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Fast vertical growth of ZnO nanorods using a modified chemical bath deposition
Journal of Alloys and Compounds 597 (2014) 85–90
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Fast vertical growth of ZnO nanorods using a modified chemical bath deposition Tae-hyun Lee a, Hyukhyun Ryu a,⇑, Won-Jae Lee b a b
Department of Nano Systems Engineering, Center for Nano Manufacturing, Inje University, Obang-dong, Gimhae, Gyeongnam 621-749, Republic of Korea Department of Materials and Components Engineering, Dong-Eui University, 995 Eomgwangno, Busanjin-gu, Busan 614-714, Republic of Korea
a r t i c l e
i n f o
Article history: Received 12 August 2013 Received in revised form 29 January 2014 Accepted 1 February 2014 Available online 7 February 2014 Keywords: ZnO nanorods Vertical growth Modified chemical bath deposition Growth rate
a b s t r a c t In this study, we grew vertical ZnO nanorods on seeded Si (1 0 0) substrates using a modified chemical bath deposition (CBD). We investigated the effects of the growth temperature, growth time and concentration on the morphological and structural properties of the ZnO nanorods using field emission gun scanning electron microscopy (FEG-SEM) and X-ray diffraction. This modified CBD method shows improved results over conventional CBD. ZnO nanorods with good structural XRD properties were grown with a very fast growth rate in a wide range of growth conditions and did not require post-growth annealing. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO) is a typical II–VI compound semiconductor. It has a wurtzite structure and a large exciton binding energy of 60 meV at room temperature [1]. Owing to its high optical transparency and conductivity, it has been used in several types of devices such as ultraviolet lasers, field-effect transistor (FET), high performance nanosensors, light-emitting diodes (LED), solar cell, and nano-piezotronics [2–4]. ZnO can also be grown with various structures, including nanorods, nanosheets, nanoflowers, nanowires, and nanorings [4]. The growth of ZnO nanostructures has been studied using various growth methods, including molecular beam epitaxy (MBE) [5], metal organic chemical vapor deposition (MOCVD) [6], Sputtering (SP) [7], vapor phase transport (VPT) [8], hydrothermal synthesis [9–11], chemical bath deposition (CBD) [12] and electrochemical deposition (ECD) [13]. Among these methods, CBD and hydrothermal synthesis have been widely studied because they can be used for large-area deposition with low growth temperatures. In addition, these methods are relatively simple and inexpensive [14]. However, CBD and hydrothermal synthesis are problematic for two reasons [15,16]. First, it is difficult to maintain the concentration of the solution in the bath during growth because the solution concentration decreases as the reaction progresses. Second, it is difficult to directly supply thermal energy to the substrate because ⇑ Corresponding author. Tel.: +82 55 320 3874; fax: +82 55 320 3631. E-mail address: [email protected] (H. Ryu). http://dx.doi.org/10.1016/j.jallcom.2014.02.003 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
the entire bath is heated, which can cause unwanted homogeneous reactions to occur in the solution, hindering growth on the substrate. To address these problems, we used a modified CBD (M-CBD) process which continuously delivers a constant concentration of solution to the substrate and reduces the likelihood of homogeneous reaction by heating the substrate directly. In this paper, we used this M-CBD process and investigated the effects of varying the growth temperature, growth time, and solution concentration on the growth of ZnO nanostructures on seeded Si (1 0 0) substrates. This M-CBD process produced vertically grown ZnO nanorods with a high growth rate and good structural XRD properties without the need for an additional annealing process. This result indicates that the M-CBD process can be favorable for flexible device fabrication. 2. Experiments details In this study, we grew vertical ZnO nanorods on seeded Si (1 0 0) substrates by using M-CBD. P-type Si (1 0 0) substrates were cleaned in piranha for 15 min at 140 °C and in 20% HF for 1 min at room temperature. After each cleaning step, substrates were sonicated in distilled water and dried with filtered air. A 50 nm-thick ZnO seed layer was deposited on the Si substrate by a radio frequency (RF) sputtering system. The sputtering power was 100 W under Ar gas at 3 m torr. A solution of zinc acetate dehydrate (Zn(O2CCH3)2(H2O)2) in ammonia and distilled water was prepared with a ratio of 10:1 for Zn(Ac):ammonia. Zinc acetate concentration was varied, as one of the experimental conditions, from 0.001 M to 0.1 M. The solution was filtered by 0.45 lm syringe filter. The substrate was placed on a hot plate which controls the reacting temperature and conveys thermal energy to substrate directly. Then a reacting Teflon chamber was put onto the substrate with O-ring, which prevents solution from leaking
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Fig. 1. The cross-section view SEM images: ZnO nanorods on seeded-Si (1 0 0) substrates with various growth temperatures: (a) 110 °C, (b) 130 °C, (c) 160 °C, (d) 180 °C, (e) 190 °C, and (f) 210 °C. outside the chamber. The prepared solution with constant concentration was delivered to the reacting chamber though silicon tube (OD = 3.7 mm, ID = 0.8 mm) using peristaltic pump (Masterflex L/S, EW-7523-90). For the solution delivery, the prepared solution was moved to the hot water bath which contained the silicon tube of 128 cm long with the temperature of 80 °C. The heated solution was fed into the reaction chamber with a constant injection rate. So, the constant concentration of solution in the reaction chamber was maintained throughout the whole process. For this experiment, the growth temperature, growth time and zinc acetate concentration were varied from 110 °C to 210 °C, from 1 min to 40 min, and from 0.001 M to 0.1 M, respectively. After the growth of ZnO nanorods, the sample was washed by distilled water and dried by filtered air. The samples were then put in an oven at 70 °C in order to remove the remaining moisture. The morphology was examined using a field emission gun scanning electron microscope (FEG-SEM, Quanta 200FEG) and the structural properties were analyzed by X-ray diffraction (XRD) with Cu K-alpha radiation.
