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The Memristor’s Fundamental Secrets Revealed

You would expect that a new fundamental passive circuit element, first postulated a mere 42 years ago, and first identified in the wild in 2008, would be as rare as hen's teeth. You'd be wrong. It turns out they're as common as cat's whiskers.

Two researchers from mLabs in India, along with Prof. Leon Chua at the University of California Berkeley, who first postulated the memristor in a paper back in 1971, have discovered the simplest physical implementation for the memristor, which can be built by anyone and everyone.

In two separate papers, one published in arXiv (“Bipolar electrical switching in metal-metal contacts”) and the other in the IEEE's own Circuits and Systems Magazine (“The First Radios Were Made Using Memristors!”), Chua and the researchers, Varun Aggarwal and Gaurav Gandhi, discovered that simple imperfect point contacts all around us act as memristors.

“Our arXiv paper talks about the coherer, which comprises an imperfect metal-metal contact in embodiments such as a point contact between two metallic balls, granular media or a metal-mercury interface,” Gandhi explained to me via e-email. “On the other hand, the CAS paper comprises an imperfect metal-semiconductor contact (Cat's Whisker) which was also the first solid-state diode. Both the systems have as their signature an imperfect point contact between two conducting/partially-conducting elements. Both act like memristor.”

Gandhi says that this ubiquitous presence of memristors in simple physical systems around us strongly points towards the fundamental nature of the memristor.

While the two papers are connected via their similarity in construction, there is also a historic connection, according to Gandhi.

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Quantum Dots in Displays Get a New Tool

Quantum dots are beginning to realize their promise for enabling the next generation of  computer displays and TVs. Just a few weeks ago, Sony put its LCD displays enabled by quantum dots provided by Massachusetts-based QD Vision on the market. And the partnership between California-based Nanosys Inc and 3M to market its Quantum Dot Enhancement Film (QDEF) technology should be on store shelves soon.

The fact that quantum dot technology has made it to market indicates just how far the technology has progressed. However, this is just their first introduction into the market so we should expect further refinements to the technology.

Some of those refinements are already in the offing. Researchers at the Massachusetts Institute of Technology say that they've developed a method that should serve to optimize quantum dots for display applications. 

The newly developed method--dubbed photon-correlation Fourier spectroscopy in solution—makes it possible to obtain the spectral properties of single particles in large groups. Up until now, if you wanted to get the spectral properties of single particles you had to look at them individually. With this new method it is possible to attain that data while looking at billions of particles at the same time.

The method, which was published in the journal Nature Chemistry ("Direct probe of spectral inhomogeneity reveals synthetic tunability of single-nanocrystal spectral line widths"),  starts by shining a laser into the quantum dots and then measuring the light that is emitted from the dots at very short time scales. This allows for dots that are not very far apart in space to be differentiated in time. Once the measurements are collected, it becomes possible to compare pairs of photons emitted by individual particles. This in itself does not provide the absolute color of particular particles, but it does allow for a statistical measure of the collection of quantum dots.

“We get the average single-particle line width in the solution, without any selection bias,” said Jian Cui, one of the authors of the paper, in a press release.

The method should make it possible to determine the quality of each quantum dot production method, serving as a kind of quality control check. This will also lead to being able to fine tune the production processes so that particular quantum dots can be synthesized for various applications.

The method has already determined that quantum dots synthesized from cadmium selenide, which are now widely used, do produce very pure colors. But it has also shown that indium phosphide is intrinsically suited for producing pure colors.

All of this should provide a useful tool in refining and improving the technology of quantum dots in displays.

Photo: Laren Aleza Kaye/MIT

Nanomaterial Introduces Zinc-Air Batteries to the EV Party

It seems both the commercial markets and the research community are coming to terms with the idea that the energy density (the amount of energy stored per unit volume) of lithium-ion (Li-ion) batteries will keep them from ever becoming a completely satisfactory solution to powering all-electric vehicles (EVs).

The market’s turn towards this realization is evidenced by the demise of two high flying manufacturers of Li-ion batteries based on nanotechnology: Ener1 and A123 Systems.  The research community is also coming around to this notion and shifting its focus to alternative battery technologies as evidenced by the recent improvements to the lithium-air battery.

Along these lines, researchers at Stanford University are now reporting that they have used nanocrystals made from non-precious metal oxide and combined them with carbon nanotubes to make a hybrid material that works as a catalyst to improve the performance of zinc-air batteries.

