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Selective-area growth of ZnO nanorod arrays via a sonochemical route
Materials Letters 62 (2008) 3673–3675
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Selective-area growth of ZnO nanorod arrays via a sonochemical route Seung-Ho Jung a, Soo-Hwan Jeong b,⁎ a b
Department of Chemical Engineering, Pohang University of Science and Technology, Hyoja-dong, Nam-gu, Pohang 790-784, Korea Department of Chemical Engineering, Kyungpook National University, 1370 Sankyuk-dong, Buk-gu, Daegu 702-701, Korea
A R T I C L E
I N F O
Article history: Received 22 January 2008 Accepted 7 April 2008 Available online 16 April 2008 Keywords: Nanomaterials Semiconductors ZnO nanorods Sonochemistry
A B S T R A C T A simple and facile sonochemical route was described for the selective-area growth of well-aligned ZnO nanorod arrays on Si wafers under ambient conditions. ZnO nanorod arrays were selectively grown on preformed various patterns of zinc seed layer, such as letters and dots of uniform or non-uniform sizes. The average diameter and length of ZnO nanorods were 76 nm and 475 nm, respectively. The crystal structure of ZnO nanorods were investigated by transmission electron microscopy and X-ray diffraction. Based on this simple sonochemical technique, highly crystalline ZnO nanorod arrays can be selectively grown on other flat substrates with various patterned morphologies. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Selective-area growth of well-aligned one-dimensional (1D) semiconductor nanostructures on a substrate is highly desirable for the integration of high-performance nanodevices because of excellent interfacial properties [1,2]. As a wide band gap semiconductor, ZnO is one of the most important multifunctional semiconductors in recent materials chemistry. Combined with conventional patterning techniques, chemical vapor deposition (CVD) [1,2], selective-area metalorganic CVD (MOCVD) [5], and a hydrothermal method [3,4] are widely-used techniques that can produce well-aligned 1D ZnO nanostructures with patterned morphologies. However, these approaches have some drawbacks. CVD and selective-area MOCVD require sophisticated equipment due to rigorous environmental conditions, such as high growth temperature (up to 1400 oC) and low pressure, which are inadequate for nanoelectronic circuit integration. Unlike vapor-phase techniques, a hydrothermal method is a low-temperature process and requires simple equipment, however, a long reaction time (usually from a few hours to several days) is the weak point of this method [3,6]. Therefore, the development of a simple and fast route to selectivearea growth of 1D ZnO nanostructures under ambient conditions remains an important topic of investigation. A sonochemical technique has been recently developed as a promising alternative technique [7,8]. Our group has developed the sonochemical method for preparing line-patterned ZnO nanorod arrays by carrying out lift-off
⁎ Corresponding author. Tel.: +82 53 950 5615; fax: +82 53 950 6615. E-mail address: [email protected] (S.-H. Jeong). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.04.041
process after growing the aligned ZnO nanorod arrays over the entire surface of substrates [9]. However, direct and selective-area growth on prescribed pattern was impossible in this case. Direct and selectivearea growth would be a preferred approach since it requires no subsequent and additional patterning steps. In this paper, we report on the selective-area growth of well-aligned ZnO nanorod arrays on Si wafers via a sonochemical route. 2. Experimental After photoresist (PR, AZ1512) was spincoated on Si wafers, Si wafers were covered with a photolithographic mask and exposed to UV in order to form PR patterns, such as letters and dots of uniform or non-uniform sizes. Ti (5 nm) and Zn (40 nm) thin films were successively sputtered on PR patterned-Si wafers and these Si wafers were immersed in acetone in order to remove PR patterns. Then, Si wafers were immersed in a mixed aqueous solution of Zn(NO3)2·6H2O and (CH2)6N4 (hexamethylenetetramine, HMT). The concentration levels of Zn(NO3)2·6H2O and HMT were both 0.01 M. 20 kHz ultrasonic wave was introduced at the intensity of 31.6 W/cm2 for 1 h under ambient conditions in order to grow well-aligned ZnO nanorod arrays with patterned morphologies. As-grown ZnO nanorods were characterized by field emission scanning electron microscope (FESEM, Hitachi S-4300), transmission electron microscope (TEM, JEOL JEM2010), and X-ray diffraction (XRD, Max Science, M18XHF). 3. Results and Discussion ZnO nanorod arrays were selectively grown on Si wafers with various patterns, such as dots of uniform size, dots of different size, and letters as shown in Fig. 1. Bright and dark areas in Fig. 1(a)-(c) correspond to the ZnO-grown area and the bare wafer surface,
3674
S.-H. Jung, S.-H. Jeong / Materials Letters 62 (2008) 3673–3675
Fig. 1. SEM images of ZnO patterns with (a) dots of uniform size, (b) dots of different size, and (c) letters. (d) A top view SEM image of the boundary between the ZnO-grown area and the bare wafer surface of a dotted circle in (c). (e) A side view SEM image of the boundary between the ZnO-grown area and the bare wafer surface. (f) A magnified view of a dotted circle in (e).
