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Nanoparticle May Drive Down Price of Photovoltaics

There is still a school of thought in the world photovoltaics that says if you can make solar cells cheaply, it will result in widespread use of solar power. Despite that fervent belief, photovoltaics really benefit from achieving a balance between cheap manufacturing costs and high energy-conversion efficiency.

Unfortunately, it’s a struggle to strike that balance with current technology. If you want to use an inexpensive spray-on photovoltaic material made from nanoparticles, you might get to around 1 percent conversion efficiency—a figure so low that it’s not clear whether the amount of electricity generated is worth the effort. However, if you raise that figure to 10 percent, you could change the game.

While the energy conversion numbers are still not in yet, researchers at the University of Alberta in Canada believe they have created one of the cheapest nanoparticles yet developed for photovoltaics. It's so cheap to make because it's based on two of the most abundant elements: phosphorous and zinc.

In addition to their low price compared to elements like cadmium, phosphorus and zinc don't bog manufacturers down with the restrictions that come with lead-based nanoparticles.

The research, which was published in the journal ACS Nano (“Solution-Processed Zinc Phosphide (α-Zn3P2) Colloidal Semiconducting Nanocrystals for Thin Film Photovoltaic Applications”), took years to complete. But all the hard work may pay huge dividends since material seems to lend itself to a variety of manufacturing processes, including roll-to-roll printing or spray coating.

“Nanoparticle-based ‘inks’ could be used to literally paint or print solar cells or precise compositions,” said Jillian Buriak, a professor at the University of Alberta, in a press release. In fact, it is this spray coating method that Buriak and her colleagues are experimenting with to determine the energy conversion efficiency levels.

As we await those final numbers, the team has applied for a provisional patent and has already secured some funding to scale up the process for manufacturing. Whether the energy conversion efficiency of the nanoparticles can reach somewhere around the 10-percent mark may determine whether there’s a market to ramp up for.

Photo: University of Alberta


Researchers Publish Cookbook for Carbon Nanotubes

Back before graphene became the favored child of the nanomaterial family, carbon nanotubes held the mantle of the “wonder material” that would replace silicon. But a succession of problems with applying carbon nanotubes to electronics led to it losing favor.

Putting them where you wanted them and connecting them was exceedingly difficult. But a perhaps more stubborn obstacle has been the difficulty of controlling their purity and quality. While all sorts of ingenious methods have been developed over the years for working around these problems, just accepting that we could never produce a set of pure carbon nanotubes and proceeding from there didn’t seem to be a satisfactory solution.

Now researchers at the University of Southern California (USC) claim to have developed a method for producing carbon nanotubes with specific and predictable atomic structures.

“We are solving a fundamental problem of the carbon nanotube,” Chongwu Zhou, a USC professor and corresponding author of the paper, said in a press release. “To be able to control the atomic structure, or chirality, of nanotubes has basically been our dream.”

The researchers, who published their work in the journal Nano Letters (“Chirality-Dependent Vapor-Phase Epitaxial Growth and Termination of Single-Wall Carbon Nanotubes”), found that if they used chirality-pure, short carbon nanotubes as “seeds,” they could essentially clone duplicates using vapor-phase epitaxial growth.

The group actually developed this growth technique last year; the latest wrinkle reported in the paper is a set of recipes for building carbon nanotubes with specific atomic structures. And having a recipe means at least one thing: the process can be repeated if you follow the instructions.

“We identify the mechanisms required for mass amplification of nanotubes,” said co-lead author Jia Liu in a press release.

Bilu Liu, another of the authors, added: “Previously it was very difficult to control the chirality, or atomic structure, of nanotubes, particularly when using metal nanoparticles. The structures may look quite similar, but the properties are very different. In this paper we decode the atomic structure of nanotubes and show how to control precisely that atomic structure.”

Zhou says that the next step will be to scale up the process.  He adds in the release: “Our method can revolutionize the field and significantly push forward the real applications of nanotube in many fields.”

Whether this new development can bring carbon nanotube research back into favor for electronic applications, after years of focus and attention being lavished on graphene, remains to be seen. Working in carbon nanotubes' favor—that is, if this work can be scaled up—is the fact that researchers looking to endow electronics with graphene's amazing characteristics are not having such an easy time of it .

