Mechanism of morphology formation and energy transfer of NaLa(WO4)2:Sm3+:Eu3+ phosphors

Abstract

A series of phosphors of Sm3+ and Eu3+ single-doped or co-doped NaLa(WO4)2 matrix are synthesized with the hydrothermal method. The structure, morphology, luminescent properties, quantum efficiency and CIE coordinates of sample are characterized and analyzed with X-ray diffraction (XRD), scanning electron microscopy (SEM and EDS), transmission electron microscope (TEM), fluorescence spectrophotometer (FL) and UV–Vis (UV) absorption spectra. The results show that all the obtained products have the scheelite structure with spherical or ellipsoidal shapes. The morphology formation mechanism is proposed as follows: nucleation—dissolution—recrystallization—crystal growth (self-assembly process). Under 393- and 403-nm photoexcitation, NaLa (WO4)2:Sm3+:Eu3+ emits intense red and orange red light. In NaLa (WO4)2: Sm3+:Eu3+ sample, the energy transfer can occur from Sm3+ to Eu3+, and the critical distance of energy transfer is between 1.655 and 2.362 nm. The energy transfer mechanism is electric dipole–electric dipole interaction. The energy transfer efficiency can be over 70%. The best product NaLa(WO4)2:2%Sm3+:4%Eu3+ has a luminous efficiency of 54.70%. The CIE coordinates are X = 0.6585, Y = 0.3404.