The well-aligned and vertical ZnO nanowires were grown on PET substrates by the hydrothermal method. The novel piezoelectric nanogenerator was fabricated from ZnO nanowires and a Pt/ZnO nanowire electrode on a flexible PET substrate. A sample was compressed and bent generating internal stress in the PET substrate, which output a current of approximately 5 × 10 −10 A without a source of vibration. The sample with 2% bending was also measured with vibration at a low frequency, yielding a maximum piezoelectric current of about 2.5 × 10 −7 A, which is 4 times the current of a nonbending sample. These results demonstrate that a little bending of a flexible substrate improves piezoelectric performance.
■ INTRODUCTIONZinc oxide (ZnO) is one of most promising II−VI compound semiconductor materials, because it has a direct band gap of 3.37 eV, a high-exciton binding energy of 60 meV, and a high electron mobility of around 100 cm 2 V −1 s −1 at room temperature. One-dimensional (1D) ZnO nanostructures can now be formed with favorable physical and chemical properties. The ZnO nanostructure has a hexagonal wurtzite lattice and exhibits favorable piezoelectricity as a result of the built-in polarization along its c-axis. Numerous exciting ZnO nanowire applications are currently available. They include the ultraviolet (UV) nanolaser, 1 field-effect transistor, 2 solar cell, 3 gas sensor, 4 UV photodetector, 5 light emitting diode, 6 and nanogenerator. 7−11 The nanogenerator is attracting substantial attention from researchers because ZnO exhibits highly efficient piezoelectricity. The function of the nanogenerator is to convert mechanical energy to electricity coupled semiconducting and piezoelectric properties.Electronic products are being miniaturized to improve portability. In nanoscale devices, the sizes of traditional rechargeable and replaceable batteries are limited. The development of a nanogenerator to convert the available form of energy from the environment into electric energy would facilitate the development of nanodevices. Such a nanogenerator can be used to develop a battery-less system for future applications. In recent years, various articles have reported upon the piezoelectric properties of ZnO nanowires, but most have focused on single ZnO nanowires or nanorods. 9,10 A few investigations have discussed ZnO nanowire arrays with glass, sapphire, Si, or other hard substrates. 9−11 However, such hard substrates suffer from the fact that it is difficult to enhance the piezoelectric effect by transferring the mechanical energy into electrical energy. Therefore, research groups have exploited the piezoelectric effect of various flexible substrates, such as paper, zinc foil, and others. 8,12 However, these substrates tend to suffer from cracking. This study suppresses cracking using a PET (polyethylene terephthalate) substrate, which is transparent, can be formed into large-area substrates, is flexible, and has a low cost. A PET substrate has a high chemical resistance and is not very effective at high temperatures...