers tried to cut the graphene sheet into a small piece of graphene nanoribbon and observed the bandgap opening successfully. However, the current of graphene nanoribbons is too low to drive the integrated circuit. In this case, the graphene nanomesh is pointed out by introducing periodical nanopores on the graphene, which is also considered as very small graphene nanoribbon array.

A research team led by Dr Fayong Liu and Professor Hiroshi MIZUTA has demonstrated in collaboration with researchers at the National Institute of Advanced Industrial Science and Technology (AIST) that large area suspended graphene nanomesh is quickly achievable by the helium ion beam microscopy with sub-10 nm nanopore diameter and well-controlled pitches. Comparing to slow speed TEM patterning, the helium ion beam milling technique overcomes the speed limitation, and meanwhile, provides a high imaging resolution. With the initial electrical measurements, it has found that the thermal activation energy of the graphene nanomesh increased exponentially by increasing the porosity of the graphene nanomesh. This immediately provides a new method for bandgap engineering beyond the conventional nanoribbon method. The team plans to continue exploring graphene nanomesh towards the application of phonon engineering.

“Graphene nanomesh is a kind of new ‘brick’ for modern micromachine systems. Theoretically, we can generate many kinds of periodical patterns on the original suspended graphene, which tunes the property of the device to the direction for a special application, in particular nanoscale thermal management” says Prof. Hiroshi Mizuta

More at link: https://scitechdaily.com/graphene-nanomesh-new-nanotechnology-brick-for-modern-micromachines/

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Post by Chromium6 Thu Jul 23, 2020 2:04 am

Researchers Create ‘Water’ That Can Corrode Diamonds

Mar 15, 2013 by Natali Anderson

Scientists from Singapore and Belgium have successfully altered the properties of water, making it corrosive enough to etch diamonds.

Diamond particles (Jason Nadler / Uic.edu / Sci-News.com)

Diamond particles (Jason Nadler / Uic.edu / Sci-News.com)

The team achieved this by attaching a layer of graphene on diamond and heating to high temperatures – water molecules trapped between them become highly corrosive.

While diamond is known to be a material with superlative physical qualities, little is known about how it interfaces with graphene, an one-atom thick substance composed of pure carbon.

The researchers sought to explore what happens when a layer of graphene, behaving like a soft membrane, is attached on diamond, which is also composed of carbon. To encourage bonding between the two rather dissimilar carbon forms, the researchers heated them to high temperatures.

At elevated temperatures, the team noted a restructuring of the interface and chemical bonding between graphene and diamond. As graphene is an impermeable material, water trapped between the diamond and graphene can’t escape. At a temperature that is above 400 degree Celsius, the trapped water transforms into a distinct supercritical phase.

“We show for the first time that graphene can trap water on diamond, and the system behaves like a ‘pressure cooker’ when heated. Even more surprising, we found that such superheated water can corrode diamond. This has never been reported,” said Prof Loh Kian Ping from the National University of Singapore, senior author of a paper published in the journal Nature Communications.

Due to its transparent nature, the graphene bubble-on-diamond platform provides a novel way of studying the behaviors of liquids at high pressures and high temperature conditions, which is traditionally difficult.

“The applications from our experiment are immense. In the industry, supercritical water can be used for the degradation of organic waste in an environmentally friendly manner. Our work can be also applicable to the laser-assisted etching of semiconductor or dielectric films, where the graphene membrane can be used to trap liquids,” Prof Loh said.

More at link: http://www.sci-news.com/othersciences/nanotechnologies/article00939.html

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Post by Chromium6 Sat Aug 29, 2020 2:11 am

New graphene super batteries charge up in seconds and last virtually forever

ENVIRONMENT

Environment  Conservation  Energy
07/25/2016 under Art, Energy, Environment, News, Science

VIEW SLIDESHOW

With the aid of one of the strongest materials on Earth, a researcher at Australia’s Swinburne University has created a battery that charges up super fast and can be used over and over and over again, without losing efficiency. Researcher Han Lin developed the battery using a form of carbon called graphene, which is commonly heralded as one of the strongest materials on the planet. The new supercapacitor addresses many of the shortcomings of traditional lithium ion batteries, beating them in charging time, lifespan, and also environmental impact.

