Photoluminescent Nano-CsPbBr3 Embedded in Cs4PbBr6 Crystals: Formation Mechanism and Properties

Abstract

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 PbBr64- 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.