Intrinsic Lead Ion Emissions in Zero-Dimensional Cs₄PbBr₆ Nanocrystals

We investigate the intrinsic lead ion (Pb²⁺) emissions in zero-dimensional (0D) perovskite nanocrystals (NCs) using a combination of experimental and theoretical approaches. The temperature-dependent photoluminescence experiments for both "nonemissive" (highly suppressed green emission) and emissive (bright green emission) Cs₄PbBr₆ NCs show a splitting of emission spectra into high- and low-energy transitions in the ultraviolet (UV) spectral range. In the nonemissive case, we attribute the high-energy UV emission at approximately 350 nm to the allowed optical transition of ³P₁ to ¹S₀ in Pb²⁺ ions and the low-energy UV emission at approximately 400 nm to the charge-transfer state involved in the 0D NC host lattice (D-state). In the emissive Cs₄PbBr₆ NCs, in addition to the broad UV emission, we demonstrate that energy transfer occurs from Pb²⁺ ions to green luminescent centers. The optical phonon modes in Cs₄PbBr₆ NCs can be assigned to both Pb-Br stretching and rocking motions from density functional theory calculations. Our results address the origin of the dual broadband Pb²⁺ ion emissions observed in Cs₄PbBr₆ NCs and provide insights into the mechanism of ionic exciton-optical phonon interactions in these 0D perovskites.