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New Layering Process Brings Graphene's 2D Properties to the 3D World

Researchers at Rensselaer Polytechnic Institute (RPI) have taken a significant step towards transforming high quality 2D graphene sheets into 3D macroscopic structures that could be used for applications such as thermal management for high power electronics, structural composites, flexible and stretchable electrodes for energy storagesensors, and membranes.

In research published in the journal Science, the RPI researchers developed a new layered structure for graphene that addresses the problem of achieving the mechanical strength of graphene in its 3D form while maintaining its attractive thermal and electrical properties in its 2D form.

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Graphene "Decorated" With Lithium Becomes a Superconductor

Graphene is a conductor unlike anything seen before. Electrons are able to travel though it without resistance at room temperature, promising a new approach to electronics.  It’s even been engineered to act like a semiconductor with a band gap for stopping and starting the flow of electrons, thus offering an alternative to silicon in electronics.

Despite these properties, and a host of others that seem to sprout up regularly, nobody had been able to make graphene behave as a superconductor, until now.

An international research team from Canada and Germany has been able to demonstrate that graphene can be made to behave as a superconductor when it’s doped with lithium atoms. The researchers believe that this new property could lead to a new generation of superconducting nanoscale devices.

Superconductors are materials that conduct electricity without resistance and without dissipating energy. In ordinary materials, electrons repel each other, but in superconductors the electrons form pairs known as Cooper pairs, which together flow through the material without resistance. Phonons, the mechanism that facilitates these electrons’ alliances are vibrations in lattice crystalline structures.

Graphene is not naturally a superconductor, and neither is its three-dimensional source—graphite. However, it was demonstrated a decade ago that graphite could be induced into behaving like a superconductor. If it’s possible with graphite, it should be with graphene, right?

Other research groups believed so and developed computer models demonstrating that combining graphene with lithium might do the trick. Lithium, they predicted, could contribute a lot of phonons to the graphene’s electrons.

In a research paper available on arXiv, the researchers demonstrated in physical experiments that the computer models were indeed correct in their predictions. Andrea Damascelli at the University of British Colombia in Vancouver, together with collaborators in Europe, grew layers of graphene on silicon-carbide substrates, then deposited lithium atoms onto the graphene in a vacuum at 8 K, creating a version of graphene known as “decorated” graphene.

In the testing and measuring of their material, the researchers found that the electrons slowed down as they travelled through the lattice, which they believe to be the result of enhanced electron–phonon coupling. The key observation was that this increased number of coupled pairs led to superconductivity, which the researchers measured by identifying an energy gap between the material's conducting and non-conducting electrons. That energy gap is equal to the amount of energy needed to break Cooper pairs.

The researchers who demonstrated last year the role phonons played in the superconductivity of graphite and calcium, Patrick Kirchmann and Shuolong Yang of the SLAC National Accelerator Laboratory, believe this latest work could usher in the fabrication of nanoscale superconducting quantum interference devices and single-electron superconductor quantum dots.

A Clearer Outlook for Quantum Dot-Enabled Solar Windows

Quantum dots have been knocking on the door of photovoltaics for a while now. But it turns out that maybe they should have been tapping on the window instead.

In joint research between the Department of Energy’s Los Alamos National Laboratory (LANL) and the University of Milan-Bicocca (UNIMIB) in Italy, researchers have spent the last 16 months perfecting a technique that makes it possible to embed quantum dots into windows so that the window itself becomes a solar panel.

In research published in the journal Nature Nanotechnology, the international team were able to improve upon their previous iteration of the technology by making the quantum dots from non-toxic materials while having the window be largely tint free and transparent.

“In these devices, a fraction of light transmitted through the window is absorbed by nanosized particles (semiconductor quantum dots) dispersed in a glass window, re-emitted at [an] infrared wavelength invisible to the human eye, and wave-guided to a solar cell at the edge of the window,” said Victor Klimov, lead researcher on the project at LANL, in a press release. 

Of course, this is not the first time someone thought that it would be a good idea to make windows into solar collectors. But this latest iteration marks a significant development in the evolution of the technology. Previous technologies used organic emitters that limited the size of the concentrators to just a few centimeters.

The energy conversion efficiency the researchers were able to acheive with the solar windows was around 3.2 percent, which stands up pretty well when compared with state-of-the-art quantum dot-based solar cells that have reached 9 percent conversion efficiency.

