Layering transitions in confined colloidal crystals: The hcp-like phase

The process of crystallization in confined colloidal suspensions due to self-assembly is very different from that in bulk systems. When a colloidal solution is confined between two plates (spatial confinement), only an integer number of crystalline layers may exist in equilibrium, resulting in many different types of layer arrangements. Previous studies have investigated the sequence of facets when the thickness of the cell gradually increases, but a complete mechanism of phase transitions between different particle orderings has not yet been published.
In this paper, the authors investigate the sequence of morphological transitions in a nearly hard sphere arrangement confined in a wedge cell. They propose a model that shows smooth transitions between different particle orderings for a small number of layers. In this model, both the buckling and the (100) hexagonal close packed (hcp) phases are particular cases of a much more general particle arrangement tendency that they call hcp-like ordering. This phase is able to adopt packing arrangements commensurate with the cell thickness. More strikingly, the hcp-like phase adapts itself to the progressive changes of the cell thickness by a smooth change in the interlayer spacing. The authors present hcp-like orderings up to six layers and a complete sequence of transformations between two and three layers. In order to complete the full transition, a new phase called the pre-square phase is introduced, as well.
The authors used a wedge cell with nearly hard sphere colloids to study the packing of spheres as a function of the number of layers. As the number of monolayers increased, they examined the evolution of the facets in the sample and the transition between them. To allow accurate identification of the different colloidal arrangements, optical experiments as well as scanning electron microscopy characterization were performed.
We found found that the buckling phase in the transition between one and two monolayers can be extended to a larger number of layers. This new phase, the hcp-like phase, is very versatile, as it easily adapts itself to the gap value of the wedge cell. Moreover, they show that the hcp-like ordering plays a very important role in smoothing the transition from n triangular to (n+1) square for small values of n.
Overall, this paper provides a new understanding of the structure sequence observed as the sample thickness progressively increases from two to six layers. It shows that the hcp-like phase appears in the transition from n triangular to (n+1) square in large surface areas because of its ability to reorganize itself according to the changing contour conditions imposed by the wedge type geometry. More importantly, it gives a unified understanding of how the system smoothly transitions between different particle orderings when the thickness value changes gradually. This research has important implications for the study of confined colloidal suspensions and could lead to the development of new materials with unique properties.

Reproduced (with the exception of the summary) from Ref. Physical Review E 76, 050401 (2007)  with permission from the American Physical Society