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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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One-Step Process Could Lead to Roll-to-Roll Production of Touchscreen Displays

Touchscreen displays have two types of conductor paths that enable a finger tap or swipe to trigger some response. There are those that cover the display so that when a finger passes over them, they open and close circuits. Then there are the larger conductor paths that are on the edges of the display, where all the smaller ones converge.

This design has always required a multiple-step manufacturing process that has made production costs high. Now researchers at the Leibniz Institute for New Materials (INM) in Germany have developed a one-step process for producing both of these conductor paths that should dramatically reduce manufacturing costs for touchscreen displays.

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White Graphene Helps Batteries Keep Their Cool in High Temperatures

While this blog has devoted a fair amount of attention to the use of nanomaterials to improve the charge capacity of electrodes in Li-ion batteries, mentions of research into the use of nanomaterials for electrolytes and separators has been more scarce on these pages. Nonetheless, a lot of research is going on with the aim of improving the thermal stability of Li-ion batteries’ electrolytes and separators.

Now, researchers at Rice University are combining their work on improving the thermal stability of electrolytes with research into separators made from a new material: hexagonal boron nitride. In research described in the journal Advanced Energy Materials, the researchers were able to produce a Li-ion battery that operated for more than a month at temperatures as high as 150 degrees Celsius (302 °F) with very little loss of efficiency.

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Graphene Filter Could Make Wireless Data Transmission 10 Times Faster

Researchers at Ecole Polytechnique Fédérale de Lausanne (EPFL) and the University of Geneva in Switzerland have developed a graphene filter for microchips that could potentially lead to wireless transmission rates 10 times as fast what chips deliver today.

In research described in the journal Nature Communications, the Swiss researchers actually fabricated what is known as a non-reciprocal isolator. The isolator filters backward radiation, preventing waves from being reflected back towards their source.

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Wafer-scale Nanotube Film Is Finally Here

Single-walled carbon nanotubes (SWCNTs) used to be the darling of those who were looking for an alternative to silicon in digital electronics. The first SWCNT-based transistors were fashioned almost twenty years ago, but scaling up the use of SWCNTs since then to very large scale integration (VLSI) processes has remained elusive.

There were two persistent problems with SWCNTs that led to much of the research community pursuing graphene instead of SWCNTs as the next great post-silicon hope: an inconsistency between semiconducting and metallic nanotubes and the frustration of trying to get all of the nanotubes to align on a wafer.

Now researchers at Rice University claim that they have struck upon a method that produces a uniform and wafer-scale film of highly aligned and densely packed SWCNTs that may finally deliver on the long-promised potential of SWCNTs.

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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
Rachel Courtland
New York City
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