Synthesis and Surface Characterization of Electrodeposited Quaternary Chalcogenide Cu2znxsn Ys1+X+2 Y Thin ﬁlm As Transparent Contact Electrode

Bull. Mater. Sci. (2020) 43:83 © Indian Academy of Sciences https://doi.org/10.1007/s12034-019-2030-y

Synthesis and surface characterization of electrodeposited quaternary chalcogenide Cu2ZnxSn yS1+x+2 y thin ﬁlm as transparent contact electrode

R A BUSARI1,4, B A TALEATU1,∗, S A ADEWINBI1,2, O E ADEWUMI1, E OMOTOSO1, KOOYEDOTUN3 and A Y FASASI4 1Department of Physics and Engineering Physics, Obafemi Awolowo University, Ile-Ife 220005, Nigeria 2Department of Physics, Osun State University, Osogbo, Osun State 210001, Nigeria 3Department of Physics, Institute of Applied Materials, Carbon Technology and Materials, University of Pretoria, Pretoria 00285, South Africa 4Centre for Energy Research and Development, Obafemi Awolowo University, Ile-Ife 220005, Nigeria ∗Author for correspondence ([email protected])

MS received 18 July 2019; accepted 7 October 2019

Abstract. A low-cost technique, electrochemical deposition has been used to grow nanocrystalline quaternary Cu–Zn– Sn–S (CZTS) on indium tin oxide (ITO)-coated glass substrate. Effects of variations in deposition potentials and sulphur content on the chemical composition, optical, morphological, structural and electrical properties of the deposited films have been investigated. The morphologies showed and confirmed the results from XRD analysis that the films are of polycrystalline grains. Average interplanar spacing of the films is 3.376 Å. The average film’s thickness as estimated from Rutherford back-scattered spectroscopy studies was 34 nm. The estimated stoichiometry was found to be that of Cu2ZnSnS4 tetragonal kesterite structure. Optical studies showed that the absorption characteristic of the deposited CZTS film across the wavelength region is significantly dependent on growth deposition potentials and electrolyte concentration. Estimated band gap is between 1.75 and 1.81 eV. The electrical studies showed that the deposited films exhibit ohmic characteristics. This study demonstrated successful deposition of tetragonal kesterite structures of CZTS using a two-electrode cell approach. It also revealed the novel route of growing CZTS thin film over the conventional three electrode cells.

Keywords. Polycrystalline; nanostructure; solar cell; electrochemical deposition; direct band gap.

1. Introduction of CZTS are non-toxic and earth-abundant compared to the elements of indium and gallium in CIGS. Based on the increase in the world population, electronic Various methods have been used to synthesize CZTS thin devices and industries; the yearly energy usage has been films, which include vacuum-based routes, such as sputtering intensively escalated. The available sources of energy are [8], thermal evaporation [9], pulsed-laser deposition [10] and being utilized fast and this has put the world in a serious so on. These techniques have been used to grow quality CZTS energy crisis. Thus, there is immediate need for inexpen- film on various substrates. However, these methods require sive and sustainable energy sources for the normalization of sophisticated equipment to sustain vacuum and high-process the world economic crisis and industrialization. Cadmium temperatures. Many non-vacuum techniques of inexpensive telluride (CdTe), copper indium selenide (CIS) and cop- production costs and satisfying the large area requirements per indium gallium selenide (CIGS) thin film solar cells have been studied. These include electrochemical deposition have attracted considerable interest in the past few decades [11], spray pyrolysis [12], sol–gel spin coating [13,14], chem- [1–3]. However, the limitations are due to the scarcity of ical bath deposition [15] and so on. In the present study, indium, gallium and tellurium and the environmental issues two-electrode deposition technique was used to synthesize associated with the toxicity of cadmium and selenium [4,5]. CZTS thin film via inorganic chemical route. To solve these issues, it is essential to search for non-toxic and earth-abundant absorbing materials. Copper-based quaternary compounds, Cu–Zn–Sn–S 2. Experimental (CZTS) have been considered as one of the most promising ‘next generation’ photovoltaic semiconductor materials due 2.1 Chemical reagents and substrates to the near optimum direct energy band gap of around 1.6 eV, p-type semiconductor and large absorption coeffi- Copper sulphate pentahydrate (CuSO4·5H2O), zinc nitrate 4 −1 cient (>10 cm ) [6,7]. Meanwhile, the constituents’ atoms (Zn(NO3)2), tin sulphate (SnSO4), sodium thiosulphate

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Table 1. Details of optimized deposition condition for electrodeposited samples.

