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Trilogy of 2-D Materials Could Constitute Future Electronics

Researchers at Rice University and Oak Ridge National Laboratory (ORNL) are aiming to remake the world of two-dimensional materials, including graphene, molybdenum disulfide (MDS) and hexagonal boron nitride (hBN), so that together they constitute a trilogy of materials for the next generation of electronics.

The ultimate goal of their research is to combine these three 2D materials: a semiconductor, insulator and conductor (MBS, hBN and graphene, respectively) to create a range of electronic devices, such as field-effect transistors, integrated logic circuits, photodetectors and flexible optoelectronics. To get there the researchers have come up with a better way of producing MDS.

When research into using MDS as a 2D material for electronics first started to gain notice, scientists suggested that it would serve as a compliment to graphene, especially in applications that require a transparent semiconductor. While some have seen a rivalry between the two 2D materials developing,  research has continued to pursue them as compliments to one another. In fact, recently graphene and MDS have been mated to create a new flash memory. And earlier this year, some of the same Rice University researchers in this current work showed that graphene could be weaved together with hBN to create nanoscale patterns.

While a trilogy of 2D materials might be the long-range aim, the team of researchers started their work by seeing if they could produce large, high-quality sheets of MDS through chemical vapor deposition (CVD) instead of employing the so-called “Scotch Tape” method in which layers of the material are peeled off from bulk samples.

In the research, which was published in the journal Nature Materials (“Vapour phase growth and grain boundary structure of molybdenum disulphide atomic layers”),  the team discovered that they could improve the CVD process by adding artificial edges to the substrate.

“The material is difficult to nucleate, unlike hBN or graphene,” said Sina Najmaei. co-author of the paper, in a press release. “We started learning that we could control that nucleation by adding artificial edges to the substrate, and now it’s growing a lot better between these structures.”

The final material built up through CVD was sent over to ORNL where microscopy tools were used to characterize it. Among the properties they discovered in the material is what they believe to be the potential for the atoms in the MDS to bind with carbon atoms in the graphene.

“We’re working on it,” said Zheng Liu, a researcher at Rice, in the press release. “We would like to stick graphene and MDS together (with hBN) into what would be a novel, 2-D semiconductor component.”

“These are very different materials, with different electronic properties and band gaps. Putting one on top of the other would give us a new type of material that we call van der Waals solids,” said Pulickel Ajayan, an engineering professor at Rice. “We could put them together in whatever stacking order we need, which would be an interesting new approach in materials science.”

Image: Oak Ridge National Laboratory

Angela Belcher: The Consummate Nanotechnologist

The first I heard about Angela Belcher, someone explained to me her work using genetically engineered viruses to build electronic circuits through self assembly. That was ten years ago and in the ensuing decade she has, to the fascination of everyone, mixed the biological with the electrical in ways that alter entire industries, including solar power, batteries, fuel cells, and fuel production.

So consistently inventive has been her work that I was surprised she had not won the US $500,000 Lemelson-MIT Prize until she was given it this week.

Belcher is the consummate nanotechnologist because she has never been tied down to one discipline, instead moving freely between biology, physics, and chemistry. This multidisciplinary approach has allowed her to consider ideas that are readily dismissed by those tied to just one of these disciplines.

Two years ago I had the occasion to speak to Michael Grätzel, himself a winner, in 2010, of the similarly-prestigious Millennium Technology Prize. I asked him about Belcher’s work in using viruses to manipulate carbon nanotubes for use in dye-sensitized solar cells. Grätzel, who invented the dye-sensitized solar cell, said, “That’s a real breakthrough. We can learn a lot from her fascinating experiment.”

Belcher’s research is a rare combination of the visionary and the practical—consistently groundbreaking, yet there have always been commercial implications. In 2002, along with Evelyn Hu of Harvard University, Belcher set up Cambrios Technologies Corporation to commercialize the use of genetically modified viruses to create transparent coatings made of silver nanowires for touch screen displays. Then in 2007 she and Hu co-founded another company, Siluria Technologies, to use the viruses to produce clean fuel.

It’s clear that when Belcher develops a method for using viruses to create a new generation of lithium-ion batteries, she is doing it with the expectation that it will someday be used in the real world.

“The full implications of Angela Belcher’s work are only beginning to be realized, and yet the applications already appear to be far-reaching,” says Hu in a press release covering Belcher’s award. “Her inventions are always linked back to her profound passion and compassion for society, and her desire to improve the quality of life for others.”

Image: Dominick Reuter/MIT

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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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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