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Synergetic effect of thermal and electromagnetic energy on electrical properties of zinc ferrite thin film
Materials Today Communications 21 (2019) 100641
Contents lists available at ScienceDirect
Materials Today Communications journal homepage: www.elsevier.com/locate/mtcomm
Synergetic effect of thermal and electromagnetic energy on electrical properties of zinc ferrite thin film S.B. Madake, K.Y. Rajpure
T
⁎
Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur, M.S., 416004, India
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnFe2O4 Thin film Deposition Thermal and optical excitation Arrhenius plot Activation energy
In this study, the nanocrystalline ZnFe2O4 thin films were deposited using spray pyrolysis technique. The structure, morphology, and optical and electrical properties of the sample were analyzed. The film shows higher conductivity for selective illumination having wavelength corresponding to the bang gap energy. In case of thermal excitation the rapid increase in conductivity was observed after 500 K. The combined exposure of heat and light affects the conductivity without changing activation energy of the material.
1. Introduction
2. Experimental
The ZnFe2O4 is one of the most trusted transition metal oxide having robust applications as photocatalysist [1], gas sensor [2], electromagnetic absorber [3], anode material for lithium-ion battery [4], microbial disinfectant [5] etc. It is an n-type semiconductor having band gap around 2 eV [6]. This lower band gap is attributed to visible light activity of the material. In the literature there is no disagreement in the visible light activeness of the material but a large controversy about type of electron transition is found. Some researchers reported the direct band gap [7–10], some reported indirect band gap [11–13] while some other reported both band gaps [14–16] for same ZnFe2O4 material. Beside these optical and electrical behaviors the zinc ferrite thin films show excellent chemical stability and mechanical hardness [17,18]. Now a days, the transition metal oxides and their binary oxides are attracted increasing attention in semiconductor device processing [19]. Various deposition techniques were reported for the thin film preparation of these oxides, including dip coating, spin coating, sol-gel, CVD, sputtering, molecular beam epitaxy, atomic layer deposition etc. [19] but most of them require high quality substrates and sophisticated instrumentation [20,21]. But the spray pyrolysis technique is simpler and require less time for film formation. The annealing plays a crucial part in oxide formation; since the spray pyrolysis includes heat treatment, it forms oxides easily. In present report, we prepared ZnFe2O4 thin films by spray pyrolysis and the effect of thermal and electromagnetic energy on electron conduction was studied.
2.1. Deposition of ZnFe2O4 thin film
⁎
The ZnFe2O4 thin films were deposited on glass substrate by spraying stoichiometric aqueous mixture of as-received AR-grade Zn (NO3)2.6H2O and Fe(NO3)3.9H2O (Thomas Baker Chemicals) at 425 °C temperature. For recovering the preset temperature and for complete decomposition of precursor solution, the intermittent spraying process was employed instead of continuous spraying [22]. After the completion of deposition the films were kept on same hot plate at same temperature for next 20 min. Finally the films were allowed to cool to the room temperature naturally. Further experimental details are summarized in Table 1. 2.2. Characterization of ZnFe2O4 thin film The deposited films were characterized by various techniques for revealing their properties. The structural properties were checked by xray diffraction analysis (XRD, Brucker, D2 Phaser), the surface morphology was observed using electron microscopy (FESEM, Tescan, Mira3), the optical properties were studied by Uv–vis spectrophotometry (Shimadzu, uv-1800). The thickness of prepared thin film was calculated using gravimetric weight difference method as; the deposited thin film was weighed then the film was dissolved in concentrated HCl and weight of bare substrate was taken. The calculated film thickness was confirmed by spectroscopic ellipsometry (SE) (J.A.Woollam, alpha-SE). The I-V measurements were recorded using
Corresponding author. E-mail address: [email protected] (K.Y. Rajpure).
https://doi.org/10.1016/j.mtcomm.2019.100641 Received 26 May 2019; Received in revised form 11 September 2019; Accepted 11 September 2019 Available online 12 September 2019 2352-4928/ © 2019 Published by Elsevier Ltd.