3. Results and discussion Fig. 1 shows SEM images of the cross-sectional view of ZnO nanorods on the Si (1 0 0) substrates grown at various temperatures: 110 °C, 130 °C, 160 °C, 180 °C, 190 °C and 210 °C. The figure insets are the corresponding top-view images of the ZnO nanorods. For these samples, the concentration of zinc acetate was 0.01 M and the growth time was 20 min. As shown in Fig. 1, ZnO nanorods were grown vertically on all samples. In addition, the ZnO nanorod shapes can be clearly seen, except for at the growth temperature of 210 °C. Fig. 2 shows the diameter, length and growth rate of the ZnO nanorods presented in Fig. 1. Fig. 2(a) shows that the ZnO nanorod diameter tends to increase with increasing growth temperature. The length increased with growth temperature from 110 °C to 190 °C and then decreased at 210 °C. The increased diameter and length at higher growth temperatures is likely a result of the increased energy supplied to the substrate. However, the 210 °C growth temperature resulted in a shorter length than the 190 °C temperature. From the SEM images, it appears that the nanorods grown at 210 °C are less distinct and much more densely packed. This high density growth, not observed at lower temperatures, likely resulted in shorter nanorods. The highest growth rate of ZnO nanorods was 490 nm/min, which was obtained from the growth temperature of 190 °C. The growth rate observed in this
Fig. 2. Effect of growth temperature on the approximate ZnO nanorod (a) diameter, (b) length, and (c) growth rate.
T.-h. Lee et al. / Journal of Alloys and Compounds 597 (2014) 85–90
Fig. 3. XRD patterns for the ZnO nanorods on Si (1 0 0) substrates with various growth temperatures: 110 °C, 130 °C, 160 °C, 180 °C, 190 °C, and 210 °C (a) XRD spectra, (b) the intensity of the (0 0 2) peak, shown as a solid line and the (0 0 2)/ (1 0 0) peak intensity ratio, shown as a dashed line.
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study exceeds the reported growth rates we have seen for other solution processes, including CBD and hydrothermal synthesis [18–23], electrochemical deposition [24–26], and CVD [6,8,27–28]. Fig. 3 shows XRD results for the ZnO nanorods grown at different temperatures. All ZnO diffraction peaks can be indexed to the hexagonal wurtzite structure. A very strong (0 0 2) peak intensity can be seen at 34.4°, which indicates that the ZnO nanorods were perpendicularly grown along the c-axis on the substrate. The (0 0 2) peak intensities are shown in Fig. 3(b). At 110 °C and 210 °C, the (0 0 2) peak intensity is relatively low because of very low and very high temperature, respectively. The (0 0 2) peak intensity was the strongest when the growth temperature was 180 °C. The (0 0 2)/ (1 0 0) peak intensity ratio has the highest value when the growth temperature was 130 °C. Mclaren et al. reported that a high (0 0 2)/ (1 0 0) peak intensity ratio value indicated vertical growth of the nanorods along the c-axis [17]. Values of the ratio are greater than 1000 for almost all samples, indicating that the samples have vertically grown ZnO nanorods that are well-aligned with the c-axis. The sample grown at 210 °C was the only exception and had a peak ratio lower than 1000. The samples grown at temperatures from 130 °C to 190 °C have a strong (0 0 2) peak intensity. In addition, the (0 0 2) peak FWHM value was 0.18–0.21° for temperatures from 110 °C to 210 °C, which indicates good ZnO nanorods crystallinity. Overall, the XRD results indicate good structural properties of the ZnO nanorods grown in the wide range of growth temperatures tested in this study. Fig. 4 shows the FEG-SEM images of the ZnO nanorods grown with different growth times: 1 min, 5 min, 10 min, 20 min and 40 min. All of these samples were grown at 190 °C, i.e., the temperature that was shown to promote the highest growth rate in the temperature investigation. The ZnO nanorods on all samples were grown vertically on the substrate. As shown in Fig. 4(d) and (e), the diameters of the ZnO nanorods for the 20 min and 40 min times are larger than the diameter of the ZnO nanorods on the other samples. It is possible that the longer durations allowed smaller diameter ZnO nanorods to combine with each other, forming the larger diameter nanorods [18].
