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Low-temperature growth of ZnO nanostructures by oxygen plasma oxidation of ZnCl2
Materials Chemistry and Physics 129 (2011) 693–695
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Low-temperature growth of ZnO nanostructures by oxygen plasma oxidation of ZnCl2 Rong Yang, Jie Zheng, Wei Li, Jianglan Qu, Xuanzhou Zhang, Xingguo Li ∗ Beijing National Laboratory for Molecular Sciences (BNLMS), The State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China
a r t i c l e
i n f o
Article history: Received 7 October 2010 Received in revised form 12 May 2011 Accepted 16 May 2011 Keywords: ZnO Thin film Nanostructure Plasma Chemical vapor deposition
a b s t r a c t The thermodynamically forbidden reaction between ZnCl2 and O2 was able to take place by using oxygen plasma, yielding cone-shaped ZnO nanostructure. In situ optical emission spectroscopy was used to identify the excited species during the plasma enhanced reaction. The determination of excited temperature suggested that the addition of O2 had great contribution to the enhanced dissociation of ZnCl2 . The successful synthesis of ZnO indicates that the chlorides may replace the organometallics as a new precursor in thin film preparation industry. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Binary semiconducting oxides have been extensively investigated, due to their potential applications as chemical and biological nanosensors, light-emitting diodes and electronic devices [1,2]. As one of the most important II–IV semiconductors, zinc oxide which is an exceeding wide band gap (3.37 eV) oxide material has attracted a great deal of interest during the past decade [3,4]. Considerable efforts have been made to obtain ZnO thin film nanostructures from various techniques, including thermal evaporation [5], chemical vapor deposition [6], reactive magnetic sputtering [7], spray pyrolysis [8], sol–gel techniques [9] and electrochemical deposition [10]. Among these techniques, chemical vapor deposition method is demonstrated to be an effective technique, owning to its inexpensiveness, easy handling, high efficiency and predominant getting high crystallized product. Most of the chemical vapor deposition methods operate at a high temperature, which is a great disadvantage from the manufacturing point of view. To overcome this problem, metal organic chemical vapor deposition (MOCVD) technique is established to deposit metal oxide semiconducting thin films [11]. However, the use of expensive precursors is a big disadvantage of MOCVD technique. Furthermore, MOCVD causes a problem with environmental pollution and safety issue, because the MOCVD vapor is always poisonous and explosive. It is highly desirable to discover a
∗ Corresponding author. Tel.: +86 10 62765930; fax: +86 10 62765930. E-mail address: [email protected] (X. Li). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.05.027
new precursor to replace the organometallics for low temperature chemical vapor deposition of oxide thin films. In this study, we used a plasma enhanced chemical vapor deposition (PECVD) approach to prepare ZnO nanostructures by the oxidation of ZnCl2 at a low temperature. The reaction between ZnCl2 and O2 is thermodynamically forbidden. By using the radio frequency plasma, ZnO nanostructures could be obtained at 350 ◦ C. The optical emission spectra (OES) results demonstrated that the chloride precursor was excited by Ar metastables in gas phase. The addition of O2 had an enhancement on the dissociation of ZnCl2 . The low-temperature synthesis method demonstrates the significance of plasma on thermodynamics of reactions. The plasma enhanced reaction presented in this paper may provide a new method to substitute MOCVD for the preparation of metal oxides at significant low temperature. 2. Experimental The experimental setup is illustrated in Fig. 1a, which consists of a tube furnace, a horizontal quartz tube (100 cm in length and 2.8 cm in diameter) and an inductively coupled copper coil (10 cm in length) surrounding the tube. The coil was driven by a 500 W radio frequency power supply with a frequency of 13.56 MHz. 0.3 g zinc dichloride anhydrous (ZnCl2 , Beijing Chemical Reagents Company, >99%) was loaded in a small ceramic boat at the centre of the furnace. Si (1 0 0) wafers were used as substrates and placed at 2 cm downstream of the ZnCl2 powder. The whole quartz tube was evacuated to 1.0 Pa by a rotary pump and flushed with Ar (99.99%) several times to remove moisture. The ZnCl2 powder was heated under a mixture gas flow of Ar and O2 (99.99%). In a typical procedure, the flow rates of Ar and O2 were 50 sccm (standard cubic centimeter per minute) and 5 sccm, respectively. The ZnCl2 evaporation temperature was controlled at 350 ◦ C, and the RF power was maintained at 100 W. The deposition process was carried out for 15 min. After deposition, the RF power was switched
694
R. Yang et al. / Materials Chemistry and Physics 129 (2011) 693–695 off and the O2 flow was cut off. The system was cooled down to room temperature under 50 sccm Ar. Deposition was found on the upper side of the Si substrate. The crystal structure of the product was characterized by X-ray powder diffraction (XRD) using an automated Rigaku X-ray diffractometer with monochromatic Cu K␣ radiation. The morphology of the as prepared samples was investigated by a Hitachi S-4800 field-emission scanning electron microscope (FESEM) at an accelerating voltage of 10 kV. Transmission electron microscopy (TEM) images were captured on the Tecani F30 instrument at an accelerating voltage of 300 kV. In situ OES were collected using a fiber spectrometer (Avantes 2048) at 1 cm upstream of the copper coil.
