A novel piezoelectric ZnO nanogenerator on flexible metal alloy substrate

A novel piezoelectric ZnO nanogenerator on flexible metal alloy substrate Venkateswarlu Gaddam, Sudeep Joshi, Mitesh Parmar and K. Rajanna Department of Instrumentation and Applied Physics Indian Institute of Science Bangalore, India [email protected], [email protected] Abstract—In this paper, we report a novel piezoelectric ZnO nanogenerator on flexible metal alloy substrate (Phynox alloy) for energy harvesting and sensing applications. The vertically aligned ZnO nanowires are sandwiched between Au electrodes. The aligned growth of ZnO nanowires have been successfully synthesized on Au coated metal alloy substrate by hydrothermal method at low temperature (95±1 °C). The as-synthesized vertically aligned ZnO nanowires were characterized using FESEM. Further, PMMA is spin coated over the aligned ZnO nanowires for the purpose of their long term stability. The fabricated nanogenerator is of size 30mm x 6mm. From energy harvesting point of view, the response of the nanogenerator due to finger tip impacts ranges from 0.9 V to 1.4V. Also for sensing application, the maximum output voltage response of the nanogenerator is found to be 2.86V due to stainless steel (SS) ball impact and 0.92 V due to plastic ball impact. Keywords: ZnO nanowires, nanogenerator, piezoelectricity I. INTRODUCTION Powering nano scale devices is the main challenge in the present day nanotechnology world. The potential power sources for nano devices are vibrational energy, mechanical energy, solar energy, hydraulic energy [1]. Moreover the miniaturization of portable electronic devices is very difficult without batteries. The problems associated with the use of batteries include limitation in the size of the device and frequent recharging process. However in place of batteries, if we adopt nanogenerators, mechanical energy is always available in and around us for powering these nano devices. Nanogenerator is a device which converts random mechanical energy into electrical energy to drive nano scale level devices. 978-1-4577-1767-3/12/$26.00 ©2012 IEEE M.M. Nayak Centre for Nano Science and Engineering Indian Institute of Science Bangalore, India [email protected] Zinc Oxide (ZnO) is one of the promising potential materials for energy harvesting and sensing applications. ZnO is a semiconducting piezoelectric material having energy band gap of 3.37 eV and large excitation binding energy of 60 meV at room temperature [2]. It is a prominent material because of its structural, semiconducting, mechanical and piezoelectric properties. The advantages of ZnO are: it exhibits both semiconducting and piezoelectric properties, environmental friendly, biocompatible, growth occurs on large variety of substrate materials [1, 3]. The applications of ZnO are plenty and it is used in electronic, electrochemical, electromechanical devices such as light– emitting diodes, field emission devices, micro/nano sensors, solar cells, nanopiezotronics and piezoelectric nanogenerators [2]. Various techniques are used for the synthesis of ZnO nanostructures such as physical vapour deposition (PVD), chemical methods, molecular beam epitaxy (MBE), pulsed laser deposition (PLD). Among these techniques, solution based chemical methods are more advantageous because they are simple, convenient, low cost, less hazardous, compatible for flexible substrates, capable of large scaling up and growth occurs at relatively low temperatures [2]. The use of these nanogenerators for sensing applications further eliminate the requirement of batteries and make them single independent system. Hence, in the present work, we have reported piezoelectric ZnO nanowires synthesized on flexible alloy substrate as independent energy harvesting and sensing system. Various literature reports on different kinds of flexible substrate materials (Kapton [4], Polythylene naphthalate (PEN) [5], PS (polyester) [6]) for the fabrication of