The US-Italy team’s first breakthrough last year was to make large-area solar concentrators that were free from reabsorption losses that had plagued previous attempts. This lastest breakthrough was in finding a way to toss out the cadmium quantum dots for a more environmentally friendly compound.

“Our new devices use quantum dots of a complex composition which includes copper (Cu), indium (In), selenium (Se) and sulfur (S),” said Klimov in the release: “these particles do not contain any toxic metals that are typically present” in similar systems, he added. And Klimov claims that because their quantum dots absorb light from throughout the spectrum of incoming sunlight, it doesn’t distort colors.  

The researchers acknowledge that we won’t see solar windows for sale in the near term, as bringing down production costs still remains an issue. But they believe this latest development should bring solar windows closer to the market, as their new quantum dots are cheaper than traditional dots.

The Universe of 2-D Materials Expands in the Real World

While research and its media coverage are quick to extol the virtues of the array of two-dimensional (2-D) materials, what is sometimes neglected is that they often come with some fairly significant obstacles that keep them from replacing silicon. Among these difficulties is the lack of a band gap. However, the potential showstoppers don’t end there. Merely exposing these materials to real-world environments with air can make these materials unstable and begin their decomposition.

Recent research has shown that encapsulation techniques in which a protective shell covers the 2-D material have proven effective at isolating materials like silicene from real-world environments, albeit briefly.

Now researchers at the University of Manchester in the U.K. have combined encapsulation with a number of other manufacturing techniques—including a different means by which the 2-D materials are cleaved away from their 3-D versions and a special method for transferring them onto a substrate and aligning them—to produce 2-D materials that can survive in real-world environments.

“This is an important breakthrough in the area of 2-D materials research, as it allows us to dramatically increase the variety of materials that we can experiment with using our expanding 2-D crystal toolbox,” said Roman Gorbachev, who led the research, in a press release.

In research published in the journal NanoLetters, the Manchester team demonstrated their production technique on two 2-D materials: niobium diselenide and black phosphorus, which has been building quite a reputation for itself over the last 18 months. The researchers fabricated field-effect devices using these materials and their technique. The results showed that the devices remained conductive and fully stable under ambient conditions.

The niobium dieselenide maintained its superconducting properties right down to a monolayer. Meanwhile, the black phosphorus, in the form of a trilayer, had such high electron mobility that it achieved Landau quantization, which is responsible for changes in the electronic properties of materials when exposed to a magnetic field.

The main impact of the research is that it shows a way towards making the full gamut of 2-D materials accessible to real-world applications.

As Andre Geim, the Nobel laureate at Manchester who, with his colleague Kostya Novoselov, first synthesized graphene, noted in the release: “The more materials we have to play with, the greater potential there is for creating applications that could revolutionize the way we live.”

Microwave Oven Is Key to Safer Quantum Dot Manufacturing

As we noted earlier this month, quantum dots have finally made a broad commercial impact on LED technology. So now it’s time to refine the processes for making them and to increase profit margins.

Now researchers at Oregon State University (OSU) have thrown their hat into the ring, presenting a manufacturing method for quantum dots that promises a new era in LED lighting.

In research published in the Journal of Nanoparticle Research, the OSU researchers employed a so-called “continuous flow” chemical reactor. Chemicals were continuously put into the reactor and came out as a continuous stream of product. This continuous flow setup makes the process cheaper, faster, and highly scalable. Meanwhile, using microwaves to heat the reagents—with a machine that operates on more or less the same principle as your microwave oven at home—addresses the issue of how to maintain precise temperature control during the chemical process.

The researchers claim that this method leads to precisely sized and shaped quantum dots that are consistent in their composition. They believe that this development will mark a big change in LED lighting.

“We may finally be able to produce low-cost, energy-efficient LED lighting with the soft quality of white light that people really want,” said Greg Herman, an associate professor of chemical engineering at OSU, in a press release. “At the same time, this technology will use nontoxic materials and dramatically reduce the waste of the materials that are used, which translates to lower cost and environmental protection.”

The environmental benefit of the quantum dots stems from the use of copper indium diselenide, which is a more environmentally benign material than the cadmium that is typically used in LED lighting systems.

Because the process will give manufacturers the ability to fine-tune the size and shape of the quantum dots, that flexibility should make them able to produce dots for a variety of applications. Smaller dots emit green light, while the larger dots emit light in the orange to red range.

