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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


Desktop Nanofabrication Becomes Much Cheaper

Chad Mirkin, director of Northwestern's International Institute for Nanotechnology, and the original developer of the technology behind NanoInk, which went bust earlier this year, is behind new research that employs beam-pen lithography to produce diverse structures at a fraction of the cost of today's nanofabrication technology.

At first, when the technology behind NanoInk was commercially launched, it was difficult to see how using an atomic force microscope-based dip-pen to execute lithography on the nanoscale would be scalable. As Tim Harper noted on this blog when NanoInk filed for bankruptcy, “NanoInk's offering was the equivalent of replacing the printing press with a bunch of monks. Illuminated manuscripts can look good but if you can't mass produce things there isn't a business.”

One of the key differences between this latest research, which was published in the journal Nature Communications (“Desktop nanofabrication with massively multiplexed beam pen lithography”), and NanoInk's original technology is that it employs relatively inexpensive components and still gives you nearly the same capabilities as an expensive AFM. It also operates pretty much like photolithographic techniques do, in that the light strikes a photosensitive substrate to generate structures. However, in this case the instrument can produce structures that range from the macro scale down to the nano scale in one go around.

“With this breakthrough, we can construct very high-quality materials and devices, such as processing semiconductors over large areas, and we can do it with an instrument slightly larger than a printer," said Mirkin in the press release. “"Instead of needing to have access to millions of dollars, in some cases billions of dollars of instrumentation, you can begin to build devices that normally require that type of instrumentation right at the point of use."

The inexpensive components used in the new instrument include beam-pen lithography (BPL) pen arrays. BPLs are structures consisting of arrays of polymeric pyramids that have been coated with an opaque layer; each has a 100-nanometer aperture at the tip. A single beam of light is projected through a digital micromirror device so that the light is broken up into thousand of individual beams. Each one of these beams of lights travels down the pyramidal pens in the array and through the apertures at their tip.

"There is no need to create a mask or master plate every time you want to create a new structure," Mirkin said. "You just assign the beams of light to go in different places and tell the pens what pattern you want generated."

The instrument is a brilliant piece of engineering and its development involved advances in the hardware and software that directs the light to go in the right places. However, it should be noted that this still is essentially top-down manufacturing and should not be confused with so-called “desk-top manufacturing”, which in theory would involve molecular assemblers building both nano-scale and macro-scale structures from the bottom up, atom by atom.

While it may not be molecular manufacturing, it does have the benefit of being available in the not-too-distant future. Mirkin believes that since the instrument uses components that are easily accessible a commercial product could be available in the next two years.

Images showing the diversity of patterns that can be created with the desktop nanofabrication system.

Image: X. Liao







Will Cooler Heads Prevail in Preliminary Graphene Toxicology Research?

After the announcement out of Brown University last week that the jagged edges of graphene can cut into cells and find themselves inside of them, the news cycle has been non stop with headlines like “Jagged Graphene Can Rip Human Cells Apart”  and “Graphene Sheet Jagged Edges Easily Pierce Cell Membranes”. Of course, it’s not clear from the research that graphene “rips human cells apart” or can “easily pierce cell membranes”, but this is how these stories are typically covered.

This is important research, and published in the prestigious journal Proceedings of the National Academy of Sciences, but it is preliminary research and is not conclusive evidence that graphene is toxic to humans. This distinction is made at least implicitly clear in the press release:

“From here, the researchers will look in more detail into what happens once a graphene sheet gets inside the cell. But Kane says this initial study provides an important start in understanding the potential for graphene toxicity.”

These stories always take the same track in the media, whether it’s the technology press, the mainstream media or the NGO-related opinions. We even have a pretty clear blueprint of how these stories play out from a news item five years ago that reported that carbon nanotubes longer than 20 micrometers lead to the same pathogenic effects in the mesothelium as asbestos fibers.

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New Form of Carbon Takes Graphene Into a New Dimension

Nearly three decades ago, our understanding that there were three basic forms of carbon—diamond, graphite, and amorphous carbon—was stood on its head with the introduction of fullerenes, then carbon nanotubes, and more recently graphene.

Now researchers at Boston College and Nagoya University have synthesized another new form of carbon unofficially dubbed “grossly warped nanographenes.” The research, which was published in the journal Nature Chemistry (“A grossly warped nanographene and the consequences of multiple odd-membered-ring defects”), has led to creating a material that is essentially defects in the two-dimensional hexagonal honeycomb-like arrangements of trigonal carbon atoms found in graphene. These defects consist of non-hexagonal rings that force distortions out of the two-dimensional plane.