Materials Today Communications 21 (2019) 100641
S.B. Madake and K.Y. Rajpure
3.4. Electrical properties
Table 1 Optimized experimental parameters used for deposition of ZnFe2O4. Solution concentration Solvent Volumetric ratio Spraying volume Substrate temperature Nozzle to substrate distance Solution flow rate Air pressure Spraying duration Spray halt time Deposition steps
The I–V characteristics of the ZnFe2O4 thin film with respect to illumination wavelength is shown in Fig. 3a. All the I–V curves are straight lines showing the ohmic nature of thin film. The slope of the line gives the conductivity of the thin film. The conductivity of film is minimum in dark and highest in 650 nm illumination. The change in conductivity with respect to illumination wavelength is not linear as shown in Fig. 3b. The higher conduction values at 460 and 650 nm can be explained by correlating them with band gap values; these wavelength values corresponds to direct and indirect band gap energies as studied in Section 3.3. The wavelengths 550 and 610 nm has more energy than observed value of indirect band gap energy so their corresponding conductivity is expected to be higher, but the observed conductivity is much lower. This phenomenon may be observed due to lack of crystal momentum corresponding to these wavelengths [25]. Therefore it can be said that the material may have dual band gap with band structure as shown in Fig. 3c. This band structure is well explained elsewhere [26] in case of germanium. The excited electrons from valence band will preferably occupy lower conduction band energy state of L-valley and those excited electrons having sufficient energy will occupy an energy state of valence band at Г-valley. The Fig. 3d shows the variation of dc conductivity with temperature. It is observed that the conductivity increases with temperature; showing the semiconducting behavior of deposited film. The conductivity found increased slowly up to Curie temperature (500 K [27]) and further it increases rapidly. At Curie point the ordered ferrimagnetic state of material converted to disordered paramagnetic state. The increased conductivity can be explained on the basis of electron exchange between Fe3+ and Fe2+ distributed randomly over equivalent crystallographic lattice sites [28]. The activation energy of thermally activated electron exchange was obtained by Arrhenius equation,
0.1 M Double distilled water 1:2 15 ml 425 °C 35 cm 4 ml/min 0.5 bar 10 s 60 s 20
electrochemical workstation (CH Instruments, 600E) and two probe kit having heating and illumination facilities. The illuminating wavelengths of LED’s were tested using spectrometer (StellarNet, LT-14). 3. Result and discussion 3.1. Structural and morphological properties The peaks observed in XRD pattern (Fig. 1) are matched with the JCPDS card 01-077-0011; showing the formation of spinel ZnFe2O4 without any impurity. The crystallite size along intense (311) peak is found nearly 27 nm which was calculated using the well known Scherrer’s formula, 0.9λ/βcosӨ. The FESEM image (inset of Fig. 1) shows the uncontrolled growth of material on substrate. The grains of different sizes and shapes are randomly oriented so the clustered morphology is observed. 3.2. Thickness of thin film
ΔE ⎞ σDC = σo exp ⎛− ⎝ kT ⎠
The thickness (t) of thin film on to the substrate was calculated using the formula t = M/Aϱ [23]; where M is absolute mass, A is area and ϱ is density of deposited ZnFe2O4. The value of density (5.37 g/ cm3) is retrieved from JCPDS card. The thickness of thin film was found to be 508 nm. The thickness was also determined by B-spline numerical modeling in SE [24], it gave the thickness 513 nm.
Where, σo is the pre-exponential factor, ΔE is activation energy, k is Boltzmann constant and T is absolute temperature. The slope of Arrhenius plot, lnσDC verses (1/T) is the measure of activation energy [29]. The Fig. 3e shows the Arrhenius plot for the prepared thin film under dark, blue and red illumination. The steepness of all three lines is found nearly equal therefore their slopes and hence activation energies are also approximately same. So it can be said that the light illumination can affect the conductivity without changing the activation energy (∼0.42 eV). From the measured conductivity data (supplementary Table 1), it was found that light illumination causes higher conductivity for all the temperatures. This data interprets that the same rise in conductivity can be obtained by proper illumination at lower temperature, and at particular temperature the conductivity can be increased by a large amount with the proper illumination. e.g. change in conductivity on blue illumination at 373 K is same as the change in conductivity on red illumination at 323 K, and the conductivity change at 523 K due to red illumination is triple than that of blue illumination.
3.3. Optical properties The Fig. 2a shows the Uv–vis absorption spectrum of ZnFe2O4 thin film. The band gap energies of the material were calculated by Tauc plots as shown in fig.2b. The values of direct and indirect bandgaps are found to be 2.7 and 1.9 eV respectively.
