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Researchers Discover New Structure Inside Nanowires

Nanowires made from III-V semiconductors like indium gallium arsenide are having a bit of run of late. Yesterday, I reported on a new method for growing them on graphene.

Now researchers at the University of Cincinnati have discovered that a newly developed architecture for semiconductor nanowires has a hidden nook in which to find electrons and holes. The discovery opens up a new understanding of the fundamental physics of nanowires.

The research, which was published in the journal Nano Letters (“Optical, Structural, and Numerical Investigations of GaAs/AlGaAs Core–Multishell Nanowire Quantum Well Tubes”), involved a host of characterization and measurement techniques.

University of Cincinnati physics professors Howard Jackson and Leigh Smith, who together led the research, believe that applications of this new structure could range from solar cells to environmental sensors.

“This kind of structure in the gallium arsenide/aluminum gallium arsenide system had not been achieved before,” Jackson said in a press release. “It’s new in terms of where you find the electrons and holes, and spatially it’s a new structure.”

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Nanowires Grow Better on Graphene

In an attempt to grow nanowires on a graphene substrate, researchers at the University of Illinois may have stumbled upon a new paradigm for epitaxy (the growth of crystals on a susbstrate).

Some believe that developing new manufacturing methods for nanoscale devices—like epitaxy—may be more crucial to meeting the demands of next generation chips than creating new materials, especially when feature sizes start falling below three nanometers. So, the Illinois researchers' development of a new method of epitaxy may ultimately be more significant than creating a new material.

The research, which was published in the journal Nano Letters (“InxGa1–xAs Nanowire Growth on Graphene: van der Waals Epitaxy Induced Phase Segregation”), produced nanowires made from III-V compound semiconductors. Generally, III-V semiconductors like gallium arsenide don't integrate well with silicon, but  recently  it was discovered that when these materials were brought down to the nanoscale that they were compatible.

Researchers have previously combined two of these semiconductors in gaseous form so that they deposit themselves on a graphene substrate (a process known as metalorganic chemical vapor deposition, or MOCVD) and self assemble into ordered crystalline form. However, the Illinois research marks the first time three of the semiconductors have been mixed together in this way.

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Nanosilver Not Harmful to Water Supply

Nanosilver is the nanoparticle that has worried people the most, when it comes to human health. Consequently, it's also been the one most scrutinized.

In fact, the seemingly endless parade of research studies examining the risk of nanosilver led two Danish researchers to publish an exasperated article in the prestigious journal Nature last year entitled “When Enough is Enough.”

One of the researchers, Steffen Foss Hansen, remarked back then in an interview with Nanowerk, “Most of these questions—and possibly all of them—have already been addressed by no less than 18 review articles in scientific journals, the oldest dating back to 2008, plus at least seven more reviews and reports commissioned and/or funded by governments and other organizations" he said. "Many of these reviews and reports go through the same literature, cover the same ground and identify many of the same data gaps and research needs."

Not only had a great deal of research gone into the toxicology of nanosilver, but there appeared to be an entire regulatory framework—at least in the US—that could control the use of nanosilver in products.  As I later pointed out, nanosilver algaecides have been regulated under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) as a pesticide since 1954. FIFRA was originally administered by the U.S. Department of Agriculture but that task was transferred to EPA when it was founded by the Nixon Administration in 1970. So nanosilver has been regulated under FIFRA for nearly 60 years, and for around  forty of those years silver has been regulated under FIFRA by EPA.

Despite a large, thorough, and still-growing body of research addressing the toxicology issue, and a regulatory framework from which to control the substance, there has been lingering concern about the lifecycle of nanosilver in our water supply through wastewater.

Now researchers at the Swiss Federal Institute of Aquatic Science and Technology (Eawag) have looked at what happens to nanosilver when it goes down our drainpipe after it has been washed off products (primarily clothes, but also the washing machines) containing them. They followed it to wastewater treatment plants and then into the environment. It turns out there is barely any nanosilver by the time it returns to the water supply.

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New Nanorod Capacitor Has High Energy and High Power Density

Researchers at Michigan Technological University (MTU) believe they have developed a new approach that will give us the best of both worlds: the quick power bursts for which capacitors are known and the long run times we get from electrochemical batteries.

While nanotechnology has had a big impact on ultracapacitors (sometimes referred to as supercapacitors) for some time now, getting ultracapacitors to behave more like batteries has remained an important research focus. Ultracapacitors are still largely relegated to applications where quick bursts of energy are needed (this is often referred to as power density, or the ability to carry large amounts of current out of the capacitor). Applications that need long runtimes of continuous energy (often referred to as energy density) has continued to be the domain of electrochemical batteries.

