Crystallization of nanocrystals in spherical confinement probed by in-situ X-ray scattering

We studied the formation of supraparticles from nanocrystals confined in slowly evaporating oil droplets in an oil-in-water emulsion. The nanocrystals consist of an FeO core, a CoFe₂O₄ shell, and oleate capping, with an overall diameter of 12.5 nm. We performed in-situ Small- and Wide-Angle X-ray Scattering experiments during the entire period of solvent evaporation and colloidal crystallization. We observed a slow increase in the volume fraction of nanocrystals inside the oil droplets up to 20%, at which a sudden crystallization occurs. This indicates that attractions between the nanocrystals are important as a driving force for the assembly. The spherical supraparticles have a diameter of about 700 nm and consist of a few crystalline face-centered-cubic domains. Nanocrystal supraparticles bear importance for magnetic and opto-electronic applications, such as color tunable bio-labels, color-tunable phosphors in LEDS, and miniaturized lasers.