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Collodial Semiconductors Challenge Amorphous Silicon

Amorphous silicon has been the “king of the hill” when it comes to thin, fast, and flexible semiconductors, but researchers at the University of Pennsylvania believe they have knocked the king off his throne and maybe right into the past.

The U Penn research team, led by doctoral students David Kim and Yuming Lai along with Professor Cherie Kagan, have used cadmium selenide nanocrystals (which are proving themselves useful in a number of areas)  to deliver devices that can move electrons 22 times faster than in amorphous silicon.

Cadmium selenide nanocrystals are within a class of colloidal semiconductor nanocrystals that have been found effective for making thin-film field-effect transistors. Essentially taking the form of ink, these colloidal nanocrystals have tantalized researchers looking to create inexpensive thin-film electronics. But until this most recent research they had not been demonstrated for use in the high-performance field-effect transistors needed in large-area integrated circuits.

The Penn research, which was published in the journal Nature Communications (“Flexible and low-voltage integrated circuits constructed from high-performance nanocrystal transistors”), may have found a way to achieve these high-performance large-area integrated circuits.

The researchers started with a flexible polymer on which they used a masking technique to stencil one level of electrodes for the circuit. Another area on the polymer was stenciled off for a conducting gold that would later serve as the electrical connection to the upper levels of the circuit. After putting down an insulating aluminum oxide layer, a spincoating deposition technique was used to deposit a 30-nanometer layer of nanocrystals on top.

What might be the main distinguishing factor between this technique and previous methods using colloidal semiconductor nanocrystals  was the use of a new ligand. These ligands extend out from the surface of the nanocrystals and aid conductivity of the nanocrystals as they are packed tightly together.

“There have been a lot of electron transport studies on cadmium selenide, but until recently we haven’t been able to get good performance out of them,” says Kim in a press release. “The new aspect of our research was that we used ligands that we can translate very easily onto the flexible plastic; other ligands are so caustic that the plastic actually melts.”

While the nanocrystal-based devices that the researchers developed are giving amorphous silicon a run for the money in terms of electron mobility, it doesn’t seem that the researchers are targeting amorphous silicon’s main application of flat-panel displays. Instead they envision these flexible and easy-to-produce circuits in pervasive sensors used in either security or biomedical applications.

Newly Developed Live Nanoscale Imaging Technique Promises Improvement in Li-ion Batteries

Much of the nanotechnology-related work going on today for improving Lithium-ion (Li-ion) batteries has focused on developing nanostructured silicon to replace graphite in the anodes of the next generation Li-ion batteries.

While this work has been encouraging, another line of research has taken a different tack. Instead of just replacing the graphite in the anodes, researchers have sought to determine why the degradation of Li-ion batteries’ storage capacity occurs in the first place.

Two years ago, I covered work conducted at Ohio State University in conjunction with both Oak Ridge National Laboratory and the National Institute of Standards and Technology that employed every microscopy tool researchers could get their hands on in the search for nanoscale phenomena that would cause this degradation. The results showed that the material from which the electrodes in Li-ion batteries are made coarsen over time; the lithium ions that need to go between the positively and negatively charged electrodes become increasingly unavailable for charge transfer.

Now researchers at the U.S. Department of Energy’s Brookhaven National Laboratory have developed a new imaging technique that allows them to observe lithium-ion reactions in real time with at the nanoscale precision.

Critical to the new imaging technique is transmission electron microscopy (TEM), which has been used to fabricate the “world’s smallest” battery. In the Brookhaven work, which was published in the journal Nature Communications (“Tracking lithium transport and electrochemical reactions in nanoparticles”), the TEM is modified with an in-situ electrochemical cell that can operate inside the TEM. This novel design gives researchers the combination of live imaging found with in-situ techniques and the spatial resolution and nanoscale precision of TEM.

