Up-conversion luminescence study of thermally stable Sr2ZnSi2O7: Er3+ phosphors

Abstract

Current research on rare-earth-doped phosphors often faces challenges related to thermal stability, energy transfer efficiency, and color purity, which limit their practical application in lighting technologies. In this study, we address these issues by synthesizing a series of thermally stable Sr₂ZnSi₂O₇: Er³⁺ (SZSi: Er³⁺) phosphors via the high-temperature solid-state reaction method. X-ray diffraction (XRD) confirmed the formation of a tetragonal crystalline phase (space group: P-421m) however, scanning electron microscopy (SEM) revealed comprehensive surface morphology and particle size distribution of irregularly shaped particles. Diffuse reflectance spectroscopy (DRS) was used to calculate the optical band gap of the synthesized phosphors. The photoluminescence (PL) studies demonstrated efficient near-ultraviolet (n-UV) excitation (λ_ex = 378 nm). Energy transfer analysis using the Dexter theory and the Inokuti-Hirayama (I–H) model indicated that dipole-dipole interactions dominate the energy transfer process between Er³⁺-Er³⁺ ions. The optimized SZSi: Er³⁺ phosphor exhibited high color purity (96 %) with Commission Internationale de l’Eclairage (CIE) chromaticity coordinates of (0.3279, 0.6651) under λ_ex = 378 nm. The upconversion luminescence intensity at 661 nm [⁴F_9/2 → ⁴I_15/2 (Er³⁺)], when excited at 980 nm, was enhanced with a higher magnitude when increasing the doping concentration from 1 to 10 mol% of Er³⁺ ions. The dependence of the laser pump power concerning the upconversion luminescence intensity depicts that the emission at 661 nm is due to a two-photon absorption process. In addition, under 980 nm, visible upconversion emissions (in the green and red regions) were observed, attributed to a two-photon absorption mechanism involving intermediate energy levels of Er³⁺ ions. Thermal quenching analysis demonstrated a moderate decrease in emission intensity (∼24 % at 100 °C and ∼36 % at 150 °C), suggesting reasonable thermal stability. These findings underscore the potential of Er³⁺-activated Sr₂ZnSi₂O₇ phosphors in non-display photonic applications, such as bioimaging, anti-counterfeiting, or infrared-pumped display technologies.