Simple Postsynthesis Thermal Treatment toward High Luminescence Performance of Rare Earth Vanadate Nanoparticles

Optical applications of colloidal oxide nanoparticles are often limited by low luminescence efficiencies caused by poor crystallinity and surface quenching. Bulk oxides prepared via conventional high-temperature annealing offer intense luminescence but commonly fail to yield stable colloidal dispersions. Coupling the best of these two situations to afford highly crystalline, dispersible nanoparticles with luminescence performance exceeding that of bulk solids is still challenging, thus requiring new, safe, scalable, and reproducible methodologies. Herein, we report a silicate-coating strategy followed by aggregate elimination to recover stable colloids of 40-150 nm single crystalline rare earth vanadates after unprotected annealing (800-1000 °C). Eu³⁺-doped nanoparticles showed enhanced photostability and ∼50% emission quantum yields in water (λ_exc = 280 nm), while Dy³⁺-, Tm³⁺-, and Yb³⁺/Er³⁺-doped vanadates provided remarkably intense multicolor emissions via downshift or upconversion luminescence. We correlated the spectroscopic properties of pristine and annealed solids to microstructural characteristics to explain the superior emission features, opening new perspectives for sensing applications.