Image: Chongwu Zhou and Jia Liu


For First Time Graphene and Metal Make Super Strong Composite

One of the characteristics of graphene that is often mentioned but seldom exploited is its strength compared to other materials. Its tensile strength has been measured at 130 GigaPascals, making it 200 times as strong as steel.

Now researchers at the Korean Advanced Institute of Science and Technology (KAIST) have put graphene’s tensile strength to work by using it in a composite consisting of copper and nickel. The graphene makes the copper 500 times as strong as it would be on its own and the nickel 180 times as strong.

This work is a significant breakthrough, since previous attempts to use graphene in a metal composite have not resulted in increased strength in the doped material. In the KAIST research, which was published in the journal Nature Communications ("Strengthening effect of single-atomic-layer graphene in metal–graphene nanolayered composites"), chemical vapor deposition (CVD) was used to grow a single layer of graphene on a metallic deposited substrate, and then another metal layer was deposited. These steps were repeated, resulting in a multilayer metal-graphene composite material.

According to the researchers, this work represents the first time a metal-graphene multilayer composite material has been successfully produced that exploits graphene's extraordinary strength.

“The result is astounding as 0.00004% in weight of graphene increased the strength of the materials by hundreds of times,” said Professor Seung Min Han in a press release. “Improvements based on this success, especially enabling mass production with roll-to-roll process or metal sintering process, in the production of automobile and spacecraft lightweight, ultra-high strength parts may become possible.”

If the process can be duplicated on an industrial scale, it would indeed be a possible way to make automobiles and aircraft lighter and therefore more fuel efficient. We’ve already seen how a nanocoating on aircraft that reduces fuel consumption by just 2 percent resulted in a whopping US $22 million in savings per year for just one airline.

This graphene-metal composite, according to Han, can also be used as a coating in nuclear reactor construction or other structural applications.

Image: iStockphoto


Researchers Pinpoint 'Triple Point' in Vanadium Dioxide

Vanadium dioxide (VO2) has both fascinated and vexed researchers ever since it was discovered at Bell Laboratories back in 1959. Because of strong interactions between its electrons, it is one of the few known materials that can switch rapidly between being an electrical insulator and a conductor—a phenomenon known as metal-insulator transition.

This quick change ability led to the material being used in devices such the first Mott transistor.  But recently, vanadium seemed to lose some of the luster that came from anticipation of it being the next-generation transistor material. While it could still eventually be that wonder material, it was revealed that physicists knew a lot less about how it operated than they had previously thought.

To overcome at least some of that knowledge gap, researchers at the University of Washington have observed the exact point at which three solid phases of VO2—the so-called triple point—can coexist stably.

The research, which was published in the journal Nature (“Measurement of a solid-state triple point at the metal–insulator transition in VO2”), represents the first time that a triple point has been accurately determined for any substance.

“These solid-state triple points are fiendishly difficult to study, essentially because the different shapes of the solid phases makes it hard for them to match up happily at their interfaces,” said David Cobden, a University of Washington physics professor, in a press release.

It has been known for the last 30 years that VO2 has two slightly different insulator phases. The University of Washington research revealed that these two phases and its conducting phase can coexist stably at 65 degrees Celsius (149 °F).

Cobden and his colleagues initially tried to pinpoint this temperature by pulling at VO2 nanowires while observing them under a microscope. But as it so happened, VO2 surprised the researchers: The triple point revealed itself when the nanowires were neither pulled nor compressed but when they were left alone.

After spending years developing the apparatus for conducting the experiments, and then carrying them out, the researchers began to realize that VO2’s triple point was key to its fascinating phase transition properties.

“If you don’t know the triple point, you don’t know the basic facts about this phase transition,” Cobden says in the release. “You will never be able to make use of the transition unless you understand it better.”

Whether this understanding will win back some of luster that the material has lost of late remains to be seen. But this work does open the possibility that researchers' understanding of vanadium's properties will yield tangible benefits in the future.

Image: David Cobden/University of Washington


Artificial Photosynthesis for Splitting Water Reaches One-Volt Milestone

Almost 18 months ago, HyperSolar, Inc., a company based in Santa Barbara, Calif., unveiled its ambitious plans for artificial photosynthesis. The announcement promised “the world’s first nanotechnology-based, zero-carbon process for the production of renewable hydrogen and natural gas.”  The company was so confident that, two months later, it promised to publicly chronicle its progress toward the lofty goal.