Fig. 4. The cross-section view SEM images: ZnO nanorods on seeded-Si (1 0 0) substrates with various growth times: (a) 1 m, (b) 5 m, (c) 10 m, (d) 20 m, and (e) 40 m.
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Fig. 6. XRD patterns for the ZnO nanorods on Si (1 0 0) substrates with various growth times: 1 min, 5 min, 10 min, 20 min, and 40 min (a) XRD spectra, (b) the intensity of (0 0 2) peak, shown as a solid line and the (0 0 2)/(1 0 0) peak intensity ratio, shown as a dashed line.
Fig. 5. Effect of growth time on the approximate ZnO nanorod (a) diameter, (b) length, and (c) growth rate.
Fig. 5 shows the diameter, length and growth rate of ZnO nanorods from Fig. 4. The change in diameter and change in length with growth time both show similar trends. For both diameter and length versus growth time, a shift in the overall trend can be observed at a growth time of 10 min. A growth time of 1 min resulted in ZnO nanorod diameter and length of 85 nm and 1600 nm, respectively. There is not a large increase in the diameter or the length for growth times of 5 min and 10 min. However, there is a dramatic increase in the diameter and the length for growth times longer than 10 min. As shown in Fig. 5(c), the sample grown for 1 min has nanorods with a length of 1600 nm grown at a very high growth rate. The growth rate decreased for growth times of 5 min and 10 min. Then the growth rate increased for growth times longer than 10 min. There is a clear difference in the diameter, length
and growth rate between growth times shorter than 10 min and growth times longer than 10 min. This is possibly because the substrate temperature as well as growth time has a significant impact on the growth. In this experiment, the substrate was located on a hot plate with the temperature maintained at 190 °C. The temperature of solution, which was fed using a peristaltic pump, was 80 °C. So, there was a temperature difference between the substrate and the solution. Consequently, the temperature of substrate likely decreased temporarily when solution flow was initialized and deposition began. We hypothesize that the growth time of 1 min was so short that the substrate had not yet cooled substantially and was still sufficiently hot to facilitate the high growth rate. The growth rate was decreased until 10 min. The temperature of substrate likely decreased due to the solution flow and the sample may have taken some time to reach steady state with the increased thermal load. In other words, it is possible that the growth times of 5 min and 10 min had a lower temperature for large portion of the growth, resulting in a decreased average growth rate. For growth times longer than 10 min, a rapid increases in diameter and length were observed. Perhaps for these longer durations, the hot plate and substrate again reached a constant temperature allowing the nanorod growth to actively occur, thereby significantly increasing the diameter, length and growth rate of ZnO nanorods. However, further study is needed to fully understand the observed trends in diameter, length and growth rate of ZnO nanorods. Fig. 6 shows XRD results for the ZnO nanorods with different growth times. The (0 0 2) peak intensity of the ZnO nanorods increased with increasing growth time. The overall value of the
T.-h. Lee et al. / Journal of Alloys and Compounds 597 (2014) 85–90
Fig. 7. The cross-section view SEM images: ZnO nanorods on seeded-Si (1 0 0) substrates with various zinc acetate concentrations: (a) 0.001 M, (b) 0.005 M, (c) 0.01 M, and (d) 0.1 M.
(0 0 2) peak FWHM value was 0.19–0.22° from 1 min to 40 min, which shows good crystallinity of the ZnO nanorods. In this study, the highest (0 0 2)/(1 0 0) peak intensity ratio value was obtained from the growth time of 10 min. All samples except for the sample with a 40 min growth time, have a (0 0 2)/(1 0 0) peak intensity ratio higher than 1000, indicating vertical growth in the (0 0 2) direction.
Fig. 8. Effect of zinc acetate concentration on the approximate ZnO nanorod (a) diameter, (b) length, and (c) growth rate.
