Green-Emitting Powders of Zero-Dimensional Cs4PbBr6: Delineating the Intricacies of the Synthesis and the Origin of Photoluminescence

A detailed investigation into the synthesis of green-emitting powders of Cs₄PbBr₆ and CsPbBr₃ materials by antisolvent precipitation from CsBr–PbBr₂ precursor solutions in dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) is reported. Various solvated lead bromide and polybromide species (PbBr₂, [PbBr₃]⁻, [PbBr₄]^2–, and possibly [PbBr₅]^3– or [PbBr₆]^4–) are detected in the precursor solutions by optical absorbance and emission spectroscopies. The solvodynamic size of the species in solution is strongly solvent-dependent: ∼1 nm species were detected in DMSO, while significantly larger species were observed in DMF by dynamic light scattering. The solvodynamic size of the lead bromide species plays a critical role in determining the Cs-Pb-Br composition of the precipitated powders: smaller species favor the precipitation of Cs₄PbBr₆, while larger species template the formation of CsPbBr₃ under identical experimental conditions. The powders have been characterized by ¹³³Cs and ²⁰⁷Pb solid-state nuclear magnetic resonance, and ¹³³Cs sensitivity toward the different Cs environments within Cs₄PbBr₆ is demonstrated. Finally, the possible origins of green emission in Cs₄PbBr₆ samples are discussed. It is proposed that a two-dimensional Cs₂PbBr₄ inclusion may be responsible for green emission at ∼520 nm in addition to the widely acknowledged CsPbBr₃ impurity, although we found no conclusive experimental evidence supporting such claims.