The MTU researchers, led by Dennis Desheng Meng, were led to their new reduction-oxidation (redox) capacitor by reevaluating the long promising but often disappointing material, manganese dioxide. Manganese dioxide has been attractive in this application because it’s abundant and environmentally friendly. But it didn’t provide the power density of carbon-based physical capacitors.

Meng believed that if the manganese dioxide could be used in the form of nanorods then the material could possess the right attributes for it to compete with carbon.

Of course, he's not the first to try to use manganese dioxide nanorods for generating electricity. In one notable example, Yi Cui, Associate Professor of Materials Science and Engineering at Stanford University, had some success in using manganese dioxide in nanorod form in a technique known as pressure-retarded osmosis in which the difference in salinity between freshwater and saltwater can be exploited to generate electricity.

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Another Two-Dimensional Replacement for Silicon

Researchers at Purdue University’s Birck Nanotechnology Center are turning toward two-dimensional materials as the future hope for meeting the demands of Moore’s Law.

The entire computer chip industry has long been engaged in a struggle to make sure that CMOS chips--which have been the cornerstone of computers for a generation--continue to perform well despite the challenges commensurate with the ever decreasing feature sizes demanded by Moore’s Law.

All the players are fighting against the “fundamental limits” of these chips, which is the point at which it becomes impossible to stop the flow of electrons between the source and the drain even when it is switched off.

Intel changed the game two years ago when it introduced its 3-D dimensional transistor, essentially moving the gate that separates the drain and source into a perpendicular position with respect to the transistor plane. Some believe that this will satisfy Moore's Law's demands for the near future. But what happens when this latest architecture is no longer able to do the job?

The Purdue researchers believe that the answer is new materials beyond silicon. 

"We are going to reach the fundamental limits of silicon-based CMOS technology very soon, and that means novel materials must be found in order to continue scaling," said Saptarshi Das, one of the Purdue researchers, in a press release. "I don't think silicon can be replaced by a single material, but probably different materials will co-exist in a hybrid technology."

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Graphene-based Li-ion Anodes Go Commercial

The big story for nanotechnology in lithium-ion (Li-ion) batteries in the past year has been the demise of high-flying nanotech startups A123 Systems and Ener1.  Both of these companies had developed nanotech-based solutions for improving the Li-ion battery and both companies announced bankruptcies in the past year after having received millions of dollars in capital investment. That’s why much of the news of late surrounding nanotechnology and the Li-ion battery has been of the R&D variety.

In that vein, research into using graphene hybrids to replace graphite in Li-ion electrodes seems to have increased in the past year and there have been some significant breakthroughs to improve both manufacturability and performance.  But there have also been some commercial ventures in the past year that have licensed some of that research to make a go of using graphene in Li-ion batteries.

One such company is XG Sciences, Inc. based in Lansing, MI. It has launched its offering, which features a graphene-hybrid material for use in the anodes of Li-ion batteries. The company claims that the anodes will result in Li-ion batteries that have four times the capacity of today's conventional anode batteries.

The basis of the anodes is a material the company has dubbed xGnP. The material uses graphene platelets that stabilizes silicon particles into a nanostructured silicon.

Nanostructured silicon anodes have become an attractive alternative to graphite-based anodes, which have a relatively small charge capacity, and silicon, which by itself starts to crack and fall apart after just a few charge/discharge cycles.  Much of the work with nanomaterials in Li-ion anodes has been to develop a hybrid material that either combines various nanomaterials with silicon or manipulates the crystalline structure of silicon on the nanoscale.

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New Germanium-Based Material Could Replace Silicon for Electronics

The old adage “what goes around comes around” is now being applied in electronics. Before silicon ruled the roost as the electronics material of choice, the first transistors were fashioned out of germanium.

Now researchers at Ohio State University (OSU) are bringing germanium back to electronics in a way that they believe could displace silicon. To achieve its new role the researchers have manipulated the germanium down into a one-atom-thick material that gives it a two-dimensional structure not unlike graphene, thereby joining a growing list of 2-D materials targeted for electronic applications.

The researchers say that electrons conduct through their germanium-based material ten times faster than through silicon and five times faster than in traditional germanium.

Joshua Goldberger, assistant professor of chemistry at Ohio State, was attracted to the material because of the more than half century that has gone into characterizing and developing electronics around germanium, such as germanium MOSFETs.

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Will the Sequester Derail U.S. Nanotech R&D?

