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Graphene Finds Its Path in Supercapacitor Commercialization

Researchers at the California NanoSystems Institute (CNSI) at UCLA have been hotly pursuing the ability to apply graphene to the electrodes of supercapacitors. While their efforts have shown progress—improving energy density for a supercapacitor to almost 40 Watt-hours per kilogram from the industry average for a standard supercapacitor of 28 Wh/kg—it apparently hasn’t provided a big enough boost for supercapacitor manufacturers to walk away from the much cheaper activated carbon.

This has not deterred the team at CNSI from continuing to work with graphene and supercapacitors. In fact, they have recently employed the ubiquitous manganese dioxide used in alkaline batteries to create a hybrid material that they believe should boost the commercial prospects for 2-D materials in supercapacitors.

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Two men in a laboratory gaze at an illuminated lightbulb mounted in a fixture they are holding in their hands

Will Graphene-based Light Bulbs Be Graphene's Commercial Break Out?

The big story this week in graphene, after taking into account the discovery of “grapene,” has to be the furor that has surrounded news that a graphene-coated light bulb was to be the “first commercially viable consumer product” using graphene.

Since the product is not expected to be on store shelves until next year, “commercially viable” is both a good hedge and somewhat short on meaning. The list of companies with a commercially viable graphene-based product is substantial, graphene-based conductive inks and graphene-based lithium-ion anodes come immediately to mind. Even that list neglects products that are already commercially available, never mind “viable”, like Head’s graphene-based tennis racquets.

So, okay, the BBC got caught up in some PR language promoting what appears to be a UK-developed technology that’s been financed by a Canadian company. That’s nothing outside standard operating procedure. But there were still some issues in the piece that had me scratching my head.

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New 2-D Material Grapene Has Spectacular Properties

Today, 1 April, researchers at the Napa Valley Research Institute announced the discovery of a new two-dimensional material—grapene—that could one-day rival silicon in computers, steel in cars, and chocolate in candybars. (Yes. We know. That’s what they all say.) The new substance exists in flat sheets, connected by strong bonds composed of cellulose and lignin. In bulk form, its natural state, it’s found hanging in chaotically-arranged bunches.

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Molybdenum Disulfide Sees the Light

Roughly two years ago, researchers at MIT started to look at the potential of molybdenum disulfide (MoS2) for photovoltaic applications. The results were somewhat mixed. They saw relatively low conversion efficiency numbers, but were encouraged by the discovery that placing just three sheets of MoS2 into a one-nanometer-thick stack makes it possible to absorb up to 10 percent of incident sunlight. That’s an order of magnitude higher than gallium arsenide and silicon.

While that was indeed an encouraging development, it was far from enough to make anyone start clamoring for MoS2 to replace silicon in photovoltaics or other photonics uses.

Now researchers at Northwestern University have employed plasmonics in combination with MoS2, boosting its ability to absorb light as well as its photolumiscence.

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Silicon Telluride Could Be the Next Big Thing in 2-D Materials

Silicon telluride is a somewhat forgotten silicon-based semiconductor material. After it was identified back in the 1960s, a paper that outlined its general properties was published over 40 years ago.  Not a lot of research has focused on the material since then, despite that fact that you can purchase just about any amount you wish. It is based on silicon after all.

Researchers at Brown University have pulled silicon telluride out of obscurity and have placed it solidly into the growing universe of two-dimensional (2-D) materials.  The result may be that it yields advances including improved electrodes in batteries and better LEDs.

“My guess as to why there is very little literature on 2-D silicon telluride is that it was simply forgotten among the class of 2-D materials,” said Kristie Koski, assistant professor of chemistry at Brown, who led the work, in an e-mail interview with IEEE Spectrum.

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"Robogerms" Spawned by Combo of Graphene and Bacterial Spores

Researchers at the University of Illinois Chicago (UIC) have combined graphene quantum dots (GQCs) with a single bacterial spore to create bio-electromechanical devices. This so-called “robotic germ” functions an electromechanical humidity sensor.

Recently, James Tours’ group at Rice University, who were the first to develop GQCs in 2013, created an improved way to manufacture them that promised to open them up to a new range of applications in optics.

However, they may not have considered the possibility of joining them to a living organism for the kind of machine the UIC researchers created. The Chicago-based team have dubbed their hybrid device NERD, standing for Nano-Electro-Robotic Device.

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Graphene Supercharges 3-D Hybrid Supercapacitor

By combining sheets of graphene with a traditional battery material, scientists have created hybrid supercapacitors that can store as much charge as lead acid batteries but can be recharged in seconds compared with hours for conventional batteries.

Supercapacitors now play an important role in hybrid and electric vehicles, consumer electronics, and military and space applications. However, they are often limited in terms of how much energy they can store.

Now researchers at the University of California, Los Angeles, have developed a hybrid supercapacitor that is based on graphene, which is made of single layers of carbon atoms. Graphene is flexible, transparent, strong and electrically and thermally conductive, qualities that have led to research worldwide into whether the material could find use in advanced circuitry and other devices.