nanogenerators. To the best of our knowledge, there are no reports on flexible alloy substrates for the nanogenerator fabrication. We are introducing for the first time, a flexible alloy substrate for the fabrication of nanogenerators. II. EXPERIMENTAL Now the tips of nanowires are clean and are ready to use for the top electrode contact purpose. The top electrode of Cr/Au (20/100 nm) was deposited on to the kapton film and was kept upside down over the tips of the nanowires for contact. The schematic diagram and the actual photograph of the fabricated nanogenerator are shown in Fig. 1. and Fig. 2. respectively. A. Deposition of ZnO seedlayer on alloy substrate Phynox metal alloy substrate was properly cleaned with standard cleaning procedure. Cr/Au (20/50 nm) layer was deposited by RF magnetron sputtering over the substrate for the purpose of bottom electrode contact. Later ZnO thin film (100 nm) as a seed layer was deposited over the Cr/Au layer for the growth of ZnO nanowires. B. Synthesis of ZnO nanowires Chemical agents used for this method are Zinc nitrate hexahydrate (Zn(NO3)2.6H2O) and Hexamethylenetetramine ((HMTA ) (CH2)6 N4)). The Zn(NO3)2 salt provides Zn2+ ions for the growth of ZnO nanowires. The HMTA during the ZnO nanowires growth would slowly hydrolyze in water solution and gradually produce OH- ions.The growth process of ZnO nanowires can be controlled by the following chemical reactions [2,7]. (CH2)6N4 + 6 H2O NH3 + H2O NH3.H2O Zn2+ + 2OHZn (OH2) Figure. 1: Schematic diagram of nanogenerator 4 NH3 + 6HCHO NH3.H2O NH4+ + OHZn (OH)2 ZnO + H2O An equi-molar concentration (25 mM) of Zn(NO3)2 and HMTA was used for the growth of nanowires. The substrate was kept upside down over the surface of solution and due to surface tension it was standing on the solution. The growth process was carried out in mechanical hot air oven at low temperature (95±1 °C) for the growth time of 5 hrs. The synthesized nanowires on substrate were cooled at room temperature and cleaned with DI water. Further, the cleaned sample was dried at room temperature Figure. 2: Fabricated nanogenerator on flexible metal alloy substrate . C. Fabrication of nanogenerator In order to fabricate the nanogenerator, 3-4 µm thick PMMA was spin coated over as-synthesized ZnO nanowires. Spin coating was done for the purpose of long term stability of nanowires and mechanical robustness of the complete structure. Also, it helps to prevent possible short-circuiting between the bottom and the top electrodes [3]. Later, oxygen plasma etching was done to etch out the fine thickness of PMMA coating for the purpose of top electrode contact. The depth of etching was 1 - 1.5 µm and the etch time was 90 sec. III. RESULTS AND DISCUSSIONS A. Characterization of ZnO nanowires The morphology of ZnO nanowires grown on alloy substrate was observed using field emission scanning electron microscopy (FE-SEM (Carl Zeiss), ULTRA 55). Fig. 3a shows the top view of the vertically grown ZnO nanowires. Fig. 3b shows the cross sectional view of grown nanowires. The average length and diameter of the synthesized vertical nanowires are found to be 3 µm and 170 nm respectively. Most of the ZnO nanowires are vertically aligned as well as highly dense over the entire surface of the alloy substrate. For the sensing application point of view, stainless steel and plastic balls of equal diameter were used. The balls were dropped from four different random heights (8.5, 11, 16 and 23 cms) on to the nanogenerator. When the balls were dropped from the maximum height of 23 cms, the response of the nanogenerator for stainless steel ball was found to be 2.86 V (Fig. 6) and 0.92 V for plastic ball (Fig. 7). Fig .8 shows the relationship between output voltage and the ball drop height for both the balls. The output voltage generated due to stainless steel ball impact is almost 3 times greater than the output voltage generated due to plastic ball impact. Figure.3: Top view of vertically grown ZnO nanowires Figure .6: Response of the nanogenerator for stainless steel ball impact. Figure.4: Cross sectional view of vertically grown ZnO nanowires B. Performance of fabricated nanogenerator The fabricated nanogenerator was directly connected to CRO (Yokogawa, DLM 2022). In order to see the suitability for energy harvesting application, dynamic force was applied on the fabricated nanogenerator using finger tip impacts. The typical response obtained from the nanogenerator due to the dynamic force of human finger impact is given in Fig. 5. The maximum output voltage obtained is 0.93V. Figure. 