Structural and optical study of erbium doped borophosphate glasses

Journal Pre-proof Structural and Optical study of Erbium Doped Borophosphate Glasses M. Amer Khan, R.J. Amjad, M.A. Ahmad, A. Sattar, Saddam Hussain, S. Yasmeen, M.R. Dousti

PII:

S0030-4026(19)31605-5

DOI:

https://doi.org/10.1016/j.ijleo.2019.163707

Reference:

IJLEO 163707

To appear in:

Optik

Received Date:

23 July 2019

Revised Date:

20 October 2019

Accepted Date:

5 November 2019

Please cite this article as: Khan MA, Amjad RJ, Ahmad MA, Sattar A, Hussain S, Yasmeen S, Dousti MR, Structural and Optical study of Erbium Doped Borophosphate Glasses, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.163707

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Structural and Optical study of Erbium Doped Borophosphate Glasses M. Amer Khan1, R. J. Amjad1*, M. A. Ahmad1, A. Sattar1, Saddam Hussain2, S. Yasmeen3, M. R. Dousti4 1

Department of Physics, COMSATS University Islamabad, Lahore 54000, Pakistan

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State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China 3

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Department of Material Science and Engineering, Incheon National University, Incheon, 22012, South Korea 4

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Academic Unit of Cabo de Santo Agostinho, Universidade Federal Rural de Pernambuco, Recife, Brazil 1

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*[email protected]

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Abstract Borophosphate (BP) glass having composition of 50P2O5 - 10B2O3 - 20ZnO - 20PbO -xEr2O3 in mole %, where x= (0, 0.005, 0.01, 0.015, 0.02, 0.025 wt. %) were prepared by the melt quench method. X-ray analysis was performed to confirm the amorphous structure of the glass network. All glasses exhibit good transparency as observed from the naked eye. UV-VIS-NIR absorption spectroscopy was performed between 300 nm to 1600 nm to probe the absorption bands of prepared samples. Ten major absorption peaks were appeared due to the excitation from ground level 4I15/2 to various excited states. The absorption spectrum was then used to calculate the direct and indirect band gap energy that lies within the range of 3.98 eV-4.32 eV and 3.86 eV4.23 eV respectively. Refractive index and density were calculated in the range of 2.125-2.197 and 3.923-4.231 g/cm3 respectively. Judd-Ofelt (JO) theory was applied to investigate the radiative properties. JO intensity parameters Ωt (t= 2, 4, 6) were calculated by least square fitting method. The result shows that studied samples are suitable candidates for green light emission solid-state laser device. Keywords: photoluminescence; Erbium; Glasses; judd Ofelt; Energy band gap

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Introduction Luminescence from various trivalent rare earth ions doped glasses have been studied extensively in the past few decades for different purposes such as optical amplifiers, solid-state laser, and temperature sensors [1]. Luminescence properties of rare earth ions depend upon the surrounding host environment and the concentration of rare earth ions as well [1]. Heavy metal ions like zinc and lead are widely used as a network modifier to enhance desired lasing parameters such as quantum efficiency. It is noticed that incorporation of PbO and ZnO in the glass network leads to improve the chemical durability and thermal stability by forming M-O-P bonds in the phosphate glass network [2-6]. Presence of heavy metals in the glass network also increases the refractive index which increases the gain of optical amplifiers by raising the

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stimulated emission cross section [1, 7]. Hence, lead and zinc-based BP glass can be considered as a potential candidate for the optoelectronics field [8]. Among all, the luminescence obtained from trivalent Erbium ions due to the transitions form I13/2 to the ground state 4I15/2 at 1530 nm is of great interest in optical communication applications. Several works on Erbium ions doped glass has been carried on. For example, N. Chanthima et al. [9] studied the luminescence properties of Er3+ doped barium boro-phosphate glasses having a composition of 25BaO-5B2O3-(70-x)P2O5-xEr2O3. Samples were excited by 521 nm which results in stronger luminescence in NIR region than 379 nm and 978 nm excitation sources. In this study prepared samples were proposed to be a promising candidate for emitting source in the NIR region and telecommunication system at 1.5 μm window. H. Linet al. [10] studied Er3+ doped Na2O.Cd3Al2Si3O12 glass. Strong luminescence at 1535 nm and up-converted green fluorescence were observed at room temperature. In present work, BP glass is prepared to probe its various luminescence properties for optoelectronics applications. BP glass is used as a host for Er3+ ions due to its low melting temperature while, ZnO and PbO are added to make the glass rigid.

