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Three-Atom Thick Material Switches Between a Conductor and an Insulator When Tugged

As chip dimensions have decreased to match the demands of Moore’s Law, insulating materials separating the transistor gate from the channel below it have had to be thinned down so much that keeping current from leaking through has been difficult. In fact, chipmakers are no longer thinning the gate oxide, and it stands now at 1 nanometer in thickness because to go thinner would allow too much current to flow through the channel when the transistor is supposed to be turned off.

Researchers at Stanford University have been running simulations with some two-dimensional materials that when sandwiched together can switch the material between conducting and insulating just by tugging on its edges.  If physical experiments on the material are successful, it could provide a way to completely shut down the leakage of current in chips and still go to smaller chip dimensions.

The researchers believe that if the material could be used in today's smart phone processors it could reduce their power consumption considerably.

The work, which was published in the journal Nature Communications, represents a growing body of knowledge on so-called transition dichalcogenide metals, which are materials that combine one of 15 transition metals with one of three members of the chalcogen family: sulfur, selenium, or tellurium.

In the computer models, the Stanford researchers took one atomic layer of molybdenum atoms and sandwiched it between two atomic layers of tellurium atoms. In the video below, you can see the three-atom thick structure switch between conductor and an insulator as it us pulled.

It does make an attractive computer model. However, whether it can be translated into an actual physical material remains to be seen. Even if they can produce the three-atom-thick sandwich, it’s not clear whether it could really be developed for large-scale production. While physical experiments have successfully demonstrated single-atom transistors, many are questioning whether such a device could ever be be made by the millions or billions.

It’s not clear that a now three-year-old challenge of Professor Mike Kelly at Cambridge University has ever been sufficiently answered. In the challenge he argues that devices with dimensions less than three nanometers cannot be mass produced using a top-down manufacturing technique. Until that question is adequately addressed, we may have here just another computer model that could lead to a physical material but not one that could be used in the mass production of electronic devices.

US Government Regulators Take on Nanomaterials

Besides a big funding gap that has prevented many nascent nanotechnologies from reaching the marketplace, two major obstacles to nanotech's advance are a seeming lack of a regulatory framework, especially in food and drugs, and environmental, health and safety (EHS) concerns.

This week, in what appears to be a coincidence, the US Food & Drug Administration (FDA) and the National Nanotechnology Initiative (NNI) have both issued formal announcements that should have an impact on both regulations and EHS concerns. The FDA has outlined a policy for overseeing nanomaterials in food, drugs and even cosmetics. Yesterday, the NNI provided an overview of progress on the implementation of the 2011 NNI Environmental, Health and Safety (EHS) Research Strategy.

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Kickstarter Comes to Nanomaterials

The IEEE Spectrum Facebook page received an exuberant, almost breathless, urging last week for the online magazine to promote a company called Aken Technologies. The firm is developing a hydrophobic nanomaterial that is designed to keep textiles from getting wet or stained.

If there is one thing that is not novel in the developing world of nanomaterials, it is stain-resistant textiles. As far back as 1998, David Soane founded Nanotex as a company that produced stain-resistant textiles. In the ensuing 16 years, the company has exhibited resilience and success, despite increasing competition, most notably from Swiss-based Schoeller Technologies. We even covered, in the pages of this blog, how the Italian cycling apparel company, Castelli, sourced Schoeller’s Nano Sphere technology to develop water repellant cycling wear.

Textiles haven't been the lone application for hydrophobic nanomaterials. The materials have also proven themselves effective for our beloved smart phones. Soon after they were developed in the labs, hydrophobic nanocoatings for smart phones were already the rage at the Consumer Electronics Show back in 2012.

So, while there may be nothing new about hydrophobic nanomaterials, there was something novel about Aken Technologies beyond its claim to be the “first in the industry” to be “100% Safe, Non-Toxic, Green, & Eco-Friendly.” The novel bit—at least to me—was that it was relying on Kickstarter, the crowdfunding site, for its initial funding.

I soon found out, however, that Aken was not the first nanotechnology-related company to use Kickstarter for its funding mechanism, or even the first company trying to fund a hydrophobic nanomaterial for textiles. A project dubbed Silic used Kickstarter late last year to fund a hydrophobic nanomaterial. Silic initially aimed to raise US $20,000, but quickly overshot its goal, securing $112,254 in funding. At least now, Silic has a website, a milestone that Aken Technologies has yet to achieve.

The funding gap remains one of the biggest obstacles for bringing nanotechnologies developed in the lab to the market place. Whether Kickstarter can become a viable method for bridging that gap remains to be seen. However, it’s hard to see how funding a few small companies so that they can develop the commercial potential for a technology that has already been in commercial markets for nearly two decades recommends it as the solution.

There’s nothing to clearly indicate that Kickstarter is offering an avenue for the unscrupulous to relieve people of their money. Unfortunately, we apparently have seen with other “nanotech investment opportunities” the potential for abuse, which has led the UK Financial Conduct Authority (FCA) to warn investors to beware of scams involving graphene.

The skepticism of the Kickstarter investors is not encouraging, if the questions that are posted on Aken Technologies’ Kickstarter page are any indication. The “investors” don’t really seem to be formulating the right questions. (Their questions are reasonable but not relevant to Aken's prospects for success.) A typical question is something along the lines of: "Will this technology work?" Yes, it works; there’s been a long commercial history of hydrophobic materials being used effectively in textiles. Instead, what potential investors should be asking is how Aken's offering is better than—or how can it even compete with—long-established companies that do the same.

After the excitement of seeing water bouncing off a t-shirt wears off, an understanding of the startup's market position needs to be soaked up.

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.

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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
New York, NY
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