Multi-layer core-shell nanoparticles (YVO<sub>4</sub>:Nd<sup>3+</sup>/mSiO<sub>2</sub>/SiO<sub>2</sub>) consisting of silica cores (SiO<sub>2</sub>), mesoporous silica (mSiO<sub>2</sub>) intermediate layers, and Neodymium doped rare-earth phosphor (YVO<sub>4</sub>:Nd<sup>3+</sup>) shell layers were successfully synthesized using the stepwise sol-gel method. The morphological structure and optical properties of the functional core-shell nanoparticles were characterized and evaluated by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) analysis. mSiO<sub>2</sub> intermediate layers were utilized as the bridge between the core and shell materials. Their porous surfaces served to anchor the YVO<sub>4</sub>:Nd<sup>3+</sup> crystals. This prevents energy loss during the energy transfer of electrons, resulting in improved optical properties. The use of intermediate layer combinations of mSiO<sub>2</sub>/SiO<sub>2</sub> in the coreshell structure also improved cost-effectiveness, because the core is filled with cheap silica, not expensive phosphors. Even though the nanoparticles used only a thin layer of the photoluminescent shell materials, the optical properties, resulting from the energy-transfer emitting mid-infrared light, were remarkably enhanced by increasing the crystallinity of the phosphor. To demonstrate the practical use of the synthesis method, the photoluminescent properties of the core-shell nanoparticles were optimized by adjusting the annealing temperature and scaling to mass production. We believe that our efficient synthetic strategy provides a facile way of obtaining functional, cost-effective core-shell nanoparticles with improved photoluminescent properties.
Near-infrared upconversion of Nd through Gd-mediated interfacial energy transfer in core-shell nanoparticles
