New research puts us closer to do-it-yourself spray-on solar cell technology

In a 2014 study, published in the journal ("Fully solution processed all inorganic nanocrystal solar cells"), St. Mary's College of Maryland energy expert Professor Troy Townsend introduced the first fully solution-processed all-inorganic photovoltaic technology. While progress on organic thin-film photovoltaics is rapidly growing, inorganic devices still hold the record for highest efficiencies which is in part due to their broad spectral absorption and excellent electronic properties. Considering the recorded higher efficiencies and lower cost per watt compared to organic devices, combined with the enhanced thermal and photo stability of bulk-scale inorganic materials, Townsend, in his 2014 study, focused on an all-inorganic based structure for fabrication of a top to bottom fully solution-based solar cell. A spray-on nanocrystal solar cell array A spray-on nanocrystal solar cell array. (Image courtesy of St. Mary's College of Maryland) A major disadvantage compared to organics, however, is that inorganic materials are difficult to deposit from solution. To overcome this, Townsend synthesized materials on the nanoscale. Inorganic nanocrystals encased in an organic ligand shell are soluble in organic solvents and can be deposited from solution (i.e., spin-, dip-, spray-coat) whereas traditional inorganic materials require a high temperature vacuum chamber. The solar devices are fabricated from nanoscale particle inks of the light absorbing layers, cadmium telluride/cadmium selenide, and metallic inks above and below. This way, the entire electronic device can be built on non-conductive glass substrates using equipment you can find in your kitchen. The outstanding challenge facing the (3-5 nm) inorganic nanocrystals is that they must be annealed or heated to form larger 'bulk scale' grains (100 nm to 1 µm) in order to produce working devices. Townsend recently teamed with Navy researchers to explore this process. "When you spray on these nanocrystals, you have to heat them to make them work," explained Townsend, "but you can't just heat the crystals by themselves, you have to add a sintering agent and that, for the last 40 years, has been cadmium chloride, a toxic salt used in commercial thin-film devices. No one has tested non-toxic alternatives for nanoscale ink devices, and we wanted to explore the mechanism of the sintering process to be able to implement safer salts." In his latest study, published this year in the ("Safer salts for CdTe nanocrystal solution processed solar cells: the dual roles of ligand exchange and grain growth"), Townsend, along with Navy researchers, found that ammonium chloride is a non-toxic, inexpensive viable alternative to cadmium chloride for nanocrystal solar cells. This discovery came after testing several different salts. Devices made using ammonium chloride (which is commonly used in bread making) had comparable device characteristics to those made with cadmium chloride, and the move away from cadmium salt treatments alleviates concerns about the environmental health and safety of current processing methods. The team also discovered that the role of the salt treatment involves crucial ligand removal reactions. This is unique to inorganic nanocrystals and is not observed for bulk-scale vacuum deposition methods. "A lot of exciting work has been done on nanocrystal ligand exchange, but, for the first time, we elucidated the dual role of the salt as a ligand exchange agent and a simultaneous sintering agent. This is an important distinction for these devices, because nanocrystals are typically synthesized with a native organic ligand shell. This shell needs to be removed before heating in order to improve the electronic properties of the film," said Townsend about the discovery. Because nanomaterials are at the forefront of emerging new properties compared to their bulk counterpart, the study is important to the future of electronic device fabrication. The research comes in the wake of the Obama Administration's announcement in July to put more solar panels on low-income housing and expand access to solar power for renters, and recent pledge to get 20 percent of the U.S. total electricity from renewable sources by the year 2030. "Right now, solar technology is somewhat unattainable for the average person," said Townsend. "The dream is to make the assembly and installation process so cheap and simple that you can go to your local home improvement store and buy a kit and then spray it on your own roof. That is why we we're working on spray-on solar cells." Townsend plans for further research to increase the efficiency of the all-inorganic nanocrystal solar cells (currently reaching five percent), while building them with completely non-toxic components.
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New electrode gives micro-supercapacitor macro storage capacity

Micro-supercapacitors are a promising alternative to micro-batteries because of their high power and long lifetime. They have been in development for about a decade but until now they have stored considerably less energy than micro-batteries, which has limited their application. Now researchers in the Laboratoire d’analyse et d’architecture des systèmes (LAAS-CNRS)1 in Toulouse and the INRS2 in Quebec have developed an electrode material that means electrochemical capacitors produce results similar to batteries, yet retain their particular advantages. This work was published on September 30, 2015 in ("3D RuO2 Microsupercapacitors with Remarkable Areal Energy"). scanning tunneling microscopy of a porous 3D-gold structur Image obtained from scanning tunneling microscopy on a porous 3D-gold structure. (Image: Anaïs Ferris – LAAS) With the development of on-board electronic systems3 and wireless technologies, the miniaturization of energy storage devices has become necessary. Micro-batteries are very widespread and store a large quantity of energy due to their chemical properties. However, they are affected by temperature variations and suffer from low electric power and limited lifetime (often around a few hundred charge/discharge cycles). By contrast, micro-supercapacitors have high power and theoretically infinite lifetime, but only store a low amount of energy. Micro-supercapacitors have been the subject of an increasing amount of research over the last ten years, but no concrete applications have come from it. Their lower energy density, i.e. the amount of energy that they can store in a given volume or surface area, has meant that they were not able to power sensors or microelectronic components. Researchers in the Intégration de systèmes de gestion de l’énergie team at LAAS-CNRS, in collaboration with the INRS of Quebec, have succeeded in removing this limitation by combining the best of micro-supercapacitors and micro-batteries. They have developed an electrode material whose energy density exceeds all the systems available to date. The electrode is made of an extremely porous gold structure into which ruthenium oxide has been inserted. It is synthesized using an electrochemical process. These expensive materials can be used here because the components are tiny: of the order of square millimeters. This electrode was used to make a micro-supercapacitor with energy density 0.5 J/cm2, which is about 1000 times greater than existing micro-supercapacitors, and very similar to the density characteristics of current Li-ion micro-batteries. With this new energy density, their long lifetime, high power and tolerance to temperature variations, these micro-supercapacitors could finally be used in wearable, intelligent, on-board microsystems. Notes 1 LAAS is part of the Instituts Carnot. Members of this renowned network conduct upstream research that refreshes their scientific and technological skills, and promote a deliberate joint research policy to benefit enterprise partners. 2 Institut national de la recherche scientifique. 3 On-board systems are wearable electronic systems. They must often meet size and consumption constraints.
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Invisibility cloak might enhance efficiency of solar cells

Success of the energy turnaround will depend decisively on the extended use of renewable energy sources. However, their efficiency partly is much smaller than that of conventional energy sources. The efficiency of commercially available photovoltaic cells, for instance, is about 20%. Scientists of Karlsruhe Institute of Technology (KIT) have now published an unconventional approach to increasing the efficiency of the panels. Optical invisibility cloaks guide sunlight around objects that cast a shadow on the solar panel, such as contacts for current extraction ("Cloaked contact grids on solar cells by coordinate transformations: designs and prototypes"). A special invisibility cloak (right) guides sunlight past the contacts for current removal to the active surface area of the solar cell A special invisibility cloak (right) guides sunlight past the contacts for current removal to the active surface area of the solar cell. (Image: Martin Schumann, KIT) Energy efficiency of solar panels has to be improved significantly not only for the energy turnaround, but also for enhancing economic efficiency. Modules that are presently mounted on roofs convert just one fifth of the light into electricity, which means that about 80% of the solar energy are lost. The reasons of these high losses are manifold. Up to one tenth of the surface area of solar cells, for instance, is covered by so-called contact fingers that extract the current generated. At the locations of these contact fingers, light cannot reach the active area of the solar cell and efficiency of the cell decreases. “Our model experiments have shown that the cloak layer makes the contact fingers nearly completely invisible,” doctoral student Martin Schumann of the KIT Institute of Applied Physics says, who conducted the experiments and simulations. Physicists of KIT around project head Carsten Rockstuhl, together with partners from Aachen, Freiburg, Halle, Jena, and Jülich, modified the optical invisibility cloak designed at KIT for guiding the incident light around the contact fingers of the solar cell. Normally, invisibility cloak research is aimed at making objects invisible. For this purpose, light is guided around the object to be hidden. This research project did not focus on hiding the contact fingers visually, but on the deflected light that reaches the active surface area of the solar cell thanks to the invisibility cloak and, hence, can be used. To achieve the cloaking effect, the scientists pursued two approaches. Both are based on applying a polymer coating onto the solar cell. This coating has to possess exactly calculated optical properties, i.e. an index of refraction that depends on the location or a special surface shape. The second concept is particularly promising, as it can potentially be integrated into mass production of solar cells at low costs. The surface of the cloak layer is grooved along the contact fingers. In this way, incident light is refracted away from the contact fingers and finally reaches the active surface area of the solar cell (see Figure). By means of a model experiment and detailed simulations, the researchers demonstrated that both concepts are suited for hiding the contact fingers. In the next step, it is planned to apply the cloaking layer onto a solar cell in order to determine the efficiency increase. The physicists are optimistic that efficiency will be improved by the cloak under real conditions: “When applying such a coating onto a real solar cell, optical losses via the contact fingers are supposed to be reduced and efficiency is assumed to be increased by up to 10%,” Martin Schumann says.
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New processes in modern ReRAM memory cells decoded

