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A scanning electron microscope image shows triangular (red) and rectangular (blue)  samples of a semimetal crystal known as cadmium arsenide.

Novel Nanomaterial Could Yield Lossless Charge and Energy Transport

Empirical evidence is continuing to pile up confirming that so-called topological insulators—materials that behave as conductors near their surfaces, but act as insulators throughout the bulk of their interiors—do exist. 

Now, researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), in cooperation with the Max Planck Institute for Chemical Physics of Solids in Germany, have produced a new type of topological matter. It can carry an electrical current on its surface at room temperture and then get that electrical current to transport itself to the other side of the material. The key to performing this trick was producing nanoscale slices of a material called cadmium arsenide, an inorganic semi-metal that is a semiconductor.

What is intriguing about cadmium arsenide is that it exhibits some of the same electronic properties as grapheneResearchers from Oxford, Stanford, and the University of California at Berkeley discovered in 2014 that electrons within cadmium arsenide act as though they have no mass at all, just as they do in graphene. But unlike graphene, cadmium arsenide is a stable 3-D material that is comparatively easy to produce in bulk and simple to fabricate for use in electronic devices. 

In research described in the journal Nature, the team used focused-ion beams to shape the cadmium arsenide so that electrons rotated around one side of the material like they were on a race track and then traveled through the bulk of the material to then rotate on the other side. One of the implications for this kind of movement is the ability to transport charge and energy through a material without loss. 

“This had been theorized by Andrew Potter [an assistant physics professor at the University of Texas at Austin] on our team and his co-workers, and our experiment marks the first time it was observed,” said James Analytis, a staff scientist at Berkeley Lab and assistant professor of physics at UC Berkeley, in a press release. Analytis, who led the research, added that, “It is very unusual—there is no analogous phenomena in any other system. The two surfaces of the material ‘talk’ to each other over large distances due to their chiral nature.”

Chirality is a quantum property in which a particle’s (in this case an electron’s) spin is coupled to its momentum, providing it with clear left-handed or right-handed properties. In this experiment, the researchers observed that the motion of the electrons possessed a dual handedness. In other words, some electrons traveled around the material in one direction while others moved in the opposite direction. The researchers believe that this experiment shows way toward exploiting this chirality to transport charge and energy through a material without loss. Researchers have predicted that this would alleviate much of the overheating experienced in today’s chips as dimensions become ever smaller.

In the experiments, the researchers applied an electric current to slices of cadmium arsenide only 150 nanometers thick; electrons began to race around in circles until their path took them through both the surface and the bulk of the material.

When the researchers applied magnetic field to the material, it pushed the electrons around the surface. When the surface electrons reached the same energy and momentum of the bulk electrons, they were pulled by the chirality of the bulk and pushed through to the other surface. This strange back and forth motion is repeated until defects in the material completely scatter the electrons.

The scientists say that, in future research, they would like to use fabrication techniques on the cadmium arsenide that build the magnetic properties directly into the material so that an external magnet will not be required to achieve this effect.

If they’re successful, then it’s possible to foresee its use in fabricating interconnects between computer chips in so-called “spintronic” devices that exploit the spin of an electron rather than its electrical charge to process data.

A molecular switch in a semiconducting polymer matrix undergoes reversible interconversion between its two forms

Flexible Nonvolatile Memory Just Got a Lot Closer

A regular stream of breakthroughs with organic nanomaterials for use in flexible electronics has observers scratching their heads as to why we aren’t seeing more of these technologies in applications such as wearable electronics. The problem has been that although organic nanomaterials have made flexible logic circuits and displays possible, they have pretty much failed to yield flexible, nonvolatile memories with write/erase speeds that would make them practical.

Now a team of researchers hailing from the University of Strasbourg and the Centre National de la Recherche Scientifique (CNRS) in France, along with collaborators from Humboldt University of Berlin and the University of Nova Gorica, in Slovenia, has developed a flexible nonvolatile optical memory thin-film transistor device made from organic nanomaterials that may change the game in wearable electronics.

To date, the major challenge in developing flexible organic memories has been creating a stable system that doesn’t lose data over time (volatility), is flexible, and offers an acceptable number of write/erase cycles (endurance).

