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Zinc-oxide Nanostructures Could Help Power Wearables

Korean-based researchers have been pushing the potential of zinc oxide (ZnO) nanomaterials as a platform for piezoelectric power generators for at least the last five years.

Once again researchers from Korea, this time out of the Korea Advanced Institute of Science and Technology (KAIST), have focused on the potential of ZnO nanomaterials to tap into their piezoelectric capability of converting mechanical energy into electrical energy to power micro devices.

In this latest research, published in Applied Physics Letters, the aim was to exploit the piezoelectric capabilities of ZnO-based nanostructures to serve as nanogenerators in wearable electronics.

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Photonic Circuits Get a Boost from Combination of 2-D Materials

Recently the field of plasmonics—which exploits the surface plasmons generated when photons hit a metal structure—has opened up the real possibility that photonic circuits could duplicate what electronic ICs do.  Previously, photonic circuits were just too large to be functional because of their need to accommodate different wavelengths of light.

Despite several advances, the plasmons still lost energy too quickly, which reduced the distance they could travel.  Now researchers in Spain, Italy, and the United States have developed a solution to this issue by combining graphene and boron nitride.

It has been known that when graphene is encapsulated in boron nitride, electrons can move ballistically for long distances without scattering, even at room temperature.

In this latest research, the international team—representing the Institute of Photonic Sciences (ICFO) in Barcelona, CIC nanoGUNE in San Sebastian, Spain, CNR/Scuola Normale Superiore in Pisa, Italy (all members of the EU Graphene Flagship), plus Columbia University in New York City—discovered, somewhat surprisingly, that the combination of graphene sandwiched between two films of hexagonal ​boron nitride (h-BN) is an excellent host for extremely strongly confined light and quite capable of suppressing of plasmon losses.

"It is remarkable that we make light move more than 150 times slower than the speed of light, and at length scales more than 150 times smaller than the wavelength of light,” said ICFO’s Frank Koppens, in a press release. “In combination with the all-electrical capability to control nanoscale optical circuits, one can envision very exciting opportunities for applications."

Columbia University has been at the forefront of creating these heterostructures by combining graphene and boron nitride since 2013. This latest research is seen as just an initial step in investigating the nano-optoelectronic properties of combining different two-dimensional materials.

"Boron nitride has proven to be the ideal 'partner' for graphene, and this amazing combination of materials continues to surprise us with its outstanding performance in many areas,” said Columbia University professor James Hone, in a press release.

In May of last year, researchers from CIC nanoGUNEand ICFO, along with the company Graphenea, demonstrated that an optical antenna made from graphene can capture infrared light and transform it into plasmons.

Rainer Hillenbrand from CIC nanoGUNE believes this latest research represents a significant advance in optoelectronics.

“Now we can squeeze light and at the same time make it propagate over significant distances through nanoscale materials, said Hillenbrand in the press release. “In the future, low-loss graphene plasmons could make signal processing and computing much faster, and optical sensing more efficient."

Nanobowl Solar Concentrator Boosts Organic Solar Cell Efficiency

Solar-concentrator technology has long held out promise as a way to increase the conversion efficiency of photovoltaics.

Now researchers at the Hong Kong University of Science and Technology (HKUST) have tackled the tricky issue of creating a solar concentrator for an organic photovoltaic (OPV) by developing a novel “nanobowl” optical concentrator fabricated on low-price aluminum foil.

Most OPV devices are based on a design that includes a glass substrate with indium tin oxide (ITO) electrodes. But there are a number of problems with this design, most notably that the solar cells are not flexible and that the ITO electrode compromises the OPV’s performance.

Aluminum foil substrates have the advantages of excellent conductivity, flexibility, inexpensiveness, and roll-to-roll manufacturing. But it is more difficult to get a uniform organic semiconductor “active layer” on aluminum foil’s textured surface than on the smooth surface of a glass substrate.

In work published in the journal Science Bulletin, the Hong Kong researchers were able to overcome this obstacle by incorporating nanobowl optical concentrators on the aluminum foil using a chemical process. The nanobowl optical concentrators improve the optical absorption in the active layer of the OPV despite the textured surface of the aluminum foil. Simulation studies carried out by the researchers revealed that such improvement was the result of the superior photon capturing capability of the nanobowls.