7: Response of the nanogenerator for plastic ball impact. Figure 5: Response of the nanogenerator due to finger tip impacts. Figure 8: Output of nanogenerator due to SS ball and plastic ball dropped from different heights. IV. CONCLUSION We have reported on the fabrication of piezoelectric ZnO nanogenerators on flexible metal alloy substrate. The vertically aligned ZnO nanowires were synthesized by hydrothermal method and were characterized using FE-SEM. The response of the nanogenerator was studied due to human finger tip impacts as well as ball drop impacts. Results obtained indicate the potential application possibilities of the nanogenerator for energy harvesting and sensing applications. ACKNOLEDGEMENT The authors are grateful to the Center for Nano Science and Engineering (CeNSE), IISc, Bangalore, for providing the fabrication and Characterization facilities. We also would like to thank Mr. Nagendra G. for his kind help in testing of the device as well as necessary electronic circuitry needed. REFERENCES [1] Zhong Lin Wang and Jinhui Song, “Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays”, Science, Vol 312, pp 242-246, 14 April 2006. [2] Shen Xu and Zhong Lin Wang, “One- dimensional ZnO Nanostructures: Solution Grwoth and Functional Properties”,Nano Res, Review Article, 2011. [3] Sheng Xu, Yong Qin, Chen Xu, Yaguang Wei, Rusen Y and Z L Wang, “Self-powered nanowire devices”, Nature nanotechnology ,Vol 5, pp 366373, May 2010. [4] Guang Zhu, Rusen Yang, Sihong Wang, and Z L Wang, “Flexible HighOutput Nanogenerator Based on Lateral ZnO Nanowire Array”, Nano Lett., 10, pp 3151–3155, 2010. [5] Brijesh Kumar and Sang-Woo Kim, “ Energy harvesting based on semiconducting Peizoelectric ZnO nanostructures”, Nano Energy, 1, pp 342– 355, 2012. [6] Youfan Hu,Yan Zhang, Chen Xu, Long Lin, Robert L. Snyder, and Z L Wang, “Self-Powered System with Wireless Data Transmission”,Nano Letters, 11, pp 2572–2577, 2011. [7] Jing-Hua Tian, Jie Hu, Si- Si Li, “Improved seedless hydrothermal synthesis of dense and ultralong ZnO nanowires”, Nanotechnology, 22, 245601, 2011. View publication stats 

A novel piezoelectric ZnO nanogenerator on flexible metal alloy substrate Venkateswarlu Gaddam, Sudeep Joshi, Mitesh Parmar and K. Rajanna Department of Instrumentation and Applied Physics Indian Institute of Science Bangalore, India [email protected], [email protected] Abstract—In this paper, we report a novel piezoelectric ZnO nanogenerator on flexible metal alloy substrate (Phynox alloy) for energy harvesting and sensing applications. The vertically aligned ZnO nanowires are sandwiched between Au electrodes. The aligned growth of ZnO nanowires have been successfully synthesized on Au coated metal alloy substrate by hydrothermal method at low temperature (95±1 °C). The as-synthesized vertically aligned ZnO nanowires were characterized using FESEM. Further, PMMA is spin coated over the aligned ZnO nanowires for the purpose of their long term stability. The fabricated nanogenerator is of size 30mm x 6mm. From energy harvesting point of view, the response of the nanogenerator due to finger tip impacts ranges from 0.9 V to 1.4V. Also for sensing application, the maximum output voltage response of the nanogenerator is found to be 2.86V due to stainless steel (SS) ball impact and 0.92 V due to plastic ball impact. Keywords: ZnO nanowires, nanogenerator, piezoelectricity