Luminescence, energy-transfer and tunable color properties of single-component Tb3+ and/or Sm3+ doped NaGd(WO4)2 phosphors with UV excitation for use as WLEDs

Abstract

Tb³⁺ and/or Sm³⁺ codoped NaGd(WO₄)₂ phosphors were prepared by a facile hydrothermal process, taking on flower-like microcrystal structures with a mean size of 3 μm. The NaGd(WO₄)₂:Sm³⁺ can efficiently absorb ultraviolet light and emit bright blue light from the WO₄²⁻ group in the host, and has yellow, orange and red emissions from the f–f transitions of Sm³⁺, due to the host sensitization effect. Under ultraviolet excitation (405 nm), individual Sm³⁺ ion-activated NaGd(WO₄)₂ phosphors generate white and red light and the emission intensities reach a maximum at 0.015 equivalents of Sm³⁺. This is due to the concentration quenching effect, occurring via a resonant-type dipole–dipole interaction, for which the critical distance (R_Sm–Sm) is calculated to be 21.24 Å. More significantly, in the Tb³⁺ and Sm³⁺ codoped NaGd(WO₄)₂ phosphors, the Tb³⁺–Sm³⁺ energy migration has been discussed to occur via a dipole–dipole mechanism. In addition, under different ultraviolet radiation, the color-tunable emissions in NaGd(WO₄)₂:Tb³⁺, Sm³⁺ microcrystals are realized, and this could make them good candidates to be used as full-color phosphors for nUV-LEDs.