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Single-Atom Sensor Offers New View of the Nanoscale

It was a eureka moment when IBM researchers first realized that they were imaging the surface of an atom with what came to be known as the scanning tunneling microscope (STM). Many believe that invention triggered the field of nanotechnology. Now researchers at the University of California Santa Barbara (UCSB) have created a next-gen microscope that can image phenomena like magnetism on the atomic scale across a huge range of temperatures. The heart of the microscope is a single atom or, perhaps more accurately, the absence of a single atom.

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Shining a Light on Phosphorene's Crystal Structure

Black phosphorus—sometimes referred to as phosphorene in a nod to its 2-D cousin, graphene—has been on a research tidal wave since 10- to 20-atom-thick sheets of the material were first exfoliated back in 2014.

The research community has been excited by a number of its properties, like its tunable band gap, which opens up all sorts of photonic applications. But they have also perceived its intrinsically strong in-plane anisotropy, which means its properties are dependent on the orientation of the crystal, as being at once a strength and a weakness.

Now a joint research project involving scientists from MIT, Tohoku University in Japan, Oak Ridge National Laboratory, the University of Pennsylvania, and Rensselaer Polytechnic Institute in New York, have developed a method for determining the orientation of the crystal that should make it easy to deduce the properties of a given sample of black phosphorous and help pave the way for greater use of phosphorene.

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DNA Creates Tiniest Thermometer Yet

Taking measurements of nanoscale objects is no easy task. Last month, researchers at IBM Zurich reported a breakthrough that leveraged an atomic force microscope (AFM) to measure the temperature of an object at the nanoscale. For the IBM researchers, the advance came when they stopped trying to make a nanoscale thermometer and instead focused on a macroscale thermometer for the nanoscale.

Now, researchers at the University of Montreal have turned that back around and produced the smallest thermometer yet. They had a skilled accomplice in accomplishing this feat: nature. The Canadian researchers have built a thermometer out of DNA that takes advantage of the molecule’s tendency to unfold in response to heat.

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Plasmonics Make Electrochromic Polymers Fast Enough for Video

Electrochromic polymers that change color when a voltage is applied already have a pretty impressive wow factor, turning windows from clear to tinted with a flip of a switch.

Now researchers at Sandia National Laboratories have put this impressive feat to shame. They have devised a way to make the usually slow responding electrochromic polymers change colors fast enough so that they could be used as a material for flat-panel TVs.

The problem that has handicapped electrochromic polymers up to now has been that in order for them to achieve a good contrast between bright and dark pixels it has been necessary for them to be relatively thick. While this is good for creating contrast, it slows down the diffusion times for ions and electrons to change the polymer’s charge state. This has limited their use to static displays or darkening windows.

The Sandia researchers overcome this limitation by making the electrochromic polymer only nanometers thick and relying on plasmonics, which exploits the waves of electrons (plasmons) that are created on the surface of a metal when it is struck by photons.

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Nanostructures Give Infrared Photodetectors Three Colors to See In

The nanostructured materials known as Type-II indium arsenide/gallium antimonide/aluminum antimonide (InAs/GaSb/AlSb) superlattices have been around since the 1970s and have served in infrared detection applications since the late 1980s. Since then, Type-II Sb-based superlattice materials have evolved drastically with many variants suited for different applications.

Now researchers at Northwestern University, led by Manijeh Razeghi, have developed a new superlattice design, called M-structure superlattice. It can be used to make devices that operate as a shortwave/mid-wave/long-wave infrared photodetector. Shortwave infrared wave (SWIR) bands make it possible to detect reflected light. Mid-wave detection picks up hot plumes and long-wave infrared detects cooler objects.

The researchers claim that a device designed around this new material can detect any of these infrared wavebands by simply adjusting the applied bias voltage. In terms of actual applications, the researchers claim this device could make possible infrared color televisions and three-color infrared imaging.

In research described in the Nature journal Scientific Reports, the researchers produced this superlattice by alternating the InAs, GaSb, and AlSb layers (with thicknesses of a few angstroms to a few nanometers) over several periods. The result is a one-dimensional periodic structure like that of the periodic atomic chain in naturally occurring crystals.

“The beauty of Type-II superlattice is the gap engineering capability which allows us to artificially manipulate and create novel ‘materials’ like the way natural semiconductors are created,” said Razeghi in an e-mail interview with IEEE Spectrum.

There are currently only a few material systems that are suitable for multi-spectral detection, according to Razeghi. The current state-of-the-art, mercury cadmium telluride (HgCdTe) and quantum well infrared photodetectors (QWIPs), are commercially available for infrared dual-band detection. However, mercury cadmium telluride technology is expensive and hard to make, while quantum well detectors suffer from low quantum efficiency and require low operating temperatures.

“In that context, Type-II InAs/GaSb/AlSb superlattices have proved to be an excellent alternative,” says Razeghi. “Controlling the electronic structure by managing the layer thicknesses as they are grown on GaSb substrate [yields] superlattices with the capability of tuning from SWIR to very-long wavelength infrared (VLWIR), covering the whole infrared spectrum.”

Despite the immense promise of the M-structure superlattices, this developing new material system has been the focus of considerably less development than II-VI based mercury cadmium telluride photodetectors, according to Razeghi.

“The current state-of-the-art in infrared detection technology is still based on HgCdTe, and relatively little effort has been expended developing dual- and triple-band T2SL based focal plane arrays (FPAs),” Razeghi told Spectrum. “There is a unique opportunity to mature this material system and realize a new generation of dual- and triple-band FPA sensors.”

However, Razeghi concedes that, responsivity, which dictates how sensitive a photodetector is, and the dark current, which is related to the noise, must be further improved. This means the optimization of many parameters, including device design, material growth, and all of the processing steps, must result in high reproducibility and high yield.

“We need to reduce the bias dependency of the long-wavelength channel to a reasonable range so that it could be compatible with the currently available readout integrated circuit,” says Razeghi.

The next step in the research, she says, will be to fabricate a three-color infrared camera. She added: “Our long term goal is to improve both electrical and optical performance of the detectors in order to make cheap high-performance infrared cameras for different applications.”

A Pinch of Salt Completes the Recipe for 2-D Materials in Supercapacitors

In joint research involving Drexel University in Philadelphia, and Huazhong University of Science and Technology (HUST) and Tsinghua University, both in China, scientists have found a way to formulate two-dimensional materials that are purer and have surface areas much closer to their theoretical maximum. The expected benefit: supercapacitors that store energy far better than ever. How did they do it? It turns out that they just needed to add a pinch of salt to coax the best out of them.

In research described in the journal Nature Communications, the international team used the surface of salt crystals to serve as growth templates for transitional metal oxides. That new ingredient, say the researchers, makes the final product bake up bigger and better. 

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Nanotubes Serve as Light Emitter in Integrated Photonic Circuit

Using fiber optic cables as waveguides for transmitting light that is ultimately converted into voice calls or data has been a mainstay for the telecommunications industry for decades.

But it’s been a massive struggle to adapt this kind of technology to the scale of a microchip so that photons carry data through an integrated circuit instead of electrons. Now researchers at Karlsruhe Institute of Technology (KIT) in Germany have tackled a major problem in making integrated optical circuits a reality by creating nanoscale photonic emitters with tailored optical properties that can be easily integrated into a chip.

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Firefly Structures Can Boost OLED Efficiency

Fireflies could help make organic light-emitting diodes (OLEDs) significantly more efficient, Korean researchers say.

Fireflies, which generate bioluminescent light more efficiently than many other animals, use their glow to attract mates. Previous research suggests that this efficiency was due not only to the chemicals that the insects use to produce light, but also to the lantern that holds these chemicals.

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Mismatched 2-D Layers Combine to Create New Optoelectronic Devices

Researchers have had success in using two-dimensional (2-D) materials to fabricate layered structures through epitaxy (the growth of crystals on a substrate), but only if they shared similar crystal lattices.

Now researchers at the U.S. Department of Energy’s Oak Ridge National Laboratory (ORNL) have shown that 2-D materials with dissimilar crystal lattices can still be grown together using epitaxy techniques. The ORNL team was able to grow a layer of gallium selenide—a p-type semiconductor—on top of molybdenum diselenide—an n-type semiconductor. This combination resulted in an atomically thin solar cell.

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Tesla Coil Remotely Induces Nanotubes to Self Assemble

Nikola Tesla conjured up all sorts of interesting experiments for his famed “Tesla Coils.” Today, however, their main use has been relegated largely to impressing visitors at science museums.

That is about to change. Researchers at Rice University have used Tesla coils to get carbon nanotubes to self-assemble into long chains, a phenomenon the scientists have dubbed “Teslaphoresis.” Controlled assembly of nanomaterials from the bottom up could be useful in applications including regenerative medicine where the nanotubes would act as nerves as well as fabricating electronic circuits without touching them.

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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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