Making single crystal colloids for photonic bandgaps in the optical domain.

We describe novel methods for making good quality colloidal crystals. We use polymethylmethacrylate (PMMA) spheres of diameter 0.8microns suspended in organic solvents between glass plates separated by about 10 microns. We have characterised the phase diagram in this confined system as a function of volume fraction and find that at volume concentrations beyond 49% a significant fraction of the suspended PMMA spheres are forced to crystallise into random close packed structures. The crystallite size is typically small (100microns) and the predominant structure is random close packed. This we confirm by single crystal light scattering measurements. We do find small areas of face centred cubic structure in these samples which we assume arises simply by chance stacking as the crystallites are typically only 10-15 layers thick. However the crystallite size can be increased simply by shearing the suspension during preparation. We also find conditions under which the formation of face centred cubic crystals can be preferential. We would present video footage of diffraction patterns formed during the shearing process where we can see clear evidence of the transition from random packing to FCC ordering.