3. Results and discussion
Fig. 1. (a) Schematic illustration of the PECVD setup and (b) XRD patterns of the ZnO product.
The typical XRD pattern of the obtained ZnO product is shown in Fig. 1b, suggesting the formation of zinc oxide. All the diffraction peaks of the XRD pattern are in accordance with the hexagonal ZnO structure. Compared with the standard pattern (JCPDS 36-1451), the strong relative intensity of the (0 0 0 2) suggests the preferential growth of ZnO along the c-axis direction, which is subsequently proved by the TEM observations. Neither cubic ZnO phase nor ZnCl2 was found in the prepared product. Without plasma, ZnCl2 could not react with O2 in vapor phase at this temperature. All the precursors condensed at low temperature region in the quartz tube. The morphology of the product was investigated by FESEM and TEM, as shown in Fig. 2. It is observed that the ZnO products have uniform cone-sharp morphology and grow vertically on the substrate (Fig. 2a and b). Fig. 2c shows the TEM images of a single ZnO cone. In the high-resolution TEM image (Fig. 2d) of a single ˚ nanocone, the lattice fringes have an inter-planar spacing of 2.6 A, which agrees with the d value of the (0 0 0 2) plane of the hexagonal ZnO, indicating that each nanocone is a single crystal. The HRTEM result also gives further evidence that the ZnO products grow alone the c-axis direction.
Fig. 2. Electron microscopy images of the ZnO nanostructures: SEM images of (a) top view and (b) 45◦ side view of the product and, (c) TEM image and (d) high-resolution TEM image of a single cone.
R. Yang et al. / Materials Chemistry and Physics 129 (2011) 693–695
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The influence of O2 addition was studied by changing the O2 flow in the reaction system. The intensity of Zn atomic lines decreased when more O2 was introduced (Fig. 3b), indicating the chloride precursor reacts with O2 efficiently. Moreover, the introduction of O2 also had a significant contribution to the precursor excitation. The excitation temperature (Tex ) can be estimated by comparing the relative intensity of spectral lines from the same elemental and ionization stage using the following equation [17].
I1 A1 g1 2 E1 − E2 = exp − I2 A2 g2 1 kB Tex
where I is the intensity of the spectra line, E is the energy of the energy level, g is the statistic weight (degeneracy degree) of the energy level, A is the spontaneous transition probability, is the wavelength of the spectra line, kB is Boltzmann constant, and subscript 1 and 2 represent two different transition. In our experiment, the spectra line 307.6 and 636.2 nm of Zn neutral atom were chosen to determine the excitation temperature. It is observed that the excitation temperature increases with the increase of O2 flow as shown in Fig. 3b. The excitation temperature represents the potential of produce excited state. Therefore, the results of the excitation temperature measurements indicate that the increase of O2 flow has a great contribution to the excitation of the chloride precursor. 4. Conclusions
Fig. 3. (a) OES spectra of Ar–ZnCl2 and Ar–ZnCl2 –O2 system and inset for the magnified spectra between 760 and 865 nm. (b) Variation of Zn atomic line (308 nm) intensity and excitation temperature with the O2 flow in the PECVD system.
