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Graphene-based Gas Membranes Promise Reduced Carbon Dioxide Emissions

Researchers at the University of Colorado Boulder have achieved the first experimental results of using graphene as a membrane to separate gases. While still a long way off from industrial use, the membranes do possess mechanical properties that should prove beneficial in  natural gas production. The ultra-thin, graphene-based membranes' highly selective pores increase flux through the membrane making the process more energy efficient, thereby reducing the plant's production of carbon dioxide.

The Univ. of Colorado research builds on work that was completed last year at Boulder that showed that graphene possessed extraordinary adhesion capabilities. That work demonstrated that if graphene were used in a multi-layer membrane, the adhesion between the layers of the membrane would be extremely strong. 

This most recent work also seems to go one step beyond research at MIT from earlier this year. In that work, scientists used computer simulation to show that nanoporous graphene could replace membrane materials currently used in the reverse osmosis water desalination processes. At the time, the MIT researchers said they expected that turning their computer models into real world membranes would be daunting. According to the researchers, getting the pores size precisely right would be difficult to do on a large scale.

While the Boulder researchers have not attempted to scale up their experimental results yet, they have managed to get the pore size correct on the graphene so it can separate a variety of gases.

The research, which was published in the journal Nature Nanotechnology (“Selective molecular sieving through porous graphene”),  was able to achieve its precisely sized pores by etching them into graphene sheets with a process involving ultraviolet light-induced oxidation. The resulting porous graphene was then tested on a range of gases including “hydrogen, carbon dioxide, argon, nitrogen, methane and sulphur hexaflouride -- which range in size from 0.29 to 0.49 nanometers.”

“These atomically thin, porous graphene membranes represent a new class of ideal molecular sieves, where gas transport occurs through pores which have a thickness and diameter on the atomic scale,” says Colorado mechanical engineering professor Scott Bunch  in a university press release.

The main technical challenge, according to the researchers, will be bringing these results up to an industrial scale. In particular, they will need to find a process by which they can create large enough sheets of graphene.  The researchers even concede that getting the pores precisely defined still needs further development.

Nanowires Show the Strain Limit of Silicon

Most nanotechnology developments targeted at electronics look ahead to a post-silicon world.  But silicon is still firmly with us and every attempt is being made to wring that last drop of capability out of the material, sometimes with the help of nanotechnology.

For the last decade, researchers have been pushing silicon's limits by straining it. Whether it be more recently the organic semiconductor variety, or just the run-of-the-mill, non-organic variety, strained silicon has been the mainstay of pushing silicon to the very edge of its capabilities. The question is how far can strained-silicon electronics take us?

Swiss researchers at Paul Scherrer Institute and the ETH Zurich may have an answer to that question. With their most recent research, they have strained silicon nanowires right up to their breaking point and still managed to integrate it into an electronic component.

Renato Minamisawa from the Paul Scherrer Institute describes the research in a press release as “"the strongest tension ever generated in silicon; probably even the strongest obtainable before the material breaks."

To accomplish this, the Swiss researchers have turned to the tried-and-true method of top-down manufacturing: etching a substrate with a silicon layer that is already under some strain. In the research, which was published in the journal Nature Communications (“Top-down fabricated silicon nanowires under tensile elastic strain up to 4.5%”), the Swiss team etched dumbbell-shaped bridges into the strained silicon, which exploit the phenomenon of strain accumulation mechanisms.

Basically, the silicon is initially strained in all directions. As you etch away the material into narrow bridges, the thin material left is pulled in just two directions. "Since all the force which was distributed over a larger area before the etching now has to concentrate in the wire, a high tension is created within it", says Minamisawa.

But just straining silicon to its maximum before it breaks is fairly straightforward process and not that noteworthy on its own.

"There is actually no magic behind building up tension in a wire - you just have to pull strongly on both ends", explains Hans Sigg of the Laboratory for Micro- and Nanotechnology at the Paul Scherrer Institute in the same press release. "The challenge is to implement such a wire in a stressed state into an electronic component."

In the method that the researchers developed, the thin wires that remain after the etching of the silicon layer are attached to the rest of the material only at its endpoints, and most importantly are perfectly uniform in dimension and strain. It would be possible to produce thousands of such wires all within a very precise strained state. "And it is even scalable, meaning that the wires can be fabricated as small as you want," Sigg points out.

