The effect of SiO2 content on structural, physical and ...

Journal of Non-Crystalline Solids 463 (2017) 118–127

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The effect of SiO2 content on structural, physical and spectroscopic properties of Er3 + doped B2O3–SiO2–Na2O–PbO–ZnO glass systems Priyanka Goyal a,⁎, Yogesh Kumar Sharma a,⁎, Sudha Pal a, Umesh Chandra Bind b, Shu-Chi Huang c,d, Shyan-Lung Chung c,d a

Department of Physics, S.B.S. Govt. P. G. College, Rudrapur (U S Nagar), Uttarakhand, India Centre of Nano Technology, I. I. T. Roorkee, India Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan d Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 70101, Taiwan b c

a r t i c l e

i n f o

Article history: Received 22 December 2016 Received in revised form 2 March 2017 Accepted 3 March 2017 Available online xxxx Keywords: Er3+ doped borosilicate glass Physical properties Absorption and fluorescence spectra Spectroscopic properties and CIE diagram

a b s t r a c t Er3+ doped borosilicate glasses were prepared by standard method with the chemical composition (50 − x) B2O3–(10 + x) SiO2–10 Na2O–20 PbO–10 ZnO–0.3 Er2O3 (where x = 0, 5, 10, 15, 20, 25, 30, 35, 40). The prepared glass samples were characterized by XRD, EDAX, FTIR, SEM and TEM. XRD reveals the amorphous nature of the glass samples. Various band positions are confirmed by the FTIR and borate network is present in the wavelength region 650–1700 cm−1. Elemental composition of the glass samples were observed by EDAX. Variation of physical and optical parameters with increasing concentration of SiO2 affects the glass structure. Decrease in the value of Oxygen Packing Density (OPD) with increasing concentration of SiO2 shows loosely packed structure of glass samples. Absorption and fluorescence spectra were recorded in Visible and NIR region at room temperature. Judd–Ofelt (JO) intensity parameters were calculated by using oscillator strength and the order of these parameters is Ω2 N Ω4 N Ω6. Radiative properties were obtained by JO parameters and fluorescence spectra. A bright green emission is observed for the transition 2H11/2, 4S3/2 → 4I15/2 and it is further confirmed by Commission Internationale de l' Eclairage (CIE) chromaticity diagram. 2H11/2, 4S3/2 → 4I15/2 is suitable for green lasers. This gives the suitability of the present glass sample for fibre optics amplifier and photonic application. © 2017 Elsevier B.V. All rights reserved.

1. Introduction In the last few decades, exploitation of rare earth ions (RE) in the field of glass materials is due to its potential applications in the fabrication of optical devices such as solid state lasers and optical fibres for communication [1]. RE ions plays an important role in modern optical technology as the active constituents of materials [2]. Er3+ ion belongs to the heavy group rare earth elements (HREE) and when erbium is added as a dopant in the glass matrix, electronic structure will be changed. This directly modifies the optical properties of dopants such as spectral substructure, spectral broadening etc. Due to this Er3+ has distinct advantage when it is used as dopant atoms to modify the optical properties of a glass. Trivalent erbium ion has special position because of its peculiar natures of frequency upconversion, broad band emission in infrared region (1.53 μ) [1]. The erbium doped glasses found ⁎ Corresponding authors. E-mail addresses: [email protected] (P. Goyal), [email protected] (Y.K. Sharma).

http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.009 0022-3093/© 2017 Elsevier B.V. All rights reserved.

applications in optical fibre amplifiers, energy upconversion and quantum informatics [3]. It is also important and interesting in the broad third communication window [4] and erbium doped fibre amplifier can be used in waveguide division multiplexing (WDM) in many channels and waveguide fibre amplifiers. For achieving a material that finds applications for fibres, choice of host matrix is very important [5], as they play a major role in obtaining the good luminescent signal [1]. Nowadays researchers are interested in borate based glasses, as they find applications such as thin amorphous film for battery applications, bioactive glasses for tissue engineering, nuclear waste disposals, photonic applications, short pulse lasers, optic fibre amplifiers and fibre amplifiers [6]. The selection of borate for the present work is due to their low melting point, high chemical durability and ease of fabrication. Another important reason for which the borate is selected due to its peculiar “Boron anomaly” character i.e. changing structural behaviour as the alkali or alkaline earth cations are introduced in the glass network. The introduction of silicate in the glass network enhances the thermal stability and optical transparency towards excitation and lasing wavelengths [7].The addition of alkali metals in

