www.afm-journal.de FULL PAPER © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2010, XX, 1–6 1 www.MaterialsViews.com wileyonlinelibrary.com By Mohammed Riaz, Jinhui Song, Omer Nur, Zhong Lin Wang, and Magnus Willander* 1. Introduction Electrical and mechanical coupling phenomena is a feature of some inorganic, organic, and biological systems. The simplest example of linear electromechanical coupling is the piezoelec- tricity. Among the tetrahedral coordinated wurtzite semicon- ductors, GaN and ZnO are typical examples. [1] ZnO can be grown in a reproducible way in a variety of nanostructures e.g., nanowires, nanorings, nanobows, platelet circular structures, Y-shape split ribbons and crossed ribbons. This variety can be unique for many applications in nanotechnology. [2] This makes ZnO a technologically important material in many practical applications that requires a large electromechanical coupling. The growth methods [3–5] and electrical and mechanical proper- ties of ZnO nanostructures [6–8] have been extensively studied and by now they are well known to the research community. [9–11] On the other hand, ZnO nanowires have demonstrated sucess in various applications e.g. opto- electronics, [12] biosensors, [13] resonators, [14] electric nanogenerators, [15] and nanola- sers. [16] Wang et al. developed a variety of piezoelectric nanogenerators with potential applications for self-powered nanodevices/ nanosystems. [17–21] Nevertheless, nano- generators made of ZnO nanostructures require more research to optimize the electromechanical coupling, thus leading to higher output power. In this report, a study and comparison of piezoelectric nanogenerators using ZnO nanowires grown on different substrates using low as well as high temperature growth methods is presented. The output potential from each nanogenerator was measured and compared under the same conditions. Finite element method calcula- tion was used to investigate the output voltage signal generated when having ZnO nanowires with different aspect ratios. 2. Results and Discussion Surface morphology and size distribution of the ZnO nanowires were characterized using LEO 1550 scanning electron micro- scope (SEM). Figure 1 shows typical SEM images of the ZnO nanowires grown by both the vapor liquid solid (VLS) and aqueous chemical growth (ACG) methods. Figure 1A,B show SEM images of ZnO nanowires grown on SiC substrate by the VLS method and Figure 1C,D show the case of ZnO nanowires grown on SiC and Si substrates by the ACG method. As can be seen, the nanowires are hexagonally arranged on the substrate. The approximate diameter, length and density of the nanowires were determined to be about 50–100 nm, 3–5 μm, and 3.369 × 10 7 cm -2 on the SiC substrates (Figure 1A) (grown by the VLS method), 200–400 nm, 3–4 μm, and 1.22 × 10 7 cm -2 on the SiC substrates (Figure 1B) (grown by VLS method), 100 nm, 1–2 μm, and 40.50 × 10 8 cm -2 on the SiC substrates (Figure 1C) (grown by ACG method) and 100 nm, 1–2 μm, and 24.0 × 10 8 cm -2 on the Si substrate (Figure 1D) (grown by the ACG method), respectively. It was found that the ZnO nanowires were vertically aligned along the c-axis and distributed uni- formly across the entire substrates. An atomic force microscopy (AFM) Molecular Force Probe MFP-3D from Asylum Research was used to perform the Study of the Piezoelectric Power Generation of ZnO Nanowire Arrays Grown by Different Methods DOI: 10.1002/adfm.201001203 The piezoelectric power generation from ZnO nanowire arrays grown on different substrates using different methods is investigated. ZnO nanowires were grown on n-SiC and n-Si substrates using both the high-temperature vapor liquid solid (VLS) and the low-temperature aqueous chemical growth (ACG) methods. A conductive atomic force microscope (AFM) is used in contact mode to deflect the ZnO nanowire arrays. No substrate effect was observed but the growth method, crystal quality, density, length, and diameter (aspect ratio) of the nanowires are found to affect the piezoelectric behavior. During the AFM scanning in contact mode without biasing voltage, the ZnO nanowire arrays grown by the VLS method produced higher and larger output voltage signal of 35 mV compared to those grown by the ACG method, which produce smaller output voltage signal of only 5 mV. The finite element (FE) method was used to investigate the output voltage for different aspect ratio of the ZnO nanowires. From the FE results it was found that the output voltage increases as the aspect ratio increases and starts to decreases above an aspect ratio of 80 for ZnO nanowires. [∗] Dr. M. Riaz, Dr. O. Nur, Prof. M. Willander Department of Science and Technology Campus Norrköping Linköping University SE-601 74 Norrköping, Sweden Email: magwi@itn.liu.se Dr. J. Song, Prof. Z. L. Wang School of Materials Science and Engineering Georgia Institute of Technology Atlanta, GA, 30332–0245, USA