Preparation and Upconversion Emission Investigation of the YVO₄:Yb³⁺:Er³⁺ Nanomaterials and Their Coupling with the Au Nanoparticles

YVO₄:Yb³⁺:Er³⁺ nanoparticles and their mixture with Au nanoparticles have been fabricated by a simple chemical method. With the excitation of a 975-nm laser, both samples had strong upconversion emissions at 2.18, 2.25, and 2.36 eV, which correspond to the transitions ⁴F_9/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ²H_11/2 → ⁴I_15/2 of Er³⁺ ions, while the emission at 2.36 eV has a large overlap with the resonant absorption energy, 2.34 eV, of the Au nanoparticles. With the coupling of the Au nanoparticles, the upconversion emission at 2.36 eV presents a strong quenching effect which is due to the energy transfer from the Er³⁺ ions to the Au nanoparticles. This depends on the energy difference between the emission of the Er³⁺ ions and the absorption of the Au nanoparticles. The upconversion power dependence shows that the slope factor of the power dependence is about 3 for a high pump power, which means that the upconversion process is a three-photon process for both samples.