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Synthesis and Photoluminescence of InGaO3(ZnO)m Nanowires with Perfect Superlattice Structure Da Peng Li, Guan Zhong Wang,* Qian Hui Yang, and Xing Xie Hefei National Laboratory for Physical Sciences at Microscale, and Department of Physics, UniVersity of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ReceiVed: July 7, 2009; ReVised Manuscript ReceiVed: NoVember 1, 2009
InGaO3(ZnO)m (m ) 3, 5) nanowires with perfect superlattice structures were synthesized via a catalystassisted vapor-liquid-solid (VLS) growth mechanism. Different from In2O3(ZnO)m nanowires maintaining wurtzite structure, the InGaO3(ZnO)m (m ) 3, 5) superlattice nanowires have a monoclinic crystal structure. High resolution transmission electron microscopy (HRTEM) indicates that the nanowires have an axial superlattice structure, which consists of an In-O layer and In, Ga/Zn-O layers stacking alternately along the nanowire length. The period of a InGaO3(ZnO)3 nanowire consists of four atom layers, which is the shortest period in quasi-one-dimensional superlattice structures. Introduction One-dimensional nano-heterostructures and, more generally, compositionally modulated (that is, superlattice) structures are ideal building blocks for potential nanoelectronic and optoelectronic devices. Compared to thin film materials, nanowires with heterostructure and superlattice structure possess unique advantages such as reduced growth temperature and novel strain relaxation mechanisms.1 Thus, the synthesis and study of the physical properties of nanowires with heterostructure and superlattice structure are of fundamental importance to reach potential applications. Recently, different types of heterostructure and superlattice nanowires have been successfully prepared,2-17 which have demonstrated potential applications in nanoelectronic and optoelectronic devices.13 M (M ) In, Fe, Al, and Ga)-doped ZnO prefers to form homologous compounds InMO3(ZnO)m with natural short period superlattice structures in film materials.18-23 According to reports, these types of compounds have demonstrated novel optical and electrical properties. High electrical/thermal conductivity, optical transparency, and excellent thermoelectric properties have been observed in In2O3(ZnO)5 film materials.21,22 The transparent field-effect transistor (TFET) fabricated by InGaO3(ZnO)5 film has shown higher performance compared to the TFET fabricated using a conventional transparent oxide semiconductor, such as SnO2 and ZnO.18 As is well-known, synthesizing semiconductors into nanostructures possibly improves their optoelectrical properties effectively.3 Thus, it is desirable to develop these materials into one-dimensional nanostructures. Jie et al. have synthesized In2O3(ZnO)m nanowires, and Na et al. prepared Sn-doped In2O3(ZnO)m nanowires.3,4 Similar results have been reported by Zhang et al. and Lu et al.5,6 However, all these nanowires of In2O3(ZnO)m have a ZnO wurtzite crystal structure and show imperfect superlattice structure. Furthermore, there is no report on the synthesis of InMO3(ZnO)m (M ) Ga, Fe, and Al) system superlattice nanowires yet. Therefore, the preparation of nanostructures with perfect superlattice structure or the development of InMO3(ZnO)m superlattices in nanoscale is desirable. Here we * Corresponding author. E-mail:
[email protected]; fax: +86 551 3606266.
report the preparation and photoluminescence of InGaO3(ZnO)m nanowires with perfect superlattice structure by a chemical vapor transport method. Like that for these materials in films, in nanowires they have monoclinic structure, which is different from that of ZnO wurtzite. Compared to the results reported,3-6 the InGaO3(ZnO)3 superlattice nanowires have the shortest period of superlattice structure, which is advantageous for quantized physical properties.4 Experiment Section InGaO3(ZnO)m superlattice nanowires are prepared by a chemical vapor transport method. First, 0.5 g of a powder mixture composed of ZnO, In2O3, and Ga2O3 (mole ratio of 10:1:1) was loaded into an alumina boat. Then the boat was put into the center of a ceramic tube which was mounted on the tube furnace. A silicon (100) slice coated with 2 nm Au used as substrate was placed 9 cm to the source. The tube furnace was heated to 1400 °C in 90 min and maintained at the peak temperature for 5 min. Finally, heating was turned off and the furnace was cooled to room temperature naturally. The as-grown products were characterized by X-ray diffraction (MAC MXPAHF with Mo KR radiation, wavelength 0.70930 Å). SEM images were obtained with field-emission scanning electron microscopy (JEOL JSM-6700F). The nanowires were examined using high-resolution transmission electron microscopy (JEOL 2010) operating at 200 kV. Photoluminescence (PL) spectra were measured at room temperature using a He-Cd laser (325 nm) as excitation source. Results and Discussion The low magnification scanning electron microscopy (SEM) image in Figure 1a reveals that most of the nanowires obtained are about 30 nm in diameter and several micrometers in length. Figure 1b is the high magnification SEM image. The facts that Au particles can be seen attached to the ends of some nanowires and that no products were formed on the Si substrate uncoated with Au catalysts suggest a vapor liquid solid (VLS) growth mechanism. Figure 2 shows the XRD pattern of as-synthesized nanowires. Besides the peaks of cubic structure In2O3, there are several
10.1021/jp906381h 2009 American Chemical Society Published on Web 12/03/2009
InGaO3(ZnO)m Nanowires
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Figure 1. SEM image of the as-synthesized nanowires: (a) low magnification; (b) high magnification.