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Fig. 7 shows the FEG-SEM images of the ZnO nanorods with zinc acetate concentrations of 0.001 M, 0.005 M, 0.01 M, and 0.1 M. The growth temperature for these samples was maintained at 190 °C and the growth time was 20 min. It can be seen that all samples, except for the sample grown with a zinc acetate concentration of 0.1 M, were grown vertically on the substrate. Shown in Fig. 7, the ZnO nanorods grown with a zinc acetate concentration of 0.1 M have the largest diameter and length, but have the worst vertical alignment when compared to the other samples. At such a high precursor concentration, ZnO nanorods grow on the substrate in relatively random directions. Fig. 8 shows the diameter, length and growth rate of ZnO nanorods from Fig. 7. The diameter of ZnO nanorods significantly increased from approximately 85 nm to 1200 nm and their length also increased from approximately 660 nm to 10,900 nm with increasing zinc acetate concentration, as shown in Fig. 8. Since the number of Zn2+ ions available to react increased, the ZnO nanorods grew better as the zinc acetate concentration was increased. Guo et al. reported that a high concentration favors the growth of nanorods with a large diameter and a low concentration favors smaller diameters. In other words, the high degree of supersaturation is responsible for the large diameter of the ZnO nanorods grown with a high concentration [19]. The growth rate of ZnO nanorods significantly increased as concentration increased from 0.001 M to 0.01 M and then increased slightly at 0.1 M, as shown in Fig. 8(c). Fig. 9 shows XRD results for the ZnO nanorods with various zinc acetate concentrations. The (0 0 2) peak was mainly observed at 34.4°. The highest (0 0 2) peak intensity was obtained from the
Fig. 9. XRD patterns for the ZnO nanorods on Si (1 0 0) substrates with various zinc acetate concentrations: 0.1 M, 0.01 M, 0.005 M and 0.001 M (a) XRD spectra, (b) the intensity of (0 0 2) peak, shown as a solid line and the (0 0 2)/(1 0 0) peak intensity ratio, shown as a dashed line.
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0.01 M sample. The (0 0 2) peak intensity for ZnO nanorods grown with a zinc acetate concentration of 0.1 M is much lower than the (0 0 2) peak intensities for the samples grown with concentrations of 0.005 M and 0.01 M. This result is possibly because of random directional growth as shown in Fig. 7(d). The overall values of the (0 0 2) peak FWHM are 0.19–0.23°, which shows good crystallinity of the ZnO nanorods. 4. Conclusion We reported the fast growth of vertical ZnO nanorods on seeded Si substrates by using a modified CBD method. The effects of growth temperature, growth time and concentration on the morphological and structural properties of the ZnO nanorods were mainly investigated. We found that all conditions resulted in vertically grown ZnO nanorods with good morphological properties from the FEG-SEM measurement. XRD measurement results show that all samples have strong (0 0 2) peak intensities and low FWHM values, 0.18–0.23, which indicates good crystallinity of the ZnO nanorods, even though no thermal annealing was performed after growth. These results suggest that the M-CBD process has great potential for processing flexible substrates that require low temperatures. Acknowledgements This work was supported by grant from Inje University, 2012. References [1] A.B. Djurišic´, Y.H. Leung, Small 2 (2006) 944–961. [2] S.H. Ko, D. Lee, H.W. Kang, K.H. Nam, J.Y. Yeo, S.J. Hong, C.P. Grigoropoulos, H.J. Sung, Nano Lett. 11 (2011) 666–671.
[3] J. Lim, C. Kang, K. Kim, I. Park, D. Hwang, S. Park, Adv Mater. 18 (2006) 2720– 2724. [4] G. Yi, C. Wang, W.I. Park, Semicond. Sci. Technol. 20 (2005) S22. [5] B. Kang, Y. Heo, L. Tien, D. Norton, F. Ren, B. Gila, S. Pearton, Appl. Phys. A 80 (2005) 1029–1032. [6] M. Rosina, P. Ferret, P. Jouneau, I. Robin, F. Levy, G. Feuillet, M. Lafossas, Microelectron. J. 40 (2009) 242–245. [7] R. Hong, J. Huang, H. He, Z. Fan, J. Shao, Appl. Surf. Sci. 242 (2005) 346–352. [8] A. Wagner, A. Behrends, A. Waag, A. Bakin, Thin Solid Films 520 (2012) 4637– 4641. [9] Y. Jeong, C. Shin, J. Heo, H. Ryu, W. Lee, J. Chang, C. Son, J. Yun, Appl. Surf. Sci. 257 (2011) 10358–10362. [10] X. Zhao, C. Kim, J. Lee, J. Heo, C. Shin, H. Ryu, J. Chang, H. Lee, C. Son, W. Lee, Appl. Surf. Sci. 255 (2009) 4461–4465. [11] C. Shin, J. Heo, Y. Jeong, H. Oh, H. Ryu, W. Lee, J. Chang, J. Kim, H. Choi, Thin Solid Films 520 (2012) 2449–2454. [12] J. Hassan, Z. Hassan, H. Abu-Hassan, J. Alloys Comp. 509 (2011) 6711–6719. [13] Z. Yin, S. Wu, X. Zhou, X. Huang, Q. Zhang, F. Boey, H. Zhang, Small 