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Graphene Combined with Quantum Dots Result in Efficient Photodetector

Graphene research has been turning increasingly towards its potential in optoelectronic applications.  This is not a surprise because graphene’s optical properties are as astounding as its electrical conductivity capabilities.

But just as graphene has the glaring Achilles Heel in electronics applications of lacking a band gap, it also suffers a couple of fatal flaws in optoelectronics. For example, while nearly every single photon the material absorbs generates an electron-hole pair, it doesn’t really absorb that much light. According to some estimates, it absorbs less than 3 percent of the photons falling on it.

So, researchers at the Institute of Photonic Sciences (ICFO) in Barcelona, Spain thought about the possibility of combining graphene with quantum dots to see if they couldn’t overcome graphene’s shortcomings.

The research, which was published in the journal Nature Nanotechnology last week,  demonstrated that the combination of the two nanomaterials did the trick. Instead of absorbing just 3 percent of the light that hits it, the graphene/quantum dot hybrid material is capable of absorbing 25 percent of the light falling on it. This new absorption capability is due to the quantum dots, and when you combine that with the graphene’s ability to make every photon into an electron-hole pair, the potential for generating current is significant.

"In our work, we managed to successfully combine graphene with semiconducting nanocrystals to create complete new functionalities in terms of light sensing and light conversion to electricity," Gerasimos Konstantatos, co-leader of the team at the Institute of Photonic Sciences (ICFO) in Barcelona, told physicsworld.com. "In particular, we are looking at placing our photodetectors on ultrathin and flexible substrates or integrating the devices into existing computer chips and cameras," added co-leader Frank Koppens in the same article.

The researchers offer a range of applications for the graphene-and-quantum-dot combination, including digital cameras and sensors. But it seems the researchers seem particularly excited about one application in particular. They expect the material will be used for night-vision technologies in automobiles—an application I have never heard trotted out before in relation to nanotech.

"We expect that most cars will be equipped with night-vision systems in the near future and our arrays could form the basis of these," Koppens told physicsworld.com.

Nanostructured Silicon Anodes Improve, But Is It Enough for EVs?

Lithium-ion (Li-ion) batteries are ubiquitous but flawed, especially for electric vehicle applications. The problem has been poor charge life. But researchers have shown that if you replace the graphite on the anodes with silicon, the charge can be increased by a factor of ten. Problem is that after a few charge-discharge cycles the silicon cracks and becomes inoperable from the expansion and contraction of the material.

One solution: nanostructured silicon anodes that improve on the traditional silicon variety in this area of charge-discharge cycles. While there’s been improvement from silicon’s poor performance, the nanostructured variety still doesn’t measure up to plain old graphite in this regard.

That's a shame, because silicon anodes just crush graphite when it comes charge life. So, if there were a way to get silicon to work, it would offer some considerable benefits to Li-ion batteries, and nanotechnology has been the prayer. At least one commercial interest believes that the nano-based solution has already been developed for enabling silicon anodes to survive a large number of charge-discharge cycles.

Now research, led by Yi Cui of Stanford, who holds the distinction of having the most cited paper at ACS journal Nano Letters over the past 10 years, has come up with a nanostructured silicon capable of 6,000 cycles while maintaining 85% of its capacity.

That sounds good…for lithium-ion batteries, but Cui demonstrated just last year that potassium or sodium ions in place of the lithium variety can create batteries capable of 40,000 cycles while maintaining 83% of its charge. The issue there was that they had the cathode sorted but hadn’t yet developed the anode.

Then there's the issue of applications. The sodium- or potassium-based ion battery was targeted for large-scale energy storage on the electrical grid. Again, the Li-ion battery Cui and his colleagues has developed is being targeted for electric vehicle applications.

