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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.

Two Great Photovoltaic Materials Brought Together Make Better LEDs

Ted Sargent at the University of Toronto has built a reputation over the years as being a prominent advocate for the use of quantum dots in photovoltaics. Sargent has even penned a piece for IEEE Spectrum covering the topic, and this blog has covered his record breaking efforts at boosting the conversion efficiency of quantum dot-based photovoltaics a few times.

Earlier this year, however, Sargent started to take an interest in the hot material that has the photovoltaics community buzzing: perovskite. Now, he and his research team at the University of Toronto have combined perovskite and quantum dots  into a hybrid that they believe could transform LED technology.

In research published in the journal Nature, Sargent’s team describes how they developed a way to embed the quantum dots in the perovskite so that electrons are funneled into the quantum dots, which then convert electricity into light.

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Perovskite Solar Cell Production Gets Environmentally Friendly

While perovskite has begun to branch out into transistors and nanowire lasers, it already has a stellar reputation as an alternative to silicon in photovoltaics.

However, perovskite’s rise to glory in the field of solar cells has not come without a few hiccups. One of the key problems has been that the production processes for making the material involves some toxic lead compounds.

Now researchers at the Karlsruhe Institute of Technology (KIT) are leading a three-year project dubbed “Nanosolar” that both seeks to address this issue and to reduce material consumption and thereby production costs.

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The Capabilities of Nanomaterials in Textiles Continue to Expand

The story of nanomaterials in textiles dates back at least to the formation of the National Nanotechnology Initiative (NNI) at the beginning of the 21st Century, when Nanotex was the NNI’s poster company for the new exciting world of nanotechnology.

The role of nanomaterials in textiles has evolved since then from comparatively simple hydrophobic materials that Nanotex continues to  produce to where we now are creating textile electrodes using graphene or weaving nanowires into t-shirts to make them into supercapacitors.

Perhaps the greatest metric of how far nanomaterials have come in textiles is the range of work being done by students at Cornell University where they are using a variety of different nanomaterials in combination with cotton to create clothing that kills bacteria, conducts electricity, and serves as a platform for electronic devices.

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First Transistor Fabricated From Black-Arsenic Phosphorus

An international team of researchers from Germany and the US have fabricated for the first time a field effect transistor made of black-arsenic phosphorus.

In research published in the journal Advanced Materials, the researchers from the Technical University of Munich (TUM) and the University of Regensburg in Germany and the University of Southern California (USC) and Yale University in the United States have developed a new method for synthesizing black-arsenic phosphorous that doesn’t require the high pressure typically needed, lowering energy requirements for the process and thereby costs.

Black phosphorus has been around for about 100 years, but recently it has been synthesized as a two-dimensional material—dubbed phosphorene in reference to its two-dimensional cousin, graphene. Black phosphorus is quite attractive for electronic applications like field-effect transistors because of its inherent band gap and it is one of the few 2-D materials to be a natively p-type semiconductor.

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Graphene-Based Microphone Could Let You Hear Like a Bat

As a species, humans have evolved to have certain strengths and weaknesses. While we don’t have the sonar-like range finding capabilities of bats or dolphins, we do have the brains to engineer a device that can give that capability to us.

Researchers at the University of California, Berkeley have done exactly that in their development of tiny ultrasonic microphones made from graphene.

“Sea mammals and bats use high-frequency sound for echolocation and communication, but humans just haven’t fully exploited that before, in my opinion, because the technology has not been there,” said UC Berkeley physicist Alex Zettl, in a press release. “Until now, we have not had good wideband ultrasound transmitters or receivers. These new devices are a technology opportunity.”

The research, which was published in the journal Proceedings of the National Academy of Sciences, uses graphene in the place of paper or plastic in the diaphragm of a microphone. In combination with the graphene-based microphone, the Berkeley researchers created an ultrasonic radio that can be used for wireless communication

At only one atom in thickness, graphene possesses the key properties of strength, stiffness, and light weight; so it is extremely sensitive to a wide-range of frequencies. In this case, the microphone can pick up frequencies from across the human hearing range—from subsonic (below 20 hertz) to ultrasonic (above 20 kilohertz)—and as high as 500 kHz.  (A bat hears in the 9 kHz to 200 kHz range.)

To prove the effectiveness of their graphene-based microphone, Zettl and colleagues used it to successfully record the sounds of bats (you can hear those recordings here).

Aside from communicating with bats, the device has demonstrated ideal flat-band frequency response—meaning that it accurately reproduces incoming sound without attenuating or delaying any particular band. The researchers claim that such flat frequency response should have significant implications for acoustics.

What makes it all very attractive is that it’s quite simple to produce the devices. “There’s a lot of talk about using graphene in electronics and small nanoscale devices, but they’re all a ways away,” said Zettl in the press release. “The microphone and loudspeaker are some of the closest devices to commercial viability, because we’ve worked out how to make the graphene and mount it, and it’s easy to scale up.”

Twisting Graphene Alters Its Electrical Properties

Graphene has developed quite a reputation as an alternative material for enabling flexible electronics, such as a replacement for indium tin oxide (ITO) as a transparent conductor in flexible displays.

But now researchers at Rice University have used computer models to demonstrate that graphene’s flexibility can be exploited in another way: by twisting it to alter its electrical properties.

