Two-step oxygen injection process for growing ZnO nanorods

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Hsu-Cheng Hsu and Wen-Feng Hsieh Institute of Electro-Optical Engineering, National Chiao Tung University, 1001 Tahsueh Road, Hsinchu 300, Taiwan, Republic of China

Kuo-Shung Liu Department of Material Science and Engineering, National Tsing-Hua University, 101, Section 2 Kuang Fu Road, Hsinchu 300, Taiwan, Republic of China

I-Cherng Chena) Materials Research Laboratories, Industrial Technology Research Institute, Bldg. 77, 195 Section 4 Chung Hsing Road, Chutung, Hsinchu 310, Taiwan, Republic of China (Received 9 June 2003; accepted 15 September 2003)

Uniform hexagonal prismatic zinc oxide rods were grown over the entire alumina substrate at 550 °C using a two-step oxygen injection process, whether the substrates were coated with a catalyst or not. X-ray diffraction showed that all of the depositions exhibited a preferred orientation in the (002) plane. The influence of oxygen concentration was investigated by changing the oxygen flow rate. Oxygen concentration affected the size of ZnO nanorods, especially the diameter. The ZnO nanorods were further checked using high-resolution transmission electron microscopy, photoluminescence, Raman spectroscopy, and room-temperature ultraviolet lasing. The results showed that the rods were single crystals and had excellent optical properties. By observing the growth process, we found that the diameter increased slowly, but the longitudinal growth rate was very high. The growth of ZnO nanorods revealed that the uniform hexagonal prismatic ZnO nanorods were synthesized through vapor deposition growth and a self-catalyzed vapor–liquid–solid (VLS) process.

I. INTRODUCTION

Semiconductor one-dimensional (1D) nanostructures have attracted much attention in recent years, especially in mesoscopic research and due to their potential application in manufacturing nanodevices. The main reasons for this are their interesting photonic and electronic properties, and their importance as a building block for interconnects of transistors, junctions of metalsemiconductors, and the tips of emitters. Many studies have been carried out on the whiskers and wires of Si and III-V systems1–9 as well as on the oxide systems, including SnO2,10 SiO2,11 GeO2,12 ZnO,13–15 indium tin oxide (ITO),16 and Al2O3.17 Among them, ZnO is an n-type semiconductor with a wide-band gap (3.30 eV). It emits short-wavelength light, shows piezoelectric properties, and is transparent in the visible range. It is also electrically conductive with appropriate dopants such as Al,

a)

Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 12, Dec 2003

http://journals.cambridge.org

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Ga, In, Sn, etc. Huang et al.18 reported the successful gas phase synthesis of ZnO nanowires on patterned Au catalyst by the vapor–liquid–solid (VLS) reaction. They used carbothermal or hydrogen reduction of ZnO as a zinc vapor source at 900–925 °C. Pan et al.19 synthesized ZnO nanobelts by thermal evaporation of ZnO powder at 1400 °C. The low-temperature gas phase processe

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