respectively. The boundary between the ZnO-grown area and the bare wafer surface (dotted circle in Fig. 1(c)) was investigated and shown in Fig. 1(d). From the SEM images, it is clearly confirmed that ZnO nanorods were selectively grown on zinc seed layer which was pre-patterned on Si wafer substrates. Well-aligned ZnO nanorods are 76 nm in average diameter and 475 nm in average length. During the selective patterned growth, ZnO nanorods had no damage and maintained good interfacial adhesion to the substrate. Therefore our sonochemical technique is thought to be compatible with standard semiconductor microfabrication technology. Fig. 2 shows XRD graphs of the uniform dot pattern before and after ZnO nanorod growth. A very sharp diffraction peak at about 33° is due to Si b100N wafer. After the ZnO nanorod growth, the XRD graph shows secondary (at about 34°) sharp peaks due to the ZnO (0002) planes. Since there was no diffraction peaks observed from other impurities, it is noted that, under the current sonochemical approach, pure hexagonal-
Fig. 2. XRD graphs of the uniform dot pattern before and after ZnO nanorod growth.
phase ZnO nanorods were vertically well-aligned on Si wafers along the [0001] direction. There are three important factors which are known to contribute to the vertical growth and alignment of ZnO nanorod arrays. First is the use of patterned zinc thin film [10]. When Zn thin film is used as nucleation sites, uniform growth and inplane alignment of ZnO nanorod arrays are improved. Second is the preferential growth of ZnO crystal due to higher crystal growth rate along the [0001] direction [11]. Third is the steric hindrance effect [12]. ZnO nanorods were detached from Si wafer for further crystal structure analysis as shown in Fig. 3(a). From the high-resolution TEM (HRTEM) image in Fig. 3(b), ZnO nanorods are highly crystalline with a lattice spacing of about 0.26 nm which
Fig. 3. (a) A low magnification TEM image, (b) a HRTEM image, and (c) a corresponding electron diffraction pattern of a single ZnO nanorod.
S.-H. Jung, S.-H. Jeong / Materials Letters 62 (2008) 3673–3675 corresponds to the d-spacing of (0002) planes in hexagonal ZnO crystal lattice. The corresponding electron diffraction pattern in Fig. 3(c) showed that ZnO nanorods grew along the [0001] direction. With the XRD and TEM analyses, we confirmed that highly crystalline ZnO nanorod arrays were well-aligned on Si wafers along the [0001] direction. The growth mechanism of ZnO nanorods were fully mentioned in our previous reports [9]. In brief, the growth mechanism can be expressed as follows [8,9]: ðCH2 Þ6 N4 þ 6H2 O→4NH3 þ 6HCHO NH3 þ
H2 O→NHþ 4
þ OH
ÞÞÞ
Zn2þ þ 2OH Y ZnO þ H2 O Zn
2þ
þ
2O 2
ÞÞÞ
3 Y ZnO þ O2 2
3675
technology, this sonochemical technique can be a useful route to facile and quick fabrication of high-performance ZnO-based nanodevices. Acknowledgements
ð2Þ
This work was supported by the both Institutional and Academic Cooperative Research Program of Korea Research Institute of Standards and Science (KRISS) and Tera-level Nanodevices (TND) Program of the Korean Ministry of Science and Technology.
