0D Cs₃ Cu₂ X₅ (X = I, Br, and Cl) Nanocrystals: Colloidal Syntheses and Optical Properties

0D lead-free metal halide nanocrystals (NCs) are an emerging class of materials with intriguing optical properties. Herein, colloidal synthetic routes are presented for the production of 0D Cs₃ Cu₂ X₅ (X = I, Br, and Cl) NCs with orthorhombic structure and well-defined morphologies. All these Cs₃ Cu₂ X₅ NCs exhibit broadband blue-green photoluminescence (PL) emissions in the range of 445-527 nm with large Stokes shifts, which are attributed to their intrinsic self-trapped exciton (STE) emission characteristics. The high PL quantum yield of 48.7% is obtained from Cs₃ Cu₂ Cl₅ NCs, while Cs₃ Cu₂ I₅ NCs exhibit considerable air stability over 45 days. Intriguingly, as X is changed from I to Br and Cl, Cs₃ Cu₂ X₅ NCs exhibit a continuous redshift of emission peaks, which is contrary to the blueshift in CsPbX₃ perovskite NCs.