Author's personal copy A large area bimaterial sheet of piezoelectric nanogenerators for energy harvesting: Effect of RF sputtering on ZnO nanorod Chun-Jie Chang a , Yi-Huan Lee a , Chi-An Dai a,b,⇑ , Chih-Chung Hsiao b , Shuh-Heng Chen c , Ni Putu Dewi Nurmalasari c , Jyh-Chien Chen f , Yao-Yi Cheng g , Wen-Pin Shih d , Pei-Zen Chang e a Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106, Taiwan b Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan c Department of Product Development, Taiwan Textile Research Institute, Taipei County 236, Taiwan d Department of Mechanical Engineering, National Taiwan University, Taipei 106, Taiwan e Institute of Applied Mechanics, National Taiwan University, Taipei 106, Taiwan f Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan g Institute of Organic and Polymeric Materials, National Taipei University of Technology, Taipei 106, Taiwan article info Article history: Available online 7 December 2010 Keywords: ZnO nanorods RF magnetron sputtering system Current-generating nanodevices Nanodevice Energy harvesting abstract In this work, we have developed an optimal radio frequency (RF) sputtering process (sputtering power, sputtering time, and target type) to grow a ZnO seed layer on an indium tin oxide (ITO) glass followed by the hydrothermal growth of a well-aligned perpendicular ZnO nanorod array for the fabrication of a piezoelectric nanogenerator. The change in nanorod morphology, crystallinity, orientation, growth rate, diameter, length, and number density of ZnO nanorods were controlled by modifying various processing parameters. A prior seeding of ZnO nanoparticle with good wurtzite structure and with preferred orien- tation along the (0 0 2) direction of the crystal is critical for the subsequent growth of well-aligned ver- tically oriented ZnO nanorods on ITO substrates. A large area (2  2 cm 2 ) of the piezoelectric nanogenerator bimaterial sheet was fabricated by combining the vertically grown ZnO nanorod array electrode with an Au-coated polyurethane (PU) film as the other electrode. By rubbing the two elec- trodes together, the nanodevice generates an optimal output current density of about 1 lA/cm 2 . The result demonstrates that the optimal seeding process and shows the potential to convert the mechanical energy to electrical current generation as a nanodevice. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Nanopiezotronics, a hot research subject in recent years, refers to a field of nanotechnology with numerous applications that in- clude piezoelectric field-effect transistors and diodes, self-pow- ered nanogenerators and biosystems, and wireless nano/ biosensors [1–4]. Based on the coupled piezoelectric and semicon- ducting characteristics, it is possible for nanostructure materials to generate rectifying piezoelectric current under applied external mechanical energies, such as body movement (handling, winding, pushing, and bending), vibrations (acoustic and ultrasonic waves), and rolling force. Among the known one-dimensional materials, ZnO has attracted an extensive research interest for use in electro- mechanical, optical and biomedical devices as a result of its versa- tile characteristics, such as coupled piezoelectric-semiconducting, biocompatible, low temperature and large-area fabrication. It has been reported that the properties of ZnO are closely dependent on their dimensions and morphologies, including crystal size, aspect ratio, crystalline density and orientation. Furthermore, highly oriented ZnO nanorods/nanowires are demonstrated to have a superior device performance as compared to nonaligned ZnO nanostructures [5–7]. Recently, Wang et al. reported that they could convert nanoscale mechanical energy into electrical energy by means of piezoelectric ZnO nanowire arrays, and developed the concept and technology of nanogenerator (NGs) for harvesting mechanical energy. The field of nanopiezotronics has now begun to attract many efforts to exploiting coupled piezoelectric-semi- conducting properties of ZnO for fabricating novel electronic de- vices. In this study, we present a simple, low-cost approach that converts low frequency (<10 Hz) friction energy into electricity using a current-generating nanodevice integrated with piezoelec- tric ZnO nanorods grown on the ITO substrate and an Au-coated polyurethane (PU) film as a top electrode. 0167-9317/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2010.12.010 ⇑ Corresponding author at: Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan. Tel.: +886 2 3366 3051; fax: +886 2 2362 3040. E-mail address: polymer@ntu.edu.tw (C.-A. Dai). Microelectronic Engineering 88 (2011) 2236–2241 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee