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, showing structural colors when particles with a size comparable to the wavelength of visible light are used. This structural color shows an angular dependence in highly ordered photonic crystals, which decreases when a low degree of disorder is present in the crystal structure. 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 with controlled dimple size and depth. In this manner, various colloidal crystals made of particles with small and large dimples have been prepared, as shown in the figures below.

The structure of these crystals was meticulously investigated using focused ion-beam scanning electron microscopy (FIB-SEM) to recover the position of all particles. This thorough examination revealed that an increase in the dimple size leads to a progressive loss of ordered structure and structural coloration. Brownian Dynamics simulations were then performed to gain a deeper understanding of the mechanism leading to the formation of disorder, confirming the progressive loss of order as the particle shape deviates from sphericity. These findings underscore the profound impact of particle shape on the crystallization of colloids, a significant contribution to our understanding of this field.