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  • richardmitnick 7:55 am on June 29, 2017 Permalink | Reply
    Tags: , Dialysis, Graphene, Hemodialysis, Material can filter nanometer-sized molecules at 10 to 100 times the rate of commercial membranes, ,   

    From MIT: “Scientists produce dialysis membrane made from graphene” 

    MIT News

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    MIT News

    June 28, 2017
    Jennifer Chu

    1
    1) Graphene, grown on copper foil, is pressed against a supporting sheet of polycarbonate. 2) The polycarbonate acts to peel the graphene from the copper. 3) Using interfacial polymerization, researchers seal large tears and defects in graphene. 4) Next, they use oxygen plasma to etch pores of specific sizes in graphene. Courtesy of the researchers (edited by MIT News)

    Material can filter nanometer-sized molecules at 10 to 100 times the rate of commercial membranes.

    Dialysis, in the most general sense, is the process by which molecules filter out of one solution, by diffusing through a membrane, into a more dilute solution. Outside of hemodialysis, which removes waste from blood, scientists use dialysis to purify drugs, remove residue from chemical solutions, and isolate molecules for medical diagnosis, typically by allowing the materials to pass through a porous membrane.

    Today’s commercial dialysis membranes separate molecules slowly, in part due to their makeup: They are relatively thick, and the pores that tunnel through such dense membranes do so in winding paths, making it difficult for target molecules to quickly pass through.

    Now MIT engineers have fabricated a functional dialysis membrane from a sheet of graphene — a single layer of carbon atoms, linked end to end in hexagonal configuration like that of chicken wire. The graphene membrane, about the size of a fingernail, is less than 1 nanometer thick. (The thinnest existing memranes are about 20 nanometers thick.) The team’s membrane is able to filter out nanometer-sized molecules from aqueous solutions up to 10 times faster than state-of-the-art membranes, with the graphene itself being up to 100 times faster.

    While graphene has largely been explored for applications in electronics, Piran Kidambi, a postdoc in MIT’s Department of Mechanical Engineering, says the team’s findings demonstrate that graphene may improve membrane technology, particularly for lab-scale separation processes and potentially for hemodialysis.

    “Because graphene is so thin, diffusion across it will be extremely fast,” Kidambi says. “A molecule doesn’t have to do this tedious job of going through all these tortuous pores in a thick membrane before exiting the other side. Moving graphene into this regime of biological separation is very exciting.”

    Kidambi is a lead author of a study reporting the technology, published today in Advanced Materials. Six co-authors are from MIT, including Rohit Karnik, associate professor of mechanical engineering, and Jing Kong, associate professor of electrical engineering.

    Plugging graphene

    To make the graphene membrane, the researchers first used a common technique called chemical vapor deposition to grow graphene on copper foil. They then carefully etched away the copper and transferred the graphene to a supporting sheet of polycarbonate, studded throughout with pores large enough to let through any molecules that have passed through the graphene. The polycarbonate acts as a scaffold, keeping the ultrathin graphene from curling up on itself.

    The researchers looked to turn graphene into a molecularly selective sieve, letting through only molecules of a certain size. To do so, they created tiny pores in the material by exposing the structure to oxygen plasma, a process by which oxygen, pumped into a plasma chamber, can etch away at materials.

    “By tuning the oxygen plasma conditions, we can control the density and size of pores we make, in the areas where the graphene is pristine,” Kidambi says. “What happens is, an oxygen radical comes to a carbon atom [in graphene] and rapidly reacts, and they both fly out as carbon dioxide.”

    What is left is a tiny hole in the graphene, where a carbon atom once sat. Kidambi and his colleagues found that the longer graphene is exposed to oxygen plasma, the larger and more dense the pores will be. Relatively short exposure times, of about 45 to 60 seconds, generate very small pores.

    Desirable defects

    The researchers tested multiple graphene membranes with pores of varying sizes and distributions, placing each membrane in the middle of a diffusion chamber. They filled the chamber’s feed side with a solution containing various mixtures of molecules of different sizes, ranging from potassium chloride (0.66 nanometers wide) to vitamin B12 (1 to 1.5 nanometers) and lysozyme (4 nanometers), a protein found in egg white. The other side of the chamber was filled with a dilute solution.