Resistive memory cells or ReRAMs for short are deemed to be the new super information-storage solution of the future. At present, two basic concepts are being pursued, which, up to now, were associated with different types of active ions. But this is not quite correct, as Jülich researchers working together with their Korean, Japanese and American colleagues were surprised to discover. In valence change memory (VCM) cells, not only are negatively charged oxygen ions active, but – akin to electrochemical metallization memory (ECM) cells – so too are positively charged metal ions. The effect enables switching characteristics to be modified as required and makes it possible to move back and forth from one concept to the other, as reported by the researchers in the journals ("Nanoscale cation motion in TaOx, HfOx and TiOx memristive systems") and ("Graphene-Modified Interface Controls Transition from VCM to ECM Switching Modes in Ta/TaOx Based Memristive Devices"). Formation of metallic Tantalum filament Formation of metallic Tantalum (Ta) filament within Ta/TaOx/Pt ReRAM memory cell. Positively charged Ta5+-ions and oxygen vacancies (VO) contribute to the process. (Image: Forschungszentrum Jülich / RWTH Aachen / Pössinger) ReRAM cells have a unique characteristic: their electrical resistance can be altered by applying an electric voltage. The cells behave like a magnetic material that can be magnetized and demagnetized again. In other words, they have an ON and an OFF state. This enables digital information to be stored, i.e. information that distinguishes between “1” and “0”. The most important advantages of ReRAMs are that they can be switched rapidly, consume little energy, and maintain their state even after long periods of time with no external voltage. The memristive behaviour of ReRAMs relay on mobile ions. These ions move in a similar manner to in a battery, flowing back and forth between two electrodes in a metal oxide layer no more than a few nanometres thick. For a long time, researchers believed that VCMs and ECMs functioned very differently. In ECMs, the ON and OFF states are achieved when metal ions move and form whisker-like filaments. This happens when an electric voltage is applied, causing such filaments to grow between the two electrodes of the cell. The cell is practically short-circuited and the resistance decreases abruptly. When the process is carefully controlled, information can be stored. The switching behaviour of VCMs, in contrast, were primarily associated with the displacement of oxygen ions. Contrary to metal ions, they are negatively charged. When a voltage is applied, the ions move out of an oxygen-containing metal compound. The material abruptly becomes more conductive. In this case as well, the process needs to be more carefully controlled. Jülich researchers working together with their partners from the Chonbuk National University, Jeonju, the National Institute for Materials Science in Tsukuba and the Massachusetts Institute of Technology (MIT) in Boston discovered an unexpected second switching process in VCMs: metal ions also help to form filaments in VCMs. The process was made visible because the scientists suppressed the movement of the oxygen ions. To do so, they modified the surface by applying a thin carbon layer directly at the interface of the electrode material with the solid electrolyte. In one case, they used the “miracle material” graphene, which comprises only one single layer of carbon. “Graphene was used to suppress the transport of oxygen ions through the phase boundary and to slow down the oxygen reactions. Suddenly, we observed a switching characteristic similar to that of an ECM cell and therefore assume that free metal ions are also active in VCMs. This was additionally verified using scanning tunnelling microscopy (STM) and diffusion experiments. It appears that the metal ions provide additional support for the switching process,” says Dr. Ilia Valov, electrochemist at Jülich’s Peter Grünberg Institute (PGI-7). A look into the Oxide Cluster A look into the Oxide Cluster at Forschungszentrum Jülich in which resistive cells and other layers of material are produced and examined in an ultrahigh vacuum. (Image: Forschungszentrum Jülich) Incorporating such a carbon interlayer would make it possible to jump from one switching process to the other in VCMs. This would lead to new options for designing ReRAMs. “Depending on the application, our findings could be exploited and the effect purposely enhanced or intentionally suppressed,” says Valov. The scientists’ findings give rise to several questions. “Existing models and studies will have to be reworked and adapted on the basis of these findings,” says the Jülich scientist. Further tests will clarify how such novel components behave in practice.
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Hopes of improved brain implants with nanowire structures

Neurons thrive and grow in a new type of nanowire material developed by researchers in Nanophysics and Ophthalmology at Lund University in Sweden ("Support of Neuronal Growth Over Glial Growth and Guidance of Optic Nerve Axons by Vertical Nanowire Arrays"). In time, the results might improve both neural and retinal implants, and reduce the risk of them losing their effectiveness over time, which is currently a problem. By implanting electrodes in the brain tissue one can stimulate or capture signals from different areas of the brain. These types of brain implants, or neuro-prostheses as they are sometimes called, are used to treat Parkinson’s disease and other neurological diseases. They are currently being tested in other areas, such as depression, severe cases of autism, obsessive-compulsive disorders and paralysis. Another research track is to determine whether retinal implants are able to replace light-sensitive cells that die in cases of and other eye diseases. However, there are severe drawbacks associated with today’s implants. One problem is that the body interprets the implants as foreign objects, resulting in an encapsulation of the electrode, which in turn leads to loss of signal. “Our nanowire structure prevents the cells that usually encapsulate the electrodes – glial cells – from doing so”, says Christelle Prinz, researcher in Nanophysics at Lund University in Sweden, who developed this technique together with Maria Thereza Perez, a researcher in Ophthalmology. “I was very pleasantly surprised by these results. In previous in-vitro experiments, the glial cells usually attach strongly to the electrodes”, she says. To avoid this, the researchers have developed a small substrate where regions of super thin nanowires are combined with flat regions. While neurons grow and extend processes on the nanowires, the glial cells primarily occupy the flat regions in between. “The different types of cells continue to interact. This is necessary for the neurons to survive because the glial cells provide them with important molecules.” So far, tests have only been done with cultured cells but hopefully they will soon be able to continue with experiments . The substrate is made from the semiconductor material gallium phosphide where each outgrowing nanowire has a diameter of only 80 nanometres.
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Electric field control of magnetic moment in palladium

Researchers at the University of Tokyo and Central Research Institute of Electric Power Industry have successfully induced a magnetic moment in palladium (Pd), usually a non-magnetic material, and demonstrated the ability to reversibly control the strength of the magnet by applying an electric field ("Electric-field control of magnetic moment in Pd"). This research has demonstrated the possibility of electrically inducing magnetism in non-magnetic materials. Induced magnetic moment in palladium (Pd) by application of voltage Induced magnetic moment in palladium (Pd) by application of voltage. One possible mechanism to explain the increased magnetic moment is that, when a voltage is applied to the structure shown in the figure, charges within the palladium layer accumulate near the surface because the surface is covered by ions. (Image: Chiba Lab) If the properties of a material could be electrically tuned after production, it would be possible to easily obtain the desired functions when needed, further increasing the range of materials that could be used in magnetic devices. In fields that employ magnetic materials, tuning of magnetic force and control of magnetization direction (together, these properties are termed the “magnetic moment”) has been demonstrated by applying a voltage to a capacitor containing a magnetic film as one electrode and charging and discharging charge carriers (electrons) from the electrode. It is expected that this method will dramatically reduce power consumption compared to conventional means of controlling magnetic moment (heating, magnetic field or electric current application). Prior studies have reported that it is possible to erase the magnetic properties of a material by the application of an electric field. However, there are no reports of successfully inducing and cancelling magnetic properties in a non-magnetic material by the same method. The research group of Associate Professor Daichi Chiba at the University of Tokyo Graduate School of Engineering and the Central Research Institute of Electric Power Industry has shown that the strength of a magnetic moment induced in palladium, a metal which is usually non-magnetic, is electrically controllable, and that application of a positive voltage induces a stronger magnetic moment than a negative voltage. The research group fabricated an ultra-thin cobalt/palladium structure in which a ferromagnetically ordered magnetic moment was induced in the top palladium layer by the ferromagnetic proximity effect. The magnetic moment in this Pd layer was reversibly controlled by applying a voltage. “This offers a new avenue for making non-magnetic materials ferromagnetic,” says Associate Professor Chiba of this latest research. He continues, “If it becomes possible to easily and reversibly induce magnetic properties in a non-magnetic material by applying an electric voltage, we may be able to make use of many materials currently not used in the field of magnetic engineering and further increase the range of materials available for use in magnetic devices.”
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New method for building on an atomic scale

UK scientists have pioneered a new way of manipulating several thousand atoms at a time, paving the way for building nanoscale electronic devices more quickly and easily at room temperature. Drawing with atoms In 1992 the very first man-made atomic structure was created by using a scanning tunnelling microscope (STM) to gently nudge individual atoms into a tiny nanometer scale logo for IBM. Scanning Tunnelling Microscope The team uses a Scanning Tunnelling Microscope (STM) to inject atoms onto a surface in a precise pattern, enabling them to build nanoscale devices more quickly and easily than before However, using this method atoms must be placed one-by-one, making the process very time-consuming, with even the most advanced microscopes taking many hours to position just a few atoms. In contrast, the new technique developed by the University of Bath in collaboration with the University of Birmingham, is able to move thousands of atoms simultaneously, but with similar precision. In their new method, the tip of the STM injects electrons onto a surface decorated with benzene molecules. The electrons can travel across the surface some tens of nanometers until they encounter one of the benzene molecules sitting on the surface, which causes the benzene to fly off into the gas phase. By carefully comparing the precise atomic position of the benzene molecules before and after the electron injections, the team was able to directly observe how high energy or “hot” electrons behave at room temperature for the first time. Hot electrons Hot electrons can leak out of silicon transistors and may limit the miniaturisation of computer circuits.They also play a critical role in transforming energy from light to electricity in photovoltaics. Their findings, published in the journal ("Atomically resolved real-space imaging of hot electron dynamics") show that instead of moving in straight lines as anticipated, they knock around like a ball in a pinball machine. Dr Peter Sloan from the University of Bath’s Department of Physics, explained: “Hot electrons are important in many processes but are really difficult to observe due to their short lifetimes, generally a millionth of a billionth of a second. “We were surprised to find that the hot electrons do not travel in straight lines, but instead behave as if they were a ball in a pin-ball machine, diffusing across the surface. “This confirms that Einstein’s theory of Brownian motion of electrons in semiconductors works even on the nanoscale. A finding that you just can’t observe with the “normal” low temperature experiments. hot electron experiment The team's experiments show that high energy or "hot" electrons don't move in straight lines as anticipated. “Our findings help us understand the fundamental physics underlying the behaviour of hot electrons and will help pave the way for building new nanotechnology devices with atomic precision.” Professor Richard Palmer at the University of Birmingham commented: "The Birmingham-Bath program is providing us with new eyes to visualise very fast electronic processes and so is relevant not just to electronics and computing but also improving the performance of solar cells designed to capture renewable energy. “It's great to see British Universities collaborating so closely together." 91 per cent of our physics research was defined as ‘world-leading’ or ‘internationally excellent’ in the REF 2014 research assessment, placing our Department of Physics 13th amongst all UK departments for its research activities. The real-world impact of our research – its influence on the economy, society, quality of life etc. - was judged to be particularly strong with 100 per cent being world-leading or internationally excellent, ranking us fourth amongst all UK physics departments for the impact of our research.
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Metamaterial absorbers for infrared inspection technologies

