| Summary: | 碩士 === 國立臺灣海洋大學 === 光電科學研究所 === 99 === ZnO nanostructures have been successfully and rapidly synthesized in large quantities from the ZnO substrate by CO2 laser direct heating the substrate under air ambient. In particular, the average growth rate of as-grown needle-shaped nanostructures exceeds 25 m s-1. By the way, ZnO nanostructures with different morphology were successfully synthesized from different growth conditions, such as different substrate, power, and heating location, by CO2 laser direct heating. The morphology and structure properties of as-grown ZnO nanostructures were further investigated by field-emission scanning electron microscope（FE-SEM）and different shapes of ZnO nanostructures from FE-SEM images were found on ZnO substrate, such as hexagonal nanorods, microtowers, nanotubes, nanorods, nanoneddles, nanowalls, flower-like shape, nanosheets, nanopins, nanoparticles.
Due to rapid synthesis of ZnO nanostructures, the as-grown ZnO nanostructures have low crystal quality with lattice defects. The lattice defects result in some defect levels to have visible photoluminescence. The furnace-annealing treatment and CO2 laser rapid-thermal-annealing treatment can enhance the crystalline and optical qualities of the ZnO nanostructures to reduce defects. The photoluminescence of the ZnO nanostructures annealed in different annealing treatment was measured. The result of IUV/IVIS in furnace-annealing treatment is 13.22（3 hr, annealing time）and in CO2 laser rapid-thermal-annealing treatment is 68.87（10 min, annealing time）, respectively. Compared with the result of IUV/IVIS in original sample which is 5.44, the green emission was successfully reduced by thermal annealing treatment. In particular, the green emission was almost reduced and the UV light emission was strongly enhanced by CO2 laser rapid-thermal-annealing treatment in few minutes.
Using CO2 laser exposure the ZnO nanostructures, we can find orange emission from the top of the nanostructures and green emission from the rest of the nanostructures, respectively. Due to oxygen atom is easily adsorbed on the top of the nanostructures, the orange emission resulted from oxygen-rich state. At high temperature condition, ZnO produces free electrons through ionized reaction. These free electrons were trapped by doubly ionized oxygen vancancy（VO**）and then show the green emission at 540nm-557nm.
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