Bright self-trapped exciton emission in alkali iodide nanocrystals via Sn(<sc>ii</sc>)-doping

Lead-free alkali halides have gained increasing attention owing to their desired optoelectronic properties across a variety of light-emitting applications. However, state-of-the-art alkali halide emitters commonly are bulky crystals functionalized with highly toxic elemental doping, hindering their integration into emerging flexible, wearable and large-area optoelectronic devices. Herein, we have developed environmental friendly and solution-processable alkali halide nanocrystals that exhibit intense self-trapped emission facilitated by Sn2+ ionic doping. We find that the presence of Sn2+-interstitial leads to the tilting of cubic structures of RbI and thus triggers the self-trapped emission. The resultant tin(ii)-doped RbI and RbxCs1-xI nanocrystals display an impressive maximum photoluminescence quantum yield of 91.9% and exhibit remarkable stability under ambient conditions. These findings pave the way for a new class of bright, solution-processable alkali halides, with diverse applications such as energy down conversion and flexible scintillation technologies.