Introducing flaws into liquid crystals by inserting microspheres and then controlling them with electrical fields: that, in a nutshell, is the rationale behind a method that could be exploited for a new generation of advanced materials, potentially useful for optical technologies, electronic displays and e-readers. A team of scientists (including research fellows at the International School for Advanced Studies, SISSA, in Trieste) has just published a paper in the journal ("Field-controlled columnar and planar patterning of cholesteric colloids") where they describe just how this approach works and provide the results of a computer simulation. 
Colloids in liquid crystals. “Generally, flaws are the last thing you’d want in a liquid crystal”, explains Giuseppe D’Adamo, postdoctoral fellow at SISSA. “However, this new method allows us to exploit the defects in the material to our advantage”. D’Adamo is first author of a paper just published in Physical Review Letters. The study made computer models of colloidal suspensions in liquid crystals subjected to electrical fields modulated over time. Colloids are particles in suspension (i.e., a condition halfway between dispersion and solution) in a liquid. These composite materials have been receiving plenty of attention for their optical properties for some time now, but the use of electrical fields to modify them at will is an absolute novelty. “Our simulations demonstrate that by switching on or off an electrical field of appropriate intensity we can re-order the colloids by arranging them into columns or planes”, comments Cristian Micheletti of SISSA, co-author of the paper. “This easy-to-control plasticity could make the material suitable for optical-electronic devices such as e-readers, for example”. Liquid crystals are particular types of liquids. In a normal liquid, molecules have no systematic arrangement and, viewed from any angle, they always appear the same. The molecules forming liquid crystals, by contrast, are arranged in precise patterns often dictated by their shape. To get an idea of what happens in a liquid crystal, imagine a fluid made up of tiny needles which, instead of being arranged chaotically, all point in the same direction. This also means that if we look at the liquid from different viewpoints it will change in appearance, for example it might appear lighter or darker (have you ever seen this happen in LCD monitors, especially the older models?). “The useful natural tendency of liquid crystal molecules to spontaneously arrange themselves in a certain pattern can be counteracted by introducing colloids in the fluid. In our case, we used microscopic spherical particles, which ‘force’ the molecules coming into contact with their surface to adapt and rotate in a different direction” explains D’Adamo. “This creates ‘defect lines’ in the material, i.e., circumscribed variations in the orientation of molecules which result in a local change in the optical properties of the medium”. More in detail... These defect lines have an important effect: they enable remote interactions among colloidal particles, by holding them together as if they were thin strings. “Liquid crystal molecules tend to align along the electrical field. By switching the field on and off we create competition between the spontaneous order of the liquid crystal, the order dictated by the surface of the colloidal particles and, finally, the order created by the electrical potential”, says Micheletti. “This competition produces many defect lines that act on the colloids by moving them or clustering them”. “It’s a bit like pulling the invisible strings of a puppet: by carefully modulating the electrical fields we can, in principle, make all the particles move and arrange them as we like, by creating defect lines with the shape we want” continues D’Adamo. “An important detail is that the colloidal configurations are metastable, which means that once the electrical field has been switched off the colloids remain in their last position for a very long time”. In brief, this implies that the system only requires energy when it changes configuration, a major saving. “In this respect, the method works like the electronic ink used in digital readers, and it would be interesting to explore its applicability in this sense”, concludes Micheletti. The study, carried out with the collaboration of SISSA, the University of Edinburgh and the University of Padova, has been included as an Editors’ Suggestion among the Highlights of the journal .
read more "Desirable defects in liquid crystals"
Colloids in liquid crystals. “Generally, flaws are the last thing you’d want in a liquid crystal”, explains Giuseppe D’Adamo, postdoctoral fellow at SISSA. “However, this new method allows us to exploit the defects in the material to our advantage”. D’Adamo is first author of a paper just published in Physical Review Letters. The study made computer models of colloidal suspensions in liquid crystals subjected to electrical fields modulated over time. Colloids are particles in suspension (i.e., a condition halfway between dispersion and solution) in a liquid. These composite materials have been receiving plenty of attention for their optical properties for some time now, but the use of electrical fields to modify them at will is an absolute novelty. “Our simulations demonstrate that by switching on or off an electrical field of appropriate intensity we can re-order the colloids by arranging them into columns or planes”, comments Cristian Micheletti of SISSA, co-author of the paper. “This easy-to-control plasticity could make the material suitable for optical-electronic devices such as e-readers, for example”. Liquid crystals are particular types of liquids. In a normal liquid, molecules have no systematic arrangement and, viewed from any angle, they always appear the same. The molecules forming liquid crystals, by contrast, are arranged in precise patterns often dictated by their shape. To get an idea of what happens in a liquid crystal, imagine a fluid made up of tiny needles which, instead of being arranged chaotically, all point in the same direction. This also means that if we look at the liquid from different viewpoints it will change in appearance, for example it might appear lighter or darker (have you ever seen this happen in LCD monitors, especially the older models?). “The useful natural tendency of liquid crystal molecules to spontaneously arrange themselves in a certain pattern can be counteracted by introducing colloids in the fluid. In our case, we used microscopic spherical particles, which ‘force’ the molecules coming into contact with their surface to adapt and rotate in a different direction” explains D’Adamo. “This creates ‘defect lines’ in the material, i.e., circumscribed variations in the orientation of molecules which result in a local change in the optical properties of the medium”. More in detail... These defect lines have an important effect: they enable remote interactions among colloidal particles, by holding them together as if they were thin strings. “Liquid crystal molecules tend to align along the electrical field. By switching the field on and off we create competition between the spontaneous order of the liquid crystal, the order dictated by the surface of the colloidal particles and, finally, the order created by the electrical potential”, says Micheletti. “This competition produces many defect lines that act on the colloids by moving them or clustering them”. “It’s a bit like pulling the invisible strings of a puppet: by carefully modulating the electrical fields we can, in principle, make all the particles move and arrange them as we like, by creating defect lines with the shape we want” continues D’Adamo. “An important detail is that the colloidal configurations are metastable, which means that once the electrical field has been switched off the colloids remain in their last position for a very long time”. In brief, this implies that the system only requires energy when it changes configuration, a major saving. “In this respect, the method works like the electronic ink used in digital readers, and it would be interesting to explore its applicability in this sense”, concludes Micheletti. The study, carried out with the collaboration of SISSA, the University of Edinburgh and the University of Padova, has been included as an Editors’ Suggestion among the Highlights of the journal .