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Graphene Image Sensor Achieves New Level of Light Sensitivity

[Editor’s note: This post was originally published with the headline “Graphene Image Sensor 1000 Times More Sensitive to Light.” The new headline reflects changes to the post that correct a factual error about the relative performance of a new graphene image sensor. A press release out of Nanyang Technological University heralded the creation of a graphene sensor by researchers there, ascribing to it a 1000-fold increase in the light sensitivity over “current imaging sensors found in today’s cameras.” But in the paper detailing their work, the researchers actually reported that the photo-responsivity (with high photoconductive gain) of the graphene sensor was three orders of magnitude greater than other graphene-based imaging sensors.]

Graphene has become an attractive material for optoelectronic applications because it can absorb light over a broad wavelength range. However, just as in electronic applications, graphene has struggled in optoelectronics because it lacks a band gap.

However, now researchers at Singapore’s Nanyang Technological University (NTU) have engineered a graphene-based image sensor so that their photoresponsivity has been improved by three orders of magnitude over previous versions of the device.

“We have shown that it is now possible to create cheap, sensitive and flexible photo sensors from graphene alone,” said Wang Qijie, assistant professor, NTU’s School of Electrical & Electronic Engineering, in a press release. “We expect our innovation will have great impact not only on the consumer imaging industry, but also in satellite imaging and communication industries, as well as the mid-infrared applications.”

The new graphene-based sensor, which is described in this month’s Nature Communications ("Broadband high photoresponse from pure monolayer graphene photodetector"), the NTU team believes can be mass produced.

By engineering a band gap into the graphene, the NTU researchers essentially trapped light-generated electron particles for a longer amount of time, which created a much stronger signal. These signals could then be processed into an image.

While dramatically improving the photoresponsivity (with high photoconductive gain), Wang still believes that the underlying technology of the graphene sensor has even more room for improvement. “The performance of our graphene sensor," Wang says, "can be further improved, such as the response speed, through nanostructure engineering of graphene, and preliminary results already verified the feasibility of our concept.”

Image: Nanyang Technological University

Quantum Dot-Enabled LCDs Draw Closer to Store Shelves

Liquid crystal displays (LCDs) enabled by quantum dots have been promising gadgets with much richer colors and better energy efficiency for years now. Nanosys, the California-based quantum dot producer, has been promoting this capability for over three years.

There seemed to be a real chance of seeing quantum dot-enabled LCDs on the market last year when Nanosys agreed with the Optical Systems Division of 3M Company to supply its Quantum Dot Enhancement Film (QDEF) for the production of a new generation of LCDs capable of displaying 50 percent more color.

Just two weeks ago, while reporting on another quantum dot enhancement to LCDs, I wondered what happened to the Nanosys/3M project. Part of my curiosity was due to the fact that there was some hint that we should have heard something within the past year about their commercial availability. At the time of the Nanosys/3M announcement last year, Jason Hartlove, CEO of Nanosys said that major LCD manufacturers are now testing the film, and a 17-inch notebook incorporating the technology should be on shelves within six months.

Now we get an update from 3M on the “3M QDEF solution” explaining that it will be available to customers (LCD manufacturers) for design cycles by the late second quarter this year (end of June). How long it takes for manufacturers to go from design cycles to products on the shelves is not made clear in the announcement. But I imagine, based on the Hartlove’s announcement from last year, that it could take anywhere from six months to never, depending on how happy manufacturers are with their evaluation of the material.

I’m not sure what’s happening here. It could be that an over zealous supplier of quantum dots over stated where the commercial process was and the big corporate conglomerate that actually knew the supply chain thought they better come in one year later and straighten out the time line. I hope that’s the case. Otherwise we might be in one of those cycles where each year we hear that the product is just six months away from our shelves for the next five years. That would would be especially disappointing because this sounds like it would make for some excellent LCDs.

Editor's Note: After the publication of this piece, it came to my attention that quantum dots supplied by Massachusetts-based QD Vision Inc. are now available in some Sony LCD displays. While these devices were promised at the CES in January to be available by the Spring of this year, I had not heard any official announcement of their availability. Nonetheless Sony does have a product that appears to be a quantum dot-enabled LCD display for sale on its website. It would seem Sony's "crystal" display did turn out to be a quantum-dot enabled LCD display.

Image: 3M

Graphene-based Ink Promises Future Flexible Electronics

Researchers at Northwestern University’s McCormick School of Engineering have developed a graphene-based ink that could be sprayed onto substrates to make flexible electronics.

While the research thus far has only extended to spraying 14-nanometer-thick layers to create precise patterns, the researchers believe that method they developed for creating the graphene-based ink could lead to flexible electronic devices in the future.

