Evaluation of optoelectronic response and Raman active modes in Tb³⁺ and Eu³⁺-doped gadolinium oxide (Gd₂O₃) nanoparticle systems

Abstract

Rare earth oxide (Tb³⁺:Gd₂O₃ and Eu³⁺:Gd₂O₃) nanophosphors are exploited through spectroscopic and microscopic tools with special emphasis on D–F mediated radiative emission and Raman active vibrational modes. Powder X-ray diffraction measurements have revealed cubic crystal structure of the nanosystems and with an average crystallite size varying between ~3.2 and 4.8 nm. Photoluminescence (PL) spectra of Tb³⁺ doped systems signify intense blue-green (~490 nm) and green (~544 nm) emissions mediated by ⁵ D ₄ → ⁷ F ₆ and ⁵ D ₄ → ⁷ F ₅ transitional events; respectively. In the PL responses of Eu³⁺ doped nanoparticle systems, we also identify magnetically-driven ⁵ D ₀ → ⁷ F ₁ (~591 nm) and electrically driven ⁵ D ₀ → ⁷ F ₂ (~619 nm) radiative features which seem to improve with increasing doping level. However, the magnitude of Judd–Ofelt (J–O) intensity parameters (Ω _{2, 4}), is significantly lowered for the high doping cases. Raman spectra of the undoped and RE doped systems exhibited several A_g and F_g modes in the range of Raman shift ~100–600 cm⁻¹. In the Raman spectra, the peaks located at ~355 cm⁻¹ are assigned to the mixed mode of F _g + A _g, the line width of which was found to increase with RE doping. Moreover, owing to the enhanced defect concentration in the doped systems than its undoped counterpart, we anticipate a faster phonon relaxation and consequently, a suppression of phonon lifetime in the former case.