Other companies have tried—and failed—to find an artificial photosynthesis process that can split water into hydrogen and oxygen and do it economically. Around the same time that HyperSolar promised to chronicle their efforts, Sun Catalytix determined that its process for splitting water just didn’t make economic sense when they crunched the numbers.

In the journal Nature, the maker of Sun Catalytix’s prototype explained the dreary economics:

“Hydrogen from a solar panel and electrolysis unit can currently be made for about US$7 per kilogram, the firm estimates; the artificial leaf would come in at US$6.50. (It costs just $1-2 to make a kilogram of hydrogen from fossil fuels.) With the prices of solar cells dropping all the time, the firm is not going to make a heavy investment that's unlikely to pay off. Instead, it is looking at cheaper designs—but these require yet-to-be-invented semiconductor materials.”

When I didn’t hear any updates  from HyperSolar on how its grand project was progressing this year, I began to suspect that they had encountered these same numbers and had a second thought. I guess I was wrong—the company just sent out a press release detailing its success in achieving the 1-volt milestone. To date inexpensive silicon solar cells are the most inexpensive and abundant, but their 0.7 volt per cell is not enough to split water.

HyperSolar's 1 volt per cell is still short of the 1.5 volts realistically needed to split water into hydrogen and oxygen, but the 1 volt number does represent a steady progression, according to the company. HyperSolar managed to increase the voltage from 0.2 volts eight months ago to .75 volts three months ago. At that rate of increase, one could expect that the company could achieve the 1.5 volts in another six months.

“Our cutting-edge research program at the University of California Santa Barbara led by Dr. Syed Mubeen Hussaini continues to make impressive progress,” stated Tim Young, CEO of HyperSolar, in the press release. “The 1.0 volt milestone is very exciting in that it provides us with a clear and encouraging roadmap to reach the 1.5 volts needed for water splitting. The semi-conductor materials used are very inexpensive, which gives us confidence that a low cost system is possible. The process to make this novel solar cell is equally exciting in that it is a simple solutions-based chemistry process. It does not require conventional expensive semiconductor processes and facilities. It was literally made in a beaker.”

The key to HyperSolar’s proposed technology will be a nanoparticle made from low-cost semiconductor materials. In the company’s original roadmap for milestones,  they were supposed to design this nanoparticle within the first year. It’s not clear from this press release whether that design has been settled upon, except to explain that each nanoparticle: “is a complete hydrogen generator that contains a novel high voltage solar cell bonded to chemical catalysts by a proprietary encapsulation coating.”

It is interesting that the company is providing key metrics for determining whether the system will work, but it would be good to have a better sense of where they are in developing the nanoparticle that forms the basis of this technology.

Image: HyperSolar


Nanoislands Simplify Structure of Resistive Memory Devices

The heralding of the memristor, or resistive memory, and the long-anticipated demise of flash memory have both been tracking on opposite trajectories with resistive memory expected to displace flash ever since the memristor was first discovered by Stanley Williams' group at Hewlett Packard in 2008.

The memristor has been on a rapid development track ever since and has been promised to be commercially available as early as 2014, enabling 10 times greater embedded memory for mobile devices than currently available.

The obsolescence of flash memory at the hands of the latest nanotechnology has been predicted for longer than the commercial introduction of the memristor. But just at the moment it appears it’s going to reach its limits in storage capacity along comes a new way to push its capabilities to new heights, sometimes thanks to a nanomaterial like graphene.

Will resistive memory displace flash memory in mobile device applications? Researchers at the University of California, Riverside Bourns College of Engineering, believe they have developed a new structure for resistive memory devices that could make the manufacturing of resistive memory easier and possibly ring the death knell for flash memory in mobile devices.

The research, which was published in the journal Scientific Reports (“Multimode Resistive Switching in Single ZnO Nanoisland System”), examined the typical structure of resistive memory devices, which involves a metal-oxide-metal structure combined with a selector device. The UC Riverside team reimagined that structure and demonstrated that one made from self-assembly zinc oxide nanoislands on a silicon substrate. This structure eliminates the need for a selector device, which is usually a diode.

