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

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

Physica B 407 (2012) 1086–1093 Contents lists available at SciVerse ScienceDirect Physica B journal homepage: www.elsevier.com/locate/physb Structu...

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Physica B 407 (2012) 1086–1093

Contents lists available at SciVerse ScienceDirect

Physica B journal homepage: www.elsevier.com/locate/physb

Structural and spectroscopic studies on 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

a b s t r a c t

Article history: Received 23 October 2011 Received in revised form 29 November 2011 Accepted 3 January 2012 Available online 9 January 2012

Er3 þ doped boro-tellurite glasses with the chemical composition (69  x)B2O3  xTeO2  15MgO  15K2O  1Er2O3 (where x ¼ 0, 10, 20, 30 and 40 wt%) have been prepared and their structural and spectroscopic behavior were studied and reported. The varying tellurium dioxide content in the host matrix that results, changes in structural and spectroscopic behavior around Er3 þ ions are explored through XRD, FTIR, UV–VIS–NIR and luminescence measurements. The XRD pattern confirms the amorphous nature of the prepared glasses and the FTIR spectra explore the fundamental groups and the local structural units in the prepared boro-tellurite glasses. The bonding parameters (b and d) have been calculated from the observed band positions of the absorption spectra to claim the ionic/covalent nature of the prepared glasses. The Judd–Ofelt (JO) intensity parameters Ol (l ¼ 2, 4 and 6) were determined through experimental and calculated oscillator strengths obtained from the absorption spectra and their results are studied and compared with reported literature. The variation in the JO parameters Ol (l ¼ 2, 4 and 6) with the change in chemical composition have been discussed in detail. The JO parameters have also been used to derive the important radiative properties like transition probability (A), branching ratio (bR) and peak stimulated emission cross-section (sEP ) for the excited state transitions 2H9/2-4I15/2 and 2H11/2 and 4S3/2-4I15/2 of the Er3 þ ions and the results were studied and reported. Using Davis and Mott theory, optical band gap energy (Eopt) values for the direct and indirect allowed transitions have been calculated and discussed along with the Urbach energy values for the prepared Er3 þ doped boro-tellurite glasses in the present study. The optical properties of the prepared glasses with the change in tellurium dioxide have been studied and compared with similar results. & 2012 Elsevier B.V. All rights reserved.

Keywords: Glasses Annealing FTIR Luminescence Optical properties

1. Introduction Nowadays good amount of work is carried out on the study of luminescent properties of rare earth (RE) doped glass materials because of their potential use in the field of optical communications. Glass materials possess different applications based on their physical, electrical and optical properties [1–4]. Already variety of rare earth doped glass hosts such as silicates, borates, phosphates and tellurites have been prepared and reported due to their good glass forming ability and wide variety of applications. These glass matrices possess good transparency, hardness, resistance towards moisture, good chemical durability and extended transmission. Among them, tellurite glasses are the most promising and attractive materials for wide variety of laser applications [5,6]. Tellurite glasses possess good optical qualities like, high refractive index, low melting temperature, corrosion resistance, resistance towards moisture and good mechanical strength and chemical durability. Also the tellurite glasses possess lower phonon energy

n

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

0921-4526/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2012.01.003

(700 800 cm  1) and larger refractive index compared to other oxide glasses which are beneficial for high radiative transition rate of RE ions [7,8]. Tellurite glasses may also be used in the production of optical fibers and planar waveguides. In fact, pure TeO2 does not form a glass and it can be combined with other oxides to prepare the glasses and some of the work on Er3 þ doped tellurite based glasses were reported 75TeO2  20ZnO  5Na2O [9], 70TeO2  30ZnO [10] and 60TeO2  25WO3  15Na2O [11]. Similarly TeO2 with other glass matrices like silicates, borates, phosphates and germinate have also been studied and reported [12,13]. Borate based glasses are the most suitable one for the design of new optical devices due to their good RE ion solubility, easy preparation on large scale shaping and cost effective properties. But it is very difficult to release efficient infrared to visible up conversion emission in borate based glasses due to their high vibrational energy. Compared to SiO2, P2O5, GeO2 host matrices B2O3 is a better glass former compound and has lower price, higher stability and larger phonon energy. Boro-tellurite glass represents favorable compromise between the requirements of low phonon energy and a relatively high thermal stability, high chemical durability and ease of fabrication [13–15]. Optical