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2. Experimental Approaches

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2.1 Sample Preparation

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Lead boro-phosphate glass of composition 50P 2O5 - 10B2O3 - 20ZnO - 20PbO-xEr2O3 in mole%, where x= (0, 0.005, 0.01, 0.015, 0.02, 0.025wt. %) were prepared by melt quenching method [11]. The ingredients of 99.9% purity were used. The powder was homogeneously mixed and poured in a platinum crucible. The mixture was placed in a muffle furnace and melted at the temperature of 11500C for 1 hour. Melt was constantly stirred for homogeneity and poured onto the aluminum mold of a thickness of 2cm [12].The melted mixture was then allowed to cool at 250C temperature.

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2.2 Characterization Techniques Samples were investigated with the help of different characterization tools. XRD patterns were obtained using PAN analytical X’pert pro super diffractometer with Cu-Kα radiations of wavelength (λ=1.5418 Å) working at high voltage 45 kV and current 40 mA with an incident beam path of 240 mm. For the collection of Raman spectra, Renishaw in Via Raman 2000 microspectrometer equipped with a 300 mW and 514nm laser diode was used. UV-VIS and near-infrared spectroscopy are done by UV-VIS/ NIR spectrometer Lambda 750 by Perkin Elmer between the wavelength ranges of 200-2000 nm. Photoluminescence was then obtained by exposing the samples to 514 nm laser for 10 seconds with laser power 1%. Inter-molecular absorption interactions between components and structure of glass were investigated by FTIR (Thermo Scientific Nicolet 8700) spectroscopy within 1600– 600 cm−1 range. 3. Results and Discussions 3. 1 X-Rays Diffraction X-Ray Diffraction analysis was performed to probe the amorphous nature of samples. Broad humps in the spectra ensure the amorphous nature of glass samples [13] as shown in figure 1while the image of studied glass is given in figure 2.

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Figure1 XRD result of Er3+ doped boro-phosphate glass samples

Figure 2 Image of studied boro-phosphate glass

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3.2 Density and molar volume

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In the studied series of glasses, the density decreased by increasing the contents of Er3+ up to 0.015 wt. % due to the formation of non-bridging oxygen (NBO) in the BP0-BP2 glass networks, which indeed opens the lattice network of amorphous structure. But the further increase in the concentration of Er3+ ions in the glass increase the density of glass. It is presumed that the addition of Er3+ ions beyond 0.015 wt. % ions will enter in the glass network by breaking the phosphate double bond. As a result, the amount of bridging oxygen’s (BO) in the glass network will increases, hence the density of glass BP3 and BP4 will increase [21]. Molar volume of the material mainly depends upon its density [3]. Decrease in the density of glass leads to an increase in the molar volume as shown in the table 1[14]. Hence the variation in molar volume with respect to density is predicted as expected [15].

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Table 1 shows Density, Direct energy band gap, indirect energy band gap, molar volume and refractive index of the glass

Base BP0 BP1

0 0.005 0.01

4.06 4.01 3.984

Direct Energy bandgap (eV) 4.23 4.19 4.17

Indirect Energy bandgap (eV) 4.170 4.165 3.94

Molar volume (cm3/mol)

Refractive Index (n)

34.179 34.618 35.410

2.137 2.147 2.167

BP2

0.015

3.923

3.98

3.861

34.858

2.197

BP3

0.02

4.228

4.29

4.187

32.868

2.134

BP4

0.025

4.231

4.32

4.237

32.857

2.125

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Density ( g/cm3)

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Glass Erbium composition Concentration (Wt. %)

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3. 3 Raman Spectra Figure 3 indicates the Raman spectra of BP glass between the frequency ranges of 100 cm-1 to 1800cm-1 [16]. The band at 339cm-1 in the graph is due to the bending oscillations of O– P–O phosphate units [17, 18]. The band near 638 cm-1 reveals the B-O-P stretching in which boron atom is incorporated in the phosphate network [17, 19]. The peak near 1106 cm-1 indicates the formation of meta-phosphate network of (PO3)n chain, rings and symmetric stretching of PO2 units [16, 20] while the peak near 1399 cm-1 is responsible for the stretching of P-O bonds in the glass [18]. The peak’s position and their respective vibrational channels are listed in Table 2

Figure 3 Raman spectra of boro- phosphate glass

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Table 2 Raman band assignments for boro-phosphate glass Raman band (cm-1) 339 638 1106 1399

Assignments Bending oscillation of O-P-O unit. Stretching vibration of B-O-P units. Symmetric stretching of PO2 units. Widening vibration of P-O.

