The dual-model up/down-conversion green luminescence of Gd6O5F8:Yb3+,Ho3+,Li+ and its application for temperature sensing

Abstract

Micro-sized Gd₆O₅F₈ particles doped with Yb³⁺, Ho³⁺ and Li⁺ ions have been synthesized by a facile hydrothermal route. The dual-model up- and down-conversion luminescence was investigated under visible blue (VIS-B) light (451 nm) and near infrared (NIR) light (980 nm) excitations. The structural properties of the products are analysed by X-ray diffraction (XRD). Upon excitation to the ²F_5/2 level of Yb³⁺ using a 980 nm laser diode, Gd₆O₅F₈:Yb³⁺,Ho³⁺ exhibits intense up-converted green and red emission at 545 nm and 660 nm, which correspond to the ⁵S₂ → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions of Ho³⁺, respectively. The nearly quadratic dependence of the green/red emission intensity on the excitation power indicates that a two-photon up-conversion process was involved to populate the green (⁵S₂) and red levels (⁵F₅) of Ho³⁺. In contrast, upon excitation Ho³⁺ to its ⁵F₁ level by 451 nm blue light, both visible and near-infrared emissions peaking at 545 nm, 660 nm, 980 nm and 1191 nm are simultaneously observed in the emission spectra, which are assigned to the ⁵S₂ → ⁵I₈ (Ho³⁺), ⁵F₅ → ⁵I₈ (Ho³⁺), ²F_5/2 → ²F_7/2 (Yb³⁺) and ⁵I₆ → ⁵I₈ (Ho³⁺) transitions, respectively. Meanwhile, the emission intensities of the up-converted green emission and down-converted emissions were further enhanced by co-doping Li⁺, which can be confirmed by the fact that the emission intensities of the up-converted green emission and down-converted NIR emission were increased 50% and 90% after co-doping 7% Li⁺. The energy transfer processes between Ho³⁺ and Yb³⁺ ions and the mechanisms of Li⁺ doping for the emission enhancement have been investigated and discussed. Additionally, the time-resolved luminescence traces are measured, and the luminescence lifetimes of the Ho³⁺ ions are calculated. Furthermore, the temperature-dependent up-conversion and down-shifted luminescence of Gd₆O₅F₈:Yb³⁺,Ho³⁺,Li⁺ are investigated, and the results show that the ratio of up-converted green to red emission as well as the ratio of down-shifted green to blue emission in Gd₆O₅F₈:Yb³⁺,Ho³⁺,Li⁺ all respond single exponentially to temperature, which could be applied to sense temperature.