Effect of silver doping on the structural, morphological, optical and electrical properties of sol–gel deposited nanostructured ZnO thin films

Effect of silver doping on the structural, morphological, optical and electrical properties of sol–gel deposited nanostructured ZnO thin films

Optik 126 (2015) 5548–5552 Contents lists available at ScienceDirect Optik journal homepage: www.elsevier.de/ijleo Effect of silver doping on the s...

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Optik 126 (2015) 5548–5552

Contents lists available at ScienceDirect

Optik journal homepage: www.elsevier.de/ijleo

Effect of silver doping on the structural, morphological, optical and electrical properties of sol–gel deposited nanostructured ZnO thin films T. Touam a,∗ , F. Boudjouan b , A. Chelouche b , S. Khodja a , M. Dehimi a , D. Djouadi b , J. Solard c , A. Fischer c , A. Boudrioua c a

Laboratoire des Semi-conducteurs, Université Badji Mokhtar-Annaba, BP 12, Annaba 23000, Algeria Laboratoire de Génie de l’Environnement, Université de Bejaia, 06000 Bejaia, Algeria c Laboratoire de Physique des Lasers, Université Paris 13, 93430 Villetaneuse, France b

a r t i c l e

i n f o

Article history: Received 24 September 2014 Accepted 6 September 2015 Keywords: Silver-doped zinc oxide Sol–gel thin film Optical transmittance Electrical properties Optoelectronic applications

a b s t r a c t In this paper, we report the study of physical and optical properties of undoped and silver-doped zinc oxide (SZO) thin films prepared by sol–gel dip-coating process onto glass substrates. The structural, morphological, optical and electrical properties of the thin films as a function of silver concentration have been investigated using X-Ray Diffraction (XRD), Scanning Electronic Microscopy (SEM), Atomic Force Microscopy (AFM), UV–visible spectrophotometry and four points probe technique. XRD spectra have shown that undoped thin film exhibit (1 0 1) orientation while doped ones display a strong c-axis orientation. SEM micrographs and AFM images have revealed that grain sizes and surface roughness decrease with increasing silver concentration. The UV–visible transmittance results show that Ag-doped ZnO thin films exhibit better transparency than undoped ones. A change in the optical bandgap of ZnO thin films is also revealed. The electrical measurements have shown that the resistivity of the thin films decrease with increasing Ag doping. The present results show that ZnO thin films doped with 4 at.% Ag have more suitable properties of high transmittance and low resistivity for optoelectronic device applications. © 2015 Elsevier GmbH. All rights reserved.

1. Introduction Zinc oxide (ZnO) is one of the most important and promising materials for a large number of areas. It has distinct advantages over its competitors such as non-toxicity, low material costs, chemical stability, high transparency in the visible and near infrared spectral region, direct wide band gap (Eg = 3.37 eV at 300 K), large exciton binding energy (60 meV). Furthermore, its related quantum well may have 100% internal quantum efficiency and most of its dopants are readily available [1–3]. These advantages are of considerable interest for practical uses in transparent conductive films, solar cell windows, surface acoustic wave devices, optoelectronics and gas sensing applications [4–10]. The structural, optical and electrical properties of ZnO thin films were governed by synthesis parameters, deposition conditions and dopants. Doping in ZnO with selective elements is the effective

∗ Corresponding author. Tel.: +213 557308598.. E-mail address: [email protected] (T. Touam). http://dx.doi.org/10.1016/j.ijleo.2015.09.066 0030-4026/© 2015 Elsevier GmbH. All rights reserved.

alternative to modify its electrical, optical, magnetic, and chemicalsensing properties without any change in the crystalline structure [11,12]. Among dopants, the silver is one of the most promising to enhance optoelectronic ZnO properties, in particular, the electrical ones. Undoped and silver-doped ZnO (SZO) nano-structured thin films have been prepared using different fabrication methods such as pulsed laser deposition [13,14], sputtering technique [15–19], spray pyrolysis [20,21], oxidation method [22,23], vapor–liquid–solid mechanism [24,25], e-beam evaporation techniques [26,27], and sol–gel methods [28–33]. The sol–gel process has attracted a great attention due to its unique advantages including low cost, simple deposition equipment, easy adjusting composition and dopants, lower crystallization temperature, and it can be used to deposit films over a large area with a very uniform thickness. Despite the fact that some results have been reported on the effect of Ag-doping on physical properties of ZnO thin films, there are still some discrepancies between them [15,17,19,28] and further study is needed. Moreover, in the majority of research works