K. Selvaraju, K. Marimuthu / Physica B 407 (2012) 1086–1093

properties of the trivalent erbium ions in various glass matrices attracted due importance because of its 1.54 mm emission which can be utilized for the optical amplification and its visible up conversion emission generally used as a solid state laser. Considering these facts it is proposed to introduce TeO2 into B2O3 host matrix and their structural and spectroscopic properties were studied and presented in the present work. The Er3 þ doped boro-tellurite glasses have been prepared and their structural and spectroscopic properties were explored through the XRD, FTIR, absorption and luminescence measurements. The aim of the present study is to (i) synthesis Er3 þ doped boro-tellurite glasses through melt quenching technique (ii) explore the various functional groups in 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 strengths and the JO parameters (Ol, l ¼2,4,6) and to compare the trends of Ol, with respect to other Er3 þ doped glasses (v) determine radiative properties of the significant energy levels and to compare with similar results and finally to determine the optical band gap energy (Eopt) and Urbach energy (DE) values to analyze the optical behavior of the prepared Er3 þ glasses.

2. Experimental High purity analytical grade (Sigma Aldrich with 99.99% purity) chemicals such as H3BO3, TeO2, Mg2CO3, K2CO3 and Er2O3 have been used as the main constitutions to prepare the Er3 þ doped boro-tellurite glasses following the conventional melt quenching technique. The chemical composition follows as (69 x)B2O3 þxTeO2 þ15MgOþ15K2Oþ1Er2O3 (where x¼0, 10, 20, 30 and 40 wt%). Each batch composition of about 10 g were taken in an agate mortar and crushed thoroughly and the mixture was converted in to a porcelain crucible and melted at a temperature between 900 and 1000 1C for 45 min. in a muffle furnace. Simultaneously, the melting mixture was stirred for getting homogeneous mixture. The melt was then poured onto a preheated thick brass plate and annealed at 300 1C for about 7 h to avoid the formation of air bubbles, to remove strains and to enhance the mechanical strength of the glass samples and then allowed to reach room temperature gradually. The prepared glasses were polished on both sides to obtain planar surfaces before measuring their optical properties. The amorphous nature of the prepared Er3 þ doped borotellurite glasses were confirmed through the X-ray diffraction studies using JEOL 8030C X-ray diffractrometer using Cuka radiation. The functional groups and the local structural units were explored through Perkin  Elmer Peragon 500 FTIR spectrophotometer with a spectral resolution of 74 cm  1. The optical absorption measurements were made using Perkin–Elmer

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Lambda 35 UV–VIS spectrometer between the wavelength 340 1050 nm with a spectral resolution of 70.1 nm and the luminescence measurements of the Er3 þ doped boro-tellurite glasses were made using Perkin–Elmer LS55 spectrophotometer in the wavelength region 400 600 nm with a spectral resolution of 71.0 nm. All these measurements were carried out at room temperature. The density of the glasses were measured employing Archimedes principle and xylene was used as the immersion liquid. The refractive index of the prepared glass samples were measured using Abbe refractometer with mono bromonapthalene as the contact liquid at sodium wavelength. The measured physical properties of the prepared glasses are presented in table 1.

3. Results and discussion 3.1. Structural analysis The X-ray diffraction pattern of the Er3 þ doped boro-tellurite glasses exhibit broad scattering at lower angles, typical of long range structural disorder which confirms the amorphous nature of the prepared glasses. Since all the spectra were alike as a representative case one due to B1TEr is shown in Fig. 1. Fig. 2 shows the FTIR spectra of the Er3 þ doped boro-tellurite glasses and the corresponding transmission band positions along with the band assignment are presented in Table 2. The FTIR spectra contains several peaks which are broad or moderate in band width

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

Physical properties

1 2 3 4

Density r (g/cm3) Refractive index nd (589.3 nm) Rare earth ion concentration N (1020 ions/cm3) ˚ Polaron radius rp (A)

5

˚ Inter ionic distance ri (A) Field strength F (1014 cm-2) Electronic polarizability ae (10  22 cm3) Molar refractivity Rm (cm3) Dielectric constant (e) Reflection losses R (%)

