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Heat Propagates as a Wave in Graphene

Thermal management is one of the biggest issues facing electronics today. While cooling fans and other system-level solutions have been the mainstay of schemes aimed at controlling heat, higher circuit densities and faster clock speeds are making chips run so hot that new solutions are needed.

Graphene has been held out as a hope for addressing these growing thermal management problems because of its very high thermal conductivity. Researchers at EPFL (École Polytechnique Fédérale de Lausanne) in Switzerland have taken a big step toward realizing this promise by demonstrating how heat actually dissipates in graphene.

In research published in the journal Nature Communications, the EPFL researchers have shown that, in graphene, heat propagates in the form of a wave, just like sound in air.

“We can show that the thermal transport is described by waves, not only in graphene but also in other materials that have not been studied yet,” explained Andrea Cepellotti, the first author of the report, in a press release. “This is extremely valuable information for engineers, who could adapt the design of future electronic components using some of these novel two-dimensional materials’ properties.”

By shedding new light on the mechanisms of thermal conductivity in graphene and other two-dimensional materials through computer modeling, the researchers believe they can help other researchers working on using graphene for thermal management solutions.

Two-dimensional (2-D) materials, like graphene, behave quite differently than their three-dimensional (3-D) cousins when it comes to the propagation of heat.

In 3-D materials, heat propagates through the vibration of atoms. These vibrations, called "phonons," keep colliding with each other, merging together, or splitting, all of which limits the heat conductivity of the material. Only when temperatures approach absolute zero (-200°C or lower) is it possible to observe quasi-lossless heat transfer.

In 2-D materials, the researchers have shown, heat propagates quite differently. Even at room temperature, heat is transmitted without significant losses due to the phenomenon of wave-like diffusion called “second sound.” With second sound, all phonons march together in unison over very long distances.

Cepellotti adds: “Our simulations, based on first-principles physics, have shown that atomically thin sheets of materials behave, even at room temperature, in the same way as three-dimensional materials at extremely low temperatures.”

Electrons Are Snake Charmed Across Graphene

While graphene continues to gain new two-dimensional (2-D) competitors, there’s no getting around its amazing ability to let electrons pass through it with so little resistance that electrons almost behave like photons.

Physicists at the University of Basel in Switzerland have been so focused on this capability that years of experimentation with the one-atom-thick sheets of carbon have led them to discover that it’s possible to direct the electrons in graphene across a predefined path.

In research published in the journal Nature Communications, the scientists discovered that when they stretched, or otherwise manipulated, the honeycomb structure of the graphene and applied both an electrical and magnetic field to it, they could direct the flow of electrons. This marks the first time that anyone has successfully switched the guidance of electrons on and off and guided them without any loss.

The researchers stretched the graphene between two silver electrical contacts and two gold control electrodes that provide the electric field. They then applied a magnetic field perpendicular to the graphene.

The mechanism by which the researchers were able to perform this on-off switching phenomenon can only be achieved in graphene, so other 2-D materials need not apply. It is, in fact, graphene’s lack of a band gap—which  has so vexed researchers trying to apply the material to electronics—that is the quality necessary for this type of switching.

By combining the electrical field and magnetic field in this way, the researchers have exploited this capability so that they can induce the electrons to move along a snake pattern: the line bends to the right, then to the left.

“A nano-switch of this type in graphene can be incorporated into a wide variety of devices and operated simply by altering the magnetic field or the electrical field,” said Christian Schönenberger, one of the researchers, in a press release.

As Walt de Heer at the Georgia Institute of Technology suggested last year when it was shown that that electrons behave like photons in graphene nanoribbons (link provided above), this could open a new way to approach the development of electronics.

“This work shows that we can control graphene electrons in very different ways because the properties are really exceptional,” de Heer said at the time about his own research. “This could result in a new class of coherent electronic devices based on room temperature ballistic transport in graphene. Such devices would be very different from what we make today in silicon.”

Black Phosphorous Demonstrates Potential in High-Speed Data Communication

The march of two-dimensional (2-D) materials continues with the 2-D version of phosphorus—known as black phosphorusbeginning to get noticed.

