Phase Transition‐Driven Highly Sensitive, NIR–NIR Band‐Shape Luminescent Thermometer Based on LiYO₂:Nd³⁺

Abstract

reliability of temperature readout, which is achievable by 1) increasing the luminescence brightness and/or 2) increasing the relative sensitivity of the luminescent thermometer. [5,6,9] For the materials that are currently most intensively used and studied in luminescence thermometry, i.e., inorganic materials doped with luminescent ions, two strategies are currently employed, 1) the utilization of singly doped materials; and 2) exploitation of Co-doped materials. [10][11][12][13][14][15][16][17][18][19][20][21][22] While the latter approach yields very promising results, there is a significant risk that scaling-up the synthesis of materials to amounts required by industry, the inhomogeneous distribution of dopant ions and their clustering may occur. This may modify the thermometric performance of the phosphor and lead to an unreliable temperature readout. [5,9] On the other hand, for ratio metric temperature measurements based on phosphors doped with one type of luminescent dopant, luminescence intensity ratio from thermally coupled levels is used as a thermometric parameter. [9,[23][24][25] In this approach, the relative sensitivity is proportional to the energy difference ΔE between these levels. [26] It is well known that above ΔE = 2000 cm -1 , the thermal energy required to populate the higher-laying state becomes too high to obtain intense emission. [26] Hence, a further increase in sensitivity is difficult to achieve. In response to the above-mentioned limitations and the requirement to develop luminescent thermometers based on single ion luminescence with high relative sensitivity, the application of materials exhibiting temperature-induced firstorder phase transitions has recently been proposed. [27] In this work, a new luminescent thermometer, whose operating principle exploits a temperature-induced first-order phase transition, is proposed. Although luminescent thermometers using structural phase transition have been described in the literature before, the main limitation of these thermometers is their relatively low sensitivity. [28,29] Therefore, here, LiYO 2 :Nd 3+ nanocrystals have been synthesized for which the temperature increase above ≈290 K induces a structural change from monoclinic (P2 1 /c, Z = 4) to tetragonal (I4 1 /amd, Z = 4) and thus a change in the point symmetry of the crystallographic site of Y 3+ (occupied by the luminescent Nd 3+ ions) from C 2 to D 2d . [30][31][32][33] This symmetry change clearly affects the energy of the Stark levels of the 4 I J mutiplets (Figure S3, Supporting Information) Almost all existing luminescent thermometers rely on the temperaturedependent processes such as multi-phonon relaxation and phonon-assisted energy transfers, thermal population, or coupling between energy levels of ground and excited states of luminescent species (lanthanides, transition metals, quantum dots, fluorescent molecules, etc.). Although such phenomena are in principle suitable for straightforward calibration, aiming to offer high temperature sensitivity, high temperature resol...

Cr3+‐Facilitated Ultra‐Sensitive Luminescence Ratiometric Thermometry at Cryogenic Temperature

“…At beginning, the lanthanide series, including Tm 3+ , Ho 3+ , Nd 3+ , Dy 3+ , Sm 3+ , Eu 3+ , etc., have presented their potential application for optical thermometry. [16][17][18][19][20][21] The most classic and well-known example belongs to the Er 3+ -doped materials. [17] The 2 H 11/2 / 4 S 3/2 states of Er 3+ are separated by a gap of 700 cm −1 , meaning that the populations at the 2 H 11/2 / 4 S 3/2 states are dominated by the Boltzmann distribution, the intensity ratio of which therefore could be considered as an indicator of temperature.…”

Energy Gap Linear Superposition of Thermally Coupled Levels toward Enhanced Relative Sensitivity of Ratiometric Thermometry

“…Specifically, S R is proportional to the energy gap of TCLs and inversely proportional to the temperature. It is noteworthy that the energy gap between two TCLs is conditional (Δ E < 2000 cm –1 ) because the particle population in the excited states should satisfy the Boltzmann distribution. , The Boltzmann equilibrium can only be preserved when the non-radiative thermalization is much faster than depopulation pathways of the excited states . The excessive energy gap between excited states leads to a small probability of thermalization from the lower to higher level, which cannot compete with the radiative decay of the lower excited state.…”

Regulating the upconversion luminescence properties of Tm3+/Yb3+-codoped ZrScW2PO12 microparticles with a negative thermal expansion effect through thermal stimulation for optical thermometry

“…LiYO 2 :Nd 3+ and LiYO 2 :Eu 3+ ). 16,17 Besides, Utochnikova et al also theoretically analyzed the S r value of terbium-europium coordination compounds in a four-level system. 18 Interestingly, researchers also tried to utilize the LIR technique to analyze the temperature-related fluorescence intensities of two emissions arising from two different emitting centers or non-thermally coupled levels (non-TCLs), 19,20 and some inspiring results were reported.…”