ð3Þ
References
ð4Þ
[1] Huang MH, Mao S, Feick H, Yan H, Wu Y, Kind H, et al. Science 2001;292:1897–9. [2] Yang P, Yan H, Mao S, Russo R, Johnson J, Saykally R, et al. Adv Funct Mater 2002;12:323–31. [3] Tak Y, Yong K. J Phys Chem B 2005;109:19263–9. [4] Hsu JWP, Tian ZR, Simmons NC, Matzke CM, Voight JA, Liu J. Nano Lett 2005;5:83–6. [5] Zhong J, Muthukumar S, Saraf G, Chen H, Chen Y, Lu Y. J Electron Mater 2004;33:654–7. [6] Yu H, Zhang Z, Han M, Hao X, Zhu F. J Am Chem Soc 2005;127:2378–9. [7] Zhang X, Zhao H, Tao X, Zhao Y, Zhang Z. Mater Lett 2005;59:1745–7. [8] Hu XL, Zhu YJ, Wang SW. Mater Chem Phys 2004;88:421–6. [9] Jung SH, Oh E, Lee KH, Park W, Jeong SH. Adv Mater 2007;19:749–53. [10] Li Q, Kumar V, Li Y, Zhang H, Marks TJ, Chang RPH. Chem Mater 2005;17:1001–6. [11] Li WJ, Shi EW, Zhong WZ, Yin ZW. J Cryst Growth 1999;203:186–96. [12] Lee CJ, Kim DW, Lee TJ, Choi YC, Park YS, Lee YH, et al. Chem Phys Lett 1999;312:461–8.
ð1Þ
where the symbol))) denotes sonication.
4. Conclusions We demonstrated a simple sonochemical route to the selectivearea growth of highly crystalline ZnO nanorod arrays on the substrate under ambient conditions. Combined with conventional patterning technologies, it is expected that ZnO nanorods can be selectively grown with various pattern features via a sonochemical route. Based on the compatibility with standard semiconductor microfabrication
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Selective-area growth of ZnO nanorod arrays via a sonochemical route Seung-Ho Jung a, Soo-Hwan Jeong b,⁎ a b
Department of Chemical Engineering, Pohang University of Science and Technology, Hyoja-dong, Nam-gu, Pohang 790-784, Korea Department of Chemical Engineering, Kyungpook National University, 1370 Sankyuk-dong, Buk-gu, Daegu 702-701, Korea
A R T I C L E
I N F O
Article history: Received 22 January 2008 Accepted 7 April 2008 Available online 16 April 2008 Keywords: Nanomaterials Semiconductors ZnO nanorods Sonochemistry
A B S T R A C T A simple and facile sonochemical route was described for the selective-area growth of well-aligned ZnO nanorod arrays on Si wafers under ambient conditions. ZnO nanorod arrays were selectively grown on preformed various patterns of zinc seed layer, such as letters and dots of uniform or non-uniform sizes. The average diameter and length of ZnO nanorods were 76 nm and 475 nm, respectively. The crystal structure of ZnO nanorods were investigated by transmission electron microscopy and X-ray diffraction. Based on this simple sonochemical technique, highly crystalline ZnO nanorod arrays can be selectively grown on other flat substrates with various patterned morphologies. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Selective-area growth of well-aligned one-dimensional (1D) semiconductor nanostructures on a substrate is highly desirable for the integration of high-performance nanodevices because of excellent interfacial properties [1,2]. As a wide band gap semiconductor, ZnO is one of the most important multifunctional semiconductors in recent materials chemistry. Combined with conventional patterning techniques, chemical vapor deposition (CVD) [1,2], selective-area metalorganic CVD (MOCVD) [5], and a hydrothermal method [3,4] are widely-used techniques that can produce well-aligned 1D ZnO nanostructures with patterned morphologies. However, these approaches have some drawbacks. CVD and selective-area MOCVD require sophisticated equipment due to rigorous environmental conditions, such as high growth temperature (up to 1400 oC) and low pressure, which are inadequate for nanoelectronic circuit integration. Unlike vapor-phase techniques, a hydrothermal method is a low-temperature process and requires simple equipment, however, a long reaction time (usually from a few hours to several days) is the weak point of this method [3,6]. Therefore, the development of a simple and fast route to selectivearea growth of 1D ZnO nanostructures under ambient conditions remains an important topic of investigation. A sonochemical technique has been recently developed as a promising alternative technique [7,8]. Our group has developed the sonochemical method for preparing line-patterned ZnO nanorod arrays by carrying out lift-off
