Portable electronics users tend to upgrade their devices frequently as new technologies offering more functionality and more convenience become available. A report published by the U.S. Environmental Protection Agency in 2012 showed that about 152 million mobile devices are discarded every year, of which only 10 percent is recycled -- a legacy of waste that consumes a tremendous amount of natural resources and produces a lot of trash made from expensive and non-biodegradable materials like highly purified silicon. Now researchers from the University of Wisconsin-Madison have come up with a new solution to alleviate the environmental burden of discarded electronics. They have demonstrated the feasibility of making microwave biodegradable thin-film transistors from a transparent, flexible biodegradable substrate made from inexpensive wood, called cellulose nanofibrillated fiber (CNF). This work opens the door for green, low-cost, portable electronic devices in future. In a paper published this week in the from AIP Publishing ("Microwave Flexible Transistors on Cellulose Nanofibrillated Fiber Substrates"), the researchers describe the biodegradable device. 
An array of microwave silicon transistors sitting on a wood-derived CNF substrate. (Image: Jung-Hun Seo, Shaoqin Gong and Zhenqiang Ma/University of Wisconsin-Madison) "We found that cellulose nanofibrillated fiber based transistors exhibit superior performance as that of conventional silicon-based transistors," said Zhenqiang Ma, the team leader and a professor of electrical and computer engineering at the UW-Madison. "And the bio-based transistors are so safe that you can put them in the forest, and fungus will quickly degrade them. They become as safe as fertilizer." Nowadays, the majority of portable electronics are built on non-renewable, non-biodegradable materials such as silicon wafers, which are highly purified, expensive and rigid substrates, but cellulose nanofibrillated fiber films have the potential to replace silicon wafers as electronic substrates in environmental friendly, low-cost, portable gadgets or devices of the future. Cellulose nanofibrillated fiber is a sustainable, strong, transparent nanomaterial made from wood. Compared to other polymers like plastics, the wood nanomaterial is biocompatible and has relatively low thermal expansion coefficient, which means the material won't change shape as the temperature changes. All these superior properties make cellulose nanofibril an outstanding candidate for making portable green electronics. To create high-performance devices, Ma's team employed silicon nanomembranes as the active material in the transistor -- pieces of ultra-thin films (thinner than a human hair) peeled from the bulk crystal and then transferred and glued onto the cellulose nanofibrill substrate to create a flexible, biodegradable and transparent silicon transistor. But to make portable electronics, the biodegradable transistor needed to be able to operate at microwave frequencies, which is the working range of most wireless devices. The researchers thus conducted a series of experiments such as measuring the current-voltage characteristics to study the device's functional performance, which finally showed the biodegradable transistor has superior microwave-frequency operation capabilities comparable to existing semiconductor transistors. "Biodegradable electronics provide a new solution for environmental problems brought by consumers' pursuit of quickly upgraded portable devices," said Ma. "It can be anticipated that future electronic chips and portable devices will be much greener and cheaper than that of today." Next, Ma and colleagues plan to develop more complicated circuit system based on the biodegradable transistors.
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An array of microwave silicon transistors sitting on a wood-derived CNF substrate. (Image: Jung-Hun Seo, Shaoqin Gong and Zhenqiang Ma/University of Wisconsin-Madison) "We found that cellulose nanofibrillated fiber based transistors exhibit superior performance as that of conventional silicon-based transistors," said Zhenqiang Ma, the team leader and a professor of electrical and computer engineering at the UW-Madison. "And the bio-based transistors are so safe that you can put them in the forest, and fungus will quickly degrade them. They become as safe as fertilizer." Nowadays, the majority of portable electronics are built on non-renewable, non-biodegradable materials such as silicon wafers, which are highly purified, expensive and rigid substrates, but cellulose nanofibrillated fiber films have the potential to replace silicon wafers as electronic substrates in environmental friendly, low-cost, portable gadgets or devices of the future. Cellulose nanofibrillated fiber is a sustainable, strong, transparent nanomaterial made from wood. Compared to other polymers like plastics, the wood nanomaterial is biocompatible and has relatively low thermal expansion coefficient, which means the material won't change shape as the temperature changes. All these superior properties make cellulose nanofibril an outstanding candidate for making portable green electronics. To create high-performance devices, Ma's team employed silicon nanomembranes as the active material in the transistor -- pieces of ultra-thin films (thinner than a human hair) peeled from the bulk crystal and then transferred and glued onto the cellulose nanofibrill substrate to create a flexible, biodegradable and transparent silicon transistor. But to make portable electronics, the biodegradable transistor needed to be able to operate at microwave frequencies, which is the working range of most wireless devices. The researchers thus conducted a series of experiments such as measuring the current-voltage characteristics to study the device's functional performance, which finally showed the biodegradable transistor has superior microwave-frequency operation capabilities comparable to existing semiconductor transistors. "Biodegradable electronics provide a new solution for environmental problems brought by consumers' pursuit of quickly upgraded portable devices," said Ma. "It can be anticipated that future electronic chips and portable devices will be much greener and cheaper than that of today." Next, Ma and colleagues plan to develop more complicated circuit system based on the biodegradable transistors.
Dipole moment The work will be of interest to those considering graphene elements in flexible touchscreens or memories that store bits by controlling electric dipole moments of carbon atoms, the researchers said. Perfect graphene – an atom-thick sheet of carbon – is a conductor, as its atoms’ electrical charges balance each other out across the plane. But curvature in graphene compresses the electron clouds of the bonds on the concave side and stretches them on the convex side, thus altering their electric dipole moments, the characteristic that controls how polarized atoms interact with external electric fields. The researchers who published their results this month in the American Chemical Society’s ("Flexoelectricity in Carbon Nanostructures: Nanotubes, Fullerenes, and Nanocones") discovered they could calculate the flexoelectric effect of graphene rolled into a cone of any size and length. The researchers used density functional theory to compute dipole moments for individual atoms in a graphene lattice and then figure out their cumulative effect. They suggested their technique could be used to calculate the effect for graphene in other more complex shapes, like wrinkled sheets or distorted fullerenes, several of which they also analyzed. “While the dipole moment is zero for flat graphene or cylindrical nanotubes, in between there is a family of cones, actually produced in laboratories, whose dipole moments are significant and scale linearly with cone length,” Yakobson said. Carbon nanotubes, seamless cylinders of graphene, do not display a total dipole moment, he said. While not zero, the vector-induced moments cancel each other out. That’s not so with a cone, in which the balance of positive and negative charges differ from one atom to the next, due to slightly different stresses on the bonds as the diameter changes. The researchers noted atoms along the edge also contribute electrically, but analyzing two cones docked edge-to-edge allowed them to cancel out, simplifying the calculations. Yakobson sees potential uses for the newly found characteristic. “One possibly far-reaching characteristic is in the voltage drop across a curved sheet,” he said. “It can permit one to locally vary the work function and to engineer the band-structure stacking in bilayers or multiple layers by their bending. It may also allow the creation of partitions and cavities with varying electrochemical potential, more ‘acidic’ or ‘basic,’ depending on the curvature in the 3-D carbon architecture.”