Figure 2. XRD pattern of the as-synthesized nanowires.
peaks corresponding to InGaO3(ZnO)3 (PCPDF 40-0253). As known for M (M ) In, Fe, Ga, and Al)-doped ZnO films,18-23 the phase of InGaO3(ZnO)3 represents a type of superlattice structure with every four Ga/Zn-O layers and one In-O layer alternately stacked along the [0001] direction. Combined with the fact that EDS shows In, Ga, Zn, and O elements in the nanowires, the XRD result suggests the formation of InGaO3(ZnO)3 superlattice nanowires. This result is different from those that have been reported previously, in which only hexagonal wurtzite ZnO diffraction peaks appeared in the XRD pattern and no In2O3(ZnO)m superlattice structure peaks were detected.3-6 Furthermore, TEM results indicated that those superlattice nanowires retain ZnO wurtzite structure and cannot be described using the In2O3(ZnO)m crystal structure. As we know, the crystal structure of InMO3(ZnO)m (M ) In, Ga, Fe, Al, etc.) is monoclinic when m is odd, which is different from that of wurtzite ZnO.20,23 According to the XRD results, the InGaO3(ZnO)3 superlattice nanowires have a monoclinic crystal structure like that of InGaO3(ZnO)3 superlattice in film. This is confirmed by TEM investigation as discussed in the next paragraph. We have also synthesized In2O3(ZnO)m planar superlattice nanoribbons with monoclinic crystal structure, which have been published elsewhere.7 To obtain direct proof for the formation of InGaO3(ZnO)3 superlattice nanowires, a TEM study was carried out, and the results are shown in Figure 3. In the TEM image, a nanowire with a diameter of about 30 nm clearly shows the alternate layered structure of the superlattice along the nanowire growth direction, which can be named as an axial superlattice nanowire.7 The inset in Figure 3a is the corresponding SAED pattern, which
Figure 3. (a) TEM image and the inset SEAD pattern, (b) HRTEM image, and (c) EDS spectrum of a InGaO3(ZnO)3 superlattice nanowire. (d) TEM image and the inset SEAD pattern. (e) HRTEM image of a InGaO3(ZnO)5 superlattice nanowire.
shows four subspots between two adjacent main spots. All the spots can be indexed using the InGaO3(ZnO)3 monoclinic structure, and the growth direction of the superlattice nanowire is along [0001]. Figure 3b gives the corresponding HRTEM image of the area marked in Figure 3a. As marked in Figure 3b, the image shows perfect periodical superlattice structure with exactly four Ga/Zn-O layers between two adjacent In-O layers, which is consistent with the SAED pattern shown in Figure 3a.
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Figure 3c shows the EDS spectra of the InGaO3(ZnO)3 axial superlattice nanowire, and the atom ratio of Zn, In, and Ga is found to be 4:3:1, which indicates excessive In atoms present in the superlattice nanowire compared to the atom ratio of InGaO3(ZnO)3. We conclude that there should be a mass of Zn and Ga atoms being substituted by In atoms in Ga/Zn-O layers, and this may cause a distance between two adjacent In-O layers to be larger than that in normal InGaO3(ZnO)3 structure, considering the radius of In3+ (0.81 Å), Zn2+ (0.74 Å), and Ga3+(0.62 Å). This is consistent with the HRTEM results. From Figure 3b, the distance between two adjacent In-O layers is 1.44 nm, which is larger than that in normal InGaO3(ZnO)3 superlattice structure, 1.38 nm. Because more than stoichiometric In atoms in the nanowires result, as shown in the EDS, the increase of the distance between two adjacent In-O layers is possibly a result of Ga2+ and Zn2+ in the Ga/Zn-O blocks being substituted by In2+ with a larger ion radius. Combining TEM investigations with the XRD pattern, we conclude that the InGaO3(ZnO)3 axial superlattice nanowires have formed in the products. Besides InGaO3(ZnO)3 superlattice nanowires, we also obtained InGaO3(ZnO)5 axial superlattice nanowires, and the results are shown in Figure 3d and Figure 3e. A clearly periodical layered structure is shown in Figure 3d, and the corresponding SAED is shown in its inset. The diameter of the InGaO3(ZnO)5 axial superlattice nanowire is modulated, which is similar to the results reported by Na et al.4 Figure 3e gives the HRTEM image of the area marked in Figure 3d, which shows a perfect superlattice structure. As displayed in Figure 3e, every six Ga/Zn-O layers are bound with two In-O layers stacking alternately along the nanowire growth direction. According to the reports, most In2O3(ZnO)m superlattice nanowires previously reported have shown imperfect superlattice structures.3-6 Those nanowires