Halide Exchange Reactions in Large Halide Perovskite Nanocrystals

We have studied halide exchange reactions in large colloidal nanocrystals of lead halide perovskites, with sizes (∼40-60 nm) that are intermediate between those of typical nanocrystals synthesized and studied by the community (5-10 nm) and those of microcrystals or thin films. This size regime still allows an investigation of the nanocrystals with techniques used for colloidal nanocrystals while enabling the observation of phenomena such as phase segregation, generation of planar defects, strain accumulation and relaxation, which are harder to identify in smaller nanocrystals, also given their high instability under the electron beam. We compare nanocubes with hollow nanocubes, and highlight how the presence of the hollow region can influence structural and morphological changes during the reactions. Notably, we report core-shell epitaxial CsPbI3/CsPbCl3 heterostructures by partial I- to Cl- exchange on CsPbI3 nanocrystals. Transient absorption spectroscopy provides evidence consistent with electronic coupling and exciton transfer between the Cl-rich shell and I-rich core in these heterostructures, evidenced by the accelerated bleach decay of the CsPbCl3 component and the delayed population of the CsPbI3 band edge.