Journal of Nanomaterials
Volume 2015 (2015), Article ID 952308, 9 pages
http://dx.doi.org/10.1155/2015/952308

Research Article

Green Emission of Tellurite Based Glass Containing Erbium Oxide Nanoparticles

Azlan Muhammad Noorazlan, Halimah Mohamed Kamari, Sharudin Omar Baki, and Daud W. Mohamad

Physics Department, Faculty of Science, University Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia

Received 28 July 2015; Revised 29 September 2015; Accepted 8 October 2015

Academic Editor: Fei Meng

Copyright © 2015 Azlan Muhammad Noorazlan et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Investigation on green emission and spectral intensity of tellurite based glass containing erbium oxide NPs is one of the crucial issues. Tellurite based glass containing erbium oxide NPs with the composition of [(TeO₂)0.70(B₂O₃)0.30]0.7(ZnO)0.30.95(Er₂O₃)0.05 has been prepared by using conventional melt-quenching method. The structural and optical properties of the glass sample were characterized by using XRD, FTIR, UV-Vis absorption, and PL spectroscopy. The amorphous structural arrangement was proved through XRD method. The formation of TeO₃ and BO₃ units was revealed by FTIR analysis. Five transition states of excitation were shown in UV-Vis spectra which arise from the ground state ⁴I15/2 to the excited states ⁴G11/2 + ²H9/2 + ⁴F5/2 + ⁴F7/2 + ²H11/2 + ⁴S3/2 + ⁴F9/2 + ⁴I9/2 + ⁴I11/2. The intensity parameters (, 4, 6) are calculated and follow the trend of . Broad green emission at 559 nm under 385 nm excitation was obtained.

1. Introduction

Judd-Ofelt theory was first introduced by Judd and Ofelt in 1962 to explore the spectral intensities at 4f-4f transitions of the rare earth ions [1, 2]. In present years, Judd-Ofelt analysis has become a very important tool to estimate the luminescence and laser efficiency of materials. Rare earth ions consist of very strong intensities and sharp spectral characteristic in 4f transitions [3]. Based on this characteristic, rare earth ions become the most needed materials to develop excellent luminescence and laser applicability. Judd-Ofelt analysis consists of three parameters which are , , and [1, 2]. The three Judd-Ofelt intensity parameters are determined empirically from the room temperature (RT) absorption spectrum by minimizing the differences between calculated and experimental transition line (or oscillator) strengths of a series of excited multiplets by standard least-squares or chi square method [4]. Lately, the luminescence and upconversion properties of Er³⁺ ions have been considered due to their intense green and red emission [5, 6]. These emissions provide an excellent application in many areas from high density optical storage and optoelectronics to medical applications.

Tellurite based glass is widely being used as the main host materials to attain excellent optical and dielectric properties. It is well known that tellurite based glass possesses a high quality of glass forming ability, wide transmission band, fast optical switches, excellent linear and nonlinear optical properties, and exceptional optical fibers for fiber-optical communications. Based on these properties, it is of interest to investigate the new tellurite glass system with various compositions. The formation of tellurite glass system acquires glass stabilizer to obtain stable and reliable glass system. Borate oxide is the most interesting compound to be used as glass stabilizer which is due to its low melting temperature and good rare earth ion solubility. Furthermore, borate matrix consists of BO₃ (trigonal structure) and BO₄ (tetrahedra structure) which generate 4 stable borate groups such as diborate and triborate. The insertion of zinc oxide in the glass system provides low rates of crystallization and contributes to a significant growth of glass forming ability. Moreover, tellurite based glass has a good compatibility with rare earth ions since they provide low phonon energy (700–800 cm⁻¹) environment to minimize the nonradiative losses which are lower compared to the other oxide glass such as silicates, borates, phosphates, and germinates [7–9].

