Synthesis and Characterization of ZnO: In Nanowires with Superlattice

New Polytypoid SnO2(ZnO:Sn)m Nanowire: Characterization and Calculation of Its ... The Journal of Physical Chemistry C 2011 115 (50), 24564-24568...
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J. Phys. Chem. B 2004, 108, 17027-17031

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Synthesis and Characterization of ZnO:In Nanowires with Superlattice Structure Jiansheng Jie,†,‡ Guanzhong Wang,*,†,‡ Xinhai Han,†,‡ and J. G. Hou† Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei 230026, China, and Department of Physics, UniVersity of Science and Technology of China, Hefei 230026, China ReceiVed: April 6, 2004; In Final Form: August 4, 2004

ZnO:In nanowires with superlattice structure have been successfully synthesized by evaporating ZnO, In2O3, and Co2O3 mixed powders. There are two types of superlattice nanowires in the product, one with longitudinal superlattice structure and another with transverse superlattice structure. Structures and components of the superlattice nanowires were investigated. It is found that they consist of In-O and In-doped Zn-O layers (In/Zn-O layers) stacked alternately perpendicular to the c axis and they have approximate In2O3(ZnO)m composition. In addition, zigzag-modulated structure was also observed and considered as the ordered arrangement of In atoms within In/Zn-O layers. A growth model based on the vapor-liquid-solid mechanism is proposed for interpreting the growth of nanowires in our work. Due to the heavy doping of In, the emission peak in photoluminescence spectra has red-shifted as well as broadened seriously.

Introduction One-dimensional (1D) semiconductor nanowires have attracted much attention due to the novel properties which make them potentially ideal functional components for nanometerscale electronics and optoelectronics.1-3 Over the past decade, component homogeneous nanowires, such as element (Si, Ge), III-VI (GaN, InP), II-VI (ZnS, CdS), and metal oxide (ZnO, In2O3, Zn2SnO4) semiconductors,4,5 have been synthesized via various methods, including vapor-phase evaporation,6,7 metalorganic vapor-phase epitaxy,8 laser ablation,9 template-assistant,10 and solution.11 However, it was not until recently that nanowires with superlattice structures were successfully synthesized. Yang and co-workers use a hybrid pulsed laser ablation/chemical vapor deposition (PLA-CVD) process to prepare Si/SiGe superlattice nanowires,12 and a similar method was employed to synthesize GaAs/GaP superlattice nanowires by Lieber and co-workers.13 Using a chemical beam epitaxy (CBE) technique, Samuelson and co-workers have also successfully synthesized the InAs/InP superlattices.14 As a common feature of these works, the vapor-liquid-solid (VLS) growth mechanism15 was used to grow superlattice structure nanowires through periodically altering the compositions of the reaction atmosphere. The achievement in the research of superlattice nanowires will significantly benefit the development of 1D nanodevices and thus “open an exciting new chapter in nanoscale research.”16,17 Although most of the superlattice structures are created through an artificial process, the natural superlattice structures are also observed in some oxide compounds, such as the In2O3ZnO compounds. Kasper first researched the structure of In2O3(ZnO)m (m ) 2-5 and 7) and reported that they have superlattice structure related closely to wurtzite structure.18 Nakamura and co-workers had studied a series of compounds * Corresponding author: Telephone: +86-551-3600075. Fax: +86-5513601073. E-mail: [email protected]. † Hefei National Laboratory for Physical Sciences at Microscale. ‡ Department of Physics.

InMO3(ZnO)m (M ) In, Fe, Ga, and Al) using X-ray diffraction analysis and suggested the superlattice structures in them were built up of a succession of InO21- (In-O layer) and MZnmOm+11+ (M/Zn-O) layers perpendicular to the c axis in the wurtzite structure,19,20 and this structure model was demonstrated by the observation of a high-resolution Z-contrast image by Yan and co-workers.21 Furthermore, as a benefit from the unique crystal structures, novel properties of these kinds of compounds have been observed. The film with a composition of Zn2In2O5 has been reported to exhibit excellent chemical stability with high electrical conductivity and optical transparency.22,23 Recently, the transparent field-effect transistor (TFET) fabricated by InGaO3(ZnO)5 was found to show higher performance than that of the TFET fabricated using a conventional transparent oxide semiconductor (TOS) such as SnO2 and ZnO.24-26 However, until now, research of the InMO3(ZnO)m (M ) In, Fe, Ga, and Al) system has been focused on their bulk materials and thin films; in no case was a one-dimensional nanostructure reported. As is well known, synthesizing semiconductors into nanostructures is an effective way to improve their optoelectrical properties. As a benefit of the unique crystal structures of compounds InMO3(ZnO)m, novel properties can be expected in their nanostructures. Herein, we report that ZnO:In nanowires with superlattice structures have been successfully synthesized by evaporating ZnO, In2O3, and Co2O3 mixed powders. This novel structure consists of In-O and In-doped ZnO layers (In/Zn-O layers) stacked alternately perpendicular to the c axis in the wurtzite structure and had approximate In2O3(ZnO)m composition. The superlattice structure observed in ZnO:In nanowire would be considered as a self-assembled micro-superlattice (or a ring of quantum boxes) in a one-dimensional structure and is expected to have novel properties which enable it to achieve a quantum confinement of carries and excitons and to have potential application in nano-optoelectronics or thermoelectricity.

