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Nanoparticles Combined with Light Reverses Rusting

Research out of the University of Michigan may have developed a method for reversing the process of rusting. But perhaps the more important potential for the development, which involves exposing copper nanoparticles to high-intensity light, will be its ability to create propylene oxide, a precursor for making many plastics.

Researchers have frequently been turning to nanomaterials to serve as catalysts for speeding up a number of industrial chemical processes—as well as other smaller scale reactions, such as those found in fuel cells—because the nanoscale of the particles creates more surface area and therefore makes them more reactive. It’s similar to cubes of sugar compared to granulated sugar. If you put two cubes of sugar on a table and then place the equivalent amount of the granulated version next to it, the latter would take up much more surface area on your table—and mix into your tea and coffee much more quickly.

For this reason, there’s been a fair amount of research into developing nanocatalysts that will improve the petrochemical process of producing plastic precursors. Just last year researchers at Utrecht University and Dow Chemical Company developed a nanocatalyst for creating the lower olefins ethylene and propylene at large petrochemical plants, both precursors in the production of plastics.

The work of the University of Michigan researchers was in this vein, but a little bit different. They were trying to develop a catalyst that would get propylene and oxygen to form propylene oxide in a direct reaction. Attempts to develop such a catalyst were falling into that unattainable-quest category of a “Holy Grail”. But sometimes the knights find the Holy Grail.

The researchers started by looking at the frustratingly promising use of metallic copper. While metallic copper did possess the electronic structure that would create the pathways for forming propylene oxide, it also tended to react with oxygen creating copper oxide, which is a poor catalyst.

To overcome this, the Michigan team structured the metallic copper into nanoparticles and dusted them with clear silica while combining them with propylene and oxygen gas. The results were encouraging, but still only capable of converting 20 percent of the gas into propylene oxide.

The eureka moment came when they exposed the reaction to high intensity light and suddenly they were able to convert 50 percent of the gas into propylene oxide.

"To our knowledge, this is the first time anyone has shown that light can be used to switch the oxidation state from an oxide to a metallic state," said Michigan's Marimuthu Andiappan in a press release.

It is unlikely that this research foretells of a future in which we can reverse the rust forming on our cars. However, it does promise to alter an array of industrial processes beyond propylene oxide production that involve changing the oxidation state, such as the production of compact discs or in electrochemical cells.

Photo: Joseph Xu/Michigan Engineering Communications & Marketing

Graphene Hybrid Material Comes to the Rescue of Li-ion Battery-Powered Vehicles

Researchers at Rice University believe a hybrid material they have developed combining vanadium oxide (VO2) and graphene could revitalize the use of lithium-ion (Li-ion) batteries for powering all-electric vehicles.

While Li-ion batteries for hybrid vehicles have enabled that car segment to grow rapidly over the years, the all-electric vehicle has languished as a niche market. This is in large part because Li-ion batteries just don’t have the charge life or short recharging capabilities for them to make sense for most people’s driving habits.  The demise of companies that have developed nanomaterials for Li-ion batteries in all-electric vehicles, like A123 Systems and Ener1, underscores just how difficult it has been to get Li-ion batteries to perform at levels necessary to make electric vehicles to take a stronger foothold in the market.

To address this shortcoming, Pulickel Ajayan, professor of engineering at Rice, and his team turned to the well-characterized use of VO2 for cathodes because of their high energy and power density. While vanadium pentoxide has been used in Li-ion batteries, oxides have not been so readily adopted because they have a low electrical conductivity that translates into slow charge and discharge rates.

Ajayan and his team overcame this problem by essentially baking graphene into the VO2, a process that imparted graphene’s high electrical conductivity into the ribbon-like hybrid material that makes up the cathodes. The graphene is able to pass its conductivity to the hybrid material even though the VO2 accounts for 84 percent of the cathode’s overall weight.

The challenge for the researchers was finding the right method for "baking" the graphene into the VO2. In a process described in the journal Nano Letters, the researchers suspended graphene oxide nanosheets along with vanadium pentoxide in water and then heated the suspension for hours in an autoclave. The result was that the vanadium pentoxide had been reduced into vanadium oxide and had taken the form of crystallized ribbons, and the graphene oxide had been reduced to graphene. When characterized, the VO2 ribbons had a web-like coating of graphene and were about 10 nanometers thick, 600 nanometers wide, and tens of micrometers in length.