Like other 2-D materials, black phosporous has an inherent band gap, a major advantage over graphene.  As a result, a fair amount research on black phosphorus has been applied to the development of electronic devices, such as field-effect transistors.

But now researchers at the University of Minnesota are exploiting black phosphorus’ optoelectronic capabilities to demonstrate its potential in in high-speed data communication that employs nanoscale optical circuits.

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Magnetic Nanoparticles Boost Polymer Solar Cells

Just about every manner of nanoparticle and nanomaterial has been applied to polymer solar cells.  Despite all of this work, conversion efficiencies for single p-n junction polymer solar cells are mired at around 9 percent, while cells with more than one p-n junction have mustered efficiencies only as high as 10.6 percent.

All those frustrated efforts made it reasonable to wonder whether nanoparticles would ever provide much of a boost to polymer solar cells.

Now, an X-ray study performed at the Deutsches Elektronen-Synchrotron (DESY) by a team from the Technical University of Munich (TUM) using DESY’s synchrotron radiation source, PETRA III, has demonstrated that magnetic nanoparticles can improve the performance of polymer solar cells—if the mix is right.

In research published in the journal Advanced Energy Materials, the German-based researchers demonstrated that by making sure the solar cell material contains just about one percent of magnetic nanoparticles by weight, they were able to boost the solar cell’s efficiency.

“The X-ray investigation shows that if you mix a large number of nanoparticles into the material used to make the solar cell, you change its structure”, explains coauthor Stephan Roth, who runs DESY’s microfocus small- and wide-angle x-ray scattering beamline at PETRA III, in a press release. “The solar cells we looked at will tolerate magnetic nanoparticle doping levels of up to one percent by mass without changing their structure.”

How to exploit the nanoparticles is where the Germany-based researchers departed from recent research. Solar cell material doped with gold nanoparticles had already been demonstrated to absorb additional sunlight—which, in turn, produced additional electrical charge carriers when the energy was released again by the gold particles.

“The light creates pairs of charge carriers in the solar cell, consisting of a negatively charged electron and a positively charged hole, which is a site where an electron is missing,” explained the main author of the current study, Daniel Moseguí González, in a press release. “The art of making an organic solar cell is to separate this electron-hole pair before they can recombine. If they did, the charge produced would be lost. We were looking for ways of extending the life of the electron-hole pair, which would allow us to separate more of them and direct them to opposite electrodes.”

To extend the life of the electron-hole pair, the researchers exploited the spin of the electrons. The positively charged hole also has a spin. If the two spins are in the same direction, they can add up to a value of one, or cancel each other out, for a value of zero, if they are oriented in opposite directions. Pairs that have an overall spin value of one last longer than those that have an overall spin of zero.

The key was finding a material capable of converting an electron-hole pair’s overall spin state from zero to one. To accomplish this, the researchers needed nanoparticles made from heavy elements, because they can flip the spin of the electron or the hole so that spins are aligned in the same direction.

The material they hit upon was iron oxide magnetite. By adding just the right amount of the magnetite (doping the substrate with 0.6 percent nanoparticles by weight) they were able to increase the energy conversion efficiency by 11 percent, from 3.05 to 3.37 percent.

“The combination of high-performance polymers with nanoparticles holds the promise of further increases in the efficiency of organic solar cells in the future,” said Peter Müller-Buschbaum of TUM in the release. “However, without a detailed examination, such as that using the X-rays emitted by a synchrotron, it would be impossible to gain a fundamental understanding of the underlying processes involved.”

Optical Nanosensor Production Only Needs CDs, Tape, and Aluminum

Researchers at the Universidad Politécnica de Madrid (UPM) in Spain have developed a way to produce optical nanosensors that can stick to uneven surfaces as well as biological surfaces such as human skin.

The researchers believe that this technique will expand the use of wearable devices for monitoring body temperature, respiration, blood pressure, and other vital signs.

