Colloidal epitaxy : a real-space analysis

Abstract

In colloidal epitaxy a patterned substrate is used to manipulate colloidal crystallization. The technique on the one hand serves as a model system to study the effects of interfaces and defects on (colloidal) crystallization and on the other hand as a means to direct colloidal self-assembly for applications like photonic crystals. In the first part of this thesis various techniques used to pattern substrates for colloidal epitaxy are described. Apart from lithographic techniques, a method was devised for positioning colloidal particles on surfaces in any designed pattern. We demonstrate the positioning of particles with a diameter ranging from 80 nm to 700 nm. The application of these 2D patterns in 3D epitaxial crystal growth is demonstrated as well. With a model system of colloidal hard spheres, growth of metastable hard-sphere hcp and 'double hcp' (dhcp) crystals by using a surface pattern that directly dictates the stacking sequence is shown. A detailed 3D analysis based on real-space measurements was performed on crystal structure as a function of template-crystal mismatch. Perfect template-induced hcp-crystal growth was found to occur at an isotropically deformed template, which is a strong indication for prefreezing at the template. At stretched and compressed lattices we observed growth of a non-close-packed superstructure and of a perfectly layered and (100)-aligned fcc crystal. In a system with long-ranged repulsive interactions, a simple 1D pattern of repulsive, charged lines was found to direct 3D crystallization. At volume fractions where the bulk phase behavior led to bcc crystallization, the 1D template was found to induce formation of a metastable fcc crystal. At lower volume fractions and different line spacing, bcc crystals were oriented with the (100)- or the (110)-plane, with two-fold twinning, parallel to the template. The template further induced prefreezing of the (100)-plane. At a large mismatch between template and interparticle spacing, 1D strings formed in the surface layer of a 3D crystal. Apart from equilibrium epitaxial crystal growth in suspension, the use of templates in directing colloidal crystallization in controlled drying techniques is examined as well. Template-induced colloidal crystallization is demonstrated in a tilted-horizontal setup of the templated substrate for crystals that are both a few layers as well as more than 100 layers thick. For vertical controlled drying, where in general there is more control over crystal thickness and uniformity, template-induced growth was examined for silica colloids ranging in diameter from about 100nm to over 1um. Our results indicate that the ability to form a templated crystal is crucially dependant on the surface topography of the template. For a square symmetric fcc(100)-template, 2D crystal growth was only observed on a pillar-shaped template. Finally, crystallization of colloids that are subjected to a relatively large gravitational field (i.e. large Peclet numbers) on a flat bottom wall is investigated. This includes a real-space study on the crystallization process itself as well as an investigation of the structure of colloidal crystals formed by sedimentation, focusing especially on the occurrence of stacking faults.