Evaporation-Driven Assembly as a Route to Photonic Materials

118 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007.The unique, structure-dependent diffraction properties of photonic crystals have inspired their proposed use in applications ranging from chemosensors, to solar power applications, to optical computing devices. The range of proposed applications for photonic crystals demands a range of techniques for fabricating them, meeting differently weighted priorities including perfection, refractive index contrast, operating wavelength, throughput, and economy. This thesis contributes to the art of preparing photonic crystals by colloidal self-assembly at an evaporating solvent-air interface, a promising approach for preparing thin films of photonic material over large areas at low cost, if some defects are permissible. The simplest possible example of evaporation-driven self-assembly, the evaporation of a stagnant colloidal suspension, is demonstrated as a practical method for colloidal crystallization; although the colloidal crystal is formed at a water-air interface, and so is difficult to handle, it can be made robust for study and use by immobilization in a hydrogel. Vertical deposition is a widely used evaporation-based method for preparing thin, dry colloidal crystal films. The thickness profiles of vertically deposited colloidal crystals are measured through the Fabry-Perot fringes in their infrared reflectance spectra: the linear shape of these thickness profiles constrains proposed mechanisms for colloidal self-assembly in vertical deposition. Optimized conditions for vertical deposition are found under which colloidal crystals of 80% peak reflectance, an exceptional optical performance for a polystyrene colloid crystal on glass, are reproducibly prepared. A process is demonstrated for preparing germanium-containing, high-index-contrast photonic crystals from a polystyrene colloidal crystal template and pre-formed germanium nanoparticles, using only room-temperature processing steps; the result is an inverse opal of air macropores in a germanium-in-photoadhesive composite with refractive index 2.05. Previously published germanium infiltration techniques rely on gas-phase chemical reactions and require temperatures well above Tg for most linear polymers. As a complementary approach to this problem, some progress has also been made towards a polymer colloidal crystal template that can withstand gasphase chemistry at temperatures up to and possibly beyond 250°C, based on highly crosslinked divinylbenzene microspheres.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD