Volume 12 Issue 10 - March 5, 2010 PDF
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Influences of ZnO seed layer characteristics on the synthesis of ZnO nanowires
Wan-Yu Wu1, Chun-Ching Yeh1 and Jyh-Ming Ting1,2,*
1Department of Materials Science and Engineering;
2Institute of Nanotechnology and Microsystems Engineering, National Cheng Kung University,Tainan, 70101, Taiwan
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Single crystalline ZnO nanowires were synthesized on sputter deposited nanoscaled ZnO seed layers using a hydrothermal process at 90℃. Influences of the sputter deposited ZnO seed layer characteristics were examined. The nanowire diameters were found to depend on the size of the (002) grains of the seed layer. TEM analysis shows epitaxially growth of the nanowires from the columnar grains of the seed layer.

Introduction
Different ZnO nanostructures are being widely investigated for a variety of applications. For the applications, it is essential and in fact critical to develop and understand the synthesis of ZnO nanostructures. There are several methods used to grow ZnO nanostructures, primarily ZnO nanowires/nanorods, which usually require processing temperatures in excess of 400℃[1,2]. Moreover, some of these methods require a metal catalyst to facilitate the growth. This thus raises compatibility issues for process integration in conventional complimentary metal oxide semiconductor technology. Due to their low growth temperatures and the potential for scale-up, recent solution approaches are appealing[3-6]. The use of a seed layer has been common in the solution methods. Unfortunately, little or no study on the effect of ZnO seed layer has been reported. In particular, there is little or no report examining the structural relation between a seed layer and the ZnO nanowires grown upon it. This study has examined such a relation and we show that ZnO nanowires grow epitaxially from the columnar grains of the seed layer.

Experimental
ZnO films were sputter-deposited on glass substrates for use as the seed layers. The power wattage and the deposition time were varied to obtain ZnO layers having different characteristics. ZnO seeded substrates were then immersed in an equimolar aqueous solutions of Zn(NO3)2•6H2O and C6H12N4 at 90 ℃ for 3hr. Specimens were then removed from the solution, rinsed with water, and dried in air. The ZnO seed layers and the resulting ZnO nanowires were examined using scanning electron microscopy (SEM), grazing incident x-ray diffractometry (GIXD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and nanobeam diffraction.

Results and discussion
Columnar ZnO seed layers having different thicknesses (35 nm to 500 nm) were obtained. The columnar structure results in a particle-like morphology. The average particle diameters range from 16 nm to 52 nm and increases with the layer thickness. The ZnO seed layers are polycrystalline and exhibit a preferred (002) orientation. Different ZnO seed layers have different degrees of preferred (002) orientation, depending on the deposition condition. The preferred (002) orientation decreases with the layer thickness as shown in Fig. 1. When the columnar growth takes place, the fastest growth (002) surface face appears first. As the growth continues, due to the high deposition rate, other surfaces, (103) and (102), appear at the expenses of the (002) surface[7]. This leads to reduced (002) preferred orientation. In general, the grain size, determined from the full width at the half maximum (FWHM) of the (002) peaks, was found to increases with the seed layer thickness. The correlation between the grain size and the (002) preferred orientation is not obvious. The grain size seemly shows a decreasing trend with (002) preferred orientation, as shown in Fig. 2. It is noted that the (002) preferred orientation represents the percentage of the (002) faces, the decrease of which, however, does not mean the reduction of the (002) grain sizes.
Fig. 2. (002) grain size decreases in general with (002) preferred orientation. The dashed lines are for visual guide.
Fig. 1. (002) preferred orientation decreases with ZnO layer thickness.
ZnO nanowires with different diameters, lengths, and area densities (number of nanowires per μm2) were obtained. Figs. 3A and 3B show respectively two SEM plain view images of ZnO nanowires grown on 106- and 191-nm thick seed layers. The former and the latter have area densities of 35/μm2 and 12/μm2, respectively. The nanowires are semi-aligned, as shown in Figs. 3C and 3D. The seed layers, as pointed out by the arrows, remain after the hydrothermal growth. The average diameters and lengths for the nanowires shown in Fig. 3C are 126 nm and 0.8μm, respectively; while for Fig. 3D, the average diameter and length are 195 nm and 1.4 m, respectively. The nanowire diameter increases with the seed layer (002) grain size. Since the correlation between the (002) preferred orientation and the grain size is not strong, the nanowire diameter seemly decreases with the (002) preferred orientation too. Also, no correlation was found between the nanowire area density and the seed layer characteristics. Regardless of the dimensions, the ZnO nanowires obtained are single crystal nanowires. Fig. 4A shows a TEM image of a ZnO nanowire. Its high resolution image is given in Fig. 4B, which shows that the growth of (002) planes is perpendicular to the wire axis. The c-axis lattice spacing was measured to be 5.35Å. The diffraction pattern shown in Fig. 4C indicates the formation of single crystalline wurtzite structure. As mentioned above, the diameter of the single-crystalline nanowire is proportional to the diameter of the (002) grains. This is ascribed to the epitaxially growth of the nanowires from the columnar grains of the seed layer.
Fig. 3. Plain views of ZnO nanowires grown on (A) 106 nm and (B) 109 nm thick seed layers. The respective cross sectional views are given in (C) and (D).

Fig. 4. TEM (A) bright-filed and (B) high resolution images of ZnO nanowire. (C) The diffraction pattern of the nanowire.
Fig. 5A shows a across sectional image of ZnO nanowires. The upper and lower circles in Fig. 5A are a root region of a nanowire and a top region of a columnar grain in the seed layer, respectively. As shown by the diffraction patters, both these two regions exhibit a single crystalline wurtzite structure. An enlarged TEM image is given in Fig. 5B. HRTEM images of three regions, representing the nanowire, the interface, and the seed layer are shown in Fig. 5C. It is seen that individual single-crystalline nanowires grow epitaxially from the columnar grains of the seed layer. In various synthesis methods, the growth of ZnO nanowires/nanorods epitaxially from a seed layer is indicated or suggested but without giving any direct evidence such as TEM analysis. A recent report shows that there is an indication of an epitaxial relationship between ZnO nanowires and the γ-AuZn catalyst particle[8]. The growth method was a vapor phase method. For other growth methods without the use of a catalyst but a seed layer such as the aqueous solution or hydrothermal method used in this study, there is no or little report that shows the epitaxial relation between ZnO nanowires/nanorods and a ZnO seed layer[9-12]. In this study, we provide TEM images showing directly the epitaxial relation between solution-synthesized ZnO nanowires and a seed layer. To our knowledge, this is the first time that such a structural relation is revealed.
Fig. 5. TEM cross-sectional images having (A) low and (B) high magnifications. (C) HRTEM lattice images of nanowire/seed layer interface region.

Conclusions
We report the synthesis of ZnO nanowires on ZnO seed layers in an aqueous solution of Zn(NO3)2•6H2O and C6H12N4 at 90 ℃. Effects of the characteristics on the growth of ZnO nanowires are investigated. TEM analysis that shows the growth of ZnO nanowires epitaxially from the columnar grains of the seed layer is reported for the first time.

Acknowledgment
This work was supported by the Top University Program at the National Cheng Kung University under R048/D97-3360 and the National Science Council in Taiwan under Grand No. NSC 97-2120-M-006 -001.

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