6 7 8 9 10

B0TEr 3.192 1.548 4.604 5.22 12.95 1.789 1.648 6.924 2.396 4.626

B1TEr 3.598 1.628 4.646 5.20 12.91 1.799 1.825 7.078 2.650 5.710

B2TEr 4.582 1.663 5.355 4.96 12.31 1.978 1.653 5.556 2.766 6.198

B3TEr 4.158 1.691 4.438 5.28 13.11 1.745 2.059 6.491 2.859 6.594

B4TEr 4.566 1.738 4.236 5.37 13.32 1.692 2.269 6.358 3.021 7.265

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specifying its local structure [16]. The vibrational modes associated with the boro-tellurite glasses are divided into three IR spectral regions. The lower energy region near 700 cm–1 contains the vibration of the B O B linkages, the middle energy region 800 1200 cm–1 of the IR spectra gives information pertaining to BO bond stretching in BO4 groups and the higher region 1200 1600 cm  1 of the IR spectra exhibit asymmetric stretching vibration of the B O bond in BO3 groups. The tellurium network usually exhibit two vibrational modes, the first one around 600 640 cm–1 corresponds to TeO vibration in trigonal bipyramid (TeO4) groups and the later one around 680 700 cm–1 due to Te O vibration in trigonal pyramid (TeO3) groups. The bands observed around 3420 3434 cm–1 [17] is attributed to hydroxyl group and the peaks around 29242922 cm–1 and 2852 cm–1 indicates the presence of hydrogen bonding. The H OH bending is experienced at 1745 cm–1 [18]. The broad band identified around 1463 cm–1 and 1382 1387 cm–1 are due to B O  bond in isolated pyroborate groups and asymmetric stretching vibration due to B  O bond in BO3 groups respectively. The peak observed around 1234 1242 cm–1 is due to the stable tetrahedral BO4 groups [19]. The BO4 stretching in pyroborate groups have been observed around 1082 cm–1 and the intensive B O B bending linkage vibrations were observed around 719 cm–1[4]. The symmetric stretching vibrations due to Te O bond in TeO3 was attributed at 686 cm–1. It is observed from the spectra that the intensity of the B–O stretching vibrations in BO3 units and the B–O–B linkage vibrations gradually decreases and the intensity of the Te–O

vibrations in the TeO3 network increases when the borate content is gradually replaced by TeO2 content. 3.2. Absorption spectra The absorption spectrum of the B3TEr boro-tellurite glass recorded at room temperature in the UV VIS  NIR region is shown in Fig. 3 as a representative case and is similar to other reported Er3 þ doped glasses [10,20,21]. Eleven transitions such as 4 I11/2, 4I9/2, 4F9/2, 4S3/2, 2H11/2, 4F7/2, 4F5/2, 4F3/2, 2G 9/2, 4G11/2 and 4 G9/2 were observed from the absorption spectrum corresponding to the band positions at 10,237, 12,516, 15,333, 18,342, 19,187, 20,471, 22,148, 22,555, 24,578, 26,356, and 27,369 cm–1. The inhomogeneously broadened bands are assigned in accordance with the transition occurred from the 4I15/2 ground state to the various excited states due to the f–f interaction of Er3 þ ion [22]. All these transitions are found to occur at the same place irrespective of the change in the chemical composition but there is only slight variation in the intensities of the bands due to the change in the chemical composition. The 4I15/2-4I13/2 transition centered around 6578 cm–1 could not be identified due to lack of instrument facility. The 4I15/2-2H11/2 transition observed at 19,187 cm  1 possess high intensity over all the other transitions and the 4I15/2-4G11/2 transition at 26,356 cm–1 follow the selection rules DL¼2 and DJ ¼2 [23]. These transitions are called as hypersensitive transition [24]. To identify the nature of the metal ligand bond the nephelauxetic ratios and bonding parameters have been calculated from the absorption band positions using the following expressions [19,25,26]. 4

G11/2

B0TEr

B3TEr

% Transmission (a.u)

B2TEr

B3TEr B4TEr

Absorption Coefficient (cm−1)

2

B1TEr

H11/2

4

G9/2

2

G9/2 4

4

4

4

F7/2

F5/2

F9/2 4

I9/2

4

S3/2

4

I11/2

F3/2

1000

2000

Wavenumber

3000

4000

400

600

(cm−1)

800

1000

Wavelength (nm)

Fig. 2. Infrared spectra of the Er3 þ doped boro-tellurite glasses.

