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DGIST's senior researcher Jeong Min-kyung

First Graphene Photodetector To Operate in the Microwave

While graphene may be losing its luster in the field of digital electronics because of its lack of an inherent band gap,  in the world of optoelectronics graphene’s gapless band structure seems to be winning a new set of acolytes. This is seen no more keenly than in photodetectors, where graphene is enabling near-terahertz-speed photodetectors that are more energy efficient.

Now, researchers at the Daegu Gyeongbuk Institute of Science and Technology (DGIST) in South Korea and the University of Basel in Switzerland have developed a new graphene-based photodetector that operates at microwave wavelengths—a departure from graphene photodetectors that detect only optical wavelengths from the near-infrared to ultraviolet light.

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Schematic of the mono- and bilayer crystal structures. Purple and red spheres correspond to indium and selenium atoms, respectively.

Indium Selenide Takes on the Mantle of the New Wonder Material

Is there a research institute with a more distinguished pedigree in graphene research than the University of Manchester? There certainly haven’t been any that have “gone all in” the way Manchester has with its construction of a $71 million graphene research facility near the campus, which is operated under the auspices of the newly established National Graphene Institute (NGI).

This dedication to graphene makes sense considering the fact that Andre Geim and Konstantin Novoselov were at Manchester when they became the first researchers to synthesize graphene—the advance for which they were awarded the 2010 Nobel Prize in Physics.

But now it appears that a new material developed at Manchester, based on indium selenide (InSe), has taken some of graphene’s spotlight at Manchester, at least in terms of meeting the demands of future super-fast electronics.

“Ultra-thin InSe seems to offer the golden middle between silicon and graphene,” said Geim in a press release. “Similar to graphene, InSe offers a naturally thin body, allowing scaling to the true nanometer dimensions. Similar to silicon, InSe is a very good semiconductor.”

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The optical image of a folded graphene oxide film looks like baked filo dough

Graphene Solar Absorber Could Enable Cheap Thermal Desalination

To those uninitiated to the costs of thermal desalination of water, the idea of simply vaporizing water to take out the impurities seems like it would offer a limitless supply of fresh water just by using it on the world’s oceans. However, the energy costs for thermal desalination has been estimated at around 80 megawatt-hours per megaliter of water produced, rendering it too costly for just about everyone except Gulf States rich in oil and desperate for fresh water.

One way around these high-energy costs has been thought to be solar-powered thermal desalination, which can help produce clean water in remote areas and developing countries. However, the solar approach to water desalination is rather limited in the amount of fresh water it can produce and is further hampered by the need for optical concentrators and for thermal insulation, both of which have limited the large-scale use of this approach.

Now researchers at Nanjing University in China have developed a solar absorber material made from graphene oxide that enables a solar approach to desalinating water without the need for solar concentrators and thermal insulation. The result could be a low-cost, portable water desalination solution ideally suited for developing countries and remote areas.

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An artist’s rendering of nonlinear light scattering by a dimer of two silicon particles with a variable radiation pattern.

Nanoantenna Changes Direction of Light and the Prospects of Optical Computing

Russian and U.S. researchers have developed a technique whereby the direction of light can be manipulated using a novel optical nanoantenna. The researchers believe that this  nanoantenna could help lead to a new era in optical information processing in telecommunications systems.

Of course, replacing electrons with photons is the basis of optical computing. However, realizing this switch is fraught with difficulties—not the least of which is the fact that because a photon has neither mass nor an electric charge, it is far more difficult to steer than an electron, which can be pushed or pulled simply by applying an electric field. For optical computing to work, there needs to a control mechanism for photons that is just as simple. Waveguides are able to contain light and guide it in a certain direction over long distances. A nanoantenna works differently. Instead of guiding light, it bounces the photons that strike it in a specific direction. That directionality is determined by the materials and geometry of the nanoantenna, just like in classical antennas.

But what sets the nanoantenna developed by the research team from ITMO University in St. Petersburg, Russia, the Moscow Institute of Physics and Technology (MIPT), and the University of Texas in Austin, apart is that this photon-scattering proerty is tunable. The researchers say they can change the direction to which it scatters the incident light without changing its physical dimensions.

