Structural and spectroscopic studies on concentration dependent Er3+ doped boro-tellurite glasses

Structural and spectroscopic studies on concentration dependent Er3+ doped boro-tellurite glasses

Journal of Luminescence 132 (2012) 1171–1178 Contents lists available at SciVerse ScienceDirect Journal of Luminescence journal homepage: www.elsevi...

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Journal of Luminescence 132 (2012) 1171–1178

Contents lists available at SciVerse ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Structural and spectroscopic studies on concentration dependent Er3 þ doped boro-tellurite glasses K. Selvaraju, K. Marimuthu n Department of Physics, Gandhigram Rural University, Gandhigram 624 302, India

a r t i c l e i n f o

abstract

Article history: Received 3 August 2011 Received in revised form 6 October 2011 Accepted 21 December 2011 Available online 5 January 2012

Er3 þ doped boro-tellurite glasses have been prepared by the conventional melt quenching technique with the chemical composition (39  x) B2O3 þ 30TeO2 þ15MgO þ15K2O þxEr2O3 (where x ¼ 0.01, 0.1, 1, 2 and 3 wt%). The structural analysis of the glasses were made through XRD, FTIR spectral measurements and the optical absorption, luminescence measurements were made to analyze the optical behavior of the prepared glasses. The bonding parameters were determined from the optical absorption spectra and were found to be ionic in nature. The experimental oscillator strengths were determined from the absorption spectra have been used to determine the Judd–Ofelt parameters. The Judd  Ofelt parameters were used to explore the important radiative parameters such as transition probability (A), stimulated emission cross-section (sEP) and branching ratios (bR) of the emission transitions 2H9/2-4I15/2 and 2H11/2 and 4S3/2-4I15/2 of the trivalent erbium ions. The optical band gap energy (Eopt) values corresponding to the direct and indirect allowed transitions and the Urbach energy values of the prepared Er3 þ doped boro-tellurite glasses have been calculated and discussed with similar studies. The spectroscopic behavior of the Er3 þ boro-tellurite glasses have been studied by varying the trivalent erbium ion content and the results were discussed and compared with similar studies. & 2011 Elsevier B.V. All rights reserved.

Keywords: Glasses Annealing FTIR Luminescence Optical properties

1. Introduction The spectroscopic properties of the Er3 þ ions have been studied in various crystalline hosts and glass matrices to explore their suitability towards lasers in the near infrared region [1–4]. Trivalent erbium doped glass hosts like silicate [5], germanate [6], phosphate [7], sulphide [8] and tellurite [9] were studied and reported for their fiber amplifier applications. Nowadays special attention is paid to explore the optical behavior of the Er3 þ ions due to its emission at 1.53 mm, which is ideal for applications in the field of optical data transmission. The trivalent erbium ion is one of the most important and efficient ion [10,11], and its solubility could be attained upto 10 mol% [12]. When the concentration of the Er3 þ ion is increased, the optical gain increases upto a certain point [13] and beyond that critical concentrations of the rare earth ions tend to form a cluster in most of the solid hosts. The studies on the clustering of the rare earth ions are of great importance, because these cluster change the local environment and the optical properties of the host matrix. The rare earth ion cluster aggregates and quench the luminescence due to the

n

Corresponding author. Tel.: þ91 451 2452371; fax: þ 91 451 2454466. E-mail address: [email protected] (K. Marimuthu).

0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.12.056

increasing ion–ion interaction between rare earth ions and at last the host matrix becomes optically inactive [14]. Nowadays the trend has been turned to add two or more glass formers to form the glass materials for various scientific and technical applications. The pure borate glasses possess low refractive index, high melting point and high phonon energies in the order of 1300–1500 cm  1 [15]. Tellurite glasses possess high non-linear refractive index, low melting point and low phonon energy  750 cm  1 [13]. Several authors have recently studied and reported the properties of Er3 þ doped boro-tellurite glasses by varying the chemical composition. Xiang Shen et al. [16] studied the effect of B2O3 on the luminescence behavior of erbium doped tellurite glasses and Yang Yanmin et al. [17] reported the modification effect of B2O3 component on the optical behavior of Er3 þ doped tellurite glasses. Purushottam Joshi et al. [18] studied the utility of Er3 þ doped boro-tellurite glasses for the optical amplification in the 1530–1580 nm region. The present work reports, a comprehensive analysis of the structural and luminescence behavior of the Er3 þ doped borotellurite glasses by varying the concentration of the Er3 þ ion in the order 0.01, 0.1, 1, 2 and 3 wt%. The aim of the present study is to (i) synthesis Er3 þ doped boro-tellurite glasses (ii) explore the various functional groups of the prepared glasses (iii) examine the energy levels and bonding parameters (b and d) to claim covalent/ ionic nature of the prepared glasses (iv) evaluate the oscillator

K. Selvaraju, K. Marimuthu / Journal of Luminescence 132 (2012) 1171–1178

strengths and the JO parameters (Ol, l ¼2,4,6) and to compare the trends of Ol, with respect to other reported Er3 þ doped glasses (v) determine radiative properties for significant energy levels and to compare the results with similar Er3 þ systems and (vii) finally to determine the optical band gap energy (Eopt) and Urbach energy (DE) values to explore the optical behavior of the prepared glasses.