The international research team overcame all of those hurdles, but they wanted more. “We wanted every single device to be able to store more than just a single bit (multilevel operation); we achieved 8 bits,” said Emanuele Orgiu, a researcher at CNRS and one of the authors of the paper, in an email interview with IEEE Spectrum.

“In addition, our devices can be made from solutions directly on a plastic substrate, and they feature very fast response times (within nanoseconds)—an intensely sought-after property for organic semiconductors, which usually exhibit very long response times (greater than a millisecond),” added Orgiu.

In a paper published in the journal Nature Nanotechnology, the team explains that it was able to achieve all of this by fabricating the device from molecules known as diarylethenes (DAEs), which can be switched between two states (called either open or closed form). Switching from writing to erasing was as simple as adjusting the wavelength of the light hitting the material (blue light for writing, green for erasing).

“The DAEs used in our work are particularly suited for nonvolatile data storage, since their two forms are stable at ambient conditions,” explained Tim Leydecker, another researcher from CNRS who is a member of the research team. “Plus, they can be switched even when embedded within a semiconducting polymer matrix, making them an ideal candidate for flexible films.”

explains that the molecules’ fast response to a 3-nanosecond laser pulse is relevant to modern electronics. Another benefit of the DAE molecules is that the amount of molecules that are switched in reaction to the light can be precisely controlled, which is a key requirement for multi-level storage that improves the data density.

Paolo Samorì, another team member from CRNS, explained that the molecules’ fast response to a 3-nanosecond laser pulse brings them right in line with modern electronics. Samorì added that another benefit of the DAE molecules is that the number of molecules that are switched in reaction to the light can be precisely controlled—a key requirement for improved data density in multilevel storage.

The devices they have fabricated so far are laboratory prototypes, and thus are relatively large at 1 square millimeter. Needless to say, miniaturization and encapsulation will need to be addressed in order for these memories to become a commercial product. However, the rearchers already have these issues in their sights, and plan to continue testing the performance and stability of the devices after encapsulation.

The team will also be examining fabrication processes compatible with industrial output, such as roll-to-roll manufacturing and inkjet printing.

Stefan Hecht, a team member from Humboldt University of Berlin, added: “Implementation into electronics featuring other organic components (organic light-emitting diodes and organic field-effect transistors) is an important step, as the entire system would benefit from the advantages of organic electronics.”

A man wearing black jeans and a grey-blue shirt sits holding a model of a ring-shaped molecule

Fusing of Organic Molecules With Graphene Opens Up New Applications

The hemoglobin-like molecule called porphyrin, which is responsible for making photosynthesis possible in plants and transporting oxygen in our blood, has been combined with graphene by researchers at the Technical University of Munich (TUM) in a new method that may make possible everything from molecular electronics to improved gas sensors.

While graphene’s properties—ranging from its electrical conductivity to its tensile strength—have made it desirable in a number of electronic applications, it still needs to be combined with so-called functional molecules to make it useful in applications such as photovoltaics and gas sensors.  To date, the addition of these other functional molecules has been carried out through “wet chemistry,” which limits the amount of control possible over the properties of the resulting material.

However, in a method described in the journal Nature Chemistry, the TUM researchers developed a highly controllable “dry” method based on exploiting the catalytic properties of a silver surface on which the graphene layer rested inside an ultra-high vacuum. 

The benefit of this technique is that it preserves all the attractive properties of the porphyrins even after being combined with the graphene, most notably their intrinsic ability to have their electronic and magnetic properties tuned by the addition of different metal atoms. In terms of real-world devices, this means that these different metal atoms can bind with gas molecules to create effective gas sensors.

More generally, the method the TUM researchers have developed could be a breakthrough for how graphene is functionalized for a range of electronic applications.

“The key to the importance of this research in terms of electronics is the complementary electronic structure in the graphene and the porphyrins,” said Wilhelm Auwärter, a professor at TUM who led the research, in an e-mail interview with IEEE Spectrum. “The porphyrins feature large electronic gaps, in contrast to graphene. The electronic, optical and magnetic properties of the porphyrins can be tuned by the choice of the metal center of the molecule.” Electronic band gaps are critical to controlling how conductive a material is, and in turn, whether or not the material can be used in an electronic switch such as a transistor.

