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Graphene-based Magnetoresistance Sensor 200 Times as Sensitive as Silicon

Most of the sensor chips that turn home appliances such as refrigerators and washing machines into smart devices do so by detecting changes in electrical resistance brought on by the presence of magnetic field—also known as magnetoresistance (MR). These sensor chips, sometimes referred to as MR sensors, have traditionally been fabricated from silicon.

Now researchers at the National University of Singapore (NUS) have produced these MR sensor chips out of graphene and boron nitride. Their version is 200 times as sensitive to electrical resistance as its silicon counterpart. 

In research published last month in the journal Nature Communications, the NUS researchers used boron nitride as a substrate for graphene sheets; the resulting chip forms an interface that allows electrons to pass through the material very quickly.

“These electrons can thus respond to magnetic fields with greater sensitivity,” said Associate Professor Yang Hyunsoo in a local report on the research.

The chip possesses high sensitivity to both high and low intensity magnetic fields, and neither its tunability nor its resistance changes substantially in varying temperatures.

Silicon sensor chips’ properties begin to change as their temperatures approach 127 degrees Celsius—the maximum temperature at which most electronics operate—causing their sensitivity to wane. Conversely, the graphene-based sensors’ sensitivity actually improves as the temperature rises. As they reach 127 °C, their ability to sense resistance changes is as much as eight times greater than it is at room temperature.

What this could mean, according to Yang, is that the costly temperature correction mechanisms that these sensors now need when they’re used in automotive applications can be eliminated.

As far as tunability, the researchers discovered that they could alter the mobility of the graphene multilayers simply by tuning the voltage across the sensor, which enables the optimization of the sensor’s properties.

The researchers, who predict that this new sensor will make a big splash in the billion-dollar magnetic sensor market—which includes not only MR sensors but also superconducting quantum interference devices, or SQUIDs—have applied for a patent to protect their technology.

As Yang told the sci-tech news service

Our sensor is perfectly poised to pose a serious challenge in the magnetoresistance market by filling the performance gaps of existing sensors, and finding applications as thermal switches, hard drives and magnetic field sensors. Our technology can even be applied to flexible applications.


Li-ion Batteries Keep Improving, But for What Application?

This blog has long chronicled the efforts to change the electrodes of lithium ion (Li-ion) batteries from graphite to silicon. If you replace graphite with silicon on the anodes of a Li-ion battery, you can increase the charge by as much as a factor of ten.

Sounds great, but after a few charge-discharge cycles the silicon cracks and becomes inoperable from the expansion and contraction of the material. This is why researchers have been hard at work trying to create a nanostructured silicon that provides most of the attractive charge capacity but doesn’t crack after a few charge/discharge cycles.

The latest attempt at creating a silicon material that won’t crack under the demands of serving as an anode material comes out of the University of Waterloo in Canada where researchers have created a hybrid material made from silicon, sulfur and graphene that promises a 40 to 60 percent increase in energy density over today’s Li-ion batteries.

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The New Wrinkle in Graphene Is Wrinkles

One of the holy grails of graphene research has been a method for achieving wafer-scale growth of wrinkle-free single-crystal monolayer graphene on a silicon wafer.

Now researchers at the RIKEN research institute in Japan have discovered that the wrinkles in graphene may be their most attractive feature.

In research published in the journal Nature Communications, the RIKEN scientists discovered that the wrinkles found in graphene create unique electronic qualities, specifically a one-dimensional electron confinement. This restriction of electron movement results in a junction-like structure that changes from a zero-gap conductor to a semiconductor and back to zero-gap conductor.

The other revelation yielded by this research is that it’s possible to manipulate the wrinkles to change graphene’s band gap using mechanical methods rather than chemical techniques.

“Up until now, efforts to manipulate the electronic properties of graphene have principally been done through chemical means, but the downside of this is that it can lead to degraded electronic properties due to chemical defects,” said Yousoo Kim, who led the RIKEN team, in a press release. “Here we have shown that the electronic properties can be manipulated merely by changing the shape of the carbon structure. It will be exciting to see if this could lead to ways to find new uses for graphene.”