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

Table 2 Functional vibrational groups (in cm  1) of the Er3 þ doped boro-tellurite glasses. Sl.No

B0TEr

B1TEr

B2TEr

B3TEr

B4TEr

Assignments

1 2 3 4 5 6 7 8 9 10

3432 2922 2852 1745 1463 1384 1234 1082 719 

3432 2924 2852 1744 1462 1387 1234 1083 718 

3434 2924 2853 1746 1460 1385 1238 1083 718 

3432 2924 2852 1744  1382 1242 1082 716 678

3420 2924 2852 1746  1382 1242 1082 717 686

Fundamental stretching of OH groups Hydrogen bonding Hydrogen bonding H  O H bending B–O  bond in isolated pyroborate groups B–O stretching (BO3)  groups Stable tetrahedral BO4 units BO4 stretching in pyroborate groups Bending of B  O  B linkage vibrations Te  O vibrations in the TeO3 network

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The nephelauxetic ratio (b) is

n b¼ c na

ð1Þ

where nc is the wavenumber (in cm  1) of a particular transition of an ion in the host matrix under selection and na is the wavenumber (in cm  1) of the same transition for the aquo  ion [26]. From the mean values of b (taken as b) the bonding parameter d can be calculated using the following expression [25]



1b

ð2Þ

b

The Er3 þ  ligand bonding may be covalent or ionic depending upon the positive or negative sign of d. The b and d values for the prepared glasses are presented in Table 3. It is observed from the table that the Er3 þ –ligand bond is of ionic in nature and the degree of ionic character changes with the change in the chemical composition. 3.3. Oscillator strengths and Judd  Ofelt analysis The experimental oscillator strengths (fexp) for the f–f induced electric dipole transitions of the each band have been determined by measuring the integrated area of each absorption transition using the following expression [27] Z Z 2:303mc2 f exp ¼ eðnÞdn ¼ 4:318  109 eðnÞdn ð3Þ 2 Npe where e(n) is the molar absorptivity of the corresponding transition (in cm  1). The calculated oscillator strengths have been determined using Judd Ofelt theory [28,29]. From the JO theory, the calculated oscillator strengths (fexp) of the electric dipole transition from the initial state (CJ) to the final state (C0 J0 ) have been calculated using the following expression. #  " 2 X 8p2 mcn ðn þ2Þ2  O ðCJ99U l 99C’ J ’ Þ2 ð4Þ f cal ¼ l ¼ 2,4,6 l 3hð2J þ 1Þ 9n

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oscillator strengths for the phospho-tellurite [12], TeO2  LiF [23], TZLFEr [30], TZN [31] and PKBAEr [32] glasses. The selection rules confirm that the 4G11/2 and 2H11/2 transitions are hypersensitive and the experimental oscillator strengths of the 4G11/2 transition is higher as reported in literature [12,31,32] which indicates that the prepared Er3 þ doped boro-tellurite glasses possess lower asymmetry. It is observed from the Table 5 that, the JO intensity parameters (O2, O4 and O6) follows the trend as O2 4 O4 4 O6. The O2 value obtained for the B3TEr glass is found to be higher among the prepared Er3 þ doped boro-tellurite glasses and is also found to be higher compared with the reported Er3 þ glasses [12,17,20,21,23,30,31,33]. Sardar et al. [35] reported that the O4/O6 intensity parameter have been used to predict the spectroscopic quality factor and the magnitude of the same has been characterized to claim good laser action among the prepared glasses. The prepared Er3 þ glasses possess the O4/O6 intensity parameter value as 1.19, 1.59, 1.95, 2.04 and 1.83 corresponding to B0TEr, B1TEr, B2TEr, B3TEr, and B4TEr glasses, respectively and these values are found to be on the higher side compared to the reported literature [17,20,21,23,31,33]. The Er3 þ doped borotellurite glasses spectroscopic quality factor follow the trend as B3TEr4B2TEr4B4TEr4B1TEr 4B0TEr.

3.4. Luminescence spectra and radiative properties The excitation spectrum recorded at room temperature for the B4TEr boro-tellurite glass is shown in Fig. 4 and the spectrum is Table 4 The experimental and calculated oscillated strengths (10  6) of the Er3 þ doped boro-tellurite glasses. Energy level 4 I15/2-

where n is the refractive index, c is the velocity of light, n is the energy of the transition in cm  1, h is Plank’s constant, J is the total angular momentum of the ground state, Ol (l ¼2, 4 and 6) are the Judd  Ofelt intensity parameters and 99Ul992is the doubly reduced matrix elements of the unit tensor operator which is evaluated from the intermediate coupling approximation for the initial state (CJ). The doubly reduced matrix elements for the Er3 þ ion in aquo solution have been taken from the reported literature [26]. A least-square fitting approximation method is adopted to determine the JO intensity parameters which gives the best fit between experimental and calculated oscillator strengths and their values are presented in table 4 along with the reported experimental

