Do the halide ions exchange and mix the same way in fresh and aged CsPbI₃-CsPbBr₃ perovskite nanocrystal mixtures?

The ultrahigh quantum yield, bandgap tunability and high absorption coefficient of CsPbI_3-xBr_x perovskites make them ideal candidates for various optoelectronic applications. Post-synthesis mixing of CsPbI₃ and CsPbBr₃ is an effective approach to tune the optical properties of CsPbI_3-xBr_x nanocrystals (NCs). The ion mixing mechanism in these perovskite NCs has attracted great interest, as it sheds valuable insights on how the halide ions are exchanged between the neighbouring NCs. The exchange, mixing and alloying of halide ions between NCs are influenced by various parameters controlling these reactions. In this study, we report two contrasting pathways, depending on the aging of the NCs, by which the halide ions are exchanged between the CsPbI₃ and CsPbBr₃ NCs. Changes in the photoluminescence spectra are monitored as a function of time to understand the dynamical changes in composition when CsPbBr₃ and CsPbI₃ NCs are mixed immediately after their preparation and after aging the NCs for 30 days. Mixing of freshly synthesized CsPbBr₃ NCs and CsPbI₃ NCs in various ratios (2 : 1, 1 : 1 and 1 : 2) lead to both the Br^- and I^- ions becoming rapidly incorporated into the CsPbI₃ and CsPbBr₃ NCs, respectively, as well as nucleation and growth of mixed-halide compositions, before the final equilibrium phase is formed. In contrast, when ≥40 days' aged CsPbBr₃ NCs are mixed with CsPbI₃, the diffusion of ions appears to proceed along two pathways, with the I^- ions from CsPbI₃ NCs primarily dissolving and becoming incorporated into the CsPbBr₃ lattice and an equilibrium phase being subsequently formed by nucleation, whereas the Br^- ions are not incorporated into the CsPbI₃ lattice. This more controlled reaction in aged samples is likely due to changes in the surface chemistry of the NCs, including the formation of a robust ligand shell on CsPbI₃ NCs, which enables one-way ion exchange.