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Graphene and Carbon Nanotubes Together Produce a Digital Switch

The two darlings of carbon nanomaterials, carbon nanotubes and graphene, increasingly are joining forces even as they are having  their obituaries read while still hardly out of the lab. We’ve seen them being used in hybrid energy storage applications and for supercapacitors.

Now researchers at the Michigan Technological University (MTU) have combined these two nanomaterials to tackle a far more difficult application field: electronics. Specifically, the researchers have created digital switches by making a sandwich of carbon nanotubes and graphene.

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A New Spin on Silicon

A group of scientists has stumbled upon a previously unknown characteristic of silicon, one that could make for faster, optical computers.

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Solution-based Process Could Produce Tuned Graphene in Bulk

Researchers in Taiwan have developed a solution-based process for producing graphene that is tuned to exhibit specific electrical and mechanical properties. While solution-based exfoliation of graphene has been possible for some time, this new approach uses pulses of an electrical voltage rather than a constant voltage to produce the desired effects.

The researchers believe that their work, which was reported in the journal Nanotechnology, could pave the way for new applications for graphene in drug delivery or electronics.

Graphene production methods have been seen as an obstacle to the use of the material in a range of applications for which it has been targeted.  The mechanical exfoliation of graphene sheets from graphite, while producing the best quality graphene for electronic applications, is decidedly un-scalable. And production methods that are more scalable lack the quality necessary for these same electronic applications. A solution-based process that can be ramped up to yield high volumes of graphene that possesses the electrical and mechanical properties one desires would cause a dramatic upshift in graphene’s commercial development.

“Whilst electrochemistry has been around for a long time it is a powerful tool for nanotechnology because it’s so finely tunable,” said Mario Hofmann, a researcher at National Cheng Kung University in Taiwan, in a press release. “In graphene production we can really take advantage of this control to produce defects.”

The trick to getting exactly the right defects in the graphene depended not only on using a pulsed voltage, but also being able to carefully monitor how the graphene was changing in the solvent process. To monitor this change, the researchers found that they could simply observe the transparency of the solution.

As part of their work, the researchers tested the quality of the graphene produced via their method as a transparent conductor (the application for which graphene is being considered as a potential replacement for indium tin oxide). The resistance of their graphene films (at 50 percent transparency) was 30 times that of other graphene-based transparent conductors.

In future research, the Taiwan-based team will look at how altering the duration of the pulses impacts the exfoliation process both in terms of producing greater quantities of final product as well as gaining greater control on the defects engineered into the graphene.

Is Graphene Really in Need of a Killer App?

The grace period for the eventual commercial success of a nanomaterial, it would seem, is limited. There’s been an abundance of hand wringing in the last few years over the fact that carbon nanotubes have offered us little more to date than advanced composites. Now we are beginning to see that same frustration play out with graphene, to the point where Nature News has published an article questioning whether graphene’s killer app is ever going to come.

It would seem that ten years is the length of time the public expects for a material to go from its first synthesis in the lab to having a huge commercial impact. That was the case for giant magnetoresistance (GMR). That effect was first produced in the lab in 1988 and made it into commercial hard disk drives by 1997.

In this somewhat unfair comparison, it must be understood that GMR-based hard drives did not need to completely uproot an entire industry to succeed. For graphene, a main target has been silicon CMOS, an entrenched technology if ever there was one. But silicon CMOS will not take us into the future indefinitely. You need no more confirmation of this than the money that is being invested by the tech giants to find a replacement.

We have passed the decade mark since graphene was first synthesized and the public is getting restless, especially about what its commercial potential actually is and how to make profitable investments in the technology.

During this past decade, we have seen the first flushes of excitement about graphene occur in electronics, where it seemed to offer a way around the problems with carbon nanotubes of getting them where you wanted and interconnecting them. But that excitement needed to be tempered with the vexing issue of graphene lacking a band gap. While a band gap has been successfully engineered into graphene, it doesn’t appear that digital logic applications are the path of least resistance for the material.

Articles like the one in Nature make all the valid points about graphene production outstripping demand, something we also saw befall carbon nanotubes. However, maybe we need not look any further for a killer app, because of the likelihood that silicon’s days are numbered. In the meantime, we seem to be finding critical applications for graphene, and other nanomaterials, that we didn’t initially consider, such as water purification membranesenergy storage, and energy generation applications.

Carbon Nanotube Speakers Promise Applications Outside of Audio Equipment

It’s been some time since we covered the use of nanomaterials in audio speakers. While not a hotly pursued research field, there is some tradition for it dating back to the first development of carbon nanotube-based speakers in 2008. While nanomaterial-based speakers are not going to win any audiophile prize anytime soon, they do offer some unusual characteristics that mainly stem from their magnet-less design.