Sample Voltage (V) CuSO4·5H2OZn(NO3)2 SnSO4 Na2S2O3·5H2O Time (min)

B1 1.2 0.25 M, 20 ml 0.15 M, 10 ml 0.15 M, 10 ml 0.3 M, 20 ml 35 B2 1.3 0.25 M, 20 ml 0.15 M, 10 ml 0.15 M, 10 ml 0.3 M, 20 ml 35 B3 1.3 0.25 M, 20 ml 0.15 M, 10 ml 0.15 M, 10 ml 0.075 M, 20 ml 45

pentahydrate (Na2S2O3·5H2O), distilled water and indium tin oxide (ITO) coated glass pieces with (20 ×15×0.7) mm3 dimension with surface resistivity of 15 m−2 were used in the deposition process. All reagents used in the experiment were analytical grade with high percentage purity (Sigma, Aldrich). They were used as received. ITO substrates were cleaned by scrubbing thoroughly with cotton bud and soap solution, rinse with running tap water, ultrasonicate with distilled water, ace- tone and methanol at 40◦C for 10 min each. The substrates were dried in open furnace at moderate temperature. All these steps were taken to remove possible impurity substances from the surface of the substrates.

2.2 Samples preparation

Electrodeposition of CZTS was done by a simple two- electrode cell. It comprises a graphite piece as counter electrode and ITO-coated glass substrate as working elec- Figure 1. Schematic preparation CZTS Film. trode. Three different samples were prepared with ITO-coated glass substrates. They were named B1, B2 and B3, respectively. For the growth of sample B1, an electrolyte formed analysis was performed by energy dispersive X-ray from the combination of 20 ml of 0.25 M of CuSO4·5H2O, spectroscopy (EDX) and the films’ thickness and composition 10 ml of 0.15 M of Zn(NO3)2, 10 ml of 0.15 M of SnSO4 were analysed using Rutherford back-scattered spectroscopy and 20 ml of 0.3 M of Na2S2O3·5H2O were fed into the cell. (RBS). Electrical properties were determined by four-point The electrodes were held firmly in the holder and then con- probe station T121 sheet resistance measurement system. nected to the power supply. A digital multimeter was used to achieve stabilized power with sensitive current and voltage modes. Cathodic potential of 1.2 V was selected to run 3. Results and discussion the experiment until optimum deposition of the sample was obtained in 35 min at an ambient temperature. For samples 3.1 Structural analysis B2 and B3, a similar procedure with slight variation in deposition potential and concentration of Na2S2O3 · 5H2Owas Figure 2 shows the XRD patterns of CZTS thin films. The used. The three samples were taken out of the bath, rinsed presence of multiple peaks in the XRD patterns indicates that in distilled water and dried in an open furnace at 120◦C the deposited films are polycrystalline in nature [16]. In sam- for 5 min to remove residual water content and other pos- ple B1, the peaks observed at 2θ values of 28.46, 32.87, 36.03, sible adsorbed surface impurities. Table 1 gives the details of 37.07, 47.34 and 56.17◦ correspond to (112), (200), (202), the deposition condition, while figure 1 gives a schematic (211), (220) and (312) diffraction planes of CZTS. In sample representation of the deposition procedure. The structural B2, the peaks observed at 2θ values of 28.46, 32.75, 36.05, analysis of CZTS films was achieved by X-ray diffraction 37.98, 47.36 and 56.46◦ correspond to (112), (200), (202), (XRD) technique on an XPERTPRO diffractometer (PAN- (211), (220) and (312) diffraction planes of CZTS. In sample alytical BV, The Netherlands) with CuKα radiation source B3, the only peak observed at 2θ = 18.92◦ corresponds to (λ = 1.5406 Å). Surface morphology of the films was (101). This suggests that sample B3 is not as crystalline as the examined by field emission scanning electron microscopy other samples. Reflections from all the planes are recognized (FESEM) and optical properties were analysed on a Shi- as features of tetragonal CZTS kesterite structure (JCPDS madzu UV-1800double beam spectrophotometer in the wave- card no. 26-0575). No peak corresponding to film segregation length range of 200–800 nm, respectively. Compositional or any secondary phase is observed from all the diffraction Bull. Mater. Sci. (2020) 43:83 Page 3 of 9 83

The induced strain in the samples can be calculated by using:

