Thermal-enhanced near-infrared upconversion luminescence of Er3+ for high-sensitive optical temperature sensing

Highlights: • Thermal-enhanced near-infrared UCL was found in Er³⁺ doped YbMoO₄ phosphors. • A new thermometry strategy based on LIR of Er³⁺ in a four-level system was proposed. • LIR between the near-infrared and green UCL of Er³⁺ was used for temperature sensing. • High-sensitive thermometry was realized based on LIR of Er³⁺ in a four-level system. In this paper, we report the thermal-enhanced near-infrared upconversion luminescence (UCL) from thermal-coupled (TC) energy levels of Er³⁺ and a new optical temperature sensing strategy in Er³⁺-doped phosphors. The near-infrared UCL of Er³⁺ in the wavelength range of 750–900 nm due to the transitions from the ²H_11/2 and ⁴S_3/2 TC energy levels to the ⁴I_13/2 metastable energy level showed a thermally enhanced luminescence behavior at elevated temperatures that is related to the enhanced temperature-dependent non-radiative relaxation. The luminescence intensity ratio (LIR) between the near-infrared (²H_11/2/⁴S_3/2 → ⁴I_13/2) and the green (²H_11/2/⁴S_3/2 → ⁴I_15/2) UCL of Er³⁺ was studied as a function of temperature and demonstrated good optical thermometry behavior based on the classic Boltzmann distribution. The absolute sensitivity based on the LIR between near-infrared and green UCL of Er³⁺ was as high as 0.061 K⁻¹, showing a nearly two orders of magnitude improvement compared to that based on the traditional LIR between two green UCL of Er³⁺. It is anticipated that the near-infrared UCL from TC energy levels of Er³⁺ can be used to construct new luminescent thermometry devices with high sensitivity, low temperature uncertainty, and good repeatability.