Two nanocylinders produce resonant electric (E) and magnetic (H) fields when excited with visible light. Earlier theoretical work had predicted the existence of such magnetic hot spots, but this is the first time that they have been observed experimentally with visible light in a nanodimer configuration (see image), according to lead author Reuben Bakker from the A*STAR Data Storage Institute. The researchers numerically calculated the expected electric and magnetic resonances and found good agreement with the experimental results. The use of light to carry information, known as photonics, is critical to the continued growth of information technology. Unfortunately, the diffraction limit of light restricts it from being directed at dimensions smaller than half its wavelength, which imposes a limit on the minimum sizes of photonics-based devices. The use of plasmon resonances in metals — resonant collective oscillations of conduction electrons — has been proposed as a way to overcome this limit. However, metals that support plasmons are often ‘lossy’, which means that the distance the light can travel in them is quite limited. “Typically in metal photonics, researchers have been studying the electric field,” says Bakker. “But we are now looking at materials in the subwavelength regime (below the diffraction limit), where we can create and manipulate the magnetic field as well. Essentially, the electric field creates a current loop inside the nanoparticle and this current loop creates the magnetic resonance.” Being able to manipulate the magnetic field close to the nanodimer provides “another lever to pull so that light does what we want it to do,” says Bakker. To exploit this effect, the nanoparticles need to be made of a high-dielectric-constant material, such as silicon. “We’ve taken the silicon direction because it has a high refractive index and doesn’t have the losses that metals do,” says Bakker. “But silicon may not be the final answer. We know how to work with silicon because of the integrated circuit industry and it is good — but is it the best? We’re still figuring that out.” Bakker sees this work as a step toward more complex systems that could potentially end up as being nanoantennas or waveguide systems. “This nanodimer is an intermediary — it’s not the most useful device in itself. We have to build up our understanding of these systems on an incremental basis,” he says.
Analysis of the electric field inside a semiconductor microparticle containing a metal nanoparticle reveals enhanced absorption of sunlight. Photocatalysts accelerate chemical reactions by absorbing light from the sun and using the energy to drive reactions on their surfaces. They are attractive for environmentally friendly applications such as generating hydrogen from water and breaking down pollutants. Experimental studies have shown that microspheres made from metal-oxide semiconductors and embedded with metal nanoparticles are particularly effective photocatalysts, but researchers have been uncertain about why this was the case. Now, Ping Bai and his colleagues at the A*STAR Institute of High Performance Computing in Singapore have performed computer simulations that reveal what makes these structures such effective photocatalysts. Their study also provides scientists with helpful guidelines for designing plasmonic photocatalysts. Bai and his colleagues used a widely employed computational technique known as the finite element method to analyze how light interacts with a semiconductor microparticle containing a single metal nanoparticle. Their analysis revealed that the refractive index difference between the semiconductor and the catalytic medium sets up an interference pattern within the semiconductor microparticle. This interference enhances the light absorption of the embedded metal nanoparticles as a result of plasmon resonance (see image). As a consequence, the microspheres with embedded metal nanoparticles drive chemical reactions by harnessing solar energy much more efficiently than other commonly used photocatalyst structures. “The broadband absorption enhancement exists everywhere inside the microspheres,” explains Bai, “and the maximum enhancement can be hundred times greater than that of metal nanoparticles or small core–shell photocatalysts.” This explains their superior catalytic rates measured in previous experiments. In addition to explaining previous experimental findings, the analysis can also be used to inform the design of photocatalysts. In particular, it suggests that using semiconductors with higher refractive indices will maximize the broadband absorption induced by the interference, while using a mix of different plasmonic nanoparticles will enable flexible energy harvesting and enhanced selectivity. Finally, the findings also imply that locating the metal nanoparticles close to the surfaces of the microspheres will increase the catalytic rate as a consequence of the very short range of the plasmon near field. Bai and his team are now seeking to join forces with others working in the field. “Our next step is to look for end users and experimental collaborators to design, optimize and fabricate particular photocatalysts,” says Bai.
High energy particles introduce severe radiation damage in metallic materials, such as Ag. This paper reports on the study on twin boundary (TB) affected zone in irradiated nanotwinned Ag wherein time accumulative defect density and defect diffusivity are substantially different from those in twin interior. In situ studies also reveal surprising resilience and self-healing of TBs in response to radiation. This study provides further support for the design of radiation-tolerant nanotwinned metallic materials. © ACS) The design of next generation nuclear reactors calls for materials with superior radiation tolerance. Under the extreme radiation conditions, a large number of defects and their clusters are generated and consequently result in mechanical instability of irradiated metallic materials. While studying radiation damage in nanotwinned Ag by in-situ irradiation technique at the IVEM facility at Argonne National Laboratory, Zhang’s students, Jin Li and Kaiyuan Yu discovered two interesting phenomena. First, they identified twin boundary affected zones wherein time accumulative defect density and defect diffusivity are substantially different from those in twin interior. Additionally, in situ studies also revealed excellent resilience of twin boundaries in response to radiation: twin boundaries continue to change their geometry to facilitate the capture, transportation and removal of defect clusters. Furthermore twin boundaries can recover by absorbing opposite type of defects. “Without performing time accumulative studies, we would have missed the existence of twin boundary affected zones”, said Li. “This study provides further support for the implementation of twin boundaries as effective defect sinks for the design of radiation tolerant nanostructured metallic materials”, added Yu, who is now an assistant professor in China. The paper, "In situ Study of Defect Migration Kinetics and Self-Healing of Twin Boundaries in Heavy Ion Irradiated Nanotwinned Metals" was published in the April 2015 issue in .
A nanostructured electrode produced from widely available iron can use sunlight to cheaply produce the energy carrier hydrogen on a large scale. (Image: ICMS / DIFFER) Blowing bubbles in metal In their research Tanyeli and her colleagues exposed different metal surfaces to a hot intense beam of charged helium gas (plasma) in DIFFER's plasma experiment Magnum-PSI. Helium easily penetrates into the metal lattice where it forms bubbles that push the surrounding metal outwards. In this way different structures of tens to hundreds of nanometres in size arise per metal. By describing the differences, Tanyeli could analyse which underlying processes formed the nanostructures such as the temperature and the structure of the metal lattice. That helium plasma can cause a metal to explode in nanostructures had previously been discovered when researchers tested wall materials for fusion energy reactors. They then discovered strange shapes on the metal wall surface. In a fusion reactor these nanostructures are undesirable because they reduce the discharge of heat, but in other applications the nanostructures are very useful, thinks co-researcher and DIFFER director Richard van de Sanden. 
Nanostructures on aluminium. Overview (a) and cross-section (b) of nanostructures on an aluminium surface. (©Tanyeli et al. / Nature Scientific Reports) Fundamental insight "Irem Tanyeli's research is important due to the fundamental insight", says Van de Sanden. "How do such nanostructures grow on a surface, which processes play a role in that, what are the bottlenecks, and how can you manage the process? If you understand that then you can produce advanced materials on a large-scale that can be given properties to order." That has a wide range of applications in sustainable energy technologies. Converting sunlight into hydrogen Tanyeli's nanostructures are interesting for catalyst applications such as the use of solar energy to produce hydrogen from water. Widely available and cheap materials can usually not compete against the efficiency of expensive but rare record holders such as platinum. But with the right nanostructures the cheaper materials can still be made competitive. That opens up possibilities for the large-scale storage and conversion of sustainable energy in the form of chemical compounds: solar fuels. Such fuels have no net CO
Figure 1: This image shows decomposition of deformation configuration: (a) Initial configuration; (b) first intermediate configuration; (c) second intermediate configuration; (d) current configuration. © Science China Press) (click on image to enlarge) In previous researches for metal crystals, some theories based on the quantum mechanics were dependent on the vibration frequencies of lattice and their derivatives, and the calculation process was time-cosuming. On the other hand, for some forthright macroscopic models, the thermal expansion was not considered in the deformation process, and the temperature effects on plastic behavior, such as initial critical resolved shear stress and hardening modulus are difficult to determine. These problems bring inconvenience to prediction of the whole deformation behavior of metal crystals at different temperatures. Now, this new research has made an effort to solve these problems and put forward a practical method to obtain the thermo-elasto-plasticity deformation of metal crystals. In this research, the new deformation decomposition is proposed, and the total deformation contains thermal, elastic and plastic parts. The thermal strain can be obtained conveniently by the coefficient of thermal expansion from experimental data. Then, the constitutive equations are established based on the new deformation decomposition. Furthermore, the temperature effects on plastic behavior are reflected by simple expressions, which make the description of the thermo-plasticity behavior more explicitly. In this work, to reflect the effect of thermal expansion in the whole deformation process, a new deformation decomposition is given firstly, which is different with kinematical theory for the mechanics of elastic-plastic deformation for crystal. As shown in Figure 1, the deformation configuration is decomposed into four parts: the initial configuration at the undeformed state of 0 K (Figure 1(a)); the first intermediate configuration after free thermal expansions at T K (Figure 1(b)); the second intermediate configuration after elastic deformation at T K (Figure 1(c)); and the current configuration after plastic deformation at T K (Figure 1(d)). The new deformation decomposition provides a good basis to establish the constitutive equations and makes the present model applicable to structural calculation with some boundary constraints in the future. Then, the increment constitutive equations are obtained at the different deformation stages: the one is established by the rate of the second Piola-Kirchhoff stress and the rate of the Green strain, and the other one is established by the Jaumann rate of the Kirchhoff stress and symmetric parts of the velocity gradient. Meanwhile, the temperature dependences of initial critical resolved shear stress and hardening modulus are considered by exponential function. The parameters can be determined easily with three uniaxial stress-strain curves at different temperature. So the plastic behavior at different temperature can be determined accurately. Lastly, the stress-strain curves of Al crystals at different temperatures are calculated using this new model, and the calculation results are compared with experimental results. As shown in the Figure 2, the comparisons verify that the new model can predict the thermo-elasto-plasticity behavior of metal crystal very effectively. "Comparing with some widely recognized models, this new model has some characteristics as follows: firstly, the new deformation decomposition considers the thermal deformation in the whole deformation process, provides a good basis to establish the constitutive equations and makes the calculation process more simple and explicit than the MD and MC methods. Then the concise expressions for temperature effects on plastic behaviors make this new model have the ability to describe the thermo-elasto-plasticity behavior more clearly and accurately." Said the authors. This research provides an applicative method to calculate the thermo-elasto-plasticity deformation at different temperature, which is more explicit and concise than some previous models.
