Few-Nanometer-Sized α-CsPbI3 Quantum Dots Enabled by Strontium Substitution and Iodide Passivation for Efficient Red-Light Emitting Diodes

Cubic phase CsPbI₃ quantum dots (α-CsPbI₃ QDs) as a newly emerging type of semiconducting QDs hold tremendous promise for fundamental research and optoelectronic device applications. However, stable and sub-5 nm-sized α-CsPbI₃ QDs have rarely been demonstrated so far due to their highly labile ionic structure and low phase stability. Here, we report a novel strontium-substitution along with iodide passivation strategy to stabilize the cubic phase of CsPbI₃, achieving the facile synthesis of α-CsPbI₃ QDs with a series of controllable sizes down to sub-5 nm. We demonstrate that the incorporation of strontium ions can significantly increase the formation energies of α-CsPbI₃ QDs and hence reduce the structure distortion to stabilize the cubic phase at the few-nanometer size. The size ranging from 15 down to sub-5 nm of as-prepared stable α-CsPbI₃ QDs allowed us to investigate their unique size-dependent optical properties. Strikingly, the few-nanometer-sized α-CsPbI₃ QDs turned out to retain high photoluminescence and highly close packing in solid state thin films, and the fabricated red light emitting diodes exhibited high brightness (1250 cd m^-2 at 9.2 V) and good operational stability (L₅₀ > 2 h driven by 6 V). The developed cation-substitution strategy will provide an alternative method to prepare uniform and finely size-controlled colloidal lead halide perovskite QDs for various optoelectronic applications.