There are many advantages to the solution phase synthesis of semiconductor nanowires, the most notable of which are the ease of scalability and the production of nanowires in higher yields than those typically obtained in chemical vapor deposition (CVD) based processes. The solution phase synthesis of high quality, high aspect ratio (>100) narrow diameter semiconductor nanowires depends sensitively on three parameters: the diameter of the nanocrystals utilized to promote (seed) nanowire growth, molecular precursor decomposition kinetics and the choice of solvent in which the nanowires are grown. Bismuth is a low melting point (270 °C) semimetal and thus an ideal candidate for the solution-liquid-solid (SLS) growth of nanowires. A bismuth nanocrystal synthesis was developed that affords nanocrystals with average diameters from 4 - 20 nm. The nanocrystal diameter is controlled by varying the capping ligand (TOPO) to bismuth molar ratio. The synthesis of Au2Bi nanocrystals was also studied as it also affords small diameter (~ 2 nm) nanocrystals that are suitable for SLS nanowire growth. Molecular precursor decomposition kinetics can have a significant impact on nanowire yield and quality. Precursors that decompose too quickly can produce relatively large diameter nanowires, while precursors that decompose too slowly can produce nanowires with a highly tortuous morphology as a result of a high density of crystallographic defects. The choice of molecular precursor for the synthesis of III-V and II-VI nanowires was investigated. The solvent in which nanowires are grown can also have a significant effect on nanowire yield, quality and morphology. Coordinating solvents such as alkylphosphine oxides and alkylamines can interact with the atoms, or atomic complexes, that constitute nanowires and thus mediate the nanowire growth rate. In some instances, for example InAs nanowires grown in TOPO, this interaction can completely quench nanowire growth. This solvent effect has been investigated for the growth of III-V and II-VI nanowires. Solvents can also affect nanowire morphology. Branched ZnSe nanowires, i.e. hybrid nanostructures in which ZnSe nanorods grow epitaxially from the surface of ZnSe nanowires, are synthesized in trioctylamine whereas TOPO suppresses this branched growth. Finally, a mechanism which allows for the synthesis of narrow diameter nanowires seeded by much larger diameter nanocrystals is investigated. Bismuth nanocrystals with an average diameter of ~ 20 nm are utilized to promote the growth of narrow diameter (~ 6 nm) CdSe.₀₈Te.₉₂ nanowires. / text

Solar cells have been traditionally developed for optimizing three key steps for charge carriers: generation, separation, and transport. Conventional solar cells are essentially PN junction based, and utilize the internal electric field near the junction interface for realizing charge carrier separation. However, this kind of structure limits material choices and device fabrication to form a working junction due to issues such as lattice mismatch, doping, and band alignment. Ferroelectric photovoltaic devices with typical capacitor structure have been developed to overcome the junction caused disadvantage but suffer from the poor charge transport issue. In this work, novel ferroelectric-semiconductor photovoltaic devices were developed and investigated in detail with experimental results and theoretical simulation. This type of solar cell is fundamentally different with traditional PN junction based solar cells, utilizing ferroelectric polarization for charge separation in semiconductor layer. Systematical works have been conducted on: (1) device working principle and mechanism study; (2) effect of electrode; (3) influence of device key dimension parameters. The new cells showed the rectifying behavior and effective photovoltaic effect after specific asymmetric polarization. Furthermore, the device performance has been improved through adjusting electrode design and semiconductor layer thickness, which is mainly due to the optimized electric field strength and distribution resulting from polarization. As low cost commercial semiconductor, the multicrystalline silicon (mc-Si) has great potential application in the novel ferroelectric-semiconductor photovoltaic devices. However, the grain boundaries with high density of defects limit the material electric properties. In order to improve the multicrystalline silicon transport property, a polar molecules system was developed to play the role in grain boundaries passivation. The small polar molecule composition and solution passivation process were carried out to optimize the passivation effect. The result showed the developed ZK series solutions reduced the Rsheet across large-angle grain boundaries by up to more than one order to be close to the bulk Rsheet. Also, the correlation between the grain misorientation and passivation effectiveness was built up. / published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

Crystal plasticity has been an active research field for several decades. The crystal plasticity of the bulk materials has its key relevance in the industrial process. Besides, the plasticity of nano-sized materials becomes a topic attracting a lot of interest recently. In the Part I of the thesis, molecular dynamics (MD) simulations were used to study the plasticity of small nanoparticles. Firstly, the coalescence process of Cu nanoparticles was explored. It was found that a peculiar type of five-fold twins in the sintered products were formed via an unseen before dislocation-free process involving a series of shear waves and rigid-body rotations. Secondly, a similar study on the heating of a single nanoparticle was conducted. The same dislocation-free shear wave mechanism was spotted again. In this mechanism, a cluster of atoms rearranges in a highly coordinated way between different geometrical configurations (e.g. fcc, decahedral, icosahedral) without involving dislocations. Thirdly, simulations on the sintering of many nanoparticles were performed, and the governing processes during the consolidation were discussed. The findings in this part of the thesis can provide some guidance for controlling the motifs of nanoparticles. In Part II of the thesis, the emphasis was switched to the crystal plasticity at larger spatial and temporal scales. A dislocation density-based model was developed in our research group. This model employs a dynamics formulation in which the force on each group of dislocation density is calculated with the Taylor and mutual elastic interactions taken into account. The motion of the dislocation densities is then predicted using a conservative law, with annihilation and generation considered. The new dislocation density-based model was used in this work to simulate the plastic deformation of single crystals under ultrasonic irradiation. Softening during vibrations as well as enhanced cell formation was predicted. This is the first simulation effort to successfully predict the cell formation phenomenon under vibratory loadings. / published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

It has been seen in physical experiments as early as the 1960’s that when a positively charged particle is injected into a crystal in certain directions and with sufficiently high energy, the particle can penetrate into the crystalline structure to a depth which is unexpectedly long. Such motions in the crystal are referred to as channelling trajectories. Herein, we consider a Hamiltonian model for particle motion in a crystal. We then proceed to show that the results of KAM theory are applicable to the model, and moreover that these result predict the existence of trajectories for the fired particle which do indeed penetrate the crystal deeply. Finally, we present the results of two series of numerical simulations which strongly suggest that this behaviour is observable in our model. / text

published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

The crystallization kinetics of hydronium jarosite have been studied in the area of the Fe₂O₃-SO₃-H₂O phase diagram where hematite is the stable phase. Hydronium jarosite has been shown to be a kinetically favored intermediate to hematite over a wide range of chemical and thermal conditions. A model useful for predicting the crystal size distribution as a function of temperature, free acid and iron concentrations, and residence time has been developed. Hydronium, sodium, and potassium jarosite have been shown to convert, via a solid phase reaction, to hematite. A model useful for predicting the conversion rate as a function of temperature, free acid concentration, and particle size has been developed. A predictive model, the growing core model, has been developed. It is useful for predicting the crystal size distribution and the product split between hydronium jarosite and hematite when both crystallization and conversion are occurring simultaneously. The cardinal assumption in this model is that crystal growth and conversion occur at separate cites on the crystal surface simultaneously. The model, with only one adjustable constant, has been verified with experimental results. The effect of double draw off (DDO) operation in this system has been demonstrated. It has been shown, via the growing core model and experimental results, that DDO operation can produce a high iron, low sulfur product. Without DDO operation, this high product quality can only be achieved through higher operating temperature, high neutralization rates, or very large vessel size.

No description available.

No description available.

No description available.