MOSK web The smallest detail a traditional optical microscope can reveal is about half the wavelength of green light, or 0.25 micrometer (a micrometer is a thousandth of a millimeter). Many interesting and important structures in biological cells and computer chips have features smaller than that. A very convenient and general method to enhance the resolution of microscopes is to structure the illumination. From several pictures under different illuminations, a single high-resolution image is constructed in the computer. So far, scientists have carefully selected the clearest glass optics for such imaging. Yet, the range of materials from which clear optics can be made is limited. In many materials random scattering takes place. New method Randomly scattered laser light appears as a finely grained speckle pattern as a result of interference of many scattered light paths. Researchers at the MESA+ Institute of the University of Twente in the Netherlands have developed a new and powerful approach to use these fine speckles for high resolution imaging. Using optimized scattering materials they produce the finest-grained speckles yet made with visible light. With this speckle illumination they obtain fluorescence images that have a very high resolution (0.12 micrometer) and a wide field of view. In the new method, the object you want to see – for instance a biological cell – is placed on the substrate of the scattering material and the laser light is shone upon the scattering surface. The lens creates a speckle pattern that can be scanned on the object. Multiple low resolution images of the object are then combined in the computer, which leads to a clear image. “The resolution improvement looks like the fog has cleared” says Hasan Yilmaz, the paper’s first author. “But in fact it is the low resolution image that is taken with clear optics. The high resolution picture is taken using scattered light!” The speckle illumination method is surface-specific and robust to environmental noise. The new high-resolution imaging method, called Speckle Correlation Resolution Enhancement (SCORE) is reported in the Optical Society’s (OSA) new high-impact journal ("Speckle correlation resolution enhancement of wide-field fluorescence imaging").
A single layer of graphene shown on a slide. Structures with these well-defined, atomic-sized gaps could be used to detect single molecules associated with certain diseases and might one day lead to microprocessors that are 100 times smaller than the ones in today’s computers. The ability to generate extremely small gaps - known as nanogaps - is highly desirable in fabricating nanoscale structures, which are typically used as components in optic and electronic devices. By decreasing the spacing between electronic circuits on a microchip, for example, one can fit more circuits on the same chip to produce a device with greater computing power. A team of Ph.D. students and undergraduate researchers led by UC San Diego nanoengineering professor Darren Lipomi demonstrated that the key to generating a smaller nanogap between two nanostructures involves using a graphene spacer, which can be etched away to create the gap. Graphene is the thinnest material known: it is simply a single layer of carbon atoms and measures approximately 0.3 nanometers (nm), which is about 100,000 times thinner than a human hair. The technique developed by Lipomi’s team overcomes some of the limitations of standard fabrication methods, such as photolithography and electron-beam lithography. By comparison, the smallest nanogaps that can be generated using the standard methods are 10–20 nm wide. “Making a nanogap is interesting from a philosophical standpoint,” said Lipomi. “While most efforts in nanotechnology focus on making materials, we’ve essentially made nothing - but with controlled dimensions.” Making “nothing” The method for making nanogaps begins with the production of thin films in which a single layer of graphene is sandwiched between two gold metal sheets. First, graphene is grown on a copper substrate, and then layered on top with a sheet of gold metal. Because graphene sticks better to gold than to copper, the entire graphene single-layer can be easily removed and remains intact over large areas. Compared to other techniques that are used to produce similar layered structures, this method allows graphene to be transferred to gold film with minimal defects or contamination. “This new method, which we developed in our lab, is called metal-assisted exfoliation. This is the only way so far in which we can place single-layer graphene between two metals and ensure that it contains no rips, cracks, folds, or unwanted chemical species,” said Alex Zaretski, a graduate student in Lipomi’s research group who pioneered the technique and is the first author of the study. “Metal-assisted exfoliation can potentially be useful for industries that use large areas of graphene.” Once the gold/graphene composite is separated from the copper substrate, the newly exposed side of the graphene layer is sandwiched with another gold sheet to produce the gold:single-layer graphene:gold thin film. The films are then sliced into 150 nm-wide nanostructures. Finally, the structures are treated with oxygen plasma to remove graphene. Scanning electron micrographs of the structures reveal extremely small nanogaps between the gold layers. Nanogap applications One potential application for this technology is in ultra-sensitive detection of single molecules, particularly those that are characteristic of certain diseases. When light is shined upon structures with extremely small gaps, the electromagnetic field that is confined within the gap becomes enormously enhanced. This enhanced electromagnetic field, in turn, increases the signal produced by any molecule within the gap. “If some disease marker comes in and bridges the gap between the nanostructures, you would observe a change in the light scattering from the nanogap that would correspond to whether the disease was present or not,” said Lipomi. While the technique reported in this study can produce nanostructures suitable for optical applications, it exhibits a major drawback for electronic applications. Raman spectroscopic measurements of the gold nanostructures reveal that small amounts of graphene still remain between the gold layers after being treated with oxygen plasma. This means that only the graphene exposed near the surfaces of the gold nanostructures can be removed so far. Having graphene still in the structures is not desirable for electronic devices, which require an entire gap between the structures. The team is working to figure out how to solve this problem. In the future, the team would also like to explore ways to vary the thickness of the well-defined gap between the structures by increasing the number of graphene layers. “For optical applications, it would be desirable to have gaps that are a little bit bigger than what we’ve generated. We just wanted to show, in principle, the smallest gap size that is possible to achieve,” said Lipomi.