6 (2010) 307–312. [14] Q. Li, J. Bian, J. Sun, J. Wang, Y. Luo, K. Sun, D. Yu, Appl. Surf. Sci. 256 (2010) 1698–1702. [15] K.M. McPeak, J.B. Baxter, Ind. Eng. Chem. Res. 48 (2009) 5954–5961. [16] J.Y. Jung, N. Park, S. Han, G.B. Han, T.J. Lee, S.O. Ryu, C. Chang, Curr. Appl. Phys. 8 (2008) 720–724. [17] A. Mclaren, T. Valdes-Solis, G. Li, S.C. Tsang, J. Am. Chem. Soc. 131 (2009) 12540–12541. [18] S. Baruah, J. Dutta, J. Cryst. Growth 311 (2009) 2549–2554. [19] M. Guo, P. Diao, S. Cai, J. Solid State Chem. 178 (2005) 1864–1873. [20] X. Zhao, J.Y. Lee, C. Kim, J. Heo, C.M. Shin, J. Leem, H. Ryu, J. Chang, H.C. Lee, W. Jung, Phys. E: Low-dimensional Syst. Nanostruct. 41 (2009) 1423–1426. [21] Z. Yang, Y. Shi, X. Sun, H. Cao, H. Lu, X. Liu, Mater. Res. Bull. 45 (2010) 474–480. [22] X. Yan, Z. Li, R. Chen, W. Gao, Cryst. Growth Des. 8 (2008) 2406–2410. [23] J. Yang, J. Lang, L. Yang, Y. Zhang, D. Wang, H. Fan, H. Liu, Y. Wang, M. Gao, J. Alloys Comp. 450 (2008) 521–524. [24] T. Pauporté, G. Bataille, L. Joulaud, F. Vermersch, J. Phys. Chem. C 114 (2009) 194–202. [25] Y. Tang, J. Chen, D. Greiner, L. Aé, R. Baier, J. Lehmann, S. Sadewasser, M.C. LuxSteiner, J. Phys. Chem. C 115 (2011) 5239–5243. [26] H.E. Belghiti, T. Pauporté, D. Lincot, Phys. Status Solidi (a) 205 (2008) 2360– 2364. [27] J.Y. Park, D.J. Lee, S.S. Kim, Nanotechnology 16 (2005) 2044. [28] B.H. Kong, H.K. Cho, J. Cryst. Growth 289 (2006) 370–375.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Fast vertical growth of ZnO nanorods using a modified chemical bath deposition Tae-hyun Lee a, Hyukhyun Ryu a,⇑, Won-Jae Lee b a b
Department of Nano Systems Engineering, Center for Nano Manufacturing, Inje University, Obang-dong, Gimhae, Gyeongnam 621-749, Republic of Korea Department of Materials and Components Engineering, Dong-Eui University, 995 Eomgwangno, Busanjin-gu, Busan 614-714, Republic of Korea
a r t i c l e
i n f o
Article history: Received 12 August 2013 Received in revised form 29 January 2014 Accepted 1 February 2014 Available online 7 February 2014 Keywords: ZnO nanorods Vertical growth Modified chemical bath deposition Growth rate
a b s t r a c t In this study, we grew vertical ZnO nanorods on seeded Si (1 0 0) substrates using a modified chemical bath deposition (CBD). We investigated the effects of the growth temperature, growth time and concentration on the morphological and structural properties of the ZnO nanorods using field emission gun scanning electron microscopy (FEG-SEM) and X-ray diffraction. This modified CBD method shows improved results over conventional CBD. ZnO nanorods with good structural XRD properties were grown with a very fast growth rate in a wide range of growth conditions and did not require post-growth annealing. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO) is a typical II–VI compound semiconductor. It has a wurtzite structure and a large exciton binding energy of 60 meV at room temperature [1]. Owing to its high optical transparency and conductivity, it has been used in several types of devices such as ultraviolet lasers, field-effect transistor (FET), high performance nanosensors, light-emitting diodes (LED), solar cell, and nano-piezotronics [2–4]. ZnO can also be grown with various structures, including nanorods, nanosheets, nanoflowers, nanowires, and nanorings [4]. The growth of ZnO nanostructures has been studied using various growth methods, including molecular beam epitaxy (MBE) [5], metal organic chemical vapor deposition (MOCVD) [6], Sputtering (SP) [7], vapor phase transport (VPT) [8], hydrothermal synthesis [9–11], chemical bath deposition (CBD) [12] and electrochemical deposition (ECD) [13]. Among these methods, CBD and hydrothermal synthesis have been widely studied because they can be used for large-area deposition with low growth temperatures. In addition, these methods are relatively simple and inexpensive [14]. However, CBD and hydrothermal synthesis are problematic for two reasons [15,16]. First, it is difficult to maintain the concentration of the solution in the bath during growth because the solution concentration decreases as the reaction progresses. Second, it is difficult to directly supply thermal energy to the substrate because ⇑ Corresponding author. Tel.: +82 55 320 3874; fax: +82 55 320 3631. E-mail address: [email protected] (H. Ryu). http://dx.doi.org/10.1016/j.jallcom.2014.02.003 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