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3.4 Fourier transforms infrared spectroscopy

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Fourier transform infrared spectroscopy analysis was performed to check the network structure of the glass samples as shown in figure 4 [21]. All samples exhibit similar bands position, which clearly shows that the addition of erbium oxide creates no effect on the glass network. The absorption peak at 754 cm-1 shows the symmetric vibrations of PO in P-O-P bonds [22]. The vibrational band at 901cm-1 is assigned to the meta-phosphate unit overlapping with non-bridging oxygen of BO4 group [21-23]. The peak centered at 1009 cm-1 is due to symmetric stretching of both O-P-O bonds and B-O of BO4 unit [24]. The vibrational bands at 1212-1522 cm-1 are attributed to the vibration of non-bridging clusters of PO2 unit and stretching vibration of BO3 in Penta, ortho, pyro, and meta-borate groups [23, 24, 26-28]. The Peak observed at 1695 cm-1 is allocated to the vibration of the H-O-H bond in the glass network [23, 25]. All observed peaks and respective vibration channels are tabulated in table 3.

Figure 4 FTIR spectra of50P 2O5 - 10B2O3 - 20ZnO - 20PbO-xEr2O3

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Table 3 FTIR Bands Assignments FTIR bands (cm-1) 754 901 1009 1212-1522 1695

Assignments Symmetric oscillations of PO in P-O-P unit Overlapping of the meta-phosphate unit with non-bridging oxygen of BO4 group Symmetric vibration of O-P-O bonds and B-O of BO4 unit The vibration of non-bridging clusters of PO2 and stretching vibration of BO3 in Penta, ortho, pyro, and meta-borate groups Oscillation of H-O-H bond within the glass network

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3.5 Photoluminescence

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Photoluminescence (PL) spectra of glass samples are given in figure 5. First peak in the PL spectra is centered at a wavelength of 526 nm is due to the 2H11/2→4I15/2 transition. The peak at 550 nm shows the 4S3/2→4I15/2transition in the glass sample. The peak of weak intensity is observed at 63 nm in the PL spectra is due to 4F9/2→4I15/2 transition. According to the spectra, Increase in the Rare earth ions increases the emission intensity of the glass samples.

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Figure 5 Photoluminescence of Er3+ doped boro-phosphate glass

3.6 UV-Vis absorption spectroscopy Figure 6 shows the UV-VIS-NIR absorption spectra of BP glass series in the wavelength range of 300-1600 nm. Ten absorption bands appeared due to electric dipole transition, which are centered at 360, 380, 410, 488, 491, 522, 649, 800, 976 and 1530 nm. All peaks could be attributed to the transitions from the fundamental state 4I15/2 to the various excited states 2G7/2, 4 G11/2,2G9/2,4F5/2,4F7/2,2H11/2, 4F9/2,4I9/2,4I11/2 and 4I11/3, respectively. The intensity increased due to an increase in the amount of Er3+ ions [15, 26-28]. Meanwhile, no shift in peaks observed in all Er3+ doped BP glass samples.

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Figure 6 a) shows the UV VIS spectra while b) indicates spectra in the infrared region of the prepared glass

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3.7 Energy band gap The graph of the direct and indirect band gap energy is given in Figures 7. First, the Energy band gap decreased by increase in the amount of Er3+ concentration up to 0.015 wt. % due to the increasing number of NBO in the BP0, BP1 and BP2 glass network but, further addition of Er3+ ions in the glass increased the energy band gap of glass BP3 and BP4 due to the formation of BO. Similar behavior is observed for In-direct energy band gap as given in table 3. Electrons are tightly bounded to the BO’s as compared to the NBO’s in the glass network hence more energy is needed to excite electrons from ground state to the excited state. Similar trend of result is observed in the density of glass [29, 30].