"These ribbons were the building blocks of the three-dimensional architecture," said Shubin Yang, lead author of the research, in a press release. "This unique structure was favorable for the ultrafast diffusion of both lithium ions and electrons during charge and discharge processes. It was the key to the achievement of excellent electrochemical performance."

As far as performance, the cathodes are capable of holding 204 milliamp hours of energy per gram and remained stable after 200 cycles even at high temperatures (75 degrees Celsius).

"We think this is real progress in the development of cathode materials for high-power lithium-ion batteries," Ajayan said in the press release. "This is the direction battery research is going, not only for something with high energy density but also high power density. It’s somewhere between a battery and a supercapacitor."

Image: Rice University/Ajayan Group

Nanowires Could Enable Solar Cells to Surpass the Shockley-Queisser Limit

Researchers at the Nano-Science Center at the Niels Bohr Institut in Denmark and the Ecole Polytechnique Fédérale de Lausanne in Switzerland have developed a single nanowire prototype device that can concentrate sunlight up to 15 times its normal intensity.  The researchers believe that if the technology can be further developed, it could lead to photovoltaics (PVs) that can surpass what's known as the Shockley-Queisser limit.

The Shockley-Queisser limit has developed into a Holy-Grail quest for conversion efficiency of PVs.  As Hans J. Queisser commented on this blog: “Exactly on October 30, 1960, Shockley and I published this paper, which initially nobody quoted. Now, merely 50 years later, twice a week.”

As the term “limit” implies, the theory posited that only 33.7 percent of all the sun’s energy hitting a solar cell could be converted into electricity for solar cells with a single p-n junction.

Achieving, or even surpassing, the Shockley-Queisser limit would overcome one of the commercial problems PVs have faced in competing with fossil fuel energy: higher conversion efficiency. While PVs have seen their costs decrease by a factor of 20 between 1978 and 2008, the efficiencies have not risen quite as dramatically. Commercially available silicon crystal-based PVs are still stuck with conversion efficiencies only in the high teens.

Various nanomaterials have  promised both lower costs and higher efficiencies and in some cases the ability to surpass the Shockley-Queisser limit. However, some of these approaches have centered on the somewhat controversial ideas of electron multiplication and hot carrier cells. Electron multiplication involves making multiple electron-hole pairs for each incoming photon while with hot carrier cells the extra energy supplied by a photon that is usually lost as heat is exploited to make in higher-energy electrons which in turn leads to a higher voltage.

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Carbon Aerogel Supersponge Could Soak Up Oil Spills

Researchers in China claim to have produced the world’s lightest aerogel. The feather-like aerogel is synthesized from a combination of carbon nanotubes and graphene and weighs in at 0.16 milligrams per cubic centimeter, a sixth that of air.

Carbon nanotubes have been applied to the production of aerogels previously. However, instead of enabling an “invisibility cloak” as in previous research, the researchers at Zhejiang University in Hangzhou, China believe this aerogel, which they have dubbed “carbon aerogel," could be used as an environmental remediation tool for cleaning up oil spills.

While aerogels have long been proposed as a solution to cleaning up oil spills, actual commercial offerings of any nanotech-based method have been few and far between.

But it’s hard to dismiss the incredible capability of this latest aerogel to absorb organic solvents. Whereas current commercial oil-absorbent products are capable of soaking up to 10 times their own weight, this carbon aerogel is reported to be capable of absorbing 900 times its own weight. This translates into 1 gram of carbon aerogel absorbing 68.8 grams of organics per second, according to the researchers.

"Carbon aerogel is expected to play an important role in pollution control such as oil spill control, water purification and even air purification," said Professor Professor Gao Chao, one of the lead researchers in the project, in a press release.

The researchers have reported the development of their carbon aerogel in the journal Advanced Materials ("Multifunctional, Ultra-Flyweight, Synergistically Assembled Carbon Aerogels").

The Chinese scientists were able to reduce the weight of their aerogel to previous carbon-nanotube versions by using freeze-dried solutions to create the carbon aerogel. This eliminated any moisture that may have been on the carbon nanotubes and graphene, but still managed to maintain the characteristics that were needed for creating the aerogel.

In addition to reducing the weight of the aerogel, the freeze-dried approach lends itself more readily to mass production, according to Gao.

Despite improved avenues to mass production and significantly improved absorption capabilities, it’s easy to be skeptical about whether this technology will be available the next time there’s a catastrophic oil spill. Let’s hope commercialization efforts start sooner rather than later.