Auwärter further explains the electronic and magnetic properties of the porphyrins can also be modified by the attachment of gaseous ligands (like oxygen or nitric monoxide), This would allow, for example turning on and off the material’s mechanical response to a magnetic field. “Such functionalities are not inherent to the pristine graphene,” he added.

Auwärter also said that it should be possible to directly incorporate porphyrins into graphene nanoribbons.  “In this way, one could achieve sequences of graphene ‘wires’ and porphyrin units. This should allow the engineering of an electronic gap in the hybrid structures,” he said. 

While Auwärter believes that this manufacturing approach provides an avenue that could lead to new device designs for a range of electronic applications, he does concede that this is preliminary research that primarily serves as a starting off point.

“We need to apply our protocol to well-defined graphene nanostructures, such as nanoribbons or nanographenes,” said Auwärter. “We need to place the hybrid structures on specific supports or to include them in layered materials and devices.”

In the future, to exploit this method for electronic applications, Auwärter points out that the hybrid material will need to be grown on insulating supports like hexagonal boron nitride.

While the electronic applications may still be somewhat far off, the novel protocol does offer an intriguing way forward for graphene-based electronics.

The puckered honeycomb lattice of monolayer phosphorene

Phosphorene Meets Expectations After Precise Measurements

Any of us who have done some do-it-yourself home improvements know that there’s a big difference between measuring “by eye” versus taking out a tape measure to get an exact measurement.

This difference in measurement approaches more or less represents what an international team of researchers from China and the United States have done in measuring the different band gaps that can be created when black phosphorous, also known as phosphorene, is layered.

Previous measurements of the band gaps in layered phosphorene employed fluorescence spectroscopy, which involves using a beam of light to excite electrons in molecules in the test sample, causing them to emit a measurable light. The new, more precise method leveraged in this latest research was optical absorption spectroscopy, in which the absorption of radiation is measured due to its interaction with a sample.

“This is the first measurement based on optical absorption of encapsulated phosphorene,” explained Steven G. Louie, professor of at the University of California Berkeley, in an e-mail interview with IEEE Spectrum. “The optical absorption data are not susceptible to defects and impurities, [unlike] the fluorescence spectroscopy used previously. The encapsulation helps to keep phosphorene from degradation.”

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SEM images of AgNW/graphene.

Silver Nanowires and Graphene Join Forces for Touch Screen Displays

The field of nanomaterials vying to replace indium tin oxide (ITO) as the transparent conductor that controls display pixels in touch screen displays is getting crowded. We’ve seen materials including carbon nanotubessilver nanowires, and graphene promoted as the heir apparent for this application.

Now, researchers at the University of Sussex in England have introduced a strong contender into the battle to replace indium tin oxide: a hybrid material consisting of silver nanowires that are linked together with graphene

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The researchers have developed a new strategy for crafting one-dimensional nanorods based on cellulose using a wide range of precursor materials.

"Hairy" Nanorods Offer Simpler Production Process

The optical, electrical, and magnetic properties of one-dimensional nanomaterials such as nanorods, nanocrystals, and nanotubes depend on their size and shape. A number of manufacturing techniques like chemical vapor deposition have been used to try and control these dimensions. However, these manufacturing techniques require tailor-made, multi-step reactions and purification procedures that are difficult to generalize.

Now, researchers at the Georgia Institute of Technology have developed a far more generalized approach that allows the production of many different kinds of one-dimensional nanorods from a wide range of precursor materials. The key to the technique is the use of block copolymer “arms” to create nanometer-scale compartments that serve as chemical reactors. The outer blocks of the arms prevent the aggregation of the nanorods.

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On the left side crystals of residual black phosphorus and tiniodide. The material is easy to produce and shows extraordinary optical and electronic properties, as well as extreme mechanical flexibility.

Novel Semiconductor Has Double-Helix Structure of DNA

The use of DNA in nanodevices has in large part been aimed at manipulating DNA to act like a semiconductor. But what if we could create an inorganic semiconductor that had some of the properties, including flexibility, of DNA?