4. Conclusion Single phase cubic spinel ZnFe2O4 thin films has been successfully deposited using intermittent spray pyrolysis. The direct and indirect optical band gaps are found to be 2.7 and 1.9 eV respectively. The film shows ohmic I–V nature. The room temperature conductivity was found higher for the illumination corresponding to both band gaps. The Arrhenius plots gave the constant activation energy 0.42 eV irrespective of light illumination. Therefore it can be said that the light illumination can change the conductivity only. The combination of thermal and optical energy gives higher conductivity at the same temperature. This synergetic effect may be applicable in designing of thermally activated
Fig. 1. XRD pattern and electron micrograph (inset) of ZnFe2O4 thin film. 2
Materials Today Communications 21 (2019) 100641
S.B. Madake and K.Y. Rajpure
Fig. 2. a) Uv–vis absorption spectrum, b) Tauc plots for band gap energies.
Fig. 3. a) I–V characteristics of ZnFe2O4 thin film in dark and illuminations. b) Change in conductivity with variation of wavelength. c) Band structure. d) Variation of conductivity with temperature. e) Arrhenius plots of sample.
electronic devices like gas sensors, where we can reduce the operational temperature using this phenomenon.
interests or personal relationships that could have appeared to influence the work reported in this paper.
Declaration of Competing Interest
Acknowledgement
The authors declare that they have no known competing financial
Authors are thankful to University Grant Commission, India for 3
Materials Today Communications 21 (2019) 100641
S.B. Madake and K.Y. Rajpure
providing research facilities at our department through DSA-SAPPhase-II programme.
[13] G. Rekhila, Y. Bessekhouad, M. Trari, Synthesis and characterization of the spinel ZnFe2O4, application to the chromate reduction under visible light, Environ. Technol. Innov. 5 (2016) 127–135, https://doi.org/10.1016/j.eti.2016.01.007. [14] D. Gao, Z. Shi, Y. Xu, J. Zhang, G. Yang, J. Zhang, X. Wang, D. Xue, Synthesis, magnetic anisotropy and optical properties of preferred oriented zinc ferrite nanowire arrays, Nanoscale Res. Lett. 5 (2010) 1289–1294, https://doi.org/10.1007/ s11671-010-9640-z. [15] Z. Wu, M. Okuya, S. Kaneko, Spray pyrolysis deposition of zinc ferrite films from metal nitrates solutions, Thin Solid Films 385 (2001) 109–114, https://doi.org/10. 1016/S0040-6090(00)01906-4. [16] A.G. Hufnagel, K. Peters, A. Müller, C. Scheu, D. Fattakhova-Rohlfing, T. Bein, Zinc ferrite photoanode nanomorphologies with favorable kinetics for water-splitting, Adv. Funct. Mater. 26 (2016) 4435–4443, https://doi.org/10.1002/adfm. 201600461. [17] M. Goodarz Naseri, E.B. Saion, A. Kamali, An overview on nanocrystalline ZnFe 2 O 4, MnFe 2 O 4, and CoFe 2 O 4 synthesized by a thermal treatment method, ISRN Nanotechnol. 2012 (2012) 1–11, https://doi.org/10.5402/2012/604241. [18] D. Sibera, U. Narkiewicz, N. Guskos, G. Zonierkiewicz, The preparation and EPR study of nanocrystalline ZnFe2O 4, J. Phys. Conf. Ser. 146 (2009) 8–13, https://doi. org/10.1088/1742-6596/146/1/012014. [19] G. He, X. Chen, Z. Sun, Interface engineering and chemistry of Hf-based high-k dielectrics on III-V substrates, Surf. Sci. Rep. 68 (2013) 68–107, https://doi.org/10. 