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Reducing Detrimental Defects for High‐Performance Metal Halide Perovskite Solar Cells
Get PDFIn several photovoltaic (PV) technologies, the presence of electronic defects within the semiconductor band gap limit the efficiency, reproducibility, as well as lifetime. Metal halide perovskites (MHPs) have drawn great attention because of their excellent photovoltaic properties that can be achieved even without a very strict film‐growth control processing. Much has been done theoretically in describing the different point defects in MHPs. Herein, we discuss the experimental challenges in thoroughly characterizing the defects in MHPs such as, experimental assignment of the type of defects, defects densities, and the energy positions within the band gap induced by these defects. The second topic of this Review is passivation strategies. Based on a literature survey, the different types of defects that are important to consider and need to be minimized are examined. A complete fundamental understanding of defect nature in MHPs is needed to further improve their optoelectronic functionalities.journal articl
Dynamic actin-mediated nano-scale clustering of CD44 regulates its meso-scale organization at the plasma membrane
Get PDFTransmembrane adhesion receptors at the cell surface, such as CD44, are often equipped with modules to interact with the extracellular matrix (ECM) and the intracellular cytoskeletal machinery. CD44 has been recently shown to compartmentalize the membrane into domains by acting as membrane pickets, facilitating the function of signaling receptors. While spatial organization and diffusion studies of membrane proteins are usually conducted separately, here we combine observations of organization and diffusion by using high spatio-temporal resolution imaging on living cells to reveal a hierarchical organization of CD44. CD44 is present in a meso-scale meshwork pattern where it exhibits enhanced confinement and is enriched in nanoclusters of CD44 along its boundaries. This nanoclustering is orchestrated by the underlying cortical actin dynamics. Interaction with actin is mediated by specific segments of the intracellular domain. This influences the organization of the protein at the nano-scale, generating a selective requirement for formin over Arp2/3-based actin-nucleation machinery. The extracellular domain and its interaction with elements of ECM do not influence the meso-scale organization, but may serve to reposition the meshwork with respect to the ECM. Taken together, our results capture the hierarchical nature of CD44 organization at the cell surface, with active cytoskeleton-templated nanoclusters localized to a meso-scale meshwork pattern.journal articl
Integrating host plant phylogeny, plant traits, intraspecific competition and repeated measures using a phylogenetic mixed model of field behaviour by polyphagous herbivores, the leaf-cutting ants
Get PDFHerbivores use a wide range of factors to choose their host, including their own physiological states, physical characteristics of plants and the degree of competition. Field observations of herbivores in their native habitats provide a means for simultaneously estimating the relative importance of these factors, but statistical analysis of all these factors may be challenging. Here we used a 7-week dataset of leaf-cutting ant (Atta cephalotes) foraging in a diverse Neotropical arboretum containing 193 tree species (822 trees) to examine the relative role of tree phylogeny, territoriality and tree functional characteristics using a phylogenetic generalized least squares (PGLS) model. We observed that 54 tree species (117 trees) were foraged by the ants. This pattern was not random, but reflected known features of leaf-cutting ant foraging behaviour, such as a preference for larger trees and the decreased likelihood of foraging at the periphery of a colony's territory. However, random effects such as tree phylogeny, the identity of individual trees and colony-specific effects explained most of the variation in foraging data. A significant phylogenetic effect on foraging likelihood (lambda = 0.28), together with repeated measures of foraging on the same tree species, allowed estimation of relative palatability for each plant species. PGLS models can be flexibly scaled to include other covariates for even more complex investigation of foraging behaviour, and the link function can be modified to include the amount of plants foraged. As a result, PGLS can be used as a flexible framework for the study of LCA foraging.journal articl
Behavioral and transcriptomic analysis of embryonic microglia during colonization and degeneration of zebrafish retina
Get PDFOkinawa Institute of Science and Technology Graduate UniversityDoctor of PhilosophyThis work was designed to understand how embryonic microglia precursors migration relates to global processes such as blood vessel development and neurogenesis and to decipher transcriptomic signatures governing embryonic microglia behavior. The zebrafish retina provides a precise anatomical and molecularly characterized extension of forebrain to study colonization by microglia precursors. In this study we decode the mechanism driving the process of the microglia precursors colonization of developing zebrafish retina and its role in early neurodegeneration of visual system. Transgenic live imaging studies and slbp1 mutant analysis of migrating microglia precursors into retina revealed that this migration is a step wise process depending on both retinal vasculogenesis and neurogenesis. These two key processes proceeds at the same time-scale as retinal colonization by microglia precursors. Initially microglia precursors uses emerging blood vessels system around the lens to enter the a space near basement membrane of the retina. As the wave of neurogenesis proceeds, these microglia precursors starts to migrate into retina, preferentially in neuronally differentiated neurons. Intraretinal microglia numbers decrease when retinal neurogenesis is delayed, suggesting crosstalk between early born neurons and microglia precursors. To explore microglia signaling during neurogenesis at the molecular level, we carried out a single-cell transcriptomic analysis of microglia precursors and newly matured microglia in the developing zebrafish brain.doctoral thesi