The discovery that it was possible to produce graphene semiconductors without the need to chemically dope the carbon sheets was the result of trying to produce graphene films using chemical vapor deposition (CVD). They were attempting to use CVD to grow graphene on a nickel substrate; they were examining how they could control the process with changes in temperature.

“We were attempting to grow graphene on a single crystalline nickel substrate, but in many cases we ended up creating a compound of nickel and carbon, Ni2C, rather than graphene,” explained Hyunseob Lim, the paper’s lead author, in a press release. “In order to resolve the problem, we tried quickly cooling the sample after the dosing with acetylene, and during that process we accidentally found small nanowrinkles, just five nanometers wide, in the sample.”

When examining the wrinkles with a scanning tunneling microscope, the researchers discovered that there were band gaps within them, which meant that they could act as semiconductors.


Much Ado About Carbon Nanotubes...Or Not

Researchers at the University of Paris-Saclay in France have discovered that fluid samples taken from the airways of 64 asthmatic children contained carbon nanotubes (CNTs). In addition, the France-based researchers determined that five other children studied also had CNTs in macrophages found in their lungs.

While this will no doubt add fuel to the fury of NGOs bent on shutting down research into nanotechnology immediately, there is little in this research that breaks new ground—at least qualitatively.

“From past studies, the conditions in combustion engines seem to favor the production of at least some CNTs (especially where there are trace metals in lubricants that can act as catalysts for CNT growth),” explained Andrew Maynard Director, Risk Innovation Lab and Professor, School for the Future of Innovation in Society at Arizona State University, in an e-mail interview. Says Maynard:

What, to my knowledge, is still not known, is the relative concentrations of CNT in ambient air that may be inhaled, the precise nature of these CNT in terms of physical and chemical structure, and the range of sources that may lead to ambient CNT. This is important, as the potential for fibrous particles to cause lung damage depends on characteristics such as their length—and many of the fibers shown in the paper appear too short to raise substantial concerns.

It’s not even clear from the research whether the nanoparticles in question are in fact carbon nanotubes. At this point, they are best described as carbon nanotube-like fibers.

Nonetheless, Maynard praises the research for establishing that these carbon nanotube-like fibers are part of the urban aerosol and therefore end up in the lungs of anyone who breathes it in. However, he cautions that the findings don’t provide information on the potential health risks associated with these exposures.

“Because of this,” Maynard told IEEE Spectrum, “it would be highly premature to draw any conclusions on health risk from the study.” He added that, “It would be appropriate to conduct further study into whether there is an association between these unusual carbon-based fibers and ill health.”

At least some of the coverage of the research has made the misleading point that “nanotubes have shown great potential in areas such as computing, clothing and healthcare technology” with the obvious implication being that the CNTs used in these applications are the ones found in the lungs of children.

While the research doesn’t draw a distinction between manufactured CNTs and the natural and incidental varieties produced by, say, car exhausts, there is little to suggest they are anything other than particles that have been around with us since the introduction of the internal combustion engine. Meanwhile, there has been little evidence showing that a manufactured CNT, once embedded in the matrix of a material, can ever be separated from that matrix so that it’s free to float around in the air.

“Some studies have indicated that occasionally single nanotubes might be released from abraded materials,” Maynard admits. “But it looks like the release rates are extremely low.  This is what would be expected given how tightly carbon nanotubes bind to polymers used in composite materials, and the amount of energy that would be required to release them.”

Maynard points that it may be possible in principle to create fingerprints for different types of CNTs based on their source, allowing scientists to determine definitively whether a sample of CNTs is from car exhaust, or tennis racquets and bicycles. He adds:

For instance, CNTs that are lab generated will often be associated with trace amounts of catalyst materials such as nickel or iron.  However, I suspect that such fingerprinting will require a level of characterization rarely used on such materials.