Tellurite glass containing nanoparticles system has a great attraction, especially to study the effect of nanosize particles on optical behavior. Nowadays, special attention has been given to enhance the luminescence properties of materials by using silver and gold nanoparticles [10–13]. However, the investigations on the tellurite glass containing rare earth nanoparticles have not been well explored. Erbium oxide has very special properties, especially in luminescence and laser applications. Lately, the investigations of luminescence and spectral intensity of tellurite glass containing erbium oxide system have been extensively studied [14, 15]. Nevertheless, the research on tellurite glass system containing erbium oxide NPs seems not to be available. Nanoparticles are very useful to improve the quantum efficiency of laser materials. Awang et al., 2013, stated that nanoparticles may enhance the weak optical transitions by generation of intense electric fields upon electromagnetic excitation where plasmonic metal nanostructures in the vicinity of the rare earth (RE) ions alter their free space spectroscopic properties [12]. Hence, it is extremely demanding to further explore the spectral intensity and green emission of tellurite based glass containing erbium oxide NPs.

2. Experimental Methodology

The tellurite based glass containing Er³⁺ NPs was prepared by using melt-quenching method with chemical composition of . The high purity of raw materials (99.99%, purity grade) of erbium (III) oxide nanoparticles, Er₂O₃ NPs (20–30 nm, Nanostructured and Amorphous Materials Inc.), tellurium (IV) oxide, TeO₂ (Puratronic, Alfa Aesar), zinc oxide, ZnO (Assay, Alfa Aesar), and boron oxide, B₂O₃ (Assay, Alfa Aesar), was used to fabricate the glass sample. The chemical composition of about 13 g was weighted and mixed thoroughly and placed in alumina crucible. The homogenous mixture was then transferred to the electrical furnace of 400°C in about 1 hour to remove the excess water molecule.

The mixture was transferred to the electrical furnace of 900°C in about 2 hours for the melting process. The melt was poured onto a preheated stainless steel split mould. The mould was kept in an electrical furnace of 400°C in about 1 hour to remove strain and improve the mechanical strength. After that, the furnace was turned off to cool down at room temperature. The glass sample was cut by using Isomet, Buehler, low speed saw machine to obtain 2 mm thickness of the glass sample. The sample was polished with various types of sand papers, 1500 grit, 1200 grit, and 1000 grit, to obtain flat and smooth surface. The density of the glass sample was measured through Archimedes principle by using acetone as immersion liquid. The FTIR, XRD, and EDX analysis were performed by using EDX-720/800 HS Shimadzu, Xpert Highscore PANalytical X-ray diffractometers and PerkinElmer Spectrum 100 Series FT-IR spectrometers. The refractive index of the glass sample was carried out by using EL X-02C high precision ellipsometer with the angle of the incident at 70° and wavelength of the beam laser,  nm. The absorption analysis of the glass sample was measured by using UV-1650PC UV-Vis Spectrophotometer (Shimadzu) with the wavelength of 190–1100 nm.

3. Result and Discussions

3.1. Transmission Electron Microscopy (TEM)

Figure 1 illustrates the TEM image for erbium oxide NPs before and after the glass formation. It is clear from the figure that the pure erbium oxide nanoparticles exhibit three-dimensional spherical-shaped structures. The average size of nanoparticles before the glass formation is found in the range 18 nm. It can be seen from Figure 1 that the erbium oxide NPs exist after the glass formation. The average size of the nanoparticles is found in the range 28 nm with three-dimensional spherical-shaped structures. It can be seen that the average size of nanoparticles is increased after the glass formation. This may be due to the particle sintering and grain growth as a result of the high-temperature thermal treatment in which the smaller particles tend to form larger particles.

(a)

(b)

Figure 1: TEM image of erbium oxide NPs before (a) and after (b) the glass formation.

