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Fiber-like Supercapacitors Could Be Woven Into Wearable Electronics

Initial hopes were that graphene and its cousin the carbon nanotube could serve as a replacements to activated carbon to push supercapacitors to the equivalent storage capacity of batteries. That hope soon waned when it became apparent that these carbon nanomaterials don’t even have the theoretical surface area—one of the key features for higher storage capacity in supercapacitors—of activated carbon.

As far back as four years ago, research started to move away from just trying to beat the energy density (the amount of energy stored per unit mass) of supercapacitors using activated carbon, but instead started looking at the interesting structures that could be built using graphene and carbon nanotubes as the electrode material for supercapacitors. This meant that new, smaller supercapacitors could be used to power microelectronic devices with unusual geometries.

Now an international team of researchers from Nanyang Technological University (NTU) in Singapore, Tsinghua University in China, and Case Western Reserve University in the United States has realized one of these potential new applications by developing a fiber-like supercapacitor made from both graphene and carbon nanotubes that could be woven right into clothing.

In keeping with the new fiber-like geometry of the supercapacitors, the researchers have released figures on the energy density of the novel supercapacitors by volume rather than by mass. They claim that the volumetric energy density is the highest yet reported for carbon-based microscale supercapacitors: 6.3 microwatt-hours per cubic millimeter, which is comparable to a 4-volt-500-microampere-hour thin-film lithium ion battery that can be used to power smart cards and RFID tags.

This record-breaking figure for volumetric energy density addresses one of the weaknesses of typical, activated carbon-based supercapacitors. Using activated carbon on the electrodes of supercapacitors could approximate the energy density of batteries by mass, but when it came to volume they were woefully deficient because they require large amounts of accessible surface area to store energy.

In research reported in journal Nature Nanotechnology, the team demonstrated that their hybrid fiber could store energy along its entire length, providing huge amounts of accessible surface area—396 square meters per gram of hybrid fiber.

The researchers produced the fiber-like supercapacitor by heating a solution of graphene and carbon nanotubes. The graphene and carbon nanotubes self assemble into an interconnected, porous network that runs the entire length of the fiber. The researchers have made flexible fibers as long as 50 meters and having a charge capacity of 300 Farad per cubic centimeter.

"We have tested the fiber device for 10 000 charge/discharge cycles, and the device retains about 93 percent of its original performance, while conventional rechargeable batteries have a lifetime of less than 1000 cycles," said Yuan Chen, a professor of chemical engineering at NTU, in a press release. "The fiber supercapacitor continues to work without performance loss, even after bending hundreds of times.”

The researchers envision the supercapcitor fibers being woven into clothing that could power biomedical monitoring devices a patient wears at home.

The next steps for the researchers will be to scale up the production method to bring down its costs to make it more attractive for commercialization. Meanwhile they will also be looking into applying the supercapacitor fibers  solar cells, biofuel production, wearable optoelectronics, and other systems.

Graphene Oxide Could Lead to Easy-to-Make Integrated Photonics

In order for us to enjoy our high-speed Internet connections, we depend on optical fiber carrying multiple beams of laser light at different wavelengths. A key property that makes optical networks function is something called optical nonlinearity, which is the ability of a medium to have its optical properties (transmission, refraction, etc.) manipulated by changing the intensity of the light traveling through it. Optical nonlinearity provides us the possibility to use light to control light so we can operate fiber optic networks.

Now, researchers at Swinburne University of Technology in Melbourne, Australia, have found that graphene oxide (GO) possesses a record-breaking optical nonlinearity that makes it suitable for use in high-performance integrated photonic devices for all-optical communications, biomedicine, and photonic computing. Associate Professor Baohua Jia at Swinburne told Nanoclast that the nonlinearity of the GO film they developed is 1000 times as large as previous results. 

In research which was published in the journal Advanced Materials (“In Situ Third-Order Non-linear Responses During Laser Reduction of Graphene Oxide Thin Films Towards On-Chip Non-linear Photonic Devices”), the Swinburne team spin coated a GO film onto a glass surface. The researchers then used a laser to create microstructures on the surface of GO film to tune the nonlinearity of the material.

