Contents lists available atScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage:www.elsevier.com/locate/jpcsEvaluating the in
fluence of polyethylene glycol as a surfactant on CdO films
grown by SILAR method
Halit Çavu
şoğlu
Department of Physics, Faculty of Sciences, Selcuk University, Konya, Turkey
A R T I C L E I N F O Keywords: Polyethylene glycol CdO SILAR Surfactant A B S T R A C T
Nanocrystalline cadmium oxide (CdO)films have been prepared on glass substrates by the Successive Ionic Layer Adsorption and Reaction (SILAR) technique in the presence of polyethylene glycol (PEG) at different levels. The influence of PEG concentrations on structural, morphological and optical characteristics of the CdO films was elucidated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–Vis spectroscopy. XRD analysis indicated that all thefilms had a polycrystalline structure with (111) preferential orientation. The SEM analysis showed that the CdOfilms prepared with different levels of surfactants presented different morphologies. From the UV–Vis analysis of the CdO films, the direct band gap energy of the CdO films was dramatically affected by the amount of PEG content. The variation of the band gap energy can be ascribed to the improvement in the crystalline quality of thefilms. The investigations revealed that the amount of PEG doping during the growth process of CdO nanostructures has a significant impact on the physical and optical char-acteristics of CdOfilms.
1. Introduction
Nanostructured metal oxide semiconductors including copper oxide, nickel oxide, zinc oxide, cadmium oxide (CdO) etc., have gained in-creasing attention over the past few years based on their potential ap-plications, including gas sensors [1–4], supercapacitors [5–8], photo-transistors [9–12] and solar cells [13–16]. On account of the fact that CdO has a narrow band gap, it has received less attention compared to the corresponding counterparts. As a substantial n-type semiconductor, CdO has a direct energy band gap of 2.2–2.7 eV [17].
Potential implementations of CdO are depended upon physical and optical characteristics of the material. To improve the intended prop-erties of CdO, various techniques have been developed by the re-searchers [18]. Structural, morphological and optical characteristics of thinfilms can be enhanced by using surfactants during the deposition process [19]. Addition of appropriate percentage of suitable surfactant can lower the surface energy and alter the growth kinetics of crystal faces. In the literature, a variety of anionic, cationic and nonionic surfactants such as polyethylene glycol (PEG) [20–22], cetyl-trimethy-lammonium bromide (CTAB) [23], sodium dodecyl sulfate [24], glycine [25] and dextrin [26] have been widely used in the deposition pro-cesses of the metal oxide thinfilms. Among these surfactants, PEG is a non-ionic water soluble and non-toxic surfactant that can easily adsorb onto the metal oxide surface which decreases the surface activity [27].
The presence of PEG in the growth solution of thinfilms is found to obtain various morphologies with different shapes and sizes and im-prove the crystal quality of the depositedfilms [28].
To date, there is a wide range of solution-based preparation methods such as hydrothermal method [29,30], chemical bath de-position (CBD) [31,32], spray pyrolysis [33,34], electrochemical de-position [35,36] and successive ionic layer adsorption and reaction (SILAR) [37–39] method to produce CdO films with different morphologies. Among these methods, SILAR method has a great number of advantages including instrumental simplicity, cost-e ffec-tiveness, facile control overfilm thickness and not to require vacuum systems.
To the best of my knowledge, there is have been no reports in the literature about the influence of low-cost coating material (PEG) on the physical characteristics of CdO films grown by SILAR method. Accordingly, in the present work, the preliminary assessment has been made to investigate the impact of PEG on the physical characteristics of CdOfilms synthesized by the simple and cost-effective SILAR method. 2. Experimental details
2.1. Materials
Cadmium acetate dehydrate (Cd(CH3COO)2·2H2O, analytical
https://doi.org/10.1016/j.jpcs.2018.08.034
Received 31 March 2018; Received in revised form 30 July 2018; Accepted 29 August 2018 E-mail address:hcavusoglu@selcuk.edu.tr.