The nanobowl optical concentrator OPV demonstrated a more than 28-percent enhancement in power conversion efficiency versus devices without a nanobowl, the researchers claim. This 28-percent increase still only brings the nanobowl solar cell to an energy conversion efficiency of 3.12 percent, well below the 8.4-percent conversion efficiency record for OPVs reached last year.

Despite this comparatively low conversion efficiency, the promise of this latest research is that it shows a way forward in the development of geometrical light trapping with low-cost, chemical-based processes for manufacturing OPVs.

Graphene Hybrid Resists High Temperatures and Humidity

Almost three years ago, researchers at the University of Exeter in the UK developed a hybrid material that had molecules of ferric chloride sandwiched between two layers of graphene. The new material had better conductivity than graphene on its own. The researchers dubbed their creation GraphExeter.

Now the team at Exeter has announced that GraphExeter is resistant to both high temperatures and humidity.

This resistance to heat came as a surprise to the researchers because the molecules in the material on their own actually melt in air at room temperature.

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Piezoelectricity in a Free-Standing 2-D Material

Researchers at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have demonstrated that a free-standing single layer of molybdenum disulfide (MoS2) can exhibit the piezoelectric effect. The result could make possible new nanoscale low-power switches for sensors and other electronics.

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 a mainstay for a range of electronic devices. But it has been limited to bulk crystals. This latest demonstration of the effect in a two-dimensional material promises to expand those applications.

 “The discovery of piezoelectricity at the molecular level not only is fundamentally interesting, but also could lead to tunable piezo-materials and devices for extremely small force generation and sensing,” said Xiang Zhang, director of Berkeley Lab’s Materials Sciences Division, in a press release.

In October 2014, joint research out of Columbia University and Georgia Tech also demonstrated that MoS2 exhibits piezoelectricity and the piezotronic effect, which is the use of the piezoelectric effect as the gate voltage in transistor or similar device.

But in that research, the team at Columbia positioned thin flakes of MoS2 on flexible plastic substrates. In the new work out of Berkeley Lab, however, no substrate was used, and the researchers showed that free-standing MoS2 could demonstrate a piezoelectric effect.

In both efforts, the piezoelectric effect in MoS2 only appeared when an odd number of layers were used (1,3,5, etc.).

“This discovery is interesting from a physics perspective since no other material has shown similar layer-number sensitivity,”  Hanyu Zhu, one of the co-authors of the research published in the journal Nature Nanotechnology,  said in a press release. “The phenomenon might also prove useful for applications in which we want devices consisting of as few as possible material types, where some areas of the device need to be non-piezoelectric.”

In addition to logic switches and bio-sensors, the researchers believe that piezoelectricity in MoS2 could have an impact on the development of so-called “valleytronics.” Valleytronics essentially moves us away from the use of charge as a means for storing information and toward a scheme where we instead we encode data in the wave “quantum number” of an electron in a crystalline material. The term valleytronics refers to the fact that if you plotted the energy of electrons relative to their momentum on a graph, the resulting curve would feature two deep valleys.

The researchers are investigating the possibility of using piezoelectricity to directly control valleytronic properties  in molybdenum disulfide.

Nanowires Made From Solar Wonder Material, Perovskite, Promise Even More Efficient Solar Cells

If you’re working with the latest in photovoltaic materials, chances are you’re doing something with perovskite, which has become all the rage in the solar world.

Now researchers at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have become the first to make nanowires out of perovskite, combining two favored materials for use in photovoltaics.

In work published in the journal Nano Letters, the Swiss researchers developed a novel method for fabricating the perovskite into nanowires, which also have developed quite the reputation in photovoltaic applications.

The road to developing the method started with attempts to produce nanostructures by getting a liquid, methylammonium lead iodide, to form into nanocrystals. From there, the researchers expected that the nanocrystals could be used in some kind of device.

“I could grow it into flakes, needles, and cubic crystals,” said Endre Horváth, one of the EPFL researchers, in a press release. “But these are large, macroscopic structures, so I tried to scale them down.”

He was able to shrink the crystals first to micrometer scale and then ultimately down to the nanoscale. Encouraged by this success, the research team started to think that perhaps the nanocrystal material could be elongated into nanowires.

“If we could texture this perovskite from loosely connected grains into nanowires, we could improve the performance of photovoltaic cells,” said László Forró, one of the EPFL researchers, in a press release.

