Layer-by-Layer Construction Strategies Towards Efficient CZTS Solar Cells

Solar energy has very high potential for ensuring the world’s energy requirements for the long-term future. Earth-abundant materials like Cu2ZnSnS4 (CZTS) are especially desirable, with a non-toxic, low-cost nature, though the quaternary nature allows for a lot of crystal structure variability, and thus underwhelming performance. This Ph.D. thesis is devoted to deepening the understanding of the CZTS material formation, and the processes that can be used to control it, to construct a low-cost, high efficiency CZTS-based solar cell. The layer-by-layer approach presented within this thesis shows great potential for rectifying the problem. CZTS nanocrystal (NC) stoichiometric control was achieved, and led to reproducible structure formation within the films (Chapter 2). Structural correlations to photoresponse for these films were established by means of synchrotron spectroscopies, and increased charge-carrier flux out of the NC film (Chapter 3). Refinement of the NC stoichiometry (Chapter 4) enhanced these results, and extended the structural correlations. CdS addition to the CZTS film to form the p-n junction was investigated, and confirmed water intercalations in the film arising during CdS deposition. Mild thermal treatments were found to purify the films, and lead to further amplification of the charge-carrier flux (Chapter 5). The CZTS/CdS films were found to not have the desired enhancement to the overall photoresponse due to surface oxides, and poor alignment in the valence/conduction bands of the materials interface. It was discovered that acetic acid etching of the CZTS layer prior to CdS addition removed the oxides, and drastically improved the charge-carrier flux (Chapter 6). In fact, the band structure was aligned favorably to create an ideal p-n junction. The band structure diagram was well established, and the electron flow in the conduction band overlap was confirmed to be favored. The full device was built by combining all refinement processes, and adding ZnO and Al-doped ZnO window layers with atomic layer deposition (Chapter 7). A high open-circuit potential of 0.85 V, and competitive device efficiency of 8.5% were achieved. The layer-by-layer approach is thus proven throughout this thesis to be a highly effective strategy and anticipated to guide intelligent solar cell designs and fabrications

The Effects of Global Climate Change on Canadian Boreal Forest Collembola Communities

Soil fauna are an integral component of terrestrial ecosystem function. The effects of global environmental change on soil biodiversity are poorly studied, particularly interactions among temperature, atmospheric CO2, precipitation intensity, and nutrient loading. Body size distributions can be used to quantify soil community responses to perturbation and consequences for ecosystem function. I quantified top-down and bottom-up effects of environmental change on the abundance, richness, and size distribution of the soil microarthropod group Collembola. I demonstrated negative effects in a lab experiment of increased precipitation on collembolan density and richness across all size groups. I demonstrated positive effects in a field experiment of N addition on collembolan richness, and a positive effect of C addition on evenness. These findings demonstrate that precipitation can act as a disturbance to soil communities, as well as the importance of bottom-up control in soils, and the responsiveness of body size distributions to environmental change

Polydnavirus Innexins Disrupt Host Cellular Encapsulation and Larval Maturation

Polydnaviruses are dsDNA viruses associated with endoparasitoid wasps. Delivery of the virus during parasitization of a caterpillar and subsequent virus gene expression is required for production of an amenable environment for parasitoid offspring development. Consequently, understanding of Polydnavirus gene function provides insight into mechanisms of host susceptibility and parasitoid wasp host range. Polydnavirus genes predominantly are arranged in multimember gene families, one of which is the vinnexins, which are virus homologues of insect gap junction genes, the innexins. Previous studies of Campoletis sonorensis Ichnovirus Vinnexins using various heterologous systems have suggested the four encoded members may provide different functionality in the infected caterpillar host. Here, we expressed two of the members, vnxG and vnxQ2, using recombinant baculoviruses in susceptible host, the caterpillar Heliothis virescens. Following intrahemocoelic injections, we observed that \u3e90% of hemocytes (blood cells) were infected, producing recombinant protein. Larvae infected with a vinnexin-recombinant baculovirus exhibited significantly reduced molting rates relative to larvae infected with a control recombinant baculovirus and mock-infected larvae. Similarly, larvae infected with vinnexin-recombinant baculoviruses were less likely to survive relative to controls and showed reduced ability to encapsulate chromatography beads in an immune assay. In most assays, the VnxG protein was associated with more severe pathology than VnxQ2. Our findings support a role for Vinnexins in CsIV and more broadly Ichnovirus pathology in infected lepidopteran hosts, particularly in disrupting multicellular developmental and immune physiology

