Formation and optical characterization of titanium dioxide photonic band gap materials

Certain periodically ordered dielectric materials are predicted to exhibit photonic band gaps in the visible spectrum. To date, research has focused on attempts to make such materials using a close-packed face-centered cubic (fcc) assembly of spherical voids in a titanium dioxide (TiO2, titania) matrix. Rutile titania, with a refractive index near 3, is an ideal material because optical band gaps appear only when the refractive index ratio of the matrix to that of the spheres exceeds 2.86. The processing and fabrication of such materials pose challenging materials problems: the matrix material must have a uniform and high refractive index and the spherical voids must form a highly ordered fcc array.; The first part of the work investigates the formation of titania photonic crystals. These crystals are formed by drying a fcc colloidal crystal of monodisperse polymer spheres suspended in aqueous slurry containing much smaller anatase titania particles. Following this stage, it was found that excluded volume effects reduce the packing fraction of titania in the interstices to ∼0.3. Subsequent heat treatment removes the polymer spheres by pyrolysis, densifies the titania matrix by sintering, and converts the titania from the anatase form (refractive index ∼2.5) to rutile. Sintering and grain growth of the titania causes the interstitial walls to thicken and distorts the shape and ordering of the spherical voids. The conversion temperature to rutile was found to be ∼100°C higher than in samples prepared without polymer spheres.; The second part of the work examines the optical characteristics of the materials. Bragg reflectivity measurements show the presence of stop bands through reflectivity peaks. The width of the reflectivity peak corresponding to a specific stop gap increases to 21% upon conversion to rutile, agreeing well with theoretical predictions. The reflectivity is attenuated by 84--99% from theory due to diffuse scattering. Random scattering from the titania nanoparticles dominates the sources of this scattering. The dependence of this diffuse scattering from titania with wavelength causes the reflectivity peak height to increase with wavelength. The reflectivity peak heights also become attenuated with temperature due to random scattering from surface roughness.