P. Goyal et al. / Journal of Non-Crystalline Solids 463 (2017) 118–127

the glass host is important as they can be thermally activated, so this can make their possibility to move from one position to other within in the glass and this helps in the easy replacement of the other ions of the same valence. If the alkali ions are added in the borate network this can increases the Tg and decreases the thermal coefficient [8]. Thus the combination of borate and silicate has low melting point, thermal stability and good optical transparency towards the lasing and excitation wavelengths. If glass host has two glass forming oxides, like borosilicate glasses, gives a wide range of practical applications. B2O3 is a typical glass former and can form a glass network alone which consists of BO3 triangles forming three-member (boroxol) rings connected by B\\O\\B linkages. It was reported that addition of network modifier in the borate glasses results in a progressive change of some of the triangular BO3 structural units into BO4 tetrahedra resulting in a more complex cyclic groups such as di-, tri-, tetra- or penta-borate groups [9]. Addition of transition metal ions such as ZnO in the glass system results in the high intense optical and spectral properties because their addition generates different sites by creating strong interactions [10]. ZnO with properties like optical, electrical and magnetic along with their nontoxicity and non-hygroscopic nature, makes ZnO doped glasses possible in the development of opto-electronic devices, solar convertors, ultraviolet emitting lasers and gas sensors [11]. Luminescence quenching on higher concentration of Er3 + doped glasses are studied and reported by Ashur et al. and Dai et al. [12,13]. Variation effect of B2O3 on JO parameters and life time and thermal stabilities of erbium doped bismuth based glasses were reported by Yang et al. [14]. The variation of B2O3 content on the luminescent properties of erbium was studied and reported by Shen et al. [15]. Number of works on erbium doped glasses such as sodium silicates [16], boro-tellurite [17], sodium aluminium phosphate [18], silicate [19], phospho tellurite [20,21], tellurofluoroborate [1] and lithium zinc borate [6] glasses have been carried out and reported. In this present study, we report a Er3+ doped borosilicate glasses prepared by conventional melt quenching technique for enhance optical properties (absorption and fluorescence spectra in visible and NIR region), to compute the spectroacopic parameters: (i.e. JO parameters, optical band gap, Urbach's energy and radiative properties) and CIE chromaticity diagram and their coordinates. 2. Experimental details The Er3+ doped borosilicate glasses were prepared by conventional melt quenching technique, mixing the high purity A R grade reagents silicon dioxide (SiO2), Borax (Na2B4O7.10H2O), Lead carbonate (PbCO3) and Zinc carbonate {ZnCO3}2. The chemical composition for present glass specimens is (50 − x) B2O3 - (10 + x) SiO2–10 Na2O– 20PbO–10 ZnO–0.3 Er2O3 (where x = 0, 5, 10, 15, 20, 25, 30, 35, 40).The chemical composition are in 10 g batches, were taken in agate mortar for proper grinding of the samples and then it was preheated in an electric furnace at 300 °C for 1 h. After that the temperature of the furnace was increased slowly up to the working temperature i.e. 1000 °C and the samples were heated for 3 h at this temperature for homogeneity. The melt was poured on to a preheated brass moulds at room temperature. The prepared samples were then annealed for 6 h. The glass samples were cut and polished by cerium oxide for optical measurements. The characterization of the borosilicate glass specimens was done to ensure the glass formation by X-ray diffraction. Composition and functional groups are justified by the EDAX and FTIR respectively. Optical absorption spectra were recorded at room temperature using UV-VIS/NIR spectrophotometer (Varian carry 5000). The density of the glass specimens were calculated using Archimedes principle with toluene as immersion liquid. Optical path lengths of the glass materials were measured using digital Vernier callipers. Fluorescence spectrum of the present glass samples were recorded by Hitachi Model F–4600FL spectrophotometer in visible region at room temperature. SEM and TEM

