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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

Nanoparticles Emitted from 3D Printers Could Pose a Risk

The migration of 3D printer technology from factory settings to people’s homes has triggered mainstream excitement about the technology's potential. In fact, it has even turned gun control laws on their ear after it was demonstrated that you could build a fully functional firearm with one of these 3-D printers.

What buyers of these systems may not have considered is that operating them could expose them to toxic nanoparticles. The hobbyist may now have to think about this potential risk when operating a 3-D printer in a garage or other indoor environment because of research out of the Illinois Institute of Technology showing that commercially available desktop 3-D printers emitted potentially harmful nanoparticles that could accumulate in indoor environments.

The research, which was published in the journal Atmospheric Environment (“Ultrafine particle emissions from desktop 3D printers”), demonstrated that common, commercially available 3-D printers available for home use emitted between 20 and 200 billion ultrafine particles (UFPs) per minute. The UFPs are, by definition, nanoparticles because their diameters are no larger than 10 nanometers.

The wide size range for the UFPs is due to the different kinds of printing technology used. A 3-D printer that uses a lower temperature polylactic acid (PLA) feedstock emits about 20 billion UFPs per minute whereas a higher temperature acrylonitrile butadiene styrene (ABS) feedstock printer produces about 200 billion UFPs per minute.

Alarming as these figures may sound, these emission rates are about on par with what is generated from “cooking on a gas or electric stove, burning scented candles, operating laser printers, or even burning a cigarette,” according to the Phys.org story covering the research. It’s hard to imagine that candles and gas stoves need to be eliminated from people’s homes. So, what gives?

The real measurement of risk involves not only the level of exposure we have to a given material but the hazard associated with that material because of its unique properties. UFPs generated by the 3-D printers the researchers studied had varying degrees of toxicity, depending on the feedstock they used. While PLA-generated UFPs have actually been shown to be biocompatible with mammals, previous studies have demonstrated that thermal decomposition byproducts from ABS processing have toxic effects in mice and rats.

The researchers remain somewhat circumspect about how this risk should be addressed. Their main advice: the operator of a home 3-D printer should ensure that the place where the machine is stationed is adequately ventilated, thereby reducing the exposure to the UFPs. Nonetheless, they note that more research needs to be performed to evaluate the risk from these 3-D printers.

Photo: Illinois Institute of Technology

The Market for Nanomaterial Solutions for ITO Replacement Gets Crowded

With the introduction of Apple’s iPhone and then all the other smart phones, and then the introduction of Apple’s iPad followed by all the other tablets, touch screen displays have experienced enormous growth over the last six years. However, from the beginning of that growth, concern was developing about what could be done about the relatively scarce resource of indium-tin oxide (ITO) that these devices need to operate.

ITO is used as a transparent conductor to control display pixels. What was a clear challenge and concern for display manufacturers actually served as a new ray of hope for nanomaterial producers. Companies like Cambrios Technologies, which had been launched back in 2002 with the aim of getting man-made viruses to pattern inorganic materials for a host of electronic applications, finally saw an application that was driven by "market pull" rather than "technology push".

Cambrios now markets itself on its homepage as a leader in silver nanowire solutions for replacing ITO. While the technology is still described at times as “the use of genetically modified viruses to create transparent coatings made of silver nanowires for touch screen displays”, the genetically modified virus bit gets left off most of the marketing and instead is replaced with descriptions like this from the website: “Our proprietary nanostructured materials can be deposited using existing production equipment.” However it's marketed, Cambrios has become a player in the replacement of ITO with nanomaterials.

Cambrios is not alone as a provider of silver nanowire materials for replacing ITO, companies such as Blue Nano and Carestream Health are just a couple of the competitors offering this solution. But nanowire technology is not the only material that tackles the ITO replacement issue.

Cima Nanotech, which has spent the last 10 years in low-profile development of its self-assembling nanoparticle coating, announced earlier this year commercial-scale production of a transparent coating.

While Cima does have its coating technology already in commercial use for EMI shielding in laptops for rugged environments, I learned after speaking to the company’s CEO, Jon Brodd, that the firm is expecting to make announcements about some of the big display companies using their technology as an ITO replacement. A description of Cima’s self-assembling nanoparticle coating can be seen in the video below.

While nanowires and nanoparticles have gotten nearly a decade head start in becoming the much-needed ITO replacement, graphene is offering itself into this market, as well. Samsung has demonstrated a display based on graphene, but the material is still largely at a research stage at this point. While graphene does possess superior transmission performance characteristics over ITO and single-walled carbon nanotubes, less costly and more repeatable manufacturing process will need to be developed if it is really to compete in this marketplace.