In the journal Laser & Photonics Reviews, the international research team described the development of a tiny (less than 200 by 200 by 500 nanometers) silicon-based nanoantenna that pushes photons in a particular direction depending on the intensity of the incoming wave of light.

“The new device will allow us to change the direction of light propagation at a much better rate compared to electronic analogues,” said Sergey Makarov, a senior researcher at ITMO University, in a press release.

As with previous research out of ITMO where researchers gained control over light scattering for optical computing, this proposed nanoantenna is built from silicon nanoparticles.

When silicon nanoparticles are subjected to laser light, they produce an electron plasma. This electron plasma in not the well-known surface plasmons we have become acquainted with in the field of plasmonics. This plasma is simply a bunch of conduction (free) electrons that are injected into the conduction band of a semiconductor when it absorbs light. They are free in a sense that they can move freely through the semiconductor (until they lose their energy and fall to the valence band from whence they originated).

“Surface plasmons are the special oscillations of these free electrons, but these oscillations may arise only when density of free electrons is quite large—and we dont have that many electrons in our situation,” explained Denis Baranov, a postgraduate student at MIPT, in an e-mail interview with IEEE Spectrum. “So, these are the same electrons that give rise to plasmons, but the density is not enough for them.”

It is this plasma excitation that serves as the mechanism for the nanoantenna to rotate the radiation pattern. Essentially, the more intense the incoming light, the greater the rotation becomes; it can change light’s direction by as much as 20-degrees.

“Once you choose specific diameters and positions of the particles and you gather them into a single antenna, then the direction of the light scattering is fixed,” says Baranov. “But, when plasma is generated within the particles (upon irradiation with a strong pulse), it changes their refractive index and, consequently, optical properties of the whole nanoantenna.”

In particular, it’s the plasma changes the direction of scattering. “Effectively, one may think of this as slightly changing the material the particles are made of: you change the material but leave the geometry unchanged—you have a different antenna with different light scattering,” added Baranov.

Also part of the nanoantenna’s design is the fact that one of the silicon nanoparticles needs to be resonant while the other is not. This is done to enhance the effect of the beam routing.

“Suppose that we have a nanoantenna composed of two identical particles. It cannot scatter light sideways; it always scatters it forward, due to symmetry,” explained Baranov.

However, when one of the particles is resonant, it experiences intense generation of plasma, while the other, non-resonant particle, does not. This provides the desired asymmetry in the behavior of the antenna.

“Now you see, that the same nanoantenna is capable of scattering light sideways or forward depending on the incident intensity,” said Baranov. “For weak pulses, there is not plasma generation, and since the antenna is not symmetric it scatters light sideways. When an intense pulse is applied, plasma is generated within the resonant particle, and the antenna becomes kind of "symmetric", so it scatters light forward.”

The optical antenna developed here can support data rates as fast as at 250 gigabits per second—a speed that could help it bridge the gulf between optical data transmission rates and electronic data processing speeds. Fiber optic cables are transmitting data at hundreds of gigabits per second but our electron-based computers can only process these signal at a fraction of those speeds.

Solar clothing from nanotextiles

"Back to the Future" Serves as Inspiration for Clothing With a Solar-Powered Battery

Jayan Thomas, an associate professor at the University of Central Florida’s (UCF’s) NanoScience Technology Center, has devoted a fair amount of his recent research to developing nanoprinting techniques to produce high-density supercapacitors. Thomas and his colleagues at UCF have been following that up with research in the area of supercapacitors aimed at creating nanowire-enabled cables that can both conduct and store energy.

Now Thomas has kept on this supercapacitor theme, but this time has taken some inspiration from the pop culture—namely the movie Back to the Future Part II—to create garments that can serve as solar-powered batteries that would never need to be plugged in.

“That movie was the motivation,” said Thomas in a press release. “If you can develop self-charging clothes or textiles, you can realize those cinematic fantasies—that’s the cool thing.”

In research published in the journal Nature Communications, Thomas and his UCF colleagues developed a ribbon-like device that can harvest light, convert it into electricity, and then store that electricity.

The work is remarkably reminescent of research presented back in September. Researchers created a woven material comprising two power-generating components: fiber solar cells and a triboelectric generator that produces energy through static electricity. However, that device did not have any energy storage element.

