The effect of Eu3+ and Gd3+ co-doping on the morphology and luminescence of NaYF4:Eu3+, Gd3+ phosphors

Abstract

β-NaYF₄:Eu³⁺ microparticles co-doped with Gd³⁺ ions were obtained by hydrothermal synthesis at 180 °C using citric acid as a stabilizing agent. All synthesized materials have a β-NaYF₄ crystalline phase, where the unit cell volume increases upon the addition of Eu³⁺ and Gd³⁺ ions. The particles have a hexagonal prism shape and a size of 40–714 nm, where Eu³⁺ and Gd³⁺ doping results in size reduction. Upon 393 nm excitation, phosphors exhibit distinct emission peaks centered at 591, 615, and 695 nm and a weak band at 650 nm attributed to ⁵D₀–⁷F_J transitions (J = 1–4). The optimum Eu³⁺ doping concentration was found to be 30% in the NaYF₄ host. Concentration quenching was realized through dipole–dipole interactions. Kinetic measurements showed a gradual decline of the ⁵D₀ lifetime from 6.7 ms to 2.2 ms along with an increase in Eu³⁺ doping concentration. Co-doping of the small Gd³⁺ number led to an increase of emission intensity and ⁵D₀ lifetime. The effects of Eu³⁺ doping and Gd³⁺ co-doping on radiative and nonradiative decay rates were studied using 4f–4f intensity theory.