119

images were recorded by using FEI Quanta 200F and Tecnai G2 20 respectively. 3. Results and discussions Erbium ion (HREE) is added as a dopant in the glass matrix results to change in its electronic structure. This directly modifies the optical properties of dopants such as spectral substructure, spectral broadening etc. Erbium ion has HCP structure and has paired electron. Due to this Er3+ has distinct advantage when it is used as dopant atoms to modify the optical properties of a glass. 3.1. Structural properties XRD spectrum of the Er3+ doped borosilicate glass specimen was shown in Fig. 1 for E specimen (representative). There is a broad diffuse hump instead of crystalline peaks, which confirms the amorphous nature of the present glass samples. Absence of the Bragg's peak confirms the glassy nature of the samples [2]. EDAX of the present glass sample is shown in Fig. 2 and shows that all the components like boron, sodium, zinc, silicon, oxygen, lead and erbium, taken initially are present in the final composition with their appropriate amount. EDAX confirms presence of all the elements in the host matrix and finalise the composition of the prepared glass samples (Table 1). The Structural changes involved by the Er2O3 content addition in glass specimens have been analyzed on the basis of the Erbium ions effect to the changes in the relative population of borate, silicate and zinc units. In the host matrix the concentration of B2O3 decreases while SiO2 increases. Till mid concentration (E specimen) the SiO2 act as a glass modifier and B2O3 as a glass former. After E specimen SiO2 work as a glass former in the host matrix and B2O3 act as a glass modifier. 3.2. FTIR analysis FTIR is the powerful tool for exploring the presence of the functional groups in the glass samples. Fig. 3 represents the FTIR spectrum of the Er3 + doped borosilicate glass samples in wave number region 400– 4000 cm−1. The corresponding transmission band positions along with their band assignments are collected in Table 2. The FTIR spectra is very much useful for analyzing the borate network and for the present glass samples, it is present in the wave number region 650–1700 cm−1. The presence of the hydroxyl group is the most dominant characteristic in all the infrared groups. For present glass samples; the band around 3330 cm−1 is due to the fundamental stretching of O\\H group [6]. The bond nearby 1650 cm−1 indicates the presence of B\\O– in isolated pyroborate [6]. The peak around 1420 is due to the B\\O stretching vibration of BO3 units in metaborate, pyroborate and orthoborate groups [22]. The peak at nearby 1330 is due to the B\\O stretch in BO3 units from varied types of borate groups [22]. Asymmetric vibrations of the silicate tetrahedral network are present at nearby 1070 cm−1 [23]. The peak around 950 cm−1 is due to the presence of Vibrations of

Fig. 1. XRD spectrum of Er3+ doped borosilicate glass of E specimen.

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where, Wa is the weight of glass sample in air, Wb is the weight of glass sample in immersion liquid (toluene) and ρt is the density of the immersion liquid (toluene). Molar volume of the glass samples were calculated by [27] Vm ¼

MAV ρ

ð2Þ

where, MAv is the average mass of the samples and ρ is the density of the glass samples. Refractive index of a glass is also an important parameter which is related to the optical feature of glass samples. Researchers had tried to find out a relation between refractive index and glass composition. Refractive index of the present glass samples were measured by using the relation

Fig. 2. EDAX spectrum of Er3+ doped borosilicate glass of E specimen.

nd ¼ 1:57376 þ pentaborate along with BO−4 tetrahedral [24] and the peak at 931 cm−1 is due to the B\\O stretch in BO4 units from di-borate groups [22]. The peak found at 706 cm−1 indicates the B\\O\\B bending of the BO3 group [6,25]. The peak at 677 cm−1 is due to the presence of υ4 vibration of BO4 tetrahedral. Symmetrical bending vibrations (υ2) of Si\\O\\Si group are present nearby 663 cm−1. The presence of Zn\\O bond in the prepared glass was confirmed with the peak positioned at 654 cm− 1 [26]. Presence of all the bending and stretching vibrations of borate network and other functional group are confirmed.

SEM image of the present glass samples is shown in Fig. 4. There are no grains in this image, which confirms the amorphous nature of the prepared glass samples. TEM image of the Er3 + doped borosilicate glass samples is shown in Fig. 5 for E specimen. The TEM image represents the internal structure of the present glass samples. The structure of the prepared glass samples is HCP. TEM image confirms the results of FTIR.

ε ¼ n2d

ρ¼

W a ρt W a −W b

ð1Þ

ð4Þ

where, nd is the refractive index of the glass. The optical dielectric constant of the glass is obtained by the relation.
 ∂t ¼ ðε−1Þ ¼ n2d −1 ∂p

ð5Þ

where, ε is the dielectric constant. The rare earth ions concentration of the glass is computed by, NA ρ ðmol%of rare earth ionÞ MAV

N¼

ð6Þ

where, NA is Avogadro's no. and ρ is the density of the glass samples. The reflection loss is calculated by using Fresnel's formula [28].

3.4. Physical and optical properties Physical and optical properties of the present glass samples were computed and collected in Table 3. Density is the effective tool for defining the structure compactness of the glass. It is measured by Archimedes principle by using toluene as immersion liquid. The relation used in the calculation of density is [27].

ð3Þ

° where, λðAÞ, for peak wavelengths. Dielectric constant is calculated by using refractive index of the glass [27].

p

3.3. SEM and TEM

153:137 λðA°Þ−686:2

R¼

  nd −1 2 nd þ 1

ð7Þ

where, nd is the refractive index of the glass. Molar refractivity of the glass samples were calculated by the well known Volf and Lorentz-Lorenz formula [29], RM ¼ VM

! n2d −1 n2d þ 2

ð8Þ

Oxygen Packing Density (OPD) of the glass samples were computed from the relation [30]. Table 1 Final composition of the Er3+ doped borosilicate glass specimens.