Carbon fiber composites are typically prized for their high strength and low weight, and carbon nanotube (CNT) composites (or nanocomposites), which have more and smaller carbon filaments, show promise for high strength as well as other properties such as the ability to conduct heat and electricity. However, according to NIST's Alex Liddle, an author on the study, while researchers previously could reliably measure a nanocomposite's bulk properties, they didn't know exactly why various formulations of the composite had different properties. "Figuring out why these materials have the properties they do requires a detailed, quantitative understanding of their complex 3-D structure," says Liddle. "We need to know not only the concentration of nanotubes but also their shape and position, and relate that to the properties of the material." Seeing the arrangement of carbon nanotubes in a composite material is tough, though, because they're surrounded by an epoxy resin which also is mostly carbon atoms. Even with sophisticated probes the contrast is too low for software image processors to pick them out easily. In such research situations, you turn to graduate students and postdocs like NIST's Bharath Natarajan, because humans generally make great image processors. But marking thousands of carbon nanotubes in an image is mighty boring, so Natarajan designed an image-processing algorithm that can distinguish CNTs from an epoxy resin as well as he can. It paid off. According to Liddle, a CNT expresses its full potential in strength and thermal and electrical conductivity when it is stretched out and straight, but … "When CNTs are suspended in an epoxy resin, they spread out, bundle and twist into different shapes," Liddle says. "Our analysis revealed that the benefits of CNTs increases in a non-linear fashion as their concentration increases. As the concentration raises, the CNTs come into contact, increasing the number of intersections, which increases their electrical and thermal conductivity, and the physical contact causes them to conform to one another, which straightens them, increasing the material's strength." The fact that increasing the concentration of CNTs enhances properties is not particularly surprising, but now researchers know how this affects the materials' properties and why earlier models of nanocomposite materials' performance never quite matched how they performed in practice. "We've really only seen the tip of the iceberg with respect to this class of material," says Liddle. "There are all sort of ways other researchers might slice and dice the data to model and eventually manufacture optimal materials for thermal management, mechanical reinforcement, energy storage, drug transport and other uses."
Drexel's microswimmer robots (bottom) are modeled, in form and motion, after the spiral-shaped bacteria, Borrelia burgdorferi (top), that cause Lyme Disease. DGIST, a government-funded research entity in Daegu, South Korea, is the leader of the 11-institution partnership, which includes some of the top engineers and roboticists in the world. Drexel’s team, the lone representatives from the United States, is already well on its way to tailoring robotic “microswimmer” technology for clearing arteries. “Microrobotics is still a rather nascent field of study, and very much in its infancy when it comes to medical applications,” Kim said. “A project like this, because it is supported by leading institutions and has such a challenging goal, is an opportunity to push both medicine and microrobotics into a new and exciting place.” Kim’s microswimmers are chains of three or more iron oxide beads, rigidly linked together via chemical bonds and magnetic force. These chains are small enough—on the order of nanometers—that they can navigate in the bloodstream like a tiny boat. The beads are put in motion by an external magnetic field that causes each of them to rotate. Because they are linked together, their individual rotations cause the chain to twist like a corkscrew and this movement propels the microswimmer. 
Using magnetic fields (visual representation at right) generated by an electromagnetic device (left) Drexel engineers are able to control the movement of their micro-swimmer robots. By controlling the magnetic field, Kim can direct the speed and direction of the microswimmers. The magnetism involve also allows the researchers to join separate strands of microswimmers together to make longer strings, which can then be propelled with greater force. This research, which was recently reported in the ("Minimal geometric requirements for micropropulsion via magnetic rotation"), is one of the reasons Kim’s lab was chosen for the ambitious project. “Our magnetically actuated microswimmer technology is the perfect fit for this project,” Kim said. “The microswimmers are composed of inorganic biodegradable beads so they will not trigger an immune response in the body. And we can adjust their size and surface properties to accurately deal with any type of arterial occlusion.” Kim’s inspiration for using the robotic swimmers as tiny drills actually came from a malicious bacterium that wreaks havoc inside the body by doing just that—burrowing through healthy tissue. Borrelia burgdorferi, the bacteria that causes Lyme’s Disease, is classified by its spiral shape, which enables both its movement and the resultant cellular destruction. DGIST researchers are planning to harness this behavior in the microswimmmers to lead the way for a vascular probe by loosening the arterial plaque that is causing the blockage. The probe, which looks like a tiny drill, is being designed by Bradley Nelson from ETH Zurich, a pioneer in the field of microrobotic surgery. The team’s plan is to use a catheter to deliver the microswimmers and the drill directly to the blocked artery. From there, the swimmers would push their way into the blockage, then the drill would clear it completely. Once flow is restored in the artery, the microswimmer chains could disperse and be used to deliver anti-coagulant medication directly to the effected area to prevent future blockage.
In this adiabatic elimination scheme, the movement of light through two outer waveguides is controlled via a "dark" middle waveguide that does not accumulate any light. (Image: Zhosia Rostomian, Berkeley Lab) Integrated electronic circuitry is approaching its limits because of heat dissipation and power consumption issues. Photonics, in which electrical signals moving through copper wires and cables are replaced by pulses of light carrying data over optical fibers, is a highly touted alternative, able to carry greater volumes of data at faster speeds, while giving off much less heat and using far less power. However, the crosstalk problem in coupled optical nanowaveguides has been a major technological roadblock. "When nanowaveguides in close proximity are coupled, the light in one waveguide impacts the other. This coupling becomes particularly severe when the separation is below the diffraction limit, placing a restriction on how close together the waveguides can be placed," Zhang says. "We have experimentally demonstrated an adiabatic elimination scheme that effectively cuts off the cross-talk between them, enabling on-demand dynamical control of the coupling between two closely packed waveguides. Our approach offers an attractive route for the control of optical information in integrated nanophotonics, and provides a new way to design densely packed, power-efficient nanoscale photonic components, such as compact modulators, ultrafast optical signal routers and interconnects." Zhang, who also holds an appointment with the Kavli Energy NanoSciences Institute (ENSI) at Berkeley, is the corresponding author of a paper describing this research in ("Adiabatic elimination based coupling control in densely packed subwavelength waveguides"). "A general approach to achieving active control in coupled waveguide systems is to exploit optical nonlinearities enabled by a strong control pulse," Zhang says. "However this approach suffers from the nonlinear absorption induced by the intense control pulse as the signal and its control propagate in the same waveguide." Zhang and his group turned to the adiabatic elimination concept, which has a proven track record in atomic physics and other research fields. The idea behind adiabatic elimination is to decompose large dynamical systems into smaller ones by using slow versus fast dynamics. "Picture three buckets side-by-side with the first being filled with water from a tap, the middle being fed from the first bucket though a hole while feeding the third bucket through another hole," says co-lead author Mrejen. "If the flow rate into the middle bucket is equal to the flow rate out of it, the second bucket will not accumulate water. This, in a basic manner, is adiabatic elimination. The middle bucket allows for some indirect control on the dynamics compared to the case in which water goes directly from the first bucket to the third bucket." Zhang and his research group apply this concept to a coupled system of optical nanowaveguides by inserting a third waveguide in the middle of the coupled pair. Only about 200 nanometers separate each of the three waveguides, a proximity that would normally generate too much cross-talk to allow for any control over the coupled system. However, the middle waveguide operates in a "dark" mode, in the sense that it doesn't seem to participate in the exchange of light between the two outer waveguides since it does not accumulate any light. "Even though the dark waveguide in the middle doesn't seem to be involved, it nonetheless influences the dynamics of the coupled system," says co-lead author Suchowski, who is now with the Tel Aviv University. "By judiciously selecting the relative geometries of the outer and intermediate waveguides, we achieve adiabatic elimination, which in turn enables us to control the movement of light through densely packed nanowaveguides. Until now, this has been almost impossible to do."
