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Smallest and Fastest Nanomotor Could Advance Drug Delivery

Engineers at the University of Texas Austin have developed what they claim is the smallest, fastest spinning and longest lasting nanomotor built to date. The engineers have demonstrated that their nanomotor is capable of rotating for 15 continuous hours at a speed of 18 000 rpm. These performance figures are on an entirely different scale to those of similar nanomotors that run anywhere from 14 rpm to 500 rpm and have only rotated for a few seconds or minutes.

The nanomotors, which are described in the journal Nature Communications, were built in a bottom-up manufacturing technique in which nanowires serve as rotors, patterned nanomagnets are the bearings, and quadrupole microelectrodes act as stators.

The entire structure of the nanomotor is smaller than 1 square micrometer, which means it is conceivable that they could fit inside some living cells. The nanomotors are also capable of rapidly mixing and pumping biochemicals as well as moving through liquids.

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What Is the Status of U.S. Nanomanufacturing?

The U.S. Government Accountability Office (GAO) made public yesterday a report on the state of nanotechnology in the United States compared to the rest of the world.

The GAO prepared the report, "Nanomanufacturing and U.S. Competitiveness: Challenges and Opportunities," at the request of the U.S. Congress, and GAO chief scientist Timothy M. Persons presented its findings in a testimony before the Subcommittee on Research and Technology of the House of Representatives.

I was interviewed extensively by two GAO economists for the accompanying report "Nanomanufacturing: Emergence and Implications for U.S. Competitiveness, the Environment, and Human Health," where I shared background information on research I helped compile and write on global government funding of nanotechnology.

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Nanotubes and Graphene Foam Make Hybrid Energy Storage Device

A paper in the journal Science earlier this year suggested that the problem of nomenclature for energy storage devices—specifically, defining the difference between what is a supercapacitor and what is a pseudocapacitor—is beginning to hold back development in the field.

To confuse matters further, researchers out of University of California Riverside have now developed an energy storage device that they define as a hybrid between a supercapacitor and a pseudocapacitor, but they prefer to term simply a supercapacitor.

The research, which is published in the journal Nature Scientific Reports, used hydrous ruthenium oxide (RuO2) nanoparticles that were modified by carbon nanotubes (CNT) and graphene foam as the electrode material for the supercapacitor. They then put the electrodes in an aqueous electrolyte. The combination not only operated safely, but also provided more energy and power density than what today’s commercially available supercapacitors can give.

“Besides high energy and power density, the designed graphene foam electrode system also demonstrates a facile and scalable binder-free technique for preparing high energy supercapacitor electrodes,” said graduate student Wei Wang in a press release. “These promising properties mean that this design could be ideal for future energy storage applications.”

The graphene-and-nanotube hybrid foam was  dipped into a slurry of the RuO2 resulting in a few-layer-thick graphene foam architecture covered with hybrid networks of RuO2 nanoparticles and anchored nanotubes. This design merges the supercapacitor's high  conductivity and pseudocapacitor's high specific capacitance, according to the researchers.

“The resulting hybrid device enables enough electrolyte access to the active materials (CNT-RuO2 network layer),” Wang told the Nanoclast in an e-mail. “And at the same time, the embedded CNTs in the CNT-RuO2 network layer work as a conductive framework.”

Supercapacitors operate in the space between batteries and traditional capacitors when it comes to the metrics of energy density (the amount of energy stored per unit mass) and power density (the maximum amount of power that can be supplied per unit mass). Batteries can store a lot more energy than supercapacitors and capacitors can deliver power far more quickly than supercapacitors. However, since supercapacitors can deliver power fast and charge up quickly in comparison to batteries, they are attractive for many applications, like electric vehicles, where they could be charged up in minutes rather than hours. But compared to batteries supercapacitors have low energy density, so the aim has been to increase how much energy they can store. If supercapacitor energy density could be increased, they could potentially replace chemical-based batteries.

While nanomaterials have been touted as good alternatives to traditional activated carbon for supercapacitor electrodes, it’s not been clear that they can provide enough surface area to make the resulting devices match the energy density of lithium-ion (Li-ion) batteries. On average those batteries have a specific energy density of 200 Watt-hour/kilogram,  whereas today’s supercapacitors can get around 28 Wh/kg. In figures released by the UC Riverside researchers, they claimed a full cell energy density of their device of 39.28 Wh/kg. Whether this will be enough to really be a breakthrough for supercapacitors in all-electric vehicles remains to be seen.

Bottom-Up Manufacturing of Nanowires on Silicon Expands Its Capabilities

With each passing day, the limitations of silicon chips become increasingly pressing. The inexorable march of Moore’s Law to smaller and smaller dimensions in order to double the number of transistors on integrated circuits every two years is beginning to sound like the footsteps of an army of giants. Despite brilliant engineering twists and tweaks, eventually silicon is not going to be able to avoid getting crushed by this onslaught, at least not without some help from other materials.

The problems with silicon are not just limited to an inability to meet the demands put on it by the shrinking dimensions of chips. It’s also not particularly good at a number of things that would be nice capabilities. For instance, you can’t really get silicon to emit light for photonic applications, and silicon circuits can’t withstand temperatures beyond 250 degrees Celsius. This last limitation requires all sorts of complicated engineering of the circuits to account for its temperature sensitivity.

But now, researchers at the University of California Davis have found a way to combine silicon with nanowires to create circuits that not only will make possible smaller dimensions, but also emit light for optoelectronic applications and withstand high temperatures found in hostile environments such as inside a jet engine. 

"Silicon can't do everything," said Saif Islam, professor of electrical and computer engineering at UC Davis, in a press release. "In the foreseeable future, society will be dependent on a variety of sensors and control systems that operate in extreme environments, such as motor vehicles, boats, airplanes, terrestrial oil and ore extraction, rockets, spacecraft, and bodily implants.”

While combining silicon with non-silicon materials has offered more robust performance, it has remained a challenge to grow non-silicon materials as layers over silicon because of incompatibilities in the crystal structures and differences in thermal properties.

To overcome this limitation, Islam has grown nanopillars from gallium arsenide, gallium nitride, or indium phosphide on top of the silicon to serve as foundations on top of which nanowires are used to bridge the gap between the nanopillars.

"We can't grow films of these other materials on silicon, but we can grow them as nanowires," Islam explained in the release. In the video below, Islam explains how nanowires can be grown on silicon and can improve its capabilities.

In research, which was published in the journal Advanced Materials (“3D-Transistor Array Based on Horizontally Suspended Silicon Nano-bridges Grown via a Bottom-Up Technique”), Islam and his colleagues developed a bottom-up technique to create these nanowire bridges. They note that it does not require the tedious nanowire alignment and contact formation processes that are typically required to connect these nanowires.

This bottom-up approach led to the researchers being able to get the nanowires to operate like transistors and form the basis of more complex circuits. The material also proved to be responsive to light.

Perhaps most attractive to the semiconductor industry is that the technology does not require any significant changes to the manufacturing of silicon integrated circuits.

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.”



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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