25,007 research outputs found

    What is moving in hybrid halide perovskite solar cells?

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    Organic-inorganic semiconductors, which adopt the perovskite crystal structure, have perturbed the landscape of contemporary photovoltaics research. In this Account, we discuss the internal motion of methylammonium lead iodide (CH3_3NH3_3PbI3_3) and formamidinium lead iodide ([CH(NH2_2)2_2]PbI3_3), covering: (i) molecular rotation-libration in the cuboctahedral cavity; (ii) drift and diffusion of large electron and hole polarons; (iii) transport of charged ionic defects. These processes give rise to a range of properties that are unconventional for photovoltaic materials, including frequency-dependent permittivity, low electron-hole recombination rates, and current-voltage hysteresis. Multi-scale simulations - drawing from electronic structure, ab initio molecular dynamic and Monte Carlo techniques - have been combined with neutron scattering and ultra-fast vibrational spectroscopy to qualify the nature and timescales of the motions. Recent experimental evidence and theoretical models for simultaneous electron transport and ion transport in these materials has been presented, suggesting they are mixed-mode conductors with similarities to metal oxide perovskites developed for battery and fuel cell applications. We expound on the implications of these effects for the photovoltaic action. The temporal behaviour found in hybrid perovskites introduces a sensitivity in materials characterisation to the time and length scale of the measurement, as well as the history of each sample. It also poses significant challenges for accurate materials and device simulations. Herein, we critically discuss the atomistic origin of the dynamic processes.Comment: 29 pages, 3 figure

    Growth of confined cancer spheroids: a combined experimental and mathematical modelling approach

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    We have integrated a bioengineered three-dimensional platform by generating multicellular cancer spheroids in a controlled microenvironment with a mathematical model to investigate\ud confined tumour growth and to model its impact on cellular processes

    Scratch hardness as a quasi-intrinsic parameter to measure the scratch resistance of polymers

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    In this work four different polymers (acrylonitrile-butadiene-styrene, high-impact polystyrene, rubber-toughened polybutylene terephthalate, linear low-density polyethylene) were characterized in terms of their bulk (modulus and yield stress) and surface (scratch hardness) mechanical properties. The intrinsic time-dependence of the materials was addressed by performing DMA and compression tests at varying testing speed/frequency, exploiting time-temperature superposition and Eyring’s model to obtain data at strain rates compatible with scratch experiments. The latter were performed by applying different loading histories (constant depth or load) and indenters. Scratch hardness was determined using Pelletier’s model; it was demonstrated that such a parameter provides a reliable and almost intrinsic (i.e. loading history independent) evaluation of scratch resistance, seen as the resistance the material opposes to indenter penetration. The material compressive yield stress (evaluated at the strain rate relevant to scratch phenomena) was found to be the key controlling factor in determining scratch hardness. It can therefore be taken as a measure of the mechanical scratch resistance when evaluating the possible effects of variables such as material composition, crystallinity, physical ageing… Its relation with other aspects of the scratch phenomenon (in particular deformation recovery) was also explored, accounting for the specific deformation regime imposed by the indenter (transitioning from elastic to predominantly plastic)

    Municipal wastewater treatment with pond technology : historical review and future outlook

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    Facing an unprecedented population growth, it is difficult to overstress the assets for wastewater treatment of waste stabilization ponds (WSPs), i.e. high removal efficiency, simplicity, and low cost, which have been recognized by numerous scientists and operators. However, stricter discharge standards, changes in wastewater compounds, high emissions of greenhouse gases, and elevated land prices have led to their replacements in many places. This review aims at delivering a comprehensive overview of the historical development and current state of WSPs, and providing further insights to deal with their limitations in the future. The 21st century is witnessing changes in the way of approaching conventional problems in pond technology, in which WSPs should no longer be considered as a low treatment technology. Advanced models and technologies have been integrated for better design, control, and management. The roles of algae, which have been crucial as solar-powered aeration, will continue being a key solution. Yet, the separation of suspended algae to avoid deterioration of the effluent remains a major challenge in WSPs while in the case of high algal rate pond, further research is needed to maximize algal growth yield, select proper strains, and optimize harvesting methods to put algal biomass production in practice. Significant gaps need to be filled in understanding mechanisms of greenhouse gas emission, climate change mitigation, pond ecosystem services, and the fate and toxicity of emerging contaminants. From these insights, adaptation strategies are developed to deal with new opportunities and future challenges

    Intermixing of InGaAs/GaAs quantum wells and quantum dots using sputter-deposited silicon oxynitride capping layers

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    Various approaches can be used to selectively control the amount of intermixing in III-Vquantum well and quantum dotstructures. Impurity-free vacancy disordering is one technique that is favored for its simplicity, however this mechanism is sensitive to many experimental parameters. In this study, a series of silicon oxynitride capping layers have been used in the intermixing of InGaAs/GaAs quantum well and quantum dotstructures. These thin films were deposited by sputter deposition in order to minimize the incorporation of hydrogen, which has been reported to influence impurity-free vacancy disordering. The degree of intermixing was probed by photoluminescence spectroscopy and this is discussed with respect to the properties of the SiOxNyfilms. This work was also designed to monitor any additional intermixing that might be attributed to the sputtering process. In addition, the high-temperature stress is known to affect the group-III vacancy concentration, which is central to the intermixing process. This stress was directly measured and the experimental values are compared with an elastic-deformation model.This work has been made possible with access to the ACT Node of the Australian National Fabrication Facility and through the financial support of the Australian Research Council

    A MEMS viscometer for unadulterated human blood

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    The design and theoretical modelling of an oscillating micro-mechanical-viscometer designed for the measurement of whole unadulterated human blood, is described. The proposed device utilises the dependence of the squeeze-film damping ratio on properties of the surrounding fluid to measure fluid viscosity using an oscillating plate structure. The optimum geometrical configuration for the device structure has been investigated and a methodology for defining the optimum configuration of the micro-mechanical sensor identified. This is then applied to calculate the predicted noise equivalent viscosity change . It was found that the device performance is limited by electronic noise within the detection circuitry rather than thermal mechanical noise. An electronic noise limited measurement resolution of , is predicted for measurement over a shear range of , at a measurement bandwidth of . The linearity of response of the micro-mechanical-viscometer is considered and the device is predicted to provide a linear measurement response

    Second-layer nucleation in coherent Stranski-Krastanov growth of quantum dots

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    We have studied the monolayer-bilayer transformation in the case of the coherent Stranski-Krastanov growth. We have found that the energy of formation of a second layer nucleus is largest at the center of the first-layer island and smallest on its corners. Thus nucleation is expected to take place at the corners (or the edges) rather than at the center of the islands as in the case of homoepitaxy. The critical nuclei have one atom in addition to a compact shape, which is either a square of i*i or a rectangle of i*(i-1) atoms, with i>1 an integer. When the edge of the initial monolayer island is much larger than the critical nucleus size, the latter is always a rectangle plus an additional atom, adsorbed at the longer edge, which gives rise to a new atomic row in order to transform the rectangle into the equilibrium square shape.Comment: 6 pages, 4 figures. Accepted version, minor change
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