Synthesis and Characterization of CaF 2 Thin Films Doped with Tb 3+
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Synthesis and Characterization of CaF2 Thin Films Doped with Tb3+ A. Méndez-Blas1, E. López-Cruz1, G. Palestino2, and M. E. Calixto1 1 Instituto de Física, Benemérita Universidad Autónoma de Puebla, Apdo. Postal J-48, Puebla, Pue. 72570, México 2 Laboratory of Biopolymers and Nanostructrures, Faculty of Chemical Sciences, Universidad Autónoma de San Luis Potosí, San Luis Potosí, S.L.P., México ABSTRACT In this work, calcium fluoride (CaF 2) and Tb3+-doped calcium fluoride (CaF2:Tb3+) thin films were prepared by electrochemical methods. According to EDS and XRD results, CaF2 thin films undergo through a phase transformation from cubic CaF2:Tb3+ to NaTbF4 when Tb3+ concentration in the electrolytic bath is increased. So that, peaks showed a very clear shifting towards lower angles. On the other hand, optical absorption results showed the typical range of transparency from IR to UV, and from photoluminescence (PL) spectra results it can be observed that it is possible to identify the f-f transition of Tb3+. The PL at low temperature allows finding the quenching concentration when the PL intensity decreases at the highest doping concentration value. However, the results also showed that the transition of Tb3+ does not seem to be affected by the coexistence of the secondary phases such as cubic NaTbF4. In the case of RE-doped CaF2 thin films materials, they can be used with a double purpose for solar cell applications, not only as antireflecting coating, but also as a host for lanthanide doping for down conversion of light. These properties could be very useful for photovoltaic applications, so that the spectral range of light conversion could be increased to achieve higher conversion efficiency values. INTRODUCTION Calcium fluoride (CaF2) has a transmission response higher than 90% between 0.25 and 7 microns, and is used commonly in laser optics due to their low absorption level and high damage threshold and as part of anti-reflecting coatings. These characteristics are associated to their relatively large thermal conductivity and hardness despite its high thermal expansion coefficient [1]. CaF2 shows an abundance of brilliant luminescence colors due to the presence of impurities or color centers, the most important of these impurities are the lanthanides ions; in fact several lasers have been constructed from lanthanide ions incorporated into calcium fluoride [2]. The radiative emission is propitiated due to the relatively low value of highest phonon frequencies of CaF2 (257, 322 and 463 cm-1) [3]. These characteristics reduce the probability of non-radiative relaxations between 4f levels of lanthanide ions. When CaF2 is doped with trivalent lanthanide ions (Ln3+), they incorporate to the lattice as substitutional defects, replacing the Ca2+ ions; so a valence mismatch occurs which must be compensated. Charge compensation is attained by the presence of one F- ion in one of the six nearest neighbor interstitial sites at a distance a/2. The interest of incorporating Ln3+ ions to the CaF2 thin films is because they could
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