O¼

Composition of the glass specimens

Nomenclature

50 B2O3–10 SiO2–10 Na2O–20 PbO–10 ZnO–0.3 Er2O3 45 B2O3–15 SiO2–10 Na2O–20 PbO–10 ZnO–0.3 Er2O3 40 B2O3–20 SiO2–10 Na2O–20 PbO–10 ZnO–0.3 Er2O3 35 B2O3–25 SiO2–10 Na2O–20 PbO–10 ZnO–0.3 Er2O3 30 B2O3–30 SiO2–10 Na2O–20 PbO–10 ZnO–0.3 Er2O3 25 B2O3–35 SiO2–10 Na2O–20 PbO–10 ZnO–0.3 Er2O3 20 B2O3–40 SiO2–10 Na2O–20 PbO–10 ZnO–0.3 Er2O3 15 B2O3–45 SiO2–10 Na2O–20 PbO–10 ZnO–0.3 Er2O3 10 B2O3–50 SiO2–10 Na2O–2 0PbO–10 ZnO–0.3 Er2O3

A B C D E F G H I

1000 n ρ M

ð9Þ

where, ρ is the density of the desired glass samples, n is the number of oxygen atoms in the composition and M is molecular weight. Polaron radius is calculated by [31]. rp ¼

1  π 1=3 2 6N

where, N is the rare earth ion concentration.

ð10Þ

P. Goyal et al. / Journal of Non-Crystalline Solids 463 (2017) 118–127

121

Fig. 3. FTIR spectrum of Er3+ doped borosilicate glass of E specimen.

Inter ionic separation were computed from [31]. ri ¼

 1=3 1 N

ð11Þ

Field strength of the glass samples were given by [32]. Z r 2p

F¼

! ð12Þ

where, Z is the atomic mass of neodymium ion and rp is the polaron radius. According to the theory of metallization of condensed matter, refractive index turn to be infinite in the condition Rm/Vm = 1 from the Lorentz-Lorenz equation [27]. This is in accordance with the metallization theory of covalent solid materials [33]. The nature of the solids can be explained on the condition that Rm/Vm b 1 (non-metal) and Rm/Vm N 1 (metal). Metallization criterion (M) is given by subtracting the ratio Rm/Vm by 1. M ¼ 1−

Rm Vm

ð13Þ

Physical and optical properties of the Er3+ doped borosilicate glass samples have been computed and collected in Table 3. Variation of OPD with increasing concentration of SiO2 is shown in Fig. 6 and its value decreases with increasing concentration of SiO2. Decrease in the value of OPD with increasing concentration of SiO2 reveals that glass structure is loosely packed. Fig. 7 shows the variation of density and molar volume with SiO2 concentration. Density decreases with increasing concentration of SiO2 while molar volume increases with SiO2 increasing concentration. It is observed that density and molar volume behaves opposite to each other [30]. Variation of rare earth ion concentration with increasing concentration of SiO2 is given in Fig. 8 and shows that rare earth ion concentration decreases with increasing concentration of SiO2. The effect of polaron radius and field strength with SiO2 concentration is shown in Fig. 9. Polaron radius decreases and field strength increases with increasing concentration of SiO2. The behaviour of polaron radius and field strength with SiO2 concentration is opposite to each other. The small values of metallization (M) indicates that the width of both valence and conduction bands becomes large, resulting in a narrow band gap and increased metallicity of the solids. Other properties like inter ionic separation, molar volume, increases with increasing concentration of SiO2. While properties like refractive index, average molecular weight, dielectric constant, optical dielectric

The molar refraction of the glass is related to the structure of the glass and it is proportional to the molar electronic polarizability of the glass samples, according to the relation [27] ∝m ¼

Rm 2:52

ð14Þ

Table 2 Band positions in FTIR spectra of Er3+ doped borosilicate glass. S. no.

Band position (cm−1)

Structural vibrations

1) 2) 3)

~3330 ~1650 ~1420

4)

~1330

5) 6) 7) 8) 9) 10) 11)

~1070 ~950 ~931 ~706 ~677 ~663 ~654

Presence of OH group B\ \O– bond in isolated pyroborate group B\ \O Stretching vibration of BO3 units in metaborate, pyroborate & orthoborate groups B\ \O stretch in BO3 units from varied types of borate groups Symmetric stretching vibration of Si\ \O\ \Si bonds Vibrations of pentaborate along with BO−4 tetrahedral B\ \O stretch in BO4 units from di-borate groups Bending vibrations B\ \O\ \B linkages υ4 vibration of BO4 tetrahedral \O\ \Si Asymmetrical bending vibrations (υ4) of Si\ Presence of ZnO tetrahedral bond

Fig. 4. SEM image of Er3+ doped borosilicate glass of E specimen.

