Intentional modification of the optical spectral response and relative sensitivity of luminescent thermometers based on Fe3+,Cr3+,Nd3+ co-doped garnet nanocrystals by crystal field strength optimization

Abstract

Among many different optically active ions used for luminescence thermometry, transition metal ions reveal unique features which make them especially attractive for this purpose like high susceptibility to luminescence thermal quenching and ease of modulation of their spectroscopic properties by modification of the crystal field strength. Therefore this work is devoted to the detailed investigation of the thermally-dependent spectroscopic properties of tetrahedrally coordinated Fe³⁺ ions (λ_max ∼ 790 nm) in nanoscale garnet host materials i.e. Y₃Al₅O₁₂ (YAG), Y₃Ga₅O₁₂ (YGG), Lu₃Ga₅O₁₂ (LuGG) and Gd₃Ga₅O₁₂ (GGG). It was proved that the spectral position of the deep red Fe³⁺ emission band can be changed via modification of the crystal field strength (CFS). Additionally it was shown that the Fe³⁺ luminescence thermal response can be significantly enhanced by incorporation of Cr³⁺ ions and thereby via thermally activated nonradiative Fe³⁺ → Cr³⁺ energy transfer. Therefore the highest relative sensitivities of ratiometric luminescent thermometers based on the emission intensity of Fe³⁺ (⁶A₁(⁶S) → ⁴T₂(⁴D)) and Nd³⁺ (⁴F_3/2 → ⁴I_9/2) were as high as 5.90% °C⁻¹ (at 100 °C) for LuGG, 1.68% °C⁻¹ (at 50 °C) for YGG and 1.57% °C⁻¹ (at 50 °C) for GGG host materials. It was proved that lowering the CFS has a beneficial impact on the performance in noncontact temperature sensing of Fe³⁺ based luminescent thermometers. This is the first study focused on thermally-affected optical properties of Fe³⁺ doped nanomaterials towards temperature sensing.