Proton permeation through flagellar motor stator complex MotA/B. Based on the model of the three-dimensional structure of MotA/B identified in this research, protons can permeate through the gate (green) of the motor by diffusion of hydronium ions (blue), which induces the formation of a water wire (red and white) that may mediate the proton transfer to the proton binding site (yellow). (Image: Yasutaka Nishihara and Akio Kitao) Bacteria such as and swim by rotating flagellar motors and filaments, which highly efficiently utilize the energy originating from the difference in ion concentration between the cell interior and exterior. Among the bacterial flagellar motors, some convert the energy by the permeation of protons through the motor stator, while others utilize sodium ions or multiple ions. However, the atomic structure of the bacterial flagellar motor remained unknown, and the mechanism of ion permeation had not been elucidated in detail. Project Researcher Nishihara Yasutaka at the Graduate School of Arts and Sciences and Associate Professor Akio Kitao at the Institute of Molecular and Cellular Biosciences constructed a three-dimensional model structure of the protein complex that comprises the flagellar motor stator MotA/B, and found that protons permeate through the transmembrane stator as hydronium ions, inducing a motion similar to a ratchet wrench (ratchet movement) limited to one directional rotation. Investigation of this type of highly efficient energy conversion mechanism is essential to understand biological mechanisms which can utilize energy efficiently.
The nuclear pore complex is comprised of several layered rings: the cytoplasmic ring (gold), the spoke ring within the pore (blue) and the nucleoplasmic ring (green). (Image: University of Zurich) (click on image to enlarge) "Molecular gate" discovered in the pore channel For their study, the scientists used shock-frozen specimens of clawed frog oocytes. With the aid of cryo-electron microscopes, Medalia's team was able to display the miniscule nuclear pores, which were merely a ten thousandth of a millimeter in diameter, at a considerably higher resolution than ever before. As a result, they uncovered new details: "We discovered a previously unobserved structure inside the nuclear pore that forms a kind of molecular gate, which can only be opened by molecules that hold the right key," explains Medalia. This "molecular gate" is the so-called spoke ring, which is sandwiched between two other rings and extends inside the nuclear pores. The gate itself consists of a fine lattice, which enables small molecules to slip through unobstructed. The new, high-resolution presentation of the nuclear pore structure leads to a better understanding of why certain molecules are allowed to pass through the nuclear pores while others are turned away. It also helps improve our understanding of the development of some diseases that involve a defective transportation to the nuclear pores - such as intestinal, ovarian and thyroid cancer.
Figure 1: Two interfering plane waves (yellow arrows) exert a force (red) and torque (blue) on a small particle (yellow sphere) perpendicular to the interfering waves. (Licensed under CC BY 3.0 © 2014 A. Y. Bekshaev et al.) Two-dimensional waves have been studied for centuries: initially to understand the intrinsic behavior of waves and more recently to understand the fundamental mechanics of quantum physics. “The interference between two plane waves has always provided an important model for understanding the basic features of waves,” notes Bliokh. “It is difficult to find a simpler and more thoroughly studied system in physics. We show that such a basic system still exhibits unexpected and unusual features.” Recent research has showed that interfering planar waves can have unusual properties on a small scale. For over a century, waves such as light beams have been known to carry both momentum and angular momentum in the direction of the propagating wave and this momentum can be used to move and rotate small particles. This is consistent with the common understanding of photons as particles carrying momentum and spin. On the local scale in non-plane-wave optical fields, however, light can also impart forces and torques perpendicular to the light beam, counterintuitive to our everyday experience. These unusual effects have been noticed in highly confined near-field radiation known as evanescent waves, but so far they have not turned up in freely propagating light waves. In a comprehensive theoretical study, the scientists, from the RIKEN Center for Emergent Matter Science and Interdisciplinary Theoretical Science Research Group (iTHES), revisited the concept of two propagating waves interfering in the same plane. Their mathematical analysis of this system revealed that even this well-studied example of interfering waves can exert a force and torque on a small particle perpendicular to both waves (Fig. 1). Both the force and torque are strongly dependent on the polarization of the two interfering waves, which differs to the conventional experience of waves carrying the same momentum irrespective of their polarizations. The possibility of realizing such an effect in an actual experimental system and to potentially control it through parameters such as polarization is attractive and, Nori predicts, practically feasible. “Our findings offer a new vision for the fundamental properties of propagating optical fields and pave the way for novel optical manipulations of small particles.”
With ultrasound, represented by curved red and blue lines, ETH Zurich researchers move micro-droplets for sorting and analysis. (Scheme: ETH Zurich / Ivo Leibacher) Laboratory experiments today tend to be wasteful. For example, in order to conduct diagnostic tests, liquids are mixed together in reaction vessels when all this task requires is a few nanolitres of liquids. With miniaturisation, it would be possible to have a higher throughput with less consumption of materials. When attempting to compress an entire experiment into the size of a chip, known as 'Lab on a Chip', there is one key question: how can minuscule amounts of liquid or individual cells be moved, merged and assessed in a controlled way? Ivo Leibacher and Peter Reichert, doctoral students at the Institute of Mechanical Systems, developed a system to move tiny droplets under the guidance of ETH Professor Jürg Dual. The concept is based on acoustophoresis, which uses a ultrasonic standing wave to move aqueous droplets through a carrier liquid of oil on a silicon-glass chip. The droplets, which have a diameter of 50 to 250 micrometres, cannot mix with the carrier liquid, nor can they evaporate. “On this scale, the droplets are very stable because they are held together by the surface tension,” Leibacher explains. 
