Optimizing Electronic Interactions in Terbium–Dysprosium-Codoped Calcium Fluoride Nanoparticles to Enhance Near-Monochromatic Green and Generate Broadband Yellow Emission

Codopants in inorganic nanoparticles (NPs) can alter electronic interactions for potential beneficial outcomes. This work systematically investigates the electronic interactions between Tb³⁺ and Dy³⁺ that are codoped in CaF₂ [Ca(TbDy)F₂] NPs in order to address questions related to (a) excitation wavelength dependence and (b) relative dopant concentration. Specifically, the Ca(TbDy)F₂ NPs are perturbed by selective excitation of chromophore(s). Varying nominal dopant amounts are considered, with Tb–Dy = 10–1, 10–10, 1–10, and 1–1. The spectral outcomes are governed by a complex interplay of (a) Dy³⁺* → Tb³⁺ energy transfer, (b) Dy³⁺-induced site symmetry breaking-mediated Tb³⁺ emission brightening, (c) aggregation effects, and (d) Tb³⁺* → Dy³⁺ back energy transfer. Counterintuitively, a Dy³⁺* → Tb³⁺ electronic interaction is found to be more favorable in the statistically unfavored NP with Tb–Dy = 10–1, compared to that in the corresponding 1–10 case. Experiments with the Ca(TbDy)F₂ NPs having Tb–Dy = 1–1 identify maximal contributions from breaking dopant local site symmetry. A spectral overlap-mediated mechanism and a charge trapping-mediated mechanism are used to correlate the multifaceted codopant electronic interactions. Finally, the optimally emissive codoped Ca(TbDy)F₂ NPs are demonstrated for their use to generate long-lived (a) enhanced near-monochromatic green emission centered at 545 nm and (b) broadband yellow emission at 575–585 nm for potential applications in biological sensing, chemical sensing, light-emitting diodes, and traffic light signaling.