Available online 31 August 2018
0022-3697/ © 2018 Elsevier Ltd. All rights reserved.
reagent), polyethylene glycol 400 (PEG400) and aqueous ammonia solution (25%, Merck) were purchased from Merck (Darmstadt, Germany) and used without any further purification. The deionized water with a resistivity of 18.2 MΩ cm−1was used in all experiments.
All glassware was washed with chromic acid and hydrochloric acid, well rinsed with deionized distilled water and then dried.
2.2. The growth mechanism of CdOfilms
For the synthesis of the nanostructured CdOfilms, 0.1 M growth solution was prepared by using Cd(CH3COO)2·2H2O as a precursor salt
and PEG at different volume percentages (1%, 2%, and 4%) as a sur-factant in 100 mL double distilled water (18.2 MΩ cm−1). Cadmium
precursor solution was maintained at pH≈ 12.0 by the addition of concentrated aqueous ammonia solution with constant stirring. Before immersing the glass substrates, the temperature of the growth solution was maintained at 85 °C. For the synthesis of CdOfilms, first of all, pre-cleaned soda-lime glass slides were immersed vertically into the growth solution containing aqueous cadmium-ammonia complex ions ([Cd (NH3)4]2+) and different proportions of PEG for 20 s. Then, the
sub-strates were immersed in hot deionized water (85 °C) for 20 s. By re-peating the deposition cycle 20 times, uniform Cd(OH)2 films were
fabricated. Before implementing the characterization process, as-de-posited Cd(OH)2 films were converted to the CdO by annealing at
350 °C for 1 h in the air atmosphere.
2.3. Characterization of CdOfilms
The phase structure and orientation of deposited films were in-vestigated by X-ray diffraction (XRD) (Bruker D8 Advanced XRD, Bruker AXS Inc., Madison, WI) using Cu Kα radiation (1.5406 Å) in the range of 2θ = 30–70°. The morphological characteristics and particle thickness of the CdOfilms were determined using a scanning electron microscope (FEI Nova NanoSEM 430). The thickness of thefilms was measured with a NanoMap-500LS contact surface profilometer. UV–Visible absorption spectra of the CdO films were obtained using a JASCO V-670 double beam spectrophotometer in the range of 190–1100 nm with 1 nm optical resolution at room temperature. 3. Results and discussion
3.1. Morphological analysis of CdOfilms
Investigation of the morphological characteristics of thefilms is of great importance since the surface morphologies of thefilms affect their optical and structural features. Based on the aforementioned properties of the films, several optoelectronic devices can be designed. In the present work, the surface characteristics of thefilms were analyzed by scanning electron microscope (SEM).Fig. 1indicates the SEM images of as-deposited CdOfilms which were synthesized with and without the addition of PEG. It can be explicitly seen from thefigure that all these films had very dense and homogeneous structure and they adhered strongly to the whole surface of the substrate. Apparent impact of the surfactant PEG on the morphology and grain size of CdOfilms might be seen in their SEM images. The change of grain size remarkably alters the crystalline quality of thefilms and controls the behavior of the grain boundary diffusion. Furthermore, the particle size of the CdO films was found to be decreased with the addition of PEG. The values of average particle size decreased from 172.4 nm (without PEG) to 104.5 nm (for 4% PEG in the growth solution) with the increase of PEG concentration. The particle size values of thefilms were listed inTable 2. From this point of view, the size of the CdO films can be further adjusted by means of the appropriate percentage of PEG. Consequently, the surface properties of the CdOfilms considerably change with the addition of PEG content.