On this small chip, researchers move droplets with the help of ultra sonic waves. (Photo: Ivo Leibacher / ETH Zürich) Toward the node When the ultrasonic standing waves are applied, the droplets move to the node of the wave. This means researchers can place two different droplets in both sides of the channel to merge them in a controlled manner. Changing the frequency, on the other hand, guides targeted drops with, for example, a light signal into a branched-off channel. By separating them in this way, they can be sorted and analysed after the conclusion of the experiment. "One of the advantages of our technology is its high biocompatibility and versatility," says Reichert. Previous methods in which researchers manipulated individual cells on a tiny scale resulted in cases of cells being damaged. This method can be used for cells as well as for DNA, reagents and chemicals.
The image shows corrosion of a silver-gold alloy spontaneously resulting in the formation of nanoscale porous structures that undergo high-speed cracking under the action of a tensile stress. It helps demonstrate a discovery by an Arizona State University research team about the stress-corrosion behavior of metals that threatens the mechanical integrity of engineered components and structures. (Provided by Karl Sieradzki/Arizona State University) Research led by Arizona State University materials science and engineering professor Karl Sieradzki is uncovering new knowledge about the causes of stress-corrosion cracking in alloys used in pipelines for transporting water, natural gas and fossil fuels -- as well as for components used in nuclear power generating stations and the framework of aircraft. Sieradzki is on the faculty of the School for Engineering of Matter, Transport and Energy, one of ASU's Ira A. Fulton Schools of Engineering. His research team's findings are detailed in an advance online publication on June 22 of the paper "Potential-dependent dynamic fracture of nanoporous gold" on the website of the journal ("Potential-dependent dynamic fracture of nanoporous gold"). Using advanced tools for ultra-high-speed photography and digital image correlation, the team has been able to closely observe the events triggering the origination of stress-corrosion fracture in a model silver-gold alloy and to track the speed at which cracking occurs. They measured cracks moving at speeds of 200 meters per second corresponding to about half of the shear wave sound velocity in the material. This is a remarkable result, Sieradzki said, given that typically only brittle materials such as glass will fracture in this manner and that gold alloys are among the most malleable metals. In the absence of a corrosive environment these gold alloys fail in the same manner as children's modeling clay, Sieradzki explained: Roll modeling clay into a cylindrical shape and you can stretch it by a by 100 percent before it slowly tears apart. In the presence of corrosive environments, silver is selectively dissolved from the alloy causing porosity to form (see photo). If this occurs while the alloy is stressed, then the material fails as if it were made of glass. These results provide a deeper understanding of the stress-corrosion behavior of metals such as aluminum alloys, brass and stainless steel that threatens the mechanical integrity of important engineered components and structures. The team's discoveries could provide a guide for "designing alloys with different microstructures so that the materials are resistant to this type of cracking," Sieradzki said.
Figure 1: 3D Gap-Plasmon Antenna Structure and the Simulation Results. (Image: KAIST) Focusing light into a point-like space is an active research field as it finds many applications. However, concentrating light into a smaller space than its wavelength is often hindered by diffraction. In order to tackle this problem, many researchers have utilized the plasmonic phenomenon in a metal where light can be confined to a greater extent by overcoming the diffraction limit. Many researchers focused on developing a two dimensional plasmonic antenna and were able to focus light under 5 nanometers. However, this two dimensional antenna reveals a challenge that the light disperses to the opposite end regardless of how small it was focused. For a solution, a three dimensional structure has to be employed in order to maximize the light intensity. Adopting the proximal focused-ion-beam milling technology, the KAIST research team developed a three dimensional 4 nanometer wide gap-plasmon antenna. By squeezing the photons into a three dimensional nano space of 4 x 10 x 10 nm
Figure 2: Constructed 3D Gap-Plasmon Antenna Structure. (Image: KAIST) This technology is expected to improve the speed of data transfer and processing up to the level of terahertz (one trillion times per second) and to enlarge the storage volume per unit area on hard disks by 100 times. In addition, high definition images of sub-molecule size can be taken with actual light, instead of using an electron microscope, while it can improve the semiconductor process to a smaller size of few nanometers. Professor Kim said, “A simple yet genuine idea has shifted the research paradigm from 2D gap-plasmon antennas to 3D antennas. This technology sees numerous applications including in the field of information technology, data storage, image medical science, and semiconductor process.”
(click on image to enlarge) The structure and movement of molecules is not feasible to observe via conventional microscopic methods. In order to get information about molecular shape and the orientation of their orbits, researchers use a process called high harmonic generation (HHG). To do this, a laser pulse tuned to a specific high frequency is directed into a jet of gas of the molecule being studied. When the pulse meets the jet of gas, plasma is generated which emits specific color light. This interaction with the molecule and light generation reveals what is called the highest-occupied molecular orbital (HOMO). The HOMO can be envisioned as the “shape” of the outside molecular orbits. The pulsed laser beam is converted to a high harmonic frequency which reaches the sensor where data from the interaction can be collected. What the researchers see allow them to gather information about the characteristics of molecule’s structure and dynamics. As useful as this technology is, researchers have been limited in what information they can obtain because they have been confined to observing the high harmonic frequency from a single laser pulse on a one-dimensional plane each time. To gather more information from the molecules during each test, Professor Yu’s team, have devised a method for resolving multiple molecular orbitals by using two-dimensional high-harmonic spectroscopy (HHS). This HHS process involves pulsing a laser at an ultra-fast interval through a polarizing lens which splits the beam in two. The team focused the laser through a thin crystal which split the beam into two polarized waves traveling in the same direction but now perpendicular to each other. When one beam traveling up and down while the other is moving side to side, the beams are moving orthogonally. When the two beams interacted with the gas sample, they revealed not only the HOMO, but simultaneously the HOMO-1, a lower lying molecular orbit. In the past these two orbits have been difficult to distinguish from one another, because HOMO-1 has been overshadowed by the more energetic HOMO. According to Yun, “In this work, we approached molecules in two dimensions. HOMO-1 can be revealed with relative ease in the orthogonal direction to the molecular axis, while HOMO does it in a parallel direction. Orthogonally polarized two waves enable us to probe both orbitals in two dimensions and to separate signals to different harmonic frequencies. Thus, we could resolve the signals from the two orbitals and could simultaneously obtained information on both orbitals.” After combining the data collected from each laser pulse the researchers were able to use a clever technique called tomography to piece the two-dimensional images together into a three-dimensional approximation. With the three-dimensional approximation, they were able to discern the shape and relative alignment of the HOMO and HOMO-1 orbitals, something that had never been done before. There is no loud applause, nobody waiting to congratulate Professor Yun on this achievement. “The ultimate goal” he says, “is to follow a chemical reaction in its own time scale. It leads us to have direct insight and to understand fundamental mechanism about transformations in molecular scale. We expect this method can be a route or be of help to achieve the goal.” This new method will advance future molecular research by allowing for independent and simultaneous observation of the structures and dynamics of multiple molecular orbitals. It will enable the observation of multi-orbital dynamics during chemical reactions of more complicated molecules.