the entire bath is heated, which can cause unwanted homogeneous reactions to occur in the solution, hindering growth on the substrate. To address these problems, we used a modified CBD (M-CBD) process which continuously delivers a constant concentration of solution to the substrate and reduces the likelihood of homogeneous reaction by heating the substrate directly. In this paper, we used this M-CBD process and investigated the effects of varying the growth temperature, growth time, and solution concentration on the growth of ZnO nanostructures on seeded Si (1 0 0) substrates. This M-CBD process produced vertically grown ZnO nanorods with a high growth rate and good structural XRD properties without the need for an additional annealing process. This result indicates that the M-CBD process can be favorable for flexible device fabrication. 2. Experiments details In this study, we grew vertical ZnO nanorods on seeded Si (1 0 0) substrates by using M-CBD. P-type Si (1 0 0) substrates were cleaned in piranha for 15 min at 140 °C and in 20% HF for 1 min at room temperature. After each cleaning step, substrates were sonicated in distilled water and dried with filtered air. A 50 nm-thick ZnO seed layer was deposited on the Si substrate by a radio frequency (RF) sputtering system. The sputtering power was 100 W under Ar gas at 3 m torr. A solution of zinc acetate dehydrate (Zn(O2CCH3)2(H2O)2) in ammonia and distilled water was prepared with a ratio of 10:1 for Zn(Ac):ammonia. Zinc acetate concentration was varied, as one of the experimental conditions, from 0.001 M to 0.1 M. The solution was filtered by 0.45 lm syringe filter. The substrate was placed on a hot plate which controls the reacting temperature and conveys thermal energy to substrate directly. Then a reacting Teflon chamber was put onto the substrate with O-ring, which prevents solution from leaking
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Fig. 1. The cross-section view SEM images: ZnO nanorods on seeded-Si (1 0 0) substrates with various growth temperatures: (a) 110 °C, (b) 130 °C, (c) 160 °C, (d) 180 °C, (e) 190 °C, and (f) 210 °C. outside the chamber. The prepared solution with constant concentration was delivered to the reacting chamber though silicon tube (OD = 3.7 mm, ID = 0.8 mm) using peristaltic pump (Masterflex L/S, EW-7523-90). For the solution delivery, the prepared solution was moved to the hot water bath which contained the silicon tube of 128 cm long with the temperature of 80 °C. The heated solution was fed into the reaction chamber with a constant injection rate. So, the constant concentration of solution in the reaction chamber was maintained throughout the whole process. For this experiment, the growth temperature, growth time and zinc acetate concentration were varied from 110 °C to 210 °C, from 1 min to 40 min, and from 0.001 M to 0.1 M, respectively. After the growth of ZnO nanorods, the sample was washed by distilled water and dried by filtered air. The samples were then put in an oven at 70 °C in order to remove the remaining moisture. The morphology was examined using a field emission gun scanning electron microscope (FEG-SEM, Quanta 200FEG) and the structural properties were analyzed by X-ray diffraction (XRD) with Cu K-alpha radiation.
3. Results and discussion Fig. 1 shows SEM images of the cross-sectional view of ZnO nanorods on the Si (1 0 0) substrates grown at various temperatures: 110 °C, 130 °C, 160 °C, 180 °C, 190 °C and 210 °C. The figure insets are the corresponding top-view images of the ZnO nanorods. For these samples, the concentration of zinc acetate was 0.01 M and the growth time was 20 min. As shown in Fig. 1, ZnO nanorods were grown vertically on all samples. In addition, the ZnO nanorod shapes can be clearly seen, except for at the growth temperature of 210 °C. Fig. 2 shows the diameter, length and growth rate of the ZnO nanorods presented in Fig. 1. Fig. 2(a) shows that the ZnO nanorod diameter tends to increase with increasing growth temperature. The length increased with growth temperature from 110 °C to 190 °C and then decreased at 210 °C. The increased diameter and length at higher growth temperatures is likely a result of the increased energy supplied to the substrate. However, the 210 °C growth temperature resulted in a shorter length than the 190 °C temperature. From the SEM images, it appears that the nanorods grown at 210 °C are less distinct and much more densely packed. This high density growth, not observed at lower temperatures, likely resulted in shorter nanorods. The highest growth rate of ZnO nanorods was 490 nm/min, which was obtained from the growth temperature of 190 °C. The growth rate observed in this
Fig. 2. Effect of growth temperature on the approximate ZnO nanorod (a) diameter, (b) length, and (c) growth rate.
