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Polymer semiconductors, which can be processed on large-area and mechanically flexible substrates with low cost, are considered as one of the main components for future plastic electronics. However, they, especially n-type semiconducting polymers, currently lag behind inorganic counterparts in the charge carrier mobility – which characterizes how quickly charge carriers (electron) can move inside a semiconductor – and the chemical stability in ambient air.

Recently, a joint research team, consisting of Prof. Kilwon Cho and Dr. Boseok Kang with Pohang University of Science and Technology, and Prof. Yun-Hi Kim and Dr. Ran Kim with Gyungsang National University, has developed a new n-type semiconducting polymer with superior electron mobility and oxidative stability. The research outcome was published in Journal of the American Chemical Society (JACS) as a cover article and highlighted by the editors in JACS Spotlights.

The team modified a n-type conjugated polymer with semi-fluoroalkyl side chains – which are found to have several unique properties, such as hydrophobicity, rigidity, thermal stability, chemical and oxidative resistance, and the ability to self-organize. As a result, the modified polymer was shown to form a superstructure composed of polymer backbone crystals and side-chain crystals, resulting in a high degree of semicrystalline order. The team explained this phenomenon is attributed to the strong self-organization of the side chains and significantly boosts charge transport in polymer semiconductors.

Prof. Cho emphasized “We investigated the effects of semi-fluoroalkyl side chains of conjugated polymers at the molecular level and suggested a new strategy to design highly-performing polymeric materials for next-generation plastic electronics”.

This research was supported by the Center for Advanced Soft Electronics under the Global Frontier Research Program and the National Research Foundation (NRF) of Korea funded by the Ministry of Science, ICT and Future Planning.

Dow Corning will present an exclusive glimpse of upcoming products and technologies at LIGHTFAIR International 2016 (Booth #3657), and showcase new advances in LED lamp and luminaire lighting that its broad commercial portfolio of cutting-edge optical silicone solutions are enabling worldwide.

“Three years ago, Dow Corning’s optical silicones technology sparked a surge of breakthrough innovations in LED lighting designs, and the demand for these uniquely advanced materials has only grown as the industry seeks to maintain the momentum they have helped build,” said Hugo da Silva, global industry director for LED lighting at Dow Corning. “Dow Corning is as committed as ever to working closely with customers to expand on their early successes, and formulate new optical silicone solutions to help them usher in the next-generation of LED illumination.”

Dow Corning will offer an early glimpse at LIGHTFAIR 2016 of at least one of those upcoming optical silicone solutions – Dow Corning MS-4002 Moldable Silicone. Planned for launch later this year, this high-performing material signals the latest advance in the company’s award-winning Moldable Silicone portfolio. Currently in development and testing, MS-4002 Moldable Silicone aims to offer the optimum balance of material toughness for reaching high IP and IK ratings, high light transmittance rate and smooth surface feel for secondary optics in LED lamp and luminaire applications for both indoor and outdoor.

As the global leader in silicone innovation and technology, Dow Corning is changing the game for LED design, and the company will show exactly how during LIGHTFAIR 2016. The booth will feature the company’s broad and growing range of proven solutions at three corner kiosks, focusing on:

  • Dow Corning Moldable Silicones, where visitors can explore how these materials are delivering proven solutions for enhancing the optical quality, efficiency and reliability of lamp and luminaire designs
  • Protection & Assembly Solutions, where customer products illustrate how Dow Corning’s innovative silicone protection, assembly and optical solutions have helped develop products with longer life cycles and greater efficiency in outdoor/architectural, interior/specialty, display and automotive lighting applications
  • Silicone-Enabled Designs demonstrating new ways to shape, direct and diffuse light more efficiently with Dow Corning Optical Silicones. Visitors can also explore how silicone materials have expanded innovative design possibilities as LumenFlow Corp. takes them step by step through the LED design ideas process

In addition to offering an exclusive sneak peek at upcoming technologies, Dow Corning Lighting experts will be on hand to discuss the unique design flexibilities, proven reliability and simpler processability enabled by Dow Corning’s optical silicones. A market leader in materials, expertise and collaborative innovation for LED lighting concepts, Dow Corning offers solutions that span the entire LED value chain, adding reliability and efficiency for sealing, protecting, adhering, cooling and shaping light across all lighting applications.

