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Junkyard Parts Lead to New Spray on Process for Quantum Dots

For years, the University of Toronto research team helmed by Edward H. Sargent has been setting the standards for the use of colloidal quantum dots—including setting conversion efficiency records in photovoltaics and pointing towards their use in infrared optoelectronics.

Now researchers from Sargent’s research team, led by Illan Kramer, have demonstrated that colloidal quantum dots (CQDs) can be sprayed onto a flexible film and used to coat just about anything.

“My dream is that one day you’ll have two technicians with Ghostbusters backpacks come to your house and spray your roof,” said Kramer, a post-doctoral fellow with Sargent’s group, in a press release.

The new process, which Kramer has dubbed “sprayLD,” a play on the term atomic layer deposition (ALD), lays a liquid containing the quantum dots directly onto flexible surfaces, such as film or plastic. The layer of CQDs is laid on in a roll-to-roll process not unlike how newspaper printing presses operate. This makes the process fairly easy to incorporate into other manufacturing methods, say the researchers.

In the research, which was published in the journals Advanced Materials and Applied Physics Letters, Kramer demonstrated that the resulting CQDs do not lose any of their energy conversion efficiency in the process.

Prior to this research, the only way to get light-sensitive colloidal quantum dots onto a substrate involved batch processing, which is a chemical coating process that is comparatively slow and expensive. For this new roll-to-roll process, Kramer was able to fashion the deposition device out of a spray nozzle used in steel mills for cooling steel with a fine mist of water, plus a few regular airbrushes found in any art store.

“This is something you can build in a Junkyard Wars fashion, which is basically how we did it,” says Kramer. “We think of this as a no-compromise solution for shifting from batch processing to roll-to-roll.”

The device can be seen in operation in the video below.

Commenting on the research, Sargent said: “As quantum dot solar technology advances rapidly in performance, it’s important to determine how to scale them and make this new class of solar technologies manufacturable. We were thrilled when this attractively-manufacturable spray-coating process also led to superior performance devices showing improved control and purity.”

Nanoparticle Reduces Charge Times for Li-ion Batteries From Hours to Minutes

The lithium-ion (Li-ion) battery that we curse under our breath every time we find ourselves needing to charge our smart phones may become less a target of our wrath in the near future.

Researchers at Purdue University have developed a new material made from tin-oxide nanoparticles that could reduce the charge time for a Li-ion battery from hours down to minutes.

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Graphene-based Fuel Cell Membrane Could Extract Hydrogen Directly from Air

In research out of the University of Manchester in the UK led by Nobel Laureate Andre Geim, it has been shown that the one-atom-thick materials graphene and hexagonal boron nitride (hBN), once thought to be impermeable, allow protons to pass through them. The result, the Manchester researchers believe, will be more efficient fuel cells and the simplification of the heretofore difficult process of separating hydrogen gas for use as fuel in fuel cells.

This latest development alters the understanding of one of the key properties of graphene: that it is impermeable to all gases and liquids. Even an atom as small as hydrogen would need billions of years for it to pass through the dense electronic cloud of graphene.  In fact, it is this impermeability that has made it attractive for use in gas separation membranes.

But as Geim and his colleagues discovered, in research that was published in the journal Nature, monolayers of graphene and boron nitride are highly permeable to thermal protons under ambient conditions. So hydrogen atoms stripped of their electrons could pass right through the one-atom-thick materials.

The surprising discovery that protons could breach these materials means that that they could be used in proton-conducting membranes (also known as proton exchange membranes), which are central to the functioning of fuel cells. Fuel cells operate through chemical reactions involving hydrogen fuel and oxygen, with the result being electrical energy. The membranes used in the fuel cells are impermeable to oxygen and hydrogen but allow for the passage of protons.

It is these proton exchange membrane fuel cells that are thought to be the most viable fuel cell design for replacing the internal combustion engine in vehicles. However, the polymer-based membranes that have been used to date suffer from fuel crossover that limits their efficiency and durability.