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dealing with electrical properties, most authors concentrated on the p-type conductivity. However, to the best of our knowledge the effect of Ag contents on resistivity of the SZO thin films prepared by sol–gel process has not been reported. In this paper, undoped and Ag-doped ZnO thin films were prepared on glass substrates by sol–gel dip-coating process. The effect of silver doping concentration on structural, morphological, optical and electrical properties was investigated. 2. Experimental All commercial reagents were used as received. Pure ZnO and silver doped ZnO (SZO) thin films were prepared by the sol–gel process. As a starting material, zinc acetate dihydrate (Zn(CH3 COO)2 ·2H2 O; Sigma–Aldrich) was dissolved in a mixture of absolute ethanol (EtOH, 100%, BioChem) and monoethanolamine (MEA; Sigma–Aldrich) yielding to a precursor concentration of 0.75 mol L−1 . MEA acts as bidentate ligands to Zn2+ to stabilize the solution against any precipitation thus producing clear solution for coating procedure [34]. The MEA to zinc acetate molar ratio was set to 1. For doped films, silver nitrate (AgNO3 ) was added to the mixture with an atomic percentage fixed at 1, 2, 3, 4 and 5 at.% Ag. The resulting ZnO and SZO sol were stirred at 50 ◦ C for 1 h, and then aged at room temperature for 36 h to get a clear and transparent homogeneous ZnO and SZO aqueous solutions. The commercial glass substrates (Esco Optics) were cleaned with detergent, rinsed in flowing deionized water and immersed in 4 M nitric acid for 1 h. Rinsing with acetone and isopropanol, drying with nitrogen and in an oven at 100 ◦ C for 30 min complete the procedure. The substrates were dipped in the prepared sols and then withdrawn at a constant dip-coating speed of 15 mm/min. Synthesized films, doped and undoped, were preheated at 200 ◦ C for 10 min after each coating. This procedure was repeated twelve times to increase the thickness. The films were subsequently heated up to 500 ◦ C for 1 h with a heating rate of 5 ◦ C/min in order to obtain crystallized ZnO. The prepared nano-structured thin films were characterized for the crystalline structure by X-ray Diffraction (XRD) with a PANalytical X´ıPert diffractometer, operating at 40 kV and 30 mA using Cu ˚ at a grazing incidence angle ω equal K␣1 radiation ( = 1.54056 A) to 0.54◦ . Scanning Electronic Microscopy (SEM) characterizations were carried out by means of a Raith PIONEER System. Surface morphology of thin films in terms of root mean squared roughness (Rrms ) was explored from the images collected in contact mode by Atomic Force Microscopy (AFM, Nanosurf easyScan 2) operated at room temperature. The AFM images were collected in four different regions on the surface of each sample. The Rrms values used throughout this paper were calculated with the SPIPTM analysis software (Image Metrology). The optical transmission spectra were analyzed at room temperature by a Safas UVmc2 UV–Visible spectrophotometer and the optical band gap energy data was then derived from the transmission spectra. Electrical resistivity characterizations of the thin films have been carried out using the four-point probe method. The film thickness of the all samples was measured by using a Veeco Dektak 150 Surface Profiler. 3. Results and discussion 3.1. Structural and morphological properties The crystallinity and the preferred crystal orientation of the undoped and Ag-doped ZnO thin films were analyzed by the X-ray diffraction. Fig. 1 represents the diffraction patterns of the pure, 1, 2, 3, 4 and 5 at.% Ag-doped 12-layer ZnO samples. The diffracted peaks have been identified using standard ZnO cards indicating that all the deposited films have a polycrystalline wurtzite hexagonal

Fig. 1. XRD patterns of pure, 1, 2, 3, 4 and 5 at.% Ag-doped ZnO thin films.