This rendering shows the atomic arrangements of a gold nanocluster as reported in a new study led by UNL chemist Xiao Cheng Zeng. The cluster measures about 1.7 nanometers long - roughly the same length that a human fingernail grows in two seconds. Zeng and his colleagues unveiled the arrangements for a molecule featuring 68 gold atoms and 32 pairs of bonded sulfur-hydrogen atoms. Sixteen of the gold atoms form the molecule's core; the remainder bond with the sulfur and hydrogen to form a protective coating that stems from the core. Differences in atomic arrangements can alter molecular energy and stability, with less potential energy making for a more stable molecule. The team calculates that one of the arrangements may represent the most stable possible structure in a molecule with its composition. "Our group has helped lead the front on nano-gold research over the past 10 years," said Zeng, an Ameritas University Professor of chemistry. "We've now found new coating structures of much lower energy, meaning they are closer to the reality than (previous) analyses. So the deciphering of this coating structure is major progress." The researchers reported their findings in the April 24 edition of ("Unraveling structures of protection ligands on gold nanoparticle Au
Translocation of “individual” MWNTs-NH
Irradiating a thin crystal of κ-Br on a monolayer of spiropyran molecules with visible (left) and ultraviolet (right) light switches the κ-Br material between insulating and superconducting states due to the formation of an electric double layer. (© AAAS) The properties of solid materials can be dramatically altered by applying an electric field. In a common electronic component called a field-effect transistor, the flow of electrons through a semiconducting channel is controlled according to the strength of an internal electric field, which is created by applying a voltage to an insulating material—a dielectric. Higher field strengths provide more efficient conduction, but at very high fields the dielectric material begins to break down and conduct itself. To overcome this limitation, scientists have turned to the use of ionic liquids. When a voltage is applied to an ionic liquid, the charged particles in the liquid move to the surface of the channel material, creating an ‘electric double layer’ (EDL) that is not susceptible to dielectric breakdown. The high electric fields enabled by this technique have previously allowed researchers to convert the channel material from an insulator into a superconductor. So far, switching of the electric field has only been possible at relatively high temperatures because the ionic motion freezes at approximately 200 kelvin. Masayuki Suda and Hiroshi Yamamoto from the IMS and RIKEN, in collaboration with RIKEN’s Reizo Kato, have now shown that a light-sensitive molecule can be used to switch on a superconducting state at temperatures as low as 5 kelvin. The team replaced the ionic liquid with a single layer of spiropyran molecules, which are ionic under ultraviolet light and non-ionic under visible light. To test this strategy, they placed an organic crystal called ?-Br, which is known to have a superconducting state, on a single layer of self-assembled spiropyran molecules mounted on a thin oxide film. They confirmed that the resistance of the ?-Br switched from a high-resistance state under visible light to a low-resistance state when illuminated with ultraviolet light, due to the formation of an EDL (Fig. 1). “Even at low temperatures, ultraviolet light induces a zwitterionic structure in spiropyran molecules and a situation very similar to an EDL without an applied voltage, leading to superconductivity,” says Suda. “In this system, light induces the superconducting state by photoisomerization of the spiropyran molecules,” says Suda. “If we could get light to generate the superconducting carriers directly, other types of light-driven devices could also be possible.”
Silicon chip used for the design of the gate sensor. (Image: TOLOP) In the same Cambridge laboratory in the United Kingdom where the British physicist J.J. Thomson discovered the electron in 1897, European scientists have just developed a new ultra-sensitive electrical-charge sensor capable of detecting the movement of individual electrons. "The device is much more compact and accurate than previous versions and can detect the electrical charge of a single electron in less than one microsecond," says M. Fernando González Zalba, leader of this research from the Hitachi Cambridge Laboratory and the Cavendish Laboratory. Details of the breakthrough have been published in the journal 'Nature Communications' and its authors predict that these types of sensors, dubbed 'gate sensors', will be used in quantum computers of the future to read information stored in the charge or spin of a single electron. "We have called it a gate sensor because, as well as detecting the movement of individual electrons, the device is able to control its flow as if it were an electronic gate which opens and closes," explains González Zalba. The researchers have demonstrated the possibility of detecting the charge of an electron with their device in approximately one nanosecond, the best value obtained to date for this type of system. This has been achieved by coupling a gate sensor to a silicon nanotransistor where the electrons flow individually. In general, the electrical current which powers our telephones, fridges and other electrical equipment is made up of electrons: minuscule particles carrying an electrical charge travelling in their trillions and whose collective movement makes these appliances work. However, this is not the case of the latest cutting-edge devices such as ultra-precise biosensors, single electron transistors, molecular circuits and quantum computers. These represent a new technological sector which bases its electronic functionality on the charge of a single electron, a field in which the new gate sensor can offer its advantages.
