Particulate mobility in vertical deposition of attractive monolayer colloidal crystals.

In the colloidal self-assembly of charged particles on surfaces with opposite polarity, disorder often dominates. In this report, we show that ionic strength, volume fraction, and solvent evaporation temperature can be optimized in the vertical deposition method to yield hexagonal close-packed monolayer arrays with positively charged colloids on negatively charged bare glass. We further extend our study to form well-defined binary two-dimensional superlattices with oppositely charged monolayers grown layer-by-layer.

Layer-by-layer growth of attractive binary colloidal particles.

We investigate the two-dimensional (2D) colloidal structures formed by oppositely charged polystyrene monolayers grown layer-by-layer, where the electrostatic forces are recruited to assist in the packing of the layers. Our results show a transition through several 2D-superlattices to more close-packed structures with increasing ionic strength. The observed geometrical packing constraints of the 2D-superlattice structures agree well with the estimated Debye screening length of the electric double layer.

Kinetic stages of single-component colloidal crystallization.

To harness the full potential of colloidal self-assembly, the dynamics of the transition between colloids in suspension to a colloidal crystalline film should be better understood. In this report, the structural changes during the self-assembly process in a vertical configuration for colloids in the size range 200-400 nm are monitored in situ, using the transmission spectrum of the colloidal assembly treated as an emergent photonic crystal. It is found that there are several sequential stages of colloidal ordering: in suspension, with a larger lattice parameter than the solid state, in a close-packed wet state with solvent in the interstices, and, finally, in a close-packed dry state with air in the interstices.

In situ monitoring of structural changes during colloidal self-assembly.

Reflectance spectroscopy is utilized to monitor structural changes during the self-assembly of a monodisperse colloidal system at the meniscus of a sessile drop on an inert substrate. Treating the ordered colloidal structure as a photonic crystal is equivalent to monitoring the changes in the photonic band gap (PBG) as the colloidal system self-assembles heterogeneously into a crystal through solvent evaporation in ambient conditions. Using a modified Bragg's law model of the photonic crystal, we can trace the structural evolution of the self-assembling colloidal system.