structure, and no other crystallized phases are observed. This means Ag-incorporation does not change the crystal structure. The results show that all the pure ZnO thin films have (1 0 1) as the preferred orientation. The intensity of the (0 0 2) peak of Ag-doped samples indicates that all the SZO thin films exhibit preferential orientation growth along (0 0 2) direction, that is, perpendicular to the substrate surface. We have to note that broadening of the diffracted peaks and overlapping of the (0 0 2) and (1 0 1) peaks are usually a result of a smaller crystallite size and lattice. It is clearly seen that the peak intensities of the Ag-doped ZnO thin films increase when the silver content were increased from 1 to 5 at.%. This feature demonstrates that the crystallinity of the samples is enhanced when Ag concentration was increased. An apparent increase in crystallinity has been also observed by Li et al. [35] as suggested by the increase of the (0 0 2) peak intensity in the XRD patterns of the sol–gel Li:ZnO thin films with increasing the Ag co-doping contents. Morphological study of the nano-structured samples has been investigated using SEM imaging. Fig. 2 presents high-magnification scanning electronic micrographs of the undoped and SZO thin films on glass substrates. The films show uniform and dense morphology overall the surface; this morphology consists of spherical aggregate grains in the 50–90 nm range in the vicinity of which one can see much finer nanoparticles, with a mean diameter around 15–25 nm. Since the average crystallites size (different orientations) was estimated by the Scherrer method [36] in the 4–8 nm range, this means that each grain consists of several crystallites with different orientations. It is clear from these micrographs that the surface morphology of SZO films was slightly modified with doping. It can be seen that grain size seems to be sensitive to the Ag content as finer particles and aggregate grains are observed. The present results seem to be in agreement with those reported by Thongsuriwong et al. [37] who showed from AFM images that the grain size of the Ag-doped ZnO thin films slightly decreased with increasing Ag concentration. Surface features of the all samples have also been studied by AFM in contact mode to examine the quality of film deposition on glass substrate. Fig. 3 displays the three-dimensional AFM images of the undoped and Ag-doped ZnO thin films scanned over a surface area of 5 × 5 ␮m2 . From Fig. 3, it can be seen that all the samples have uniform and dense ZnO grains. Furthermore, the grains have columnar shapes which grow preferentially along the c-axis orientation. This observation is in good agreement with the results of XRD and SEM analyses. The films exhibit different surface roughness which seems to be dependent on the Ag-doping. The root mean squared roughness was calculated and found to be as 19.57, 18.87, 17.76, 12.64, 10.52 and 15.44 nm for 0, 1, 2, 3, 4 and 5 at.%

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Fig. 4. Transmittance spectra of pure and Ag-doped ZnO thin films. The inset shows the magnification of absorption edge region.

It is possibly connected with the decrease of ZnO grain as revealed by SEM micrographs. Our results and observations agree with previous work [38], where it has been demonstrated that the doping of Ag can reduce the scattering or absorption loss at the surface and thus change the properties of ZnO thin films significantly.

Fig. 2. SEM micrographs of Ag-doped ZnO thin films: (a) 0 at.% Ag, (b) 1 at.% Ag, (c) 2 at.% Ag, (d) 3 at.% Ag, (e) 4 at.% Ag and (f) 5 at.% Ag.

Ag-doped ZnO thin films, respectively, which shows that good quality of films was achieved by our present sol–gel process. Undoped ZnO thin film has the largest surface roughness, but when Agdoping concentration is increased, the surface roughness decreases gradually reaching its minimum value at 4 at.% silver contents, then slightly increase when the Ag concentration is increased to 5 at.%.

Fig. 3. AFM images of Ag-doped ZnO thin films: (a) 0 at.% Ag, (b) 1 at.% Ag, (c) 2 at.% Ag, (d) 3 at.% Ag, (e) 4 at.% Ag and (f) 5 at.% Ag.

