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Thin Is in for Invisibility Cloaks

When Xiang Zhang first invented an invisibility cloak that worked at optical wavelengths, his son complained that it wouldn’t allow him to easily stroll around unseen, like Harry Potter sneaking through the library.

“My son always joked that, ‘Daddy, if I want to make myself disappear I have to carry a huge cylinder around myself,’” says Zhang, a professor of mechanical engineering at the University of California, Berkeley. The boy wondered why he couldn’t have something more like a sunblock, a spray-on material that rendered him invisible. That may not be such a far-fetched vision, Zhang says.

He and his colleagues have created an extremely thin cloak that works in visible light, made of a metasurface, a special type of metamaterial that is thin enough to be considered two-dimensional. They described the device in last week’s Science.

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Carbon Nanotubes Can Outperform Other Carbon Capture Materials

Whether you see carbon capture tools as solutions to environmental remediation or power generation, it has become increasingly clear that nanomaterials are pretty good at it.

Now researchers at Technische Universität Darmstadt in Germany and the Indian Institute of Technology Kanpur have found that they can tailor the gas adsorption properties of vertically aligned carbon nanotubes (VACNTs) by altering their thickness, height, and the distance between them.

“These parameters are fundamental for 'tuning' the hierarchical pore structure of the VACNTs,” explained Mahshid Rahimi and Deepu Babu, doctoral students at the Technische Universität Darmstadt who were the paper's lead authors, in a press release. “This hierarchy effect is a crucial factor for getting high-adsorption capacities as well as mass transport into the nanostructure. Surprisingly, from theory and by experiment, we found that the distance between nanotubes plays a much larger role in gas adsorption than the tube diameter does.”

Previously, carbon materials in gas adsorption-desorption suffered from hysteresis effects in which the property of the material was somewhat behind the factors that were changing it. In the case of carbon nanotubes, the sizes, structure, and distribution of the pores in the material were slow to react to these changes.

In research published in The Journal of Chemical Physics, the researchers first set out through computer modeling to gain a better theoretical understanding of adsorption and selectivity in carbon materials.

Then through experimentation, the researchers validated their models and demonstrated that VACNTs adsorbed the gases of carbon dioxide (CO2) and sulfur dioxide (SO2) better than traditional adsorption materials, such as porous carbon, zeolites, and metal organic frameworks within the mid-pressure (30 bars) regime. “This adsorption range is important for technologically relevant processes like gas storage for automotive purposes,” noted Rahimi in the release.

In future research, the plan is to introduce specific atoms to the carbon nanotubes for elemental doping.

Rahimi added: “This will allow us to further tune the gas selectivity. Another area we'll also explore is ‘controlled carbon nanotube openings’ in such VACNTs to increase the gas adsorption.”

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New Avenue Proposed for Bringing Carbyne Into the Real World

A couple of years ago, a material dubbed carbyne—which is a chain of carbon atoms held together by either double or alternating single and triple atomic bonds—was awarded the title of the world’s strongest material. Later, scientists also demonstrated that it has the unusual property of being able to change from being a conductor to an insulator when it’s stretched by as little as 3 percent.

Both are great properties to have. But thus far carbyne has proven nearly impossible to make in the real world. As a result, its exploits have remained firmly planted in the realm of computer models. While carbyne has been found in highly compressed graphite, and it has even been synthesized at room temperature, no one has devised a way to produce the material in bulk.

While researchers at Lawrence Livermore National Laboratory (LLNL) looking at carbyne’s properties in computer models and discovering some new ones that should benefit nanoelectronics, they discovered a potential avenue for producing the material.

In research published in the Journal of Physical Chemistry, the LLNL researchers demonstrated through computer models that it was possible to form carbyne fiber bundles by melting graphite with a laser.

“There’s been a lot of speculation about how to make carbyne and how stable it is,” said Nir Goldman, an LLNL scientist, in the press release. “We showed that laser melting of graphite is one viable avenue for its synthesis.” And depending on how you cook the graphite, the resulting carbyne could have applications, including tunable semiconductors or even hydrogen storage materials. Says Goldman:

Our method shows that carbyne can be formed easily in the laboratory or otherwise. The process also could occur in astrophysical bodies or in the interstellar medium, where carbon-containing material can be exposed to relatively high temperatures and carbon can liquefy.

