Tunable blue-green emission and energy transfer properties in β-Ca3(PO4)2:Eu2+, Tb3+ phosphors with high quantum efficiencies for UV-LEDs

Abstract

A series of Eu²⁺ and Tb³⁺ singly-doped and co-doped β-Ca₃(PO₄)₂ phosphors have been synthesized via the high-temperature solid-state reaction method. Thermogravimetric (TG) analysis, fourier transform infrared (FT-IR) spectra, X-ray diffraction (XRD) patterns and Rietveld refinements, photoluminescence (PL) spectra including temperature-dependent PL and quantum efficiency, and fluorescence decay lifetimes have been used to characterise the as-prepared samples. Under UV excitation, β-Ca₃(PO₄)₂:Eu²⁺ presents a broad emission band centered at 415 nm, which can be decomposed into five symmetrical bands peaking at 390, 408, 421, 435 and 511 nm based on the substitution of five kinds of Ca²⁺ sites by Eu²⁺ ions. β-Ca₃(PO₄)₂:Tb³⁺ shows characteristic emission lines under Tb³⁺ 4f–5d transition excitation around 223 nm. In β-Ca₃(PO₄)₂:Eu²⁺, Tb³⁺ phosphors, similar excitation spectra monitored at 415 and 547 nm have been observed, which illustrates the possibility of energy transfer from Eu²⁺ to Tb³⁺ ions. The variations in the emission spectra and decay lifetimes further demonstrate the existence of energy transfer from Eu²⁺ to Tb³⁺ ions under UV excitation. The energy transfer mechanism has been confirmed to be dipole–quadrupole, which can be validated via the agreement of critical distances obtained from the concentration quenching (12.11 Å) and spectrum overlap methods (9.9–13.2 Å). The best quantum efficiency can reach 90% for the β-Ca₃(PO₄)₂:0.01Eu²⁺, 0.15Tb³⁺ sample under 280 nm excitation. These results show that the developed phosphors may possess potential applications in UV-pumped white light-emitting diodes.