Raman spectroscopy studies of Er3+-doped zinc tellurite glasses

Raman spectroscopy studies of Er3+-doped zinc tellurite glasses

Journal of Non-Crystalline Solids 351 (2005) 833–837 www.elsevier.com/locate/jnoncrysol Raman spectroscopy studies of Er3+-doped zinc tellurite glass...

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Journal of Non-Crystalline Solids 351 (2005) 833–837 www.elsevier.com/locate/jnoncrysol

Raman spectroscopy studies of Er3+-doped zinc tellurite glasses N. Jaba a

b

a,*

, A. Mermet b, E. Duval b, B. Champagnon

b

Laboratoire de Physique des Semiconducteurs et des Composants Electroniques, De´partement de Physique, Faculte´ des Sciences, 5019, Monastir, Tunisia Laboratoire de Physico-Chimie des Mate´riaux Luminescents, UMR 5620 CNRS, Universite´ Lyon 1, 69622 Villeurbanne, France Received 13 February 2004; received in revised form 31 December 2004

Abstract Er3+-doped tellurite-based glasses have been investigated using Raman spectroscopy. The study is aimed to determine the structural characteristics of systems with broad Er3+ emission bands. In the Er2O3 concentration 0.5–4 mol% range studied, experimental results showed that adding erbium ions to zinc tellurite glass converts TeO4 trigonal bipyramid (tbp) units into TeO3 trigonal pyramid (tp) groups. A Raman band, associated with the Te–O–Te vibration mode has been observed near 450 cm1. It was found that this band shows a significant decrease in intensity as the Er3+ ion concentration increases, indicating a structural disruption in the glass network due to the erbium increasing. On the other hand, the addition of erbium ions results in an increase in the Boson peak amplitude as compared to the respective undoped glasses which means that the contrast of the cohesive nanodomains relative to the softer interfacial zones increases. Further addition of more than 1 mol% of rare-earth ions, however, leads to a reduction of the Boson peak which is ascribed to clustering of dopants and/or a decrease in the contrast within the glass network. Ó 2005 Elsevier B.V. All rights reserved.

1. Introduction Erbium-doped tellurite glasses have optical and chemical properties suitable for optical applications [1,2]. High linear and non-linear indices of refraction, relatively low phonon energy spectra, many valence states of tellurium, good infrared transmittance, and chemical durability make them promising candidates for fiber laser and optical amplifier devices [1–3]. Many available techniques, the Raman scattering is one of them, were used to provide information on the local structure of glasses, more especially for determining the molecular units present in the lattice network and

characterizing the atom arrangements around active rare-earth dopants. To our knowledge, no Raman study has been performed on Er-doped 70TeO2–30ZnO glasses as a function of the doping in the concentration range 0.5–4 mol% Er2O3. In the present work, we will discuss the effect of Er doping on the glass structure change based on existing data in the literature. The paper also reports on a study of structural properties of Er3+ with the aim to compare the effect of erbium ions incorporated in a tellurite glass network to modifier oxides. We will attempt to describe, at least qualitatively, the microscopic mechanism for the Boson peak with increasing erbium content in the tellurite glass network.

2. Experimental * Corresponding author. Tel.: +216 98 676 057/73 500 280; fax: +216 73 500 278. E-mail addresses: [email protected], [email protected] (N. Jaba).

0022-3093/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.02.003

Glasses were prepared from oxide powders of TeO2, ZnO and Er2O3 as starting materials using the conventional melt-quenching method. The amount of dopant

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was varied between 0.5 and 4 mol% Er2O3. Raman measurements were performed using a double grating spectrometer (XY Dilor) with the 457.9 nm Ar+ ion laser exciting line. All Raman spectra were recorded at room temperature, in the wavenumber range 10–1000 cm1 under a vertical–vertical (VV) polarization, with a spectral slit width of 0.6 cm1.

3. Results Fig. 1 shows VV Raman spectra at room temperature of both undoped and Er3+-doped 70TeO2–30ZnO glass. The doping concentration ranges from 0.5 to 4 mol% Er2O3. The Raman spectra were normalized relative to the maximum intensity of the band at 675 cm1. The shape of Raman spectra is more or less similar among themselves and to those found in tellurium-oxide based glasses [4–8]. No additional vibration bands have been found beyond the wavenumber of 900 cm1. In analyzing the Raman spectra, two distinct regions can be distinguished. The first one that ranges from 1000 to 250 cm1 exhibits two bands centered around 675 and 750 cm1 and a third smaller amplitude band located at 450 cm1. The intensity of the 750 cm1 Raman band, increases with the erbium concentration. In contrast, the intensity of the Raman band near 450 cm1 decreases with increased Er2O3 content. Note that the peak frequency of the band at 750 cm1 shifts towards larger wavenumbers when increasing Er3+ concentration. Controversly, a shift towards smaller wavenumbers has been observed for the peak frequency of the 450 cm1 Raman band. The second region in the 250–