In this latest research, a power generation layer is joined to an energy-storage layer. The ribbon integrates a perovskite solar cell with a supercapacitor via a copper ribbon which functions as a shared electrode for direct charge transfer.

Supercapacitors are increasingly being considered a viable alternative for powering many of the things that now depend on batteries. However, there is a bit of a tradeoff. Though supercapacitors can release a large amount of energy very quickly and can be rapidly recharged, they pale in comparison to batteries when it comes to  storing large amounts of energy and discharging it over a long period of time. A lot of research has gone into maintaining that quick charge-discharge capability, while beefing up supercapacitors’ energy density.

In measurements taken by the UCF researchers, when the flexible solar ribbon was exposed to simulated sunlight, the perovskite solar cell achieved a 10 percent energy conversion ratio and the supercapacitor had a specific capacitance of 1193 farads per gram (F/g). To give you some context, a commercially available supercapacitor has a specific capacitance of around 100 F/g.

This means that the ribbon, when acting as a solar cell, can produce electricity pretty efficiently. And when it becomes a supercapacitor, it stores energy much longer than typical supercapacitors but still remains pretty short on capacity compared to chemical-based batteries.

In demonstrating the technology, the researchers made filaments from the ribbon and then weaved these filaments into a square of yarn using a loom. In real application, more advanced weaving techniques would be used. But the principle would be the same: the filaments would be woven into the material.

“A major application could be with our military,” Thomas said. “When you think about our soldiers in Iraq or Afghanistan, they’re walking in the sun. Some of them are carrying more than 30 pounds of batteries on their bodies. It is hard for the military to deliver batteries to these soldiers in this hostile environment. A garment like this can harvest and store energy at the same time if sunlight is available.”

In a telephone interview with IEEE Spectrum, Thomas did concede that at this point, the supercapacitor was not capable of storing enough energy to replace the batteries entirely, but could be used to make a hybrid battery that would certainly reduce the load a soldier carries.

Thomas added: “By combining a few sets of ribbons (2-3 ribbons) in parallel and connecting these sets (3-4) in a series, it’s possible to provide enough power to operate a radio for 10 minutes. Currently these devices are not optimized for providing the highest energy and power density. However, we are working on improving the energy density so that it can work as a hybrid battery-supercapacitor device.”

At just one atom thick, tungsten disulfide allows energy to switch off and on, but it also absorbs and emits light, which could find applications in optoelectronics, sensing, and flexible electronics.

Highest Performing Tungsten Disulfide Yet Brings Flexible 2D Circuits Closer

Layered two-dimensional (2D) transition metal dichalcogenides (TMDs)—like tungsten disulfide or molybdenum disulfide—are attractive for electronics applications because you can manipulate their band gap simply by adjusting the number of layers used.

But there’s a catch: it’s tricky to develop a processes that will lead to large-area synthesis of device quality TMDs. Now researchers at New York University’s (NYU) Tandon School of Engineering may have taken a big step toward closing down this issue with a new manufacturing process for tungsten disulfide that resulted in highest quality ever reported for the material.

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What looks like foil and wires wrapped around the back of a hand is a terahertz scanner made from carbon nanotubes

Flexible, Portable Terahertz Scanner Made From Carbon Nanotubes

Terahertz radiation can peer through objects to spot hidden items and analyze their chemistry, but today’s terahertz detectors are typically inflexible and bulky. Now scientists in Japan have for the first time created a portable, flexible, wearable terahertz scanner in order to better image objects with curves, including the human body.

Terahertz rays, which lie between the infrared and microwave bands of the electromagnetic spectrum, can pass through a wide variety of materials without damaging them. As such, terahertz cameras have great potential for noninvasive, high-resolution imaging. Promising applications include revealing hidden weapons, identifying explosives, and checking for defects in machined parts, among others.

However, conventional terahertz imaging technologies “use inflexible materials and therefore are adaptable only to flat samples,” says Yukio Kawano at the Tokyo Institute of Technology. So these imagers encounter difficulties when scanning most real-life samples—which possess 3D curvature—greatly limiting their use, he says. For instance, terahertz scanners at security checkpoints need to rotate detectors 360 degrees around human bodies to image them, a necessity that makes these systems very bulky.