The implication of this latest research is that graphene and hBN could be used to create a thinner membrane that would be more efficient while reducing fuel crossover and cell poisoning. The end result is that it could give the fuel cell the technological push that it has needed to make hydrogen a viable alternative to fossil fuels.

Another, even more remarkable prospect highlighted by this discovery is that these one-atom-thick materials could be used to extract hydrogen from a humid atmosphere. This could be a huge bend in the road that points us towards the so-called hydrogen economy.

One of the inconvenient truths about fuel cells for powering automobiles is that it is extremely costly and energy intensive to isolate hydrogen gas. The main push in nanomaterials for hydrogen gas separation has been artificial photosynthesis in which sunlight rather than electricity is used to split the hydrogen from a water molecule. In fact, another two-dimensional material, molybdenum sulfide (MoS2), has been used as a somewhat effective catalyst for producing hydrogen gas in a solar water-splitting process.

But what Geim and his colleagues are suggesting with this latest research stands this paradigm on its head. It is conceivable, based on this research, that hydrogen production could be combined with the fuel cell itself to make what would amount to a mobile electric generator fueled simply by hydrogen present in air.

“When you know how it should work, it is a very simple setup,” said Marcelo Lozada-Hidalgo, a PhD student and corresponding author of this paper, in a press release. “You put a hydrogen-containing gas on one side, apply a small electric current, and collect pure hydrogen on the other side. This hydrogen can then be burned in a fuel cell.”

Lozada-Hidalgo added: “We worked with small membranes, and the achieved flow of hydrogen is of course tiny so far. But this is the initial stage of discovery, and the paper is to make experts aware of the existing prospects. To build up and test hydrogen harvesters will require much further effort."

While some have been frustrated that Geim has focused his attention on fundamental research rather than becoming more active in the commercialization of graphene, he may have just cracked open graphene’s greatest application possibility to date.

Graphene and Ammonia Combine for Novel Ferroelectric Tunnel Junction

The next generation of high-speed and high-capacity random-access memory (RAM) has seen a number of competitive approaches take center stage of late, from spin-transfer torque (STT) Magnetic RAM to resistive RAM.

Now, researchers at the University of Nebraska at Lincoln have shown a way forward that could offer a general improvement to RAM. They’ve made improvements to the ferroelectric tunnel junction by combining graphene with ammonia so that it is capable of switching on and off the flow of electrons more completely. The result is a distinct improvement in the reliability of RAM devices and the ability to read data without having to rewrite it.

“This is one of the most important differences between previous technology that has already been commercialized and this emergent ferroelectric technology,” said Alexei Gruverman, a physics professor who co-authored the study, in a press release.

The researchers, who published their findings in the journal Nature Communications, tackled the ferroelectric tunnel junction (FTJ), which consists of a layer of material so thin that electrons can tunnel through it. This ferroelectric layer is positioned between two electrodes so that when an electric field is applied to them, it can reverse the direction of the junction’s polarization. This reversal of polarization serves to change the alignment of positive and negative charges, which in turn is used to represent zero and one in binary computing.

The new wrinkle the Nebraska researchers brought to this setup was making the electrodes out of graphene for the first time. While graphene is primarily useful in electronic applications for its conductivity, the Nebraska-Lincoln team was more interested in its ability to accommodate just about any molecule. In this case, they used an ammonia molecule, which sat between the electrodes and the ferroelectric layer.

Typically, the polarity of a junction determines its resistance to the tunneling current; in one direction, the current is allowed to flow and in the other direction the current is greatly reduced. The researchers determined that the graphene-ammonia combination made the difference between these “on” and “off” states even more dramatic.

Ferroelectric materials do have the advantage of being non-volatile, in that they can maintain their polarization even in the absence of an external power source and thereby keep their stored information. Nonetheless, the space between the positive and negative charges is so small in these junctions that it becomes difficult to maintain the polarization.

