This thesis describes the synthesis and characterisation of certain mesostructured and mesoporous materials, as well as their use for optical device applications. Such materials possess regularly arranged pores, which provide high surface areas able to better disperse optically active components such as laser dyes. Thin film geometries and patterned microstructures of rhodamine 6G doped mesostructured silica and titanium dioxide, which hold great promise for applications such as optical coatings, emissive displays and optical sensor devices, have been investigated. The novelty of the research described in this thesis lies in the synthesis methods developed, as well as the resulting material properties, which could be advantageous for optical devices.
Methods such as transmission electron microscopy. X-ray diffraction and nitrogen adsorption/desorption were applied to characterise the nanoscale structure (mesostructure) of these composite materials, while spectroscopic methods were used to examine the interactions between the dye molecules and their aggregation behaviour.
The synthesis of unsupported, free-standing mesoporous silica based films was carried out and it was shown that ammonia vapour treatment provides a simple and convenient method for stabilising mesostructured thin silica films. For the fabrication of silica based films, purely cationic as well as mixed cationic-anionic surfactant assemblies have been used. The mixed surfactant synthesis resulted in a change of the mesostructure, and made the thin films macroscopically more robust. Although these surfactant based thin silica films could have potential for use in diverse applications. such as membranes and sensors, their use for optical applications appears to be rather limited.
Optically transparent, mesostructured titanium dioxide thin films were fabricated which were better suited for optical device applications than the silica films, discussed above. The synthesis was based on amphiphilic poly(alkylene oxide) block copolymer templates in combination with retarded hydrolysis of titanium dioxide precursors. The dye rhodamine 6G was incorporated into the mesostructure during the templating process. The polymer proved to be a suitable solvation environment for rhodamine 6G in that it prevented dye aggregation, which is known to degrade the fluorescence quantum yield of the composite material. In this way, novel dye-doped mesostructured titanium dioxide films with a high refractive index were synthesised. The dye-doped films exhibited optical threshold-like behavior characteristic of amplified spontaneous emission, which is the basis for laser applications.
Soft lithography techniques were successfully applied to micropattern block copolymers, silica and titanium dioxide based mesostructured materials into line, disk, donut and hexagonally shaped patterns. These results pave the way for the fabrication and demonstration of novel microlaser structures and other active optical structures. Future work could be concerned with further investigation and exploitation of the dye doped microcavities or other device applications, such as membranes, sensors or solar, electrochromic and photochromic cells.