(61b) Controlling the Order in Colloidal Crystals By Progressively Changing the Shape of Particles

Monodisperse colloidal particles are known to self-organize into an ordered crystalline phase with a face-centered cubic structure when subject to strong repulsive interactions at sufficiently high concentrations. These colloidal crystals can be formed by controlling the evaporation of colloidal suspensions of such particles or by exposing them to centrifugal forces to accelerate their deposition and crystallization. Because of their ordered structure, when particles with a size comparable to the wavelength of visible light are assembled in colloidal crystals, structural coloration emerges, similar to what is observed in natural opals. Because of these optical properties, colloidal crystals have been investigated as sensors and as the simplest system mimicking the structural colors observed in various animals (butterfly wings, bird feathers, and sea shells). However, the structural color shows an angular dependence in highly ordered photonic crystals. At the same time, a low degree of disorder leads to structural colors with a lower angular dependence. Once the disorder in the crystal becomes too large, the structural color is lost.

To control the extent of disorder in colloidal crystals, we decided to investigate the formation of colloidal crystals using slightly non-spherical particles. This has been achieved by preparing highly monodisperse dimpled particles synthesized from standard polystyrene colloids and coating them with a layer of carbonized sugar to create a rigid shell. When these colloids are first swollen and then de-swollen with ethanol at high temperatures, the shell buckles upon deswelling, forming a uniform dimple. The size and depth of this dimple can be controlled by tuning the rigid shell formation conditions. In this manner, a variety of colloids with small dimples and large dimples have been prepared, and the formation of their colloidal crystals has been investigated. An SEM image of two of these crystals is shown in the included figure.

The structure of these crystals has been investigated using focused ion-beam scanning electron microscopy (FIB-SEM) to recover the position of all particles in a crystal. From this, the order of the structure has been investigated. It was discovered that an increase in the dimple size leads to a progressive loss of ordered structure and structural coloration. Brownian Dynamic simulations have been performed to understand further the mechanism leading to the formation of disorder, confirming the progressive loss of order as the particle shape deviates from sphericity. The work sheds light on the strong impact of particle shape on the crystallization of colloids.