NANO LETTERS
The Role of Thickness Transitions in Convective Assembly
2006 Vol. 6, No. 10 2249-2253
Linli Meng, Hong Wei, Anthony Nagel, Benjamin J. Wiley, L. E. Scriven, and David J. Norris* Department of Chemical Engineering & Materials Science, UniVersity of Minnesota, 421 Washington AVenue SE, Minneapolis, Minnesota 55455 Received July 14, 2006; Revised Manuscript Received August 17, 2006
ABSTRACT Here we examine the microscopic details of convective assembly, a process in which thin colloidal crystals are deposited on a substrate from suspensions of nearly monodisperse spheres. Previously, such crystals have been shown to exhibit a strong tendency toward the facecentered cubic structure, which is difficult to explain on thermodynamic grounds. Using real-time microscopic visualization, electron microscopy, and scanning confocal microscopy, we obtain clues about the crystallization mechanism. Our results indicate that the regions at which a growing crystal transitions from n to n + 1 layers can play an important and previously unrecognized role in the crystallization. For thin crystals, we show both from experiment and through simple modeling that these transition regions can generate specific crystal structures. In thicker crystals, the crystallization is more complicated, but the transition regions must still be considered before a complete understanding of convective assembly can be obtained.
Weakly interacting colloidal spheres will organize spontaneously into crystals under the appropriate conditions.1,2 In the idealized case of noninteracting “hard” spheres, calculations indicate that the face-centered cubic (fcc) structure is more stable than hexagonally close-packed (hcp) at thermodynamic equilibrium.3-6 Initially, it may be surprising that these two structures have different free energies. When the spheres are close-packed, both exhibit the same volume fraction (74%) and both are constructed simply by stacking planes of hexagonally packed spheres.7 The only difference is in the sequence of the layers. The fcc structure is achieved with an ABCABC... sequence while hcp is obtained with an ABAB... sequence. Nevertheless, this leads to a change in entropy due to differences in third nearest neighbors. Consequently, the fcc crystal is predicted to be lower in energy by
