Unidirectional Crystallization of Charged Colloidal Silica under Temperature Gradient

Submicron-sized charged colloidal particles self-assemble into "crystal" structures in an aqueous medium due to long-range electrostatic interparticle interaction. Since colloidal crystals usually have a Bragg wavelength in the visible-light regime, they have attracted considerable attention as photonic materials. Here, we describe our recent study on the novel crystallization of charged colloids under a temperature gradient. We could obtain well-oriented and large (1 x 10 x similar to 30 mm) single-domain crystals in a short time (<10 min) under gravity; moreover, the crystals have sharp and deep transmission dips as well as good spatial uniformity at the Bragg wavelength lambda(B) (similar to 0.1%). However, the fabrication of larger crystals has been difficult because of thermal convection, which inevitably arises under the temperature gradient, and particle sedimentation. We anticipate that microgravity environments will be quite effective in improving the size of crystals as well as their optical quality.