Recrystallization and Zone Melting of Charged Colloids by Thermally Induced Crystallization

Abstract

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.

An empirical correction for moderate multiple scattering in super-heterodyne light scattering

“…We therefore believe, that such experiments contain interesting information about grain boundary dynamics. Future experiments will complement existing structural studies on coarsening samples [61,62,63,64].…”

Nonclassical Recrystallization

“…[12] Additionally,r ecrystallization and zone-meltingo fc harged silica particlesl eads to an improvement of crystal quality and crystal size. [13] A" self-cleaning effect" demonstrated by microscopy techniques due to "colloidal recrystallization" was reported for metallicn anoparticles by rinsing impurities from the surface of as uperstructure, [14] while in case of semiconducting nanoparticles asize-selective formation waso bserved. [15] Applicability to chemically differentn anocrystals or scientific considerations provided by sufficient analytical examination of the purification process itself are, however,missing for the reported approaches.…”

“…Shape separation of nanoparticles can be enabled by different solubilities of the building block shapes as well as by DNA‐programmable nanoparticles [12] . Additionally, recrystallization and zone‐melting of charged silica particles leads to an improvement of crystal quality and crystal size [13] . A “self‐cleaning effect” demonstrated by microscopy techniques due to “colloidal recrystallization” was reported for metallic nanoparticles by rinsing impurities from the surface of a superstructure, [14] while in case of semiconducting nanoparticles a size‐selective formation was observed [15] .…”

Melting of Colloidal Crystals

“…Besides being model systems, colloidal crystals have important technical applications as photonic crystals. Melting a defective region by a local heating followed by the recrystallization is effective in the fabrication of large, high‐quality colloidal crystals …”