I. INTRODUCTION Powering nano scale devices is the main challenge in the present day nanotechnology world. The potential power sources for nano devices are vibrational energy, mechanical energy, solar energy, hydraulic energy [1]. Moreover the miniaturization of portable electronic devices is very difficult without batteries. The problems associated with the use of batteries include limitation in the size of the device and frequent recharging process. However in place of batteries, if we adopt nanogenerators, mechanical energy is always available in and around us for powering these nano devices. Nanogenerator is a device which converts random mechanical energy into electrical energy to drive nano scale level devices. 978-1-4577-1767-3/12/$26.00 ©2012 IEEE M.M. Nayak Centre for Nano Science and Engineering Indian Institute of Science Bangalore, India [email protected] Zinc Oxide (ZnO) is one of the promising potential materials for energy harvesting and sensing applications. ZnO is a semiconducting piezoelectric material having energy band gap of 3.37 eV and large excitation binding energy of 60 meV at room temperature [2]. It is a prominent material because of its structural, semiconducting, mechanical and piezoelectric properties. The advantages of ZnO are: it exhibits both semiconducting and piezoelectric properties, environmental friendly, biocompatible, growth occurs on large variety of substrate materials [1, 3]. The applications of ZnO are plenty and it is used in electronic, electrochemical, electromechanical devices such as light– emitting diodes, field emission devices, micro/nano sensors, solar cells, nanopiezotronics and piezoelectric nanogenerators [2]. Various techniques are used for the synthesis of ZnO nanostructures such as physical vapour deposition (PVD), chemical methods, molecular beam epitaxy (MBE), pulsed laser deposition (PLD). Among these techniques, solution based chemical methods are more advantageous because they are simple, convenient, low cost, less hazardous, compatible for flexible substrates, capable of large scaling up and growth occurs at relatively low temperatures [2]. The use of these nanogenerators for sensing applications further eliminate the requirement of batteries and make them single independent system. Hence, in the present work, we have reported piezoelectric ZnO nanowires synthesized on flexible alloy substrate as independent energy harvesting and sensing system. Various literature reports on different kinds of flexible substrate materials (Kapton [4], Polythylene naphthalate (PEN) [5], PS (polyester) [6]) for the fabrication of nanogenerators. To the best of our knowledge, there are no reports on flexible alloy substrates for the nanogenerator fabrication. We are introducing for the first time, a flexible alloy substrate for the fabrication of nanogenerators. II. EXPERIMENTAL Now the tips of nanowires are clean and are ready to use for the top electrode contact purpose. The top electrode of Cr/Au (20/100 nm) was deposited on to the kapton film and was kept upside down over the tips of the nanowires for contact. The schematic diagram and the actual photograph of the fabricated nanogenerator are shown in Fig. 1. and Fig. 2. respectively. A. Deposition of ZnO seedlayer on alloy substrate Phynox metal alloy substrate was properly cleaned with standard cleaning procedure. Cr/Au (20/50 nm) layer was deposited by RF magnetron sputtering over the substrate for the purpose of bottom electrode contact. Later ZnO thin film (100 nm) as a seed layer was deposited over the Cr/Au layer for the growth of ZnO nanowires. B. Synthesis of ZnO nanowires Chemical agents used for this method are Zinc nitrate hexahydrate (Zn(NO3)2.6H2O) and Hexamethylenetetramine ((HMTA ) (CH2)6 N4)). The Zn(NO3)2 salt provides Zn2+ ions for the growth of ZnO nanowires. The HMTA during the ZnO nanowires growth would slowly hydrolyze in water solution and gradually produce OH- ions.The growth process of ZnO nanowires can be controlled by the following chemical reactions [2,7]. (CH2)6N4 + 6 H2O NH3 + H2O NH3.H2O Zn2+ + 2OHZn (OH2) Figure. 