LIGHTFAIR International is the world’s largest annual architectural and commercial lighting trade show and conference. Held at San Diego’s Convention Center from April 26-28, this year’s edition is expected to attract over 28,000 design, lighting, architectural, design, engineering, energy, facility and industry professionals from around the world to set future trends for lighting, design and technology innovation.

Epitaxy, or growing crystalline film layers that are templated by a crystalline substrate, is a mainstay of manufacturing transistors and semiconductors. If the material in one deposited layer is the same as the material in the next layer, it can be energetically favorable for strong bonds to form between the highly ordered, perfectly matched layers. In contrast, trying to layer dissimilar materials is a great challenge if the crystal lattices don’t match up easily. Then, weak van der Waals forces create attraction but don’t form strong bonds between unlike layers.

In a study led by the Department of Energy’s Oak Ridge National Laboratory, scientists synthesized a stack of atomically thin monolayers of two lattice-mismatched semiconductors. One, gallium selenide, is a “p-type” semiconductor, rich in charge carriers called “holes.” The other, molybdenum diselenide, is an “n-type” semiconductor, rich in electron charge carriers. Where the two semiconductor layers met, they formed an atomically sharp heterostructure called a p-n junction, which generated a photovoltaic response by separating electron-hole pairs that were generated by light. The achievement of creating this atomically thin solar cell, published in Science Advances, shows the promise of synthesizing mismatched layers to enable new families of functional two-dimensional (2D) materials.

The idea of stacking different materials on top of each other isn’t new by itself. In fact, it is the basis for most electronic devices in use today. But such stacking usually only works when the individual materials have crystal lattices that are very similar, i.e., they have a good “lattice match.” This is where this research breaks new ground by growing high-quality layers of very different 2D materials, broadening the number of materials that can be combined and thus creating a wider range of potential atomically thin electronic devices.

“Because the two layers had such a large lattice mismatch between them, it’s very unexpected that they would grow on each other in an orderly way,” said ORNL’s Xufan Li, lead author of the study. “But it worked.”

The group was the first to show that monolayers of two different types of metal chalcogenides–binary compounds of sulfur, selenium or tellurium with a more electropositive element or radical–having such different lattice constants can be grown together to form a perfectly aligned stacking bilayer. “It’s a new, potential building block for energy-efficient optoelectronics,” Li said.

Upon characterizing their new bilayer building block, the researchers found that the two mismatched layers had self-assembled into a repeating long-range atomic order that could be directly visualized by the Moiré patterns they showed in the electron microscope. “We were surprised that these patterns aligned perfectly,” Li said.

Researchers in ORNL’s Functional Hybrid Nanomaterials group, led by David Geohegan, conducted the study with partners at Vanderbilt University, the University of Utah and Beijing Computational Science Research Center.

“These new 2D mismatched layered heterostructures open the door to novel building blocks for optoelectronic applications,” said senior author Kai Xiao of ORNL. “They can allow us to study new physics properties which cannot be discovered with other 2D heterostructures with matched lattices. They offer potential for a wide range of physical phenomena ranging from interfacial magnetism, superconductivity and Hofstadter’s butterfly effect.”

Li first grew a monolayer of molybdenum diselenide, and then grew a layer of gallium selenide on top. This technique, called “van der Waals epitaxy,” is named for the weak attractive forces that hold dissimilar layers together. “With van der Waals epitaxy, despite big lattice mismatches, you can still grow another layer on the first,” Li said. Using scanning transmission electron microscopy, the team characterized the atomic structure of the materials and revealed the formation of Moiré patterns.

The scientists plan to conduct future studies to explore how the material aligns during the growth process and how material composition influences properties beyond the photovoltaic response. The research advances efforts to incorporate 2D materials into devices.

For many years, layering different compounds with similar lattice cell sizes has been widely studied. Different elements have been incorporated into the compounds to produce a wide range of physical properties related to superconductivity, magnetism and thermoelectrics. But layering 2D compounds having dissimilar lattice cell sizes is virtually unexplored territory.

“We’ve opened the door to exploring all types of mismatched heterostructures,” Li said.

The title of the paper is “Two-dimensional GaSe/MoSe2 misfit bilayer heterojunctions by van der Waals epitaxy.”

Insights from pure mathematics are lending new insights to material physics, which could aid in development of new devices and sensors. Now an international team of physicists has discovered that applying a magnetic field to a non-magnetic metal made it conduct 70% more electricity, even though basic physics principles would have predicted the opposite.

