| Summary: | 碩士 === 國立虎尾科技大學 === 光電與材料科技研究所 === 96 === ZnO has some of the greatest potential among semiconductor materials for application in ultraviolet regions and nanotechnology. It has large exciton binding energy of about 60 meV, which is much greater than the thermal energy at room temperature, makes it a promising candidate for applications in blue-UV light emission and room-temperature UV lasing. ZnO is known to have wurtzite strucuture with lattice constant a = 3.249 Å, c = 5.207 Å. Furthermore, its highly piezoelectric constant makes it a highly valuable material for fabricating mechanical devices. For instance, ZnO thin film structures can be utilized as piezoelectrical device and ultraviolet light emitting diode. Several physical or chemical methods have been developed in succession for the preparation of ZnO nanorod/nanowire array, including vapor-liquid-solid process untilizing gold or tin as catalyst, metalorganic vapor-phase epitaxial growth, seed-layer assisted solution route, electrochemical deposition based on anodic alumina membranes, and so on. Compared with physical vapor methods, the solution based approaches exhibited obvious advantages in cost, facilities, complexity, energy consumption, and large scale up production. Self-powered nanosystems are of great importance for real-time and implantable biosensing, environmental monitoring, and electromechanical systems. We have developed a direct-current nanogenerator that is driven by ultrasonic wave. The basic principle is to use piezoelectric and semiconducting coupled nanorods(NRs), such as ZnO, to convert mechanical energy into electricity.
The ZnO nanostructures were symthesized on different substrates using chemical depostion methods. In this experiment, ZnO nanostructrues grown included vapor and liquid solution epitaxial methods. In vapor epitaxial, effect of growth temperatures, Zn/C powder ratios, of gas ratios on the morphology and characteristics of ZnO naonowires were carried out. In liquid epitaxial, effect of growth temperatures, growth times, and of solutions of pH on the morphology and characteristics of ZnO naonorods were discused. The photoluminescence(PL) and transmittance of the ZnO nanostructures were measured by UV-VIS spectrophotometer and fluorescence spectrophotometer.
The sanning electron microscope(SEM) results showed when the temperatures increased, the diameters of the ZnO grains increased. The ZnO nanowires and nanorods had a mean diameter of ~80 nm. The XRD results found that the ZnO nanorods had monocrystalline(002) structure by low temperature that the highest intensity at 90℃ and concentration ratio 2：4, and the ZnO nanowires exhibited polycrystalline structure by high -temperature method that the highest intensity at 800℃ and oxygen ratio 12：1. ZnO is a II-VI semiconductor with a band gap of 3.2 eV at room temperature. The photoluminescence measurements showed that the high-temperature epitaxial ZnO nanostructures had good ultraviolet emission at 382 nm and blue emission at 500 nm. The high-temperature epitaxial method PL characteristic quality more than low-temperature epitaxial method. Raman scattering spectrum was used to measure substance structure of the ZnO nanostructure. The raman scattering spectrum appeared two peaks at 438 cm-1 and 582 cm-1. The transmittance and absorption spectrums measurements showed that the ZnO nanorods had high transmittance 90% at 900 nm and good ultraviolet absorption at 350 nm. Transmission electron microscope(TEM) was used to measure crystal image and inner structure.
Finally, making on top of electrode to fabricate nanogenerator(NG) with ZnO nanorods and measured micro-current driven by ultrasonic waves with a frequency of 43 kHz. When the ultrasonic wave was on for an extended period of time, the generated current was ~25 nA for a NG with 25 mm2 in size, corresponding to an output current density of 0.1μA/cm2 .
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