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Nanograss is Greener on the Photovoltaic Side

Nanopillars — sometimes referred to as “nanograss” because of their resemblance to blades of grass — have offered a way to increase the light absorption of thin films of silicon.

Now nanograss has been used by researchers at the University of Massachusetts Amherst in cooperation with others from Stanford University and Dresden University of Technology in Germany to overcome the discontinuous pathways — or dead-ends — that compromise the ability of positive-negative (p-n) junctions to extract energy in organic solar cells.

“For decades scientists and engineers have placed great effort in trying to control the morphology of p-n junction interfaces in organic solar cells,” said Alejandro Briseno of the University of Massachusetts Amherst in a news release. “We report here that we have at last developed the ideal architecture composed of organic single-crystal vertical nanopillars.”

The research, which was published in the journal Nano Letters, found a simple crystallization technique for growing vertically oriented nanopillars. The technique essentially builds on thermal evaporation by using a fast deposition rate.

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Process for Producing Layered 2-D Materials Determines Their Electronic Properties

This year, we’ve seen the emergence of different types of transistors being produced entirely from layered two-dimensional (2-D) materials featuring the dichalcogenides, tungsten diselenide (WSe2) and molybdenum disulfide (MoS2). Researchers at Argonne National Laboratory in Illinois produced a transparent thin-film transistor (TFT) with WSe2 as the semiconducting layer, graphene for the electrodes, and hexagonal boron nitride as the insulator. At about the same time, researchers at Lawrence Berkeley National Laboratory in California built an all 2-D transistor that took the shape of a field emission transistor (FET), with MoS2 as the semiconducting layer.

Now, in a collaborative effort, researchers at Rice University, Oak Ridge National Laboratory, Vanderbilt University, and Pennsylvania State University have developed a novel method for producing these hybrid layered 2-D structures. Their technique, they report, provides a high degree of control on how the resulting devices perform.

In research published in the journal Nature Materials, the researchers demonstrated how, by altering the temperature the materials are exposed to during the chemical vapor deposition (CVD) process used to produce these 2-D layered devices, they could yield either an in-plane monolayer composite, which has a small but stable band gap, or a stacked layered hybrid, which exhibits enhanced photoluminescence. At high temperatures, the researchers got vertically stacked bilayers of MoS2 and WSe2, with the tungsten on top. At lower temperatures, the two 2-D materials grew side by side.

“With the advent of 2-D layered materials, people are trying to build artificial structures using graphene and now dichalcogenides as building blocks,” said Pulickel Ajayan of Rice University in a press release. “We show that depending on the conditions, we can combine two dichalcogenides to grow either in-plane hybrid or in stacks.”

“What’s even more interesting is that the layered structure has a particular lock-in stacking order,” said Wu Zhou of Oak Ridge National Laboratory in the news release. “When you stack 2-D materials by transferring layers, there’s no way to control their orientation to one another. That impacts their electronic properties. In this paper, we demonstrate that in a certain window, we can get a particular stacking order during growth, with a particular orientation.”

Ajayan has characterized the development as “pixel engineering” because atomically thin semiconductors could be manipulated in production so that their potential uses in optoelectronics are almost limitless.

“We should be able to tweak certain regions to control certain functions, like light or terahertz emission,” said Rice's Robert Vajtai, another of the study's coauthors, in the release. “The whole idea, really, is to create domains with different electronic characters within a single layer.”

Cheaper Production Costs Could Usher in Graphene-based Flexible Displays

Three years ago, we started to highlight research that seemed to indicate that graphene had a real commercial opportunity in replacing indium tin oxide (ITO) for touch screen displays.

Just earlier this month, we saw that Plastic Logic, with assistance from the Cambridge Graphene Center located at the University of Cambridge, had developed the world’s first graphene-based flexible display.

