Up-Conversion Luminescence in Sr₉Y₂W₄O₂₄:Ho³⁺, Yb³⁺ Phosphors for applications in Temperature Sensing and w-LEDs

Ho³⁺-Yb³⁺ activated Sr₉Y₂W₄O₂₄ (SYW) phosphors were synthesized using a traditional solid-state reaction method. X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FT-IR) investigations were conducted to assess the phase, morphology and bond structure in the as-synthesised samples. To determine the optical bandgap values, diffuse reflectance spectra (DRS) for the as-titled phosphors have been obtained. Preliminary photoluminescent studies on the as prepared phosphors focus on UV–visible excitation and emission spectra, as well as observations of decay lifetimes. Secondly, the up-conversion (UC) emission spectra of as-titled phosphors, excited by a 980 nm laser were also recorded. The UC findings show that there are three bands at 488 nm and 551 nm, and 668 nm, corresponding to transitions of Ho³⁺ ions from ⁵F₃ to ⁵I₈, ⁵F₄/⁵S₂ to ⁵I₈, and ⁵F₅ to ⁵I₈, respectively. UC luminescence is entirely a two-photon absorption process, as demonstrated by the fitting results of the curve exhibiting the relation between pump power and UC luminescence intensity. Furthermore, the emission spectra data yielded CIE coordinates that are located in the green region. The high thermal activation energy of the SYW phosphor substantially improves its thermal stability, as indicated by temperature dependent PL (TD-PL) spectra. Furthermore, the fluorescence intensity ratio (FIR) was employed to explore the optical sensing properties. Through the research, the bidirectional energy transmission in the UC mechanism was better understood, potentially leading to applications in solid state lighting and temperature sensors and UC applications of this phosphor.