The Swinburne researchers believe that their approach to laser writing structures on a GO film can serve as a method for tuning the nonlinearity of every optical component of integrated photonic devices.

This stands in contrast to today’s integrated photonic devices, in which multiple photonic functions are integrated in one device by building each component separately and then putting them together.

“Now we can provide a film, on which everything can be fabricated with laser and then it is automatically integratable,” said PhD student Xiaorui Zheng in a press release.

This should make the manufacturing process for integrated photonic devices, which still require clean rooms to be fabricated, dramatically easier. “Using this new method, we have demonstrated the possibility of manufacturing a scalable and cheap material,” Professor Jia said in the press release.

Over the past year, Swinburne has taken on graphene oxide as a research target for optics. Last October, a Swinburne team discovered that GO’s giant refractive index could be exploited for merging data storage with holography for improved security coding.

In this most recent research, the aim now is to fabricate a functional device.

Cyborg Beetles Detect Nerve Gas

The rival rock stars of nanotech—carbon nanotubes and graphene—have joined forces to become a super group of late. They are now being combined to make supercapacitors or just to make the manufacturing process for one of them less arduous.

Now researchers in South Korea have joined them together to create one monolithically integrated flexible electronic device that can be synthesized in a single step and be attached to, among other things, live stag beetles that can be set loose to detect a range of environmental conditions or nerve gas agents.

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Brownian Motion Helps Reveal Temperature of Nanoscale Objects

One of the key physical forces on the nanoscale is Brownian motion in which particles suspended in a gas or liquid seem to move around randomly as they are pushed to and fro by collisions with the atoms that comprise the gas or liquid. So, if you were to look at the world on the nanoscale, everything would be appear to be in a state of random movement.

Understanding and working with these forces are of great interest to those in the field of molecular nanotechnology (MNT), where mechanical engineering meets the nanoscale. MNT involves the movement of gears and motors, just like you would find in a large-scale factory, but Brownian motion has persistently presented a problem because it prevented engineers from achieving the tight tolerances required for those systems to work.

Lately, however, researchers have been able to make Brownian motion a feature rather than a bug, so to speak. A few years back, scientists in Japan demonstrated the conversion of information into energy by using Brownian motion to cause a pair of particles to rotate clockwise. The UK-based company Nanosight Ltd. is another company that exploits Brownian motion to with a method for visualizing particles in a liquid.

Now researchers at the University of Exeter and the University College London have developed a method employing Brownian motion to measure the temperature of nanoscale objects.

“This [Brownian] motion is caused by the collisions with the air molecules," said Dr. Anders from the University of Exeter in a press release. "We found that the impact of such collisions carries information about the object's surface temperature, and have used our observation of its Brownian motion to identify this information and infer the temperature."

The research, which was published in the journal Nature Nanotechnology (“Nanoscale temperature measurements using non-equilibrium Brownian dynamics of a levitated nanosphere”), involved catching a glass nanosphere in a laser beam and suspending it in air. By then heating the sphere, the researchers were able to observe rising temperatures on the nanoscale until the glass got so hot that it melted. The technique is not limited to glass; it could be used to determine the surface temperatures of any tiny sphere.

"When working with objects on the nanoscale, collisions with air molecules make a big difference", said James Millen from the team at University College London in a press release. "By measuring how energy is transferred between nanoparticles and the air around them we learn a lot about both.”

This method should give engineers more accurate temperature measurements with spatial resolution on the nanoscale. This new capability to measure the temperature of nanoscale objects should prove useful in the operation of nanoscale systems because engineers will now likely be able to exert control over them, with thermal energy as the lever for fine-tuning their activity.

Novel 2-D Material Offers a Band Gap and Self Assembly

The competitive field of two-dimensional materials has added another rival to graphene to its ranks. A collaboration between MIT and Harvard University researchers has yielded what observers are heralding as a major advance in the synthetic design of novel semiconducting materials. The Boston-area researchers have developed a new 2-D material that not only has an inherent band gap—which graphene lacks—but self-assembles, promising easier avenues to mass production.

