Luminescence and energy transfer in β-NaGdF4:Eu3+,Er3+ nanocrystalline samples from a room temperature synthesis

Abstract

Phase pure hexagonal β-NaGdF₄:Eu³⁺,Er³⁺ samples of less than 10 nm in crystallite size were synthesized in anhydrous ethylene glycol within 24 hours at room temperature. The materials were characterized by powder X-ray diffraction, electron microscopy, and luminescence spectroscopy. After Gd³⁺ excitation at 273 nm Eu³⁺ luminescence from the ⁵D_J states dominates over Gd^{3+ 6}P_J emission and only weak Er³⁺ emissions could be observed due to strong Gd→Eu and weak Gd→Er energy transfer. Excitation at 377 nm results in Er^{3+ 2}H_9/2 and Eu^{3+ 5}D_J emissions. The concentration dependence of the Er³⁺ and Eu³⁺ luminescence intensities points to independent behavior of the dopant ions without Er→Eu or Eu→Er energy transfer. Eu³⁺ excitation at 394 nm results in Eu^{3+ 5}D_J emissions only and confirms the absence of Eu→Er energy transfer. Eu³⁺ luminescence from the ⁵D_J states prevails for all investigated excitations. The optimum Eu³⁺ doping level is close to 5%. Significant Er³⁺ luminescence was observed for 377 nm excitation only. The strong ²H_9/2 and weak ⁴S_3/2 Er³⁺ emissions indicate small losses by multiphonon relaxation and a good quality of the nanomaterial. Er co-doping hinders the energy migration on the Gd³⁺ and Eu³⁺ sublattices which results in higher Gd³⁺/Eu³⁺ and Eu^{3+ 5}D₁/⁵D₀ emission ratios.