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Nanoscale Structures Enable Magnetic Mirrors Without Metals

Researchers at Sandia National Laboratory have developed a two-dimensional array of non-metallic nanoscale structures that interact with the magnetic component of incoming light in such a way that they could enable a new generation of chemical sensors, solar cells, lasers, and other optoelectronic devices.

We are, of course, familiar with traditional mirrors, which interfere with incoming light so that they reflect back a reverse image that our eyes can see. However, what we don’t see is that these mirrors also interact with the incoming light so that its electrical field is reversed, essentially cancelling out the light’s electrical component.

But what if you didn’t want to cancel out that electrical component of the light and instead have nanoscale antennas at a mirror’s surface capture it so it could be used for a host of optoelectronic devices? The way to do that has been something called “magnetic mirrors,” which interact with light’s magnetic field and preserve light’s original electrical properties.

Since nature does not provide a material for producing magnetic mirrors, researchers have turned to metamaterials, which can be broadly defined as an artificially structured material fabricated by assembling different objects so as to replace the atoms and molecules that one would see in a conventional material. The resulting material has very different electromagnetic properties than those that occur naturally or are chemically synthesized.

Prior to this most recent research, metamaterials could only produce this magnetic mirror effect for microwave frequencies; this limited it to a small number of applications such as microwave antennas.

At wavelengths shorter than those at microwave frequencies, there has been some success in using “fish-scale” shaped metallic components. However, this approach has suffered from loss of signal and uneven responses.

In the research, which was published in the journal Optica, the Sandia team used tellurium to reduce the signal loss that is associated with metals. The telluride-based nanostructures also resulted in magnetic mirrors that were reflective at infrared wavelengths and led to a much stronger electrical field at the mirror’s surface.

“We have achieved a new milestone in magnetic mirror technology by experimentally demonstrating this remarkable behavior of light at infrared wavelengths,” said Michael Sinclair, a Sandia scientist who co-authored the Optica paper, in a press release.

“Our breakthrough comes from using a specially engineered, non-metallic surface studded with nanoscale resonators.”

What may make the approach most attractive is the fact that it can be produced with widely used deposition lithography and etching processes.

In future research, the team intends to investigate other materials to reveal their magnetic mirror behavior with an eye towards getting to shorter optical wavelengths. This should expand the optoelectronic applications.

“If efficient magnetic mirrors could be scaled to even shorter wavelengths, then they could enable smaller photodetectors, solar cells, and possibly lasers,” said Sheng Liu, Sandia postdoctoral associate and lead author on the Optica paper, in the release.

Platinum Catalysts Are Outshined By Graphene Quantum Dots

Platinum is widely used as a catalyst for oxygen reduction reactions in fuel cells, but its high cost is a major obstacle to making fuel cell vehicles more affordable and more popular. Graphene, however, may be just what automotive and energy companies are looking for

Researchers from Rice University attached graphene quantum dots to a graphene base, resulting in a hybrid material that operates as an excellent– and cheap–catalyst for fuel cell reactions.

Quantum dots are nanocrystals that exhibit quantum mechanical properties, making them very useful in experimental transistors, solar cells, imaging chips, and other things. James Tour, a Rice chemistry professor, and his colleagues created graphene quantum dots (GQDs) from coal last year, and followed up on that breakthrough with this latest experiment.

This time, those same GQDs were combined with microscopic sheets of graphene into self-assembling nanoscale platelets. Tour explains that in the hybrid GQD/graphene material, the quantum dots provide a high abundance of edges where chemical reactions can occur, while the graphene is a plane of conductivity between GQDs. (The material was also treated with boron and nitrogen–co-catalysts.)

The lab discovered that the new material actually outperforms platinum-based catalysts for fuel cell reactions. Compared to platinum, the GQD/graphene material showed an oxygen reduction reaction with about 15 millivolts more in positive onset potential, or the start of the reaction. “You don’t need to apply as high a voltage as platinum to get the oxygen reduction reaction to occur,” says Tour. “We also get about 70 percent higher current than what platinum would offer.”