4

I13/2 I11/2 4 I9/2 4 F9/2 4 S3/2 4 H11/2 4 F7/2 4 F5/2 4 F3/2 (4G,4F)9/2 4 G11/2 4 G9/2 4

B3TEr

fexp

fcal

Phosphotellurite P1[12] fexp

– 0.684 0.565 2.523 0.699 15.856 1.873 0.594 0.313 0.732 22.173 1.362

– 0.689 0.461 2.457 0.437 13.78 2.003 0.533 0.309 0.694 24.38 1.728

0.86 0.54 0.23 1.44 0.39 8.41 1.47 – – 0.54 15.41 –

TeO2–LiF TZLFEr [23] [30]

TZN [31]

PKBAEr [32]

fexp

fexp

fexp

fexp

– – – 2.43 1.83 2.56 2.26 – – – 5.33 –

1.19 0.39 0.27 3.45 1.02 13.96 2.57 1.15 0.18 1.16 25.31 3.25 1

2.64 0.89 0.35 3.21 0.65 13.73 2.32 – – – – –

2.48 0.79 0.30 2.41 0.53 11.41 2.11 0.59 0.31 0.76 21.36 3.11

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

B0TEr

B1TEr

B2TEr

B3TEr

B4TEr

Aquo[26]

4

10,244 12,507 15,272 18,319 19,180 20,470 22,128 22,489 24,563 26,106 27,377 0.9999

10,236 12,507 15,350 18,342 19,180 20,460 22,143 22,569 24,563 26,219 27,377 1.0011

10,236 12,514 15,343 18,334 19,199 20,461 22,138 22,564 24,569 26,181 27,323 1.0016

10,237 12,516 15,333 18,342 19,187 20,471 22,148 22,555 24,578 26,356 27,369 1.0017

10,233 12,595 15,333 18,334 19,161 20,447 22,135 22,560 24,566 26,342 27,397 1.0019

10,250 12,400 15,250 18,350 19,150 20,450 22,100 22,500 24,550 26,400 27,400

0.0029

 0.1064

 0.1571

 0.1725

 0.1965

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

b d

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K. Selvaraju, K. Marimuthu / Physica B 407 (2012) 1086–1093

4

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

O2

O4

O6

O4/O6

Reference

B0TEr B1TEr B2TEr B3TEr B4TEr Phospho-tellurite (P1) Phosphate glass LPG Tellurite glass Teo2–LiF TZLFEr TZN Soda lime silicate

3.843 6.199 6.106 8.318 6.651 3.97 5 5.89 5.98 1.187 6.07 6.20 2.72

0.821 1.541 1.832 1.961 1.390 1.27 1.5 0.97 1.32 1.240 2.48 1.30 2.31

0.691 0.970 0.941 0.959 0.756 0.41 1.07 0.79 1.47 1.962 0.72 1.10 1.28

1.19 1.59 1.95 2.04 1.84 3.09 1.40 1.23 0.89 0.63 3.44 1.18 1.80

Present Present Present Present Present [12] [17] [20] [21] [23] [30] [31] [33]

S3/2

I15/2

H9/2

2

Intensity (a.u)

H11/2

400 4

G11/2

I15/2 B3TEr

4

2

work work work work work

4

450

4

500

I15/2

550

600

Wavelength (nm)

B4TEr λem=550 nm

Fig. 5. Luminescence spectrum of the Er3 þ doped boro-tellurite glass.

Intensity (a.u)

A particular transition of the radiative transition probability could be expressed using the following equation AðCJ, C’J’Þ ¼ Aed þ Amd

F7/2

4

G9/2

2

G9/2

4

F5/2

4

F3/2

2

G7/2

and Amd ¼

350

400

ð5Þ

where Aed and Amd are the electric and magnetic dipole radiative transition probabilities [27] where ! 64p4 n3 nðn2 þ 2Þ2 Sed  Aed ¼ ð6Þ 3 9 3hl ð2J þ 1Þ

4

450

500

Wavelength (nm) Fig. 4. The excitation spectrum of the Er3 þ doped boro-tellurite glass.

