Photoluminescent nano-CsPbBr₃ embedded in Cs₄PbBr₆ crystals: formation mechanism and properties

Luminescent crystalline cesium lead bromide has been synthesized by using an antisolvent method with the nominal ratio of Cs:Pb in the precursors varying in a wide range from 4.5:1 to 1:1. Although the powder X-ray diffraction patterns of all the specimens show Cs₄PbBr₆ as a pure phase or a main phase, high-resolution transmission electron microscopy images reveal a large amount of CsPbBr₃ nanocrystallites embedded in all the Cs₄PbBr₆ crystals. A formation mechanism of these perovskite nanocrystallites serving as actual active centers of photoluminescence is proposed. The most crucial step in the crystal growth is the deposition of a noncrystalline coating layer containing polymerized PbBr₆^4– linked by Cs⁺ with the Cs:Pb ratio of about 3:1, and therefore, the actual crystal growth sites are at the interface between the crystal and the coating layer, instead of the crystal/solution interface. The local lack of Cs during the formation of Cs₄PbBr₆ results in the formation of CsPbBr₃ nanocrystallites inside the parent crystals of Cs₄PbBr₆. The photoluminescence quantum yield and stability of the embedded CsPbBr₃ nanocrystallites are significantly improved in comparison with bare CsPbBr₃ crystals. Such simultaneous growth of parent crystals and the embedded nanocrystallites sheds light on further development of cesium lead halide-based photoluminescent materials.