It is apparent that the following reaction took place with the assistance of plasma. ZnCl2 (s) + O2 (g) → ZnO(s) + Cl2 (g) The above reaction cannot perform without plasma, the reaction is thermodynamically forbidden because the Gibbs free energy change is a positive value (48.93 kJ mol−1 , the free energy of formation f G◦ at 298 K of ZnCl2 and ZnO is −369.45 kJ mol−1 and −320.52 kJ mol−1 , respectively) [12]. The ZnCl2 precursor can only evaporate and condense at low temperature region without the introduction of plasma. The conventional non-plasma approaches have to heat up to 700 ◦ C to get the products. When the plasma was introduced, the excited states created by plasma have to be taken into account. OES is an effective technique to identify the excited species during the plasma enhanced reaction. Fig. 3a shows the OES spectra of Ar–ZnCl2 and Ar–ZnCl2 –O2 system. The peaks originated from Zn atoms (303.9, 325.6, 410.1 and 451.1 nm) [13,14] were observed in Ar–ZnCl2 system, while the peak intensity of Ar 4p lines (700–900 nm) had decreased [15], indicating the energy transfer from Ar* metastables to ZnCl2 . The energy transfer process is responsible for the dissociation ZnCl2 during the plasma enhanced chemical reaction. When O2 was introduced into the reaction system, the dissociated precursor reacts with O2 , leading to the formation of ZnO product. The inset shows the magnified spectra between 760 and 865 nm. The peaks of Cl atoms (774.5, 821.2, 837.5 and 858.5 nm) [16] which were ascribed to atomic Cl appeared in Ar–ZnCl2 –O2 system. This result indicates that O2 has a great promotion for the decomposition of the chloride precursor during the PECVD system.
The PECVD technique was used to obtain ZnO nanostructures at low temperature. Due to the species in excited electronic states created by plasma, the reaction which was thermodynamically forbidden in conventional condition took place. The energy transfer from Ar* metastables to ZnCl2 precursor is significant to the change in the thermodynamics. When more O2 was introduced in the system, the excitation temperature increased, indicating the enhancement of the precursor excitation. The successful synthesis of ZnO by oxidation of ZnCl2 suggests the O2 plasma enhanced reaction presented in this paper may provide a new method to substitute MOCVD in thin film preparation industry. Acknowledgements This work was supported by NSFC (nos. 20971009 and 20821091), MOST of China (no. 2009CB939902 and 2010CB631301). References [1] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, Y.Q. Yan, Adv. Mater. 15 (2003) 353–389. [2] G. Korotcenkov, Sens. Actuators B-Chem. 107 (2005) 209–232. [3] M.H. Huang, Y.Y. Wu, H. Feick, N. Tran, E. Weber, P.D. Yang, Adv. Mater. 13 (2001) 113–116. [4] L. Vayssieres, Adv. Mater. 15 (2003) 464–466. [5] P.X. Gao, Z.L. Wang, J. Phys. Chem. B 106 (2002) 12653–12658. [6] P.C. Chang, Z.Y. Fan, D.W. Wang, W.Y. Tseng, W.A. Chiou, J. Hong, J.G. Lu, Chem. Mater. 16 (2004) 5133–5137. [7] S. Singh, R. Kumar, T. Ganguli, R.S. Srinivasa, S.S. Major, J. Cryst. Growth 310 (2008) 4640–4646. [8] A. Bashir, P.H. Wobkenberg, J. Smith, J.M. Ball, G. Adamopoulos, D.D.C. Bradley, T.D. Anthopoulos, Adv. Mater. 21 (2009) 2226–2231. [9] Y. Masuda, K. Kato, Cryst. Growth Des. 9 (2009) 3083–3088. [10] A. Ashida, A. Fujita, Y.G. Shim, K. Wakita, A. Nakahira, Thin Solid Films 517 (2008) 1461–1464. [11] A.N. Gleizes, Chem. Vap. Deposition 6 (2000) 155–173. [12] J.G. Speight (Ed.), Lange’s Handbook of Chemistry, Mc Graw-Hill, London, 2005. [13] J.R. Fuhr, W.L. Wiese, in: D.R. Lide (Ed.), NIST Atomic Transition Probability Tables, CRC Handbook of Chemistry & Physics, 77th ed., CRC Press, Inc., Boca Raton, FL, 1996. [14] D.C. Morton, Astrophys. J. Suppl. Ser. 149 (2003) 205–238. [15] A. Bogaerts, R. Gijbels, J. Vlcek, Spectrochim. Acta, Part B 53 (1998) 1517–1526. [16] J.L.J. Radziemski, V. Kaufman, J. Opt. Soc. Am. 64 (1974) 366–389. [17] F. Iza, J. Hopwood, Plasma Sources Sci. Technol. 11 (2002) 229–235.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Low-temperature growth of ZnO nanostructures by oxygen plasma oxidation of ZnCl2 Rong Yang, Jie Zheng, Wei Li, Jianglan Qu, Xuanzhou Zhang, Xingguo Li ∗ Beijing National Laboratory for Molecular Sciences (BNLMS), The State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China
a r t i c l e
i n f o
Article history: Received 7 October 2010 Received in revised form 12 May 2011 Accepted 16 May 2011 Keywords: ZnO Thin film Nanostructure Plasma Chemical vapor deposition
