(351d) Directed Self-Assembly of Colloidal Crystals with Electric Fields

Directed Self Assembly (DSA) methods can create useful structures and devices using particles ranging in size from nanoparticles to micron sized particles. In this work, we explore the fundamental mechanics of electrical field driven (dielectrophoretic) DSA with the goal of enabling the engineering of methods for generating two and three-dimensional ordered structures of high fidelity and controlled architecture.

Highly ordered three-dimensional colloidal crystals are assembled from a dilute colloidal suspension by AC electrical fields with frequencies on the order of a kilohertz. This technique exploits dielectrophoretic forces induced in the double layers surrounding the particles by the applied field. Using the standard electrokinetic model, the induced DEP force is predicted to depend on the particle size, charge and electrolyte concentration. Studies on a model colloidal dispersion are compared to theoretical predictions (Hill et al., J Colloid Interface Sci., 258, 56 (2003)) through a predictive model that incorporates double layer polarization. Measurement of fundamental colloidal properties, such as electrophoretic mobility, validates the standard electrokinetic model. Modeling identifies appropriate ranges of solution conditions favoring particle ordering.

These crystalline structures are observed and the extent of order quantified both optically and with scattering techniques. In situ Small Angle Light Scattering (SALS) characterizes the degree of ordering and crystal structure for micron-sized particles. A new Small Angle Neutron Scattering (SANS) sample environment is developed for studying the ordering and crystal structure of nanoparticle assemblies. The kinetics of ordering are measured and compared to predictions based on the forces derived from the standard electrokinetic model. Dielectrophoretic ordering is demonstrated to form both 2D and 3D structures with micron lengthscale features where ordering conditions are chosen to dictate final crystalline structure and strength.