10 cm1 spectral range is composed of a large and intense band with a maximum at 50 cm1 (Boson peak) and a broad shoulder around 120 cm1 (see Fig. 2). As also found, this shoulder disappears in HV polarization, as illustrated in Fig. 3. The amplitude of the Boson peak as observed in the Raman spectra is relatively higher compared to that in undoped glass. Further addition of rare-earth ions (>1 mol% Er2O3) results in a reduction in the amplitude of this band (see Fig. 2). The peak energy of Boson band in Er-doped TeO2–ZnO glasses shifts towards higher frequencies as the proportion of erbium increases between 1 and 4 mol% Er2O3 (Table 1).

4. Discussion 4.1. High-frequency Raman measurements Spectroscopic investigations have shown that TeO2– MO glasses (where MO is a modifier oxide: ZnO, PbO, MgO) are formed by a three-dimensional network composed of asymmetrical TeO4 trigonal bipyramids (tbps) when the modifier content is relatively low [9]. An increase in the modifier concentration leads to the progressive formation of distorted TeO3+1 units (where the subscript Ô3 + 1Õ indicates the existence of a longer bond compared to the three others [10]), followed by the creation of regular trigonal TeO3 pyramids (tps) associated with non-bridging oxygen atoms (NBO) [11]. The Raman bands around 675 and 750 cm1 are assigned to stretching vibrations in TeO4 and TeO3 and/ or TeO3+1 groups, respectively. The increase in intensity observed for the 750 cm1 band with erbium concentra-

1.8

4% Er2O3

3% Er2O3

1.5

1% Er2O3 undoped

Intensity (a.u.)

1.2

0.9

0.6

0.3

0.0 0

200

400

600

800

1000

Raman shift (cm-1) Fig. 1. Room-temperature VV Raman spectra of the 70TeO2–30ZnO glass doped with various concentrations of erbium. The excitation line used is 457.9 nm.

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1.0

4% Er2O3 (51.0 cm-1)

3% Er2O3 (49.0 cm-1)

1% Er2O3 (45.0 cm-1)

0.8

Intensity (a.u)

undoped (45.5 cm-1)

0.6

0.4

0.2 0

50

100

150

200

250

300

Raman Shift (cm-1) Fig. 2. Low-frequency Raman spectra (VV) at room-temperature of Er3+ doped-70TeO2–30ZnO glass.

1.0

VV polarization HV polarization

Normalized intensity (a.u.)

0.8

0.6

0.4

0.2

0.0 0

50

100

150

200

250

300

Raman shift (cm-1) Fig. 3. Normalized VV and HV Raman spectra of undoped 70TeO2–30ZnO glass. Table 1 Position of the Boson peak maximum, xmax, and size of nanodomains, 2a, of the 70TeO2–30ZnO glass doped with various concentrations of erbium Concentration (Er2O3)

xmax (cm1)

2a (nm)

0 1 3 4

45.5 45.0 49.0 51.0

1.57 1.58 1.46 1.40

tion is consistent with the destruction of TeO4 groups. The spectra show a monotonic dependence on the er-

bium content for all of the concentrations studied. The introduction of erbium oxide into the glass network induces a gradual reduction of tellurium coordination (4 ! 3 + 1 ! 3), thus leading to a substantial change in the glass structure. Comparing the Raman spectra of Er-doped glass with those of binary or ternary tellurite based glasses, it appears that erbium ions influence the Te–O local structure [4,7–9]. Therefore, it can be concluded that TeO4 tbps units can be converted into TeO3 tp units as the number of erbium ions increases in the Er-doped 70TeO2–30Zno glass network. As reported in Ref. [8], the authors showed that the large