After discovering that the typical method for texturing perovskite, a process known as “blade coating,” wasn’t effective, the researchers hit upon the idea of pressing the perovskite between two glass coverslips typically used to view samples under a microscope.

The process of pressing the two glass coverslips and then sliding them apart created needle-like structures that were observable within seconds. Playing off the term “blade coating,” the researchers dubbed their process “slip-coating.” In the video below, you can see how the perovskite forms into the nanowires.

“If we can make nanowires like this, it will open up a whole new subfield of technology, where we can make a number of optical tools, such as detector antennas, lasers or diffraction gratings,” added Horváth.

Multiferroic Memory Promises Low-Power, Instant-on Computing

An international team of researchers, led by a group at Cornell University, have developed a room-temperature magneto-electric memory device that can be switched with just voltage rather than current. If developed further, the technology could lead to low-power, instant-on computing, they claim.

The memory device is based on a multiferroic material that combines both magnetic and ferroelectric properties. The result of joining these two properties is that it becomes possible to control charges using magnetic fields and spins simply by applying a voltage.

“The advantage here is low energy consumption,” said Cornell postdoctoral associate John Heron, in a press release. “It requires a low voltage, without current, to switch it. Devices that use currents consume more energy and dissipate a significant amount of that energy in the form of heat. That is what’s heating up your computer and draining your batteries.”

In research published in the journal Nature, the international team—consisting of researchers from the University of Connecticut; University of California, Berkeley; Tsinghua University, in China; and the Swiss Federal Institute of Technology, in Zurich—made the multiferroic material out of bismuth ferrite. That substance has the rare property of possessing a permanent local magnetic field while always posessing an electric polarization that can be switched by applying an electric field.

Research at Cornell, led by Darrell Schlom, had previously demonstrated that multiferroic materials other than bismuth ferrite could serve as a basis for nonvolatile memory devices.  However, in that research the multiferroic materials had to be kept at super low temperatures of 4 Kelvin (-269 Celsius).

The breakthrough for the new research was first theorizing and then demonstrating that the kinetics of the switching in a bismuth ferrite device occurs in a two-step process. They found an intermediate state between the 0 and 180-degree rotated states under the electric field.

In addition to this two-step process allowing for room temperature switching, the researchers discovered that such a multiferroic memory device requires an order of magnitude lower energy than spin-transfer-torque (STT) memory devices, which just earlier this year were promising to upend the memory business.

However, STT memory is already available commercially, albeit in limited scope, so this latest one-off multiferroic memory device has a ways to go before it pushes any other memory technologies aside.

Nonetheless, getting multiferroics to operate at room temperature is a major development. “Ever since multiferroics came back to life around 2000, achieving electrical control of magnetism at room temperature has been the goal,” Schlom said.

Graphene Wraps Up Lithium-Sulfur Batteries

Earlier this year, researchers at the Department of Energy's Pacific Northwest National Laboratory (PNNL) developed a nanomaterial powder called a metal organic framework (MOF) that could be added to the cathode of lithium-sulfur batteries, promising to hold a charge longer and increase the number of charge-discharge cycles.

Now, in joint research between the University of Cambridge and the Beijing Institute of Technology, an MOF powder has been used to improve the cathodes of lithium-sulfur batteries but in a different way. The new wrinkle to this research was that the team wrapped the sulfur-carbon energy storage unit in a thin sheet of flexible graphene, resulting in faster transport of electrons and ions.

In work that was published in the journal APL Materials, the international team used the MOF powder as a template for creating a conductive porous carbon cage in which sulfur acts as the host and each sulfur-carbon nanoparticle behaves as an energy storage unit where electrochemical reactions occur.

“Our carbon scaffold acts as a physical barrier to confine the active materials within its porous structure,” explained Kai Xi, a research scientist at Cambridge, in a press release. "This leads to improved cycling stability and high efficiency."

The researchers report that the graphene in this design is used as a kind of bridge to form interconnected networks that can reduce the internal resistances of each component. The graphene and MOF-derived microporous carbons form a composite structure—sort of a porous scaffold—that has conductive connections, making it a promising electrode structure design for rechargeable batteries.