“We never expected that magnetoresistance could be lowered even further in the compound we tested, because in theory it should have increased,” says Kyoto University study author Shingo Yonezawa.

Applying a magnetic field to metals affects how well they are able to conduct electricity. Resistance arising from the magnetic field — magnetoresistance — is used in contexts like writing data in hard discs. Because of its wide application potential, material physicists are constantly striving to find new materials that show large-scale magnetoresistance.

Exposing a non-magnetic metal to a magnetic field typically increases its resistance and reduces the amount of electric current that is able to pass through it. Researchers at Kyoto University and the National Institute for Materials Science, in collaboration with researchers at National High-Magnetic Field Laboratory in the US, observed otherwise, however; when they applied a magnetic field to the compound PdCoO2, its resistance actually decreased, consequently increasing electrical current.

“Oxides tend not to deliver currents so readily, but PdCoO2 is one the oxides that actually conduct electricity beautifully,” says Yonezawa. “It already has low resistance relative to other oxides.”

The phenomenon remained unexplained until colleagues from the United States made a link with an analogy from topology, a mathematics discipline concerning continuous deformations.

“Electrons in some classes of materials have topological characteristics that lead them to be ‘boosted’ by magnetic fields, ultimately decreasing resistance,” continues Yonezawa. Although PdCoO2 was believed to lack such topological characteristics, it turns out that in the magnetic field this material can exhibit a phenomenon similar to these, aided by its very ‘clean’, layered crystal structure.”

Resistance also decreased in compounds PtCoO2 and Sr2RuO4, which have similar layered structures to PdCoO2.

“From these observations we now know that the phenomenon generally applies to other oxides with a layered structure,” explains Yoshiteru Maeno, a senior author also at Kyoto University. “Further developments in stratified non-magnetic metals with good conductivity should bring about new devices and sensors that have large magnetoresistance even when exposed to weak magnetic fields.”

The transistor is the most fundamental building block of electronics, used to build circuits capable of amplifying electrical signals or switching them between the 0s and 1s at the heart of digital computation. Transistor fabrication is a highly complex process, however, requiring high-temperature, high-vacuum equipment.

Now, University of Pennsylvania engineers have shown a new approach for making these devices: sequentially depositing their components in the form of liquid nanocrystal “inks.”

Their new study, published in Science, opens the door for electrical components to be built into flexible or wearable applications, as the lower-temperature process is compatible with a wide array of materials and can be applied to larger areas.

The researchers’ nanocrystal-based field effect transistors were patterned onto flexible plastic backings using spin coating but could eventually be constructed by additive manufacturing systems, like 3-D printers.

The study was lead by Cherie Kagan, the Stephen J. Angello Professor in the School of Engineering and Applied Science, and Ji-Hyuk Choi, then a member of her lab, now a senior researcher at the Korea Institute of Geoscience and Mineral Resources. Han Wang, Soong Ju Oh, Taejong Paik and Pil Sung Jo of the Kagan lab contributed to the work. They collaborated with Christopher Murray, a Penn Integrates Knowledge Professor with appointments in the School of Arts & Sciences and Penn Engineering; Murray lab members Xingchen Ye and Benjamin Diroll; and Jinwoo Sung of Korea’s Yonsei University.

The researchers began by taking nanocrystals, or roughly spherical nanoscale particles, with the electrical qualities necessary for a transistor and dispersing these particles in a liquid, making nanocrystal inks.

Kagan’s group developed a library of four of these inks: a conductor (silver), an insulator (aluminum oxide), a semiconductor (cadmium selenide) and a conductor combined with a dopant (a mixture of silver and indium). “Doping” the semiconductor layer of the transistor with impurities controls whether the device transmits a positive or negative charge.

“These materials are colloids just like the ink in your inkjet printer,” Kagan said, “but you can get all the characteristics that you want and expect from the analogous bulk materials, such as whether they’re conductors, semiconductors or insulators.

“Our question was whether you could lay them down on a surface in such a way that they work together to form functional transistors.”

The electrical properties of several of these nanocrystal inks had been independently verified, but they had never been combined into full devices.

“This is the first work,” Choi said, “showing that all the components, the metallic, insulating, and semiconducting layers of the transistors, and even the doping of the semiconductor could be made from nanocrystals.”

Such a process entails layering or mixing them in precise patterns.