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Fig. 3. XRD patterns for the ZnO nanorods on Si (1 0 0) substrates with various growth temperatures: 110 °C, 130 °C, 160 °C, 180 °C, 190 °C, and 210 °C (a) XRD spectra, (b) the intensity of the (0 0 2) peak, shown as a solid line and the (0 0 2)/ (1 0 0) peak intensity ratio, shown as a dashed line.
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study exceeds the reported growth rates we have seen for other solution processes, including CBD and hydrothermal synthesis [18–23], electrochemical deposition [24–26], and CVD [6,8,27–28]. Fig. 3 shows XRD results for the ZnO nanorods grown at different temperatures. All ZnO diffraction peaks can be indexed to the hexagonal wurtzite structure. A very strong (0 0 2) peak intensity can be seen at 34.4°, which indicates that the ZnO nanorods were perpendicularly grown along the c-axis on the substrate. The (0 0 2) peak intensities are shown in Fig. 3(b). At 110 °C and 210 °C, the (0 0 2) peak intensity is relatively low because of very low and very high temperature, respectively. The (0 0 2) peak intensity was the strongest when the growth temperature was 180 °C. The (0 0 2)/ (1 0 0) peak intensity ratio has the highest value when the growth temperature was 130 °C. Mclaren et al. reported that a high (0 0 2)/ (1 0 0) peak intensity ratio value indicated vertical growth of the nanorods along the c-axis [17]. Values of the ratio are greater than 1000 for almost all samples, indicating that the samples have vertically grown ZnO nanorods that are well-aligned with the c-axis. The sample grown at 210 °C was the only exception and had a peak ratio lower than 1000. The samples grown at temperatures from 130 °C to 190 °C have a strong (0 0 2) peak intensity. In addition, the (0 0 2) peak FWHM value was 0.18–0.21° for temperatures from 110 °C to 210 °C, which indicates good ZnO nanorods crystallinity. Overall, the XRD results indicate good structural properties of the ZnO nanorods grown in the wide range of growth temperatures tested in this study. Fig. 4 shows the FEG-SEM images of the ZnO nanorods grown with different growth times: 1 min, 5 min, 10 min, 20 min and 40 min. All of these samples were grown at 190 °C, i.e., the temperature that was shown to promote the highest growth rate in the temperature investigation. The ZnO nanorods on all samples were grown vertically on the substrate. As shown in Fig. 4(d) and (e), the diameters of the ZnO nanorods for the 20 min and 40 min times are larger than the diameter of the ZnO nanorods on the other samples. It is possible that the longer durations allowed smaller diameter ZnO nanorods to combine with each other, forming the larger diameter nanorods [18].
Fig. 4. The cross-section view SEM images: ZnO nanorods on seeded-Si (1 0 0) substrates with various growth times: (a) 1 m, (b) 5 m, (c) 10 m, (d) 20 m, and (e) 40 m.
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Fig. 6. XRD patterns for the ZnO nanorods on Si (1 0 0) substrates with various growth times: 1 min, 5 min, 10 min, 20 min, and 40 min (a) XRD spectra, (b) the intensity of (0 0 2) peak, shown as a solid line and the (0 0 2)/(1 0 0) peak intensity ratio, shown as a dashed line.
Fig. 5. Effect of growth time on the approximate ZnO nanorod (a) diameter, (b) length, and (c) growth rate.
Fig. 5 shows the diameter, length and growth rate of ZnO nanorods from Fig. 4. The change in diameter and change in length with growth time both show similar trends. For both diameter and length versus growth time, a shift in the overall trend can be observed at a growth time of 10 min. A growth time of 1 min resulted in ZnO nanorod diameter and length of 85 nm and 1600 nm, respectively. There is not a large increase in the diameter or the length for growth times of 5 min and 10 min. However, there is a dramatic increase in the diameter and the length for growth times longer than 10 min. As shown in Fig. 5(c), the sample grown for 1 min has nanorods with a length of 1600 nm grown at a very high growth rate. The growth rate decreased for growth times of 5 min and 10 min. Then the growth rate increased for growth times longer than 10 min. There is a clear difference in the diameter, length
and growth rate between growth times shorter than 10 min and growth times longer than 10 min. This is possibly because the substrate temperature as well as growth time has a significant impact on the growth. In this experiment, the substrate was located on a hot plate with the temperature maintained at 190 °C. The temperature of solution, which was fed using a peristaltic pump, was 80 °C. So, there was a temperature difference between the substrate and the solution. Consequently, the temperature of substrate likely decreased temporarily when solution flow was initialized and deposition began. We hypothesize that the growth time of 1 min was so short that the substrate had not yet cooled substantially and was still sufficiently hot to facilitate the high growth rate. The growth rate was decreased until 10 min. The temperature of substrate likely decreased due to the solution flow and the sample may have taken some time to reach steady state with the increased thermal load. In other words, it is possible that the growth times of 5 min and 10 min had a lower temperature for large portion of the growth, resulting in a decreased average growth rate. For growth times longer than 10 min, a rapid increases in diameter and length were observed. Perhaps for these longer durations, the hot plate and substrate again reached a constant temperature allowing the nanorod growth to actively occur, thereby significantly increasing the diameter, length and growth rate of ZnO nanorods. However, further study is needed to fully understand the observed trends in diameter, length and growth rate of ZnO nanorods. Fig. 6 shows XRD results for the ZnO nanorods with different growth times. The (0 0 2) peak intensity of the ZnO nanorods increased with increasing growth time. The overall value of the
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Fig. 7. The cross-section view SEM images: ZnO nanorods on seeded-Si (1 0 0) substrates with various zinc acetate concentrations: (a) 0.001 M, (b) 0.005 M, (c) 0.01 M, and (d) 0.1 M.