A distinctive graphene framework structure coated by an in situ formed solid electrolyte interphase (SEI) with Li depositing in the pores as the anode of Li-S batteries. (© ACS) Among various promising battery candidates with high energy densities, Li-S batteries, with a high theoretical capacity of 1675 mAh g
Side-view optical image of bulk graphene framework on a flower. (© ACS) Based on this concept, Xin-Bing Cheng, a graduate student and the first author, proposed a nanostructured graphene framework with Li depositing to be a high-efficiency and high-stability Li metal anode for Li-S batteries. In a routine configuration of Li metal anode without graphene framework, Li dendrites easily grew on routine 2D substrates (such as Cu foil). As the root of dendrites can receive the electron easily and then dissolve earlier, Li dendrites easily fractured and were detached from the substrate to form dead Li. If there is a pre-existed conductive framework such as self-supported graphene foam, the deposited Li will be well accommodated. Free-standing graphene foam affords several promising features as underneath layer for Li anode, including (1) relative larger surface area than 2D substrates to lower the real specific surface current density and the possibility of dendrite growth, (2) interconnected framework to support and recycle dead Li, and (3) good flexibility to sustain the volume fluctuation during repeated incorporation/extraction of Li. "We hope that the wise combination of the nanoscale engineering and electrochemistry can help improve Coulombic efficiency and ion conductivity of Li metal anode for the applications of Li-S batteries," said Xin-Bing. Future research is required to investigate the diffusion of Li ions before and after crossing the SEI. The results indicated that nanoscale interfacial electrode engineering could be a promising strategy to tackle the intrinsic problems of lithium metal anodes and the concepts described herein shed a new light toward high-energy-density LMBs, such as Li-S and Li-O
Scanning electron microscope image of a self-assembled platinum lattice, false-colored to show the two-layer structure. Each inner square of the nanoscale grid is just 34 nanometers on each side. The scientists synthesized the materials at Brookhaven Lab's Center for Functional Nanomaterials (CFN) and characterized the nanoscale architectures using electron microscopy at CFN and x-ray scattering at the National Synchrotron Light Source—both DOE Office of Science User Facilities. The new technique relies on polymer self-assembly, where molecules are designed to spontaneously assemble into desired structures. Self-assembly requires a burst of heat to make the molecules snap into the proper configurations. Here, an intensely hot laser swept across the sample to transform disordered polymer blocks into precise arrangements in just seconds. "Self-assembled structures tend to automatically follow molecular preferences, making custom architectures challenging," said lead author Pawel Majewski, a postdoctoral researcher at Brookhaven. "Our laser technique forces the materials to assemble in a particular way. We can then build structures layer-by-layer, constructing lattices composed of squares, rhombuses, triangles, and other shapes." Laser-assembled nano-wires For the first step in grid construction, the team took advantage of their recent invention of laser zone annealing (LZA) to produce the extremely localized thermal spikes needed to drive ultra-fast self-assembly. To further exploit the power and precision of LZA, the researchers applied a heat-sensitive elastic coating on top of the unassembled polymer film. The sweeping laser's heat causes the elastic layer to expand—like shrink-wrap in reverse—which pulls and aligns the rapidly forming nanoscale cylinders. "The end result is that in less than one second, we can create highly aligned batches of nano-cylinders," said study coauthor Charles Black, who leads the Electronic Nanomaterials group at CFN. "This order persists over macroscopic areas and would be difficult to achieve with any other method." 
Scanning electron microscope image of a three-layer platinum mesh. The colored inset shows each distinct layer of the nanoscale grid. To make these two-dimensional grids functional, the scientists converted the polymer base into other materials. One method involved taking the nano-cylinder layer and dipping it into a solution containing metal salts. These molecules then glom onto the self-assembled polymer, converting it into a metallic mesh. A wide range of reactive or conductive metals can be used, including platinum, gold, and palladium. They also used a technique called vapor deposition, where a vaporized material infiltrates the polymer nano-cylinders and transforms them into functional nano-wires. Layer-by-layer lattice The first completed nano-wire array acts as the foundation of the full lattice. Additional layers, each one following variations on that same process, are then stacked to produce customized, crisscrossing configurations—like chain-link fences 10,000 times thinner than a human hair. "The direction of the laser sweeping across each unassembled layer determines the orientation of the nano-wire rows," Yager said. "We shift that laser direction on each layer, and the way the rows intersect and overlap shapes the grid. We then apply the functional materials after each layer forms. It's an exceptionally fast and simple way to produce such precise configurations." 
Illustration of the experiment showing the sweeping laser inducing intense heat that both accelerates polymer self-assembly and precisely aligns the nano-cylinders that form the foundation of the final grid. Study coauthor Atikur Rahman, a CFN postdoctoral researcher, added, "We can stack metals on insulators, too, embedding different functional properties and interactions within one lattice structure. "The size and the composition of the mesh make a huge difference," Rahman continued. "For example, a single layer of platinum nano-wires conducts electricity in only one direction, but a two-layer mesh conducts uniformly in all directions." LZA is precise and powerful enough to overcome interface interactions, allowing it to drive polymer self-assembly even on top of complex underlying layers. This versatility enables the use of a wide variety of materials in different nanoscale configurations. "We can generate nearly any two-dimensional lattice shape, and thus have a lot of freedom in fabricating multi-component nanostructures," Yager said. "It's hard to anticipate all the technologies this rapid and versatile technique will allow."