3.2. Optical and electrical properties Fig. 4 shows the UV–Vis–IR transmittance spectra of Ag-doped ZnO thin films for various Ag-doping concentrations. The maximum optical transmissions in the visible region were observed higher than 80% and SZO films show better transparency than the undoped ZnO, but the transmission decreases substantially at short wavelengths near the ultraviolet range. With increasing silver dopants up to 4 at.%, the maximum transmission increases to 91.64% but then decreases to 82.7% when the Ag concentration is increased to 5 at.%. The increase in the transmittance of the thin films with increase of Ag-doping may be attributed to the enhancement of the crystallinity and the decrease of the surface roughness. From the magnification of absorption edge region shown by inset Fig. 4, it is found that by increasing silver contents up to 4 at.%, the absorption edge and maximum transmittance peak position shift toward shorter wavelengths which may indicate an increase in the bandgap and a decrease in the film thickness. However, at 5 at.% doping, opposite behavior is observed by a slight shift to a longer wavelength region. In order to determine the allowed direct optical bandgaps of undoped and Ag-doped ZnO thin films, we have used the technique based on the derivative of the transmittance with respect to energy, dT/dE. This very accurate method, experimentally validated, has been successfully used for the analysis of bandgaps by several authors [39–41]. According to the transmittance spectra measured, the dT/dE curves of ZnO films with different Ag-doping concentration are presented in Fig. 5. Bandgap values of 3.20, 3.21, 3.22, 3.23, 3.24 and 3.22 eV were obtained for pure ZnO, 1, 2, 3, 4 and 5 at.% of Ag, respectively. It can be seen that the band gap initially increases as Ag contents increase then slightly decreases when the Ag concentration is increased to 5 at.%. Several reports indicated controversial results concerning the energy gap (Eg ) change of silver doped ZnO. Some authors have reported that the optical bandgap increased with increasing Agdoping concentration in ZnO nanocrystals [15,17], while others have found that optical bandgap of ZnO thin films decreased with increasing Ag-doping content [19,28]. For example, Xue et al. [15] have investigated the influence of Ag-doping on the optical properties of ZnO films prepared by rf magnetron sputtering technique.

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Table 1 Root mean squared roughness, maximum transmittance, peak position, bandgap energy, film thickness and electrical resistivity of undoped and Ag-doped ZnO thin films. Ag-doping percentage (%)

RMS roughness (nm)

Maximum transmittance (%)

Peak position (nm)

Bandgap energy, Eg (eV)

Film thickness (nm)

Film resistivity ( cm)

0 1 2 3 4 5

19.6 18.8 17.7 12.6 10.5 15.4

80.1 80.6 80.7 85.9 91.6 82.7

757 754 743 702 656 677

3.204 3.212 3.221 3.229 3.238 3.221

450 412 382 372 371 382

0.0456 0.0416 0.0395 0.0327 0.0273 0.0096

Fig. 6. Measured electrical resistivity of pure and Ag-doped ZnO thin films.

Fig. 5. Plots of the derivative of the transmittance with respect to energy of pure and Ag-doped ZnO thin films.

It was demonstrated that the optical band edge shifted to a shorter wavelength first as Ag is incorporated, and then to a longer wavelength with the increasing of Ag doping concentration. Sahu [19] has studied the effect of Ag-doping on the structural, optical and electrical properties of ZnO thin films prepared by simultaneous rf magnetron sputtering of ZnO and dc magnetron sputtering of Ag on glass substrate. It was found that the absorption edge shifted slightly to a longer wavelength and the band gap narrowed with increasing Ag content. The present results seem to be in very good agreement with the work of Xue et al. [15]. According to our obtained results, the increment in the band gap by silver doping may be attributed, on the one hand, to the quantum size effect as indicated by the work of Marotti et al. [42], where it was suggested that the band gap and the absorption edge of nano-structured materials shift due to quantum size effects. On the other hand, this widening of the optical band gap is well known as Burstein–Moss effect [43,44] and was discussed in many semiconducting materials of low density of state effective mass and doped semiconductors [45,46]. This increment in the bandgap of the Ag-doped ZnO thin films is considered as advantage property since the optical transmittance window is broadened. To enrich the present investigation, the electrical resistivity was measured at room temperature by four-probe method. The thickness of all thin films was determined from surface profile analysis. Thickness values of 450, 412, 382, 372, 371 and 382 nm were obtained for pure ZnO, 1, 2, 3, 4 and 5 at.% of silver, respectively. Fig. 6 depicts the variation of resistivity of Ag doped ZnO films with doping concentration. The resistivity of undoped ZnO is about 4.56 10−2  cm; this value decreases with increasing Ag concentration to reach its lowest value of 9.6 10−3  cm for Agdoping concentration of 5 at.%. This behavior can be attributed to an increase of not only in carrier concentration but also in mobility. All the obtained results from the different characterization techniques of undoped and Ag-doped ZnO thin films prepared by the sol–gel process are summarized in Table 1; it can readily be noticed

