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Nanowires for Tougher Touchscreens

Many of us have experienced that sinking feeling after dropping an expensive smart phone on the asphalt and realizing that the screen is shattered.

That heartbreak may be a thing of the past due to research out of the University of Akron: a new transparent electrode material that makes the screen virtually shatterproof.

There has been a huge push in nanomaterial research with the aim of finding a replacement for indium tin oxide (ITO), which is the material from which transparent conductors that control screen pixels are made.

One of the problems with ITO is that it’s a relatively scarce resource, and with the market for tablets and smart phones exploding, that scarcity has become more acute. This market shortage, combined with the brittleness of ITO-based screens, explains why a variety of nanomaterials have been given a “market pull” opportunity rather than merely a “technology push” prayer.

“These two pronounced factors drive the need to substitute ITO with a cost-effective and flexible conductive transparent film,” said Yu Zhu, an assistant professor at the University of Akron, in a press release. “We expect this film to emerge on the market as a true ITO competitor. The annoying problem of cracked smartphone screens may be solved once and for all with this flexible touchscreen.”

Xu and his colleagues published their results in the journal ACS Nano; the paper describes the process they used to create their transparent film.

They started with conductive metal films (copper, in this case) on which they patterned transparent metal nanowire networks with electrospun fibers as a mask. Then, with the metal nanowires, they fabricated transparent electrodes on both rigid glass and polymer (polyethylene terephthalate (PET)) substrates.

The researchers claim that both the transmittance (the amount of light that passes through a material) and the sheet resistance (a measurement of a thin film's resistance to electrical current) of the metal nanowire-based electrodes they have developed are better than ITO-based electrodes.

Two years ago, Samsung made a transparent conductor from graphene, and there are a number of companies already out there—like Cambrios, and Blue Nano, to name a couple—that are marketing silver-nanowire-based transparent electrodes. If this copper-nanowire-based transparent electrode solution is going to be the next ITO, it’s got a lot of competitors fighting for the same role.

Li-ion Batteries with Nanotube Anodes Charge Phones in Ten Minutes

The introduction of portable electronics pretty much spelled the end for graphite as the anode material for lithium-ion (Li-ion) batteries. We could no longer get through a day of regular usage of some smart phones without having to recharge their batteries.

The hope had been that silicon could replace graphite. Silicon anode material has a theoretical capacity (i.e., Li storage capability) of 4000 milliamp-hours per gram (mAh/g). This represented an enormous increase over graphite that was coming in at 372mAh/g. However, there was a big problem: silicon would start to crack after a relatively small number of charge/discharge cycles, rendering the material useless.

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Nanoparticle Self Assembly of Wafer-Scale Thin Films Done in Minutes

Researchers at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a technique the rapidly builds wafer-scale thin films through nanoparticle self-assembly.

Prior to this work, it would take hours for nanoparticles to self assemble into a film that was just barely able to cover a microscopic chip. Now a film covering a full-sized silicon wafer can assemble itself in just a few minutes.

Because this new technique should be compatible with today's manufacturing processes, the Berkeley Lab researchers believe that it could lead to new types of optical coatings for applications in photovoltaics and data storage.

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Nanowires Enable a Cable To Both Conduct and Store Electricity

Jayan Thomas, an assistant professor at the University of Central Florida’s NanoScience Technology Center, thought it would be interesting to see if the copper cable we use to conduct electricity could also be used to store energy. So he and his graduate student, Zenan Yu, set out to see if they could make it so.

Thomas and his team had previously been working on inexpensive nanoprinting techniques for producing supercapacitors with highly ordered nanoelectrodes. When they looked for a solution to both conducting and storing electricity in a single cable they again turned to supercapacitors.

In this latest research, which will be reported in the 30 June edition of Advanced Materials, Thomas and Yu essentially wrapped a supercapacitor around a copper cable. The problem they had to overcome was how to create the two electrodes necessary for a supercapacitor.