Controlling the self-assembly of nanoparticles into superlattices is an important approach to build functional materials. The Brookhaven team used nanosized building blocks—cubes or octahedrons—decorated with DNA tethers to coordinate the assembly of spherical nanoparticles coated with complementary DNA strands. (click on image to enlarge) "If we want to take advantage of the promising properties of nanoparticles, we need to be able to reliably incorporate them into larger-scale composite materials for real-world applications," explained Brookhaven physicist Oleg Gang, who led the research at Brookhaven's Center for Functional Nanomaterials (CFN), a DOE Office of Science User Facility. "Our work describes a new way to fabricate structured composite materials using directional bindings of shaped particles for predictable assembly," said Fang Lu, the lead author of the publication. The research builds on the team's experience linking nanoparticles together using strands of synthetic DNA. Like the molecule that carries the genetic code of living things, these synthetic strands have complementary bases known by the genetic code letters G, C, T, and A, which bind to one another in only one way (G to C; T to A). Gang has previously used complementary DNA tethers attached to nanoparticles to guide the assembly of a range of arrays and structures. The new work explores particle shape as a means of controlling the directionality of these interactions to achieve long-range order in large-scale assemblies and clusters. Spherical particles, Gang explained, normally pack together to minimize free volume. DNA linkers—using complementary strands to attract particles, or non-complementary strands to keep particles apart—can alter that packing to some degree to achieve different arrangements. For example, scientists have experimented with placing complementary linker strands in strategic locations on the spheres to get the particles to line up and bind in a particular way. But it's not so easy to make nanospheres with precisely placed linker strands. "We explored an alternate idea: the introduction of shaped nanoscale 'blocks' decorated with DNA tethers on each facet to control the directional binding of spheres with complementary DNA tethers," Gang said. When the scientists mixed nanocubes coated with DNA tethers on all six sides with nanospheres of approximately the same size, which had been coated with complementary tethers, these two differently shaped particles did not segregate as would have been expected based on their normal packing behavior. Instead, the DNA "glue" prevented the separation by providing an attractive force between the flat facets of the blocks and the tethers on the spheres, as well as a repulsive force between the non-pairing tethers on same-shaped objects. 
The DNA tethers lead cubic blocks and spheres to self assemble so that one sphere binds to each face of a cube, resulting in a regular, repeating arrangement. "The DNA permits us to enforce rules: spheres attract cubes (mutually); spheres do not attract spheres; and cubes do not attract cubes," Gang said. "This breaks the conventional packing tendency and allows for the system to self-assemble into an alternating array of cubes and spheres, where each cube is surrounded by six spheres (one to a face) and each sphere is surrounded by six cubes." Using octahedral blocks instead of cubes achieved a different arrangement, with one sphere binding to each of the blocks' eight triangular facets. The method required some thermal processing to achieve the most uniform long-range order. And experiments with different types of DNA tethers showed that having flexible DNA strands was essential to accommodate the pairing of differently shaped particles. "The flexible DNA shells 'soften' the particles, which allows them to fit into arrangements where the shapes do not match geometrically," Lu said. But excessive softness results in unnecessary particle freedom, which can ruin a perfect lattice, she added. Finding the ideal flexibility for the tethers was an essential part of the work. 
Changing the shape of the blocks from cubes to octahedrons results in a different 3-D arrangement. The scientists used transmission and scanning electron microscopy at the CFN and also conducted x-ray scattering experiments at the National Synchrotron Light Source, another DOE Office of Science User Facility at Brookhaven Lab, to reveal the structure and take images of assembled clusters and lattices at various length scales. They also explained the experimental results with models based on the estimation of nanoscale interactions between the tiny building blocks. "Ultimately, this work shows that large-scale binary lattices can be formed in a predictable manner using this approach," Gang said. "Given that our approach does not depend on the particular particle's material and the large variety of particle shapes available—many more than in a child's building block play set—we have the potential to create many diverse types of new nanomaterials."
Mechanical cloak of invisibility: In a regular honeycomb structure (left), a hole is compensated by a distortion (right). External forces act as if the hole would not exist. (Image: T. Bückmann/KIT) The conception of “coordinate transformation” may sound complex, but such mathematical transformations are rather helpful: A mesh of connected points is drawn onto a rubber skin. Coordinate transformation is simulated by extending and distorting this rubber surface. When the assumed mesh can be mapped onto material distribution, a rather universal design approach results. It can be used to direct e.g. mechanical forces acting on the material along the tracks desired. For light, such transformations are based on the mathematics of transformation optics. So far, however, it has been impossible to transfer this principle to real materials and components in mechanics. The mathematics made impossible requirements on the material. To overcome these difficulties, researchers of the KIT Institute of Applied Physics around first author Tiemo Bückmann found a new, simple method. “We imagined a network of electric resistors,” Bückmann explains. “The wire connections between the resistors may be chosen to be of variable length, but their value does not change. Electric conductivity of the network even remains unchanged, when it is deformed.” The researchers transferred this thought experiment to practice. “In mechanics, this principle is found again when imagining small springs instead of resistors,” Tiemo Bückmann says. “We can make single springs longer or shorter when adapting their shapes, such that the forces between them remain the same. This simple principle saves computation expenditure and allows for the direct transformation of real materials.” 
Analysis of a hole in a hexagonal structure: External forces strongly deform the structure, the structure is unstable. By means of the new construction method, this error can be reduced strongly. (Image: T. Bückmann) (click on image to enlarge) The researchers tested their method in a model experiment with a material made of printed polymer. A stable hexagonal honeycomb structure was provided with a hole. Due to its reduced stability, the distorting forces first caused an error of more than 700 percent. After application of the newly developed transformation, the error amounted to 26 percent only. The results have just been published in the ("Mechanical cloak design by direct lattice transformation"). Applications are manifold, as the new method can be used to calculate known composite materials or mechanical support constructions. Even special designs will react as stably as possible to external forces – as if the support construction would not have been deformed.