Plasmonic metamaterials are man-made substances whose structure can be manipulated to influence the way they interact with light. As such, metamaterials offer an attractive platform for sensing applications, including infrared (IR) absorption spectroscopy – a technique used to uncover details of the chemical make-up and structure of substances. Now, Atsushi Ishikawa at Okayama University and colleagues have fabricated a novel plasmonic metamaterial absorber comprised of gold and magnesium fluorine capable of high sensitivity IR detection ("Metamaterial absorbers for infrared detection of molecular self-assembled monolayers"). The metamaterial could prove invaluable in the development of next-generation IR inspection technologies. self-assembling monolayer of 16-MHDA acid Researchers at Okayama University have created a new IR spectroscopic technique utilizing the properties of a metamaterial-based absorber to enhance spectral output. Trials on a self-assembling monolayer of 16-MHDA acid showed distinct peaks corresponding to carbon-hydrogen stretching in the monolayer. The researchers carefully designed their absorber to maximise the IR signal and minimise background noise. The metamaterial consists of 50 nm gold ribbons on a thick gold film, separated by a layer of magnesium fluorine (see image). The wavelength of IR is longer and has less energy than visible light, meaning that it is not strong enough to excite electrons, unlike other types of spectroscopy. IR absorption spectroscopy therefore exploits the ability of IR to induce vibrations in bonded atoms. Organic compounds will absorb IR radiation corresponding to the different types of molecular vibrations present; the resulting absorption spectra tell scientists about the unique chemical structure of the compounds. To test the capabilities of the new metamaterial, the team decided to identify the stretching vibrational modes of carbon-hydrogen bonds in 16-Mercaptohexadecanoic (16-MHDA) acid. They dipped the absorber in 16-MHDA ethanol solution to encourage a self-assembling monolayer of the acid molecules to develop. Under IR radiation at different incident angles, the metamaterial-monolayer spectral output displayed distinct peaks corresponding to carbon-hydrogen stretching, with the most pronounced peaks under IR at an angle of 40°. The new metamaterial approach gave highly-detailed measurements pertaining to tiny molecular details (at the attamole level) in the 16-MHDA monolayer. The researchers hope their new technique will open doors to the development of ultrasensitive IR inspection technologies for material science and security applications. Background Metamaterials The ability to manipulate the light absorption of materials could revolutionize many technologies, such as photovoltaic cells and thermal devices. Research into the design and development of plasmonic metamaterials is still relatively new. These materials are synthetic, and scientists can design their surface structures to exploit the behavior of surface plasmons – quasiparticles that exist on metal surfaces and interact with light – to achieve tuneable optical properties. Infrared absorption spectroscopy could be dramatically enhanced by the introduction of tuneable metamaterial-based absorbers designed to enable high-resolution detection of tiny molecular details. Methodology The metamaterial absorber built by the team comprised gold nano-ribbons (measuring 50 nm thick) on a gold film base, with a thin layer of magnesium fluorine separating the two gold layers. As molecular monolayers self-assemble on noble metal surfaces, they hypothesized that the gold-based metamaterial would prove a strong candidate for enabling the high-resolution measurement of IR-induced vibrational modes in self-assembling monolayers. Their approach involved covering their metamaterial absorber with an ultrathin self-assembling monolayer of 16-MDHA acid molecules. They also covered a bare gold film sample with the same monolayer for comparison. The researchers subjected the two monolayers to IR radiation at different incident angles. The monolayer on bare gold exhibited a low signal-noise ratio, and it was very difficult to see the absorption dips on the spectral output corresponding to the IR-induced carbon-hydrogen stretching in the monolayer. In contrast, the absorption dips were very well-pronounced in the spectral read-out for the metamaterial-monolayer, because the vibrational modes of the 16-MHDA molecules resonated with the plasmonic modes of the metamaterial. This so-called ‘resonant coupling’ produced distinct peaks corresponding to IR-induced carbon-hydrogen stretching in the 16-MHDA molecular structure. The resonant coupling was dependent on the angle of the incident light, with the clearest, strongest signal at an angle of 40°. Future work The researchers believe their absorber may open doors to new ultra-sensitive IR detection technologies. Further, their technique could be exploited in other ways – by optimizing the surface structure of other metamaterials, they could enhance resonant coupling still further and enable sensitivities down to the zeptomole level.
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Biomimetic dental prosthesis

There are few tougher, more durable structures in nature than teeth or seashells. The secret of these materials lies in their unique fine structure: they are composed of different layers in which numerous micro-platelets are joined together, aligned in identical orientation. Although methods exist that allow material scientists to imitate nacre, it was a challenge to create a material that imitates the entire seashell, with comparable properties and structural complexity. Now a group of researchers led by André Studart, Professor of Complex Materials, has developed a new procedure that mimics the natural model almost perfectly. The scientists were able to produce a tough, multi-layered material based on the construction principle of teeth or seashells, and which compares well. The ETH researchers managed, for the first time, to preserve multiple layers of micro-platelets with differing orientation in a single piece. Artificial Tooth The left structure is showing the natural tooth in its gypsum mold, the middle structure is the artificial tooth (sintered but not yet polymer infiltrated). The model on the right has been sintered and polymer infiltrated. It is embedded in a "puck" to enable polishing and coated with platinum to prevent charging in the electron microscope. (Photo: Tobias Niebel/ETH Zurich) It is a procedure the ETH researchers call magnetically assisted slip casting (MASC). "The wonderful thing about our new procedure is that it builds on a 100-year-old technique and combines it with modern material research," says Studart's doctoral student Tobias Niebel, co-author of a study just published in the specialist journal ("Magnetically assisted slip casting of bioinspired heterogeneous composites"). Revival of a 100-year-old technique This is how MASC works: the researchers first create a plaster cast to serve as a mould. Into this mould, they pour a suspension containing magnetised ceramic platelets, such as aluminium oxide platelets. The pores of the plaster mould slowly absorb the liquid from the suspension, which causes the material to solidify and to harden from the outside in. The scientists create a layer-like structure by applying a magnetic field during the casting process, changing its orientation at regular intervals. As long as the material remains liquid, the ceramic platelets align to the magnetic field. In the solidified material, the platelets retain their orientation. Through the composition of the suspension and the direction of the platelets, a continuous process can be used to produce multiple layers with differing material properties in a single object. This creates complex materials that are almost perfect imitations of their natural models, such as nacre or tooth enamel. "Our technique is similar to 3D printing, only 10 times faster and much more cost-effective," says Florian Bouville, a post-doc with Studart and co-lead author of the study. Artificial teeth from casting moulds To demonstrate the potential of the MASC technique, Studart's research group produced an artificial tooth with a microstructure that mimics that of a real tooth. The surface of the artificial tooth is as hard and structurally complex as a real tooth, while the layer beneath is softer, just like the dentine of the natural model. The co-lead author of the study, doctoral student Hortense Le Ferrand, and her colleagues began by creating a plaster cast of a human wisdom tooth. They then filled this mould with a suspension containing aluminium oxide platelets and glass nanoparticles as mortar. Using a magnet, they aligned the platelets perpendicular to the surface of the object. Once the first layer was dry, the scientists poured a second suspension into the same mould. This suspension, however, did not contain glass particles. The aluminium oxide platelets in the second layer were aligned horizontally to the surface of the tooth using the magnet. This double-layered structure was then 'fired' at 1,600 degrees to compress and harden the material: the term sintering is used for this process. Finally, the researchers filled the pores that remained after the sintering with a synthetic monomer used in dentistry, which subsequently polymerised. Artificial teeth behave just like real teeth The researchers are very happy with the result. "The profile of hardness and toughness obtained from the artificial tooth corresponds exactly with that of a natural tooth," says a pleased Studart. The procedure and the resulting material lend themselves for applications in dentistry. However, as Studart points out, the current study is just an initial proof-of-concept, which shows that the natural fine structure of a tooth can be reproduced in the laboratory. "The appearance of the material has to be significantly improved before it can be used for dental prostheses." Nonetheless, as Studart explains, the artificial tooth clearly shows that a degree of control over the microstructure of a composite material can be achieved, previously the sole preserve of living organisms. One part of the MASC process, the magnetisation and orientation of the ceramic platelets, has already been patented. However, the new production process for such complex biomimetic materials also has other potential applications. For instance, copper platelets could be used in place of aluminium oxide platelets, which would allow the use of such materials in electronics. "The base substances and the orientation of the platelets can be combined as required, which rapidly and easily makes a wide range of different material types with varying properties feasible," says Studart.
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Designed defects in liquid crystals can guide construction of nanomaterials