The design of the modified TEM may be novel, but it’s not overly complex. “The entire setup for the in-situ TEM measurements was assembled from commercially available parts and was simple to implement," said Brookhaven Lab physicist and lead author Feng Wang in a press release. "We expect to see a widespread use of this technique to study a variety of high-energy electrodes in the near future,” says Wang.

The new imaging technique allowed the researchers to observe the lithium ion reaction that occurs across iron fluoride (FeF2) nanoparticles. They watched the lithium ions move quickly across the surface of the nanoparticles and then observed the compounds being broken down into different regions in a layer-by-layer process—all in real time. The Brookhaven team saw that the lithium-ion reaction leaves in its wake a trail of new molecules.

“Although many questions remain regarding the true mechanisms behind this conversion reaction, we now have a much more detailed understanding of electron and lithium transport in lithium-ion batteries,” said Brookhaven physicist and study coauthor Jason Graetz in the release. “Future studies will focus on the charge reaction in an attempt to gain new insights into the degradation over time that plagues most electrodes, allowing for longer lifetimes in the next generation of energy storage devices.”

Block Copolymers Lead to Five-fold Increase of Disk Drive Storage Capacity

Earlier this year nanoscientists in Ireland took their first steps towards realizing the promise of block copolymers  for next generation computing. Their research, which included scientists from both the University of Wisconsin and Intel, developed a method for fabricating large-area arrays of silicon nanowires through the directed self-assembly (DSA) of block copolymer nanopatterns.

Now researchers at the University of Texas Austin in collaboration with the disk drive company HGST have exploited the DSA characteristics of block copolymers to create a new type of disk drive with up to five times the storage capacity of today’s models.

The new research, which was published in the journal Science (“Polarity-Switching Top Coats Enable Orientation of Sub–10-nm Block Copolymer Domains”),  was not only able to push the boundaries of storage capacity, it created a method that is well matched with today’s manufacturing processes.

While the method’s compatibility with current high-throughput techniques is critical for it to be adopted into commercial applications, it is the extraordinary speed at which the block copolymers self assemble that has amazed even the researchers.

“I am kind of amazed that our students have been able to do what they’ve done,” says co-author C. Grant Willson, a professor of chemistry at U Texas Austin, in a press release. “When we started, for instance, I was hoping that we could get the processing time under 48 hours. We’re now down to about 30 seconds. I’m not even sure how it is possible to do it that fast. It doesn’t seem reasonable, but once in a while you get lucky.”

In addition to its speed and compatibility with current manufacturing techniques, the newly developed method addresses a real need in computing. Data storage on disk drives is approaching its limits. In the past, we've always stored more by packing the magnetic dots that make up the data on disk drives closer together. But the industry now has reached about a terabit of data per square inch (2.54 cm) of disk. If you bring them much closer, the magnetic fields of each dot begin to interfere with each other and data can be corrupted.

This use of block copolymers makes it possible to make the disk so that there are no magnetic fields between the dots but they are still isolated from one another. This means you can push the dots closer together without any magnetic fields interfering with the dots and corrupting the data.

The key to the process the U Texas researchers developed is a spin-on top coat that neutralizes surface energy at the top interface of a block copolymer film. This allows the polymers to orient themselves to the plane of the disk with just heat.

“The patterns of super small dots can now self-assemble in vertical or perpendicular patterns at smaller dimensions than ever before,”  saysThomas Albrecht, manager of patterned media technology at HGST, in the release. “That makes them easier to etch into the surface of a master plate for nanoimprinting, which is exactly what we need to make patterned media for higher capacity disk drives.”

As with the research coming out of Ireland earlier this year, this work was conducted in close collaboration with industry, suggesting that commercial applications of the technology are a real possibility in a fairly short time—much shorter than typically seen in this kind of lab research.

Sunlight and Nanoparticles Make Steam Without Boiling Water

According to some sources, steam-driven turbines still account for between 80 and 90 percent of the electricity generated in the world.  Of course, the method for producing that steam can vary from nuclear power to burning fossil fuels.

