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Engineering of Photophysical Properties in Halide Perovskites: From Nano to Bulk for Optoelectronic Applications(2019-05-20) [Dissertation]
Advisor: Bakr, Osman
Committee members: Mohammed, Omar F.; Alshareef, Husam N.; Malko, AntonHalide perovskites have attracted the attention of a broad segment of the optoelectronics field, owing to their outstanding optical and electrical properties; simple low-temperature solution processing; low-cost raw materials; and tunable bandgaps. The main objective of this dissertation is engineering the materials’ properties of halide perovskites – their crystallinity, composition, and dimensionality – in order to understand the fundamental photophysical processes leading to their extraordinary behavior and to translate this understanding into optoelectronic applications. This dissertation is divided into two parts: the first focuses primarily on halide perovskites as a photonic source from an emission perspective, whereas the second is devoted to fundamental investigation of emergent photophysical concepts in halide perovskite materials including photon recycling and hot carriers. In the first part of this dissertation, we studied the synthesis and characterization of Cs-Pb-Br-based perovskite-related single crystals to elucidate the origin of the materials’ emission properties. After that, we presented perovskite nanocrystals (NCs) as a color converter in solid state lighting and visible light communication. Perovskites NCs’ converted white light (with a high color rendering index of 89 and a color correlated temperature of 3236 K) exhibits an extraordinary modulation bandwidth of 491 MHz, and data transmission rate of 2 Gbit/s. In the second part of this dissertation, we developed a facile synthesis method for perovskite microwires and demonstrate efficient photon recycling in those microwires with conclusive spectroscopic evidence. Subsequently, we investigated hot charge carriers in halide perovskites solar cells by a combination of laser spectroscopy and density functional modelling. Furthermore, we presented that hot holes were extracted at the device interface between the perovskite absorber and a hole transport layer. The findings and methodologies described in this dissertation represent a significant advance for utilizing the optical properties of halide perovskites, bring new fundamental photophysical insights to the field of halide perovskites, and provide a new powerful approach for designing the interface of perovskite solar cells to efficiently extract the hot charge carriers.
Investigating Semiconductor Nanostructures Functionalized by Emerging Materials for Optoelectronic Devices(2019-05-19) [Dissertation]
Advisor: Roqan, Iman S.
Committee members: Zhang, Xixiang; Han, Yu; El-Bashir, SamahWide and direct bandgap semiconductors (WBSs) are promising materials for many deep UV (DUV) applications. However, several challenges presently hinder the enhancement of DUV optoelectronics, such as low crystal quality, as well as complex and costly fabrication and growth processes that prevent production of high-performance devices, especially for large-scale applications. As a part of the study reported in this dissertation, I demonstrate several novel WBS-based devices with improved or novel functionalities, for the first time. The first part of work reported in this dissertation is designated for the novel, highly ordered and well-defined hexagonal ZnO nanotube (NT) arrays that were obtained without a catalyst. These arrays were grown on a p-GaN template using pulsed laser deposition (PLD), resulting in a highly bright and cost-effective UV light emitting diode (LED). In the second part, Gd-doped ZnO NRs grown on cost-effective metal substrate by PLD are presented and it is demonstrated that these can be functionalized by CH3NH3PbI3 perovskite to extend the functionality of ZnO photodetector from the ultraviolet to the infrared region (λ > 1000 nm), for the first time. The work reported in the third part demonstrates that the PLD method adopted in the present study can be extended to other high-quality metal oxide nanostructures. For this purpose, uniform p-type CuO pyramids were grown by PLD on Si substrate without a metal catalyst. Moreover, laser ablation method was advanced from vacuum based (PLD) to liquid based (femtosecond-laser ablation in liquid − FLAL) method to synthesize high-quality ZnO quantum dots (QDs). Adoption of this novel strategy allows producing high-performance self-powered DUV photodetectors based on p-CuO pyramids/n-ZnO QDs heterojunction device. In the last part, this research field is further advanced by exploring the functionality of other metal oxides synthesized by FLAL to fabricate a high-performance self-powered DUV photodetector. Such photodetector was fabricated using p-MnO QDs that were synthesized by FLAL and functionalized by high-quality mechanically exfoliated n-β-Ga2O3 nanoflakes as an active heterojunction layer grown on SiO2, confirming its superior response. All fabrication strategies, including use of heterojunction structures (mainly p−n junction), adopted in this work overcome the aforementioned issues related to the currently available WBS devices.
