Fabrication of Nanostructured Copper Indium Di ...

Copyright © 2013 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Nanoscience and Nanotechnology Vol. 13, 1–8, 2013

Fabrication of Nanostructured Copper Indium Di-Selenide (CIS) Thin Films by Electrohydrodynamic Atomization Technique Navaneethan Duraisamy1 , Nauman Malik Muhammad1 3 , Jeongdai Jo2 , and Kyung-Hyun Choi1 ∗ 1

Department of Mechatronics Engineering, Jeju National University, Jeju 690-756, Republic of Korea 2 Korea Institute of Machinery and Materials, Daejon 305-343, Republic of Korea 3 Department of Mechanical Engineering, Mohammad Ali Jinnah University, Islamabad 44000, Pakistan

Keywords: Copper-Indium-Diselenide, Electrohydrodynamic Atomization, Surface Morphology, Optical Properties.

In recent years, a lot of attention has been drawn by the researchers towards the fabrication of semiconductor materials (especially nanostructures) due to their potential applications in thin films and electronic industry.1 2 The semiconducting materials at nanoscale (nanostructured thin films for device applications) provide excellent electrical and optical properties compared to their bulk nature which is attributed to the size and quantum effects.3 4 The nanostructured semiconducting thin films have significant interest in electronic industries due to their widespread applications such as transistors, capacitors, photochromism, solar cells, light emitting diodes, etc.5 6 The performance of semiconducting thin film based devices are highly influenced by crystalline phase, size, surface morphology, energy band gap, and purity.7 8 These are physico-chemical properties of the thin films, which significantly vary with respect to the fabrication technique employed for the material preparation.9 So far, ∗

Author to whom correspondence should be addressed.

J. Nanosci. Nanotechnol. 2013, Vol. 13, No. xx

there is lot of techniques available for the fabrication of semiconducting thin films for electronic applications, viz. spin coating, chemical vapour deposition (CVD), physical vapour deposition (PVD), Langmuir–Blodgett (LB) technique, lithography, sputtering, etc.10–13 In the case of CVD, PVD and sputtering are capable of producing highly uniform and transparent thin films but these techniques are needed high vacuum system and temperature controller.14 15 The concentration of starting precursors, chemical reaction on the substrate surface, thickness of thin film and surface purity are difficult to control. The nano/micro sized thin films/patterns are easily achieved by lithographic techniques for electronic device application but the mask aligner is an important tool required to achieve desired patterns or thin films and also not suitable for large scale deposition. Other methods such as spin coating and dip coating, controlling the film thickness and surface morphology is one of the biggest challenge until now.16 17 These drawbacks limit their applications towards large scale production which directly limits the

1533-4880/2013/13/001/008

doi:10.1166/jnn.2013.7930

1

RESEARCH ARTICLE

In this article, we report a non-vacuum electrohydrodynamic atomization (EHDA) technique for deposition of CuInSe2 (CIS) thin films. The CIS ink has been prepared with three different concentrations (7.5 wt.%, 12.5 wt.% and 15 wt.%) by using suitable solvent mixture (ethanol:terpineol as 1:1 molar ratio) with surfactant to achieve a stable dispersions. The important physical parameters for achieving homogeneous with non-agglomerated CIS layers through EHDA technique are investigated in detail. The X-ray diffraction pattern confirms the crystalline structure of CIS layers oriented in the chalcopyrite phase. The film uniformity has been investigated by field emission scanning electron microscopy. Different thickness of CIS layers has been achieved by varying the concentration of CIS particles in the precursor ink solution. The optical properties of CIS layers show the two optical band gaps in UV-visible and near infra-red (NIR) region with band gap of about 2.67–2.49 eV and 1.34–1.29 eV respectively. The energy band gap of CIS thin films have been decreased with the increase of film thickness. The X-ray photoelectron spectra confirmed presence of binding energy corresponding to CuInSe2 . The electrical study observed the sheet resistivity 76∼33 cm with respect to film thickness.

