Elsevier

Journal of Luminescence

Volume 131, Issue 8, August 2011, Pages 1747-1752
Journal of Luminescence

Magneto-optical investigations of rare earth doped sol–gel derived silicate xerogels

https://doi.org/10.1016/j.jlumin.2011.04.031Get rights and content

Abstract

Rare-earth doped xerogels (Eu3+, Sm3+, Ho3+, Pr3+) were prepared by using the sol–gel method and their magneto-optical and optical properties have been studied. The Magnetic Circular Dichroism (MCD) spectra are quite similar to those recorded in the RE-doped fluorozirconate glasses; the fine structures shown by the MCD spectra are better resolved compared to the optical absorption spectra. The MCD technique has been correlated with optical measurements in order to investigate the site symmetry in the particular case of Eu3+-doped xerogel and oxyfluoride glasses. In the xerogel, coordination symmetry around the Eu3+ ions is close to D3h and is lower in the oxyfluoride glass.

Highlights

► Rare-earth doped xerogels (Eu3+, Sm3+, Ho3+, Pr3+) were prepared by the sol–gel method. ► MCD and luminescence methods were applied to probe the site symmetry in europium doped xerogel and oxyfluoride glasses. ► Rare-earth site symmetry is close to D3h in xerogel and is lower in oxyfluoride glass.

Introduction

There is considerable interest in sol–gel technology with regard to rare-earth (RE) luminescent materials for various applications in luminescent solar concentrators, tunable lasers, active waveguide sensors for environmental and biological impurities, waveguides and materials for non-linear optics ([1] and references therein). The sol–gel method has several advantages over conventional “melt-quenching” glass preparation methods such as the wide range of compositions achievable, lower processing temperature, easier composition control and better chemical homogeneity of the product.

Optical spectroscopy methods such as absorption and photoluminescence (PL) are well known and used for the study of the optical properties associated with the RE dopant ions. The absorption spectra are due to superposition of different electric-dipole (ED) and magnetic-dipole (MD) transitions of the RE ions [2]. Additional effects due to the multiplets degeneracies, inhomogeneous broadening and crystal field splittings make the analysis difficult. Magnetic Circular Dichroism (MCD) is a very useful method for the detection of the overlapping transitions in the absorption spectra [3]. Compared with the optical absorption the MCD spectrum contains more information since in addition to the intensity the MCD signal is characterized by its sign (positive or negative), which is related to the symmetry of the RE environment [3]. The usefulness of the MCD technique has been proved in the particular case of RE3+-doped fluorozirconate (ZBLAN) glasses and aqueous solution [4], [5], [6], [7], [8], [9], [10].

In this paper, we report MCD and optical absorption investigations on RE3+-doped xerogels. The MCD and photoluminescence measurements have been used to investigate the site symmetry in the particular case of Eu3+-doped xerogel and oxyfluoride glasses.

Section snippets

Samples preparation

Transparent bulk silicate xerogels doped with (1%) RE=Eu3+, Sm3+, Ho3+ and Pr3+ were prepared by the sol–gel method according to the methods described in Ref. [11]; concentration of the RE ions diluted in the xerogel was 0.291 mol l−1 [12]. Tetraethyl orthosilicate (>99%), Si(OC2H5)4 (Aldrich), trifluoroacetic acid (>99%), CF3COOH (Aldrich), barium acetate (>99%), (Ba(CH3COO)2 (Aldrich) and rare-earth (III) acetate hydrate Eu(CH3COO)3·xH2O) (>99%; Aldrich) were used as starting materials.

In the

Theoretical details

The MCD signal gives the difference between the left and right circularly polarized light, more precisely between the optical densities or absorbances of the polarized components of light. The difference between the optical densities of left and right circularly polarized lights for an absorption band can be written as [13]ΔD=(Af(E)E+(B+CkT)f(E))Hwhere f(E) is the shape function of the absorption band and H is the applied magnetic field. These three terms are connected with the lifting of

FTIR spectroscopy

The FTIR spectra of Eu3+-doped xerogel and oxyfluoride glass samples in the spectral region 400–1400 cm−1 are shown in Fig. 1; the spectra for the other RE3+-doped xerogels are essentially the same. In the xerogel sample the bands at 455, 800, 1085, 1150 and 1200 cm−1 are attributed to the Si–O–Si bonds; the band at 950 cm−1 was assigned to the Si–O stretching mode vibration [14] and a small peak at 725 cm−1 is due to absorbed CO2. The shoulder at around 550–600 cm−1 was observed in un-doped xerogel

Discussion

The optical absorption and MCD spectra are due to the RE3+ ions transitions from the RE ground state and are quite complex because of the superposition between different ED and MD transitions of the RE ions (Fig. 2, Fig. 3, Fig. 4, Fig. 5). An exception is the Eu3+ ion doped samples, which show pure magnetic-dipole (MD) and electric-dipole (ED) transitions.

Conclusions

RE-doped xerogels with RE=Eu3+, Sm3+, Ho3+ and Pr3+ were prepared by the sol–gel method and their magneto-optical and optical properties have been studied. The MCD spectra are quite similar to those recorded in RE-doped fluorozirconate glasses; the fine structures shown by the MCD spectra are better resolved compared to the optical absorption spectra. The MCD technique together with luminescence measurements was applied to probe the site symmetry in the particular case of Eu3+-doped xerogel and

Acknowledgements

The authors gratefully acknowledge the Romanian Research Ministry (“Core Program no. PN09-450102”) for the financial support of this work.

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