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Magnetic Nanoparticles Promise to Prevent Strokes and Heart Attacks

Magnetic nanoparticles have served as the foundation for a number of medical technologies, including drug delivery, medical imaging contrast agents and cancer diagnosis and treatment

Now researchers at Houston Methodist are loading up magnetic nanoparticles with drugs and camouflaging them from the immune systems so that they can  destroy blood clots at a rate about 100 to 1000 times faster than a commonly used clot-busting technique.

The researchers believe that if the technique proves successful in human trials that it could help prevent strokes, heart attacks and pulmonary embolisms.

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Silicon Nanofibers Boost Li-ion Batteries for EVs

Last summer, Mihri and Cengiz Ozkan, both professors at the University of California Riverside, put a small twist on all the attempts to use nanostructured silicon on the anodes of lithium-ion (Li-ion) batteries. They dispersed silicon particles onto nanostructures rather than making nanostructures on silicon.

Now the Ozkans are at it again. This time they and their colleagues at UC Riverside have created a paper-like nanofiber material that can be applied to the anodes of Li-ion batteries, boosting by several times a battery’s specific energy—the amount of energy that can be delivered per unit weight.

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Crumpling Graphene Could Expand Its Applications

Last October, researchers at MIT showed that graphene could be crumpled and then flattened again and still remain effective for use in the electrodes of supercapacitors that could be used to power flexible electronics.

Now a team at the University of Illinois at Urbana-Champaign are showing that if you keep the graphene crumpled, you increase its surface area. This 3D surfaced graphene could open new application areas for the material in electronics and biomaterials. 

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Nanowire Brushes Usher in New Generation of Smoke Detectors

Zinc oxide's' ability to absorb and emit ultraviolet light has long been the operational foundation of photoelectric smoke detectors.

While this technology has proved effective in detecting larger smoke particles found in dense smoke, it’s not quite as sensitive in detecting the small smoke particles produced by fast burning fires.

Now researchers at the University of Surrey’s Advanced Technology Institute have dramatically increased the effective surface area of zinc oxide by fashioning the material into what amounts to nanowire "brushes," making the smoke detectors they’re used in 10,000 times more sensitive to UV light than a traditional zinc-oxide detector.

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DNA Data Storage Just Got a Bit More Practical

For two years now, researchers have been storing digital information in the form of DNA, but there has remained some question as to whether it’s a practical solution for digital storage.

Now researchers at the Swiss Federal Institute of Technology in Zurich (ETHZ) have addressed a number of the problems associated with using DNA as data storage—enough so that they believe it can be used for error-free storage of information.  If their solution proves successful, it could open the door for data storage that lasts for a million years.

Researchers around the world have been investigating a variety of new methods for storing digital information because we’ll be lucky if the solutions we have now, like hard drives and servers, can keep faithful records for fifty years. DNA has been among those potential alternatives. But errors during data retrieval have been the method’s bugaboo. Gaps and false information in the encoded data result from chemical degradation and mistakes in DNA sequencing.

In research published in the journal Angewandte Chemie, the Swiss team was able to overcome the problem of chemical degradation of the DNA by encapsulating the genetic material in silica (glass) spheres with diameters of around 150 nanometers.

In order to test the quality of their encapsulation, the researchers simulated a long period of time by storing the information-encoded DNA at temperatures between 60 and 70 degrees Celsius for up to a month. This simulated, within a few weeks, the degradation that would occur over hundreds of years under normal conditions.

After finding that the silica capsules outperformed several other materieals they tested, the researchers then moved on to ensuring that the DNA remained error free.

While advances in DNA sequencing make it possible to read stored data on DNA affordably, affordability and exactitude don’t always go hand in hand. To overcome this problem, the researchers have developed a way to correct any errors based on the Reed-Solomon Codes, error-correcting codes normally used to ensure accurate data recovery after long-distance data transmission.

The scheme adds just a bit more data to ensure that what’s encoded is error free. “In order to define a parabola, you basically need only three points,” said Reinhard Heckel from ETH Zurich’s Communication Technology Laboratory in a press release. “We added a further two in case one gets lost or is shifted.”



IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

Dexter Johnson
Madrid, Spain
Rachel Courtland
Associate Editor, IEEE Spectrum
New York, NY
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