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Fig. 6. Variation of OPD with increasing concentration of SiO2.

and changes in the local ligand field. The intensity of the transition I15/2 → 2H11/2 is high as compared to the other transitions and obeys the selection rules |ΔL| ≤ 2, |ΔJ| ≤ 2 and ΔS = 0, so that this transition is called the hypersensitive transition. Hypersensitivity in the glass network is defined as, “enhanced sensitivity of the intensity of specific electronic transitions to the chemical environment with respect to normal f–f transitions”. The nature of the Er\\O bond in the prepared glass sample is determined by the Nephlauxetic ratio (β) and bonding parameter (δ) and calculated by the expressions.

4

Fig. 5. TEM image of Er3+ doped borosilicate glass of E specimen.

constant, molar polarizability and reflection losses decreases with increasing concentration of SiO2. 3.5. Absorption studies

β¼

Optical absorption studies are the useful method in the investigation of induced transitions. It gives information about the band structure and energy band gap of the amorphous material. Optical absorption spectra of the Er3+ doped borosilicate glasses have been shown in Fig. 10(a & b) in visible and NIR region. Nine absorption bands were observed in absorption spectra and arises from ground state 4I15/2 to various excited states such as 4I13/2, 4I11/2, 4I9/2, 4F9/2, 4S3/2, 2H11/2, 4F7/2, (4F5/2, 4F3/2) and 2G9/2 corresponding to the wavelength positions 1531, 1006, 827, 681, 573, 550, 518, 481 and 407 nm respectively. These bands are assigned according to the reported data [1,6], which indicates the homogeneity of Er3+ ions in the glass network without forming clusters

δ¼

νc νa

ð15Þ

1−β

ð16Þ

β

where, νc is the observed wave number (in cm−1) of a particular transition of the Erbium ion in the host matrix and νa is the wave number (in cm−1) of the aqua-ion for the corresponding transition [34]. The value of δ determines the nature of the bonding, for the present case value of δ is b 1 (Table 4) and positive, which shows the covalent bonding of the prepared glass samples.

Table 3 Physical and optical parameters of Er3+ doped borosilicate glass specimens. Physical parameters

Name of the glass specimens A

B

C

D

E

F

G

H

I

Density(ρ) (gm/cm3) Thickness (cm) Average molecular weight (MAV)(gm) Rare earth ion concentration N (1022) Oxygen Packing Density (OPD) Polaron radius rp Inter ionic separation ri Field strength (F) (1016)

6.73 0.15 100.944 1.606 140.609 7.265 8.539 3.169

6.67 0.2 100.467 1.599 136.697 7.256 8.551 3.177

6.59 0.11 99.990 1.587 132.406 7.237 8.572 3.193

6.52 0.2 99.513 1.578 128.351 7.224 8.589 3.205

6.45 0.2 99.036 1.569 124.328 7.210 8.606 3.218

6.38 0.2 98.560 1.559 120.337 7.195 8.623 3.231

6.3 0.21 98.083 1.547 116.195 7.176 8.646 3.247

6.23 0.19 97.606 1.537 112.274 7.161 8.664 3.261

6.15 0.21 97.129 1.525 108.210 7.155 8.687 3.267

Optical parameters Refractive index nd Dielectric constant (Є) Optical dielectric constant Molar volume Vm Reflection Losses R Molar Refractivity Rm Metallization (M) Molar polarizability (αm) Electronic polarazability (αe) (10−24)

1.613 2.602 1.602 14.999 5.504 5.222 0.652 2.072 0.05177

1.609 2.590 1.590 15.062 5.456 5.219 0.653 2.071 0.05175

1.607 2.582 1.582 15.173 5.420 5.239 0.654 2.079 0.05195

1.605 2.577 1.577 15.263 5.398 5.259 0.655 2.087 0.05214

1.599 2.558 1.558 15.354 5.315 5.248 0.658 2.082 0.05204

1.594 2.541 1.541 15.448 5.245 5.243 0.661 2.080 0.05198

1.589 2.528 1.528 15.569 5.187 5.253 0.662 2.084 0.05209

1.584 2.508 1.508 15.667 5.105 5.243 0.665 2.080 0.05197

1.583 2.507 1.507 15.793 5.101 5.282 0.666 2.095 0.05237

P. Goyal et al. / Journal of Non-Crystalline Solids 463 (2017) 118–127

123

Fig. 7. Variation of density and molar volume with increasing concentration of SiO2.

Fig. 9. Variation of polaron radius and field strength with SiO2 concentration.