The MIT researchers' prototype for a chip measuring 3 millimeters by 3 millimeters. The magnified detail shows the chip's main control circuitry, including the startup electronics; the controller that determines whether to charge the battery, power a device, or both; and the array of switches that control current flow to an external inductor coil. This active area measures just 2.2 millimeters by 1.1 millimeters. (click on image to enlarge) Moreover, the researchers’ chip achieves those efficiency improvements while assuming additional responsibilities. Where its predecessors could use a solar cell to either charge a battery or directly power a device, this new chip can do both, and it can power the device directly from the battery. All of those operations also share a single inductor — the chip’s main electrical component — which saves on circuit board space but increases the circuit complexity even further. Nonetheless, the chip’s power consumption remains low. “We still want to have battery-charging capability, and we still want to provide a regulated output voltage,” says Dina Reda El-Damak, an MIT graduate student in electrical engineering and computer science and first author on the new paper. “We need to regulate the input to extract the maximum power, and we really want to do all these tasks with inductor sharing and see which operational mode is the best. And we want to do it without compromising the performance, at very limited input power levels — 10 nanowatts to 1 microwatt — for the Internet of things.” The prototype chip was manufactured through the Taiwan Semiconductor Manufacturing Company's University Shuttle Program. Ups and downs The circuit’s chief function is to regulate the voltages between the solar cell, the battery, and the device the cell is powering. If the battery operates for too long at a voltage that’s either too high or too low, for instance, its chemical reactants break down, and it loses the ability to hold a charge. To control the current flow across their chip, El-Damak and her advisor, Anantha Chandrakasan, the Joseph F. and Nancy P. Keithley Professor in Electrical Engineering, use an inductor, which is a wire wound into a coil. When a current passes through an inductor, it generates a magnetic field, which in turn resists any change in the current. Throwing switches in the inductor’s path causes it to alternately charge and discharge, so that the current flowing through it continuously ramps up and then drops back down to zero. Keeping a lid on the current improves the circuit’s efficiency, since the rate at which it dissipates energy as heat is proportional to the square of the current. Once the current drops to zero, however, the switches in the inductor’s path need to be thrown immediately; otherwise, current could begin to flow through the circuit in the wrong direction, which would drastically diminish its efficiency. The complication is that the rate at which the current rises and falls depends on the voltage generated by the solar cell, which is highly variable. So the timing of the switch throws has to vary, too. Electric hourglass To control the switches’ timing, El-Damak and Chandrakasan use an electrical component called a capacitor, which can store electrical charge. The higher the current, the more rapidly the capacitor fills. When it’s full, the circuit stops charging the inductor. The rate at which the current drops off, however, depends on the output voltage, whose regulation is the very purpose of the chip. Since that voltage is fixed, the variation in timing has to come from variation in capacitance. El-Damak and Chandrakasan thus equip their chip with a bank of capacitors of different sizes. As the current drops, it charges a subset of those capacitors, whose selection is determined by the solar cell’s voltage. Once again, when the capacitor fills, the switches in the inductor’s path are flipped. “In this technology space, there’s usually a trend to lower efficiency as the power gets lower, because there’s a fixed amount of energy that’s consumed by doing the work,” says Brett Miwa, who leads a power conversion development project as a fellow at the chip manufacturer Maxim Integrated. “If you’re only coming in with a small amount, it’s hard to get most of it out, because you lose more as a percentage. [El-Damak’s] design is unusually efficient for how low a power level she’s at.” “One of the things that’s most notable about it is that it’s really a fairly complete system,” he adds. “It’s really kind of a full system-on-a chip for power management. And that makes it a little more complicated, a little bit larger, and a little bit more comprehensive than some of the other designs that might be reported in the literature. So for her to still achieve these high-performance specs in a much more sophisticated system is also noteworthy.”
Xenon islands on a copper substrate (Cu 111), simulation. (Image: SISSA) (Nano)islands that slide freely on a sea of copper, but when they become too large (and too dense) they end up getting stuck: that nicely sums up the system investigated in a study just published in Nature Nanotechnology. "We can suddenly switch from a state of superlubricity to one of extremely high friction by varying some parameters of the system being investigated. In this study, we used atoms of the noble gas xenon bound to one another to form two-dimensional islands, deposited on a copper surface (Cu 111). "At low temperatures these aggregates slide with virtually no friction," explains Giampaolo Mistura of the University of Padua. "We increased the size of the islands by adding xenon atoms and until the whole available surface was covered the friction decreased gradually. Instead, when the available space ran out and the addition of atoms caused the islands to compress, then we saw an exceptional increase in friction." The study was divided into an experimental part (mainly carried out by the University of Padua and Nano-Cnr/University of Modena and Reggio Emilia) and a theoretical part (based on computer models and simulations) conducted by SISSA/Iom-Cnr-Democritos/ICTP. "To understand what happens when the islands are compressed, we need to appreciate the concept of 'interface commensurability'," explains Roberto Guerra, researcher at the International School for Advanced Studies (SISSA) in Trieste and among the authors of the study. 
Sample of crystalline copper used as a ‘sliding’ substrate. (Image: Nano-Cnr, Modena) "We can think of the system we studied as one made up of Lego bricks. The copper substrate is like a horizontal assembly of bricks and the xenon islands like single loose bricks," comments Guido Paolicelli of the CNR Nanoscience Institute. "If the substrate and the islands consist of different bricks (in terms of width and distance between the studs), the islands will never get stuck on the substrate. This situation reproduces our system at temperatures slightly above absolute zero where we observe a state of superlubricity with virtually no friction. However, the increase in surface of the islands and the resulting compression of the material causes the islands to become commensurate to the substrate – like Lego bricks having the same pitch – and when that happens they suddenly get stuck." The study is the first to demonstrate that it is possible to dramatically vary the sliding properties of nano-objects. "We can imagine a number of applications for this," concludes Guerra. "For example, nanobearings could be developed that, under certain conditions, are capable of blocking their motion, in a completely reversible manner."
Simplified sketch of electron oscillations (plasmons) at the metal/air interface. Orange and yellow clouds indicate regions with lower and higher electron concentration, respectively. Arrows show electric field lines in and outside of the metal. (Image: Hans-Peter Wagner and Masoud Kaveh-Baghbadorani, CC BY-ND) These free electrons collectively start to oscillate with respect to the fixed position of positively charged nuclei in the metal lattice. Like the density of air molecules in a sound wave, the electron density fluctuates in the metal lattice as a plasmon wave. Visible light, which has a wavelength of approximately half a micrometer, can thus be concentrated by a factor of nearly 100 to travel through metal films just a few nanometers (nm) thick. That’s 1,000 times smaller than a human hair. The new mixed light-electron-wave-state empowers intense light-matter interactions with unprecedented optical properties. What can plasmonics do? Plasmonics could revolutionize the way computers or smartphones transfer data within their electronic integrated circuits. Data transfer in current electronic integrated circuits happens via the flow of electrons in metal wires. In plasmonics, it’s due to oscillatory motion about the positive nuclei. Data transfer is therefore more time-consuming in the old technology. Since plasmonic data transfer happens with light-like waves and not with a flow of electrons (electrical current) as in conventional metal wires, the data transmission would be superfast (close to the speed of light) – similar to present glass fiber technologies. But plasmonic metal films are more than 100 times thinner than glass fibers. This could lead to faster, thinner and lighter information technologies. Surface plasmons also are exceptionally sensitive to any material next to the metal film. A low concentration of atoms, molecules or bacteria bound to the metal surface can change the property of its plasmons. This feature can be used for biological and chemical sensing at extremely low concentrations – for instance, to examine polluted water. If properly designed, multilayers of plasmonic metal/insulator nanostructures form artificial metamaterials, where the Greek word “meta” means “beyond.” Unlike any other material in nature, these metamaterials have a negative index of refraction. That’s a measure of how much light changes its direction when it enters a transparent insulator. Insulators, including glass, have a positive refractive index; they bend light that enters at a certain angle closer to perpendicular to the insulator surface. 
