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SEM image of the repeater array

Carbon Nanotubes Make Big Push in Plasmonic Circuits

The field of plasmonics—which exploits the waves of electrons generated when photons strike a metal structure in order to carry out optoelectronic processes—has been building momentum in the research community over the past half decade. This interest is well placed. Plasmonics has made all sorts of interesting things possible, such as confining wavelengths of light to design smaller photonic devices. During that time, a range of two-dimensional materials, including black phosphorus and graphene, has enabled this growing interest. But the granddaddy of nanomaterials—the single-walled carbon nanotube—may still have a role to play in this exploding field.

Researchers at Peking University in China have gone back to single-walled carbon nanotubes (SWNTs) and used them as the active channel materials in the construction of surface plasmon polariton (SPP)-based plasmonic interconnect circuits. (Just as a bit of a primer, the waves of electrons that are generated when photons hit a metal structure are called either surface plasmons when referring to the oscillations in charge alone, or surface plasmon polaritons when referring to both the charge oscillations and the electromagnetic wave.)

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Microscopic image of the device

Integrating Silicon With Two-Dimensional Material Could Shake Up Photonics

Researchers at MIT have developed a fabrication method for integrating silicon photonics with layered two-dimensional material molybdenum ditelluride (MoTe2) to create a single device that acts as both a light-emitting diode and a photodetector.

This work could have a dramatic impact on the field of silicon photonics, which has become a leading architecture in chip-integrated optical interconnects. This popularity stems, in part, from the promise that many components, such as waveguides, couplers, interferometers and modulators, could someday be directly integrated on silicon-based processors.

This latest MIT research could smooth the path to this level of integration because it represents the first time that an electrically powered light source enabled by a 2D material has been integrated on a passive silicon photonic crystal waveguide, according to the researchers.

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Three researchers in white lab coats look at a tiny photodetector that one holds up under an array of colored lights.

Quantum Mechanical Process Could Double Efficiency of Photodetectors

Researchers at the University of California (UC) Riverside have discovered that the combination of two inorganic two-dimensional (2D) materials produces a quantum mechanical process that could significantly increase the efficiency of photodetectors, leading to revolutionary new ways of collecting solar energy.

In research described in the journal Nature Nanotechnology, the UC Riverside researchers have used the transition metal dichalcogenides  tungsten diselenide and molybdenum diselenide to achieve the effect known as electron-hole multiplication. Electron multiplication involves making multiple electron-hole pairs for each incoming photon. This can dramatically increase the efficiency of a photovoltaic cell in converting light into electricity.

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A close-up image of a tiny memristor that resembles a gray storm cloud with its funnel shape.

Memristor-Driven Analog Compute Engine Would Use Chaos to Compute Efficiently

When you’re really harried, you probably feel like your head is brimful of chaos. You’re pretty close. Neuroscientists say your brain operates in a regime termed the “edge of chaos,” and it’s actually a good thing. It’s a state that allows for fast, efficient analog computation of the kind that can solve problems that grow vastly more difficult as they become bigger in size.

The trouble is, if you’re trying to replicate that kind of chaotic computation with electronics, you need an element that both acts chaotically—how and when you want it to—and could scale up to form a big system.

“No one had been able to show chaotic dynamics in a single scalable electronic device,” says Suhas Kumar, a researcher at Hewlett Packard Labs, in Palo Alto, Calif. Until now, that is.

He, John Paul Strachan, and R. Stanley Williams recently reported in the journal Nature that a particular configuration of a certain type of memristor contains that seed of controlled chaos. What’s more, when they simulated wiring these up into a type of circuit called a Hopfield neural network, the circuit was capable of solving a ridiculously difficult problem—1,000 instances of the traveling salesman problem—at a rate of 10 trillion operations per second per watt.

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Simulation of the temperature induced self-folding process of a functionalized graphene flower.

Adding Wrinkles to Graphene Just Got Easier

Graphene’s two-dimensional physical attributes have offered some of its most attractive properties. But over the past couple of years, it’s been shown that adding a little wrinkle to the material—effectively making it three-dimensional—offers some new possibilities for the wonder material in wearable electronics and biological or dispensable sensors. However, adding those wrinkles comes at a price: the manipulation is performed under harsh conditions that compromise precision or tunability.

Now, in joint research between Johns Hopkins University and MIT, a team of researchers has developed a benign approach to self-folding graphene that lets it scrunch up into well-defined 3D microstructures.

Prior to this most recent work, graphene has been crumpled into relatively disorganized folded geometries by manipulating the substrate or etching pre-patterned catalysts. Folding has also been achieved with manual probes and by transferring graphene on to thick polymer substrates.

But in research described in the journal Science Advances, David Gracias and Weinan Xu from Johns Hopkins, and Markus Buehler from MIT developed an entirely new approach to making graphene thermally responsive while preserving its intrinsic properties and its ultrathin and flexible nature.

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Illustration of the multiple-SWNT device with PMo12 molecules (red and blue clusters)

Carbon Nanotubes Bring Background Noise to Computation

It seems counterintuitive that background noise—like a white noise—could actually be used to help detect faint signals. But that’s what happens in an unexpected phenomenon known as stochastic resonance (SR). We see it in use helping people keep their equilibrium or paddlefish locating plankton in muddy waters.