Kawano and his colleagues devised their new flexible terahertz imaging device from films of carbon nanotubes, which are pipes of carbon only nanometers or billionths of a meter wide. At room temperature, their imager could detect a wide band of terahertz rays, ranging in frequency from 0.14 to 39 terahertz. This work marks "the first realization of a flexible terahertz camera," Kawano says.

The scientists developed portable terahertz scanners that they could wrap around objects. Using these scanners, they could image hidden items such as metal washers or paper clips concealed behind paper sheets or germanium plates or find a piece of chewing gum hidden in a plastic box. They could also identify metal impurities in a plastic bottle and a break in a syringe. These findings suggest this scanner could find use in “high-speed and multi-view inspections of industrial products, especially non-flat samples,” such as plastic bottles and pharmaceutical products, Kawano says.

In addition, the scientists developed a wearable scanner that could detect terahertz rays emitted by a human hand. “The wearable terahertz imaging of human hand without external terahertz sources is an important step for future medical applications,” Kawano says. For instance, this scanner could help inspect a broad range of things including cancer cells, sweat glands, and tooth decay, enhancing “real-time monitoring of daily health conditions,” Kawano says.

"We are planning to integrate our terahertz camera with a signal read-out circuit and a wireless communication device into a single chip and to develop a high-speed terahertz inspection system," Kawano says. "Real-time medical monitoring applications are our next step."

The scientists detailed their findings online 14 November issue of Nature Photonics.

DNA-based nanowires

Nanoscale Interconnects Come to Self-Assembling DNA Origami

DNA origami structures are essentially DNA strands that have been folded into structures using the techniques of the Japanese art of paper folding for which it is named. These DNA origami structures have been hotly pursued as a way to keep shrinking the feature sizes of chips. 

One could really get a sense of how seriously researchers were taking this approach for electronics when IBM researchers reported seven years ago that they were able to use such folded nanostructures to create a scaffold that served as a kind of quasi circuit board. That board allowed them to assemble components with features as small as 6 nanometers.

Despite this research interest, some aspects of the DNA origami technique have not been fully developed for electronics applications. One issues pressing the brakes has been interconnects: Nobody has produced well-defined electrical contacts between macroscopic electrodes and the DNA-based origami nanodevices.

Now researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and Paderborn Universities in Germany have taken on this gap in research and have developed a technique that will make it possible to build interconnects on the nanoscale between electronic components.  The researchers believe that their technique represents an important first step on the way to building electronic circuits by self-assembly.

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Basic setup that enables researchers to use lasers as optical “tweezers” to pick individual atoms out from a cloud and hold them in place

Large Number of Atoms Trapped In an Array Bolsters Quantum Computing

Digital logic depends on bits. The binary states of “0” or “1” form the basis of computing. In quantum computers, the bit is replaced by something called a quantum bit (or, qubit), which is an atomic particle that can be coerced into being both 0 and 1 simultaneously, at least for a time.

But one of the problems for quantum computing has been how to get restless atomic particles, like electrons, to sit down together in large groups long enough so that they can be used to carry out calculations.

Researchers at MIT and Harvard University have devised a way to capture atomic particles using optical “tweezers” and hold them in place long enough to take a picture of them so that their locations can be determined and lasers can be directed at them based on that information. Optical tweezers—more formally known as “single-beam gradient force traps”—have been a key instrument in manipulating matter in biology and quantum optic applications since Bell Labs first described that instrument in 1986.

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A nanostructured transistor for a transparant glucose sensor.

Nanostructred Transistor Enables Glucose Sensing Contact Lens

Various nanomaterials have been enlisted for creating improved glucose sensors that help diabetics determine when their blood sugar levels are too high or low. There have also been various ways in which nanomaterials have been incorporated into contact lenses to enable a variety of new capabilities.

Now, a research team at Oregon State University (OSU) has brought together nano-enabled contact lenses and glucose sensors into a single device that may someday do double duty as a blood glucose monitor and a contro mechanism for deciding when to deliver insulin injections. The device, say the researchers, will be a transparent sensor embedded in a contact lens.

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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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