Light changes its direction when it enters a transparent insulator with positive refractive index or a metamaterial with negative refractive index. (Image: Hans-Peter Wagner and Masoud Kaveh-Baghbadorani, CC BY-ND) In contrast, multilayered metamaterials bend light to the “opposite” direction. This fascinating property can be used to cloak objects by covering them with a metamaterial wrap. The foil guides the light smoothly around the object instead of reflecting it. Almost unbelievably, the cloaked object becomes invisible. Other applications include optical superlenses with significantly higher resolution compared to regular optical microscopes. They could allow scientists to see objects as small as about 100 nm in size. That’s about one-tenth as big as a typical germ. A few proof-of-principle optical cloaks and superlenses do exist. But high resistivity losses in the metal layers which convert the light-electron-wave energy into heat currently limit the feasibility of many applications. 
Simplified sketch of a plasmonic metal/organic/semiconductor nanowire heterostructure. The emission from the nanowire generated by the exciting laser beam is used as an energy pump to compensate for resistivity losses in the metal shell. An organic spacer layer of few 10 nm thickness is inserted to control this energy transfer. (Image: Hans-Peter Wagner and Masoud Kaveh-Baghbadorani, CC BY-NC-ND) Manufacturing plasmonic nanowires High resistivity losses are the major issue with plasmonics. To overcome these limitations, we design and fabricate unique plasmonic metal/organic/semiconductor nanowire heterostructures. Our goal is to excite the semiconductor nanowires with an external light source, then use the internal radiation in the nanowires as an energy-pump source to compensate for metallic losses. This way, the nanowires couple light energy in concert with the light-electron-oscillations to the metal film, thus restoring the amplitude of the damped plasmon wave. We use the organic molecular beam deposition (OMBD) method to coat the semiconductor nanowires with metal/organic multilayers. In the OMBD chamber, organic and metal materials reside in heatable cylindrical cells. We evaporate both organic molecules and metal atoms in heated cells at ultra-high vacuum (which is hundreds of billion times lower than atmosphere pressure). Then we direct the molecular and atom beams we have produced toward the semiconductor nanowire sample. The thickness of the resulting deposited film on the nanowire is controlled by mechanical shutters at the cell openings. 
Transmission electron microscope (HRTEM) image of a GaAs-AlGaAs core-shell nanowire coated with nominally 10 nm aluminum quinoline and a 5 to 10 nm thick gold cluster film on top. (Image: Melodie Fickenscher (Advanced Materials Characterization Center College of Engineering and Applied Science) University of Cincinnati, CC BY-ND) The energy-transfer processes from the optically excited semiconductor nanowire to the plasmon oscillations in the surrounding metal film are studied with ultrafast spectroscopic techniques ("Exciton emission from hybrid organic and plasmonic polytype InP nanowire heterostructures"). Results from our studies will provide a new understanding of light-electron-waves in the novel and unique metal-semiconductor environment. Hopefully, we will open new prospects for designing low-loss or loss-free plasmonic devices. Ideally we want to enable new and important applications in information technologies, biological sensing and national defense. We further envision our investigations having a strong impact in other research fields: for instance, by utilizing the biocompatibility of our hybrid organic/metal structures, by enhancing the light emission in light-emitting diodes and laser structures or by improving light harvesting in photovoltaic devices.
Organic solar cells made of metal-organic frameworks are highly efficient in producing charge carriers. (Figure: Wöll/KIT) “We have opened the door to a new room,” says Professor Christof Wöll, Director of KIT Institute of Functional Interfaces (IFG). “This new application of metal-organic framework compounds is the beginning only. The end of this development line is far from being reached,” the physicist emphasizes. Metal-organic frameworks, briefly called MOFs, consist of two basic elements, metal node points and organic molecules, which are assembled to form microporous, crystalline materials. For about a decade, MOFs have been attracting considerable interest of researchers, because their functionality can be adjusted by varying the components. “A number of properties of the material can be changed,” Wöll explains. So far, more than 20,000 different MOF types have been developed and used mostly for the storage or separation of gases. The team of scientists under the direction of KIT has now produced MOFs based on porphyrines. These porphyrine-based MOFs have highly interesting photophysical properties: Apart from a high efficiency in producing charge carriers, a high mobility of the latter is observed. Computations made by the group of Professor Thomas Heine from Jacobs University Bremen, which is also involved in the project, suggest that the excellent properties of the solar cell result from an additional mechanism – the formation of indirect band gaps – that plays an important role in photovoltaics. Nature uses porphyrines as universal molecules e.g. in hemoglobin and chlorophyll, where these organic dyes convert light into chemical energy. A metal-organic solar cell produced on the basis of this novel porphyrine-MOF is now presented by the researchers in the journal Angewandte Chemie (Applied Chemistry). The contribution is entitled “Photoinduzierte Erzeugung von Ladungsträgern in epitaktischen MOF-Dünnschichten: hohe Leistung aufgrund einer indirekten elektronischen Bandlücke?“ (photo-induced generation of charge carriers in epitactic MOF-thin layers: high efficiency resulting from an indirect electronic band gap?). “The clou is that we just need a single organic molecule in the solar cell,” Wöll says. The researchers expect that the photovoltaic capacity of the material may be increased considerably in the future by filling the pores in the crystalline lattice structure with molecules that can release and take up electric charges. By means of a process developed at KIT, the crystalline frameworks grow in layers on a transparent, conductive carrier surface and form a homogeneous thin film, so-called SURMOFs. “The SURMOF process is suited in principle for a continuous manufacturing process and also allows for the coating of larger plastic carrier surfaces,” Wöll says. Thanks to their mechanical properties, MOF thin films of a few hundred nanometers in thickness can be used for flexible solar cells or for the coating of clothing material or deformable components. While the demand for technical systems converting sunlight into electricity is increasing, organic materials represent a highly interesting alternative to silicon that has to be processed at high costs before it can be used for the photoactive layer of a solar cell.
