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"). 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. (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) 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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