64p4 n3 3

3hl ð2J þ 1Þ

 ðn3 Smd Þ

ð7Þ

where Sed and Smd are the electric and magnetic dipole line strengths, respectively. The Sed and Smd are defined as  2 X Sed ¼ e2 O CJ:U l :C’ J’ ð8Þ l ¼ 2,4,6 l and

quiet similar to other reported Er3 þ glasses [30,34]. The excitation transitions such as 2G7/2, 4G9/2, 4G11/2, 2G9/2, 4F3/2, 4F5/2 and 4 F7/2 corresponding to the band positions at 357, 365, 378, 407, 443, 451 and 488 nm were observed from the spectrum. The 4G11/2 transition possess higher intensity compared to all other transitions. This transition have been used as the excitation wavelength (378 nm) to record the emission spectra for the Er3 þ doped borotellurite glasses and the emission spectrum of the B3TEr glass is shown in Fig. 5 as a representative case. Three transitions such as 2 H9/2-4I15/2, 2H11/2-4I15/2, and 4S3/2-4I15/2 were observed corresponding to the band center at 410 (blue), 530 (green) and 554 (green)nm. The later and former transitions are having more prominent intensity than the other one and the similar trend has been observed in all the prepared Er3 þ doped boro-tellurite glasses. It is observed from the emission spectra that while adding the TeO2 content in to B2O3 host matrix, the emission peak intensity gradually increases upto 30% TeO2 content in the host matrix and after that the luminescent intensity decreases. All the glasses are equally doped with 1 wt% erbium oxide. The JO parameters have been used to find the important radiative properties like radiative transition probability (A), peak stimulated emission cross-section (sEP ) and branching ratio (bR) for the 2H9/2-4I15/2, 2H11/2 and 4S3/2-4I15/2 transitions.

 2 e2 h O CJ:Lþ 2S:C’ J ’ 16p2 m2 c2 l 2

Smd ¼

ð9Þ

The sum of the radiative transition probability (A) for all the terminal level gives the total radiative transition probability (AT) X AðCJ, C0 J 0 Þ ð10Þ AT ðCJÞ ¼ The reciprocal of the total radiative transition probability (AT) gives the predicted radiative lifetime of the CJ level

tR ðCJÞ ¼ ½AT ðCJÞ1

ð11Þ

The branching ratios (bR) for the respective emission transition is defined as

bR ðCJ, C’ JÞ ¼

AðCJ, C’ J’Þ AT CJ

ð12Þ

The peak stimulated emission cross-section (sEP ) is an essential property, which characterize the laser performance of the corresponding transition and is related to the radiative transition probability (A) [27] as

sEP ¼

l4p A 8pcn2 Dleff

ð13Þ

K. Selvaraju, K. Marimuthu / Physica B 407 (2012) 1086–1093

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

4

H9/2- I15/2

lp

Dleff A

sEP bR(Exp) bR(Cal)

tcal 2

H11/2 &4S3/2-4I15/2

lp

Dleff A

sEP bR(Exp) bR(Cal)