The OSU researchers believe that their precision manufacturing method will yield better color control than other quantum-dot-making techniques. So much so that they foresee quantum dots produced via this method providing a cheaper alternative in an array of applications including optics, electronics, and biomedicine.

Carbon Fiber Cloth Can Generate Hydrogen

Splitting water can yield clean-burning hydrogen fuel, but catalysts that generate hydrogen are often expensive, and unstable in water. Now a team from Singapore and Taiwan have shown that carbon fiber cloths coated in inexpensive catalysts can generate hydrogen, and perform not only in water but in seawater as well. The researchers detailed their findings online August 21 in the journal Science Advances.

The most effective catalyst for generating hydrogen is platinum. However, this metal is scarce and expensive, limiting its use in large-scale hydrogen generation. Instead, the researchers investigated molybdenum sulfide as a catalyst. Molybdenum and sulfur are respectively about 300 and more than 100,000 times more abundant than platinum.

"One hundred grams of pure molybdenum metal costs $44, while the same amount of platinum costs $3,211.86 today," says study co-author Bin Liu, a materials scientist at Nanyang Technological University in Singapore.

Normally, molybdenum disulfide is bad at generating hydrogen. However, prior studies found that while its flat surfaces are not catalytically active, its edges are. The researchers incorporated nickel into the catalyst, which essentially punched holes into the molybdenum sulfide, increasing the amount of its exposed edges.

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Can Big Data Shed Light on Nanotech Research?

It can take days for a supercomputer to unravel all the data contained in a single human genome. So it wasn’t long after mapping the first human genome that researchers coined the umbrella term “bioinformatics” in which a variety of methods and computer technologies are used for organizing and analyzing all that data.

Now teams of researchers led by scientists at Duke University believe that the field of nanotechnology has reached a critical mass of data and that a new field needs to be established, dubbed “nanoinformatics.”

If you’re not convinced that there’s as much data being produced in nanotechnology research as there is genetic research, you can choose to use the other buzz word of our time “big data,” which seems to be used interchangeably in the research, which was published in the journal Beilstein Journal of Nanotechnology.

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New Method for Layering 2-D Materials Offers Breakthrough in Energy Storage

For the past couple of years, researchers have been excited at the prospect of what they can make from all the possible instantiations of transition metal dichalcogenides (TMDs). TMDs are combinations of one of 15 transition metals such as molybdenum or tungsten, with one of the three members of the chalcogen family, which includes sulfur, selenium and tellurium. To date, only a small number of combinations have been tested, raising hope that there may be some as yet unidentified combination out there that will prove to be a viable alternative for silicon.

Now researchers at Drexel University have multiplied the number of potential silicon replacements by demonstrating that they can combine two transition metals—in this case molybdenum and titanium—using carbon atoms as the glue bonding the two together. With the method they have developed for combining the two-dimensional (2-D) versions of these materials, the researchers are testing various combinations to see what combinations might be applicable to energy storage, electronics, or wear-resistant materials.

“By sandwiching one or two atomic layers of a transition metal like titanium between monoatomic layers of another metal, such as molybdenum, with carbon atoms holding them together, we discovered that a stable material can be produced,” said Babak Anasori, the post-doctoral researcher who led the research, in a press release. “It was impossible to produce a 2-D material having just three or four molybdenum layers in such structures, but because we added the extra layer of titanium as a connector, we were able to synthesize them.”

The researchers believe that their work, which was published in the journal ACS Nano, will prove to be significant in the future because it provides a way to combine elemental materials in a stable compound. The Drexel team claims that each new combination should exhibit new properties. Eventually, they say, a combination will be discovered that could revolutionize a variety of different technologies, including thermoelectrics, batteries, catalysis, solar cells, electronic devices, and structural composites.

“While it’s hard to say, at this point, exactly what will become of these new families of 2-D materials we’ve discovered, it is safe to say that this discovery enables the field of materials science and nanotechnology to move into an uncharted territory,” Anasori said in the release.

The combining of 2-D versions of elements that resist being joined on the atomic scale has proven difficult and has required over a decade of research at Drexel.

“Due to their structure and electric charge, certain elements just don’t ‘like’ to be combined,” Anasori said. “It’s like trying to stack magnets with the poles facing the same direction—you’re not going to be very successful and you’re going to be picking up a lot of flying magnets.”

