tailieunhanh - Wave Propagation 2010 Part 14

Tham khảo tài liệu 'wave propagation 2010 part 14', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 382 Wave Propagation Fig. 3. a Low magnification TEM image of a tetraleg ZnO nanostructure b A high-resolution TEM image recorded from the center of the tetraleg structure c A Fourier transform of the image given in b and the indexes corresponding to one of the bottom-left grain in b . The incident beam direction is 24 23 nanostructure. The octa-twin has four positively charged 0001 surfaces and four negatively charged 0001 surfaces. The positively charged surfaces are likely to be terminated with Zn which may be the favorable sites to attracting vapor species resulting in the growth of whiskers along four 0001 directions that have a geometrical configuration analogous to the diamond bonds in diamond. The growth mechanism is believed to be a solid-vapor process. Fig. 4. a A pyramid formed by three 11 2 2 and one 0001 facets b The octa-twin model composed of eight pyramidal inversion twin crystals Fabrication of three-dimensional ZnO networks The three-dimensional ZnO netlike micro- nanostructures were fabricated by the following procedure. First high pure Zn and graphite powders with molar ratio of 10 1 were ground fully into a mixture before being loaded into a quartz boat. The Si substrate with the polished side facing the powder was fixed upon the boat and the boat with the mixture was placed at the center of the furnace. The vertical distance between the zinc source and the substrate was about 4-6 mm. And then the alumina ceramics boat was inserted into a quartz tube 30 mm inside diameter of a tubular furnace under a constant flow of argon and oxygen. The flow rate of Argon was 100 standard cubic centimeters per minute seem and the fraction of oxygen was 4 seem. The quartz tube was heated up to 910 C and retain reaction temperature for 30 minutes. After the evaporation finished a layer of woollike product was formed on the walls of the boat and the surface of the substrate. The SEM images in Figure 5 show the morphologies of ZnO netlike .