Now a group of researchers at Michigan Technological University (MTU) have continued this tradition of tinkering with carbon nanotubes and speakers and walked away winning the Best of Show Award at SAE International’s Noise and Vibration Conference and Exhibition.

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Spintronic Devices Possible Without Magnetic Material

Spintronics, which has offered a promising alternative to electronics, may have just been given a boost that moves it from mere promise to likely future backbone of computing.

A young researcher at Argonne National Laboratory has stumbled upon the amazing discovery that a magnetic material may not be required in order to generate spin current from insulators. The implications of this discovery, which are described in the journal Physical Review Letters, could be far reaching and make possible the continued trend towards ever more powerful computing.

“This is a discovery in the true sense,” said Anand Bhattacharya, a physicist in Argonne's Materials Science Division and the project's principal investigator, in a press release. “There’s no prediction of anything like it.”

The hope of spintronics stems from its use of the spin of electrons to encode information rather than the transport of electrical charge of electrons. This fundamental difference is believed to be a solution to many of the problems associated with electronics, such as high power consumption and the need for more space on a chip.

To date, to be able read the spin of the electrons, which is either “up” or “down,” electrons have had to be held in place in a ferromagnetic insulator material, like yttrium iron garnet (YIG). With the electrons held up momentarily, a heat gradient is applied to the material to set the spin of the electrons in motion again. Once they are spinning across the lattice of the crystal insulator, they start communicating information about the orientation of their spin. In this way, just like an electrical current is a stream of electrons moving through a conductor, a current of pure spin can be achieved in magnetic insulators.

Stephen Wu, the postdoctoral researcher who made the discovery, was looking at different materials that would make it possible to produce smaller spintronic devices and provide greater control over the thermal gradients that needed to be applied to the material to start the current spin of the electrons.

It was during this experimentation with different materials that Wu found himself working with YIG on a substrate of paramagnetic gadolinium gallium garnet (GGG). Because the GGG is a paramagnet and not a ferromagnet, Wu didn’t expect to see any spin because the paramagnet doesn’t generate a magnetic field.

“The spins in the system were not talking to each other. But we still found measurable spin current,” said Wu in the release. “This effect shouldn’t happen at all.”

What the researchers observered, in fact, was that the spin current was stronger in the GGG than in the YIG. It is an amazing scientific discovery, but at this point, the best the scientists can do is speculate regarding why the phenomenon occurs at all.

“We think that there may be other new physics working here,” said Bhattacharya in the release. “Because, since the material is not a ferromagnet, the objects that are moving the spin are not what we typically understand.”

Even while understanding of the physics that explain the phenomenon plays catch up, the researchers see an opportunity to push ahead the state-of-the-art in spintronics.

“We’ve just taken ferromagnetism off its pedestal,” said Wu in the release. “In a spintronic device you don’t have to use a ferromagnet. You can use either a paramagnetic metal or a paramagnetic insulator to do it now.”

Stretchable Conducting Fiber Provides Super Hero Capabilities

The list of potential applications for a new electrically conducting fiber—artificial muscles,  exoskeletons and morphing aircraft—sounds like something out of science fiction or a comic book. With a list like that, it’s got to be a pretty special fiber… and it is. The fiber, made from sheets of carbon nanotubes wrapped around a rubber core, can be stretched to 14 times its original length and actually increase its electrical conductivity while being stretched, without losing any of its resistance.

An international research team based at the University of Texas at Dallas  initially targeted the new super fiber for artificial muscles and for capacitors whose storage capacity increases tenfold when the fiber is stretched. However, the researchers believe that the material could be used as interconnects in flexible electronics and a host of other related applications.

In research published in the journal Science, the team describes how they devised a method for wrapping electrically conductive sheets of carbon nanotubes around the rubber core in such a way that the fiber's resistance doesn’t change when stretched, but its conductivity increases.

You can watch a demonstration in the video below:

“We make the inelastic carbon nanotube sheaths of our sheath-core fibers super stretchable by modulating large buckles with small buckles, so that the elongation of both buckle types can contribute to elasticity, said Ray Baughman, senior author of the paper and director of the Alan G. MacDiarmid NanoTech Institute at UT Dallas, in a press release. “These amazing fibers maintain the same electrical resistance, even when stretched by giant amounts, because electrons can travel over such a hierarchically buckled sheath as easily as they can traverse a straight sheath.”

