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Tightening Graphene Like a Drumhead Changes Its Electrical Properties

 

The main preoccupation with graphene research has been trying to impart a band gap to the wonder material. Researchers at the University of Wisconsin-Milwaukee were able to get graphene to behave like a semiconductor earlier this year by making a new variety of graphene dubbed “graphene monoxide”. But now researchers at National Institute of Standards and Technology (NIST) and the University of Maryland discovered they could do it by just treating graphene like a drumhead

The research was published in the journal Science under the title “Electromechanical Properties of Graphene Drumheads”  and is available with a subscription.

Initially the aim of the NIST and University of Maryland team was to see if they could just slow electrons down with the use of a substrate. So they suspended a layer of graphene over a substrate of silicon dioxide that contained shallow holes. When the graphene was laid on top of these holes, it formed a kind of graphene drumhead.

The next step, of course, was to measure their new graphene drumhead. It was then that they discovered when the tip of the scanning probe microscope (SPM) approached the graphene, it rose up to meet the tip. The attraction of the graphene to the SPM tip was caused by van der Waal force, in which a weak electrical force is created that attracts objects together. 

"While our instrument was telling us that the graphene was shaped like a bubble clamped at the edges, the simulations run by our colleagues at the University of Maryland showed that we were only detecting the graphene's highest point," says NIST scientist Nikolai Zhitenev in the NIST press release covering the research. "Their calculations showed that the shape was actually more like the shape you would get if you poked into the surface of an inflated balloon, like a teepee or circus tent."

The researchers then tinkered with the graphene drumhead a little more and soon discovered that they could tighten the graphene like one would with the skin on a real drum. But instead of changing the sound as with the real drum, the tightening of the graphene drumhead resulted in changing the electrical properties of the graphene.

If you tightened the graphene drumhead enough so that it actually went into the shallow hole and created a kind of tent like shape, the graphene started to behave like a quantum dot.

This result could open up an entirely new avenue of research in which by simply altering the shape of graphene you can maintain its high conductivity but create a band gap as well.

Zhitenev further noted in the release: "Normally, to make a graphene quantum dot, you would have to cut out a nanosize piece of graphene," says NIST fellow Joseph Stroscio. "Our work shows that you can achieve the same thing with strain-induced pseudomagnetic fields. It's a great result, and a significant step toward developing future graphene-based devices."

Nanostructures Modeled on the Moth Eye Reduce Radiation in Medical Imaging

Typically it’s the butterfly that serves as the bio-inspiration for nanotechnology advances.  But now the butterfly’s modest cousin, the moth, is serving as the model.

Yasha Yi, a physics professor at the City University of New York, has attempted to duplicate the moth’s anti-reflective eyes with a nanostructured material that should improve medical imaging

This is not the first time that the anti-reflective qualities of moth eyes have been used as models for devices. Researchers have attempted to duplicate this feature to create more efficient coatings for solar panels and some military devices. But Yi and his research team, which published their work in the journal Optics Letters,  were looking at improving “scintillation” materials used in medical imaging technologies. These scintillation materials absorb incoming X-rays and reemit the energy as light of wavelengths that can be picked up by a detector.

If you want the detector to pick up more light, the technique has usually been to increase the intensity of the X-rays. But this obviously has associated health risks. Yi and his team believed that if they could improve the scintillation material so that it reemitted more light from the same amount of X-rays, then they could create safer medical imaging devices.

To do this, the researchers needed to create a new class of materials. What they came up with is based on a thin film made from cerium-doped lutetium oxyorthosilicate crystals. They were then able to cover these crystals with pyramid-shaped bumps made of silicon nitride. It is these bumps that make the scintillator appear like the moth’s eye and give the structures its ability to extract more light.

The results have been pretty dramatic. Yi and his team measure that adding their moth-eye-inspired thin film to the scintillator of an X-ray mammographic unit increases the amount of reemitted light by 175 percent.