2. Experimental The trivalent erbium doped boro-tellurite glasses (xErBT) with the chemical composition 40B2O3 þ30TeO2 þ15MgOþ15K2Oþ xEr2O3 (where x¼0.01, 0.1, 1, 2 and 3 wt%) labeled as 0.01ErBT, 0.1ErBT, 1ErBT, 2ErBT and 3ErBT, respectively were prepared by following conventional melt quenching technique. The high purity analytical grade (99.99% purity) chemicals such as H3BO3, TeO2, Mg2CO3, K2CO3 and Er2O3 were used to prepare the glasses following the above said chemical composition in wt%. All the weighed chemicals (each batch composition of about 10 g) were thoroughly mixed and ground in an agate mortar to attain homogeneity and transferred to a porcelain crucible, melted in an electrical furnace in the temperature range 800–900 1C for 45 min. The melts were air quenched by pouring it onto a preheated brass plate kept in another furnace. The glasses were annealed at 300 1C for 7 h in order to remove the formation of air bubbles, thermal strains and to improve the mechanical strength. The prepared glasses were slowly allowed to reach room temperature and then polished on both sides before further optical measurements. The X-ray diffraction pattern of the prepared glasses were recorded using JEOL 8530 X-ray diffractometer employing Cuka radiation to confirm its amorphous nature. To identify the functional groups of the Er3 þ doped boro-tellurite glasses, the FTIR spectral measurements were carried out with a resolution of 74.0 cm  1 using Perkin–Elmer paragon 500 FTIR spectrophotometer in the wave number range 400–4000 cm  1 following KBr Pellet technique. Optical absorption measurements were recorded with a resolution of 70.1 nm using CARY 500 UV–vis spectrophotometer in the wavelength range 350–1100 nm. Luminescence measurements were recorded with a resolution of 71.0 nm using Perkin Elmer LS 55 spectrophotometer in the wavelength range 400–600 nm. All these measurements were carried out at room temperature. The refractive indices (n), were measured using Abbe refractometer at sodium wavelength (589.3 nm) by using mono bromonaphthalin as the contact liquid. The density of the prepared glasses were determined the following Archimedes principle by using xylene as an immersion liquid. The calculated physical properties of the Er3 þ doped boro-tellurite glasses were presented in Table 1.

3. Results and discussion 3.1. Structural analysis The X-ray diffraction pattern of the xErBT boro-tellurite glasses were recorded in the range 51r y r801 and as a representative case XRD pattern of the 0.01ErBT glass is shown in Fig. 1. It is observed from the figure that, the XRD pattern follows a broad diffused scattering at lower angles suggesting the existence of the long range structural disorder in the prepared glasses under investigation. This confirms the amorphous nature of the prepared boro-tellurite glasses. The FTIR spectroscopy is one of the essential tools to explore the fundamental and functional groups in crystalline and noncrystalline matrices [19]. The FTIR spectra of the Er3 þ doped boro-tellurite glasses were recorded between 400 and 4000 cm  1 against percentage of transmission and is shown in Fig. 2. The observed broad bands are due to the combination of higher degeneracy of vibrational states, thermal broadening of the lattice dispersion and mechanical scattering of the powdered samples and the corresponding band assignments are presented in Table 2. The fundamental groups such as OH bond, Hydrogen bond, and H–O–H bending have been observed in the higher region. The broad shoulder corresponds to the fundamental stretching of hydroxyl groups and hydrogen bonds are observed around 3421 cm  1 and 2852 cm  1, respectively. The H–O–H bending

800 Sample : 0.01ErBT

600

Count

1172

400

200

0 20

40 2θ ° →

60

80

Fig. 1. XRD pattern of the Er3 þ doped boro-tellurite glass.