The material is a combination of nickel and an organic compound called 2,3,6,7,10,11-hexaiminotriphenylene (HITP). The resulting material belongs to a class of materials known as metal-organic frameworks (MOFs) that are compounds in which metal ions are coordinated to rigid organic molecules to form a porous material that can be one-, two-, or three-dimensional.

The research, which was published in the Journal of the American Chemical Society ("High Electrical Conductivity in Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2, a Semiconducting Metal–Organic Graphene Analogue"),  demonstrated that the new compound, Ni3(HITP)2, has the same hexagonal honeycomb structure as graphene.

One of the attractive characteristics the researchers demonstrated with this particular MOF is that its properties can be tuned to a desired capability simply by adding more or less of the two constituent parts. This could lead to the development of photovoltaics in which the solar cell could be manipulated to capture different wavelengths of light that match the solar spectrum.

The MIT-Harvard team performed their studies of the material in its bulk form rather than as flat sheets, making the record-breaking measurements for the MOF all the more impressive. By using two-probe and van der Pauw electrical measurements, the researchers revealed that the bulk (pellet) and surface (film) specific conductivity values of the materials were 2 Siemens/centimeter-1 (S/cm-1) and 40 S/cm-1, respectively—both records for MOFs, and among the best for any coordination polymer.

“There’s every reason to believe that the properties of the particles are worse than those of a sheet,” said MIT assistant professor of chemistry Mircea Dincă in a press release. “But they’re still impressive.”

In addition to the material’s potential applications to photovoltaics, the researchers envision that it could be used in the creation of exotic materials such as magnetic topological insulators, or materials that exhibit quantum Hall effects.

“They’re in the same class of materials that have been predicted to have exotic new electronic states,” said Dincă in the release. “These would be the first examples of these effects in materials made out of organic molecules. People are excited about that.”

Should We Worry About Graphene Oxide in Our Water?

Researchers at University of California, Riverside have measured the mobility of graphene oxide (GO) in water and have determined that it could move around easily if it were released into lakes and streams.

While the UC Riverside did not look at the toxicity of GO in their study, researchers at the Hersam group from Northwestern University did report in a paper published in the journal Nano Letters (“Minimizing Oxidation and Stable Nanoscale Dispersion Improves the Biocompatibility of Graphene in the Lung”) that GO was the most toxic form of graphene-based materials that were tested in mice lungs. In other research published in the Journal of Hazardous Materials (“Investigation of acute effects of graphene oxide on wastewater microbial community: A case study”), investigators determined that the toxicity of GO was dose dependent and was toxic in the range of 50 to 300 mg/L. So, below 50 mg/L there appear to be no toxic effects to GO. To give you some context, arsenic is considered toxic at 0.01 mg/L.

Graphene oxide is synthesized under extreme conditions (exposure to highly concentrated sulfuric acid, high temperatures, ultra sonication). This results in oxygen functional groups being present on the surface of the graphene oxide flakes. These oxygen functional groups make the material more stable than graphene and also more toxic, according to the researchers.

While GO is quite different from graphene in terms of its properties (GO is an insulator while graphene is a conductor), there are many applications that are similar for both GO and graphene. This is the result of GO’s functional groups allowing for different derivatives to be made on the surface of GO, which in turn allows for additional chemical modification. Some have suggested that GO would make a great material to be deposited on additional substrates for thin conductive films where the surface could be tuned for use in optical data storage, sensors, or even biomedical applications.

In addition to being a conductor before it is functionalized, GO is also known to be easily dispersed in water and other organic solvents, which begs the question of how does this research add to the understanding of GO’s known fundamental properties.

As Jake Lanphere, a UC Riverside graduate student who co-authored the paper, which was published in the journal Environmental Engineering Science (“Stability and Transport of Graphene Oxide Nanoparticles in Groundwater and Surface Water”), explained to Nanoclast in an email interview: “Other studies have looked at ideal lab conditions that do not necessarily reflect the conditions one might find in aquatic environments. Our study investigated the effects of environmentally relevant parameters and different water types that would be found in groundwater and surface waters. Our study is the first to look at the effects of these environmentally relevant parameters on the fate and transport in porous media.”