Irregularity of the size and distribution of nanostructures on the surface of the butterfly wing causes low reflection of light at all view angles. (Photo: Radwanul Hasan Siddique, KIT) Transparent materials such as glass, always reflect part of the incident light. Some animals with transparent surfaces, such as the moth with its eyes, succeed in keeping the reflections small, but only when the view angle is vertical to the surface. The wings of the glasswing butterfly that lives mainly in Central America, however, also have a very low reflection when looking onto them under higher angles. Depending on the view angle, specular reflection varies between two and five percent. For comparison: As a function of the view angle, a flat glass plane reflects between eight and 100 percent, i.e. reflection exceeds that of the butterfly wing by several factors. Interestingly, the butterfly wing does not only exhibit a low reflection of the light spectrum visible to humans, but also suppresses the infrared and ultraviolet radiation that can be perceived by animals. This is important to the survival of the butterfly. For research into this so far unstudied phenomenon, the scientists examined glasswings by scanning electron microscopy. Earlier studies revealed that regular pillar-like nanostructures are responsible for the low reflections of other animals. The scientists now also found nanopillars on the butterfly wings. In contrast to previous findings, however, they are arranged irregularly and feature a random height. Typical height of the pillars varies between 400 and 600 nanometers, the distance of the pillars ranges between 100 and 140 nanometers. This corresponds to about one thousandth of a human hair. In simulations, the researchers mathematically modeled this irregularity of the nanopillars in height and arrangement. They found that the calculated reflected amount of light exactly corresponds to the observed amount at variable view angles. In this way, they proved that the low reflection at variable view angles is caused by this irregularity of the nanopillars. Hölscher’s doctoral student Radwanul Hasan Siddique, who discovered this effect, considers the glasswing butterfly a fascinating animal: “Not only optically with its transparent wings, but also scientifically. In contrast to other natural phenomena, where regularity is of top priority, the glasswing butterfly uses an apparent chaos to reach effects that are also fascinating for us humans.” The findings open up a range of applications wherever low-reflection surfaces are needed, for lenses or displays of mobile phones, for instance. Apart from theoretical studies of the phenomenon, the infrastructure of the Institute of Microstructure Technology also allows for practical implementation. First application tests are in the conception phase at the moment. Prototype experiments, however, already revealed that this type of surface coating also has a water-repellent and self-cleaning effect.
The different circles represent the studied surfaces of the Si membranes: crystalline, rough, flat with native SiO2, and rough with native SiO2. The right image shows a representative thermal map on the membranes upon a localized thermal excitation used to measure the thermal conductivity. The advent of nanotechnology, where the rules of classical physics gradually fail as the dimensions shrink, is challenging Fourier's theory of heat in several ways. A paper published in ("Tuning Thermal Transport in Ultrathin Silicon Membranes by Surface Nanoscale Engineering") and led by researchers from the Max Planck Institute for Polymer Research (Germany), the Catalan Institute of Nanoscience and Nanotechnology (ICN2) at the campus of the Universitat Autònoma de Barcelona (UAB) (Spain) and the VTT Technical Research Centre of Finland (Finland) describes how the nanometre-scale topology and the chemical composition of the surface control the thermal conductivity of ultrathin silicon membranes. The work was funded by the European Project Membrane-based phonon engineering for energy harvesting (MERGING). The results show that the thermal conductivity of silicon membranes thinner than 10 nm is 25 times lower than that of bulk crystalline silicon and is controlled to a large extent by the structure and the chemical composition of their surface. Combining state-of-the-art realistic atomistic modelling, sophisticated fabrication techniques, new measurement approaches and state-of-the-art parameter-free modelling, researchers unravelled the role of surface oxidation in determining the scattering of quantized lattice vibrations (phonons), which are the main heat carriers in silicon. Both experiments and modelling showed that removing the native oxide improves the thermal conductivity of silicon nanostructures by almost a factor of two, while successive partial re-oxidation lowers it again. Large-scale molecular dynamics simulations with up to 1,000,000 atoms allowed the researchers to quantify the relative contributions to the reduction of the thermal conductivity arising from the presence of native SiO2 and from the dimensionality reduction evaluated for a model with perfectly specular surfaces. Silicon is the material of choice for almost all electronic-related applications, where characteristic dimensions below 10 nm have been reached, e.g. in FinFET transistors, and heat dissipation control becomes essential for their optimum performance. While the lowering of thermal conductivity induced by oxide layers is detrimental to heat spread in nanoelectronic devices, it will turn useful for thermoelectric energy harvesting, where efficiency relies on avoiding heat exchange across the active part of the device. The chemical nature of surfaces, therefore, emerges as a new key parameter for improving the performance of Si-based electronic and thermoelectric nanodevices, as well as of that of nanomechanical resonators (NEMS). This work opens new possibilities for novel thermal experiments and designs directed to manipulate heat at such scales.
Perspective view of the zipping area with 17 skin cells "zipping". Membranes are colored in shades of brown and green to discriminate individual skin cells coming from the left or the right. The cells expand various types of protrusions in all directions to find their respective neighbor. The scientists have used a top-of-the-range electron microscope to study exactly how this zipping of the skin works. “Our electron microscope allows us to distinguish the molecular components in the cell that act like small machines to fuse the skin. When we look at it from a distance, it appears as if skin cells simply fuse to each other, but if we zoom in, it becomes clear that membranes, molecular machines, and other cellular components are involved", explains Eltsov. “In order to visualize this orchestra of healing, a very high-resolution picture of the process is needed. For this purpose we have recorded an enormous amount of data that surpasses all previous studies of this kind”, says Mikhail Eltsov. As a first step, as the scientists discovered, cells find their opposing partner by “sniffing” each other out. As a next step, they develop adherens junctions which act like a molecular Velcro. This way they become strongly attached to their opposing partner cell. The biggest revelation of this study was that small tubes in the cell, called microtubules, attach to this molecular Velcro and then deploy a self-catastrophe, which results in the skin being pulled towards the opening, as if one pulls a blanket over. Damian Brunner who led the team at the University of Zurich has performed many genetic manipulations to identify the correct components. The scientists were astonished to find that microtubules involved in cell-division are the primary scaffold used for zipping, indicating a mechanism conserved during evolution. “What was also amazing was the tremendous plasticity of the membranes in this process which managed to close the skin opening in a very short space of time. When five to ten cells have found their respective neighbors, the skin already appears normal”, says Achilleas Frangakis from the Goethe University Frankfurt, who led the study. The scientists hope that their results will open new avenues into the understanding of epithelial plasticity and wound healing. They are also investigating the detailed structural organization of the adherens junctions, work for which they were awarded a starting grant from European Research Council (ERC).
