Structure and optical properties of chalcohalide glasses doped with Pr3+ and Yb3+ ions

Journal of Non-Crystalline Solids 355 (2009) 1865–1868

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Structure and optical properties of chalcohalide glasses doped with Pr3+ and Yb3+ ions B. Frumarova a,*, M. Frumar b, J. Oswald c, M. Kincl a, M. Vlcek a a

Joint Laboratory of Solid State Chemistry of Institute of Macromolecular Chemistry of the ASCR, v.v.i., University of Pardubice, Czech Republic Research Center and Department of Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, 53210 Pardubice, Czech Republic c Institute of Physics of the ASCR, v.v.i., Prague, Czech Republic b

a r t i c l e

i n f o

Article history: Available online 23 July 2009 PACS: 78.55.Qr 81.05.Kf 78.30.Hv Keywords: Chalcohalides Luminescence Raman spectroscopy

a b s t r a c t Glasses of systems 100-y((GeS2)80(Sb2S3)20x(PbI2)x)yPr2S3, x = 0; 2; 5, 8; y = 0; 0.01; 0.1; 0.5 and 99.9-z((GeS2)80(Sb2S3)18(PbI2)2)0.1Pr2S3zYb2S3, z = 0.05; 0.1; 0.15) were synthesized in high purity. Optically well transparent glasses were obtained for x 6 5 mol.% PbI2, for y 6 0.1 mol.% Pr2S3 and for z 6 0.15 mol.% Yb2S3. The glasses were stable and homogeneous, as confirmed by X-ray diffraction and electron microscopy, with high optical transmittivity from visible (red) region up to infrared region (900 cm1). The density of the glasses was 3.26–3.33 gcm3 for PbI2 containing glasses. The glass transition temperature, Tg, was 320–336 °C. The optical absorption bands in rare-earth doped glasses corresponded to 3H4–3F4, 3H4–3F3, 3H4–(3F2 + 3H6) f–f electron transitions of Pr3+ ions and to 2F7/2–2F5/2 f–f electron transitions of Yb3+ ions. Strong luminescence band with maximum near 1340 nm (electron transition 1G4–3H5) was found in Pr2S3 doped glasses. The intensity of this band was rising with doping by Yb3+ ions. The possible mechanism of the luminescence enhancement is suggested. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Rare earth (RE) doped chalcogenide glasses are promising materials for active optical devices in near- and mid-infrared spectral regions (see, e.g. [1–4]). The solubility of RE ions in glassy matrix is often small and many RE doped chalcogenide glasses are unstable. The addition of heavy metal halides to chalcogenide glasses could increase the solubility of RE ions in chalcogenide glassy matrix. Doping by halides can also increase the index of refraction and influence the non-radiative electron transitions. Optimization of glass composition and of its microstructure, especially in the first coordination sphere of RE ions, can increase the quantum efficiency of photoluminescence, which is important for possible applications. In our previous work [5], the near- and mid-infrared Pr3+ luminescence in new stable and homogeneous GeS2–Sb2S3–PbI2(PbCl2) glasses doped with Pr3+ ions was observed. This result made the Pr doped GeS2–Sb2S3–PbI2(PbCl2) glasses potential candidate for near and mid-IR applications. In this paper the structure and the luminescence properties of the GeS2–Sb2S3–PbI2 glasses doped with Pr3+ and Yb3+ ions were studied. Co-doping with Yb3+ ions was used as a possible method for increasing the intensity of near-IR luminescence of Pr3+ doped

* Corresponding author. Tel.: +420 46 603 6148; fax: +420 46 603 6011. E-mail address: [email protected] (B. Frumarova). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.05.063

non-chalcogenide glasses (see, e.g. [6]). Presently, the Pr3+/Yb3+ co-doping was applied also for GeS2–Sb2S3–PbI2 glasses. 2. Experimental part All glasses were synthesized from high purity elements (Ge, Sb and S) all of 5 N-purity, Pr (99.9%), Yb (99.9%), PbI2 (99.999%) in sealed and evacuated (p ffi 104 Pa) silica ampoules in a rocking furnace (970 °C, 20 h). After the synthesis, the ampoules with the melt were quenched in under cooled mixture of water with ice and then annealed at temperatures near glass transition. Thermal analysis was performed in the range of 20–900 °C with the heating rate 10 K/min. The density (d) of the samples was measured by a hydrostatic method using toluene as an immersion medium. The transmission spectra of cut and polished plan-parallel plates of prepared glasses were measured using spectrophotometer JASCO V-570 (VIS, NIR) and FT spectrophotometer BIORAD FTS 175C (NIR, MID). The room temperature Raman spectra were measured by FT-IR spectrophotometer IFS 55 provided with Raman FRA-106 accessory (Bruker) for back scattering method. The YAG:Nd3+ laser line (1064 nm) was used for excitation. For correction of temperature dependent population of phonon levels, the intensities of the bands in Raman spectra were reduced using Gammon-Shuker formula [7].