tcal

B0TEr

B1TEr

B2TEr

B3TEr

B4TEr

412 9.74 600.56 0.099

412 8.38 1021.56 0.177

411 6.27 1091.36 0.24

411 6.62 1255.31 0.253

410 6.41 776.8 0.24

0.528 0.330 549 553 7.69 575.52 0.388

0.315 0.315 308 553 7.44 954.28 0.602

0.317 0.31 284 554 7.55 991.78 0.595

0.274 0.288 229 554 8 1143.72 0.629

0.337 0.240 309 553 6.37 679.74 0.444

0.472 0.676 1174

0.673 0.673 704

0.683 0.671 676

0.726 0.671 586

0.663 0.669 984

where n is the refractive index, lp is the emission transition peak wavelength and Dleff is the effective line width calculated by dividing the area of the emission band width by its average height. The above equation is more helpful to calculate various spectroscopic properties and the derived results are presented in Table 6. By using the Judd–Ofelt intensity (Ol) parameters, the radiative properties of the emission bands 2H9/2-4I15/2 and 2H11/2 and 4 S3/2-4I15/2 of the Er3 þ doped boro-tellurite glasses have been computed and presented in Table 6. The radiative properties of the prepared glasses depend on the JO parameters O4 and O6 and the ratio (O4/O6) is known as spectroscopic quality factor that describes the strength of the emission transition. From the luminescence spectra it is observed that, the luminescence intensity of the glasses increase upto 30 wt% of TeO2 content and beyond that the luminescence intensity decreases. No change in the shape or peak position of the broad emission could be observed. The observed line widths are large, probably due to inhomogeneous local fields in the glass. Peak wavelength (lp), transition probability (A), peak stimulated emission cross-section (sEP ), effective band width (Dleff) and branching ratios (bR) for the two transitions 2H9/2-4I15/2, and 2H11/2 and4S3/2-4I15/2 are presented in Table 6. An efficient laser transition is characterized by a large stimulated emission cross-section value. The peak stimulated emission cross-section values (sEP ) have been calculated for the emission transitions using the relevant expressions reported in literature [27,33]. The sEP values corresponding to the 2 H11/2 and 4S3/2-4I15/2 transition is found to be 0.388, 0.602, 0.595, 0.629 and 0.444 (  10  20 cm2) respectively for the B0TEr, B1TEr, B2TEr, B3TEr and B4TEr glasses and these values are found to be higher than the reported Er3 þ glasses [23,30,33]. Among the prepared five glasses B3TEr glass exhibit higher stimulated emission cross-section value. The experimental branching ratio (bR) values were calculated from the emission spectra and compared with the predicted values derived using JO theory. It is observed that their values are in close agreement with each other. The bR values follow the trend as 4I15/2-2H11/2 and 4S3/2 4 2H9/2 for all the prepared glasses. The bR values of the prepared Er3 þ glasses are in agreement with the reported Er3 þ glasses [30,33]. It is observed from the tabulated results that, the magnitude of the stimulated emission cross-section and the branching ratio values of the 2H11/ 4 4 2 and S3/2- I15/2 transition is found to be higher for the prepared boro-tellurite glasses and is favorable for green laser applications. Among the prepared five glasses B3TEr glass is more suitable for green laser applications since it exhibits higher stimulated emission cross-section and branching ratio values.

B0TEr B1TEr B2TEr B3TEr B4TEr

(αhν)1/2 (cm)-1/2 (eV)1/2

Transition parameters 2

2.0

2.2

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.

3.5. Optical band gap energy (Eopt) and Urbach energy (DE) analysis Absorption spectra have been used as a powerful tool to analyze the optical transition and electronic band structure. Several researchers reported that, the direct and indirect inter band transitions are the possible transitions that can occur at the fundamental absorption edge of the amorphous materials [36–42]. In both transitions, the electromagnetic waves interact with the electrons in the valence band which are raised across the fundamental gap to reach the conduction band. According to Mott and Davis [36] for amorphous materials, the absorption coefficient a(n) at the fundamental edge having a value greater than 104 cm  1 is represented as a function of photon energy (hn) and is expressed as

aðnÞ ¼

BðhnEopt Þn hn

ð14Þ

where n is the index number, which is used to decide the type of electronic transition causing the absorption and Eopt is the optical band gap energy. Fig. 6 shows the tauc’s plot which represents the variation of (ahn)1/2 with photon energy (hn), by substituting the value n ¼2 in Eq. (14) corresponding to indirect

K. Selvaraju, K. Marimuthu / Physica B 407 (2012) 1086–1093

Sample ledge code (nm)

n¼22

n¼ 21/2

Eopt (eV) B (cm eV)

B0TEr B1TEr B2TEr B3TEr B4TEr

366 370 374 378 383

3.07 2.96 2.85 2.77 2.63

2.19 2.36 1.89 1.80 1.72

 1/2

DE(eV)

Eopt (eV)

B (cm  2 eV)

3.17 3.12 3.01 2.98 2.88

9.58 13.65 10.41 20.19 12.83

0.58 0.57 0.63 0.64 0.68

3.1

0.7

3.0

2.9

Band tail (eV)

Table 7 The fundamental absorption edge (ledge), optical band gap (Eopt), band tailing parameter (B) and Urbach energy (DE ) corresponding to n ¼21/2 and 2 for the prepared Er3 þ doped boro-tellurite glasses.

Indirect band gap energy (eV)