Photo:  Imaginechina/AP Photo

Graphene and Molybdenite Join Forces for a New Flash Memory

As noted here earlier this week, yet another attempt has been mounted to overthrow the non-volatile memory king: flash. It's easy to be skeptical; the landscape of non-volatile memory is littered with pretenders to the throne. One of the biggest reasons for this carnage has been flash memory's ability to consistently evolve into a more powerful memory storage medium than it had been originally.

Now researchers at École polytechnique fédérale de Lausanne (EPFL) in Switzerland have combined graphene, which has already been shown to be effective as a basis for flash memory, with molybdenum disulfide (MoSor molybdenite), which is developing into graphene’s biggest two-dimensional material rival, into a flash memory prototype with improved performance.

The research, which was published in the journal ACS Nano (“Nonvolatile Memory Cells Based on MoS2/Graphene Heterostructures”), builds on the work EPFL had done in using molybdenite to create a working transistor. Since the development of a working transistor, the EPFL team has continued to focus its attention on the two-dimensional material to explore its potential applications.

To demonstrate the versatility of molybdenite, the Swiss researchers have combined it with graphene to create a flash memory prototype that is at least theoretically capable of being faster and with greater power efficiency than conventional silicon designs.

"Combining these two materials enabled us to make great progress in miniaturization, and also using these transistors we can make flexible nanoelectronic devices," says Andras Kis, author of the study, in a press release.

The flash memory prototype was built around field-effect geometry that forms the basis of field-effect transistors (FETs) used in complementary metal-oxide semiconductor (CMOS) electronics. In this case, the Swiss researchers replaced the silicon that would make up the middle layer of the sandwich-like device with the molybdenite. Graphene electrodes reside beneath this layer of molybdenite to transmit electricity into the molybdenite. The top layer of the device is several layers of graphene, which capture electrical charge and thereby stores memory.

The researchers believe this architecture should make for a more efficient flash memory design. The graphene is a much better at conducting electricity than silicon and the molybdenite is more sensitive to charge because it is far thinner than silicon.

This is all pretty early-stage at this point, so it's understandable that reports on this research seem to lack any discussion of its potential for commercialization. It’s not clear that marrying graphene and molybdenum will keep flash memory the king of the hill, but at a minimum, the king may have enlisted some new allies.

Novel Nanostructures Give Boost to Phase Change Memory

Flash memory is that non-volatile (NV) memory that everyone believes is ripe to be displaced in mobile electronics. But instead it has remained remarkably resilient undergoing just enough incremental improvements over the years to maintain its dominant position in the field.

Among the alternatives attempting to mount a commercial challenge to flash is phase-change random access memory (PCRAM or PRAM).  PRAM operates by heating a material in the memory cell that switches between a conductive crystalline phase and a resistive amorphous phase, imparting the binary characteristic necessary for memory.

While Samsung has commercialized PRAM with a 512Mb device, its writing current has to be reduced by at least one-third if it is to find wide adoption in mobile electronics, according to Korean Advanced Institute of Science and Technology (KAIST). Now those researchers claim to have taken a significant step towards achieving this lower power consumption by employing a new type of nanostructure for the PRAM.

The material, which is described in the journal ACS Nano ("Self-Assembled Incorporation of Modulated Block Copolymer Nanostructures in Phase-Change Memory for Switching Power Reduction"), employs self-assembled block copolymer silica nanostructures.  Block copolymers are particularly attractive in this application because they can produce self-ordered arrays with sub-20 nm features through relatively simple spin-coating and plasma treatments.

The real payoff with the new material is that it has resulted in a five-fold decrease in the required writing current, which corresponds to a power reduction of five percent.

One strategy for reducing switching power consumption in PRAM has been by decreasing the size of the contact area between the heating layer of the PRAM and the actual phase-change materials. The KAIST researchers achieved this by incorporating the silica nanostructures on top of the phase-change material.

"This is a very good example that self-assembled, bottom-up nanotechnology can actually enhance the performance of electronic devices,” says Keun-Jae Lee, one of the lead KAIST researchers, in a press release. “We also achieved a significant power reduction through a simple process that is compatible with conventional device structures and existing lithography tools."

Whether this will be the feature that can help PRAM compete with flash is not clear. But one can imagine that KAIST's national neighbor--Samsung--will be intrigued by this latest development.