Judd–Ofelt theory explains the probability of forced electric dipole transitions of rare earth ions in different environments. According to JO theory, electric dipole transitions between two states of 4fN configuration of rare earth ions, which are forbidden when the ions are free, become allowed in the crystal field by mixing into 4fN configuration with another configuration having opposite parity [36,37]. The JO parameters i.e. Ω2, Ω4 and Ω6 were computed by oscillator strength, absorption spectral measurements and refractive index. The calculated values of omega parameters were given in Table 5. These Ωλ parameters are associated to the covalency, structural changes and symmetry of the ligand field around the Er3+ ion site. In general Ω2 parameter gives information about the covalency and asymmetry in the present glass samples. The omega parameters follow the trend Ω2 N Ω4 N Ω6 and similar to the reported data [6,38–42]. The dependence of Ω2 parameter on the host matrix and Er3 + ion site is higher than the other two omega parameters.

respectively. Optical band gap of the glass samples were calculated by the expression. αðν Þ ¼


r B hν−Eopt hν

ð18Þ

where α(ν) is the absorption coefficient, hν is the photon energy, Eopt is the optical band gap, B is the band tailing parameter and r is the index number which determines the type of transitions i.e. r = ½ for direct transition and r = 2 for indirect transition. The optical band gap for the direct and indirect transitions are obtained by extrapolating the linear part of the curves to the zero absorption at (αhν)1/2 = 0 and (αhν)2 = 0 respectively. Fig. 11(a & b) shows the Tauc's plot for

3.6. Band gap and Urbach's energy analysis The optical band of the Er3+ doped borosilicate glasses was calculated from the absorption edge of the absorption spectra by using Davis and Mott relation [43]. The direct and indirect band gaps were calculated from the absorption edge, which is not sharp in the present glass due to the absence of long range order. The absorption coefficient is computed by the expression. αðνÞ ¼ ð1=tÞ ln ðI0 =It Þ

ð17Þ

where, t is the thickness of the glass samples and I0 and It are the intensities of the incident and transmitted radiations

Fig. 8. Variation of Rare earth ion concentration with increasing concentration of SiO2.

Fig. 10. (a): Absorption spectra of Er3+ doped borosilicate glass in the range 400– 1100 nm. (b): Absorption spectra of Er3+ doped borosilicate glass specimens in the range 1100–2300 nm.

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Table 4 Observed band positions (cm−1) and bonding parameters (β and δ) of the Er3+ doped borosilicate glasses. Transition

A

B

C

D

E

F

G

H

I

Aqua ion [35]

4

6531 9940 12,091 14,684 17,452 18,181 19,305 20,790 24,570 0.964809

6531 9940 12,121 14,684 17,452 18,181 19,305 20,790 24,570 0.965077

6531 9940 12,106 14,684 17,452 18,181 19,305 20,790 24,570 0.964943

6531 9940 12,091 14,684 17,421 18,181 19,305 20,790 24,570 0.964621

6531 9940 12,106 14,684 17,421 18,181 19,305 20,790 24,630 0.965027

6531 9950 12,106 14,684 17,452 18,181 19,305 20,790 24,570 0.964943

6531 9940 12,091 14,684 17,452 18,181 19,305 20,833 24,630 0.965296

6531 9940 12,106 14,684 17,421 18,181 19,342 20,790 24,570 0.964956

6531 9940 12,091 14,684 17,452 18,181 19,342 20,833 24,630 0.965497

6600 10,250 12,400 15,250 18,350 19,150 20,450 22,100 24,550 –

0.036475

0.036186

0.036331

0.036677

0.036241

0.036331

0.035951

0.036316

0.035736

–

I13/2 I11/2 I9/2 4 F9/2 4 S3/2 2 H11/2 4 F7/2 4 F5/2 , 4F3/2 2 G9/2 4 4

β δ

calculating the direct and indirect band gap. The values for direct and indirect optical band gap lies in the range 0.966 to 0.984 eV and 0.941 to 0.967 eV respectively and collected in Table 6. Absorption coefficient of the amorphous material, below the absorption edge varies exponentially with the photon energy indicating the presence of Urbach's tail due to static disorder in the present glass. Urbach's energy is originated from the phonons which are related to the indirect transitions and characterize the exponential tail of the absorption edge. The exponential absorption tail and Urbach's energy are related with the relation. αðν Þ ¼ α0 exp

hν ΔE

ð19Þ

where α0 is a constant and ΔE is the Urbach's energy associated with the optical transition between the localized tail states adjacent to the valance band and the conduction bands which extend into the band gap. A graph is plotted between log α and photon energy and shown in Fig. 12. The Urbach's energy is determined by extrapolating the linear part of the curve to the zero. The value of Urbach's energy is given in Table 6 along with the optical band gap. The value of Urbach's energy lies in between 1.047 and 1.069 eV. Values of Urbach's energy indicate the disorderness in the material and the changes in the values of Urbach's energy are due to the formation of defects in the prepared glass. These defects are related to the production of localized states in the glasses, due to which width of the localized states decreases in the optical band gap.