Such characterization may also be a moot point from a health perspective. Maynard notes that while ambient carbon nanotubes may be analytically difficult to distinguish from engineered carbon nanotubes, it’s reasonable to assume that our lungs will also find it hard to make the distinction.


Memristor Capable of Three Stable Resistive States Could Challenge Flash Memory

The imminent demise of flash memory at the hands of some new technological upstart has been predicted at least for the last decade.  The latest pretender to the throne is the so-called memristor (also called resistive RAM, ReRAM, or RRAM).  Of course, if you don’t like the term “memristor”, you can alternatively refer to it as “two-terminal non-volatile memory devices based on resistance switching.”

Now researchers at ETH Zurich have designed a memristor device out of perovskite just 5 nanometres thick that has three stable resistive states, which means it can encode data as 0,1 and 2, or a “trit” as opposed to a “bit.”

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Black Phosphorus Adds Thermal Management to Its Quiver

Much has been made of the fact that black phosphorus has an inherent band gap that graphene lacks, making it a more natural candidate than graphene to step into the role of silicon as a semiconductor material for electronics.

While that’s a pretty big advantage over graphene, black phosphorus possesses another property that nearly all of its 2-D cousins lack: in-plane anisotropy, which means its properties are dependent on the direction of the crystal.

Now researchers at the U.S. Department of Energy’s (DOE’s) Lawrence Berkeley National Laboratory (Berkeley Lab) have demonstrated experimentally what had previously only been theorized in computer models: black phosphorous has opposite anisotropy in thermal and electrical conductivities. This means that in black phosphorus, heat flows more easily in a direction in which electricity flows with more difficultly.

“Our study shows that in a similar manner heat flow in the black phosphorous nanoribbons can be very different along different directions in the plane,” said Junqiao Wu, one of the Berkeley Lab researchers, in a press release. “This thermal conductivity anisotropy has been predicted recently for 2-D black phosphorous crystals by theorists but never before observed.”

What this means is that when a microelectronic device fabricated from black phosphorus starts to heat up, it will be able to dissipate that heat extremely efficiently.

“This anisotropy can be especially advantageous if heat generation and dissipation play a role in the device operation,” said Wu in the press release. “For example, these orientation-dependent thermal conductivities give us opportunities to design microelectronic devices with different lattice orientations for cooling and operating microchips. We could use efficient thermal management to reduce chip temperature and enhance chip performance.”

In research published in the journal Nature Communications, the Berkeley Lab team found that when they increased the temperature of the black phosphorus above 100 Kelvin, the thermal conductivity anisotropy grew by a factor of two.

They also discovered that at 300 Kelvin, the thermal conductivity of black phosphorus nanoribbons decreased as the nanoribbons’ thickness decreased from 300 nanometers to 50 nanometers.

In the future, the researchers plan to look at how other physical parameters such as stress and pressure impact the thermal and electrical properties of black phosphorus nanoribbons.


The Artificial Skin That Could Deliver the Sense of Touch Directly to the Brain

Zhenan Bao at Stanford University has invested a lot of her research into building flexible circuits out of carbon nanotubes.

Her team’s latest feat is an “artificial skin” that’s capable of providing the sense of touch directly into the brain cells of mice and is initially aimed for use in prosthetic limbs to give the users the full sense of touch.

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Graphene-Coated Fabric Makes for a Wearable Gas Sensor

One of graphene’s key properties is its surface area—as a two-dimensional material it really is just all surface. This has advantages in a number of applications, one of which is sensors.

Now researchers at the Electronics and Telecommunications Research Institute and Konkuk University in South Korea have found that if they coat fabrics with graphene, they can detect dangerous gases and alert the wearer of their presence by triggering an LED light.

In research published in Scientific Reports, the Korean researchers coated commercially available yarn with reduced graphene oxide (RGO)—which refers to graphene oxide being stripped of most of its oxide to make graphene—using electrostatic self assembly and molecular glue to produce a bendable and washable electronic textile (e-textile) gas sensor.

