Benefiting from their large compositional space, direct bandgap nature, and outstanding structural stability,^[1–4] vacancy-ordered layered double perovskites with formula Cs₄M(II)M(III)₂X₁₂ (M(II): Cu²⁺, Mn²⁺; M(III): Bi³⁺, Sb³⁺, In³⁺; X: Cl^-, Br^-, I^-), have recently attracted increasing attention as substitutive materials of lead-based halide perovskites for potential commercial optoelectronic applications. The layered double perovskite structure comprises one layer of [M(II)X₆]^4- octahedra inserted in between two layers of [M(III)X₆]^3- octahedra. As a representative, Cs₄CuSb₂Cl₁₂ has been successfully synthesized both in the form of single-crystalline powder^[2] and NCs,^[3,5] exhibiting a narrow direct bandgap (1.0–1.8 eV) and impressive stability, which however still suffers from the high toxicity of Sb element and the absence of emission at room temperature. To overcome these drawbacks, Cs₄CuIn₂Cl₁₂, with toxic Sb³⁺ substituted by relatively non-toxic In³⁺, could be a promising candidate to fulfill the requirement of compositional engineering and optical tunability particularly in the UV range. Yet, to date, the synthesis of Cs₄CuIn₂Cl₁₂ layered double perovskites has not yet been reported for neither bulk film nor NCs.

Herein, we report the first-ever colloidal synthesis of lead-free Cs₄CuIn₂Cl₁₂ layered double perovskite NCs using a modified hot-injection method.^[5,6] The synthetic details are described in the Supporting Information (SI). While a standard hot-injection reaction in moisture-free environment resulted in Cs₄CuIn₂Cl₁₂ with a very low photoluminescence quantum yield (PLQY) of 0.12%, we found that the handling of synthesis precursors in the presence of moisture (RH~40%) enhances PLQY by more than one order of magnitude up to 1.70 %. Water-assisted in-situ synthesis has been recognized as an effective strategy to tune the optical properties and stability for both lead-based^[7,8] and lead-free halide perovskite NCs^[9,10] via the controlling of NCs size, shape, and crystallinity. Nevertheless, there is still a lack of deep understanding of how water influences NCs growth regime and corresponding PL property, especially for lead-free double perovskite NCs. The introduction of moisture in the precursor (namely “wet” precursor conditions) of Cs₄CuIn₂Cl₁₂ NCs (w-Cs₄CuIn₂Cl₁₂) induces the morphological transformation from 3D nanocubes (NCus) to 2D nanoplatelets (NPLs), driven by the ionized H₃O⁺ and OH^- from the water content as additional capping ligands. The ultrafast transient absorption studies suggest a strengthened self-trapped exciton (STE) effect for w-Cs₄CuIn₂Cl₁₂ NCs compared to the NCs synthesized in “dry” conditions (d-Cs₄CuIn₂Cl₁₂ NCs), resulting in the conversion of dark transitions into radiative transitions, which directly contributes to the PLQY.