“The moth eye has been considered one of the most exciting bio structures because of its unique nano-optical properties,” Yi says in Nanomagazine article. “Our work further improved upon this fascinating structure and demonstrated its use in medical imaging materials, where it promises to achieve lower patient radiation doses, higher-resolution imaging of human organs, and even smaller-scale medical imaging. And because the film is on the scintillator,” he adds, “the patient would not be aware of it at all.”

We shouldn’t expect to see this scintillator material on the market in the near future. Yi expects that it will be another three to five years to evaluate and perfect the film.

Nanoscale Vacuums Speed Semiconductors

When you want to make the point of how far electronics and computer technology have come in the last sixty years, you likely refer to the old computers that used vacuum tubes for circuitry.

So, it’s a bit counterintuitive to see the latest research that suggests vacuums may be the way forward to help semiconductor electronics keep pace with Moore’s law. Researchers at the University of Pittsburgh have developed a method for generating a vacuum within a semiconductor device to transport electrons more efficiently through it. 

“Physical barriers are blocking scientists from achieving more efficient electronics,” said Hong Koo Kim, principal investigator on the project and Bell of Pennsylvania/Bell Atlantic Professor in the University of Pittsburgh’s Swanson School of Engineering, in a press release. “We worked toward solving that road block by investigating transistors and its predecessor—the vacuum.”

Of course, there already exist vacuum electronic devices, but these require high voltage. The researchers, who published their findings in the journal Nature Nanotechnology, designed an entirely new vacuum electronic device that requires minimal voltage to operate.

The key to the design was the discovery by Kim's team that it was fairly easy to pull electrons out into the air when they are trapped at the interface of an oxide or metal layer inside a semiconductor. These trapped electrons form a two-dimensional electron gas.

The researchers exploited the phenomenon known as Coulombic Repulsion--the repulsive force between two positive or negative charges--to emit the electrons from this electron gas layer. By then applying a small voltage of  1V to the silicon structure, the electrons were extracted into the air, which made it possible for them to travel ballistically in a nanometer-scale vacuum channel without the scattering typically seen in conventional devices.

Kim further noted in the release, “The emission of this electron system into vacuum channels could enable a new class of low-power, high-speed transistors, and it’s also compatible with current silicon electronics, complementing those electronics by adding new functions that are faster and more energy efficient due to the low voltage."

Analysis of Nanosilver Risk Reaching Point of Diminished Return?

It's been clear for several years that we need to move beyond merely conducting more studies on nanotechnology and risk and start looking at how regulation of nanotechnologies should be established, something I first wrote about nearly three years ago.

In the intervening years, serious observers have expressed concern on how such regulations would be enacted. Then the European Union (EU) further fueled that concern by their awkward process of defining nanotechnology in order to establish its regulatory policy. Creating regulatory policy is never an elegant process.

But this is the way of governments, I suppose. When in doubt about what course of action to take, establish a committee, or, better yet, a public outreach project

Now this grinding pace of governance has worn thin the patience of some scientists. In a recent editorial piece in the journal Nature Nanotechnology, entitled “When Enough is Enough,”  authors Steffen Foss Hansen and Anders Baun, both of Technical University of Denmark's Department of Environmental Engineering, have reached their breaking point. In the editorial, they explain their frustration with a request from the European Commission (EC) for another study on the environmental, health and safety considerations of nanosilver.

If you don’t have a Nature Nanotechnology subscription, Nanowerk has a story on the editorial and an interview with the two authors. They do not hide their irritation with the EC and its “paralysis by analysis,” as Nanowerk calls it.

"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" Hansen tells Nanowerk. "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."

What may make the matter even worse is that we may already have a pretty substantial framework—in the US, at least—on which to base nanosilver regulations, which dates back to the 1950s. It concerned what was called at the time collodial silver, which is essentially what today is called nanosilver.

But getting back to current stagnant state of affairs, it’s hard to know exactly what’s causing the paralysis. It could be concern over implementing regulations in a depressed economy, or just a fear of taking a position. But in both these instances, the lack of action is making the situation worse. Investment is scared off by a market that appears ripe for regulation but none have been implemented. Taking no action may seem politically expedient now, but it will come back to haunt the timid bureaucrats. Surely, we can base regulations on what we know now about nanosilver risk, and amend them later, if the situation demands it.