“In all memory devices, there is a gradual relaxation, or decrease, of this polarization,” says Gruverman in the press release. “The thinner the ferroelectric layer is, the more difficult it is to keep these polarization charges separate, as there is a stronger driving force in the material that tries to get rid of it.”

This most recent research promises a way to eliminate that weakness for ferrolectric materials.

A Single Nanoparticle Enables Two Medical Imaging Techniques

Researchers at the Massachusetts Institute of Technology (MIT) have developed a nanoparticle that enables both magnetic resonance imaging (MRI) as well as fluorescent imaging in living animals.  The researchers believe that a single nanoparticle capable of performing these two functions should be able to help track specific molecules through the body, monitor a tumor’s environment, and determine whether drugs have reached their intended target.

In research published in the journal Nature Communications, the MIT team combined an MRI contrasting agent called nitroxide and a fluorescent molecule called Cy5.5 to produce a nanostructure called a branched bottlebrush polymer. The ratio of the two materials in the nanoparticle is 99 percent nitroxide and 1 percent Cy5.5.

This combination enables both MRIs and fluorescent imaging because of the interesting way these materials interact with each other. The nitroxides are reactive molecules in which a nitrogen atom is bound to an oxygen atom with one unpaired electron. Typically, the nitroxides suppress the Cy5.5’s fluorescence, except when the nitroxides are in the presence of molecule from which they can grab an electron, which, in the case of this study, was a vitamin C molecule. Once the free electrons in the nitroxides bind with the free electrons from another molecule, the MRI signal switches off and the Cy5.5 fluoresces.

In the study, which used mice as subjects, the researchers found that the nanoparticles headed towards the liver, where nanoparticles typically end up. It was there that the nanoparticles came in contact with vitamin C (which is produced by the mouse's liver), the MRI signals were turned off, and the Cy5.5 began to fluoresce. Vitamin C ultimately finds its way to a mouse’s brain, and so the researchers were able to detect fluorescence there. Meanwhile, in areas where the concentration of vitamin C was low, the MRI contrast proved to be strong.

The researchers continue to work on enhancing the signal differences that occur when the nanoparticles encounter a target molecule, such as vitamin C. They have also experimented with adding up to three different drugs to the nanostructure. The aim of having these drugs hitch a ride on the nanostructure is to give diagnosticians the ability to track whether they reach their intended target. A fair amount of research is currently being done in this area, wherein nanoparticles behave as both diagnostic tools and therapeutic devices—a capability that has led to the nanoparticles being dubbed “theranostic” nanoparticles.

“That’s the advantage of our platform—we can mix and match and add almost anything we want,” said Jeremiah Johnson, an assistant professor of chemistry at MIT and senior author of the study, in a press release.

The researchers also believe that this platform should allow detection of oxygen radicals inside a patient’s tumor—a biomarker that can indicate how aggressive the tumor is.

“We think we may be able to reveal information about the tumor environment with these kinds of probes, if we can get them there,” said Johnson. “Someday you might be able to inject this in a patient and obtain real-time biochemical information about disease sites and also healthy tissues, which is not always straightforward.”

Topological Insulators Move Closer to Practical Applications

Nearly a decade ago, theorists predicted the upside-down world of topological insulators, which were supposed to possess the peculiar property of being insulators on the inside but conductors on the outside.  While the theories were experimentally confirmed just a couple of years later, making practical devices out of the material has remained a largely unsolved challenge.

Now researchers at Purdue University claim they have found evidence that is the “smoking gun” proving that topological insulators are indeed a path towards realizing practical quantum computers as well as “spintronic” devices that are far more powerful than today’s number crunchers.

The “smoking gun” in this case comes in the form of something called a half-integer quantum Hall effect on the surface of a topological insulator. This is an effect that is seen in graphene in which there is a no resistance plateau at a zero magnetic field.

