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2-D Material Could Lead the Way to "Valleytronics"

Earlier this year, we reported on researchers at the Massachusetts Institute of Technology (MIT) who had demonstrated that two-dimensional materials could be an alternative to diamonds in the esoteric world known as “valleytronics.”

Now, once again, researchers at MIT have shown that the 2-D material known as tungsten disulfide (WS2)—which belongs to a class of 2-D crystals known as transition metal dichalcogenides—could lead the way to valleytronics replacing conventional electronics.

Valleytronics essentially moves us away from the use of electrons’ electrical charge as a means for storing information to a scheme where we instead employ the wave quantum number of an electron in a crystalline material to encode data. The term valleytronics refers to the fact that if you plotted the energy of electrons relative to their momentum on a graph, the resulting curve would feature two deep valleys.

Manipulating these two valleys so that one is deeper than the other, would yield a way for the electrons to populate one of the two valleys. The positions into which electrons fall is a way to represent the zeroes and ones in digital computing.

But to get to the point where you could create stored information, you need to create a difference in the energies of the electrons populating the two valleys. The problem is that the electrons naturally want to settle into the lowest energy value, and they can achieve that in either of the two valleys.

You need to find some way to induce a difference in the energies of the two electron valleys. To date, the idea has been to achieve this change through the use of magnetic fields. However, to trigger that change you need a very powerful magnetic field, in the range of hundreds of tesla, to get even the most miniscule change. This limits the technique’s use to the lab.

“We discovered a way to directly control this valley by using light,” said Edbert Jarvis Sie, a MIT graduate student, in a press release.

The MIT researchers were able to achieve a much greater energy shift in the electrons by using a relatively conventional laser pulse with a special polarization.

“Being able to manipulate the valley degree of freedom in two-dimensional transition metal dichalcogenides would enable their application in the field of valleytronics,” said David Hsieh, an assistant professor of physics at Caltech, who was not connected to this research, in a press release. “This experiment makes a large step toward realizing this goal by demonstrating a method to control the energy difference between two valleys in tungsten disulfide for the first time.”

Self-Assembly Technique Offers Potential Scalable Production of Optical Memory

Researchers at RIKEN in Japan have developed a self-assembly method for creating well-ordered molecular structures that could eventually lead to the scalable production of organic optoelectronic devices such as optical memory.

Previous research has indicated that organic molecules can reversibly change their state in response to pulses of light—a quality that would be useful for storing digital ones and zeros in a different form. However, for it to work in an optoelectroic device, the molecules have to be arranged into highly ordered, single-molecule layers that are bonded to a metal surface. This bonding is where the problems begin. Once bonded, the optical properties of the molecules are altered so that getting the desired optoelectronic properties becomes difficult.

In research that was published in the journal Angewandte Chemie, the RIKEN team used interactions between electric dipoles (objects with a positive pole and a negative one) of molecules and alkali metal ions to create a homogeneous monolayer of diarylethene molecules on a copper surface.

In general, the property that the diarylethene molecules have that is useful in optoelectronics is that they are photochromic, which allows them to change color reversibly when irradiated with light. But what distinguishes the diarylethene molecules the RIKEN researchers created is that they also have electric dipoles. This means the molecules can self-assemble on the copper substrate and maintain their attractive photochromic properties.

“With homogeneous and close placing of individual molecules on a solid surface, we might be able to develop a memory device with several hundred to a thousand times the density achievable using current technology,” said Tomoko Shimizu, one of the lead researchers, in a press release. “We now want to study ways to achieve on–off switching of individual molecules in the superstructure in a controlled manner.”

This is pretty early stage research, especially when measured against the immense competition to develop optical memory technologies. But when you see terms like “scalable production”, you have to consider the long-term potential of the technology.

Quantum Dots Enable 3-D Printing of Contacts Lens With LEDs

While the research may have only aimed to demonstrate what is possible for 3D printing of electronic devices, researchers at Princeton University have used 3D printing to create an entire contact lens with light-emitting diodes (LEDs) embedded into it.

For the contact lens to actually work, it would require an external energy source, making it impractical as a real-world device. However, the real point for the Princeton team was to show that it’s possible to produce electronic devices into complex shapes using equally complex materials.

"This shows that we can use 3D printing to create complex electronics including semiconductors," said Michael McAlpine, an assistant professor of mechanical and aerospace engineering, in a press release. "We were able to 3D print an entire device, in this case an LED."

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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.”



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
Rachel Courtland
Associate Editor, IEEE Spectrum
New York, NY
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