3.2. X-Ray Diffraction and EDX Analysis

The noncrystallinity of the glass system was confirmed by using X-ray diffraction (XRD) method. The X-ray diffraction pattern of the tellurite based glass containing Er³⁺ NPs was recorded at room temperature in the range of . The XRD spectra are shown in Figure 2 and it is clear from the figure that the spectra possess broad diffusion at lower scattering angle indicating the long range disorder arrangement. This is in accordance with the characteristic of glass materials which possess amorphous structural arrangement. The absence of sharp peaks recommends that the glass sample exhibit noncrystalline phase. The energy dispersion X-ray (EDX) analysis was performed to determine the exact composition of the glass materials. The EDX spectra are shown in Figure 3 and the measured weight composition of the glass sample is tabulated in Table 1. It can be seen from Figure 3 that all the elements of zinc, erbium oxide NPs, boron, and tellurite exist in the glass system. There is no sign of foreign elements in the EDX spectra which indicates that the glass sample is free from contamination.

Table 1: Calculated and EDX-measured weight of oxides of [(TeO₂)0.70(B₂O₃)0.30]0.7(ZnO)0.30.95(Er₂O₃)0.05.

Oxides
Experimental weight%
Calculated weight%

TeO₂
58.062
56.929

B₂O₃
17.073
17.772

ZnO
8.806
10.643

Er₂O₃
16.059
14.656

Figure 2: XRD spectra of [(TeO₂)0.70(ZnO)0.3(Er₂O₃)0.05.

Figure 3: EDX spectra of [(TeO₂)0.70(B₂O₃)0.30]0.7(ZnO)0.3(Er₂O₃)0.05.

3.3. Fourier Transform Infrared Analysis

Fourier transform infrared analysis (FT-IR) is used to understand the characteristic of the local structure and functional groups for particular materials. The transmission spectra were recorded at room temperature in the range of 280–2400 cm⁻¹. The obtained data of transmission spectra were plotted in Figure 4 and tabulated in Table 2. It can be seen from Figure 4 that the existence of intense absorption bands was centered at 645 cm⁻¹, 1223 cm⁻¹, and 1331 cm⁻¹. The transmission band of the local structure of pure TeO₂ glass was centered at 640 cm⁻¹ [22]. Tellurite oxide containing glass possesses two types of structural arrangement which are trigonal pyramidal TeO₃ and trigonal bipyramidal TeO₄. These two types of structural arrangements can be identified through the transmission band at 600–700 cm⁻¹. The transmission band centered at 600–650 cm⁻¹ is due to the formation of trigonal bipyramidal TeO₄ while that at 650–700 cm⁻¹ corresponds to the formation of trigonal pyramidal TeO₃, respectively. Based on Figure 4, the existence of transmission band located at 656–664 cm⁻¹ correlates to the formation of trigonal pyramidal TeO₃ structural arrangement. This is the indication of the formation of nonbridging oxygen in the glass network, which contributes to the high frequency position of TeO₃ compared to TeO₄.

Table 2: Assignment of infrared transmission bands of [(TeO₂)0.70(B₂O₃)0.30]0.7(ZnO)0.30.95(Er₂O₃)0.05.

Number
Position (cm⁻¹)
Assignments

1
1331
Trigonal B-O bond stretching vibrations in isolated trigonal BO₃ units

2
1223
Trigonal B-O bond stretching vibrations of BO₃ units from boroxyl groups

3
645
TeO₃ group exists in tellurite containing glass

Figure 4: FTIR spectrum of [(TeO₂)0.70(B₂O₃)0.30]0.7(ZnO)0.3(Er₂O₃)0.05.