1016/j.surfrep.2013.01.002. [20] G. He, J. Liu, H. Chen, Y. Liu, Z. Sun, X. Chen, M. Liu, L. Zhang, Interface control and modification of band alignment and electrical properties of HfTiO/GaAs gate stacks by nitrogen incorporation, J. Mater. Chem. C Mater. Opt. Electron. Devices 2 (2014) 5299–5308, https://doi.org/10.1039/c4tc00572d. [21] G. He, J. Gao, H. Chen, J. Cui, Z. Sun, X. Chen, Modulating the interface quality and electrical properties of HfTiO/InGaAs gate stack by atomic-layer-deposition-derived Al2O3 passivation layer, ACS Appl. Mater. Interfaces 6 (2014) 22013–22025, https://doi.org/10.1021/am506351u. [22] A. Sutka, G. Strikis, G. Mezinskis, A. Lusis, J. Zavickis, J. Kleperis, D. Jakovlevs, Properties of Ni-Zn ferrite thin films deposited using spray pyrolysis, Thin Solid Films 526 (2012) 65–69, https://doi.org/10.1016/j.tsf.2012.11.017. [23] S.K. Shaikh, S.I. Inamdar, V.V. Ganbavle, K.Y. Rajpure, Chemical bath deposited ZnO thin film based UV photoconductive detector, J. Alloys. Compd. 664 (2016) 242–249, https://doi.org/10.1016/j.jallcom.2015.12.226. [24] V. Zviagin, P. Richter, T. Böntgen, M. Lorenz, M. Ziese, D.R.T. Zahn, G. Salvan, M. Grundmann, R. Schmidt-Grund, Comparative study of optical and magnetooptical properties of normal, disordered, and inverse spinel-type oxides, Phys. Status Solidi Basic Res. 253 (2016) 429–436, https://doi.org/10.1002/pssb. 201552361. [25] T. Wang, B. Daiber, J.M. Frost, S.A. Mann, E.C. Garnett, A. Walsh, B. Ehrler, Indirect to direct bandgap transition in methylammonium lead halide perovskite, Energy Environ. Sci. 10 (2017) 509–515, https://doi.org/10.1039/c6ee03474h. [26] R. Geiger, T. Zabel, H. Sigg, Group IV direct band gap photonics: methods, challenges, and opportunities, Front. Mater. 2 (2015), https://doi.org/10.3389/fmats. 2015.00052. [27] C. Park, R. Wu, P. Lu, H. Zhao, J. Yang, B. Zhang, W. Li, C. Yun, H. Wang, J.L. MacManus-Driscoll, S. Cho, Use of mesoscopic host matrix to induce ferrimagnetism in antiferromagnetic spinel oxide, Adv. Funct. Mater. 28 (2018) 1–8, https://doi.org/10.1002/adfm.201706220. [28] G.S.Y. Kumar, H.S.B. Naik, A.S. Roy, K.N. Harish, R. Viswanath, Synthesis, Optical and Electrical Properties of ZnFe 2 O 4 Nanocomposites, Int. J. Nanomater. Nanotechnol. Nanomed. 2 (2013) 19, https://doi.org/10.5772/56169. [29] Z.Z. Lazarević, C. Jovalekić, A. Milutinović, D. Sekulić, M. Slankamenac, M. Romčević, N.Z. Romčević, Study of NiFe 2O 4 and ZnFe 2O 4 spinel ferrites prepared by soft mechanochemical synthesis, Ferroelectrics. 448 (2013) 1–11, https://doi.org/10.1080/00150193.2013.822257.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mtcomm.2019. 100641. References [1] L. Han, X. Zhou, L. Wan, Y. Deng, S. Zhan, Synthesis of ZnFe2O4 nanoplates by succinic acid-assisted hydrothermal route and their photocatalytic degradation of rhodamine B under visible light, J. Environ. Chem. Eng. 2 (2014) 123–130, https:// doi.org/10.1016/j.jece.2013.11.031. [2] N. Van Hoang, C.M. Hung, N.D. Hoa, N. Van Duy, I. Park, N. Van Hieu, Excellent detection of H 2 S gas at ppb concentrations using ZnFe 2 O 4 nanofibers loaded with reduced graphene oxide, Sens. Actuators B Chem. (2019) 876–884, https:// doi.org/10.1016/j.snb.2018.11.157. [3] G. Wu, H. Zhang, X. Luo, L. Yang, H. Lv, Investigation and optimization of Fe/ZnFe 2 O 4 as a Wide-band electromagnetic absorber, J. Colloid Interface Sci. 536 (2019) 548–555, https://doi.org/10.1016/j.jcis.2018.10.084. [4] Y. Gao, L. Yin, S.J. Kim, H. Yang, I. Jeon, J.P. Kim, S.Y. Jeong, H.W. Lee, C.R. Cho, Enhanced