Eu3+- activated Sr2GdF7 colloid and nano-powder for horticulture LED applications

Abstract

A series of multifunctional Sr₂Gd_1-xEu_xF₇ (x = 0, 0.05, 0.10, 0.40, 0.60. 0.80, and 1.00) phosphors in stable colloidal form and as nanopowders have been prepared using a hydrothermal method. Powder X-ray diffraction analysis confirmed that the materials crystallize in a cubic crystal structure. Transmission electron microscopy shows quasi-spherical nanoparticles with an average particle size of ~24 nm. Photoluminescence measurements show highly efficient red emission in both colloids and nanopowders, with intensity continually increasing up to 80 mol% of Eu³⁺ content without concentration quenching. The most prominent emission peaks are around 600 nm (orange/red) and 700 nm (deep red), with the latter more pronounced. Quantum efficiency follows a similar trend, and reaches 60% for the sample with 80 mol% of Eu³⁺ content. In addition, similar asymmetry ratio values and CIE coordinates show that there is not a big change in the local symmetry around Eu³⁺ ions or emission color across the series. This confirms that Eu³⁺ resides in the same crystalline environment in samples. The observed ⁵D₀-level lifetimes gradually decrease from 12.0 ms to 6.9 ms as the Eu³⁺ concentration increases. Judd-Ofelt parameters show slight variation with Eu³⁺ concentration with Ω₄ always larger than Ω₂. The temperature-dependent steady-state and time-resolved photoluminescence measurements demonstrate high stability of nanopowders’ emission up to 100 °C. The combination of temperature stability and high efficiency of emission, as well as the untypical dominant deep-red emission at 700 nm labels these nanoparticles as potential nanophosphors for various applications.