One thing is for certain with all of these nanomaterial companies: selling a nanomaterial by itself is just not going to work. For a nanomaterial business to succeed it needs to develop the upstream product. It either can be a specially formulated emulsion used in the coating, or the coating itself, but it can be assured that selling only nanoparticles of any kind is largely a doomed business model.

It’s really difficult to say at this point which of these materials will be the ITO replacement of choice in touch screen displays. But one can say with some confidence that consumers will be getting a product with better performance and endurance characteristics at a likely lower price.

Photo: Merve Karahan/iStockphoto

IBM Demonstrates a Competitive Graphene Infrared Detector

Earlier this year, researchers at IBM’s Nanoscale Science and Technology group revealed some of the fundamental photoconductivity mechanisms of graphene.

The IBM researchers demonstrated that graphene can either be positive or negative depending on its gate bias. The positive is due to a photovoltaic effect and the negative is due to a bolometric effect.

The bolometric effect involves photo-generated carriers that, while propagating across graphene, emit quanta of lattice vibrations called phonons and thereby transfer their energy into the lattice. Heating up the lattice implies enhancing the electron-phonon scattering process and reducing the carrier’s mobility. The IBM researchers discovered this effect was dominant in the photo response of graphene and is what leads to the photocurrent flowing in the opposite direction of the source-drain current.

In new research, which was published both in Nature Communications (“Photocurrent in graphene harnessed by tunable intrinsic plasmons”) and Nature Photonics (“Damping pathways of mid-infrared plasmons in graphene nanostructures”), the IBM team has begun to explore ways to amplify this bolometric effect in graphene.

The research team, which includes Hugen Yan, Tony Low, Wenjuan Zhu, YanqingWu, Marcus Freitag, Xuesong Li, Francisco Guinea, Phaedon Avouris, and Fengnian Xia, began by first studying the fundamental property of plasmons in graphene metamaterials by purely optical methods, revealing important information about its dispersion and damping mechanisms. This knowledge guided them in their design of graphene photodetectors, leading to the first demonstration of a graphene infrared detector driven by intrinsic plasmons.

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Full Steam Ahead With Nanoscale Heating

Late last year, thermal-based solar systems were forever reimagined when researchers at Rice University developed a method of placing nanoparticles in water so that sunlight would heat them to create steam but not boil the water.

At the time, the research, which was funded by a Grand Challenges grant from the Bill and Melinda Gates Foundation, had a host of applications suggested for it, large and small.

The grandest ambition for the technology, that of using it to power the electrical grid with steam turbines, will have to wait, but in the meantime the Rice researchers have announced the development of a system aimed at sterilizing medical equipment, even in places where there is no electricity.

This latest follow-up research, which was published in the journal Proceedings of the National Academy of Sciences Early Edition (“Compact solar autoclave based on steam generation using broadband light-harvesting nanoparticles”), envisions a system that could serve a dual purpose of not only cleaning medical instruments but also sanitizing human waste.

“Sanitation and sterilization are enormous obstacles without reliable electricity,” said Naomi Halas, the director of Rice’s Laboratory for Nanophotonics (LANP) and lead researcher on the project, in a press release. “Solar steam’s efficiency at converting sunlight directly into steam opens up new possibilities for off-grid sterilization that simply aren’t available today.”

The technology can use a range of materials, including metallic and carbon nanoparticles, all of which absorb light. These nanoparticles are then dispersed into water, directing most of the energy into creating steam rather than heating up the water. The system already meets existing standards for medical sterilization.

Halas originally described this technology when it was first announced as: “We’re going from heating water on the macro scale to heating it at the nanoscale. Our particles are very small—even smaller than a wavelength of light—which means they have an extremely small surface area to dissipate heat. This intense heating allows us to generate steam locally, right at the surface of the particle, and the idea of generating steam locally is really counterintuitive.”

A video describing the technology in its application to sanitation can be viewed below.

“Sanitation technology isn’t glamorous, but it’s a matter of life and death for 2.5 billion people,” Halas said in the latest press release. “For this to really work, you need a technology that can be completely off-grid, that’s not that large, that functions relatively quickly, is easy to handle and doesn’t have dangerous components. Our Solar Steam system has all of that, and it’s the only technology we’ve seen that can completely sterilize waste. I can’t wait to see how it performs in the field.”

Photo: Jeff Fitlow

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Nanoclast

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

 
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Dexter Johnson
Madrid, Spain
 
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Rachel Courtland
Associate Editor, IEEE Spectrum
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