Tunable blue-green emitting and energy transfer of a Eu2+/Tb3+ codoped Sr3La(PO4)3 phosphor for near-UV white LEDs

Abstract

A series of Eu²⁺ and Tb³⁺ doped Sr₃La(PO₄)₃ phosphors have been synthesized via the high-temperature solid-state reaction method. X-ray diffraction (XRD) patterns, luminescence spectra including temperature-dependent luminescence spectra, and fluorescence decay lifetimes have been used to characterize the as-prepared samples. Under ultraviolet excitation, Sr₃La(PO₄)₃:Eu²⁺ shows a strong blue emission around 418 nm and a shoulder centered at 500 nm, which is based on the substitution of two kinds of Sr²⁺ sites by Eu²⁺ ions (Eu1 and Eu2). Sr₃La(PO₄)₃:Tb³⁺ shows characteristic emission lines of Tb³⁺ under 376 nm excitation. For Sr₃La(PO₄)₃:Eu²⁺,Tb³⁺ phosphor, similar excitation spectra monitored at 418, 500 and 545 nm have been observed, which illustrates the possibility of energy transfer from Eu²⁺ to Tb³⁺ ions. Compared with the Tb³⁺ singly doped phosphor, the codoped phosphors have more intense absorption in the n-UV range and stronger emission of the Tb³⁺ ions, which are attributed to the effective energy transfer from the Eu²⁺ to Tb³⁺ ions. The variations in the emission spectra, emission color and decay lifetimes further demonstrate the existence of energy transfer from Eu²⁺ to Tb³⁺ ions under ultraviolet excitation. For Eu1 and Eu2, the energy transfer mechanism has been confirmed to be quadrupole–quadrupole and dipole–quadrupole interaction, respectively. The results can be validated via the agreement of critical distances obtained from the concentration quenching (21.62 Å). These results show that the phosphors may possess potential application in ultraviolet-based white light-emitting diodes.