Borate glass, B₂O₃, possesses boroxyl ring structural arrangement located at 806 cm⁻¹. However, this band disappeared after the glass formation which indicates the absence of boroxyl ring in the glass system. Furthermore, the trigonal BO₃ and tetrahedral BO₄ are taking place after the glass formation. Previous research reported that the transmission band of borate network is mainly active in only three spectral regions [23, 24]. The first band of borate network is located in the range of 1200–1600 cm⁻¹. This correlates with the asymmetric stretching vibration of the B-O band in trigonal BO₃ units [25]. The second band of borate network lies in the range of 800–1200 cm⁻¹ which corresponds to the stretching vibrations of B-O band in tetrahedral BO₄ units. The third group of borate network is positioned in the range of 700 cm⁻¹ which correlates to bending vibrations of B-O-B in trigonal BO₃ units. It can be seen from Figure 4 that the intense absorption bands of borate network are located at 1233 cm⁻¹ and 1343 cm⁻¹. These two bands are attributed to the symmetric stretching vibrations of B-O in trigonal BO₃ units.

The characteristic of ZnO₄ unit is located at 418 cm⁻¹. However, the ZnO₄ unit is absent from the present glass system. This indicates that the zinc lattice is completely broken down and may be formulated as ZnB₄O₇ [26]. It can be seen from Figure 4 that no sign of erbium unit appeared. This is due to the low concentration of erbium ions that could not be detected by the instrument.

3.4. Optical Density and Extinction Coefficient

The optical absorption studies give information to understand the electronic transitions of the materials. The absorption spectra of tellurite based glass containing erbium oxide NPs recorded at room temperature in the UV-Vis region are shown in Figure 5. It can be seen from the figure that the absorption spectra consist of several bands which is due to the characteristic of Er³⁺ ions. Furthermore, erbium ions consist of 4f electrons which are shielded by the outer 5s and 5p bonding electrons which result from the sharp absorption bands. These bands correspond to the 10 transitions originating from the ground state. The transitions arise from the intraconfigurational (f-f) transitions from the ground state to the excited states [27]. The absorption band below 300 nm could not be determined which is due to the rapid increases of the electronic absorption edge. The absorption coefficient has been obtained by using the following relation:

where is the absorbance and is the thickness of the glass sample. The obtained value of absorption coefficient is presented in Table 3. The absence of clear sharp absorption coefficient edge recommends that the glass sample is amorphous in nature. Besides that, the absorption edge depends on the oxygen bond strength of the glass sample. The variety of oxygen bond strength will affect the absorption characteristic of the materials.

Table 3: Optical coefficient and extinction coefficient of [(TeO₂)0.70(B₂O₃)0.30]0.7(ZnO)0.30.95(Er₂O₃)0.05.

Samples
Absorption coefficient (cm⁻¹)
Extinction coefficient () 10⁻⁸

0.05
1.3359
66.110

Figure 5: Optical density spectra of .

The hallmark of the Er³⁺-ligand bonds can be determined through the nephelauxetic ratio and bonding parameters of the glass sample. The value of nephelauxetic ratio can be expressed by the following relation:

where correspond to the wavenumber (in cm⁻¹) for the single excited states transition of Er³⁺ and is the wavenumber (in cm⁻¹) for the same position of excited states transitions in aquo-ion [27]. The bonding parameter of the glass sample can be determined by considering the average values of through the following formula:

The obtained values of nephelauxetic ratio and bonding parameter for the title glass were tabulated in Table 4. The ionic or covalent characteristic of the materials can be predicted by negative or positive sign value of the bonding parameter. It can be seen from the table that the bonding parameter is in negative sign which indicates that the glass sample is ionic in nature. The ionic nature of the metal-ligand is affected by the chemical composition of the glass materials. The existence of trivalent electron of erbium oxide NPs contributes to the strong ionic characteristic of the glass sample. Previous research on glass containing erbium oxide reported the same ionic behavior with this work [28].

Table 4: Band positions and bonding parameters of [(TeO₂)0.70(B₂O₃)0.30]0.7(ZnO)0.30.95(Er₂O₃)0.05.

