enhanced charge generation of the zno nanowirespzt hetero-junction based

Enhanced charge generation of the ZnO nanowires/PZT hetero-junction based nanogenerator Im-Jun No a , Dae-Yong Jeong b , Sunwoo Lee c , Sung-Hyun Kim d , Jin-Woo Cho d , Paik-Kyun Shin a, ⇑ a Department of Electrical Engineering, INHA University, Incheon City 402-751, Republic of Korea b School of Material Science and Engineering, INHA University, Incheon City 402-751, Republic of Korea c Department of Electrical Engineering, INHA Technical College, Incheon City 402-752, Republic of Korea d Energy Nano Material Center, Korea Electronics Technology Institute, Seongnam City 463-816, Republic of Korea article info Article history: Available online 26 February 2013 Keywords: ZnO PZT Piezoelectricity Hetero-junction Nanogenerator abstract We fabricated an alternative nanogenerator device with distinguished structure. Representative piezo- electric materials of ZnO nanowires and PZT thin films were tried to be combined to form a hetero-junc- tion structure for fabrication of an alternative nanogenerator device to possibly obtain a synergy effect and then improved performance. The ZnO nanowires were grown by a hydrothermal synthesis technique and then PZT thin films were deposited on the surface of the ZnO nanowires by rf magnetron sputtering process. The PZT thin films were annealed to be crystallized with different conditions for post-deposition thermal treatment process. The hetero-junction structure was polarized by a Corona poling process to obtain a unidirectional orientation of dipole moments to enhance their piezoelectric property. Structure and morphology of the grown ZnO nanowires were investigated to achieve appropriate characteristics to achieve performance improvement for the resulting nanogenerator device. To confirm effect of the het- ero-junction structure on improvement of power generation performance of the resulting nanogenerator device, current generating properties were comparatively investigated with those of nanogenerator device with only ZnO nanowires or PZT thin films as active piezoelectric component, respectively. The nanogenerator device with a hetero-junction structure of ZnO nanowires/PZT revealed distinctively improved average currents of 270 nm, which is quite higher than those of the nanogenerator devices with pristine ZnO nanowires, or with pristine PZT thin films, respectively. Possible factors contributed to improvement of the current generation properties were discussed for the presented nanogenerator device. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Recent advances of technologies have been accompanied by drastic increase of energy consumption and various kinds of re- ports have been devoted for introduction of alternative energy gen- eration technologies. Technologies of green energy harvesting and self-powered energy sources would be one of those promising methodologies for the alternative energy sources. Among them, piezoelectric materials have been studied to be prepared by nano- technologies for possibly enhancing their piezoelectric effect in various energy-harvesting devices. A new power-generator device applying enhanced charge carrier generation of one dimensional piezoelectric nanomaterials has been reported as so-called nanopi- ezoelectronics by numerous research groups including the Wang group at the Georgia Institute of Technology [1–4]. The nanopiezo- electronics are based on an energy conversion mechanism of nano- generator device, by which mechanical energies are converted to an electrical energy. To date, ZnO, PZT, and BaTiO 3 are representa- tive materials for preparation of one-dimensional structure to combine piezoelectricity and semiconducting property for obtain- ing a synergy effect to enhance power generation performance [4– 6]. Up to now, the Prof. Wang and his group have reported several research reports for nanopiezoelectronics using combination of ZnO and PZT nanowires [7–10]. Nanogenerators based on ZnO nanowires can be prepared relatively easily by hydrothermal syn- thesis process at low temperature, and their power generation per- formance could be improved by controlling material properties of grown crystalline nanowires as well as by alteration of those de- vice structure designs. However, the base material of ZnO has rel- atively low piezoelectric coefficient of $12pC/N, and hence, realization of a high performance nanogenerator using ZnO nano- wire might be quite