“This is a significant step as the electronics industry is considering wide-scale adoption of resistive memory as an alternative for flash memory,” said Jianlin Liu, a professor of electrical engineering at UC Riverside, in a press release. “It really simplifies the process and lowers the fabrication cost.”

While this structure will not likely be incorporated into the initial commercially available resistive memory devices, it could possibly provide a design for next generation devices.

Photo: The University of California, Riverside


A Simple Twist Changes Graphene's Fate

Ever since graphene was first fabricated, research focused on how it might be applied to electronics to serve as an alternative to silicon and meet the crushing demands of Moore’s Law. The problem was that graphene does not possess an inherent band gap. While it can conduct electrons faster than other materials, there is no way to stop that flow of electrons, making it useless for electronic applications that require an on/off capability.

The research community was undeterred: Surely there could be a way to engineer such a band gap into the material. Eventually, a method to do this was proposed. But the engineered band gap didn’t always operate the way the researchers expected; the flow of electrons was not fully halted.

Now, researchers at the Lawrence Berkeley National Laboratory in California and the Fritz Haber Institute in Berlin have discovered why these engineered band gaps in graphene don’t measure up to expectations. It turns out that when monolayers of graphene are stacked to create bilayers, they are slightly misaligned, resulting in a twist that changes the bilayer graphene’s electronic properties.

“The introduction of the twist generates a completely new electronic structure in the bilayer graphene that produces massive and massless Dirac fermions,” Aaron Bostwick, a scientist at Berkeley Lab’s Advanced Light Source (ALS), said in a press release. “The massless Dirac fermion branch produced by this new structure prevents bilayer graphene from becoming fully insulating even under a very strong electric field. This explains why bilayer graphene has not lived up to theoretical predictions in actual devices that were based on perfect or untwisted bilayer graphene.”

Massless Dirac fermions are essentially electrons that act as if they were photons. As a result, they are not restricted by the same band gap constraints as conventional electrons.

The research, which was published in the journal Nature Materials (“Coexisting massive and massless Dirac fermions in symmetry-broken bilayer graphene”), employed angle-resolved photoemission spectroscopy (ARPES) and Beamline 7.0.1 techniques to resolve the electronic spectrum given off by the twist in the bilayer graphene.

“The spectrum we observed was very different from what has been assumed and contains extra branches consisting of massless Dirac fermions," Eli Rotenberg, who oversees the research at ALS Beamline 7.0.1, said in the press release. “These new massless Dirac fermions move in a completely unexpected way governed by the symmetry twisted layers.”

Of course, the researchers have merely identified the problem; they haven’t yet solved it. But figuring out the source of the problem is the first fundamental step.

As lead author of the research, Keun Su Kim of the Fritz Haber Institute in Berlin, noted in the press release: “Now that we understand the problem, we can search for solutions. For example, we can try to develop fabrication techniques that minimize the twist effects, or reduce the size of the bilayer graphene we make so that we have a better chance of producing locally pure material.”

Photo: Roy Kaltschmidt


Nanowires Give Off Light Under Pressure

Nanomaterials have offered the tantalizing possibility of lifelike artificial skin. Now researchers at the Georgia Institute of Technology have developed a use for zinc-oxide (ZnO) nanowires to serve as tiny LEDs whose emission intensity is dependent on the local strain put on them.

The Georgia Tech researchers believe that this work offers a new approach to imaging force and could lead to a new approach for human-machine interfaces.

“You can write with your pen and the sensor will optically detect what you write at high resolution and with a very fast response rate,” said Zhong Lin Wang, Regents’ professor and Hightower Chair in the School of Materials Science and Engineering at Georgia Tech, in a press release. “This is a new principle for imaging force that uses parallel detection and avoids many of the complications of existing pressure sensors.”

The research, which was published in the journal Nature Photonics ("High resolution electroluminescent imaging of pressure distribution using a piezoelectric nanowire-LED array"), builds on Wang’s previous work in applying ZnO nanowires to uses that can exploit their piezoelectric properties.

The ZnO nanowires used in this latest device exploit a phenomenon Wang has dubbed piezo-phototronics because they operate on the same principal as piezoelectric materials, but emit different light intensities based on the level of pressure applied to them.

