Unraveling exciton kinetics of electroluminescence in colloidal quantum dot LEDs

We systematically studied emission kinetics of colloidal quantum dots in active light-emitting diodes using time-correlated single photon counting and conclusively revealed the combined effect of both electric field and charging acting together in reducing the quantum efficiency during operation. � 2016 OSA

Azimuthally Polarized, Circular Colloidal Quantum Dot Laser Beam Enabled by a Concentric Grating

Since optical gain was observed from colloidal quantum dots (CQDs), research on CQD lasing has been focused on the CQDs themselves as gain materials and their coupling with optical resonators. Combining the advantages of a CQD gain medium and optical microcavity in a laser device is desirable. Here, we show concentric circular Bragg gratings intimately incorporating CdSe/CdZnS/ZnS gradient shell CQDs. Because of the strong circularly symmetric optical confinement in two dimensions, the output beam CQD-based circular grating distributed feedback laser is found to be highly spatially coherent and azimuthally polarized with a donut-like cross section. We also observe the strong modification of the photoluminescence spectrum by the grating structures, which is associated with modification of optical density of states. This effect confirmed the high quality of the resonator that we fabricated and the spectral overlap between the optical transitions of the emitter and resonance of the cavity. Single mode lasing has been achieved under a quasi-continuous pumping regime, while the position of the lasing mode can be conveniently tuned via adjusting the thickness of the CQD layer. Moreover, a unidirectional output beam can be observed as a bright circular spot on a screen without any collimation optics, presenting a direct proof of its high spatial coherence. © 2016 American Chemical Society

A Secure Cloud-based Platform to Host Healthcare Applications

Digital technologies, such as Big Data analytics, artificial intelligence, cloud and high-performance computing are presenting new opportunities to transform healthcare systems, increase connectivity of hospitals and other providers, and therefore potentially and significantly improve patient care. However, such networked computing infrastructures also raise significant cybersecurity risks, especially in the healthcare domain, where protecting sensitive personal information is of paramount importance. Project ASCLEPIOS aims at strengthening the trust of users in cloud-based healthcare services by utilizing trusted execution environment and several modern cryptographic approaches such as attribute based encryption, searchable encryption, functional encryption to build a cloud-based e-health framework that protects users’ privacy, prevents both internal and external attacks, verifies the integrity of medical devices before application, and runs privacy-preserving data analytics on encrypted data. The project investigates modern encryption techniques and their combination in order to provide increased security of e-health applications that are then presented towards end-users utilizing a cloud-based platform. Although some topics such as security and privacy are already investigated through block-chain related technologies, it has been decided that the selected approaches would be more suitable for these particular challenges. In order to prototype its security services, ASCLEPIOS develops and deploys three large-scale healthcare demonstrators, provided by three leading hospitals from Europe. These demonstrators are rooted in the practice-based problems and applications provided by the project’s healthcare partners. The Amsterdam University Centers, University of Amsterdam, plans to improve stroke hyper-acute care through secure information sharing on a cloud computing platform to improve patient management. Additionally, they are also building prediction models to enable earlier discharge of patients from hospitals with lower risk factors. Charité Berlin plans to improve inpatient and outpatient sleep medication by remotely controlling the quality of the collected data and transferring it on-line for further analysis. Finally, the Norwegian Centre for e-health Research, University Hospital of North Norway is developing a system for privacy-preserving monitoring and benchmarking of antibiotics prescription of general practitioners. The common characteristics of these three scenarios are the increased demand for high levels of security in data transfer, storage and privacy preserving analytics on cloud infrastructures. In order deploy, operate and further develop these applications to increase their security with the ASCLEPIOS framework, a cloud computing testbed is being setup. The testbed uses state-of-the-art technologies for cloud application deployment and run-time orchestration in order to ensure the optimized deployment and execution of the demonstrator applications. As the data sources do not require the local execution (albeit in one case data may remain on the data source) of processing, there is no need for fog or edge computing, but the testbed is based on private OpenStack cloud computing infrastructures and utilizes the MiCADO framework which is compatible with different containers such as Docker and Kubernetes. The project started only recently, and currently it is in the early stages of systems design and specification. This presentation will provide a short introduction to the ASCLEPIOS project and its demonstrators and will present early results of the currently ongoing requirements specification and platform design processes