Table 1 Physical properties of the Er3 þ doped boro-tellurite glasses. Sl. no 1 2 3 4 5 6 7 8 9 10

Physical properties 3

Density r (g/cm ) Refractive index nd (589.3 nm) Rare earth ion concentration N (1020 ions/cm3) Polaron radius rp (A1) Inter ionic distance ri (A1) Field strength F (1014 cm  2) Electronic polarizability ae (10  22 cm3) Molar refractivity Rm (cm3) Dielectric constant (e) Reflection losses R (%)

0.01ErBT

0.1ErBT

1ErBT

2ErBT

3ErBT

5.422 1.676 0.0595 22.23 55.17 0.099 150.92 4.755 2.809 6.381

4.827 1.685 0.5288 10.73 26.64 0.423 17.17 5.412 2.839 6.509

4.337 1.691 4.7925 5.15 12.78 1.837 1.91 6.011 2.859 6.594

4.352 1.734 9.0345 4.17 10.34 2.804 1.05 6.287 3.007 7.208

4.682 1.792 14.187 3.59 8.89 3.788 0.71 6.358 3.211 8.047

K. Selvaraju, K. Marimuthu / Journal of Luminescence 132 (2012) 1171–1178

3.2. Absorption spectra The optical absorption spectra of the trivalent erbium doped boro-tellurite glasses recorded at room temperature in the wavelength range 350–1100 nm are quite similar to each other and as a representative case the absorption spectrum of 1ErBT glass is shown in Fig. 3. The absorption spectrum consists of several inhomogeneously broadened bands due to the f–f interaction of the Er3 þ ions [23]. Eleven transitions such as 4I11/2, 4I9/2, 4F9/2, 4S3/2, 2 H11/2, 4F7/2, 4F5/2, 4F3/2, (2G,4F)9/2, 4G11/2 and 4G9/2 were observed in the absorption spectrum of the Er3 þ ion corresponding to the energy positions at 10237, 12516,15333, 18342, 19187, 20471, 22148, 22555, 24578, 26356 and 27369 cm  1, respectively [24–28]. Since the electronic absorption edge of the glass matrix rises rapidly and obscures the upper levels of Er3 þ ion, the absorption transitions if any, could not be observed below 350 nm. The position and intensity of certain transitions of

1ErBT

% Transmission (a.u)

0.01ErBT

0.1ErBT 2ErBT

lanthanide ions are found to be very sensitive to the environment around the ion, and such transitions are termed as hypersensitive transitions. The hypersensitive transitions obey the selection rule 9DS9¼ 0, 9DL9 r2 and 9DJ9r2 [29]. In the case of Er3 þ (4f11) ion, the 4I15/2-4G11/2 and 4I15/2-2H11/2 hypersensitive transitions are found to be more intense than the other transitions. Bonding properties of the prepared glasses can be investigated from the nephelauxatic ratio (b) given by [30].



nc na

ð1Þ

where nc the wave number (in cm  1) of a particular transition of an ion in the host matrix under investigation and na is the wave number (in cm  1) of the same transition in the aquo  ion [31]. From the average values of b (taken as b) the bonding parameter d can be calculated using the following expression [30,32].



1b

ð2Þ

b

The bonding will be ionic or covalent depending upon the positive or negative sign of d. The d value indicates the bonding between the Ln ions and their surrounding ligands. The bonding parameter (b, d) values of the prepared Er3 þ doped boro-tellurite glasses are presented in Table 3, and it is observed from the table that the Er3 þ –ligand bond is of ionic in nature. Further it is observed from Table 3 that, the ionic nature of the Er3 þ –ligand 4G 11/2

Absorption coefficient (cm−1)

vibrations were observed at 1744 cm  1 [20]. Due to the asymmetric stretching relaxation in trigonal BO3 groups, B–O  bond in isolated pyroborate group at 1434 cm  1 and (BO3)– group due to B–O stretching at 1352 cm  1 have been observed. The broad spectral peaks due to the B–O stretching vibrations of the tri, tetra and pentaborate in BO4 groups were observed at 1083 cm  1 [21]. Symmetrical stretching vibration of Te–O bond in trigonal bipyramids (TeO4) and Te–O bending vibrations in trigonal pyramids (TeO3) in the tellurium network were observed around 612 cm  1 and 686 cm  1 [22].

1173

2H

Sample:1ErBT 11/2

4G 9/2

2G 9/2 4F 7/2 4F 9/2

4F 5/2

3ErBT

4S 3/2

4I 9/2

4I 11/2

4F 3/2

1000

2000

3000

Wavenumber



Fig. 2. Infrared spectra of the Er

400

4000

600

800

1000

Wavelength (nm)

(cm-1)

Fig. 3. Absorption spectrum of the Er3 þ doped boro-tellurite glass.

doped boro-tellurite glasses.