The key to making this all work was the discovery of a material called MAX phase by a Drexel researcher over two decades ago. MAX phase (the M is for transition metal, the A for "A group" metal, and the X for carbon and/or nitrogen) is described as a kind of primordial goo from which all things came—it contains all the elements but needs to be organized by the researcher.

The first order placed with MAX phase as the source material was a material developed in 2011 called “MXene.” It derived its name from the process of etching and exfoliating atomically thin layers of aluminum from MAX phases.

Two years ago, this blog covered research that showed MXenes were effective in energy storage applications.

The Drexel researchers were continuing their research into the use of MXenes in energy storage applications, working their way through the periodic table. When they reached the transition metals, they were stymied by molybdenum.

“We had reached a bit of an impasse, when trying to produce a molybdenum containing MXenes,” Anasori said in the release. “By adding titanium to the mix we managed to make an ordered molybdenum MAX phase, where the titanium atoms are in center and the molybdenum on the outside.”

Michel W. Barsoum, one of the Drexel researchers who first synthesized MXene over four years ago, added in the release: “This new layering method gives researchers an unimaginable number of possibilities for tuning materials’ properties for a variety of high-tech applications.”

Black Phosphorus Takes a Step Toward CMOS

Researchers working at the Institute for Basic Science Center for Integrated Nanostructure Physics at Sungkyunkwan University (SKKU) in South Korea have discovered that they can manipulate black phosphorus to behave as an n-type (excess electrons) semiconductor, a p-type (excess holes), or as if it were ambipolar (both n- or p-type) simply by changing its thickness and its bandgap or by using a different metal to contact it with. (Today’s digital logic, CMOS, requires both n-type and p-type transistors.)

With this knowledge, the Korean researchers were able to fabricate a transistor from the material that can operate at lower voltages than a silicon-based transistor. (Though it wasn’t the first black phosphorus transistor ever made.)

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Scalable Production for Graphene Nanoribbons Boosts Potential in Electronics

The form of graphene that holds possibly the most promise for use in electronics is the graphene nanoribbon. The narrowness of these ribbons is the key. At widths ranging from a few nanometers to no more than few hundred nanometers the edge of the graphene determines its properties: Very narrow nanoribbons act as a semiconductors and wider ones behave as conductors.

There are a number of methods for producing graphene nanoribbons that can be categorized as either “top down” approaches, like lithographic techniques, or “bottom up” methods where the graphene nanoribbons self assemble into a desired form.

Researchers at the University of Wisconsin-Madison recently developed a technique based on a bottom-up approach and managed to do it without the limitation of previous methods. The result looks to be a production technique that is compatible with semiconductor manufacturing methods and can be scaled up to bulk production.

In research published in the journal Nature Communications, the Wisconsin team were able to grow graphene on a conventional germanium semiconductor wafer.

"Graphene nanoribbons that can be grown directly on the surface of a semiconductor like germanium are more compatible with planar processing that's used in the semiconductor industry, and so there would be less of a barrier to integrating these really excellent materials into electronics in the future," said Michael Arnold, an associate professor UW-Madison, in a press release.

Previous bottom-up approaches only worked on metal substrates, and, to date, have not been able to produce nanoribbons with the necessary length to be useful for electronics.

The UW-Madison researchers overcame this limitation by putting a bit of a twist on chemical vapor deposition (CVD)—a technique that has served as a kind of bridge of late between scalability and purity in graphene production. In CVD techniques, the graphene is grown from a vaporous precursor on a metal substrate like copper or nickel in a furnace. What the UW-Madison team did was to start the process with methane, which attaches to the germanium surface and breaks down into various hydrocarbons. These different hydrocarbons then begin to react with each other to form graphene.

The key to the technique is its tenability. The researchers can slow the growth rate of the graphene and thereby both lengthen the nanoribbons and make them narrower by decreasing the amount of methane in the CVD furnace chamber.

"What we've discovered is that when graphene grows on germanium, it naturally forms nanoribbons with these very smooth, armchair edges," Arnold said in the release. "The widths can be very, very narrow and the lengths of the ribbons can be very long, so all the desirable features we want in graphene nanoribbons are happening automatically with this technique."

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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
 
Contributor
Rachel Courtland
Associate Editor, IEEE Spectrum
New York, NY
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