    The team then measured the flow of molecules as they diffused through each graphene membrane.

    Membranes with very small pores let through potassium chloride but not larger molecules such as L-tryptophan, which measures only 0.2 nanometers wider. Membranes with larger pores let through correspondingly larger molecules.

    The team carried out similar experiments with commercial dialysis membranes and found that, in comparison, the graphene membranes performed with higher “permeance,” filtering out the desired molecules up to 10 times faster.

    Kidambi points out that the polycarbonate support is etched with pores that only take up 10 percent of its surface area, which limits the amount of desired molecules that ultimately pass through both layers.

    “Only 10 percent of the membrane’s area is accessible, but even with that 10 percent, we’re able to do better than state-of-the-art,” Kidambi says.

    To make the graphene membrane even better, the team plans to improve the polycarbonate support by etching more pores into the material to increase the membrane’s overall permeance. They are also working to further scale up the dimensions of the membrane, which currently measures 1 square centimeter. Further tuning the oxygen plasma process to create tailored pores will also improve a membrane’s performance — something that Kidambi points out would have vastly different consequences for graphene in electronics applications.

    “What’s exciting is, what’s not great for the electronics field is actually perfect in this [membrane dialysis] field,” Kidambi says. “In electronics, you want to minimize defects. Here you want to make defects of the right size. It goes to show the end use of the technology dictates what you want in the technology. That’s the key.”

    This research was supported, in part, by the U.S. Department of Energy and a Lindemann Trust Fellowship.

    See the full article here .

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  • richardmitnick 3:45 pm on April 20, 2017 Permalink | Reply
    Tags: , Graphene, How Graphene Could Cool Smartphone Computer and Other Electronics Chips,   

    From Rutgers: “How Graphene Could Cool Smartphone, Computer and Other Electronics Chips” 

    Rutgers University
    Rutgers University

    March 27, 2017 [Nothing like beimg timely.]
    Todd B. Bates

    Rutgers scientists lead research that discovers potential advance for the electronics industry.

    1
    Graphene, a one-atom-thick layer of graphite, consists of carbon atoms arranged in a honeycomb lattice. Photo: OliveTree/Shutterstock

    With graphene, Rutgers researchers have discovered a powerful way to cool tiny chips – key components of electronic devices with billions of transistors apiece.

    “You can fit graphene, a very thin, two-dimensional material that can be miniaturized, to cool a hot spot that creates heating problems in your chip, said Eva Y. Andrei, Board of Governors professor of physics in the Department of Physics and Astronomy. “This solution doesn’t have moving parts and it’s quite efficient for cooling.”

    The shrinking of electronic components and the excessive heat generated by their increasing power has heightened the need for chip-cooling solutions, according to a Rutgers-led study published recently in Proceedings of the National Academy of Sciences. Using graphene combined with a boron nitride crystal substrate, the researchers demonstrated a more powerful and efficient cooling mechanism.

    “We’ve achieved a power factor that is about two times higher than in previous thermoelectric coolers,” said Andrei, who works in the School of Arts and Sciences.

    The power factor refers to the effectiveness of active cooling. That’s when an electrical current carries heat away, as shown in this study, while passive cooling is when heat diffuses naturally.

    Graphene has major upsides. It’s a one-atom-thick layer of graphite, which is the flaky stuff inside a pencil. The thinnest flakes, graphene, consist of carbon atoms arranged in a honeycomb lattice that looks like chicken wire, Andrei said. It conducts electricity better than copper, is 100 times stronger than steel and quickly diffuses heat.

    The graphene is placed on devices made of boron nitride, which is extremely flat and smooth as a skating rink, she said. Silicon dioxide – the traditional base for chips – hinders performance because it scatters electrons that can carry heat away.

    See the full article here .

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  • richardmitnick 10:54 am on March 27, 2017 Permalink | Reply
    Tags: , , Graphene, , Power factor,   

    From Rutgers: “How Graphene Could Cool Smartphone, Computer and Other Electronics Chips” 

    Rutgers University
    Rutgers University

    March 27, 2017
    Todd B. Bates

    1
    Graphene, a one-atom-thick layer of graphite, consists of carbon atoms arranged in a honeycomb lattice. Photo: OliveTree/Shutterstock

    Rutgers scientists lead research that discovers potential advance for the electronics industry.