The world of electronics is going to find out soon. Researchers at the Technical University of Munich (TUM) have discovered a double helix structure similar to DNA’s in an inorganic semiconductor material. The material consists of tin (Sn), iodine (I) and phosphorus (P), resulting in its chemical name SnIP. These three elements form in the SnIP around a double-helix configuration.

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Graphene speaker

Graphene Enables Flat Speakers for Mobile Audio Systems

Nanomaterials have been responsible for all sorts of innovation of audio speaker designs. We’ve seen magnetic nanoparticles used to eliminate the need for a magnet in speakers.  Carbon nanotubes have also been demonstrated to produce sound with heat. While these designs have been innovative, they were developed to demonstrate the capabilities of nanomaterials rather than to produce a piece of audio equipment.

Now, researchers at Korea Advanced Institute of Science and Technology (KAIST) have developed a new speaker design specifically targeted for the mobile audio market that draws its capabilities from nanomaterials. The KAIST researchers have used graphene to produce a speaker that does not require an acoustic box to produce sound.

In research described in the journal ACS Applied Materials & Interfaces, the researchers used graphene in a relatively simple process that yielded the long-elusive thermoacoustic speaker. Thermoacoustics is based on the century-old idea that sound can be produced by the rapid heating and cooling of a material instead of through vibrations.

While graphene has previously been shown to enable thermoacoustics (and carbon nanotubes have even been used previously to create thermoacoustic speakers), what sets the KAIST researchers’ work apart is the ease with which the graphene-based speakers are fabricated. The simple, two-step process, they say, will make commercial applications more likely. 

They started by first freeze-drying a solution of graphene oxide flakes. They then reduced and doped the oxidized graphene to improve its electrical properties. (The process does not require any templates to complete the fabrication.) The end result: an N-doped, three-dimensional, reduced graphene oxide aerogel (N-rGOA) that is freestanding.

The final aerogel sound element has a porous macroscopic structure can be easily modulated. The speaker the KAIST researchers produced consists of an array of 16 of the aerogels; it operates on 40 watts of power and produce a sound quality comparable to that of other graphene-based sound systems.

The researchers believe that because of the simplicity of their fabrication method, speakers can be mass-produced for use in mobile devices and other applications. As you can see in the video below, the fact that speakers are flat and don’t vibrate means that can be placed against walls and even curved surfaces.

Transmission electron micrographs of brain thin sections, identifying two distinct types of magnetite nanoparticles within frontal brain cells

Nanoparticles Found in Brains Come From External Sources

An international team of researchers, led by Barbara Maher, a professor at Lancaster University, in England, has found evidence that suggests that the nanoparticles that were first detected in the human brain over 20 years ago may have an external rather an internal source.

In research described in the Proceedings of the National Academy of Sciences, the scientists leveraged electron microscopy and magnetic analyses to not only discover the abundant presence of magnetite nanoparticles in the brain, but also determine that these nanoparticles are consistent with high-temperature formation, which means that they were likely not produced inside the body but were manufactured outside of it.

These magnetite nanoparticles are an airborne particulate that are abundant in urban environments and formed by combustion or friction-derived heating. In other words, they have been part of the pollution in the air of our cities since the dawn of the Industrial Revolution.

However, according to Andrew Maynard, a professor at Arizona State University, and a noted expert on the risks associated with nanomaterials,  the research indicates that this finding extends beyond magnetite to any airborne nanoscale particles—including those deliberately manufactured .

“The findings further support the possibility of these particles entering the brain via the olfactory nerve if inhaled.  In this respect, they are certainly relevant to our understanding of the possible risks presented by engineered nanomaterials—especially those that are iron-based and have magnetic properties,” said Maynard in an e-mail interview with IEEE Spectrum. “However, ambient exposures to airborne nanoparticles will typically be much higher than those associated with engineered nanoparticles, simply because engineered nanoparticles will usually be manufactured and handled under conditions designed to avoid release and exposure.”

While the results do seem to confirm previous research that indicates that airborne nanoparticles can reach our brains if inhaled, Maynard cautions that we should be careful not to extrapolate the data too far. He says that the paper had insufficient evidence to establish a causal link between the nanoparticles and neurodegenerative disease.

