Quantification of upconversion photon and thermosensitive feedback in Er3+/Yb3+ doped fluorotellurite glasses
Graphical abstract
Introduction
Optical thermometry technique provides non-contact measurement and imaging capability owing to its fast response, electromagnetic passivity and high thermal sensitivity, among them, luminescence thermometry has already confirmed its uncommon value in conventional, harsh and nanoscale environments [[1], [2], [3], [4], [5], [6]]. In addition, optical thermometer based on the fluorescence intensity ratio (FIR) technique of UC rare earth ions (RE3+) is not impacted by the fluctuation of pumping power or the induction of surrounding electromagnetic, which has shown unique potential for tissue thermal sensing [[7], [8], [9], [10], [11], [12], [13], [14], [15]]. Especially, the enhancement of thermometer sensitivity is of great significance to monitor the slight temperature change during some special processes, such as photothermal therapy and temperature-dependent drug release. Therefore, the application of lanthanide doping materials to optical temperature sensors by FIR technique is a new field worth exploring [[16], [17], [18], [19], [20], [21]]. Based on above, it is still necessary to explore thermosensitive feedback property and luminescence characteristics of material in the purpose of fitting the high sensitive temperature sensing applications of UC materials.
The requirement for UC emissions are strongly influenced on the phonon energy of the glass matrixes. In order to achieve powerful UC emissions, TeO2 based glass family has been attempted and proved to be good vitreous host due to the its low phonon-energy, high RE solubility, low melting temperature, superior corrosion resistance and great physicochemical stability [[22], [23], [24], [25], [26], [27], [28], [29], [30]]. Furthermore, the addition of fluorine ions can modify the ligand field structure of glass system and reduce the content of residual hydroxyl as the fluorescence quenching center, which will be benefit to the improvement of shortwave transmittance and a macro upgrade for the luminous efficiency of the radiative emissions in RE3+ ions for tellurite glasses [[31], [32], [33], [34], [35], [36], [37], [38]]. Hence, it is of significance to give considerable interest for RE3+ doped fluorotellurite glass systems. Among them, Er3+ has gained increasing attention due to its two thermally coupled levels, 2H11/2 and 4S3/2, from which a transition to the 4I15/2 ground state takes place from 500 to 570 nm [[39], [40], [41], [42], [43]]. Thus, the quantitative evaluation for Er3+ doped fluorotellurite glasses will give an intuitive reference for thermosensitive behavior of intense visible emissions as temperature sensor.
In this work, the quantification of excitation-power dependency and thermal sensing behavior in Er3+/Yb3+ doped fluorotellurite (BZLFT) glasses have been confirmed. Efficient two-photon-excited green and red UC fluorescence was observed and recorded under the excitation of 978 nm laser. The quantum yields (QYs) and luminous fluxes for visible UC emissions of Er3+ are identified as a positive dependency with pumping powers. When the excitation power increases to 983 mW, the total QY value for total UC emissions is up to 1.815 × 10−4. The maximum sensitivity of 0.4wt% Er2O3 - 0.4 wt% Yb2O3 co-doped sample is about 4.77 × 10−3 K−1 at 587.5 K, indicating an admirable thermal sensitivity of BZLFT glass material. Large absolute spectral parameter and high maximum sensitivity demonstrate that Er3+/Yb3+ doped BZLFT glasses have a promising application in temperature sensor field.
Section snippets
Materials and methods
Er3+/Yb3+ doped fluorotellurite glasses were prepared from high-purity BaF2, AlF3, ZnO, La2O3, BaCO3 and TeO2 powders according to the molar host composition 4BaF2–3AlF3–8ZnO–6La2O3–9BaO–70TeO2 (BZLFT). Additional 0.4 wt% Er2O3, 0.2 wt% Er2O3 and 0.4 wt% Yb2O3, and 0.4 wt% Er2O3 and 0.4 wt% Yb2O3 as dopants were introduced based on the host weights. The well-mixed raw materials in platinum crucibles were heated at 850 °C for 20min using an electric furnace, and then the molten glasses were
Radiative transition properties of Er3+ and Yb3+ co-doped BZLFT glasses
Er3+/Yb3+ doped BZLFT glasses are visually transparent and homogeneous as shown in inset of Fig. 1(a). The absorption spectrum of Er3+ doped BZLFT glasses presents nine exposed inhomogeneous bands, which are attributed to the transitions from the 4I15/2 ground state to specific excited states as labeled in Fig. 1. The XRD pattern of 0.4 wt% Er2O3-0.4 wt% Yb2O3 co-doped BZLFT glass powder exhibits two characteristic broad humps for crystal phase without sharp diffraction peaks, and the amorphous
Conclusions
Visible UC photon quantification have been revealed in Er3+/Yb3+ doped BZLFT glasses by absolute spectral characterization. Intense green and weak red multi-photon-excited UC emissions of Er3+ are exhibited under the excitation of 978 nm laser with different power densities. The emission photon number and quantum yield are confirmed to be positive dependency with different pumping powers. When the excitation power increases to 983 mW, the QYs for detected 526 nm, 545 nm green and 658 nm red
Author statement
Jiaxin Yang conceived and designed the experiments, Jiaxin Yang and Peijian Lin analyzed data, Edwin Yue Bun Pun and Jinliang Yuan provided guidance, and Xin Zhao and Hai Lin commented on the manuscript. All authors have read and approved the final manuscript.
Declaration of competing interest
There are no conflicts of interest to declare.
Acknowledgments
The research work was funded by the Scientific Research Funding Project from the Educational Department of Liaoning Province, China (Grant No. J2019021) and the Research Grants Council of the Hong Kong Special Administrative Region, China (Grant No. CityU 11218018).
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