First, the conductive silver nanocrystal ink was deposited from liquid on a flexible plastic surface that was treated with a photolithographic mask, then rapidly spun to draw it out in an even layer. The mask was then removed to leave the silver ink in the shape of the transistor’s gate electrode. The researchers followed that layer by spin-coating a layer of the aluminum oxide nanocrystal-based insulator, then a layer of the cadmium selenide nanocrystal-based semiconductor and finally another masked layer for the indium/silver mixture, which forms the transistor’s source and drain electrodes. Upon heating at relatively low temperatures, the indium dopant diffused from those electrodes into the semiconductor component.

“The trick with working with solution-based materials is making sure that, when you add the second layer, it doesn’t wash off the first, and so on,” Kagan said. “We had to treat the surfaces of the nanocrystals, both when they’re first in solution and after they’re deposited, to make sure they have the right electrical properties and that they stick together in the configuration we want.”

Because this entirely ink-based fabrication process works at lower temperatures than existing vacuum-based methods, the researchers were able to make several transistors on the same flexible plastic backing at the same time.

“Making transistors over larger areas and at lower temperatures have been goals for an emerging class of technologies, when people think of the Internet of things, large area flexible electronics and wearable devices,” Kagan said. “We haven’t developed all of the necessary aspects so they could be printed yet, but because these materials are all solution-based, it demonstrates the promise of this materials class and sets the stage for additive manufacturing.”

Researchers from the University of Illinois at Urbana-Champaign have demonstrated a new approach to modifying the light absorption and stretchability of atomically thin two-dimensional (2D) materials by surface topographic engineering using only mechanical strain. The highly flexible system has future potential for wearable technology and integrated biomedical optical sensing technology when combined with flexible light-emitting diodes.

“Increasing graphene’s low light absorption in visible range is an important prerequisite for its broad potential applications in photonics and sensing,” explained SungWoo Nam, an assistant professor of mechanical science and engineering at Illinois. “This is the very first stretchable photodetector based exclusively on graphene with strain-tunable photoresponsivity and wavelength selectivity.”

Graphene–an atomically thin layer of hexagonally bonded carbon atoms–has been extensively investigated in advanced photodetectors for its broadband absorption, high carrier mobility, and mechanical flexibility. Due to graphene’s low optical absorptivity, graphene photodetector research so far has focused on hybrid systems to increase photoabsorption. However, such hybrid systems require a complicated integration process, and lead to reduced carrier mobility due to the heterogeneous interfaces.

According to Nam, the key element enabling increased absorption and stretchability requires engineering the two-dimensional material into three-dimensional (3D) “crumpled structures,” increasing the graphene’s areal density. The continuously undulating 3D surface induces an areal density increase to yield higher optical absorption per unit area, thereby improving photoresponsivity. Crumple density, height, and pitch are modulated by applied strain and the crumpling is fully reversible during cyclical stretching and release, introducing a new capability of strain-tunable photoabsorption enhancement and allowing for a highly responsive photodetector based on a single graphene layer.

“We achieved more than an order-of-magnitude enhancement of the optical extinction via the buckled 3D structure, which led to an approximately 400% enhancement in photoresponsivity,” stated Pilgyu Kang, and first author of the paper, “Crumpled Graphene Photodetector with Enhanced, Strain-tunable and Wavelength-selective Photoresponsivity,” appearing in the journal, Advanced Materials. “The new strain-tunable photoresponsivity resulted in a 100% modulation in photoresponsivity with a 200% applied strain. By integrating colloidal photonic crystal–a strain-tunable optomechanical filter–with the stretchable graphene photodetector, we also demonstrated a unique strain-tunable wavelength selectivity.”

“This work demonstrates a robust approach for stretchable and flexible graphene photodetector devices,” Nam added. “We are the first to report a stretchable photodetector with stretching capability to 200% of its original length and no limit on detection wavelength. Furthermore, our approach to enhancing photoabsorption by crumpled structures can be applied not only to graphene, but also to other emerging 2D materials.”

A finely tuned carbon nanotube thin film has the potential to act as a thermoelectric power generator that captures and uses waste heat, according to researchers at the Energy Department’s National Renewable Energy Laboratory (NREL).

The research could help guide the manufacture of thermoelectric devices based on either single-walled carbon nanotube (SWCNT) films or composites containing these nanotubes. Because more than half of the energy consumed worldwide is rejected primarily as waste heat, the idea of thermoelectric power generation is emerging as an important part of renewable energy and energy-efficiency portfolios.