lithium storage by ZnFe 2 O 4 nanofibers as anode materials for lithiumion battery, Electrochim. Acta 296 (2019) 565–574, https://doi.org/10.1016/j. electacta.2018.11.093. [5] Q. Liu, Y. Xu, J. Wang, M. Xie, W. Wei, L. Huang, H. Xu, Y. Song, H. Li, Fabrication of Ag/AgCl/ZnFe 2 O 4 composites with enhanced photocatalytic activity for pollutant degradation and E. Coli disinfection, Colloids Surf. A Physicochem. Eng. Asp. 553 (2018) 114–124, https://doi.org/10.1016/j.colsurfa.2018.05.019. [6] D. Peeters, D.H. Taffa, M.M. Kerrigan, A. Ney, N. Jöns, D. Rogalla, S. Cwik, H.W. Becker, M. Grafen, A. Ostendorf, C.H. Winter, S. Chakraborty, M. Wark, A. Devi, Photoactive zinc ferrites fabricated via conventional CVD approach, ACS Sustain. Chem. Eng. 5 (2017) 2917–2926, https://doi.org/10.1021/acssuschemeng. 6b02233. [7] N. Sapna, V. Budhiraja, S.K. Kumar, Singh, Nanoparticles-assembled ZnFe 2 O 4 mesoporous nanorods for physicochemical and magnetic properties, J. Mater. Sci. Mater. Electron. 30 (2019) 3078–3087, https://doi.org/10.1007/s10854-01800587-0. [8] A. Silambarasu, A. Manikandan, K. Balakrishnan, Room-temperature superparamagnetism and enhanced photocatalytic activity of magnetically reusable spinel ZnFe2O4 nanocatalysts, J. Supercond. Nov. Magn. 30 (2017) 2631–2640, https://doi.org/10.1007/s10948-017-4061-1. [9] P.A. Vinosha, L.A. Mely, J.E. Jeronsia, S. Krishnan, S.J. Das, Synthesis and properties of spinel ZnFe2O4 nanoparticles by facile co-precipitation route, Optik (Stuttg). 134 (2017) 99–108, https://doi.org/10.1016/j.ijleo.2017.01.018. [10] M.A. Golsefidi, M. Abrodi, Z. Abbasi, A. Dashtbozorg, M.E. Rostami, M. Ebadi, Hydrothermal method for synthesizing ZnFe2O4 nanoparticles, photo-degradation of Rhodamine B by ZnFe2O4 and thermal stable PS-based nanocomposite, J. Mater. Sci. Mater. Electron. 27 (2016) 8654–8660, https://doi.org/10.1007/s10854-0164886-6. [11] L.J. Hoong, Y.C. Keat, A. Chik, T.P. Leng, Band structure and thermoelectric properties of inkjet printed ZnO and ZnFe2O4 thin films, Ceram. Int. 42 (2016) 12064–12073, https://doi.org/10.1016/j.ceramint.2016.04.135. [12] A.A. Tahir, K.G.U. Wijayantha, M. Mazhar, V. McKee, ZnFe2O4 thin films from a single source precursor by aerosol assisted chemical vapour deposition, Thin Solid Films 518 (2010) 3664–3668, https://doi.org/10.1016/j.tsf.2009.09.104.
4
Contents lists available at ScienceDirect
Materials Today Communications journal homepage: www.elsevier.com/locate/mtcomm
Synergetic effect of thermal and electromagnetic energy on electrical properties of zinc ferrite thin film S.B. Madake, K.Y. Rajpure
T
⁎
Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur, M.S., 416004, India
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnFe2O4 Thin film Deposition Thermal and optical excitation Arrhenius plot Activation energy
In this study, the nanocrystalline ZnFe2O4 thin films were deposited using spray pyrolysis technique. The structure, morphology, and optical and electrical properties of the sample were analyzed. The film shows higher conductivity for selective illumination having wavelength corresponding to the bang gap energy. In case of thermal excitation the rapid increase in conductivity was observed after 500 K. The combined exposure of heat and light affects the conductivity without changing activation energy of the material.
