Comparative Investigation on Nanocrystal Structure and Luminescence Properties of Gadolinium Molybdates Codoped with Er3+/Yb3+

This paper reports on the comparative investigation of nanocrystal structure and luminescence properties of Er³⁺/Yb³⁺-codoped gadolinium molybdate nanocrystals Gd₂(MoO₄)₃ and Gd₂MoO₆ synthesized by the Pechini method with citric acid and ethylene glycol. Their crystallization, structure transformation, and morphologies have been investigated by X-ray diffraction, thermogravimetric/differential scanning calorimetry, and transmission electron microscopy. It is noticed that Er³⁺/Yb³⁺-codoped monoclinic Gd₂(MoO₄)₃ nanocrystals have shown an intense upconversion through a sintering of the organic complex precursor at 600°C. Furthermore, it transforms to orthorhombic Gd₂(MoO₄)₃ when the precursor is sintered at 900°C. In counterpart of monoclinic Gd₂MoO₆, however, the monoclinic structure remains unchanged when the precursor is sintered at a temperature ranging from 600°C to 900°C. Intense visible emissions of Er³⁺ attributed to the transitions of ²H_11/2, ⁴S_3/2–⁴I_15/2 at 520 and 550 nm, and ⁴F_9/2–⁴I_15/2 at 650 nm have been observed upon an excitation with a UV source and a 980 nm laser diode, and the involved mechanisms have been explained. It is quite interesting to observe obvious differences both in the excitation and the upconversion emission spectra of Er³⁺/Yb³⁺-codoped Gd₂(MoO₄)₃ respectively with monoclinic and orthorhombic structure. The quadratic dependence of fluorescence on excitation laser power has confirmed that two-photons contribute to upconversion of the green–red emissions.