Green Stimulated Emission Boosted by Nonradiative Resonant Energy Transfer from Blue Quantum Dots

Thanks to their tunability and versatility, the colloidal quantum dots (CQDs) made of II-VI semiconductor compound offer the potential to bridge the "green gap" in conventional semiconductors. However, when the CQDs are pumped to much higher initial excitonic states compared to their bandgap, multiexciton interaction is enhanced, leading to a much higher stimulated emission threshold. Here, to circumvent this drawback, for the first time, we show a fully colloidal gain in green enabled by a partially indirect pumping approach assisted by Förster resonance energy transfer process. By introducing the blue CQDs as exciton donors, the lasing threshold of the green CQDs, is reduced dramatically. The blue CQDs thus serve as an energy-transferring buffer medium to reduce excitation energy from pumping photons in a controlled way by injecting photoinduced excitons into green CQDs. Our newly developed colloidal pumping scheme could enable efficient CQD lasers of full visible colors by a single pump source and cascaded exciton transfer. This would potentially pave the way for an efficient multicolor laser for lighting and display applications. © 2016 American Chemical Society

Temperature-dependent optoelectronic properties of quasi-2D colloidal cadmium selenide nanoplatelets

Colloidal cadmium selenide (CdSe) nanoplatelets (NPLs) are a recently developed class of efficient luminescent nanomaterials suitable for optoelectronic device applications. A change in temperature greatly affects their electronic bandstructure and luminescence properties. It is important to understand how and why the characteristics of NPLs are influenced, particularly at elevated temperatures, where both reversible and irreversible quenching processes come into the picture. Here we present a study of the effect of elevated temperatures on the characteristics of colloidal CdSe NPLs. We used an effective-mass envelope function theory based 8-band k·p model and density-matrix theory considering exciton-phonon interaction. We observed the photoluminescence (PL) spectra at various temperatures for their photon emission energy, PL linewidth and intensity by considering the exciton-phonon interaction with both acoustic and optical phonons using Bose-Einstein statistical factors. With a rise in temperature we observed a fall in the transition energy (emission redshift), matrix element, Fermi factor and quasi Fermi separation, with a reduction in intraband state gaps and increased interband coupling. Also, there was a fall in the PL intensity, along with spectral broadening due to an intraband scattering effect. The predicted transition energy values and simulated PL spectra at varying temperatures exhibit appreciable consistency with the experimental results. Our findings have important implications for the application of NPLs in optoelectronic devices, such as NPL lasers and LEDs, operating much above room temperature. © 2017 The Royal Society of Chemistry

Anomalous spectral characteristics of ultrathin sub-nm colloidal CdSe nanoplatelets

We demonstrate high quantum yield broad photoluminescence emission of ultrathin sub-nanometer CdSe nanoplatelets (two-monolayer). They also exhibit polarization-characterized lateral size dependent anomalous heavy hole and light/split-off hole absorption intensities. © 2017 IEEE

High performance infrared photodetectors up to 2.8 μm wavelength based on lead selenide colloidal quantum dots

The strong quantum confinement effect in lead selenide (PbSe) colloidal quantum dots (CQDs) allows to tune the bandgap of the material, covering a large spectral range from mid- to near infrared (NIR). Together with the advantages of low-cost solution processability, flexibility and easy scale-up production in comparison to conventional semiconductors especially in the mid- to near infrared range, PbSe CQDs have been a promising material for infrared optoelectronic applications. In this study, we synthesized monodisperse and high purity PbSe CQDs and then demonstrated the photodetectors working at different wavelengths up to 2.8 μm. Our high quality PbSe CQDs show clear multiple excitonic absorption peaks. PbSe CQD films of different thicknesses were deposited on interdigitated platinum electrodes by a simple drop casting technique to make the infrared photodetectors. At room temperature, the high performances of our PbSe CQD photodetectors were achieved with maximum responsivity, detectivity and external quantum efficiency of 0.96 A/W, 8.13 × 109 Jones and 78% at 5V bias. Furthermore, a series of infrared LEDs with a broad wavelength range from 1.5 μm to 3.4 μm was utilized to demonstrate the performance of our fabricated photodetectors with various PbSe CQD film thicknesses. © 2017 Optical Society of America