Increased efficiency aside, the hybrid material is also cheaper to make and install in fuel cells. The average cost of a fuel cell for an automobile has gone down from $275 per kW capability in 2002, to $55 per kW in 2013. Yet the fuel cell industry has yet to make a profit. The U.S. Department of Energy believes automotive fuel cell costs will need to fall to $30 per kW before we can expect to see real consumer interest. Toyota began selling its hybrid hydrogen fuel cell car in Japan this year for around $68,600. Unfortunately, with a 2014 Corolla going for between $16,000 and $21,000, fuel cell vehicles have yet to be noticed by most consumers.

The researchers believe the new findings, published in ACS Nano, could pave the way toward that goal. “This allows one of those two key reactions in the fuel cell to happen very well for the formation of water,” says Tour. “With fuel cells, the concern is always the cost of platinum electrode. This obviously needs no platinum electrode.”

However, Tour is very hesitant to say the fuel cell era is just over the horizon, and with good reason. “We don’t have an infrastructure as a country or global system for exploiting hydrogen-based systems well. You have to have a buy-in from everyone involved in that system for things to start taking off.”

Bottom-Up Self Assembly of Graphene Holds Promise for Spintronics

Not all graphene is alike. The way in which graphene is produced determines in large measure how it can be applied. The aim, of course, has been to produce the best quality graphene in large quantities.

However, these bulk production methods come at a price, which usually involves compromising those astounding electronic properties that make graphene so attractive in the first place.

Now researchers at the University of California Los Angeles (UCLA) and Tohoku University in Japan may have found a way around these limitations by abandoning “top-down” manufacturing techniques like lithography for a bottom-up approach in which the graphene nanoribbons self assemble exactly into the desired form.

The researchers were looking for a way to produce graphene nanoribbons that have the zigzag edges that give the material a strong magnetic property, making it attractive for spintronics. Spintronics exploits the way in which the spin of particles respond to magnetic fields so that the spin is either parallel or antiparallel to the magnetic field. These two possibilities make it useful for creating a digital signal that can be used in computing.

“To make devices out of graphene, we need to control its geometric and electronic structures,” said Paul Weiss of UCLA in a press release. “Making zigzag edges does both of these simultaneously, as there are some special properties of graphene nanoribbons with zigzag edges. Having these in hand will enable us to test theoretical predictions about them, such as magnetic properties.”

The typical lithographic method for producing graphene nanoribbons with these zigzag edges resulted in too many defects in the final product for the material to be useful.

In the research, which was published in the journal ACS Nano, the team exploited the properties of a copper substrate to alter the way the graphene precursor molecules reacted to each other as they assembled into graphene nanoribbons. With this method, the researchers were able to control the length, edge configuration, and location of the nanoribbons on the substrate.

This isn’t the first time that graphene nanoribbons were produced by self-assembly, but in earlier efforts the end results were bundles of ribbons that needed to go through another process to untangle them and position them in a device.

“Previous strategies in bottom-up molecular assemblies used inert substrates, such as gold or silver, to give molecules a lot of freedom to diffuse and react on the surface,” said Patrick Han of Tohoku University in the press release. “But this also means that the way these molecules assemble is completely determined by the intermolecular forces and by the molecular chemistry. Our method opens the possibility for self-assembling single-graphene devices at desired locations, because of the length and the direction control.”

2-D Molybdenum Disulfide Shows Piezoelectric Properties

Joint research out of Columbia University and the Georgia Institute of Technology has demonstrated for the first time that the two-dimensional (2-D) material molybdenum disulfide (MoS2) exhibits piezoelectricity and the piezotronic effect.

The research, which was published in the journal Nature, adds another dimension to the possible applications of 2-D materials like MoS2—notably the construction of new kinds of mechanically controlled electronic devices.

“This material–just a single layer of atoms–could be made as a wearable device, perhaps integrated into clothing, to convert energy from your body movement to electricity and power wearable sensors or medical devices, or perhaps supply enough energy to charge your cell phone in your pocket,” said James Hone, professor of mechanical engineering at Columbia and co-leader of the research, in a press release.

