Real-Time Observation of Anion-Exchange Reaction in Cs₃Cu₂Cl₅ Single Crystal

Cs3Cu2Cl5 has attracted great attention in many photonic applications owing to its high photoluminescence quantum yield (PL QY), nontoxicity, and large Stokes shift. However, its labile nature to air and single-color emission have limited its practical use in display-related applications. In this work, a single crystal of Cs3Cu2Cl5 was grown with a user-friendly Cu2+ precursor. To tune the emission color and stability, the anion-exchange reaction was employed with trace amounts of I ions. The anion-exchanged crystals not only enabled a fine color tuning from sky-blue to emerald-green but also dramatically improved the chemical stability of Cs3Cu2Cl5. Microscopic imaging of an individual crystal flake demonstrated a uniform emission of cross section, which was quite different from conventional core/shell structures. Importantly, the anion-exchanged crystals retained a high PL QY up to 98%. Our work brings insights into the manipulation of self-trapped excitons, opening many avenues for stable photoelectric devices.