Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2005. / Includes bibliographical references (leaves 124-141). Also available in electronic version.

Thesis (Ph.D.)--Brown University, 2005. / Vita. Thesis advisor: Clyde L. Briant. Includes bibliographical references (leaves 95-99). Also available online.

Thesis (Ph.D.)--Brown University, 2005. / Vita. Thesis advisor: Vivek B. Shenoy. Includes bibliographical references (leaves 191-200). Also available online.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

The fabrication process of two-dimensional photonic crystals in an AlGaInP/GaInP multi-quantum-well membrane structure is developed. The process includes high resolution electron-beam lithography, pattern transfer into SiO₂ etch mask by reactive ion etching, pattern transfer through AlGaInP/GaInP layer by inductively coupled plasma (ICP) etching and a selective undercut wet etch to create the freestanding membrane. The chlorine-based ICP etching conditions are optimized to achieve a vertical sidewall. The photonic crystal structures with periods of a=160-480nm are produced. / Singapore-MIT Alliance (SMA)

Bibliography: p. 174-183. Aims to contribute to the understanding of the first excited electronic state of NaI

A basic introduction to liquid crystals and surfactants has been given, along with a description of the main techniques employed in the study of liquid crystals (in particular optical polarising microscopy, differential scanning calorimetry, nuclear magnetic resonance spectroscopy and x-ray diffraction studies). Conventional surfactants comprise a polar head group and a hydrophobic, hydrocarbon chain ie. they are amphiphilic. Because long chains have high melting points the length of the alkyl chain in these compounds is limited to < ca C[18], as long chain surfactants are usually insoluble. Therefore, in this project the alkyl group has been replaced by a long, hydrophobic polydimethylsiloxane chain. Polydimethylsiloxanes are low melting materials (glass transition at ca -120&deg;C) with very flexible chains, hence surfactants based on them might be readily soluble in water. This project involves chemical attachment of amphiphilic mesogens to alpha-SiH terminated siloxanes of varying lengths and the examination of their surfactant properties. The following type of structure was successfully synthesised: CH[3]CH[2]CH[2]CH[2](Si (CH[3]) [2]O)n Si(CH[3])[2] - m where n = integer; m = amphiphilic mesogen. The amphiphilic head groups of these novel surfactants contained the salts of either a mono-, or a dicarboxylic acid. After the synthesis of these surfactants, the liquid crystal and micelle properties of the sodium and calcium salts, were investigated utilising a number of physical techniques eg. optical microscopy and differential scanning calorimetry. Finally, some work on synergism has been described. When different types of surfactants are purposely mixed, what is sought is synergism, the condition when the properties of the mixture are better than those attainable with the individual components by themselves.

Results are presented from a series of simulations undertaken to investigate the effect of adding small spherical particles to a fluid of rods which would otherwise represent a liquid crystalline (LC) substance. Firstly, a bulk mixture of Hard Gaussian Overlap particles with an aspect ratio of 3:1 and hard spheres with diameters equal to the breadth of the rods is simulated at various sphere concentrations. Both mixing-demixing and isotropic-nematic transition are studied using Monte Carlo techniques. Secondly, the effect of adding Lennard-Jones particles to an LC system modelled using the well established Gay-Berne potential is investigated. These rod-sphere mixtures are simulated using both the original set of interaction parameters and a modified version of the rod-sphere potential proposed in this work. The subject of interest is the internal structure of the binary mixture and its dependence on density, temperature, concentration and various parameters characterising the in-termolecular interactions. Both the mixing-demixing behaviour and the transitions between the isotropic and any LC phases have been studied for four systems which differ in the interaction potential between unlike particles. A range of contrasting microphase separated structures including bicontinuous, cubic, and micelle-like arrangement have been observed in bulk. Thirdly, the four types of mixtures previously studied in bulk are subjected to a static magnetic field. A variety of novel phases are observed for the cases of positive and negative anisotropy in the magnetic susceptibility. These include a lamellar structure, in which layers of rods are separated by layers of spheres, and a configuration with a self-assembling hexagonal array of spheres. Finally, two new models are presented to study liquid crystal mixtures in the presence of curved substrates. These are implemented for the cases of convex and concave spherical surfaces. The simulation results obtained in these geometries indicate segregation of spheres at the topological defect of the director field induced by the surface.

No description available.

The study of the following compounds is based on the premise that high symmetry in molecules should lead to low barriers to molecular motion even in the solid state i.e. the principle of least distress. The study of 3,3-diethylpentane is a very nice example of a phase change which results in no chemical shift changes but, produces a sharp discontinuity in the rate of ethyl group rotation. A large discontinuity in rate of molecular motion is also observed in 4,4-dipropylheptane which is also due to a phase change. The following study of tetraalkyl ammonium halides revealed both alkyl group rotation and cation tumbling to be occurring. This is most significant in the tetramethyl ammonium halides where the effects of methyl rotation and cation tumbling are shown to overlap considerably. The activation energies for the cation tumbling in the tetramethyl ammonium halides are Ea= 30 - 36 kJ mol−1 and for ethyl group rotation in tetraethyl ammonium chloride are, △H†; = 57.8 kJ mol−1 and △S†; = 45.9 J K−1 mol−1. The chemical shift differences in each chain due to different chain lengths is seen in tetrabutyl ammonium iodate which also shows a sudden coalescence and introduction of rapid molecular motion after two small phase changes. This compound has a large phase change which also affects the rate of molecular motion. The tetraalkyl phosphonium halides have a greater degree of molecular motion than the ammoniums in the temperature range studied and also offer the chance to use Tip measurements. The tetramethyl phosphonium halides show a distinct cation tumbling as opposed to the ammoniums. The process in the chloride and bromide which have hexagonal structures gives △H†; ~ 30 kJ mol−1 and negative △S†; values and the lower symmetry iodide has △H†; = 45.2 kJ mol−1 and a positive value of △S†;. The tetraethyl phosphonium halides show ethyl group rotations measurable to varying degrees by 13CT1p. The tetrabutyl phosphonium halides also have the different chain lengths as seen for the ammonium compounds. They also show bond rotations which are quite difficult to measure by 13CT1p values. The study of bis-(hydroxymethyl)cyclopentane (BHMCP) in an attempt to study ring puckering/pseudorotation in cyclopentane derivatives in the solid state reveals a hydrogen bond exchange process (△Gc†;~ 60 kJ mol−1). The bicyclic derivatives of BHMCP showed no such molecular motions but did reveal some interesting solid- solid phase transitions and chemical shift changes. The study of trans-cyclopentanediol yields △H†; = 77.0 and △S&†; = 184 J K−1 mol−1 for the processes involved. The study of 4,4-dimethyl-trans−1,2-cyclopentanediol reveals two processes. One results in the coalescence of resonances and appears to be a hydrogen bonding exchange process. The other is a much lower energy process (Ea ~ 26 kJ mol−1) which could be a ring puckering process. The study of sulpholane also shows significant molecular motion with △H&†; = 50.6 kJ mol−1 and △S&†; = 77.9 J K−1 mol−1.