Rapid and efficient microwave-assisted synthesis of Mn-doped cesium bromide to phase engineered cesium manganese bromide nanocrystals with color-tunable RGB emission†

Lead-free perovskite nanocrystals (NCs) have attracted considerable attention due to their excellent optoelectronic properties. However, they generally exhibit broadband emission with poor color purity. Similarly, obtaining tunable red/green/blue (RGB) emissions from lead-free perovskite NCs is highly desirable for several display applications. However, directly synthesizing lead-free perovskite NCs with tunable RGB emission and high color purity remains challenging. Herein, we have developed a simple and rapid microwave-assisted (MW-AT) synthesis strategy for synthesizing Mn-doped CsBr [Mn²⁺:CsBr] to phase engineering of non-toxic and stable all-inorganic cesium manganese bromide perovskite NCs in a nonpolar solvent with tunable blue-green-red emission color with high color purity. The phase transition was triggered by changing the MnBr₂ concentration during the microwave synthesis: from Mn²⁺:CsBr (blue emission) to zero dimensional (0D) Cs₃MnBr₅ NCs (green emission) to one dimensional (1D) CsMnBr₃ NCs (red emission). In addition, in a controlled moisture environment, both the 0D Cs₃MnBr₅ and 1D CsMnBr₃ NCs were transformed into 0D Cs₂MnBr₄·2H₂O NCs (blue emission), which could be inversely transformed back to their respective original phase via thermal annealing. Thus, our work highlights for the first time a rapid and efficient MW-AT synthesis strategy to obtain phase-pure tunable optical properties with high color purity from Mn-doped CsBr to cesium manganese bromide perovskite NCs via phase engineering, which can be further utilized in designing anti-counterfeiting and encryption materials for coding with high security and information concealment. Our work also provides a new avenue for exploring efficient MW-AT synthesis of other earth-abundant eco-friendly highly luminescent Pb-free perovskite NCs for future endeavors.