Tunable thermal activation energy of paramagnetic defects and Eu3+ in organic auxiliary modified fluorapatite for optical applications

Abstract

Luminescence thermometry is a reliable temperature indicator by the way of spectral responses to thermal stimuli. It is of great significance to realize the flexible adjustment of its high thermal sensitivity for broadening its application range. Herein, we adopt the thermal activation energy of dual emission centers for the modulation of fluorapatite-type high-sensitivity luminescence thermometers. The organic auxiliary modified compound, Eu³⁺ doped Ca₅(PO₄)₃F (CAF): sodium citrate (SC) matrix, is synthesized by hydrothermal method. On this basis, metal ions Sr²⁺ and Ba²⁺ with different radii are introduced to enhance the luminescence of CO₂^∙- paramagnetic defects by crystal field distortion and morphology adjustment. The change of local crystal field environment symmetry is characterized by the luminescence properties of Eu³⁺. Specifically, the high local environmental symmetry is beneficial to gain excellent thermal stability of the CO₂^∙- with big thermal activation energy. Mechanistic investigations reveal that thermal activation energy of CO₂^∙- and Eu³⁺ as key parameter can adjust the relative sensitivity (S_r) of the thermos-sensitive material, yielding the maximum S_r and temperature uncertainty (δT) of 1.92 %K⁻¹ at 298 K and 0.23 K at 453 K, respectively. Besides, Carbon dots (CDs) are introduced to form other compound and further verify the feasibility of the above sensitivity adjustment. Meanwhile, a flexible-film with long afterglow information encoding is also achieved for optical anti-counterfeiting. This work not only provides a new strategy for the design of tunable fluorapatite luminescent thermometers, but also constructs promising anti-counterfeiting candidate materials.