Article pubs.acs.org/crystal
Single Crystal Kinked ZnO [001] and [110] Nanowires: Synthesis, Characterization, and Growth/Kinking Mechanism Peite Bao, Rongkun Zheng,* Sichao Du, Li Li, Wai Kong Yeoh, Xiangyuan Cui, and Simon P. Ringer Australian Centre for Microscopy & Microanalysis, The University of Sydney, New South Wales 2006, Australia ABSTRACT: We report the fabrication of single crystal kinked ZnO [001] and [100] nanowires (NWs). From the detailed morphology and microstructure analyzed by transmission electron microscopy, we discuss the growth characteristics, including nucleation and kinking, from a growth process point of view. We have discovered that the crystallographic property of the substrate surface played a critical role in determining the growth directions, which in turn determine the characteristics of single crystal kinking. A mechanism based on the unbalanced facet growth was proposed to explain the kinking of the single crystal kinked nanowires. Photoluminescence spectra confirm that the optical property of the ZnO NWs was not altered by the single crystal kinking.
1. INTRODUCTION Owing to the low dimensionality and quantum-confinement effects, semiconductor nanowires (NWs) exhibit distinctive and novel properties that differ from those of conventional bulk and thin-film materials.1−3 NWs have been intensively studied for fundamental science as well as for potential applications in electronics and photonics, energy conversion and storage, and interfacing with living cells.4 Most of the applications require the precise control over the composition,1,3 doping,1,3,5 crystal structure,6,7 and morphology8−10 of NWs. However, NWs growth is often complicated by various of behaviors,11 including kinking from one direction to another,12,13 which is suggested to affect the physical properties of NWs.14,15 On the other hand, kinked NWs could also be used as crucial interconnection parts in NW-based nanodevices.16 Therefore, it is important to understand the underlying mechanisms of various growth morphologies, including kinking.17,18 ZnO is known to exhibit the most diverse morphology of nanostructures known so far.8 Numerous studies have demonstrated various novel applications of ZnO nanostructures, due to their attractive semiconducting and piezoelectric properties.1,5,19 However, there are only a few reports on the kinked ZnO NWs,9,12 and none with the single crystallinity at the kink corners. Therefore the mechanism of kinking and its effect on the properties of ZnO NWs require further exploration. In this paper, we report the controllable fabrication and detailed morphology and microstructure analysis of single crystal kinked ZnO NWs. The crystallinity was analyzed by transmission electron microscopy. A mechanism was proposed to explain the kinking of the single crystal kinked NWs. Finally, the physical property of the single crystal kinked ZnO NWs was examined by photoluminescence (PL) spectroscopy. © 2012 American Chemical Society
2. EXPERIMENTAL SECTION Single crystal kinked ZnO NWs were synthesized via a typical vapor transport method,8,20 with a few modifications in the growth conditions. Pure ZnO nanopowder (⩾99.0%, Sigma-Aldrich) was loaded in an alumina boat and placed in the center of a horizontal tube furnace as the source powder. Si (100) wafers coated with 3 nm Au film by magnetron sputtering were used as substrates and placed around 25 cm down flow from the center. The source powder was first heated up to 1050 °C with a ramping rate of 10 °C/min under a vacuum with no gas feed and kept at the temperature for 1 h growth, and then the system was allowed to cool down naturally to room temperature. Different from most similar methods, no carrier gas was used during the whole process and one end of the porcelain worktube was simply sealed by a valve; the mechanical pump was kept on to maintain the pressure within the tube, which was 1.5 mbar during growth. The morphology of the kinked ZnO NWs was characterized by Zeiss Ultra Plus field emission scanning electron microscope (FESEM). The investigation of the microstructures was carried out by X-ray diffraction (XRD) on an X-ray diffractometer with a Cu−Kα1 source (λ = 0.15418 nm), and high-resolution transmission electron microscopy (HRTEM) on JEOL 3000F with a field emission gun operated at 300 kV. JEOL 2200FS field emission TEM at scanning transmission electron microscopy (STEM) mode was used for energydispersive X-ray spectroscopy (EDX) elemental mapping of the NWs. PL measurements were tested on a Horiba Jobin Yvon T64000 microRaman system at room temperature, using linearly polarized excitation from a 325 nm He−Cd laser onto a spot of 5−10 μm2. The roomtemperature PL spectrum was measured using either a 2400/mm grating in a 1 m spectrometer, with signal detected by liquid nitrogen cooled CCD.
3. RESULTS AND DISCUSSION Figure 1a shows a typical FESEM image for the as-grown kinked ZnO NWs. The diameter of a majority of the NWs is Received: March 9, 2012 Revised: April 19, 2012 Published: April 25, 2012 3153
dx.doi.org/10.1021/cg300328c | Cryst. Growth Des. 2012, 12, 3153−3157
Crystal Growth & Design
Article
Figure 1. (a) A FESEM image of as-grown kinked ZnO NWs, top-view. Inset is the straight NWs grown at 1300 °C source temperature (scale bar: 2 μm). (b) A bright field STEM image of the tip of a NW and EDX mapping for (c) Au, (d) zinc, and (e) oxygen.