Cui himself started a company, Amprius, in which he was going to use his silicon nanowire anode technology with the aim of doubling the energy density of Li-ion batteries. I hope the company succeeds, but even if you achieve a doubling of energy density in the Li-ion battery you still only get to 400Wh/kg for powering vehicles. According to Stephen Chu, Li-ion batteries will need to get to 1000Wh/kg to really be competitive with fossil fuel-powered vehicles. Why can’t powering laptops be good enough for these batteries?

Nanocatalyst Splits Water Molecules at a Fraction the Cost of Platinum

Nanotechnology-based solutions for improving fuel cells have fallen a bit short of expectations. So  recent research has focused instead on using nanotech to produce hydrogen gas for existing fuel cells more cheaply and efficiently.

Some of these solutions—like those from University of California, San Diego, or those of Angela Belcher of MIT—have been aimed at breaking down a water molecule into its constituent parts of hydrogen and oxygen by replicating photosynthesis. This is really cutting edge stuff and pretty far removed from the process currently used to create hydrogen gas, which involves applying electricity to water in the presence of a catalyst.

One of the main problems with this current method has been the cost of platinum, which is the best material to serve as a catalyst for the process. With platinum going for about $50,000 per kilogram, it’s pretty clear why this gets to be a very expensive process.

To address this issue researchers at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have developed a new nanomaterial that can duplicate the capabilities of platinum at a fraction of the cost.

“We wanted to design an optimal catalyst with high activity and low costs that could generate hydrogen as a high-density, clean energy source,” said Brookhaven Lab chemist Kotaro Sasaki in a press release covering the research. “We discovered this exciting compound that actually outperformed our expectations.”

The researchers—whose results were initially published online yesterday in the journal Angewandte Chemie International Edition—determined early on that nickel can take the reactive place of platinum, but didn’t have the same electron density. While the introduction of metallic molybdenum to the nickel improved its reactivity, it still wasn’t up to platinum standards.

Sasaki and his colleagues believed that they could push the nickel-molybdenum material up to platinum levels by applying nitrogen, based on the understanding that this had been done with bulk materials. They weren’t quite sure what to expect when you applied the nitrogen to nanoscale nickel-molybdenum but they suspected that it would change the structure of the material into discrete, sphere-like nanostructures. That’s not what they got.

To the surprise of the researchers, the infusing of nitrogen with nickel-molybdenum material produced two-dimensional nanosheets.

“Despite the fact that metal nitrides have been extensively used, this is the first example of one forming a nanosheet,” said research associate Wei-Fu Chen, the paper’s lead author in the Lab’s press release. “Nitrogen made a huge difference – it expanded the lattice of nickel-molybdenum, increased its electron density, made an electronic structure approaching that of noble metals, and prevented corrosion.”

While the researchers are realistic in their understanding that this new catalyst doesn’t answer all the issues facing the production of hydrogen gas, it does have the advantage over other solutions in that it can be directly substituted into current processes that use platinum as a catalyst to cut the costs dramatically. Whether this will usher in the age of the hydrogen economy is impossible to say, but it's a step in the right direction.

Nanogenerators Easier and Cheaper to Produce than Ever Before

Professor Zhong Lin Wang at Georgia Tech has been championing his work in exploiting the piezoelectric qualities of zinc oxide nanowires for years now with his so-called "nanogenerators".

Now researchers at the Korea Advanced Institute of Science and Technology  (KAIST) have taken up the mantle of Wang’s work by creating a piezoelectric “nanogenerator” more easily and cheaply than ever before.

The research, which was initially published in the Wiley journal Advanced Materials, produced a piezoelectric nanocomposite through relatively simple processes such as spin-casting and the bar-coating method. So this new generation of “nanogenerators” is not restricted by a complicated and high-cost process or even size.

Even Wang himself is impressed by this work. “This exciting result first introduces a nanocomposite material into the self-powered energy system, and therefore it can expand the feasibility of nanogenerator in consumer electronics, ubiquitous sensor networks, and wearable clothes," says Wang.