The piezoelectric effect, in which compressing or stretching a material produces a voltage or where a voltage can cause a material to expand or contract, has been demonstrated in a number of nanomaterials, including nanowires. (Piezotronics is the use of the piezoelectric effect as the gate voltage in transistor or similar device.) And researchers at Stanford demonstrated two years ago through computer modeling that the 2-D material graphene should exhibit piezoelectric properties. Producing the effect in the lab, the expectation is, will lead to new applications.

“Proof of the piezoelectric effect and piezotronic effect adds new functionalities to these two-dimensional materials,” says Zhong Lin Wang, Regents’ Professor at Georgia Tech and a co-leader of the research, in a press release. “The materials community is excited about molybdenum disulfide, and demonstrating the piezoelectric effect in it adds a new facet to the material.”

Wang added that in bulk form, or in samples with six or more atomic layers, MoS2 has no piezoelectricity. This means that bringing the material down to atomic thickness is the key to triggering its piezoelectric property.

The piezoelectric effect in the MoS2 only occurs if an odd number of layers are used and it’s flexed in the right direction. Because the material is highly polar, if you use an even number of layers, the effect is cancelled out.

In the demonstration, Hone’s research team at Columbia positioned thin flakes of MoS2 on flexible plastic substrates and determined if their crystal lattices were lined up. Then, Hone and his team patterned metal electrodes onto the flakes.

Wang and his team installed measurement electrodes on samples provided by Hone’s group. They then measured the flow of current as the samples were mechanically deformed.

Wang, who has been championing the piezoelectric characteristics of nanowires for years now,  believes that this research could lead to complete atomically-thick nanosystems that are self-powered by harvesting mechanical energy from the environment.

When asked how 2-D material’s piezoelectric effect compared with that of nanowires, Wang said that no such comparisons were measured. Wang also offered no indications of what inherent benefits there might be in using MoS2 over nanowires to achieve a piezoelectric effect. But at least we now know that it can be achieved outside of a computer model.

3-D Mapping of Electrons Moving on a Material's Surface

Terahertz spectroscopy uses infrared light to probe matter and is widely used to investigate the electrical properties and behavior of semiconductor materials. By sending very short laser pulses, called probe pulses, in quick succession, it is possible to follow how these properties change over time when the material responds to a light pulse, for example. However, unlike x-rays, terahertz waves have relatively long wavelengths ranging from 3 to 3000 micrometers. And just as with optical microscopes, structures smaller than the pulses’ wavelength remain invisible.

Now researchers at the University of Regensburg in Germany have demonstrated that it is possible to dramatically increase the spatial resolution of terahertz spectroscopy by focusing the probe pulses on the needle of an atomic force microscope (AFM). They published this research in Nature Photonics yesterday.

AFMs are widely used tools in solid-state research. Scanning with the sharp tip of the AFM at a slight distance from the surface of a material and measuring the variation of the force between the tip and the material—which can be an electrostatic or van der Waals force, for example—allows the creation of an image of the surface in which individual atoms are visible.

But the Regensburg researchers took a different tack. Instead of measuring the force between the tip and the surface of the material, they measured the intensity of light scattered by the tip with a series of probe pulses. According to Rupert Huber, the physicist who led the research, this method, “Is a little like a conventional radio antenna, just downscaled to wavelengths of infrared radiation.”

The light pulse, which is an oscillating electromagnetic field, shifts electrons up and down along the shaft of the metallic tip.  However, because the electrons cannot travel beyond the tip, they accumulate at the very tip apex during each half cycle of the oscillating electromagnetic field. Since electrons are charged, they give rise to an intense burst light at the tip a very small area, called the near field, with roughly the size of the tip apex, explains Huber.

This setup, which the Regensburg team is calling a scattering-type near-field scanning optical microscope, or s-NSOM, confines light to an area that is only 10 nanometers across. Without the needle, one would be limited to about half the wavelength of the light, the diffraction limit that curtails the resolution of every conventional microscope. "In contrast, the near-field occupies a volume that is approximately nine orders of magnitude smaller than the usual diffraction limit,” Huber told IEEE Spectrum.

For their experiment, the Regensburg team used indium arsenide nanowires prepared by a group—who are coauthors of the Nature Photonics paper—at the Pisa site of CNRNano, a nanoscience institute of the Italian Research Council. Indium arsenide is a semiconducting material holding a promise for terahertz sources and infrared lasers.