limited. To overcome such shortage of material property of ZnO, material property alteration via appropriate impurity doping and/or multi-layer stacking using different mate- rial with higher piezoelectric coefficient could be tried to obtain higher power generation performance. On the other hand, nano- wires of crystalline PZT could be prepared by hydrothermal 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.01.058 ⇑ Corresponding author. Tel.: +82 32 860 7402; fax: +82 32 863 5822. E-mail address: shinsensor@inha.ac.kr (P K. Shin). Microelectronic Engineering 110 (2013) 282–287 Contents lists available at SciVerse ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee synthesis technique to achieve high power generation performance utilizing higher piezoelectric coefficient of PZT. However, hydro- thermal synthesis of PZT might be quite difficult due to several limitation factors: choice of appropriate substrate; deteriorated reproducibility; complicated synthesis process. Therefore, several approaches have been tried for realization of ZnO/PZT hetero-junc- tion structure [11–13]. Those hetero-junctions have been mainly either a composite of power-to-powder, or a stacking of layer-to- layer [14–16]. Distinct enhancement of polarization characteristic and capacitance value have been reported up to now [17,18]. Nev- ertheless, practical application for realization of those electrical characteristic has not yet been reported for nanogenerator devices based on those studies [17]. This study aims for realization of a new hetero-junction structure of ZnO nanowires and PZT thin films to fabricate a nanogenerator device to achieve a synergy effect, by which disadvantages of each material could be minimized and improved power generation performance could be achieved. Electrical characteristics of the resulting nanogenerator devices were studied and improved power generation performance was discussed to be confirmed. 2. Experimental Commercially available ITO coated glass (Corning; 200 nm thick ITO) substrates were used. The substrates were cleansed by using conventional semiconductor cleansing process and then seed lay- ers were coated on the ITO coated glass substrates by rf magnetron sputtering process using an Al-doped ZnO target (AZO; 2 wt.% Al + 98 wt.% ZnO). Material characteristic of the seed layer is criti- cal for density and diameter of synthesized nanowires [19,20]. Seed layer of a 40-nm-thick AZO thin film was prepared in this re- port to minimize density of the grown nanowires and to obtain diameter of those as smaller than 50 nm. The thickness was decided after experimental results. Fig. 1. FE-SEM image of the ZnO nanowires grown by hydrothermal synthesis. Fig. 2. TEM image (a) and SAED pattern (b) of the ZnO nanowires grown by hydrothermal synthesis. Fig. 3. SEM image of the hetero-junction structure of ZnO nanowires/PZT. I J. No et al. /Microelectronic Engineering 110 (2013) 282–287 283 2.1. Synthesis of ZnO nanowires by hydrothermal technique ZnO nanowires were grown by a hydrothermal technique. A reaction solution was prepared for the hydrothermal synthesis of ZnO nanowires by adding the following ingredients in de-ionized (DI) water and then stirring: 0.015 mol/L of zinc nitrate hexahy- drate (Zn(NO 3 ) 2 Á6H 2 O); 0.015 mol/L of hexamethylenetetramine (HMTA); 0.09 mol/L of ammonium chloride; 0.003 mol/L of poly- ethyleneimine (PEI). Seed layer coated ITO-glass substrates were immersed in the prepared reaction solution (400 ml) contained in cultivation bottle, and then the cultivation bottle was immersed for 3 h in a water bath of constant temperature of 90 °C. The sub- strates with grown ZnO nanowires were then cleansed carefully with ethanol, so that tips of the thin ZnO nanowires would not be tied by possible capillary force. Then the ZnO nanowires were then cleansed ultrasonically to remove residues which are possibly remained during the hydrothermal synthesis. 