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For First Time, Researchers Demonstrate Heat and Sound Are Magnetic

Earlier this month, we reported on research demonstrating that heat propagates as a wave through graphene rather than as vibrations of atoms the way it does in 3-D materials.  In 3-D materials, the collective state of those vibrating atoms is known as phonons.

For the first time, researchers at Ohio State University (OSU) have demonstrated that acoustic phonons, which can carry both heat and sound, have magnetic properties that allow them to be manipulated with magnetism. 

In research published in the journal Nature Materials, the OSU researchers applied a magnetic field equivalent to that inside a magnetic resonance imaging (MRI) device (in this case, the magnet was reported to be fairly powerful at seven Tesla). They discovered that they could reduce the amount of heat flowing through a semiconductor by 12 percent.

“This adds a new dimension to our understanding of acoustic waves,” said Joseph Heremans, professor of mechanical engineering at Ohio State, in a press release. “We’ve shown that we can steer heat magnetically. With a strong enough magnetic field, we should be able to steer sound waves, too.”

Before anyone starts thinking about the discovery’s applicability to heat management in computers, they should keep in mind that the semiconductor had to be kept at temperatures very close to absolute zero (specifically, -268 degrees Celsius) in order for the researchers to measure the movements of the phonons.

In fact, it was the complexity of taking the measurements that had prevented researchers from recognizing the magnetic properties of phonons previously. In order to take thermal measurements at such a low temperature, Hyungyu Jin, a postdoctoral researcher and lead author of the study, used the semiconductor indium antimonide and shaped it into a lopsided tuning fork in which one arm was 4 millimeters wide and the other was 1 mm wide. Then he placed a heater at the base of each arm.

At normal temperatures, the ability of the material to transfer heat would be solely dependent on the kind of atoms in the material. But near absolute zero, the ability of the material to transfer heat can be determined by the physical size of the material. In this case, the difference in the sizes of the fork arms was significant. Phonons more easily filled the wider arm.

“Imagine that the tuning fork is a track, and the phonons flowing up from the base are runners on the track,” explained Heremans in the press release. “The runners who take the narrow side of the fork barely have enough room to squeeze through, and they keep bumping into the walls of the track, which slows them down. The runners who take the wider track can run faster, because they have lots of room.”

Eventually they all end up at their respective finish lines. But the track’s geometry determines just how quickly. 

With this understanding, Jin was able to compare the temperature changes in the two fork arms. He first took the measurements without a magnet and then with one. With the magnet on, the heat flow through the larger arm slowed down by 12 percent.

Now that the researchers have measured magnetism’s effect on heat, they want to move on to see if they can use it to deflect sound waves.

New Production Twist for Graphene Quantum Dots Opens Up Applications

Ever since the end of 2013, when James Tour and his colleagues at Rice University created graphene quantum dots (GQDs) from coal, they have been busily looking for applications for the material.

Whenever a nanomaterial needs an application, offering it up as a replacement for platinum catalysts in fuel cells is a sure fire way to generate some attention. Late last year, Tour’s group proved to be no exception to this rule when they did exactly that.

Not to say that finding a cheaper alternative to platinum in fuel cells isn’t necessary, but isolating hydrogen and then creating a distribution infrastructure for it is a far more critical factor in realizing the so-called “hydrogen economy.”

So, Tour and his team have taken a new tack with their GQDs and developed a simple manufacturing technique that can sort out the GQDs according to their size—and therefore, their semiconducting properties.

In research published in the journal Applied Materials & Interfaces, the Rice team focused renewed attention on their method for producing the GQDs from coal. They discovered that if they carefully controlled the reaction temperature in the oxidation process that turned the coal into quantum dots—the nanoscale semiconducting crystals with properties that make them attractive for several optoelectronic applications—they could control the size of the dots they produced. Hotter temperatures produced smaller dots; different size dots have different semiconducting properties. The researchers then used an ultrafiltration system, which is often used in industrial water filtration systems, to start the sorting of the quantum dots according to their size.

Quantum dots can absorb and emit photons at specific wavelengths, from visible colors into infrared. The size of the dot can determine which wavelength, or color, is absorbed. Smaller dots emit green light, while the larger dots emit light in the orange to red range. Tour and his team determined that the tiniest quantum dots, which emit blue light, were the easiest to produce from coal.

The ability to do this sorting is critical for the production of optoelectronic devices based around their fluorescence. It is for this reason that the Rice researchers are now discussing metal or chemical detectors that let engineers tune the fluorescence of the quantum dots so the devices can avoid interference with the target materials’ emissions. Of course, the Rice team still mentions catalytic reactions, but this latest production development seems to place the graphene quantum dots on a broader, if not more adoptable, application path.

Magnetized Graphene Could Lead to a Million-Fold Increase in Data Storage Capacity

Lately there has been a trend in graphene research to imbue the material with both magnetic and electric properties. But just two years ago, the research world was pretty impressed that a team in Spain was able to make graphene magnetic alone.

Now researchers at the U.S. Naval Research Laboratory (NRL) have gone back to imparting just magnetic properties into graphene, and in so doing may have developed a method that could lead to graphene becoming a new data storage medium capable of a million-fold increase in capacity over today’s hard drives.

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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
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