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On the dual role of ZnO in zinc-borate glasses
Journal of Non-Crystalline Solids 432 (2016) 406–412
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
On the dual role of ZnO in zinc-borate glasses S. Cetinkaya Colak ⁎, I. Akyuz, F. Atay Eskisehir Osmangazi University, Department of Physics, 26480 Eskisehir, Turkey
a r t i c l e
i n f o
Article history: Received 8 July 2015 Received in revised form 12 October 2015 Accepted 26 October 2015 Available online 4 November 2015 Keywords: ZnO; Zinc-borate glass; FT-IR; Temperature dependent electrical measurements; Optical constants; Spectroscopic ellipsometry
a b s t r a c t ZnO incorporated borate glass structures have been produced by classical melt-quench technique. The effect of ZnO concentration on the behavior of ZnO in glass network as a modifier or network former has been clearly defined by combining various characterization tools. FT-IR transmittance spectra have been used to study the structure of glass systems. Temperature dependent electrical measurements have been performed to investigate the conduction mechanisms, especially at low temperatures. Activation energies determined by considering classical Arrhenius relation and small polaron hopping mechanism support the comments on the behavior of ZnO in glass network. Optical band gap and Urbach energy values have been calculated using optical measurements. Also, Spectroscopic Ellipsometry technique has been used to determine optical constants (refractive index and extinction coefficient) by Cauchy–Urbach model. Results showed that refractive index of borate glasses can be controlled by the composition of ZnO in the structure. Combining all of the outputs from different characterizations, it has been concluded that the amount of ZnO in borate glass structure has a dramatic effect on the number of non-bridging oxygen atoms that determines the role of ZnO as a modifier/network former. ZnO acted as a network former in samples having more than 5% (weight%) incorporation. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In recent years, borate based glass structures have attracted the attention of researchers because of their unique properties such as high transmittance, thermal stability, low melting point and high glass forming ability. These glass structures are widely used in applications such as opto-acoustical electronics, piezoelectric actuators, solid-state laser materials, optical glasses, luminescent materials and microelectronics [1–8]. In addition to such attractive properties and application fields, studies related to the development of physical properties and diversification of the practical applications of borate glass structures are required. Exhibiting a suitable structure for the addition of different alkaline metals, alkaline earths and transition metals makes these glass structures promising materials for various technological applications [9–14]. In particular, ZnO containing borate glass structures draw attention as noteworthy materials [15–20]. ZnO is a material known for its low cost, being non-toxic and good environmental acceptability [21–23]. ZnO may join the glass structure as a modifier or network former. When acting as a network modifier, it breaks B–O–B bonds and leads to the formation of non-bridging oxygen (NBO) atoms together with the defects known as dangling bonds. However, ZnO, when played a network former role, joins the glass network as ZnO4 structural units, where zinc is linked to four oxygen ions in a covalent bond configuration [24–25]. ⁎ Corresponding author. E-mail address: [email protected] (S. Cetinkaya Colak).
http://dx.doi.org/10.1016/j.jnoncrysol.2015.10.040 0022-3093/© 2015 Elsevier B.V. All rights reserved.
Studies are carried out with different characterization methods to brighten this complex behavior of ZnO in borate glass structures. However, a systematic examination of this problem with respect to the composition of ZnO in the structure and interpretation of a detailed data in terms of optical, structural and electrical properties are required. With this aim, in our work, ZnO with different amounts has been incorporated into the borate glass structures and behavior of ZnO in these structures has been investigated. In this work, the dual role of ZnO in borate glass structures has been evaluated using the outputs taken from the combination of optical measurements including UV–VIS absorption spectroscopy and determination of optical constants by spectroscopic ellipsometry, temperature dependent electrical measurements and FT-IR transmittance data with a different perspective.
2. Experimental 2.1. Glass preparation ZnO incorporated borate glass structures have been prepared with the chemical composition of 1% TiO2–3% CaO–1% Al2O3–15% Na2O–(80 − x)% B2O3: x ZnO (x = 0, 5, 10 and 15 weight %), respectively. The details of the used compositions for the current study are listed in Table 1. These glasses have been prepared in a pure platinum crucible containing high purity H3BO3 (≥ 99.5%), CaO (reagent grade), Al2O3 (99.9%), Na2CO3 (99.9%), TiO2 (99.9%) and ZnO (99.9%) chemicals at 1300 °C by melt-quenching technique. All samples have been annealed at 350 °C for 1 h.
