Green and red up-conversion emissions of Er–Yb Co-doped TiO2 nanocrystals prepared by sol–gel method

In this paper, green and red up-conversion emissions of Er³⁺–Yb³⁺ co-doped TiO₂ nanocrystals were reported. The phase structure, particle size and optical properties of Er³⁺–Yb³⁺ co-doped TiO₂ nanocrystals samples were characterized by using X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–vis–NIR absorption spectra and photoluminescence (PL) spectra. Green and red up-conversion emissions in the range of 520–570 nm (²H_11/2, ⁴S_3/2→⁴I_15/2) and 640–690 nm (⁴F_9/2→⁴I_15/2) were observed for the Er³⁺–Yb³⁺ co-doped TiO₂ nanocrystals. The visible up-conversion mechanism and temperature dependence of up-conversion emission for Er³⁺ in TiO₂ nanocrystals were discussed in detail.