While Lanphere believes that this information will be critical for the Environmental Protection Agency (EPA) to understand the risk posed by GO, he doesn’t see that the EPA has to make any changes to its current approach for dealing with graphene in its various forms.

“I believe the EPA is doing a great job making sure that we maximize the benefits of nanotechnology while reducing the negative impacts it might have on society,” said Lanphere. “I do not have any specific suggestions.”

Ultimately, the question of danger of any material or chemical comes down to the simple equation: Hazard x Exposure=Risk. To determine what the real risk is of GO reaching concentrations equal to those that have been found to be toxic (50-300 mg/L) is the key question.

The results of this latest study don’t really answer that question, but only offer a tool by which to measure the level of exposure to groundwater if there was a sudden spill of GO at a manufacturing facility.

“As a result of our transport studies, you could determine the distance GO will travel in a specific environment as a function of the soil matrix conditions,” said Lanphere. “This information could help you understand, for example, if your well water would be at risk if there was a contaminant spill with GO nearby.”

Nanoporous Material Combines the Best of Batteries and Supercapacitors

Researchers at Rice University in Houston, Texas, have developed a nanoporous material that has the energy density (the amount of energy stored per unit mass) of an electrochemical battery and the power density (the maximum amount of power that can be supplied per unit mass) of a supercapacitor. It's important to note that the energy storage device enabled by the material is not claimed to be either of these types of energy storage devices.

The research community has wearied of claims that some new nanomaterial enables a “supercapacitor," when in fact the energy storage device is not a supercapacitor at all, but a battery. However, in this case, the Rice University researchers, led by James Tour, who is known for having increased the storage capacity of lithium-ion (Li-ion) batteries with graphene, don’t make any claims that the device they created is a supercapacitor. Instead it is described as an electrochemical capacitor with nanoporous nickel-fluoride electrodes layered around a solid electrolyte that is flexible and relatively easy to scale up for manufacturing.

The issue of appropriate nomenclature aside, the reported performance figures for this energy storage material are very attractive. In the Journal of the American Chemical Society ("Flexible Three-Dimensional Nanoporous Metal-Based Energy Devices"),  the researchers report energy density of 384 watt-hours per kilogram (Wh/kg), and power density of 112 kilowatts per kilogram (kW/kg).

To give some context to these numbers, a typical energy density for a Li-ion battery is 200Wh/kg, whereas commercially available supercapacitors store around 5- to 25 Wh/kg and research prototype supercapacitors have made claims of anywhere from 85 to 164 Wh/kg. In terms of power density, the numbers for the new nanoporous material is in line with those of supercapacitors, which range from 10 to 100 kW/kg—far higher than the 0.005 to 0.4kW/kg that batteries can deliver.

“The numbers are exceedingly high in the power that it can deliver, and it’s a very simple method to make high-powered systems,” Tour said in a press release. “We’re already talking with companies interested in commercializing this.”

To make the battery-supercapacitor hybrid, the Rice team deposited a nickel layer on a backing material. They then etched the nickel layer to create pores five nanometers in diameter. The result is high surface area for storing ions. After removing the backing, the nickel-based electrode material is wrapped around a solid electrolyte of potassium hyrodroxide in polyvinyl alcohol. In testing, the researchers found that there was no degradation of the pore structure after 10 000 charge-discharge cycles, or any significant degradation of the electrode-electrolyte interface.

“Compared with a lithium-ion device, the structure is quite simple and safe,” said Yang Yang, lead author of the paper, in the press release. “It behaves like a battery but the structure is that of a supercapacitor. If we use it as a supercapacitor, we can charge quickly at a high current rate and discharge it in a very short time. But for other applications, we find we can set it up to charge more slowly and to discharge slowly like a battery.”

With the device’s flexibility and high charge-up rate, it’s possible to imagine this storage device powering flexible mobile devices. However, charging rates for the battery/supercapacitor will be limited by the typical 200-amp 240V single-phase residential service, which is only capable of providing (absent any other load) only 48 kW.