Transition
(from )
Wavenumber (cm⁻¹)
Wavenumber in aquo-ion (cm⁻¹) [16]

22,201
22,300

20,435
20,600

19,120
19,270

18,349
18,550

15,291
15390

12,516
12,400

2.4830

−59.726

3.5. Judd-Ofelt Analysis

The introduction of Judd-Ofelt theory [2, 16] provides the information of transition behaviour between 4f-4f electronic configuration and calculation of transition probabilities, branching ratio, oscillator strength, and intensity parameters (, , and ). Judd-Ofelt theory is an important approach to analyze and investigate the spectral properties of tellurite glass system containing erbium ions. The Judd-Ofelt analysis acquires the precise integrated absorption cross section measurement over the range of wavelength and transition state of excitation. The experimental oscillator strength for each transition state of excitation can be expressed by the following relation:

where is the concentration of Er³⁺ ions in cm⁻¹ and is the molar absorptivity in L/(mol·cm) obtained from the measured absorbance of the glass system. The molar absorptivity at a given energy is computed from Beer-Lambert Law as shown in the following:

where is the concentration of Er³⁺ ion (mol%), is the thickness of the glass sample (cm), and is the optical density (OD). According to the Judd-Ofelt theory, the estimation of theoretical oscillator strength of an electric dipole transition from to is determined by the following expression:

where is Plank’s constant, is the refractive index, and is the doubly reduced matrix elements of the unit tensor operator. The obtained values of experimental and calculated oscillator strength were tabulated in Table 5. The Judd-Ofelt parameter is computed by using least-square fitting procedure which gives the best fit between experimental and calculated oscillator strength. Meanwhile, according to the Judd-Ofelt theory, the line strength can be found from an integrated absorption cross section by the following expression [29]:

where is the total angular momentum of the lower state, is the mean wavelength, and is the optical density over the range of wavelength. The theoretical expression of electric dipole line strength is given by

where is the Judd-Ofelt parameters. The reduced matrix element can be calculated in the intermediate-coupling approximation and is invariant of environment. A Judd-Ofelt analysis minimizes the square of the difference between and with as adjustable parameters [29]. The validity of fitting has been obtained by comparing the experimental and calculated line strength which is listed in Table 5. Using the least-square fitting method, the Judd-Ofelt parameters () of erbium oxide NPs together with various types of glass system from earlier reported literature [17–21] were summarized in Table 6. The data of Judd-Ofelt parameters from previous literature will be used for comparison with the present glass. It can be seen from Table 6 that the obtained values of Judd-Ofelt parameters are as follows: , 0.400, and 0.07, respectively, in units of 10−20 cm².

Table 5: Integrated areas, dipole line strengths , oscillator strength , and calculated JO intensity parameters of [(TeO₂)0.70(B₂O₃)0.30]0.7(ZnO)0.30.95(Er₂O₃)0.05.

Transition

Line strength (×10⁻²⁰ cm²)
Oscillator strengths (×10⁻²⁰ cm²)

(from )
(nm)
(10⁻⁷) cm
Measured
Calculated
Measured
Calculated

452
5.05
0.051
0.051
0.144
0.144

489
11.60
0.108
0.066
0.285
0.173

527
21.97
0.191
0.202
0.473
0.499

654
25.97
0.180
1.037
0.354
2.039

800
10.80
0.061
0.075
0.096
0.119

977
14.71
0.069
0.008
0.091
0.011

Ω₂ = (1.223 ± 0.021) × 10⁻²⁰ cm², Ω₄ = (0.400 ± 0.104) × 10⁻²⁰ cm², Ω₆ = (0.037 ± 0.112) × 10⁻²⁰ cm², and = 0.0537 × 10⁻²⁰ cm².

Table 6: JO intensity parameters for various glass systems.