The trick was to grow electrochemically active nanowires (or nanowhiskers) on a copper wire that had been coated with copper oxide. This layer of nanowires sticking out from the cable created the first electrode for the supercapacitor. The researchers then covered the copper cable and nanowhiskers with a polymer. Next they surrounded the polymer with a nanowire coated copper coil, forming the second electrode.

The insulation of the separator allows the inner copper wire to continue conducting electricity while the layers around the wire can independently store the energy.

In an article published in the journal Nature discussing the work, it is pointed out that design is limited to direct current (DC), which could prove useful for powering small electronic devices and automotive electronics, but not for household or industrial uses that need alternating current (AC).

While this is still pretty preliminary research, Thomas believes that the technology could be transferred to other types of materials such as fibers that could be woven into clothing to power electronic devices. Others believe that the supercapacitor cables might be effective for storing the electrical energy produced by solar panels or wind turbines. In any case, the manufacturing costs of the cables will need to be kept low if they are realistically going to be able to offer an alternative to today's supercapacitor devices.

Transistors Made From 2-D Materials Promise New Class of Electronic Devices

Last April, two separate research projects reported building transistors made entirely from two-dimensional (2-D) materials. Researchers at Argonne National Laboratory described in the journal Nano Letters that they had produced a transparent thin-film transistor (TFT) made from tungsten diselenide (WSe2) as the semiconducting layer, graphene for the electrodes and hexagonal boron nitride as the insulator.

Then, one week later, the journal ACS Nano published work from researchers at Lawrence Berkeley National Laboratory who had also built an all 2-D transistor that took the shape of a field emission transistor (FET). The Berkeley Lab FET had the same materials for its electrode and insulator layers as Argonne's TFT, but used molybdenum disulfide (MoS2) as the semiconducting layer.

While the fabrication of transparent TFTs made entirely from 2-D materials could lead to flexible displays with a super-high density of pixels, the impact of an all-2-D FET could have a broader impact. That's because FETs are nearly ubiquitous, used in computers, mobile devices and just about every other electronic system you can think of.

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Cancer Bursting Nanobubbles Prove Effectiveness in Preclinical Trials

Two years ago, researchers at Rice University, led by Dmitri Lapotko, a physicist and biochemist, developed a novel method for killing cancer cells. The technique relies on gold nanoparticles infiltrating cancerous cells. When a laser is shone on those cells, tiny bubbles surround them and explode, thereby ripping the cancerous cells apart. If the bursting bubbles don't completely destroy the cancer cell, the weakened state it's left in by the explosions makes it more susceptible to chemotherapy drugs.

Now Lapotko and his colleagues are reporting the results of pre-clinical trials using the technique, dubbed “quadrapeutics.” The term stems from the use of four tools in the destruction of the cancer cells: gold nanoparticles, laser pulse, x-ray, and a drug.

Chemotherapy is actually the first step in the four-pronged attack. In the case of the Rice pre-clinical trials, doxorubicin and paclitaxel were used.

After the drugs are introduced, the protocol works by tagging the gold nanoparticles with antibodies that target specific cancer cells and attach to the cells' surfaces.  The cancer cells begin to ingest the nanoparticles, which are stored just beneath the cells’ protective outer membranes.

The cancer cells are then fired upon with near-infrared laser pulses. The near-infrared light is able to penetrate human tissue but the gold nanoparticles can’t absorb that wavelength of light. Instead, the light excites the free electrons on the gold nanoparticles so that there are collective oscillations that generate excess heat. This material effect is known as plasmonics.

Unlike recent research out of ETH Zurich in Switzerland in which this plasmonic effect with near-infrared light and gold nanoparticles was used to turn up the heat on the cancer cells to kill them, the Rice approach doesn’t depend on heat. Instead, Lapotko's team focuses on the destruction of cancer cells through intracellular explosions. In this way, only the cancer cells are destroyed and not nearby healthy cells that might otherwise be killed by the heat.

A video describing the method and presenting images of how the cancer cells are blown apart are provided in the video below.

