Unexpected Red Emission From Cs4PbI6 Nanocrystals

Luminescent zero-dimensional perovskites of Cs4PbBr6 have spurred a great deal of interest on their photoluminescence (PL) origin in the last few years. Regardless of the ongoing debate, it is interesting to realize that only a Br-based zero-dimensional phase was found to be highly luminescent, while its Cl and I counterparts remain elusive. In this study, we report a luminescent Cs4PbI6 zero-dimensional perovskite having both a sharp red emission and a sharp excitation peak. The phase-pure nanocrystal exhibited a polycrystalline nature whereby lattice boundary or active defects may form, leading to an Urbach tail in the absorption peak. A single-particle imaging technique reveals the heterogeneous emission behavior from particle to particle. A temperature-dependent photoluminescence measurement was performed to evaluate the exciton binding energy (172 meV), which classified the nature of the exciton as a Frenkel type. Compared to that of its 3D-counterpart CsPbI3 nanocrystals, the PL of Cs4PbI6 exhibits a robust resistance to both the ion exchange and photobleaching effect. Our study brings a new luminescent member to the zero-dimensional perovskite family, which may inspire more endeavors to exploit those relative phosphors and to unravel the corresponding luminescent origins.