Er3+-Yb3+-Na+:ZnWO4 phosphors for enhanced visible upconversion and temperature sensing applications☆
Graphical abstract
Enhancement observed in the UC emission intensity as well as in temperature sensing on codoping with Yb3+ and Na+ ions is due to the efficient energy transfer from Yb3+ to Er3+ ions and distortion of the local field symmetry in the ZnWO4 phosphors. Thus, the developed phosphors can be used in display devices, NIR to green upconverter, optical heater and as temperature sensing probe.
Introduction
Nowadays, upconverting phosphors are promising materials in different fields such as displays, sensors, agriculture and medicine.1, 2, 3, 4, 5, 6, 7 Among the rare earth ions, Er3+-Yb3+ have been proven for exhibiting efficient green UC emissions due to large oscillator strength of Yb3+ ions corresponding to the 2F7/2→2F5/2 transition.8, 9, 10, 11, 12 Moreover, by introducing some non-rare earth ions (viz. Li+, Na+, Zn2+, Mg2+, etc.) the luminescence intensity has been progressed by distorting the crystal field symmetry and improving the crystalline nature of the material.13, 14, 15 Another possible way to enhance the luminescence intensity arising from the rare earth ions is to choose suitable host materials which are self luminescent so that they can act as sensitizers to the activator ions. Metal tungstate has been shown to be of much interest in photoluminescence, microwave applications, optical fibers, X-ray and γ-ray scintillators, photocatalysis, etc. due to low phonon energy, high thermal stability, non-hygroscopic and non-toxicity nature, radio-resistance, high photo damage threshold, increase in the quenching concentration range, etc.15, 16, 17, 18, 19, 20 UC luminescence and temperature sensing properties of different rare earth ions doped ZnWO4 phosphors have been studied by various researchers.15, 16, 17, 18, 19, 20 In modern science and industries temperature is the most important physical property and thus it is very important to monitor the temperature for controlling the quality of the material. The noncontact thermometry methods overcome large sizes and non-invasiveness problem and takes considerable attention because of its flexibility of utilization in oil refineries, mines, etc.10,21 Among different noncontact methods, luminescence thermometry based on FIR technique is of special interest. In this technique, power fluctuations of the excitation source, variations in the concentration of luminescent materials and inhomogeneity of the material are avoided. In Er3+ ions, 2H11/2 and 4S3/2 thermally coupled levels (TCLs) (energy difference is 200–2000 cm−1) are commonly used in temperature sensing because the transitions arising from these levels are independent of luminescence loss and fluctuations in excitation density.9,17,20
In the present work, the Er3+-Yb3+-Na+:ZnWO4 phosphors were synthesized through solid state reaction method. The effect of Na+ ions on the structural, optical and temperature sensing ability of the developed Er3+-Yb3+:ZnWO4 phosphors was investigated under 980 nm diode laser excitation.
Section snippets
Experimental
Triply ionized Er3+/Er3+-Yb3+/Er3+-Yb3+-Na+:ZnWO4 phosphors were synthesized by conventional solid state reaction method. Initially, Er2O3, Yb2O3, Na2CO3, ZnO, WO3 were taken as starting materials. For preparation of the host material the chemical equations are given as follows,The different compositions of dopant elements are taken as follows,where, x = 0.1 mol%, 0.2 mol%, 0.3 mol%, 0.4 mol%, 0.6 mol%, 0.8 mol%where, x = 0.2 mol%, y
Structural study
The peaks observed in XRD pattern are well indexed to standard reference (ICSD card No. 98-008-4540) with space group P12/c1 of monoclinic ZnWO4 crystal system (Fig. 1(a)). The lattice constants are a = 0.4683 nm, b = 0.5709 nm, c = 0.4923 nm and α = 90°, β = 90.54°, γ = 90°. The several diffraction peaks are exhibited in the XRD pattern and maximum intense peak is observed at ∼30.62° corresponding to (11̅1) plane. The average crystallite size of the Er3+-Yb3+:ZnWO4 and Er3+-Yb3+-Na+:ZnWO4
Conclusions
The monoclinic ZnWO4 phosphors codoped with the Er3+-Yb3+-Na+ synthesized via solid state reaction method were successfully structurally and optically characterized. The enhancement observed in the UC emission intensity on codoping with Yb3+ and Na+ ions in the Er3+:ZnWO4 phopshors was explained on the basis of efficient energy transfer from Yb3+ to Er3+ ions and distortion of the local field symmetry. The maximum temperature sensitivity ∼15.92×10−3 K−1 for the Er3+-Yb3+-Na+ codoped materials
Acknowledgements
Authors are grateful to the Council of Scientific and Industrial Research (CSIR), New Delhi, India, project No. 03(1354)/16/EMR-II for providing the financial assistance in the form of a research project.
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Foundation item: Project supported by the Council of Scientific and Industrial Research (CSIR), New Delhi, India (03(1354)/16/EMR-II).