“What kills the most-resistant cancer cells is the intracellular synergy of these components and the events we trigger in cells,” Lapotko said in a press release. “This synergy showed a 100-fold amplification of the therapeutic strength of standard chemoradiation in experiments on cancer cell cultures.”

In the research, which was published in the journal Nature Medicine, the team applied the technique to head and neck squaomous cell carcinoma, which is a lethal form of cancer that recently had grown immune to traditional chemotherapies. The results showed that the quadrapeutic therapy caused cancerous tumors in mice to be destroyed within one week—even though the researchers used only 3 percent of the typical drug dosages and 6 percent of the typical radiation doses.

The effectiveness of the quadrapeutic therapy should not be limited to just this particular form of cancer, say the researchers. Lapotko believes that the treatment could be applied to various solid tumors, especially those that have proven hard-to-treat, such as brain, lung, and prostate cancer.

Quantum Dot Solar Cells Break Conversion Efficiency Record

Quantum dots have offered an attractive option for photovoltaics. Multijunction solar cells made from colloidal quantum dots (CQD) have been able to achieve around 7-percent conversion efficiency in the lab. While figures like this may not seem too impressive when compared to silicon solar cells, their promised theoretical conversion efficiency limit is an eye-popping 45 percent. This is possible because when a single photon is absorbed by a quantum dot, it produces more than one bound electron-hole pair, or exciton, thereby doubling normal conversion efficiency numbers seen in single-junction silicon cells.

Now researchers at the Massachusetts Institute of Technology (MIT) have raised the bar for quantum dot-based solar cells by producing one that changes light to electricity with 9-percent conversion efficiency. Furthermore, says the MIT team, it can be produced using an inexpensive production method that promises to keep manufacturing costs down.

The researchers, who published their findings in the journal Nature Materials, hit upon a way to produce quantum dot solar cells through a solution processing technique that doesn’t require high temperatures or a vacuum atmosphere to achieve stability for the solar cells when they are exposed to air. By using ligand treatments, which involve molecules or ions that bind to a central metal, the researchers were able to align the bands of the quantum dot layers, improving the performance of the films.

“Every part of the cell, except the electrodes for now, can be deposited at room temperature, in air, out of solution. It’s really unprecedented,” said graduate student Chia-Hao Chuang in a press release.

The processing technique for the quantum dot layers allows for the dots to do what they do well individually and also to work together in the transport of electrical charge to the edges of the film where it can then be collected to provide an electrical current.

Nine-percent efficiency may still seem low to casual observers, but the development of quantum dots for photovoltaics has been so rapid that researchers are impressed by the latest development.

“Silicon had six decades to get where it is today, and even silicon hasn’t reached the theoretical limit yet. You can’t hope to have an entirely new technology beat an incumbent in just four years of development,” said professor Vladimir Bulović in the release.

The researchers still need to determine why these films are so stable and there’s still a long way to go before they are commercially viable. But they now hold the National Renewable Energy Lab (NREL) record for quantum dot solar efficiency.

Graphene Plasmonic Circuits Take a Critical Step Foward

When one takes into account the rough and tumble world for electrons in electronic devices, one would think it might make more sense just to use photons instead. The problem has been that light takes up a lot of space. The components for such a photonic system can’t get any smaller than a wavelength of light, which would translate to devices many times larger than today’s.

However, the field of plasmonics, which takes advantage of the surface plasmons that are generated when photons hit a metal structure, has looked to be a way forward for confining electromagnetic energy to subwavelength scales and create smaller photonic logic circuits. Recent research has shown that graphene and other two-dimensional materials produce plasmons too. These so-called graphene plasmons have an advantage over surface plasmons because their confinement of electromagnetic energy at subwavelength scales can be tuned and controlled by a gate voltage.

Now Spanish researchers at CIC nanoGUNE outside of San Sebastian, the Institute of Photonic Sciences (ICFO) near Barcelona, and the company Graphenea located at the CIC nanoGUNE research center have demonstrated that an optical antenna made from graphene can capture infrared light and transform it into graphene plasmons.

