Complex Colloidal Structures by Self-assembly in Electric Fields

Abstract

The central theme of this thesis is exploiting the directed self-assembly of both isotropic and anisotropic colloidal particles to achieve the fabrication of one-, two-, and three-dimensional complex colloidal structures using external electric fields and/or a simple in situ thermal annealing method. Colloids are typically defined as objects having at least one dimension in the size range of a few nanometers to several micrometers that form a dispersed phase when suspended in a continuum phase. As a result of Brownian motion, the colloidal particles are able to explore configurational space, and eventually reach the equilibrium configuration that minimizes the free energy. An important feature of the colloidal particles is the possibility of controlling the size, shape, and composition. The assembly of colloidal particles has long been a rich and continuously growing area of materials science, with great potential for a broad range of applications including electronics, optics, and biotechnology. Within this field, the bulk of the research has been devoted to studying the assembly of isotropic spherical particles. Recently, there has been growing interest in the design of more complex structures to see how such a change in microstructure could influence certain material properties, especially optical properties, but also to answer the demand for more realistic model systems for molecular analogues. In this thesis, we used external electric fields to impart anisotropy into systems consisting of both isotropic and an-isotropic particles. If there is a mismatch in permittivity between the particles and the suspending medium, the colloids acquire an induced dipole moment. A major advantage of this approach is that the interactions are tunable and fully reversible. Moreover, a large number of parameters can be used to control and tune particle interactions and subsequent self-assembly in AC electric fields, including field strength and frequency, particle shape, particle and solvent dielectric properties. Interestingly, the relatively simple anisotropic dipolar interaction already gives rise to several new phases in a uniaxial field. We developed methods to produce model systems that are essentially colloidal analogues of polymer chains in all three stiffness regimes that can be observed on a single particle level, even in concentrated systems without using molecular tracers. Moreover, we obtained control over the length, and the flexibility of the bead chains. We exploited our simple thermal sintering method further for bonding polymeric colloidal particles after they have been assembled into various three-dimensional structures. Next, we discussed the generality of our method by implementing this method to close and non-close packed structures. We used our thermal annealing method to synthesize more complex shape particles such as rhombic dodecahedron particles and also we discuss the stability of the particles. We controlled the lateral position of the strings of particles with micrometer-scale precision by a combination of structured wall and electric dipoles. We investigated the self-assembly of gold nano-sheets as a function of salt in electric fields. Finally, we studied the effect of external electric fields on the phase behavior of sharp-edged colloidal cubes using optical microscopy and Monte Carlo simulations.