1. Introduction
2. Experimental
The ZnFe2O4 is one of the most trusted transition metal oxide having robust applications as photocatalysist [1], gas sensor [2], electromagnetic absorber [3], anode material for lithium-ion battery [4], microbial disinfectant [5] etc. It is an n-type semiconductor having band gap around 2 eV [6]. This lower band gap is attributed to visible light activity of the material. In the literature there is no disagreement in the visible light activeness of the material but a large controversy about type of electron transition is found. Some researchers reported the direct band gap [7–10], some reported indirect band gap [11–13] while some other reported both band gaps [14–16] for same ZnFe2O4 material. Beside these optical and electrical behaviors the zinc ferrite thin films show excellent chemical stability and mechanical hardness [17,18]. Now a days, the transition metal oxides and their binary oxides are attracted increasing attention in semiconductor device processing [19]. Various deposition techniques were reported for the thin film preparation of these oxides, including dip coating, spin coating, sol-gel, CVD, sputtering, molecular beam epitaxy, atomic layer deposition etc. [19] but most of them require high quality substrates and sophisticated instrumentation [20,21]. But the spray pyrolysis technique is simpler and require less time for film formation. The annealing plays a crucial part in oxide formation; since the spray pyrolysis includes heat treatment, it forms oxides easily. In present report, we prepared ZnFe2O4 thin films by spray pyrolysis and the effect of thermal and electromagnetic energy on electron conduction was studied.
2.1. Deposition of ZnFe2O4 thin film
⁎
The ZnFe2O4 thin films were deposited on glass substrate by spraying stoichiometric aqueous mixture of as-received AR-grade Zn (NO3)2.6H2O and Fe(NO3)3.9H2O (Thomas Baker Chemicals) at 425 °C temperature. For recovering the preset temperature and for complete decomposition of precursor solution, the intermittent spraying process was employed instead of continuous spraying [22]. After the completion of deposition the films were kept on same hot plate at same temperature for next 20 min. Finally the films were allowed to cool to the room temperature naturally. Further experimental details are summarized in Table 1. 2.2. Characterization of ZnFe2O4 thin film The deposited films were characterized by various techniques for revealing their properties. The structural properties were checked by xray diffraction analysis (XRD, Brucker, D2 Phaser), the surface morphology was observed using electron microscopy (FESEM, Tescan, Mira3), the optical properties were studied by Uv–vis spectrophotometry (Shimadzu, uv-1800). The thickness of prepared thin film was calculated using gravimetric weight difference method as; the deposited thin film was weighed then the film was dissolved in concentrated HCl and weight of bare substrate was taken. The calculated film thickness was confirmed by spectroscopic ellipsometry (SE) (J.A.Woollam, alpha-SE). The I-V measurements were recorded using
Corresponding author. E-mail address: [email protected] (K.Y. Rajpure).
https://doi.org/10.1016/j.mtcomm.2019.100641 Received 26 May 2019; Received in revised form 11 September 2019; Accepted 11 September 2019 Available online 12 September 2019 2352-4928/ © 2019 Published by Elsevier Ltd.
Materials Today Communications 21 (2019) 100641
S.B. Madake and K.Y. Rajpure
3.4. Electrical properties
Table 1 Optimized experimental parameters used for deposition of ZnFe2O4. Solution concentration Solvent Volumetric ratio Spraying volume Substrate temperature Nozzle to substrate distance Solution flow rate Air pressure Spraying duration Spray halt time Deposition steps
The I–V characteristics of the ZnFe2O4 thin film with respect to illumination wavelength is shown in Fig. 3a. All the I–V curves are straight lines showing the ohmic nature of thin film. The slope of the line gives the conductivity of the thin film. The conductivity of film is minimum in dark and highest in 650 nm illumination. The change in conductivity with respect to illumination wavelength is not linear as shown in Fig. 3b. The higher conduction values at 460 and 650 nm can be explained by correlating them with band gap values; these wavelength values corresponds to direct and indirect band gap energies as studied in Section 3.3. The wavelengths 550 and 610 nm has more energy than observed value of indirect band gap energy so their corresponding conductivity is expected to be higher, but the observed conductivity is much lower. This phenomenon may be observed due to lack of crystal momentum corresponding to these wavelengths [25]. Therefore it can be said that the material may have dual band gap with band structure as shown in Fig. 3c. This band structure is well explained elsewhere [26] in case of germanium. The excited electrons from valence band will preferably occupy lower conduction band energy state of L-valley and those excited electrons having sufficient energy will occupy an energy state of valence band at Г-valley. The Fig. 3d shows the variation of dc conductivity with temperature. It is observed that the conductivity increases with temperature; showing the semiconducting behavior of deposited film. The conductivity found increased slowly up to Curie temperature (500 K [27]) and further it increases rapidly. At Curie point the ordered ferrimagnetic state of material converted to disordered paramagnetic state. The increased conductivity can be explained on the basis of electron exchange between Fe3+ and Fe2+ distributed randomly over equivalent crystallographic lattice sites [28]. The activation energy of thermally activated electron exchange was obtained by Arrhenius equation,