2.2. Preparation of PZT thin films An rf magnetron sputtering process was used to deposit PZT thin film on top of the prepared ZnO nanowires. A self-made 2 inch target of Pb(Zr 0.52 Ti 0.48 )O 3 was used for the sputter deposition of the PZT thin film. Deposition rate is a critical process condition regarding extremely narrow gaps of the underlying ZnO nano- wires. Inferior step coverage of the being deposited PZT thin film due to inappropriate deposition rate should be prevented, so that the PZT thin films could cover entire surface of the ZnO nanowires. Fig. 4. XRD patterns for PZT thin films annealed at different temperatures and for different annealing times: (a) annealing temperature; (b) annealing time. 284 I J. No et al. /Microelectronic Engineering 110 (2013) 282–287 A 1.5- l m-thick PZT thin film was deposited with a deposition rate of 80 Å/min. The deposited PZT thin films were then thermally treated in an electric oven, so that an amorphous phase PZT thin film could be crystallized. The post-deposition annealing was car- ried out by using a PLC control with three steps: (1) Step 1: up to 600 °C with temperature elevation rate of 8.3 °C/min; (2) Step 2: constant temperature of 600 °C for 1 h; (3) Step 3: cooling to room temperature with temperature descending rate of 8.3 °C/ min. After the post-deposition annealing, the PZT thin film depos- ited on the ZnO nanowires were then polarized by a Corona poling process regarding the device structure in our study. Voltage of 11 kV was applied during the poling process for 30 min and the substrate was heated at a constant temperature of 80 °C to achieve a homogeneous aligning of dipoles in crystallized PZT molecules. Finally, a thin Pt upper electrode was coated to fabricate a nano- generator device: Pt/PZT-ZnO nanowires/ITO. 2.3. Analysis for characterization of ZnO nanowires, PTZ thin films, and nanogenerator device Structure of the PZT thin films were investigated by X-ray dif- fraction (XRD). Field-emission scanning electron microscopy (FE- SEM) was used to investigate the structure of the nano power-gen- erator device. Structural characteristic of the ZnO nanowires were investigated by selected area diffraction (SAED), energy dispersive spectrometer (EDS), and tunneling electron microscopy (TEM). To investigate power generation characteristic, a fixed force of 0.9 kgf was applied to the nanogenerator device by a linear motor and current generating characteristics were measured with a picoammeter (Keithley 6485). 3. Results and discussions In this study, the hydrothermally synthesized ZnO nanowires might have two functions: (1) charge transfer layer for appropri- ate transportation of charge carriers generated by piezoelectric effect to electrode; (2) additional piezoelectric element to con- vert mechanical energy to electrical energy. Moreover, the underlying one-dimensional ZnO nanowires were thought to be served as a template for the overlying PZT thin films to have lar- ger surface area like a pseudo one-dimensional structure, so that the PZT would reveal more enhanced piezoelectric charge gener- ation property due to enhanced elasticity. Fig. 1 shows a FE-SEM image of the hydrothermally grown ZnO nanowires, which re- veals vertically grown ZnO nanowires perpendicular to substrate surface. Tips of the ZnO nanowires reveal no tied feature due to capillary force, and residue from the hydrothermal process can- not be observed, which could be contributed to a defect in the resulting nanogenerator device. The grown ZnO nanowires reveal length of approximately 3 l m and average diameter of 50 nm. In general, hydrothermally grown ZnO nanowires have been known to have crystallinity of either poly-crystalline or single- crystalline [21,22]. To investigate crystal characteristic of the ZnO nanowires grown in this study, TEM and SAED analyses were car- ried out and Fig. 2 shows the results. Two sets of lattices with dis- tances of 0.264 nm and 0.484 nm can be observed for the hydrothermally grown ZnO nanowires. The TEM image shown in the Fig. 2(a) reveal a well-oriented crystal structure for the grown ZnO nanowires. The SAED pattern presented in the Fig. 2(b) con- firms that the grown ZnO nanowires have single crystalline struc- ture with relatively high crystallinity. A Fig. 5. A schematic of the power generation performance evaluation setup. I J. No et al. /Microelectronic Engineering 110 (2013) 282–287 285 Crystalline characteristic of ZnO nanowires is especially essen- tial in this study, because effective piezoelectric conversion could hardly be obtained for ZnO nanowires with amorphous or inferior crystalline property: (1) piezoelectric field would be formed by io- nic charges due to polarization of atoms in lattice; (2) charge car- riers generated by piezoelectric mechanism could not be effectively transferred to electrode. The superior crystalline prop- erties of the ZnO nanowires confirmed in the Fig. 2 might serve for eventual improvement of power generation performance for the