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3-D Mapping of Electrons Moving on a Material's Surface

Terahertz spectroscopy uses infrared light to probe matter and is widely used to investigate the electrical properties and behavior of semiconductor materials. By sending very short laser pulses, called probe pulses, in quick succession, it is possible to follow how these properties change over time when the material responds to a light pulse, for example. However, unlike x-rays, terahertz waves have relatively long wavelengths ranging from 3 to 3000 micrometers. And just as with optical microscopes, structures smaller than the pulses’ wavelength remain invisible.

Now researchers at the University of Regensburg in Germany have demonstrated that it is possible to dramatically increase the spatial resolution of terahertz spectroscopy by focusing the probe pulses on the needle of an atomic force microscope (AFM). They published this research in Nature Photonics yesterday.

AFMs are widely used tools in solid-state research. Scanning with the sharp tip of the AFM at a slight distance from the surface of a material and measuring the variation of the force between the tip and the material—which can be an electrostatic or van der Waals force, for example—allows the creation of an image of the surface in which individual atoms are visible.

But the Regensburg researchers took a different tack. Instead of measuring the force between the tip and the surface of the material, they measured the intensity of light scattered by the tip with a series of probe pulses. According to Rupert Huber, the physicist who led the research, this method, “Is a little like a conventional radio antenna, just downscaled to wavelengths of infrared radiation.”

The light pulse, which is an oscillating electromagnetic field, shifts electrons up and down along the shaft of the metallic tip.  However, because the electrons cannot travel beyond the tip, they accumulate at the very tip apex during each half cycle of the oscillating electromagnetic field. Since electrons are charged, they give rise to an intense burst light at the tip a very small area, called the near field, with roughly the size of the tip apex, explains Huber.

This setup, which the Regensburg team is calling a scattering-type near-field scanning optical microscope, or s-NSOM, confines light to an area that is only 10 nanometers across. Without the needle, one would be limited to about half the wavelength of the light, the diffraction limit that curtails the resolution of every conventional microscope. "In contrast, the near-field occupies a volume that is approximately nine orders of magnitude smaller than the usual diffraction limit,” Huber told IEEE Spectrum.

For their experiment, the Regensburg team used indium arsenide nanowires prepared by a group—who are coauthors of the Nature Photonics paper—at the Pisa site of CNRNano, a nanoscience institute of the Italian Research Council. Indium arsenide is a semiconducting material holding a promise for terahertz sources and infrared lasers.

They first hit the nanotube with a terahertz pump pulse, which caused the liberation of charge carriers in the nanowire. This allowed them to detect the presence of electrons moving at the surface of nanowire—a little like what you would see if you disturb an ants' nest. Immediately after the pump pulse, they sent a series of terahertz probe pulses lasting few femtoseconds each. The pulses produced the 10-nm flashes of confined light on the tip. The intensity of these flashes depends on how many electrons are roaming around on the surface of the nanowire. These electrons don't stay around long, however; so, for every new probe pulse, the intensity of scattered light by the tip decreases.

By moving the tip to a new location for each pump cycle, the researchers created a complete "film" of how the nanowire reacts to the pump pulse. Among the phenomena they documented was that the free electrons first disappeared at the ends of the nanowires.

There is no reliable alternative, as of now, for measuring local carrier densities on the few-femtosecond time scale with 10-nm resolution, says Huber. "It is extremely important to understand how carriers behave locally. This insight is crucial for future high-speed integrated electronics and lasers based on semiconductor nanowires," he adds.

Nanotube-based Li-ion Batteries Can Charge to Near Maximum in Two Minutes

The prospects for ubiquitous all-electric vehicles (EVs) powered by lithium-ion (Li-ion) batteries took a bit of a hit back in 2010, when then U.S. Secretary of Energy Steven Chu addressed the United Nations Climate Change Conference in Cancun and suggested that, for battery powered cars to replace those powered by fossil fuels, some pretty significant improvements would need to be made to current technology.

Chu said at the time: “It will take a battery, first that can last for 15 years of deep discharges. You need about five as a minimum, but really six- or seven-times higher storage capacity and you need to bring the price down by about a factor of three.” Chu suggested it might take another five years before such a battery would be developed, and he was almost exactly right in his prediction.

Researchers at the Nanyang Technology University (NTU) in Singapore have achieved at least some of those criteria by developing a Li-ion battery capable of 20 years of deep discharges, more than 10 times that of existing Li-ion batteries.

In addition to longer battery life, the new battery design can be charged up quickly so that it can reach 70 percent of its maximum charge in just two minutes.

These features tick at least two of the metrics that Chu and others have indicated are key to making all-EVs compete with those running on fossil fuels. This would mean that EV owners would not have to spend roughly $5000 every two years for a completely new set of batteries. It could also allow for a quick stop of just a couple of minutes to significantly increase the driving range of the vehicle.

The key to the new Li-ion battery is the replacement of graphite at the anode with nanotubes synthesized from titanium dioxide. This is a departure from a lot of recent work toward improved anodes; other research teams have been using nanostructured silicon in place of graphite.

“With our nanotechnology, electric cars would be able to increase their range dramatically with just five minutes of charging, which is on par with the time needed to pump petrol for current cars,” said Chen Xiaodong, an associate professor at NTU Singapore, in a press release.

The new nanotube material, which is described in the journal Advanced Materials, is produced relatively easily, according to the researchers, by taking titanium dioxide nanoparticles and mixing them with sodium hydroxide. The real key to getting the long titanium dioxide nanotubes the nanoparticles yield is conducting the stirring process at the right temperature.