0.1 M Double distilled water 1:2 15 ml 425 °C 35 cm 4 ml/min 0.5 bar 10 s 60 s 20
electrochemical workstation (CH Instruments, 600E) and two probe kit having heating and illumination facilities. The illuminating wavelengths of LED’s were tested using spectrometer (StellarNet, LT-14). 3. Result and discussion 3.1. Structural and morphological properties The peaks observed in XRD pattern (Fig. 1) are matched with the JCPDS card 01-077-0011; showing the formation of spinel ZnFe2O4 without any impurity. The crystallite size along intense (311) peak is found nearly 27 nm which was calculated using the well known Scherrer’s formula, 0.9λ/βcosӨ. The FESEM image (inset of Fig. 1) shows the uncontrolled growth of material on substrate. The grains of different sizes and shapes are randomly oriented so the clustered morphology is observed. 3.2. Thickness of thin film
ΔE ⎞ σDC = σo exp ⎛− ⎝ kT ⎠
The thickness (t) of thin film on to the substrate was calculated using the formula t = M/Aϱ [23]; where M is absolute mass, A is area and ϱ is density of deposited ZnFe2O4. The value of density (5.37 g/ cm3) is retrieved from JCPDS card. The thickness of thin film was found to be 508 nm. The thickness was also determined by B-spline numerical modeling in SE [24], it gave the thickness 513 nm.
Where, σo is the pre-exponential factor, ΔE is activation energy, k is Boltzmann constant and T is absolute temperature. The slope of Arrhenius plot, lnσDC verses (1/T) is the measure of activation energy [29]. The Fig. 3e shows the Arrhenius plot for the prepared thin film under dark, blue and red illumination. The steepness of all three lines is found nearly equal therefore their slopes and hence activation energies are also approximately same. So it can be said that the light illumination can affect the conductivity without changing the activation energy (∼0.42 eV). From the measured conductivity data (supplementary Table 1), it was found that light illumination causes higher conductivity for all the temperatures. This data interprets that the same rise in conductivity can be obtained by proper illumination at lower temperature, and at particular temperature the conductivity can be increased by a large amount with the proper illumination. e.g. change in conductivity on blue illumination at 373 K is same as the change in conductivity on red illumination at 323 K, and the conductivity change at 523 K due to red illumination is triple than that of blue illumination.
3.3. Optical properties The Fig. 2a shows the Uv–vis absorption spectrum of ZnFe2O4 thin film. The band gap energies of the material were calculated by Tauc plots as shown in fig.2b. The values of direct and indirect bandgaps are found to be 2.7 and 1.9 eV respectively.
4. Conclusion Single phase cubic spinel ZnFe2O4 thin films has been successfully deposited using intermittent spray pyrolysis. The direct and indirect optical band gaps are found to be 2.7 and 1.9 eV respectively. The film shows ohmic I–V nature. The room temperature conductivity was found higher for the illumination corresponding to both band gaps. The Arrhenius plots gave the constant activation energy 0.42 eV irrespective of light illumination. Therefore it can be said that the light illumination can change the conductivity only. The combination of thermal and optical energy gives higher conductivity at the same temperature. This synergetic effect may be applicable in designing of thermally activated
Fig. 1. XRD pattern and electron micrograph (inset) of ZnFe2O4 thin film. 2
Materials Today Communications 21 (2019) 100641
S.B. Madake and K.Y. Rajpure
Fig. 2. a) Uv–vis absorption spectrum, b) Tauc plots for band gap energies.
Fig. 3. a) I–V characteristics of ZnFe2O4 thin film in dark and illuminations. b) Change in conductivity with variation of wavelength. c) Band structure. d) Variation of conductivity with temperature. e) Arrhenius plots of sample.
electronic devices like gas sensors, where we can reduce the operational temperature using this phenomenon.
interests or personal relationships that could have appeared to influence the work reported in this paper.
Declaration of Competing Interest
Acknowledgement
The authors declare that they have no known competing financial
Authors are thankful to University Grant Commission, India for 3
Materials Today Communications 21 (2019) 100641
S.B. Madake and K.Y. Rajpure
providing research facilities at our department through DSA-SAPPhase-II programme.
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