Identifying the Evolutionary Conditions for the Emergence of Alternative Reproductive Tactics in Simulated Robot Colonies
Get PDFOkinawa Institute of Science and Technology Graduate UniversityDoctor of PhilosophyAlternative reproductive tactics (ARTs), phenomena in which individuals within one sex adopt different tactics for accessing mates or raising offspring, are commonly observed in all major taxa. In order to study the ecological conditions for the emergence of ARTs, we developed an embodied evolution framework incorporating ecological features, such as body size and energy maintenance, where male and female robotic agents naturally face both intersexual and intrasexual interactions for survival and reproduction. Each agent has a decision neural network with extrinsic and intrinsic sensory inputs to choose one of four basic behaviors: mating, foraging, approaching and waiting. The reproductive success depends on the body size and the energy level of both male and female upon mating and it is assumed that only female carries the reproduction cost, as in nature the cost of male’s sperm production is negligible relative to that of female’s eggs. We performed simulation experiments in environments with different conditions (food density, reproductive cost, and male-female ratio) and found ARTs emerged both in males and females. Males evolved three kinds of alternative tactics - fixed genetically distinct ARTs (dominant and sneaker males that differ in body size and the tactic for getting access to female), conditionally flexible ARTs (individuals change tactics according to body size), and mixed ARTs (combination of genetically fixed and conditionally flexible ARTs). Females evolved to have two genetically distinct ARTs (quality oriented female, QoF, and number oriented female, NoF), where they increase fitness either by offspring quality or quantity. Analysis of the results confirms the experimental notions that male genetically fixed ARTs are strongly affected by intensity of sexual selection, male conditionally flexible ARTs are significantly affected by competition level, and female ARTs are mainly affected by food density. Analysis of ESS shows male ARTs are evolutionary stable with negative frequency dependent selection and female ARTs are evolutionary stable with both frequency and density dependent selection. To our knowledge, this study is the first to show the emergence of ARTs in both male and female from initially continuous characteristics in a simulated embodied evolution framework. The evolved ARTs are quite similar to the ARTs found in nature and provide insights about how interactions between the sexes are affected by and affect the evolution of ARTs within each sex. This framework is flexible enough to further analyze species of different sexual mechanisms (hermaphrodite, androdioecious, gynodioecious, etc.) and can be used as an important tool to understand the ecology of social interaction.doctoral thesi
Conditional and biased regeneration of cone photoreceptor types in the zebrafish retina
Get PDFA major challenge in regenerative medicine is replacing cells lost through injury or disease. While significant progress has been made, much remains unknown about the accuracy of native regenerative programs in cell replacement. Here, we capitalized on the regenerative capacity and stereotypic retinal organization of zebrafish to determine the specificity with which retinal Müller glial cells replace lost neuronal cell types. By utilizing a targeted genetic ablation technique, we restricted death to all or to distinct cone photoreceptor types (red, blue, or UV‐sensitive cones), enabling us to compare the composition of cones that are regenerated. We found that Müller glia produce cones of all types upon nondiscriminate ablation of these photoreceptors, or upon selective ablation of red or UV cones. Pan‐ablation of cones led to regeneration of the various cone types in relative abundances that resembled those of nonablated controls, that is, red > green > UV ~ blue cones. Moreover, selective loss of red or UV cones biased production toward the cone type that was ablated. In contrast, ablation of blue cones alone largely failed to induce cone production at all, although it did induce cell division in Müller glia. The failure to produce cones upon selective elimination of blue cones may be due to their low abundance compared to other cone types. Alternatively, it may be that blue cone death alone does not trigger a change in progenitor competency to support cone genesis. Our findings add to the growing notion that cell replacement during regeneration does not perfectly mimic programs of cell generation during development.journal articl
Fabrication of optical nanofibre-based cavities using focussed ion-beam milling: a review