“This sensor can bring a significant change to our daily life since it was developed with flexible and widely used fibers, unlike the gas sensors invariably developed with the existing solid substrates,” said Dr. Hyung-Kun Lee, who led this research initiative, in a press release.

The researchers discovered that the graphene-coated yarn was extremely sensitive to nitrogen dioxide, which is produced through the burning of fossil fuels. The sensor operates by the nitrogen oxide molecules changing  the electrical resistance of the graphene, which in turn triggers an LED light to turn on.

Within 30 minutes of exposure to 0.25 parts per million of nitrogen dioxide (just under five times above the acceptable standard set by the U.S. Environmental Protection Agency), the sensor detected the toxic gas.

While this fabric-based sensor compares favorably with previous RGO sensors prepared on a flat material, and offers three times the sensitivity to nitrogen oxide, it’s not clear why we would want clothing made from such a fabric. Wearable sensors have all sorts of potentially useful purposes, but this one seems somewhat obscure.

Nonetheless, the research does demonstrate that responsive gas sensors need not be fabricated on a solid substrate.


Memristors Don't Work the Way We Thought

What’s going to replace flash? The R&D arms of several companies including Hewlett Packard, Intel, and Samsung think the answer might be memristors (also called resistive RAM, ReRAM, or RRAM). These devices have a chance at unseating the non-volatile memory champion because, they use little energy, are very fast, and retain data without requiring power. However, new research indicates that they don’t work in quite the way we thought they do.

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Developments in Magnetic Skyrmions Come in Bunches

About two years ago, researchers in Germany introduced a new alternative in the world of magnetic storage media: skyrmions. A short description of skyrmions is that they are tiny, swirling magnetic spin patterns in thin films.  What makes them important is that they could change the face of data storage, hugely increasing the storage capacity over today’s hard disk drives.

Now, within a span of week, three separate research teams, one at the National Institute of Standards and Technology (NIST) in the United States, another at KTH The Royal Institute of Technology in Sweden and still another at the RIKEN Center for Emergent Matter Science in Japan have all announced breakthroughs that may bring these magnetic spin patterns one step closer to being the basis of real-world data storage applications.

In most magnetic materials, the magnetic force of each atom—known as the magnetic moment—lines up in the same direction as those of all the others. However, some magnetic materials such as manganese silicon (MnSi), when under extreme conditions, can develop areas where the magnetic moments curve and twist; these areas are known as magnetic skyrmions. Once skyrmions are formed, they are pretty resilient to outside influence, which makes them an ideal storage medium because they are not easily corrupted.

The extreme conditions are the key. To first form skyrmions, some of the researchers exposed the magnetic material to extremely low temperatures. But generally speaking, the conditions for forming skyrmions involve either magnetic, thermal, or electrical stimuli.

All three research teams published their findings in the journal Nature Communications.  The NIST researchers demonstrated that they could produce skyrmions by patterning asymmetric magnetic nanodots in a controlled circle on top of a thin film made of cobalt and palladium. The NIST researchers were able to create their skyrmions not only at room temperature, but also without an external magnetic field.

The Riken team out of Japan demonstrated that a new stimuli could create skyrmions, namely a mechanical forceWhile this method still involves creating the skyrmions in extremely low temperatures, it does offer the potential of both creating and deleting skyrmions with a small push. This, they say, could lead to new, low-cost memory devices that consume very little energy. 

The KTH researchers demonstrated not only a novel way to produce the swirling magnetic regions, but also presented a way to maintain their stability. The Swedish group formed its skyrmions under a nanocontact in which a spin-polarized current had been injected into the magnetic thin film. This provided a so-called spin torque to the film’s  magnetic moments so that the skyrmions remains more stable.

Observed as a group, this latest flurry of research suggests that skyrmions will become a mainstay of spintronics research for data storage. However, we may have a bit of a wait before we see this become the basis for magnetic data storage in our devices.



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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