The research, which was recently published in the journal Science, demonstrated that a metal rod on graphene can act as an antenna for infrared light and transform it into graphene plasmons in much the same way a radio antenna converts radio waves into electromagnetic waves in a metal cable.

“We introduce a versatile platform technology based on resonant optical antennas for launching and controlling of propagating graphene plasmons, which represents an essential step for the development of graphene plasmonic circuits”, said team leader Rainer Hillenbrand in a press release.

The researchers believe that the graphene-based optical antennas provide a number of advantages. “The excitation of graphene plasmons is purely optical, the device is compact and the phase and wavefronts of the graphene plasmons can be directly controlled by geometrically tailoring the antennas,” said Pablo Alonso-González, who performed the experiments at nanoGUNE, in a press release. “This is essential to develop applications based on focusing and guiding of light.”

Graphene Grown Directly on Insulator Substrate

Initially, laboratory-produced graphene was the result of what has been dubbed the “Scotch Tape” method, in which graphene is pulled off in single-layer flakes directly from graphite. The method does produce a high quality, single crystal graphene that possesses many attractive properties like high electron mobility, but it is decidedly not scalable.

Chemical Vapor Deposition (CVD) has been hotly pursued as a potentially scalable way to grow graphene on a metal substrate like copper or nickel. However, it has struggled with the step of peeling the graphene off of the metal substrate without ruining or contaminating it.

Now researchers at the Massachusetts Institute of Technology (MIT) and the University of Michigan have devised a way in which graphene can be grown directly onto an insulator like glass or silicon, eliminating the need to peel the graphene off of a metal substrate.

In the research, which was published in Nature’s Scientific Reports, the team was able to grow the graphene at the interface of nickel and silicon dioxide. They followed that step by mechanical removal of the nickel from the substrate.

This process eliminates the need for the wet etching steps that researchers from the National University of Singapore (NUS) employed in a similar process where graphene was grown at the interface between copper and a silicon substrate. Some researchers believe that the Singaporean method would suffer from contamination and wouldn’t be able to produce large, uniform films.

In contrast to this NUS method, the new process grows graphene  on both sides of the nickel. In this way, it is possible to peel away the nickel and it will leave the graphene behind on top of the silicon substrate.

The researchers developed the method while working with a glass manufacturer so the aim was always to have a scalable production method.

“To meet their manufacturing needs, it must be very scalable,” says MIT's A. John Hart, in a press release. “We were inspired by the need to develop a scalable manufacturing process that could produce graphene directly on a glass substrate.”

While Hart cautions that the work is still at a preliminary stage due to a lack of consistent uniformity in the graphene films, it does open up some attractive potential applications.

Hart adds: “The ability to produce graphene directly on nonmetal substrates could be used for large-format displays and touch screens, and for ‘smart’ windows that have integrated devices like heaters and sensors.”

Smallest and Fastest Nanomotor Could Advance Drug Delivery

Engineers at the University of Texas Austin have developed what they claim is the smallest, fastest spinning and longest lasting nanomotor built to date. The engineers have demonstrated that their nanomotor is capable of rotating for 15 continuous hours at a speed of 18 000 rpm. These performance figures are on an entirely different scale to those of similar nanomotors that run anywhere from 14 rpm to 500 rpm and have only rotated for a few seconds or minutes.

The nanomotors, which are described in the journal Nature Communications, were built in a bottom-up manufacturing technique in which nanowires serve as rotors, patterned nanomagnets are the bearings, and quadrupole microelectrodes act as stators.

The entire structure of the nanomotor is smaller than 1 square micrometer, which means it is conceivable that they could fit inside some living cells. The nanomotors are also capable of rapidly mixing and pumping biochemicals as well as moving through liquids.

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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
 
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Rachel Courtland
Associate Editor, IEEE Spectrum
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