Faceting and Commensurability in Crystal Structures of Colloidal Thin Films

This Letter investigates the influence of finite size effects on the particle arrangement of thin film colloidal crystals. A rich variety of crystallographic faceting with large single domain microcrystallites is shown. Optical reflectance experiments together with scanning electron microscopy permit the identification of the crystal symmetry and the facet orientation, as well as the exact number of monolayers. When the cell thickness is not commensurable with a high symmetry layering, particles arrange themselves in a periodic distribution of (111)- and (100)-orientated face centered cubic (fcc) microcrystallites separated by planar defects. These structures can be described as a fcc ordering orientated along a vicinal surface, modified by a periodic distribution of fcc (111) stacking faults.

Figure 1

Optical spectra and the corresponding SEM images of different crystalline arrangements as the film thickness increases from 7 to 8 monolayers. The figure shows how the system changes from the (100) fcc prismatic phase [SEM image (1) and the bottom spectrum] to the fcc (111) facet [SEM image (4) and the top spectrum]. Spectra have been shifted for the sake of comparison.

Figure 2

Optical spectra and the corresponding SEM images of different particle orderings for a 9 monolayers thick colloidal crystal. The figure shows how the system changes from a long-range (111)-oriented fcc arrangement [SEM image (1) and the bottom spectrum] to (100) oriented hcp facet [SEM image (4) and the top spectrum]. Spectra have been shifted for the sake of comparison.

Figure 3

(a) Top and side views of a vicinal fcc (100) crystal ordering. Models (b) and (c) show how the system produces stacking faults planes to maximize filling fraction. The gray and black colors in model (c) highlight the periodic distribution of fcc (111) triangular based prisms. (d) shows the side and top views of the prismatic phases with one, two, and three stacking faults layers between them. Top and bottom SEM images of the cleft edges of the sample correspond to (c) and (d) models, respectively. The spheres have also been colored with gray and black colors to facilitate comparison with the proposed models. Triangles and broken lines in the bottom SEM image highlight the prismatic phases and the planar defects, respectively.

Figure 4

(a) Top and side views of a vicinal fcc (111) crystal ordering. (b–d): Side views of the prismatic (111) phases. The gray and the black colors highlight the periodic distribution of fcc (111) microcrystallites shaped as prisms with a rhomboid base. (e) shows the side and top views of a hcp (100) ordering. Top and bottom SEM images of the cleft edges of the sample correspond to the (b) and top view (e) models, respectively. The spheres have also been colored to facilitate the comparison to the models.