Abstract
We have fabricated the cylindrical and hexagonal-shaped ZnO nanowires by using thermal evaporation with different growth mechanism. Firstly, we will focus on the difference of crystal characterization, structure defects, and physic properties between the two types of ZnO nanowires, which were inspected by using scanning electron microscopy, x-ray diffraction, high-resolution transmission electron microscopy, photoluminescence, four-probe electric measurement and two-probe TEM-STM instrument. From the results of this study, the hexagonal ZnO nanowires yield better stoichiometry and optical property. The electrical measurement of TEM-STM technique indicates that the resistivity of hexagonal nanowires will be ten times higher than those in cylindrical nanowires. In contrast, the cylindrical ZnO nanowires display lower resistivity and higher density of green defect emission. The above findings are closely related to the structure defects, including surface roughness and point defects such as O-vacancies or Zn-interstitials, which results from the non-stoichiometric compounds. Additionally, I will discuss three kinds of mechanism of forming stacking faults in cylindrical ZnO nanowires which are closely related to its growth mechanism. Furthermore, we prepared Zn1−xCoxO nanowires by using Co ion implantation, and study its magnetic properties. The bombardments by Co ions produced a good number of structural defects (stacking faults and orientational variations) in the nanowires. The as-implanted Zn1−xCoxO nanowires were paramagnetic. We performed two types of thermal annealing, one in 1 atm argon flow and the other in a high vacuum, at 600 ◦C, and studied the effects of annealing on the magnetic properties of these nanowires. Argon annealing removed structural defects in the nanowires and the nanowires then revealed ferromagnetic ordering. This result suggests that structure defects are harmful to the occurrence of ferromagnetism in the Co-implanted ZnO. Noticeably, the nanowires even displayed largely enhanced ferromagnetism after annealing in a high vacuum. A subsequent annealing in oxygen has also been performed on those vacuum-annealed nanowires to study the roles played by the O vacancies in determining the ferromagnetic properties of the nanowires. Our results indicate that both the improved structural quality and the increased number of O vacancies are key factors for the occurrence of ferromagnetic ordering in the Zn1−xCoxO nanowires. The furthermore investigation will discuss the magnetic properties of size effect on cylindrical Zn1−xCoxO nanowires. The important conclusion is high surface-to-volume ratio in thin Zn1−xCoxO nanowires also facilitated enhancement in ferromagnetism through the high-vacuum annealing process, which is closely corrected the O vacancies. The results provide a unified basis for better understanding mechanism of ferromagnetism in Zn1-xCoxO system.