Tunable luminescence properties of the novel Tm3+- and Dy3+-codoped LiLa(MoO4)x(WO4)2−x phosphors for white light-emitting diodes

Abstract

A series of Tm³⁺- and Dy³⁺-codoped LiLa(MoO₄)_x(WO₄)_2−x phosphors were prepared via the conventional solid-state reaction method. It was discovered that the optimum single doped concentrations of both Tm³⁺ and Dy³⁺ are 0.04 for the LiLa(WO₄)₂ host; moreover, LiLa(WO₄)₂:0.04Tm³⁺ and LiLa(WO₄)₂:0.04Dy³⁺ emit blue and yellow light, respectively. On the other hand, the emission colors of Tm³⁺- and Dy³⁺-codoped LiLa(WO₄)₂ could be tuned from blue to white by tuning the energy transfer. With increase in the Dy³⁺ doped concentration, the energy transfer efficiency of LiLa(WO₄)₂:0.005Tm³⁺,yDy³⁺ increased gradually and reached as high as 88% when the concentration of Dy³⁺ was 0.05. The energy transfer from Tm³⁺ to Dy³⁺ was revealed to be resonant via the quadrupole–quadrupole mechanism. As for LiLa(MoO₄)_x(WO₄)_2−x:0.005Tm³⁺,0.03Dy³⁺ phosphors, the emission intensities of both Tm³⁺ and Dy³⁺ were found to reach a maximum when the molar ratio of Mo/W was 0 : 2. In addition, with increase in the proportion of MoO₄²⁻, the location of the chromaticity coordinates changed from the edge to the center of the white area. When the relative ratio of Mo/W was 2 : 0, the chromaticity coordinates were much closer to the standard chromaticity coordinates for white light. The results indicate that these phosphors may have potential applications in the fields of UV-excited white light-emitting diodes.