808 nm-triggered optical thermometry based on up-conversion luminescence of Nd3+/Yb3+/Er3+ doped MIn2O4 (M = Ca, Sr and Ba) phosphors

Abstract

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 (⁴S_3/2/²H_11/2) 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.