Synthesis of highly uniform Cu2O spheres by a two-step approach and their assembly to form photonic crystals with a brilliant color

Abstract

Monodisperse semiconductor colloidal spheres with a high refractive index hold great potential for building photonic crystals with a strong band gap, but the difficulty in separating the nucleation and growth processes makes it challenging to prepare highly uniform semiconductor colloidal spheres. Herein, real monodisperse Cu₂O spheres were prepared via a hot-injection & heating-up two-step method using diethylene glycol as a milder reducing agent. The diameter of the as prepared Cu₂O spheres can be tuned from 90 nm to 190 nm precisely. The SEM images reveal that the obtained Cu₂O spheres have a narrow size distribution, which permits their self-assembly to form photonic crystals. The effects of precursor concentration and heating rates on the size and morphology of the Cu₂O spheres were investigated in detail. The results indicate that the key points of the method include the burst nucleation to form seeds at a high temperature followed by rapid cooling to prevent agglomeration, and appropriate precursor concentration as well as a moderate growth rate during the further growth process. Importantly, photonic crystal films exhibiting a brilliant structural color were fabricated with the obtained monodisperse Cu₂O spheres as building blocks, proving the possibility of making photonic crystals with a strong band gap. The developed method was also successfully applied to prepare monodisperse CdS spheres with diameters in the range from 110 nm to 210 nm.