resulting nanogenerator device. In addition, step-coverage of the deposited PZT thin films on the ZnO nanowires would have also an important role to achieve im- proved performance for the resulting nanogenerator device. Fig. 3 shows a SEM image of the PZT coated ZnO nanowires. Observing the cross-sectional image of the hetero-junction of ZnO nano- wires/PZT, the sputter deposited PZT thin films cover almost entire surface of the underlying ZnO nanowires even in those of bottom area, although some minute portion of the bottom area is not still covered by the PZT thin film, which could possibly influence the current generation property of the resulting nanogenerator. It could be expected that the obtained step-coverage of the PZT thin films would contribute to a wider surface area of the PZT due to underlying ZnO nanowires possibly working as a template, so that quantity of generated charge carrier could be increased. In this study, the deposited PZT thin film was annealed by post- deposition thermal treatment in an electric oven in order to im- prove crystallinity of the as-deposited PZT, which normally still in an amorphous state. To obtain an appropriate annealing condi- tion, temperature and treatment time were varied for the post- deposition thermal treatment. Fig. 4 with XRD patterns shows ef- fect of the temperature and treatment time on crystalline proper- ties of the resulting PZT thin films. It can be confirmed that the PZT thin films prepared in this study has a typical perovskite crystalline structure. The XRD pattern shown in the Fig. 4(a) reveal that crys- tallization of the PZT thin films crystallization is started at anneal- ing temperature of 500 °C. Higher crystallinity could be expected for higher annealing temperature, but a moderate annealing tem- perature of 600 °C was decided in this study regarding following factors: (1) the substrate (Corning glass) used in this study could not sustain high temperature of over 700 °C; (2) property of the lower electrode (ITO) could be eventually degraded at higher temperature. Fig. 4(b) shows then influence of annealing time on crystalline properties of the PZT thin films at the annealing tem- perature of 600 °C. It confirms that annealing time would have no impact on the crystalline properties of the PZT thin films. There- fore, an annealing time of 1 h was decided for the post-deposition thermal treatment for PZT thin films to fabricate the nanogenerator device with hetero-junction of ZnO nanowires/PZT. Dipole moments of grain domains in crystallized PZT are nor- mally not unidirectional. A poling process for unidirectional orien- tation of the dipole moments is therefore inevitable: anisotropic orientation of dipole moments ( P P s =0;P s = spontaneous polariza- tion) before poling can be changed to isotropic after appropriate poling process [23]. Two kinds of typical poling process can be 02468101214 0 100 200 300 400 Current [nA] Time [s] (a) (b) (c) 0 2 4 6 8 10 12 14 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Current [nA] Time [s] 02468101214 0 2 4 6 8 10 12 14 16 Current [nA] Time [s] Fig. 6. Current generation property of the nanogenerator devices: (a) device with pristine ZnO nanowires; (b) device with pristine PZT thin film; (c) device with hetero- junction structure of ZnO nanowires/PZT. 286 I J. No et al. /Microelectronic Engineering 110 (2013) 282–287 used: (1) poling with direct electrode contacting; (2) Corona pol- ing. For thin films and bulk type piezoelectric materials, direct electrode contact method is favorable for unidirectional orienta- tion of the dipole moments. However, the piezoelectric material prepared in thin study has rather a nearly one-dimensional struc- ture, and the direct electrode contact poling would cause non-uni- form distribution of electric field or breakdown. Therefore, a Corona poling method was used in this study for poling the het- ero-junction of ZnO nanowires/PZT. Fig. 5 shows a schematic of the power generation evaluation setup used for the nanogenerator device fabricated in this study. The samples were shielded from outside interference in a shield box. Spacer between the bottom and top electrode can be served as a mechanical buffer at applied force, so that the sample being measured could be protected from mechanical damage and a space could be secured for the measurement. A defined stable mechani- cal force was applied by a linear motor successively, and generated strain of active layer due to