Figure 7 a) shows the direct energy band gap b) indicates indirect energy band gap of the boro-phosphate glass

3.8 Refractive index Variation of refractive index with varying the erbium oxide is given in table 3. Initially refractive index increased for the glass samples then decreased. Incorporating of erbium oxide reconstructs the boro-phosphate network and grows NBO’s in the network. The ionic character of NBO’s decreases bond energy and increases the polarizability and hence, refractive index

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increased. Addition of erbium ions beyond 0.15 wt.% Er ions increase the formation of nonbridging oxygen in the glass which reduced the polarizability of glass, as a result, the refractive index decreased [30].

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4. Theoretical Insight 4.1 Judd Ofelt analysis Radiative analysis of rare earth embedded glass could be evaluated by its absorption spectra. Sharp absorption lines in the spectra are due to the inner band 4f-4f transitions. Ligand environment around rare earth ions is responsible for splitting and mixing of 4f energy level with opposite parity (4fn-15d) [31]. The intensity of line strength decreased by increase in concentration Er3+ ions in the glass network. It is noted that the experimental and calculated line strength for hypersensitive transition (4I15/2→2H11/2) and ( 4I15/2→4G11/2) is more than other transitions which make it suitable for green emission [32]. The root means square (rms) values indicate the deviations between experimental and calculated line strength. In this work, rms deviation shows a good agreement. Experimental, theoretical line strength and root mean square error (10-2 cm2) for Er3+ doped BP0, BP1, BP2, BP3, and BP4 glass samples are given in Table 4 [17].

λ

Sexp

Stheo

Sexp

2G 7/2

360

0.68

0.04

0.22

4G 11/2

380

0.791

0.87

0.45

2G 9/2

410

0.488

0.09

2F 5/2

448

0.441

0.03

2H 11/2 4F 9/2 4I

9/2

11/2

4I

11/3

rms →

Stheo

Sexp

Stheo

Sexp

Stheo

0.01

0.21

0.01

0.2

0.06

0.54

0.42

0.44

0.45

0.47

0.40

0.46

0.11

0.04

0.16

0.09

0.16

0.08

0.19

0.07

0.19

0.03

0.13

0.05

0.15

0.04

0.18

0.03

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0.24

491

0.397

0.22

0.17

0.18

0.16

0.11

0.14

0.06

0.15

0.05

522

0.805

0.69

0.50

0.41

0.46

0.38

0.45

0.37

0.44

0.36

649

0.57

0.57

0.18

0.15

0.17

0.16

0.18

0.15

0.15

0.16

800

0.203

0.01

0.05

0.08

0.07

0.09

0.02

0.08

0.01

0.05

976

0.421

0.17

0.08

0.07

0.07

0.06

0.08

0.03

0.03

0.02

153

0.199

0.58

0.12

0.22

0.14

0.14

0.13

0.13

0.18

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4I

Sexp

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7F 4/2

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Transitions

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Table 4 shows the Experimental and theoretical line strength (10-20 cm2) for each glass sample. Samples→ BP0 BP1 BP2 BP3 BP4

0.0000

0.0997

0.1123

0.099

0.0958

JO intensity parameters Ωt (t=2, 4, 6) are very useful parameters to determine the rigidness, local structure, and asymmetry around rare earth ions. These parameters can be estimated by leastsquare fit of experimental and calculated line strength [33]. Parameter Ω2 indicates the symmetry of surrounding around the rare earth ions. Increase in the Ω2 value is assigned to the decrease in the symmetry around the rare earth ions. Hence an increase in the concentration of rare earth

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ions in the glass network leads to an increase in the symmetry of around rare earth ions [32]. In this work the trend of JO intensity parameters is Ω2> Ω4 > Ω6 which is similar to the Er3+ doped phosphate glass [34] and different form Er3+ doped boro–tellurite glass [32]. The ratio Ω4 /Ω6 is known as a spectroscopic quality factor which predicts the potentiality of glass for lasing operation. The spectroscopic ratio is calculated for the present Er3+ doped BP glass, which is closed to the other Erbium-doped phosphate glass[34] and higher than Er3+ doped boro–tellurite glass [32]. JO intensity parameters and spectroscopic quality factors for present work are tabulated in Table 5. Table 5 JO intensity parameters (10-20 cm2) and RMS (10-20 cm2) values for studied glass Ω6 0.284 0.136 0.112 0.106 0.105 1.55 1.79