⁎ Corresponding author. Tel.: +82 53 950 5615; fax: +82 53 950 6615. E-mail address: [email protected] (S.-H. Jeong). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.04.041
process after growing the aligned ZnO nanorod arrays over the entire surface of substrates [9]. However, direct and selective-area growth on prescribed pattern was impossible in this case. Direct and selectivearea growth would be a preferred approach since it requires no subsequent and additional patterning steps. In this paper, we report on the selective-area growth of well-aligned ZnO nanorod arrays on Si wafers via a sonochemical route. 2. Experimental After photoresist (PR, AZ1512) was spincoated on Si wafers, Si wafers were covered with a photolithographic mask and exposed to UV in order to form PR patterns, such as letters and dots of uniform or non-uniform sizes. Ti (5 nm) and Zn (40 nm) thin films were successively sputtered on PR patterned-Si wafers and these Si wafers were immersed in acetone in order to remove PR patterns. Then, Si wafers were immersed in a mixed aqueous solution of Zn(NO3)2·6H2O and (CH2)6N4 (hexamethylenetetramine, HMT). The concentration levels of Zn(NO3)2·6H2O and HMT were both 0.01 M. 20 kHz ultrasonic wave was introduced at the intensity of 31.6 W/cm2 for 1 h under ambient conditions in order to grow well-aligned ZnO nanorod arrays with patterned morphologies. As-grown ZnO nanorods were characterized by field emission scanning electron microscope (FESEM, Hitachi S-4300), transmission electron microscope (TEM, JEOL JEM2010), and X-ray diffraction (XRD, Max Science, M18XHF). 3. Results and Discussion ZnO nanorod arrays were selectively grown on Si wafers with various patterns, such as dots of uniform size, dots of different size, and letters as shown in Fig. 1. Bright and dark areas in Fig. 1(a)-(c) correspond to the ZnO-grown area and the bare wafer surface,
3674
S.-H. Jung, S.-H. Jeong / Materials Letters 62 (2008) 3673–3675
Fig. 1. SEM images of ZnO patterns with (a) dots of uniform size, (b) dots of different size, and (c) letters. (d) A top view SEM image of the boundary between the ZnO-grown area and the bare wafer surface of a dotted circle in (c). (e) A side view SEM image of the boundary between the ZnO-grown area and the bare wafer surface. (f) A magnified view of a dotted circle in (e).
respectively. The boundary between the ZnO-grown area and the bare wafer surface (dotted circle in Fig. 1(c)) was investigated and shown in Fig. 1(d). From the SEM images, it is clearly confirmed that ZnO nanorods were selectively grown on zinc seed layer which was pre-patterned on Si wafer substrates. Well-aligned ZnO nanorods are 76 nm in average diameter and 475 nm in average length. During the selective patterned growth, ZnO nanorods had no damage and maintained good interfacial adhesion to the substrate. Therefore our sonochemical technique is thought to be compatible with standard semiconductor microfabrication technology. Fig. 2 shows XRD graphs of the uniform dot pattern before and after ZnO nanorod growth. A very sharp diffraction peak at about 33° is due to Si b100N wafer. After the ZnO nanorod growth, the XRD graph shows secondary (at about 34°) sharp peaks due to the ZnO (0002) planes. Since there was no diffraction peaks observed from other impurities, it is noted that, under the current sonochemical approach, pure hexagonal-
Fig. 2. XRD graphs of the uniform dot pattern before and after ZnO nanorod growth.