“What is lacking is any indication of how much exposure is needed to lead to harmful effects, and how the severity and probability of possible effects increases with increased exposure,” explains Maynard.

The formula for determining the risk of any substance is Hazard x Exposure = Risk. In this formula you can see that a highly hazardous substance like an acid may have restricted access, limiting its exposure and in so doing reducing its risk. When this formula is applied to the difference between engineered nanoparticles and those found in the air because of air pollution, we can begin to put the risks into perspective.

“In most workplaces, exposure to intentionally made nanoparticles is likely be small compared to ambient nanoparticles, and so it’s reasonable to assume—at least without further data—that this isn’t a priority concern for engineered nanomaterial production,” said Maynard. 

While deliberate nanoscale manufacturing may not carry much risk, Maynard does believe that the research raises serious questions about other manufacturing processes where exposure to high concentrations of airborne nanoscale iron particles is common—such as welding, gouging, or working with molten ore and steel.

UW–Madison engineers coated the entire surface of this substrate with aligned carbon nanotubes in less than 5 minutes. The breakthrough could pave the way for carbon nanotube transistors to replace silicon transistors,

Carbon Nanotube Transistors Finally Outperform Silicon

Back in the 1990s, observers predicted that the single-walled carbon nanotube (SWCNT) would be the nanomaterial that pushed silicon aside and created a post-CMOS world where Moore’s Law could continue its march towards ever=smaller chip dimensions. All of that hope was swallowed up by inconsistencies between semiconducting and metallic SWCNTs and the vexing issue of trying to get them all to align on a wafer.

The introduction of graphene seemed to take the final bit of luster off of carbon nanotubes’ shine, but the material, which researchers have been using to make transistors for over 20 years, has experienced a renaissance of late.

Now, researchers at the University of Wisconsin-Madison (UW-Madison) have given SWCNTs a new boost in their resurgence by using them to make a transistor that outperforms state-of-the-art silicon transistors.

“This achievement has been a dream of nanotechnology for the last 20 years,” said Michael Arnold, a professor at UW-Madison, in a press release. “Making carbon nanotube transistors that are better than silicon transistors is a big milestone,” Arnold added.  “[It’s] a critical advance toward exploiting carbon nanotubes in logic, high-speed communications, and other semiconductor electronics technologies.”

In research described in the journal Science Advances, the UW-Madison researchers were able to achieve a current that is 1.9 times as fast as that seen in silicon transistors. The measure of how rapidly the current that can travel through the channel between a transistor’s source and drain determines how fast the circuit is. The more current there is, the more quickly the gate of the next device in the circuit can be charged .

The key to getting the nanotubes to create such a fast transistor was a new process that employs polymers to sort between the metallic and semiconducting SWCNTs to create an ultra-high purity of solution.

“We’ve identified specific conditions in which you can get rid of nearly all metallic nanotubes, [leaving] less than 0.01 percent metallic nanotubes [in a sample],” said Arnold.

The researchers had already tackled the problem of aligning and placing the nanotubes on a wafer two years ago when they developed a process they dubbed “floating evaporative self-assembly.” That technique uses a hydrophobic substrate and partially submerges it in water. Then the SWCNTs are deposited on its surface and the substrate removed vertically from the water.

“In our research, we’ve shown that we can simultaneously overcome all of these challenges of working with nanotubes, and that has allowed us to create these groundbreaking carbon nanotube transistors that surpass silicon and gallium arsenide transistors,” said Arnold.

In the video below, Arnold provides a little primer on SWCNTs and what his group’s research with them could mean to the future of electronics. 

In continuing research, the UW-Madison team will be aiming to replicate the manufacturability of silicon transistors. To date, they have managed to scale their alignment and deposition process to 1-inch-by-1-inch wafers; the longer-term goal is to bring this up to commercial scales.

Arnold added: “There has been a lot of hype about carbon nanotubes that hasn’t been realized, and that has kind of soured many people’s outlook. But we think the hype is deserved. It has just taken decades of work for the materials science to catch up and allow us to effectively harness these materials.”



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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