    With graphene, Rutgers researchers have discovered a powerful way to cool tiny chips – key components of electronic devices with billions of transistors apiece.

    “You can fit graphene, a very thin, two-dimensional material that can be miniaturized, to cool a hot spot that creates heating problems in your chip, said Eva Y. Andrei, Board of Governors professor of physics in the Department of Physics and Astronomy. “This solution doesn’t have moving parts and it’s quite efficient for cooling.”

    The shrinking of electronic components and the excessive heat generated by their increasing power has heightened the need for chip-cooling solutions, according to a Rutgers-led study published recently in Proceedings of the National Academy of Sciences. Using graphene combined with a boron nitride crystal substrate, the researchers demonstrated a more powerful and efficient cooling mechanism.

    “We’ve achieved a power factor that is about two times higher than in previous thermoelectric coolers,” said Andrei, who works in the School of Arts and Sciences.

    The power factor refers to the effectiveness of active cooling. That’s when an electrical current carries heat away, as shown in this study, while passive cooling is when heat diffuses naturally.

    Graphene has major upsides. It’s a one-atom-thick layer of graphite, which is the flaky stuff inside a pencil. The thinnest flakes, graphene, consist of carbon atoms arranged in a honeycomb lattice that looks like chicken wire, Andrei said. It conducts electricity better than copper, is 100 times stronger than steel and quickly diffuses heat.

    The graphene is placed on devices made of boron nitride, which is extremely flat and smooth as a skating rink, she said. Silicon dioxide – the traditional base for chips – hinders performance because it scatters electrons that can carry heat away.

    In a tiny computer or smartphone chip, billions of transistors generate lots of heat, and that’s a big problem, Andrei said. High temperatures hamper the performance of transistors – electronic devices that control the flow of power and can amplify signals – so they need cooling.

    Current methods include little fans in computers, but the fans are becoming less efficient and break down, she said. Water is also used for cooling, but that bulky method is complicated and prone to leaks that can fry computers.

    “In a refrigerator, you have compression that does the cooling and you circulate a liquid,” Andrei said. “But this involves moving parts and one method of cooling without moving parts is called thermoelectric cooling.”

    Think of thermoelectric cooling in terms of the water in a bathtub. If the tub has hot water and you turn on the cold water, it takes a long time for the cold water below the faucet to diffuse in the tub. This is passive cooling because molecules slowly diffuse in bathwater and become diluted, Andrei said. But if you use your hands to push the water from the cold end to the hot, the cooling process – also known as convection or active cooling – will be much faster.

    The same process takes place in computer and smartphone chips, she said. You can connect a piece of wire, such as copper, to a hot chip and heat is carried away passively, just like in a bathtub.

    Now imagine a piece of metal with hot and cold ends. The metal’s atoms and electrons zip around the hot end and are sluggish at the cold end, Andrei said. Her research team, in effect, applied voltage to the metal, sending a current from the hot end to the cold end. Similar to the case of active cooling in the bathtub example, the current spurred the electrons to carry away the heat much more efficiently than via passive cooling. Graphene is actually superior in both its passive and active cooling capability. The combination of the two makes graphene an excellent cooler.

    “The electronics industry is moving towards this kind of cooling,” Andrei said. “There’s a very big research push to incorporate these kinds of coolers. There is a good chance that the graphene cooler is going to win out. Other materials out there are much more expensive, they’re not as thin and they don’t have such a high power factor.”

    The study’s lead author is Junxi Duan, a Rutgers physics post-doctoral fellow. Other authors include Xiaoming Wang, a Rutgers mechanical engineering post-doctoral fellow; Xinyuan Lai, a Rutgers physics undergraduate student; Guohong Li, a Rutgers physics research associate; Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science in Tsukuba, Japan; Mona Zebarjadi, a former Rutgers mechanical engineering professor who is now at the University of Virginia; and Andrei. Zebarjadi conducted a previous study on electronic cooling using thermoelectric devices.

    See the full article here .

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    Founded in 1766, Rutgers teaches across the full educational spectrum: preschool to precollege; undergraduate to graduate; postdoctoral fellowships to residencies; and continuing education for professional and personal advancement.

