Single‐Photon Superradiance in Giant Rhombicuboctahedral CsPbI ₃ Nanocrystals

ABSTRACT Increasing the volume of halide perovskite nanocrystals (NCs) is generally accompanied by lower photoluminescence quantum yield (PLQY). Here, we overcame this trade‐off with giant (25 nm) rhombicuboctahedral CsPbI 3 NCs that retain an exceptional PLQY of 87%. Their distinct size and morphology were induced by using phenacyl iodide as a novel precursor and subsequently characterized by x‐ray diffraction and transmission electron microscopy. In these giant NCs, the non‐radiative Auger process is suppressed, giving rise to a high biexciton PLQY of 55%, while a high single‐photon purity up to 95% was achieved using a time‐gating method. Benefiting from their enlarged volume, these NCs achieve one of the largest reported absorption cross‐sections of 5.3 × 10 −13 cm 2 , along with emission tunable to the near‐infrared region (>700 nm) and a prolonged room‐temperature lifetime of 465 ns—nearly an order of magnitude longer than conventional NCs. In contrast, at cryogenic temperatures, these enlarged NCs exhibit narrow and ultrafast emission ( τ = 467 ps) arising from the coherent coupling of dipoles within a single NC, which induces a giant oscillator strength and leads to single‐photon superradiance. These results position the unique rhombicuboctahedral, giant CsPbI 3 NCs as novel near‐infrared emitters and promising candidates for high‐speed quantum photon sources.