In research published in the Journal of Physical Chemistry Letters, the Rice researchers, in collaboration with a scientist in Moscow, used computer models to show how to produce in graphene the so-called flexoelectric effect in which a material exhibits a spontaneous electrical polarization brought on by a strain.

It is well known that graphene is a great conductor when it is laid flat on a plane so that all of its atoms have a balanced electrical charge. However, if you put a curve in that plane of graphene, the electron clouds of the bonds on the concave side compress while on the convex side they stretch. This changes the electric dipole moment, which is a measure of the overall polarity and determines how polarized atoms interact with external electric fields.

The researchers determined how each possible curvature in graphene could impact its dipole moment. In so doing, they have provided a way to calculate how graphene’s electrical properties change in any given geometry.

“While the dipole moment is zero for flat graphene or cylindrical nanotubes, in between there is a family of cones, actually produced in laboratories, whose dipole moments are significant and scale linearly with cone length,” said Boris Yakobson, who led the research, in a press release.

Yakobson believes that this research could help with a number of engineering issues with graphene.

“One possibly far-reaching characteristic is in the voltage drop across a curved sheet,” he said. “It can permit one to locally vary the work function and to engineer the band-structure stacking in bilayers or multiple layers by their bending. It may also allow the creation of partitions and cavities with varying electrochemical potential, more ‘acidic’ or ‘basic,’ depending on the curvature in the 3-D carbon architecture.”

Nanogenerator Produces Energy From Tires Rolling on Pavement

Researchers at the University of Wisconsin-Madison, in collaboration with a scientist in China, have developed a nanogenerator that can harvest energy from a tire rolling against the pavement.

In research published in the journal Nano Energy, the international team created a single-electrode nanogenerator that could be incorporated into a tire. When the part of the tire that contains the electrode comes into contact with the pavement, the friction between the rubber and the road creates an electrical charge, which is known as the tribolectric principle. The researchers believe that capturing the energy that is typically lost in the friction between a tire and the road surface could be a new avenue for greater energy efficiency in automobiles.

The researchers also determined that the amount of energy that the nanogenerator produces is proportional to the weight of the car and the speed at which it’s traveling.

“The friction between the tire and the ground consumes about 10 percent of a vehicle’s fuel,” said Xudong Wang, associate professor at Wisconsin-Madison, in a press release. “That energy is wasted. So if we can convert [some of it to electricity], it could give us very good improvement in fuel efficiency.”

To test their device, the researchers used a toy car with LED lights. When the electrode in the tires of the toy car ran over the surface, the energy harnessed by the generator would turn the LED lights on.

While the release doesn’t speculate where the energy might be used in a full-scale vehicle, it’s conceivable that the energy could be stored in the batteries of hybrid electric vehicles or full EVs.

“Regardless of the energy being wasted, we can reclaim it, and this makes things more efficient,” Wang said in the release. “I think that’s the most exciting part of this, and is something I’m always looking for: how to save the energy from consumption.”

Graphene Magnetic Sensor Hundred Times More Sensitive Than Silicon

Two years after it was launched, the European Commission’s $1-billion investment in graphene research, The Graphene Flagship, is holding a conference in Manchester, UK, called Graphene Week 2015.

While there’s been a lot of graphene news coming out of the conference, it appears that one advance has everyone a bit stunned. Researchers at Bosch and the Max-Planck Institute for Solid State Research, both in Germany, have developed a magnetic sensor that is reportedly 100 times as sensitive than the silicon variety.

This blog has covered research using graphene in biosensors, chemical sensors and pressure sensors, but we haven’t seen a lot of news about graphene in magnetic sensors. What we have noticed, however, is a lot of research effort devoted to making graphene magnetic. It started two years ago with research out of Spain.  That work led to a recent advance in the magnetization of graphene by the U.S. Naval Research Laboratory that could lead to a million-fold increase in storage capacity over today’s hard drives.

Instead of enabling improved hard drives, the work out Bosch labs aims to see what effect two-dimensional materials like graphene could have on magnetic sensors of the kind used in the automotive sector.

The first thing the researchers realized was that top-down approaches for producing graphene, such as chemical vapor deposition (CVD) or exfoliation of graphite, commonly referred to as the “Scotch Tape” Method, would not meet their needs.  These techniques were just not going to be scalable enough for a company like Bosch, which is doing this research with an eye towards production in five to 10 years.

For the substrate material, the researchers employed hexagonal boron nitride, a semiconductor with such a wide band gap that it essentially serves as an insulator. The researchers said they chose it for cost considerations as well as its performance.

The graphene sensors are based on the Hall effect, in which a magnetic field focused on a conductor causes a Lorentz force that deflects charge carriers in a current and leads to a measurable voltage. The key performance parameters for a Hall effect sensor are sensitivity, which is based on the number of charge carriers, and power consumption, which depends on charge carrier mobility. One of graphene’s most attractive properties is its extremely high charge carrier mobility, so it would seem to be a fit for this application.

The researchers claim that in the worst case scenario, the graphene-based sensor performs about as well as sensors using silicon as the conductor. But in the best case, the source current for the graphene sensor can be much lower than that needed by the silicon sensor, and the sensitivity of the graphene sensor is two orders of magnitude greater than that of its silicon-based counterpart.



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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