Imperfections running through liquid crystals can be used as miniscule tubing, channeling molecules into specific positions to form new materials and nanoscale structures, according to engineers at the University of Wisconsin-Madison. The discovery could have applications in fields as diverse as electronics and medicine. "By controlling the geometry of the system, we can send these channels from any one point to any other point," says Nicholas Abbott, a UW-Madison professor of chemical and biological engineering. "It's quite a versatile approach." So far, Abbott and his collaborators at UW-Madison's Materials Research Science and Engineering Center (MRSEC) have been able to assemble phospholipids — molecules that can organize into layers in the walls of living cells — within liquid crystal defects. Their technique may also be useful for assembling metallic wires and various semiconducting structures vital to electronics. There's also potential for mimicking the selective abilities of a membrane, designing a defect so that one type of molecule can pass through while others can't. "This is an enabling discovery," Abbott says. "We're not looking for a specific application, but we're showing a versatile method of fabrication that can lead to structures you can't make any other way." The researchers — including UW-Madison graduate students Xiaoguang Wang, Daniel S. Miller and Emre Bukusoglu, and Juan J. de Pablo, a former UW-Madison engineering professor now at the University of Chicago — published details of their advance this week in the journal ("Topological defects in liquid crystals as templates for molecular self-assembly"). For about 20 years, Abbott's research has examined the surfaces of soft materials, including liquid crystals — a particular phase of matter in which liquid-like materials also exhibit some of the molecular organization of solids. "We've done a lot of work in the past at the interfaces of liquid crystals, but we're now looking inside the liquid crystal," he says. "We're looking at how to use the internal structure of liquid crystals to direct the organization of molecules. There's no prior example of using a defect in a liquid crystal to template molecular organization." When the researchers manipulate the geometry of a liquid crystalline system, a variety of different defects can result. Abbott's group assembled liquid crystals with defects shaped like ropes or lines they call "disclinations," that formed templates they could fill with amphiphilic (water- and fat-loving) molecules. Then they can link together assemblies of molecules and remove the liquid crystal templates, leaving behind the amphiphilic building blocks in a lasting, nanoscale structure. The research is an example of how liquid crystal research is taking us from the nano to macro world, says Dan Finotello, program director at the National Science Foundation, which funds the MRSEC. "It is also an exquisite demonstration of MRSEC programs' high impact," Finotello says. "MRSECs bring together several researchers of varied experience and complementary expertise who are then able to advance science at a considerably faster rate."
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Organic electronics with an edge

Using sophisticated theoretical tools, A*STAR researchers have identified a way to construct topological insulators — a new class of spin-active materials — out of planar organic-based complexes rather than toxic inorganic crystals ("Topological insulators based on 2D shape-persistent organic ligand complexes"). The unique crystal structure of topological insulators makes them insulating everywhere, except around their edges. Because the conductivity of these materials is localized into quantized surface states, the current passing through topological insulators acquires special characteristics. For example, it can polarize electron spins into a single orientation — a phenomenon that researchers are exploiting to produce ‘spin–orbit couplings’ that generate magnetic fields for spintronics without the need for external magnets. Many topological insulators are made by repeatedly exfoliating inorganic minerals, such as bismuth tellurides or bismuth selenides, with sticky tape until flat, two-dimensional (2D) sheets appear. “This gives superior properties compared to bulk crystals, but mechanical exfoliation has poor reproducibility,” explains Shuo-Wang Yang from the A*STAR Institute of High Performance Computing. “We proposed to investigate topological insulators based on organic coordination complexes, because these structures are more suitable for traditional wet chemical synthesis than inorganic materials.” Coordination complexes are compounds in which organic molecules known as ligands bind symmetrically around a central metal atom. Yang and his team identified novel ‘shape-persistent’ organic ligand complexes as good candidates for their method. These compounds feature ligands made from small, rigid aromatic rings. By using transition metals to link these organic building blocks into larger rings known as ‘macrocycles’, researchers can construct extended 2D lattices that feature high charge carrier mobility. Pinpointing 2D organic lattices with desirable topological insulator properties is difficult when relying only on experiments. To refine this search, Yang and colleagues used a combination of quantum calculations and band structure simulations to screen the electronic activity of various shape-persistent organic complexes. The team looked for two key factors in their simulations: ligands that can delocalize electrons in a 2D plane similar to graphene and strong spin–orbit coupling between central transition metal nodes and ligands. The researchers’ new family of potential organic topological insulators has a 2D honeycomb macrocycles containing tri-phenyl rings, palladium or platinum metals, and amino linking groups. With promising quantum features and high theoretical stability, these complexes may serve as topological insulators in real world applications. “These materials are easy to fabricate, and cheaper than their inorganic counterparts,” says Yang. “They are also suitable for assembling directly onto semiconductor surfaces, which makes nanoelectronic applications more feasible.”
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A magnetic memory bubbling with opportunity

RIKEN researchers have used ultrafast laser pulses to poke, stretch and position tiny magnetic domains on demand ("Photodrive of magnetic bubbles via magnetoelastic waves"). New technology that exploits the spin of electrons, known as spintronics technology, requires rapid and non-invasive methods to manipulate magnetic fields without generating excess heat. Current approaches focus on the interfaces, or ‘domain walls’, between regions with different ferromagnetic spin—applying a spin-polarized electric current, or spin wave, to such areas produces a twisting force that can be used to flip magnetic bits on or off. A three-dimensional image of a photoexcited magnetoelastic wave Fig. 1: A three-dimensional image of a photoexcited ‘magnetoelastic wave’—a combination of spin waves and acoustic vibrations—that can optically manipulate magnetic structures. (Image: Naoki Ogawa, RIKEN Center for Emergent Matter Science) The recent discovery of skyrmions—nanoscale, vortex-like magnetic textures that are more stable and information rich than conventional domains—has heightened the demand for improved control over small-scale domain walls. Ultrafast laser pulses are promising for achieving this control since they can generate effective magnetic fields at the desired location with a sub-micrometer spatial resolution. But they usually only excite a fraction of magnetic sites due to the weak interaction between light photons and spin. Naoki Ogawa and his colleagues at the RIKEN Center for Emergent Matter Science explored an optical manipulation strategy of exploiting optically excited ‘magnetoelastic’ waves—hybridized propagations of collective spins and acoustic vibrations of the crystal lattice (Fig. 1) – rather than conventional spin waves. This strong spin excitation was found to exert forces strong enough to attract magnetic domain walls. The researchers tested their concept on films of iron garnet, an insulator known for its long-lived spin waves and distinctively shaped domains, which include narrow stripes and ‘magnetic bubbles’—cylinder-like domains that exhibit similar properties to skyrmions. On spotting a bubble domain with their microscope, the team focused a laser spot close to it and then gradually increased the laser power. Eventually, they generated magnetoelastic waves that were so powerful that the domains followed the laser spot by moving against the collective spin propagation. This technique also worked on stripe domains, causing them to kink in the middle, or, if an endpoint was targeted, to stretch and bend in the film. “These interactions really depend on the curvature of the domain walls,” explains Ogawa. “Smaller structures with steep curvatures such as bubbles can be manipulated easier than larger domains.” The team notes that magnetic bubbles, which were deployed in nonvolatile memory devices in the 1970s, can now help establish the rules for manipulating skyrmions at the nanoscale. “This work is a proof of concept: it shows that magnetoelastic waves have the potential to deliver enough force to magnetic domains,” says Ogawa.
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New cathode material creates possibilities for sodium-ion batteries

Led by the inventor of the lithium-ion battery, a team of researchers in the Cockrell School of Engineering at The University of Texas at Austin has identified a new safe and sustainable cathode material for low-cost sodium-ion batteries. During the past five years, sodium-ion batteries have emerged as a promising new type of rechargeable battery and an alternative to lithium-ion batteries because sodium, better known as the main element of salt, is abundant and inexpensive. In contrast, lithium-ion batteries are limited by high production costs and availability of lithium. If researchers can figure out how to improve the performance and safety of sodium-ion batteries enough to widely commercialize them, then they could one day be used for wind and solar energy storage and to power electric vehicles. To that end, professor John Goodenough, the inventor of the lithium-ion battery, and his team have identified a new cathode material made of the nontoxic and inexpensive mineral eldfellite, presenting a significant advancement in the race to develop a commercially viable sodium-ion battery. The researchers reported their findings Aug. 27 in the journal ("Eldfellite, NaFe(SO4)2: an intercalation cathode host for low-cost Na-ion batteries"). Sodium-ion Cathode This illustration showcases the crystal structure of the eldfellite cathode for a sodium-ion battery. (Image: Cockrell School of Engineering) "At the core of this discovery is a basic structure for the material that we hope will encourage researchers to come up with better materials for the further development of sodium-ion batteries," said Preetam Singh, a postdoctoral fellow and researcher in Goodenough's lab. Sodium-ion batteries work just like lithium-ion batteries. During the discharge, sodium ions travel from the anode to the cathode, while electrons pass to the cathode through an external circuit. The electrons can then be used to perform electrical work. Although sodium-ion batteries hold tremendous potential, there are obstacles to advancing the technology including issues related to performance, weight and instability of materials. The team's proposed cathode material addresses instability. Its structure consists of fixed sodium and iron layers that allow for sodium to be inserted and removed while retaining the integrity of the structure. One challenge the team is currently working through is that their cathode would result in a battery that is less energy dense than today's lithium-ion batteries. The UT Austin cathode achieved a specific capacity (the amount of charge it can accommodate per gram of material) that is only two-thirds of that of the lithium-ion battery. "There are many more possibilities for this material, and we plan to continue our research. " Singh said. "We believe our cathode material provides a good baseline structure for the development of new materials that could eventually make the sodium-ion battery a commercial reality."
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New diamond structures produce bright luminescence for quantum cryotography and biomarkers applications