RESEARCH ARTICLE

Fabrication of Nanostructured CIS Thin Films by Electrohydrodynamic Atomization Technique

commercialization. In this regards, the development of a unique method for mask less techniques is highly encouraged for the fabrication of thin film devices with low cost, simple processing and also can be utilized for large scale application is an area of potential interest.18 In these aspects, EHDA becomes a versatile technique with the merits of being low cost, solution processed and low temperature deposition of nanostructured thin films.19 Moreover, this technique can be employed for large scale production which is an important strategy for the commercialization of any device applications for their potential utility in human welfare. The working principle of EHDA technique relies on applying an electric potential, which is used to overcome the liquid surface tension (precursors or dispersed nanoparticle inks) into stable cone-jet or Taylor cone, which is further disintegrated into mono-dispersed droplets and deposited on a substrate.20 21 The formation of Taylor cone is one of the important parameter needs to be optimized for achieving a better thin film quality.20 Since, EHDA relies on the solution processing; the physico-chemical properties such as surface tension, electrical conductivity, viscosity and stability of the precursor ink directly impact on the Taylor cone formation.22 Hence, the optimization of the precursor solution to achieve a stable cone-jet/Taylor cone formation is a significant parameter prior to the deposition of thin films. Among the nanostructured chalcogenide materials such as CdTe, Cu(In, Ga)Se2 , CuInS2 , and Cu2 ZnSnS4 used for electronic devices, CuInSe2 (CIS) is more promising with the advantage of good energy band gap with reasonable work functions, high photo-absorption coefficient compared to the others.23 24 CIS has the advantage of altering their electronic properties by varying the material concentration which made this material as hot compared to the others.25 The previous report published in the literature demonstrated that CIS thin films have good stability under high energy light irradiation.25 Hence, the optimization process of CIS thin film deposition using EHDA is one of the significant area need to be investigated in detail. In this paper, the EHDA deposition of nanostructured CIS thin films has been optimized and discussed in detail by varying the concentration of CIS nanoparticle in the precursor solution. Totally, three different concentrations of CIS nanoparticles such as 7.5 wt.%, 12.5 wt.% and 15 wt.% in the precursor solution were investigated in this study. A detailed anlaysis has been performed both in the physico-chemical properties of the precursor ink and also in the optimization process for the formation of stable cone-jet. The changes in thin film quality such as crystalline phase, surface morphology, chemical bonding and surface states, optical band gap and their electrical properties were also discussed in detail. Copper indium diselenide (CuInSe2 , CIS) nanoparticles, polyvinylpyrrolidone (PVP), ethanol and terpineol were purchased from Sigma-Aldrich, South Korea. All the 2

Duraisamy et al.

chemicals were used in this experiment without further purifications. In this work, CIS inks were prepared with three different nanoparticle concentrations. At first, 7.5 wt.% of CIS nanoparticles was dispersed in solvent mixture (ethanol:terpineol as 1:1 molar ratio) under ultrasonication (using probe type sonicator with frequency of 10 kHz) for 5 min. Then PVP was added into the above solution and the sonication was continued for 10 min with same frequency to achieve a uniform dispersion of CIS ink and denoted as CIS-1. The above process was repeated by changing concentration of CIS nanoparticle (12.5 and 15 wt.%). The concentration of CIS inks such as 12.5 wt.% and 15 wt.%, which was corresponds to the CIS-2 and CIS-3 respectively. Finally, the uniform dispersion CIS inks were achieved and used for further experiments. EHDA system consisted of a stainless steel needle with inner diameter of 410 m installed on the nozzle adapter (NanoNC) which was mounted on a y–z robotic plunk. A syringe pump (Harvard Apparatus, PHD 2000 Infusion), which holds the CIS ink containing syringe (Hamilton, Model 1001GASTIGHT syringe), was connected to the inlet of the nozzle and keeps the minimum required flow rate. Teflon tubing is used for the CIS ink supply from the syringe to the nozzle. A high voltage power source (NanoNC, 30 kV) connected between the capillary and copper plate ground electrode generates highly concentrated electric field at the capillary outlet and generates the largely mono-dispersed electrospray of the CIS ink. The ground electrode plate was mounted on a stage capable of moving in x-direction. A high resolution CCD camera (Motion Pro X) was connected to the main computer and provided the high resolution capturing of the events going on at the capillary tip i.e., dripping and Taylorcone formation etc. A fiber optic light source was used for illumination.20 21 The CIS inks were deposited on glass substrates through EHDA technique. Before the deposition process, the substrates were cleaned with acetone, ethanol, DI-water and then irradiated with UV light in the UV cleaner for 15 min. The deposited CIS thin films were sintered at 200 C for 24 h. During the sintering, the solvent was evaporated completely without affecting the deposited CIS thin films. The deposited CIS thin films were used to further characterization. The CIS inks are prepared by using CIS nanoparticles evenly dispersed in solvent mixture (ethanol:terpineol) with 1:1 molar ratio with PVP as a stabilizer. The terpineol plays a key role to control the rate of solvent evaporation during electrohydrodynamic atomization for achieving a uniform thin film and also stabilize the CIS ink. Addition of PVP is used to control the agglomeration of nanoparticles and also enhance the film uniformity.26 Figure 1(a) shows the stable dispersion of CIS inks. The ink stability is disturbed after 4 months in CIS-3, which is due to the influence of higher concentration of nanoparticles but other two inks reveal uniform dispersion (Fig. 1(b)). J. Nanosci. Nanotechnol. 13, 1–8, 2013

