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Much Ado About Carbon Nanotubes...Or Not

Researchers at the University of Paris-Saclay in France have discovered that fluid samples taken from the airways of 64 asthmatic children contained carbon nanotubes (CNTs). In addition, the France-based researchers determined that five other children studied also had CNTs in macrophages found in their lungs.

While this will no doubt add fuel to the fury of NGOs bent on shutting down research into nanotechnology immediately, there is little in this research that breaks new ground—at least qualitatively.

“From past studies, the conditions in combustion engines seem to favor the production of at least some CNTs (especially where there are trace metals in lubricants that can act as catalysts for CNT growth),” explained Andrew Maynard Director, Risk Innovation Lab and Professor, School for the Future of Innovation in Society at Arizona State University, in an e-mail interview. Says Maynard:

What, to my knowledge, is still not known, is the relative concentrations of CNT in ambient air that may be inhaled, the precise nature of these CNT in terms of physical and chemical structure, and the range of sources that may lead to ambient CNT. This is important, as the potential for fibrous particles to cause lung damage depends on characteristics such as their length—and many of the fibers shown in the paper appear too short to raise substantial concerns.

It’s not even clear from the research whether the nanoparticles in question are in fact carbon nanotubes. At this point, they are best described as carbon nanotube-like fibers.

Nonetheless, Maynard praises the research for establishing that these carbon nanotube-like fibers are part of the urban aerosol and therefore end up in the lungs of anyone who breathes it in. However, he cautions that the findings don’t provide information on the potential health risks associated with these exposures.

“Because of this,” Maynard told IEEE Spectrum, “it would be highly premature to draw any conclusions on health risk from the study.” He added that, “It would be appropriate to conduct further study into whether there is an association between these unusual carbon-based fibers and ill health.”

At least some of the coverage of the research has made the misleading point that “nanotubes have shown great potential in areas such as computing, clothing and healthcare technology” with the obvious implication being that the CNTs used in these applications are the ones found in the lungs of children.

While the research doesn’t draw a distinction between manufactured CNTs and the natural and incidental varieties produced by, say, car exhausts, there is little to suggest they are anything other than particles that have been around with us since the introduction of the internal combustion engine. Meanwhile, there has been little evidence showing that a manufactured CNT, once embedded in the matrix of a material, can ever be separated from that matrix so that it’s free to float around in the air.

“Some studies have indicated that occasionally single nanotubes might be released from abraded materials,” Maynard admits. “But it looks like the release rates are extremely low.  This is what would be expected given how tightly carbon nanotubes bind to polymers used in composite materials, and the amount of energy that would be required to release them.”

Maynard points that it may be possible in principle to create fingerprints for different types of CNTs based on their source, allowing scientists to determine definitively whether a sample of CNTs is from car exhaust, or tennis racquets and bicycles. He adds:

For instance, CNTs that are lab generated will often be associated with trace amounts of catalyst materials such as nickel or iron.  However, I suspect that such fingerprinting will require a level of characterization rarely used on such materials.

Such characterization may also be a moot point from a health perspective. Maynard notes that while ambient carbon nanotubes may be analytically difficult to distinguish from engineered carbon nanotubes, it’s reasonable to assume that our lungs will also find it hard to make the distinction.

Memristor Capable of Three Stable Resistive States Could Challenge Flash Memory

The imminent demise of flash memory at the hands of some new technological upstart has been predicted at least for the last decade.  The latest pretender to the throne is the so-called memristor (also called resistive RAM, ReRAM, or RRAM).  Of course, if you don’t like the term “memristor”, you can alternatively refer to it as “two-terminal non-volatile memory devices based on resistance switching.”

Now researchers at ETH Zurich have designed a memristor device out of perovskite just 5 nanometres thick that has three stable resistive states, which means it can encode data as 0,1 and 2, or a “trit” as opposed to a “bit.”

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Black Phosphorus Adds Thermal Management to Its Quiver

Much has been made of the fact that black phosphorus has an inherent band gap that graphene lacks, making it a more natural candidate than graphene to step into the role of silicon as a semiconductor material for electronics.

While that’s a pretty big advantage over graphene, black phosphorus possesses another property that nearly all of its 2-D cousins lack: in-plane anisotropy, which means its properties are dependent on the direction of the crystal.

Now researchers at the U.S. Department of Energy’s (DOE’s) Lawrence Berkeley National Laboratory (Berkeley Lab) have demonstrated experimentally what had previously only been theorized in computer models: black phosphorous has opposite anisotropy in thermal and electrical conductivities. This means that in black phosphorus, heat flows more easily in a direction in which electricity flows with more difficultly.

“Our study shows that in a similar manner heat flow in the black phosphorous nanoribbons can be very different along different directions in the plane,” said Junqiao Wu, one of the Berkeley Lab researchers, in a press release. “This thermal conductivity anisotropy has been predicted recently for 2-D black phosphorous crystals by theorists but never before observed.”