Germanium defects in a diamond crystal lattice act as a reliable source for single photons, new research shows. The results are reported in Scientific Reports and provide a promising new route to building components for quantum cryptography and biomarkers. Pure diamonds are naturally colorless, but gaps in the crystal structure or impurities of other elements can create colors and even emit fluorescence. Recently, researchers have shown that the fluorescent lattice defects could be useful as single photon sources for quantum cryptography and as bright luminescent makers in living cells. Now, Takayuki Iwasaki and co-workers at Tokyo Institute of Technology (Tokyo Tech), together with scientists across Japan and Germany, have demonstrated a new type of diamond crystal defect that fluoresces to produce single photons in a narrow, high energy wavelength band. The defects, which have been named germanium-vacancy (GeV) centres, are relatively easy to fabricate in a reliable, reproducible way ("Germanium-Vacancy Single Color Centers in Diamond"). Single GeV center in diamond Single GeV center in diamond. (left) Fluorescence mapping and (right) atomic structure model. Iwasaki and co-workers were inspired by recent work that demonstrated fluorescence from nitrogen-vacancy (NV) and silicon-vacancy (SiV) defects in diamond. They used an ion implantation method to insert germanium atoms into diamond films, before heating the films at 800 °C. The resulting samples showed fluorescence only after heating, which induces diffusion of vacancies in the diamond lattice. The researchers therefore concluded that the fluorescence was produced by combined defects, each comprising a germanium atom side-by-side with a vacancy. The GeV centres produced single-photon bursts of fluorescence centred at a wavelength of around 602 nm, representing a higher energy fluorescence than SiV centres. Moreover, the researchers were also able to create the films through the less destructive method of chemical vapor deposition, producing films with narrower and more stable emission peaks of ensemble GeV centres which are useful for biomarkers. Overall, the work opens up a promising new avenue for developing sources of single photons, which are essential for quantum cryptography. Iwasaki and co-workers are also hopeful that they could incorporate GeV centres in nanodiamonds for use as biological markers. Background Quantum computing and cryptography Our current digital computers encode information in bits, which can have values of either 0 or 1. In quantum computers, data will instead be stored in 'qubits', which can take on values of not only 0 or 1, but also a superposition of the two states. This small difference represents a huge change in functionality, and allows information and data to be encrypted in ways that are impossible to decode using only classical methods-this is known as quantum cryptography. Indeed, quantum-encoded data cannot be copied or read without changing its state, meaning that it is impossible for third parties to eavesdrop on communications without being discovered. Single photon generation To achieve the secure data transmission by quantum cryptography, individual photons of known wavelengths must be used but are difficult to generate. Herein lies the motivation behind the work of Iwasaki and co-workers. Defects in diamond have been shown to produce fluorescence — emitting photons of fixed-wavelength light when illuminated by higher energy light — but these are often unreliable or difficult to fabricate. The search is on for new defect structures that not only produce strong, consistent fluorescence, but can also be made in a reproducible way. Biomarker To monitor individual proteins and the interior of living cells, nanometer sized markers such as fluorescent proteins and quantum dots are used. Due to the high biological compatibility of diamond, fluorescent defects in diamond nanostructures are stable biomarkers without optical bleaching. The bright emission from the GeV centres could be suitable for such biological applications. Methodology Iwasaki and co-workers began with an ion implantation method, which involved firing germanium atoms at high speed into pure diamond surfaces. They then heated the samples at 800 °C to induce diffusion of vacancies — gaps in the diamond crystal lattice where a carbon atom is missing. By using Raman spectroscopy and confocal microscopy they observed fluorescent light emerging from the samples at a wavelength of around 602 nm. The team used theoretical calculations to deduce that this fluorescence resulted from combined defects, each comprising a germanium atom next to a lattice vacancy. The biggest step forward in the work was when Iwasaki and co-workers managed to create the same types of defects through a different method, microwave plasma chemical vapor deposition (MPCVD). MPCVD involves reactions of volatile chemicals on a substrate, and is often used to make synthetic diamonds. The defects in the sample prepared using MPCVD gave off more consistent fluorescence with a narrower and more stable peak. Moreover, MPCVD provides closer control over the fabrication process, and is less likely to produce unwanted damage to the samples than ion implantation. Future work Further work is needed to refine the fabrication process so that diamond films with germanium defects could be incorporated into devices for reliable single photon generation on demand.
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Nanowire quantum dot solar cells: oxide layer boosts performance

Attempts to improve solar cells can seem a balancing act, as optimising one variable can compromise another. The introduction of nanowires to colloidal quantum-dot solar cells (CQDSCs) aroused interest as a means of improving a limitation in the charge-collection layer thickness. However the high nanowire surface area brings other inhibiting factors. Now Jin Chang, Qing Shen and colleagues demonstrate how a further modification using an oxide layer can reduce the nanowire surface area effects for better-performing solar cells. A schematic illustration of the solar cells with zinc oxide (ZnO) nanowire heterojunctions (a) A schematic illustration of the solar cells with zinc oxide (ZnO) nanowire heterojunctions passivated with titanium oxide (TiO2) and lead sulphide (PbS) colloidal-quantum-dot charge separation layers (ZnO@TiO2/PbS solar cells);(b) a photograph of standard PbS CQDSCs fabricated in Shen's lab; (c) a typical cross-section scanning electron microscope image of the ZnO@TiO2/PbS solar cells. Colloidal quantum dots offer a number of advantages for solar cells: they provide effective charge separation layers for producing a photocurrent; have tunable band gaps; and can be solution-processed at low temperatures. However the low diffusion length for charge carriers generated in colloidal quantum dots limits the maximum layer thickness - it must be no thicker than the distance the carriers can travel to reach the heterojunction before recombining. This limited thickness caps the energy absorption capacity. Penetrating the quantum-dot layers with nanowire heterojunctions can allow greater thicknesses. But since recombination occurs at interfaces, the higher surface of nanowire heterojunctions undermines the advantage made. Chang, Shen and colleagues at the University of Electro-Communications and CREST in Japan, Universitat Jaume I in Spain, Kyushu Institute of Technology and King Abdulaziz University in Saudi Arabia show that a titanium oxide layer can passivate the surface of the nanowires thereby reducing recombination (" High reduction of interfacial charge recombination in colloidal quantum dot solar cells by metal oxide surface passivation"). The oxide layer allows a 40% improvement in the energy conversion efficiency of the devices and they are stable in air for over 130 days. "This work highlights the significance of metal oxide passivation in achieving high performance bulk heterojunction solar cells," conclude the authors. "The charge recombination mechanism uncovered in this work could shed light on the further improvement of PbS CQDSCs and/or other types of solar cells."
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Ultrathin graphene oxide lens could revolutionise next-gen devices

Researchers at Swinburne University of Technology, collaborating with Monash University, have developed an ultrathin, flat, ultra-lightweight graphene oxide optical lens with unprecedented flexibility. The ultrathin lens enables potential applications in on-chip nanophotonics and improves the conversion process of solar cells. It also opens up new avenues in:
  • – non-invasive 3D biomedical imaging
  • – photonic chips
  • – aerospace photonics
  • – micromachines
  • – laser tweezing – the process of using lasers to trap tiny particles.
graphene oxide lens Optical lenses are indispensable components in almost all aspects of technology including imaging, sensing, communications, and medical diagnosis and treatment. The rapid development in nano-optics and on-chip photonic systems has increased the demand for ultrathin flat lenses with three-dimensional subwavelength focusing capability – the ability to see details of an object smaller than 200 nanometres. Recent breakthroughs in nanophotonics have led to the development of a number of ultrathin flat lens concepts, however their real-life application is limited due to their complex design, narrow operational bandwidth and time consuming manufacturing processes. “Our lens concept has a 3D subwavelength capability that is 30 times more efficient, able to tightly focus broadband light from the visible to the near infrared, and offers a simple and low-cost manufacturing method,” research leader in nanophotonics at Swinburne’s Centre for Micro-Photonics (CMP), Associate Professor Baohua Jia, said. The researchers produced a film that is 300 times thinner than a sheet of paper by converting graphene oxide film to reduced graphene oxide through a photoreduction process. “These flexible graphene oxide lenses are mechanically robust and maintain excellent focusing properties under high stress,” lead author of the research, PhD candidate Xiaorui Zheng said. “They have the potential to revolutionise the next-generation integrated optical systems by making miniaturised and fully flexible photonics devices.” CMP Director, Professor Min Gu, said: “The newly demonstrated laser nano-patterning method in graphene oxides holds the key to fast processing and programming of high capacity information for big data sectors.” Professor Dan Li, Co-director of the Monash Centre for Atomically Thin Material, which provided the graphene oxide film for this research said this work opens up a new high-tech application for graphene oxide and demonstrates how nanotechnology can add significant value to natural graphite. The research is published in ("Highly efficient and ultra-broadband graphene oxide ultrathin lenses with three-dimensional subwavelength focusing") and has been funded by the Australian Research Council under its Discovery Early Career Researcher Award, Discovery Project and Laureate Fellowship scheme.
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Low-cost nanomembrane a new option for high-temperature fuel cells

Obtaining energy from fuel cells is an important issue nowadays to conserve the environment and membranes play the role of electrolyte in fuel cells and are solid electrolytes in proton exchanging fuel cells, which allow the pass of ions. Researchers from New Energies Research Center of Amirkabir University of Technology produced nanomembranes that can be used in the production of high temperature fuel cells ("Fabrication BaZrO3/PBI-based nanocomposite as a new proton conducting membrane for high temperature proton exchange membrane fuel cells"). The membrane has been made of a cheap nanocomposite through a simple method. The production and evaluation of the nanomembrane have been carried out at laboratorial scale. Nefion is one of the most common polymeric membranes used in fuel cells. Despite its numerous advantages, Nefion membrane has a weak performance at temperatures higher than 80°C. Therefore, it cannot be used in polymeric fuel cells that work at temperatures higher than 100°C. In addition, it is very expensive to buy Nefion. The nanocomposite membrane produced in this research has appropriate thermal stability and performance at high temperatures, and its production cost is much cheaper. Therefore, it can be considered as a promising option in the production of membranes. A simple and cost-effective solution casting method was used to produce the membrane, without the need for complicated processes. The cost to produce polymeric membranes to be used in fuel cells can be reduced in this method so their application has an economic justification. In case of being mass produced, the nanocomposite membrane presented in this research can help the development of fuel cells as an option for reducing pollution.
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Molecular diagnostics at home: Chemists design rapid, simple, inexpensive tests using DNA