3.2. Structural analysis of CdOfilms
The quality of the crystal growth and the crystallite sizes of as-produced CdOfilms were investigated by XRD analysis.Fig. 2(a) de-monstrates the X-ray diffraction patterns of the nanostructured CdO film acquired by the addition of varying proportions of PEG. According to the XRD results, all diffraction peaks of CdO films were poly-crystalline in nature with cubic structure (JCPDS Card No.:005-0640 data). It can be clearly seen from thefigure that the insertion of dif-ferent amounts of PEG dramatically influenced the preferential or-ientation of thefilms. As the PEG content increased, the (111) peak intensities proportionally increased as well, which denoted the incre-ment of (111) preferential growth. Other XRD peaks such as (200), (220) and (311) also increased with the addition of PEG in a similar manner. This situation may have arisen from the internal strain sti-mulated by the insertion of PEG. The recorded peak intensities of the (111), (200), (220) and (311) planes were given inTable 1. In the XRD pattern of CdOfilms having different PEG concentrations, (111) dif-fraction peak, which is the strongest one in the pattern of samples. The intensities of (111), (200) (220) and (311) diffraction peaks were get-ting stronger obviously with the increase of PEG concentration. Espe-cially the relative intensity of (111)/(200) and (111)/(220) planes in-creased greatly. The results demonstrate the possible preferential orientation of CdO films, which is associated with the anisotropic crystallographic characteristics. Moreover, the thickness of the CdO films having different PEG concentrations affected the peak intensities. Thefilm thicknesses were measured to be 545, 673, 855 and 1444 nm for the CdOfilms with PEG concentrations of 0 (without PEG), 1%, 2% and 4%, respectively. As one can see that the diffraction peak intensities of thefilms increased with increasing film thicknesses. The thicknesses of the CdOfilms were given inTable 2.
Fig. 2(b) indicates the magnified patterns of (111) plane at 2θ ≅ 33°. The Bragg position for the direction of (111) reflections deviated from 33.36° to 32.96°, respectively, without and with 4% PEG added to the growth solution of CdOfilms. A shift of (111) plane towards a smaller angle was observed when PEG was used as the surfactant. This variation leads to a change in the lattice constants of CdOfilms.
The texture coefficient of planes corresponds to the texture of a particular plane, whose deviation from the standard sample points out the preferential growth orientation. The texture coefficient (TC (hkl)) of each (hkl) plane have been expressed from the XRD data using fol-lowing formula [40];
∑
= ⎧ ⎨ ⎩ ⎫ ⎬ ⎭ − TC hkl I hkl I hkl N I hkl I hkl ( ) ( ) ( ) 1 ( ) ( ) n 0 0 1 (1) where I (hkl) is the measured relative intensity of (hkl) diffraction peak, I0(hkl) is the standard intensity of the (hkl) plane taken from thedif-fractionfile JCPDS data, N is the number of reflections and n is the number of diffraction peaks considered in the analysis. By utilizing this equation, the preferential orientation growth of thefilms can be un-derstood. TC (hkl) is anticipated to be unity for the facet, which does not have preferential orientation. In case it is higher than unity, it is a preferentially grown facet. It can be seen that the highest TC (hkl) was in (111) plane for the CdOfilms. The usually favorable (111) preferred orientation growth of CdO films which were prepared by various methods [41–44] was obtained in here through the texture coefficient (TC). The variations of texture coefficient calculated for the diffraction peaks of CdOfilms were presented inTable 1. It is obvious from the results that the texture coefficient increased gradually with the increase of the PEG concentration, and then reached a maximum value; even-tually, decreasing at higher PEG concentration. This behavior was at-tributed to the interstitial inclusion of the surfactant atoms [45]. Be-sides, the thickness of thefilm directly affects the texture of the growing material. Moreover, the physical characteristics of the material and also the trustworthiness and performance of the produced devices are
substantially influenced by the texture of the material [46].
The crystallite size of the CdOfilms was estimated using the well-known Scherrer's equation [47];
=
D λ
β cosθ
0.94.
. (2)
where D is the crystallite size,λ is the wavelength of X-rays (1.5406 Å), β is the broadening of diffraction line which is measured at half its maximum intensity in radians andθ is the angle of Bragg diffraction. The crystallite size values of the CdOfilms, prepared with and without PEG, was reported inTable 2. The crystallite size values of CdOfilms were influenced by surfactant material and deviated from 17.28 nm to 14.60 nm with the addition of surfactant. These variations were also in accordance with the estimation of the particle size from the SEM ana-lysis.