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  • richardmitnick 1:06 pm on January 8, 2017 Permalink | Reply
    Tags: 3-D forms, Graphene, , Researchers design one of the strongest lightest materials known   

    From MIT: “Researchers design one of the strongest, lightest materials known” 

    MIT News
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    January 6, 2017
    David L. Chandler

    1
    This illustration shows the simulation results of tensile and compression tests on 3-D graphene. Image: Zhao Qin

    Porous, 3-D forms of graphene developed at MIT can be 10 times as strong as steel but much lighter.


    Access mp4 video here .
    A team of MIT engineers has successfully designed a new 3-D material with five percent the density of steel and ten times the strength, making it one of the strongest, lightweight materials known.
    Video: Melanie Gonick/MIT

    A team of researchers at MIT has designed one of the strongest lightweight materials known, by compressing and fusing flakes of graphene, a two-dimensional form of carbon. The new material, a sponge-like configuration with a density of just 5 percent, can have a strength 10 times that of steel.

    In its two-dimensional form, graphene is thought to be the strongest of all known materials. But researchers until now have had a hard time translating that two-dimensional strength into useful three-dimensional materials.

    The new findings show that the crucial aspect of the new 3-D forms has more to do with their unusual geometrical configuration than with the material itself, which suggests that similar strong, lightweight materials could be made from a variety of materials by creating similar geometric features.

    The findings are being reported today in the journal Science Advances, in a paper by Markus Buehler, the head of MIT’s Department of Civil and Environmental Engineering (CEE) and the McAfee Professor of Engineering; Zhao Qin, a CEE research scientist; Gang Seob Jung, a graduate student; and Min Jeong Kang MEng ’16, a recent graduate.Other groups had suggested the possibility of such lightweight structures, but lab experiments so far had failed to match predictions, with some results exhibiting several orders of magnitude less strength than expected. The MIT team decided to solve the mystery by analyzing the material’s behavior down to the level of individual atoms within the structure. They were able to produce a mathematical framework that very closely matches experimental observations.

    Two-dimensional materials — basically flat sheets that are just one atom in thickness but can be indefinitely large in the other dimensions — have exceptional strength as well as unique electrical properties. But because of their extraordinary thinness, “they are not very useful for making 3-D materials that could be used in vehicles, buildings, or devices,” Buehler says. “What we’ve done is to realize the wish of translating these 2-D materials into three-dimensional structures.”

    The team was able to compress small flakes of graphene using a combination of heat and pressure. This process produced a strong, stable structure whose form resembles that of some corals and microscopic creatures called diatoms. These shapes, which have an enormous surface area in proportion to their volume, proved to be remarkably strong. “Once we created these 3-D structures, we wanted to see what’s the limit — what’s the strongest possible material we can produce,” says Qin. To do that, they created a variety of 3-D models and then subjected them to various tests. In computational simulations, which mimic the loading conditions in the tensile and compression tests performed in a tensile loading machine, “one of our samples has 5 percent the density of steel, but 10 times the strength,” Qin says.

    Buehler says that what happens to their 3-D graphene material, which is composed of curved surfaces under deformation, resembles what would happen with sheets of paper. Paper has little strength along its length and width, and can be easily crumpled up. But when made into certain shapes, for example rolled into a tube, suddenly the strength along the length of the tube is much greater and can support substantial weight. Similarly, the geometric arrangement of the graphene flakes after treatment naturally forms a very strong configuration.

    The new configurations have been made in the lab using a high-resolution, multimaterial 3-D printer. They were mechanically tested for their tensile and compressive properties, and their mechanical response under loading was simulated using the team’s theoretical models. The results from the experiments and simulations matched accurately.

    The new, more accurate results, based on atomistic computational modeling by the MIT team, ruled out a possibility proposed previously by other teams: that it might be possible to make 3-D graphene structures so lightweight that they would actually be lighter than air, and could be used as a durable replacement for helium in balloons. The current work shows, however, that at such low densities, the material would not have sufficient strength and would collapse from the surrounding air pressure.

    But many other possible applications of the material could eventually be feasible, the researchers say, for uses that require a combination of extreme strength and light weight. “You could either use the real graphene material or use the geometry we discovered with other materials, like polymers or metals,” Buehler says, to gain similar advantages of strength combined with advantages in cost, processing methods, or other material properties (such as transparency or electrical conductivity).