The lead researcher on this project, Keon Jae Lee, a Professor at the Department of Materials Science and Engineering at KAIST, has a comprehensive website that covers much of the work.

One of the videos from that website (below) manages to demonstrate just how uncomplicated the process is to create this nanocomposite and shows how it works in generating an electric charge from the movement of a finger.

While Wang has been expert at getting this research into the media, it hasn't yet found itself in commercial products. It should be interesting to see if these manufacturing simplifications and cost reductions will help push this technology into consumer electronics where Wang always envisioned it could be.

NIST Develops Laser Feedback Method for Nudging Nanoparticles Just Enough

The example of the Lycurgus Cup, as a paradigm of nanotechnology, has always bothered me. Yes, the glass of the cup changes colors depending on the light because of the presence of nanoparticles, but it's not really nanotechnology. While the  Romans knew the process that would make this glass, they didn’t know why it performed the way it did. Without that kind of knowledge—and the concomitant ability to manipulate an effect—what you have is at most craft, and not science.

That's the reason many trace the real beginning of nanotechnology to the development of microscopy tools like the scanning tunneling microscope (STM). Without the array of tools we have for seeing, measuring and manipulating the world on the scale of atoms and molecules, there would have been little to no advancement in nanotechnology. In other words, nanotechnology is not just the world of the very small but having the tools to know what we’re doing at that scale and setting about to do it.

Now researchers at the National Institute of Standards & Technology (NIST) have added a new tool to the arsenal that should allow researchers to better user lasers in manipulating nanoparticles inside biological cells without damaging them. The researchers have developed a control and feedback system that lets the laser nudge nanoparticles to exactly where they are needed for analysis.

In the NIST press release covering the technology, Thomas LeBrun, one of the researchers on the project, explains: "You can think of it like attracting moths in the dark with a flashlight. A moth is naturally attracted to the flashlight beam and will follow it even as the moth flutters around apparently at random. We follow the fluttering particle with our flashlight beam as the particle is pushed around by the neighboring molecules in the fluid. We make the light brighter when it gets too far off course, and we turn the light off when it is where we want it to be. This lets us maximize the time that the nanoparticle is under our control while minimizing the time that the beam is on, increasing the particle's lifetime in the trap."

The researchers report two key results:

  • 100-nanometer gold particles remained trapped 26 times longer than had been seen in previous experiments
  • Silica particles 350 nanometers in diameter lasted 22 times longer, but with the average beam power reduced by 33 percent.

LeBrun characterizes this development as pushing the field an order of magnitude further than it was before. He says the researchers will now begin work in building complex nanoscale devices and testing nanoparticles as sensors and drugs in living cells.

Nanocomposite Fillings Kill Bacteria and Regenerate the Tooth

Earlier this year, I was teased by news that graphene could be ‘tattooed’ to one’s tooth to detect harmful bacteria. I say teased because the bacteria that could be detected was not the Streptococcus mutans bacteria that causes tooth decay. Instead the device could detect just about every other bacteria except that one. So, what appeared to be a story that might be able to redeem me in the eyes of the dental community—after I echoed others’ doubts about the efficacy of a nanofilm in promoting cell growth in decayed teeth—just wasn’t the opportunity I had hoped for.

This week, however, researchers at the University of Maryland have offered up a technology that should be of keen interest to the dentists of the world, and those who visit them. The researchers have developed a nanocomposite that can be used not only as a filling for the cavity, but also can kill any remaining bacteria in the tooth and regenerate that tooth’s structure that had been lost due to the decay. You can access a PDF file that offers up a poster presentation of the research here.

The basis of the nanocomposite are calcium phosphate nanoparticles that regenerate tooth minerals. The ingredient that kills off the remaining bacteria in the tooth is made up of silver nanoparticles and quaternary ammonium along with a high pH. According to the news release, the alkaline pH is the feature that limits the acid production by the bacteria. This begs the question: why not just develop a mouthwash with these ingredients so you don’t need the cavity filling in the first place?

