Structure, luminescence and energy transfer in Ce3+ and Mn2+ codoped γ-AlON phosphors

Abstract

Mn²⁺ doped γ-AlON is an interesting narrow-band green emitting phosphor which can be used in wide color gamut white LED backlights for liquid crystal displays (LCDs). However, the spin-forbidden transition of 3d⁵ in Mn²⁺ results in a quite low absorption efficiency, γ-AlON:Mn²⁺ thus has a small external quantum efficiency. Besides Eu²⁺,Ce³⁺ is another promising sensitizer and is expected to improve the emission of Mn²⁺ through a distinctive energy transfer process. In this work, a series of Ce³⁺–Mn²⁺ codoped γ-AlON phosphors were synthesized by gas pressure sintering at 1800 °C for 2 h under 0.5 MPa nitrogen gas pressure. The Rietveld refinement analysis shows that Al atoms occupy both 8a and 16d sites, while codoped Ce³⁺ and Mn²⁺ ions substitute the 8a Al atoms only in the tetrahedral sites. The ²⁷Al solid state NMR spectrum further confirms the chemical shifts of AlO₄ and AlO₆ sites at 71.4 and 0 ppm, respectively. The measured PL, CL and decay times have evidenced that a brisk energy transfer occurs between the Ce³⁺ and Mn²⁺ ions. The energy transfer mechanism from Ce³⁺ to Mn²⁺ in γ-AlON is a quadrupole–quadrupole interaction and the critical distance is calculated to be 15.97 Å. Upon UV or blue light excitation, the codoped γ-AlON phosphor has higher luminescence intensity, quantum efficiency and thermal stability than the Mn²⁺-doped γ-AlON due to the energy transfer and low total concentration of Ce³⁺ and Mn²⁺ ions. Meanwhile, the absorption, internal quantum efficiency and external quantum efficiency are raised to 60.2%, 61.3% and 36.9% upon 310 nm excitation.