Duraisamy et al.


Fig. 1. (a) Photograph of uniform dispersion of CIS inks, (b) Stability test of CIS inks after 4 months.

Table I. Physical properties of CIS inks. Solvent Sample CIS-1 CIS-2 CIS-3

Ethanol (%)

Terpineol (%)

Particle wt.%

Conductivity (S cm−1 )

Surface tension (N m−1 )

Viscosity (cP)

Dielectric constant

Density (Kg /m3 )

50 50 50

50 50 50

7.5 12.5 15

6.8 7.1 7.2

9.9 17.3 27.2

2.9 4.52 4.58

19.5 21.2 23.4

1247 1285 1298

J. Nanosci. Nanotechnol. 13, 1–8, 2013

3

RESEARCH ARTICLE

The physical properties of CIS inks are very much impartment in EHDA process for deposition of uniform thin films. The important physical properties such as electrical conductivity, surface tension, viscosity and dielectric constant are significantly influenced by nanoparticle concentrations and solvent of CIS inks. Herein, nanoparticle concentrations of CIS ink are significantly changing the physical properties of CIS ink with constant solvent mixtures (ethanol:teripineol-50:50%). Table I revealed that the ink conductivity, dielectric constant, surface tension and viscosity are gradually increased with respect to CIS concentrations. In EHDA, stability of cone-jet, droplet diameter and spray ability are mainly depends upon the ink concentration and its physical properties. The main advantage with the cone-jet mode of the EHDA is the larger mono-dispersity of the sprayed particles, which carries a lot of importance when dealing with thin films.26 Therefore the emphasis of this study is to achieve the stable spray of the CIS ink with cone-jet mode. Figure 2(a) shows the Taylor cone or stable conejet for CIS inks (CIS-1, CIS-2 and CIS-3 with different

