Mathis on Graphene? Any hints?

Page 4 of 4 Previous  1, 2, 3, 4

Go down

Re: Mathis on Graphene? Any hints?

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

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

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

Experimental observation

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

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

Cr6
Admin

Posts : 1080
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

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

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

Jared Magneson

Posts : 440
Join date : 2016-10-11

View user profile

Back to top Go down

Re: Mathis on Graphene? Any hints?

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

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

Columbia researchers observe exotic quantum particle in bilayer graphene

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

Columbia team observes exotic quantum particle in graphene image

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

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

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

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

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

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

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

Cr6
Admin

Posts : 1080
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

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

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

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

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

Jared Magneson

Posts : 440
Join date : 2016-10-11

View user profile

Back to top Go down

Re: Mathis on Graphene? Any hints?

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

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

----------

This article was invited by Rolf J Haug.

Abstract

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

-----------

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

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

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

Significance

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

Abstract

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

   moiré crystalgraphenetwisted bilayermoiré bandHofstadter butterfly

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

Cr6
Admin

Posts : 1080
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

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

Surprise Graphene Discovery Could Unlock Secrets of Superconductivity

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

By Elizabeth Gibney, Nature magazine on March 7, 2018

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Cr6
Admin

Posts : 1080
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

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

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

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

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

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

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

hole filling process in suspended graphene

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

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

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

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

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

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

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

Cr6
Admin

Posts : 1080
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

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

Scientists predict green energy revolution after incredible new graphene discoveries

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

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

Steve Connor

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

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

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

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

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

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

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

A computer generated illustration of graphene cells (Corbis)

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

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

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

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

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

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

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

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

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

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


Cr6
Admin

Posts : 1080
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

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

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

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

By Thomas Hornigold -
Aug 05, 2018 5,888

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Cr6
Admin

Posts : 1080
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

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

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

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

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

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

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


Magic-angle graphene superlattice
Graphene moiré superlattice

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

Cr6
Admin

Posts : 1080
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

The Effective mass in graphene

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

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

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

The effective mass in graphene

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

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

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

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

-------

Graphene membrane

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

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

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

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

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

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

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

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

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

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

--------

Theory of resonant photon drag in monolayer graphene

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


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

Cr6
Admin

Posts : 1080
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

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


MIT may have just solved how to mass-produce graphene

by Colm Gorey

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

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

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

Video on Process:

...

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

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

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

‘Like a continuous bed of pizza’

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

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

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

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

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

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

Related: materials science, MIT, graphene

Colm Gorey is a journalist with Siliconrepublic.com

editorial@siliconrepublic.com

........


Cr6
Admin

Posts : 1080
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

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


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

Credit: University of Manchester

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

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

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

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

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

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

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

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

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

Cr6
Admin

Posts : 1080
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

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


Cr6
Admin

Posts : 1080
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

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

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

But here are a few more:

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

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

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

Abstract


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



Cr6
Admin

Posts : 1080
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

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

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

http://milesmathis.com/fermi.pdf

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

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

http://milesmathis.com/drude.pdf

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


...

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


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

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

Abstract

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

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

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

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

Cr6
Admin

Posts : 1080
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Sponsored content


Sponsored content


Back to top Go down

Page 4 of 4 Previous  1, 2, 3, 4

Back to top


 
Permissions in this forum:
You cannot reply to topics in this forum