Glass system
(×10⁻²¹ cm²)
References

Ω₂
Ω₄
Ω₆

TeO₂-ZnO-Na₂O-B₂O₃-GeO₂-Er₂O₃
5.75
5.02
1.11
[17]

TeO₂-B₂O₃-CdO-Li₂O-Er₂O₃
5.21
1.93
1.05
[18]

B₂O₃-TeO₂-MgO-Er₂O₃
3.74
2.74
1.86
[19]

PbBr₂-TeO₂-Er₂O₃
3.13
1.25
0.73
[20]

Er/Yb-GeO₂-PbO-Bi₂O₃
3.29
1.34
3.02
[21]

Er₂O₃ (NPs)-ZnO-B₂O₃-TeO₂
1.223
0.400
0.037
This work

The values of and parameters correspond to the asymmetry of the local environment of Er³⁺ ions sites which depends on the covalency between Er³⁺ ions and ligand anions. Meanwhile, the value of parameter is linked to the local basicity of Er³⁺ ions and inversely proportional to the covalency of the Er-O bond. It can be seen that the Judd-Ofelt parameters behavior of most of the glass system is following the trend of . The relatively small value of and was found for tellurite glass containing erbium oxide NPs compared to the other glass system. According to the Judd-Ofelt theory, the and parameters are strongly sensitive to the local environment symmetry of rare earth ions. The small value of and indicates that the glass system possesses the lower asymmetric nature of the local environment around Er³⁺ sites. This has also shown the ionic nature of the chemical bond between Er³⁺ ions and the ligands. Furthermore, this effect is reflected to the inorganic ligand field character of the glass matrix [30].

Compared with and parameters, parameter does not depend on the local structure. It can be seen from Table 6 that the obtained value of parameter of present glass is lower compared to the other glass system containing erbium oxide. This indicates that the prepared glass system possesses a high number in Er-O covalency compared to the other glass system. The high number of covalency is due to the high number of nonbridging oxygen ions (NBOs) around the host matrix. In addition, the presence of a high number of NBOs leads to producing higher number of electron density of the ligand ions. It can be concluded that the tellurite glass containing Er³⁺ (NPs) possesses a relatively strong covalency and lower asymmetry around Er³⁺ sites.

The Judd-Ofelt parameters (, ) can be used to compute the radiative transition probability (electric dipole transition probability and magnetic dipole transition probability ), fluorescence branching ratio β, and radiative lifetime of Er³⁺ (NPs) ions [31]. The radiative transition probability (also called Einstein coefficient for spontaneous emission) for any excited transition state can be expressed by the following relation:

The magnetic dipole line strength is neglected since the excitation bands are essentially electric dipole in nature. The calculated radiative transition probabilities were tabulated in Table 7. It can be seen that the radiative probability of Er³⁺ :  transition possesses a high value which is beneficial to the green emission.

Table 7: Calculated transition probabilities and branching ratio of [(TeO₂)0.70(B₂O₃)0.30]0.7(ZnO)0.30.95(Er₂O₃)0.05.

Transition
λ (nm)
(S⁻¹)
(S⁻¹)
β
Lifetime (ms)

1535
17.063
11.77
1.000
13.070

2680

3.070
0.443

976

22.25
0.557
25.014

4406
10.02
0.190
0.037

1666

3.370
0.051

799

60.26
0.912
15.136

3604
11.99
1.710
0.016

1982
4.791
8.910
0.049

1139

23.38
0.056

654

368.65
0.879
2.384

1714

9.27
0.090

1234

2.83
0.027

845

26.96
0.261

545

64.17
0.622
9.688

2611

10.64
0.003

1514

34.76
0.011

1127

27.39
0.009

793

43.26
0.014

523

2986.46
0.963
0.322

1938

2.36
0.003

1260

21.78
0.031

980

68.67
0.099

718

201.15
0.289

489

402.70
0.578
1.435

2532

4.57
0.007

1894

7.72
0.011

1098

7.27
0.010

841

17.15
0.024

707

121.87
0.174

559

432.97
0.617

410

110.25
0.157
1.425

The quality of the fitting between and was performed by the following expression [30]:

where is the number of the spectral bands analyzed and is the number of JO parameters calculated. The obtained value of rms deviation was 0.0537 × 10⁻²⁰ cm², which shows a good agreement with calculated and experimental data and consequently a good precision in the determination of intensity parameters. The branching ratio and radiative lifetime of Er³⁺ can be evaluated by using the following equation:

The obtained results for fluorescence branching ratio β and radiative lifetime of Er³⁺ (NPs) ions are tabulated in Table 7. The lifetime is an important information for optical amplifiers and lasers application, especially at the 1.5 μm band. The longer lifetime at transition of level gives advantage to the population inversion between and levels [32].