What this means is that when a microelectronic device fabricated from black phosphorus starts to heat up, it will be able to dissipate that heat extremely efficiently.

“This anisotropy can be especially advantageous if heat generation and dissipation play a role in the device operation,” said Wu in the press release. “For example, these orientation-dependent thermal conductivities give us opportunities to design microelectronic devices with different lattice orientations for cooling and operating microchips. We could use efficient thermal management to reduce chip temperature and enhance chip performance.”

In research published in the journal Nature Communications, the Berkeley Lab team found that when they increased the temperature of the black phosphorus above 100 Kelvin, the thermal conductivity anisotropy grew by a factor of two.

They also discovered that at 300 Kelvin, the thermal conductivity of black phosphorus nanoribbons decreased as the nanoribbons’ thickness decreased from 300 nanometers to 50 nanometers.

In the future, the researchers plan to look at how other physical parameters such as stress and pressure impact the thermal and electrical properties of black phosphorus nanoribbons.

The Artificial Skin That Could Deliver the Sense of Touch Directly to the Brain

Zhenan Bao at Stanford University has invested a lot of her research into building flexible circuits out of carbon nanotubes.

Her team’s latest feat is an “artificial skin” that’s capable of providing the sense of touch directly into the brain cells of mice and is initially aimed for use in prosthetic limbs to give the users the full sense of touch.

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Graphene-Coated Fabric Makes for a Wearable Gas Sensor

One of graphene’s key properties is its surface area—as a two-dimensional material it really is just all surface. This has advantages in a number of applications, one of which is sensors.

Now researchers at the Electronics and Telecommunications Research Institute and Konkuk University in South Korea have found that if they coat fabrics with graphene, they can detect dangerous gases and alert the wearer of their presence by triggering an LED light.

In research published in Scientific Reports, the Korean researchers coated commercially available yarn with reduced graphene oxide (RGO)—which refers to graphene oxide being stripped of most of its oxide to make graphene—using electrostatic self assembly and molecular glue to produce a bendable and washable electronic textile (e-textile) gas sensor.

“This sensor can bring a significant change to our daily life since it was developed with flexible and widely used fibers, unlike the gas sensors invariably developed with the existing solid substrates,” said Dr. Hyung-Kun Lee, who led this research initiative, in a press release.

The researchers discovered that the graphene-coated yarn was extremely sensitive to nitrogen dioxide, which is produced through the burning of fossil fuels. The sensor operates by the nitrogen oxide molecules changing  the electrical resistance of the graphene, which in turn triggers an LED light to turn on.

Within 30 minutes of exposure to 0.25 parts per million of nitrogen dioxide (just under five times above the acceptable standard set by the U.S. Environmental Protection Agency), the sensor detected the toxic gas.

While this fabric-based sensor compares favorably with previous RGO sensors prepared on a flat material, and offers three times the sensitivity to nitrogen oxide, it’s not clear why we would want clothing made from such a fabric. Wearable sensors have all sorts of potentially useful purposes, but this one seems somewhat obscure.

Nonetheless, the research does demonstrate that responsive gas sensors need not be fabricated on a solid substrate.

Memristors Don't Work the Way We Thought

What’s going to replace flash? The R&D arms of several companies including Hewlett Packard, Intel, and Samsung think the answer might be memristors (also called resistive RAM, ReRAM, or RRAM). These devices have a chance at unseating the non-volatile memory champion because, they use little energy, are very fast, and retain data without requiring power. However, new research indicates that they don’t work in quite the way we thought they do.

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Developments in Magnetic Skyrmions Come in Bunches

About two years ago, researchers in Germany introduced a new alternative in the world of magnetic storage media: skyrmions. A short description of skyrmions is that they are tiny, swirling magnetic spin patterns in thin films.  What makes them important is that they could change the face of data storage, hugely increasing the storage capacity over today’s hard disk drives.

Now, within a span of week, three separate research teams, one at the National Institute of Standards and Technology (NIST) in the United States, another at KTH The Royal Institute of Technology in Sweden and still another at the RIKEN Center for Emergent Matter Science in Japan have all announced breakthroughs that may bring these magnetic spin patterns one step closer to being the basis of real-world data storage applications.

In most magnetic materials, the magnetic force of each atom—known as the magnetic moment—lines up in the same direction as those of all the others. However, some magnetic materials such as manganese silicon (MnSi), when under extreme conditions, can develop areas where the magnetic moments curve and twist; these areas are known as magnetic skyrmions. Once skyrmions are formed, they are pretty resilient to outside influence, which makes them an ideal storage medium because they are not easily corrupted.

The extreme conditions are the key. To first form skyrmions, some of the researchers exposed the magnetic material to extremely low temperatures. But generally speaking, the conditions for forming skyrmions involve either magnetic, thermal, or electrical stimuli.