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Swinburne University, australia, melbourne, han lin, battery, graphene, supercapacitor, carbon, new battery technology
Researchers around the globe have worked on expanding the capabilities of supercapacitors for many years, but they are typically limited in storage capacity. Han overcame this problem by adding sheets of graphene, which have a large surface area for energy storage due to the material’s honeycomb structure. The material is also strong and flexible at the same time. The researcher used a 3D printer to create the graphene sheets, resulting in a cost-effective energy storage method that could someday replace the batteries in our cell phones and electric cars.

Related: Melbourne’s Advanced Technologies Centre by H2O Architects looks like a gigantic LEGO brick

The new supercapacitor’s ultra-quick charging time—just seconds compared to the minutes or hours needed by a lithium-based battery—is its primary selling point, as it eliminates the inconvenience of long charging times. The graphene-enhanced battery also costs less than a traditional lithium ion battery over the course of its lifetime, due to its unique ability to withstand more recharges without losing strength.

Han presented his new supercapacitor at Fresh Science Victoria 2016 earlier this year.

+ Swinburne University

Via Phys.org

https://inhabitat.com/new-graphene-super-batteries-charge-up-in-seconds-and-last-virtually-forever/

Also:

A New Battery Built by Samsung Uses Graphene to Charge Five Times Faster

https://futurism.com/a-new-battery-built-by-samsung-uses-graphene-to-charge-five-times-faster


Last edited by Chromium6 on Sat Aug 29, 2020 2:14 am; edited 1 time in total (Reason for editing : https://futurism.com/a-new-battery-built-by-samsung-uses-graphene-to-charge-five-times-faster)

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Post by Chromium6 Thu Sep 03, 2020 12:32 am

Korean Researchers Develop Device That Can Control Speed of Light
By Yoon Young-sil May 16, 2018, 10:21

Korean researchers have developed a device that can control the speed of light. The new device is expected to accelerate the development of next-generation optical communication equipment.

Professor Kim Teun-teun of the Center for Integrated Nanostructure Physics (CINAP) of the Institute for Basic Science (IBS) announced that his team has developed a graphene-based metamaterial device that can control the speed of light, in cooperation with a research team led by Professor Min Beom-ki of KAIST.

Korean researchers have developed a device that can control the speed of light.
Korean researchers have developed a device that can control the speed of light.

Traveling faster than anything in the world, light is the most useful means for data transmission. However, light should be converted into electrical signals to transmit data. In this process, the speed of light is slowed down due to the limitations of electronic signal processors.

By combining graphene with a metamaterial, the researchers have developed a device that can slow down the speed of light and then accelerate it. The new device is expected to help develop next-generation optical communication devices.

A metamaterial is a material engineered to have a property that is not found in nature. Graphene is a single, tightly packed layer of carbon atoms that are bonded together in a hexagonal honeycomb lattice.

http://www.businesskorea.co.kr/news/articleView.html?idxno=22292

......

New graphene-based metasurface capable of independent amplitude and phase control of light

A KAIST research team in collaboration with the University of Wisconsin-Madison theoretically developed a graphene-based active metasurface capable of independent amplitude and phase control of mid-infrared light. This research gives a new insight into modulating the mid-infrared wavefront with high resolution by solving the problem of the independent control of light amplitude and phase, which has remained a long-standing challenge.

Light modulation technology is essential for developing future optical devices such as holography, high-resolution imaging, and optical communication systems. Liquid crystals and a microelectromechanical system (MEMS) have previously been utilized to modulate light. However, both methods suffer from significantly limited driving speeds and unit pixel sizes larger than the diffraction limit, which consequently prevent their integration into photonic systems.