1: Schematic diagram of nanogenerator 4 NH3 + 6HCHO NH3.H2O NH4+ + OHZn (OH)2 ZnO + H2O An equi-molar concentration (25 mM) of Zn(NO3)2 and HMTA was used for the growth of nanowires. The substrate was kept upside down over the surface of solution and due to surface tension it was standing on the solution. The growth process was carried out in mechanical hot air oven at low temperature (95±1 °C) for the growth time of 5 hrs. The synthesized nanowires on substrate were cooled at room temperature and cleaned with DI water. Further, the cleaned sample was dried at room temperature Figure. 2: Fabricated nanogenerator on flexible metal alloy substrate . C. Fabrication of nanogenerator In order to fabricate the nanogenerator, 3-4 µm thick PMMA was spin coated over as-synthesized ZnO nanowires. Spin coating was done for the purpose of long term stability of nanowires and mechanical robustness of the complete structure. Also, it helps to prevent possible short-circuiting between the bottom and the top electrodes [3]. Later, oxygen plasma etching was done to etch out the fine thickness of PMMA coating for the purpose of top electrode contact. The depth of etching was 1 - 1.5 µm and the etch time was 90 sec. III. RESULTS AND DISCUSSIONS A. Characterization of ZnO nanowires The morphology of ZnO nanowires grown on alloy substrate was observed using field emission scanning electron microscopy (FE-SEM (Carl Zeiss), ULTRA 55). Fig. 3a shows the top view of the vertically grown ZnO nanowires. Fig. 3b shows the cross sectional view of grown nanowires. The average length and diameter of the synthesized vertical nanowires are found to be 3 µm and 170 nm respectively. Most of the ZnO nanowires are vertically aligned as well as highly dense over the entire surface of the alloy substrate. For the sensing application point of view, stainless steel and plastic balls of equal diameter were used. The balls were dropped from four different random heights (8.5, 11, 16 and 23 cms) on to the nanogenerator. When the balls were dropped from the maximum height of 23 cms, the response of the nanogenerator for stainless steel ball was found to be 2.86 V (Fig. 6) and 0.92 V for plastic ball (Fig. 7). Fig .8 shows the relationship between output voltage and the ball drop height for both the balls. The output voltage generated due to stainless steel ball impact is almost 3 times greater than the output voltage generated due to plastic ball impact. Figure.3: Top view of vertically grown ZnO nanowires Figure .6: Response of the nanogenerator for stainless steel ball impact. Figure.4: Cross sectional view of vertically grown ZnO nanowires B. Performance of fabricated nanogenerator The fabricated nanogenerator was directly connected to CRO (Yokogawa, DLM 2022). In order to see the suitability for energy harvesting application, dynamic force was applied on the fabricated nanogenerator using finger tip impacts. The typical response obtained from the nanogenerator due to the dynamic force of human finger impact is given in Fig. 5. The maximum output voltage obtained is 0.93V. Figure. 7: Response of the nanogenerator for plastic ball impact. Figure 5: Response of the nanogenerator due to finger tip impacts. Figure 8: Output of nanogenerator due to SS ball and plastic ball dropped from different heights. IV. CONCLUSION We have reported on the fabrication of piezoelectric ZnO nanogenerators on flexible metal alloy substrate. The vertically aligned ZnO nanowires were synthesized by hydrothermal method and were characterized using FE-SEM. The response of the nanogenerator was studied due to human finger tip impacts as well as ball drop impacts. Results obtained indicate the potential application possibilities of the nanogenerator for energy harvesting and sensing applications. ACKNOLEDGEMENT The authors are grateful to the