applied mechanical force would be re- sulted in a chare generation. The generated charges could be trans- ferred and collected to electrodes. Fig. 6 shows current generating properties of three different nanogenerator devices. Current generating properties of ZnO nano- wires based nanogenerator (Fig. 6a) and PZT based nanogenerator (Fig. 6b) were also investigated to compare the performance of the nanogenerator with hetero-junction structure of ZnO nanowires/ PZT (Fig. 6c): The same process conditions for ZnO nanowires and PZT thin films were used. The nanogenerator with ZnO nanowires shows the smallest average currents of 0.5 nA, and the nanogenerator with PZT thin films shows average currents of approximately 9 nA. On the contrary, the nanogenerator with the hetero-junction structure of ZnO nanowires/PZT reveals distinc- tively improved average currents of 270 nA. The quite improved current generating property could be caused firstly by effects reported for hetero-junction structure based nanogenerators through enhanced polarization and capacitance characteristics [11,16,18]. Secondly, a possible increase of surface area of PZT thin films could be contributed to increase of generated currents. Compared to a two-dimensional structure of thin film piezoelectric material, one-dimensional piezoelectric structure could have increased surface area. It could be thought that the PZT thin films could be quasi one-dimensional possibly by a tem- plate effect of the underlying ZnO nanowires, which might have quite larger surface area comparable to a one-dimensional nano- wires. The increased surface area of the piezoelectric PZT layer would produce increased charge carriers, and hence, currents pro- portional to increased surface area. Thirdly, a larger energy genera- tion could be possible for the quasi one-dimensional PZT piezoelectric thin film coated on the ZnO nanowires than that of PZT thin films with flat surface structure, because smaller applied force could attribute to larger deformation. It is generally known that current generation in piezoelectric material is proportional to applied strain rate: i / d @F @t ¼ dAE @  @t i = current, d = charge constant, E = Young’s modulus of piezoelectric element, F = applied load, A = load area and e = strain [24,25]. 4. Summary and conclusions In this report, we tried to fabricate an alternative high perfor- mance nanogenerator device with distinguished structure. Repre- sentative piezoelectric materials of ZnO nanowires and PZT thin films were tried to be combined to form a hetero-junction struc- ture for fabrication of alternative nanogenerator device to possibly obtain a synergy effect and then improved performance. The ZnO nanowires were grown by a hydrothermal synthesis technique and then PZT thin films were deposited on the surface of the ZnO nanowires with careful consideration of step-coverage, so that the deposited PZT thin films would have a quasi one-dimensional structure with increased surface area due to a template effect of the underlying ZnO nanowires. The PZT thin films were annealed to be crystallized and the hetero-junction structure was polarized by a Corona poling process to obtain a unidirectional orientation of dipole moments to enhance their piezoelectric property. The grown ZnO nanowires were confirmed to have well-oriented single crystalline structure and vertically preferred growth features. The PZT thin films prepared by rf magnetron sputtering process re- vealed a typical perovskite structure after post-deposition anneal- ing treatment. The nanogenerator device with a hetero-junction structure of ZnO nanowires/PZT revealed distinctively improved average currents of 270 nA, which is quite higher than average cur- rents of 0.5 nA for the nanogenerator device with pristine ZnO nanowires and average currents of 9 nA for the nanogenerator de- vice with pristine PZT thin films as active piezoelectric component, respectively. It was confirmed that the concept and preparation processes for the hetero-junction structure of ZnO nanowires/PZT presented in this report was promising for performance improve- ment of nanogenerator device. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (No. 2012- 0001596). One of the authors (D Y. 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Observing the cross-sectional image of the hetero-junction of ZnO nano- wires/PZT, the sputter deposited PZT thin films cover almost entire surface of the underlying ZnO nanowires