Chemists at the University of Montreal used DNA molecules to developed rapid, inexpensive medical diagnostic tests that take only a few minutes to perform. Their findings, which will officially be published tomorrow in the ("A highly selective electrochemical DNA-based sensor that employs steric hindrance effects to detect proteins directly in whole blood"), may aid efforts to build point-of-care devices for quick medical diagnosis of various diseases ranging from cancer, allergies, autoimmune diseases, sexually transmitted diseases (STDs), and many others. portable sensor that enables fast and easy detection of multiple diagnostically relevant proteins The sensing principle is straightforward: the diagnostically relevant protein (green or red), if present, binds to an electro-active DNA strand and limits the ability of this DNA to hybridize to its complementary strand located on the surface of a gold electrode. This causes a reduction of electrochemical signal, which can be easily measured using inexpensive devices similar to those used in the home glucose self-test meter. Using this sensor, the researchers were able to detect several proteins directly in whole blood in less than 10 minutes. (Image: Ryan & Peter Allen) The new technology may also drastically impact global health, due to its low cost and easiness of use, according to the research team. The rapid and easy-to-use diagnostic tests are made of DNA and use one of the simplest force in chemistry, steric effects - a repulsion force that arises when atoms are brought too close together - to detect a wide array of protein markers that are linked to various diseases. The design was created by the research group of Alexis Vallée-Bélisle, a professor in the Department of Chemistry at University of Montreal. "Despite the power of current diagnostic tests, a significant limitation is that they still require complex laboratory procedures. Patients typically must wait for days or even weeks to receive the results of their blood tests," Vallée-Bélisle said. "The blood sample has to be transported to a centralised lab, its content analyzed by trained personnel, and the results sent back to the doctor's office. If we can move testing to the point of care, or even at home, it would eliminates the lag time between testing and treatment, which would enhance the effectiveness of medical interventions." The key breakthrough underlying this new technology came by chance. "While working on the first generation of these DNA-base tests, we realised that proteins, despite their small size (typically 1000 times smaller than a human hair) are big enough to run into each other and create steric effect (or traffic) at the surface of a sensor, which drastically reduced the signal of our tests," said Sahar Mashid, postdoctoral scholar at the University of Montreal and first author of the study. "Instead of having to fight this basic repulsion effect, we instead decided to embrace this force and build a novel signaling mechanism, which detects steric effects when a protein marker binds to the DNA test." The sensing principle is straightforward: the diagnostically relevant protein (green or red), if present, binds to an electro-active DNA strand, and limits the ability of this DNA to hybridize to its complementary strand located on the surface of a gold electrode. Francesco Ricci, a professor at University of Rome Tor Vergata who also participated in this study, explains that this novel signaling mechanism produces sufficient change in current to be measured using inexpensive electronics similar to those in the home glucose test meter used by diabetics to check their blood sugar. Using this DNA-base assay, the researchers were able to detect multiple protein markers directly in whole blood in fewer than 10 minutes, even if their concentration is 1,000 000 times less concentrated than glucose. "A great advantage of this DNA-based electrochemical test is that its sensing principle can be generalized to many different targets, allowing us to build inexpensive devices that could detect dozens of disease markers in less than five minutes in the doctor's office or even at home," concludes Vallée-Bélisle. A patent has been submitted for this invention, and many other applications are envisaged, including pathogen detection in food or water and therapeutic drug monitoring at home, a feature which could drastically improve the efficient of various class of drugs and treatments.
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Diverse set of Turing nanopatterns coat corneae of insects

In 1952, the legendary British mathematician and cryptographer Alan Turing proposed a model, which assumes formation of complex patterns through chemical interaction of two diffusing reagents. Russian scientists managed to prove that the corneal surface nanopatterns in 23 insect orders completely fit into this model. Their work is published in the ("Diverse set of Turing nanopatterns coat corneae across insect lineages"). Diversity of Corneal Nanostructural Patterns among Arthropod Groups The diversity of corneal nanostructural patterns among arthropod groups: (AandB) Corneal nanostructures of Trichoptera. Merged as well as undersized nipples in an irregular nipple array of the Phryganeidaefamily (A) and maze-like nanocoating of the Limnephilidae family (B). (C) Clearly expressed parallel strands in a true spider. (D) Dimpled nanopattern of an earwig (Dermaptera). (E) Nipples merging into maze on stonefly (Plecoptera) corneae. (FandG) Merging of individual Dipteran nipples into parallel strands and mazes: full merging of nipples into strands and mazes on the entire corneal surface in Tabanidae (F); partial merging of nipples in the center of Tipulidae cornea into elongated protrusions and then complete fusion into an array of parallel strands near the ommatidial edge (G). (H) Merging of individual burrows and dimples into a maze-like structure on bumblebee (Apidae, Hymenoptera) corneae. All image dimensions are 5×5?m, except forH, which is 3×3?m. Surface height in nanometers is indicated by the color scale shown next to 2-D images. (Image: Artem Blagodatsky et al) (click on image to enlarge) The work was done by a team working in the Institute of Protein Research of the Russian Academy of Sciences, (Pushchino, Russia) and the Department of Entomology at the Faculty of Biology of the Lomonosov Moscow State University. It was supervised by Professor Vladimir Katanaev, who also leads a lab in the University of Lausanne, Switzerland. Artem Blagodatskiy and Mikhail Kryuchkov performed the choice and preparation of insect corneal samples and analyzed the data. Yulia Lopatina from the Lomonosov Moscow State University played the role of expert entomologist, while Anton Sergeev performed the atomic force microscopy. The initial goal of the study was to characterize the antireflective three-dimensional nanopatterns covering insect eye cornea, with respect to the taxonomy of studied insects and to get insight into their possible evolution path. The result was surprising as the pattern morphology did not correlate with insect position on the evolutionary tree. Instead, Russian scientists have characterized four main morphological corneal nanopatterns as well as transition forms between them, omnipresent among the insect class. Another finding was that all the possible forms of the patterns directly matched to the array of patterns predicted by the famous Turing reaction-diffusion model published in 1952, what Russian scientists confirmed not by mere observation, but by mathematical modeling as well. The model assumes formation of complex patterns through chemical interaction of two diffusing reagents. The analysis of corneal surface nanopatterns in 23 insect orders has been performed by means of atomic force microscopy with resolution up to single nanometers. "This method allowed us to drastically expand the previously available data, acquired through scanning electron microscopy; it also made possible to characterize surface patterns directly, not based upon analysis of metal replicas. When possible, we always examined corneae belonging to distinct families of one order to get insight into intra-order pattern diversity," Artem Blagodatskiy says. The main implication of the work is the understanding of the mechanisms underlying the formation of biological three-dimensional nano-patterns, demonstrating the first example of Turing reaction-diffusion model acting in the bio-nanoworld. Interestingly, the Turing nanopatterning mechanism is common not only for the insect class, but also for spiders, scorpions and centipedes in other words - universal for arthropods. Due to the antireflective properties of insect corneal nanocoatings, the revealed mechanisms are paving the way for design of artificial antireflective nanosurfaces. "A promising future development of the project is planned to be a genetic analysis of corneal nanopattern formation on platform of a well studied Drosophila melanogaster (fruitfly) model. The wild-type fruitflies possess a nipple array type nanocoating on their eyes," Artem Blagodatskiy summarized. Different combinations of overexpressed and underexpressed proteins known to be responsible for corneal development in Drosophila may alter the nipple pattern to another pattern type and thus shed the light on chemical nature of compounds, forming the Turing-type structures upon insect eyes. Revealing of proteins and\or other agents responsible for nanopattern formation will be a direct clue to artificial design of nanocoatings with desired properties. Another direction of project development will be the comparison of antireflective features of different types of characterized nanocoatings.
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Permanent data storage with light

The first all-optical permanent on-chip memory has been developed by scientists of Karlsruhe Institute of Technology (KIT) and the universities of Münster, Oxford, and Exeter. This is an important step on the way towards optical computers. Phase change materials that change their optical properties depending on the arrangement of the atoms allow for the storage of several bits in a single cell. The researchers present their development in the journal ("On-chip integratable all-photonic nonvolatile multi-level memory"). All-optical data memory All-optical data memory: Ultra-short light pulses make the GST material change from crystalline to amorphous and back. Weak light pulses read out the data. (Image: C. Rios/Oxford University) Light determines the future of information and communication technology: With optical elements, computers can work more rapidly and more efficiently. Optical fibers have long since been used for the transmission of data with light. But on a computer, data are still processed and stored electronically. Electronic exchange of data between processors and the memory limits the speed of modern computers. To overcome this so-called von Neumann bottleneck, it is not sufficient to optically connect memory and processor, as the optical signals have to be converted into electric signals again. Scientists, hence, look for methods to carry out calculations and data storage in a purely optical manner. Scientists of KIT, the University of Münster, Oxford University, and Exeter University have now developed the first all-optical, non-volatile on-chip memory. “Optical bits can be written at frequencies of up to a gigahertz. This allows for extremely quick data storage by our all-photonic memory,” Professor Wolfram Pernice explains. Pernice headed a working group of the KIT Institute of Nanotechnology (INT) and recently moved to the University of Münster. “The memory is compatible not only with conventional optical fiber data transmission, but also with latest processors,” Professor Harish Bhaskaran of Oxford University adds. The new memory can store data for decades even when the power is removed. Its capacity to store many bits in a single cell of a billionth of a meter in size (multi-level memory) also is highly attractive. Instead of the usual information values of 0 and 1, several states can be stored in an element and even autonomous calculations can be made. This is due to so-called phase change materials, novel materials that change their optical properties depending on the arrangement of the atoms: Within shortest periods of time, they can change between crystalline (regular) and amorphous (irregular) states. For the memory, the scientists used the phase change material Ge2Sb2Te5 (GST). The change from crystalline to amorphous (storing data) and from amorphous to crystalline (erasing data) is initiated by ultrashort light pulses. For reading out the data, weak light pulses are used. Permanent all-optical on-chip memories might considerably increase future performance of computers and reduce their energy consumption. Together with all-optical connections, they might reduce latencies. Energy-intensive conversion of optical signals into electronic signals and vice versa would no longer be required.
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Pushing the limits of lensless imaging