that ZnO thin films doped with 4 at.% Ag exhibited interesting properties of high transmittance and low resistivity that could be very suitable for optoelectronic applications. 4. Conclusions Pure and Ag-doped ZnO thin films were successfully prepared by the sol–gel dip-coating method. The effects of Ag-doping concentration (1–5 at.% Ag) on the structural, morphological, optical and electrical properties of the obtained films were investigated by X-ray diffraction, scanning electronic microscopy, atomic force microscopy, UV–visible spectrophotometer and four points probe technique. It was found that pure ZnO thin films exhibit (1 0 1) orientation while doped ones display a strong c-axis orientation perpendicular to the substrates with enhanced crystallinity. SEM micrographs and AFM images have revealed that the grain size and surface roughness tends to decrease with increasing silver concentration. The maximum optical transmissions in the visible region were found to be higher than 80% and Ag-doped ZnO thin films show better transparency with a maximum value of 91.6% obtained at 4 at.% concentration. Moreover, it is found that the absorption edge is shifted toward UV region indicating a widening of optical bandgap with silver doping. The measured resistivity of thin films decreases with increasing Ag doping. Hence, it can be concluded that all obtained results put into evidence that ZnO thin films doped with 4 at.% Ag have the best physical properties that may be well suited for optoelectronic applications. Acknowledgements The authors are grateful to Mr. L. Benidiri for his help in the experimental characterization. The authors would like to express their thanks to Prof. A. Doghmane for his careful reading of the manuscript and English language corrections. References [1] M. Hass, H. Weller, A. Henglein, Photochemistry and radiation chemistry of colloidal semiconductors. 23. Electron storage on ZnO particles and size quantization, J. Phys. Chem. 92 (1988) 482–487.