Nanoporous Graphene Promises Affordable Water Desalination

For some parts of the world desalination of seawater is an important option for accessing fresh drinking water. In 2007 the estimates were that worldwide desalination reached 30 billion liters a day.  But the cost of that desalination was at the exorbitant levels of $0.50 to $0.85 per cubic meter.

Because of this huge expense, most desalination production remains in the oil-producing countries of the Persian Gulf, where they can afford the huge energy costs of running the multi-stage flash (MSF) processes. But outside of the Middle East, the predominant method for desalination is Reverse Osmosis (RO), which is only slightly less energy consuming and expensive than MSF processes. 

Researchers at MIT are looking to replace the membrane materials used now in RO with nanoporous graphene

Currently, RO depends on comparatively thick membranes that effectively block the salt ions when water molecules are hydraulically pushed through them. In the process envisioned by the MIT researchers, which was published in the journal Nano Letters, one-atom-thick grapheme with nanometer-sized pores would replace those membranes. Because the graphene is a thousand times thinner than the traditional membrane materials it requires far less force—and therefore energy—to push the water molecules through it. A video describing the benefits can be seen below.

The key to making nanoporous graphene work in this desalination process is getting the size of the pores just right. If the pores are too big, the salt can pass right through; and, conversely, if the they are too small, the water will be blocked. According to Jeffrey Grossman, the Carl Richard Soderberg Associate Professor of Power Engineering in MIT’s Department of Materials Science and Engineering, the ideal size range is extremely limited and looks to be 1 nanometer. If the pores are slightly smaller, 0.7 nanometers, the water won’t pass through the membrane at all.

At this point, the research seems largely centered around computer modeling of an RO process using the nanoporous graphene. And in the MIT press release Joshua Schrier, an assistant professor of chemistry at Haverford College, points out that translating this research from computer models to the real world will not be an easy step.

“Manufacturing the very precise pore structures that are found in this paper will be difficult to do on a large scale with existing methods,” he says. However, he also believes that “the predictions are exciting enough that they should motivate chemical engineers to perform more detailed economic analyses of … water desalination with these types of materials.”

Simple Process Turns T-Shirt into a Supercapacitor

The idea of combing electronics with our clothing has been around for a while, but, so far, it has produced mixed results. Nanotechnology has often been mentioned as an ingredient to add to the electronics-textile mix, to help push concept beyond mere novelty.

One way to bring nanotechnology into electronic textiles is to make the textile itself into a big battery for powering our personal electronics. Two years ago, researchers at the University of California Berkeley pursued this line of research. Their approach was to weave nanowires into the textile and to rely on the piezoelectric properties of the nanowires to develop power. While I have not followed up to see where that research ended up, it did seem a bit complicated—weaving nanowires into a textile sounded like a daunting task.

Now, researchers at University of South Carolina (USC), led by Xiaodong Li, a professor of mechanical engineering, has developed what appears to be a much easier way to make a cotton t-shirt into a supercapacitor

The technique, which was described in the Wiley journal Advanced Materials, started by soaking a regular cotton t-shirt a in a solution of fluoride. After dying it, the researchers examined the material with infrared spectroscopy and discovered that the cellulose of the cotton t-shirt had been converted into activated carbon. Not only had they made it into activated carbon, but also the t-shirt still maintained its flexibility and could be rolled and folded without breaking.

While the activated carbon now could serve as a capacitor and store electrical charge, Li and post-doctoral associate Lihong Bao decided to take it a step further. They took the individual fibers from the treated t-shirt and coated them with “nanoflowers” of manganese oxide. The result was a “stable, high-performing supercapacitor,” said Li in the USC press release. "By stacking these supercapacitors up, we should be able to charge portable electronic devices such as cell phones."

While Li makes mention of the environmentally friendly chemicals used to impart this capability to a t-shirt, it is perhaps the simplicity of the process that will likely be the most intriguing aspect to manufacturers.