So, over the past three years, we’ve seen steady development culminating in a real device being produced. As a result, it comes with a bit of surprise that researchers at the University of Surrey and AMBER, the materials science center based at Trinity College Dublin, are just now letting us know that graphene offers a real alternative to ITO in flexible low-cost touchscreen displays.

Don't be confused. While this may seem like old news, the researchers make their claim based on the method they have developed for producing graphene-treated silver nanowires, which could significantly reduce production costs for nanowire-based displays.

"Our work has cut the amount of expensive nanowires required to build such touchscreens by more than fifty times as well as simplifying the production process,” said Izabela Jurewicz, a researcher at the University of Surrey, in a press release. “We achieved this using graphene, a material that can conduct electricity and interpret touch commands whilst still being transparent."

In research published in the journal Advanced Functional Materials, the researchers were able to overcome the typical cost issues associated with multilayer networks of silver nanowires by modifying the electrical properties of the nanowire network through local deposition of conducting graphene platelets.

The key to the solution-based process was the use of pristine graphene instead of graphene oxide. Since the graphene was free of oxygen functional groups, it was electrically conducting without any further chemical treatment. The result was a more than 50-fold reduction in the number of nanowires needed to produce viable transparent electrodes.

The result of this reduction in the number of nanowires led to significant savings in production costs.

"This is a real alternative to ITO displays and could replace existing touchscreen technologies in electronic devices,” said Jonathan Coleman of AMBER in a press release. “Even though this material is cheaper and easier to produce, it does not compromise on performance."

Needless to say, industry has already taken note. Coleman added: "We are currently working with industrial partners to implement this research into future devices and it is clear that the benefits will soon be felt by manufacturers and consumers alike."

LED Displays Get 400 Percent Clearer With Nanomaterial

Back in 2012, Stephen Chou of Princeton University developed a nanostructure that, if incorporated in solar cells, would let them absorb 96 percent of the light that hit them and increase their efficiency by 175 percent. The nanostructure, which was a sandwich of metal and plastic configured to behave as a subwavelength plasmonic cavity, simultaneously dampened the reflection of light and trapped it.

Chou and his Princeton colleagues were eventually struck by another possibility: If the material could absorb light, they thought, maybe it could radiate light as well. With that in mind, the team has used this same configuration of materials to improve light emitting diodes (LEDs) so that they can achieve greater brightness and better efficiency. This, they say, is true for both organic and inorganic LEDs. This advance could lead to LED displays in whose picture clarity is five times better than that provided by conventional approaches.

"From a view point of physics, a good light absorber, which we had for the solar cells, should also be a good light radiator," Chou said in a press release. "We wanted to experimentally demonstrate this is true in visible light range, and then use it to solve the key challenges in LEDs and displays."

In research published in the journal Advanced Functional Materials, the nanostructured material exploited the phenomenon known as plasmonics, which involves oscillations in the density of electrons that are generated when photons hit a metal surface, to pump more light out of the LEDs.

While LEDs are much more efficient than incandescent light, a lot of light is still trapped inside the structure. In the case of cheap LEDs, only about 2 to 4 percent of the light the device generates is actually emitted.

"It is exactly the same reason that lighting installed inside a swimming pool seems dim from outside – because the water traps the light," said Chou in the release. "The solid structure of an LED traps far more light than the pool's water."

Current methods for extracting more light from LEDs involve the use of mirrors or lenses. While these methods can increase the amount of light put to good use to around 38 percent, they come at a cost of reducing the contrast, resulting in hazy images.

To overcome the limitations of these light extraction techniques, the researchers employed their nanostructure, called a plasmonic cavity with subwavelength hole-array (PlaCSH). The device comprises a layer of light-emitting material, about 100 nanometers thick, that is sandwiched between a cavity whose surface is made from a thin-metal film and another cavity that has a metal-mesh surface made from wires that are 15 nanometers thick, 20 nanometers wide, and spaced 200 nanometers apart on center.

This design essentially guides the light out of the LED and focuses it towards the viewer.  An added benefit to the design is that it replaces the brittle transparent indium tin oxide electrodes that are used as a transparent conductor to control display pixels.