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number component in Raman spectra varies from 730 cm1 in PbO–TeO2 to 790 cm1 in MgO–TeO2. They concluded that this band must include stretching vibrations involving oxygen and dopant metal atoms (PbO or MgO . . .). Hence, the large wavenumber component is related to stretching vibration in TeO3+1, TeO3 and MO groups. Here, this band shifts monotonically from 745 cm1 in the undoped glass to 764 cm1 in the highly doped glass. So that, this band may be assigned to stretching vibration in TeO3+1, TeO3 and Er2O3 groups. The Raman band at 450 cm1 is usually ascribed to bending vibrations of Te–O–Te linkages [12–14], which are formed by vertex sharing of TeO4, TeO3+1 and TeO3 polyhedra. The fact that the intensity of this band decreases with increased amount of doped erbium reveals that doping with erbium deforms the Te–O–Te linkages. According to that reported in the literature, a decrease in the 450 cm1 Raman band intensity suggests the destruction of Te–O–Te linkages. The decrease in intensity of this band in erbium-doped tellurium glasses indicates a cleavage of Te–O–Te linkage and a formation of NBO atoms which is consistent with the conversion of TeO4 tbps into TeO3 polyhedra having one NBO atoms. So, the addition of Er2O3 to TeO2-based glasses breaks Te–O–Te bonds, thus leading to a decrease in the Te coordination number. 4.2. Low-frequency Raman data According to that reported in Ref. [15] the glass network is assumed to be not continuous, but composed of domains or blobs, of size 1–2 nm. The bonding between atoms of nearest-neighbor blobs is certainly weaker than the bonding inside the blobs. Consequently the vibrational dynamics can be divided into two regimes: (i) the low-energy regime concerns the motion of blobs with respect to each other, and (ii) the high-energy regime that accounts for the vibrations in blobs. In the region of the Boson peak, the observed vibrational modes are localized in blobs or structural domains, named more cohesive domains, that are separated from each other by less cohesive interfacial zones [16]. The intensity of the Boson peak is related to the inhomogeneous cohesion; the more contrast in the cohesion on the nanoscale, the more intense is the boson peak [17]. Rare-earth ions are assumed to be incorporated at non-bridging anion bond sites (interfacial zones), which dominate the rareearth environment. The presence of non-bridging anion atoms influences the energy and the amplitude of the Boson band, as well as the solubility of rare-earth ions in glasses [6]. As shown in Fig. 2, the amplitude of the Boson band in the glass doped with 1 mol% Er2O3 is increased compared to that of undoped glass. This trend indicates that Er3+ ions modifies the glass nanostructure. This can be understood as due to an increase in the con-

trast of the rigid nanodomains relative to the softer interfacial zones [18] which can be weakened by the doping. Addition of rare-earth ions beyond 1 mol% Er2O3 results in the reduction in the amplitude of the Boson band. This reduction can be explained as originating from the clustering of rare-earth dopants due to the lack of suitable sites in the host glass [6]. In fact, this observation is expected to be occurring at relatively high concentrations of rare-earth ions. For doping levels larger than 1 mol% Er2O3, the erbium ions located between cohesive domains, induces an hydrostatic pressure in the glass network – like the effect of an applied hydrostatic pressure [19]. The ion-induced internal compression decreases the boson peak intensity and shifts its peak energy to higher frequency as well. This is in agreement with the proposal of Tikhomirov et al. [6] that the concentration of rare-earth ions giving the most large Boson band amplitude corresponds to the limit of rare-earth ion solubility without clustering, i.e., 1 mol% Er2O3 in the case of erbium-doped tellurite glasses. The variation in the energy location of the Boson band reflects a structural modification induced by the Er2O3 incorporation into the glassy matrix. The decrease of the Boson band intensity is due to the decrease of contrast between cohesive domains and less cohesive interfacial zones due to the pressure induced by the dopant ions (the less cohesive interfacial zones are more deformed than the cohesive domains under pressure). On the other hand the shift to high frequency is presumably due to an increase in the elastic constant of the cohesive domains and less cohesive zones. According to Ref. [20], the unusually narrow band at 120–140 cm1 in the Raman spectra of lead silicate, lead borate and lead gallate glasses arises from optical modes of network-forming Pb atoms, namely to symmetric stretching vibrations of Pb–O either in square pyramids [21] or in trigonal pyramids [22] with Pb atoms in the apex. Accordingly, the Raman band at 120 cm1 in tellurite glasses can be attributed to symmetric stretching vibration of Te–O.

5. Conclusion A Raman spectroscopy study has been made on erbium-doped zinc tellurite glasses. The results obtained showed that Er3+ behaves like an alkali or an other metal cation in binary tellurite glasses, i.e., in converting TeO4 tbp units into primarily TeO3 tp units, and possibly some terminal TeO3+1 polyhedra. The quenching of the 450 cm1 band at relatively high concentrations of erbium has been explained as due to a structure disruption in the tellurite network and to a decrease in the Te coordination number as well. Variations in intensity and in the energy position of the Boson band were also re-

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ported from the Raman spectra of undoped and rareearth doped zinc tellurite glasses. The observations reveal that doping with rare-earth ions first induces an increase in the amplitude of the Boson peak and then leads to a quenching of this band for concentrations exceeding 1 mol% Er2O3. Results are interpreted in terms of a change in the contrast of the cohesive nanodomains relative to the softer interfacial zones. Proposal of explanations are consistent with the inhomogeneous nanostructure model.

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