S. Cetinkaya Colak et al. / Journal of Non-Crystalline Solids 432 (2016) 406–412 Table 1 Compositions (weight %) for the prepared glass structures. Sample
TiO2
CaO
Al2O3
Na2O
B2O3
ZnO
BZ0 BZ5 BZ10 BZ15
1 1 1 1
3 3 3 3
1 1 1 1
15 15 15 15
80 75 70 65
– 5 10 15
2.2. Characterization tools The FT-IR transmission spectra of the glasses have been recorded on a Perkin Elmer 100 FT-IR spectrometer in the range of 2000–400 cm−1 by ATR (Attenuated Total Reflection), with a resolution of 4 cm− 1. Temperature dependent electrical measurements of the samples have been taken in the range of 65–420 K with 0.1 K intervals by using a cryostat system combined with a Keithley 6514 hot-stage high temperature electrical measurement set-up. The optical transmittance spectra of the samples have been taken by Schimadzu UV 2550 Spectrophotometer in the wavelength range of 300–900 nm at room temperature, with sampling interval of 2 nm. OPT-S9000 Spectroscopic Ellipsometer has been used to determine the optical constants (refractive index and extinction coefficient) of the samples. Measurements have been taken in the range of 1200–1600 nm by 75° angle of incidence. Required precautions related to the operator or instruments have been taken to eliminate the random errors. Systematic errors for a specific result have been calculated by combining the errors arising from instrument and measurements in the related mathematical equations. 3. Results and discussion 3.1. FT-IR analysis FT-IR spectroscopy has been used to study the structure of the glasses in detail. Fig. 1 shows the FT-IR transmittance spectra of the zinc borate glass structures. Generally, IR spectra of borate glasses contain three main regions. These are: i) the band which occurs at 1600–1200 cm−1 which is attributed to the B–O stretching vibrations of trigonal BO3 units. ii) the second band between 1200 and 800 cm−1
Fig. 1. FT-IR spectra of zinc-borate glass structures.
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which belongs to the B–O stretching vibrations of tetragonal BO4 units and iii) the third band at ~700 cm−1 which corresponds to the B–O–B bending vibrations of bridging oxygen atoms [17,26,27]. It is clear from Fig. 1 that FT-IR spectra of the glass structures produced in this work exhibit a similar behavior (labeled as a, b, c, and d). In ZnO containing borate glass structures, ZnO may act as a network former or modifier. These can be deduced from the FT-IR spectra. It is known that ZnO which acts as a network former in borate glasses shows itself with a band between 400 and 550 cm−1 which belongs to ZnO4 units [18,28]. In our study, we can see some traces of these for samples BZ10 and BZ15 with the peak located at ~ 450 cm−1. These confirm that ZnO play a network former role for these glasses. The absence of this peak for BZ5 glass calls to mind the idea of ZnO acting as a modifier for this incorporation rate. Another remarkable point in Fig. 1 is changing relative intensity of two peaks indexed as b and d. According to the literature, peak b is attributed to the B–O stretching vibrations containing linkage oxygen connecting different groups [3,29]. However, peak d is assigned to the B–O–B bending vibrations of bridging oxygen atoms [17]. Our results show that relative intensity of these peaks is lower for sample BZ5, which supports the idea of ZnO acting as modifier for this glass structure. For higher ZnO concentrations (sample BZ10 and BZ15), these peaks play noticeable and active role in FT-IR spectra. This indicates the dominant role of bridging oxygen atoms in glass structure where ZnO behaves as a network former. 3.2. Electrical properties Electrical conduction mechanisms of glass structures have been investigated by using temperature dependent electrical measurements. Fig. 2 shows the ln(σ) ~ 1000/T plots in the range of 65– 420 K. There are two regions in these plots where conductivity exhibits different behaviors. Linear region at high temperatures has been labeled as region I, while the region with the deviation from linearity has been labeled as region II. For each glass structure, the mechanism in region I has been examined using the classical Arrhenius relation given below:
σ ðT Þ ¼ σ o exp½−ΔE=kT
ð1Þ
where σo is a pre-exponential value, ΔE is the activation energy, k is Boltzmann's constant and T is the absolute temperature [30]. Activation energy (Ea) values calculated from region I are given in Table 2. Ea values have been calculated to be between 225 and 269 meV. As seen from Table 2, there is a decrease in the Ea value of sample BZ5. ZnO may play two different roles in glass structures as a modifier or network former depending on its concentration. If ZnO behaves as a modifier, the number of NBOs will increase and this will cause the expansion of glass network [31,32]. In this case, alkali metal ions will be able to move more freely and this effect will show itself as a decrease in Ea values. Unlike this, for sample BZ10 that contains more ZnO than sample BZ5, this downward trend has not continued and even for sample BZ15 an increase in Ea value has been observed. ZnO, in addition to acting as a modifier in borate glass structures, could play a role as an oxide that joined the glass network. In such a structure, NBO density will be less and an increase will be observed in E a values. This refers that samples BZ10 and BZ15 composed of tighter network structure where diffusion of Na+ ions become difficult and their mobilities decreased. For the second region where hopping mechanism is active, small polaron hopping (SPH) and variable range hopping (VRH) conduction mechanisms have been considered and a better fit has been found for SPH mechanism. So, electrical conductivity data of region II has been
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S. Cetinkaya Colak et al. / Journal of Non-Crystalline Solids 432 (2016) 406–412
Fig. 2. Temperature dependent conductivity variations of zinc-borate glass structures used for activation energy calculations.