They first hit the nanotube with a terahertz pump pulse, which caused the liberation of charge carriers in the nanowire. This allowed them to detect the presence of electrons moving at the surface of nanowire—a little like what you would see if you disturb an ants' nest. Immediately after the pump pulse, they sent a series of terahertz probe pulses lasting few femtoseconds each. The pulses produced the 10-nm flashes of confined light on the tip. The intensity of these flashes depends on how many electrons are roaming around on the surface of the nanowire. These electrons don't stay around long, however; so, for every new probe pulse, the intensity of scattered light by the tip decreases.

By moving the tip to a new location for each pump cycle, the researchers created a complete "film" of how the nanowire reacts to the pump pulse. Among the phenomena they documented was that the free electrons first disappeared at the ends of the nanowires.

There is no reliable alternative, as of now, for measuring local carrier densities on the few-femtosecond time scale with 10-nm resolution, says Huber. "It is extremely important to understand how carriers behave locally. This insight is crucial for future high-speed integrated electronics and lasers based on semiconductor nanowires," he adds.

Nanotube-based Li-ion Batteries Can Charge to Near Maximum in Two Minutes

The prospects for ubiquitous all-electric vehicles (EVs) powered by lithium-ion (Li-ion) batteries took a bit of a hit back in 2010, when then U.S. Secretary of Energy Steven Chu addressed the United Nations Climate Change Conference in Cancun and suggested that, for battery powered cars to replace those powered by fossil fuels, some pretty significant improvements would need to be made to current technology.

Chu said at the time: “It will take a battery, first that can last for 15 years of deep discharges. You need about five as a minimum, but really six- or seven-times higher storage capacity and you need to bring the price down by about a factor of three.” Chu suggested it might take another five years before such a battery would be developed, and he was almost exactly right in his prediction.

Researchers at the Nanyang Technology University (NTU) in Singapore have achieved at least some of those criteria by developing a Li-ion battery capable of 20 years of deep discharges, more than 10 times that of existing Li-ion batteries.

In addition to longer battery life, the new battery design can be charged up quickly so that it can reach 70 percent of its maximum charge in just two minutes.

These features tick at least two of the metrics that Chu and others have indicated are key to making all-EVs compete with those running on fossil fuels. This would mean that EV owners would not have to spend roughly $5000 every two years for a completely new set of batteries. It could also allow for a quick stop of just a couple of minutes to significantly increase the driving range of the vehicle.

The key to the new Li-ion battery is the replacement of graphite at the anode with nanotubes synthesized from titanium dioxide. This is a departure from a lot of recent work toward improved anodes; other research teams have been using nanostructured silicon in place of graphite.

“With our nanotechnology, electric cars would be able to increase their range dramatically with just five minutes of charging, which is on par with the time needed to pump petrol for current cars,” said Chen Xiaodong, an associate professor at NTU Singapore, in a press release.

The new nanotube material, which is described in the journal Advanced Materials, is produced relatively easily, according to the researchers, by taking titanium dioxide nanoparticles and mixing them with sodium hydroxide. The real key to getting the long titanium dioxide nanotubes the nanoparticles yield is conducting the stirring process at the right temperature.

The technology has been patented and has been licensed by a company that says it could get a new generation of fast-charging batteries to market in two years.

While battery life and recharging have been significantly improved with the new battery design, it’s not clear that new batteries have a longer charge life, or what is known as gravimetric energy density (the amount of energy stored per unit mass). Instead, they have improved Li-ion’s relatively weak gravimetric power density (the maximum amount of power that can be supplied per unit mass) by eliminating the additives that are used to bind the electrodes to the anode. This allows the battery to transfer electrons and ions in and out of the battery more quickly. This translates into batteries that will last about the same amount of time on a charge as today’s current batteries, but can be charged up to near maximum very quickly.

NTU professor Rachid Yazami, who was the co-inventor of the lithium-graphite anode 34 years ago but not involved in this most recent research, has noted the significant improvement to Li-ion batteries this work represents.

