Structural, photoluminescence and energy transfer investigations of novel Dy3+ → Sm3+ co-doped NaCaPO4 phosphors for white-light-emitting diode applications†

In this study, Dy³⁺-doped and Dy³⁺/Sm³⁺ co-doped NaCaPO₄ white-emitting polycrystalline phosphor samples were synthesized using a solid-state reaction method. The samples were characterized by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Field-emission scanning electron microscopy (FE-SEM), and Photoluminescence (PL) analysis. The phase purity characterization and crystal structural analysis were done using the Rietveld refinement-based FullProf Suite software. The Rietveld refinement result confirms single-phase formation for both Sm³⁺ and Dy³⁺/Sm³⁺ co-doped NaCaPO₄ samples with an orthorhombic structure and with a monotonic change in lattice parameters with doping. The PL studies of the Dy³⁺-doped samples revealed two emission bands. However, at 352 nm, the Dy³⁺/Sm³⁺-co-doped samples revealed distinctive emission bands for both ions. The emission peaks at 480 nm (blue) and 573 nm (yellow) are related to the ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions of Dy³⁺ ions; however, the emission peaks at 600 nm and 647 nm are attributed to the ⁴G_5/2 → ⁶H_7/2 and ⁴G_5/2 → ⁶H_7/2 transitions of Sm³⁺ ions. The intensity of the Dy³⁺ emissions decreased as the Sm³⁺ levels increased but the emission intensity of the Sm³⁺ ions increased. The co-doping of Sm³⁺ ions in Dy³⁺-doped phosphors results in unique characteristics due to the energy transfer (ET) from Dy³⁺ → Sm³⁺ ions. The effectiveness of this ET from Dy³⁺ → Sm³⁺ ions is positively correlated with the dopant amounts of the Sm³⁺ ions. The interaction mechanisms have been identified as dipole–dipole based on Dexter's energy transfer and Readfield's approaches. All decay curves can be adequately fitted via bi-exponential functions, suggesting the movement of energy between Dy³⁺ → Sm³⁺ ions. Temperature-dependent PL measurements and CIE color coordinate analysis reveal excellent luminescent properties, making these Dy³⁺/Sm³⁺ co-doped phosphors advantageous for white light-emitting diode (WLED) technologies.