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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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Another Report Laments the Status of Carbon Nanotube Development

In the last half-a-decade we have witnessed once-beloved carbon nanotubes (CNTs) slowly being eclipsed by graphene as the “wonder material” of the nanomaterial universe.

This changing of the guard has occurred primarily within the research community, where the amount of papers being published about graphene seems to be steadily increasing. But in terms of commercial development, CNTs still have a leg up on graphene, finding increasing use in creating light but strong composites. Nonetheless, the commercial prospects for CNTs have been taking hits recently, with some producers scaling down capacity because of lack of demand.

With this as the backdrop, the National Nanotechnology Initiative (NNI), famous for its estimate back in 2001 that the market for nanotechnology will be worth $1 trillion by 2015,  has released a report based on a meeting held last September. The report, called “Realizing the Promise of Carbon Nanotubes: Challenges, Opportunities, and the Pathway to Commercialization,” offers recommendations on the commercialization path for CNTs.  

None of the recommendations should come as a surprise to those who have followed the commercial travails of CNTs over the years. While one of the recommendations of the report seemingly incongruously urges the scaling up of CNT production, it would appear the report is recommending a particular kind of increase. The aim of the recommendation is to support a scaled-up manufacturing that would impart the same kind of functionality seen in individual CNTs for CNT-based bulk materials.

Also, for those who measure all nanomaterial research by the degree to which it addresses environmental concerns, the report ticks that box by highlighting the need for life-cycle assessments as products based on CNTs reach commercialization.

Over the years, there has been a regular stream of research that has improved upon CNT production, whether it’s for electronics applications or for advanced composites.

Despite these advances, it doesn’t seem that anyone has been able to translate them into real-world products. That’s why the report contains what has come to be a fixture in any review on the status of nanomaterials: a lamentation of the innovation ecosystem.

It makes perfect sense that the report offers this recommendation: “Use of public-private partnerships or other collaboration vehicles to leverage resources and expertise to solve these technical challenges and accelerate commercialization.”

While urging the creation of a more effective innovation infrastructure is incumbent upon any report dealing with nanotechnology,  it might be time for one of these groups to not only identify the need for it but also to outline what that infrastructure would look like and actually begin buidling it. Until then, we’re likely to see more reports such as these, which tell those who are likely to be paying attention all the things they already know.

Graphene Makes Copper Nanowires Useful for Flexible Displays

What happens when you coat copper nanowires with graphene? According to research out of Purdue University, you lower the resistance and susceptibility to heating of the copper wires. This could allow copper nanowires to be used in a wider range of electronics.

“Highly conductive copper nanowires are essential for efficient data transfer and heat conduction in many applications like high-performance semiconductor chips and transparent displays,” said doctoral student Ruchit Mehta in a press release. She has been working as part of a team led by Zhihong Chen, an associate professor of electrical and computer engineering at Purdue University.

In research published in the journal Nano Letters,  the Purdue team developed a method for encapsulating the wires with graphene. Compared to uncoated wires, the encapsulated wires can transmit data 15 percent faster while reducing the peak temperature by 27 percent.

“This is compelling evidence for improved speed and thermal management by adapting the copper-graphene hybrid technology in future silicon chips and flexible electronic applications,” Mehta added in the release.

As we learned earlier this week, graphene is effective at dissipating heat in part because heat propagates as a wave in it, which is quite different from the all-direction-vibration of atoms as seen in three-dimensional materials.

By combining graphene with nanowires, it could become possible to address some of heat issues that arise from the high packing density of today’s electronic components in chips.

As the wires on these chips become smaller to accommodate the packing density, both their electrical and thermal conductivity are compromised due to oxidation. With the graphene coating the copper wires are resistant to oxidation, maintaining low resistance and dissipating heat more effectively. In particular, the researchers believe that the hybrid wires could make the copper nanowires applicable to transparent flexible displays, where previously they were a poor fit because of oxidation problems.

“If the surface is covered with oxide then a lot of the electrical and thermal conductive properties of copper are lost,” Mehta said. “This is very important because you want as much current as possible going through these wires to increase speed, but they cannot take too much current because they will melt. But if the copper has good electrical and thermal conductivity you can push more current through it.”