Chemical phase map showing how the electrochemical discharge of iron fluoride microwires proceeded from 0 percent discharge (left), to 50 percent (middle), to 95 percent. (Image: Linsen Li) "In the past, we weren't able to truly understand what is happening to iron fluoride during battery reactions because other battery components were getting in the way of getting a precise image," says Li. By accounting for the background signals that would otherwise confuse the image, Li was able to accurately visualize and measure, at the nanoscale, the chemical changes iron fluoride undergoes to store and discharge energy. Thus far, using iron fluoride in rechargeable lithium ion batteries has presented scientists with two challenges. The first is that it doesn't recharge very well in its current form. "This would be like your smart phone only charging half as much the first time, and even less thereafter," says Li. "Consumers would rather have a battery that charges consistently through hundreds of charges." By examining iron fluoride transformation in batteries at the nanoscale, Jin and Li's new X-ray imaging method pinpoints each individual reaction to understand why capacity decay may be occurring. "In analyzing the X-ray data on this level, we were able to track the electrochemical reactions with far more accuracy than previous methods, and determined that iron fluoride performs better when it has a porous microstructure," says Li. The second challenge is that iron fluoride battery materials don't discharge as much energy as they take in, reducing energy efficiency. The current study yielded some preliminary insights into this problem and Jin and Li plan to tackle this challenge in future experiments. Some implications of this research are obvious — like using portable electronic devices for longer before charging — but Jin also foresees a bigger and broader range of applications. "If we can maximize the cycling performance and efficiency of these low-cost and abundant iron fluoride lithium ion battery materials, we could advance large-scale renewable energy storage technologies for electric cars and microgrids," he says. Some implications of this research are obvious — like using portable electronic devices for longer before charging — but Jin also foresees a bigger and broader range of applications. Jin also believes that the novel X-ray imaging technique will facilitate the studies of other technologically important solid-state transformations and help to improve processes such as preparation of inorganic ceramics and thin-film solar cells. The experiments were performed with the help of Yu-chen Karen Chen-Wiegart, Feng Wang, Jun Wang and their co-workers at Beamline X8C, National Synchrotron Light Source, Brookhaven National Laboratory, and supported by the U.S. Department of Energy Basic Energy Sciences and a seed grant from the Wisconsin Energy Institute. The synthesis of the battery materials in Jin's lab was supported by National Science Foundation Division of Materials Research.
Light on – molecule on. For the first time a light beam switches a single molecule to closed state (red atoms). At the ends of the diarylethene molecule gold electrodes are attached. This way, the molecule functions as an electrical switch. (Image: HZDR/Pfefferkorn ) This requires a molecule in which an otherwise strong bond between individual atoms dissolves in one location – and forms again precisely when energy is pumped into the structure. Dr. Jannic Wolf, chemist at the University of Konstanz, discovered through complex experiments that a particular diarylethene compound is an eligible candidate. The advantages of this molecule, approximately three nanometres in size, are that it rotates very little when a point in its structure opens and it possesses two nanowires that can be used as contacts. The diarylethene is an insulator when open and becomes a conductor when closed. It thus exhibits a different physical behaviour, a behaviour that the scientists from Konstanz and Dresden were able to demonstrate with certainty in numerous reproducible measurements for the first time in a single molecule. A computer from a test-tube A special feature of these molecular electronics is that they take place in a fluid within a test-tube, where the molecules are contacted within the solution. In order to ascertain what effects the solution conditions have on the switching process, it was therefore necessary to systematically test various solvents. The diarylethene needs to be attached at the end of the nanowires to electrodes so that the current can flow. “We developed a nanotechnology at the HZDR that relies on extremely thin tips made of very few gold atoms. We stretch the switchable diarylethene compound between them,” explains Dr. Erbe. When a beam of light then hits the molecule, it switches from its open to its closed state, resulting in a flowing current. “For the first time ever we could switch on a single contacted molecule and prove that this precise molecule becomes a conductor on which we have used the light beam," says Dr. Erbe, pleased with the results. "We have also characterized the molecular switching mechanism in extremely high detail, which is why I believe that we have succeeded in making an important step toward a genuine molecular electronic component.” Switching off, however, does not yet work with the contacted diarylethene, but the physicist is confident: “Our colleagues from the HZDR theory group are computing how precisely the molecule must rotate so that the current is interrupted. Together with the chemists from Konstanz, we will be able to accordingly implement the design and synthesis for the molecule.” However, a great deal of patience is required because it’s a matter of basic research. The diarylethene molecule contact using electron-beam lithography and the subsequent measurements alone lasted three long years. Approximately ten years ago, a working group at the University of Groningen in the Netherlands had already managed to construct a switch that could interrupt the current. The off-switch also worked only in one direction, but what couldn't be proven at the time with certainty was that the change in conductivity was bound to a single molecule. Nano-electronics in Dresden One area of research focus in Dresden is what is known as self-organization. “DNA molecules are, for instance, able to arrange themselves into structures without any outside assistance. If we succeed in constructing logical switches from self-organizing molecules, then computers of the future will come from test-tubes," Dr. Erbe prophesizes. The enormous advantages of this new technology are obvious: billion-euro manufacturing plants that are necessary for manufacturing today’s microelectronics could be a thing of the past. The advantages lie not only in production but also in operating the new molecular components, as they both will require very little energy.