Analysts are still trying to determine the impact of the US budget sequestration, which went into effect on 1 March.The sequester—as it has been dubbed—requires across-the-board mandatory federal budgets cuts.

Some programs are already feeling the pinch and now one of the premier U.S. nanotechnology scientists, A. Paul Alivisatos, in comments he made at the National Meeting & Exposition of the American Chemical Society (ACS) in New Orleans this week expressed concern about the budget cuts imposed by the sequester on nanotechnology funding.

“The National Science Foundation announced that they will issue a thousand fewer new grants this year because of sequestration,” said Alivisatos, in an ACS press release. “What it means in practice is that an entire generation of early career scientists, some of our brightest and most promising scientists, will not have the funding to launch their careers and begin research properly, in the pathway that has established the United States as leader in nanotechnology research. It will be a setback, perhaps quite serious, for our international competitiveness in this key field.”

If Alivisatos is correct, this would be an unfortunate way to make cuts in federal funding of nanotechnology research. Of course, this has been the concern about the sequester from the time it was first proposed: It would make cuts indiscriminately, leaving wasteful programs largely intact while hacking into useful ones.

To give some context, US government funding of nanotechnology research through the National Nanotechnology Initiative (NNI) has been on a fairly steady yearly increase since 2001 (except for a brief hiccup in 2010),  cumulatively accounting for US $14 billion in spending between 2001-2011.

The timing is indeed unfortunate. Much of the initial funding was used to build out the laboratory infrastructure to support the new line of research. So far, this has mainly enriched the construction industry and the microscopy companies that equip these facilities, investments that inevitably take time to bear fruit in terms of nanotechnology research and products. Now that the funding can actually go to supporting research projects, the crunch is on and the new labs will likely run below their full capacity.

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Combining Nanowires and Memristors Could Lead to Brain-like Computing

For decades now, researchers have been trying to get computers to behave like artificial brains instead of merely binary data crunchers. One of the obstacles in creating this capability has been that computers are based on silicon CMOS chips rather than the dendrites and synapses found in the human brain. One of the drawbacks with silicon chips is that they lack what is known  as "plasticity" in which the brain's neurons adapt in order to learn and remember.

To overcome such limitations, nanotechnology has been offering alternatives to silicon chip architecture that will more closely resemble the human brain. DARPA’s SyNAPSE project is one example.

Now researchers at the Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) at Trinity College in Dublin are pursuing a new nanomaterial-based approach to neural networks that combines work in nanowires and memristors. The aim of the project, for which the researchers have just received a  €2.5 million research grant from the European Research Council (ERC), is to develop a new computing paradigm that mimics the neural networks of the human brain.  A video describing the CRANN research can be seen below.

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Any Mobile Device Can Display Glasses-Free 3-D Images With This Novel Plastic Film

Glasses-free 3D imaging on mobile devices is nothing new; the Nintendo 3DS has been around since 2010 impressing users with its 3D graphics sans spectacles. Since that breakthrough, ensuing generations have seen incremental improvements.

The latest development comes via a plastic film produced through nanoimprinting technology that enables mobile devices to display 3D content without using 3D glasses in both portrait and landscape mode. And because it is a film, it can be put on the screens of mobile devices with normal screens to enable them to produce 3D imaging.

The thin film technology was jointly developed at Temasek Polytechnic (TP) and A*STAR’s Institute of Materials Research and Engineering’s (IMRE), both located in Singapore. The start-up Nanoveu Pte Ltd. film will commercialize and market the technology.

The film is basically a lenticular lens, which is a series of tiny lens elements that direct light to each eye.  The nanoimprinting technology developed at IMRE makes it possible to create this type of lens on a plastic film.

“The filter is essentially a piece of plastic film with about half a million perfectly shaped lenses engineered onto its surface using IMRE’s proprietary nanoimprinting technology,” said Jaslyn Law, the IMRE scientist who worked with TP on the nanoimprinting R&D since 2010, in a press release.

The Singapore team apparently recognized that a stronger business model is to sell products rather than just a nanomaterial. So in addition to the plastic film, the researchers developed a few applications that will operate on both Apple iOS and Android. The applications will make it possible to convert 2D images into 3D content. They will also provide a software development kit to help game developers convert their existing games into 3D.

“The team’s expertise in both hardware and software development in 3D technology has enabled high quality 3D to be readily available to consumers,” said Mr. Frank Chan, the TP scientist who led the overall NRF-funded project, in the press release. “We have taken age-old lenticular lens technology that has been around for the last hundred years, modernized it, and patented it, using nanotechnology.”

Image: A*Star



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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