The metasurface platform is considered a strong candidate for the next generation of light modulation technology. Metasurfaces have optical properties that natural materials cannot have, and can overcome the limitations of conventional optical systems, such as forming a high-resolution image beyond the diffraction limit. In particular, the active metasurface is regarded as a technology with a wide range of applications due to its tunable optical characteristics with an electrical signal.

However, the previous active metasurfaces suffered from a correlation between light amplitude control and phase control. This problem is caused by the modulation mechanism of conventional metasurfaces. Conventional metasurfaces have been designed such that a metaatom only has one resonance condition, but a single resonant design inherently lacks the degrees of freedom to independently control the amplitude and phase of light.

The research team made a metaunit by combining two independently controllable metaatoms, dramatically improving the modulation range of active metasurfaces. The proposed metasurface can control the amplitude and phase of the mid-infrared light independently with a resolution beyond the diffraction limit, thus allowing complete control of the optical wavefront.

The research team theoretically confirmed the performance of the proposed active metasurface and the possibility of wavefront shaping using this design method. Furthermore, they developed an analytical method that can approximate the optical properties of metasurfaces without complex electromagnetic simulations. This analytical platform proposes a more intuitive and comprehensively applicable metasurface design guideline.

https://www.graphene-info.com/new-graphene-based-metasurface-capable-independent-amplitude-and-phase-control


Last edited by Chromium6 on Thu Sep 03, 2020 12:48 am; edited 1 time in total

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Post by Chromium6 Thu Sep 03, 2020 12:45 am

NEWS AND VIEWS  05 MARCH 201
Novel electronic states seen in graphene



A simple system made from two sheets of graphene has been converted from an insulator to a superconductor. The finding holds promise for opening up studies of an unconventional form of superconductivity.


In two papers in Nature, Cao et al.1,2 report the discovery of new electronic ground states in twisted bilayer graphene — a pair of single-atom-thick sheets of carbon atoms, stacked with their honeycomb lattices rotated out of alignment. The authors interpret one of these states2 as a correlated Mott insulator, a non-conducting state produced by strong repulsive interactions between electrons. The other1 is a superconductor, a state of zero electrical resistance produced by effective attractive interactions between electrons. The insulator turns into the superconductor when a small number of charge carriers are added to the graphene. This connection between the states is unlikely to be a coincidence — as Sherlock Holmes might have commented, “the universe is rarely so lazy”.


Read the paper: Unconventional superconductivity in magic-angle graphene superlattices
Cao et al. show that the stacking of graphene sheets allows access to a new family of materials with electronic behaviours that are exquisitely sensitive to the atomic alignment between the layers, which affects interlayer electron motion. This finding might surprise physicists, because electronic behaviour is usually dominated by whichever of the associated processes has the largest energy scale. But, in this case, there’s a conundrum: the energy associated with electron motion between atoms within a layer is of the order of electronvolts, whereas the energy for electron motion between layers3 is, at most, hundreds of millielectronvolts.
https://www.nature.com/articles/d41586-018-02660-4


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Post by Chromium6 Thu Sep 03, 2020 12:56 am

Stanford team finds novel form of magnetism in twisted bi-layer graphene

July 19 2019

(The Hall Effect mentioned again....Cr6)

Stanford physicists recently observed a novel form of magnetism, predicted but never seen before, that is generated when two graphene sheets are carefully stacked and rotated to a special angle. The researchers suggest the magnetism, called orbital ferromagnetism, could prove useful for certain applications, such as quantum computing.

bi-layer graphene between hBN gives off orbital ferromagnetism image

Optical micrograph of the assembled stacked structure, which consists of two graphene sheets sandwiched between two protective layers made of hexagonal boron nitride. (Image: Aaron Sharpe)

“We were not aiming for magnetism. We found what may be the most exciting thing in my career to date through partially targeted and partially accidental exploration,” said study leader David Goldhaber-Gordon, a professor of physics at Stanford’s School of Humanities and Sciences. “Our discovery shows that the most interesting things turn out to be surprises sometimes.”

