Colloidal Synthesis\nof Blue-Emitting Cs<sub>3</sub>TmCl<sub>6</sub> Nanocrystals via Localized\nExcitonic Recombination\nfor Down-Conversion White Light-Emitting Diodes

Lead-halide perovskite nanocrystals (NCs) have gained\nsignificant\nattention for their promising applications in lighting and display\ntechnologies. However, blue-emitting NCs have struggled to match the\nhigh efficiency of their red and green counterparts. Moreover, many\nreported blue-emitting perovskite NCs contain heavy metal lead (Pb),\nwhich poses risks to human health and the environment. In this study,\nwe synthesized rare-earth-based Cs<sub>3</sub>TmCl<sub>6</sub> NCs\nvia the hot injection method, which exhibit a broadband blue emission\nat 440 nm. Combined experimental and theoretical studies indicate\nthat the broadband emission in Cs<sub>3</sub>TmCl<sub>6</sub> arises\nfrom self-trapped excitons due to the excited-state structural distortion\nof the [TmCl<sub>6</sub>]<sup>3–</sup> cluster. Furthermore,\nthe ultrafast dynamics of charge carriers were analyzed using time-resolved\nphotoluminescence and transient absorption measurements. Encouraged\nby the remarkable thermal, light, and water stabilities of Cs<sub>3</sub>TmCl<sub>6</sub> NCs, as evidenced by experimental and theoretical\nresults, a white light-emitting diode was further designed and fabricated\nusing the Cs<sub>3</sub>TmCl<sub>6</sub> NCs as the color converter.\nThe device exhibits outstanding performance, achieving a long half-lifetime\nof 336 h and a large color-rendering index of 87.0. Combining eco-friendly\nfeatures and a facile synthesis method, the rare-earth-based Cs<sub>3</sub>TmCl<sub>6</sub> NCs mark a significant breakthrough as a\nreliable blue emitter, showcasing their future potential in lighting\nand display applications.