Thermally coupled energy levels of Eu 3+ within the BaHfO3 matrix, excited with UV radiation
Introduction
Optical nanothermometry based on luminescent materials that exhibit changes in their emission when their temperature changes within the physiological temperature range (288–323 K), are of particular interest for biological applications. The most common temperature measurement methods are based on the expansion of metals or liquids or changes in electrical properties that presents some materials; however, due to dimensional limitations, these sensors cannot perform adequate measurements in areas such as microelectronics, micromedicine, microchemistry, microbiology, etc. where micrometer size sensors are required (Khalid and Kontis, 2008; Brbach et al., 2011; Wang et al., 2013; Childs et al., 2000). In recent years, submicron sensing techniques like micro-thermocouple, liquid crystal thermography, infrared thermography, and thermos reflectance have been explored; some of the most promising being those that use fluorescent optical thermometry, particularly those based on lanthanide ions (Brites et al., 2012). The luminescent emission of some lanthanide-doped materials is dependent on temperature, which has been attributed to the presence of thermally coupled energy levels, namely energy levels with a difference between them from 200 cm−1 to 2000 cm−1, which allows electronic transitions between both levels to through the action of temperature (Xu et al., 2012; Blasse and Grabmaier, 1994). When temperature raises, the electrons come to the highest energy level, increasing their electronic occupancy, at the expense of the electron population of the lowest energy level; this leads to an increase in the number of transitions from the highest energy level and a decrease in the number of transitions from the lowest energy level. Thus, by detecting these changes and characterizing them by the luminescent emission intensity, it is possible to determine the temperature to which the material is exposed and a temperature sensor can be obtained. Luminescent thermometry requires a suitable host matrix for optically active ions, this host must meet some specifications such as low phononic vibration, a wide energy gap, chemical stability, and thermal stability (Gnach and Bednarkiewicz, 2012). Metal oxide matrices usually fulfill these requirements, some of the most used are YVO4, KLaP4O12, and CaZrO3, which have been doped with lanthanide ions to obtain a temperature dependent luminescent emission (Getz et al., 2019; Drabik and Marciniak, 2020; Tian et al., 2020).
Among the different metal oxides, barium hafnate (BaHfO3) has a phononic vibration lower than 700 cm−1, an energy gap of 3.5 eV, and a melting point of 2893 K, in addition, in the BaHfO3 there are three different sites of symmetry where the Eu ion can be introduced, making its incorporation into the crystal lattice more likely. These factors make barium hafnate a potential candidate to be a good luminescent matrix. On the other hand, the trivalent europium lanthanide ion (Eu3+) has the energy levels 5D0 and 5D1 with an energy difference of ΔE ≈ 1700 cm−1. They present the characteristic to be considered thermally coupled energy levels (B ü nzli, 2016), despite this, this material has been scarcely studied in luminescent thermometry (Liu et al., 2010). The objective of this work was to investigate the emission intensity at the thermally coupled energy levels of the Eu3+ ion (5D0 and 5D1) within the matrix of barium hafnate with perovskite structure, in the physiological temperature range.
Section snippets
Materials and methods
Doped and undoped BaHfO3:Eu3+ were synthesized by the hydrothermal route using BaCl2 * 6H2O, HfCl4 and EuCl3 * 6H2O precursors from Sigma-Aldrich Co. (99.99%). Four solutions were obtained by dissolving the barium and hafnium precursors in 5 ml of deionized water at a concentration of 0.4 M in a 1:1 ratio. To determine the Eu3+ concentration with the maximum emission intensity, EuCl3*6H2O was added to these solutions at concentrations of 1, 3, and 5 at% with respect to hafnium and barium. All
X-ray diffraction (XRD)
XRD pattern of the undoped sample is shown in Fig. 1. Based on powder diffraction card files 22–0084 and 78–0050, the sample consists of a mixture of cubic perovskite phase of BaHfO3 and monoclinic phase of HfO2. Previous reports on the synthesis of BaHfO3 at the nanoscale give an account of the inherent formation of HfO2, depending on the synthesis temperature (Dobrowolska and Zych, 2009). Our results replicate the reported results, where synthesis temperature below 1473 K promotes the growth
Conclusion
The material obtained by the hydrothermal route from chlorides corresponds to a mixture of two phases, the cubic perovskite phase of HfBaO3 and the monoclinic phase of HfO2. HRTEM analysis confirms the presence of these two phases and shows the formation of a polycrystalline system, with an average crystallite size of 10 nm, making it suitable for use in possible submicrometric applications. The photoluminescent study of the samples doped with different amounts of europium presenting the
CRediT authorship contribution statement
R.I. López-Esquivel: Formal analysis. I.A. Garduño-Wilches: Investigation. J.C. Guzmán-Olguín: Methodology. T. Rivera Montalvo: Supervision. J. Guzmán-Mendoza: Writing – original draft.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors thank the Secretaria de Investigación y Posgrado, Instituto Politécnico Nacional, for the support through the project 20210363. Also thanks to Nicolas Cayetano, from Centro de Nanociencias, Micro y Nanotecnologías, for his support in the high-resolution transmission electron microscopy images.
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Fluorescence intensity ratio for BaHfO<inf>3</inf>:Eu<sup>3+</sup> temperature sensor
2023, Applied Radiation and IsotopesCitation Excerpt :Emission spectra showed significant variations in luminescent intensity at all transitions, replicating the results of Boronat (Boronat et al., 2017). The sample doped with 3% Eu was the one that presented the highest luminescent intensity (López Esquivel et al., 2022). Regarding the thermometric sensitivity, the analysis photoluminescent as a function of temperature was performed at seven different temperatures in the range of 328 to 289 K, Table 1 shows the results obtained for the 5D0 → 7F2 transition.