Luminescence and energy transfer of a color tunable phosphor: Dy3+-, Tm3+-, and Eu3+-coactivated KSr4(BO3)3 for warm white UV LEDs

Abstract

A series of Dy³⁺-, Tm³⁺-, Eu³⁺-coactivated KSr₄(BO₃)₃ phosphors were synthesized via a standard solid-state reaction under normal ambient air, and the emission colors could be tuned from blue to yellow and then to red, including almost all the white light region, through tuning the energy transfer. It was discovered that the energy is transferred from Tm³⁺ to Dy³⁺ by: directly observing overlap of the excitation spectrum of Dy³⁺ and the emission spectrum of Tm³⁺; the systematic relative decline and growth of emission bands of Tm³⁺ and Dy³⁺, respectively; and faster decay times of the blue emissions from energy donors. The resonance-type energy transfer from Tm³⁺ to Dy³⁺ was demonstrated to be via the dipole–quadrupole mechanism and the critical distance of energy transfer was calculated to be 20.7 Å. Rietveld refinements of the crystal structures of the products obtained from powder X-ray diffraction (XRD) elucidated a preferable occupancy in the crystal unit for the doped rare-earth cations, which well explained the formation of the Tm³⁺–Dy³⁺ close pair. By utilizing the principle of energy transfer, we have demonstrated that with appropriate tuning of activator content, KSr₄(BO₃)₃:Dy³⁺,Tm³⁺,Eu³⁺ phosphors exhibit great potential for use as single-component phosphors for warm white ultraviolet light-emitting diodes (UV LEDs).