We examined the application of recrystallization and zone-melting crystallization methods, which have been used widely to fabricate large, high-purity crystals of atomic and molecular systems, to charged colloidal crystals. Our samples were aqueous dispersions of colloidal silica (with particle diameters of d = 108 or 121 nm and particle volume fractions of ϕ = 0.035-0.05) containing the weak base pyridine. The samples crystallized upon heating because of increases in the particle charge numbers, and they melted reversibly on cooling. During the recrystallization experiments, the polycrystalline colloids were partially melted in a Peltier cooling device and then were crystallized by stopping the cooling and allowing the system to return to ambient temperature. The zone-melting crystallization was carried out by melting a narrow zone (millimeter-sized in width) of the polycrystalline colloid samples and then moving the sample slowly over a cooling device to recrystallize the molten region. Using both methods, we fabricated a few centimeter-sized crystals, starting from millimeter-sized original polycrystals when the crystallization rates were sufficiently slow (33 μm/s). Furthermore, the optical quality of the colloidal crystals, such as the half-band widths of the diffraction peaks, was significantly improved. These methods were also useful for refining. Small amounts of impurity particles (fluorescent polystyrene particles, d = 333 nm, ϕ = 5 × 10(-5)), added to the colloidal crystals, were excluded from the crystals when the crystallization rates were sufficiently slow (∼0.1 μm/s). We expect that the present findings will be useful for fabricating large, high-purity colloidal crystals.

Charged colloids form "crystal" structures, wherein the colloidal particles are regularly arranged as a result of strong electrostatic interparticle interactions. We have reported that silica colloids containing pyridine (Py) form colloidal crystals upon heating. Herein, we report that in Py + silica colloids including small numbers of fluorescent polystyrene (PS) particles, the PS particles are arranged in stripe patterns upon stepwise unidirectional crystallization when the temperature is increased by 1°C every 10 and 15 min. We think that the present stripe patterns are closely analogous to the thermally induced striations found in a number of crystalline materials.Charged colloidal particles self-assemble to form "crystal" structures when the electrostatic interactions among the particles are sufficiently strong. In the crystal states, the colloidal particles are regularly arranged in body-centered or face-centered cubic lattices. 14We have reported that colloidal silica containing pyridine (Py) exhibits thermally induced crystallization. 5,6 Silica particles have weakly acidic silanol groups (SiOH) on their surfaces, which partly dissociate to provide surface charges. On addition of Py, the dissociation of silanol groups is enhanced, resulting in an increase in the particle charge number Z, that is, the electrostatic interaction becomes stronger. Since dissociation of Py increases with temperature (T), the silica + Py colloids crystallize upon heating, under appropriate conditions. Furthermore, we reported that impurity particles added to the silica crystal are excluded from the crystal grains and accumulate in the crystal grain boundaries. 7 In particular, stripe patterns were observed in charged silica + polystyrene (PS) binary colloids after unidirectional crystallization driven by diffusion of Py. 812 This pattern formation was explainable in terms of a combination of (1) development of thin, lamellashaped crystal grains and (2) exclusion of the PS particles from the grains to the crystal grain boundaries.In the present study, we report the formation of another type of stripe pattern in silica + PS binary colloids, which is observed after unidirectional crystallization when the temperature is varied in a stepwise manner. We think that the stripe patterns observed here are closely analogous to the "striation" patterns observed in crystalline materials that results from the short-range composition fluctuations found in nearly all crystals. 13Aqueous dispersions of colloidal silica particles (KE-W10, Japan Catalyst Co., Ltd., Tokyo, Japan) were used after they were first purified by dialysis for four months and then purified using an ion-exchange method. The particle diameter, determined by the dynamic light-scattering method, was 105 nm. An aqueous dispersion of fluorescent PS particles (G300, Thermo Scientific Inc., Waltham, MA, USA, diameter: 333 nm) was purified using an ion-exchange method. In the binary colloids used for the experiments, the volume fractions of silica (º silica ) and PS particles (º...

We examined the performance of charged colloidal crystals immobilized in a polymer gel as tunable optical filters. The colloidal crystals of charged silica particles (particle diameter = 121 nm; particle concentration = 3.5 vol %; and Bragg wavelength λB = 630–720 nm) were produced by unidirectional crystallization under a temperature gradient. Photocurable gelation reagents were dissolved in the sample beforehand; this enabled gel immobilization of the crystals under ultraviolet illumination. The crystals had dimensions of more than 25 mm2 in area and 1 mm in thickness, and spatial λB variations of less than 1%. Upon mechanical compression, λB values shifted linearly and reversibly over almost the entire visible spectrum. Using the gelled crystals as tunable optical filters, we measured the transmittance spectra of various samples and found them to be in close agreement with those determined using a spectrophotometer equipped with optical gratings.