“There have not been many examples where people have really looked at the intrinsic thermoelectric properties of carbon nanotubes and that’s what we feel this paper does,” said Andrew Ferguson, a research scientist in NREL’s Chemical and Materials Science Center and co-lead author of the paper with Jeffrey Blackburn.

The research, “Tailored Semiconducting Carbon Nanotube Networks with Enhanced Thermoelectric Properties,” appears in the journal Nature Energy, and is a collaboration between NREL, Professor Yong-Hyun Kim’s group at the Korea Advanced Institute of Science and Technology, and Professor Barry Zink’s group at the University of Denver. The other authors from NREL are Azure Avery (now an assistant professor at Metropolitan State University of Denver), Ben Zhou, Elisa Miller, Rachelle Ihly, Kevin Mistry, and Sarah Guillot.

Nanostructured inorganic semiconductors have demonstrated promise for improving the performance of thermoelectric devices. Inorganic materials can run into problems when the semiconductor needs to be lightweight, flexible, or irregularly shaped because they are often heavy and lack the required flexibility. Carbon nanotubes, which are organic, are lighter and more flexible.

How useful a particular SWCNT is for thermoelectrics, however, depends on whether the nanotube is metallic or a semiconductor, both of which are produced simultaneously in SWCNT syntheses. A metallic nanotube would harm devices such as a thermoelectric generator, whereas a semiconductor nanotube actually enhances performance. Furthermore, as with most optical and electrical devices, the electrical band gap of the semiconducting SWCNT should affect the thermoelectric performance as well.

Fortunately, Blackburn, a senior scientist and manager of NREL’s Spectroscopy and Photoscience group, has developed an expertise at separating semiconducting nanotubes from metallic ones and his methods were critical to the research, Ferguson said.

“We are at a distinct advantage here that we can actually use that to probe the fundamental properties of the nanotubes,” he said.

To generate highly enriched semiconducting samples, the researchers extracted nanotubes from polydisperse soot using polyfluorene-based polymers. The semiconducting SWCNTs were prepared on a glass substrate to create a film, which was then soaked in a solution of oxidant, triethyloxonium hexachloroantimonate (OA), a process known as “doping.” Doping increases the density of charge carriers, which flow through the film to conduct electricity. The researchers found the samples that performed the best were exposed to a higher concentration of OA, but not at the highest doping levels. They also discovered an optimum diameter for a carbon nanotube to achieve the best thermoelectric performance.

When it comes to thermoelectric materials, a trade-off exists between thermopower (the voltage obtained when subjecting a material to a temperature gradient) and electrical conductivity because thermopower decreases with increasing conductivity. The researchers discovered, however, that with carbon nanotubes you can retain large thermopowers even at very high electrical conductivities. Furthermore, the researchers found that their doping strategy, while dramatically increasing the electrical conductivity, actually decreased the thermal conductivity. This unexpected result is another benefit of carbon nanotubes for thermoelectric power generation, since the best thermoelectric materials must have high electrical conductivity and thermopower, while maintaining low thermal conductivity.

A simple filtration process helped Rice University researchers create flexible, wafer-scale films of highly aligned and closely packed carbon nanotubes.

Scientists at Rice, with support from Los Alamos National Laboratory, have made inch-wide films of densely packed, chirality-enriched single-walled carbon nanotubes through a process revealed today in Nature Nanotechnology.

In the right solution of nanotubes and under the right conditions, the tubes assemble themselves by the millions into long rows that are aligned better than once thought possible, the researchers reported.

Rice University researchers discovered a method to make highly aligned nanotube films. The films may become valuable for flexible electronics and photonic devices. Credit: Jeff Fitlow/Rice University

Rice University researchers discovered a method to make highly aligned nanotube films. The films may become valuable for flexible electronics and photonic devices. Credit:
Jeff Fitlow/Rice University

The thin films offer possibilities for making flexible electronic and photonic (light-manipulating) devices, said Rice physicist Junichiro Kono, whose lab led the study. Think of a bendable computer chip, rather than a brittle silicon one, and the potential becomes clear, he said.

“Once we have centimeter-sized crystals consisting of single-chirality nanotubes, that’s it,” Kono said. “That’s the holy grail for this field. For the last 20 years, people have been looking for this.”

