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Graphene Paper Transforms Into Tiny Origami Robots

A tiny sheet of graphene “paper” smaller than a human fingernail can behave like an origami robot that folds and walks on command. The inspired work by Chinese researchers could pave the way for such self-folding devices as tiny robots and artificial muscles, or even help with biological tissue engineering on the smallest scales.

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Researchers Develop Fastest and Most Flexible Silicon Phototransistor Ever

Researchers at the University of Wisconsin-Madison (UW-Madison) have developed a flexible phototransistor based on single-crystalline silicon nanomembranes (Si NM). They claim that this phototransistor is the fastest and most flexible one ever produced. 

The flexible phototransistor could be incorporated into a wide range of applications. In a digital camera, for example, it could result in a thinner lens that would capture images faster and yield higher quality still photos and videos.

In research published in the journal Advanced Optical Materials, the silicon nanomembrane is used as the top layer of the phototransistor; it enables full exposure of the active region of the device to any light. The researchers used a technique known as  “flip-transfer” in which they essentially flip the nanomembrane onto a reflective metal layer.

This arrangement allowed the researchers to boost the light absorption capabilities of the phototransistor without the need of an external amplifier. They simply placed electrodes under the nanomembrane layer; both the electrodes and the metal layer serve as reflectors.

“In this structure—unlike other photodetectors—light absorption in an ultrathin silicon layer can be much more efficient because light is not blocked by any metal layers or other materials,” said Zhenqiang “Jack” Ma, a professor at UW-Madison, in a press release.

It is the combination of the device’s high sensitivity and flexibility that are unique in a phototransistor.

“This demonstration shows great potential in high-performance and flexible photodetection systems,” said Ma, in the press release. “It shows the capabilities of high-sensitivity photodetection and stable performance under bending conditions, which have never been achieved at the same time.”

The upshot, say the researchers is that the flexibility allows the photodetector to better mimic mammalian vision by curving to fit the  shape of the camera’s optical system. “Currently, there's no easy way to do that,” says Ma.

Simple Nano Trick Purifies Silicon for Energy Applications

Silicon is the second most abundant element on earth. Unfortunately, it’s mostly found in impure forms, not the refined elemental stuff needed for integrated circuits and solar cells.

The existing technology for making pure silicon is mature but it’s expensive and dirty, and it’s optimized for making electronics—not battery electrodes, thermoelectrics, and solar cells, which have different requirements, says Nanjing University’s Jia Zhu. He worked with Yi Cui, a materials scientist at Stanford University, to develop a simple and cheap nanotech trick to get 99.999 percent pure silicon from cheap bulk ferrosilicon.

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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.



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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