Elsevier

Optical Materials

Volume 35, Issue 12, October 2013, Pages 2456-2460
Optical Materials

Eu3+ probe ion for rare-earth dopant site structure in sol–gel derived LiYF4 oxyfluoride glass–ceramic

https://doi.org/10.1016/j.optmat.2013.06.053Get rights and content

Highlights

  • Annealing of Eu3+-doped SiO2–LiYF4 xerogel induces the crystallization of LiYF4 nanocrystals.

  • Eu3+-ions are embedded in the nanocrystals most probably as Eu–O centers and/or dimer centers.

  • Eu3+-ions site symmetry is C2v and is lower than in the crystal S4.

Abstract

Sol–gel route using metal alkoxides and trifluoroacetic acid as precursors has been used to prepare oxyfluoride glass–ceramic containing Eu3+-doped LiYF4 nanocrystals of about tens of nm size embedded in a silica matrix through controlled crystallization at higher temperatures of the xerogel. Photoluminescence spectra and decay curves recorded in the Eu3+-doped LiYF4 polycrystalline pellet and glass ceramic have been discussed using group-theoretical arguments. In the glass–ceramic Eu3+ ions are embedded dominantly inside the LiYF4 nanocrystals most probably as Eu–O center and/or dimer centers in low symmetry (C2v) sites; oxygen ions were incorporated in their neighborhood during the glass ceramization.

Introduction

New hybrid materials based on stabilized RE3+-doped nanoscaled particles embedded in various matrix (i.e. solution, sol–gel-materials, nanoporous, etc. [1] and references therein) represents a high potential for applications in various fields (optical amplifiers, optical waveguides, OLEDs, etc.) since they allow the exploitation of the optical phenomena with the optical transparency due to lack of scattering. Among them, RE3+-doped nanofluorides shows efficient UP-conversion effects (i.e. near-infrared (NIR) conversion into the visible spectral range) [2] are opening up new alternative as fluorescent labels with great potential for imaging and biodetection assays in both in vitro and in vivo applications ([3] and references therein). Therefore in the last years there has been a increased interest toward the nanochemistry of the fluoride compounds various synthesis methods being proposed [4], [5].

Recently RE3+-doped NaYF4, KYF4 and LiYF4 nanocrystals have received considerable attention for up-conversion (UC) based applications [6], [7], [8] and monodisperse colloidal solution [9], [10], [11], normal microemulsion [12], core–shell [13], [14] or embedded in silica glass matrix (so called oxyfluoride glass–ceramic [15], [16], [17]) have been successfully produced. In the last case a controlled crystallization of the precursor glass (with appropriate chemical composition and during a heat-treatment) it can be obtained and the partition of the optically active RE3+-ions into the precipitated fluoride nanocrystals with low phonon frequencies (i.e. reduced non-radiative relaxation rate and therefore intense luminescence) but keeping in the same time good chemical and mechanical stability of the oxide glass (see the review of de Pablos-Martin et al. [18]). In order to avoid the difficulties related to the this method a sol–gel route has been proposed using metal alkoxides and trifluoroacetic acid as precursors [19] and recently glass ceramic containing RE-doped nanocrystals: NaYF4 [20], LiYF4 [21] KYF4 [22] have been prepared. In the oxyfluoride glass ceramic containing Nd3+-doped LiYF4 nanocrystals it was performed a preliminary site analysis using PL and PL lifetime data and Prony series approximation that has indicated the incorporation of Nd3+ ions into the LiYF4 nanocrystals [17].

The aim of this paper is to extract new information and to get a deeper understanding of the local environment around the RE3+ ions in LiYF4 nanoparticles embedded in sol–gel derived oxyfluoride glass ceramic taking the advantages of Eu3+ ion to probe the site symmetry and using group-theoretical arguments.

Section snippets

Samples preparation

Transparent bulk silicate undoped and (1%) Eu3+ xerogels were prepared by using the sol–gel method (Aldrich reagents) according to the method described in [20], [21]. In the first step tetraethoxysilane (TEOS) was hydrolyzed under constant stirring with a mixed solution of ethanol and water and using glacial acetic acid as catalyst; molar ratio was 1:4:10:0.5. Then another solution of Eu(CH3COO)3, Y(CH3COO)3, Li(CH3COO) and CF3COOH with the molar ratio for Eu:Y:Li:F of 1:5:20:255 was prepared

Thermal analysis

Thermal analyses (TG and DSC) of gels dried at 120 °C are presented in Fig. 1. The TG curve indicates three steps of the gel weight loss. In the 120–200 °C temperature range the volatilization of water, ethanol and acetic acid take place and this can be observed by the weight loss observed in the TG curve in that domain.

In the temperature range from 280 to 360 °C we observed a DSC peak at 360 °C that was assigned to the decomposition of yttrium trifluoacetate Y(CF3COO)3 [23] and lithium

Conclusion

Glass ceramic containing Eu3+-doped LiYF4 nanocrystals of about 16 nm size have been made by using the controlled crystallization at higher temperatures. Photoluminescence spectra and decay curves recorded in the Eu3+-doped LiYF4 polycrystalline pellet and glass ceramic have been discussed using group-theoretical arguments. In the oxyfluoride glass–ceramic the Eu3+ ions are embedded dominantly inside the LiYF4 nanocrystals most probably in association with oxygen impurity as Eu–O center and/or

Acknowledgements

We acknowledge the financial support of the Romanian Education and Research Ministry (IDEI Project No. 290/05.11.2011).

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