Defects in Colloidal Crystals. Identification, Characterization, and Manipulation

Colloids are particles with a size roughly between 1 nm and 1 μm in size. Such particles are small enough to exhibit Brownian motion, like molecules and atoms, but move much slower and are directly observable using visible light. This makes them ideal for studying processes related to crystallization, such as nucleation and growth dynamics, as well as annealing and final crystal structure. Their size also makes them attractive building blocks for photonic materials. If the periodicity of a colloidal crystal is right and a large refractive index contrast exists between the particles and their surroundings, a photonic band gap opens up; a band of photon energies that cannot propagate in the crystal. Photonic crystals are under investigation for application in solar cells, low threshold lasing, optical communication and even optical computing, but all of these require highly perfect crystals. Colloidal self-assembly, however, results in crystals that are far from perfect. The occurrence of point, linear and planar defects is frequent and hard to control. By studying the properties of these defects and the mechanisms by which they grow, we have gained fundamental knowledge on their behavior and their influence on crystal growth. Additionally, we have found ways to control the growth of certain defects and are able to incorporate them at predetermined positions to selectively influence crystal properties.