The determination of the microstructural parameter, micro-strain, is very significant for the physical properties of nanostructured films. The micro-strain (ε) developed in CdO film was calculated from the fol-lowing relation [48];
=
ε β cosθ.
4 (3)
The values obtained using the above-mentioned equation were listed inTable 2. In order to clarify the relationship between the crys-tallite size and micro-strain of CdOfilms with varied PEG concentra-tions were plotted inFig. 3. It can be clearly observed fromFig. 3that the micro-strain (ε) values of the CdO films increased as the crystallite sizes presented a decreasing tendency with the increasing PEG con-centration. This case may arise from the movement of interstitial Cd atoms from inside the crystal structure [49]. Moreover, it was obvious fromTable 2andFig. 3that the maximum crystallite size values of the CdOfilms corresponded to the minimum micro-strain values, and vice Fig. 1. SEM illustrations of CdOfilms with different rates of PEG concentrations of (a) 0 (without PEG), (b) 1%, (c) 2% and (d) 4%.
Table 1
XRD peak intensity values of the preferentially oriented planes and calculated TC (hkl) values of CdOfilms with different PEG concentrations.
PEG Concentration XRD Peak Intensity TC (hkl)
CdO Film (111) (200) (220) (311) (111) (200) (220) (311) 0% 1338 815 375 305 1.58 1.05 0.48 0.71 1% 3460 917 470 339 2.38 0.68 0.48 0.46 2% 5070 844 454 334 2.77 0.50 0.37 0.36 4% 5110 1280 600 405 2.50 0.68 0.44 0.39 Table 2
Important structural and optical parameter values of the CdOfilms with varied PEG concentrations. PEG Concentration Average Particle Size Film Thickness Crystallite size (nm) Micro-strain × − ε ( ) 10 3 Band gap (eV) CdO Film (nm) (nm) 0% 172.4 545 17.28 2.10 2.12 1% 120.83 673 16.10 2.27 2.07 2% 140.8 855 16.71 2.28 2.03 4% 104.5 1444 14.60 2.48 2.00
versa. Similar results were reported by Mahalingam et al. (2004) [50]. The opposite variations among the structural parameters with PEG content were in accordance with the results of Venkateswarlu et al. (2014) [51]. As a result, the crystalline quality was significantly af-fected by the PEG concentration of the growth solution which was used for the deposition of CdOfilms.
3.3. Optical analysis of CdOfilms
To clarify the influence of PEG concentration on the optical char-acteristics of CdO film, optical absorption measurements were per-formed at room temperature. The optical absorption spectra of the CdO films with different PEG concentrations was given inFig. 4. As seen in Fig. 4, the absorption edge shifted toward shorter wavelengths with decreasing PEG concentration of the growth solution. It is understood that the optical band gap of the CdO films was broadened with
decreasing PEG concentration of the precursor solution.
The optical band gap energy values (Eg) of CdOfilms were
calcu-lated by using the following equation [52];
= −
α C hν E
hν
( g)m
(4) whereα is the absorption coefficient, C is a constant and hν is the photon energy. For direct band gap semiconductors, m is equal to 1/2. By the extrapolation of the linear portion of the horizontal energy axis (hν) gives the optical band gap energy of Eg, as shown inFig. 5. As can
be seen in Fig. 5and in Table 2, Egvalues decreased from 2.12 to
2.00 eV with increasing PEG concentrations.
The relationship between the band gap values and thefilm thick-nesses as a function of PEG concentration was also examined. As can be clearly seen inFig. 6andTable 2, the optical band gap energy values of the CdOfilms decreased from 2.12 to 2.00 eV with increasing the film thickness. The decrease in the optical band gap energy value with in-creasing the thickness of thefilms can be originated from the role of structural defects, morphological changes of the surface structure of the films and changes in the grain sizes of the films [53–56]. As a Fig. 2. (a) XRD patterns of CdOfilms having varied PEG concentrations (b) Magnified patterns of CdO films with PEG concentrations of 0 (without PEG), 1%, 2% and 4% at 2θ = 33°.