1092

2.8 0.6 2.7

2.6

transition. The band gap values of the prepared glasses were obtained through the linear region of the curves extrapolating at (ahn)1/2 ¼0. The band tailing parameter is obtained through the slope of the linear region of the curves. Optical band gap and the band tail parameter of the Er3 þ doped boro-tellurite glasses are measured using tauc’s plot and the results are presented in table 7. The fundamental absorption edge of the spectra slightly shift towards the higher wavelength side with the increase in TeO2 content which is due to the structural rearrangement in the glass network former and modifier [37]. The optical band gap values of the prepared glasses are found to be in the range 2.63–3.07 eV which is higher than the reported Er3 þ doped phosphate glasses (1.78–1.66 eV) [38] and antimony-borosilicate glasses (2.65– 2.52 eV) [39] and lower than Ga–Ge–S:Er3 þ system (1.6–3.5 eV) [40], the germinate-borate glasses (5.5–4.1 eV) [41]. The absorption coefficient a(n) of the optical absorption near the band edge exhibit an exponential dependence on the photon energy (hn) which obeys the empirical relation given by Urbach [42]   hn ð15Þ aðnÞ ¼ a0 exp DE where a0 a constant and DE is the width of the band tail of the localized states in the forbidden band gap associated with the amorphous materials. The Urbach energy values of the prepared Er3 þ doped boro-tellurite glasses are presented in Table 7. It is observed from the table that, the width of the band tail DE, anges from 0.58 to 0.68 eV. Materials with larger Urbach energy use to have greater tendency to convert weak bonds into defects. The DE values found to increase slightly with the increase in TeO2 content. The band gap energy and the Urbach energy values with reference to varying TeO2 content have been presented in Fig. 7. It is observed from Fig. 7 that, the band gap energy decreases with the addition of TeO2 content whereas, the Urbach energy found to increase. When an alkali oxide modify the pure boron oxide, the additional oxygen causes a conversion from the BO3 trigonal boron atoms into 4-fold BO4 coordinated boron atoms. Each BO4 structural group is negatively charged and the four oxygens are included in the network as bridging oxygen. These units are responsible for the increase in the connectivity of the glass network. As a result, the degree of the structural compactness and modification in the geometrical configuration of the glass network vary with the change in chemical composition. On the other hand, the structure of the tellurite glass is a laminar network based on triangular TeO3 pyramids or square TeO4 pyramids [42,43]. In boro-tellurite glasses, the TeO4 units and BO4 units have a strong tendency to link with each other to form BTeO3 and BTeO5 units, which results in a higher connectivity in the glass network which indicates the presence of minimum defects in the title glasses with lower TeO2 content. As the TeO2 content increases the Urbach energy is increasing almost linearly

0

10

20

30

40

TeO2 content (wt %) Fig. 7. The optical band gap energy Eopt (Empty square) and the Urbach energy DE (Solid stars) values as a function of TeO2 content for the Er3 þ doped boro-tellurite glasses.

and it may be due to the considerable increase in the TeO4 pyramids in the boro-tellurite glasses, making the structure less stable than the borate glasses. The effect of Urbach energy on the boro-tellurite glasses can be examined further by extending similar investigations on other RE ions in order to obtain multifunctional materials which combine the optical properties of the RE ions and the host matrix.

4. Conclusion Er3 þ doped boro-tellurite glasses have been prepared and their structural and spectroscopic behavior were studied and reported. The XRD pattern confirms the amorphous nature of the prepared glasses and the FTIR spectra reveal the presence of quite significant OH– functional groups, B O bond in BO3 units, B O B bending linkage vibrations and the symmetric stretching vibration of Te  O bonds in the TeO3 units. The bonding parameter studies suggests that, the Er3 þ –ligand bond is of ionic in nature and the degree of ionic character changes with the change in chemical composition. The prepared Er3 þ doped boro-tellurite glasses possess lower asymmetry which is confirmed through the higher experimental oscillator strength values for the 4G11/2 transition. The luminescent studies confirms that the emission peak intensity gradually increases upto 30 wt% TeO2 content in the host matrix and after that decreases while adding TeO2 content in to the B2O3 host matrix. The branching ratio values for the 2H9/2-4I15/2, 2H11/2 and 4 S3/2-4I15/2 transitions are found to be in close agreement with the calculated values, and the B3TEr glass possess maximum bR value among the prepared Er3 þ glasses. Among the prepared five glasses B3TEr glass exhibit higher stimulated emission crosssection and maximum bR value which in-turn can be used as a green laser medium with normal pumping. Optical band gap studies have been carried out using Davis and Mott theory and the Eopt values for the direct and indirect allowed transitions are found to be 3.17 to 2.88 eV and 3.07 to 2.63 eV respectively for the Er3 þ doped boro-tellurite glasses. The Urbach energy values of the prepared Er3 þ doped boro-tellurite glasses are found to be in the range 0.58 to 0.68 eV. The DE. lues are found to increase slightly with the increase in TeO2 content. The present study suggest that, the band gap energy decreases with the addition of TeO2 content whereas, the Urbach energy found to

K. Selvaraju, K. Marimuthu / Physica B 407 (2012) 1086–1093

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