Grain Reynolds number scale effects in dry granular slides

©2020. The Authors. Scale effects are differences in physical behavior that manifest between a large event and a geometrically scaled laboratory model and may cause misleading predictions. This study focuses on scale effects in granular slides, important in the environment and to industry. A versatile 6 m long laboratory setup has been built following Froude similarity to investigate dry granular slides at scales varied by a factor of 4, with grain Reynolds numbers Rein the range of 102 to 103. To provide further comparison, discrete element method simulations have also been conducted. Significant scale effects were identified; the nondimensional surface velocity increased by up to 35%, while the deposit runout distance increased by up to 26% from the smallest to the largest model. These scale effects are strongly correlated with Re, suggesting that interactions between grains and air are primarily responsible for the observed scale effects. This is supported by the discrete element method data, which did not show these scale effects in the absence of air. Furthermore, the particle drag force accounted for a significant part of the observed scale effects. Cauchy number scale effects caused by unscaled particle stiffness resulting in varying dust formation with scale are found to be of secondary importance. Comparisons of the laboratory data to that of other studies and of natural events show that data normalization with Re is an effective method of quantitatively comparing laboratory results to natural events. This upscaling technique can improve hazard assessment in nature and is potentially useful for modeling industrial flows

Multi-spatial-mode quadrature squeezing from four-wave mixing in a hot atomic vapour

Noise suppression across multiple spatial modes of a light field would serve to improve the field's imaging capabilities and allow it to act as a more effective carrier of quantum information. This thesis describes a scheme that utilises a nonlinear process (four wave mixing (4WM)) in Rubidium 85 in order to generate a single beam of light that demonstrates sub quantum noise limit (QNL) quadrature fluctuations of up to -4dB across a multitude of spatial modes simultaneously. Included is a description of sub-QNL (squeezed) light, followed by a breakdown of how this 4WM arrangement generates the desired quantum noise suppression. Analysis has been performed that displays explicitly how the phase matching arrangement that maximises the noise suppression differs from that which optimises the efficiency of the process. This consideration is crucial to understanding why the setup achieves the levels of squeezing observed despite the presence of a strong absorption feature. Finally, the multi-spatial-mode nature of the generated squeezing is observed directly via homodyne detection using local oscillators with a range of transverse profiles. These profiles select the mode of the signal to be analysed and as such the ability to detect squeezing using a range of them demonstrates its presence across all of these modes simultaneously

Observation of Localized Multi-Spatial-Mode Quadrature Squeezing

Quantum states of light can improve imaging whenever the image quality and resolution are limited by the quantum noise of the illumination. In the case of a bright illumination, quantum enhancement is obtained for a light field composed of many squeezed transverse modes. A possible realization of such a multi-spatial-mode squeezed state is a field which contains a transverse plane in which the local electric field displays reduced quantum fluctuations at all locations, on any one quadrature. Using a traveling-wave amplifier, we have generated a multi-spatial-mode squeezed state and showed that it exhibits localized quadrature squeezing at any point of its transverse profile, in regions much smaller than its size. We observe 75 independently squeezed regions. The amplification relies on nondegenerate four-wave mixing in a hot vapor and produces a bichromatic squeezed state. The result confirms the potential of this technique for producing illumination suitable for practical quantum imaging

Role of the NS segment of Influenza A virus in setting host range and pathogenicity