Using ultrafast beams of extreme ultraviolet light streaming at a 100,000 times a second, researchers from the Friedrich Schiller University Jena, Germany, have pushed the boundaries of a well-established imaging technique. Not only did they make the highest resolution images ever achieved with this method at a given wavelength, they also created images fast enough to be used in real time. Their new approach could be used to study everything from semiconductor chips to cancer cells. The team will present their work at the Frontiers in Optics, The Optical Society's annual meeting and conference in San Jose, California, USA, on 22 October 2015. Custom-Built Ultrafast Laser In coherent diffraction imaging, extreme ultraviolet light scatters off the target and produces a diffraction pattern. A computer analyzes the pattern to reconstruct an image of the target material. (Image: Dr. Michael Zürch, Friedrich Schiller University Jena, Germany) The researchers' wanted to improve on a lensless imaging technique called coherent diffraction imaging, which has been around since the 1980s. To take a picture with this method, scientists fire an X-ray or extreme ultraviolet laser at a target. The light scatters off, and some of those photons interfere with one another and find their way onto a detector, creating a diffraction pattern. By analyzing that pattern, a computer then reconstructs the path those photons must have taken, which generates an image of the target material -- all without the lens that's required in conventional microscopy. "The computer does the imaging part -- forget about the lens," explained Michael Zürch, Friedrich Schiller University Jena, Germany and lead researcher. "The computer emulates the lens." Without a lens, the quality of the images primarily depends on the radiation source. Traditionally, researchers use big, powerful X-ray beams like the one at the SLAC National Accelerator Laboratory in Menlo Park, California, USA. Over the last ten years, researchers have developed smaller, cheaper machines that pump out coherent, laser-like beams in the laboratory setting. While those machines are convenient from the cost perspective, they have drawbacks when reporting results. The table-top machines are unable to produce as many photons as the big expensive ones which limits their resolution. To achieve higher resolutions, the detector must be placed close to the target material -- similar to placing a specimen close to a microscope to boost the magnification. Given the geometry of such short distances, hardly any photons will bounce off the target at large enough angles to reach the detector. Without enough photons, the image quality is reduced. Zürch and a team of researchers from Jena University used a special, custom-built ultrafast laser that fires extreme ultraviolet photons a hundred times faster than conventional table-top machines. With more photons, at a wavelength of 33 nanometers, the researchers were able to make an image with a resolution of 26 nanometers -- almost the theoretical limit. "Nobody has achieved such a high resolution with respect to the wavelength in the extreme ultraviolet before," Zürch said. The ultrafast laser also overcame another drawback of conventional table-top light sources: long exposure times. If researchers have to wait for images, they can't get real-time feedback on the systems they study. Thanks to the new high-speed light source, Zürch and his colleagues have reduced the exposure time to only about a second -- fast enough for real-time imaging. When taking snapshots every second, the researchers reached a resolution below 80 nanometers. The prospect of high-resolution and real-time imaging using such a relatively small setup could lead to all kinds of applications, Zürch said. Engineers can use this to hunt for tiny defects in semiconductor chips. Biologists can zoom in on the organelles that make up a cell. Eventually, he said, the researchers might be able to cut down on the exposure times even more and reach even higher resolution levels. About the Presentation The presentation, "Approaching the Abbe Limit in the Extreme Ultraviolet: Ultrafast Imaging Using a Compact High Average Power High Harmonic," by Michael Zürch, will take place from 13:00 - 14:45, Thursday, 22 October 2015, in The Fairmont Hotel, San Jose, California, USA. Media Registration: A media room for credentialed press and analysts will be located on-site in The Fairmont Hotel, 18-22 October 2015. Media interested in attending the event should register on the FiO website media center: Media Center.
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Darwin on a chip

Researchers of the MESA+ Institute for Nanotechnology and the CTIT Institute for ICT Research at the University of Twente in The Netherlands have demonstrated working electronic circuits that have been produced in a radically new way, using methods that resemble Darwinian evolution. The size of these circuits is comparable to the size of their conventional counterparts, but they are much closer to natural networks like the human brain. The findings promise a new generation of powerful, energy-efficient electronics, and have been published in the leading British journal ("Evolution of a Designless Nanoparticle Network into Reconfigurable Boolean Logic"). Learning from Nature One of the greatest successes of the 20th century has been the development of digital computers. During the last decades these computers have become more and more powerful by integrating ever smaller components on silicon chips. However, it is becoming increasingly hard and extremely expensive to continue this miniaturisation. Current transistors consist of only a handful of atoms. It is a major challenge to produce chips in which the millions of transistors have the same characteristics, and thus to make the chips operate properly. Another drawback is that their energy consumption is reaching unacceptable levels. It is obvious that one has to look for alternative directions, and it is interesting to see what we can learn from nature. Natural evolution has led to powerful ‘computers’ like the human brain, which can solve complex problems in an energy-efficient way. Nature exploits complex networks that can execute many tasks in parallel. Moving away from designed circuits The approach of the researchers at the University of Twente is based on methods that resemble those found in Nature. They have used networks of gold nanoparticles for the execution of essential computational tasks. Contrary to conventional electronics, they have moved away from designed circuits. By using 'designless' systems, costly design mistakes are avoided. The computational power of their networks is enabled by applying artificial evolution. This evolution takes less than an hour, rather than millions of years. By applying electrical signals, one and the same network can be configured into 16 different logical gates. The evolutionary approach works around - or can even take advantage of - possible material defects that can be fatal in conventional electronics. Powerful and energy-efficient It is the first time that scientists have succeeded in this way in realizing robust electronics with dimensions that can compete with commercial technology. According to prof. Wilfred van der Wiel, the realized circuits currently still have limited computing power. “But with this research we have delivered proof of principle: demonstrated that our approach works in practice. By scaling up the system, real added value will be produced in the future. Take for example the efforts to recognize patterns, such as with face recognition. This is very difficult for a regular computer, while humans and possibly also our circuits can do this much better." Another important advantage may be that this type of circuitry uses much less energy, both in the production, and during use. The researchers anticipate a wide range of applications, for example in portable electronics and in the medical world.
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Exploring catalytic reactions at the nanoscale

The National Physical Laboratory (NPL) has used a novel imaging capability - tip-enhanced Raman spectroscopy - to map catalytic reactions at the nanoscale for the first time. Catalysts are substances that facilitate chemical reactions without being consumed, enabling industry to produce chemicals which would otherwise be uneconomic or even impossible. Catalysts are used in over 90% of industrial chemical processes, from the production of pharmaceuticals to energy generation, and are thought to contribute to over 35% of global GDP. Pressure for greener, cheaper and more sustainable chemistry in industry is driving the search for new catalysts with improved efficiency and selectivity. Rational design of catalyst materials with tailored properties relies on our ability to identify active sites at reacting surfaces in order to understand structure-performance relationships. However, conventional analytical techniques often lack the required sensitivity at the necessary length-scales for this to be achieved. Tip-enhanced Raman spectroscopy (TERS) has emerged as a powerful and reliable technique for characterising surfaces at the nanoscale, combining the high chemical sensitivity of surface-enhanced Raman spectroscopy and nanoscale spatial resolution of scanning probe microscopy Together, these properties make TERS ideally-suited to the characterisation of catalytic reactions at the nanometre length-scale. Schematic of a TERS apparatus and a catalytic reaction Schematic of the TERS apparatus and the catalytic reaction studied. A team from NPL has taken the lead in using TERS to identify catalytic nanoparticles on a surface and has achieved nanoscale mapping of catalytic activity for the first time. The nanometre resolution of this reactive spectroscopic imaging, published in the Royal Society of Chemistry journal ("Nanoscale mapping of catalytic activity using tip-enhanced Raman spectroscopy"), has yet to be matched by any other analytical technique. The team's work is hoped to pave the way for the routine use of TERS to study catalytic reactions with nanoscale resolution. In the future, the spatial variations identified using this technique could provide powerful new insights into molecular adsorption and reaction dynamics at surfaces, ultimately enabling improved control and efficiency of chemical processes through informed catalyst optimisation.
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How to make large 2-D sheets

