Nearly Monodisperse Insulator CsPbX (X = Cl, Br, I) Nanocrystals, Their Mixed Halide Compositions, and Their Transformation into CsPbX Nanocrystals.

We have developed a colloidal synthesis of nearly monodisperse nanocrystals of pure CsPbX (X = Cl, Br, I) and their mixed halide compositions with sizes ranging from 9 to 37 nm. The optical absorption spectra of these nanocrystals display a sharp, high energy peak due to transitions between states localized in individual PbX octahedra. These spectral features are insensitive to the size of the particles and in agreement with the features of the corresponding bulk materials. Samples with mixed halide composition exhibit absorption bands that are intermediate in spectral position between those of the pure halide compounds. Furthermore, the absorption bands of intermediate compositions broaden due to the different possible combinations of halide coordination around the Pb ions. Both observations are supportive of the fact that the [PbX] octahedra are electronically decoupled in these systems. Because of the large band gap of CsPbX (>3.2 eV), no excitonic emission in the visible range was observed. The CsPbBr nanocrystals can be converted into green fluorescent CsPbBr nanocrystals by their reaction with an excess of PbBr with preservation of size and size distributions. The insertion of PbX into CsPbX provides a means of accessing CsPbX nanocrystals in a wide variety of sizes, shapes, and compositions, an important aspect for the development of precisely tuned perovskite nanocrystal inks.