Growth Mechanism, Photoluminescence, and Field-Emission Properties of ZnO Nanoneedle Arrays Zengxing Zhang, Huajun Yuan, Jianjun Zhou, Dongfang Liu, Shudong Luo, Yanming Miao, Yan Gao, Jianxiong Wang, Lifeng Liu, Li Song, Yanjuan Xiang, Xiaowei Zhao, Weiya Zhou, and Sishen Xie* Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Graduate School of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100080, P. R. China ReceiVed: NoVember 26, 2005; In Final Form: March 8, 2006 ZnO nanoneedle arrays have been grown on a large scale with a chemical vapor deposition method at 680 °C. Zn powder and O 2 gas are employed as source materials, and catalyst-free Si plates are used as substrates. Energy-dispersive X-ray and X-ray diffraction analyses show that the nanoneedles are almost pure ZnO and preferentially aligned in the c-axis direction of the wurtzite structure. The growth mechanism of ZnO nanoneedle arrays is discussed with the thermodynamic theory and concluded to be the result of the co-effect of the surface tension and diffusion. Photoluminescence spectrum of the as-grown products shows a strong emission band centering at about 484 nm, which originates from oxygen vacancies. Field-emission examination exhibits that the ZnO nanoneedle arrays have a turn-on voltage at about 5.3V/μm. Introduction Recently, one-dimensional (1-D) materials on nanoscale have been extensively studied due to their novel fundamental properties and wide potential applications in many fields. 1-4 They are expected to play important roles in the future technology. So far, various methods, including chemical vapor deposition (CVD), 5,6 laser ablation, 7,8 soft solution route, 9 and template-assisted, 10 have been developed to grow 1-D nano- structures. Vapor-liquid-solid 7,11 and vapor-solid 12,13 mech- anisms are commonly employed to explain the growth process. Among these materials, ZnO nanostructures have been paid much more attention. Zinc oxide (ZnO), as one of the most important functional semiconductor materials, has a direct wide band-gap of 3.37 eV at room temperature and a large exciton binding energy of 60 meV. It is widely applied in photonics devices, ultraviolet (UV) lasers, sensors, etc. Up to now, abundant ZnO nanostructures, such as nanowires, 14,15 nano- belts, 5,16 nanotubes, 17,18 etc., have been grown and investigated intensively by some groups. In fact, different morphologies have strong effects on properties and applications. For example, well- aligned ZnO nanowire/nanorod arrays exhibit excellent optical 9,19-21 and field-emission properties. 22-25 They are pro- posed to be applied for UV lasers and field-emission displays. This stimulates further investigation for controlling the growth of well-aligned ZnO nanowire/nanorod arrays. Thus, it is necessary to study the growth mechanism and understand the thermodynamic process. In the present work, we show an effective way to grow well- aligned ZnO nanoneedle arrays on a large scale with a CVD method at 680 °C. Here we discussed the growth mechanism from the thermodynamic theory by careful investigation of the growth details. The photoluminescence (PL) and field-emission properties of the as-grown well-aligned ZnO nanoneedle arrays were also studied. Experimental Section Well-aligned ZnO nanoneedle arrays were grown in a horizontal quartz tube inserted in a furnace. Zn powder and O 2 gas were employed as source materials. Catalyst-free Si (001) plates were used as substrates, which were ultrasonically cleaned in alcohol for 20 min previously. First, Zn powder, placed on a quartz boat, was loaded in the middle of an inset quartz tube. Then the Si substrates were placed on the upstream of the Zn powder with a distance of about 0.5 cm, where the deposition of ZnO products is much more suitable in our experiment. As is well-known, the position on the upstream of the Zn powder is a head-to-head meeting site of two airflows (Zn vapor and oxygen flow) from opposite directions, whereas the position after the Zn powder is a head-to-tail meeting site of the two airflows from the same directions. Obviously, the former site is beneficial to reaction and deposition, where the two opposite airflows encounter head-to-head and react into ZnO molecules more easily. While the temperature of the furnace was raised to 680 °C, the inset quartz tube with the Zn powder and Si substrates in was loaded in. Then the quartz tube was enclosed and pumped. At the same time, the mixture of O 2 (99.99%, 7 sccm) and Ar (99.99%, 21 sccm) was introduced into the system. During the whole growth process, the system was maintained at the pressure of 10 -2 Pa. After 30 min, the inset quartz tube was taken out and cooled to the room temperature under the protection of Ar ambience. The products were found to be semitransparent films depositing on the substrates. Furthermore, field-emission scanning electron microscopy (FESEM) was employed to study morphologies of the as-grown products. Energy-dispersive X-ray (EDX) and X-ray diffraction (XRD) were used to characterize composition and crystal structure in sequence. The PL spectrum was examined with a He-Cd laser of 325 nm, and the field-emission property was measured in a vacuum chamber at a pressure of about 10 -7 Pa. * To whom correspondence should be addressed. Tel: +86-10-82649081. Fax: +86-10-82640215. E-mail: ssxie@aphy.iphy.ac.cn. 8566 J. Phys. Chem. B 2006, 110, 8566-8569 10.1021/jp0568632 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/08/2006