Design and construction of a very lively bridge

In recent years, an increasing number of light structures has been reported to exhibit substantial vertical vibrations when exposed to pedestrian-induced dynamic loading. It is believed that pedestrians interact with lively structures by altering their walking style and changing the dynamic properties of the vibrating system. As the existing vibration serviceability guidelines do not address these pedestrian-structure interaction effects, they cannot predict the structural dynamic response accurately. Fundamental understanding of the pedestrian-structure interaction is currently limited since most reported observations are of qualitative nature. To improve understanding and develop models of human interaction with lively structures, a purpose-built experimental facility that can be excited by human walking is required. This paper describes design and construction of a 19.9 m long, low-frequency and lightly damped experimental bridge for studying pedestrian-structure interaction. The challenge to design a relatively heavy and low-frequency footbridge in the limited space of the Structures Laboratory at the University of Warwick, UK, was met by adopting a traditional steel-concrete composite structural system. The experimental data collected on the "Warwick Bridge" during first six-months of structural life are presented to characterise both its static and dynamic behaviour. Dynamic testing of the bridge revealed that, with an achieved fundamental natural frequency of 2.4 Hz, the corresponding damping ratio of 0.5%, and an opportunity to tune the dynamic properties as required, the key design criteria were successfully met

Using chlorate as an analogue to nitrate to identify candidate genes for nitrogen use efficiency in barley

Nitrogen (N) is one of the most important macronutrients for crop growth and development. Large amounts of N fertilizers are applied exogenously to improve grain yield and quality, which has led to environmental pollution and high cost of production. Therefore, improvement of N use efficiency (NUE) is a very important aspect for sustainable agriculture. Here, a pilot experiment was firstly conducted with a set of barley genotypes with confirmed NUE to validate the fast NUE screening, using chlorate as an analogue to nitrate. High NUE genotypes were susceptible to chlorate-induced toxicity whereas the low NUE genotypes were tolerant. A total of 180 barley RILs derived from four parents (Compass, GrangeR, Lockyer and La Trobe) were further screened for NUE. Leaf chlorosis induced by chlorate toxicity was the key parameter observed which was later related to low-N tolerance of the RILs. There was a distinct variation in chlorate susceptibility of the RILs with leaf chlorosis in the oldest leaf ranging from 10 to 80%. A genome-wide association study (GWAS) identified 9 significant marker-trait associations (MTAs) conferring high chlorate sensitivity on chromosomes 2H (2), 3H (1), 4H (4), 5H (1) and Un (1). Genes flanking with these markers were retrieved as potential targets for genetic improvement of NUE. Genes encoding Ferredoxin 3, leucine-rich receptor-like protein kinase family protein and receptor kinase are responsive to N stress. MTA4H5468 which exhibits concordance with high NUE phenotype can further be explored under different genetic backgrounds and successfully applied in marker-assisted selection

Genetic dissection of the interactions between semi-dwarfing genes sdw1 and ari-e and their effects on agronomic traits in a barley MAGIC population

The introduction of semi-dwarf cereal crop varieties in the 1960s enabled extensive increases in global food production. Semi-dwarf barley cultivars have shorter stature and improved stress resistance, grain yield and quality than tall varieties. The major semi-dwarf genes, sdw1 and ari-e, have been widely used in barley breeding programmes worldwide. In this study, we identified a novel sdw1 allele in cv. Lockyer and investigated the effect of sdw1 and ari-e and their interactions on plant height, flowering time and grain yield using a Multi-parent Advanced Generation Inter-cross (MAGIC) population generated from four elite barley cultivars. Allele-specific markers combined with Kompetitive Allele Specific Polymerase Chain Reaction (KASP) assays were used for fast and accurate screening of plants with different allele combinations. The results showed that the novel sdw1 allele from cv. Lockyer had similar effects on plant growth and phenology as the known sdw1.d allele. The two semi-dwarf genes sdw1 and ari-e had an additive effect for plant height with height reductions up to 27 cm. The ari-e gene triggered earlier flowering up to 6 days compared to WT and reduced the delay in flowering time caused by the sdw1 gene. The two semi-dwarf genes boosted grain yields by 20% in the sdw1 plants and up to 28% in plants with the two combined semi-dwarf genes. The results presented will benefit the development of higher-yielding and better-adapted barley cultivars