The technology has been patented and has been licensed by a company that says it could get a new generation of fast-charging batteries to market in two years.

While battery life and recharging have been significantly improved with the new battery design, it’s not clear that new batteries have a longer charge life, or what is known as gravimetric energy density (the amount of energy stored per unit mass). Instead, they have improved Li-ion’s relatively weak gravimetric power density (the maximum amount of power that can be supplied per unit mass) by eliminating the additives that are used to bind the electrodes to the anode. This allows the battery to transfer electrons and ions in and out of the battery more quickly. This translates into batteries that will last about the same amount of time on a charge as today’s current batteries, but can be charged up to near maximum very quickly.

NTU professor Rachid Yazami, who was the co-inventor of the lithium-graphite anode 34 years ago but not involved in this most recent research, has noted the significant improvement to Li-ion batteries this work represents.

Yazami said: “There is still room for improvement and one such key area is the power density—how much power can be stored in a certain amount of space—which directly relates to the fast charge ability. Ideally, the charge time for batteries in electric vehicles should be less than 15 minutes, which Prof Chen’s nanostructured anode has proven to do.”

Organic Coating Could Boost Photovoltaics' Conversion Efficiencies Far Beyond Today's Limits

Researchers at the University of Cambridge in the U.K. have developed a hybrid material made from pentacene (an organic semiconductor) and lead selenide (PbSe) nanocrystals (an inorganic semiconductor) that is capable of harvesting dark spin-triplet excitons at 100-percent efficiency.

This research, which was published in the journal Nature Materials, marks the first time that the energy from triple excitons has been transferred from organic to inorganic semiconductors. Prior to this work, that kind of transfer had only been shown to be possible with spin-singlet excitons.

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New Theranostic Nanoparticle Diagnoses and Treats Cancer

So-called “theranostic” nanoparticles are capable of providing both a diagnostic as well as a therapeutic function in the same nanoparticle. Such theranostic nanoparticles have been primarily developed to address cancer diagnosis and treatment.

Now researchers at Singapore’s A*STAR Institute of Materials Research and Engineering and colleagues at the National University of Singapore have developed a theranostic nanoparticle that has the added benefit of being able to offer two distinct cancer therapies.

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Crumpled Graphene Offers Inexpensive Way to Achieve Flexible Supercapacitors

It was recently reported that used cigarette butts provide a material that yields far better energy density (the amount of energy stored per unit mass) than do many nanomaterials proposed for superconductors. So, it appeared that if graphene were really going to be a replacement for activated carbon on the electrodes of supercapacitors, it had better be able to compete on something more than energy density alone.

Now, researchers at MIT have used graphene to make stretchable supercapacitors to power flexible electronics.

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Carbon Nanotube Yarns Could Replace Copper Windings in Electric Motors

A staggering fact is that motors and motor driven systems account for between 43 percent and 46 percent of all global electricity consumption. Needless to say, if electric motors could be made to  run more efficiently, energy consumption would fall. With research out of Rice University back in 2011 demonstrating that carbon nanotubes braided into wires could outperform copper in conducting electricity, it looked like there would soon be a new way to create those improved efficiencies.

Building on that research, a team at the Lappeenranta University of Technology (LUT) in Finland hasreplaced the copper windings used to conduct electricity in electric motors with a woven material made from threads of carbon nanotubes and achieved remarkable new efficiencies in the motors.

"If we keep the electrical machine design parameters unchanged and only replace copper with future carbon nanotube wires, it is possible to reduce the Joule losses in the windings to half of the present-day machine losses,” said Professor Juha Pyrhönen, who has led the design of the prototype at LUT, in a press release.

Copper windings have traditionally been used because they are the second best metal at conducting electricity at room temperature, and they come relatively cheap. However, despite their high conductivity, they do offer some resistance—to the point where Joule losses are often referred to as “copper losses.”

Meanwhile carbon nanotubes have conductivity far beyond the best metals and their limits of conductivity have not been found: some have been measured at 100 megasiemens/meter, compared to ultra-pure copper at 58.65 MS/m. With this kind of conductivity, carbon nanotube-based windings could result in double the conductivity of today’s copper windings, according to the Finnish researchers. And as Pyrhönen claimed, Joule losses could be cut in half with the carbon nanotube yarn.

In the prototype motor made by the Finnish team, the carbon nanotube yarns are spun and converted into an isolated tape by a Japanese-Dutch company Teijin Aramid. (The actual spinning technology was developed in collaboration with Rice University.)  Since this industrial use of the carbon nanotube yarn is still at its early stages, the production capacity has not been ramped up. This will have to be addressed if the new wire is to replace the ubiquitous copper windings.

However, the performance improvements are significant enough to warrant at least an investigation into whether it can become a realistic replacement for copper wiring.

"There is a significant improvement potential in the electrical machines, but we are now facing the limits of material physics set by traditional winding materials,” said Dr. Marcin Otto, business development manager of Teijin Aramid, in the press release. “We expect that in the future, the conductivity of carbon nanotube yarns could be even three times the practical conductivity of copper in electrical machines. In addition, carbon is abundant while copper needs to be mined or recycled by heavy industrial processes."

Sounds as though there’s a company that has a firm belief in the potential of their product in this application. And with the potential to put a big dent in the 45 percent of global electricity consumption that comes from the use of electrical motors, why not be confident?

A video of the motor equipped with the carbon nanotube windings is below.

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.



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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