Center for Nano Science and Engineering (CeNSE), IISc, Bangalore, for providing the fabrication and Characterization facilities. We also would like to thank Mr. Nagendra G. for his kind help in testing of the device as well as necessary electronic circuitry needed. REFERENCES [1] Zhong Lin Wang and Jinhui Song, “Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays”, Science, Vol 312, pp 242-246, 14 April 2006. [2] Shen Xu and Zhong Lin Wang, “One- dimensional ZnO Nanostructures: Solution Grwoth and Functional Properties”,Nano Res, Review Article, 2011. [3] Sheng Xu, Yong Qin, Chen Xu, Yaguang Wei, Rusen Y and Z L Wang, “Self-powered nanowire devices”, Nature nanotechnology ,Vol 5, pp 366373, May 2010. [4] Guang Zhu, Rusen Yang, Sihong Wang, and Z L Wang, “Flexible HighOutput Nanogenerator Based on Lateral ZnO Nanowire Array”, Nano Lett., 10, pp 3151–3155, 2010. [5] Brijesh Kumar and Sang-Woo Kim, “ Energy harvesting based on semiconducting Peizoelectric ZnO nanostructures”, Nano Energy, 1, pp 342– 355, 2012. [6] Youfan Hu,Yan Zhang, Chen Xu, Long Lin, Robert L. Snyder, and Z L Wang, “Self-Powered System with Wireless Data Transmission”,Nano Letters, 11, pp 2572–2577, 2011. [7] Jing-Hua Tian, Jie Hu, Si- Si Li, “Improved seedless hydrothermal synthesis of dense and ultralong ZnO nanowires”, Nanotechnology, 22, 245601, 2011. 

A novel piezoelectric ZnO nanogenerator on flexible metal alloy substrate Venkateswarlu Gaddam, Sudeep Joshi, Mitesh Parmar and K. Rajanna Department of Instrumentation and Applied Physics Indian Institute of Science Bangalore, India [email protected], [email protected] Abstract—In this paper, we report a novel piezoelectric ZnO nanogenerator on flexible metal alloy substrate (Phynox alloy) for energy harvesting and sensing applications. The vertically aligned ZnO nanowires are sandwiched between Au electrodes. The aligned growth of ZnO nanowires have been successfully synthesized on Au coated metal alloy substrate by hydrothermal method at low temperature (95±1 °C). The as-synthesized vertically aligned ZnO nanowires were characterized using FESEM. Further, PMMA is spin coated over the aligned ZnO nanowires for the purpose of their long term stability. The fabricated nanogenerator is of size 30mm x 6mm. From energy harvesting point of view, the response of the nanogenerator due to finger tip impacts ranges from 0.9 V to 1.4V. Also for sensing application, the maximum output voltage response of the nanogenerator is found to be 2.86V due to stainless steel (SS) ball impact and 0.92 V due to plastic ball impact. Keywords: ZnO nanowires, nanogenerator, piezoelectricity I. INTRODUCTION Powering nano scale devices is the main challenge in the present day nanotechnology world. The potential power sources for nano devices are vibrational energy, mechanical energy, solar energy, hydraulic energy [1]. Moreover the miniaturization of portable electronic devices is very difficult without batteries. The problems associated with the use of batteries include limitation in the size of the device and frequent recharging process. However in place of batteries, if we adopt nanogenerators, mechanical energy is always available in and around us for powering these nano devices. Nanogenerator is a device which converts random mechanical energy into electrical energy to drive nano scale level devices. 978-1-4577-1767-3/12/$26.00 ©2012 IEEE M.M. Nayak Centre for Nano Science and Engineering Indian Institute of Science Bangalore, India [email protected] Zinc Oxide (ZnO) is one of the promising potential materials for energy harvesting and sensing applications. ZnO is a semiconducting piezoelectric material having energy band gap of 3.37 eV and large excitation binding energy of 60 meV at room temperature [2]. It is a prominent material because of its structural, semiconducting, mechanical and piezoelectric properties. The advantages of ZnO are: it exhibits both semiconducting and piezoelectric properties, environmental