phase ZnO nanorods were vertically well-aligned on Si wafers along the [0001] direction. There are three important factors which are known to contribute to the vertical growth and alignment of ZnO nanorod arrays. First is the use of patterned zinc thin film [10]. When Zn thin film is used as nucleation sites, uniform growth and inplane alignment of ZnO nanorod arrays are improved. Second is the preferential growth of ZnO crystal due to higher crystal growth rate along the [0001] direction [11]. Third is the steric hindrance effect [12]. ZnO nanorods were detached from Si wafer for further crystal structure analysis as shown in Fig. 3(a). From the high-resolution TEM (HRTEM) image in Fig. 3(b), ZnO nanorods are highly crystalline with a lattice spacing of about 0.26 nm which
Fig. 3. (a) A low magnification TEM image, (b) a HRTEM image, and (c) a corresponding electron diffraction pattern of a single ZnO nanorod.
S.-H. Jung, S.-H. Jeong / Materials Letters 62 (2008) 3673–3675 corresponds to the d-spacing of (0002) planes in hexagonal ZnO crystal lattice. The corresponding electron diffraction pattern in Fig. 3(c) showed that ZnO nanorods grew along the [0001] direction. With the XRD and TEM analyses, we confirmed that highly crystalline ZnO nanorod arrays were well-aligned on Si wafers along the [0001] direction. The growth mechanism of ZnO nanorods were fully mentioned in our previous reports [9]. In brief, the growth mechanism can be expressed as follows [8,9]: ðCH2 Þ6 N4 þ 6H2 O→4NH3 þ 6HCHO NH3 þ
H2 O→NHþ 4
þ OH
ÞÞÞ
Zn2þ þ 2OH Y ZnO þ H2 O Zn
2þ
þ
2O 2
ÞÞÞ
3 Y ZnO þ O2 2
3675
technology, this sonochemical technique can be a useful route to facile and quick fabrication of high-performance ZnO-based nanodevices. Acknowledgements
ð2Þ
This work was supported by the both Institutional and Academic Cooperative Research Program of Korea Research Institute of Standards and Science (KRISS) and Tera-level Nanodevices (TND) Program of the Korean Ministry of Science and Technology.
ð3Þ
References
ð4Þ
[1] Huang MH, Mao S, Feick H, Yan H, Wu Y, Kind H, et al. Science 2001;292:1897–9. [2] Yang P, Yan H, Mao S, Russo R, Johnson J, Saykally R, et al. Adv Funct Mater 2002;12:323–31. [3] Tak Y, Yong K. J Phys Chem B 2005;109:19263–9. [4] Hsu JWP, Tian ZR, Simmons NC, Matzke CM, Voight JA, Liu J. Nano Lett 2005;5:83–6. [5] Zhong J, Muthukumar S, Saraf G, Chen H, Chen Y, Lu Y. J Electron Mater 2004;33:654–7. [6] Yu H, Zhang Z, Han M, Hao X, Zhu F. J Am Chem Soc 2005;127:2378–9. [7] Zhang X, Zhao H, Tao X, Zhao Y, Zhang Z. Mater Lett 2005;59:1745–7. [8] Hu XL, Zhu YJ, Wang SW. Mater Chem Phys 2004;88:421–6. [9] Jung SH, Oh E, Lee KH, Park W, Jeong SH. Adv Mater 2007;19:749–53. [10] Li Q, Kumar V, Li Y, Zhang H, Marks TJ, Chang RPH. Chem Mater 2005;17:1001–6. [11] Li WJ, Shi EW, Zhong WZ, Yin ZW. J Cryst Growth 1999;203:186–96. [12] Lee CJ, Kim DW, Lee TJ, Choi YC, Park YS, Lee YH, et al. Chem Phys Lett 1999;312:461–8.
ð1Þ
where the symbol))) denotes sonication.
4. Conclusions We demonstrated a simple sonochemical route to the selectivearea growth of highly crystalline ZnO nanorod arrays on the substrate under ambient conditions. Combined with conventional patterning technologies, it is expected that ZnO nanorods can be selectively grown with various pattern features via a sonochemical route. Based on the compatibility with standard semiconductor microfabrication