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[2] D.C. Look, G.C. Farlow, P. Reunchan, S. Limpijumnong, S.B. Zhang, K. Nordlund, Evidence for native-defect donors in n-type ZnO, Phys. Rev. Lett. 95 (2005) 225502–225504. [3] L. Béaur, T. Bretagnon, B. Gil, A. Kavokin, T. Guillet, C. Brimont, D. Tainoff, M. Teisseire, J.-M. Chauveau, Exciton radiative properties in nonpolar homoepitaxial ZnO/(Zn,Mg)O quantum wells, Phys. Rev. B 84 (2011) 165312. [4] T.L. Yang, D.H. Zhang, J. Ma, H.L. Ma, Y. Chen, Transparent conducting ZnO:Al films deposited on organic substrates deposited by r.f. magnetron-sputtering, Thin Solid Films 326 (1998) 60–62. [5] K.L. Chopra, S. Major, D.K. Pandya, Transparent conductors – a status review, Thin Solid Films 102 (1983) 1–46. [6] C.G. Granqvist, Window coatings for the future, Thin Solid Films 193/194 (1988) 730–741. [7] S. Major, K.L. Chopra, Indium-doped zinc oxide films a transparent electrodes for solar cells, Sol. Energ. Mater. Sol. C 17 (1988) 319–327. [8] Y.I. Alivov, Ü. Özgür, S. Do˘gan, D. Johnstone, V. Avrutin, N. Onojima, C. Liu, J. Xie, Q. Fan, H. Morkoc¸, P. Ruterana, High efficiency n-ZnO/p-SiC heterostructure photodiodes grown by plasma-assisted molecular-beam epitaxy, Superlattices Microstruct. 38 (2005) 439–445. [9] Y.-D. Ko, K.-C. Kim, Y.-S. Kim, Effects of substrate temperature on the Ga-doped ZnO films as an anode material of organic light emitting diodes, Superlattices Microstruct. 51 (2012) 933–941. [10] S.T. Shishiyanu, T.S. Shishiyanu, O.I. Lupan, Sensing characteristics of Tin-doped ZnO thin films as NO2 gas sensor, Sens. Actuat. B Chem. 107 (2005) 379–386. [11] L. Znaidi, T. Touam, D. Vrel, N. Souded, S. Ben Yahia, O. Brinza, A. Fischer, A. Boudrioua, AZO thin films by sol–gel process for integrated optics, Coatings 3 (2013) 126–139. [12] S.W. Shin, I.Y. Kim, K.S. Jeon, J.Y. Heo, G.-S. Heo, P.S. Patil, J.H. Kim, J.Y. Lee, Wide band gap characteristic of quaternary and flexible Mg and Ga co-doped ZnO transparent conductive thin films, J. Asian Ceram. Soc. 1 (2013) 262–266. [13] M.A. Myers, J.H. Lee, Z. Bi, H. Wang, High quality p-type Ag-doped ZnO thin films achieved under elevated growth temperatures, J. Phys. Condens. Matter. 24 (2012) 145802–145808. [14] M.E. Koleva, A.O. Dikovska, N.N. Nedyalkov, P.A. Atanasov, I.A. Bliznakova, Enhancement of ZnO photoluminescence by laser nanostructuring of Ag underlayer, Appl. Surf. Sci. 258 (2012) 9181–9185. [15] Y. Chen, X.L. Xu, G.H. Zhang, H. Xue, S.Y. Ma, A comparative study of the microstructures and optical properties of Cu- and Ag-doped ZnO thin films, Physica B 404 (2009) 3645–3649. [16] W.J. Li, C.Y. Kong, H.B. Ruan, G.P. Qin, G.J. Huang, T.Y. Yang, W.W. Liang, Y.H. Zhao, X.D. Meng, P. Yu, Y.T. Cui, L. Fang, Electrical properties and Raman scattering investigation of Ag doped ZnO thin films, Solid State Commun. 152 (2012) 147–150. [17] H. Xue, X.L. Xu, Y. Chen, G.H. Zhang, S.Y. Ma, Influence of Ag-doping on the optical properties of ZnO films, Appl. Surf. Sci. 255 (2008) 1806–1810. [18] X.B. Wang, C. Song, K.W. Geng, F. Zeng, F. Pan, Luminescence and Raman scattering properties of Ag-doped ZnO films, J. Phys. D Appl. Phys. 39 (2006) 4992–4996. [19] D.R. Sahu, Studies on the properties of sputter-deposited Ag-doped ZnO films, Microelectron. J. 38 (2007) 1252–1256. [20] K. Liu, B.F. Yang, H. Yan, Z. Fu, M. Wen, Y. Chen, J. Zuo, Effect of Ag doping on the photoluminescence properties of ZnO films, J. Lumin. 129 (2009) 969–972. [21] K. Liu, B.F. Yang, H. Yan, Z. Fu, M. Wen, Y. Chen, J. Zuo, Strong room-temperature ultraviolet emission from nanocrystalline ZnO and ZnO:Ag films grown by ultrasonic spray pyrolysis, Appl. Surf. Sci. 255 (2008) 2052–2056. [22] R. Chen, C. Zou, J. Bian, A. Sandhu, W. Gao, Microstructure and optical properties of Ag-doped ZnO nanostructures prepared by a wet oxidation doping process, Nanotechnology 22 (2011) 105706–105714. [23] X. Li, Y. Wang, Structure and photoluminescence properties of Ag-coated ZnO nano-needles, J. Alloys Compd. 509 (2011) 5765–5768. [24] Y.-W. Song, K. Kim, S.Y. Lee, Morphology transition of Ag-doped ZnO nanostructures in hot-walled pulsed laser deposition, Thin Solid Films 518 (2009) 1318–1322.