What Makes for Better CdTe Solar Cells

Cadmium-telluride (CdTe) solar cell materials have had a bumpy ride ever since they were first introduced as an alternative to silicon-based photovoltaics. They were never quite as efficient at converting sunlight into electricity as silicon.

As a result, back in 2002, British Petroleum, which at the time was billing itself as the world’s biggest solar company, terminated U.S. production of CdTe and amorphous silicon cells. However, the fortunes of CdTe started to turnaround back in 2010 when General Electric announced its plans to enter the business. Since then, GE has been announcing ever-increasing conversion efficiency numbers, with a 19.6-percent conversion efficiency reached last year. First Solar Inc. recently broke this record by achieving a conversion efficiency of 20.4 percent.

As good as these numbers are, they still fall short of the 18 to 21 percent conversion efficiency of conventional silicon. (Recently, Panasonic announced that it had achieved a conversion efficiency of 25.6% for its silicon-based solar cells, a new record.)

To see if the latest conversion efficiency numbers of CdTe solar cells could be improved upon, and to find out what was behind the escalating numbers of the recent past, researchers at the Department of Energy’s Oak Ridge National Laboratory along with colleagues from the University of Toledo and DOE’s National Renewable Energy Laboratory used electron microscopy to peer into cadmium-telluride solar cell materials to see what made them tick.

Specifically, the researchers wanted to examine CdTe solar cell materials that had been treated with cadmium-chloride, which had been improving the efficiency numbers of the cadmium-based solar cells since the 1980s, though no one knows why.

“We knew that chlorine was responsible for this magical effect, but we needed to find out where it went in the material’s structure,” said ORNL’s Chen Li in a press release. “Only by understanding the structure can we understand what’s wrong in this solar cell—why the efficiency is not high enough, and how can we push it further.”

In research published in the journal Physical Review Letters (“Grain-Boundary-Enhanced Carrier Collection in CdTe Solar Cells”), the research team discovered atom-scale grain boundaries were involved in the enhanced performance. Grain boundaries are essentially tiny defects, which, in the case of solar cells, typically result in reduced efficiency numbers.

Using electron microscopy, the researchers saw that chlorine atoms were replacing tellurium atoms within these grain boundaries. The substitution was creating local electric fields at the grain boundaries that were improving the photovoltaic performance rather than worsening it.

The researchers believe that this understanding could lead future research into CdTe solar cells that could push their conversion efficiency closer to their theoretical maximum of 32 percent.

“We think that if all the grain boundaries in a thin film material could be aligned in same direction, it could improve cell efficiency even further,” Li added.

Graphene and Carbon Nanotubes Join Forces to Tackle Supercapacitors

Graphene and carbon nanotubes have been competing for many of the same applications for years, especially in the broad area of electronics. The jockeying for supremacy between these two carbon materials has been fierce in energy storage applications as well. In fact, both carbon nanotubes and graphene have been proposed as a replacement material for activated carbon on the electrodes of supercapacitors.

Now, following a newly developing trend where graphene and carbon nanotubes join forces to create an even better material than they could on their own, researchers at George Washington University have combined the two materials to create a supercapacitor that is claimed to be both low cost and high performance.

In research published in the Journal of Applied Physics ("Paper-based ultracapacitors with carbon nanotubes-graphene composites"), the GWU researchers mixed graphene flakes with single-walled carbon nanotubes through an arc discharge under various magnetic conditions.

The resulting combination takes advantage of the high-surface area and good in-plane conductivity of graphene flakes while the carbon nanotubes connect all the structures to make a uniform network. The device’s specific capacitance—its ability to store a charge—was reported as 100 Farads per gram (F/g), three times higher than the specific capacitance of a supercapacitor made by carbon nanotubes alone.

“In our lab we developed an approach by which we can obtain both single-walled carbon nanotubes and graphene, so we came up with the idea to take advantage of the two promising carbon nanomaterials together," said Michael Keidar, a professor at GWU and director of the Micro-propulsion and Nanotechnology Laboratory, in a press release.