3.6. Green Photoluminescence

Figure 6 shows the room temperature photoluminescence spectra of zinc borotellurite glass system containing erbium oxide NPs under 385 nm excitation wavelength. Two main peaks were observed at 559 nm and 539 nm, which are attributed to and levels to the ground state at . The observed bands are due to the stark splitting effects which correspond to the low symmetry of the local environment around Er³⁺ sites [33]. This can be proved by the previous data of intensity parameter at the lower number. The electronic configuration schematic diagram is shown in Figure 7 to determine the mechanism of emission and energy transfer. The emission peaks at 559 nm and 539 nm can be ascribed to the visible light emissions by transitions of excited optical centers in the deep levels [33].

Figure 6: Green emission spectrum of .

Figure 7: The schematic diagram of and possible mechanism for visible emissions.

It can be seen from Figure 7 that the excitation occurs from the ground state () by absorbing a photon (25 974 cm⁻¹) from the excitation beam (GSA (ground state absorption) 385 nm) and makes a transition to level. The electrons at decay nonradiatively (NR) by multiphonon relaxation to populate , , and levels. The electrons at level were then relaxed nonradiatively and populate level , while the latest state () is in thermal equilibrated population with excitation levels [34]. The electrons at the excited states of then decay radiatively to the ground states, , and produce green emission. The mechanism of red emission could be also explained by the energy transfer (ET) process between two adjacent Er³⁺ electrons through this process:

 ET₁: , ET₂: .

The energy transfer rate strongly depends on the distance of two Er³⁺ ions which means that the concentration of Er³⁺ affects the efficiency of energy transfer. Furthermore, the emission peaks of red emission are strongly influenced by energy transfer process. However, there is absence of red emission peak of the graph in the present glass system which is due to the low concentration of Er³⁺ ions.

4. Conclusion

In summary, the quaternary composition of TeO₂-B₂O₃-ZnO-Er₂O₃ NPs glass was successfully prepared and analyzed for structural and optical properties. The noncrystallinity of the glass sample was confirmed by XRD analysis. The existence of all glass elements with their exact composition was proved by EDX analysis. FT-IR analysis revealed the formation of TeO₃ indicating the existence of nonbridging oxygen. The bands of BO₃ units at 1233 and 1343 cm⁻¹ were also shown which correspond to the symmetric stretching vibrations of B-O in trigonal BO₃ units. The absorption spectra consist of 10 transitions originating from the ground state to the excited states . The extinction coefficient is found to be decreased with increasing wavelength due to the decreasing number of absorption coefficient. The obtained value of nephelauxetic ratio and bonding parameter suggest that the present glass system is ionic in nature. The Judd-Ofelt parameter was shown to follow the trend of . The obtained value of Judd-Ofelt parameter recommends that the present glass system possesses a relatively strong covalency and lower asymmetry around Er³⁺ sites. The quenched green emission of the present glass system is shown by the photoluminescence spectra. The existence of green emission peaks at 559 nm and 539 nm, which are attributed to ⁴S3/2 and ²H11/2, is observed which is due to the stark splitting effect. The obtained result of Judd-Ofelt and photoluminescence shows that the glass sample is very useful in green laser application with high lifetime and strong spectral intensity.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

The authors appreciate the financial support for the work from the Ministry of Higher Education of Malaysia through GPIBT (9411800).

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