Nanoclast iconNanoclast

Gold Nanoparticles Make Molybdenum Disulfide Extra Special

While graphene has been on a nearly decade long surge of research, its two-dimensional (2D) rival molybdenum sulfide (MoS2) has enjoyed an equal rush of interest over a mere two-and-a-half years. Ever since researchers at Ecole Polytechnique Federale de Lausanne’s (EPFL) Laboratory showed it could be used in replacing silicon in transistors in early 2011, the competitive landscape for 2D materials has gotten a little crowded. Despite the growing competition between 2D materials, MoS2 still holds a special place among the competition because it can be used to further enable graphene or work on its own in the next generation of nanocircuits.

Now researchers Kansas State University have raised the prospects of MoS2 a little bit higher by combining it with gold nanoparticles. The researchers believe that the incorporation of gold nanoparticles with MoS2 will open greater possibilities for the material in diverse applications such as transistors and biochemical sensors.

The research, which was published in the journal NanoLetters ("Controlled, Defect-Guided, Metal-Nanoparticle Incorporation onto MoS2 via Chemical and Microwave Routes: Electrical, Thermal, and Structural Properties"), focused on the surface structure of MoS2. The team decided that MoS2's strong chemical bond with noble metals, like gold, could be an avenue for investigation.

They were not disappointed. They quickly discovered that once a bond had been established between the MoS2 and gold nanostructures, the bond behaved like a highly coupled gate capacitor. Following on this discovery, the Kansas State team was able to further enhance the transistor characteristics of MoS2 by manipulating it with the gold nanostructures.

"The spontaneous, highly capacitive, lattice-driven and thermally-controlled interfacing of noble metals on metal-dichalcogenide layers can be employed to regulate their carrier concentration, pseudo-mobility, transport-barriers and phonon-transport for future devices," Vikas Berry, a professor at Kansas State and a leader of the research, said in a press release (though it does stretch the bounds of credulity to imagine him actually speaking these words aloud without pausing numerous times for breath).

Among the transistor characteristics of MoS2 that the researchers were able to manipulate with the gold was its power requirements. The team also demonstrated a direct route for attaching electrodes to a MoS2 tunneling gate.

"The research will pave the way for atomically fusing layered heterostructures to leverage their capacitive interactions for next-generation electronics and photonics," Berry said. "For example, the gold nanoparticles can help launch 2-D plasmons on ultrathin materials, enabling their interference for plasmonic-logic devices."

In further research, the team intends to create more complex nanoscale structures with MoS2, leading to the building of logic devices and structures.

With MoS2 having the advantage of an inherent band gap—unlike graphene—and the recent flood of research that’s turning up new ways to work with, it may have a slight advantage over graphene at the moment for transistor applications.

Image: Vikas Berry

Dye-based Solar Cells Get Bump in Conversion Efficiency and Lifespan

As I pointed out this week, inexpensive photovoltaics are good, but inexpensive and efficient ones are much better. For years now, the dye-sensitized solar cell (DSSC) has been one of the least expensive photovoltaic devices on the market. But even one of the inventors of the technology conceded just a couple of years ago that there needed to be a big push to improve the energy conversion efficiency of the devices.

While new manufacturing techniques have recently been proposed that should further reduce the manufacturing costs associated with producing DSSCs, a bit of a bump up in conversion efficiency would perhaps be a more welcome development.

To meet this need, researchers at the KTH Royal Institute of Technology in Sweden have developed a method for making DSSCs that are not only are more efficient but longer lasting. The foundation of the improvement is a new, quasi-liquid, polymer-based electrolyte that increases the solar cells' voltage and current and lowers resistance between electrodes.

DSSCs are essentially a photochemical system in which a photo-sensitized anode and an electrolyte form a semiconductor. In their commercial incarnation, today's DSSCs consist of a porous layer of titanium dioxde (TiO2) nanoparticles that have been covered with a molecular dye that absorbs sunlight, and a platinum-based catalyst. The TiO2 , which is immersed in an electrolyte solution that acts as a conductor, is the device's anode; the platinum, which sits atop the electrolyte, is the cathode.

A more efficient DSSC would use a material like acetonitrile for the electrolyte. However, this material does not lend itself to the production of a stable solar cell that could be commercially marketed. Instead, a low-volatility solvent is typically used, but this comes at the price of being more viscous and impeding the flow of ions.

The novel quasi-liquid electrolyte that the KTH researchers have developed delivers the best of all worlds: overcoming the viscosity problem, improving the flow of electrons, and doing so at a much lower volatility than can be achieved with acetonitrile.

“We now have clear evidence that by adding [a special] ion-conducting polymer to the solar cell’s cobalt redox electrolyte, the transport of oxidized electrolytes is greatly enhanced,” said James Gardner, a professor of photoelectrochemistry at KTH, in a press release. “The fast transport increases solar cell efficiency by 20 percent.”

With conversion efficiencies for DSSCs already having already reached the 10 percent mark, this would boost efficiency to around 12 percent.

These are impressive numbers, but perhaps a more beneficial characteristic—at least for the economics of DSSCs—would be a longer lifespan. With this new electrolyte, DSSCs could have a longer lifespan, making it possible to amortize the cost of the initial installation over a greater period of time. It's not clear how much more life could be added to the DSSCs, but every bit counts.

Photo: David Callahan

 

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, 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

Advertisement

Nanoclast

IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

 
Editor
Dexter Johnson
Madrid, Spain
 
Contributor
Rachel Courtland
Associate Editor, IEEE Spectrum
New York, NY
Advertisement
Load More