Synthesis and Upconversion Luminescence Fine-tuning of Yb³⁺/Ho³⁺-Doped Indium and Gallium Oxide Nanoparticles

Rare earth (RE) dopants can modulate the bandgap of oxides of indium and gallium and provide extra upconversion luminescence (UCL) abilities. However, relevant UCL fine-tuning strategies and energy mechanisms have been less studied. In this research, InGaO, Ho³⁺ monodoped and Yb³⁺/Ho³⁺ codoped In₂O₃, and Ho³⁺ monodoped Yb₃Ga₅O₁₂ nanoparticles (NPs) were synthesized by a solvothermal method. The effects of Yb³⁺ and Ho³⁺ dopants on the crystal structures, UCL properties, and optical bandgaps of the oxides were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UCL spectroscopy, and measurements of decay times, pump power dependence, and transmittance spectra. The crystal structures of oxide products of indium and gallium were significantly modified with RE dopants. In₂O₃ and Yb₃Ga₅O₁₂ were selected as the host materials. For Yb³⁺/Ho³⁺ codoped In₂O₃ NPs, there existed energy transfers from the defect states of In₂O₃ to Ho³⁺ and from Yb³⁺ to Ho³⁺. With a fixed Ho³⁺ concentration, In₂O₃:0%Yb³⁺,2%Ho³⁺ NPs showed the optimal UCL properties mainly due to In₂O₃-Ho³⁺ energy transfer and Ho³⁺-Yb³⁺ energy-back-transfer, while with a fixed Yb³⁺ concentration, In₂O₃:5%Yb³⁺,3%Ho³⁺ NPs with a slight Yb₂O₃ impurity and Yb₃Ga₅O₁₂:2%Ho³⁺ NPs did mainly due to Ho³⁺-Ho³⁺ cross-relaxation. Besides, the optical bandgaps of In₂O₃ and Yb₃Ga₅O₁₂ were noticeably broadened with RE dopants. These findings can offer feasible directions for the synthesis and UCL fine-tuning of RE-doped oxides of indium and gallium and improve their multifunction application prospects in the fields of semiconductor and UCL nanomaterials.