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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.

Graphene You Can Whip Up In A Blender

The first graphene was made by pulling layers off of graphite using Scotch tape. Now, in keeping with the low-tech origins of the material, a team at Trinity College Dublin has found that it should be possible to make large quantities of the stuff by mixing up some graphite and stabilizing detergent with a blender.

The graphene produced in this manner isn't anything like the wafer-scale sheets of single-layer graphene that are being grown by Samsung, IBM and others for high-performance electronics. Instead, the blender-made variety consists of small flakes that are exfoliated off of bits of graphite and then separated out by centrifuge. But small-scale graphene has its place, the researchers say. Solutions of the stuff could be used in printed electronics and conductive coatings. The flakes could also be used as filler to boost the mechanical, thermal, or electrical properties of composite materials.

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Photoluminescent Nanoparticles Kill Cancer

In a case of serendipity, researchers at the University of Texas at Arlington started out trying to develop new security-related radiation detection and stumbled upon a potential breakthrough in cancer treatment.

In the research, which will be published in the August edition of the Journal of Biomedical Nanotechnology (“A New X-Ray Activated Nanoparticle Photosensitizer for Cancer Treatment”), Wei Chen, professor of physics at the UT Arlington, was exposing copper-cysteamine (Cu-Cy) nanoparticles to X-rays when he noticed an unusual luminescence over a time-lapse exposure. When he investigated the cause of the Cu-Cy nanoparticle luminescence, he realized that the particles were losing energy as they emitted singlet oxygen, a toxic byproduct that also happens to be used to in photodynamic cancer therapy to damage cancer cells.

“This new idea is simpler and better than previous photodynamic therapy methods. You don’t need as many steps. This material alone can do the job,” Chen, who is also leading federally-funded cancer research, said in a press release. “It is the most promising thing we have found in these cancer studies and we’ve been looking at this for a long time.”

Photodynamic therapy (PDT) is a technique in which some photosensitizer is introduced into tumor tissue. When exposed to light, the photosensizer produces singlet oxygen that kills the cancer cells. Two years ago, this blog covered research from Rice University related to a similar PDT technique in which gold nanoparticles were introduced near a tumor and then subjected to light. This light exposure to the cluster of nanoparticles created bubbles that burst, temporarily ripping open small pores in the cell membranes that allow drugs to penetrate. 

The problem with PDT techniques is that the light cannot penetrate very far into human tissue to activate the nanoparticles. Chen and his colleagues have discovered that Cu-Cy the nanoparticle's sensitivity to X-rays and the ability of X-rays to penetrate deeply into tissue means that the tumor can be deep inside tissue and still be effective. Another advantage of the Cu-Cy nanoparticle in combination with the X-rays is that no other photosensitizer needs to be used, making it convenient, efficient and cost-effective, according to the researchers.

Like some of the “theranostic” nanoparticles that are being developed, in which both therapeutic and diagnostic functions are combined into one nanoparticle, the Cu-Cy nanoparticle can serve both as treatment for cancer as well as for cell imaging.

In experiments carried out thus far, Chen and his team have used the Cu-Cy nanoparticle and X-ray combination to treat a tumor. The results over a 13-day period showed that the tumor had not grown at all with the treatment while a tumor that had not been treated grew three times in size.

The UT Arlington has applied for a patent on the technology. Meanwhile Chen and his team are working on ways to shrink the size of the Cu-Cy nanoparticles, which currently are around 250 nanometers, so that the tumor tissue can more easily absorb them. If they can reduce the nanoparticles down to dimensions below 200nm, this should improve cell uptake. Ideally, they would like to bring those dimensions down to around 50 to 100nm.

Nanomaterials Keep Pushing Lithium-Sulfur Battery Capabilities for EVs

Researchers at the Department of Energy's Pacific Northwest National Laboratory (PNNL) have developed a nanomaterial powder that can be added to the cathode of lithium-sulfur batteries to capture problematic polysulfides that usually cause them to fail after a few charges.

The nanomaterial powder is a metal organic framework (MOF) in which metal ions are coordinated with rigid organic molecules to form a porous material that can be one-, two-, or three-dimensional. The research paper was published in the journal Nano Letters.

"Lithium-sulfur batteries have the potential to power tomorrow's electric vehicles, but they need to last longer after each charge and be able to be repeatedly recharged," said materials chemist Jie Xiao at PNNL in a press release. "Our metal organic framework may offer a new way to make that happen."

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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
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