Now researchers at Rice University believe that they have found a completely new way for generating steam by placing light-absorbing nanoparticles in water and focusing sunlight on the water so that steam is produced without actually boiling the water.

In this new method not only is it not necessary to boil the water, but the Rice researchers have also demonstrated that steam can be produced in water that remains near the freezing point with this sunlight/nanoparticle combination.  According to the researchers, the steam is produced at very high efficiency in which 80 to 90 percent of the energy absorbed from the sun is actually converted to steam.

When these figures are translated into the energy conversion measurements used for photovoltaics it has an overall energy efficiency of 24 percent, significantly higher than photovoltaics that typically measure around 15 percent energy conversion efficiency.

A video demonstrating and describing the technology can be seen below:

The research, which was published in the journal ACS Nano (“Solar Vapor Generation Enabled by Nanoparticles") , made use of a range of materials including metallic and carbon nanoparticles. The key feature for all of them was that they needed to absorb light. When dispersed into water, these nanoparticles direct most of the energy into creating steam rather than heating up the water. 

“We’re going from heating water on the macro scale to heating it at the nanoscale,” says Naomi Halas, the lead scientist on the project, in a press release. “Our particles are very small — even smaller than a wavelength of light — which means they have an extremely small surface area to dissipate heat. This intense heating allows us to generate steam locally, right at the surface of the particle, and the idea of generating steam locally is really counterintuitive.”

While the technology is a tantalizing alternative to the way most industrial steam is produced in large boilers, the first prototypes of the technology have taken on a more modest scale.

Funded by a Grand Challenges grant from the Bill and Melinda Gates Foundation, the research team built a small-scale system for treating human waste in areas without sewer systems or electricity. The Rice team have also created a system based on the technology that could sterilize medical and dental instruments in places lacking electricity.

A small-is-beautiful approach to this technology may be the way to proceed initially, but the big hope certainly has to be that it could make large-scale electricity production cheaper and more efficient.

A Twist and Some Wax Turns Carbon Nanotubes into Super Muscles

Carbon nanotubes have already been demonstrated to be a useful material in the development of artificial muscles. But an international team of researchers led by the University of Texas at Dallas has discovered that if you twist carbon nanotubes into a yarn and infuse them with paraffin wax their capabilities as artificial muscles become staggering.

The researchers claim that the wax-infused muscles can lift 100 000 times their own weight and produce 85 times more mechanical power than natural muscle of equivalent size.

“The artificial muscles that we’ve developed can provide large, ultrafast contractions to lift weights that are 200 times heavier than possible for a natural muscle of the same size,” says Dr. Ray Baughman, team leader, Robert A. Welch Professor of Chemistry and director of the Alan G. MacDiarmid NanoTech Institute at UT Dallas in a press release. “While we are excited about near-term applications possibilities, these artificial muscles are presently unsuitable for directly replacing muscles in the human body.”

You can see Baughman further describe the carbon nanotube-based muscles in the video below:

While Baughman concedes that replacing artificial muscles in humans is out of the application list for this material at the moment, he does believe that it could be used in “robots, catheters for minimally invasive surgery, micromotors, mixers for microfluidic circuits, tunable optical systems, microvalves, positioners, and even toys.”

Baughman further believes that the material can make its way into marketable uses fairly quickly. He notes in the release: “The remarkable performance of our yarn muscle and our present ability to fabricate kilometer-length yarns suggest the feasibility of early commercialization as small actuators comprising centimeter-scale yarn length. The more difficult challenge is in upscaling our single-yarn actuators to large actuators in which hundreds or thousands of individual yarn muscles operate in parallel.”

Whether Baughman can tackle that next challenge remains to be seen, but the research, which was published in the journal Science (“Electrically, Chemically, and Photonically Powered Torsional and Tensile Actuation of Hybrid Carbon Nanotube Yarn Muscles"), is impressive in its elegant simplicity.

