Preparation of ZnO Sub-micrometer Rod Arrays on Bent Surface of Zn Microspheres

ZnO sub-micrometer rod arrays have been prepared on bent surface of Zn microspheres by thermal evaporating the mixture of ZnS and Zn powder to metal Zn slice. In a typical preparation process, 0.01 mol ZnS and 0.01 mol Zn powder were mixed in an aluminum boat and positioned at the center of a horizontal quarts tube furnace. A slice of metal Zn substrate in a small quartz boat was located in downstream region of the tube. The whole system was sealed and pumped to a pressure of 100 Pa. High-purity Ar was aerated into the tube at a flow rate of 100 standard cubic centimeters per minute. The pressure in the tube was kept at 0.04 MPa. The furnace temperature was heated up to 900 ℃ at a 20 ℃/min rate. The system was kept at 900 ℃ for 2 h. After the furnace was cooled to room temperature naturally, white wool-like matter was found to depo-sit on the Zn substrate. Finally, the product was collected for further characterizations by field-emission scanning electron microscope (FE-SEM, Hitachi S-4800) and LEO 1550 SEM attached with an Oxford Inca Drycool EDS detector. The results show ZnO sub-micrometer rod arrays can self-organized grow on surfaces of formed Zn microspheres. These ZnO rods with hexagonal cross-section have an average diameter of 500 nm and length of 1 μm. Top of the ZnO rod is very flat. Energy dispersive X-ray spectrum of the ZnO rod array demonstrates that only Zn and O elements exist with a ratio of about 1∶1. No S element has been found. ZnO crystal exhibits hexagonal wurtzite structure, which belongs to the space group C64V. According to selection rule of phonon mode, Raman active modes for the wurtzite ZnO are: 1A1+2B2+1E1+2E2. A1 and E1 modes are polar and can split into the transverse optical (TO) and longitudinal optical (LO) phonon modes. E2 mode is non-polar optical phonon mode, which is composed of two modes with low and high frequency. The vibration peaks at 331, 381, 430 and 580 cm-1 can be clearly observed in Raman spectrum of the ZnO sub-micrometer rod array. The peaks at 381 and 580 cm-1 correspond to polar transverse A1 and longitudinal E1 optical phonon modes, respectively. The strong peak at 430 cm-1 can be assigned to the nonpolar optical phonon (E2) modes of the ZnO rod at high frequency, which is associated with oxygen deficiency. The peak at 331 cm-1 is attributed to the 2E2 mode. Photoluminescence spectra of the ZnO rod arrays show two peaks at the wavelength of 387 nm and 509 nm, respectively. The stronger peak at 387 nm with the full width at half maximum (FWHM) of 16 nm can be attributed to near band-edge emission. Another weaker peak at 509 nm probably is due to defect state luminescence. The sub-micrometer ZnO rod arrays have a potential application in ultraviolet laser devices.