Piezoelectric Nanostructures: From Growth Phenomena to Electric Nanogenerators
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Nanostructures: From Growth Phenomena to Electric Nanogenerators Zhong Lin Wang
Abstract Zinc oxide is a unique material that exhibits semiconducting, piezoelectric, and pyroelectric multifunctionalities. By controlling the size and orientation of the polar surfaces of ZnO nanobelts, single-crystal nanocombs, nanorings, nanohelices, nanosprings, and nanobows of ZnO have been synthesized. This article centers on the fundamental growth mechanism and fabrication of electromechanical devices based on piezoelectric ZnO nanostructures, including a nanogenerator using aligned ZnO nanowires for converting nanoscale mechanical energy into electric energy. The mechanism of the electric nanogenerator relies on the unique coupling of the piezoelectric and semiconducting properties of ZnO, which is the fundamental principle of nano-piezotronics, a new field using the piezoelectric effect for fabricating electronic devices and components. The approach has the potential of converting biological mechanical energy, acoustic/ultrasonic vibration energy, and biofluid hydraulic energy into electricity, demonstrating a new pathway for self-powering of wireless nanodevices and nanosystems.
Introduction Since the first synthesis of oxide nanobelts in 2001,1 research in functional oxide-based one-dimensional (1D) nanostructures has rapidly expanded due to their potential applications in optics, optoelectronics, catalysis, and piezoelectricity. Semiconducting oxide nanobelts are a unique group of quasi1D nanomaterials that have been systematically studied for a wide range of materials with distinct chemical compositions and crystallographic structures.2 Field-effect transistors3 and ultrasensitive nano-sized gas sensors,4 as well as nanoresonators5 and nanocantilevers,6 have been fabricated based on individual nanobelts. Thermal transport along nanobelts has also been measured.7
Zinc oxide (ZnO) is a material that has diverse nanostructures. Owing to its noncentral symmetry, ZnO is not only semiconducting but also piezoelectric and pyroelectric. ZnO is a versatile smart material that has key applications in catalysts, sensors, piezoelectric transducers,8 transparent conductors,9 and surface acoustic wave devices.10 The structure of ZnO can be described as a number of alternating planes composed of tetrahedrally coordinated O2– and Zn2+ ions stacked along the c-axis (Figure 1a). The oppositely charged ions produce positively charged Zn(0001) – and negatively charged O(0001 ) polar surfaces, resulting in a normal dipole moment and spontaneous polarization along
MRS BULLETIN • VOLUME 32 • FEBRUARY 2007 • www.mrs.org/bulletin
the c-axis (Figures 1a and 1b). If the elastic deformation energy is largely suppressed by reducing the thickness of a nanobelt, the polar nanobelt could self-assemble into different shapes, such as a nanospring (Figure 1c), by minimizing the electrostatic energy coming from the ionic charges on the polar surfaces.11 The existence of polar charges on the basal plane is the driving force for forming nanorings12 and na
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