“When you have a zinc oxide nanowire under strain, you create a piezoelectric charge at both ends which forms a piezoelectric potential,” Wang explained in the press release. “The presence of the potential distorts the band structure in the wire, causing electrons to remain in the p-n junction longer and enhancing the efficiency of the LED.”

Wang and his team fabricated the devices by growing the nanowires on a gallium nitride thin film substrate with the c-axis pointing upward. A polymer is then added, filling the space between the nanowires. A nickel-gold electrode is then attached to the gallium-nitride film to form an ohmic contact, and a transparent indium-tin oxide (ITO) film is deposited on top of the array to serve as a common electrode.

When pressure is applied to the device, some of the nanowires are compressed along their axial directions, resulting in a negative piezo-potential; the uncompressed nanowires have no potential.

The researchers demonstrated that when they used an ordinary pen to write on the material, light is emitted from the bottom of the material that corresponds to the letter written on the top.

“The response time is fast, and you can read a million pixels in a microsecond,” said Wang. “When the light emission is created, it can be detected immediately with the optical fiber.”

While the light turning on and off is completed in a mere 90 milliseconds, Wang believes that the spatial resolution, which currently stands at 2.7 micrometers, can be improved. He thinks the team can achieve this by reducing the diameter of the nanowires, making it possible to fit more of them into a given area.

Photo: Georgia Institute of Technology


Carbon Nanotubes Could Solve Problems With Silicon in Li-ion Batteries

Replacing graphite in the anodes of lithium-ion (Li-ion) batteries with silicon, or variations of nanostructured silicon, in order to boost the time between charges has been the main focus of research for years now.  I have been covering these developments for the last six years because of my interest in having a smart phone that can manage to get through half a day without needing a recharge. 

The big buzz of late has been the use of graphene in the anodes of Li-ion batteries, with the aim of powering all-electric vehicles.

Researchers at North Carolina State University have not only returned to our former favorite nanomaterial, carbon nanotubes, for solving the problems of replacing graphite with silicon, but have also taken the sensible approach of making mobile devices the target application for the improved batteries.  Okay, they do mention electric vehicles in the press release announcing the results, but I can at least hope that they are swayed by the indications that Li-ion batteries, no matter how good we make them, will not be entirely effective in all-electric vehicle applications. And there has been no stronger indicator of this than the consumer marketplace, which has not been too friendly to attempts to make Li-ion batteries work for all-electric vehicles.

The researchers, who published their findings in the journal Advanced Materials (“Aligned Carbon Nanotube-Silicon Sheets: A Novel Nano-architecture for Flexible Lithium Ion Battery Electrodes”), used silicon as a coating on the carbon nanotubes (CNTs). The CNTs are aligned in one direction; this structure ensures controlled expansion of the silicon so that when it expands and contracts it’s less likely to start breaking apart.

“There’s a huge demand for batteries for cell phones and electric vehicles, which need higher energy capacity for longer driving distances between charges,” said Xiangwu Zhang, associate professor of textile engineering, chemistry and science at N.C. State, in a press release. “We believe this carbon nanotube scaffolding potentially has the ability to change the industry, although technical aspects will have to be worked out. The manufacturing process we’re using is scalable and could work well in commercial production.”

I know I am supposed to be impressed by waterproof mobile phones, but I would give up that feature in a second for a smart phone that lasted an entire day without recharging—maybe even two days.

Photo: Wiley-VCH Verlag/KGaA


Dye-Sensitized Solar Cells Produced Without Iodine

Researchers at the University of Basel have developed a method by which iodine is replaced in copper-based, dye-sensitized solar cells (DSSCs) (also known as the Grätzel cell) with more abundant and less expensive cobalt.

The replacement of iodine should make the future of DSSC more sustainable because its manufacture will no longer depend on a relatively scarce element.

“Iodine is a rare element, only present at a level of 450 parts per billion in the Earth, whereas cobalt is 50 times more abundant,” explained the Project Officer Biljana Bozic-Weber in a press release.

Perhaps more important than improving the sustainability of manufacturing DSSCs, the replacement of the iodine should lengthen the lifetime of DSSCs, which have been criticized for their short lifespans. Typically in copper-based DSSCs the copper reacts with the iodine in the electrolyte to create copper iodide, which degrades the DSSCs.

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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
Associate Editor, IEEE Spectrum
New York, NY
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