Phase Stabilization and Luminescence Quenching in Sol-Gel Synthesized Er-Doped Al₂O₃ Nanopowder via Supercritical Drying

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Undoped and Er-doped alumina nanostructures at concentrations of 0.5, 1 and 3 at.%. were synthesized via the sol-gel method under supercritical isopropanol drying. X-ray diffraction (XRD) analysis demonstrated that undoped alumina consists of two crystalline phases: α and θ. In contrast, the Er-doped samples exhibited only the θ phase of alumina. This suggests that the presence of Er ions in the alumina matrix delays the transformation temperature from the θ phase to the α phase. UV-visible analysis revealed that the doped samples showed absorption bands in the blue-near infrared (NIR) region, confirming the incorporation of Er ions into the alumina matrix. Fourier-transform infrared (FTIR) spectroscopy spectra displayed bands corresponding to O-Al-O and Al-O functional groups in both θ- and α-alumina. Scanning electron microscopy (SEM) analysis showed that pure alumina featured a smooth surface with large interconnected grains and a uniform microstructure. The photoluminescence (PL) spectra revealed that Er doping introduced luminescent centers associated with Er³+ 4f-4f transitions, while also influencing the density of oxygen vacancies. At higher Er concentrations, self-quenching effects became significant. Additionally, an excitation wavelength of 600 nm was identified as optimal for Er-doped alumina, as it aligned with the energy levels of Er³+ ions and minimized interference from the host material, thereby enhancing luminescent efficiency. Overall, this study establishes a systematic correlation between erbium concentration, θ-phase stabilization, defect formation, and photoluminescent response in sol–gel-derived alumina nanostructures, clearly distinguishing it from previous reports and providing new insights into the role of Er–host interactions during thermal evolution.

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