Broad-Scope Thermometry Based on Dual-Color Modulation up-Conversion Phosphor "Ba₅Gd₈Zn₄O₂₁:Er³⁺/Yb³⁺"

Yb³⁺/Er³⁺ codoped Ba₅Gd₈Zn₄O₂₁ up-conversion (UC) phosphors with tunable emission were synthesized using a facile sol-gel method. UC spectra are composed of green emission from ²H_11/2/⁴S_3/2 → ⁴I_15/2 transitions and red emission from ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺ ion with the excitation of 980 nm laser diodes. Modulation of emitting color from green to red could be achieved by adjusting dopant concentrations or pulse width of 980 nm laser. The mechanism of the former strategy was figured out through analyzing visible and near-infrared (NIR) down-conversion emission spectra together with the corresponding green level (⁴S_3/2) lifetimes under excitation of 490 nm light, and the latter method was explained by the non-steady-state up-converison process. Temperature detection range was expanded to low temperature region by utilizing red-emitting stark levels of Er³⁺ ion as thermally coupled levels. Thermal sensing performances based on green-emitting levels (²H_11/2/⁴S_3/2) and red emitting stark levels (⁴F_9/2(1)/ ⁴F_9/2(2)) of Er³⁺ ion were estimated and the maximum sensitivity are 0.0032 K^-1 at 490 and 0.0029 K^-1 at 200 K in our experimental range, respectively. Moreover, the effects of UC emission color from different dopant concentrations and pulse widths of lasers on sensor sensitivity were also investigated in detail. Results imply that the present phosphor Ba₅Gd₈Zn₄O₂₁:Er³⁺/Yb³⁺ exhibits high and stable sensitivity in a wide temperature detection scope, which makes it an excellent candidate for an optical thermometer.