Whether this technique can be scaled up to producing carbyne in bulk remains to be seen. But if it proves to be a viable method for carbyne synthesis, it does buoy hope for the material’s use in nanoelectronics, where it could perform some amazing feats such as adjusting the amount of electrical current traveling through a circuit, according to the user’s need.

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Graphene Filament Enables Fabrication of Electronic Devices with 3-D Printing

Earlier this month, we reported on research that was bringing the attractive qualities of graphene in its 2-D form to the fabrication of 3-D objects.

Now a start-up based in Calverton, NY, Graphene 3D Lab, Inc., has made commercially available a graphene-based conductive polymer filament for use in 3-D printing to fabricate electronic devices. The graphene-based filament, which is targeted for both industry and hobbyists, has been dubbed Black Magic 3D.

“Our material is the most electrically conductive material on the market right now and is the best option for 3-D printing of electronics,” claimed Daniel Stolyarov, who along with Elena Polyakova, are Co-CEOs, in an e-mail interview. “Even though our material is more expensive, you only need a very small amount (a few grams), which would cost as low as $1, along with regular material to make 3-D printed electronics. Without graphene this is not possible.”

Stolyarov believes that this graphene-enabled polymer filament is unique on the market in its ability to impart electrical conductivity. Stolyarov argues that their product compares favorably to other 3-D printing filaments that have at best a volume resistivity of 15 Ohms-centimeter (Ohms-cm), whereas Black Magic 3D’s volume resistivity measures at 0.6 Ohms-cm—25 times better. According to Stolyarov, 15 Ohms-cm is just not good enough for most of electronic applications. If electrical properties are poor, the device will not work properly.

Stolyarov has pointed to the emerging trend of 3-D printed electronics, which he believes may soon show explosive growth. An indication of this potential was the recent launch of a new 3-D printer from a company called Voxel8 that specifically targets the printing of electronics and circuitry.

However, Stolyarov is quick to note that his company’s graphene-based filament can be used with just about any 3-D printer on the market now, from hobbyist to industrial.

To demonstrate how the graphene-based filament can fabricate devices requiring high thermal and electrical conductivity, the company produced a battery. It seems that this battery design remains primarily to demonstrate the capabilities of the graphene 3-D printing filament.

“The 3-D printed graphene battery project is still being developed and we are very much looking forward to offering more details on the technology in the future,” said Stolyarov.

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Silicon Nanoparticle Could be Heart of New Optical Transistor

The quest to transform the basis of computing from electrons to photons has been full of challenges. The aim has been to get photonic circuits to do what electronic ICs do but do it much faster—at the speed of light, achieving it has remained elusive. 

Researchers at ITMO University in St. Petersburg, Russia suggest that a new technique could be a big step toward photonic ICs and optical computing. It uses a single silicon nanoparticle as an optical transistor.

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Researchers Create Quantum Dots that Transmit Identical Single Photons

Single photons will play an important role in quantum communication. This will require quantum repeaters, in much the same way that optical amplifiers are required for the transmission of digital data through optical fibers. However, these quantum repeaters will work only if all the single photons that they receive have the exact same wavelength. A team of researchers at universities in Switzerland, Germany, and France reported on 8 September in Nature Communications that they’ve developed quantum dots that can produce streams of photons with identical wavelengths. 

To create the photons, the researchers used self-assembled indium gallium arsenide (InGaAs) quantum dots embedded in gallium arsenide. Each quantum dot, although it consists of about a hundred thousand atoms, traps a single electron that can occupy two energy levels. By illuminating the quantum dot with laser light, the electron is moved into the higher energy state. When the electron drops down to its lower energy level, it emits a single photon whose wavelength is determined by the difference in the energy of the two levels. “In many ways it behaves like a single atom, and this is why it is often called an 'artificial atom,’” says Andreas Kuhlmann a post-doctoral researcher at the University of Basel who was the paper’s lead author. “But because it is inside a semiconductor it is quite robust, and that is, of course, nice if you want at one point to develop a product.”  

