Preparation, electronic structure and photoluminescence properties of RE (RE = Ce, Yb)-activated SrAlSi4N7 phosphors

Abstract

The electronic structure of SrAlSi₄N₇ was calculated using the CASTEP code and SrAlSi₄N₇ is an intermediate band gap semiconductor with an indirect energy gap of 3.6 eV. Ce³⁺ and Yb²⁺-activated SrAlSi₄N₇ samples were prepared by a solid-state reaction method at high temperature, and their photoluminescence properties were investigated. SrAlSi₄N₇:Ce³⁺ shows a broad band emission in the wavelength range of 450–700 nm, originating from the 5d¹–4f¹ transition of Ce³⁺. The emission band of Ce³⁺ shifts to longer wavelength with an increase of Ce³⁺ concentration due to the increased Stokes shift and reabsorption by Ce³⁺. SrAlSi₄N₇:Yb²⁺ can be excited efficiently over a broad spectral range between 300 and 550 nm, and exhibits a single intense red emission at 600 nm with a full width at half maximum of 95 nm due to the electronic transitions from the excited state of 4f¹³5d to the ground state 4f¹⁴ of Yb²⁺. The low energy of Yb²⁺ emission in SrAlSi₄N₇ can be attributed to the large nephelauxetic effect and crystal field splitting due to the coordination of Yb²⁺ by nitrogen. In addition, Sr_1−2xCe_xLi_xAlSi₄N₇ shows higher thermal stability in air than that of Sr_1−yYb_yAlSi₄N₇ (0 ≤ x, y ≤ 0.1). A white-light LED can be generated by using single SrAlSi₄N₇:Ce³⁺ as the wavelength conversion phosphor combined with a blue LED chip (InGaN). The obtained LED exhibits a luminous efficiency of 74.3 lm W⁻¹ at a corrected color temperature (CCT) up to 6350 K, and the color rendering index (CRI Ra) is around 78. These novel developed yellow-red phosphors have potential applications in spectral conversion materials for white-LEDs.