Extrinsic Green Photoluminescence from the Edges of 2D Cesium Lead Halides

Since the first report of the green emission of 2D all-inorganic CsPb₂Br₅, its bandgap and photoluminescence (PL) origin have generated intense debate and remained controversial. After the discovery that PL centers occupy only specific morphological structures in CsPb₂Br₅, a two-step highly sensitive and noninvasive optical technique is employed to resolve the controversy. Same-spot Raman-PL as a static property–structure probe reveals that CsPbBr₃ nanocrystals are contributing to the green emission of CsPb₂Br₅; pressure-dependent Raman-PL with a diamond anvil cell as a dynamic probe further rules out point defects such as Br vacancies as an alternative mechanism. Optical absorption under hydrostatic pressure shows that the bandgap of CsPb₂Br₅ is 0.3–0.4 eV higher than previously reported values and remains nearly constant with pressure up to 2 GPa in good agreement with full-fledged density functional theory (DFT) calculations. Using ion exchange of Br with Cl and I, it is further proved that CsPbBr_{3− x} X_x (X = Cl or I) is responsible for the strong visible PL in CsPb₂Br_{5− x} X_x. This experimental approach is applicable to all PL-active materials to distinguish intrinsic defects from extrinsic nanocrystals, and the findings pave the way for new design and development of highly efficient optoelectronic devices based on all-inorganic lead halides.