Er³⁺/Yb³⁺ co-doped ZnS quantum dots: structure, optical properties and up-conversion luminescence

Er³⁺/Yb³⁺ co-doped ZnS quantum dots (QDs) were synthesized using a wet chemical approach and studied for their structural and optical properties, as well as upconversion (UC) luminescence. X-ray diffraction (XRD) confirmed the formation of phase cubic ZnS, where rare-earth Er³⁺ and Yb³⁺ ions were effectively incorporated into the host lattice without secondary phases. X-ray photoelectron spectroscopy (XPS) analyses further verified the trivalent states of Er and Yb ions in the QDs. For the first time, the UC luminescence phenomenon in Er³⁺/Yb³⁺ co-doped ZnS QDs was studied and explained in detail. Under 980 nm excitation, the Er³⁺/Yb³⁺ co-doped ZnS QDs exhibited green and red UC bands, dominated by the green emission, whose intensity strongly depended on Yb³⁺ concentration. Power-dependent and lifetime measurements indicated that the UC process was primarily governed by a two-photon mechanism facilitated by efficient Yb³⁺ → Er³⁺ energy transfer. Chromaticity analyses demonstrated a distinct emission color shift from deep-blue (pure ZnS QDs) to stable green-yellow in co-doped QDs. These results highlight the potential of Er³⁺/Yb³⁺ co-doped ZnS QDs as efficient UC nanomaterials for applications in photonic and optoelectronic devices.