A far-red-emitting ZnAl_1.95Cr_0.05O₄ phosphor for plant growth LED applications

Abstract

ZnAl_2-<i>x</i>Cr_<i>x</i>O₄ (<i>x</i> = 0 and 0.05) samples were synthesized <i>via</i> a high-temperature solid-state reaction method. The structure, photoluminescence properties, EPR measurements, thermal stability, and chromaticity diagram of the far-red phosphor ZnAl_1.95Cr_0.05O₄ were investigated. These measurements have enabled us to study the Cr³⁺ transitions and the site symmetry of Cr³⁺ in the ZnAl₂O₄ host lattice and examine the suitability of ZnAl_1.95Cr_0.05O₄ for plant growth application. According to optical and EPR measurements, Cr³⁺ ions substitute Al³⁺ ions with <i>D</i>_3d symmetry in the ZnAl₂O₄ host. PLE measurement indicates that upon excitation at 390 nm and 530 nm, the far-red phosphor ZnAl_1.95Cr_0.05O₄ exhibited bright far-red emission around 687 nm. Photoluminescence phenomena show a sharp R line origin from the sublevels of the ²E_g(²G) → ⁴A₂(⁴F) transition in Cr³⁺ ions. The ²E_g level was split into ²E_g (Ē_g) and ²E_g (2Ā_g) levels in the distorted crystal field environment, and the sharp R line in the ZnAl₂O₄ matrix was split into R₁ and R₂ lines. In this paper, the temperature-dependent luminescence characteristics of ZnAl_1.95Cr_0.05O₄ have been investigated. As the temperature increased from 300 K to 440 K, a slight decrease in the intensity of the R1 and R2 lines was observed under excitation at 390 nm. The experimental results show that the ZnAl_1.95Cr_0.05O₄ phosphors exhibit a nearly zero-thermal-quenching behavior. The CIE chromaticity coordinates of the ZnAl_1.95Cr_0.05O₄ phosphor were located at the boundary of the chromaticity diagram, signifying that the phosphors possessed high color purity. The emissions of the ZnAl_1.95Cr_0.05O₄ phosphor match well with the PFR absorption of phytochromes in plants. The investigation indicates that ZnAl_1.95Cr_0.05O₄ is a potential far-red phosphor matching ultraviolet (UV) LED chips for plant growth applications.