3.7. Fluorescence spectra and radiative properties Fluorescence spectra of the Er3 + doped borosilicate glasses is recorded at excitation wavelength 407 nm in visible region and shown in Fig. 13. In the present case we find two transitions for fluorescence spectra i.e. 2H11/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2. A bright green emission was observed for the thermally coupled energy levels 2H11/2, 4S3/ 4 (2) 2 || and 2 → I15/2, which is purely electric dipole in nature. Since ||U (4) 2 4 ||U || of the transition S3/2 are zero and the intensity value of this emission transition is dominated by Ω6 intensity parameter values. The radiative properties such as transition probability (A), branching ratio (β′) and radiative lifetimes were determined by Judd-Ofelt theory and fluorescence data, were listed in Table 7. The transition probability was calculated by,

A¼

64π4 3

3hλ ð2 J þ 1Þ


2 ! n n2 þ 1 S 9

ð20Þ

where S is the line strength and obtained by ||U(t)||2 matrix elements. The radiative lifetime is obtained by,

τ¼

1 A

ð21Þ

Table 5 Judd-Ofelt parameters of Er3+ doped borosilicate glasses. Glass code

Ω2(10−20)

Ω4(10−20)

Ω6(10−20)

Trend in omega parameters

References

A B C D E F G H I LZB0.01E LZB0.05E LZB0.1E LZB0.5E LZB1.0E LZB2.0E SGS01 TBZNbEr10 Sodalime silicate Lithium borate Bismuth borate

1.970 1.749 2.667 2.170 1.468 1.318 1.732 1.776 1.517 2.33 2.47 3.71 4.31 4.72 4.76 2.11 4.41 2.72 3.24 3.87

1.464 1.435 0.708 1.457 1.362 1.520 2.043 1.799 1.353 1.38 1.87 0.34 0.26 0.70 1.06 1.67 1.07 2.31 0.92 1.52

0.940 1.225 1.380 0.654 0.830 0.710 0.239 0.477 0.300 1.23 1.74 2.89 2.13 1.53 1.03 1.22 1.57 1.28 0.82 1.17

Ω2 Ω2 Ω2 Ω2 Ω2 Ω2 Ω2 Ω2 Ω2 Ω2 Ω2 Ω2 Ω2 Ω2 Ω2 Ω2 Ω2 Ω2 Ω2 Ω2

Present work Present work Present work Present work Present work Present work Present work Present work Present work [6] [6] [6] [6] [6] [6] [38] [39] [40] [41] [42]

N N N N N N N N N N N N N N N N N N N N

Ω4 Ω4 Ω4 Ω4 Ω4 Ω4 Ω4 Ω4 Ω4 Ω4 Ω4 Ω4 Ω4 Ω4 Ω4 Ω4 Ω6 Ω4 Ω4 Ω4

N N N N N N N N N N N N N N N N N N N N

Ω6 Ω6 Ω6 Ω6 Ω6 Ω6 Ω6 Ω6 Ω6 Ω6 Ω6 Ω6 Ω6 Ω6 Ω6 Ω6 Ω4 Ω6 Ω6 Ω6

P. Goyal et al. / Journal of Non-Crystalline Solids 463 (2017) 118–127

125

Fig. 12. ln ⍺ plotted against photon energy, ħω, for glass sample E.

transition observed. Effective line width of the particular transition is given by, Δλeff ¼

Fig. 11. (a): Tauc's plot for the direct band gap of Er3+ doped borosilicate glasses. (b): Tauc's plot for the indirect band gap of Er3+ doped borosilicate glasses.

The calculated branching ratio is given by, β0 ¼

A AT

ð22Þ

It gives information about the relative intensity of all the emission lines. The most important factor stimulated emission cross-section is determined by Fuchtbauer–Ladenburg (FL) formula [44,45], σP ¼

λ4P A 8πcn2 Δλeff

ð23Þ

1 ∫IðλÞdλ Imax

ð24Þ

where I is the fluorescence intensity and Imax is the band intensity at maximum. Other properties like gain band width and gain line width or figure of merit (FOM) were calculated by product of effective band width (Δλeff) and stimulated emission cross-section (σP) and radiative lifetime (τ) and stimulated emission cross-section ( σP) values respectively. The high value of FOM shows the high performance in compact lasers for display and data storage devices [1]. The value of transition probability, radiative life time, branching ratio, stimulated emission cross section and gain band width for transition 4F9/2 → 4I15/2 was 1662.40, 6015.38, 0.181, 22.99 and 321.96 respectively. The computed values of radiative parameters, gain band width and FOM were given in Table 7 for transition 2H11/2, 4S3/2 → 4I15/2 and compared with other glasses. From these studies it was observed that the laser transition 2 H11/2, 4S3/2 → 4I15/2 have higher gain band width ( σP × Δ λeff) and gain line width ( σP × τ) in comparison to same transition of other glasses, so that this transition is suitable for laser applications, display devices, fabrication of storage devices and fibre optics amplifiers. The prepared glass specimens give the green emission, suggesting that the present glass is suitable for photonic applications. For measuring the colour of visible emission that the human eye perceives the Commission Internationale de l’ Eclairage, (CIE) coordinates were calculated. It is a standard method for defining the colours and obtained by considering the sensitivity of human eye for different colours

where n is the refractive index, c is the velocity of light, λp is the peak wavelength of the transition and Δλeffis the effective line width of the