Yazami said: “There is still room for improvement and one such key area is the power density—how much power can be stored in a certain amount of space—which directly relates to the fast charge ability. Ideally, the charge time for batteries in electric vehicles should be less than 15 minutes, which Prof Chen’s nanostructured anode has proven to do.”

Organic Coating Could Boost Photovoltaics' Conversion Efficiencies Far Beyond Today's Limits

Researchers at the University of Cambridge in the U.K. have developed a hybrid material made from pentacene (an organic semiconductor) and lead selenide (PbSe) nanocrystals (an inorganic semiconductor) that is capable of harvesting dark spin-triplet excitons at 100-percent efficiency.

This research, which was published in the journal Nature Materials, marks the first time that the energy from triple excitons has been transferred from organic to inorganic semiconductors. Prior to this work, that kind of transfer had only been shown to be possible with spin-singlet excitons.

 

To put this in perspective, a little background is in order. The process that occurs in solar cells involves photons from a light source (typically the sun) being absorbed and the photons generating excited states referred to as excitons. The excitons travel to positive-negative (p-n) interfaces where the charges are separated; positive charges are collected at anodes and negative charges are collected at cathodes. From there the charges are sent to an external circuit to produce electricity.

One critical note on this paradigm is that excitons come in two flavors: spin-singlet and spin-triplet. Spin-singlet excitons are for the most part what photovoltaic cells harvest to create electricity. On the other hand, spin-triplet excitons, known as “dark” triplet excitons, have an electron spin that makes it difficult for photovoltaics to harvest their energy.

“The key to making a better solar cell is to be able to extract the electrons from these dark triplet excitons,” says Maxim Tabachnyk, a Gates Cambridge Scholar at the University’s Cavendish Laboratory, and the paper’s lead author, in a press release. “If we can combine materials like pentacene with conventional semiconductors like silicon, it would allow us to break through the fundamental ceiling on the efficiency of solar cells.”

This fundamental constraint, known as the Shockley-Quessier limit, is based on the belief that solar cells with a single p-n junction can convert no more than 33.7 percent of the sun’s energy into electricity. This limit has pretty much stood the test of time, with state-of-the-art single p-n junction photovoltaics not reaching much above a 20-percent conversion efficiency.

However, Tabachnyk said via e-mail that these spin-triplet-exciton-harnessing hybrid solar cells could reach an energy conversion efficiency of 46 percent, well above the Shockley-Quessier limit.

While some research with nanowires has suggested an actual theoretical energy conversion efficiency of 65 percent, the best these nanowire solar cells have so far been able to achieve is around 14 percent conversion efficiency.

Whether this hybrid material can fare better in actual prototypes remains to be seen. Tabachnyk suggested that there has been some interest in commercializing the technology, but provided no further details.

Presumably, to this end, the research team is now developing a cheap organic coating that could be used to boost the power conversion efficiency of solar cells made from silicon.

New Theranostic Nanoparticle Diagnoses and Treats Cancer

So-called “theranostic” nanoparticles are capable of providing both a diagnostic as well as a therapeutic function in the same nanoparticle. Such theranostic nanoparticles have been primarily developed to address cancer diagnosis and treatment.

Now researchers at Singapore’s A*STAR Institute of Materials Research and Engineering and colleagues at the National University of Singapore have developed a theranostic nanoparticle that has the added benefit of being able to offer two distinct cancer therapies.

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Crumpled Graphene Offers Inexpensive Way to Achieve Flexible Supercapacitors

It was recently reported that used cigarette butts provide a material that yields far better energy density (the amount of energy stored per unit mass) than do many nanomaterials proposed for superconductors. So, it appeared that if graphene were really going to be a replacement for activated carbon on the electrodes of supercapacitors, it had better be able to compete on something more than energy density alone.

Now, researchers at MIT have used graphene to make stretchable supercapacitors to power flexible electronics.

 

While graphene has not been able to beat activated carbon in terms of surface area for electrodes of supercapacitors, it offers two other key features that activated carbon electrodes lack: because of its superior conductivity, it can operate at higher frequencies, making it useful for filtering circuits in AC rectifiers; and it can be made into engineered structures that enable use in applications that require thinner or just smaller dimensions.