All three research teams published their findings in the journal Nature Communications.  The NIST researchers demonstrated that they could produce skyrmions by patterning asymmetric magnetic nanodots in a controlled circle on top of a thin film made of cobalt and palladium. The NIST researchers were able to create their skyrmions not only at room temperature, but also without an external magnetic field.

The Riken team out of Japan demonstrated that a new stimuli could create skyrmions, namely a mechanical forceWhile this method still involves creating the skyrmions in extremely low temperatures, it does offer the potential of both creating and deleting skyrmions with a small push. This, they say, could lead to new, low-cost memory devices that consume very little energy. 

The KTH researchers demonstrated not only a novel way to produce the swirling magnetic regions, but also presented a way to maintain their stability. The Swedish group formed its skyrmions under a nanocontact in which a spin-polarized current had been injected into the magnetic thin film. This provided a so-called spin torque to the film’s  magnetic moments so that the skyrmions remains more stable.

Observed as a group, this latest flurry of research suggests that skyrmions will become a mainstay of spintronics research for data storage. However, we may have a bit of a wait before we see this become the basis for magnetic data storage in our devices.

The First Two-Qubit Logic Gate in Silicon

Qubits come in several flavors: atoms suspended by laser beams, photons trapped in microwave cavities, superconducting rings in which currents can run in two directions simultaneously.  David DiVincenzo (a theoretical physicist at RWTH Aachen University in Germany, recently interviewed by Spectrum) and Daniel Loss proposed in 1998 using the spin state of electrons trapped in quantum dots to store quantum bits, and many view this as the most promising approach to quantum computing

Last week researchers at the University of New South Wales (UNSW) in Sidney, Australia, reported in the journal Nature that such trapped electrons can be integrated with existing CMOS technology, and that it might be possible to create quantum computer chips that could store thousands, even millions of qubits on a single silicon processor chip.

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Nanoscale Photodetector Promises Next Generation Photonic Circuits

Last year, we covered joint research between the University of Rochester and the Swiss Federal Institute of Technology in Zurich in which a primitive circuit consisting of a silver nanowire and single-layer flake of molybdenum disulfide (MoS2) was developed that could guide both electricity and light along the same wire.

Now researchers at the University of Rochester are continuing their work with nanowires and MoS2 to create a nanoscale photodetector that the researchers believe could be a step towards a new type of photonic circuit.

"Our devices are a step towards miniaturization below the diffraction limit," said Kenneth Goodfellow, a graduate student at the University of Rochester, in a press release. "It is a step towards using light to drive, or, at least complement electronic circuitry for faster information transfer."

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Disappearing Circuits Move From Spy Thrillers to Reality

Generally speaking, the issue that most electronic circuit research is aimed at is making them smaller yet still functional. It would seem that creating circuits that change over time, or even disappear entirely, is an endeavor that has been largely neglected, outside of a TV spy show from the 1960s that gets periodically rebooted into films.

Now researchers from the Georgia Institute of Technology have taken up the challenge of creating circuits than change over time, and may have come up with a technology that could have some attractive biomedical applications.

In research published in the journal Nanoscale, the Georgia Tech researchers deposited carbon atoms onto graphene using a focused electron beam process to create patterns that evolve over time on the graphene.

“We will now be able to draw electronic circuits that evolve over time,” said Andrei Fedorov, a professor at Georgia Tech, in a press release. “You could design a circuit that operates one way now, but after waiting a day for the carbon to diffuse over the graphene surface, you would no longer have an electronic device. Today the device would do one thing; tomorrow it would do something entirely different.”

Providing further evidence that intentionally setting out to create disappearing circuits is rare at best, the Georgia Tech researchers admit that they were initially just trying to see how they could remove hydrocarbon contaminants from graphene.

What they soon discovered was that when they deposited the carbon atoms on the graphene, they could create patterns and these patterns served to create negatively charged areas in the graphene.

But what really grabbed their attention was the discovery that these patterns would change over time as the carbon atoms moved around the surface of the graphene until they were spread uniformly across the entire surface. This change, which occurs over tens of hours, converts positively charged (p-doped) surface regions into surfaces with a uniformly negative charge (n-doped). It also manages to form an intermediate p-n junction domain during this transformation.

“The electronic structures continuously change over time,” Fedorov explained. “That gives you a reconfigurable device, especially since our carbon deposition is done not using bulk films, but rather an electron beam that is used to draw where you want a negatively-doped domain to exist.”

While the security applications for such a capability have been demonstrated vividly in the TV and movie series Mission: Impossible, the researchers have suggested that such a capability could prove useful in biomedicine.

“Perhaps there could be certain activated, triggered processes that could benefit from this type of behavior in which the electronic state changes continuously over time,” said Fedorov in the press release.

In further research, the Georgia Tech team will be aiming to find an application that could not be achievable without this capability.

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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.

 
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Dexter Johnson
Madrid, Spain
 
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Rachel Courtland
Associate Editor, IEEE Spectrum
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