interpreted according to SPH mechanism. For this region, ln (σT) ~ 1000/T plots are given in Fig. 3 and activation energies for SPH mechanism (Es1 and Es2) have been calculated using the relation given below [30,33]:
σ ðT Þ ¼ ðσ o =T Þ exp½−ΔE=kT
localized regions to decrease and Es1 and Es2 values to increase. The concepts that we have used in the above discussion, on the density of NBOs are in agreement with the peaks representing the vibrations related to the bridging oxygen in FT-IR spectra which can be recognized clearly especially for samples BZ10 and BZ15.
ð2Þ 3.3. Optical properties
These values are also given in Table 2. With Es1 = 46 meV and Es2 = 14 meV, sample BZ5 has the lowest activation energy values among other samples. ZnO, as a modifier in the glass structure for sample BZ5, would probably increase the possibility of hopping conduction in localized levels because of the higher number of NBOs and this will in turn cause Es1 and Es2 values to decrease. As can be seen from Table 2, Es1 and Es2 values for BZ10 and BZ15 are higher. Lower number of NBOs in these glass structures causes the width of the defect levels in
Table 2 Activation energy values of zinc-borate glass structures.
3.3.1. UV–VIS spectroscopy Transmittance spectra of zinc borate glasses are given in Fig. 4. It is clear from these spectra that there is an increase in the transmittance values of borate glasses containing high amounts of ZnO in the visible region. Optical band gap values, width of the localized tail states and defect levels may be determined by investigating the dependence of absorption coefficient on photon energy. In the region with strong absorption above the mobility edge, absorption coefficient is given by [34,35]: αhv ∝ hv Eopt
Sample
Ea (meV)
ES1 (meV)
ES2 (meV)
BZ0 BZ5 BZ10 BZ15
248.90 ± 0.09 225.20 ± 0.08 226.00 ± 0.08 268.80 ± 0.09
59.00 ± 0.02 46.00 ± 0.01 108.10 ± 0.03 132.10 ± 0.04
17.000 ± 0.006 14.000 ± 0.004 20.400 ± 0.007 28.000 ± 0.009
2
ð3Þ
where α is absorption coefficient, hv is absorbed photon energy and Eopt is optical band gap. Optical band gap values of glass structures determined by using (αhv)1/2 ~ hv plots for borate based structures are given in Fig. 5. Also, in the region below the strong absorption,
S. Cetinkaya Colak et al. / Journal of Non-Crystalline Solids 432 (2016) 406–412
409
Fig. 3. ln (σT) ~ 1000/T plots of zinc-borate glass structures used for Es1 and Es2 values.
absorption coefficient (α) exhibits an exponential variation. In this region, α is given by [34,36]: α ∝ expðhv=EU Þ
where Eu represents the width of the localized tail states. Similarly, absorption coefficient can be expressed as another exponential function of photon energy;
ð4Þ a ∝ expðhv=Ed Þ
Fig. 4. Transmittance spectra of zinc-borate glass structures at room temperature.
ð5Þ
here, Ed is known as the width of the defect states and generally takes higher values than Eu. It is considered that the energy values calculated in this regime are closely related to the structural properties of materials [37]. Urbach energies (Eu) and defect related energies (Ed) for borate based glasses have been calculated using the ln (α) ~ hν plots given in Fig. 6. The calculated values of Eopt, EU and Ed are given in Table 3. Sample BZ5 has the lowest optical band gap value of 3.709 eV. It is known that defects like dangling bonds or NBOs in glasses cause static atomic structural disorder and this affect the absorption characteristic. Higher NBO concentration in these systems causes lower optical band gap and higher Urbach energy values [38–41]. When we consider the energy values in Table 3 and higher NBO density of sample BZ5 inferred from FT-IR and electrical characterizations, as expected, this sample has lower optical band gap and higher Urbach energy values with respect to samples BZ10 and BZ15. This explanation is another proof of ZnO acting as a modifier at low concentrations and as a network former at high concentrations. Lower Ed values of samples BZ10 and BZ15, as 1.112 eV and 0.962 eV in Table 3, also support this comment considering the
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S. Cetinkaya Colak et al. / Journal of Non-Crystalline Solids 432 (2016) 406–412
Fig. 5. (αhν)1/2 ~ hν plots used to determine optical band gap values of zinc-borate glass structures.
decrease in the width of localized states and the SPH activation energy data given in Table 2. 3.3.2. Spectroscopic ellipsometry Spectroscopic ellipsometry technique has been used to determine the optical constants (refractive index and extinction coefficient) of borate based glasses. Psi spectra taken from the device and theoretical values obtained using Cauchy–Urbach model have been evaluated. After the successful fitting of the data, refractive index and extinction coefficient values have been obtained. Refractive index spectra are given in Fig. 7 and Cauchy–Urbach model parameters and optical constants are given in Table 4. Refractive index values of samples are determined to be between 1.39 and 1.48. It is clear from Fig. 7 that ZnO containing borate glasses have higher refractive index values. Sample BZ5 has lower content of ZnO than other samples. However, it has a refractive index value comparable to sample BZ15 which has the highest ZnO content. We know that ZnO acts as a modifier for sample BZ5, which causes an increased number of NBO's in the structure. This will result in a higher refractive index value than expected for this sample.
4. Conclusion Zinc borate glass structures have been obtained by melt-quenching technique for three different ZnO concentrations. Two different roles of ZnO in these structures have been examined in detail with the outputs of various characterization techniques. ZnO acted as a network former in samples having ZnO more than 5%. FT-IR data showed traces for the behavior of ZnO as a modifier/network former. Wavenumbers and relative intensities of the peaks in FT-IR spectra have been used to brighten the role of ZnO in borate glass structures. Activation energy values calculated from temperature dependent electrical measurements exhibited similar results. Also, it has been determined that width of the localized tail states for sample BZ5 is larger than those of other samples because of the increased number of NBO's. Spectroscopic ellipsometry measurements combined with Cauchy–Urbach model enabled the determination of optical constants and results showed that refractive index value can be controlled with the amount of ZnO in borate structures. High NBO in sample BZ5 caused this sample to have the highest refractive index among others. Sample BZ15, including ZnO at the highest rate, has the highest transmittance in visible
S. Cetinkaya Colak et al. / Journal of Non-Crystalline Solids 432 (2016) 406–412
411
Fig. 6. Urbach plots of zinc-borate glass structures.
region which may decrease the losses in light related applications arising from substrate. Acknowledgments Authors are thankful to Eskisehir Osmangazi University Scientific Research Projects Commission for providing financial support under the project number of 201319013. Authors also wish to express their gratitude to Prof. Dr. G. S. Kurkcuoglu for her help in FT-IR measurements. Table 3 Optical band gap (Eopt), Urbach energy (EU) and defect energy (Ed) values of zinc-borate glass structures. Sample
Eopt (eV)
EU (eV)
Ed (eV)
BZ0 BZ5 BZ10 BZ15
3.820 ± 0.021 3.709 ± 0.021 3.800 ± 0.025 3.816 ± 0.022
0.222 ± 0.021 0.364 ± 0.021 0.222 ± 0.025 0.213 ± 0.022
1.984 ± 0.021 2.226 ± 0.021 1.112 ± 0.025 0.962 ± 0.022
Fig. 7. Refractive index spectra of zinc-borate glass structures obtained by the help of Spectroscopic ellipsometry measurements.