Under a magnetic field, 1D arrays of protein-encased superparamagnetic nanoparticles reversibly assemble into bundles. (© American Chemical Society) The lateral assembly of superparamagnetic nanoparticles (SNPs) into 1D structures has been predicted theoretically, but scientists have found it difficult to synthesize assemblies that are both thermodynamically stable and sufficiently rigid to inhibit entanglement. Yet such 1D SNP arrays continue to attract broad scientific attention due to their potentially useful properties such as a large magnetic susceptibility in one dimension, which could allow them to self-assemble under a moderate magnetic field, and a thermally fluctuating magnetic spin under ambient conditions that makes them magnetically isotropic—features that are not found in more conventional ferromagnetic nanoparticles. Taking an unusual approach, Daigo Miyajima, Takuzo Aida and colleagues from the RIKEN Center for Emergent Matter Science and other institutions in Japan have now succeeded where others have not by encasing the nanoparticles in protein nanotubes. The researchers formed their protein nanotubes by a process called supramolecular polymerization using the barrel-shaped protein, GroEL. This polymerization results in the formation of micrometer-long protein nanotubes with hydrophobic internal cavities housing the SNPs. These protein ‘jackets’ both protect the superparamagnetic nanoparticles, which are formed from iron oxide, and permit the self-assembly of ordered 1D nanoparticle arrays. In the 1D structure, the magnetic moments of the individual SNPs arrange into a tip-to-tip configuration that induces a large magnetic moment along the long axis of the 1D SNP array. Under a magnetic field, this causes the 1D structures to assemble into thick bundles (Fig.). The protein jackets provide the physical separation needed to allow the bundles to completely dissociate when the magnetic field is turned off to give the original, isolated 1D arrays. The bundles formed are also quite selective, with arrays of similar lengths tending to bundle together. These characteristics give the system excellent homogeneity and high stability under ambient conditions. “The most interesting aspect of our system is that the nanoparticle arrays reside inside the protein nanotubes,” says Miyajima. “We envisage broadening this concept to functional materials, dual-responsive materials and even a magneto-induced gelation system. Also, because the magnetic field is a non-invasive directing force, we are interested in making biomaterials, such as smart magnetic resonance imaging contrast agents controlled by chemical signals.”
The CVD process begins with tanks containing an initiator material (red) and one or more monomers (purple and blue), which are the building blocks of the desired polymer coating. These are vaporized, either by heating them or reducing the pressure, and are then introduced into a vacuum chamber containing the material to be coated. The initiator helps to speed up the process in which the monomers link up in chains to form polymers on the surface of the substrate material. (Illustration courtesy of Karen Gleason) (click on image to enlarge) This vapor deposition of polymers has opened the door to a variety of materials that would be difficult, and in some cases impossible, to produce in any other way. For example, many useful polymers, such as water-shedding materials to protect industrial components or biological implants, are made from precursors that are not soluble, and thus could not be produced using conventional solution-based methods. In addition, says Gleason, the Alexander and I. Michael Kasser Professor at MIT, the CVD process itself induces chemical reactions between coatings and substrates that can strongly bond the material to the surface. Gleason’s work on polymer-based CVD began in the 1990s, when she did experiments with Teflon, a compound of chlorine and fluorine. That work led to a now-burgeoning field detailed in a new book Gleason edited, titled “CVD Polymers: Fabrication of Organic Surfaces and Devices” (Wiley, 2015). At the time, the thinking was that the only way to make CVD work with polymer materials was by using plasma — an electrically charged gas — to initiate the reaction. Gleason tried to carry out experiments to prove this, beginning by running a control experiment without the plasma in order to demonstrate how important it was for making the process work. Instead, her control experiment worked just fine with no plasma at all, proving that for many polymers this step was not necessary. But the equipment Gleason used allowed the temperature of the gas to be controlled separately from that of the substrate; having the substrate cooler turned out to be key. She went on to demonstrate the plasma-free process with more than 70 different polymers, opening up a whole new field of research. The process can require a lot of fine-tuning, but is fundamentally a simple set of steps: The material to be coated is placed inside a vacuum chamber — which dictates the maximum size of objects that can be coated. Then, the coating material is heated, or the pressure around it is reduced until the material vaporizes, either inside the vacuum chamber or in an adjacent area from which the vapor can be introduced. There, the suspended material begins to settle onto the substrate material and form a uniform coating. Adjusting the temperature and duration of the process makes it possible to control the thickness of the coating. With metals or metal compounds, such as those used in the semiconductor industry, or the silvery coatings inside snack bags, the heated metal vapor deposits on a cooler substrate. In the polymer process, it’s a bit more complex: Two or more different precursor compounds, called monomers, are introduced into the chamber, where they react to form polymers as they deposit on the surface. Even high-temperature CVD processing has evolved, with great potential for commercial applications. For example, the research group of John Hart, an associate professor of mechanical engineering, has built a roll-to-roll processing system using CVD to make sheets of graphene, a material with potential applications ranging from large-screen displays to water-filtration systems. Hart’s group and others have used CVD to produce large arrays of carbon nanotubes, materials with potential as new electrodes for batteries or fuel cells. “It’s a very versatile and widely used manufacturing process,” Hart says, “and a very general process that can be tailored to many different applications.” One great advantage of CVD processing is that it can create coatings of uniform thickness even over complex shapes. For example, CVD can be used to uniformly coat carbon nanotubes — tiny cylinders of pure carbon that are far more slender than a hair — such as to modify their mechanical properties and make them react chemically to certain substances. “By combining two CVD processes — one to grow the carbon nanotubes, and another to coat the nanotubes — we have a scalable way to manufacture nanomaterials with new properties,” Hart says. Much progress in CVD research in recent years traces back to Gleason’s unexpected discovery, back in the 1990s, that the process could work without plasma — and her follow-up on that finding. “You need to pay attention when a new thing happens,” she says. “That’s sort of the key.”
Snapshot of correlation of particle structure and dynamics at density of 0.97. Disks are colored according to the following criteria: white, low mobility and high order; black, high mobility and low order; cyan, low mobility and low order; and magenta, high mobility and high order. (Image: John Russo, Hajime Tanaka) Normally, a liquid changes to a solid when its temperature becomes lower than the melting point. However, some materials remain liquid even below the melting point, finally solidifying with further cooling (supercooling) at what is called the glass-transition point. Despite intensive research over the years, its physical mechanism has remained elusive. One possibility is that increasing structural order develops in a supercooled liquid upon cooling, increasing the size of that structure and thus slowing down the dynamics and leading to the glass transition. Because the structure of liquids that undergo a glass transition is disordered, it was difficult to detect fluctuations of such a structure, but a new method has been proposed recently. This method does not depend on the type of liquid structure and has attracted much attention as it may enable extraction of structure size, which is key to understanding slow dynamics, for all liquids. The research group of Professor Hajime Tanaka and Project Research Associate John Russo at the Institute of Industrial Science, the University of Tokyo, were only able to retrieve the separation distance of two particles using this method, finding instead that this method fails at extracting the correlation between more than two particles (many-body correlations) which are key for understanding the glass transition. In a liquid composed of disk-shaped particles that do not deform no matter how much force is applied (a hard disc liquid), it is apparent that the dynamics of the liquid are dominated by a hexagonal lattice structure that is impossible to extract using this method. “These findings not only support the physical mechanism proposed by this group that slow glassy dynamics is a consequence of the development of structural fluctuations in a supercooled liquid, but also provides a new insight into the liquid phase, which was believed to be uniform and random, and leads to a deeper understanding of the very nature of the supercooled liquid state,” says Professor Tanaka.