B. Frumarova et al. / Journal of Non-Crystalline Solids 355 (2009) 1865–1868

The luminescence spectra were measured in a spectral region between 880 and 1500 nm at room temperature. The YAG:Nd3+ laser (1064 nm) and laser diode (980 nm) were used for luminescence excitation. 3. Results

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Ground State Absorption Cross Section [cm2]

Glasses of systems 100-y((GeS2)80(Sb2S3)20x(PbI2)x)yPr2S3, x = 0; 2; 5, 8; y = 0; 0.01; 0.1; 0.5 and 99.9-z((GeS2)80(Sb2S3)18 (PbI2)2)0.1Pr2S3zYb2S3, z = 0.05; 0.1; 0.15) were prepared in high purity. For the simplicity, the prepared glasses are labelled GSbS for x = 0, and GSbSxPbI for x – 0, respectively. Homogenous and well transparent glasses were obtained for x 6 5 mol.% PbI2, for y 6 0.1 mol.% Pr2S3 and for z < 0.15 mol.% Yb2S3. The homogeneity of glasses and absence of any observable crystalline phase was confirmed by optical and electron microscopy and by X-ray diffraction. The densities of the GSbS glasses were near 3.24 gcm3 and were not practically influenced by Pr2S3 content. Addition of PbI2 increased the density values to 3.26 gcm3 for 2 mol.% PbI2 up to 3.33 gcm3 for 5 mol.% PbI2. The GSbSxPbI glass density was slightly increased by addition of Pr2S3. The glass transition temperatures, Tg, were found near 336 °C for glasses 100-y(GSbS)yPr2S3 and between 320 and 323 °C for glasses containing PbI2. Glasses doped with 0.01 mol.% Pr2S3 did not show any crystallisation peak for temperatures up to 900 °C (heating rate of 10 °C/min). The prepared glasses are transparent in a broad spectral region from visible to IR one. The short-wavelength absorption edge lies between 550 and 700 nm and is shifted toward longer wavelengths by addition of PbI2 (Fig. 1a). The additions of Pr2S3 did not influence position of this edge, but the addition of Yb2S3 shifted the longwavelength part of absorption edge in more complex way (Fig. 1b). The reason is under study. The long-wavelength absorption edge of studied glasses is located near 900 cm1 due to first harmonic Ge-S vibrations. No absorption bands caused by stretching vibrations of impurity hydroxyl (O-H) groups were observed in optical infrared spectrum, only very weak bands of S-H stretching vibrations (2500 cm1) were found. Doping of the glasses by the Pr2S3 creates new absorption bands near 4760, 2020, 1570 and 1475 nm, which can be assigned to the f–f electron transitions from the 3H4 level of Pr3+ to the 3H5, 3H6, 3 F2, 3F3 and 3F4 levels. Electron transitions in the PbI2 modified glasses were shifted to longer wavelengths, caused probably by

the nephelauxetic effect. Absorption band near 1020 nm corresponding to 3H4 ? 1G4 transition, which is important for excitation of 1340 nm luminescence, is very weak (Fig. 2), because this transition is spin forbidden and Pr3+ concentration in glasses is small. Low absorption of Pr3+ ions in this spectral region is the reason, why co-doping with Yb3+ was used. The strong optical absorption band between 900 and 1050 nm in Yb3+ co-doped glasses (Fig. 2) corresponds to 2F7/2 ? 2F5/2 f–f electron transition of Yb3+ ions. The 2F7/2 ? 2F5/2 transition of Yb3+ has a large absorption cross-section of 3.68  1020 cm2 for 980 nm in glass (GSbS2PbI)0.1 Pr2S30.1Yb2S3, whereas the 3H4 ? 1G4 transition of Pr3+ has a much smaller cross-section (8.17  1022 cm2 for 1020 nm in glass (GSbS2PbI)0.1Pr2S3). The absorption cross-section of Yb3+ at 980 nm is about 45 times larger than that of Pr3+ (Fig. 2). The Raman spectra of pure GSbS glass, the spectra of this glass doped with 0.01 mol% of Pr2S3, as well as the spectra of glasses containing PbI2 and doped with Pr, or co-doped with Yb are similar in the region of 260–480 cm1, where the main Raman band and its shoulders are located. The maximum of the main band is shifted a little to the higher wavenumbers in glasses containing PbI2 (Fig. 3a). The broad luminescence band near 1343 nm was observed in all Pr doped glasses (Fig. 4). It is assigned to the 1G4 ? 3H5 electron transitions between the discrete energy levels of Pr3+ ions.

