Green upconversion luminescence in Yb 3+/Er 3+ co-doped ALn(MoO 4) 2 (A = Li, Na and K; Ln = La, Gd and Y)

Yb ³⁺ and Er ³⁺ co-doped ALn(MoO ₄) ₂ (A = Li, Na and K; Ln = La, Gd and Y) phosphors were synthesized by sol-gel method. The structures and the phase purities of the samples were characterized by X-ray diffraction (XRD). The shapes and size distribution of the phosphor particles were also investigated by the field emission scanning electron microscopy (FE-SEM). The phosphors showed intense green emission peaked at 530 and 552 nm due to the ²H _11/2 → ⁴I _15/2 and ⁴S _3/2 → ⁴I _15/2 transitions of Er ³⁺ under the excitation of a 980 nm laser diode. The dependences of their UC luminescence properties on the hosts, the concentration of the dopants and annealing temperature were investigated. The phosphor showed the brightest green emission as the concentration of Er ³⁺ fixed at 0.5 mol% and the annealing temperature is 800 °C. In the range from 5 to 20 mol%, the more Yb ³⁺ was doped, the more intense UC luminescence can be achieved. The UC excitation mechanism for green emission of Er ³⁺ was also investigated based on the power dependence of UC luminescence intensity.