friendly, biocompatible, growth occurs on large variety of substrate materials [1, 3]. The applications of ZnO are plenty and it is used in electronic, electrochemical, electromechanical devices such as light– emitting diodes, field emission devices, micro/nano sensors, solar cells, nanopiezotronics and piezoelectric nanogenerators [2]. Various techniques are used for the synthesis of ZnO nanostructures such as physical vapour deposition (PVD), chemical methods, molecular beam epitaxy (MBE), pulsed laser deposition (PLD). Among these techniques, solution based chemical methods are more advantageous because they are simple, convenient, low cost, less hazardous, compatible for flexible substrates, capable of large scaling up and growth occurs at relatively low temperatures [2]. The use of these nanogenerators for sensing applications further eliminate the requirement of batteries and make them single independent system. Hence, in the present work, we have reported piezoelectric ZnO nanowires synthesized on flexible alloy substrate as independent energy harvesting and sensing system. Various literature reports on different kinds of flexible substrate materials (Kapton [4], Polythylene naphthalate (PEN) [5], PS (polyester) [6]) for the fabrication of nanogenerators. To the best of our knowledge, there are no reports on flexible alloy substrates for the nanogenerator fabrication. We are introducing for the first time, a flexible alloy substrate for the fabrication of nanogenerators. II. EXPERIMENTAL Now the tips of nanowires are clean and are ready to use for the top electrode contact purpose. The top electrode of Cr/Au (20/100 nm) was deposited on to the kapton film and was kept upside down over the tips of the nanowires for contact. The schematic diagram and the actual photograph of the fabricated nanogenerator are shown in Fig. 1. and Fig. 2. respectively. A. Deposition of ZnO seedlayer on alloy substrate Phynox metal alloy substrate was properly cleaned with standard cleaning procedure. Cr/Au (20/50 nm) layer was deposited by RF magnetron sputtering over the substrate for the purpose of bottom electrode contact. Later ZnO thin film (100 nm) as a seed layer was deposited over the Cr/Au layer for the growth of ZnO nanowires. B. Synthesis of ZnO nanowires Chemical agents used for this method are Zinc nitrate hexahydrate (Zn(NO3)2.6H2O) and Hexamethylenetetramine ((HMTA ) (CH2)6 N4)). The Zn(NO3)2 salt provides Zn2+ ions for the growth of ZnO nanowires. The HMTA during the ZnO nanowires growth would slowly hydrolyze in water solution and gradually produce OH- ions.The growth process of ZnO nanowires can be controlled by the following chemical reactions [2,7]. (CH2)6N4 + 6 H2O NH3 + H2O NH3.H2O Zn2+ + 2OHZn (OH2) Figure. 1: Schematic diagram of nanogenerator 4 NH3 + 6HCHO NH3.H2O NH4+ + OHZn (OH)2 ZnO + H2O An equi-molar concentration (25 mM) of Zn(NO3)2 and HMTA was used for the growth of nanowires. The substrate was kept upside down over the surface of solution and due to surface tension it was standing on the solution. The growth process was carried out in mechanical hot air oven at low temperature (95±1 °C) for the growth time of 5 hrs. The synthesized nanowires on substrate were cooled at room temperature and cleaned with DI water. Further, the cleaned sample was dried at room temperature Figure. 2: Fabricated nanogenerator on flexible metal alloy substrate . C. Fabrication of nanogenerator In order to fabricate the nanogenerator, 3-4 µm thick PMMA was spin coated over as-synthesized ZnO nanowires. Spin coating was done for the purpose of long term stability of nanowires and mechanical robustness of the complete structure. Also, it helps to prevent possible short-circuiting between the bottom and the top electrodes [3]. Later, oxygen plasma etching was done to etch out the fine thickness of PMMA coating for the purpose of top electrode contact. The depth of etching was 1 - 1.5 µm and the etch time was 90 sec. III. RESULTS