Ground State Absorption Cross Section [cm2]

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Wavelenght [nm] Fig. 2. Ground state absorption cross-sections of (GSbS2PbI)0.1Pr2S3 and (GSbS2PbI)0.1Pr2S30.1Yb2S3 glasses.

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Fig. 1. Spectral dependence of absorption coefficient of GSbS, GSbS2PbI and GSbS5PbI glasses doped with 0.1 mol.% Pr2S3 (a) and of GSbS2PbI glasses co-doped with 0.01 mol.% Pr2S3 and 0; 0.05; 0.1; 0.15 mol.% Yb2S3 (b).

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B. Frumarova et al. / Journal of Non-Crystalline Solids 355 (2009) 1865–1868

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Fig. 3. Reduced Raman spectra of undoped GeSbS glass and GSbS, GSbS5PbI glasses doped with 0.01 mol.% Pr2S3 and GSbS5PbI co-doped with 0.1 mol.% Pr2S3 and 0.5 mol.% Yb2S3 (a); Stokes and anti-Stokes part of Raman spectra of (1) undoped GSbS2PbI glass, (2) doped glasses GSbS2PbI0.1Pr2S3 and (3) co-doped GSbS2PbI0.1Pr2S30.1Yb2S3. Inserted figure shows theoretical fit of emission bands (b); deconvoluted Raman spectra (c,d).

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Fig. 4. Luminescence spectra under excitation 1064 nm (a,b,c) and under excitation 980 nm (d).

4. Discussion The Raman spectra of pure and doped glasses are very similar (Fig. 3a) and their assignment is in accordance with those given

in papers [8,9]. The spectra of all glasses can be deconvoluted into several bands (see, e.g. Fig. 3c and d). The main (most intensive) Raman band near 337–340 cm1 can be assigned to A1 vibration mode of GeS4/2 tetrahedra. Its shoulder at 416–423 cm1 can be

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B. Frumarova et al. / Journal of Non-Crystalline Solids 355 (2009) 1865–1868

assigned to vibrations of two tetrahedra connected by S bridging atom at the corner [S3Ge-S-GeS3], the band with maximum near 373–376 cm1 can be assigned to the T2 mode of 2 edge-sharing tetrahedra [Ge2S4S2/2], the band at 390–393 cm1 to T2 mode of corner sharing GeS4/2 tetrahedra. The band near 296–302 cm1 can be assigned to SbS3 pyramids vibrations. This band is of lower amplitude than the main band of GeS4 symmetric stretching vibrations (337–340 cm1) because the content of SbS3 pyramids is relatively low in studied glasses. The glass matrix is then formed mainly by [GeS4/2], [S3Ge-S-GeS3], [SbS3/2] and [Ge2S4S2/2] structural units. When we compare the Raman spectra of undoped glasses with those of identical glasses doped with Pr2S3 or Yb2S3, the only difference is in the intensity of the broad band between 80 and 250 cm1. There is also a difference in the intensities of bands in anti-Stokes region of spectra (Fig. 3b). Broad band in the range 80–250 cm1 is nearly without any structure and could be formed by overlapping of several bands. In this region can be located bands corresponding to Ge–Ge vibrations (250 cm1, [10]), vibrations S8 (208 cm1), vibrations Sb–Sb (170 cm1 [11]), S–S vibrations (150 cm1 [10]) and bending vibrations of GeS4 tetrahedra (147 and 114 cm1 [12]). The presence of Ge–Ge and Sb–Sb structural units is however unprobable and, if they were present their densities should be very low, because the sulfur content in studied glasses is relatively high. The amplitude of the band at lower wavenumbers (80– 250 cm1) increases in glasses doped with RE elements. This is evidently caused by overlapping of this part of Raman spectrum with luminescence band of Pr3+ and Yb3+ ions, corresponding to 1 G4 ? 3H4 transitions with maximum near 1060 nm and to 2 F5/2 ? 2F7/2 transition with maxima near 990, 1000 and probably 1040 nm. Large part of these luminescence bands is situated in anti-Stokes part of the spectrum (Fig. 3b). Emission band in the luminescence spectra in the wavelengths ranging between 1225 and 1475 nm corresponds to 1G4 ? 3H5 electron transition of Pr3+ ions (Fig. 4). At the excitation 1064 nm (3H4 ? 1G4 transition), the intensity of this Pr3+ luminescence band (1342 nm) is higher in GSbSxPbI glasses than in GSbS glasses (Fig. 3a and b). The intensity of this band further increases with co-doping with Yb3+ ions. For sample containing 0.05 mol.% Yb2S3, this band is more than 1.5 times higher than those in sample without Yb co-doping. When the content of Yb2S3 is further increased, the luminescence intensity is decreasing. If the exciting light of 980 nm is used, the intensity of this band is steeply increasing with increasing content of Yb2S3. The question of sensitizing Pr3+ doped glasses with addition of Yb3+ is under exploration. There are two possibilities, either the direct transfer of excited electrons from 3F5/2 level of Yb3+ to 1G4 level of Pr3+, or the excitation of electrons of Pr to the 1G4 level by

