The influence of the Er3+ dopant concentration in LaPO4:Nd3+, Er3+ on thermometric properties of ratiometric and kinetic-based luminescent thermometers operating in NIR II and NIR III optical windows

In this work, properties of the near infrared absorbing and near infrared emitting Er³⁺ and Nd³⁺ co-doped LaPO₄ nanocrystals luminescent thermometer were investigated by exploiting luminescence spectra ratiometric and luminescence lifetime. The unique configuration of the energy levels of Nd³⁺ and Er³⁺ ions and the energy transfer between them requires temperature dependent phonon assistance. Since, the probability of this process is dependent on the distance between interacting ions, the thermometric parameters of luminescent thermometers were investigated as a function of Er³⁺ dopant concentration. Strong susceptibility of its probability to temperature changes enables to quantify temperature with relative sensitivity as high as S_R = 1.15%/K for LaPO₄:1%Nd³⁺, 20%Er³⁺ nanocrystals in the ratiometric approach and S_R = 2.3%/K at 600 K for LaPO₄:1%Nd³⁺, 5%Er³⁺ in the luminescence lifetime based mode. The obtained results confirm the high applicative potential of this phosphor for remote temperature measurements.