Despite making every effort to ensure that the process is perfectly compatible with current fabrication methods and materials, the researchers seem curiously unconcerned whether the process ever makes it into industry. As Minamisawa notes about the silicon nanowires: "But even if they do not end up in microelectronic applications, our research could show what the limits of silicon electronics really are."

Silicon Oxide as Resistive Memory Goes Transparent

Since 2008, James Tours’ research group at Rice University has been championing its discovery that silicon oxide can form the basis of resistive memory. In the past four years, there have been a fair number of doubters who just couldn’t believe that silicon oxide could switch its resistance, but there has also been a growing body of research from other labs that corroborates that four-year old work.

Now Tour and his team--with Jun Yao at the fore—have pushed their research one step further with their latest paper in the journal Nature Communications (“Highly transparent nonvolatile resistive memory devices from silicon oxide and graphene”) in which they demonstrate a non-volatile, transparent memory chip resistant to both heat and radiation.

The core of the new memory chip is the use of silicon oxide that has been sandwiched between layers of graphene and placed on flexible plastic sheets. While the Rice researchers have been looking at the use of graphene in flexible displays of late, in that work nanowires were sandwiched between the layers of graphene. In this latest research, silicon oxide is sandwiched between the graphene layers.

In fact, it has been the Rice team’s nanowire-and-graphene research—and its dogged belief in silicon oxide’s ability to switch its resistance and serve as a resistive memory—that has made this latest development possible. In the 2010 paper that followed their 2008 discovery of silicon oxide’s capabilities, Yao observed that running a voltage through silicon oxide removed oxygen atoms from the material creating a 5-nanometer-wide channel, and turned it into pure silicon filaments.

When Yao reversed the voltage, he discovered that the silicon filaments could be broken and reformed, creating a “0” for a broken circuit and a “1” for the healed circuit—the basis of computer memory. Since then Yao has been attempting to build up evidence that the switching effect he witnessed was not a result of the breaking graphite but because of the underlying crystalline silicon.

“Jun was the first to recognize what he was seeing. Nobody believed him,” says Tour in the press release. “Jun quietly continued his work and stacked up evidence, eventually building a working device with no graphite.”

To quiet the doubters, who believed that the switching had to be due to some carbon in the system, Yao created a device that had no exposure to the carbon at all. The devices that the Rice team is producing now contain no metals except for the leads attached to the graphene electrodes.

Tour further notes: “Now we’re making these memories with about an 80 percent yield of working devices, which is pretty good for a non-industrial lab When you get these ideas into industries’ hands, they really sharpen it up.”

The history of nano-enabled, non-volatile memory's challenges to flash memory have proven to be less than successful in the past, but this could be an architecture that changes that trend.

New Study Indicates Nanoparticles Do Not Pass Through Skin

For at least the last several years, NGO’s like Friends of the Earth (FoE) have been leveraging preliminary studies that indicated that nanoparticles might pass right through our skin to call for a complete moratorium on the use of any nanomaterials in sunscreens and cosmetics.

Even if you’re a world-renowned expert on assessing the risk of nanomaterials, you had better not challenge the orthodoxy of that line of thinking.  Or if you’re an NGO that investigated the issue of nanoparticles in sunscreens and found no risk, you should be prepared to be so marginalized by other NGOs that you will hardly warrant a mention.

On the contrary, the prevailing argument was that if you’re a company using nanoparticles in your sunscreen you had to prove your product was safe, even if there was no conclusive evidence that your product was risky and a fair amount of evidence that it was safe. So frightening and compelling was this scare screed that in a poll of Australians they said they would prefer to risk getting skin cancer rather than use a sunscreen that might contain nanoparticles. Fear almost always wins out over reason.

Unfortunately for the fear mongers, the evidence is mounting that nanoparticles cannot penetrate the skin. Researchers at the University of Bath in the UK found that even the smallest nanoparticles are not capable of passing through the skin barrier.

The research, which was published in the Journal of Controlled Release (“Objective assessment of nanoparticle disposition in mammalian skin after topical exposure”),  employed laser scanning confocal microscopy to see whether fluorescently tagged polystyrene beads on the scale of 20 to 200 nanometers were absorbed into the skin.