AND DISCUSSIONS A. Characterization of ZnO nanowires The morphology of ZnO nanowires grown on alloy substrate was observed using field emission scanning electron microscopy (FE-SEM (Carl Zeiss), ULTRA 55). Fig. 3a shows the top view of the vertically grown ZnO nanowires. Fig. 3b shows the cross sectional view of grown nanowires. The average length and diameter of the synthesized vertical nanowires are found to be 3 µm and 170 nm respectively. Most of the ZnO nanowires are vertically aligned as well as highly dense over the entire surface of the alloy substrate. For the sensing application point of view, stainless steel and plastic balls of equal diameter were used. The balls were dropped from four different random heights (8.5, 11, 16 and 23 cms) on to the nanogenerator. When the balls were dropped from the maximum height of 23 cms, the response of the nanogenerator for stainless steel ball was found to be 2.86 V (Fig. 6) and 0.92 V for plastic ball (Fig. 7). Fig .8 shows the relationship between output voltage and the ball drop height for both the balls. The output voltage generated due to stainless steel ball impact is almost 3 times greater than the output voltage generated due to plastic ball impact. Figure.3: Top view of vertically grown ZnO nanowires Figure .6: Response of the nanogenerator for stainless steel ball impact. Figure.4: Cross sectional view of vertically grown ZnO nanowires B. Performance of fabricated nanogenerator The fabricated nanogenerator was directly connected to CRO (Yokogawa, DLM 2022). In order to see the suitability for energy harvesting application, dynamic force was applied on the fabricated nanogenerator using finger tip impacts. The typical response obtained from the nanogenerator due to the dynamic force of human finger impact is given in Fig. 5. The maximum output voltage obtained is 0.93V. Figure. 7: Response of the nanogenerator for plastic ball impact. Figure 5: Response of the nanogenerator due to finger tip impacts. Figure 8: Output of nanogenerator due to SS ball and plastic ball dropped from different heights. IV. CONCLUSION We have reported on the fabrication of piezoelectric ZnO nanogenerators on flexible metal alloy substrate. The vertically aligned ZnO nanowires were synthesized by hydrothermal method and were characterized using FE-SEM. The response of the nanogenerator was studied due to human finger tip impacts as well as ball drop impacts. Results obtained indicate the potential application possibilities of the nanogenerator for energy harvesting and sensing applications. ACKNOLEDGEMENT The authors are grateful to the Center for Nano Science and Engineering (CeNSE), IISc, Bangalore, for providing the fabrication and Characterization facilities. We also would like to thank Mr. Nagendra G. for his kind help in testing of the device as well as necessary electronic circuitry needed. REFERENCES [1] Zhong Lin Wang and Jinhui Song, “Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays”, Science, Vol 312, pp 242-246, 14 April 2006. [2] Shen Xu and Zhong Lin Wang, “One- dimensional ZnO Nanostructures: Solution Grwoth and Functional Properties”,Nano Res, Review Article, 2011. [3] Sheng Xu, Yong Qin, Chen Xu, Yaguang Wei, Rusen Y and Z L Wang, “Self-powered nanowire devices”, Nature nanotechnology ,Vol 5, pp 366373, May 2010. [4] Guang Zhu, Rusen Yang, Sihong Wang, and Z L Wang, “Flexible HighOutput Nanogenerator Based on Lateral ZnO Nanowire Array”, Nano Lett., 10, pp 3151–3155, 2010. [5] Brijesh Kumar and Sang-Woo Kim, “ Energy harvesting based on semiconducting Peizoelectric ZnO nanostructures”, Nano Energy, 1, pp 342– 355, 2012. [6] Youfan Hu,Yan Zhang, Chen Xu, Long Lin, Robert L. Snyder, and Z L Wang, “Self-Powered System with Wireless Data Transmission”,Nano Letters, 11, pp 2572–2577, 2011. [7] Jing-Hua Tian, Jie Hu, Si- Si Li, “Improved seedless hydrothermal synthesis of dense and ultralong ZnO nanowires”, Nanotechnology, 22, 245601, 2011. View publication stats