Understanding of the Photoluminescence Mechanism Based on Zero-Dimensional Cs4PbBr6–mXm (X = Cl, I) Single Crystals

The isolated [PbBr6] octahedra in the zero-dimensional (0-D) perovskite Cs4PbBr6 crystal lattice trigger localized optical transitions, which exhibits extremely unique optical properties and brings controversial topics concerning its optical properties, particularly about its photoluminescence (PL) nature. Herein, we successfully grown a series of large-scale Cs4PbBr6–mXm (X = Cl, I) single crystals (SCs) from Cs-rich solutions, partially replacing Br with Cl and I, taking advantage of superiorities from SCs to explore optical behavior and eliminate the mutual transformation distraction between Cs4PbBr6 and CsPbBr3. No CsPbBr3 is detected in our Cs4PbBr6–mXm (X = Cl, I) large SCs, whereas they exhibit obvious PL phenomena. It is revealed that the Cs4PbBr6 SC exhibits intense PL, and the replacements of Br with Cl and I can regularly alter the crystal lattice, optical absorption cutoffs, and PL positions and bring multiple PL peaks, among which one is attributed to the Br vacancy (VBr) and another originates from crystal lattice distortions triggered by both halogen replacements. Therefore, we proposed that the PL nature of CsPbBr3–mXm (X = Cl, I) SCs originates from crystal lattice distortions that can be induced by both halogen replacements and VBr. Severe local lattice distortion, for example, in Cs4PbBr6 films and powders, definitely can induce Cs4PbBr6–mXm to transform toward CsPbBr3 and therefore enhance the PL yield.