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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 O2 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
Experimental Section
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 nanostructures. Vapor-liquid-solid7,11 and vapor-solid12,13 mechanisms 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 nanobelts,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, wellaligned ZnO nanowire/nanorod arrays exhibit excellent optical9,19-21 and field-emission properties.22-25 They are proposed 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 wellaligned 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.
Well-aligned ZnO nanoneedle arrays were grown in a horizontal quartz tube inserted in a furnace. Zn powder and O2 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 O2 (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:
[email protected].
10.1021/jp0568632 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/08/2006
Growth and Properties of ZnO Nanoneedle Arrays
J. Phys. Chem. B, Vol. 110, No. 17, 2006 8567
Figure 1. (a) FESEM image of the well-aligned ZnO nanoneedle arrays. The inset is the high-magnification FESEM image. (b) Corresponding EDX spectrum. Figure 3. (a) Cross-sectional FESEM image of the well-aligned ZnO nanoneedle arrays. The drawn line arrowed in the image shows that the interface of the nanoneedle arrays and the film has a nearly periodic structure. (b) FESEM image of the obvious growth steps observed in our experiment. (c) Schematic plane of the growth model.
Figure 2. XRD spectrum of the well-aligned ZnO nanoneedle arrays. Two obvious peaks can be observed, which are indexed to (002) and (004) planes of the ZnO wurtzute structure, respectively.
Results and Discussions Morphologies and Crystal Structure. Figure 1a shows a representative FESEM image of the as-grown products. As shown in the image, the nanoneedles are well-aligned and uniform on a large scale. The inset in Figure 1a is a highmagnification FESEM image, which shows that the nanoneedles have small tips with the diameter of about 30 nm. The composition of the nanoneedle arrays was examined with EDX equipped on FESEM. The EDX spectrum is shown in Figure 1b. The EDX spectrum exhibits that the nanoneedle arrays are mainly composed of Zn and O. They are 52.87% and 47.13% in sequence, and they are nearly 1:1 in atoms. This indicates that the as-grown nanoneedle arrays are almost pure ZnO. The XRD spectrum of the acquired ZnO nanoneedle arrays is shown in Figure 2. The result exhibits that there are two obvious peaks at 34.44° and 72.58°, respectively. They are identified to match up to the (002) and (004) planes of the ZnO wurtzite structure in sequence. Nearly no other obvious peaks can be observed in the XRD spectrum. We can conclude that
the as-grown ZnO nanoneedle arrays are wurtzite structure, vertical to the substrates, and preferentially aligned in the c-axis direction. Growth Mechanism. The growth mechanism of the wellaligned ZnO nanoneedle arrays was further investigated carefully. The products were cut into parts, and the cross-sectional FESEM image is shown in Figure 3a. Obviously, the wellaligned nanoneedle arrays grow from a film. The interface of the well-aligned nanoneedle arrays and the film has a periodicity-like structure (as described by the line arrowed in Figure 3a). The growth of the well-aligned nanoneedle arrays should be the result of the co-effect of the surface tension and diffusion. As we all know, there is an important characteristic in employing Zn powder as source material. It is that Zn powder continuously vaporizes and then decreases rapidly during the growth process. In the initial growth stage, Zn vapor is adequate. It reacts with O2 into ZnO and deposits on the substrates as a film. This can be explained with the oxide-assisted growth mechanism proposed by Lee et al.26,27 First, ZnO clusters form on the substrates. They serve as seeds for further growth. Second, Zn atoms and O atoms react continuously on these seeds. In the end, they grow into a film. Because of the perturbation, the surface of the film is instable and sinuate. Generally, the surface tension and diffusion are two dominating effects on the film growth. The surface tension makes the surface of the film sinuate, but on the contrary, the diffusion makes it flat. In the reported literature, the local thermodynamic equilibrium theory is employed to explain a similar problem in solidification.28 Due to the fact that the viscidity of the liquid is not considerable in the theory, it should be reasonable to discuss our growth process. Here we suppose the surface to be the morphology of a sinusoidal curve (as well-known, every curve can be transformed
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Zhang et al.
into a serial of sinusoidal curves with Fourier transform). The periodicity λ of the sinusoidal curve is
λ ) 2π
xΓφ
φ ) mGc - G, Γ )
σ ∆S
where σ is the surface energy, ∆S is the entropy, m is the slope coefficient of the liquid curve, Gc is the concentration gradient (here supposing Gc ) 0), and G is the temperature gradient. Supposing half of one periodic of the sinusoidal curve is hemisphere and the diameter is R, i.e., R ≈ λ/2, and the temperature difference between the top and bottom of the sinusoidal curve is ∆T, then the temperature gradient is G ) 2∆T/R. So
σ R ) - π2 2∆T∆S
Figure 4. PL spectrum of the well-aligned ZnO nanoneedle arrays at room temperature.
where R is also the diameter of the bottom of the ZnO nanoneedles. As we all know, the concentration of the atoms N has a relation of the curvature radius of surface 1/r,
(2σΩ rkT )
N ∝ p ) p0 exp
where p is the pressure, p0 is the equilibrium pressure on plane surface, Ω is the atomic volume, k is the Boltzmann’s constant, and T is the absolute temperature. Then many more atoms deposit on the point of the wave crest than others (where 1/r is bigger than others). But nearly no atoms deposit on the point of the wave hollow, (where 1/r