β cos θ ε = . (4) 4

The number of crystallites per unit area can be calculated using the equation:

t N = . (5) D3

From table 2, considering samples B1 and B2, it was observed that the dislocation density decreases from 19.34 to 16.58 with increase in deposition potential. This behaviour can be assigned to the improvement in crystallinity of the films owing Figure 2. XRD patterns of deposited CZTS thin films. to the proper arrangements of atom-to-atom in the crystal lattice [18]. Also, in samples B2 and B3, it is revealed that the patterns. This confirmed that the prepared thin film is homoge- dislocation density increases from 16.58 to 23.61 with equal nous and single-phase [16]. The dominant peaks observed at deposition potential, but decrease in the sulphur concentra- 2θ values of 28.46◦ in samples B1 and B2 indicate that their tion of sample B3. It is evident that deposition potential and preferred growth orientation is along (112) plane, while the solution concentration influence the quality of the crystalline peak at 18.92◦ in sample B3 shows its preferred orientation is CZTS film. along (101) plane. The structural parameters of CZTS films are given in table 2. The lattice parameters of the CZTS thin 3.2 Surface morphology film were estimated using the relations for tetragonal crystal structure given as Surface morphology of the film deposited at different potentials and solution concentrations are presented in figure 3a–c. 1 h2 + k2 l2 From figure 3a, it can be seen that the film adhered well = + , (1) to the substrate with flake-like structure. The micrographs d2 a2 c2 revealed the polycrystalline nature of the film with large where d is the interplanar spacing; h, k andl are Miller indices; densely packed grains [19]. Figure 3a and c displays the lay- and a and c are the lattice constants. ers with crystals dispersed over the substrate. The observable The crystallite size (D) of the film was calculated from the grains size with small voids and pores confirmed the nanos- XRD peaks using the Debye–Scherrer’s equation [17]: tructural nature of the films. It has been observed that the conversion efficiency of polycrystalline solar cells increases 0.94λ with the increase in grain size of the absorber materials. This D = , (2) β cos θ is because film layer with larger grains maximizes the diffu- sion length of minority carrier and the intrinsic potential of where β is the full width at half maximum (FWHM) of the polycrystalline cells [20,21]. Therefore, larger grain size can peak in radians, θ is the peak angle and λ is the X-ray wave- improve the photovoltaics performance. length. The dislocation density, δ is determined from the William- 3.3 Films composition son and Smallman’s relation [17]: EDX spectra of deposited CZTS film are shown in figure 4. 1 From the spectra, the signals of constituent elements and ITO δ = . (3) D2 glass substrate were revealed. However, the signal due to Zn

Table 2. Some calculated parameters from XRD patterns.

Lattice constant (Å) ◦ − Sample 2θ( ) hkl FWHM (β) acd (Å) D (nm) δ (nm) 2 ε N

B1 28.46 112 0.657 5.385 10.761 3.134 22.74 19.34 0.16 2721.09 B2 28.46 112 0.693 5.385 10.761 3.134 21.56 21.51 0.17 3493.01 B3 23.01 110 0.483 4.354 8.700 3.376 30.58 10.70 0.12 1188.81 83 Page 4 of 9 Bull. Mater. Sci. (2020) 43:83

Figure 3. FESEM image of (a) CZTS thin film, (b) CZTS thin film and (c) CZTS thin film.

Figure 4. EDX spectrum of the deposited CZTS film. atom was not revealed. From the literature, the loss of tin was backscattered data and elemental profiles were fitted within a also reported after annealing the process [22]. But from the range of 300–1200 fit region using SIMNRA software code XRD studies of the samples in the present work, the patterns with 0.001 accuracy. The analysis confirmed the presence of confirmed the polycrystallinity and kesterite structure (I 4) Cu, Zn, Sn and S in the samples. of the film. Therefore, the absence of Zn may be attributed to the quantity of zinc in the film or the overlapping of zinc spectrum with another element. Other signals in the spectra 3.4 Films thickness and stoichiometry may probably originate from the glass substrate. Figure 5 shows the RBS spectra of the film. The atomic density Furthermore, constituent elements of the samples and sto- was calculated using the relation below: ichiometry as well as film thicknesses were examined by RBS studies. The spectra data were acquired with 4He incident ion beam operating at 2200 keV energy with exit and nNA ◦ Atomic density (ρ) = × C , (6) scattering angles of 12 and 168 , respectively. The collected M x Bull. Mater. Sci. (2020) 43:83 Page 5 of 9 83