“Previous studies have reached conflicting conclusions over whether nanoparticles can penetrate the skin or not,” says Professor Richard Guy from the University’s Department of Pharmacy & Pharmacology, in the press release. “Using confocal microscopy has allowed us to unambiguously visualize and objectively assess what happens to nanoparticles on an uneven skin surface. Whereas earlier work has suggested that nanoparticles appear to penetrate the skin, our results indicate that they may in fact have simply been deposited into a deep crease within the skin sample.”

This latest UK research certainly won’t put this issue to rest. These experiments will need to be repeated and the results duplicated. That’s how science works. We should not be jumping to any conclusions that this research proves nanoparticles are absolutely safe any more than we should be jumping to the conclusion that they are a risk. Science cuts both ways.

 

Graphene Replaces Traditional Silicon Substrates in Future Devices

Researchers at the Norwegian University of Science and Technology (NTNU) have patented and are commercializing a method by which gallium arsenide (GaAs) nanowires are grown on graphene.

The method, which was described and published in the journal Nano Letters (“Vertically Aligned GaAs Nanowires on Graphite and Few-Layer Graphene: Generic Model and Epitaxial Growth”), employs Molecular Beam Epitaxy (MBE) to grow the GaAs nanowires layer by layer. A video describing the process can be seen below.

"We do not see this as a new product," says Professor Helge Weman, a professor at NTNU's Department of Electronics and Telecommunications in the press release. "This is a template for a new production method for semiconductor devices. We expect solar cells and light emitting diodes to be first in line when future applications are planned."

Whether it is a method or a product, Weman and his colleagues have launched a new company called Crayonano that will be commercializing the hybrid material that the researchers developed.

The researchers contend that replacing traditional semiconductor materials as a substrate will reduce material costs. The silicon materials are fairly expensive and are usually over 500µm thick for 100mm wafers. As the video explains, using graphene reduces the substrate thickness to the width of one atom. Obviously reduction in material is really only a side benefit to the use of graphene. The real advantage is that the electrode is transparent and flexible, thus its targeting for solar cells and LEDs.

Interestingly Weman sees his team's work as a compliment to the work of companies like IBM that have used graphene “to make integrated circuits on 200-mm wafers coated with a continuous layer of the atom-thick material.”

Weman notes: "Companies like IBM and Samsung are driving this development in the search for a replacement for silicon in electronics as well as for new applications, such as flexible touch screens for mobile phones. Well, they need not wait any more. Our invention fits perfectly with the production machinery they already have. We make it easy for them to upgrade consumer electronics to a level where design has no limits."

As magnanimous as Weman’s invitation sounds, one can’t help but think it comes from concern. The prospect of a five-year-development period before a product gets to market might be somewhat worrying for a group of scientists who just launched a new startup. A nice licensing agreement from one of the big electronics companies must look appealing right about now.

Graphene Proves To Be One Hundred Times Better at Rustproofing Metals

While the wonder material graphene continues to come under pressure from other two-dimensional materials in electronics applications, it has continued to build up applications far afield from electronics.

One of the applications that has opened up over the last year is rustproofing. In May of this year, researchers from the University of Buffalo demonstrated that they could use graphene in rustproofing steel.

Now researchers at Monash University in Australia and Rice University in the USA have used graphene to rustproof copper.

The research, which was published in the journal Carbon (“Protecting copper from electrochemical degradation by graphene coating”), claims that the graphene-based coating renders copper nearly 100 times more resistant to corrosion than if left unprotected.

“We have obtained one of the best improvements that has been reported so far,” says study co-author Dr Mainak Majumder in the university press release. “At this point we are almost 100 times better than untreated copper. Other people are maybe five or six times better, so it’s a pretty big jump.”

To achieve the atomic-scale rustproof coating, the researchers simply heated the graphehe to temperatures between 800 and 900 degrees celcisus and then applied the graphene to the copper through chemical vapor deposition. Seeing whether they can apply the graphene coating at a lower temperature will be a focus for future research.

The University of Buffalo researchers explained that their research into rustproofing steel was in part motivated by a desire to find a more environmentally friendly method than the chrome electroplating that is typically used. But the Monash and Rice team see their graphene film replacing polymer coatings used in metals, so the environmental aspect is less acute in this case.