The combination of twisting carbon nanotubes into a yarn and infusing them with wax made it possible to simply add a bit of electrical charge to the material to get the wax to expand and then the yarn volume to increase, causing the yarn to shorten. This volume increasing and length decreasing is directly related to the twisting of the carbon nanotube yarn.

In operation, when the wax-filled yarn is heated electrically it untwists, but when the heating is stopped the yarn winds back up. What is remarkable is how fast this twisting and untwisting occurs. The researchers claim that yarn can rotate a paddle that is attached to the yarn at 11 500 revolutions per minute. Perhaps more importantly, it can repeat this cycle more than 2 million times.

Another attractive feature of the material is that fact that it can be treated like a textile. So it could be sewn or woven into clothing to react to outside environmental factors such as heat (a fireman’s coat is given an as an example in the video) and actuate (like a muscle) a change to the textile’s porosity. This change in porosity could provide thermal protection, or chemical protection in the presence of poisonous substances.

Paper and Scissors Key in Latest Development of Nanofluidics

When one recalls that graphene was first produced by placing scotch tape on top of the graphite found in pencils and then pulling the tape off, it may not sound so strange that the next breakthrough in nanofluidic devices may come from using paper and scissors.

Two researchers at Northwestern University have discovered that if you stack up layers of graphene on top of one another it creates a flexible paper-like material that forms tens of thousands of nanoscale channels between the layers.  In keeping with the school supplies theme, the researchers further discovered that they could cut the paper-like material into any shape they wished with a pair of scissors.

“In a way, we were surprised that these nanochannels actually worked, because creating the device was so easy,” said Jiaxing Huang, quoted in a university press release. Huang, a Junior Professor in Materials and Manufacturing, who conducted the research with postdoctoral fellow Kalyan Raidongia, said, “No one had thought about the space between sheet-like materials before. Using the space as a flow channel was a wild idea. We ran our experiment at least 10 times to be sure we were right.”

The material could potentially have applications in batteries, water purification, harvesting energy and DNA sorting. While listing a range of applications for lab technologies is always a fairly easy matter, this material stands out in these application areas because of how cheaply and easily it is produced.

Typically nanofluidic devices require slow and expensive lithography techniques to carve out the channels. But this technique lends itself to the building of massive arrays of nanochannels simply by staking sheets of graphene oxide (GO) on top of one another. To create more nanochannels, simply stack more layers on top of each other.

The research, which was published in the Journal of the American Chemical Society (“Nanofluidic Ion Transport through Reconstructed Layered Materials’), demonstrated a working device using the material by cutting a piece of the GO paper into a centimeter-long rectangle. Huang and Raidongia covered the paper in a polymer. They then drilled either end of the rectangle to fashion holes in which an electrolyte solution was placed.

In tests, the researchers discovered that the rectangle conducted a higher than normal amount of current, whether it was laid out flat or bent.

The next step is to test the nanoscale properties of papier-mâché. Just kidding—but maybe someone should try it.


Hybrid Nanomaterial Converts Both Light and Heat to Electricity

Nanomaterials are becoming an attractive proposition for realizing the hopes of future thermoelectric devices, which would derive power just from differences in temperature. And, of course, the future of photovoltaics is increasingly dependent on developments in nanomaterials.

What nanomaterials haven’t been used for to date is to combine thermoelectrics with photovoltaics. But now in joint research between Louisiana Tech University and the University of Texas at Arlington that combination has been achieved. The joint research team developed a new hybrid nanomaterial that combines single-walled carbon nanotubes (SWNTs) with copper sulfide (CuS) nanoparticles and is capable of converting both light and thermal radiation into electricity.

The research, which was published in the UK’s Institute of Physics’ journal Nanotechnology (“Optical thermal response of single-walled carbon nanotube-copper sulfide nanoparticle hybrid nanomaterials”), builds on previous work that demonstrated that SWNTs are excellent materials for absorbing both light and thermal energy.

