School of Chemistry - Theses
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ItemUnderstanding cadmium telluride nanocrystal solar cells( 2016)Solution-processed solar cells use cheaper manufacturing methods that could potentially bring down the cost of solar energy to be cost-competitive with fossil fuels. However, solution processed solar cells still have many issues that keep them from being commercially viable. While several solution processed techniques have resulted in efficiencies comparable to solar cells available on the market, there are many issues with long-term performance and stability. The work presented here is focused on one of the major candidates for commercial development of solution-processed solar cells. Nanocrystal (NC) cadmium telluride (CdTe) solar cells have reached power conversion efficiencies of over 12 % which is comparable to commercially available CdTe solar cells that use high cost vacuum depositions. However, these efficiencies are reached with a biasing treatment and degrade over time. We refer to the improvement in PCE through biasing treatments as “the biasing effect”. The work presented in this thesis will give a comprehensive analysis of NC CdTe solar cells by understanding the role of processing steps on chemical and electronic properties of CdTe films. Following this, we will show the major cause of degradation of NC CdTe films and devices. We will give comprehensive understanding and show the cause of the biasing effect. Finally we will show some possibilities for stabilizing and improving the current nanocrystal CdTe device which is an integral part of commercializing this technology. This thesis will answer the following questions: 1. What is the purpose of the cadmium chloride treatment and temperature annealing in NC CdTe films? 2. What is the cause of degradation and the biasing effect? 3. Can we stabilize and improve the device?
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ItemSurface engineering for mechanically robust superhydrophobic films( 2016)The inherent surface roughness of superhydrophobic surfaces renders them mechanically fragile and limits their use in many applications from self-cleaning to anti-fouling. With the view of improving the mechanical durability of these films several steps have been taken to both identify and understand the underlying principles for the apparent dichotomy between superhydrophobicity and mechanical durability. Rough surface coatings with variable surface roughness have been developed and examined using atomic force and electron microscopy, contact angle goniometry nanoindentation as well as industry based mechanical testing. Prepared predominately by bottom up strategies such as sol-gel processing, a diverse variety of superhydrophobic surfaces were prepared exhibiting contact angles greater than 150° and sliding angles less than 10°. Subsequently, several synthetic protocols have been developed to overcome these difficulties. Within conventional sol-gel derived coatings, by normalizing against the surface topology, the enhancement in abrasion resistance can be correlated to crosslinked polymer material property ratios H/E and H3/E2, providing a rationale for polymer choice to wear improve wear behavior in future coatings. Understanding of geometric limitations led to the development of polymer spheres prepared through emulsion synthesis which were utilized as sacrificial templates within a siloxane matrix to yield films with crater-like surface roughness. Surface roughness was controlled through the template geometry and concentration. The intrinsic hydrophobicity of the MTMS matrix provides enhanced longevity towards wear. This was subsequently improved through the development of polyhedral silsequioxane chemistry. Further design of the crater-like surface was inspired by mimicking the fascinating assembly of particles in natural materials. Hierarchical assembly of anisotropic particles to achieve mutually exclusive properties inspired work toward the preparation of biomimetic, superhydrophobic coatings predominantly from the incorporation of silica and polyaniline fibers and rods into craterlike surfaces.
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ItemSurface plasmon spectroscopy of redox processes on single gold nanocrystals( 2016)Gold nanocrystals (Au NCs) are important materials for catalysis, sensing and photonics. Au NCs exhibit strong scattering signals in the visible and near infrared range due to localised surface plasmon resonances. Due to statistical averaging, when measurements are made on ensembles of particles, the precise determination of the effect of gold nanocrystal size, shape and local environment on specific application performance is not feasible. This ensemble problem is overcome by applying a combination of dark field imaging with surface plasmon spectroscopy, enabling the scattering spectra of individual nanocrystals to be measured. This approach allows changes in the electron concentration of a single Au NC to be observed via localised surface plasmon resonance shifts. In this thesis, the dark field microscopy technique has been expanded to study single gold nanocrystal electrodeposition, gas-phase adsorption, photoreduction, and solid-state charging. The first key objective was to understand how nanocrystal morphology and surface properties influence underpotential deposition. The second key objective was to understand how metal oxide supports influence charge transport during hydrogen adsorption and photoexcitation. These studies clearly demonstrate that the above parameters are crucial to the electrochemical and catalytic properties of Au NCs. By performing the measurements on single nanocrystals, the chemical kinetics and charging rates could be uncovered with detail never before achieved at this scale. In addition to using standard dark field microscopy techniques in this work, an upgraded laser illuminated dark field system was developed to optically reveal single particle charging rates by measuring electron transfer in real-time. The added sensitivity of this new approach has enabled the optical detection of fewer than 150 electrons as they are transferred to a single gold nanorod. In order to characterise the performance of the laser system, a reliable and reproducible method to rapidly charge single gold nanocrystals was developed. Au NCs were integrated in an ion gel capacitor, enabling them to be charged in a solid, transparent and highly capacitive device, ideal for transmission microscopy.
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ItemSynthesis and optical properties of CdSe core and core/shell nanocrystals( 2008)The synthesis of nanocrystals is unique compared to the formation of larger micron-sizesspecies as the final crystal sizes are not much larger than the primary nuclei. As a consequencethe final outcome of a nanocrystal synthesis i.e mean crystal size, concentrationand standard deviation is almost solely determined by the end of the nucleation phase. Directingthe growth of crystals beginning from aggregates of only tens of atoms into maturemonodisperse nanocrystals requires that the governing kinetics are strictly controlled at everymoment of the reaction. To effect this task various different ligands need to be employed,each performing a particular function during both nucleation and growth. (For complete abstract open document)
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