concentrations 7.5, 12.5 and 15 wt.% respectively) at constant flow rate of 100 l/h with a minimum required voltage. An almost symmetric and largely equilateral triangle is observed emanating from nozzle end in all the cases. As can be seen, there is a hint of spray emanating at the end of the jet. There is little wetting on surface of the nozzle due to hydrophilicity of the nozzle surface. The wetness is influenced in atomization process and results in pulsations of the stable cone-jet. However, the amount of wetness in this case is not high and the stability of cone does not affect the cone-stability. A little skewness is visible in the cone for CIS-3 (Fig. 2(a)(iii)) which occurs if the applied voltage is higher than the required onset voltage. This higher voltage is sometimes required to provide constant stability for atomization process due to the influence of physical properties of ink. The flow rate and applied voltage for achieving Taylor cone have been clearly investigated by operating envelops (Fig. 2(b)) at standoff distance (the distance between capillary outlet and ground) of 12 mm. The operating envelop confirms that the all CIS inks have working on same parameters range with slight deviation, which is due to the ink physical properties. However, the operating envelop of CIS-2 is wider than that of the other two inks and required voltage is also lesser in this case. Therefore CIS-2 is good for atomization process. It might be good physical properties as compared with CIS-1 and uniform dispersion stability in the long period of time as compared with CIS-3. The X-ray diffraction analysis (Rikagu D/MAX 2200H, Bede model 200) is used to investigate the crystalline properties of CIS thin films. Figure 3 shows the diffraction pattern of as-deposited CIS thin films. The X-diffraction pattern for a typical CIS (CIS-1, CIS-2 and CIS-3) thin films are oriented in the (1 1 2), (2 1 1), (2 0 4)/(2 2 0), (1 1 6)/(3 1 2), (0 0 8) and (3 3 2)/(3 1 6) direction and the crystalline peaks indicate a chalcopyrite structure (JCPDS card No. 81-1936). In Figure 3, the crystalline peak observed at 26.7 shows slight intensity difference due to influence of CIS thin films. The XRD analysis evidences that there are no characteristic peaks of any impurities observed, thereby suggesting the high quality of thin films. The deposited CIS thin films are measured by thin film thickness measurement (ellipsometry- K-MAC ST4000DLX).27 28 Figure 4(a) provides the film thickness of CIS layers such as CIS-1, CIS-2 and CIS-3 respectively. The thickness is measured at 10 different points on deposited

RESEARCH ARTICLE


Duraisamy et al.

Fig. 2. (a) Taylor cone or stable cone-jet mode of CIS inks with three different concentrations, (b) Operating envelops of CIS inks showing stable cone jet regimes at different flow rates and applied voltages.

thin film. The average thickness of deposited CIS thin films was 1∼2 m which is due to the influence of particle concentration as shown in Figure 4(b). If we take into account layer morphology from FE-SEM, we can observe that 12.5 wt.% of CIS ink is the comparatively good and the corresponding film thickness is ∼1.6 m thickness with good surface morphology. The surface morphology of deposited CIS thin films is investigated through FE-SEM (JEOL, JEM 1200EX II). Figure 5 shows the morphology image of CIS-1, CIS-2 and CIS-3 with low and high magnification to investigate the surface of CIS layer with respect to nanoparticle concentration. In EHDA, the surface morphology of deposited thin films are not only depends upon the nanoparticle concentrations but also some other parameters such as ink flow rate, stand-off distance, substrate velocity or spray 4

time. Hence, the surface morphology of CIS thin film deposition has been examined by changing the nanoparticle concentrations. Therefore the other important parameters such as stand-off distance (12 mm) and substrate velocity (1 mm/s) to be constant throughout the experiment. The observed high magnification FE-SEM image shows that the deposited layers are homogeneous with little cracks on the surface (Figs. 5(a), (c) and (e)) but the high magnification FE-SEM (Figs. 5(b), (d) and (f)) analysis confirms that the deposited layers are interconnected with each other. The image shows that the deposited layers are uniform with void free nature due to densely packed CIS nanostructure. The FE-SEM image of CIS-2 ink was observed that the close packed structure of CIS layers with void free as compared with CIS-1 and also almost same denser of nanostructured thin film with less agglomeration J. Nanosci. Nanotechnol. 13, 1–8, 2013

Duraisamy et al.

Fig. 3.


X-ray diffraction pattern of deposited CIS thin films.