World Smallest Optical Cavities Lead to Most Intense Nanolaser Beams

Metamaterials can produce astounding effects. They possess quite different electromagnetic properties from conventional materials that allow them—among other things—to make objects appear invisible. Physics as magic, if you will.

If you manipulate the structure of a metamaterial, it will change how it interacts with electromagnetic waves. Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley have taken advantage of this phenomenon at the nanoscale to create “the world’s smallest three-dimensional optical cavities with the potential to generate the world’s most intense nanolaser beams.” 

The development of the “world’s smallest optical cavities” should have applications across a number of optical fields, including LEDs, optical sensing, nonlinear optics, quantum optics and photonic integrated circuits. It seems that metamaterials are on a bit of a roll when it comes to optoelectronics applications as evidenced by research coming out of the UK and Spain earlier this year.

Key to this technology’s operation is the optical property known as negative refraction, which makes it possible for some kinds of metamaterials to bend light in the opposite direction from what we would expect based on typical refraction. Achieving this negative refraction involves reversing the electrical component (permittivity) and the magnetic component (permeability) of a material’s refractive index. This is accomplished by constructing the material so that it has structures with dimensions smaller than the wavelengths of the light it is intended to refract.

“Due to the unnaturally high refractive index supported in the metamaterials, our 3D cavities can be smaller than one tenth of the optical wavelength,” says Xiaodong Yang, lead author of the Nature Photonics paper,  in a Berkeley Lab press release. “At these nanoscale dimensions, optical cavities compress the optical mode into a tiny space, increasing the photon density of states and thereby enhancing the interactions between light and matter.”

Xiang Zhang, a principal investigator with Berkeley Lab’s Materials Sciences Division and director of UC Berkeley’s Nano-scale Science and Engineering Center (SINAM), believes this research should make possible a new generation of high-performance photonic devices.

“Our work opens up a new approach for designing a truly nano-scale optical cavity,” Zhang says in the Berkeley Lab press release. “By using metamaterials, we show intriguing cavity physics that counters conventional wisdom. For example, the quality factor of our optical mode rapidly increases with the decrease of cavity size. The results of this study provide us with a tremendous opportunity to develop high performance photonic devices for communications.”

Carbon Nanotube-enabled ‘Strain Paint’ Could Replace Strain Gauges

 

Rice University appears committed to the development of the carbon nanotube (CNT). Ever since its chemistry professor Richard Smalley spun out his research with CNTs into the start-up Carbon Nanotechnologies Inc. (CNI), the university has been inextricably tied to CNTs.

CNI eventually merged with Unidym to create a “nano blockbuster” in 2007,and then it went belly up in 2009. But CNI’s demise had been written on the wall long before that. 

The cautionary tale of CNI is not a knock against the use of carbon nanotubes in commercial applications, but simply a reminder that market pull needs to be more critical to a company’s story than IP position or Nobel Prizes

So, it was pleasing to see that Rice University researchers have not abandoned commercial aspirations for CNTs but instead re-focused their commercial strategies. They have developed a polymeric varnish infused with CNTs that when painted on a surface can act as a strain sensor. The researchers, who have dubbed their material “strain paint,” believe that its early applications could include structures such as airplane wings.

After the “strain paint” has been applied to the surface of a structure and allowed to cure, it is then possible to excite the CNTs that are in the film by focusing a laser beam on the paint. The excited CNTs fluoresce in a way that indicates the amount of strain. More about the research, which was initially published in the American Chemical Society journal Nano Letters, can be found in the video below.

 

“For an airplane, technicians typically apply conventional strain gauges at specific locations on the wing and subject it to force vibration testing to see how it behaves,” says Satish Nagarajaiah, a Rice professor of civil and environmental engineering and of mechanical engineering and materials science, in a Rice University press release. “They can only do this on the ground and can only measure part of a wing in specific directions and locations where the strain gauges are wired. But with our non-contact technique, they could aim the laser at any point on the wing and get a strain map along any direction,” says Nagarajaiah.