a b s t r a c t The thermodynamically forbidden reaction between ZnCl2 and O2 was able to take place by using oxygen plasma, yielding cone-shaped ZnO nanostructure. In situ optical emission spectroscopy was used to identify the excited species during the plasma enhanced reaction. The determination of excited temperature suggested that the addition of O2 had great contribution to the enhanced dissociation of ZnCl2 . The successful synthesis of ZnO indicates that the chlorides may replace the organometallics as a new precursor in thin film preparation industry. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Binary semiconducting oxides have been extensively investigated, due to their potential applications as chemical and biological nanosensors, light-emitting diodes and electronic devices [1,2]. As one of the most important II–IV semiconductors, zinc oxide which is an exceeding wide band gap (3.37 eV) oxide material has attracted a great deal of interest during the past decade [3,4]. Considerable efforts have been made to obtain ZnO thin film nanostructures from various techniques, including thermal evaporation [5], chemical vapor deposition [6], reactive magnetic sputtering [7], spray pyrolysis [8], sol–gel techniques [9] and electrochemical deposition [10]. Among these techniques, chemical vapor deposition method is demonstrated to be an effective technique, owning to its inexpensiveness, easy handling, high efficiency and predominant getting high crystallized product. Most of the chemical vapor deposition methods operate at a high temperature, which is a great disadvantage from the manufacturing point of view. To overcome this problem, metal organic chemical vapor deposition (MOCVD) technique is established to deposit metal oxide semiconducting thin films [11]. However, the use of expensive precursors is a big disadvantage of MOCVD technique. Furthermore, MOCVD causes a problem with environmental pollution and safety issue, because the MOCVD vapor is always poisonous and explosive. It is highly desirable to discover a
∗ Corresponding author. Tel.: +86 10 62765930; fax: +86 10 62765930. E-mail address: [email protected] (X. Li). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.05.027
new precursor to replace the organometallics for low temperature chemical vapor deposition of oxide thin films. In this study, we used a plasma enhanced chemical vapor deposition (PECVD) approach to prepare ZnO nanostructures by the oxidation of ZnCl2 at a low temperature. The reaction between ZnCl2 and O2 is thermodynamically forbidden. By using the radio frequency plasma, ZnO nanostructures could be obtained at 350 ◦ C. The optical emission spectra (OES) results demonstrated that the chloride precursor was excited by Ar metastables in gas phase. The addition of O2 had an enhancement on the dissociation of ZnCl2 . The low-temperature synthesis method demonstrates the significance of plasma on thermodynamics of reactions. The plasma enhanced reaction presented in this paper may provide a new method to substitute MOCVD for the preparation of metal oxides at significant low temperature. 2. Experimental The experimental setup is illustrated in Fig. 1a, which consists of a tube furnace, a horizontal quartz tube (100 cm in length and 2.8 cm in diameter) and an inductively coupled copper coil (10 cm in length) surrounding the tube. The coil was driven by a 500 W radio frequency power supply with a frequency of 13.56 MHz. 0.3 g zinc dichloride anhydrous (ZnCl2 , Beijing Chemical Reagents Company, >99%) was loaded in a small ceramic boat at the centre of the furnace. Si (1 0 0) wafers were used as substrates and placed at 2 cm downstream of the ZnCl2 powder. The whole quartz tube was evacuated to 1.0 Pa by a rotary pump and flushed with Ar (99.99%) several times to remove moisture. The ZnCl2 powder was heated under a mixture gas flow of Ar and O2 (99.99%). In a typical procedure, the flow rates of Ar and O2 were 50 sccm (standard cubic centimeter per minute) and 5 sccm, respectively. The ZnCl2 evaporation temperature was controlled at 350 ◦ C, and the RF power was maintained at 100 W. The deposition process was carried out for 15 min. After deposition, the RF power was switched
694
R. Yang et al. / Materials Chemistry and Physics 129 (2011) 693–695 off and the O2 flow was cut off. The system was cooled down to room temperature under 50 sccm Ar. Deposition was found on the upper side of the Si substrate. The crystal structure of the product was characterized by X-ray powder diffraction (XRD) using an automated Rigaku X-ray diffractometer with monochromatic Cu K␣ radiation. The morphology of the as prepared samples was investigated by a Hitachi S-4800 field-emission scanning electron microscope (FESEM) at an accelerating voltage of 10 kV. Transmission electron microscopy (TEM) images were captured on the Tecani F30 instrument at an accelerating voltage of 300 kV. In situ OES were collected using a fiber spectrometer (Avantes 2048) at 1 cm upstream of the copper coil.