Fig. 4. (a) Thin film thickness measurement of deposited CIS layers, (b) Different film thickness with respect to concentration of CIS.

coefficient of CuInSe2 influences the film transmittance with respect to film thickness. The deposited CIS layers exhibit two different optical band gaps, which is calculated by using well-known Tauc model relation32 as given below AEg − hn (1) h where is the absorption coefficient, A is a constant, Eg is the energy band gap, h is the Plank’s constant, is frequency of light and n is depends on the transition nature (n = 1/2, for direct transition). The energy band gap is calculated from a (h 2 ) versus energy (eV). The liner plot to the energy axis was used to calculate band gap energy of CIS. Figure 8(b) reveals that the calculated energy band gap values are in the range of 1.34–1.29 eV, which corresponds to CIS phase (CIS-1, CIS-2, CIS-3) in NIR region and the energy band gap in the UV-visible region (Fig. 8(c)) is in the range of 2.67–2.49 eV. The CIS-2 thin films reveal the energy band gap such as 2.6 eV and 1.3 eV corresponds to UV-vis and NIR region, which is a better optical band gap due to influence of crystalline size effect, film thickness and film uniformity 33 34 as compared with other two CIS layers. More over the deposited CIS thin films through EHDA process are more suitable for electronic device application. =

5

RESEARCH ARTICLE

as compared with CIS-3 thin film. In CIS-3 thin films show some agglomeration, which is due to influence of nanoparticles concentrations, substrate moving speed during atomization process. The FE-SEM analysis confirms that the 12.5 wt.% of CIS-2 ink shows good surface morphology with well and closely packed nanostructured thin film. The X-ray photoelectron spectrum (XPS-ESCA 2000 VG Microtech) is used to analyze the surface composition of deposited CIS thin films. Figure 6 is showing the chemical bonding and purity of deposited CIS layers. The survey spectrum (Fig. 6) of deposited CIS-2 layers confirm the presence of copper (Cu 2p), Indium (In 3d) and selenide (Se 2p). Figure 7 reveals the deconvoluted spectrum of In3d, Se3d and Cu2p. In Figure 7(a), the In3d exhibits the binding energies 444.5 eV and 451.7 eV corresponding to In3d5/2 and In3d3/2 respectively.29 The spectrum of Se3d revealed the spectrum of Se3d5/2 with corresponding binding energy 53.9 eV 30 as shown in Figure 7(b). The Cu2p exhibits Cu2p3/2 and Cu2p1/2 with energy peaks observed at 932.6 eV and 952.2 eV (Fig. 7(c)).31 However, the survey spectrum of CIS-1 and CIS-3 are shown in the inset Figure 6. The optical properties of deposited thin films are analyzed by UV/Vis/NIR spectroscopy (Shimadzu UV-3150). Figure 8(a) shows transmittance spectra of CIS thin films annealed at 200 C. The CIS thin films exhibit an absorption in the range of 300–700 nm (UV-visible range), which is corresponding to Cu–Se phase. The absorption


Duraisamy et al.

RESEARCH ARTICLE

Fig. 5. Surface morphology of CIS thin films: Low magnification FESEM image of CIS layers (a), (c), (e), high magnification of deposited CIS layers (b), (d), (f) reveals the grain shape and nature of layer surface.

The current–voltage (I–V ) measurement (B1500A, Agilent, USA) of deposited CIS thin films with various film thicknesses are shown in Figure 9(a). The aluminum electrode is made by E-beam evaporation for contact purpose on CIS thin films. The I–V analysis reveals the ohmic behavior of deposited CIS thin films with metal electrodes. The electrical study confirms that the deposited CIS layers

Fig. 7. Deconvoluted X-ray photoelectron spectroscopy of CIS-2 thin films, (a) Indium (In3d), (b) Selenide (Se3d) and (c) Copper (Cu2p).

Fig. 6. X-ray photoelectron spectroscopy of deposition of CIS thin films (survey spectrum of CIS-2 thin film), the inset shows the survey spectrum of CIS-1 and CIS-3 films.

6

are having proper matching work functions with electrode as shown in semi-log scale (Fig. 9(b)). The current density is increased with an increasing film thickness. However, the sheet resistivity of deposited CIS thin films are calculated by the following resistivity formula21 t V = (2) ln 2 I J. Nanosci. Nanotechnol. 13, 1–8, 2013

Duraisamy et al.