One answer may be the use of silver nanoparticles, which are not without some controversy when associated with anything that might be consumed by humans. Sure enough, just a quick perusal to see how this story was being covered turned up a blog called “Beyond Pesticides” that offered up this headline: “New Dental Fillings Utilize Controversial Nanotechnology to Kill Bacteria”. The blog goes into some detail about the potential risks of silver nanoparticles.

The researchers are continuing with their animal and human testing with the nanocomposite. Given that some sectors of the public are concerned about the potential risks of silver nanoparticles, they should probably take a look at the issue as part of their research.

President's Council Wants a Few More Things from the National Nanotechnology Initiative

Just four months after the National Nanotechnology Initiative (NNI) responded to the President’s Council of Advisors on Science and Technology’s (PCAST) 2010 report on the status of nanotechnology, PCAST has offered up a new assessment.

While the PCAST report on the NNI in 2010 wanted to see greater efforts towards commercialization and some attempt to address environmental, health and safety (EHS) concerns, this time they just wanted to see how well the NNI had done in meeting their previous recommendations.

In the Whitehouse.gov blog covering the announcement it seems PCAST are satisfied:

“PCAST found that the Federal agencies in the NNI have made substantial progress in addressing many of the 2010 recommendations that were aimed at maintaining U.S. leadership in nanotechnology… The PCAST assessment particularly commends the expanded efforts of the NNCO in the area of commercialization and coordination with industry, and the NNCO’s release of a focused research strategy for addressing environmental, health, and safety (EHS) implications of nanotechnology.”

Okay, pat on the back, job well done…uh, wait, there are still some new recommendations that PCAST would like to see addressed.  You can find them in the PDF of the full report on page vii. They fall into the areas of strategic planning, program management, metrics for assessing nanotechnology’s commercial and societal impacts, and…wait for it…increased support for EHS research.

Additional support for EHS research might be a required element for every PCAST report in the future. More interesting to me, however, is this continued emphasis on improved “metrics for assessing nanotechnology’s commercial and societal impacts.”

It seems to me that this is an area in which everyone from governments to corporations wants a formula that will churn out a sense of what kind of impact nanotechnology is really having. While nobody is satisfied with the metrics that we have,  I would suggest that there are few number-counting options that will really be able to sort out the full impact of nanotechnology. But again, it should be interesting to see what they come up with.

Inventors Claim Graphene Hybrid Could Revolutionize Electronics Industry

The latest flavor of graphene is something the researchers who invented it are calling GraphExeter. While the name itself is not big on originality—just combining the words graphene with Exeter (the name of the University where the researchers are affiliated)—it does seem to have some superlative characteristics.

The researchers, led by University of Exeter Engineer Monica Craciun, claim that the material is the most transparent, lightweight and flexible material ever for conducting electricity. GraphExeter is actually two layers of graphene sandwiching molecules of ferric chloride. The ferric chloride increases the electrical conductivity of the graphene without compromising its transparency.

“GraphExeter could revolutionize the electronics industry,” says Craciun in the university press release. “It outperforms any other carbon-based transparent conductor used in electronics and could be used for a range of applications, from solar panels to ‘smart’ teeshirts. We are very excited about the potential of this material and look forward to seeing where it can take the electronics industry in the future.”

Craciun and her colleagues initially published their work in the Wiley journal Advanced Materials where the description is not quite as bold as the university press release. Nonetheless the abstract does say that GraphExeter “outperforms the current limit of transparent conductors such as indium tin oxide, carbon-nanotube films, and doped graphene materials.”

It seems that the material’s capability as a superior transparent conductor makes it an ideal candidate for optoelectronic devices. So this latest news represents quite a recent run for graphene in optoelectronics applications following IBM’s announcement earlier in the week.

