Chemical transformation mechanism for blue-to-green emitting CsPbBr₃ nanocrystals

Recently, metal-halide perovskites have rapidly emerged as efficient light emitters with near-unity quantum yield and size-dependent optical and electronic properties, which have attracted considerable attention from researchers. However, the ultrafast nucleation rate of ionic perovskite counterparts severely limits the in-depth exploration of the growth mechanism of colloidal nanocrystals (NCs). Herein, we used an inorganic ligand nitrosonium tetrafluoroborate (NOBF₄) to trigger a slow post-synthesis transformation process, converting non-luminescent Cs₄PbBr₆ NCs into bright green luminescent CsPbBr₃ NCs to elucidate the concrete transformation mechanism via four stages: (i) the dissociation of pristine NCs, (ii) the formation of Pb-Br intermediates, (iii) low-dimensional nanoplatelets (NPLs) and (iv) cubic CsPbBr₃ NCs, corresponding to the blue-to-green emission process. The desorption and reorganization of organic ligands induced by NO⁺ and the involvement of BF₄^- in the ligand exchange process played pivotal roles in this dissolution-recrystallization of NCs. Moreover, controlled shape evolution from anisotropic NPLs to NCs was investigated through variations in the amount of NOBF₄. This further validates that additives exert a decisive role in the symmetry and growth of nanostructured perovskite crystals during phase transition based on the ligand-exchange mechanism. This finding serves as a source of inspiration for the synthesis of highly luminescent CsPbBr₃ NCs, providing valuable insights into the chemical mechanism in post-synthesis transformation.