Fig. 8. (a) UV/VIS/NIR spectrum of CIS thin films showing transmittance, (b)–(c) Tauc model curves of deposited CIS layers.

where, t is the film thickness, V is the voltage and I is current. Current and voltage derived from I–V plot. The sheet resistivity of CIS-1, CIS-2 and CIS-3 correspond to 76 cm, 40 cm and 33 cm respectively. The electrical study reveals that the deposited CIS-2 and CIS-3 films are minimum sheet resistivity as compared with CIS-1 films. However, the sheet resistivity has been decreased with respect to increase the film thickness. These films have good charge transfer mechanisms with J. Nanosci. Nanotechnol. 13, 1–8, 2013

minimum charge trapping due to lower sheet resistivity of deposited CIS thin films. The CIS-2 is the more appropriate for EHDA process, which is due to longer stability of ink as compared with CIS-3 and better nanoparticle concentrations with good physical properties than that of CIS-1. The CIS-2 inks has wide range of operating envelop (applied voltage range) due to good physical properties of ink. The FE-SEM reveals the less agglomerated uniform film thickness with void free morphology and good optical band gap in UV-vis and NIR region. The electrical study confirms the lower sheet resistivity of CIS thin films achieved through EHDA process. In conclusion, the CIS nanoparticle ink with different concentration was deposited through simple and environmental friendly electrohydrodynamic atomization technique. The uniform thin films were achieved through Taylor cone mode. The XRD analysis confirmed that the CIS thin films have chalcopyrite crystalline structure. The surface morphology of CIS thin films revealed homogeneous deposition with void free morphology. The film thickness of CIS layers were varied due to the influence of 7

RESEARCH ARTICLE

Fig. 9. (a) Current–voltage analysis of deposited CIS thin films through EHDA technique, (b) Semi-log scale of CIS layers with different film thickness.


nanoparticle concentrations. The XPS analysis confirmed that the binding energy of chemical composition of CIS layers and purity of thin films. The optical properties of CIS layers exhibited good absorption coefficient in the range of UV-visible and NIR. It suggests that the CIS layers have two optical energy band gaps, which was found to be 1.34–1.29 eV (NIR) and 2.67–2.49 eV (UV-visible). The band gap of CIS layers is mainly due to the influence of crystalline size effect and film thickness. The electrical study of CIS-2 confirmed that the resistivity of 40 cm. The CIS-2 layer exhibited uniform dispersion of CIS nanoparticle for long period with homogeneous thin film, void free morphology and also good electrical, optical properties. The optimization and deposition of CIS layers through EHDA technique will provide new insights into electronic applications.

RESEARCH ARTICLE

Acknowledgment: The study is supported by Ministry of knowledge economy, Korea through project “Strategy Technology Development Project” and a grant (B55117908-03-00) from the cooperative R&D Program funded by the Korea Research Council Industrial Science and Technology, Republic of Korea. This study was supported by a grant from the cooperative R&D Program (B551179-1001-00) funded by the Korea Research Council Industrial Science and Technology, Republic of Korea.

References and Notes 1. T. Chasse, C.-I. Wu, I. G. Hill, and A. Kanh, J. Appl. Phys. 85, 6589 (1999). 2. X. Fang, Y. Bando, U. K. Gautam, C. Ye, and D. Golberg, J. Mater. Chem. 18, 509 (2008). 3. T. Saga, NPG Asia Mater. 2, 96 (2010). 4. K. Okamoto, A. Scherer, and Y. Kawakami, Phys. Stat. Sol. C 5, 2822 (2008). 5. Y. Qin, X. Wang, and Z. L. Wang, Nature 451, 809 (2008). 6. H. Li, Z. Yin, Q. He, H. Li, X. Huang, G. Lu, D. W. H. Fam, A. I. Y. Tok, Q. Zhang, and H. Zhang, Small 8, 63 (2012). 7. A. S. Zoolfakar, R. A. Rani, A. J. Morfa, S. Balendhran, A. P. O’Mullane, S. Zhuiykov, and K. Kalantar-Zadeh, J. Mater. Chem. 22, 21767 (2012). 8. M. Koubaaa, A. M. Haghiri-Gosnet, R. Desfeux, Ph. Lecoeur, W. Prellier, and B. Mercey, J. Appl. Phys. 93, 5227 (2003).