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Table 4 Cauchy–Urbach model parameters and optical constants of zinc-borate glass structures. Sample An
Bn (nm)2
Cn (nm)4
Ak
Bk (eV)−1
kort (×10−5)
nort
MSE
BZ0 BZ5 BZ10 BZ15
0.011 0.010 0.010 0.011
0.022 0.020 0.019 0.017
0.002 0.002 0.001 0.001
1.12 1.12 1.12 1.12
4.65 4.80 4.45 4.10
1.39 1.48 1.45 1.47
0.01 0.01 0.03 0.02
1.385 1.467 1.440 1.467
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Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
On the dual role of ZnO in zinc-borate glasses S. Cetinkaya Colak ⁎, I. Akyuz, F. Atay Eskisehir Osmangazi University, Department of Physics, 26480 Eskisehir, Turkey
a r t i c l e
i n f o
Article history: Received 8 July 2015 Received in revised form 12 October 2015 Accepted 26 October 2015 Available online 4 November 2015 Keywords: ZnO; Zinc-borate glass; FT-IR; Temperature dependent electrical measurements; Optical constants; Spectroscopic ellipsometry
a b s t r a c t ZnO incorporated borate glass structures have been produced by classical melt-quench technique. The effect of ZnO concentration on the behavior of ZnO in glass network as a modifier or network former has been clearly defined by combining various characterization tools. FT-IR transmittance spectra have been used to study the structure of glass systems. Temperature dependent electrical measurements have been performed to investigate the conduction mechanisms, especially at low temperatures. Activation energies determined by considering classical Arrhenius relation and small polaron hopping mechanism support the comments on the behavior of ZnO in glass network. Optical band gap and Urbach energy values have been calculated using optical measurements. Also, Spectroscopic Ellipsometry technique has been used to determine optical constants (refractive index and extinction coefficient) by Cauchy–Urbach model. Results showed that refractive index of borate glasses can be controlled by the composition of ZnO in the structure. Combining all of the outputs from different characterizations, it has been concluded that the amount of ZnO in borate glass structure has a dramatic effect on the number of non-bridging oxygen atoms that determines the role of ZnO as a modifier/network former. ZnO acted as a network former in samples having more than 5% (weight%) incorporation. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In recent years, borate based glass structures have attracted the attention of researchers because of their unique properties such as high transmittance, thermal stability, low melting point and high glass forming ability. These glass structures are widely used in applications such as opto-acoustical electronics, piezoelectric actuators, solid-state laser materials, optical glasses, luminescent materials and microelectronics [1–8]. In addition to such attractive properties and application fields, studies related to the development of physical properties and diversification of the practical applications of borate glass structures are required. Exhibiting a suitable structure for the addition of different alkaline metals, alkaline earths and transition metals makes these glass structures promising materials for various technological applications [9–14]. In particular, ZnO containing borate glass structures draw attention as noteworthy materials [15–20]. ZnO is a material known for its low cost, being non-toxic and good environmental acceptability [21–23]. ZnO may join the glass structure as a modifier or network former. When acting as a network modifier, it breaks B–O–B bonds and leads to the formation of non-bridging oxygen (NBO) atoms together with the defects known as dangling bonds. However, ZnO, when played a network former role, joins the glass network as ZnO4 structural units, where zinc is linked to four oxygen ions in a covalent bond configuration [24–25]. ⁎ Corresponding author. E-mail address: [email protected] (S. Cetinkaya Colak).
http://dx.doi.org/10.1016/j.jnoncrysol.2015.10.040 0022-3093/© 2015 Elsevier B.V. All rights reserved.
Studies are carried out with different characterization methods to brighten this complex behavior of ZnO in borate glass structures. However, a systematic examination of this problem with respect to the composition of ZnO in the structure and interpretation of a detailed data in terms of optical, structural and electrical properties are required. With this aim, in our work, ZnO with different amounts has been incorporated into the borate glass structures and behavior of ZnO in these structures has been investigated. In this work, the dual role of ZnO in borate glass structures has been evaluated using the outputs taken from the combination of optical measurements including UV–VIS absorption spectroscopy and determination of optical constants by spectroscopic ellipsometry, temperature dependent electrical measurements and FT-IR transmittance data with a different perspective.
2. Experimental 2.1. Glass preparation ZnO incorporated borate glass structures have been prepared with the chemical composition of 1% TiO2–3% CaO–1% Al2O3–15% Na2O–(80 − x)% B2O3: x ZnO (x = 0, 5, 10 and 15 weight %), respectively. The details of the used compositions for the current study are listed in Table 1. These glasses have been prepared in a pure platinum crucible containing high purity H3BO3 (≥ 99.5%), CaO (reagent grade), Al2O3 (99.9%), Na2CO3 (99.9%), TiO2 (99.9%) and ZnO (99.9%) chemicals at 1300 °C by melt-quenching technique. All samples have been annealed at 350 °C for 1 h.