Still, unlike a single atom, the atoms in the quantum dot are an unruly bunch.  The fluctuating nuclear spins of the atoms in the quantum dot interact with the electron spin, and fluctuating electric fields created by electrons hopping around cause the two energy levels of the quantum dot to wobble. This results in the emission of photons with wavelengths that vary, if ever so slightly. “In order to get indistinguishable single photons, which all have exactly the same color, we needed to find a way to suppress the noise,” explains Kuhlmann.

How did they do it? They cooled the quantum dot to 4.2 Kelvin. They could reduce the noise even further by cooling it even more, an option that is not that attractive.  Despite what one might think at first blush, “4.2 Kelvin is in some ways quite warm; at some point you want to develop a product, and it makes sense to stay as warm as possible,” says Kuhlmann.

That wasn’t the only trick they employed to reduce the noise, says Kuhlmann. “The samples are grown layer by layer using molecular beam epitaxy, and when you grow these samples you will get some defects.” Crucial to limiting noise, he says, is limiting these defects because they become populated by electric charges that fluctuate. “It is extremely important that you have high-quality material and that the people who grow these samples know what they are doing,” says Kuhlmann.

The researchers overcame several hurdles. Though nuclear spins are an intrinsic property of the InGaAs semiconductor, the researchers found a way to reduce the influence of the nuclear spins.  Because of its indirect band gap, it can’t emit photons when irradiated by light, explains Kuhlmann. But he and his collaborators created a quantum dot that transmits photons. And because they were able to precisely control the number of electrons trapped in the quantum dot by applying a voltage to a gate, they were able to reduce this number to one. Over a broad voltage range, the quantum dot is empty. However, at a particular voltage, the dot’s sweet-spot, the noise from the nuclear spins is strongly suppressed. “We don’t know why yet”, says Kuhlmann, “but the improvement in the photons is immediately noticeable.” 

The result, says Kuhlmann:

If we switch on our laser, and there is a single electron inside this quantum dot, then this permanent excitation suppresses the fluctuations of the nuclear spins in the sample. When no electron is trapped in the quantum dot, we found that by changing the electrical field that we apply to the device, we also can suppress the spin noise. 

One of the problems that still has to resolved is that a number of photons are lost when the photons exit the device via the gallium arsenide layer that covers the quantum dot. “There are ways to engineer these devices; for example, you put a quantum dot inside a nanowire, and you can send almost all the photons along the waveguide,” says Kuhlmann.  There should be applications galore for the device in quantum communication and computing.  “A single photon is an ideal flying qubit,” says Kuhlmann.

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Peering Into Nanoparticles One at a Time Reveals Hidden World

Imagine you could single out individuals in a large group and see what each was doing instead of observing a large number of them all grouped together. You could detect how each is distinct within the group.

This is essentially what researchers at Chalmers University in Sweden have been able to achieve with a new microscopy technique that is capable of looking at a single nanoparticle rather than just a mass of them all clumped together.

“We were able to show that you gain deeper insights into the physics of how nanomaterials interact with molecules in their environment by looking at the individual nanoparticle as opposed to looking at many of them at the same time, which is what is usually done,” said Associate Professor Christoph Langhammer, who led the project, in a press release.

In their findings, published in the journal Nature Materials, the researchers leveraged an imaging technique known as plasmonic  nanospectroscopy, which involves exploiting the oscillations in the density of electrons that are generated when photons hit a metal surface.

The researchers applied the experimental spectroscopy technique to examine hydrogen absorption in single palladium nanoparticles. The observations were surprising. They discovered that despite various nanoparticles having the same size and shape, they would absorb hydrogen at pressures as different as 40 millibars.

In real world applications, this observation could help lead to more sensitive hydrogen sensors for detecting leaks in fuel-cell-powered vehicles.

“One main challenge when working on hydrogen sensors is to design materials whose response to hydrogen is as linear and reversible as possible. In that way, the gained fundamental understanding of the reasons underlying the differences between seemingly identical individual particles and how this makes the response irreversible in a certain hydrogen concentration range can be helpful,” added Langhammer in the release.

While others have been able to image single nanoparticles previously, those efforts came at a rather high cost of heating the nanoparticles up, or impacting them in some other way that eliminates the ability to observe them accurately.

“When studying individual nanoparticles you have to send some kind of probe to ask the particle ‘what are you doing?’,” said Langhammer. “This usually means focusing a beam of high-energy electrons or photons or a mechanical probe onto a very tiny volume. You then quickly get very high energy densities, which might perturb the process you want to look at.”

Not only is this effect is minimized in their new approach, according to Langhammer, but it is also compatible with ambient conditions, so that it is possible to study nanoparticles one at a time in their actual environments. This ability to observe nanoparticles outside the lab could prove to be a key development for studies on the impact of nanoparticles in the environment.

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Graphene's Killer App? Measuring Electrical Resistance

Graphene’s merits in electronic devices and as a light bulb coating are still being debated. But new results suggest the atom-thick carbon sheet has one clear advantage: precise but practical calibrations of electrical resistance.

This might seem like a minor use for the world’s most celebrated wonder material, but it’s one that sits at the very base of the electrical engineering pyramid. A more practical calibration could help national standards laboratories and industries that depend on those standards. It may also help disseminate the International System of Units (SI), which could be overhauled as early as 2018.

The most exacting metrology laboratories calibrate their electrical units based on quantum mechanical phenomena. The ohm, the SI unit of electrical resistance, is calibrated by taking advantage of the quantum Hall effect. The Hall effect occurs when a magnetic field is applied perpendicular to the flow of current. The resulting force on electrons causes them to migrate to the side, which in turn raises a voltage perpendicular to the flow of current.

In the quantum version of the Hall effect, which occurs in a thin layer of material, the voltage and resulting resistance are “quantized” and so take on discrete integer values. Conveniently, the quantum Hall resistance is expected to be completely independent of the kind of device that’s built. Instead it depends only on two unvarying constants of nature: the fundamental charge of the electron and a quantum mechanical measure dubbed the Planck constant. 

Unfortunately, the physical conditions required to take advantage of the quantum resistance standard are exacting. State-of-the-art measurements, which are taken using a device made of thin layers of gallium arsenide and aluminum gallium arsenide, can require a 10-Tesla magnetic field (and so a massive superconducting magnet) and temperatures within a few degrees of absolute zero. 

Researchers have long suspected that the unique behavior of electrons in graphene, namely the big spacing between electron energy levels when the material is exposed to a magnetic field, could be exploited to produce precise measurements of resistance under less extreme physical conditions. 

Several recent results support that idea. In August, Jan-Theodoor Janssen  at the UK’s National Physical Laboratory and colleagues reported a way to build a small system for a graphene resistance standard that can operate at a higher temperature and lower magnetic field. This week, a team based at France’s National Metrology and Testing Laboratory and various departments at the National Center for Scientific Research showed that graphene can indeed be used to calibrate resistance with great accuracy, rivalling that of gallium devices, and that it can do so over an even wider range of operational conditions.

The French team constructed its resistance device from a high-quality sheet of graphene grown on a silicon carbide wafer. The resulting 100-by-420-micrometer “Hall bar” contained a source and drain to raise a voltage across the device, as well as a set of metallic pads along the sides that were used to measure resistance.

The team found they could measure resistance with a level of accuracy rivaling those yielded by gallium arsenide devices, but with a magnetic field one-third as strong, with a temperature as high as 10 Kelvin, or with measurement currents 10 times as high (though not all three at the same time).

The temperature the graphene device operates at is high enough that a lab could accurately measure resistance without needing liquid helium as a refrigerant. “These results support graphene as the material of choice for the next generation of easy-to-use, helium-free, and affordable quantum electrical standards, approaching an ideal standard that would be invariant, available to anyone, at any place and any time,” the French team wrote this week in Nature Nanotechnology.