Now researchers at Osaka University in Japan have developed a SR-based electronic device that could potentially usher in a new era in bio-inspired sensors and new approaches to computing. The key to the device is the use of single-walled carbon nanotubes (SWNTs) and phosphomolybdic acid (PMo12) molecules. The combination creates a constant buzzing of movement.

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Nanofibers May Give Battery Electrodes a Needed Boost

In the annals of nanomaterial development, it’s hard to think of a research focus to which more effort has been devoted than creating an alternative to activated carbon for the electrodes in energy storage devices.

Graphene has been wrestled with for years, with the aim of producing electrodes for supercapacitors that will give these storage devices something close to the energy storage capabilities of lithium-ion (Li-ion) batteries. And we’ve seen the travails of researchers trying to use nanostructured silicon to make Li-ion batteries better suited to powering the next generation of all-electric vehicles. Despite all this effort, activated carbon has yet to cede its position as the material of choice—even though it is often little more than charred coconut husks.

Now researchers at Drexel University and Temple University have taken on the challenge of replacing activated carbon. But they decided that, rather than dispensing with it altogether, they’d use nanomaterials to make it better.

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STM image of all the initial β-form molecules in the middle row that changed into the α-form owing to a single manipulation

For the First Time, Signal Transfer Between Molecules Has Been Achieved

The history of molecular computing and electronics has been a long and twisting road—one that was meticulously catalogued on the pages of IEEE Spectrum two years ago. While the future of molecular electronics and computing remains somewhat up in the air, a great deal of research is still being focused on the field.

There have been proof-of-concept molecular switches, molecular data storage bits, and diodes.  However, one fundamental issue that has not been resolved is the transfer and exchange of signals between molecular devices for complex signal processing at room temperature.

Now researchers at Nanchang University in China have described, in the journal Nature Nanotechnology, a device that uses a particular kind of molecule that takes on two specific geometries when in contact with a copper surface. These two geometries can serve as the “0” and “1” of digital logic.

The work is based on a phenomenon known as in-plane molecular orientation, which occurs when an organic molecule lands on a solid surface. This adsorbed molecule might take different adsorption geometries. These adsorption geometries can be classified into several groups.

“In our case, the molecule we used has two distinguished adsorption geometries on a copper surface,” explained Li Wang, professor of physics at Nanchang University, in an e-mail interview with IEEE Spectrum. “One is left-handed, the other is right handed.” For the purposes of data storage or transfer, “We define left-handed geometry as ‘1’ and the right-handed geometry as ‘0’,” added Wang.

Wang and his colleagues discovered that the in-plane orientation of a molecule could be controlled by the in-plane orientations of two neighboring molecules due to their intermolecular interactions. The researchers exploited this intermolecular interaction as a way to build a logic gate in which an output signal is controlled by two input signals.

“For the first time, we have succeeded in realizing signal transfer and operation between molecules,” said Wang. “Our findings prove that a single molecule can present a certain signal and such signal can be utilized as a conventional signal to carry useful information to transfer and take part in complex operation processing.”

From this stepping stone, Wang believes in principle that as long as the molecules are coupled to each other in some way, much more complicated functions can be achieved, such as molecular computing.

The molecular devices that Wang and his colleagues fabricated in the lab were built by manipulating molecules one by one. For this kind of work to go beyond a mere prototype, it will de necessary to assemble the molecules into designed configurations with the expected intermolecular interactions, according to Wang.

In ongoing research, Wang and his colleagues intend to build more molecular devices with which they can exploit the intermolecular interactions in order to carry out different functions. “We will try to connect variable molecular devices into a whole system to achieve computing as common electronic devices can do,” he added.

Pseudo-coloured scanning electron microscope images of a fabricated PMP

Plasmonics Enables Sensing on Demand

Researchers at Northeastern University in Boston have developed an infrared sensor based on plasmonics that is capable of turning itself on when it needs to perform its sensing duties and then turns itself off when not needed to decrease energy demands and increase its lifetime.

The result is a sensor that performs only event-driven sensing. The sensor is dormant, yet is always alert and it awakes only in the presence of a signal of interest. As a result, the sensor only consumes power when there is something to be detected.

In research described in the journal Nature Nanotechnology, the Northeastern researchers used plasmonic nanostructures that take the form of nanoscale gold patches to act as tiny mechanical switches that take energy from the signal of interest—in this case a specific wavelength of infrared light—and mechanically close the contacts of the switches to create a low-resistance electrical connection.

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Conceptual illustration of a DNA robot sorting two types of cargo molecules.

DNA Robots Can Deliver Molecular Packages

Miniature robots with arms and legs made of DNA can sort and deliver molecular cargo, a new study finds. Such DNA robots could be used to shuffle nanoparticles around on circuits, assemble therapeutic compounds, separate molecular components in trash for recycling, or deliver medicines where they need to go in the body, researchers from the California Institute of Technology in Pasadena say.

“Just like electromechanical robots have been sent to places that are perhaps too far for humans to go to—for example, on another planet—if we truly master the ways of engineering molecular machines, we would be able to build molecular robots and send them to places that are perhaps too small for humans to go to—for example, inside the bloodstream,” says study senior author Lulu Qian, an assistant professor of bioengineering at Caltech.

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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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