3. Results and discussion
Fig. 1. (a) Schematic illustration of the PECVD setup and (b) XRD patterns of the ZnO product.
The typical XRD pattern of the obtained ZnO product is shown in Fig. 1b, suggesting the formation of zinc oxide. All the diffraction peaks of the XRD pattern are in accordance with the hexagonal ZnO structure. Compared with the standard pattern (JCPDS 36-1451), the strong relative intensity of the (0 0 0 2) suggests the preferential growth of ZnO along the c-axis direction, which is subsequently proved by the TEM observations. Neither cubic ZnO phase nor ZnCl2 was found in the prepared product. Without plasma, ZnCl2 could not react with O2 in vapor phase at this temperature. All the precursors condensed at low temperature region in the quartz tube. The morphology of the product was investigated by FESEM and TEM, as shown in Fig. 2. It is observed that the ZnO products have uniform cone-sharp morphology and grow vertically on the substrate (Fig. 2a and b). Fig. 2c shows the TEM images of a single ZnO cone. In the high-resolution TEM image (Fig. 2d) of a single ˚ nanocone, the lattice fringes have an inter-planar spacing of 2.6 A, which agrees with the d value of the (0 0 0 2) plane of the hexagonal ZnO, indicating that each nanocone is a single crystal. The HRTEM result also gives further evidence that the ZnO products grow alone the c-axis direction.
Fig. 2. Electron microscopy images of the ZnO nanostructures: SEM images of (a) top view and (b) 45◦ side view of the product and, (c) TEM image and (d) high-resolution TEM image of a single cone.
R. Yang et al. / Materials Chemistry and Physics 129 (2011) 693–695
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The influence of O2 addition was studied by changing the O2 flow in the reaction system. The intensity of Zn atomic lines decreased when more O2 was introduced (Fig. 3b), indicating the chloride precursor reacts with O2 efficiently. Moreover, the introduction of O2 also had a significant contribution to the precursor excitation. The excitation temperature (Tex ) can be estimated by comparing the relative intensity of spectral lines from the same elemental and ionization stage using the following equation [17].
I1 A1 g1 2 E1 − E2 = exp − I2 A2 g2 1 kB Tex
where I is the intensity of the spectra line, E is the energy of the energy level, g is the statistic weight (degeneracy degree) of the energy level, A is the spontaneous transition probability, is the wavelength of the spectra line, kB is Boltzmann constant, and subscript 1 and 2 represent two different transition. In our experiment, the spectra line 307.6 and 636.2 nm of Zn neutral atom were chosen to determine the excitation temperature. It is observed that the excitation temperature increases with the increase of O2 flow as shown in Fig. 3b. The excitation temperature represents the potential of produce excited state. Therefore, the results of the excitation temperature measurements indicate that the increase of O2 flow has a great contribution to the excitation of the chloride precursor. 4. Conclusions
Fig. 3. (a) OES spectra of Ar–ZnCl2 and Ar–ZnCl2 –O2 system and inset for the magnified spectra between 760 and 865 nm. (b) Variation of Zn atomic line (308 nm) intensity and excitation temperature with the O2 flow in the PECVD system.
