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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.

Graphene's Prospects in Biosensing Just Got a Boost

In research that may pave the way for graphene to be used as an effective biosensor, scientists at Helmholtz-Zentrum Berlin for Material and Energy in Germany have developed a way to both measure and control the thickness of an organic compound bound to a graphene layer.

Graphene has presented an attractive option for developing biosensors because of its large surface-to-volume ratio and also because its electrical conductivity quickly decreases as soon as molecules begin to bind to it. 

While this change in electrical conductivity should be a strength in biosensors, the problem has been that graphene’s conductivity changes like this for just about any molecule it comes in contact with, so it lacks the ability to differentiate between molecules—poor “selectivity” as it’s called in the biosensor business.

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"Valleytronics" Development Could Lead to New Approaches for Spintronics and Quantum Computing

Researchers at the University of Bath in England have discovered, contrary to expectations, that in the field of “Valleytronics,” valley polarization can make electron spin polarization in silicon transistors easier. This discovery could have an impact on the development of silicon-based “spintronic” devices and quantum computing.

Despite this blog having covered the field dubbed “Valleytronics” with increasing frequency over the past year, it remains a fairly esoteric research area. A shorthand definition for valleytronics would be a movement away from exploiting the electrical charge of electrons as a means for storing information, and instead using the wave quantum number of an electron in a crystalline material to encode data. The “valley” of the portmanteau “valleytronics” refers to the shape of the graph you get when you plot the energy of electrons relative to their momentum: the resulting curve features two valleys.

Our coverage of the field has focused on efforts to achieve this effect with two-dimensional semiconductors such as graphene and tungsten diselenide.  However, the UK researchers have focused their research on valleytronics in silicon.

The history of valleytronics in silicon is not one of achievement but more of an annoying curse. In silicon transistors, valleys cause electrons to lose speed. And in research for quantum-information-based devices, the valleys lead to decoherence, which can ruin the quantum state of so-called quantum computers.

In research published in the journal Nature Communications, the UK researchers looked at the behavior of electrons in the valleys of silicon-on-insulator quantum wells when exposed to a magnetic field. Conventional wisdom suggested that it would be more difficult to polarize the electrons after having polarized the valley, but the researchers discovered that the opposite was true.

Kei Takashina, lead author of the research said in a press release: 

Our paper establishes the effect valley-polarization has on spin polarization in silicon transistors by using our unique capability to polarize valleys in the steady state. According to a simplistic way in which electrons are often thought about—that they move around independently of each other—it should become twice as difficult to polarize spins when valleys are polarized. In stark contrast, we find that at low enough electron density, it becomes easier to align their spin when valleys are frozen. This is a striking demonstration of how interactions between electrons lead to qualitatively new behavior.

Takashina believes that being able to control the valley polarization electrically will lead to new approaches to the development of either new silicon-based “spintronic” devices or interface complementary metal-oxide semiconductor (CMOS) technology with silicon-based quantum information processing.

Graphene Wraps Up Wires, Boosting Chip Speeds by Thirty Percent

While graphene is often suggested as a replacement for silicon in next-generation computer chips, it could make a more immediate impact as better wrapping for copper wires on today’s silicon chips.

Researchers at Stanford University have found that by wrapping graphene instead of tantalum nitride around the copper wires of computer chips boosts speeds in the chips by 30 percent.

“Researchers have made tremendous advances on all of the other components in chips, but recently there hasn’t been much progress on improving the performance of the wires,” said H.-S Wong, who led the research, in a press release.

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Paper-based Origami Battery Operates on the Respiration of Microbes

The Japanese art of folding paper into objects, known as origami, has taken on new life in nanotechnology research. The latest incarnation of this ancient art comes to us via research out of Binghamton University in New York. The technique has been applied to a battery technology that uses bacteria as the power source.

In the journal Nano Energy, the Binghamton researchers demonstrate how they have captured the respiration of microbes to generate enough energy to power a paper-based biosensor. All the microbes that were needed could be provided in a single drop of liquid teeming with bacteria.