The PlaCSH organic LEDs can be produced very cheaply using a nanoimprint technology invented by Chou himself back in 1995.

Princeton has applied for patents for both organic and inorganic LEDs using the PlaCSH design. With a cheap and simple manufacturing process and a 400 percent improvement in picture clarity, it’s clear why the university was quick to file patents.

Graphene and Germanium: A Happy Marriage With Exceptional Conductivity

Graphene became the subject of much research because its electrical, mechanical, and optical properties make it an excellent material for electronics. The conductivity of freestanding graphene is comparable to that of copper. However, using graphene in electronic components requires a substrate to support it, and researchers were faced with a problem: graphene's electrical properties degrade when bonded to most substrates. For example, bonded to silicon dioxide, a material widely used in electronics because of its good insulating properties, graphene's conductivity decreases by two to three orders of magnitude.

Now a team of researchers has shown that graphene, when deposited on a germanium substrate covered with a thin germanium oxide layer, acquires excellent electrical properties, and its conductivity even improves compared to pure graphene. The team, from the University of Wisconsin-Madison and University of Notre Dame, reported their findings in ACS Nano earlier this month.

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Graphene Biosensor Is Faster and More Sensitive Than ELISA

With its attractive electrical conductivity properties and its large surface to volume ratio, graphene has always presented an attractive possibility for researchers looking to develop new generations of biosensors.

Researchers at Swansea University in the UK have been able to exploit those properties in graphene by developing a technique to produce it over a large area with consistent quality.

In research published in the Institute of Physics journal 2D Materials, the researchers achieved improved size and quality by abandoning the traditional exfoliation technique and instead producing it with an epitaxial growth method that deposits the graphene on a large, semi-insulating substrate of silicon carbide.

After creating device patterns on the graphene with semiconductor processing techniques, the researchers attached bioreceptor molecules. The molecules serve to bind target molecules that are found in blood, saliva or urine. In this case, the target molecule was 8-hydroxydeoxyguanosine (8-OHdG), which is produced when DNA is damaged. When it appears at elevated levels, it is a reliable indicator of an increased risk of developing several cancers.

When the 8-OHdG molecules are present in a sample, they cause a change in the channel resistance in the biosensor. Based on this method, the researchers were able to detect the molecule at concentrations as low as 0.1 nanograms per milliliter. This five times as sensitive as enzyme-linked immunosorbent assays (ELISAs), which are currently used for biomarker analysis. Not only was the graphene-based nanosensor more sensitive, it was a good deal faster than an ELISA test, completing its analysis of a sample in minutes.

“Now that we’ve created the first proof-of-concept biosensor using epitaxial graphene, we will look to investigate a range of different biomarkers associated with different diseases and conditions, as well as detecting a number of different biomarkers on the same chip," said Dr. Owen Guy, a co-author of the study, in a press release.

Are Multiferroics the Ultimate Replacement for Flash Memory?

Researchers at The City College of New York with collaborators from Drexel, Columbia, Brookhaven National Laboratory, and China’s South University of Science and Technology, have developed a new kind of material, called a complex oxide, that one researcher described as potentially leading to the “ultimate replacement for flash memory”.

The work, which was published in the Nature online journal Scientific Reports, involved the development of a single material that combines both magnetic and ferroelectric properties—a multiferroic. By joining these two properties it becomes possible to control charges using magnetic fields and spins simply by applying a voltage. This could lead to new designs in both logic circuits and spintronics, the materials' discoverers claim.

A few years back, research out of Tyndall National Institute in Ireland suggested that it could be possible to use atomic layer deposition to lay down rare earth oxides and create “a one terabyte USB stick in the near future.”

This latest research appears to further the prospects of that outcome by developing a process to build the new complex oxides using common elements: barium, titanium, and manganese. The novel material belongs to the Hollandite crystal group, which is a mineral composed of manganate of barium and manganese. 

