Xinyue Li

Strategy for thermometry via Tm 3+ -doped NaYF 4 core-shell nanoparticles

Optical thermometers usually make use of the fluorescence intensity ratio of two thermally coupled energy levels, with the relative sensitivity constrained by the limited energy gap. Here we develop a strategy by using the upconversion (UC) emissions originating from two multiplets with opposite temperature dependences to achieve higher relative temperature sensitivity. We show that the intensity ratio of the two UC emissions, ³F(2,3) and ¹G₄, of Tm³⁺ in β-NaYF₄:20%Yb³⁺, 0.5%Tm³⁺/NaYF₄:1%Pr³⁺ core-shell nanoparticles under 980 nm laser excitation exhibits high relative temperature sensitivity between 350 and 510 K, with a maximum of 1.53%u2009u2009K⁻¹ at 417 K. This demonstrates the validity of the strategy, and that the studied material has the potential for high-performance optical thermometry.

Luminescence properties of Er3+-doped transparent NaYb2F7 glass-ceramics for optical thermometry and spectral conversion

Novel Er3+-doped transparent NaYb2F7 glass-ceramics (GCs) were successfully fabricated for the first time by a conventional melt-quenching technique with subsequent heat treatment. The formation of NaYb2F7 nanocrystals (NCs) was confirmed by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected-area electron diffraction (SAED), and photoluminescence emission spectra. Moreover, the appearance of Stark level splitting of Er3+ emission bands and the variation of the decay curves demonstrate the accumulation of active centers into the NaYb2F7 NCs lattice. Hence, the photoluminescence emission intensities of Er3+ doped GC680 are greatly enhanced relative to those in precursor glass. Furthermore, the temperature dependent fluorescence intensity ratio (FIR) of thermally coupled emitting states (4S3/2, 2H11/2) in Er3+ doped GCs was studied under 980 nm laser excitation with a very low power density of 13 mW mm−2 to avoid the possible laser induced heating. A high temperature sensitivity of FIR of 1.36% K−1 is obtained at 300 K and the corresponding effective energy difference (ΔE) is 852 cm−1. Besides, laser induced heating at several excitation power densities was measured to evaluate the laser induced heating effect and the accuracy of temperature sensing for our sample. The GCs with relatively high sensitivity under low excitation power density are promising for temperature sensing. Moreover, the study on down-conversion (DC) spectra of the GC samples shows their ability to convert a high energy photon into two low energy photons, implying that they may also have important application as DC materials.

Optical thermometry based on thermal population of low-lying levels of Eu3+ in Ca2.94Eu0.04Sc2Si3O12

Non-contact optical thermometry using rare-earth materials has attracted a lot of attention due to its realization of non-invasive and real-time temperature determination. In the current work, a new mechanism, differing from the conventional approach utilizing the ratio of intensities emitted from two thermally coupled excited levels, was developed and demonstrated for temperature sensing using Eu3+-doped Ca3Sc2Si3O12 (CSSO). Under the excitation of 610.6 nm-wavelength light, Eu3+ ions at the 7F2 multiplet became excited to the 5D0 state, and then the luminescence intensity originating from the 5D0 state increased significantly as the temperature was increased from 123 K to 273 K. The thermoequilibrium of the 7F2 multiplet with the 7F0 ground state at a weak excitation ensured a steady increase of the luminescence intensity I with temperature T, which well fit the equation I = Aexp(−B/T) for the transitions to both 7F1 and 7F4 multiplets. A relative sensitivity SR of 1008/T2 was obtained for the 7F1 case, with a value of 1.35% at 273 K. This scheme, as a result of detecting the blue-shifted emission, has the advantages of being less disturbed by stray light from the host and the object of the thermometry. In addition, the high quantum efficiency of a one-photon excited photoluminescence scheme has the advantage of improving the resolution of the thermometry. Furthermore, a near-infrared broadband emission observed in the sample can be adopted as a reference, so as to transform the scheme into one using a luminescence intensity ratio. These results indicated that CSSO:Eu3+ may be used in practical temperature sensing applications.

Optical thermometry of a Tm3+/Yb3+ Co-doped LiLa(MoO4)2 up-conversion phosphor with a high sensitivity

Well-crystallized LiLa(MoO4)2 co-doped with 20% Yb3+and 0.5% Tm3+ was successfully synthesized by a sol–gel method and the temperature dependence of its up-conversion (UC) luminescence was investigated systematically. The fluorescence intensity ratio between the UC emission bands centered at around 700 (Tm3+:3F2,3 → 3H6) and 650 nm (Tm3+:1G4 → 3F4) from Tm3+ was measured as a function of temperature in the range of 303–543 K under 980 nm diode laser excitation. The maximum of the relative temperature sensitivity reaches 3.85% K−1 at 300 K, which is superior to other previously reported results based on the FIR technique using the thermally coupled energy levels. This result indicates that the phosphor LiLa(MoO4)2: 0.5% Tm3+, 20% Yb3+ is a promising candidate for accurate optical temperature sensors with a higher relative sensitivity.