[25] Y.-W. Song, K. Kim, J.P. Ahn, G.-E. Jang, S.Y. Lee, Physically processed Agdoped ZnO nanowires for all-ZnO p–n diodes, Nanotechnology 20 (2009) 275606–275610. [26] D.R. Sahu, S.-Y. Lin, J.-L. Huang, Study on the electrical and optical properties of Ag/Al-doped ZnO coatings deposited by electron beam evaporation, Appl. Surf. Sci. 253 (2007) 4886–4890. [27] Y. Wei, L. Ke, J. Kong, H. Liu, Z. Jiao, X. Lu, H. Du, X.W. Sun, Enhanced photoelectrochemical water-splitting effect with a bent ZnO nanorod photoanode decorated with Ag nanoparticles, Nanotechnology 23 (2012) 235401–235409. [28] F. Xian, K. Miao, X. Bai, Y. Ji, F. Chen, X. Li, Characteraction of Ag-doped ZnO thin film synthesized by sol–gel method and its using in thin film solar cells, Optik 124 (2013) 4876–4879. [29] J. Xu, Z.-Y. Zhang, Y. Zhang, B.-X. Lin, Z.-X. Fu, Effect of Ag doping on optical and electrical properties of ZnO thin films, Chinese Phys. Lett. 22 (2005) 2031–2034. [30] A. Chelouche, D. Djouadi, H. Merzouk, A. Aksas, Influence of Ag doping on structural and optical properties of ZnO thin films synthesized by the sol–gel technique, Appl. Phys. A 115 (2013) 613–616. [31] X.D. Zhou, X.H. Xiao, J.X. Xu, G.X. Cai, F. Ren, C.Z. Jiang, Mechanism of the enhancement and quenching of ZnO photoluminescence by ZnO–Ag coupling, Europhys. Lett. 93 (2011) 57009–57015. [32] C.-S. Hong, H.-H. Park, H.-H. Park, H.J. Chang, Optical and electrical properties of ZnO thin film containing nano-sized Ag particles, J. Electroceram. 22 (2009) 353–356. [33] M.H. Habibi, R. Sheibani, Preparation and characterization of nanocomposite ZnO–Ag thin film containing nano-sized Ag particles: influence of preheating, annealing temperature and silver content on characteristics, J. Sol–Gel Sci. Technol. 54 (2010) 195–202. [34] S. Fujihara, C. Sasaki, T. Kimura, Crystallization behavior and origin of c-axis orientation in sol–gel-derived ZnO: Li thin films on glass substrates, Appl. Surf. Sci. 180 (2001) 341–350. [35] J.-C. Li, Q. Cao, X.-Y. Hou, Effects of Ag-induced acceptor defects on the band gap tuning and conductivity, J. Appl. Phys. 113 (2013) 203518-1–203518-7. [36] J.I. Langford, A.J.C. Wilson, Scherrer after sixty years: a survey and some new results in the determination of crystallite size, J. Appl. Cryst. 11 (1978) 102–113. [37] K. Thongsuriwong, P. Amornpitoksuk, S. Suwanboon, Photocatalytic and antibacterial activities of Ag-doped ZnO thin films prepared by a sol–gel dipcoating method, J. Sol–Gel Sci. Technol. 62 (2012) 304–312. [38] C.S. Hong, H.H. Park, J. Moon, H.H. Park, Effect of metal (Al, Ga, and In)-dopants and/or Ag-nanoparticles on the optical and electrical properties of ZnO thin films, Thin Solid Films 515 (2006) 957–960. [39] M. Wang, E.J. Kim, S. Kim, J.S. Chung, I. Yoo, E.W. Shin, S.H. Hahn, C. Park, Optical and structural properties of sol–gel prepared Mg ZnO alloy thin films, Thin Solid Films 516 (2008) 1124–1129. [40] R. Viswanatha, S. Chakraborty, S. Basu, D.D. Sarma, Blue-emitting copper-doped zinc oxide nanocrystals, J. Phys. Chem. B 110 (2006) 22310–22312. [41] C.A. Parker, J.C. Roberts, S.M. Bedair, M.J. Reed, S.X. Liu, N.A. El-Masry, L.H. Robins, Optical band gap dependence on composition and thickness of Inx Ga1−x N (0  x  0.25) grown on GaN, Appl. Phys. Lett. 75 (1999) 2566–2568. [42] R.E. Marotti, P. Giorgi, G. Machado, E.A. Dalchiele, Crystallite size dependence of band gap energy for electrodeposited ZnO grown at different temperatures, Sol. Energ. Mater. Sol. C 90 (2009) 2356–2361. [43] E. Burstein, Anomalous optical absorption limit in InSb, Phys. Rev. 93 (1954) 632–633. [44] T.S. Moss, The interpretation of the properties of indium antimonide, Proc. Phys. Soc. Lond. B 67 (1954) 775–782. [45] S. Chirakkara, K.K. Nanda, S.B. Krupanidhi, Pulsed laser deposited ZnO: In as transparent conducting oxide, Sol. Energ. Mater. Sol. C 90 (2011) 2356–2361. [46] S. Gad, M.A. Rafea, Y. Badr, Optical and photoconductive properties of Pb0.9Sn0.1Se nano-structured thin films deposited by thermal vacuum evaporation and pulsed laser deposition, J. Alloys Compd. 515 (2012) 101–107.