Illustration: KAIST

Fungi Feasting on Daguerreotypes Help Reveal Nanoparticles

It’s always a bit astonishing to discover that ancient technologies like the Lycurgus Cup achieved their seemingly miraculous effects because of nanoparticles. Strictly speaking, these examples are not “nanotechnology” because the engineers who developed them did not create them intentionally or even understand how they worked. Nevertheless, these examples underline how nanotechnologies can impact the products that are around us—in some cases for centuries.

The latest addition to this list of old technologies enabled by nanotechnology was the recent discovery that daguerreotypes are in fact made up of nanoparticles. The revelation occurred after curators at George Eastman House International Museum of Photography and Film in Rochester, NY, started to notice that several of the daguerreotypes in their museum were fading or had a white haze that obscured the images.

The preservation of these images is critical because daguerreotypes represent the first form of photography when Louis-Jacques-Mandé Daguerre invented it in 1839. In addition to their historical significance, the incredible resolution that can be achieved with these images could instruct future optical technologies.

When the conservators at the Eastman House were unable to explain why the degradation was happening, they called upon Nicholas Bigelow, a physicist at the University of Rochester to determine the cause.  Bigelow confirmed earlier speculation that the damage to the images was being caused by fungi interacting with the surface of the daguerreotypes.

After discovering the cause, the Eastman House conservators started to store the images in argon gas to keep them in an a sort-of suspended animation to prevent the fungi from spreading until a more permanent solution can be developed. A video of this project can be seen below:

What may be even more significant about the work that Bigelow and his fellow researchers conducted is that it has shed further insight into the self-assembly of nanoparticles. By increasing control over the self-assembly of nanoparticles, researchers are developing better cancer treatments and a new generation of electronics.

Bigelow and his research team employed an arsenal of microscopy tools, including a focused ion beam, a scanning electron microscope and a transmission electron microscope, to determine what lay beneath the nanoparticles. They discovered there were holes, pores, and cavities that were at least part of the problem with the degradation of the daguerreotypes. But what may be a bust for daguerreotypes could be a boon for other nanoparticle applications in areas such as medicine for medical capsules, according to Bigelow in the press release.

The truth is that despite the latest microscopy tools, the researchers at the University of Rochester still don’t fully understand the fundamental physics at work with the nanoparticles in the daguerreotypes. If they can solve that riddle, a more permanent solution to preserving the images and developing new applications for the nanoparticles could be reached.

“Nobody really robustly understands what’s happening, either to create the image or what’s happening as the image degrades,” says Brian McIntyre, a senior engineer on the project. “Understanding the fundamental chemistry and physics of the daguerreotype process is seminal to understanding how to preserve them.”

Researchers Cross the 'Valley of Death' in Nanocomposite Design

In the early days of nanotech commercialization, it was popular to add nanomaterials to composites-- like carbon nanotubes (CNTs) in bicycles— in order for them to be “stronger than steel”. But that was mostly just marketing copy. The CNTs merely replaced the resins that had previously filled out the material matrix. It was pretty unclear whether they actually imparted any of the capabilities that nanomaterials possessed, like strength and flexibility.

The issue has come to be known as the “Valley of Death” in composite material design. Essentially, materials scientists were finding that the extraordinary properties of nanowires were disappearing—the properties of an entire composite were limited by the properties of other materials found in the material’s matrix.

A couple of years back some joint research between industry and academia attempted to address this shortcoming and take advantage of nanomaterials' superior strength. It looks like it's finally happened.

Researchers at the University of Western Australia have found a way for composite materials to actually match the strength and flexibility of the nanowires that have been placed inside them.

"In a normal metal matrix-nanowire composite, when we pull the composite to a very high stress, the nanowires will experience a large elastic deformation of several percent,” explains Yinong Liu, a professor at the University of Western Australia, in a press release. “That is okay for the nanowires, but the normal metals that form the matrix cannot.  They can stretch elastically to no more than 1 per cent.  Beyond that, the matrix deforms plastically.”

In research published in the journal Science (“A Transforming Metal Nanocomposite with Large Elastic Strain, Low Modulus, and High Strength”), Liu and his colleagues discovered that the shape-memory alloy nickel-titanium (NiTi) had enough elasticity to be used with nanowires in a material matrix without losing the superior functionality of the nanowires.