Louisiana Tech University assistant professor Long Que along with UT Arlington associate physics professor Wei Chen took that knowledge one step further by combining the SWNTs with CuS nanoparticles and getting an 80 percent increase in light absorption with the hybrid material versus the pure SWNTs.

In devices that the researchers made from the hybrid material, they measured a clear optical and thermal switching effect. The researchers further discovered that this switching effect could be enhanced by a factor of 10 by using asymmetric illumination in which a polydimethylsiloxane (PDMS) slab covers one of the electrodes so the light source illuminates one of the electrode regions and the other is covered. The difference in temperature between the illuminated electrode and the one in the PDMS shadow creates a thermoelectric effect that increases the electricity generated by the light.

The research team were able to use the material to create a thermoelectric generator that they believe will be able to generate milliwatts of power.  If these nanogenerators could be used in chips it could lead to a range of self-powered devices, according to the researchers.

“If we can convert both light and heat to electricity, the potential is huge for energy production,” Chen says in a press release. “By increasing the number of the micro-devices on a chip, this technology might offer a new and efficient platform to complement or even replace current solar cell technology.”

Water-Splitting Catalyst Revealed

If you wanted to do an imitation—at least an accurate one—you would be well served to carefully observe the original. With artificial photosynthesis being hotly pursued by some of the most renowned nanotechnology researchers, a team at Umeå University in Sweden thought an important step for improving artificial photosynthesis would be to peer deeply into real photosynthesis to reveal the factors that make it work.

The Swedish team examined a manganese complex with spectroscopy in conjunction with the Linac Coherent Light Source (LCLS), a free-electron x-ray laser facility at Stanford University. Manganese is a transitional metal that when combined with calcium and oxygen creates the same water-splitting catalyst found in photosynthesis.

While this may sound like an approximation of photosynthesis, the research group had already used LCLS to perform structural analyses of isolated photosynthesis complexes in plants. The new wrinkle this time was to bring spectroscopy into the imaging process.

To accomplish this imaging, the LCLS emits a laser beam with wavelengths that are the breadth of an atom at pulses that last 50 femtoseconds. When the LCLS was combined with the spectrometer, the x-rays emitted from the manganese complex after being hit with the laser pulses are diffracted by the spectrometer and picked up by a detector array.

With this set-up the researchers observed detailed information about the compound’s electronic structure before the laser beam destroyed it.

"Having both structural information and spectroscopic information means that we can much better understand how the structural changes of the whole complex and the chemical changes on the active surface of the catalysts work together to enable the enzymes to perform complex chemical reactions at room temperature,” says Johannes Messinger, professor at the Department of Chemistry at Umeå University, in a university press release.

This detailed imaging of how photosynthesis splits water into its constituent parts has been held out as a way to help engineers more cheaply synthesize hydrogen gas to power hydrogen fuel cells—and possibly the automobiles powered by them. Research efforts to split water molecules into hydrogen gas have been taken on both by commercial entities and the academics. Perhaps this new information on the electronic structure of the water-splitting process of photosynthesis can further inform both these lines of research.

How Far Can IT Take Material Science?

Last year the White House announced an oddly entitled plan called the “Materials Genome Initiative”. The aim of the initiative—and thus the title—was to apply the same kind of data crunching fire power that was used on the mapping of DNA in the so-called Genome Project to the field of material science.

While one could argue that the White House dubbing this project the Materials Genome Initiative was more a metaphorical flourish than a scientific aim, it does raise the question of whether we can map all of material science in a way that will improve manufacturing as the plan has set out to do.

To answer that question, Richard Jones has penned a piece on his blog Soft Machines, which starts by posing the rhetorical question, "Do materials even have genomes?"

Jones raises many of the questions and problems that result from depending on—or expecting—computer simulation to help us design materials to perform tasks we have designed for them. Some of the points he makes remind me of a piece I wrote five years ago: Materials By Design: Future Science or Science Fiction?