The researchers have also been able to add a thin coat of rubber to the sheath-core fibers and then another carbon nanotube sheath to create strain sensors and artificial muscles. In this setup, the buckled nanotube sheets act as electrodes and the thin rubber coating serves as the dielectric. Voilà! You have a fiber capacitor.

“This technology could be well-suited for rapid commercialization,” said Raquel Ovalle-Robles, one of the paper’s authors, in the press release. “The rubber cores used for these sheath-core fibers are inexpensive and readily available. The only exotic component is the carbon nanotube aerogel sheet used for the fiber sheath.”

Nanostructured Glass Can Switch Between Blocking Heat and Blocking Light

Electrochromic glass essentially uses electric charge to switch a window from allowing sunlight in to blocking it out. Some have estimated that such “smart windows” could cut lighting needs by about 20 percent and the cooling load by 25 percent at peak times.

Now researchers at the University of Texas Austin have found a way to make them even better. They developed a novel nanostructure architectcure for electrochromic materials that enables a highly selective cool mode and warm mode—something thought to be impossible a few years back.

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Non-Stick Pans Lead the Way to More Efficient Solar Cells

Researchers at the University of Nebraska-Lincoln have discovered that the same principle that makes some pans non-stick can be exploited to improve the performance of solar cells.

In research published in the journal Nature Communications, the researchers have built a perovskite-based solar cell on a “non-wetting” plastic, making it 1.5 times as efficient at energy conversion as such cells had been previously.

Previously, the best way to get better efficiency in solar cells was to use single crystal material because it has fewer grains—the tiny pieces whose barriers trap and prematurely recombine electrons and holes—than polycrystalline materials. This made for a tough choice: single crystal materials, which are costly and difficult to produce, or polycrystalline materials, whose energy conversion efficiency is comparatively low.

But the Nebraska team has come up with a third choice: polycrystalline materials with a reduced number of grains. That had been tried before, with limited success; the size of the grains was locked in direct proportion to the thickness of the polycrystalline film. But their new production method, featuring the non-wetting surface, dramatically improved the area-to-thickness ratio. The researchers discovered that if they grew the polycrystalline material on this non-wetting surface, the grains grew up to eight times larger than the cell is thick.

“We found that the difference is huge,” said Jingsong Huang, an associate professor of mechanical and materials engineering, in a press release. “When you have two small grains merge into a larger grain, what happens is that a boundary actually moves from (the middle of two grains) to the end of one or the other. How easily these boundaries move will determine how fast these grains can merge and grow.”

Huang added that “A non-wetting surface is slippery, like when you pour oil on a floor. It's easier for the grain boundary to move because we're reducing some of the drag force on it.”

The researchers believe that this use of a non-wetting surface can be exploited in other areas as well, such as aster transistors and more sensitive photodetectors.

Huang added: “When it comes to electronic properties, crystallinity and grain size determine a lot. So this is a simple method with a lot of potential applications.”

Self-Assembly Trick Embeds Quantum Dots Inside Nanowires

Self-assembling nanowires could give them a role in touch-screen displayssmoke detectors, and other applications.

Now researchers at the University of Cambridge in the UK in collaboration with IBM have developed a self-assembly process for nanowires that makes it possible to embed quantum dots within them, expanding their range of potential applications.

“The key to building functional nanoscale devices is to control materials and their interfaces at the atomic level,” said Stephan Hofmann of the University of Cambridge and one of the paper’s senior authors, in a press release. “We’ve developed a method of engineering inclusions of different materials so that we can make complex structures in a very precise way.”

The new self-assembly technique, which is described in the journal Nature Materials, is based on the typical process for producing nanowires: vapor-liquid-solid (VLS) synthesis.  VLS offers a fast way for producing nanowires based on chemical vapor deposition.

In VLS, chemical vapors disolve into a droplet of liquid catalyst. The chemicals crystalize at the base of the droplet, forming the nanowire, which pushes the catalyst up as it grows. Over the years, VLS has developed into a highly controlled process in which every detail of the nanowires from its size to its crystal structure can be precisely controlled.

The Cambridge researchers were able to build upon the VLS technique by using the catalyst droplet as a “mixing bowl” to add materials that lead to new growth phases. These new phases take the shape of faceted nanocrystals, or quantum dots.

“The technique allows two different materials to be incorporated into the same nanowire, even if the lattice structures of the two crystals don’t perfectly match,” said Hofmann. “It’s a flexible platform that can be used for different technologies.”

The inclusion of quantum dots in the nanowires would seem to indicate potential optoelectronic applications, for which the nanocrystals are well known. For example, the researchers anticipate that these new nanowires could find use in semiconductor lasers and other light emitters.



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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