Xi notes in the release that this work provides, “a basic, but flexible, approach to both enhance the use of sulfur and improve the cycle stability of batteries.” He added that, “Modification of the unit or its framework by doping or polymer coating could take the performance to a whole new level.”

The researchers believe that this novel design, in which energy storage and an ion-electron framework are integrated, could open the door to creating high-performance energy storage systems that are non-topotactic (meaning that the chemical reactions don’t cause structural changes to the crystalline solids involved).

2-D Material Could Lead the Way to "Valleytronics"

Earlier this year, we reported on researchers at the Massachusetts Institute of Technology (MIT) who had demonstrated that two-dimensional materials could be an alternative to diamonds in the esoteric world known as “valleytronics.”

Now, once again, researchers at MIT have shown that the 2-D material known as tungsten disulfide (WS2)—which belongs to a class of 2-D crystals known as transition metal dichalcogenides—could lead the way to valleytronics replacing conventional electronics.

Valleytronics essentially moves us away from the use of electrons’ electrical charge as a means for storing information to a scheme where we instead employ the wave quantum number of an electron in a crystalline material to encode data. The term valleytronics refers to the fact that if you plotted the energy of electrons relative to their momentum on a graph, the resulting curve would feature two deep valleys.

Manipulating these two valleys so that one is deeper than the other, would yield a way for the electrons to populate one of the two valleys. The positions into which electrons fall is a way to represent the zeroes and ones in digital computing.

But to get to the point where you could create stored information, you need to create a difference in the energies of the electrons populating the two valleys. The problem is that the electrons naturally want to settle into the lowest energy value, and they can achieve that in either of the two valleys.

You need to find some way to induce a difference in the energies of the two electron valleys. To date, the idea has been to achieve this change through the use of magnetic fields. However, to trigger that change you need a very powerful magnetic field, in the range of hundreds of tesla, to get even the most miniscule change. This limits the technique’s use to the lab.

“We discovered a way to directly control this valley by using light,” said Edbert Jarvis Sie, a MIT graduate student, in a press release.

The MIT researchers were able to achieve a much greater energy shift in the electrons by using a relatively conventional laser pulse with a special polarization.

“Being able to manipulate the valley degree of freedom in two-dimensional transition metal dichalcogenides would enable their application in the field of valleytronics,” said David Hsieh, an assistant professor of physics at Caltech, who was not connected to this research, in a press release. “This experiment makes a large step toward realizing this goal by demonstrating a method to control the energy difference between two valleys in tungsten disulfide for the first time.”

Self-Assembly Technique Offers Potential Scalable Production of Optical Memory

Researchers at RIKEN in Japan have developed a self-assembly method for creating well-ordered molecular structures that could eventually lead to the scalable production of organic optoelectronic devices such as optical memory.

Previous research has indicated that organic molecules can reversibly change their state in response to pulses of light—a quality that would be useful for storing digital ones and zeros in a different form. However, for it to work in an optoelectroic device, the molecules have to be arranged into highly ordered, single-molecule layers that are bonded to a metal surface. This bonding is where the problems begin. Once bonded, the optical properties of the molecules are altered so that getting the desired optoelectronic properties becomes difficult.

In research that was published in the journal Angewandte Chemie, the RIKEN team used interactions between electric dipoles (objects with a positive pole and a negative one) of molecules and alkali metal ions to create a homogeneous monolayer of diarylethene molecules on a copper surface.

In general, the property that the diarylethene molecules have that is useful in optoelectronics is that they are photochromic, which allows them to change color reversibly when irradiated with light. But what distinguishes the diarylethene molecules the RIKEN researchers created is that they also have electric dipoles. This means the molecules can self-assemble on the copper substrate and maintain their attractive photochromic properties.

“With homogeneous and close placing of individual molecules on a solid surface, we might be able to develop a memory device with several hundred to a thousand times the density achievable using current technology,” said Tomoko Shimizu, one of the lead researchers, in a press release. “We now want to study ways to achieve on–off switching of individual molecules in the superstructure in a controlled manner.”

This is pretty early stage research, especially when measured against the immense competition to develop optical memory technologies. But when you see terms like “scalable production”, you have to consider the long-term potential of the technology.

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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.

 
Editor
Dexter Johnson
Madrid, Spain
 
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Rachel Courtland
Associate Editor, IEEE Spectrum
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