At the time, I noted that “Any useful software modeling would need to be able to reveal how an alteration in a material’s structure—for example, a change in a crystal’s lattice structure—affect its properties and functions. Such a program would also need to be able to do that in a range of scales, because we also don’t know whether we must look at the atomic or particle level to find out where effects are taking place.”

This concern about problems of scale is reflected in Jones’ piece but he also raises the question of on what time scale this kind of endeavor would proceed:

"Even with the fastest computers, you can’t simulate the behavior of a piece of metal by modelling what the atoms are doing in it—there’s just too big a spread of relevant length and timescales. If you wanted to study the way different atoms cluster together as you cast an alloy, you need to be concerned with picosecond times and nanometer lengths, but then if you want to see what happens to a turbine blade made of it in an operating jet engine, you’re interested in meter lengths and timescales of days and years (it is the slow changes in dimension and shape of materials in use—their creep—that often limits their lifetime in high temperature situations)."

Jones points out that developing multi-scale modeling like this is nothing new; he refers to Masao Doi’s Octa project as an example. But such projects remain problematic. There are so many variables involved with material science, he says, that it is not clear how generic you can make the processes seem, at least for computer modeling. He further argues that researchers would quickly turn to physical experiments outside of the computer models.

He notes: “I’m skeptical that anyone trying to test out how to shape and weld big structures out of an oxide dispersion strengthened steel (these steels, reinforced with 2 nm nanoparticles of yttrium oxide, are candidate materials for fusion and fourth-generation fission reactors, due to their creep resistance and resistance to radiation damage) without getting someone to make a big enough batch to try it out.”

There is no doubt that computer modeling is a fantastic tool--a view Jones seems to support in the piece--but it should be clear we had better not be expecting material science to reveal itself the way the DNA molecule was mapped by the Genome Project. Whether the Materials Genome Initiative will prove beneficial to US manufacturing is something that can only be determined over the scale of time.

Plasmonic Nanolasers Shrink Down to Size of a Virus

When lasers start getting down to the nanoscale, they run up against the diffraction limit where the size of the laser cannot be smaller than the wavelength of light it emits. But researchers have shown that nanoscale plasmonic lasers can reach an optical mode well below this limit by confining light of very short wavelengths through the use of surface plasmons—oscillations of electrons that occur at the junction of a metal and an insulator. This has revitalized the hope that chips populated with these plasmonic nanolasers could make possible computer processors run by light rather than electrons.

Now researchers at Northwestern University have developed a new design for plasmonic nanolasers that are the size of a virus particle and capable of operating at room temperature. They described the discovery in the journal Nano Letters, (“Plasmonic Bowtie Nanolaser Arrays”).

"The reason we can fabricate nano-lasers with sizes smaller than that allowed by diffraction is because we made the lasing cavity out of metal nanoparticle dimers -- structures with a 3-D 'bowtie' shape," says Teri Odom, the leader of the research and professor of Chemistry at Northwestern, in a press release.

The bowtie geometry allowed the nanoparticles to achieve an antenna effect and suffer only minimal metal “losses”. Typically, plasmon nanolaser cavities have suffered from both metal and radiation losses that required them to be operated at cryogenic temperatures.

Odom also explains that the antenna effect  allows for lasing to occur from an "electromagnetic hot spot"—a capability not demonstrated previously. "Surprisingly, we also found that when arranged in an array, the 3-D bowtie resonators could emit light at specific angles according to the lattice parameters," Odom adds in the release.

Of course, nanolasers that are capable of operating at room temperature are not unique. Researchers at the University of California, San Diego reported earlier this year on a room temperature nanolaser design that requires less power to generate a coherent beam than other designs. The key difference between the two plasmonic nanolasers seems to be the bowtie geometry the Northwestern team developed.

At least one of the aims of both lines of research seems to be to integrate these nanolasers with CMOS electronics.  Whether they can reach this lofty goal remains to be seen, but these nanolasers are a key step in their realization.



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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