Ω4 /Ω6 2.733193 1.123348 1.897849 1.857143 1.919771 1.53 0.82

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Ω4 0.777 0.153 0.212 0.198 0.201 2.38 1.45

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BP0 BP1 BP2 BP3 BP4 [34] [32]

Ω2 0.4953 0.4766 0.4027 0.3935 0.3759 5.64 3.13

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Sample

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It is shown through this work that the contribution of magnetic dipole transition probability is zero for this glass. The transition probability is totally electric dipole in nature. The total transition probability for (4F7/2→4I15/2), (2H11/2→4I15/2) and ( 4F9/2→4I15/2) transitions is more than other transition channels. Branching ratio is the ratio of transition between excited state J’ to the lower state J among other transition channels, the branching ratio of (2H11/2→4I15/2), (4F9/2→4I15/2) and (4I11/3→4I15/2) channels are prominent in present work.

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Table 4 shows the total transition probabilities (1/Sec), wavelength (nm), radiative lifetime (ms) and branching ratio. Transitions BP0 BP1 BP2 λ A β t A β t A β t 4F →4I15/2 492.3 1386.2 0.6992 0.504 232.12 0.7359 3.1704 222.13 0.687 3.093 7/2 2H 4 15/2 522.2 1827.7 0.8852 0.484 499.32 0.8499 1.7021 472.35 0.8407 1.78 11/2→ I 4F →4I15/2 660.7 812.08 0.9163 1.128 103.18 0.8467 8.2062 120.03 0.8637 7.196 9/2 4I →4I15/2 814.9 106.23 0.8515 8.016 10.579 0.6835 64.613 14.663 0.7726 52.69 9/2 4I 4 15/2 989 44.952 0.6899 15.35 11.917 0.5821 48.847 10.038 0.5393 53.72 11/2→ I 4I 4 15/2 1538 99.901 0.9233 10.01 40.863 0.9122 24.445 40.527 0.8993 24.67 11/3→ I Transitions BP3 BP4 λ A β t A β t 4F →4I15/2 492.3 201.17 0.6865 3.4124 198.98 0.6833 3.434 7/2 2H 4 15/2 522.2 436.56 0.8376 1.9187 421.14 0.834 1.981 11/2→ I 4F →4I15/2 660.7 107.81 0.8598 7.9749 107.99 0.8611 7.974 9/2 4I →4I15/2 814.9 13.09 0.7671 58.599 13.235 0.772 58.33 9/2 4I 4 15/2 989 9.197 0.5304 57.671 8.951 0.5253 58.69 11/2→ I 4I 4I15/2 1538 38.414 0.8332 26.021 38.185 0.9833 26.21 → 11/3

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The total radiative lifetime increased by increasing the concentration of rare earth ions in the glass network. Several properties such as emission probability, branching ratio and radiative lifetime are given in Table 7.

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5. Conclusion The Er3+ doped boro-phosphate glass was prepared by the melt quench technique. The density of glass decreased by addition of erbium contents up to 0.015 wt. % due to the formation of non-bridging oxygen’s in the network while further addition of Erbium ions decreased the density by increasing the amount of bridging oxygen’s. A similar trend is observed in the band gap energy, as long as the concentration of Er3+ contents increased up to 0.015 wt. %, the band gap energy decreased due to the formation of non-bridging oxygen’s which reduced the energy of the system. Increase in the Erbium concentration beyond 0.015 wt. % in glass increased the energy band gap of glass due to formation bridging oxygen’s.

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Large value to Judd Ofelt intensity parameter Ω2 as compared to Ω4 and Ω6 shows the covalency of Er-O bond and asymmetry across the Erbium ions. While the large value of Ω4/Ω6 shows that the prepared samples are highly rigid. Branching ratios for 4F9/2, 2H11/2 to 4I15/2 are high as compared to other transition channels. The emission at 550 nm is more intense as compared to other emission channels. The result shows that present glass can be used as a green solid-state laser medium and for the down-conversion process.

Declaration of interests

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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[6]

[21] [22]

[23]

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[28] [29] [30]

[31]

[32]

[33]

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[34]

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[26]

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