Table 2 Peak table (in cm  1) of FTIR spectra of the Er3 þ doped boro-tellurite glasses. Sl. no

0.01ErBT

0.1ErBT

1ErBT

2ErBT

3ErBT

Assignments

1 2 3 4 5 6 7 8 9 10 11

3421 2923 2852 1744 1434 1352 1244 1083 717 686 612

3434 2924 2852 1744  1354 1241 1084 716 686 611

3432 2924 2852 1744  1382 1242 1082 716 688 610

3433 2924 2852 1744  1378 1220 1083 717 689 604

3434 2919 2852 1745 1460 1382 1217 1083 716 689 603

Fundamental stretching of OH groups Hydrogen bonding Hydrogen bonding H–O–H bending B–O  bond in isolated pyroborate groups B–O Stretching (BO3)  units Stable tetrahedral BO4 BO4 stretching in the Tri, tetra and pentaborate groups B–O–B bending vibrations Te–O bending vibrations in TeO3 Symmetric stretching vibrations of Te–O in TeO4

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K. Selvaraju, K. Marimuthu / Journal of Luminescence 132 (2012) 1171–1178

bond increases linearly with the increase in Er3 þ concentration in the prepared boro-tellurite glasses.

elements of the unit tensor operator of the rank l ¼2, 4 and 6, respectively. A least square fitting approximation method is followed to determine the JO intensity parameters, which gives the best fit between experimental and calculated oscillator strengths. The experimental and calculated oscillator strengths for various transitions along with the root mean square deviation (s) values for the prepared xErBT glasses are presented in Table 4. The rms deviation (s) of the xErBT glasses corresponding to 0.01ErBT, 0.1ErBT, 1ErBT, 2ErBT and 3ErBT glasses are found to be 7 0.53, 70.47, 70.38, 70.59 and 70.44, respectively. Some of the absorption band transitions corresponding to the Er3 þ ions are very sensitive to the environment around the RE ions and are known as hypersensitive transitions (HSTs), which follow the selection rules 9DS9¼0, 9DL9r2 and 9DJ9r2. The absorption bands of the 4I15/2-4G11/2 and 4I15/2-2H11/2 transitions are more intense due to its hypersensitive nature. It is observed from Table 4 that, the oscillator strengths of these HSTs are larger than other transitions. This indicates that the site symmetry around Er3 þ ion is lower (higher asymmetry) in the prepared xErBT boro-tellurite glasses. The oscillator strengths of the boro-tellurite glasses HSTs are larger than the reported soda lime silicate [41], PKBAEr [39], TeO2–LiF [29], TZN [18] glasses which indicates that the prepared glass materials possess more of lower site symmetry as reported in literature [40]. The JO intensity parameters (Ol, l ¼2, 4 and 6) derived following the least-squares fitting procedure for the Er3 þ doped boro-tellurite glasses are presented Table 5 along with the reported Er3 þ doped 44.5P2O5–25CaO–15BaO–15SrO–0.5Er2O3 [10], TeO2–14ZnO–10Na2O–1Er2O3 [25], LPG [26], TZLFEr [27], GPB2 [28], TeO2–LiF [29], PbO–PbF2–B2O3 [35], Phospho-tellurite (P1) [38], TZN [18] and soda lime silicate [41] glasses. It is interesting to observe from Table 5 that, the magnitude of JO parameters follows the trend as O2 4 O4 4 O6 for all the xErBT boro-tellurite glasses. It is also observed that, a maximum of 3.98  10  20 cm2 and minimum of 0.41  10  20 cm2 variation corresponding to O2 and O6 JO parameters, and a moderate variation of 0.70  10  20 cm2 corresponding to O4 parameter were observed from Table 5. This is due to the presence of two HSTs 4I15/2-4G11/2 and 4I15/2-2H11/2 in the absorption spectra of Er3 þ ions, which gives largest influence on O2, least influence on O6 and a moderate influence on O4 through 99Ul992 square reduced matrix elements. The ratio between O4/O6 known as spectroscopic quality factor is an important predictor to claim a good laser material among the prepared glasses. The O4/O6 value has been calculated for the present xErBT glasses and compared with the reported Er3 þ

3.3. Oscillator strengths and Judd–Ofelt analysis The experimental oscillator strengths (fexp) of the f–f induced electric dipole transitions of each band of the absorption spectra of the Er3 þ doped boro-tellurite glasses have been calculated using the following expression and used in the frame work of Judd–Ofelt (JO) theory [33,36,37]. Z Z 2:303mc2 f exp ¼ eðnÞdn ¼ 4:318  109 eðnÞdn ð3Þ 2 Npe where m and e are the mass and charge of electron respectively, c is the velocity of light, N is the Avogadro’s number, e(n) is the molar absorptivity and n is the transition energy (in cm–1). The calculated oscillator strength (fcal) of an electric dipole transition from the ground state (CJ) to an excited state (C0 J0 ) for the Er3 þ ions within the 4f configuration have been calculated using the following equation #  " 2 X 8p2 mcn ðn þ 2Þ2 O ðCJ99U l 99C0 J0 Þ2 ð4Þ f cal ¼  l ¼ 2,4,6 l 3hð2J þ 1Þ 9n where n is the refractive index, J is the angular momentum of the ground state, Ol (l ¼2, 4 and 6) are the Judd–Ofelt intensity parameters, which are used to characterize the metal–ligand band in the host matrix and 99Ul992 are the square reduced matrix