Supercapacitors, also known as ultracapcitors or electrochemical double-layer capacitors (EDLCs), have held out the promise that they could store as much energy as an electrochemical battery like a lithium-ion battery, but charge up in a matter of seconds and provide quick bursts of a large amount of power as they do now for applications such as powering cranes or buses.

This potential has fueled the hope that supercapacitors could be used to power all-electrical vehicles, providing as much range as a lithium-ion battery does but charge up faster than the time it takes to fill up a car with gasoline. The interest in applying nanomaterials to these devices has become so intense that the lines between batteries and supercapacitors are becoming blurred as new materials are proposed.

In the race to practical—and potentially lucrative—applications, a promising approach in giving supercapacitors the same storage capacity as an electrochemical battery is increasing the surface area of the electrodes. More surface area translates into more ions being stored on the electrodes and the greater specific capacitance. While much is made of graphene’s theoretical surface area of 2630 squared meters per gram, so far the largest surface area anyone has produced with graphene has been 1520 squared meters per gram, which is pretty typically found in today’s activated carbon made from crushed coconuts.

So, the jury is still out on whether graphene or carbon nanotubes are viable alternatives to activated carbon for today's supercapacitor applications, even if you lower the cost of the material (it’s hard to compete with crushed coconuts).

Nanometer-Scale Magnet Makes Tiny, Powerful MRI

The trend in making more powerful magnetic resonance imagining (MRI) devices has been to produce larger magnets. A European consortium, for example, is building what will be the most powerful MRI, capable of producing a field of 11.75 teslas using a superconducting magnet strong enough to lift a 60-metric-ton battle tank.

However, researchers at Harvard University have gone in the opposite direction and built a device with a magnet only 20 nanometers across, or approximately 1/300th the size of a red blood cell. Despite its small size, the researchers claim that the magnet can produce a magnetic field gradient 100 000 times larger than even the most powerful conventional systems.

The trick is that this nanoscale magnet can be brought within nanometers of the object being imaged to produce a spatial resolution down to the nanoscale. Most hospital MRI scanners can only reach a spatial resolution of 1 millimeter. With this capability, the Harvard researchers someday hope to produce detailed images of individual molecules.

“What we’ve done, essentially, is to take a conventional MRI and miniaturize it,” said Amir Yacoby, professor of physics in a press release. “Functionally, it operates in the same way, but in doing that, we’ve had to change some of the components, and that has enabled us to achieve far greater resolution than conventional systems.”

In research published in the journal Nature Nanotechnology (“Subnanometre resolution in three-dimensional magnetic resonance imaging of individual dark spins”), Yacoby and his colleagues used a combination of their nanoscale magnet with a bit of quantum computing.

First, the quantum computing part: The Harvard team milled lab-grown diamonds into super fine tips and embedded an impurity into each, called a nitrogen vacancy (NV). This impurity acted as a quantum bit, or qubit, which is the key to the operation of quantum computers.

When the tip is scanned over the surface of a diamond crystal, the qubit interacts with the electrons on the surface of the crystal. It is these interactions that serve as the basis for images of the electrons spins. While making a quantum bit magnetometer sensitive enough to detect the spin of individual electrons was groundbreaking work in its own right, the distance between the qubit sensor and the object being imaged limited the system's spatial resolution.

To overcome this limitation, Yacoby and his colleagues brought the nanoscale magnet close to both the qubit sensor and the sample being examined. With this combination, the team was able to detect distribution of spins surrounding the sensor so that they were able to image the three-dimensional landscape of electronic spins at the diamond surface and achieve a spatial resolution of  0.8 nm laterally and 1.5 nm vertically.

“This is really a game of bringing both the magnet very close to generate large gradients, and bringing the detector very close to get larger signals,” Yacoby said. “It’s that combination that gives us both the spatial resolution and the detectability."

The researchers are looking to push the technique beyond the ability to image the individual spin of electrons in 3-D and make it capable of imaging components within a molecule, such as the nuclear spins of the atoms making up the molecule.

"This is by no means an easy task, since the nuclear spin generates a signal that is 1/1000th that of the electron spin … but that’s where we’re headed,” said Yacoby.



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
New York City
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