Nematic liquid crystals are mesogenic materials that are popular working materials for optical displays. There has been an increased interest in bistable liquid crystal devices which support two optically distinct stable equilibria. These devices typically exploit a complex geometry or anchoring conditions, which often induces defects in the equilibria. There remains a great deal to be understood about the structure of the defects and how they stabilize multiple equilibria in modern devices. This thesis focuses on four problems: the first three explore the effect of confinement and defects on nematic equilibria in simple geometries, with the aim of exploring multistability in these geometries; the fourth problem concerns the fine structure of point defects, essential for future modelling of nematic equilibria in more complex geometries. Firstly, we study nematic liquid crystals confined to two-dimensional rectangular wells using the Oseen-Frank theory. Secondly, we study equilibria within a semi-infinite rectangular domain with weak tangential anchoring on the surfaces. Thirdly, we study nematic equilibria within two-dimensional annuli. We derive explicit expressions for the director fields and free energies of equilibria within these geometries and discuss the stability of the predicted states. These three problems are motivated by the experimental work on colloidal nematic liquid crystals, which we interpret in the context of our results. Finally, we study the fine structure and stability of the radial hedgehog defect in the Landau-de Gennes theory with a sixth order bulk potential, relevant to the observability of global biaxial phases in a model with higher order potential terms.

M.Phil. (Energy Studies) / Non-destructive investigations of unusual natural polycrystalline diamond samples are reported. The samples originate from various international locations discovered in soils and sediments and may have been formed by di erent mechanisms which are the subject of debate. Analysis techniques include scanning electron microscopy, with energy dispersive spectroscopy and X-ray di raction. Sample composition, structure and surface stresses were investigated. The samples (two Egyptian nodules, two Brazillian carbonados, two Venetian ballas diamonds and a polycrystalline diamond compact sample consisting of a polycrystalline diamond layer sintered onto a Co-cemented tungsten carbide support) are cubic diamonds, with varying amounts of minority phases. In most cases the minority phases could not be identi ed unambiguously due to their low intensities and the overlap of Bragg peaks. The Egyptian nodule samples in particular showed a large presence of oxygen that could not be linked to speci c metal oxides. The Ballas samples had the lowest impurity content. Using a Rietveld based quanti cation analysis, the crystalline cubic diamond contents were determined respectively as 99 wt.%, 98 wt.% and 76 wt.% in the Ballas, Brazilian carbonado and Egyptian nodule samples. A signi cant result from this study is the substantial inhomogeneous peak broadening observed in the X-ray diffraction patterns of the Egyptian samples. This is indicative of the presence of nano-crystalline diamond grains in addition to substantial mechanical deformation with extensive accumulation of dislocations and crystalline defects. These observations are supportive of a recently proposed impact mechanism for their formation.

No description available.

ESR results for DEM(TCNQ)₂ and for MEM(TCNQ)₂ are presented as examples of TCNQ salts with and without level crossing effects. In the case of DEM(TCNQ)₂ we are performing ESR on a system with two types of spins. The latter are due to the two kinds of stacks of the acceptor TCNQ molecules in DEM(TCNQ)₂. The level crossing occurs when the g value of these two kinds of electrons are the same. This is compared to the case of MEM(TCNQ)₂ where there is only one kind of TCNQ stack and consequently no level crossing effect. It is found that g values and susceptibilities of the ESR spectra for DEM(TCNQ)₂ can be fitted very well to an interaction between the stacks of the form J ∑i,j,Si,Sj; with J > 0. This gives an antiferromagnetic coupling between every spin on one type of stack with every spin on the other type of stack. This type of interaction is also in qualitative agreement with the published results for HMM(TCNQ)₂. The experimental results in DEM(TCNQ)₂ and MEM(TCNQ)₂ also show that there are small but significant differences in the g tensors when the temperature or the crystalline environment are changed, and consequently the g tensor in these compounds does not depend only on the orientation of TCNQ molecule in the magnetic field as previously assumed. These differences in g are typically less than 2 x 10⁻⁴. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