The Rice lab is closing in, he said, but the films reported in the current paper are “chirality-enriched” rather than single-chirality. A carbon nanotube is a cylinder of graphene, with its atoms arranged in hexagons. How the hexagons are turned sets the tube’s chirality, and that determines its electronic properties. Some are semiconducting like silicon, and others are metallic conductors.

A film of perfectly aligned, single-chirality nanotubes would have specific electronic properties. Controlling the chirality would allow for tunable films, Kono said, but nanotubes grow in batches of random types.

For now, the Rice researchers use a simple process developed at the National Institute of Standards and Technology to separate nanotubes by chirality. While not perfect, it was good enough to let the researchers make enriched films with nanotubes of different types and diameters and then make terahertz polarizers and electronic transistors.

The Rice lab discovered the filtration technique in late 2013 when graduate students and lead authors Xiaowei He and Weilu Gao inadvertently added a bit too much water to a nanotube-surfactant suspension before feeding it through a filter assisted by vacuum. (Surfactants keep nanotubes in a solution from clumping.)

The film that formed on the paper filter bore further investigation. “Weilu checked the film with a scanning electron microscope and saw something strange,” He said. Rather than drop randomly onto the paper like pickup sticks, the nanotubes – millions of them – had come together in tight, aligned rows.

“That first picture gave us a clue we might have something totally different,” He said. A year and more than 100 films later, the students and their colleagues had refined their technique to make nanotube wafers up to an inch wide (limited only by the size of their equipment) and of any thickness, from a few to hundreds of nanometers.

Further experiments revealed that each element mattered: the type of filter paper, the vacuum pressure and the concentration of nanotubes and surfactant. Nanotubes of any chirality and diameter worked, but each required adjustments to the other elements to optimize the alignment.

The films can be separated from the paper and washed and dried for use, the researchers said.

They suspect multiwalled carbon nanotubes and non-carbon nanotubes like boron nitride would work as well.

Co-author Wade Adams, a senior faculty fellow at Rice who specializes in polymer science, said the discovery is a step forward in a long quest for aligned structures.

“They formed what is called a monodomain in liquid crystal technology, in which all the rigid molecules line up in the same direction,” Adams said. “It’s astonishing. (The late Rice Nobel laureate) Rick Smalley and I worked very hard for years to make a single crystal of nanotubes, but these students have actually done it in a way neither of us ever imagined.”

Why do the nanotubes line up? Kono said the team is still investigating the mechanics of nucleation — that is, how the first few nanotubes on the paper come together. “We think the nanotubes fall randomly at first, but they can still slide around on the paper,” he said. “Van der Waals force brings them together, and they naturally seek their lowest-energy state, which is in alignment.” Because the nanotubes vary in length, the researchers suspect the overhangs force other tubes to line up as they join the array.

The researchers found their completed films could be patterned with standard lithography techniques. That’s yet another plus for manufacturers, said Kono, who started hearing buzz about the discovery months before the paper’s release.

“I gave an invited talk about our work at a carbon nanotube conference, and many people are already trying to reproduce our results,” he said. “I got so much enthusiastic response right after my talk. Everybody asked for the recipe.”

By chemically modifying and pulverizing a promising group of compounds, scientists at the National Institute of Standards and Technology (NIST) have potentially brought safer, solid-state rechargeable batteries two steps closer to reality.

These compounds are stable solid materials that would not pose the risks of leaking or catching fire typical of traditional liquid battery ingredients and are made from commonly available substances.

Since discovering their properties in 2014, a team led by NIST scientists has sought to enhance the compounds’ performance further in two key ways: Increasing their current-carrying capacity and ensuring that they can operate in a sufficiently wide temperature range to be useful in real-world environments.

Considerable advances have now been made on both fronts, according to Terrence Udovic of the NIST Center for Neutron Research, whose team has published a pair of scientific papers that detail each improvement.

The first advance came when the team found that the original compounds — made primarily of hydrogen, boron and either lithium or sodium — were even better at carrying current with a slight change to their chemical makeup. Replacing one of the boron atoms with carbon improved their ability to conduct charged particles, or ions, which are what carry electricity inside a battery. As the team reported in February in their first paper, the switch made the compounds about 10 times better at conducting.

But perhaps more important was clearing the temperature hurdle. The compounds conducted ions well enough to operate in a battery — as long as it was in an environment typically hotter than boiling water. Unfortunately, there’s not much of a market for such high-temperature batteries, and by the time they cooled to room temperature, the materials’ favorable chemical structure often changed to a less conductive form, decreasing their performance substantially.

