Nearly Monodisperse Insulator Cs 4 PbX 6 (X = Cl, Br, I) Nanocrystals, Their Mixed Halide Compositions, and Their Transformation into CsPbX 3 Nanocrystals.

We have developed a colloidal synthesis of nearly monodisperse nanocrystals of pure Cs ₄ PbX ₆ (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 ₆ ^4- 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 ²⁺ ions. Both observations are supportive of the fact that the [PbX ₆ ] ^4- octahedra are electronically decoupled in these systems. Because of the large band gap of Cs ₄ PbX ₆ (>3.2 eV), no excitonic emission in the visible range was observed. The Cs ₄ PbBr ₆ nanocrystals can be converted into green fluorescent CsPbBr ₃ nanocrystals by their reaction with an excess of PbBr ₂ with preservation of size and size distributions. The insertion of PbX ₂ into Cs ₄ PbX ₆ provides a means of accessing CsPbX ₃ nanocrystals in a wide variety of sizes, shapes, and compositions, an important aspect for the development of precisely tuned perovskite nanocrystal inks.