Symmetry Control in 3D Colloidal Crystallization
The 3D crystalline layers assembled by simple sedimentation or filtration are built up of domains of different color. SEM shows that these domains are essentially crystals with different orientations, each domain including between a few hundred and many thousand particles. Most of these domains exhibit hexagonal packing on their surface, but square arrays of particles are occasionally seen (as shown also in slide 5.5 further). Previous research on 3D colloidal crystallization (see e. g. K. E. Davis, W. B. Russel and W. J. Glantschnig, Science, 245, 507, 1989) has shown that in laboratory conditions the microspheres crystallize in mixtures of both randomly stacked hexagonal close-packed planes (r.h.c.p.) and face-centered cubic (f.c.c.) lattices. The existence of square arrays may be attributed to the orientation of the stacked crystalline planes (parallel or perpendicular to the surface) and occasionally to the formation of cubic lattices in the crevices between the bigger domains.
In the assembly of colloidal crystals for high tech applications, one needs to obtain large, non-fragmented single crystals of defined symmetry and orientation. Interesting and encouraging steps in this direction are the studies on the assembly of particles onto pre-patterned surfaces, where "landing sites" for the lower layer of particles have been created by photolithography. Two notable references are suggested below:
A. van Blaaderen, R. Ruel and P. Wiltzius, Nature, 385, 321 (1997).
A. D. Dinsmore, J. C. Crocker and A. G. Yodh, Current Opinion Colloid Interface Sci., 3, 5 (1998).
© Copyright O. D. Velev 1998