Nonetheless, the Australian-U.S. research team believes that this use of graphene could change the rustproofing methods for products as varied as ocean-going vessels and electronics.

Carbon Nanotubes Form Smallest Pixels for 3D Holographic Imaging

The holograms we have seen for the past 50 years have at once fascinated and disappointed us. If we had been hoping to see something along the lines of the projected image of Princess Leia from Star Wars, or the holodeck from Star Trek Next Generation, disappointment would likely have overwhelmed our sense of fascination.

Two years ago researchers at the University of Arizona for the first time “demonstrated an optical material that can display "holographic video," as opposed to static holograms found in credit cards and product packages.” Since then it seemed our hopes for holograms have been getting brighter.

Now researchers at the University of Cambridge’s Centre of Molecular Materials for Photonics and Electronics (CMMPE) have used carbon nanotubes to create 3D hologram images with an extremely wide field of view and the highest possible resolution.

The research, which was published in the journal Advanced Materials (“Carbon Nanotube Based High Resolution Holograms”), essentially used the carbon nanotubes as diffractive elements that turn the carbon nanotubes into optical projectors. The small size of the carbon nanotubes created smaller pixels thus boosting the resolution of the image.

“Smaller pixels allow the diffraction of light at larger angles – increasing the field of view. Essentially, the smaller the pixel, the higher the resolution of the hologram,” says Dr. Haider Butt from CMMPE in a press release.

The demonstration of their new carbon nanotube-based pixels involved spelling out the name “Cambridge” using various colors of laser light that had been scattered through the carbon nanotube pixels. While initially a fairly modest display and dependent on the prohibitively expensive carbon nanotubes, Butt believes that some kind of nanomaterials will form the basis of a new approach to holographic images.

Butt adds in the release: “A new class of highly sensitive holographic sensors can be developed that could sense distance, motion, tilt, temperature and density of biological materials. What’s certain is that these results pave the way towards utilizing nanostructures to producing 3D holograms with wide field of view and the very highest resolution.”

To replace the carbon nanotubes, the researchers are looking at the prospect of using zinc oxide nanowires, which Zhong Lin Wang at Georgia Tech has been using over the years for its of piezoelectric qualities.

The other big issue that the researchers still we need to address is investigating “holographic video” because currently the carbon nanotube pixels can only project static holograms. Looks like there’s still some work to be done before Princess Leia holograms are projected, at least with a nanomaterial as the pixel.

Wearable Health Monitoring Project Turns to Nanotechnology for Power Sources

Sometimes significant innovations result just from aggregating a number of different innovations into one product. So it is with a multi-institution research effort to exploit recent developments in wireless health monitoring systems and couple them with thermoelectric and piezoelectric nanomaterials to power them.

The research is being led by the Nanosystems Engineering Research Center for Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST) headquartered at North Carolina State University in collaboration with partner institutions Florida International University, Pennsylvania State University and the University of Virginia.

“Currently there are many devices out there that monitor health in different ways,” says Dr. Veena Misra, the center’s director and professor of electrical and computer engineering at NC State in the university press release covering the research. “What’s unique about our technologies is the fact that they are powered by the human body, so they don’t require battery charging.”

While Misra may be correct in her assertion that this combination is unique within health monitoring systems, both thermoelectric and piezoelectric nanomaterials for powering devices is an area being vigorously pursued.

In the area of thermoelectric nanomaterials, we have seen significant developments this year. One coming from Wake Forest University involved using multi-walled carbon nanotubes to fabricate a thin film that the researchers claim can convert differences in temperature into electrical energy. In that case, the researchers were targeting the powering of cell phones.

A month after the Wake Forest research was announced, an international team of researchers from the California Institute of Technology, the Chinese Academy of Science's Shanghai Institute of Ceramics, Brookhaven National Laboratory and the University of Michigan developed a liquid-like material in which selenium atoms make a crystal lattice and copper atoms flow through the crystal structure like a liquid. This unusual behavior of the copper ions around the selenium lattice resulted in very low thermal conductivity (bad at conducting heat) in what is otherwise a fairly simple semiconductor (good at conducting electricity), making it an excellent candidate as a thermoelectric material.

