Green Emission of Tellurite Based Glass Containing Erbium Oxide Nanoparticles

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 {[(TeO 2 ) 0.70 (B 2 O 3 ) 0.30 ] 0.7 (ZnO) 0.3 } 0.95 (Er 2 O 3 ) 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 XRDmethod.The formation of TeO 3 and BO 3 units was revealed by FTIR analysis. Five transition states of excitation were shown inUV-Vis spectra which arise from the ground state I 15/2 to the excited states G 11/2 + H 9/2 + F 5/2 + F 7/2 + H 11/2 + S 3/2 + F 9/2 + I 9/2 + I 11/2 . The intensity parameters Ω t (t = 2, 4, 6) are calculated and follow the trend of Ω 2 > Ω 4 > Ω 6 . Broad green emission at 559 nm under 385 nm excitation was obtained.


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 Ω 2 , Ω 4 , and Ω 6 [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 leastsquares or chi square method [4].Lately, the luminescence and upconversion properties of Er 3+ 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 fiberoptical 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 BO 3 (trigonal structure) and BO 4 (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][8][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][11][12][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.
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,  = 635 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.

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 hightemperature thermal treatment in which the smaller particles tend to form larger particles.

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 Er 3+ NPs was recorded at room temperature in the range of 20 ∘ ≤  ≤ 80 ∘ .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.

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 TeO 2 glass was centered at 640 cm −1 [22].Tellurite oxide containing glass possesses two types of structural arrangement which are trigonal pyramidal TeO 3 and trigonal bipyramidal TeO 4 .These two types of structural arrangements can be identified through the transmission band at 600-700 cm −1 .The transmission Assignments Trigonal B-O bond stretching vibrations of BO 3 units from boroxyl groups 3 6 4 5 TeO 3 group exists in tellurite containing glass band centered at 600-650 cm −1 is due to the formation of trigonal bipyramidal TeO 4 while that at 650-700 cm −1 corresponds to the formation of trigonal pyramidal TeO 3 , respectively.Based on Figure 4, the existence of transmission band located at 656-664 cm −1 correlates to the formation of trigonal pyramidal TeO 3 structural arrangement.This is the indication of the formation of nonbridging oxygen in the glass network, which contributes to the high frequency position of TeO 3 compared to TeO 4 .Borate glass, B 2 O 3 , 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 BO 3 and tetrahedral BO 4 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 BO 3 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 BO 4 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 BO 3 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 BO 3 units.The characteristic of ZnO 4 unit is located at 418 cm −1 .However, the ZnO 4 unit is absent from the present glass system.This indicates that the zinc lattice is completely broken down and may be formulated as ZnB 4 O 7 [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.

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 Er 3+ 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 4   [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.The hallmark of the Er 3+ -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 V  correspond to the wavenumber (in cm −1 ) for the single excited states transition of Er 3+ and V  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 metalligand 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].

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 (Ω 1 , Ω 2 , and Ω 3 ).Judd-Ofelt theory is an important approach to analyze and investigate the spectral properties of tellurite glass system containing where  is the concentration of Er 3+ 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 Er 3+ ion (mol%),  is the thickness of the glass sample (cm), and log( 0 /) 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 ℎ 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 Ω  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  ED with Ω  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 Ω  ( = 2, 4, 6) of erbium oxide NPs together with various types of glass system from earlier reported literature [17][18][19][20][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: Ω 2,4,6 = 1.223, 0.400, and 0.07, respectively, in units of 10 −20 cm 2 .The values of Ω 2 and Ω 4 parameters correspond to the asymmetry of the local environment of Er 3+ ions sites which depends on the covalency between Er 3+ ions and ligand anions.Meanwhile, the value of Ω 6 parameter is linked to the local basicity of Er 3+ 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 Ω 2 > Ω 4 > Ω 6 .The relatively small value of Ω 2 and Ω 4 was found for tellurite glass containing erbium oxide NPs compared to the other glass system.According to the Judd-Ofelt theory, the Ω 2 and Ω 4 parameters are strongly sensitive to the local environment symmetry of rare earth ions.The small value of Ω 2 and Ω 4 indicates that the glass system possesses the lower asymmetric nature of the local environment around Er 3+ sites.This has also shown the ionic nature of the chemical bond between Er 3+ ions and the ligands.Furthermore, this effect is reflected to the inorganic ligand field character of the glass matrix [30].
Compared with Ω 2 and Ω 4 parameters, Ω 6 parameter does not depend on the local structure.It can be seen from Table 6 that the obtained value of Ω 6 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 Er 3+ (NPs) possesses a relatively strong covalency and lower asymmetry around Er 3+ sites.
The Judd-Ofelt parameters (Ω  ,  = 2, 4, 6) can be used to compute the radiative transition probability  rad (electric dipole transition probability  ED and magnetic dipole transition probability  MD ), fluorescence branching ratio , and radiative lifetime  rad of Er 3+ (NPs) ions [31].The radiative transition probability  rad (also called Einstein coefficient for spontaneous emission) for any excited transition state can be expressed by the following relation: The quality of the fitting between  ED and  meas 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 cm 2 , 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  rad of Er 3+ can be evaluated by using the following equation: The obtained results for fluorescence branching ratio  and radiative lifetime  rad of Er 3+ (NPs) ions are tabulated in Table 7.The lifetime is an important information for optical amplifiers and lasers application, especially at the 1.5 m band.The longer lifetime at transition of 4 I 13/2 level gives advantage to the population inversion between 4 I 13/2 and 4 I 15/2 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 4 S 3/2 and 2 H 11/2 levels to the ground state at 4 I 15/2 .The observed bands are due to the stark splitting effects which correspond to the low symmetry of the local environment around Er 3+ sites [33].This can be proved by the previous data of Ω 2 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].
It can be seen from Figure 7 that the excitation occurs from the ground state ( 4 I 15/2 ) by absorbing a photon (25 974 cm −1 ) from the excitation beam (GSA (ground state absorption) 385 nm) and makes a transition to and possible mechanism for visible emissions.[34].The electrons at the excited states of 2 H 11/2 + 4 S 3/2 then decay radiatively to the ground states, 4 I 15/2 , and produce green emission.The mechanism of red emission could be also explained by the energy transfer (ET) process between two adjacent Er 3+ electrons through this process: The energy transfer rate strongly depends on the distance of two Er 3+ ions which means that the concentration of Er 3+ 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 Er 3+ ions.

Conclusion
In summary, the quaternary composition of TeO 2 -B 2 O 3 -ZnO-Er 2 O 3 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 TeO 3 indicating the existence of nonbridging oxygen.The bands of BO 3 units at 1233 and 1343 cm −1 were also shown which correspond to the symmetric stretching vibrations of B-O in trigonal BO 3 units.The absorption spectra consist of 10 transitions originating from the ground state 4 I 15/2 to the excited states 4 G 11/2 + 2 H 9/2 + 4 F 5/2 + 4 F 7/2 + 2 H 11/2 + 4 S 3/2 + 4 F 9/2 + 4 I 9/2 + 4 I 11/2 .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 Ω 2 > Ω 4 > Ω 6 .The obtained value of Judd-Ofelt parameter recommends that the present glass system possesses a relatively strong covalency and lower asymmetry around Er 3+ 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 4 S 3/2 and 2 H 11/2 , 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.

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

Table 7 .
It can be seen that the radiative probability  rad of Er 3+ : 2 H 11/2 [ 4 I 15/2 transition possesses a high value which is beneficial to the green emission.