Energy [keV] 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 60 58 B1-RBG.dat 56 Simulated 54 O Na 52 Mg 50 Al 48 Si 46 S K 44 Ca 42 Fe 40 Cu 38 Zn In 36 Sn 34 32 30 28

Counts 26 24 22 20 18 16 14 12 10 8 6 4 2 0 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1,000 1,050 1,100 1,150 1,200 1,250 1,300 1,350 1,400 1,450 1,500 Channel

LAYER 1: THICKNESS 1088.07 E 15 Atoms/cm2 Compo: Cu 23.70% Zn 13.15% Sn 17.40% S 45.75% LAYER 2: THICKNESS 1953.83 E 15 Atoms/cm2 Compo: In 10.58% Sn 10.29% O 79.13% LAYER 3: THICKNESS 12264.70 E 15 Atoms/cm2 Compo: Si 19.84% O 63.15% Ca 2.88% Na 10.04% Mg 2.03% Al 0.19% K 0.87% Fe 1.02%

Figure 5. RBS spectrum of the deposited CZTS ﬁlm.

where n is the density of element at room temperature, NA the 3.5 Optical characterization 23 Avogadro’s constant (6.023×10 ), M the atomic weight, Cx the % composition and x = 1, 2, 3,.... 3.5a Films absorbance and transmittance: Optical investi- By substituting the physical parameters of the constituent gation of quaternary CZTS film depends on the measurement elements and all the respective percentage compositions in of absorbance, A (λ) and transmittance, T (λ) in the spectra equation (6), the film thicknesses were estimated as 32, 35 range between 200 and 800 nm. These data were utilized to and 34 nm for the samples B1, B2 and B3, respectively. obtain absorption coefficient (α), optical energy gap (Eg), Approximate calculated stoichiometry of the film is presented skin-depth (δ) and Urbach energy (Eu). The absorbance in table 3. From the table, the ratio of Cu, Zn, Sn and S can be spectra of the samples are presented in figure 6. It can be written as Cu1.8Zn1.0Sn1.3S3.5. This shows that the obtained deduced from the spectra that sample B2 exhibits higher stoichiometry is very close to tetragonal kesterite structure of absorption power than B3 across the wavelength of consid- Cu2ZnSnS4 thin film. eration. This behaviour can be attributed to higher sulphur 83 Page 6 of 9 Bull. Mater. Sci. (2020) 43:83

Table 3. Approximate thickness and the stoichiometry of electrodeposited ﬁlm.

Sample Thickness (nm) Element % Composition Ratio

B1 32 Cu 23.70 1.8 B2 35 Zn 13.15 1.0 B3 34 Sn 17.40 1.3 S 45.75 3.5

Figure 7. UV–Vis transmittance spectra of the samples.

Tauc’s plot for estimating band gap energy was obtained using the equation below:

n αhν = B(hν − Eg) , (8)

= 1 where n 2 for direct allowed transition, B is an empirical constant, hν is the photon energy and Eg is the energy band gap. Values of (αhν)2 were plotted against hν and energy band Figure 6. UV–Vis absorbance spectra of the samples. gap was determined by extrapolating the linear portion of the plot to the hν axis at (αhν)2 = 0. Figure 8 showed the plots of the absorption coefficient concentration in sample B2. When compared samples B1 and (α) vs. wavelength. It was observed that the absorption coef- B2, B2 showed higher absorption power than B1. This can ficients of all the films were sufficiently >104 cm−1 in the be attributed to its higher deposition potential. It can, there- UV–Vis region. This suggests that films can effectively absorb fore, be suggested that solution concentration and deposition photon energy from the visible down to infrared region [16]. potential can influence the absorption power of the film. Opti- Therefore, this material can be considered in the fabrica- cal transmittances of the samples are shown in figure 7.It tion of a good absorber material for thin film solar devices. can be seen that film transmittance dropped as the applied Figure 9 shows Tauc’s plot for the estimation of band gap. voltage increased to 1.3 V in samples B2 and B3. It is also Direct allowed band gap values of 1.82, 1.75 and 1.79 eV observed that B3 transmitted slightly higher than B2. This were obtained for samples B1, B2 and B3, respectively. These behaviour corroborates our earlier suggestion of low sulphur values matched well with some reported values of a good concentration in sample B3 in the discussion of XRD. In gen- absorber electrode in optoelectronic devices [24–27]. eral, the behaviour of these samples suggested that as the deposition potential and sulphur content increase, film’s thick- Skin depth and Urbach energy ness increases. This is in agreement with an earlier report on 3.5c : In any semiconduc- chalcogenide compounds [23]. tor materials, conductivity is strongly a function of the optical band gap. Therefore, correlation between the optical properties and the skin depth of semiconductor is desired [28]. The 3.5b Absorption coefficient and band gap energy:The skin (penetration) depth (δ) is related to absorption coefficient α absorption coefficient, was generated from the measured (α) by the following equation: optical data using the relation below: 1 A δ = . (9) α = 2.303 , (7) α t The dependency of penetration depth on incident photon where t is the film thickness and A the absorbance. energy is presented in figure 10. It was observed from Bull. Mater. Sci. (2020) 43:83 Page 7 of 9 83

