Three-dimensional ordered colloidal systems with lattice constants comparable to the wavelength of visible light might find important application as photonic crystals, optic filters and switches, and chemical sensors. Colloidal crystallization has been actively studied, leading to the development of several methods to control the self-assembly of the colloidal particles; examples include colloidal epitaxy and space-based reduced-gravity techniques. 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.
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