S. Cetinkaya Colak et al. / Journal of Non-Crystalline Solids 432 (2016) 406–412 Table 1 Compositions (weight %) for the prepared glass structures. Sample
TiO2
CaO
Al2O3
Na2O
B2O3
ZnO
BZ0 BZ5 BZ10 BZ15
1 1 1 1
3 3 3 3
1 1 1 1
15 15 15 15
80 75 70 65
– 5 10 15
2.2. Characterization tools The FT-IR transmission spectra of the glasses have been recorded on a Perkin Elmer 100 FT-IR spectrometer in the range of 2000–400 cm−1 by ATR (Attenuated Total Reflection), with a resolution of 4 cm− 1. Temperature dependent electrical measurements of the samples have been taken in the range of 65–420 K with 0.1 K intervals by using a cryostat system combined with a Keithley 6514 hot-stage high temperature electrical measurement set-up. The optical transmittance spectra of the samples have been taken by Schimadzu UV 2550 Spectrophotometer in the wavelength range of 300–900 nm at room temperature, with sampling interval of 2 nm. OPT-S9000 Spectroscopic Ellipsometer has been used to determine the optical constants (refractive index and extinction coefficient) of the samples. Measurements have been taken in the range of 1200–1600 nm by 75° angle of incidence. Required precautions related to the operator or instruments have been taken to eliminate the random errors. Systematic errors for a specific result have been calculated by combining the errors arising from instrument and measurements in the related mathematical equations. 3. Results and discussion 3.1. FT-IR analysis FT-IR spectroscopy has been used to study the structure of the glasses in detail. Fig. 1 shows the FT-IR transmittance spectra of the zinc borate glass structures. Generally, IR spectra of borate glasses contain three main regions. These are: i) the band which occurs at 1600–1200 cm−1 which is attributed to the B–O stretching vibrations of trigonal BO3 units. ii) the second band between 1200 and 800 cm−1
Fig. 1. FT-IR spectra of zinc-borate glass structures.
407
which belongs to the B–O stretching vibrations of tetragonal BO4 units and iii) the third band at ~700 cm−1 which corresponds to the B–O–B bending vibrations of bridging oxygen atoms [17,26,27]. It is clear from Fig. 1 that FT-IR spectra of the glass structures produced in this work exhibit a similar behavior (labeled as a, b, c, and d). In ZnO containing borate glass structures, ZnO may act as a network former or modifier. These can be deduced from the FT-IR spectra. It is known that ZnO which acts as a network former in borate glasses shows itself with a band between 400 and 550 cm−1 which belongs to ZnO4 units [18,28]. In our study, we can see some traces of these for samples BZ10 and BZ15 with the peak located at ~ 450 cm−1. These confirm that ZnO play a network former role for these glasses. The absence of this peak for BZ5 glass calls to mind the idea of ZnO acting as a modifier for this incorporation rate. Another remarkable point in Fig. 1 is changing relative intensity of two peaks indexed as b and d. According to the literature, peak b is attributed to the B–O stretching vibrations containing linkage oxygen connecting different groups [3,29]. However, peak d is assigned to the B–O–B bending vibrations of bridging oxygen atoms [17]. Our results show that relative intensity of these peaks is lower for sample BZ5, which supports the idea of ZnO acting as modifier for this glass structure. For higher ZnO concentrations (sample BZ10 and BZ15), these peaks play noticeable and active role in FT-IR spectra. This indicates the dominant role of bridging oxygen atoms in glass structure where ZnO behaves as a network former. 3.2. Electrical properties Electrical conduction mechanisms of glass structures have been investigated by using temperature dependent electrical measurements. Fig. 2 shows the ln(σ) ~ 1000/T plots in the range of 65– 420 K. There are two regions in these plots where conductivity exhibits different behaviors. Linear region at high temperatures has been labeled as region I, while the region with the deviation from linearity has been labeled as region II. For each glass structure, the mechanism in region I has been examined using the classical Arrhenius relation given below:
σ ðT Þ ¼ σ o exp½−ΔE=kT
ð1Þ
where σo is a pre-exponential value, ΔE is the activation energy, k is Boltzmann's constant and T is the absolute temperature [30]. Activation energy (Ea) values calculated from region I are given in Table 2. Ea values have been calculated to be between 225 and 269 meV. As seen from Table 2, there is a decrease in the Ea value of sample BZ5. ZnO may play two different roles in glass structures as a modifier or network former depending on its concentration. If ZnO behaves as a modifier, the number of NBOs will increase and this will cause the expansion of glass network [31,32]. In this case, alkali metal ions will be able to move more freely and this effect will show itself as a decrease in Ea values. Unlike this, for sample BZ10 that contains more ZnO than sample BZ5, this downward trend has not continued and even for sample BZ15 an increase in Ea value has been observed. ZnO, in addition to acting as a modifier in borate glass structures, could play a role as an oxide that joined the glass network. In such a structure, NBO density will be less and an increase will be observed in E a values. This refers that samples BZ10 and BZ15 composed of tighter network structure where diffusion of Na+ ions become difficult and their mobilities decreased. For the second region where hopping mechanism is active, small polaron hopping (SPH) and variable range hopping (VRH) conduction mechanisms have been considered and a better fit has been found for SPH mechanism. So, electrical conductivity data of region II has been
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Fig. 2. Temperature dependent conductivity variations of zinc-borate glass structures used for activation energy calculations.
