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he graphene superlattice in which Stanford researchers measured conduction behaviors. Two-dimensional material is shown in green.

Long-Theorized Material Poised to Close the Terahertz Gap

Electronic devices are designed around how electrons travel through different kinds of solids. This movement of electrons can determine the material’s conductivity, its band gap, and its optical properties.

Nobel Laureate and one-time Stanford University professor Felix Bloch  theorized decades ago that it should be possible to get electrons to travel through a material in a very particular way that scientists hadn’t previously considered. Bloch predicted that it should be possible to structure a material in a way that causes electrons to oscillate in the terahertz band of the electromagnetic spectrum

This terahertz region has become known as the terahertz gap, because it has been so difficult to access even though it sits right between the radio wave and infrared radiation bands that we’ve been able to exploit in myriad ways.

Now, researchers at Stanford have realized Bloch’s theorized material. They’ve created heterostructures made of graphene and hexagonal-boron nitride that form a periodic lattice pattern of atoms known as a superlattice. In the process, they may have put us on a path to making a wide range of electronic devices including better airport scanners and solar cells.

In research described in the journal Science, the Stanford researchers demonstrated that layering graphene-hexagonal-boron nitride heterostructures in this superlattice structure makes it possible for the electrons to travel relatively long distances through the material before they are deflected by anything such as small impurities. The superlattice structure also ensured that the electrons were restricted to very narrow energy bands. This confinement, combined with their long-distance travel, cause the electrons oscillate at the frequencies they do.

It should be noted that this research did not actually produce a so-called Bloch oscillator. But it did yield a material that makes it possible for an electron to maintain its momentum and velocity over long periods—a key to eventually making a terahertz oscillator device.

What benefits would such a material provide? In the case of a solar cell, when a photon hits the superlattice material at a p-n junction, instead of the photon pushing out one electron, it could cause several several electrons to be emitted. If this material can eventually be made into a terahertz oscillator, it could eventually lead to the replacement of microwave-based scanners at airport security checkpoints. The shorter wavelength of terahertz-based scanner could reveal non-metal objects that can’t be detected currently.

Interestingly, while all of this comports perfectly with Bloch’s theory, the researchers discovered something new: another property of the material that turns the understanding of how electrons behave in semiconductors upside down.

“In semiconductors, like silicon, we can tune how many electrons are packed into this material,” said David Goldhaber-Gordon, a professor at Stanford and co-author of the paper, in a press release. “If we put in extra, they behave as though they are negatively charged. If we take some out, the current that moves through the system behaves as if it’s instead composed of positive charges, even though we know it’s all electrons.”

The graphene–hexagonal boron nitride offers a twist on this property: When more electrons are added to this material, it produces particles of positive charge; removing electrons has the opposite effect.

This reversal in the way electrons behave as they are added or subtracted from the material could lead to more efficient p-n junctions, the points at which p-type and n-type semiconductors are joined together to form the basis of many of the electronic devices we are familiar with. Menyoung Lee, a collaborator on the study who conducted the research as a graduate student in the Goldhaber-Gordon Group, believes that this nano-engineered material offers a very clear application of Bloch’s theory of solid-state physics.

Goldhaber-Gordon further envisions this work leading to a better understanding of how electrons travel through a material, and plans to leverage that understanding to create extremely narrow beams of electrons that are then send through superlattices. He believes that this could open up an entirely new field that he has dubbed “electron optics in 2-D materials.” The beams of electrons would travel in straight lines like a beam of light following the laws of refraction.

Goldhaber-Gordon added: “This is going to be an area that opens up a lot of new possibilities, and we’re just at the start of exploring what we can do.”

Visual representation of a fractal pattern

Mimicking the Veins in a Leaf, Scientists Hope to Make Super-Efficient Displays and Solar Cells

If you take a close look at a leaf from a tree and you’ll notice the veins that run through it. The structure these veins take are what’s called a quasi-fractal hierarchical networks. Fractals are geometric shapes in which each part has the same statistical character of the whole. Fractal science is used to model everything from snowflakes and the veins of leaves to crystal growth.

Now an international team of researchers led by Helmholtz-Zentrum Berlin have mimicked leaves’ quasi-fractal structure and used it to create a network of nanowires for solar cells and touch screen displays.

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NIST's proposed design for a DNA sequencer based on an electronic motion sensor.

A Superfast DNA Sequencer Based on Motion Detection

For more than 20 years, the practice of using a low-intensity electric current to pull long strands of DNA through nanometer-scale pores in a membrane and measure the electric field variations of the four nucleic acids—A, C, G, T—has been growing as the main approach for DNA sequencers. 

We’ve seen the development of this technology reach the point where U.K.-based Oxford Nanopore has been offering portable DNA sequencers based on this fundamental measurement principle for more than a year. Meanwhile, in the research labs, scientists have been tinkering with better materials for the membrane and have started to work with the “wonder material” graphene to see what benefits it might provide in these types of devices.

Now researchers at the National Institute of Standards and Technology (NIST) may have changed the technology paradigm for DNA sequencers in their proposal for an entirely new material architecture that would represent the first DNA sequencer based on sensing motion in the membrane as the DNA thread passes through it.

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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 temperature 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 semimetal 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, 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 3D 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 as if they were on a racetrack and then traveled through the bulk of the material to 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 coworkers, 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 a 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 a 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, which 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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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
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