The Stanford researchers inadvertently made their discovery while trying to reproduce a previous finding - in early 2018, Pablo Jarillo-Herrero’s group at MIT announced that they had coaxed a stack of two subtly misaligned sheets of graphene – twisted bilayer graphene – to conduct electricity without resistance, a property known as superconductivity.

The discovery was a confirmation of a nearly decade-old prediction that graphene sheets rotated to a very particular angle should exhibit interesting phenomena.

When stacked and twisted, graphene forms a superlattice with a repeating interference, or moiré, pattern. “It’s like when you play two musical tones that are slightly different frequencies,” Goldhaber-Gordon said. “You’ll get a beat between the two that’s related to the difference between their frequencies. That’s similar to what you get if you stack two lattices atop each other and twist them so they’re not perfectly aligned.”



Physicists theorized that the particular superlattice formed when graphene rotated to 1.1 degrees causes the normally varied energy states of electrons in the material to collapse, creating what they call a flat band where the speed at which electrons move drops to nearly zero. Thus slowed, the motions of any one electron becomes highly dependent on those of others in its vicinity. These interactions lie at the heart of many exotic quantum states of matter.

“I thought the discovery of superconductivity in this system was amazing. It was more than anyone had a right to expect,” Goldhaber-Gordon said. “But I also felt that there was a lot more to explore and many more questions to answer, so we set out to try to reproduce the work and then see how we could build upon it.”

While attempting to duplicate the MIT team’s results, Goldhaber-Gordon and his group introduced two seemingly unimportant changes.

First, while encapsulating the graphene sheets in thin layers of hexagonal boron nitride, the researchers inadvertently rotated one of the protective layers into near alignment with the twisted bilayer graphene.

“It turns out that if you nearly align the boron nitride lattice with the lattice of the graphene, you dramatically change the electrical properties of the twisted bilayer graphene,” said study co-first author Aaron Sharpe, a graduate student in Goldhaber-Gordon’s lab.

Secondly, the group intentionally overshot the angle of rotation between the two graphene sheets. Instead of 1.1 degrees, they aimed for 1.17 degrees because others had recently shown that twisted graphene sheets tend to settle into smaller angles during the manufacturing process.

“We figured if we aim for 1.17 degrees, then it will go back toward 1.1 degrees, and we’ll be happy,” Goldhaber-Gordon said. “Instead, we got 1.2 degrees.”

The consequences of these small changes didn’t become apparent until the Stanford researchers began testing the properties of their twisted graphene sample. In particular, they wanted to study how its magnetic properties changed as its flat band – that collection of states where electrons slow to nearly zero – was filled or emptied of electrons.

While pumping electrons into a sample that had been cooled close to absolute zero, Sharpe detected a large electrical voltage perpendicular to the flow of the current when the flat band was three-quarters full. Known as a Hall voltage, such a voltage typically only appears in the presence of an external magnetic field – but in this case, the voltage persisted even after the external magnetic field had been switched off.

This anomalous Hall effect could only be explained if the graphene sample was generating its own internal magnetic field. Furthermore, this magnetic field couldn’t be the result of aligning the up or down spin state of electrons, as is typically the case for magnetic materials, but instead must have arisen from their coordinated orbital motions.

“To our knowledge, this is the first known example of orbital ferromagnetism in a material,” Goldhaber-Gordon said. “If the magnetism were due to spin polarization, you wouldn’t expect to see a Hall effect. We not only see a Hall effect, but a huge Hall effect.”

The researchers estimate that the magnetic field near the surface of their twisted graphene sample is about a million times weaker than that of a conventional refrigerator magnet, but this weakness could be a strength in certain scenarios, such as building memory for quantum computers.