The grossly warped nanographene consists of 80 carbon atoms joined together in a network of 26 rings, with 30 hydrogen atoms on the outside rim. In contrast to graphene sheets, which typically have planar two-dimensional geometries, the new material juts out from a single plane because of the five 7-membered rings and one 5-membered ring embedded in the hexagonal lattice of carbon atoms.

Pushing its geometry out of planarity has altered the new material's physical, optical, and electronic properties vis-à-vis its carbon cousins.

“Our new grossly warped nanographene is dramatically more soluble than a planar nanographene of comparable size,” said Lawrence T. Scott, professor at Boston College and one of the principal authors of the research, in a press release.  “The two differ significantly in color, as well. Electrochemical measurements revealed that the planar and the warped nanographenes are equally easily oxidized, but the warped nanographene is more difficult to reduce.”

These are just the initial differentiating characteristics of this new form of carbon. If the near-decade-long history of graphene is any guide, we can expect new properties to be discovered on a regular basis. (Just this week, for example, researchers discovered in measurements that the response rate of an optical switch using graphene is 100 times faster than traditional materials.)

Whether the so-called “grossly warped nanographenes” will offer the same wealth of new characteristics as graphene remains to be seen. However, it would seem that graphene is now not only facing competition from other 2-D materials, but from an entirely new form of carbon.

Illustration: Boston College


Graphene Optical Switches One Hundred Times Faster Than Current Devices

Just two years ago, Andre Geim and Konstantin Novoselov, the two scientists who won the Nobel Prize for discovering graphene, succeeded in improving photodetectors to the degree that they could boost optoelectronic data transfer rates by a factor of 20. That breakthrough relied on using graphene combined with plasmonic nanostructures.

Now researchers at the University of Bath in the U.K. are reporting measurements indicating that graphene could lead to optical switches that are nearly a hundred times faster than materials used in today’s current switches.

The research, which was published in the journal Physical Review Letters (“Carrier Lifetime in Exfoliated Few-Layer Graphene Determined from Intersubband Optical Transitions”), found that the response rate of an optical switch using graphene to be around 100 femtoseconds, which is about a hundred times faster than the few picoseconds measured in today’s devices.

“We’ve seen an ultrafast optical response rate, using few-layer graphene, which has exciting applications for the development of high speed optoelectronic components based on graphene,” said lead researcher Dr. Enrico Da Como in a press release. “This fast response is in the infrared part of the electromagnetic spectrum, where many applications in telecommunications, security, and also medicine are currently developing and affecting our society.”

In addition to photodetectors and optical switches, graphene is proving attractive for tunable notch filters, an area where IBM has made some interesting progress. Also, researchers have been able to exploit graphene’s wide spectral range for different kinds of tunable lasers that are used in optoelectronic systems.

In fact, the research team’s long-range goal is to apply this discovery to the development of graphene-based quantum cascade lasers that could be used for pollution monitoring, security, and spectroscopy applications.

Image: Martin McCarthy/iStockphoto

Graphene Protects Metal Silicides From Oxidation

While much research into graphene for electronics applications has focused on ways to have it replace silicon, a research group at the University of Vienna is looking at ways to integrate graphene into current silicon-based technologies.

The Vienna researchers along with colleagues in Germany and in Russia took an approach to integrating silicon within graphene that involved building a semiconducting or an insulating buffer between graphene and a metallic substrate.

With this aim, the international team have successfully built a structure of high-quality metal silicides covered and protected underneath a graphene layer. Metal silicides, which are a compound of silicon with a more electropositive element, are used extensively in applications including complementary metal oxide semiconductor (CMOS) devices, thin film coatings and photovoltaics as interconnects and barriers.

The research published in Nature’s new open-access journal Scientific Reports (“Controlled assembly of graphene-capped nickel, cobalt and iron silicides”) used monocrystalline layers of films of nickel, cobalt and iron as the substrate on top of which high-quality graphene produced through chemical vapor deposition (CVD) was deposited. The resulting structure is protected against oxidation because of the graphene capping the metal silicide layers.

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Seven or Never: On-Off Adhesives Make Progress Towards Commercial Applications

Sometimes the biggest inspiration for technological development is nature. Biomimicry—as it is known—is leading to advancements in wind power where the small bumps on the leading edges of the fins of humpback whales are duplicated on the blades of wind turbines.  Another example of this biomimicry is the tiny hairs on the petals of the lotus flower that repel water being mimicked in the waterproof coatings for mobile phones.