interpreted according to SPH mechanism. For this region, ln (σT) ~ 1000/T plots are given in Fig. 3 and activation energies for SPH mechanism (Es1 and Es2) have been calculated using the relation given below [30,33]:
σ ðT Þ ¼ ðσ o =T Þ exp½−ΔE=kT
localized regions to decrease and Es1 and Es2 values to increase. The concepts that we have used in the above discussion, on the density of NBOs are in agreement with the peaks representing the vibrations related to the bridging oxygen in FT-IR spectra which can be recognized clearly especially for samples BZ10 and BZ15.
ð2Þ 3.3. Optical properties
These values are also given in Table 2. With Es1 = 46 meV and Es2 = 14 meV, sample BZ5 has the lowest activation energy values among other samples. ZnO, as a modifier in the glass structure for sample BZ5, would probably increase the possibility of hopping conduction in localized levels because of the higher number of NBOs and this will in turn cause Es1 and Es2 values to decrease. As can be seen from Table 2, Es1 and Es2 values for BZ10 and BZ15 are higher. Lower number of NBOs in these glass structures causes the width of the defect levels in
Table 2 Activation energy values of zinc-borate glass structures.
3.3.1. UV–VIS spectroscopy Transmittance spectra of zinc borate glasses are given in Fig. 4. It is clear from these spectra that there is an increase in the transmittance values of borate glasses containing high amounts of ZnO in the visible region. Optical band gap values, width of the localized tail states and defect levels may be determined by investigating the dependence of absorption coefficient on photon energy. In the region with strong absorption above the mobility edge, absorption coefficient is given by [34,35]: αhv ∝ hv Eopt
Sample
Ea (meV)
ES1 (meV)
ES2 (meV)
BZ0 BZ5 BZ10 BZ15
248.90 ± 0.09 225.20 ± 0.08 226.00 ± 0.08 268.80 ± 0.09
59.00 ± 0.02 46.00 ± 0.01 108.10 ± 0.03 132.10 ± 0.04
17.000 ± 0.006 14.000 ± 0.004 20.400 ± 0.007 28.000 ± 0.009
2
ð3Þ
where α is absorption coefficient, hv is absorbed photon energy and Eopt is optical band gap. Optical band gap values of glass structures determined by using (αhv)1/2 ~ hv plots for borate based structures are given in Fig. 5. Also, in the region below the strong absorption,
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409
Fig. 3. ln (σT) ~ 1000/T plots of zinc-borate glass structures used for Es1 and Es2 values.
absorption coefficient (α) exhibits an exponential variation. In this region, α is given by [34,36]: α ∝ expðhv=EU Þ
where Eu represents the width of the localized tail states. Similarly, absorption coefficient can be expressed as another exponential function of photon energy;
ð4Þ a ∝ expðhv=Ed Þ
Fig. 4. Transmittance spectra of zinc-borate glass structures at room temperature.