The researchers say that potential applications include photovoltaics and wearable electronics among many others. An ambitious list to say the least, but the researchers do point out that looming bottleneck of using indium tin oxide (ITO) for transparent conducting electrodes provides a strong motivation to find an alternative.

Whether a shortage of ITO will be enough to get the electronics industry to adopt a material that has just been invented remains to be seen. So I think we will see some evolutions to this material’s development before we see any revolution occur to the electronics industry.

IBM Researchers Build Multi-Layer Graphene for Photonics Applications

While new breakthroughs continue to come in using graphene (or cousins of graphene) for electronics applications, research in the area of using graphene for photonics applications is growing.

The latest use of graphene for photonics comes from IBM where researchers have created a graphene/insulator superlattice capable of serving as a terahertz frequency notch filter and linear polarizer, according to an article in EE Times.

IBM has certainly been a trailblazer in using graphene for electronics applications over the years, such as their graphene transistor work and then later an integrated circuit.  But IBM—along with other researchers outside of Big Blue—has also been hard at work looking at how graphene could be used in optoelectronics.

IBM Fellow Phaedon Avouris told EE Times that "In addition to its good electrical properties, graphene also has exceptional optical properties. In particular, it absorbs light from the far-infrared to the ultra-violet."

It is in the terahertz frequency that graphene’s optical properties were of particular interest to the IBM team. Tunable notch filters have become fairly ubiquitous in optoelectronics, but they didn’t really operate at the terahertz frequency. The new application seemed that a good way to take advantage of graphene's terahertz frequency capabilities.

“Unfortunately, today there are very few ways of manipulating terahertz waves such as polarizing and filtering it, but because graphene operates well at terahertz frequencies we have concentrating on creating these types of devices," said Avouris.

As good as graphene is at operating at terahertz frequencies, when single-layer graphene is used carrier concentration and resonant frequency are too weak for it to be used in photonics applications. This is where the IBM researchers' work began and they developed a multi-layer graphene/insulator superlattice that improved the carrier density and the resonant frequency.

“We have found that graphene interaction with electromagnetic radiation is particularly strong in the terahertz range, however with a single layer of graphene the interaction was still not strong enough," Hugen Yan, a member of the Nanoscale Science and Technology Group at IBM's T. J. Watson Research Lab (Yorktown Heights, N.Y.) was quoted as saying. "But by using a multi-layer stack structure in microdisk arrays we achieved frequency selectivity in the terahertz range, allowing us to tune the desired resonant frequency."

The devices that IBM have developed could find their way into future mid- and far-infrared photonic devices, such as detectors and modulators. The next step for the researchers will be to tune the devices for infrared frequencies typically used in optical communications equipment.

FDA Invites Industry Collaboration on Nanotech Safety in Food and Cosmetics

Last Friday the US Food & Drug Administration (FDA) issued draft guidance documents on the use of nanotechnology by food and cosmetics industries.  Since then the reports covering this story have characterized it as “FDA opts for stricter role on nanotechnology”  or “FDA proposes new rules for nanotechnology.”

While "draft guidance documents" may not garner as much interest as these more confrontational headlines, the guidelines come closer to the FDA's invitation to industry on how together they can look at the issue of nanomaterials in food and cosmetics. And from someone who has been accused of being too dismissive of government attempts at regulation of nanotech, I believe this to be a sensible way to approach the issue.

A misconception that has circulated in a number of the stories covering this news is that the guidelines are limited to food packaging. This is not true. The guidelines cover food ingredients and additives in addition to packaging as the blog Frogheart rightly points out.

But getting back to why I believe this collaborative approach makes sense: Both the food and cosmetics industries could quite easily retreat into silence about their use of nanotechnology in their products if a more hard-line approach was taken. This reticence has made sense to me considering the hyped up rhetoric that seemed to preclude the possibility for a reasoned, balanced and scientific discussion of nanotechnology in food as well as cosmetics.

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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
New York, NY
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