    “You can replace the material itself with anything,” Buehler says. “The geometry is the dominant factor. It’s something that has the potential to transfer to many things.”

    The unusual geometric shapes that graphene naturally forms under heat and pressure look something like a Nerf ball — round, but full of holes. These shapes, known as gyroids, are so complex that “actually making them using conventional manufacturing methods is probably impossible,” Buehler says. The team used 3-D-printed models of the structure, enlarged to thousands of times their natural size, for testing purposes.

    For actual synthesis, the researchers say, one possibility is to use the polymer or metal particles as templates, coat them with graphene by chemical vapor deposit before heat and pressure treatments, and then chemically or physically remove the polymer or metal phases to leave 3-D graphene in the gyroid form. For this, the computational model given in the current study provides a guideline to evaluate the mechanical quality of the synthesis output.

    The same geometry could even be applied to large-scale structural materials, they suggest. For example, concrete for a structure such a bridge might be made with this porous geometry, providing comparable strength with a fraction of the weight. This approach would have the additional benefit of providing good insulation because of the large amount of enclosed airspace within it.

    Because the shape is riddled with very tiny pore spaces, the material might also find application in some filtration systems, for either water or chemical processing. The mathematical descriptions derived by this group could facilitate the development of a variety of applications, the researchers say.

    “This is an inspiring study on the mechanics of 3-D graphene assembly,” says Huajian Gao, a professor of engineering at Brown University, who was not involved in this work. “The combination of computational modeling with 3-D-printing-based experiments used in this paper is a powerful new approach in engineering research. It is impressive to see the scaling laws initially derived from nanoscale simulations resurface in macroscale experiments under the help of 3-D printing,” he says.

    This work, Gao says, “shows a promising direction of bringing the strength of 2-D materials and the power of material architecture design together.”

    The research was supported by the Office of Naval Research, the Department of Defense Multidisciplinary University Research Initiative, and BASF-North American Center for Research on Advanced Materials.

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  • richardmitnick 9:59 am on June 16, 2016 Permalink | Reply
    Tags: Compatible with current microchip technology, Converting electricity into visible radiation, Graphene, , Researchers discover new way to turn electricity into light, using graphene, Čerenkov effect   

    From MIT: “Researchers discover new way to turn electricity into light, using graphene” 

    MIT News

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    June 13, 2016
    David L. Chandler | MIT News Office

    1
    This illustration depicts the process of light emission from a sheet of graphene, which is represented as the blue lattice on the top surface of a carrier material. The light-colored arrow moving upwards at the center depicts a fast-moving electron. Because the electron is moving faster than light itself, it generates a shock wave, which spews out plasmons, shown as red squiggly lines, in two directions. Courtesy of the researchers

    By slowing down light to a speed slower than flowing electrons, researchers create a kind of optical “sonic boom.”

    When an airplane begins to move faster than the speed of sound, it creates a shockwave that produces a well-known “boom” of sound. Now, researchers at MIT and elsewhere have discovered a similar process in a sheet of graphene, in which a flow of electric current can, under certain circumstances, exceed the speed of slowed-down light and produce a kind of optical “boom”: an intense, focused beam of light.

    This entirely new way of converting electricity into visible radiation is highly controllable, fast, and efficient, the researchers say, and could lead to a wide variety of new applications. The work is reported today in the journal Nature Communications, in a paper by two MIT professors — Marin Soljačić, professor of physics; and John Joannopoulos, the Francis Wright Davis Professor of physics — as well as postdoc Ido Kaminer, and six others in Israel, Croatia, and Singapore.

    The new finding started from an intriguing observation. The researchers found that when light strikes a sheet of graphene, which is a two-dimensional form of the element carbon, it can slow down by a factor of a few hundred. That dramatic slowdown, they noticed, presented an interesting coincidence. The reduced speed of photons (particles of light) moving through the sheet of graphene happened to be very close to the speed of electrons as they moved through the same material.

    “Graphene has this ability to trap light, in modes we call surface plasmons,” explains Kaminer, who is the paper’s lead author. Plasmons are a kind of virtual particle that represents the oscillations of electrons on the surface. The speed of these plasmons through the graphene is “a few hundred times slower than light in free space,” he says.

