Judd-Ofelt modelling of the dual-excited single band ratiometric luminescence thermometry
Graphical abstract
Introduction
Both non-contact temperature measuring devices by the luminescent probes and the number of research papers on such sensors have seen an exponential growth in the last decades (as similarly reported in Ref. [22]; see Fig. 1). Their recent popularity is no surprise, as the new technological advancement enables for their successful adoption by the industry, which has long desired for the non-contact sensors that are mechanically, thermally and chemically stable, can be used in harsh environments or in high electromagnetic fields, cause no interference, no perturbation to the measurand, and have reliable and self-referencing read-out [1]. Luminescent sensors can be incorporated within or at the tip of the fiber-optic cable [2], placed as coatings on any surface [3], or exist as the nanomaterials [4], to be used for biomedical purposes [5], etc. Thus, there is a growing need not only for investigation of the new materials, but the new read-out methods and their theoretical interpretation.
Currently, the most employed method for temperature read-out from the luminescent sensors is the Luminescence Intensity Ratio (LIR), which exploits the different temperature dependence of two emission peaks [6]. This definition of LIR has been recently expanded to the LIR of excitation peaks, by monitoring the intensity of the single emission peak created by two different excitations. As this is the recent method, it is explored in a scanty number of articles [7,8]. However, the method shows an outstanding potential as it allows for the cheapest read-out devices possible for luminescent sensing. An example of the fiber-optic temperature probe that uses this excitation LIR is given in Fig. 2. The two excitation beams from LEDs or lasers are transmitted to the tip by the fiber-optic cable. The excitation pulses are sequential. Due to the luminescence, the signal travels back from the tip through the cable. At the end of the cable, a filter is placed in order to eliminate any reflectance. Thus, only the luminescence is detected by the detector. The detector records the pulsating signal, with differences in amplitude depending upon the temperature, i.e. LIR can be defined. This method is simpler and cheaper even than the LIR of emissions, as only a single detector is needed and just one filter, at only a higher expense of employing the second light source.
At 0 K only the 4f ground state of trivalent lanthanide is populated by the N optical centres. As the temperature rises, thermal energy populates the higher levels, and the initial N centres get shared among the levels depending on their distance from the ground state, redistributed according to the Boltzmann distribution. The fractional thermal population of the level A, from which the absorption originates is then given by Ref. [9]:where g = 2J + 1 is the level degeneracy. In the case of the Eu3+, the temperature dependent optical centre relative populations are presented in Fig. 3.
Thus, the excitation to the higher excited levels can be executed on the higher levels of the ground multiplet for temperatures above 0 K, i.e., in the case of Eu3+, this would mean the possibility of the excitation of 5D0 and higher levels from any of the 7FJ levels. This fact was exploited by monitoring the 5D0→7F4 emission and using the luminescence intensity ratio (LIR) of the 5D0←7F0,2 excitations [7]. However, with 5D0←7F0 any theoretical analysis is arduous as the only analytical tool for the rare-earth spectroscopy [10], Judd-Ofelt (JO) analysis, is not possible for that transition. In contrast, 5D0←7F1 is a magnetic-dipole transition and is thus allowed for an 4f-4f transition by conventional quantum mechanics, while 5D0←7F2 induced-electric dipole transition is JO allowed. Thus, the LIR can be theoretically explained by the JO if those transitions are used, in the analogous manner to the JO thermometric model presented in our previous work [11]. Although 7F0 is more populated than 7F1,2 up to 496 K, the efficiency for populating the 5D0 level are lower via the former. Thus, the excitation via the 7F1,2 transitions is more efficient even at the room temperature. Secondly, the probability for the transition is proportional to the JO parameters [12]. As 5D0←7F2 transition's oscillator strength is proportional to the Ω2 parameter [9], for efficient population of the 5D0 level it is desirable that the Ω2 is large for Eu3+ in a given host. Sesquioxides are known to be an excellent hosts for RE3+, and doped by Eu3+ they produce an intense 5D0→7F2 emission [13]. Thus, for the testing of the LIR thermometry by two excitations and validation of the modified JO thermometric model for this application (originally presented in Ref. [11] for two-emission LIR), the Lu2O3 host matrix was chosen. The model should allow for the prediction of the temperature invariant LIR parameter, which is conventionally obtained by fitting to the experimental data. The thermometric figure of merit, the absolute sensitivity, is to be compared to the absolute sensitivity predicted by this model. The success of the model would allow for the primal selection of a sensor material based solely on the single emission spectrum obtained at the room temperature, saving researcher's time and resources in comparison to the conventional thermometric process.
Section snippets
Experimental
The sample of 0.5 mol% Eu3+-doped Lu2O3 was synthesized using the polymer complex solution (PCS) method. Firstly, water solution of stoichiometric quantities of rare-earth (RE) nitrates was prepared by dissolving appropriate quantities of RE oxides in hot nitric acid. Polyethylenglycol (PEG, molecular weight 200) was added to the prepared RE-nitrates solutions in 1:1 mass ratio and after stirring at 80 °C for a few hours, metal-PEG solid complex was formed. Subsequently, the complex was
Results
The structure of prepared Eu3+ activated Lutetium oxide (Lu2O3) powders is confirmed by XRD (see Fig. 4) [14]; neither other phase peaks nor traces of impurities were detected. Fig. 5presents the strategy for dual-excitation LIR. The Eu3+ is excited into thermally populated 7F1,2 levels (sequentially), while the 5D0→7F4 emission intensity is observed (Fig. 5a). The trend of ratio of emissions excited by 7F1,2 levels depends on the temperature according to the Boltzmann distribution. Thus, LIR
Conclusion
The temperature invariant B parameter for LIR by two-excitation thermometry can be predicted with the JO thermometric model from the single spectra at room temperature by calculating the JO parameters and using the correction factors and the energy difference between utilized Stark sublevels. This hypothesis had been tested on Lu2O3:Eu3+, by the LIR of 5D0→7F4 emissions after 5D0→7F1,2 excitations, with an excellent matching (99.9%) of the LIR B parameter. Thus, this model may be used as a tool
CRediT authorship contribution statement
Aleksandar Ćirić: Methodology, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization. Ivana Zeković: Investigation. Mina Medić: Resources. Željka Antić: Project administration, Funding acquisition. Miroslav D. Dramićanin: Conceptualization, Validation, Writing - original draft, Writing - review & editing, Supervision, Project administration.
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.
Acknowledgements
This work is supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia.
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