ABSTRACT
Before preparation of ZnO network, a ZnO buffer layer was fabricated on (0001) sapphire by filtered cathodic vacuum arc technology.16, 17 Using a copper grid with 400 mesh as the mask, a patterned array of thin gold film was prepared on the ZnO buffer layer by thermal evaporation. ZnO nanofiber network was fabricated on the patterned substrate by vapor-phase transport method.18, 19 The growth was carried out in a quartz tube using a mixture of ZnO and graphite powders as source materials. The source temperature was 1100°C and the growth temperature was about 700°C. A white layer of product was observed on the collector after sintering for 30 min. Figure 3.1 shows the XRD patterns of the ZnO buffer layer and the nanofiber network. Only (0002) peak was observed from the ZnO thin film, which demonstrates that the ZnO buffer layer grows along [0001] direction with good crystal quality. The XRD spectrum illustrates multipeaks after the ZnO nanostructure was fabricated on the buffer layer. As indexed in Fig. 3.1, all diffraction peaks match the wurtzite ZnO structure with the lattice constants of a = 3.250 Å and c = 5.207 Å. Figure 3.2 (a) shows the scanning electron microscopy (SEM) image of the product. Due to the catalyst dominated position-selected growth, the surface was covered with patterned arrays of ZnO nanostructures corresponding to the inserted gold pattern (inset of Fig. 3.2 (a)). It is clearly seen that the nanofibers grow along six specific directions parallel to the substrate surface. The size of the nanofibers is about 500-800 nm in diameter and several tens of microns in length. Each cell is composed of sixfold symmetric
branches that interconnect adjacent cells to form a network with regular structure. Figure 3.2 (b) shows the enlarged SEM image of the nanowhisker morphology corresponding to the center of a cell as circled in Fig. 3.2 (a). Each whisker is composed of a stem with hexagonal cross section and several branches grown radially around the sides of the stem. It is noted that the stems are perpendicular to the substrate, and the branches from either the same or different stems are parallel to the substrate along the predefined sixfold symmetric directions as indicated by the arrows in Fig. 3.2 (b).
