Thulium Doped Phosphors Under VUV Excitation

Abstract

Development of highly efficient mercury free fluorescent lamps and plasma display panels has been a challenging task due to the need for a combination of phosphor properties that are difficult to obtain in a single material (high efficiency, short emission lifetime and weak sensitivity to aging process under VUV excitation). Quantum cutting mechanism is a way to improve the fluorescence efficacy. Here we describe quantum cutting involving pairs of Tm³⁺ ions in KY₃F₁₀. Efficient excitation in the vacuum UV is initiated to the 5d state of Tm³⁺. This is followed by a cross relaxation energy transfer (CRET) involving the excited ion in the 5d state and nearby Tm³⁺ in the ground state, producing a pair of Tm³⁺ in excited states of the 4f¹³ configuration. Both ions can then emit photons. The excitation and reflection spectra are studied as a function of Tm³⁺ concentration and temperature. An unusual enhancement of the reflectivity at excitation wavelengths corresponding to the Tm³⁺ 5d absorption peaks is shown to arise from strong 5d→4f emission which is confirmed from the VUV emission spectra. The strong reduction of the integrated 5d emission intensity and shortening of its lifetime with Tm³⁺ concentration indicates the effective presence of the desired CRET process that is required for the first step of the quantum cutting. High Tm³⁺ concentrations are required for efficient quantum cutting. Whereas the CRET from the 5d state is estimated to be quite efficient, the 4f¹³ states of Tm³⁺ also undergo a strong CRET and therefore, emission from the 4f¹³ excited states that are created from the first step are strongly quenched at high Tm³⁺ concentrations. As a result, quantum yields greater than unity are not achieved.