According to Rice chemistry professor Bruce Weisman, some reproducibility and long-term stability of the spectral shifts need to be ironed out before market considerations can be fully explored. “We’ll need to optimize details of its composition and preparation, and find the best way to apply it to the surfaces that will be monitored,” says Weisman in the same press release. “These fabrication/engineering issues should be addressed to ensure proper performance, even before we start working on portable read-out instruments.”

What I like best about the story covering this technology is the final quote from Weisman: “I’m confident that if there were a market, the readout equipment could be miniaturized and packaged. It’s not science fiction.” Exactly.

A123 Introduces New Battery Technology Amid Recent Financial Troubles

 

You have to hand it to A123 Systems. After experiencing an embarrassing and costly manufacturing snafu this past March that required a recall costing the company US $55 million, then having to report a first quarter loss of $125 million in May, one might have expected the company to retrench in order to sort out how its revenues last quarter had decreased by 40 percent from the same quarter in 2011. But A123 didn’t do that. Instead the company announced an update to its Nanophosphate® lithium iron phosphate battery technology.

This would be an understandable approach to managing dwindling fortunes, especially if the company had suddenly devised a Li-ion battery that could compete head to head with internal combustion engines. As U.S. Energy Secretary Steven Chu once declared, "a rechargeable battery that can last for 5000 deep discharges, [and offer] 6 or 7 times as much storage capacity (3.6 Mj/kg = 1,000 Wh) at [one-third of today's costs] will be competitive with internal combustion engines (400-500 mile range).”  A press release proclaiming that would certainly change the conversation and forever alter the company's market fortunes.

However, we didn’t get that. Instead, we got A123's same battery technology, but updated so that it can operate at extreme temperatures. "We believe Nanophosphate EXT is a game-changing breakthrough that overcomes one of the key limitations of lead acid, standard lithium-ion, and other advanced batteries. By delivering high power, energy, and cycle life capabilities over a wider temperature range, we believe Nanophosphate EXT can reduce or even eliminate the need for costly thermal management systems, which we expect will dramatically enhance the business case for deploying A123's lithium ion battery solutions for a significant number of applications," said David Vieau, the company's CEO, in a press release.

Will removing or just reducing the need for cooling systems be a game changer for both the transportation and telecommunications markets? Both applications will certainly benefit, but I imagine it will make more of a difference in the telecommunications market, where it will be used to power cell tower sites built off-grid or in regions with unstable power. After all, it’s hard to see how this brings Li-ion battery technology any closer to propelling a car for 800 kilometers on a single charge while lowering the price of the battery system by a factor of three. I am not sure this changes the conversation, never mind the game.

 

 

“Plastic Paint” Magnetic Field Sensor Based on Spintronics Takes Aim at Consumer Electronics

 

Ever since researchers discovered that magnetic field sensors could be produced on organic thin-film materials, there has been the hope the discovery would lead to inexpensive sensors on flexible substrates. If you could get it right, there was huge potential simply due to the ubiquity of magnetic field sensors in consumer electronics.

There were just a couple of rather large problems. A very narrow magnetic field range limited these sensors usefulness and they required continuous calibration to compensate for changes in temperature and the degradation of the material.

Now researchers at the University of Utah have developed a “spintronic” organic thin-film semiconductor that serves as an inexpensive magnetic field sensor capable of detecting intermediate to strong magnetic fields and never needs to be calibrated.

The magnetic sensing film, which is described in the June 12th edition of the journal Nature Communications,  also resists heat and degradation and operates at room temperatures. The thin film is an organic semiconductor polymer called MEH-PPV.

Christoph Boehme, Associate Professor at the University of Utah, and one of the named authors of the Nature paper, describes the thin film in the Institute of Physics’ nanotechweb.org website story linked to above as an orange-colored "electrically conducting, magnetic field-sensing plastic paint that is dirt cheap. We measure magnetic fields highly accurately with a drop of plastic paint, which costs just as little as drop of regular paint."

The researchers are so enthusiastic about their discovery that they are considering launching a spinoff company to commercialize the technology. The commercial applications for magnetic-field sensors are quite broad, so let’s hope the scientists get some good business advice on which application space to target their technology. Further, they should refrain from three-year projections to having devices on the market, managing investors’ expectations is often the key to success.

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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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