Get PDFNanofibre-based optical cavities are particularly useful for quantum optics applications, such as the development of integrated single-photon sources, and for studying fundamental light-matter interactions in cavity quantum electrodynamics (cQED). Although several techniques have been used to produce such cavities, focussed ion beam (FIB) milling is becoming popular; it can be used for the fabrication of complex structures directly in the nanofibre. However, it is challenging to mill insulating materials with highly curved geometries and large aspect ratios, such as silica nanofibres, due to charge accumulation in the material. In this article, we highlight the main features of nanofibres and briefly review cQED with nanofibre-based optical cavities. An overview of the milling process is given with a summary of different FIB milled devices and their applications. Finally, we present our technique to produce nanofibre cavities by FIB milling. To overcome the aforementioned challenges, we present a specially designed base plate with an indium tin oxide (ITO)-coated Si substrate and outline our procedure, which improves stability during milling and increases repeatability.journal articl
Hydration Properties of HnPO4n−3 (n = 0−3) From Ab Initio Molecular Dynamics Simulations
Get PDFFor a comprehensive and detailed microscopic understanding of the hydration properties of primary aqueous phosphorus species of valence states V (viz., H3PO4, H2PO4–, HPO42–, and PO43–), a series of extensive ab initio molecular dynamics simulations is conducted at ambient temperature. In each of these cases, the spatially resolved, three-dimensional hydration shells are computed, allowing for a direct microscopic visual understanding of the hydration shells around the species. Since these species are excellent agents for the formation of hydrogen bonds (H-bonds) in water, which determine a wide range of their structural, dynamic, and spectroscopic features, a detailed analysis of the qualitative and quantitative aspects of the H-bonds, including their lifetime calculations, is performed. Vibrational density of states (VDOS) is calculated for each of the species in solute phases, resolved for each H-bonding site, and compared against the gas-phase normal modes of H3PO4 for the purpose of understanding the signatures of the peaks in VDOS plots and, in particular, the effects of solvation and H-bonding mechanisms. The results are well in line with available experimental data and other recent computer-aided studies in the literature.journal articl
Enhancement of coercive field in atomically-thin quenched Fe5GeTe2
Get PDFWe have fabricated thin films of a van der Waals (vdW) ferromagnetic metal Fe₅GeTe₂ and characterized them by measuring the anomalous Hall effect. While the bulk Fe₅GeTe₂ does not exhibit a perpendicular magnetic anisotropy (PMA) unlike Fe₃GeTe₂, PMA emerges in the thin film devices. Furthermore, the PMA is enhanced with decreasing thickness of Fe₅GeTe₂. In particular, a thin film (5 unit-cell layer) device fabricated with Fe₅GeTe₂ quenched at 1050 K has two times larger coercive field than that prepared without quenching. Such a PMA should be useful for future vdW spintronic devices.journal articl
Improvements in optical techniques to investigate the behavior and neuronal network dynamics over long timescales
Get PDFOkinawa Institute of Science and Technology Graduate UniversityDoctor of PhilosophyDevelopments in optical technology have produced an important shift in experimental neuroscience from electrophysiological methods for observation and stimulation to all-optical solutions. One expects this trend to continue as future developments continue to deliver, and improve upon, the original promises of the technology: 1) minimally invasive actuation and recording of neurons, and 2) a drastic increase in targets that can be treated simultaneously. Moreover, as the high costs of the technology are reduced, one may expect its larger-scale adoption in the neuroscience community. In this thesis, I describe the development and implementation of two alloptical solutions for the analysis of behavior, neuronal signaling, and stimulation, which improve on previous state-of-the-art: (1) A minimally-invasive, high signal-to-noise twophoton microscopy setup capable of simultaneous, live-imaging of a large subset of sensory neurons post activation, and (2) a low-cost tracking solution to stimulate and record behavior. I begin this thesis with a review of recent advances in optical neuroscience techniques for the study of neuronal networks with the focus on work done in Caenorhabditis elegans. Then, in chapter 2, I describe my implementation of a two-photon temporal focusing microscopy setup and show significant improvements through the use of a high power/ high pulse repetition rate excitation system, enabling live imaging with high resolution for extended periods of time. I model temperature increase during a physiological imaging scenario for different repetition rates at fixed peak intensities and find range centered around 1 MHz to be optimal. Lastly, I describe the low-cost tracking setup with the ability to stimulate and record behavior over the course of hours. The setup is capable of two-color stimulation of optogenetic proteins over the area of the behavioral arena in combination with volatile chemicals. To showcase the utility of the system, I demonstrate behavioral analysis of integration of contradictory cues. In summary, I present a set of techniques for the interrogation of neural networks from animal behavior to neuronal activity, over timescales of potentially hours and days. These techniques can be used to address a new dimension of scientific questions.doctoral thesi