Faceting and Commensurability in Crystal Structures of Colloidal Thin Films

F. Ramiro-Manzano; F. Meseguer; E. Bonet; I. Rodriguez

doi:10.1103/PhysRevLett.97.028304

Research · Publication & note

Faceting and Commensurability in Crystal Structures of Colloidal Thin Films

Research publication

F. Ramiro-Manzano, F. Meseguer, E. Bonet and I. Rodriguez

Phys. Rev. Lett. 97, 028304 (2006) - Published 13 July 2006

Abstract

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.

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Figures

Complete Figure 1 composition with reflectance spectra and SEM panels — **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.

Complete Figure 2 composition with reflectance spectra and SEM panels — **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.

Complete Figure 3 composition with vicinal fcc model and SEM panels — **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.

Complete Figure 4 composition with vicinal fcc model and SEM panels — **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.

Research fields

Bottom-up

Colloidal spheres assemble under wedge confinement into films whose preferred orientation changes with layer number. When the gap is incommensurate with an ideal facet, the crystal introduces prismatic domains, terraces and periodic stacking faults to preserve dense packing.

Simulations & fits

Geometrical models and filling-fraction arguments reconstruct the fcc(100), fcc(111) and intermediate prismatic arrangements. They explain how the number and spacing of stacking faults evolve as the cell thickness moves between commensurate states.

Characterization

Position-dependent reflectance spectra are matched with top and cleaved-edge SEM images. This correlation assigns each spectral region to a three-dimensional facet and distinguishes a true structural transition from a gradual optical shift caused only by thickness.

RESEARCH TOPICS

MaterialsNearly hard polystyrene spheres crystallise in a wedge cell whose thickness varies continuously. Optical reflectance and SEM reveal large single-domain facets and determine both orientation and exact monolayer number. When the gap is incommensurate with a pure fcc(111) or fcc(100) stack, the film forms a periodic mixture of those orientations separated by planar defects; structurally, it can be described as an fcc vicinal surface decorated by regularly spaced (111) stacking faults. The work turns finite-size frustration into a reproducible crystal architecture rather than treating the defects as random disorder.