The MIT researchers, whose work was published in the journal Scientific Reports, addressed another shortcoming typical of supercapacitors: the fact that they are very rigid. Making them flexible required a complicated and expensive process.

The trick to achieving supercapacitor flexibility was crumpling up the graphene so that when the supercapacitor is stretched the ridges in the material unfold. When completely flattened out, the supercapacitor’s dimensions can expand by up to eight times its size in its crumpled state.

This crumpling up and flattening can be repeated 1000 times without any loss of performance, according to the researchers. “The graphene paper is pretty robust,” said Xuanhe Zhao, an assistant professor at MIT, in a press release. “And we can achieve very large deformations over multiple cycles.”

The process for producing the graphene paper is relatively simple and low-cost compared to the flexible supercapacitors that are produced today. It requires taking a sheet of the material and placing it in a mechanical device that can compress it one direction, creating parallel folds or pleats. Then this folding is repeated so as to give the material its chaotic, rumpled surface.

An insulating layer, made from a hydrogel that is also highly flexible, is sandwiched between two layers of conductive, crumpled graphene.

Dan Li, a professor at Monash University in Australia who was not involved in this research, commented in the release: “Making supercapacitors stretchable has been a great challenge. This paper provides a very smart way to tackle this challenge, which I believe will bring wearable energy storage devices closer.”

Carbon Nanotube Yarns Could Replace Copper Windings in Electric Motors

A staggering fact is that motors and motor driven systems account for between 43 percent and 46 percent of all global electricity consumption. Needless to say, if electric motors could be made to  run more efficiently, energy consumption would fall. With research out of Rice University back in 2011 demonstrating that carbon nanotubes braided into wires could outperform copper in conducting electricity, it looked like there would soon be a new way to create those improved efficiencies.

Building on that research, a team at the Lappeenranta University of Technology (LUT) in Finland hasreplaced the copper windings used to conduct electricity in electric motors with a woven material made from threads of carbon nanotubes and achieved remarkable new efficiencies in the motors.

"If we keep the electrical machine design parameters unchanged and only replace copper with future carbon nanotube wires, it is possible to reduce the Joule losses in the windings to half of the present-day machine losses,” said Professor Juha Pyrhönen, who has led the design of the prototype at LUT, in a press release.

Copper windings have traditionally been used because they are the second best metal at conducting electricity at room temperature, and they come relatively cheap. However, despite their high conductivity, they do offer some resistance—to the point where Joule losses are often referred to as “copper losses.”

Meanwhile carbon nanotubes have conductivity far beyond the best metals and their limits of conductivity have not been found: some have been measured at 100 megasiemens/meter, compared to ultra-pure copper at 58.65 MS/m. With this kind of conductivity, carbon nanotube-based windings could result in double the conductivity of today’s copper windings, according to the Finnish researchers. And as Pyrhönen claimed, Joule losses could be cut in half with the carbon nanotube yarn.

In the prototype motor made by the Finnish team, the carbon nanotube yarns are spun and converted into an isolated tape by a Japanese-Dutch company Teijin Aramid. (The actual spinning technology was developed in collaboration with Rice University.)  Since this industrial use of the carbon nanotube yarn is still at its early stages, the production capacity has not been ramped up. This will have to be addressed if the new wire is to replace the ubiquitous copper windings.

However, the performance improvements are significant enough to warrant at least an investigation into whether it can become a realistic replacement for copper wiring.

"There is a significant improvement potential in the electrical machines, but we are now facing the limits of material physics set by traditional winding materials,” said Dr. Marcin Otto, business development manager of Teijin Aramid, in the press release. “We expect that in the future, the conductivity of carbon nanotube yarns could be even three times the practical conductivity of copper in electrical machines. In addition, carbon is abundant while copper needs to be mined or recycled by heavy industrial processes."

Sounds as though there’s a company that has a firm belief in the potential of their product in this application. And with the potential to put a big dent in the 45 percent of global electricity consumption that comes from the use of electrical motors, why not be confident?

A video of the motor equipped with the carbon nanotube windings is below.

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Nanoclast

IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

 
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Dexter Johnson
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