Fig. 3. Variation of crystallite size and micro-strain for CdOfilms with different PEG concentrations.
Fig. 4. The absorption spectra of the CdOfilms with different PEG concentra-tions.
consequence of the increment of film thickness, the localized states which locate in the band structure may combine with the band edges, lead to the decrease of the optical band gap. These results are consistent with previous studies [57–59].
4. Conclusions
In summary, nanostructured CdOfilms with and without surfactant
were successfully fabricated by relatively simple and low-cost SILAR method under non-vacuum conditions. The influence of adding PEG in different proportions on the characteristics of deposited CdO films has been extensively evaluated. Structural analysis by XRD confirmed the nanocrystallinity and cubic crystalline structure formation of CdOfilms. The doping of PEG concentration changed the crystallinity dramati-cally. Surface examination by SEM revealed that surface morphological characteristics and particle size of the CdO films were varied re-markably as a function of PEG concentrations. Optical absorption stu-dies showed that optical band gap energy was found to decrease from 2.12 eV to 2.00 eV with the increase in the PEG concentration of the CdOfilms. Consequently, it can be indicated that through careful ad-dition of PEG has a vital impact on the structural and optical char-acteristics of metal oxide films. By controlling the PEG addition can improve the quality of thefilms so that functional solar films can be produced by SILAR deposition technique.
Acknowledgements
This work was partially supported by the Scientific Research and Project Council of Selcuk University (BAP) project no: 18703003. Moreover, the author would like to thank Prof. Dr. HalukŞafak for his helpful advice on the optical part of this paper.
Appendix A. Supplementary data
Supplementary data related to this article can be found athttps:// doi.org/10.1016/j.jpcs.2018.08.034.
References
[1] J.K. Rajput, T.K. Pathak, V. Kumar, H.C. Swart, L.P. Purohit, Tailoring and opti-mization of optical properties of CdO thinfilms for gas sensing applications, Physica
Fig. 5. (αhν)2vs. hν plots of CdO films with different PEG concentrations.
Fig. 6. Film thickness and band gap variation of the CdO thinfilms as a func-tion of PEG concentrafunc-tions.
B 535 (2018) 314–318.
[2] A. Mhamdi, M.S. Alkhalifah, S. Rajeh, A. Labidi, M. Amlouk, S. Belgacem, Electrical and gas sensing investigations on the sprayed ZnO:Cu thinfilms, Physica B 521 (2017) 178–187.
[3] J. Zhang, Z. Qin, D. Zeng, C. Xie, Metal-oxide-semiconductor based gas sensors: screening, preparation, and integration, Phys. Chem. Chem. Phys. 19 (2017) 6313–6329.
[4] A. Dey, Semiconductor metal oxide gas sensors: a review, Mater. Sci. Eng. B 229 (2018) 206–217.
[5] C.D. Lokhande, D.P. Dubal, O.-S. Joo, Metal oxide thinfilm based supercapacitors, Curr. Appl. Phys. 11 (2011) 255–270.
[6] T. Kou, B. Yao, T. Liua, Y. Li, Recent advances in chemical methods for activating carbon and metal oxide based electrodes for supercapacitors, J. Mater. Chem. 5 (2017) 17151–17173.
[7] F. Shi, L. Li, X.-L. Wang, C.-D. Gua, J.-P. Tu, Metal oxide/hydroxide-based materials for supercapacitors, RSC Adv. 4 (2014) 41910–41921.
[8] F.E. Atalay, D. Asma, H. Kaya, E. Ozbey, The fabrication of metal oxide nanos-tructures using Deinococcus radiodurans bacteria for supercapacitor, Mater. Sci. Semicond. Process. 38 (2015) 314–318.
