Manipulation of colloidal crystallization

Abstract

Colloidal particles (approximately a micrometer in diameter) that are dispersed in a fluid, behave thermodynamically similar to atoms and molecules: at low concentrations they form a fluid, while at high concentrations they can crystallize into a colloidal crystal to gain entropy. The analogy with molecular crystallization, combined with the easy access to real-space data (because of longer time and length scales as compared to molecules), makes this system ideal for studying crystallization and nucleation processes. Furthermore, colloidal crystals are studied because of their applicability in new developing technologies based on light: the length scale of the periodically varying refractive index in the crystals is of the same order as the wavelength of visible light, giving the possibility to open up an optical band gap, similar to the energy band gap in semiconductors. Whether or not a photonic band gap opens depends sensitively on the crystal lattice, the refractive index contrast and the quality of the colloidal crystals. Increased control over crystallization and nucleation processes can increase the tunability of the colloidal crystals and therefore increase the possibilities for producing photonic crystals. In this thesis, we present techniques to manipulate colloidal crystallization such that crystals are formed with a well-defined stacking, a low amount of defects and with a crystal lattice that is suitable for forming a 3D photonic band gap in the visible. Sedimentation of colloids on a templated surface (epitaxial growth) is combined with other external fields, such as electric fields and dielectrophoresis to reach optimal control. Using these techniques we were able to grow and dry a binary colloidal crystal with a NaCl structure. This crystal consists of two types of colloids with different sizes, which has a band gap when the dielectric contrast is inverted. Another binary colloidal crystal, with the MgCu2-structure, consists of small spheres on a pyrochlore lattice and large spheres on a diamond lattice. Both lattices can have a large photonic band gap. A procedure was designed to selectively grow the colloidal MgCu2-structure on a templated surface and photonic band structures were calculated of related dielectric and metallo-dielectric structures. Optical tweezers were used to fix a seed structure of tracer particles in the bulk of the (refractive index matched) host dispersion, inducing nucleation of the host dispersion at the seed structure. Using confocal microscopy, the effect of the seed structure on the host dispersion was studied for different configurations of the seed structure. The volume fraction of the dispersion was changed in-situ by dielectrophoretic compression. The nucleus grown on a square seed consisted of multiple domains and was non-spherical. These findings disagree with the assumptions in classical nucleation theory. Preliminary results are presented of a method, combining dielectrophoresis and epitaxial growth, to grow large oriented colloidal crystals using only a small template structure. The last chapter discusses the mechanism behind the formation of colloidal crystals by spin coating colloids from volatile media. The structure of the crystals was studied using real-space imaging and scattering techniques, demonstrating well-defined orientational correlation, but no long-range positional order.