UV/VUV switch-driven color-reversal effect for Tb-activated phosphors

Abstract

The remarkable narrow-band emission of trivalent lanthanide-doped phosphors excited by the vacuum ultraviolet (VUV) radiation lines of Xe atoms/Xe₂ molecules at 147/172 nm are extensively investigated in the development of plasma display panels and Hg-free fluorescent lamps, which are frequently used in our daily lives. Numerous solid materials, particularly Tb³⁺-doped oxides, such as silicates, phosphates and borates, are efficient green/blue sources with color-tunable properties. The excitation wavelength and rare earth concentration are usually varied to optimize efficiency and the luminescent properties. However, some underlying mechanisms for the shift in the emission colors remain unclear. The present study shows that a UV/VUV switch systematically controls the change in the phosphor (Ba₃Si₆O₁₂N₂:Tb) photoluminescence from green to blue, resulting in a green emission when the system is excited with UV radiation. However, a blue color is observed when the radiation wavelength shifts to the VUV region. Thus, a configurational coordinate model is proposed for the color-reversal effect. In this model, the dominant radiative decay results in a green emission under low-energy UV excitation from the ⁵D₄ state of the f–f inner-shell transition in the Tb system. However, under high-energy VUV excitation, the state switches into the ⁵D₃ state, which exhibits a blue emission. This mechanism is expected to be generally applicable to Tb-doped phosphors and useful in adjusting the optical properties against well-known cross-relaxation processes by varying the ratio of the green/blue contributions.

Phosphorescence: model of green—blue emission shift

A model explains why a new inorganic phosphor emits green on ultraviolet excitation but blue on vacuum ultraviolet excitation. Inorganic phosphors are used in liquid-crystal displays, plasma displays and white light emitting diodes. In particular, oxides doped with terbium ions (Tb³⁺) are efficient green–blue sources whose properties – including emission wavelength – can be tuned by varying the excitation wavelength and doping concentration. But the mechanisms responsible for the color shifting are not fully understood. Now, Ru-Shi Liu at Taiwan University and co-workers have experimentally investigated the green–blue color shift of Tb-doped Ba₃Si₆O₁₂N₂ and have proposed a model that describes this shift in terms of the electronic states of the system. The researchers anticipate that the model can be extended to other Tb-doped phosphors.