Piezoelectric nanomaterials have been dominated until late by the use of nanowires, and specifically the research of Professor Zhong Lin Wang, Director of the Center for Nanostructure Characterization at Georgia Tech, who has almost singlehandedly kept the somewhat obscure topic of piezoelectric qualities of zinc oxide nanowires in the news. But recently graphene has entered into the area of piezoelectric materials with research coming out of Stanford University. While the Stanford research was only conducted in modeling and simulation software, it did promise to open up the fairly new conceptual field of “straintronics”.

It is not clear from either the NC State press materials or even the video that they have produced (see below), what nanomaterials they intend to use to bring on either the thermoelectric or piezoelectric effects. It will be interesting to see which direction they go with their materials.

Directed Self-Assembly Accomplished with Magnets

As chips shrink ever smaller, traditional lithography techniques have begun to lose their ability to remain accurate and the hopes of maintaining Moore's Law have dimmed as a result. But directed self-assembly, a way to build integrated circuits from the bottom up, has been held out as a possible way to keep shrinking chip feature sizes and and sustain Moore’s Law.

Basically, directed self-assembly is a way of exploiting the ability of molecules to arrange themselves into ordered structures—with a little direction from our end. Previously, H.-S. Philip Wong used lithography to carve indentations that served as a template for the molecules to self-assemble themselves. This combination of traditional lithography with self-assembly techniques has been a line of research that has seemed to be the most promising in the field. 

A less traditional line of research for directed self-assembly has been the work of Angela Belcher, in which DNA serves as the template and viruses actually build up the integrated circuit

A new addition to the field comes from the University of Delaware. Eric M. Furst and his colleagues have demonstrated how paramagnetic colloids can be directed to self-assemble in the presence of a magnetic field. 

The research, which was published in the Proceedings of the National Academies of Science (PNAS) online edition (“Multi-scale kinetics of a field-directed colloidal phase transition”),  was able to observe how the particles transitioned from a solid-like material into an organized crystalline structure. Furst claims that this represents the first time that anyone has observed this guided “phase separation” of particles.

“This development is exciting because it provides insight into how researchers can build organized structures, crystals of particles, using directing fields and it may prompt new discoveries into how we can get materials to organize themselves,” Furst says in the university press release covering the research.

An interesting twist to the research was the enlistment of NASA to see how the particles would react in a weightless environment. To realize this some of the experiments were conducted in the International Space Station. The weightless environment allowed for some fresh observations on the self-assembly process. The particles first developed into its solid-like form and then coarsened to the point of breaking apart. After separating—not unlike the way water and oil separate when combined—the particles formed themselves into the crystalline lattice structure.

Furst further notes in the article: “This is the first time we've presented the relationship between an initially disordered structure and a highly organized one and at least one of the paths between the two. We’re excited because we believe the concept of directed self-assembly will enable a scalable form of nanotechnology.”

Nanotechnology Takes Aim at Improving Beer

The motivations for certain applications of nanotechnology can run the gamut from ending our dependence on fossil fuels to providing clean drinking water in poor, remote regions of the world.  But these high-minded aspirations are not always the goal for nanotechnology applications; sometimes we just want to have a better beer drinking experience.

Australian researchers created a better way to keep beer cool two-and-a-half years ago. But  it seems that scientists in Ireland were not entirely satisfied and are developing a new material for extending the shelf life of beer

Until quite recently, it was unheard of to use plastic for beer containers because oxygen and carbon monoxide would escape through the relatively porous plastic and take away the taste of the beer.  But for some years now Nanocor Inc., which is wholly owned subsidiary of AMCOL International Corporation, has been selling its nanoclay materials to create gas-proof plastic composites for beer packaging.

Researchers at CRANN--the Science Foundation Ireland-funded nanoscience institute based at Trinity College Dublin--have decided on a different approach. Instead of using a nanoclay, the researchers will exfoliate nanosheets of boron nitride and mix them into a polymer.

The CRANN team have partnered with the brewing company SABMiller, which has agreed to invest in the research over the next two years, so we're pretty sure to have a commercial product at the end.

So, if we combine the Australian and Irish research we should be able to enjoy a plastic bottle of beer that remains cold over a much longer period of time. The joys of science.

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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.

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