I introduce the concept of modular materials and give a brief overview of their history and widespread occurrence in many areas of chemistry. I then discuss some of the many applications in which modular materials may find a use and link them to the following chapters. Chapter 1 describes the layered superatomic material Re₆Se₈Cl₂ and the induction of superconducting behavior in its single crystals through a current annealing technique. We suggest that this superconductivity arises through electron doping, as a result of dissociation of the apical Cl atoms from the clusters. Chapters 2-4 explore other types of superatomic materials and their properties, centered on the well-studied Co₆X₈ unit, where X is a chalcogen. Chapter 2 describes a Co₆Te₈-C₇₀ co-crystal that exhibits multiple phase changes with temperature, each giving rise to unique electronic, thermal, and structural properties. Chapter 3 describes a series of “solid solutions” of Co₆Se₈ and Cr₆Te₈ units. By varying the ratios of the component superatoms, transport properties of the crystals can be tuned, and unexpected behavior arises as a result of structural heterogeneity. Chapter 4 presents another study of Co₆Se₈ co-crystallized with rod-shaped C₁₄₀ fullerenes. The packing and electronic properties are found to be greatly affected by the degree of solvent inclusion. Chapters 5-6 examine another class of cluster-based materials: atomically precise gold nanoparticles. In Chapter 5 the cluster Au₂₁ is shown to self-assemble depending on the surface “hook” ligands, with corresponding differences in electronic transport. Chapter 6 discusses an interesting phase transition and thermally-induced hysteresis observed in crystals of the Au₁₀₃ cluster, also related to the surface ligand configuration. Chapters 7-8 take a different approach to modular materials, in the form of organic polymers. Using the robust, electroactive pigment molecule PDI as a common building block, we synthesize extended networks that are found to be exceptional pseudocapacitive energy storage materials. Chapter 7 introduces the honeycomb-shaped PDI-triptycene polymer, establishes its pseudocapacitive nature, and explores the role of cyclization in tuning its behavior. Chapter 8 expands upon the concept by combining PDI with hexaazatrinaphthalene to create a “contorted” network with best-in-class energy storage performance. In addition to in-depth kinetic analyses to elucidate the mechanism of storage, we fabricate two-electrode cells to demonstrate the material’s potential in real-world devices.

The characteristics of seventy-six calix[4]arene crystal structures derived from the Cambridge Crystallographic Database are presented. This survey is a discussion of the inter and intramolecular effects on the solid state cavity shape and molecular recognition ability of the compounds. In addition to this survey, four new calix[4]arene crystal structures are presented. The conformational characteristics of these four calixarenes are determined by a complicated array of inter and intramolecular interactions in the crystal packing.

No description available.

No description available.

No description available.

Assuming that its deformation is both static and homogeneous, a method was devised within the framework of finite strain theory to calculate the elastic constants of a crystal at zero pressure as well as functions of pressure. As this method can be used for a crystal of any symmetry with a variety of potentials, elastic constant calculations were completed for both quartz and cristobalite using the potentials derived by Lasaga and Gibbs (1987, 1988), Tsuneyuki et al. (1988), van Beest et al. (1990) and Boisen and Gibbs (1993). Among the four theoretical potentials derived from force fields calculated for molecule H₆Si₂Q₇, two potentials associated with the SiO bond stretching as well as the OSiO and the SiOSi bond angle bending terms yield reasonable agreements with the experimental data, supporting the assumption that the binding forces in crystals are similar to those in small molecules (Gibbs 1982). Using the semi-empirical potentials, including the SQLOO potential derived with the covalent model (Boisen and Gibbs 1993) and two derived with the ionic model (Tsuneyuki et al. 1988; van Beest et al. 1990), calculations of the elastic constants reproduce the experimental results for both quartz and cristobalite. The pressure derivatives of the elastic constants calculated with these potentials also agree with the experimental results measured for quartz at low pressures and yield pressure derivatives of the bulk modulus that are in close agreement with that observed for quartz. Using the SQLOO potential, although the results of the calculations do not agree with the experimental data as well as those calculated using the two ionic potentials, the agreement of the calculations made with the theoretical potentials was improved significantly. Finally, using the three semi-empirical potentials, the elastic constants were calculated as functions of pressure for quartz and cristobalite up to their transitional pressures. Calculated for both quartz and cristobalite, different behaviors of the elastic constants were found using different potentials. For cristobalite, the variations of its elastic constants and bulk modulus are better modeled by the SQLOO potential as its structural behaviors calculated with the SQLOO potential are consistent with the X-ray diffraction studies. / Master of Science