One solution turned out to be crushing the compound’s particles into a fine powder. The team had been exploring particles that are measured in micrometers, but as nanotechnology research has demonstrated time and again, the properties of a material can change dramatically at the nanoscale. The team found that pulverizing the compounds into nanometer-scale particles resulted in materials that could still perform well at room temperature and far below.

“This approach can remove worries about whether batteries incorporating these types of materials will perform as expected even on the coldest winter day,” says Udovic, whose collaborators on the most recent paper include scientists from Japan’s Tohoku University, the University of Maryland and Sandia National Laboratories. “We are currently exploring their use in next-generation batteries, and in the process we hope to convince people of their great potential.”

As electronic components are becoming ever smaller, the industry is gradually approaching the limits of what is achievable using the traditional approach with silicon as a semiconductor material.

Graphene, the material with a number of “miraculous” properties, is considered a possible replacement. The one atom thin carbon film is ultra-light, extremely flexible and highly conductive. However, in order to be able to use graphene for electronic components such as field effect transistors, the material has to be “transformed” into a semiconductor. This was achieved by Empa scientists some time ago using a newly developed method – in 2010, they presented, for the first time, graphene nanoribbons (GNR) only a few nanometres wide with precisely shaped edges. For this, the ribbons were grown on a metal surface from specifically designed precursor molecules. The narrower the ribbons, the larger their electronic band gap – i.e. the energy range in which no electrons can be located, which is responsible for ensuring that an electronic switch (for example, a transistor) can be turned on and off. The Empa researchers were then also able to “dope” the nanoribbons, i.e. to furnish the ribbons with impurity atoms such as nitrogen at certain points, in order to influence the electronic properties of the graphene ribbons even more.

The perfect blueprint

In the paper now published in Nature, the Empa team led by Roman Fasel reports, together with colleagues from the Max Planck Institute for Polymer Research in Mainz, headed by Klaus Müllen, and from the Technical University of Dresden led by Xinliang Feng, how it managed to synthesise GNR with perfectly zigzagged edges using suitable carbon precursor molecules and a perfected manufacturing process. The zigzags followed a very specific geometry along the longitudinal axis of the ribbons. This is an important step, because researchers can thus give graphene ribbons different properties via the geometry of the ribbons and especially via the structure of their edges.

As with floor tiling, the right tiles – or precursor molecules – for the synthesis on the surface first had to be found for the specific pattern of the zigzag graphene ribbons. Unlike in organic chemistry, which takes the occurrence of by-products into account on the path to achieving a pure substance, everything had to be designed for the surface synthesis of the graphene ribbons so that only a single product was produced. The scientists repeatedly switched back and forth between computer simulations and experiments, in order to design the best possible synthesis. With molecules in a U-shape, which they allowed to grow together to form a snake-like shape, and additional methyl groups, which completed the zigzag edges, the researchers were able to finally create a “blueprint” for GNR with perfect zigzag edges. To check that the zigzag edges were exact down to the atom, the researchers investigated the atomic structure using an atomic force microscope (AFM). In addition, they were able to characterise the electronic states of the zigzag edges using scanning tunnelling spectroscopy (STS).

Using the internal spin of the electrons

And these display a very promising feature. Electrons can spin either to the left or to the right, which is referred to as the internal spin of electrons. The special feature of the zigzag GNR is that, along each edge, the electrons all spin in the same direction; an effect which is referred to as ferro-magnetic coupling. At the same time, the so-called antiferromagnetic coupling ensures that the electrons on the other edge all spin in the opposite direction. So the electrons on one side all have a “spin-up” state and on the other edge they all have a “spin-down” state.

Thus, two independent spin-channels with opposite “directions of travel” arise on the band edges, like a road with separated lanes. Via intentionally integrated structural defects on the edges or – more elegantly – via the provision of an electrical, magnetic or optical signal from the outside, spin barriers and spin filters can thus be designed that require only energy in order to be switched on and off – the precursor to a nanoscale and also extremely energy efficient transistor.

Possibilities such as this make GNR extremely interesting for spintronic devices; these use both the charge and the spin of the electrons. This combination is prompting scientists to forecast completely new components, e.g. addressable magnetic data storage devices which maintain the information that has been fed in even after the power has been turned off.