“Dirty water has a lot of organic matter,” said Seokheun “Sean” Choi, who led the research, in a press release. “Any type of organic material can be the source of bacteria for the bacterial metabolism.”

Origami’s role in the battery design comes into play with the folding of two-dimensional sheets to create a three-dimensional battery structure that is about the size of a matchbook. The air-breathing cathode was produced by spraying nickel onto one side of a typical piece of paper. The anode is screen printed with carbon paints. The bacteria-laced water was added into a folded battery stack. In operation, this stack is unfolded, exposing all the cathodes to the air, maximizing their cathodic reactions.

The point of developing this simple battery was to find a way to power a separate paper-based biosensor without depending on an external handheld device to run the analysis. Choi believes that the simple battery he and his colleagues have developed can produce the microwatts required to run a biosensor in a self-contained system.

This simple, self-contained device should prove particularly useful in remote locations where resources—especially money—are limited; the entire device would cost only five cents.

Having been awarded a US $300,000 three-year grant from the National Science Foundation to develop a commercially viable origami battery, Choi is confident that we could see such a device in the field in the not-too-distant future.

Choi added: “Paper is cheap and it’s biodegradable. And we don’t need external pumps or syringes [for a paper-based biosensor] because paper can suck up a solution using capillary force.”

Graphene Shines in World's Thinnest Light Bulb

Back in April, we covered news that graphene was going to make a commercial breakout of sorts as a coating in an LED light bulb to reduce its energy consumption and lengthen its lifetime.

Now researchers at Columbia University from James Hone’s lab,  in cooperation with a team at Korea Research Institute of Standards and Science (KRISS), have taken this huge step forward by creating the first on-chip incandescent visible light source using graphene as the filament.

“We've created what is essentially the world's thinnest light bulb,” says Hone, a professor at Columbia Engineering, in a press release. “This new type of 'broadband' light emitter can be integrated into chips and will pave the way towards the realization of atomically thin, flexible, and transparent displays, and graphene-based on-chip optical communications.”

In work published in the journal Nature Nanotechnology, researchers suspended graphene above a silicon substrate by attaching it to two metal electrodes and then passed current through the graphene-based filament, causing it to heat up.

In the video below you can see an animated depiction of how the graphene filament operate.

The aim of creating integrated circuits that use photons rather than electrons, sometimes called integrated photonic circuits,  depends on being able to generate light on the chip itself. While a number of approaches have been developed for generating this light, this research marks the first time that anyone has done it with the simplest artificial light source: incandescent light.

The main reason that this was never achieved before is because of the amount of heat that incandescent light generates. In order for these micro-scale metal wires to glow in the visible light range, they must be able to withstand temperatures reaching thousands of degrees Celsius. Getting that level of heat to transfer out of the micro-scale wires was always a problem and often led to damaging the surrounding chip.

But graphene makes all the difference. The international team demonstrated heating the graphene-based filament to 2500 degree Celsius, so that it would glow brightly enough to be seen by the naked eye. The material’s success in this application depended on two of its properties: transparency and the fact that it acts as a poorer heat conductor the hotter it gets.

Graphene’s unusual heat conduction was key to keeping the light emitter from destroying the chip it was built on. The lack of conduction confines the heat within a small “hot spot” in the center of the graphene filament.

Graphene’s transparency was behind the discovery that emitted spectrum of light emitted had peaks at certain wavelengths. This, they found, occurred because of interference between the light emitted from the graphene filament and the light reflecting off the silicon substrate beneath it. So it becomes possible to tune the emission spectrum by altering the distance between the filament and the substrate.

Novel Nanomaterial Makes Carbon Dioxide Sensors Portable

Researchers at ETH Zurich, Switzerland and the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany have leveraged a new type of nanomaterial to fabricate a novel carbon dioxide (CO2) sensor that is much smaller in scale and requires less energy than previous devices.

A new operating principle allows the sensor to be both small and energy efficient. The researchers envision portable applications, such as scuba diving or high-altitude mountaineering.

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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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