Synthesis of titanium dioxide photonic band gap structures

Calculations predict certain material structures to exhibit photonic band gaps in the visible spectrum of light. These composites consist of different dielectric materials arranged into period structures. Up to now research efforts have focussed on the synthesis of bulk structures consisting of a face-centered cubic arrangement of interconnected spherical voids within a high dielectric titania matrix.; In the work here, two synthesis approaches were investigated to contributed to this area of research. Firstly the synthesis of spherical 5 to 50 μm sized titania photonic crystals is presented. An emulsion based technique is used to form micron sized droplets, containing a PMMA particles in oil dispersion and Ti-alkoxide, the PMMA particles providing a template for the later voids and the alkoxide being the precursor for the high dielectric phase. After consolidation the emulsion droplets form spherical PMMA agglomerates with titania precursor remaining in the interstices. A heat treatment followed, removing the polymer and crystallizing the titania matrix to produce titania photonic crystals. It was found that the behavior of the PMMA particles within the oil emulsion droplets can be controlled by controlling the wetting of the PMMA particles by their solvent, making it possible to pack the PMMA spheres into ordered arrangements.; The second part of the work deals with the synthesis of nano-sized rutile titania powders. The rutile structure possesses the highest index of refraction of all titania polymorphs (n ≈ 2.9) and is therefore the desired material for the high dielectric phase in a photonic band gap crystal. The rutile powder particles are synthesized via a sol-gel/hydrothermal process using Ti-alkoxides as the precursor and nitric acid as the catalyst. A particle break-up process was observed leading from initially spherical 1 μm large agglomerates to 100 nm large broom-like agglomerates at temperatures below 50°C. Powders hydrothermally treated at 150°C caused the initially large, spherical agglomerates to break up into sub 100 nm large rutile single crystals. Dissolution-precipitation processes were identifies to be the main driving force for the break-up mechanism.