Structural and luminescence properties of tunable white-emitting Sr0.5Ca0.5Al2O4:Eu2+, Dy3+ for UV-excited white-LEDs

Abstract

Tunable white-emitting Sr_0.5Ca_0.5Al₂O₄:Eu²⁺, Dy³⁺ phosphors were synthesized via solid-state reactions and evaluated as suitable candidates for white light emitting diodes in this study. The crystal structure of Sr_0.5Ca_0.5Al₂O₄:Eu²⁺, Dy³⁺ was refined and determined from X-ray diffraction (XRD) profiles via the Rietveld refinement method. The as-prepared phosphors exhibit a monoclinic structure with the P121/n1 space group. Under ultraviolet (UV) excitation, these phosphors display two emission bands, which peak at around 445 and 540 nm, corresponding to the 5d → 4f transition of Eu²⁺ ions. The photoluminescence results indicate that the Eu²⁺ ions tend to occupy three different crystallographic sites of the Ca²⁺ and Sr²⁺ ions in the Sr_0.5Ca_0.5Al₂O₄ structure. By the increment of Eu²⁺ ion concentration in Sr_0.5Ca_0.5Al₂O₄:Eu²⁺, Dy³⁺, the emission colors can be tuned from bluish white to warm white and eventually to yellowish white. Such color tuning has been achieved due to the resonance type energy transfer among the Eu²⁺ ions located at various crystallographic sites. The non-radiative energy transfer between Eu²⁺ ions is attributed to dipole–dipole interactions. The critical distances of the energy transfer are calculated to be 2.98 and 2.86 nm using concentration quenching and spectral overlap methods, respectively. The present research suggests that tunable white-emitting Sr_0.5Ca_0.5Al₂O₄:Eu²⁺, Dy³⁺ phosphors have the potentiality to be applied for UV excited white LEDs.