808 nm-triggered optical thermometry based on up-conversion luminescence of Nd/Yb/Er doped MInO (M = Ca, Sr and Ba) phosphors.

Optical thermometry based on up-conversion (UC) fluorescent intensity ratio (FIR) with 808 nm excitation is preferable in water-rich environments, and investigation of the ambiguous intrinsic influencing factors on host-dependent sensitivity is a prerequisite for the development of highly sensitive thermometry. Herein, MIn2O4:Nd3+/Yb3+/Er3+ (M = Ca, Sr, and Ba) microcrystals with low phonon energy are synthesized via a sol-gel method. Intense UC luminescence with tunable emission color from green to red is obtained by controlling the Yb3+ content, and the UC mechanisms and successive energy transfer of Nd3+ → Yb3+ → Er3+ are elaborated using lifetime measurements. The thermal sensing properties of the samples based on the thermally coupled levels (4S3/2/2H11/2) of Er3+ are assessed, and their sensitivities increase gradually with an increase in temperature and reach the maximum of about 0.0048, 0.0033 and 0.0058 K-1 at 490 K for M = Ca, Sr, and Ba, respectively. By analysing the host structure, site symmetry of M2+ ions and characteristics of the M-O bonds, it is proposed that the higher Ca-O bond covalency in CaIn2O4 leads to better sensitivity than SrIn2O4 with the same structure, and the optimal sensitivity in BaIn2O4 is mainly attributed to the specific local crystal field of the Ba2+ site with higher ligancy and longer chemical bonds. These results provide insight for the selection of appropriate matrix materials to achieve higher temperature detection sensitivity.