Self-assembly of polyhedral hybrid colloidal particles
Non-spherical or polyhedral-shaped particles have potential applications in new photonic crystal materials where the bandgap is in the visible range. This paper introduces a technique for producing polyhedral-shaped particles with high yield. The technique can be used to create complex geometries. In addition to electron microscopy, optical microscopy can be used to study these particles using their optical properties.
Since the technique uses emulsion polymerization, it allows for high yield, and can create colloidal particles with crosslinked polymer which can be index matched under certain solvent to tailor the optical properties. Polymer beads were grown onto silica particles by first surface modifying the silica particles with methacryloxymethyltriethoxysilane (MMS). The surface modified silica particles are then mixed with surfactants and monomers to undergo polymerization. Crystallization happens through sedimentation. The polyhedral colloidal particles surrounded by polystyrene beads are shown in the figure below, where both transmission and scanning electron microscopy were used.
There is a linear relationship between the radius of the silica particles and the number of polystyrene beads surrounding them. This mathematical relationship can be used to predict the morphology for a given silica particle size.
In order to study the optical properties of the particles, PMMA-co-EGDMA is used instead of polystyrene. The surface modified particle can be index matched in toluene, as seen in the figure below. The EGDMA is helpful at crosslinking the silica particles, and allowing them to be swollen by the solvent but not dissolved. Thus the resulting colloidal suspension is transparent and has a slight blue color due to Rayleigh-Gan scattering under visible light.
Optical microscope is used to reveal the presence of crystallites, and the face-centered cubic morphology, as seen in the figure below.
When they prepare dimer crystal structures, optical microscope revealed that the crystallites were cauliflower-shaped, and there are dislocation lines around the central point, as seen in figure below.
Overall, they demonstrated a technique that can fabricate polyhedral hybrid colloids, and since the particles can be index matched using surfactants, optical microscopy can be used to study their morphologies. When dimers were fabricated, the crystallite morphology was not completely understood, and further work involving fluorescence labeling of the silica particles is needed.