Graphene could also help bring about the realization of a simplified ampere, one of the seven SI base units. In the new SI, this unit of current will be redefined in terms of the fundamental charge of the electron, and quantum electrical standards will play a closer, more integrated role.

The kilogram will also be redefined. Instead of being measured against a physical artifact locked in a vault, it will be realized directly through the watt balance, which measures the weight of an object against an opposing electromagnetic force. That force is calibrated against quantum electrical standards. (Read the strange story of the kilogram change here.)

In the long term, it could be possible for any national metrology laboratory to measure the kilogram from scratch, says Jon Pratt of the U.S. National Institute for Standards and Technology, which is also investigating graphene’s potential as a resistance standard. “If we are to make watt balances available to everyone, we will need quantum standards that are low cost and easy to use,” he says. “Graphene appears to move us towards this, at least for resistance.”

This article was updated on 14 September 2015 to clarify the NPL and LNE/CNRS findings.

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Graphene and Perovskite Lead to Inexpensive and Highly Efficient Solar Cells

Perovskite is the new buzzword in photovoltaics. And graphene is the buzzword for just about every other high-tech application, including photovoltaics.

Now researchers at Hong Kong Polytechnic University have combined these two materials to make a semi-transparent solar cell capable of power conversion efficiencies around 12 percent, a significant improvement over the roughly 7-percent efficiency of traditional semi-transparent solar cells.

The semi-transparent design of these solar cells means that they can absorb light from both sides and could allow them to be used as windows that serve the dual function of letting light into a building and generating electricity.

In the design of the Hong Kong researchers’ solar cell, the perovskite serves as active layer for harvesting the light, and the graphene acts as the transparent electrode material. Graphene has long been pursued as a potential replacement for indium tin oxide (ITO) as a transparent electrode material for displays.

Here again, graphene’s transparency, high conductivity, and potentially low cost seemed attractive to the researchers. The researchers improved on the conductivity of the graphene by coating it with a thin layer of a polymer that also served as an adhesion layer to the perovskite active layer during the lamination process.

The researchers were able to improve the energy conversion capability of the solar cells by employing a multi-layer chemical vapor deposition process in which the graphene formed the top transparent electrodes. This approach maintained the transparency of the electrodes while increasing their sheet resistance.

A big concern for the researchers was lowering costs. They claim that their solar cells cost less than US$.06/watt, which they reckon is more than a 50 percent reduction in the costs of silicon solar cells. They believe that the whole process is ripe for scaling up because the mechanical flexibility of the graphene enables the possibility of roll-to-roll processing.

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One-step Fabrication Method Produces Nanofibers Ideal for Energy Storage

Researchers at Case Western Reserve University have tackled the problem of getting carbon nanotubes and graphene to exhibit their extraordinary thermal, electrical and mechanical properties as three-dimensional nanostructures. As a result of their efforts, they have walked away with an improved one-step fabrication method for producing these nanostructures and carbon fibers that could lead to highly efficient energy storage systems.

In research published in the journal Science Advances, the Case Western researchers employed an anodic oxidation of aluminum (AAO) template to directly grow hollow fibers made of carbon nanotubes in a simple one-step process without the need for metal catalysts.

The researchers believe that the flexible hollow fibers could improve energy storage devices and could even be woven into textiles as power sources for wearable electronics. Exploiting the flexibility of carbon nanomaterials by using them as a power source in wearable electronics has been the focus of a fair amount of research.

“The porous 3-D carbon structure provides a huge surface/interface area for charge storage,” explained Liming Dai, a professor at Case Western and one of the lead researchers, told IEEE Spectrum via e-mail. “Furthermore, the presence of holes facilitates the electrolyte diffusion while the seamless nodal junction allows fast electron transport, and hence the fast charging and discharging rates.”

In the research, Dai and his colleagues fashioned wire capacitors from the 3-D carbon nanotubes; these energy storage devices could potentially be woven into different fabrics.

“We will try to make textiles from the fibers, but we also don't mind others to work along this line as long as our publications are properly cited,” said Dai.

In future research, Dai and his colleagues plan to develop 3-D multilayered structures.

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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
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