Of course, others have looked at the potential of coating nanowires with graphene, but the process proved too daunting because it required chemical vapor deposition (CVD), which operates at 1000 degrees Celsius, which can ruin both copper thin films and small-dimension wires.

The breakthrough achieved by the Purdue team was to use a plasma-enhanced CVD that can be run at 650 degrees Celsius, keeping the small wires intact.

Fear of Nanoparticles Takes the White Out of Dunkin' Donuts

An advocacy group called As You Sow has managed to get Dunkin' Donuts to remove titanium dioxide (TiO2) from its powdered sugar formula. The move stems from fears that the TiO2 was in a nanoparticle form that had not yet been determined to be safe.

TiO2 is fairly ubiquitous in foodstuffs and is used anywhere that whiteness and opacity are desired characteristics, such as powdered sugar.  Based on this industry-wide use, it was a safe bet that some kind of TiO2 was present in Dunkin’ Donuts powdered sugar donuts.

To ensure there was no doubt, As You Sow reportedly hired an independent consultant to test the donuts for the presence of TiO2 and its use was confirmed in the tests.

The suspicion that TiO2 was in fact in nanoparticle form likely stemmed from research back in 2012 in which Paul Westerhoff and his colleagues at the University of Arizona tested random foodstuffs and personal care products and found that 5 percent of the TiO2 particles contained in the food products were less than 100 nanometers (nm) in at least one dimension.

According to Andrew Maynard, Chair of UM Department of Environmental Health Sciences and Director, University of Michigan Risk Science Center, this size of the nanoparticle is likely incidental to the manufacturing process since the ideal size for these particles from the manufacturers’ perspective is around 200 to 300 nm—the size at which the particles can most effectively do their job of reflecting and blocking light.

Whether the size of the TiO2 particles are in fact nanoscale could be seen as quibbling, but the real question is whether TiO2 nanoparticles are in fact toxic.

As Maynard explains, it is complicated. On the one hand, TiO2 particles that are ingested orally and work their way through our digestive system have thus far shown few signs of toxicity.  However, if TiO2 particles are inhaled, they can lead to pulmonary toxicity.  When the science is applied to donuts, one would hope that people are eating them and not inhaling them.

The move by As You Sow and other advocacy groups to eliminate an ingredient that has been in our food supply for generations is supported under the so-called “precautionary principle” in which producers have to take on the burden of proof regarding the level of risk of their products.

With decades of TiO2 being in our food supply and no reports of toxic reactions, it would seem that the threshold for proof is extremely high, especially when you combine the term “nano” with “asbestos”.

As You Sow makes sure to point out that asbestos is a nanoparticle. While the average diameter of an asbestos fiber is around 20 to 90 nm, their lengths varied between 200 nm and 200 micrometers.

The toxic aspect of asbestos was not its diameter, but its length. The pathogenic quality of asbestos occurs when the body's phagocytes attempt to engulf the fiber, and when unable to get around the entire length of the fiber, the phagocytes try to kill the fiber with toxic products. The attempt fails to kill the fiber but succeeds in damaging the surrounding tissue leading to mesothelioma.

It is, in fact, the longer asbestos fibers—not those nanoscale in length—that lead to the lung disease, mesothelioma. It is a bit of scare tactic to be sure to reference asbestos, but a clearly effective one.

What may turn out to be the most important part of the story is how As You Sow managed to twist Dunkin’ Donuts’ arm to get them to make the move: it was an inside job.

As You Sow’s strategy has been to introduce shareholder proposals at various companies calling for the elimination of the TiO2. At a Dunkin’ Brands’ shareholders meeting last year 19 percent of the shareholders supported a resolution to eliminate TiO2.

When asked about this method, Maynard said: “People will always find avenues to get their agenda on the table. Whatever the method, what is important is that the science is always respected. Sometimes in these efforts, it’s easy to lose sight of what the science indicates on balance.”

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.”

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Nanoclast

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

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