Graphene’s s one atom thick, two-dimensional characteristics have been tantalizing to those interested in flexible electronics.  Just last year, researchers at Cornell University performed work with graphene that suggested that circuits could be made so thin that they would be both flexible and transparent.

The Cornell researchers' approach used traditional chip manufacturing processes like lithography, whereas the Northwestern team has developed a graphene-based ink that could be used for ink-jet printing of electronic circuits.

This use of ink-jet printing means that the Northwestern team had to focus much of its work on finding ways of creating an ink-powder form of graphene without losing any of its attractive electrical characteristics, such as high conductivity.

The research, which was initially published in the Journal of Physical Chemistry Letters (“Inkjet Printing of High Conductivity, Flexible Graphene Patterns”), yielded a method for exfoliating graphite so that it produces small bits of graphene whose electrical conductivity has not been compromised. The method can be done at room temperature using a combination of ethanol and ethyl cellulose to exfoliate the graphite. This technique, say the researchers, reduces residues and leaves a high concentration of graphene flakes that are subsequently put into a solvent, creating the ink.

While the graphene-based ink leads to patterns that are 250 times as conductive as previous attempts to print graphene-based electronic patterns, the paper doesn't discuss whether the material can be engineered to act as a semiconductor. Engineering a band gap into graphene remains a critical prerequisite for applying the material in electronics, and was a preoccupation for the Cornell team in its attempts to use graphene in flexible electronics.

Of course, the focus of the Northwestern team was just to find a way to extract a large amount of graphene during an exfoliation process on graphite and have the resulting material remain highly conductive. But if a method is developed that allows for the doping of the graphene ink so it can be a semiconductor as well as a conductor, we may indeed be moving towards flexible electronics made from graphene.

Image: PARC

Heinrich Rohrer: The Modest Pioneer of Nanotechnology

By now, just about everyone with an interest in the field of nanotechnology has heard that Heinrich Rohrer, who won the 1986 Nobel Prize in Physics for his co-invention of the scanning tunneling microscope (STM), passed away this week at the age of 79 from natural causes.

It would be hard to overstate the impact that Rohrer and his colleague at IBM Zurich, Gerd Binnig, have had on the field of nanotechnology. The STM has become a cornerstone tool for characterizing and manipulating the world on the nanoscale. Through ever more refined iterations of the device, we are peering into the atomic scale with greater and greater clarity. Even the lay-est of laypersons can appreciate the STM’s feats of prowess when they're put on display in videos in which atoms are made to perform stunts as if they're children in a home movie.

For a description of how the STM came to be and how it works, IBM Zurich’s reporting on Rohrer’s life is both thorough and poignant and I recommend you take a look at it.

All I would add are my own personal recollections of Rohrer from a one-on-one interview I had with him and from joint interviews I and other journalists had with him and Binnig back in 2011 while attending the grand opening of IBM Zurich’s new nanotechnology research facility, which IBM aptly named the  “Binnig and Rohrer Nanotechnology Center.“

In these interviews, I was struck by three things.

First, Rohrer’s absolute humility in his role in the development of the STM. He characterized himself as simply wanting to see if it would be possible to eliminate approximations of inhomogenities on surfaces and measure them precisely. Beyond his genius of simply asking the right question, he also had the good sense to hire a brilliant young scientist—Binnig—who could help him in his quest.

Second, Rohrer was funny. Nearly everything he said during our brief time together had a wry twist of humor to it. It seemed to be humor borne of humility (not taking himself too seriously), pragmatism, and his sense that his role as a leader in a technology revolution was so unexpected that he just had to laugh at it.

Finally, I was struck by the chemistry between the two men. They expressed unflagging admiration for one another, despite being in some ways polar opposites. Rohrer was the pragmatist, while Binnig seems to have the touch of the poet. Interestingly, though, in the development of the STM those roles were reversed in that Rohrer was the idea guy and Binnig was the engineer who got the device built.

In any event, their contrasting personalities, humor, and chemistry were on clear display the day of the opening of the lab named after them.

After Binnig had carefully answered a question about their co-discovery of the STM, Rohrer quipped, "If you didn't quite understand what Gerd just told you, you are not alone."

The audience laughed with relief that it was okay that they didn’t understand the carefully thought out explanation—I among them. But the truth was that Rohrer understood Binnig’s explanation perfectly and said that to put the audience at ease. Rohrer was both a great scientist and a true gentleman.

Image: IBM

“Seven or Never”: Emerging Technology’s Seven-Year Odyssey

Technology writers often hear complaints from readers that go something like: “All you ever talk about is this technology 'would,' 'could,' or 'might'.” Fair enough. But when the field is an emerging one, such as nanotechnology, most of the good stories are about just that—a development in the lab, or just coming out of it, that may or may not have an impact in the years to come.

