Phase transition and luminescent properties of the Eu3+ ions-doped NaYF4:Yb, Er nanoparticles

Monodisperse cubic NaYF4: 20%Yb, 2%Er nanoparticles (NPs) were successfully synthesized using the high-temperature thermal decomposition method. It is found that the crystal structure of NaYF4: 20%Yb, 2%Er, x%Eu (x = 0, 2, 4) NPs changes from cubic to hexagonal phase with the increase in Eu3+ concentration, and the upconversion (UC) luminescent intensities of green and red emissions were decreasing. The emission peaks of Eu-3+ (D-5(0) -> F-7(J) (J = 1, 2)) ions and the extra peaks of Er3+ (F-4(5/2) -> I-4(15/2)) ions at 450 nm are found, which suggests that the energy transfer between Er3+ ions and Eu3+ ions occurred with the near infrared light excitation. The probable mechanism related to the cubic-to-hexagonal phase transition of NaYF4 and energy transfer between Eu3+ and Er3+ ions or Yb3+ ions are discussed.