Tuning the optical and magnetic properties of Tb3+-doped ZnO quantum dots: structural investigation and Judd-Ofelt theoretical analysis.

Tb3+-doped ZnO quantum dots (ZnO:Tb3+ QDs) with Tb3+ concentrations of 0-7 mol% were synthesized by a wet-chemical precipitation method to tune their optical and magnetic properties. XRD and Rietveld refinement confirmed the formation of hexagonal wurtzite ZnO without detectable secondary phases, while lattice expansion and increased microstrain suggested the incorporation of Tb3+ ions into the ZnO matrix. XPS and EDX mapping verified the presence and homogeneous distribution of Tb species. UV-vis absorption spectra showed a red shift of the absorption edge and bandgap narrowing with increasing Tb3+ content, indicating enhanced defect-related disorder. The PL spectra exhibited characteristic Tb3+ emissions, dominated by the green 5D4 → 7F5 transition at approximately 546 nm. The emission intensity increased up to 5 mol% Tb3+ and then decreased due to concentration quenching. Judd-Ofelt analysis revealed enhanced local asymmetry and covalent character around Tb3+ ions, with ZnO:Tb5% showing favorable radiative parameters for green emission. Lifetime analysis confirmed increased non-radiative relaxation at higher Tb3+ concentrations. Magnetic measurements showed that pristine ZnO QDs were diamagnetic, whereas Tb3+ doping induced weak room-temperature ferromagnetic-like behavior, with the highest saturation magnetization obtained for ZnO:Tb3%. These results demonstrate that Tb3+ doping is an effective strategy for simultaneously tailoring the luminescent and magnetic functionalities of ZnO QDs for photonic, magneto-optical, and spintronic applications.