Table 3 Observed band positions (cm  1) and bonding parameters (b and d) of the Er3 þ doped boro-tellurite glasses. Energy level 4

I15/2-4I11/2 I15/2-4I9/2 4 I15/2-4F9/2 4 I15/2-4S3/2 4 I15/2-2H11/2 4 I15/2-4F7/2 4 I15/2-4F5/2 4 I15/2-4F3/2 4 I15/2-(2G,4F)9/2 4 I15/2-4G11/2 4 I15/2-4G9/2 4

b d

0.01ErBT 0.1ErBT

1ErBT

2ErBT

3ErBT

Aquo [25]

 12509  18334 19173 20384 22123 22554 24576 26342  1.00169 –0.102

10237 12516 15333 18342 19187 20471 22148 22555 24578 26356 27369 1.00173 –0.173

10243 12518 15336 18352 19190 20475 22154 22558 24580 26443  1.00258 –0.256

10252 12516 15342 18358 19198 20476 22162 22564 24586 26445  1.00292 –0.285

10250 12400 15250 18350 19150 20450 22100 22500 24550 26400 27400

 12513 15325 18344 19182 20380 22138 22554 24576 26348  1.00167 –0.168

Table 4 Experimental and calculated oscillator strengths (10  6) of the Er3 þ doped boro-tellurite glasses. Energy level

0.01ErBT

4

fexp

fcal

fexp

fcal

fexp

fcal

fexp

fcal

fexp

fcal

– 1.071 – 2.560 8.644 2.395 1.357 0.849 0.717 13.146 – 8 70.53

– 0.839 – 1.742 7.992 3.204 0.904 0.525 0.947 12.949 – 9 7 0.47

– 0.699 2.877 0.729 11.356 3.327 0.987 0.349 0.817 15.626 – 11 70.38

– 0.469 3.036 0.772 10.543 3.047 0.941 0.546 1.151 14.624 – 10 7 0.59

1.034 0.566 3.294 0.701 14.383 2.795 0.823 0.249 1.417 21.550 2.303 10 7 0.44

0.938 0.565 3.287 0.704 13.398 2.979 0.878 0.498 1.079 20.874 2.244

1.103 0.659 3.505 0.829 12.679 2.864 0.663 0.351 0.913 18.920 –

0.895 0.615 3.477 0.707 11.437 3.056 0.861 0.499 1.092 20.264 –

1.134 0.578 3.317 0.739 10.840 2.592 0.720 0.327 0.886 16.667 –

0.849 0.561 3.245 0.688 9.957 2.922 0.838 0.486 1.055 17.633 –

I15/2-

4

I11/2 I9/2 4 F9/2 4 S3/2 2 H11/2 4 F7/2 4 F5/2 4 F3/2 (2G,4F)9/2 4 G11/2 2 G9/2 N 4

s

0.1ErBT

1ErBT

2ErBT

3ErBT

K. Selvaraju, K. Marimuthu / Journal of Luminescence 132 (2012) 1171–1178

1175

Table 5 Judd–Ofelt parameters (Ol, 10  20 cm2) and Spectroscopic quality factor (O4/O6) of the Er3 þ doped boro-tellurite glasses and some reported Er3 þ doped glasses. Glass

O2

O4

O6

O4/O6

Reference

0.01ErBT 0.1ErBT 1ErBT 2ErBT 3ErBT 44.5P2O5–25CaO–15BaO–15SrO–0.5Er2O3 TeO2–14ZnO–10Na2O–1Er2O3 LPG TZLFEr GPB2 Teo2–LiF PbO–PbF2–B2O3 Phospho-tellurite (P1) TZN Soda lime silicate glass

3.74 5.99 7.72 6.02 4.92 5.00 5.98 5.78 6.07 5.95 1.187 1.20 3.97 6.20 2.72

2.74 2.57 2.88 2.51 2.18 1.50 1.32 0.97 2.48 1.70 1.240 0.59 1.27 1.30 2.31

1.86 1.62 1.45 1.64 1.48 1.07 1.47 0.79 0.72 1.50 1.962 0.42 0.41 1.10 1.28

1.47 1.58 1.98 1.53 1.47 1.40 0.89 1.23 3.44 1.09 0.63 1.40 3.09 1.18 1.80

Present Present Present Present Present [10] [25] [26] [27] [28] [29] [35] [38] [18] [41]

λem= 550 nm Sample: 3ErBT

4G 11/2

4S

4G 9/2

9/2

4I 15/2 2H

11/2

2G 9/2

4I 15/2

4I 15/2

intensity (a.u)

Intensity (a.u)

2H

3/2

work work work work work

1ErBT

4F 7/2

3ErBT

4F 5/2

2G 7/2

2ErBT

4F 3/2

0.1ErBT 0.01ErBT

400

350

400

450

500

Wavelength (nm)

450

500

550

600

Wavelength (nm) Fig. 5. Luminescence spectra of the Er3 þ doped boro-tellurite glasses.

