Color tunable Ca8ZnM(PO4)7 (M = Lu/Tb, Lu/Eu, Tb/Eu) phosphors: luminescence, energy transfer and thermal stability studies for n-UV white LEDs

Abstract

A series of Tb³⁺- and Eu³⁺-doped Ca₈ZnLu(PO₄)₇ (CZLP:Tb³⁺ and CZLP:Eu³⁺) as well as Ca₈ZnTb(PO₄)₇:Eu³⁺ (CZTP:Eu³⁺) phosphors have been prepared via the traditional high-temperature solid-state reaction. X-ray powder diffraction (XRD) patterns of the as-prepared phosphors indicate that the introduction of Tb³⁺ or Eu³⁺ affects neither the phase impurity nor the crystal structure of the CZLP host lattice. The concentration dependent photoluminescence (PL) spectra reveal that even if Lu³⁺ was fully substituted by the dopants, Tb³⁺ or Eu³⁺, the phenomenon of concentration quenching would not occur. Color tunable emissions from green to red can be realized by adjusting the type of doping ion (Tb³⁺ and Eu³⁺) and their relative concentration. Furthermore, the energy transfer from Tb³⁺ to Eu³⁺ was confirmed and the mechanism was determined to be the dipole–quadrupole interaction. In addition, the quantum efficiencies were found to be 0.61, 0.58 and 0.85 for CZTP, CZTP:0.2Eu³⁺ and CaZnEu(PO₄)₇ (CZEP), respectively. As a result, a white light emitting diode (WLED) device was fabricated using the optimal CZTP:0.2Eu³⁺ yellow phosphor, the BaMgAl₁₀O₁₇:Eu²⁺ (BAM:Eu²⁺) blue phosphor and a 370 nm near-ultraviolet (n-UV) chip. The obtained device displays a suitable color rendering index (CRI, ∼81.3) and correlated color temperature (CCT, ∼2634 K) value, indicating its potential application in n-UV LEDs.