[9] A. Sharma, N.K. Chourasia, A. Sugathan, Y. Kumar, S. Jit, S.-W. Liu, A. Pandey, S. Biringd, B.N. Pal, Solution processed Li5AlO4dielectric for low voltage transistor
fabrication and its application in metal oxide/quantum dot heterojunction photo-transistors, J. Mater. Chem. C 6 (2018) 790–798.
[10] H.-W. Zan, W.-T. Chen, H.-W. Hsueh, S.-C. Kao, M.-C. Ku, C.-C. Tsai, H.-F. Meng, Amorphous indium-gallium-zinc-oxide visible-light phototransistor with a poly-meric light absorption layer, Appl. Phys. Lett. 97 (2010) 203506.
[11] S.E. Ahn, I. Song, S. Jeon, Y.W. Jeon, Y. Kim, C. Kim, B. Ryu, J.H. Lee, A. Nathan, S. Lee, G.T. Kim, U.I. Chung, Metal oxide thinfilm phototransistor for remote touch interactive displays, Adv. Mater. 24 (2012) 2631–2636.
[12] J. Yang, H. Kwak, Y. Lee, Y.-S. Kang, M.-H. Cho, J.H. Cho, Y.-H. Kim, S.-J. Jeong, S. Park, H.-J. Lee, H. Kim, MoS2–InGaZnO heterojunction phototransistors with
broad spectral responsivity, ACS Appl. Mater. Interfaces 8 (2016) 8576–8582. [13] T.T.T. Pham, N. Mathews, Y.-M. Lam, S. Mhaisalkar, Influence of size and shape of
sub-micrometer light scattering centers in ZnO-assisted TiO2photoanode for
dye-sensitized solar cells, Physica B 532 (2018) 225–229.
[14] I. Concina, A. Vomiero, Metal oxide semiconductors for dye- and quantum-dot-sensitized solar cells, Small 11 (2015) 1744–1774.
[15] X. Bao, A. Yang, Y. Yang, T. Wang, L. Sun, N. Wang, L. Han, Low-temperature solution-processed ZnO nanocrystalline interfacial layer with antireflective effect for efficient inverted polymer solar cells, Physica B 432 (2014) 1–4.
[16] J. Chung, J. Lee, S. Lim, Annealing effects of ZnO nanorods on dye-sensitized solar cell efficiency, Physica B 405 (2010) 2593–2598.
[17] Y.-S. Choi, C.-G. Lee, S.M. Cho, Transparent conducting ZnxCd1−xO thinfilms prepared by the sol-gel process, Thin Solid Films 289 (1996) 153–158. [18] R.R. Salunkhe, C.D. Lokhande, Effect of film thickness on liquefied petroleum gas
(LPG) sensing properties of SILAR deposited CdO thinfilms, Sens. Actuators, B 129 (2008) 345–351.
[19] N. Li, W. Huang, Q. Shi, Y. Zhang, L. Song, A CTAB-assisted hydrothermal synthesis of VO2(B) nanostructures for lithium-ion battery application, Ceram. Int. 39 (2013)
6199–6206.
[20] L.M. Bertus, C. Faure, A. Danine, C. Labrugere, G. Campet, A. Rougier, A. Duta, Synthesis and characterization of WO3thinfilms by surfactant assisted spray
pyr-olysis for electrochromic applications, Mater. Chem. Phys. 140 (2013) 49–59. [21] S. Cholan, N. Shanmugam, N. Kannadasan, K. Sathishkumar, K. Deivam, Effect of
poly ethylene glycol (PEG) as surfactant on cerium doped ZnS nanoparticles, J. Mater. Res. Technol. 3 (2014) 222–227.
[22] Z. Deng, X. Fang, S. Wu, W. Dong, J. Shao, S. Wang, M. Lei, The morphologies and optoelectronic properties of delafossite CuFeO2thinfilms prepared by PEG assisted
sol–gel method, J. Sol. gel Sci. Technol. 71 (2014) 297–302.
[23] B. Sahin, Improved optical performance of CdOfilms prepared with CTAB as sur-factant, Philos. Mag. A 94 (2014) 2750–2757.
