Preparation and Upconversion Emission Investigation of the YVO4: Yb³⁺/Tb³⁺/Eu³⁺ Nanomaterials and Their Coupling with the Au Nanoparticles

The YVO₄ nanomaterials doped with the Yb³⁺/Tb³⁺/Eu³⁺ ions and their mixture with the Au nanoparticles are synthetized with a classical sol-gel method. With the excitation of a 975 nm laser, both the doped YVO₄ sample and the corresponding mixture sample have a series of upconversion emission peaks of Eu³⁺ and Tb³⁺, which includes the ⁵D₀→⁷F_j transitions of the Eu³⁺ ions and the ⁵D₄→⁷F_j transitions of the Tb³⁺ ions. The slope values of the log-log plots of laser power dependence for the upconversion emission intensities range from 1 to 2, which confirms that the upconversion process is a two-photon process. With the plasmon coupling, the slope value for the power dependence of the magnetic dipole transition ⁵D₀→⁷F₁ of the Eu³⁺ ions decreases from 1.4 to 1.1, while it increases for other electric dipole transitions. Because of the plasmon coupling, the emission peak of the Eu³⁺ at about 2.095 eV decreases significantly and almost cannot be observed. Meanwhile, the emission peak of the Tb³⁺ at about 2.113 eV increases significantly and becomes the main emission peak in the yellow band. This phenomenon confirms that there is different mechanism for the magnetic and electric dipole emission with the plasmon coupling.