Synthesis and Upconversion Luminescence Fine-tuning\nof Yb<sup>3+</sup>/Ho<sup>3+</sup>-Doped Indium and Gallium Oxide\nNanoparticles

Rare\nearth (RE) dopants can modulate the bandgap of oxides of indium\nand gallium and provide extra upconversion luminescence (UCL) abilities.\nHowever, relevant UCL fine-tuning strategies and energy mechanisms\nhave been less studied. In this research, InGaO, Ho<sup>3+</sup> monodoped\nand Yb<sup>3+</sup>/Ho<sup>3+</sup> codoped In<sub>2</sub>O<sub>3</sub>, and Ho<sup>3+</sup> monodoped Yb<sub>3</sub>Ga<sub>5</sub>O<sub>12</sub> nanoparticles (NPs) were synthesized by a solvothermal method.\nThe effects of Yb<sup>3+</sup> and Ho<sup>3+</sup> dopants on the\ncrystal structures, UCL properties, and optical bandgaps of the oxides\nwere characterized by X-ray diffraction (XRD), transmission electron\nmicroscopy (TEM), UCL spectroscopy, and measurements of decay times,\npump power dependence, and transmittance spectra. The crystal structures\nof oxide products of indium and gallium were significantly modified\nwith RE dopants. In<sub>2</sub>O<sub>3</sub> and Yb<sub>3</sub>Ga<sub>5</sub>O<sub>12</sub> were selected as the host materials. For Yb<sup>3+</sup>/Ho<sup>3+</sup> codoped In<sub>2</sub>O<sub>3</sub> NPs,\nthere existed energy transfers from the defect states of In<sub>2</sub>O<sub>3</sub> to Ho<sup>3+</sup> and from Yb<sup>3+</sup> to Ho<sup>3+</sup>. With a fixed Ho<sup>3+</sup> concentration, In<sub>2</sub>O<sub>3</sub>:0%Yb<sup>3+</sup>,2%Ho<sup>3+</sup> NPs showed the\noptimal UCL properties mainly due to In<sub>2</sub>O<sub>3</sub>–Ho<sup>3+</sup> energy transfer and Ho<sup>3+</sup>–Yb<sup>3+</sup> energy-back-transfer, while with a fixed Yb<sup>3+</sup> concentration,\nIn<sub>2</sub>O<sub>3</sub>:5%Yb<sup>3+</sup>,3%Ho<sup>3+</sup> NPs\nwith a slight Yb<sub>2</sub>O<sub>3</sub> impurity and Yb<sub>3</sub>Ga<sub>5</sub>O<sub>12</sub>:2%Ho<sup>3+</sup> NPs did mainly due\nto Ho<sup>3+</sup>–Ho<sup>3+</sup> cross-relaxation. Besides,\nthe optical bandgaps of In<sub>2</sub>O<sub>3</sub> and Yb<sub>3</sub>Ga<sub>5</sub>O<sub>12</sub> were noticeably broadened with RE dopants.\nThese findings can offer feasible directions for the synthesis and\nUCL fine-tuning of RE-doped oxides of indium and gallium and improve\ntheir multifunction application prospects in the fields of semiconductor\nand UCL nanomaterials.