expected to exhibit intriguing optical properties. [22,23] This is because the coordinated metal octahedra are spatially isolated by surrounding inorganic or organic cations, resulting in strong exciton confinement and self-trapped exciton (STE) emission effects. The STE emission in 0D lead-free metal halides originates from the lattice deformation of metal halide structures and possesses some typical features, such as broadband PL emission and large Stokes shift. [24][25][26] Recently, a series of 0D lead-free metal halides have been reported to exhibit intriguing STE emissions. [27][28][29][30][31][32][33][34] 0D all-inorganic copper halides are one kind of enthralling materials due to their low toxicity and earth-abundant elements. In particular, Cu atoms are fourfold coordinated with halide ions to form tetrahedral structure, [35] which favors the Jahn-Teller distortion to produce strong STE emission. [36,37] Although the synthesis and optical properties of 0D Cs 3 Cu 2 I 5 NCs have been reported, [38] it is still challenging to prepare high-quality colloidal NCs of 0D all-inorganic copper(I) halides materials with isostructural series (such as Cs 3 Cu 2 X 5, X = I, Br, and Cl), and systematically investigate their halogen-dependent optical properties. Herein, we present the colloidal syntheses and optical characterizations of Cs 3 Cu 2 X 5 (X = I, Br, and Cl) NCs. These colloidal cesium cuprous halide NCs possess well-defined shapes, narrow size distributions, and tunable emissions. Intriguingly, with halogen ions changing from I â to Br â , and Cl â , Cs 3 Cu 2 X 5 NCs exhibit gradually redshifted emission peaks. Meanwhile, the high PL quantum yield (PLQY) of 48.7% is achieved on Cs 3 Cu 2 Cl 5 NCs, while Cs 3 Cu 2 I 5 NCs exhibit considerable air stability over 45 days.In our work, Cs 3 Cu 2 X 5 NCs were obtained via a hot injection method. [39,40] In particular, Cs 3 Cu 2 I 5 NCs were prepared based on a hot plate route in air. Typically, oleylammonium iodide (OLA-I) precursor was injected into 1-octadecene (ODE) solution containing cupric acetate, cesium carbonate and oleic acid (OA) (see details in supporting information, SI) to initiate the reaction. The OLA-I precursor not only offers iodine ion, but also reduces Cu(II) to Cu(I) cations. Subsequently, Cu + combines with Cs + and I â to form Cs 3 Cu 2 I 5 NCs stabilized by OA and OLA ligands. The feeding ratio of inorganic salt precursors and the amount of OA are revealed to play key roles in producing Cs 3 Cu 2 I 5 NCs with pure orthorhombic phase ( Figures S1 and S2, Supporting Information). Figure 1a presents a representative transmission electron microscopy (TEM) image of as-prepared Cs 3 Cu 2 I 5 NCs, 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 Cs 3 Cu 2 X 5 (X = I, Br, and Cl) NCs with orthorhombic structure and well-defined morphologies. All these Cs 3 Cu 2 X 5 NCs exhibit broadband blue-green photolum...