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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.”

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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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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