Ag-to-Cu template conversion enables sharp red emission from (Cu,Ag)–In–Ga–S quantum dots with unity quantum yield

Cu-based I–III–VI quantum dots (QDs) have long been constrained by defect-dominated emission, limiting narrowband, high-efficiency red output. Here we report a template-conversion strategy that transforms sharp-emissive Ag–In–Ga–S (AIGS) nanocrystals into (Cu,Ag)–In–Ga–S ((C,A)IGS) cores via post-synthetic Ag-to-Cu cation exchange, yielding excitonic red emission at 629 nm with a small Stokes shift (60 meV) and a narrow linewidth (41 nm). Sequential shell growth produces (C,A)IGS/GaS_x/ZnS heterostructures with unity photoluminescence (PL) quantum yield and linewidth down to 37 nm. The ZnS outer shell suppresses oxidation-driven reconstruction of GaS_x, enabling near-complete PL retention under thermal aging (85 °C) and markedly improved UV stability, corroborated by nearly invariant Ga 3d and S 2p X-ray photoelectron spectroscopy features after UV exposure. As color converters, these (C,A)IGS QDs outperform red InP/ZnSe/ZnS benchmarks, delivering 24%–52% higher blue absorption and 30%–44% higher blue-to-red color-conversion efficiency at identical loadings. Finally, (C,A)IGS QDs-based electroluminescent devices demonstrate pure red emission, reaching a maximum luminance of 14007 cd/m² and an external quantum efficiency up to 13.8%.