23 research outputs found
Recrystallization and Zone Melting of Charged Colloids by Thermally Induced Crystallization
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 <i>d</i> = 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, <i>d</i> = 333 nm, ϕ = 5 ×
10<sup>–5</sup>), 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