Fig. 4. Excitation spectrum of the Er3 þ doped boro-tellurite glass.

glasses and presented in Table 5. Among the prepared glasses, the O4/O6 value for the 1ErBT glass possess a maximum value of 1.98, which is higher than the reported Er3 þ glasses excepting TZLFEr [27] and Phospho-tellurite [38] glasses. The calculated JO intensity parameters have been used to predict the radiative properties like radiative transition probability (A), stimulated emission cross-section (sEP), radiative lifetime (tR) and branching ratios (bR) for the 2H9/2-4I15/2 and 2H11/2 and 4S3/2-4I15/2 transitions of the Er3 þ doped boro-tellurite glasses. 3.4. Luminescence spectra and radiative properties Excitation spectrum for the Er3 þ doped boro-tellurite glasses were recorded at room temperature in the wavelength range 350–550 nm and were quite similar to other reported Er3 þ glasses [10,39]. Since all the spectra are alike as a representative case, the excitation spectrum of the 3ErBT glass is shown in Fig. 4. Seven excitation transitions such as 2G7/2, 4G9/2, 4G11/2, 2G9/2, 4F3/2, 4 F5/2 and 4F7/2 with the corresponding band positions at 357, 365, 378, 407, 443, 451 and 488 nm were observed. Among the observed bands, the 4I15/2-4G11/2 transition positioned at 378 nm is comparatively more intense than the other transitions. This

excitation spectrum provides different possibilities of excitation to get green luminescence from the prepared xErBT boro-tellurite glasses. The 4G11/2 transition positioned at 378 nm have been used as the excitation wavelength to measure luminescence spectra in the wavelength range 400–600 nm for the prepared xErBT borotellurite glasses at room temperature and the same is shown in Fig. 5. The luminescence spectra recorded at room temperature under 378 nm excitation exhibits bright green emission. The visible emission spectra of the xErBT glasses exhibit three emission transitions such as 2H9/2-4I15/2, 2H11/2-4I15/2 and 4 S3/2-4I15/2 observed at 410 (blue), 530 (green) and 554 (green) nm, respectively. It is observed from the luminescence spectra that among the observed three emissions transitions, the 4 S3/2-4I15/2 transition located at 554 nm is more intense and is suitable for green laser applications. It is observed from the Fig. 5 that, the intensity of the luminescence spectra increases linearly with the dopant concentration of the Er3 þ ions upto 1 wt% and after that, it gradually decreases due to energy transfer process between Er3 þ ions. The JO theory has been used to derive the radiative properties such as radiative transition probability (A), stimulated emission cross-section (sEP) branching ratios (bR) and radiative lifetime (tR)

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of the prepared glasses using the following expressions reported in literature [42,43]. The electric (Aed) and magnetic (Amd) dipole radiative transition probabilities are calculated from the below given expressions #  " 64p4 n3 nðn2 þ 2Þ2 Sed Aed ¼ ð5Þ 3hð2J þ 1Þ 9 Amd ¼



 64p4 n3 n3 Smd 3hð2J þ1Þ

ð6Þ

The sum of Aed and Amd gives the radiative transition probability (A) for a particular transition CJ-C0 J0 as AðCJ, C0 J0 Þ ¼ Aed þ Amd

ð7Þ

The total radiative transition probability (AT) of an excited state is the sum of the A(CJ,C0 J0 ) terms calculated over all the terminal levels and is given by AT ðCJÞ ¼ SAðCJ, C0 J 0 Þ

ð8Þ

The predicted radiative life time (tR) of an excited state in terms of AT, the total radiative transition probability of an excited state is given by

tR ðCJÞ ¼ ½AT ðCJÞ1

ð9Þ

and it gives the rate of depopulation of an excited state. The radiative branching ratios (bR) can be obtained from the below given equation

bR ðCJ, C0 J0 Þ ¼

AðCJ, C0 J 0 Þ AT ðCJÞ

ð10Þ

The branching ratio values are used to predict the relative intensity of all the emission lines originates from a given excited state, and the experimental branching ratios are determined from the relative area of the emission lines. The other important radiative property, the stimulated emission cross-section sEP (CJ, C0 J0 ) is an essential property which predicts the laser performance of the prepared glasses and is related in terms of the radiative transition probability (A) of a transition as " # 4

sEP ðCJ, C0 J0 Þ ¼

lp

8pcn2 Dlef f

AðCJ, C0 J0 Þ

ð11Þ

Where lp is the emission transition peak wavelength, and Dleff is the effective line width found by dividing the area of the emission band by its average height.

