Precise characterization of grain structures, stacking disorders, and lattice disorders of a close-packed colloidal crystal

Abstract

The perpendicular fracture surface of a dried colloidal crystal with pillar-like grains, obtained by centrifugation of a dispersion of polystyrene particles, was observed using a scanning electron microscope. Many grain boundaries on the fracture surface were observed at the particle level. Most of the particles on the surface showed a face-centered cubic (FCC) array. Although some grains were single FCC ones, other FCC grains contained some stacking disorders. Most of the surface was covered with such grains, and the grain boundaries formed a mosaic-like pattern. From these results, we confirmed that the colloidal crystals obtained by centrifugation formed a bundle structure of pillar-like FCC grains. A fracture surface adjacent to the side wall of the growth cell was also observed. The surface was composed of several layers. In the uppermost layer closest to the wall, numerous point defects and mismatches of triangular lattices between the neighboring two-dimensional islands were observed. These mismatches and point defects probably generated several lattice defects in the crystal. Similar generation of lattice defects probably occurred at the bottom of the container or the growth front of the crystals. Screw dislocations were also found in the layers, although they were not observed frequently. From these results, it was concluded that two-dimensional nucleation growth and spiral growth probably occurred on the crystal-dispersion interfaces of colloidal crystals as well as on the surface of atomic crystals.

Introduction

A face-centered cubic (FCC) lattice of a close-packed colloidal crystal is useful as a template for an inverse opal with a perfect three-dimensional photonic bandgap [1], and the close-packed colloidal crystals are easily fabricated by drying highly concentrated colloidal crystals without inducing cracks [2]. Colloidal crystallization processes are suitable for fabricating a large three-dimensional photonic crystal because of their self-assembly processes; on the other hand, the self-assembly processes easily induce lattice defects and sometimes degrade the quality of the crystals.

Although many trials on the control of lattice defects and grain boundaries of colloidal crystals have been conducted [2], [3], [4], controlled fabrication of crystals with sufficiently large and high-quality grains for device applications is still difficult. Yin et al. [3] fabricated large two-dimensional close-packed crystals by templating against regular two-dimensional arrays of pyramidal pits, but their thickness was much less than 100 μm. Davis et al. [4] obtained sequentially grown highly concentrated colloidal crystals with pillar-like grains by gravitational sedimentation of organophilic silica particles in cyclohexane. Although their grains were large (∼1 cm-long), and the potential for fabrication of large three-dimensional grains seemed high, further studies on grain size controls have not been conducted. In addition, the control of gravitational sedimentation rates is somewhat bothersome. We obtained highly concentrated colloidal crystals with pillar-like grains by centrifugation of polystyrene particles in water [2]. Relatively large (∼1 cm-long) pillar-like grains were obtained simply by controlling centrifugal acceleration. Larger grains can be obtained by slower centrifugation; however, further reduction of grain boundaries is still difficult, and the control of lattice defects remains an unsolved problem.

As the first step towards the precise control of lattice defects and grain boundaries, precise characterization of the fracture surface of close-packed colloidal crystals using a scanning electron microscope (SEM) will help us clarify the mechanisms of the generation of the grain boundaries and lattice defects during the crystallization processes. Although Miguez et al. [5] and Cheng et al. [6] have already observed the fracture surface of a silica colloidal crystal, they did not focus on the crystallization processes. Miguez et al. did not discuss lattice defects, while Cheng et al. concluded that the lattice defects in their crystals were caused mainly by irregular spheres. Hoogenboom et al. [7] observed three-dimensional colloidal crystals and characterized stacking disorders using a fluorescence confocal microscope. They also used a patterned substrates, and found that the substrates drastically reduced the number of stacking faults. Schall et al. [8] also observed a colloidal crystal at the particle level and found a Shockley partial dislocation in the crystal with a laser scanning confocal microscope. They also characterized dislocation dynamics at the microscopic level using a laser diffraction microscope. Although both groups successfully characterized stacking disorders in crystals at the particle level, they did not fully discuss the mechanisms of the generation of the disorders at the particle level.

In this study, we obtained highly concentrated polystyrene colloidal crystals with pillar-like grains by centrifugation, dried the crystals without inducing cracks, characterized the fracture surface of the dried crystals, and tried to clarify the mechanisms of the generation of grain boundaries, stacking disorders, and lattice defects of colloidal crystals at the particle level.