    This effect dovetailed with another of graphene’s exceptional characteristics: Electrons pass through it at very high speeds, up to a million meters per second, or about 1/300 the speed of light in a vacuum. That meant that the two speeds were similar enough that significant interactions might occur between the two kinds of particles, if the material could be tuned to get the velocities to match.

    That combination of properties — slowing down light and allowing electrons to move very fast — is “one of the unusual properties of graphene,” says Soljačić. That suggested the possibility of using graphene to produce the opposite effect: to produce light instead of trapping it. “Our theoretical work shows that this can lead to a new way of generating light,” he says.

    Specifically, he explains, “This conversion is made possible because the electronic speed can approach the light speed in graphene, breaking the ‘light barrier.’” Just as breaking the sound barrier generates a shockwave of sound, he says, “In the case of graphene, this leads to the emission of a shockwave of light, trapped in two dimensions.”

    The phenomenon the team has harnessed is called the Čerenkov effect, first described 80 years ago by Soviet physicist Pavel Čerenkov. Usually associated with astronomical phenomenon and harnessed as a way of detecting ultrafast cosmic particles as they hurtle through the universe, and also to detect particles resulting from high-energy collisions in particle accelerators, the effect had not been considered relevant to Earthbound technology because it only works when objects are moving close to the speed of light. But the slowing of light inside a graphene sheet provided the opportunity to harness this effect in a practical form, the researchers say.

    There are many different ways of converting electricity into light — from the heated tungsten filaments that Thomas Edison perfected more than a century ago, to fluorescent tubes, to the light-emitting diodes (LEDs) that power many display screens and are gaining favor for household lighting. But this new plasmon-based approach might eventually be part of more efficient, more compact, faster, and more tunable alternatives for certain applications, the researchers say.

    Perhaps most significantly, this is a way of efficiently and controllably generating plasmons on a scale that is compatible with current microchip technology. Such graphene-based systems could potentially be key on-chip components for the creation of new, light-based circuits, which are considered a major new direction in the evolution of computing technology toward ever-smaller and more efficient devices.

    “If you want to do all sorts of signal processing problems on a chip, you want to have a very fast signal, and also to be able to work on very small scales,” Kaminer says. Computer chips have already reduced the scale of electronics to the points that the technology is bumping into some fundamental physical limits, so “you need to go into a different regime of electromagnetism,” he says. Using light instead of flowing electrons as the basis for moving and storing data has the potential to push the operating speeds “six orders of magnitude higher than what is used in electronics,” Kaminer says — in other words, in principle up to a million times faster.

    One problem faced by researchers trying to develop optically based chips, he says, is that while electricity can be easily confined within wires, light tends to spread out. Inside a layer of graphene, however, under the right conditions, the beams are very well confined.

    “There’s a lot of excitement about graphene,” says Soljačić, “because it could be easily integrated with other electronics” enabling its potential use as an on-chip light source. So far, the work is theoretical, he says, so the next step will be to create working versions of the system to prove the concept. “I have confidence that it should be doable within one to two years,” he says. The next step would then be to optimize the system for the greatest efficiency.

    This finding “is a truly innovative concept that has the potential to be the key toward solving the long-standing problem of achieving highly efficient and ultrafast electrical-to-optical signal conversion at the nanoscale,” says Jorge Bravo-Abad, an assistant professor at the Autonomous University of Madrid, in Spain, who was not involved in this work.

    In addition, Bravo-Abad says, “the novel instance of Čerenkov emission discovered by the authors of this work opens up whole new prospects for the study of the Čerenkov effect in nanoscale systems, without the need of sophisticated experimental set-ups. I look forward to seeing the significant impact and implications that these findings will surely have at the interface between physics and nanotechnology.”

    The research was supported by the U.S. Army Research Laboratory and the U.S. Army Research Office, through the Institute for Soldier Nanotechnologies at MIT. The team included researchers Yichen Shen, Ognjen Ilic, and Josue Lopez at MIT; Yaniv Katan at Technion, in Haifa, Israel; Hrvoje Buljan at the University of Zagreb in Croatia; and Liang Jie Wong at the Singapore Institute of Manufacturing Technology.

    Science paper:
    Efficient plasmonic emission by the quantum Čerenkov effect from hot carriers in graphene

    See the full article here .

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