Rapid and Direct Liquid-Phase Synthesis of Luminescent Metal Halide Superlattices.

The crystallization of nanocrystal building blocks into artificial superlattices has emerged as an efficient approach for tailoring the nanoscale properties and functionalities of novel devices. To date, ordered arrays of colloidal metal halide nanocrystals have mainly been achieved by using post-synthetic strategies. Here, a rapid and direct liquid-phase synthesis is presented to achieve a highly robust crystallization of luminescent metal halide nanocrystals into perfect face-centered-cubic (FCC) superlattices on the micrometer scale. The continuous growth of individual nanocrystals is observed within the superlattice, followed by the disassembly of the superlattices into individually dispersed nanocrystals owing to the highly repulsive interparticle interactions induced by large nanocrystals. Transmission electron microscopy characterization reveals that owing to an increase in solvent entropy, the structure of the superlattices transforms from FCC to hexagonal close-packed (HCP) and the nanocrystals disassemble. The FCC superlattice exhibits a single and slightly redshifted emission, due to the reabsorption-free property of the building block units. Compared to individual nanocrystals, the superlattices have three times higher quantum yield with improved environmental stability, making them ideal for use as ultrabright blue-light emitters. This study is expected to facilitate the creation of metamaterials with ordered nanocrystal structures and their practical applications.