Controlled growth of hard-sphere colloidal crystals

Three-dimensional ordered colloidal systems with lattice constants comparable to the wavelength of visible light might find important application as photonic crystals1, optic filters and switches2, and chemical sensors3. Colloidal crystallization has been actively studied4,5,6,7,8, leading to the development of several methods to control the self-assembly of the colloidal particles; examples include colloidal epitaxy9 and space-based reduced-gravity techniques10,11. Here we report a method to control the nucleation and growth of hard-sphere colloidal crystals that relies on the use of temperature gradients to define a density gradient. This is somewhat counterintuitive as temperature does not play a role in determining the hard-sphere phase diagram. We obtain hard-sphere single crystals (size ∼3 mm) from a sample in a concentration regime that would remain in the liquid state in the absence of a temperature gradient. We expect the method to have applications in controlling the ordering and growth of various ‘soft’ systems including colloids, copolymers, emulsions and proteins.