Figure 8. Absorption coefﬁcient of the CZTS ﬁlm. Figure 10. Skin depth upon the photon energy.

Figure 9. Energy band gap of the deposited thin films. Figure 11. Plot of Urbach energy for the samples. all the samples that the penetration depth decreases as the incident photon energy increases (wavelength decreases) It can be seen from the graph that the value of Eu increases from 0.40 to 0.67 eV with increase in the deposition potential. [29]. For λ>λcut-off, where hν = 4.18 eV, the absorption effect vanishes and at a longer wavelength, the reduction in amplitude occurs [28]. 3.6 Electrical characterization Relationship shown in equation (10a) between (α) and (hν) − is known as Urbach empirical rule [29]: Figure 12 showed the I V characteristics of the samples. − Linear response was observed in the I V curve of all the ν samples. This shows that the grown films exhibited ohmic α = α h . 0 exp (10a) characteristics. Similar ohmic behaviour has been reported Eu in the literature [30]. Sheet resistance and resistivity were Linearization of equation (10a)gives: calculated from these relations [31]: hν = . ( / ), ln α = ln α0 + , (10b) RSheet 4 53 V I (11a) Eu ρ = tRSheet, (11b) where α0 is a constant and Eu is the Urbach energy. Behaviour of ln(α) against (hν) is shown in figure 11.The where RSheet is the sheet resistance, V the voltage, I the reciprocal of the slope of the plot gives the Urbach energy, Eu. current and V/I determined from the slope, t the film’s 83 Page 8 of 9 Bull. Mater. Sci. (2020) 43:83

Figure 12. I −V characteristic plots of (a) ITO blank glass substrate, (b) sample B1, (c) sample B2 and (d) sample B3, respectively.

Table 4. I −V characteristics of electrodeposited samples. deposited film is polycrystalline. Reflections from all the prominent planes are peculiarities of Cu ZnSnS tetrag- ( ) ( ) ( )−1 2 4 Sample RSheet Resistivity m Conductivity m onal kesterite structure. The calculated lattice constants were in good agreement with many reports on CZTS thin ITO 14.86 6.731 × 105 1.486 × 10−6 − film. The micrographs revealed that the films were well B1 135.43 1.788 × 10 5 5.612 × 104 B2 59.50 8.032 × 10−6 2.524 × 105 adhered to the substrate and continuous. The observed B3 189.22 2.536 × 10−5 4.650 × 104 nanopores on the film’s surface may enhance its performance for solar cell and gas sensing devices. EDX spectra showed peaks corresponding to elements in the film and the substrate. Optical absorption coefficients of the thickness and ρ the resistivity. The calculated sheet resistance, − deposited films were observed to be >104 cm 1. Estimated resistivity and conductivity are presented in table 4. The result direct energy gaps suggested that the films can function is in conformity with some reported works on CZTS film as good absorber electrode in optoelectronic devices. The [30,32]. obtained stoichiometry from the RBS data revealed that the deposited films were very close to the reported ratio 2:1:1:4 4. Conclusion kesterite structure. The films showed ohmic behaviour.

In this study, two-electrode electrochemical cell has been suc- Acknowledgements cessfully used to grow CZTS on ITO-coated glass substrates at room temperature. The occurrence of multiple This work is based on research supported by The World characteristic peaks in the XRD patterns showed that the Academy of Science (TWAS) of the Abdus Salam Bull. Mater. Sci. (2020) 43:83 Page 9 of 9 83