cannot be described by a precisely defined In2O3(ZnO)m unit cell but rather by a polytypoid structure. Therefore, it is diffraction peaks of wurtzite ZnO, and not that of In2O3(ZnO)m that emerge in the XRD pattern. According to the XRD results and TEM investigations, the InGaO3(ZnO)m axial superlattice nanowires obtained here have a perfect superlattice structure that can be described by a precisely defined InGaO3(ZnO)m unit cell. A VLS growth process of InGaO3(ZnO)m superlattice nanowires is suggested as follows: ZnO, In2O3, and Ga2O3 mixed powders were vaporized at elevated temperature, and then the vapor diffused to the substrate. Zn, In, and Ga vapor reacted with Au and formed small alloy droplets. When the compound system reached the supersaturated state, solid InGaO3(ZnO)m precipitated from the droplet in the form of nanowires, and continued feeding of Zn, In, and Ga atoms into the droplet sustained the growth of the nanowires. In the formation of a perfect superlattice structure, we suggest that Ga2O3 played a very important role. Ga atoms in the crystal structure of InGaO3(ZnO)m nanowires result in a perfect superlattice structure that is more stable than the imperfect superlattice of nanowires. Without Ga2O3 powder added to the source, the product contains only In-doped ZnO nanobelts with (001) as the wide surface and In2O3(ZnO)m superlattice nanoribbons with In/Zn-O layers and In-O layers stacking along the nanoribbon height direction.7 The In2O3(ZnO)m superlattice nanoribbons do not have perfect superlattice structure. In order to investigate the optical properties of InGaO3(ZnO)m axial superlattice nanowires, PL spectrum measurements were carried out, and the results are shown in Figure 4. As shown in Figure 4, two kinds of PL spectra, labeled a and b, were obtained in different positions of the substrate. For curve a, the dominant
Li et al.
Figure 4. PL spectra of as-synthesized nanowires excited by a He-Cd laser (325 nm).
peak is positioned at 3.26 eV; for curve b, the corresponding peak is positioned at 3.23 eV. Furthermore, a broad peak emerges in the visible region around 2.3 eV for both curves. According to the reports, the peak centered at 3.26 eV could be attributed to near band edge emission (NBE) of the In2O3 nanowires and the broad peak corresponds to the deep level emission.24,25 Curve b is different from the typical emission of In2O3 nanowires. Considering the two types of PL spectra obtained at different sites, we suggest heterogeneous distribution of nanowires on the substrate. Because only two kinds of nanowires, In2O3 and InGaO3(ZnO)m, are observed in the product according to XRD results, curve b possibly reflects the emission from the perfect superlattice InGaO3(ZnO)m nanowires. However, 3.23 eV is smaller than the bandgap of homogeneous compounds InGaO3(ZnO)m,26 which is possibly induced by the heavy doping of In in Ga/Zn-O layers in the InGaO3(ZnO)m superlattice nanowires. The doping also induces the increase of the distance between two adjacent In-O layers. Because the product contains two kinds of nanowires, the curve b in Figure 4 cannot be certainly attributed to the emission of InGaO3(ZnO)m. To clarify this problem, the preparation of superlattice nanowires with higher purity or a spectrum obtained from a single nanowire is needed for advanced research. Conclusion InGaO3(ZnO)m (m ) 3, 5) nanowires with pefect superlattice structures have been prepared by a chemical vapor transport method. The XRD pattern suggests that the nanowires have a monoclinic crystal structure, described using a InGaO3(ZnO)m unit cell, that is different from the wurtzite structure of the In2O3(ZnO)m quasi-one-dimensional superlattice nanostructures so far reported. HRTEM images demonstrate clearly alternate layers of In-O and In, and Ga/Zn-O stacking along the nanowire growth direction. EDS results suggest that an amount of Zn and Ga atoms are substituted by In atoms, which results in the broadening of adjacent In-O layers and the shift of the diffraction peaks in the XRD pattern. A period of a InGaO3(ZnO)3 nanowire consists of four atom layers, which is the shortest period in quasi-one-dimensional superlattice structures. Acknowledgment. This work was supported by Natural Science Foundation of China (Grant No. 10574122, 50772110, 50721091), the National Basic Research Program of China (2006CB922000, 2007CB925202, 2009CB939901). References and Notes (1) Lensch-Falk, J. L.; Hemesath, E. R.; Lauhon, L. J. Nano Lett. 2008, 8, 2669.
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