“Our magnetic bilayer graphene can be switched on with very low power and can be read electronically very easily,” Goldhaber-Gordon said. “The fact that there’s not a large magnetic field extending outward from the material means you can pack magnetic bits very close together without worrying about interference.”

https://www.graphene-info.com/stanford-team-finds-novel-form-magnetism-twisted-bi-layer-graphene

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Post by Chromium6 Fri Sep 04, 2020 12:31 am

Twisted bilayer graphene responds strongly to infrared light

02 Sep 2020 Isabelle Dumé

Graphene. Credit: University of Texas at Dallas

The list of surprising behaviours in “twisted bilayer” graphene (TBG) just keeps getting longer. The material – which is made by stacking two sheets of graphene on top of one another, and then rotating one of them so that the sheets are slightly misaligned – was already known to support a wide array of insulating and superconducting states, depending on the strength of an applied electric field. Now researchers in the US have uncovered yet another oddity: when TBG is exposed to infrared light, its ability to conduct electricity changes. According to Fengnian Xia of Yale University, Fan Zhang of the University of Texas at Dallas, and colleagues, this finding could make it possible to develop a new class of infrared detectors using these stacked carbon sheets.

A single layer of graphene consists of a simple repetition of carbon atoms arranged in a two-dimensional hexagonal lattice. In this pristine state, the material does not have an electronic bandgap – that is, it is a gapless semiconductor. However, when two graphene sheets are placed on top of each other and slightly misaligned, they form a moiré pattern, or superlattice. In this new arrangement, the unit cell of the 2D crystal expands to a huge extent, as if it were artificially “stretched” in the two in-plane directions. This stretching dramatically changes the material’s electronic interactions.

From magic angle to twistronics
The misalignment angle in TBG is critically important. For example, at a so-called “magic” misalignment angle of 1.1°, the material switches from an insulator to a superconductor (that is, able to carry electrical current with no resistance below 1.7 K), as a team at the Massachusetts Institute of Technology (MIT) discovered in 2018.

The existence of such strongly correlated effects – which were first theoretically predicted in 2011 by Allan MacDonald and Rafi Bistritzer of the University of Texas at Austin – kick-started the field of “twistronics”. In this fundamentally new approach to device engineering, the weak coupling between different layers of 2D materials, like graphene, can be used to manipulate the electronic properties of these materials in ways that are not possible with more conventional structures, simply by varying the angle between the two layers.

Infrared light affects TBG’s conductance
Xia and Zhang’s teams have now studied how TBG interacts with infrared light – something that has never been investigated before. In their experiments, they shone light in the mid-infrared region of the spectrum, with a wavelength of between 5 and 12 microns, onto samples of TBG and measured how the electrical conductance varied at different twist angles. They found that the conductance reached a peak at 1.81° and that the photoresponse of the material was much stronger compared to untwisted bilayer graphene. This is because the twist significantly enhances the interactions between light and the material and induces a narrow bandgap (as well as superlattice-enhanced density of states). They also found that this strong photoresponse fades at a twist angle of less than 0.5°, as the bandgap closes.

Further investigations by the team revealed that the TBG absorbs the incident energy of the photons from the infrared light. This increases its temperature, which, in turn, produces an enhanced photocurrent.

Ferromagnetism appears in twisted bilayer graphene

The results suggest that the conducting mechanism in TBG is fundamentally connected to the period of the moiré pattern, and the superlattice produced, which is itself connected to the twist angle between the two graphene layers, Zhang explains. The twist angle is thus clearly very important in determining the material’s electronic properties, Xia adds, with smaller twist angles producing a larger moiré periodicity.

Towards a new class of infrared detectors
The researchers, who report their work in Nature Photonics, now hope to find out whether they can combine photoresponsivity and superconductivity in TBG. “Can shining a light induce or somehow modulate superconductivity? That will be very interesting to study,” Zhang says.