(0 0 2) peak FWHM value was 0.19–0.22° from 1 min to 40 min, which shows good crystallinity of the ZnO nanorods. In this study, the highest (0 0 2)/(1 0 0) peak intensity ratio value was obtained from the growth time of 10 min. All samples except for the sample with a 40 min growth time, have a (0 0 2)/(1 0 0) peak intensity ratio higher than 1000, indicating vertical growth in the (0 0 2) direction.
Fig. 8. Effect of zinc acetate concentration on the approximate ZnO nanorod (a) diameter, (b) length, and (c) growth rate.
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Fig. 7 shows the FEG-SEM images of the ZnO nanorods with zinc acetate concentrations of 0.001 M, 0.005 M, 0.01 M, and 0.1 M. The growth temperature for these samples was maintained at 190 °C and the growth time was 20 min. It can be seen that all samples, except for the sample grown with a zinc acetate concentration of 0.1 M, were grown vertically on the substrate. Shown in Fig. 7, the ZnO nanorods grown with a zinc acetate concentration of 0.1 M have the largest diameter and length, but have the worst vertical alignment when compared to the other samples. At such a high precursor concentration, ZnO nanorods grow on the substrate in relatively random directions. Fig. 8 shows the diameter, length and growth rate of ZnO nanorods from Fig. 7. The diameter of ZnO nanorods significantly increased from approximately 85 nm to 1200 nm and their length also increased from approximately 660 nm to 10,900 nm with increasing zinc acetate concentration, as shown in Fig. 8. Since the number of Zn2+ ions available to react increased, the ZnO nanorods grew better as the zinc acetate concentration was increased. Guo et al. reported that a high concentration favors the growth of nanorods with a large diameter and a low concentration favors smaller diameters. In other words, the high degree of supersaturation is responsible for the large diameter of the ZnO nanorods grown with a high concentration [19]. The growth rate of ZnO nanorods significantly increased as concentration increased from 0.001 M to 0.01 M and then increased slightly at 0.1 M, as shown in Fig. 8(c). Fig. 9 shows XRD results for the ZnO nanorods with various zinc acetate concentrations. The (0 0 2) peak was mainly observed at 34.4°. The highest (0 0 2) peak intensity was obtained from the
Fig. 9. XRD patterns for the ZnO nanorods on Si (1 0 0) substrates with various zinc acetate concentrations: 0.1 M, 0.01 M, 0.005 M and 0.001 M (a) XRD spectra, (b) the intensity of (0 0 2) peak, shown as a solid line and the (0 0 2)/(1 0 0) peak intensity ratio, shown as a dashed line.
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0.01 M sample. The (0 0 2) peak intensity for ZnO nanorods grown with a zinc acetate concentration of 0.1 M is much lower than the (0 0 2) peak intensities for the samples grown with concentrations of 0.005 M and 0.01 M. This result is possibly because of random directional growth as shown in Fig. 7(d). The overall values of the (0 0 2) peak FWHM are 0.19–0.23°, which shows good crystallinity of the ZnO nanorods. 4. Conclusion We reported the fast growth of vertical ZnO nanorods on seeded Si substrates by using a modified CBD method. The effects of growth temperature, growth time and concentration on the morphological and structural properties of the ZnO nanorods were mainly investigated. We found that all conditions resulted in vertically grown ZnO nanorods with good morphological properties from the FEG-SEM measurement. XRD measurement results show that all samples have strong (0 0 2) peak intensities and low FWHM values, 0.18–0.23, which indicates good crystallinity of the ZnO nanorods, even though no thermal annealing was performed after growth. These results suggest that the M-CBD process has great potential for processing flexible substrates that require low temperatures. Acknowledgements This work was supported by grant from Inje University, 2012. References [1] A.B. Djurišic´, Y.H. Leung, Small 2 (2006) 944–961. [2] S.H. Ko, D. Lee, H.W. Kang, K.H. Nam, J.Y. Yeo, S.J. Hong, C.P. Grigoropoulos, H.J. Sung, Nano Lett. 11 (2011) 666–671.
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