“This is unambiguous smoking-gun evidence to confirm theoretical predictions for the conduction of electrons in these materials,” said Purdue doctoral student Yang Xu, lead author of the paper detailing the discovery, in a press release.

The research, which appears in the journal Nature Physics, demonstrated for the first time that a three-dimensional material’s electrical resistance did not have to be dependent on the thickness of the material.

The researchers discovered further evidence that the conduction of electrons were topologically protected in these materials, which means their surfaces are guaranteed to be robust conductors. In the experiments, the researchers would slice off thin layers from the surface of the material and after each thinning of the material, the surface maintained its conductance without the slightest change.

“For the thinnest samples, such topological conduction properties were even observed at room temperature, paving the way for practical applications,” Xu said in the release.

This conduction on the surface of topological insulators is important for spintronic devices. The unique applicability of topological insulators to spintronic devices comes from the fact that the conducting electrons on the surface have no mass and are automatically “spin polarized,” leading to the unique half-integer quantum Hall effect that was observed in this research.

This past summer, researchers at Penn State and Cornell University provided one of the first promising indications that it might actually be possible to derive practical applications such as spintronic devices from these topological insulator materials.

In addition to spintronics, researchers believe that topological insulators, when combined with superconductors, could lead to a practical quantum computer

“One of the main problems with prototype quantum computers developed so far is that they are prone to errors,” said Yong P. Chen, a Purdue associate professor, in the press release. “But if topologically protected, there is a mechanism to fundamentally suppress those errors, leading to a robust way to do quantum computing.”

Chen added: "This experimental system provides an excellent platform to pursue a plethora of exotic physics and novel device applications predicted for topological insulators.”

A Drop of Water and a Push Transfers 2-D Material Between Substrates

The production of molybdenum disulfide (MoS2) films has been getting better of late. Recently researchers at Rice University found a way to orient the two-dimensional (2-D) material on its side so that the maximum amount of edge is exposed—a good thing for fuel cell catalysts and the supercapcitor electrodes.

Now researchers at North Carolina State University (NCSU) have developed a method for transferring these MoS2 films onto any substrate without causing any damage to the 2-D material  in the process. The researchers believe that this comparitively easy process could lead to the material’s use in flexible electronics for computing, photonic systems, and more.

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Nanofilm Serves as Artificial Retina

Israel-based Nano Retina made some noise a few years back with its development of so-called “nanoelectrodes” that “interface with the eye’s bipolar neurons” and restart neural stimulation, allowing for messages to go to the brain. The nanoelectrodes served as a kind of artificial retina.

Since then it doesn’t appear that Nano Retina has had much more to report on the development of its implantable device, at least from what its website reveals. But it other Israeli researchers do, in a paper published in the journal Nano Letters. The team from Tel Aviv University, the Hebrew University of Jerusalem Centers for Nanoscience and Nanotechnology and Newcastle University combined semiconductor nanorods and carbon nanotubes to create a wireless, light-sensitive, flexible film that could potentially act in the place of a damaged retina.

The researchers used a plasma polymerized acrylic acid midlayer to make covalent bonds with the semiconductor nanorods directly onto neuro-adhesive, three-dimensional carbon nanotube surfaces.

The researchers have tested the device in a chick’s retina that in normal conditions would not have responded to light. These tests demonstrated that the flexible film absorbed light, which then triggered neuronal activity in the chick.

The researchers claim that this film is more durable, flexible and efficient, as well as better able to stimulate neurons, compared to other competitive devices. While there have been a number of approaches to creating artificial retinas to address diseases of the retina, such as macular degeneration, the stumbling block has largely been getting the device to fit inside the eye itself.

This is why solutions such as Nano Retina’s nanoelectrodes have been so attractive. It would seem, based Nano Retina’s reticence since its initial announcement, that the engineering issues are significant even with a nanoscale solution. Whether this latest research can find a way around them remains to be seen.

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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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