Sheets of graphene and other materials that are virtually two-dimensional hold great promise for electronic, optical, and other high-tech applications. But the biggest limitation in unleashing this potential has been figuring out how to make these materials in the form of anything larger than tiny flakes. Now researchers at MIT and elsewhere may have found a way to do so. The group has determined a way to make large sheets of one such material, called molybdenum telluride, or MoTe2. The team says their method is also likely to work for many similar 2-D materials, and could make widespread applications feasible. The findings have been published in the ("Large-Area Synthesis of High-Quality Uniform Few-Layer MoTe2") by a team including MIT postdoc Lin Zhou; professors Mildred Dresselhaus, Jing Kong, and Tomás Palacios; and eight others at MIT, the China University of Petroleum, Central South University in China, the National Tsing-hua University in Taiwan, and Saitama University and Tohoku University in Japan. Millie Dresselhaus and Lin Zhou Millie Dresselhaus and Lin Zhou. “This material has a similar bandgap to silicon” — a characteristic needed in order to make transistors and solar cells — “and in single-layer form it has a direct bandgap,” Zhou says, which allows better light emission. “It also has strong absorption for solar radiation,” which is key to making practical solar cells, she says. Molybdenum telluride can exist in two different forms; one is metallic, meaning it conducts electricity well, and the other is a natural semiconductor, lending itself to applications in electronics. Controlling how the material is made allows the researchers to create whichever form is needed for a particular use. The new method is based on chemical vapor deposition (CVD), and makes it possible to create sheets of any thickness, and of a size limited only by the dimensions of the CVD chamber used for deposition. One challenge the team had to overcome was that the atoms of molybdenum telluride are very weakly bound to each other, so the tendency of the two precursor materials to form molybdenum telluride is low. “This makes it more challenging to make, compared to other similar materials,” Zhou says. The researchers were able to overcome this by using several stages of deposition, beginning with a layer of pure molybdenum. “This method makes it easy, because you only need to control one material,” Zhou says. This step is followed by oxidation of that layer; this material is then removed and powdered tellurium is added, vaporized in a carrier gas of hydrogen and argon, at a temperature of 700 degrees Celsius. The use of hydrogen in the process, the team found, is crucial to producing a uniform MoTe2 film. The material should be immediately usable to create electronic devices including field-effect transistors, which the team has already demonstrated in the lab. “Our process can grow sheets that have a very large area, are very homogeneous, and have high quality,” Zhou says. The team now aims to explore adapting this process to create large sheets of other promising thin materials: “2-D materials are a big family with different properties,” Zhou says. She and her colleagues will examine whether versions of the process can work with other compounds. Molybdenum telluride also lends itself to applications in spintronics, Zhou says, an emerging technology based on the spins of electrons rather than their charge, as in conventional electronics. Physicist Ado Jorio of the Federal University of Minas Gerais, Brazil, who was not involved in this work, says, “What is most impressive is that this group has been able to consecutively develop new formulae to produce almost any low-dimensional material they want, always scalable with the highest quality worldwide.” And Vincent Meunier, a physicist at Rensselaer Polytechnic Institute who was also not associated in this research, adds, “One of the many advantages of the proposed approach stems from its simplicity. The consequences of this development are likely to be numerous, as it provides a versatile and scalable technique to develop macroscopic amounts of atomically thin films, thereby surmounting major roadblocks faced by layered-materials based research so far.”
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Nano-thin invisibility cloak makes 3D objects disappear (w/video)

Invisibility cloaks are a staple of science fiction and fantasy, from Star Trek to Harry Potter, but don't exist in real life, or do they? Scientists at the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley have devised an ultra-thin invisibility "skin" cloak that can conform to the shape of an object and conceal it from detection with visible light. Although this cloak is only microscopic in size, the principles behind the technology should enable it to be scaled-up to conceal macroscopic items as well. Invisibility Skin Cloak This image shows a 3-D illustration of a metasurface skin cloak made from an ultrathin layer of nanoantennas (gold blocks) covering an arbitrarily shaped object. Light reflects off the cloak (red arrows) as if it were reflecting off a flat mirror. (Image courtesy of Xiang Zhang group, Berkeley Lab/UC Berkeley) Working with brick-like blocks of gold nanoantennas, the Berkeley researchers fashioned a "skin cloak" barely 80 nanometers in thickness, that was wrapped around a three-dimensional object about the size of a few biological cells and arbitrarily shaped with multiple bumps and dents. The surface of the skin cloak was meta-engineered to reroute reflected light waves so that the object was rendered invisible to optical detection when the cloak is activated. "This is the first time a 3D object of arbitrary shape has been cloaked from visible light," said Xiang Zhang, director of Berkeley Lab's Materials Sciences Division and a world authority on metamaterials - artificial nanostructures engineered with electromagnetic properties not found in nature. "Our ultra-thin cloak now looks like a coat. It is easy to design and implement, and is potentially scalable for hiding macroscopic objects." Zhang, who holds the Ernest S. Kuh Endowed Chair at UC Berkeley and is a member of the Kavli Energy NanoSciences Institute at Berkeley (Kavli ENSI), is the corresponding author of a paper describing this research in ("An ultrathin invisibility skin cloak for visible light"). It is the scattering of light - be it visible, infrared, X-ray, etc., - from its interaction with matter that enables us to detect and observe objects. The rules that govern these interactions in natural materials can be circumvented in metamaterials whose optical properties arise from their physical structure rather than their chemical composition. For the past ten years, Zhang and his research group have been pushing the boundaries of how light interacts with metamaterials, managing to curve the path of light or bend it backwards, phenomena not seen in natural materials, and to render objects optically undetectable. In the past, their metamaterial-based optical carpet cloaks were bulky and hard to scale-up, and entailed a phase difference between the cloaked region and the surrounding background that made the cloak itself detectable - though what it concealed was not.

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"Creating a carpet cloak that works in air was so difficult we had to embed it in a dielectric prism that introduced an additional phase in the reflected light, which made the cloak visible by phase-sensitive detection," says co-lead author Xingjie Ni, a recent member of Zhang's research group who is now an assistant professor at Penn State University. "Recent developments in metasurfaces, however, allow us to manipulate the phase of a propagating wave directly through the use of subwavelength-sized elements that locally tailor the electromagnetic response at the nanoscale, a response that is accompanied by dramatic light confinement." In the Berkeley study, when red light struck an arbitrarily shaped 3D sample object measuring approximately 1,300 square microns in area that was conformally wrapped in the gold nanoantenna skin cloak, the light reflected off the surface of the skin cloak was identical to light reflected off a flat mirror, making the object underneath it invisible even by phase-sensitive detection. The cloak can be turned "on" or "off" simply by switching the polarization of the nanoantennas. "A phase shift provided by each individual nanoantenna fully restores both the wavefront and the phase of the scattered light so that the object remains perfectly hidden," says co-lead author Zi Jing Wong, also a member of Zhang's research group. The ability to manipulate the interactions between light and metamaterials offers tantalizing future prospects for technologies such as high resolution optical microscopes and superfast optical computers. Invisibility skin cloaks on the microscopic scale might prove valuable for hiding the detailed layout of microelectronic components or for security encryption purposes. At the macroscale, among other applications, invisibility cloaks could prove useful for 3D displays.
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New diamond structures produce bright luminescence for quantum cryotography and biomarkers applications

Germanium defects in a diamond crystal lattice act as a reliable source for single photons, new research shows. The results are reported in ("Germanium-Vacancy Single Color Centers in Diamond") and provide a promising new route to building components for quantum cryptography and biomarkers. Pure diamonds are naturally colorless, but gaps in the crystal structure or impurities of other elements can create colors and even emit fluorescence. Recently, researchers have shown that the fluorescent lattice defects could be useful as single photon sources for quantum cryptography and as bright luminescent makers in living cells. Now, Takayuki Iwasaki and co-workers at Tokyo Institute of Technology (Tokyo Tech), together with scientists across Japan and Germany, have demonstrated a new type of diamond crystal defect that fluoresces to produce single photons in a narrow, high energy wavelength band. The defects, which have been named germanium-vacancy (GeV) centres, are relatively easy to fabricate in a reliable, reproducible way. Single GeV center in diamond Single GeV center in diamond. (left) Fluorescence mapping and (right) atomic structure model. Iwasaki and co-workers were inspired by recent work that demonstrated fluorescence from nitrogen-vacancy (NV) and silicon-vacancy (SiV) defects in diamond. They used an ion implantation method to insert germanium atoms into diamond films, before heating the films at 800°C. The resulting samples showed fluorescence only after heating, which induces diffusion of vacancies in the diamond lattice. The researchers therefore concluded that the fluorescence was produced by combined defects, each comprising a germanium atom side-by-side with a vacancy. The GeV centres produced single-photon bursts of fluorescence centred at a wavelength of around 602 nm, representing a higher energy fluorescence than SiV centres. Moreover, the researchers were also able to create the films through the less destructive method of chemical vapor deposition, producing films with narrower and more stable emission peaks of ensemble GeV centres which are useful for biomarkers. Overall, the work opens up a promising new avenue for developing sources of single photons, which are essential for quantum cryptography. Iwasaki and co-workers are also hopeful that they could incorporate GeV centres in nanodiamonds for use as biological markers. Background Quantum computing and cryptography Our current digital computers encode information in bits, which can have values of either 0 or 1. In quantum computers, data will instead be stored in ‘qubits’, which can take on values of not only 0 or 1, but also a superposition of the two states. This small difference represents a huge change in functionality, and allows information and data to be encrypted in ways that are impossible to decode using only classical methods-this is known as quantum cryptography. Indeed, quantum-encoded data cannot be copied or read without changing its state, meaning that it is impossible for third parties to eavesdrop on communications without being discovered. Single photon generation To achieve the secure data transmission by quantum cryptography, individual photons of known wavelengths must be used but are difficult to generate. Herein lies the motivation behind the work of Iwasaki and co-workers. Defects in diamond have been shown to produce fluorescence – emitting photons of fixed-wavelength light when illuminated by higher energy light – but these are often unreliable or difficult to fabricate. The search is on for new defect structures that not only produce strong, consistent fluorescence, but can also be made in a reproducible way. Biomarker To monitor individual proteins and the interior of living cells, nanometer sized markers such as fluorescent proteins and quantum dots are used. Due to the high biological compatibility of diamond, fluorescent defects in diamond nanostructures are stable biomarkers without optical bleaching. The bright emission from the GeV centres could be suitable for such biological applications. Methodology Iwasaki and co-workers began with an ion implantation method, which involved firing germanium atoms at high speed into pure diamond surfaces. They then heated the samples at 800°C to induce diffusion of vacancies – gaps in the diamond crystal lattice where a carbon atom is missing. By using Raman spectroscopy and confocal microscopy they observed fluorescent light emerging from the samples at a wavelength of around 602 nm. The team used theoretical calculations to deduce that this fluorescence resulted from combined defects, each comprising a germanium atom next to a lattice vacancy. The biggest step forward in the work was when Iwasaki and co-workers managed to create the same types of defects through a different method, microwave plasma chemical vapor deposition (MPCVD). MPCVD involves reactions of volatile chemicals on a substrate, and is often used to make synthetic diamonds. The defects in the sample prepared using MPCVD gave off more consistent fluorescence with a narrower and more stable peak. Moreover, MPCVD provides closer control over the fabrication process, and is less likely to produce unwanted damage to the samples than ion implantation. Future work Further work is needed to refine the fabrication process so that diamond films with germanium defects could be incorporated into devices for reliable single photon generation on demand.
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