Photoluminescence properties of Er3+ and Er3+/Yb3+ doped tellurite glass and glass-ceramics containing Bi2Te4O11 crystals

Glass and glass-ceramics containing nanocrystals of Bi₂Te₄O₁₁ cubic phase co-doped with Er³⁺ and Yb³⁺ were prepared by heat treatment of the precursor tellurite glass and investigated for optical applications. Lanthanide doped tellurite glass and glass-ceramics have been extensively investigated because of their optical and photoluminescence performance for technological photonic applications. Er³⁺ and Er³⁺/Yb³⁺ doped TeO₂-GeO₂-K₂O-Bi₂O₃ tellurite glass compositions were prepared by the conventional melt-quenching method. Photoluminescence results showed the important role played by Yb³⁺ ions when co-doping with Er³⁺ ions in comparison with the Er³⁺ single-doped glass. Due to their larger absorption cross-section, Yb³⁺ species significantly absorbs 980 nm photons and effectively transfers them to Er³⁺ ions via a set of mechanisms including ground-state absorption (GSA), excited-state absorption (ESA), and energy transfer upconversion (ETU). Er³⁺/Yb³⁺ co-doped sample was chosen for the synthesis of transparent glass-ceramics by controlled heat treatment above T_g for 5 to 120 min. X-ray diffraction patterns, high-resolution transmission electron microscopy (TEM) images, and selected area electron diffraction (SAED) from Er³⁺/Yb³⁺ co-doped glass-ceramic samples were used to verify the nanocrystal precipitation, crystalline phase, and chemical nature. The structural change resulting from the crystallization of Bi₂Te₄O₁₁ nanocrystals was evaluated by the Raman shift of the bands between 300-500 cm^-1, which are assigned to the formation of Bi-O-Te linkages and the reduction of [TeO₃] depolymerized units. The effects of HT time on the glass-ceramic's optical and upconversion photoluminescence properties were studied in the visible range under excitation at 980 nm in terms of the energy transfer mechanisms from Yb³⁺ to Er³⁺. Results indicate that Er³⁺/Yb³⁺ co-doped tellurite glass and glass-ceramics are potential candidates for photonic applications in lighting, energy conversion, and luminescent solar cell concentrators.