absorption of emission light from 2F5/2 to 2F7/2 transition of Yb ions, since the 2F5/2 ? 2F7/2 emission band of Yb3+ overlaps the 3 H4 ? 1G4 absorption band of Pr3+. It can be supposed that the light of k = 1064 nm excites the electron from 3H4 to 1G4 level of Pr3+, while the light of k = 980 nm excites mainly the Yb3+ and the excitation is transferred from Yb3+ to Pr3+ as we mentioned above. 4. Conclusion Glasses of systems 100-y((GeS2)80(Sb2S3)20x(PbI2)x)yPr2S3, x = 0; 2; 5, 8; y = 0; 0.01; 0.1; 0.5 and 99.9-z((GeS2)80(Sb2S3)18(PbI2)2)0.1Pr2S3zYb2S3, z = 0.05; 0.1; 0.15) were synthesized in high purity and optically homogenous state. Their structure, as derived from the Raman spectra, is formed by SbS3 pyramids and GeS4 tetrahedra connected mostly via corner shared S atoms. The short-wavelength absorption edge is slightly shifted towards lover energies by addition of PbI2 and also by co-doping with Yb2S3. The co-doping with Yb3+ ions increases the optical absorption in the region between 900 and 1040 nm due to 2F7/2 ? 2F5/2 transition of Yb3+ ions. The strong luminescence of Pr3+ions (electron transition 1G4–3H5) is enhanced in Yb3+ co-doped glasses, probably by energy transfer from Yb3 to Pr3+ ions. The co-doped glasses can be applied in active optoelectronic elements. Acknowledgements The support of Grant Agency of Czech Rep., project 203/06/ 0627, of Academy of Sci. of the Czech Rep. projects AV0Z 40500505 and AV0Z 10100521 and of Ministry of Education of the Czech Republic, projects VZ002167501 and LC523, is highly acknowledged. References [1] J.S. Sanghera, L.B. Shaw, I.D. Aggarwal, C. R. Chim. 5 (2002) 873. [2] T. Schweizer, D.W. Hewak, B.N. Samson, D.N. Payne, J. Lumin. 72–74 (1997) 419. [3] B. Cole, L.B. Shaw, P.C. Pureza, R. Mossadegh, J.S. Sanghera, I.D. Aggarwal, NonCryst. Solids 256$257 (1999) 253. [4] S.R. Zakery, J. Elliott, Non-Cryst. Solids 330 (2003) 1. [5] J. Oswald, K. Kuldova, B. Frumarova, M. Frumar, Mater. Sci. Eng. B 146 (2008) 107. [6] Q.Y. Zhang, Z.M. Yang, G.F. Yang, Z.D. Deng, Z.H. Jiang, J. Phys. Chem. Solids 66 (2005) 1281. [7] R. Shuker, R.W. Gammon, Phys. Rev. Lett. 25 (1970) 222. [8] B. Frumarova, P. Nemec, M. Frumar, J. Oswald, M. Vlcek, J. Non-Cryst. Solids 256/257 (1999) 266–270. [9] L. Petit, N. Carlie, F. Adamietz, M. Couzi, V. Rodriguez, K.C. Richardson, Mater. Chem. Phys. 97 (2006) 64. [10] L. Koudelka, M. Frumar, M. Pisárcˇik, J. Non-Cryst. Solids 41 (1980) 171. [11] I. Watanabe, S. Noguchi, T. Shimizu, J. Non-Cryst. Solids 58 (1983) 35. [12] L. Koudelka, J. Horák, M. Pisárcˇik, Chem. Zvesti 35 (1981) 327.