ð5Þ
here, Ed is known as the width of the defect states and generally takes higher values than Eu. It is considered that the energy values calculated in this regime are closely related to the structural properties of materials [37]. Urbach energies (Eu) and defect related energies (Ed) for borate based glasses have been calculated using the ln (α) ~ hν plots given in Fig. 6. The calculated values of Eopt, EU and Ed are given in Table 3. Sample BZ5 has the lowest optical band gap value of 3.709 eV. It is known that defects like dangling bonds or NBOs in glasses cause static atomic structural disorder and this affect the absorption characteristic. Higher NBO concentration in these systems causes lower optical band gap and higher Urbach energy values [38–41]. When we consider the energy values in Table 3 and higher NBO density of sample BZ5 inferred from FT-IR and electrical characterizations, as expected, this sample has lower optical band gap and higher Urbach energy values with respect to samples BZ10 and BZ15. This explanation is another proof of ZnO acting as a modifier at low concentrations and as a network former at high concentrations. Lower Ed values of samples BZ10 and BZ15, as 1.112 eV and 0.962 eV in Table 3, also support this comment considering the
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Fig. 5. (αhν)1/2 ~ hν plots used to determine optical band gap values of zinc-borate glass structures.
decrease in the width of localized states and the SPH activation energy data given in Table 2. 3.3.2. Spectroscopic ellipsometry Spectroscopic ellipsometry technique has been used to determine the optical constants (refractive index and extinction coefficient) of borate based glasses. Psi spectra taken from the device and theoretical values obtained using Cauchy–Urbach model have been evaluated. After the successful fitting of the data, refractive index and extinction coefficient values have been obtained. Refractive index spectra are given in Fig. 7 and Cauchy–Urbach model parameters and optical constants are given in Table 4. Refractive index values of samples are determined to be between 1.39 and 1.48. It is clear from Fig. 7 that ZnO containing borate glasses have higher refractive index values. Sample BZ5 has lower content of ZnO than other samples. However, it has a refractive index value comparable to sample BZ15 which has the highest ZnO content. We know that ZnO acts as a modifier for sample BZ5, which causes an increased number of NBO's in the structure. This will result in a higher refractive index value than expected for this sample.
4. Conclusion Zinc borate glass structures have been obtained by melt-quenching technique for three different ZnO concentrations. Two different roles of ZnO in these structures have been examined in detail with the outputs of various characterization techniques. ZnO acted as a network former in samples having ZnO more than 5%. FT-IR data showed traces for the behavior of ZnO as a modifier/network former. Wavenumbers and relative intensities of the peaks in FT-IR spectra have been used to brighten the role of ZnO in borate glass structures. Activation energy values calculated from temperature dependent electrical measurements exhibited similar results. Also, it has been determined that width of the localized tail states for sample BZ5 is larger than those of other samples because of the increased number of NBO's. Spectroscopic ellipsometry measurements combined with Cauchy–Urbach model enabled the determination of optical constants and results showed that refractive index value can be controlled with the amount of ZnO in borate structures. High NBO in sample BZ5 caused this sample to have the highest refractive index among others. Sample BZ15, including ZnO at the highest rate, has the highest transmittance in visible
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Fig. 6. Urbach plots of zinc-borate glass structures.
region which may decrease the losses in light related applications arising from substrate. Acknowledgments Authors are thankful to Eskisehir Osmangazi University Scientific Research Projects Commission for providing financial support under the project number of 201319013. Authors also wish to express their gratitude to Prof. Dr. G. S. Kurkcuoglu for her help in FT-IR measurements. Table 3 Optical band gap (Eopt), Urbach energy (EU) and defect energy (Ed) values of zinc-borate glass structures. Sample
Eopt (eV)
EU (eV)
Ed (eV)
BZ0 BZ5 BZ10 BZ15
3.820 ± 0.021 3.709 ± 0.021 3.800 ± 0.025 3.816 ± 0.022
0.222 ± 0.021 0.364 ± 0.021 0.222 ± 0.025 0.213 ± 0.022
1.984 ± 0.021 2.226 ± 0.021 1.112 ± 0.025 0.962 ± 0.022
Fig. 7. Refractive index spectra of zinc-borate glass structures obtained by the help of Spectroscopic ellipsometry measurements.
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Table 4 Cauchy–Urbach model parameters and optical constants of zinc-borate glass structures. Sample An
Bn (nm)2
Cn (nm)4
Ak
Bk (eV)−1
kort (×10−5)
nort
MSE
BZ0 BZ5 BZ10 BZ15
0.011 0.010 0.010 0.011
0.022 0.020 0.019 0.017
0.002 0.002 0.001 0.001
1.12 1.12 1.12 1.12
4.65 4.80 4.45 4.10
1.39 1.48 1.45 1.47
0.01 0.01 0.03 0.02
1.385 1.467 1.440 1.467
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