Among the prepared xErBT boro-tellurite glasses, 1ErBT glass exhibit higher stimulated emission cross-section corresponding to 2H11/2 and 4S3/2-4I15/2 transition. The stimulated emission cross-section (sEP) obtained for the 2H11/2 and 4S3/2-4I15/2 transition for the 1ErBT glass is 0.982, which is comparatively higher than the reported Er3 þ doped glasses viz, 0.48 for TeO2–LiF [29], 0.271 for TZLFEr [27] and 0.255 for soda lime silicate glasses [41]. The relative areas under the emission transition peaks are called as experimental branching ratios (bR) and are compared with the predicted values derived using JO theory and the same is presented in Table 6 for the xErBT boro-tellurite glasses. The branching ratio values for the 0.01ErBT, 0.1ErBT, 1ErBT, 2ErBT and 3ErBT glasses are found to be 0.640, 0.665, 0.733, 0.707 and 0.658, respectively for the 2H11/2 and 4S3/2-4I15/2 transition. It is observed from the spectra that, there is no significant changes in branching ratios (bR) due to change in glass composition. However, the branching ratio value for the 1ErBT boro-tellurite glass corresponding to 2H11/2 and 4S3/2-4I15/2 transition is relatively higher among the prepared five glasses. Among the prepared five boro-tellurite glasses, the stimulated emission cross-section and the branching ratio values for the 1ErBT borotellurite glass is found to be higher, and this glass may be suggested for suitable green laser applications with normal pumping. 3.5. Optical band gap energy (Eopt) and Urbach energy (DE) analysis The fundamental absorption edges of the absorption spectra were used to explore the optical transition and electronic band structure of the crystalline and non-crystalline materials. The Davis and Mott theory [44] has been used to evaluate the optical band gap energy of the amorphous materials through direct and indirect allowed transitions [34,45–48]. In both of the transitions, electromagnetic waves interact with the electrons in the valence band, which are raised across the fundamental gap to reach the conduction band. The absorption coefficient a(n) as a function of photon energy (hn) for the direct and indirect allowed transition can be expressed using the below given equation.

aðnÞ ¼

BðhnEopt Þn hn

ð12Þ

where a(n) is the absorption coefficient, B is the band tailing parameter, hn is the photon energy, n is the index number, which is used to decide the type of electronic transition causing the

Table 6 Emission band position (lp, nm), effective band width (Dleff, nm), radiative transition probability(A, s  1), stimulated emission cross-section (sEP  10  20 cm2), experimental, calculated branching ratios (bR) and radiative life time (tcal, ms) of the Er3 þ doped boro-tellurite glasses. Transition parameters 2

0.1ErBT

1ErBT

2ErBT

3ErBT

413 9.44 2039 0.499

412 8.54 2073 0.327

412 7.13 1953 0.366

412 5.22 1600 0.389

412 5.63 1742 0.368

0.237 0.409 200

0.235 0.391 263

0.277 0.352 180

0.293 0.402 251

0.342 0.379 210

551 6.57 1103 0.731

551 6.6 1257 0.842

558 8.44 1841 0.982

555 7.37 1434 0.817

555 6.72 1307 0.764

0.640 0.673 501

0.665 0.676 461

0.733 0.674 365

0.707 0.673 436

0.658 0.650 402

H9/2- I15/2

lp Dleff A

sEP bR(Exp) bR(Cal)

tcal 2

0.01ErBT

4

H11/2 and 4S3/2-4I15/2

lp Dleff A

sEP bR(Exp) bR(Cal)

tcal

Table 7 Fundamental absorption edge (ledge), Optical band gap (Eopt), Band Tailing parameter (B) and Urbach energy (DE) values corresponding to n ¼1/2 and 2 for the prepared Er3 þ doped boro-tellurite glasses. Sample code

0.01ErBT 0.1ErBT 1ErBT 2ErBT 3ErBT

ledge

n ¼2

n ¼1/2

DE(eV)

(nm)

373 377 381 382 385

Eopt (eV)

B (cm eV)  1/ Eopt 2 (eV)

B (cm  2 eV)

