Up-conversion luminescence in Yb³⁺-Er³⁺/Tm³⁺ co-doped Al₂O₃-TiO₂ nano-composites

The sol gel method was used to prepare rare-earths (Yb³⁺-Er³⁺ and Yb³⁺-Tm³⁺) co-doped Al₂O₃-TiO₂ nano-composite powder phosphors and their up-conversion luminescence properties were investigated. Mixed oxides of titania (TiO₂) rutile phase and an early stage crystallization of alumina (Al₂O₃) phase were confirmed from the X-ray diffraction data with the average crystallite size of ∼36 nm. The rutile phase TiO₂ was further confirmed by selected area diffraction analysis of the composites. Microscopy analysis showed interesting rod-like morphologies with rough surfaces indicating that a spherulitic growth process took place during the crystal growth. Photoluminescence characterization of the phosphors was carried out under near infra-red excitation at 980 nm and the prominent emission bands were observed in the visible region at 523, 548 and 658 nm for the Yb³⁺-Er³⁺ co-doped systems. Emission in bands extending from the visible to near infra-red regions were observed at 480, 650, 693 and 800 nm for the Yb³⁺-Tm³⁺ co-doped systems. These upconverted emissions and energy transfer mechanisms taking place are discussed in detail.