Let's face it. A tech blog isn't the Daily Racing Form, and even in horseracing, good breeding is no guarantee of crossing the finish line first. First comes the struggle to secure funding, and then come any number of opportunities for management to make some tragic blunder or to fail to dislodge the incumbent competition, which often successfully blocks the technology from ever coming to market. The bottom line is that not only is success in the marketplace the exception and not the rule, but discerning the few winners from the many losers at a technology's earliest stages can make picking the ponies feel like child's play.

Then there's the frustration of time. Going to the racetrack offers immediate gratification, but handicapping high-tech requires quite a bit of patience. I have been writing about emerging technologies for over 15 years. In that time, I've chronicled some successes and failures and a common rule of thumb I picked up early on was that it typically takes seven years to bring a laboratory technology to market.

The seven-year rule is something of a shibboleth. Try as I might, I have not been able to determine where that notion originates, but I thought I should at least try to see how accurate it is as a barometer as to whether a new technology can make a commercial impact.

Thus this post starts a new series within The Nanoclast that looks back on some of the technologies that we have covered with words like “would”, “could” or “might.” How far along have On-Off Super Glue or Junctionless Transistors, to name just two of my favorites, progressed?

We're calling the series “Seven or Never,” a reference to the seven-year time-to-market timescale shibboleth. But first, I thought I might see if that timeline really holds true by asking a couple of tech jockeys who have put in their time riding fast horses down a seven-furlong track.

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Nanotube Supply Glut Claims First Victim

Just three years after announcing a huge capacity increase to its multi-walled carbon nanotube (MWNT) production, Bayer Material Science has announced that it will completely close down its MWNT production to focus on its core business.

This is no surprise since there was a huge glut of product resulting in industry utilization rates that must have been in the single digits. This oversupplied market was the result of a MWNT capacity arms race that started in the mid-2000s. While this steep ramping up of production capacity reduced pricing from $700/kg in 2006 to below $100/kg in 2009—with some estimates putting the price at $50/kg as of last year—the problem seemed to be that no matter how cheap you made the stuff nobody was buying it because there were no applications for it. This resulted in stories, at once humorous and worrisome, of big chemical companies that had gotten themselves caught up in this arm race making desperate phone calls to laboratory researchers pitching application ideas for the material.

While some observers believed that this price cut would result in the applications being developed, most people recognized that this was a case of putting the cart before the horse, or “technology push” ahead of the preferable “market pull.”

This is not to say strategically it was wrong for a company like Bayer Material Science to build out capacity for a product that nobody seemed to want at that moment but may in the future. A company like Bayer can ramp up production with relatively little capital cost and manage to price everyone else out of the market. It was worth the risk.

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New Quantum Dots Make Colors in LCD Even Brighter

Quantum dots have been promoted as a technology that is poised to transform the LCD (liquid-crystal display) market for years now. This promise looked to be taking shape when California-based Nanosys Inc. announced last year that it had worked out a deal with the Optical Systems Division of 3M Company to produce an LCD capable of displaying 50 percent more color.

The Nanosys/3M pairing was intended to improve the color and performance efficiency of LCD displays by using the quantum dots as an improved back light.

In the current display market landscape, LCDs are both inefficient and don’t produce the vibrant colors of organic light-emitting diodes (OLEDs). However, LCDs are far cheaper to produce in large screen sizes, and consumers often choose the right price over the right color. Quantum dots were supposed to give us the best of both worlds.

In work that appears to tip the scales further for quantum dot-enabled LCDs, researchers at the University of Illinois at Chicago (UIC) have developed a method for doping quantum dots that will give LCDs a color vibrancy not seen before.

In research published in the ACS journal Nano Letters ("Cluster-Seeded Synthesis of Doped CdSe:Cu4 Quantum Dots"), the UIC team reveal a method for introducing precisely four copper ions into each and every quantum dot. This doping with copper ions opens up the potential for fine-tuning the optical properties of the quantum dots and producing extraordinarily bright colors.

“When the crystallinity is perfect, the quantum dots do something that no one expected—they become very emissive and end up being the world’s best dye,” says Preston Snee, assistant professor of chemistry at UIC and principal investigator on the study, in a press release.

Whether UIC's doped quantum could be a compliment to the Nanosys/3M technology or a competition is not known. Likewise, it remains to be seen if they can keep LCDs at or near their current price point while bringing picture quality up to that of OLEDs. In other words, it'll take a few more years worth of Consumer Electronics Shows to sort out the winners and losers.

Image: University of Illinois, Chicago



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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