A far-red-emitting ZnAl1.95Cr0.05O4 phosphor for plant growth LED applications

ZnAl2-xCrxO4 (x = 0 and 0.05) samples were synthesized via a high-temperature solid-state reaction method. The structure, photoluminescence properties, EPR measurements, thermal stability, and chromaticity diagram of the far-red phosphor ZnAl1.95Cr0.05O4 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 ZnAl1.95Cr0.05O4 for plant growth application. According to optical and EPR measurements, Cr3+ ions substitute Al3+ ions with D-3d symmetry in the ZnAl2O4 host. PLE measurement indicates that upon excitation at 390 nm and 530 nm, the far-red phosphor ZnAl1.95Cr0.05O4 exhibited bright far-red emission around 687 nm. Photoluminescence phenomena show a sharp R line origin from the sublevels of the E-2(g)((2)G) & RARR; (4)A(2)(F-4) transition in Cr3+ ions. The E-2(g) level was split into E-2(g) (E-g) and E-2(g) (2A(g)) levels in the distorted crystal field environment, and the sharp R line in the ZnAl2O4 matrix was split into R-1 and R-2 lines. In this paper, the temperature-dependent luminescence characteristics of ZnAl1.95Cr0.05O4 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 ZnAl1.95Cr0.05O4 phosphors exhibit a nearly zero-thermal-quenching behavior. The CIE chromaticity coordinates of the ZnAl1.95Cr0.05O4 phosphor were located at the boundary of the chromaticity diagram, signifying that the phosphors possessed high color purity. The emissions of the ZnAl1.95Cr0.05O4 phosphor match well with the PFR absorption of phytochromes in plants. The investigation indicates that ZnAl1.95Cr0.05O4 is a potential far-red phosphor matching ultraviolet (UV) LED chips for plant growth applications.