International Centre for Theoretical Physics (ICTP) Trieste, [12] Bhosale S M, Suryawanshi M P, Gaikwad M A, Bhosale P N Italy. The Grant number is 15-049 RG/PHYS/AF/AC_I- and Kim J H 2014 Mater. Lett. 129 153 FR3240287081. We kindly appreciate the support of Mr [13] Tanaka K, Fukui Y,Moritake N and Uchiki H 2011 Sol. Energy M A Rahman of Chemistry Laboratory, in the Department of Mater. Sol. Cells 95 838 Chemistry, Obafemi Awolowo University. Efforts of members [14] Swami S K, Kumar A and Dutta V 2013 Energy Proc. 33 198 of staff of Materials Science Laboratory in the Department of [15] Gao C, Schnabel T, Abzieher T, Krammer C, Powalla M, Kalt H et al 2014 Thin Solid Films 562 621 Physics & Engineering Physics are also acknowledged. We [16] Parthibaraj V, Tamilarasan K, Pugazhvadivu K S and Ran- are also thankful to members of staff of Carbon-Technology gasami C 2015 Int. J. Innov. Res. Sci. Eng. Technol. 4 670 Laboratory at Department of Physics, University of Pretoria, [17] Cullity B D and Stock S R (eds) 1956 The directions Pretoria, South Africa, for their assistance on SEM and XRD of diffracted beams: elements of X-ray diffraction (USA: characterizations. Addison-Wesley Publishing Co.) p 78 [18] Yıldırım M A and Ate¸s A 2009 Sens. Actuators A Phys. 155 272 [19] Aldalbahi A, Mkawi E M, Ibrahim K and Farrukh M A 2016 Sci. Rep. 6 32431 References [20] Tanaka K, Fukui Y,Moritake N and Uchiki H 2011 Sol. Energy Mat. Sol. Cells 95 838 [1] Repins I, Contreras M A, Egaas B, DeHart C, Scharf J, [21] Shin S W, Pawar S M, Park C Y, Yun J H, Moon J H, Kim J H PerkinsCLet al 2008 Prog. Photovoltaics Res. Appl. 16 et al 2011 Sol. Energy Mat. Sol. Cells 95 3202 235 [22] Sakthivel S and Baskaran V 2013 Nano Vision 3 123 [2] Mitzi D B 2009 Adv. Mater. 21 3141 [23] Chaudhari S, Palli S, Kannan P K and Dey S R 2016 Thin Solid [3] Mitzi D B, Yuan M, Liu W, Kellock A J, Chey S J, Deline V Films 600 169 et al 2008 Adv. Mater. 20 3657 [24] Katagiri H, Jimbo K, Maw W S, Oishi K, Yamazaki M, Araki [4] Britt J and Ferekides C 1993 Appl. Phys. Lett. 62 2851 H et al 2009 Thin Solid Films 517 2455 [5] Green M A, Emery K, Hishikawa Y, Warta W and Dunlop [25] Sarswat P K, Snure M, Free M L and Tiwari A 2012 Thin Solid W D 2012 Prog. Photovoltaics Res. Appl. 20 12 Films 520 1694 [6] Guo Q, Hillhouse H W and Agrawal R 2009 J. Am. Chem. [26] Chen H, Ye Q, He X, Ding J, Zhang Y, Han J et al 2014 Green Soc. 131 11672 Chem. 16 3841 [7] Steinhagen C, Panthani M G, Akhavan V, Goodfellow B, Koo [27] Sani R, Manivannan R and Victoria S N 2017 Chalcogenide B and Korgel B A 2009 J. Am. Chem. Soc. 131 12554 Lett. 14 165 [8] Pawar S M, Inamdar A I, Pawar B S, Gurav K V, Shin S W, [28] Chiad S S 2015 Int. Lett. Chem. Phys. Astron. 6 50 Xiao Y et al 2014 Mater. Lett. 118 76 [29] Hassanien A S and Akl A A 2015 J. Alloys Compd. 648 280 [9] Wang K, Gunawan O, Todorov T, Shin B, Chey S J, Bojarczuk [30] Henry J, Mohanraj K and Sivakumar G 2016 J. Asian Ceram. NAet al 2010 Appl. Phys. Lett. 97 143508 Soc. 4 81 [10] Vanalakar S A, Agawane G L, Shin S W, Suryawanshi M P, [31] Mwathe P, Musembi R, Munji M, Odari V, Munguti L, Nti- Gurav K V, Jeon K S et al 2015 J. Alloys Compd. 619 lakigwa A et al 2014 Coatings 4 747 109 [32] Digraskar R, Dhanayat S, Gattu K, Mahajan S, Upadhye D, [11] Lee S G, Kim J M, Woo H S, Jo Y C, Inamdar A I, Pawar S M Ghule A et al 2013 in AIP Conference Proceedings, vol 1512, et al 2014 Curr. Appl. Phys. 14 254 p 236