Influenza A virus (IAV) circulates in waterfowl, causing mostly asymptomatic infections. IAV can undergo host adaptation and evolve to cause significant disease and mortality in domestic poultry and mammals, applying an enormous socio-economic burden on society. Sporadically, IAV causes global pandemics in man due to its zoonotic nature, and this can result in millions of deaths worldwide during a single outbreak. Host adaptation of IAV is an incompletely understood phenomenon, but is known to involve both host and viral determinants. It is essential to improve the understanding of the factors governing host range and pathogenicity of avian IAV, especially given the absence of a universal influenza vaccine and a limited weaponry of effective antiviral compounds. This study set out to improve the understanding of host adaptation of avian IAV, focussing on segment 8 (NS segment) of the virus genome. The NS segment of non-chiropteran IAV circulates as two phylogenetically distinct clades – the ‘A-’ and ‘B-alleles’. The A-allele is found in avian and mammalian viruses, but the B-allele is considered to be almost exclusively avian. This might result from one or both of the major NS gene products (NS1 and NEP) being non-functional in mammalian host cells, or from an inability of segment 8 RNA to package into mammalian-adapted strains. To investigate this, the NS segments from a panel of avian A- and B-allele strains were introduced into human H1N1 and H3N2 viruses by reverse genetics. A- and B-allele reassortant viruses replicated equally well in a variety of mammalian cell types in vitro. Surprisingly, the consensus B-allele segment 8 out-competed an A-allele counterpart when reassortant H1N1 viruses were co-infected, with the parental WT segment 8 being most fit in this system. A- and B-allele NS1 proteins were equally efficient at blocking the mammalian IFN response both in the context of viral infection and in transfection-based reporter assays. Consensus A- and B-allele H1N1 viruses also caused disease in mice and replicated to high virus titre in the lung. Interestingly, the B-allele virus induced more weight-loss than the A-allele, although the parental WT virus was most pathogenic in vivo. To re-address the hypothesis that B-allele NS genes really are avian-restricted, the relative rates of independent Aves to Mammalia incursion events of A- and B-allele lineage IAV strains was estimated and compared using phylogenetic analyses of all publically available segment 8 sequences. 32 A-allele introduction events were estimated compared to 6 B-allele incursions, however the total number of avian Aallele sequences outnumbered B-allele sequences by over 3.5 to 1, and the relative rates of introduction were not significantly different across the two lineages suggesting no bias against avian B-allele NS segments entering mammalian hosts in nature. Therefore, this study provides evidence that avian B-allele NS genes are not attenuating in mammalian hosts and are able to cause severe disease. Thus, this lineage of IAV genes, previously assumed to be avian-restricted, should be considered when assessing zoonotic potential and pandemic risk of circulating avian IAVs

Ground-Based Fabry-Perot Interferometry of the Terrestrial Nightglow with a Bare Charge-Coupled Device: Remote Field Site Deployment

The application of Fabry-Perot interferometers (FPIs) to the study of upper atmosphere thermodynamics has largely been restricted by the very low light levels in the terrestrial airglow as well as the limited range in wavelength of photomultiplier tube (PMT) technology. During the past decade, the development of the scientific grade charge-coupled device (CCD) has progressed to the stage in which this detector has become the logical replacement for the PMT. Small fast microcomputers have made it possible to "upgrade" our remote field sites with bare CCDs and not only retain the previous capabilities of the existing FPls but expand the data coverage in both temporal and wavelength domains. The problems encountered and the solutions applied to the deployment of a bare CCD, with data acquisition and image reduction techniques, are discussed. Sample geophysical data determined from the FPI fringe profiles are shown for our stations at Peach Mountain, Michigan, and Watson Lake, Yukon Territory

Bringing the Kok effect to light: A review on the integration of daytime respiration and net ecosystem exchange

Net ecosystem exchange (NEE) represents the difference between carbon assimilated through photosynthesis, or gross primary productivity (GPP), and carbon released via ecosystem respiration (ER). NEE, measured via eddy covariance and chamber techniques, must be partitioned into these fluxes to accurately describe and underst and the carbon dynamics of an ecosystem. GPP and daytime ER may be significantly overestimated if the light inhibition of foliar mitochondrial respiration, or "Kok effect," is not accurately estimated and further integrated into ecosystem measurements. The light inhibition of respiration, a composite effect of multiple cellular pathways, is reported to cause between 25-100% inhibition of foliar mitochondrial respiration, and for this reason needs to be considered when estimating larger carbon fluxes. Partitioning of respiration between autotrophic and heterotrophic respiration, and applying these scaled respiratory fluxes to the ecosystem-level proves to be difficult, and the integration of light inhibition into single and continuous measures of ecosystem respiration will require new interpretations and analysis of carbon exchange in terrestrial ecosystems