It is apparent that the following reaction took place with the assistance of plasma. ZnCl2 (s) + O2 (g) → ZnO(s) + Cl2 (g) The above reaction cannot perform without plasma, the reaction is thermodynamically forbidden because the Gibbs free energy change is a positive value (48.93 kJ mol−1 , the free energy of formation f G◦ at 298 K of ZnCl2 and ZnO is −369.45 kJ mol−1 and −320.52 kJ mol−1 , respectively) [12]. The ZnCl2 precursor can only evaporate and condense at low temperature region without the introduction of plasma. The conventional non-plasma approaches have to heat up to 700 ◦ C to get the products. When the plasma was introduced, the excited states created by plasma have to be taken into account. OES is an effective technique to identify the excited species during the plasma enhanced reaction. Fig. 3a shows the OES spectra of Ar–ZnCl2 and Ar–ZnCl2 –O2 system. The peaks originated from Zn atoms (303.9, 325.6, 410.1 and 451.1 nm) [13,14] were observed in Ar–ZnCl2 system, while the peak intensity of Ar 4p lines (700–900 nm) had decreased [15], indicating the energy transfer from Ar* metastables to ZnCl2 . The energy transfer process is responsible for the dissociation ZnCl2 during the plasma enhanced chemical reaction. When O2 was introduced into the reaction system, the dissociated precursor reacts with O2 , leading to the formation of ZnO product. The inset shows the magnified spectra between 760 and 865 nm. The peaks of Cl atoms (774.5, 821.2, 837.5 and 858.5 nm) [16] which were ascribed to atomic Cl appeared in Ar–ZnCl2 –O2 system. This result indicates that O2 has a great promotion for the decomposition of the chloride precursor during the PECVD system.
The PECVD technique was used to obtain ZnO nanostructures at low temperature. Due to the species in excited electronic states created by plasma, the reaction which was thermodynamically forbidden in conventional condition took place. The energy transfer from Ar* metastables to ZnCl2 precursor is significant to the change in the thermodynamics. When more O2 was introduced in the system, the excitation temperature increased, indicating the enhancement of the precursor excitation. The successful synthesis of ZnO by oxidation of ZnCl2 suggests the O2 plasma enhanced reaction presented in this paper may provide a new method to substitute MOCVD in thin film preparation industry. Acknowledgements This work was supported by NSFC (nos. 20971009 and 20821091), MOST of China (no. 2009CB939902 and 2010CB631301). References [1] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, Y.Q. Yan, Adv. Mater. 15 (2003) 353–389. [2] G. Korotcenkov, Sens. Actuators B-Chem. 107 (2005) 209–232. [3] M.H. Huang, Y.Y. Wu, H. Feick, N. Tran, E. Weber, P.D. Yang, Adv. Mater. 13 (2001) 113–116. [4] L. Vayssieres, Adv. Mater. 15 (2003) 464–466. [5] P.X. Gao, Z.L. Wang, J. Phys. Chem. B 106 (2002) 12653–12658. [6] P.C. Chang, Z.Y. Fan, D.W. Wang, W.Y. Tseng, W.A. Chiou, J. Hong, J.G. Lu, Chem. Mater. 16 (2004) 5133–5137. [7] S. Singh, R. Kumar, T. Ganguli, R.S. Srinivasa, S.S. Major, J. Cryst. Growth 310 (2008) 4640–4646. [8] A. Bashir, P.H. Wobkenberg, J. Smith, J.M. Ball, G. Adamopoulos, D.D.C. Bradley, T.D. Anthopoulos, Adv. Mater. 21 (2009) 2226–2231. [9] Y. Masuda, K. Kato, Cryst. Growth Des. 9 (2009) 3083–3088. [10] A. Ashida, A. Fujita, Y.G. Shim, K. Wakita, A. Nakahira, Thin Solid Films 517 (2008) 1461–1464. [11] A.N. Gleizes, Chem. Vap. Deposition 6 (2000) 155–173. [12] J.G. Speight (Ed.), Lange’s Handbook of Chemistry, Mc Graw-Hill, London, 2005. [13] J.R. Fuhr, W.L. Wiese, in: D.R. Lide (Ed.), NIST Atomic Transition Probability Tables, CRC Handbook of Chemistry & Physics, 77th ed., CRC Press, Inc., Boca Raton, FL, 1996. [14] D.C. Morton, Astrophys. J. Suppl. Ser. 149 (2003) 205–238. [15] A. Bogaerts, R. Gijbels, J. Vlcek, Spectrochim. Acta, Part B 53 (1998) 1517–1526. [16] J.L.J. Radziemski, V. Kaufman, J. Opt. Soc. Am. 64 (1974) 366–389. [17] F. Iza, J. Hopwood, Plasma Sources Sci. Technol. 11 (2002) 229–235.