"NiTi is a shape memory alloy, a fancy name but not totally new,” says Liu in the press release. “It is no stronger than other common metals but it has one special property that is its martensitic transformation. The transformation can produce a deformation compatible to the elastic deformation of the nanowires without plastic damage to the structure of the composite. This effectively gives the nanowires a chance to do their job, that is, to bear the high load and to be super strong. With this we have crossed the ‘Valley of Death'!”

The resulting composite has proved to be twice as strong as high-strength steels and it enjoys elastic strain limits that are 5 to 10 times greater than the best spring steels currently available.

One obvious application is in medical implants. More intriguingly, the material's high elastic strain levels could enable breakthroughs in electronics, optoelectronics, piezoelectrics, piezomagnetics, photocatalytics, and chemical sensing properties. Even better (from my perspective), it might lead to bike frames that actually benefit from the nanomaterial used in them.

Image: Shijie Hao/China University of Petroleum

Electrically Powered Nanolasers Capable of Being Operated at Room Temperature

Much of the research conducted to date with nanolasers has involved units powered by larger light sources rather than directly from electrical current.  While these light-powered nanolasers have been successfully operated at room temperature, the fact that they don't run on electricity makes them impractical for many electronic applications. For instance, in many embedded system designs adding an additional laser light source is just not feasible.

Nanolasers could serve as another key step in keeping Moore’s Law chugging along if they could run continuously at room temperature and be powered by an electric current instead of a beam of light. But heretofore, such lasers were prone to overheating and failure. Now researchers at Arizona State University (ASU) believe they have hit upon a design for an electrically powered nanolaser that has overcome that hurdle.

The laser, which was described in the journal Optics Express (“Record performance of electrical injection sub-wavelength metallic-cavity semiconductor lasers at room temperature”), is the culmination of nearly seven years of work by ASU professor Cun-Zheng Ning and his colleagues. 

“In terms of fundamental science, it shows for the first time that metal heating loss is not an insurmountable barrier for room-temperature operation of a metallic cavity nanolaser under electrical injection,” said Ning in a press release. “For a long time, many doubted if such operation is even possible at all.”

In this latest research, Ning and his team employed the same indium phosphide/indium gallium arsenide/indium phosphide (InP/InGaAs/InP) rectangular core and the same silicon nitride (SiN) insulating layer—encapsulated in a silver shell—that they used in an earlier iteration of the device that suffered from the heating problem. But this time, they adjusted the thickness of the SiN layer and refined the fabrication process.

“It is extremely challenging to get everything correct at the nanometer scale. At such a small scale, any fabrication error becomes relatively large, and there are many fabrication steps, each of which is rather complex,” Ning explains in the press release.

Other recent nanolaser designs had already shown that it's possible to operate them at room temperature if you carefully choose the driving laser that powers them. But the ASU team's breakthrough shows the way towards a nanolaser that could be operated with a simple battery rather than another complex laser light source. The impact of this capability could be felt across the entire field of electronics, with nanolasers that speed up computers, broaden Internet bandwidth and serve as light sources for computer-chip-based sensing technologies that are integral to embedded computing.

Image: K. Ding and C.Z. Ning/Arizona State University

Nanoparticle Carrying Bee Venom Could Prevent HIV Infection and Cure It

Nanoparticles are proving themselves effective carriers, delivering therapies within the human body.

Last year, nanoparticles carrying biological components proved effective as anti-viral treatments for Hepatitis C. And some years back, research also demonstrated that attaching short-interfering RNA (siRNA) molecules to a biodegradable polymer nanoparticle was effective in the treatment of sexually transmitted diseases, like the human immunodeficiency virus (HIV).

Now researchers at the Washington University in St. Louis have attacked HIV more directly. They have shown that a nanoparticle carrying bee venom effectively destroys HIV while leaving surrounding cells unharmed.

The research, which was published in the journal Antiviral Therapy ("Cytolytic nanoparticles attenuate HIV-1 infectivity"), employed a nanoparticle that had previously been abandoned when it proved ineffective for delivering oxygen to blood cells. But in its new role, carrying the toxin melittin, a poison found in bee venom, it is extremely effective at breaking down the essential structure of HIV.

“We are attacking an inherent physical property of HIV,” says Joshua L. Hood, MD, PhD, a research instructor in medicine at Washington University in a press release. “Theoretically, there isn’t any way for the virus to adapt to that. The virus has to have a protective coat, a double-layered membrane that covers the virus.”

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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
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