In the field of nanotechnology and biomimicry there has been one creature of nature that has been of particular interest: The gecko. Researchers have been fascinated with the gecko’s gravity-defying ability to walk on ceilings and not fall. The gecko’s exploits are accomplished by hundreds of thousands of tiny hairs, called setae, covering their feet. Each one of these setae itself has nanoscale projections, which are so small that they produce the weak molecular interactions known as van der Waals between themselves and the substrate.

Over three years ago, I covered research coming out of the University of California Santa Barbara (UCSB) that was exploiting the gecko’s design to potentially produce industrial adhesives that could be actuated—or turned on and off, like a switch—by magnetism.  I joked at the time that it could enable Spiderman-like super powers. But I also suggested that this was a technology that was commercially attractive.

With this commercial opportunity suggested by none other than myself, I thought I should investigate how far along the technology had developed as part of the ongoing series: “Seven or Never,” which looks back at technologies I have covered over the years to check on the current state of development.

Professor Kimberly L. Turner, who has been leading this research for the last decade at UCSB, gave me the update.

“We began working on this research in about 2003,” Turner told me via email. “My student at the time, Michael Northen, got interested in synthetic adhesives, and we were the first group to really focus on the hierarchical nature of the problem by using active MEMS technology.  We were able to integrate meso-, micro-, and nano- scales into an active device that could be changed from an adhesive state to a non-adhesive state.  Later on, we were able to use magnetic actuation to 'release' the adhesives from a stuck state.  It was a very exciting time.  That was back in 2006.  The work has grown in many directions since that early work.”

In addition to Northen, Turner has worked with three other PhD candidates in this area, including Abhishek Srivastava, Sathya Chary, and John Tamelier.

The work of Tamelier has been aimed at scaling up the adhesive patches that the team has been developing. He is also designing and building a test mechanism to study the best ways to approach surfaces with the adhesives in order to maximize adhesion.

“John Tamelier is about to have a paper come out in Langmuir which focuses on that work,” said Turner. “This is an essential result for achieving high adhesion with micro-robots.  How the foot of the robot approaches the surface it is running on is key to how much adhesion is generated.  The adhesives we are using for this are passive adhesives, meaning that they work without actuation.  However, they are anisotropic, meaning if moved in one direction they are sticky, but in another they are not sticky, thus creating more flexibility and ease in removal.”

Turner is quick to mention that the work in active devices (those that can be actuated to turn on or off) that were mentioned in my initial blog post covering their technology has continued.

“In a collaboration with the Army Research Lab and a local MEMS foundry, IMT Inc., we integrated thermal actuators and piezoelectric actuators into the adhesives,” explained Turner in an e-mail. “We were able to show that by adding force (from the thermal actuators) in the direction perpendicular to the pull-off direction, we could enhance the adhesion.  This was a very challenging fabrication, and was limited as to how much surface area we could generate, but it is a promising result.”

The aim of the “Seven or Never” series is to find out how far along a hopeful technology gets over a period of time. While I first covered the technology just three years ago, the first successful results were achieved back in 2006—right in line with our seven-year time frame. But as we learned from our first installment of this series, seven years is hardly enough time to bring an emerging technology to market and this one appears to be no exception. Nonetheless, the researchers are pursuing real-world, commercial applications.

“We do hold a patent on the magnetic technology, and have had quite a lot of interest from industry on commercializing,” said Turner.  “We have not been focused on that as of late, as it was not quite to the stage where it is useful for many of the applications of interest, but it is close.  We are excited about the future of this technology.”

With efforts being made in commercializing their technology, I asked Turner what her thoughts were on the challenges facing innovation and bringing an emerging technology to market.

“Easier industrial partnerships would certainly make this a lot smoother,” said Turner. “ There always seems to be a problem with intellectual property agreements, and this takes a lot of time and effort to iron out.  If there was an easier way to get this done, I think more industry would partner with universities, and it would lead to faster innovation.”

Despite these challenges, Turner added: “I bet there will be some products in the next 5 years…the possibilities are vast.”

Photo: Canebisca/iStockphoto


Nanoparticle Ink Enables 3-D Printing of Microbattery Electrodes

Over two-and-a-half years ago, researchers at Sandia National Laboratories developed what they claimed was the smallest battery yet produced. The lithium-based battery they produced was no bigger than a grain of sand, so small that one of its anodes consisted of nothing more than a single nanowire.