[24] R. Aydin, B. Sahin, F. Bayansal, Sodium dodecyl sulfate-assisted SILAR synthesis of nanostructured cadmium oxidefilms, Ceram. Int. 42 (2016) 11822–11826. [25] R. Aydin, The critical mission of glycine as a surfactant in the improvement of
structural, morphological and optoelectronic features of CdOfilms, Superlattice. Microst. 117 (2018) 46–56.
[26] B. Sahin, F. Bayansal, M. Yüksel, A novel approach to enhancement of surface properties of CdOfilms by using surfactant: dextrin, Philos. Mag. A 95 (2015) 3888–3895.
[27] P.V. Adhyapakn, S.P. Meshram, D.P. Amalnerkar, I.S. Mulla, Structurally enhanced photocatalytic activity offlower-like ZnO synthesized by PEG-assisted hydro-thermal route, Ceram. Int. 40 (2014) 1951–1959.
[28] C.C. Vidyasagar, Y. Arthoba Naik, Surfactant (PEG 400) effects on crystallinity of ZnO nanoparticles, Arabian J. Chem. 9 (2016) 507–510.
[29] K. Sirohi, S. Kumar, V. Singh, A. Vohra, Synthesis and characterization of CdO–SnO2
Nanocomposites prepared by hydrothermal method, Acta Metall. Sin. 31 (2018) 254–262.
[30] W. Xia, Y. Liu, J. Li, C. Chen, Investigation of CdO hexagonal nanoflakes synthe-sized by a hydrothermal method for liquefied petroleum gas detection, Anal. Methods 8 (2016) 6265–6269.
[31] M. Ocampo, A.M. Fernandez, P.J. Sebastian, Transparent conducting CdOfilms formed by chemical bath deposition, Semicond. Sci. Technol. 8 (1993) 750–751. [32] H. Khallaf, C.-T. Chen, L.-B. Chang, O. Lupan, A. Dutta, H. Heinrich, A. Shenouda,
L. Chow, Investigation of chemical bath deposition of CdO thinfilms using three different complexing agents, Appl. Surf. Sci. 257 (2011) 9237–9242.
[33] R. Kumaravel, K. Ramamurthi, V. Krishnakumar, Effect of indium doping in CdO thinfilms prepared by spray pyrolysis technique, J. Phys. Chem. Solid. 71 (2010) 1545–1549.
[34] P. Velusamy, R. Ramesh Babu, K. Ramamurthi, E. Elangovan, J. Viegas, M. Sridharan, Spray deposited ruthenium incorporated CdO thinfilms for opto-electronic and gas sensing applications, J. Phys. Chem. Solid. 112 (2018) 127–136. [35] A.K. Yıldırım, B. Altıokka, Effects of concentration on CdO films grown by
elec-trodeposition, Appl. Nanosci. 7 (2017) 131–135.
[36] B. Sarma, Y.R. Smith, S.K. Mohanty, M. Misra, Electrochemical deposition of CdO on anodized TiO2nanotube arrays for enhanced photoelectrochemical properties,
Mater. Lett. 85 (2012) 33–36.
[37] M.A. Yıldırım, A. Ateş, Structural, optical and electrical properties of CdO/Cd(OH)2
thinfilms grown by the SILAR method, Sens. Actuators, A 155 (2009) 272–277. [38] A.-H.K. Elttayef, H.M. Ajeel, A.I. Khudiar, Effect of annealing temperature and
doping with Cu on physical properties of cadmium oxide thinfilms, J. Mater. Res. Technol. 2 (2013) 182–187.
[39] A. Shameem, P. Devendran, V. Siva, M. Raja, S.A. Bahadur, A. Manikandan, Preparation and characterization studies of nanostructured CdO thinfilms by SILAR method for photocatalytic applications, J. Inorg. Organomet. Polym. 27 (2017) 692–699.