Duraisamy et al.

9. M. Dhanam, B. Kavitha, B. Maheswari, and G.R. Jesna, Acta Physica. Polonica. A 119, 885 (2011). 10. A. Ramadoss, K. Krishnamoorthy, and S. J. Kim, Appl. Phys. Exp. 5, 085803 (2012). 11. A. Gupta and S. Isomura, Sol. Energ. Mat. Sol. C 53, 385401 (1998). 12. R. H. Bari, L. A. Patil, and P. P. Patil, Bull. Mater. Sci. 29, 529 (2006). 13. K. Ariga, Q. Ji, J. P. Hill, Y. Bando, and M. Aono, NPG Asia Mater. 4, e17 (2012) 14. K. Sakakibara, J. P. Hill, and K. Ariga, Small 7, 1288 (2011). 15. L. S. C. Pingree, B. A. MacLeod, and D. S. Ginger, J. Phys. Chem. C 112, 7922 (2008). 16. A. M. Ganan-Calvo, J. Davila, and A. Barrero, J. Aerosol Sci. 28, 249 (1997). 17. I. B. Rietveld, N. Suganuma, K. Kobayashi, H. Yamada, and K. Matsushige, Macromol. Mater. Eng. 293, 387 (2008). 18. K. S. Kim and D. J. Kim, Aerosol. Sci. 37, 1532 (2006). 19. N. Duraisamy, S. J. Hong, and K. H. Choi, Che. Eng. J doi: 10.1016/j.cej.2013.04.007. 20. N. Duraisamy, N. M. Muhammad, H.-C. Kim, J. Jo, and K. H. Choi, Thin Solid Films 520, 5070 (2012). 21. N. M. Muhammad, A. M. Naeem, N. Duraisamy, D.-S. Kim, and K. H. Choi, Thin Solid Films 520, 1751 (2012). 22. N. Duraisamy, N. M. Muhammad, A. Ali, J. Jo, and K. H. Choi, Mater. Lett. 83, 80 (2012). 23. A. Fischereder, T. Rath, W. Haas, H. Amenitsch, D. Schenk, A. Zankel, R. Saf, F. Hofer, and G. Trimmel, ACS. Appl. Mater. Interfaces 4, 382 (2012). 24. K. Bindu, C. SudhaKartha, K. P. Vijayakumar, T. Abe, and Y. Kashiwaba, Sol. Energy. Mater. Sol. Cells 79, 67 (2003). 25. S. J. Ahn, K. Kim, A. Cho, J. Gwak, J. H. Yun, K. Shin, S. K. Ahn, and K. Yoon, ACS Appl. Mater. Interfaces 4, 1530 (2012). 26. K. H. Choi, N. Duraisamy, N. M. Muhammad, I. Kim, H. Choi, and J. Jo, Appl. Phys. A. 107, 715 (2012). 27. N. M. Muhammad, N. Duraisamy, H.-W. Dang, J. Jo, and K.-H. Choi, Thin Solid Films 520, 6398 (2012) 28. G. Venugopal, K. Krishnamoorthy, R. Mohan, and S.-J. Kim, Mater. Chem. Phys. 132, 29 (2012). 29. M. S. Park, S. Y. Han, E. J. Bae, T. J. Lee, C. H. Chang, and S. O. Ryu, Curr. Appl. Phys. 10, S379 (2010). 30. K. S. Ramaiah and V. S. Raja, Scripta. Mater. 44, 771 (2001). 31. Y. Shi, Z. Jin, C. Li, H. An, and J. Qiu, App. Surf. Sci. 252, 3737 (2006). 32. K. Krishnamoorthy, J. Y. Moon, H. B. Hyun, S. K. Cho, and S.-J. Kim, J. Mater. Chem. 22, 24610 (2012). 33. K. Mogyorosi, I. Dekany, and J. H. Fendler, Langmuir 19, 2938 (2003). 34. G. Lei, X. Mingxia, F. Haibo, and S. Ming, Appl. Surf. Sci. 253, 720 (2006).

Received: 17 March 2013. Accepted: 26 April 2013.

8
