Up/down conversion luminescence and energy transfer of Er/Tb activated NaGd(WO) green emitting phosphors.

A series of double scheelite-type tungstate green phosphors NaGd(WO):Er, Tb were synthesized by a hydrothermal route and subsequent calcination process, and they were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectrometry (EDS), photoluminescence spectroscopy and fluorescence lifetime measurements. The phosphors take on octahedral microcrystals with a mean side length of ~2 μm. In the single doped phosphors system, the energy transfer processes from WO to Er or Tb were discussed. The quenching concentrations of Er and Tb are 0.05 and 0.07, respectively. The critical distances for Er and Tb ions are calculated to be 14.28 Å and 12.76 Å, respectively. When doping Er/Tb is applied in the single compound, the concentration quenching effect of Tb ions occurs via a resonant-type dipole-dipole interaction as well as that of Er ions. Under the excitation with ultraviolet (378 nm) or infrared (980 nm) light, the Er/Tb co-doped NaGd(WO) phosphors emit strong green emission. The obtained samples with bright emission intensity and appropriate decay time are suitable for use as green phosphors in the near ultraviolet LEDs and bioimaging applications.