Colloidal epitaxy: playing with the boundary conditions of colloidal crystallization.

We have studied, with quantitative confocal microscopy, epitaxial colloidal crystal growth of particles interacting with an almost hard-sphere (HS) potential in a gravitational field and density matched colloids interacting with a long-range (LR) repulsive potential with a body-centred cubic (BCC) equilibrium crystal phase. We show that in both cases it is possible to grow thick, stacking fault-free metastable crystals: close-packed crystals with any stacking sequence, including hexagonal close packed (HCP), for the HS particles and face-centred cubic (FCC) in the case of the LR colloids. In accordance with recent computer simulations done for HS particles it was found that the optimal lattice constant to grow HS HCP crystals was larger than that of equilibrium FCC crystals. In addition, because of the absence of gravity, pre-freezing could be observed for the particles with the LR potential on a template of charged lines. We also argue that the ability to manipulate colloids with highly focused light, optical traps or tweezers, will become an important tool in both the study of colloidal crystallization and in making new structures. We show how cheap 2D and 3D templates can be made with optical tweezers and demonstrate, in proof of principle experiments with core-shell colloids, how light fields can generate crystal nuclei and other structures in the bulk of concentrated dispersions and how the effect of these structures on the rest of a dispersion can be studied quantitatively in 3D.