Impact of membrane biofouling in the sequential development of performance indicators: Feed channel pressure drop, permeability, and salt rejection(Journal of Membrane Science, Elsevier BV, 2019-05-18) [Article]Biofouling development is affected by a variety of factors that change over the length of reverse osmosis (RO) membrane modules in pressure vessels. Spatially resolved biofouling formation was studied under conditions representative to practice using four one-meter Long Channel Membrane Test Cells (LCMTCs) in series, simulating an industrial pressure vessel. Biofouling was induced by dosing an easily assimilable substrate to the feed water. The impact of biofouling on the sequential decline of RO membrane performance indicators (feed channel pressure drop, permeability and salt rejection) was investigated. Also, the temporal organic carbon (DOC) consumption was assessed spatially over the four test cells. Results showed that all membrane performance indicators were impacted by biofouling formation. The feed channel pressure (FCP) drop increase was impacted earliest and strongest followed by permeability and salt rejection decline, underlining that FCP drop is a sensitive and early biofouling monitoring indicator. Spatially resolved biofouling investigations revealed that most biofouling was formed in the lead sections of membrane installation with a decreasing gradient over length, linked to DOC availability in the system. In this study, FCP drop played a crucial role: the FCP drop increase at the lead test cell of the membrane installation caused performance losses for the downstream test cells. Minimizing the effect of biofouling on membrane performance should be pursued by a combination of strategies involving (i) early detection and preventive cleaning, (ii) substrate limitation for delaying biofouling built-up and (iii) optimized (early) cleaning procedures for more effective biofilm removal.
Net community production in a productive coastal ocean from an autonomous buoyancy-driven glider(Journal of Geophysical Research: Oceans, American Geophysical Union (AGU), 2019-05-17) [Article]Net community production (NCP), an analog of carbon export out of the surface ocean, is often estimated using budgets of dissolved oxygen. Accurate estimates of oxygen-based NCP, especially in dynamic coastal regions, require constraints on vertical transport of water with O2 out of equilibrium with the atmosphere, non-steady state change in the oxygen inventory, heating/cooling-driven O2 disequilibrium, and the rate of bubble injection from wave activity. The latter two are typically evaluated by using discrete measurements of the O2/Ar ratio in lieu of O2 only. Because sophisticated sampling and measurement techniques are required to make these measurements, they are often limited in spatiotemporal resolution. However, high-resolution estimates of NCP may be useful in determining small-scale patchiness in export. In this study, we calculated high-resolution NCP in coastal Southern California using dissolved oxygen measurements made by an autonomous buoyancy-driven Slocum glider and an empirical relationship derived using discrete measurements of O2/Ar in the surface mixed layer to remove the influence of bubble injection, which accounted for ~1/4 of the O2 supersaturation observed. Using estimates of vertical transport from wind-speed based parameterizations, previously validated using a 7Be budget, we were able to correct for the physical biases to the signal, which are known to significantly influence dissolved oxygen budgets in this region. Our results agree well with previously published NCP estimates for the study area, but also reveal higher-frequency variability that discrete sampling was unable to resolve, suggesting that this approach may be useful in other regions with well-constrained vertical transport rates.
The role of fungi in heterogeneous sediment microbial networks(Scientific Reports, Springer Science and Business Media LLC, 2019-05-17) [Article]While prokaryote community diversity and function have been extensively studied in soils and sediments, the functional role of fungi, despite their huge diversity, is widely unexplored. Several studies have, nonetheless, revealed the importance of fungi in provisioning services to prokaryote communities. Here, we hypothesise that the fungal community plays a key role in coordinating entire microbial communities by controlling the structure of functional networks in sediment. We selected a sediment environment with high niche diversity due to prevalent macrofaunal bioturbation, namely intertidal mangrove sediment, and explored the assembly of bacteria, archaea and fungi in different sediment niches, which we characterised by biogeochemical analysis, around the burrow of a herbivorous crab. We detected a high level of heterogeneity in sediment biogeochemical conditions, and diverse niches harboured distinct communities of bacteria, fungi and archaea. Saprotrophic fungi were a pivotal component of microbial networks throughout and we invariably found fungi to act as keystone species in all the examined niches and possibly acting synergistically with other environmental variables to determine the overall microbial community structure. In consideration of the importance of microbial-based nutrient cycling on overall sediment ecosystem functioning, we underline that the fungal microbiome and its role in the functional interactome cannot be overlooked.