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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

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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Nanoprinting Technique Enables High Energy Density Supercapacitors

Supercapacitors—alternatively known as ultracapacitors—are increasingly being considered as a viable alternative for powering many of the things that now depend on batteries. But it’s always been a trade off with supercapacitors. On the one hand, they can release a large amount of energy very quickly and can be rapidly recharged, but they are pretty poor in comparison to chemical-based batteries at storing large amounts of energy and discharging it over a long period.

Research has been intense to try and maintain supercapacitors' big bursts of energy and quick recharge while also managing to get them to perform more like batteries.

Now researchers at the University of Central Florida have developed a new nanoprinting technique that produces highly ordered nanoelectrodes without the need for templates or any expensive tools. The result appears to be a supercapacitor with a significantly higher energy storage capacity.

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Graphene's Discoverer Weighs In on Its Commercialization

Upon receiving the prestigious Copley medal from the Royal Society this week, Sir Andre Geim, the co-discoverer of graphene, said during an interview:

"I'm not interested in going into industry or property development or creating 'graphene valley' as the government would like me to. It's a bit silly for society to throw a little bit of money at something and expect it to change the world. Everything takes time."

Thank you, Professor Geim. Somebody had to say it.

Geim’s comments come in response to questions about the UK’s large financial commitment to making the country a “graphene hub" so there won't be a repeat of the UK supposedly losing its leadership role in nanotechnology. The UK government announced earlier this year that it plans to invest approximately US $71 million in a single research facility at the University of Manchester.

In the six years that I have been writing this blog, I have consistently said that attempts by regional governments to throw money at nanotechnology to create a so-called “Silicon Valley” of nanotechnology in their respective regions were largely misguided. To the extent that building facilities and attracting top-flight scientists to one specific region can lead to an economic boom, as some believe is possible or at least helpful, that positive movement is often offset by the lack of patience and ensuing frustration people succumb to when they don’t see immediate results.

By many estimates, the bulk of the commercial applications that will come from graphene won’t arrive until around 2020. With the large investments that are going into graphene now, and the long wait before those investments show return, one can’t help but think we’re headed for the same kind of disappointment many felt about carbon nanotubes when graphene started to emerge as a competitive form of carbon.

Geim also vents his frustration at the confusion that results when a new social media outlet is equated with actual technological breakthroughs. Geim characterizes social media sites as “utterly time wasting” and their developers as “making billions from a few lines of computer code."

This confusion between a new iPhone app and developing post-silicon electronics lies at the heart of people’s frustration with the development of nanotechnology. When they see a new app offered up every day, they wonder why nanotechnology hasn’t changed their lives, at least in ways that they recognize. It also demonstrates the inefficiency of technology investment when billions can go into a company whose long-term business model is still unclear, while the technologies that will maintain computer hardware technology to keep these social media sites available tend to languish, desperate to find any funding. Of course, from an investment perspective, you want to invest a little and get a big return and get that return back quickly.

Investment perspectives notwithstanding, Sir Andre Geim is right to point out the cognitive dissonance on display when all the investment goes into an industry that has little in the way of capital costs but ambiguous revenue streams, while nanotechnology, with its high capital costs but clear business model (selling stuff) goes unfunded. And then when nanomaterials fail to perform because of this lack of investment, everybody clamors, “What went wrong?” You don’t have to wait seven years from now to get your answer, Geim has already given it to you.

Photo: Friedrun Reinhold/Koerber-Stiftung/Reuters


Water-Enabled Lithography Creates Long Graphene Nanoribbons

James Tour and his lab at Rice University have been tinkering with graphene nanoribbons (GNR) since they started unzipping carbon nanotubes to create them back in 2009. In the ensuing four years, they have been hard at work developing applications for the material, such as increasing the storage capacity of Li-ion batteries.

Tour, along with two of his graduate students,  Vera Abramova and Alexander Slesarev, have developed a method for producing GNRs that ensures the creation of long nanowires of the material simply by using water.

The research, which was published in the journal ACS Nano, (“Meniscus-Mask Lithography for Narrow Graphene Nanoribbons”),  essentially uses water as the mask in a lithography process that—when followed by ion etching—cuts up graphene into nanoribbons. The process does not require any high-resolution lithography tools—just atmospheric water collected at the edge of a lithography pattern.

Under the influence of surface tension water is forced to curve, forming a meniscus. Because the meniscus serves as the mask for the lithography, the researchers have dubbed the process: meniscus-mask lithography (MML).

Tour believes that because this process can generate long graphene nanoribbons it should be of interest to anyone working in microelectronics.

“They can never take advantage of the smallest nanoscale devices if they can’t address them with a nanoscale wire,” Tour said in the press release covering the research. “Right now, manufacturers can make small features, or make big features and put them where they want them. But to have both has been difficult. To be able to pattern a line this thin right where you want it is a big deal because it permits you to take advantage of the smallness in size of nanoscale devices.”

In ironic twist, the water that most lithography processes try avoid and eliminate at great cost is the same water that makes this new lithography process work.

“There are big machines that are used in electronics research that are often heated to hundreds of degrees under ultrahigh vacuum to drive off all the water that adheres to the inside surfaces,” Tour added in the release. “Otherwise there’s always going to be a layer of water. In our experiments, water accumulates at the edge of the structure and protects the graphene from the reactive ion etching (RIE). So in our case, that residual water is the key to success.

This counter-intuitive use of water in a lithography process, as one might suspect, was developed when another method was not working out as hoped.

Tour’s graduate students, Abramova and Slesarev, had actually intended to duplicate another process for creating GNRs that had been developed at Rice. This method, which is also new, exploits the ability of certain metals to form a native oxide layer. This layer expands and protects the material at the edge of the metal mask.

After observing their results with this method, they discovered that some metals didn’t expand as much as others and others showed no expansion at all. Desperate to find something that would change their results, the researchers worked on the project for two years before they tested and developed their meniscus theory. They confirmed in that time that MML method produces sub-10-nanometer wires from different materials, including platinum.

In further research the aim is to gain better control over the width of the nanoribbons and to better refine the edges of the nanoribbons, because it's those edges that dictate the nanoribbon's electronic properties.

Credit: Tour Group/Rice University



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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