More at link: https://physicsworld.com/a/twisted-bilayer-graphene-responds-strongly-to-infrared-light/

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Post by Chromium6 Fri Sep 04, 2020 1:13 am

Polycyclic Aromatic Hydrocarbons as Potential Building Blocks for Organic Solar Cells

Cyril Aumaitre et al. Chem Rec. 2019 Jun.

Abstract

Since the discovery of graphene in the early 2000's, polycyclic aromatic hydrocarbons (PAHs) have been resurrected and new synthetic tools have been developed to prepare unprecedented structures with unique properties. One application that has been overlooked for this class of molecules is organic solar cells (OSCs). In this account, we present the recent development in the preparation of moderate to low band gap PAHs that could potentially be used as semiconducting materials in OSCs. Our focus is directed toward all-carbon PAHs as well as their polymeric analogs.

Keywords: Anthanthrene; Carbon-rich molecules; Organic semiconductors; Polycyclic aromatic hydrocarbons; Solar cells.

© 2019 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

https://pubmed.ncbi.nlm.nih.gov/31106986/

.........

Polycyclic Aromatic Hydrocarbons Adsorption onto Graphene: A DFT and AIMD Study

Bing Li et al. Materials (Basel). 2018.
Free PMC article Show details

Abstract

Density functional theory (DFT) calculations and ab-initio molecular dynamics (AIMD) simulations were performed to understand graphene and its interaction with polycyclic aromatic hydrocarbons (PAHs) molecules. The adsorption energy was predicted to increase with the number of aromatic rings in the adsorbates, and linearly correlate with the hydrophobicity of PAHs. Additionally, the analysis of the electronic properties showed that PAHs behave as mild n-dopants and introduce electrons into graphene; but do not remarkably modify the band gap of graphene, indicating that the interaction between PAHs and graphene is physisorption. We have also discovered highly sensitive strain dependence on the adsorption strength of PAHs onto graphene surface. The AIMD simulation indicated that a sensitive and fast adsorption process of PAHs can be achieved by choosing graphene as the adsorbent. These findings are anticipated to shed light on the future development of graphene-based materials with potential applications in the capture and removal of persistent aromatic pollutants.

https://pubmed.ncbi.nlm.nih.gov/29751556/

.........

Adsorption of Polycyclic Aromatic Hydrocarbons by Graphene and Graphene Oxide Nanosheets

Jun Wang†‡, Zaiming Chen†‡, and Baoliang Chen†‡*
View Author Information
Cite this: Environ. Sci. Technol. 2014, 48, 9, 4817–4825
Publication Date:March 31, 2014
https://doi.org/10.1021/es405227u
Copyright © 2014 American Chemical Society

Abstract

Abstract Image

The adsorption of naphthalene, phenanthrene, and pyrene onto graphene (GNS) and graphene oxide (GO) nanosheets was investigated to probe the potential adsorptive sites and molecular mechanisms. The microstructure and morphology of GNS and GO were characterized by elemental analysis, XPS, FTIR, Raman, SEM, and TEM. Graphene displayed high affinity to the polycyclic aromatic hydrocarbons (PAHs), whereas GO adsorption was significantly reduced after oxygen-containing groups were attached to GNS surfaces. An unexpected peak was found in the curve of adsorption coefficients (Kd) with the PAH equilibrium concentrations. The hydrophobic properties and molecular sizes of the PAHs affected the adsorption of G and GO. The high affinities of the PAHs to GNS are dominated by π–π interactions to the flat surface and the sieving effect of the powerful groove regions formed by wrinkles on GNS surfaces. In contrast, the adsorptive sites of GO changed to the carboxyl groups attaching to the edges of GO because the groove regions disappeared and the polar nanosheet surfaces limited the π–π interactions. The TEM and SEM images initially revealed that after loading with PAH, the conformation and aggregation of GNS and GO nanosheets dramatically changed, which explained the observations that the potential adsorption sites of GNS and GO were unusually altered during the adsorption process.

https://pubs.acs.org/doi/suppl/10.1021/es405227u/suppl_file/es405227u_si_001.pdf

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