High-sensitive nonthermally coupled upconversion for ultralow temperature sensing

Upconversion based nanothermometry has received much attention due to its merits of stability, narrow band emission and rich emission peaks. However, the previous works are mainly based on the emissions from thermally coupled energy levels which is theoretically limited by Boltzmann distribution theory with resultant low temperature sensitivity in particular at ultralow temperatures. Here we report a LiYF4:Yb/Ho@LiYF4 core-shell nanostructure to improve the sensitivity at low temperatures by taking advantage of non-thermally coupled energy levels of Ho3+. In detail, the green upconversion emission of Ho3+ shows an increase with reducing temperature while its red upconversion emission presents a decline during the same process. This is primarily due to the suppression of the non-radiative multi-phonon relaxation occurred at the green emitting levels (5F4,5S2) and the intermediate level (5I8) at low temperatures. Such a feature contributes to a high relative sensitivity of 7.17%/K at 11 K, much higher than reported values. Our results provide a promising candidate for the development of nanothermometer with high-sensitive low-temperature sensing performance.