There didn’t seem to be any real-world applications for the battery, because it was created inside a transmission electron microscope (TEM). Instead it was intended to demonstrate a way forward in the miniaturization of batteries to satisfy a market in which gadgets are becoming smaller and smaller but the batteries used to run them remain rather large.

Now researchers at Harvard University are following up on Sandia's battery miniaturization by using a 3-D inkjet printing process enabled by nanoparticles made from lithium metal. This research brings 3-D printing to a new level, according to the researchers.

“Not only did we demonstrate for the first time that we can 3D-print a battery; we demonstrated it in the most rigorous way,” said Jennifer A. Lewis a professor at the Harvard School of Engineering and Applied Sciences (SEAS), in a press release.

The research, which was published in Advanced Materials (“3D Printing of Interdigitated Li-Ion Microbattery Architectures”), updates the traditional method of building electrodes that involves depositing thin films of solid materials. On it's own, this long-used technique results in solid-state micro-batteries that don’t store enough energy for today’s devices.

Instead the process Lewis and her colleagues employed used 3-D printing to build tightly interlaced, ultrathin electrodes. To do this, the team had to develop a special type of ink that would be electrochemically active and harden into layers as narrow as those produced by the thin-film manufacturing methods. The researchers developed ink for the anodes made from one compound of lithium metal oxide nanoparticles and from a different compound for the cathodes.

After depositing the inks onto two gold combs (a process you can watch on the video below), the stacks of interlaced anodes and cathodes were packaged into a container filled with electrolyte. The researchers were impressed with the performance measurements of the resulting battery.

“The electrochemical performance is comparable to commercial batteries in terms of charge and discharge rate, cycle life, and energy densities,” said Shen Dillon, an assistant professor of materials science and engineering at Harvard and one of Lewis' collaborators, in the press release. “We’re just able to achieve this on a much smaller scale.”

If the miniature batteries can be produce on a bulk scale, this could change the way in which small devices are powered and open up entirely new possibilities for electronic devices in both medial and non-medical applications.

Image: Jennifer A. Lewis, Harvard University


2-D Nanomaterials Put Photovoltaics on a Diet

Soon after graphene was discovered back in 2004, a number of other two-dimensional (2-D) materials appeared, vying for the same attention. By now, the list has grown to a veritable catalogue of 2-D materials.

The story of these 2-D materials will ultimately be about how well they can adapt to different applications that we consider for them today, or may discover in the future. Of them, perhaps no area has been as hotly pursued as photovoltaics (PVs).

One interesting new potential application of graphene is in creating what is known as “hot carrier” cells in which the graphene produces, after absorbing one photon, is capable of generating multiple electrons instead of just a single one. But the main focus of applying graphene to PVs has been as a replacement for indium tin oxide (ITO) used in the electrodes of organic solar cells.

Now researchers at MIT are looking at 2-D materials, such as molybdenum disulfide and molybdenum diselenide, to make the thinnest and lightest PVs ever made. While this may not produce the highest energy conversion efficiency or be the cheapest material for PVs—the two typical metrics most sought after—they do expect that their lightness should create some possibilities in this application.

Graphene's own energy conversion capabilities are not what you would call impressive, and more generally, two-dimensional materials don’t really compete with the 18-19 percent conversion efficiencies of standard silicon cells already on the market. But in the research, which was published in the ACS journal Nano Letters (“Extraordinary Sunlight Absorption and 1 nm-Thick Photovoltaics using Two-Dimensional Monolayer Materials”),  the MIT team demonstrated that if you stacked just three sheets on top of each other, a 1nm-thick stack can absorb up to 10 percent of incident sunlight, which is one order of magnitude higher than gallium arsenide and silicon. While the actual conversion efficiency is still pretty low at 1 percent, it does correspond to power densities being 100 to 1000 times higher than the best existing ultrathin solar cells.

“Stacking a few layers could allow for higher efficiency, one that competes with other well-established solar cell technologies,” said Marco Bernardi, a postdoc in MIT’s Department of Materials Science, in a press release.

Jeffrey Grossman, Associate Professor of Power Engineering at MIT and the paper's senior author, added in the release: “It’s 20 to 50 times thinner than the thinnest solar cell that can be made today. You couldn’t make a solar cell any thinner.”

Whether thinner and lightness will really translate into something that the market demands remains to be seen. The problem, as the researchers seem to concede, is that at this point there is no way to produce molybdenum disulfide and molybdenum diselenide in bulk. Until manufacturing techniques are developed that make that possible—and economical—the thinnest solar cells will likely remain laboratory curiosities.

Illustration: Jeffrey Grossman and Marco Bernard/MIT News



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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