For nearly two decades, scientists have predicted that inorganic substances like this had a ferroelectric nature, and this work has confirmed that prediction.

“The Holy Grail in this field is the combination of both magnetic and ferroelectric elements at room temperature with a sufficient magnitude of interaction,” said Stephen O’Brien, associate professor of chemistry at The City College, in a press release. He added that the material could be the “ultimate replacement for flash memory” or smaller devices with massive storage capacities.

O’Brien is apparently not alone in his optimism for this material, with the noted “father of integrated ferroelectrics,” J.F. Scott of the University of Cambridge, making it known that he believes that multiferroics might hold the future for the ultimate memory device.

Weird New Graphene Effect Makes Electrons Scoot Sideways

Electrons are like people – they follow the path of least resistance. In a conductive material, this means running in the same direction as the electric field.

But like people, electrons sometimes ignore the rules. Physicists from MIT and the University of Manchester have developed a new graphene-based material in which electrons move at controllable angles. The research could spawn new types of energy efficient transistors and have huge implications for how electronics are developed, they claim.

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Wine Critics Watch Out: Artificial Tongues Are Getting Better

As it turns out we humans are not as good as we think at discerning differences in wine. While some argue considerable expertise does exist around wine tasting, others have branded that expertise junk (or should that be drunk?) science.

To overcome the junk science aspect of wine tasting, artificial tongue technologies, sometimes referred to as electronic tongues, have been advanced over the years as an objective way to discern wines based on their taste, free from the human wine critic's personal prejudices.

To further the state-of-the-art in artificial tongue technologies, researchers at the Interdisciplinary Nanoscience Centre (iNANO), at Aarhus University, have developed a nanosensor that is capable of measuring the effect of astringency in your mouth when you drink wine.

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Electricity Makes Mortar for Nanotube Bricks

Each allotrope of carbon—diamond, graphite, graphene, and fullerenes—has its unique set of interesting properties. So finding a way to get carbon to form a hybrid of these allotropes has been an enticing concept. The problem with making such hybrids is that it usually entails extreme chemical, temperature, or pressure conditions, leading to a lack of control over the final product.

Now researchers from Northeastern University, MIT, and the Korea Advanced Institute of Science and Technology (KAIST) have developed a simple, highly-scalable method for creating inter-allotropic transformations and hybridizations of carbon that appear across large-area ​carbon networks. Using alternating pulses of electricity across single-walled carbon nanotubes (SWNTs) they transform them into larger-diameter SWNTs, multi-walled CNTs of varying morphologies, or multi-layered graphene nanoribbons. They reported the details in  the journal Nature Communications.

The key feature of the method is that it produces molecular junctions for the carbon nanotubes that have superior electrical and thermal conductivity compared to carbon nanotubes arrays that are junction-free.

To visualize the difference between a CNT array with molecular junctions and one without, the researchers say that the one without is like a wall of bricks without mortar, while the one with molecular junctions is like a brick wall made using mortar.

“We have filled in the gaps with cement,” said co-​​author Swastik Kar, an assistant pro­fessor of physics at Northeastern, in the press release. “We started with single-​​walled carbon nanotubes,” he added, “and then used this pioneering method to bring them together.”

The researchers believe that CNT arrays using these junctions could be useful for reinforcing composite materials. In the last few years, we have begun to see the use of CNTs in composites that actually improve the strength of the composite as opposed to just replacing a regular resin material. (In research back in 2012, scientists in Switzerland demonstrated how using magnetic forces could orient the carbon nanotubes in the composite to impart even greater strength.)

While stronger composites are indeed an attractive characteristic for these new CNT arrays, their improved electrical and thermal conductivity properties should be attractive for electronic applications as well.

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Nanoclast

IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

 
Editor
Dexter Johnson
Madrid, Spain
 
Contributor
Rachel Courtland
Associate Editor, IEEE Spectrum
New York, NY
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