Table 6 Direct and Indirect optical energy band gap and Urbach's energy for Er3+ doped borosilicate glasses. Direct transitions (αħω)1/2

Sample Code

Indirect transitions ((αħω)2 Eopt (eV)

Eopt (eV)

A B C D E F G H I

0.986 0.980 0.981 0.978 0.984 0.977 0.966 0.969 0.970

0.967 0.966 0.962 0.958 0.965 0.951 0.943 0.941 0.948

Urbach energy (ΔE) (eV)

1.051 1.047 1.051 1.055 1.061 1.064 1.069 1.067 1.064

Fig. 13. Fluorescence spectra of Er3+ doped borosilicate glass samples in the range 565– 815 nm.

126

P. Goyal et al. / Journal of Non-Crystalline Solids 463 (2017) 118–127

Table 7 Radiative properties of Er3+ doped borosilicate glass of E specimen. Transitions

Parameters

E [present work]

BTFB0.5E [1]

LZB0.5Er [6]

TZLF1.0Er [46]

LiTFPEr [47]

0.5ETFB [48]

2

(λ) nm A(s−1) β′ τ (μ s) Δλeff (nm) σp (cm2) Gain band width Gain line width

581 7523 0.819 1329 16 27.32 (10−18) 437.21 363.22

553 2066 0.672 325 14 45.4 (10−22) 90.8 1.38

547 2176 0.68 310 14 68.0 (10−22) 95.7 2.69

550 1510 0.66 435 16.7 27.1 (10−22) 45.4 1.18

550 1169 0.67 575 16 34.09 (10−22) 64.05 1.96

548 495 0.86 – 6.05 32.61(10−22) 19.73 –

H11/2, 4S3/2 → 4I15/2

(wavelengths) [49]. The CIE chromaticity diagram is shown in Fig. 14(a). This diagram was obtained from fluorescence spectra. The CIE coordinates for this diagram is (x, y) = (0.3608, 0.6287) which lies in the green region of Fig. 14(a). Spectral graph is shown in Fig. 14(b) for CIE chromaticity diagram for verifying the position of colour of emission. Hence it verifies the result of fluorescence according to which Er3+ ion gives the green emission and suitable for fibre optic amplifiers and photonic applications.

4. Conclusion Er3+ doped borosilicate glasses were prepared by conventional melt quenching technique. The prepared glass samples were characterized by XRD, EDAX and SEM. Different stretching and vibrational groups of the prepared glass samples was confirmed by FTIR and these vibrational modes give information about the structure of the glass samples. In present glass samples, till mid concentration (E specimen) SiO2 act as a glass modifier while B2O3 as a glass former. After mid concentration SiO2 work as a glass former and B2O3 as glass modifier. TEM defines the internal structure of the glass samples and verifies the results of the FTIR. Various physical and optical properties of the present glass samples were calculated. The increasing concentration of SiO2 affects the physical and optical parameters of the glass specimens and decrease in the value of physical and optical parameters shows that structure of glass specimens is loosely packed. Among all the physical and optical properties OPD has its great role in defining the structure of the glass specimens. Absorption and fluorescence spectra of the prepared glass samples were recorded at room temperature. Spectroscopic parameters such as JO parameters, direct and indirect optical band, Urbach's energy and radiative properties were computed. The transition 2 H11/2, 4S3/2 → 4I15/2 gives green laser emission, which is suitable for fibre optic amplifiers and photonic applications. These results were confirmed by CIE chromaticity diagram and their coordinates lie in the green region of this diagram. Acknowledgements The authors would like to thanks to Dr. Jagdish Prasad, Principal, S. B. S. Govt. P. G. College, Rudrapur for lab facilities and Dr. R. K. Dutta, Centre of Nanoscience and Nanotechnology, IIT Roorkee for characterization of the present glass specimens. One of the author's is thankful to the UGC grant commission for financial support and Dr. L. P. Verma, M. B. Govt. P. G. College, Haldwani for valuable suggestions. References

Fig. 14. (a): CIE chromaticity diagram of Er3+ doped borosilicate glass of E specimen. (b): Spectral graph for Verification of CIE diagram for Er3+ doped borosilicate glass of E specimen.

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