[40] C.S. Barret, T.B. Massalski, Structure of Metals, Pergamon Press, Oxford, 1980. [41] A.A. Dakhel, Electrical and optical properties of iron-doped CdO, Thin Solid Films
518 (2010) 1712–1715.
[42] T.K. Subramanyam, S. Uthanna, B.S. Naidu, Preparation and characterization of CdOfilms deposited by dc magnetron reactive sputtering, Mater. Lett. 35 (1998) 214–220.
[43] K.T.R. Reddy, C. Sravani, R.W. Miles, Characterisation of CdO thinfilms deposited by activated reactive evaporation, J. Cryst. Growth 184/185 (1998) 1031–1034. [44] G. Phatak, R. Lal, Structural and electrical properties of cadmium oxidefilms de-posited by the activated reactive evaporation technique, Thin Solid Films 209 (1992) 240–249.
[45] D. Goyal, P. Solanki, B. Marathi, M. Takawale, V.G. Bhide, Deposition of aluminum-doped zinc oxide thinfilms by spray pyrolysis, Jpn. J. Appl. Phys. 31 (1992) 361–364.
[46] S. Kumar, P. Sharma, V. Sharma, CdS nanofilms: effect of film thickness on mor-phology and optical band gap, J. Appl. Phys. 111 (2012) 043519.
[47] B.E. Warren, X-ray Diffraction, Addison Wesley Publishing Co., London, 1969. [48] M. Ashraf, S.M.J. Akhtar, A.F. Khan, Z. Ali, A. Qayyum, Effect of annealing on
structural and optoelectronic properties of nanostructured ZnSe thinfilms, J. Alloy. Comp. 509 (2011) 2414–2419.
[49] M. Bedir, M. Öztaş, Ö.F. Bakkaloğlu, R. Ormanci, Investigations on structural,
op-tical and electrical parameters of spray deposited ZnSe thinfilms with different substrate temperature, Eur. Phys. J. B 45 (2005) 465–471.
[50] T. Mahalingam, J.S.P. Chitra, J.P. Chu, P.J. Sebastian, Preparation and micro-structural studies of electrodeposited Cu2O thinfilms, Mater. Lett. 58 (2004)
1802–1807.
[51] K. Venkateswarlu, T.A. Nellaippan, N. Rameshbabu, Estimation of crystallite size, lattice strain and dislocation density of nanocrystalline carbonate substituted hy-droxyapatite by X-ray peak variance analysis, Procedia Mater. Sci. 5 (2014) 212–221.
[52] E. Rosencher, B. Vinter, Optoelectronics, Cambridge University Press, 2002. [53] Y. Akaltun, M.A. Yıldırım, A. Ateş, M. Yıldırım, The relationship between refractive
index-energy gap and thefilm thickness effect on the characteristic parameters of CdSe thinfilms, Optic Commun. 284 (2011) 2307–2311.
[54] Y. Akaltun, T. Çayır, Fabrication and characterization of NiO thin films prepared by SILAR method, J. Alloy. Comp. 625 (2015) 144–148.
[55] A. Ateş, M.A. Yıldırım, M. Kundakçı, A. Astam, Annealing and light effect on optical and electrical properties of ZnS thinfilms grown with the SILAR method, Mater. Sci. Semicond. Process. 10 (2007) 281–286.
[56] M.A. Yıldırım, A. Ateş, Influence of films thickness and structure on the photo-response of ZnOfilms, Optic Commun. 283 (2010) 1370–1377.
[57] D. Bao, Band-gap energies of sol-gel-derived SrTiO3thinfilms, Appl. Phys. Lett. 79
(2001) 3767.
[58] K. Mageshwari, R. Sathyamoorthy, Physical properties of nanocrystalline CuO thin films prepared by the SILAR method, Mater. Sci. Semicond. Process. 16 (2013) 337–343.
[59] A.U. Ubale, Effect of complexing agent on growth process and properties of na-nostructured Bi2S3thinfilms deposited by chemical bath deposition method, Mater.
Chem. Phys. 121 (2010) 555–560.
