Effect of homogeneous coating on K+-doped NaGdF4:Er3+,Yb3+ upconversion materials

The K+-doped (Na1−xKx)GdF4:Er3+,Yb3+ upconversion materials were successfully synthesized by solvothermal method. The doping of K+ was testified to have great influences on improving upconversion properties as it directly affects the morphology of the material matrix. In addition, by coating the doped material with a homogeneous shell layer, a newly developed core–shell (Na0.9K0.1)GdF4:Er3+,Yb3+@NaGdF4 nanomaterials were produced. Surprisingly, its light emission intensity was 2 times of that for the uncoated material, although the crystal structure and the emission color of the nanoparticles did not change. The upconversion mechanism has also been studied, and the results show that there is a two-photon mechanism of (Na1−x Kx)GdF4:Er3+,Yb3+. The work provides a new strategy for enhancing the luminescence density of upconversion materials by coating a homogeneous shell after doping.