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

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 5DJ states dominates over Gd3+ 6PJ 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 Er3+ 2H9/2 and Eu3+ 5DJ 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 Eu3+ 5DJ emissions only and confirms the absence of Eu→Er energy transfer. Eu3+ luminescence from the 5DJ 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 2H9/2 and weak 4S3/2 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 Eu3+ 5D1/5D0 emission ratios.