40 ± 7 nm with the length ranging from 1 to 1.5 μm, while some thin NWs with a diameter around 20 nm could also be seen. No other structures, such as belts, were found. A catalyst cap can be clearly seen on the tip of each NW, and further study by STEM-EDX mapping confirms that the composition of the cap is gold and a very small amount of oxygen, as shown in Figure 1b−e. This suggests that the growth mechanism for the NWs is the vapor−liquid−solid (VLS) mechanism. The wide-scan (10−90°) XRD pattern in Figure 2 obviously exhibits ZnO characteristic peaks and the substrate Si (400)
growth directions exist, along the [001] and [100] axis, respectively. HRTEM images and selected-area electron diffraction (SAED) patterns demonstrate that, in both cases, the NWs are of wurtzite ZnO structure and free of obvious structural defects at the straight parts. It is a general understanding that in a typical process of VLS growth, the supersaturation of the source material in the catalyst cap stimulates the growth, the lattice, and the orientation relationship between the substrate surface and ZnO NW plays a critical role in determining the growth characteristic of the NWs.21 Initially, melted Au droplets start to absorb the Zn/O vapor from the environment, and the overall solubility of Zn inside Au is reasonably low, which was confirmed by the elemental mapping results in Figure 1.22 The growth started and the crystallographic property at the substrate−liquid interface has a significant impact on the NW growth direction. From a microscopic point of view, the Si (100) substrate has a 4-fold symmetry on the surface, and the lattice spacing for Si {110} is around 3.84 Å. For ZnO NWs grew along the [001] and [100] directions, the lattice mismatch with the Si (100) substrate is similar, but neither of them is 4-fold symmetrical; this provides the possibility of the coexistence of both types of the NWs. In contrast, if α-sapphire is used as the substrate, c-axis growth would dominate.20,23 Figure 3c,d and insets reveal the orientation relationships of the ZnO NWs and Au in both cases when the growth is finished, which can be expressed as ZnO(002) (010)[100]//Au(010)(10̅ 1)[101] and ZnO(100)(12̅ 0)[001]// Au(101)̅ (121̅ )[111]. The overlap of the Au and ZnO patterns under FFT indicates a good match of the structures. Along the interfaces, the mismatch value between ZnO(010)[100] and Au(10̅ 1)[101] is ∼2.25%, while the mismatch for ZnO(12̅ 0) [001] and Au(12̅ 1)̅ [111] is ∼1.96%, calculated from the standard cell parameters. The orientation relationships were shown by a diagram in Figure 3e−j. Such results can be easily understood that when the growth process is finished, suggested by the VLS mechanism, the liquid Au particles solidified and the crystallinity of the particle is decided by the NW as the energy is lowest, in this case, the mismatch values is the smallest. To sum up, the mechanism can be generalized as: as the temperature of the whole system rises, the liquid Au aggregates from film into particles, and then the catalyst Au starts to absorb the Zn/O vapor and ZnO layers start form at the interface of Si/Au once the Zn/O solvent saturates. The orientation of the ZnO NW is determined by the Si substrate surface property; that is, the lattice mismatch value and the symmetry, and both [001] and
Figure 2. Wide-scan XRD pattern for as-grown kinked ZnO NWs on silicon (100) substrate, in comparison with the standard ZnO powder diffraction pattern. The standard powder diffraction intensity has been normalized to the ZnO (101) peak in the experimental data.
peak at 69.14°; the broadness of the 69.14° peak results from the overlap of the Si peak and a few adjacent ZnO peaks. Unlike the straight vertical NWs on substrates, which shows a texture XRD pattern, the appearance of multiple characteristic peaks and the change of the relative intensity compared to powder diffraction may be a result of either the randomness of the NW growth direction out of substrate plane or the kinkings of the NW. A detailed TEM study was carried out in order to understand the crystallographic information as well as the growth mechanism of the NWs. As shown in Figure 3a,b, two different 3154
dx.doi.org/10.1021/cg300328c | Cryst. Growth Des. 2012, 12, 3153−3157
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Article
Figure 3. HRTEM images for ZnO NW grown along (a) the [001] direction and (b) [100] direction, and the insets are SAED patterns. Panels (c) and (d) show the HRTEM imaging of the tip of the [001] and [100] NWs, respectively. The insets of (c) and (d) are FFT patterns of the corresponding square areas, showing the crystallographic relationship between the catalyst Au particle and the body of the NWs. (e−j) The orientation relationship of the Au−ZnO−Si substrate interfaces. (e) Schematic for NWs grown on the substrate. (f) Au ⟨010⟩ and (g) ZnO ⟨001⟩ refer to NWs grown along the [001] direction, (h) Au ⟨101̅⟩ and (i) ZnO ⟨100⟩ refer to NWs along [100], and (j) Si ⟨001⟩. Triangles and dashed squares show the orientation relationships.
[100] NW were grown. At the end of the growth, the Au liquid solidified form the crystal at the lowest energy level, which is as observed in our case. Kinking was also carefully investigated by HRTEM. In contrast to previous reports on kinked ZnO NWs, our NWs are single crystal without any twinning or other structural defects.9,12 This was carefully examined by using FFT on HRTEM images of the kink corners, as shown in Figure 4. FFT patterns are mostly consistent throughout the NW for both types of growth directions, while occasionally there is a very small rotation angle (