3.038 2.950 2.893 2.827 2.616

1.122 1.256 1.385 1.414 1.254

8.575 10.478 14.028 11.175 9.285

3.079 3.043 3.016 2.877 2.669

0.548 0.569 0.574 0.583 0.590

(αhν)1/2 (cm)-1/2 (eV)1/2

0.01ErBT 01ErBT 1ErBT 2ErBT 3ErBT

1.8

2.0

2.2

1177

3.1

0.60

3.0

0.59

2.9

0.58 0.57

2.8

0.56

Band tail (eV)

absorption and Eopt is the optical band gap energy. The tauc’s plot can be drawn between (ahn)1/n and photon energy (hn), by substituting the value n¼1/2 in Eq. (12) for direct allowed transitions and n ¼2 for indirect allowed transitions. The optical band gap values (Eopt) were obtained from the linear portion of the curves extrapolating at (ahn)2 ¼0 and (ahn)1/2 ¼0 for direct and indirect transitions, respectively. The calculated optical band gap (Eopt) and the band tailing parameter (B) values of the prepared Er3 þ doped boro-tellurite glasses are presented in Table 7. The dependence of (ahn)1/2 on the photon energy (hn) corresponding to the indirect allowed transition of the prepared boro-tellurite glasses are presented in Fig. 6. When the Er2O3 content was increased systematically in the boro-tellurite host matrix, it is observed that the fundamental cut off wavelength slightly shift towards the longer wavelength region [45] and it may be due to the structural rearrangement in the glass network of the prepared Er3 þ doped boro-tellurite glasses. The optical band gap values are found to be in the range 2.616–3.038 eV and are found to be higher than the reported Er3 þ doped borate (1.70–1.36 eV) [46], phosphate (1.78–1.66 eV) [47] and antimony–borosilicate glasses (2.65–2.52 eV) [48] and is lower than the germinate–borate glasses (5.5–4.1 eV) [34]. This suggests that the non-bridging oxygen ion content increases with increasing Er2O3 content and there by shifts the band edge to lower energies and in-turn lead to have a fall in the Eopt values.

Optical band gap (Indirect) energy (eV)

K. Selvaraju, K. Marimuthu / Journal of Luminescence 132 (2012) 1171–1178

2.7 0.55 2.6 0.54 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Er2O3 content Fig. 7. Optical band gap energy Eopt (Empty square) and Urbach energy DE (Solid circles) as a function of Er2O3 content for the Er3 þ boro-tellurite glasses.

The absorption coefficient a(n) near the absorption band edge exhibit an exponential behavior on the photon energy (hn) and obeys the empirical relation given by Urbach [49]   hn ð15Þ aðnÞ ¼ a0 exp DE where a0 is a constant and DE is the Urbach energy. This exponential behavior is due to the band tails associated with the valence and conduction bands which extend in to the band gap. The magnitude of the Urbach energy (DE)r the width of localized states can be used to characterize the degree of disorderness in the amorphous/crystalline materials. Materials with larger Urbach energy would have greater tendency to convert weak bonds into defects. Urbach energy values of the prepared Er3 þ doped boro-tellurite glasses are found to be in the range 0.548–0.590 eV. The optical band gap energy (Eopt) and Urbach energy as a function of Er2O3 content were shown graphically in Fig. 7. While increasing the Er2O3 content in the host matrix, the non-bridging oxygen ion content increases there by shifting the band edge to lower energies, which in-turn decreases the optical band gap energy values and increases the Urbach energy values.

4. Conclusion

2.4

2.6

2.8

3.0

3.2

hν (eV) Fig. 6. Dependence of (ahn)1/2 on photon energy (hn) for the Er3 þ doped borotellurite glasses.

Er3 þ doped boro-tellurite glasses were prepared and their structural and spectroscopic behavior has been studied and reported. The XRD pattern confirms the amorphous nature and the FTIR spectral studies confirms the presence of functional groups like, the B–O  bond in isolated pyroborate group, (BO3)  group due to B–O stretching, B–O stretching vibration of the tri, tetra and pentaborate groups in BO4 units, the bending of B–O–B linkage vibrations, Te–O bond in trigonal bipyramids (TeO4) and the Te–O bending vibrations of the trigonal pyramids (TeO3) in the prepared boro-tellurite glasses. The absorption spectral studies reveal the ionic nature of the prepared Er3 þ doped boro-tellurite glasses. The higher oscillator strengths of the hypersensitive transitions 4I15/2-4G11/2 and 4 I15/2-2H11/2 indicate that the site symmetry around Er3 þ ion is lower in the prepared glasses. The magnitude of the JO parameters follow the trend as O2 4 O4 4 O6 for all the glasses and the higher O2 value further confirm the lower site symmetry around Er3 þ ions. The luminescence spectral studies reveals that the peak intensity gradually increases up to 1 wt% Er2O3 doped boro-tellurite glasses and after that decreases and it may be due

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K. Selvaraju, K. Marimuthu / Journal of Luminescence 132 (2012) 1171–1178

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