Phase engineering of Cs-Pb-Br nanoparticles for tunable optical behavior

We report tunable structural and optical properties of Cs-Pb-Br nanomaterials, including the classic CsPbBr3 perovskite, and perovskite-inspired CsPb2Br5 and Cs4PbBr6 compounds. By systematically varying precursor composition, reaction conditions, and external stimuli, such as moisture and solvents, we achieve controlled formation and reversible phase transitions among these structures. Optical spectroscopy, combined with density functional theory, reveals that the degree of octahedral connectivity governs both absorption and emission behavior: CsPbBr3 exhibits bright green photoluminescence due to parity-allowed direct transitions, CsPb2Br5 shows weak orange-red emission associated with indirect or symmetry-forbidden transitions, while Cs4PbBr6 remains non-emissive due to isolated [PbBr₆]^4- octahedra and parity-forbidden electronic transitions. We demonstrate how external stimuli can induce structural reorganization into the emissive CsPbBr3 phase, enabling dynamic modulation of luminescence. The reported findings highlight the tunability of structural phases and emission properties in perovskite-inspired systems, offering new opportunities for switchable optoelectronic devices and stimuli-responsive luminescent materials.