Salt-assisted size-controlled synthesis and luminescence studies of single phase CaWO4:Dy3+: an insight into its morphological evolution, energy transfer and colour evaluation

Abstract

Although there are literature reports available on the use of salt as a flux in the solid state synthesis of phosphors, there are not many reports on the synthesis of nanophosphors using a salt-assisted wet chemical method. Here, we report the synthesis of a nearly spherical single phase CaWO₄:Dy³⁺ nanophosphor using a solvothermal method with the addition of NH₄Cl and KCl. It was observed that the addition of salt has no adverse effects on the crystal structure of the nanophosphor, however, it increases its size. A plausible explanation for the variation in size of the particles upon the addition of salt is presented. The photoluminescence emission, energy transfer and lifetimes of the nanophosphor were found to be dependent on its size. The blue emission from the host increases with increasing size. The average lifetime of the 480 nm (⁴F_9/2 → ⁶H_15/2) emission for the as-prepared CaWO₄:Dy³⁺ sample was found to be 1.628 ms, with the lifetime found to decrease with an increase in size. The CIE chromaticity indicates either bluish white or near white emission from the samples. The quantum yields of the various samples prepared were found to vary from 12 to 25%. Furthermore, the optimization of energy transfer was studied by changing the concentration of Dy³⁺ (3–15 at%) doping and Sm³⁺ (5 at%) codoping at 50 mM NH₄Cl addition during synthesis. As the concentration of Dy³⁺ increases, the emission intensity decreases, whereas the energy transfer increases. Thus, this report will provide a significant contribution in the field of synthesis and optimizing of the luminescence characteristics of nanophosphors for display devices and optical imaging.