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

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 2H11/2 and 4S3/2 TC energy levels to the 4I13/2 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 (2H11/2/4S3/2 → 4I13/2) and the green (2H11/2/4S3/2 → 4I15/2) 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.