10.1021/jp0484783 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/07/2004

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Experimental Section ZnO (purity, 99.999%) and In2O3 (purity, >99%) powder were mixed in a molar ratio of 18:1, and then pressed into small pills under 10 MPa pressure. The pills were calcined at 500 °C for 5 h and sintered at 900 °C for 12 h in air. After being heattreated, the pills were grinded into powder (0.2 g) again and mixed with Co2O3 (analytically pure, >99%, 0.8 g) powder, and then put in an alumina boat and inserted into a horizontal tube furnace as an evaporation source. Several pieces of SiO2 (100 nm)/Si (100) plates coated with 2 nm Au used as substrates were located at downstream positions of source materials. The powder was heated to 1400 °C within 1 h and maintained at this temperature for 30 min. The temperature of the substrates was approximately 800-1000 °C during the growth. Ar was used as carrier gas and flowed constantly at a rate of 50 standard cubic centimeters per minute (sccm). The chamber pressure was kept at 150 Torr during the experimental process. After the furnace slowly cooled to room temperature, substrates were taken out from the furnace tube. A white color product was found on the Au-coated area of the substrates. The morphologies and structures of the as-deposited products were characterized by field-emission scanning electron microscopy (FE-SEM) (JEOL model JSM-6700F), X-ray diffraction (XRD) (MAC Science, model MXPAHF), high-resolution transmission electron microscopy (HRTEM) (JEOL model 2010, operating at 200 kV), and selected area electron diffraction (SAED). Their components were measured via energy-dispersive X-ray spectroscopy (EDS) attached in the HRTEM system. A photoluminescence (PL) spectrum was measured using a He-Cd laser (325 nm) as the excitation source at 13 K. Results and Discussion FE-SEM studies of as-grown products, as shown in Figure 1a, demonstrate that the product consists almost exclusively of wire-like structures with typical lengths in the range of several tens to several hundreds of micrometers. The diameters of the nanowires normally range from 20-200 nm. Catalyst particles can be observed in the ends of nanowires, indicating a VLS growth process of the nanowires. The XRD pattern is shown in Figure 1b; except for the Au, Si, and SiO2 peaks from the substrate, all of the peaks can be indexed basing on the ZnO wurtzite (hexagonal) structure. Nevertheless, the diffraction angles of the peaks in the pattern are significant smaller than that of the undoped ZnO nanowire [see the inset in Figure 1b]. As determined from analysis of the XRD pattern, the product has lattice constants of a ) 3.281 Å and c ) 5.243 Å, which are remarkably larger than those of undoped ZnO nanowires (a ) 3.251 Å and c ) 5.204 Å). The difference could not be explained by the measurement error of the XRD equipment, which is less than 0.005 Å. Considering the fact that the additional element, indium, had been used in the action process and In3+ (0.81 Å) has a bigger ionic radius than Zn2+ (0.74 Å), it is reasonably expected that In substitutes Zn in the product and leads to the significant change of ZnO lattice constants. To obtain more detail about the structures and compositions of the product, HRTEM, SAED, and EDS measurements were performed to characterize the nanowires. It was found that most of the product is In-doped ZnO nanowires and superlattice structure is observed in a part of the ZnO:In nanowires (20-40%). Figure 2a,b shows the TEM images of a single nanowire. From the images, we have found that the nanowire has a uniform diameter along its entire length and a smooth surface without an amorphous shell. Detailed investigation shows the nanowire has superlattice structure, and each block

Figure 1. (a) FE-SEM image of the as-grown nanowires prepared by evaporating ZnO, In2O3, and Co2O3 mixed powders. The catalyst particles are located at the ends of nanowires, and some of them are indicated by white arrows. (b) XRD pattern of the sample. For comparing, enlarged spectra of the In-doped sample (black line) and the undoped ZnO nanowires (dashed line) are shown in the inset. Smaller diffraction angles imply a lattice dilatation of the In-doped sample.

has near equal thickness and aligns along the growth direction. Furthermore, in the enlarged image of Figure 2b, shown in Figure 2c, modulated structures with zigzag shape also appear within each layer. On the basis of previous studies of In2O3-ZnO compounds,19 the longitudinal superlattice structure shown in Figure 2b can be analyzed in detail, and the results are illustrated in Figure 2c: each bright line corresponds to one In-O layer, and a few pieces of In-doped Zn-O layers (In/Zn-O layers), that is, (0002) crystal planes of ZnO, are interposed in two adjacent In-O layers. Hence, the superlattice structure consists of In-O and In/Zn-O layers stacked alternately along the c axis. This superlattice structure is obviously reflected in the SAED pattern, in which a series of small diffraction spots occur between two adjacent main spots [see Figure 2d,e]. In addition, further investigations of the superlattice structure reveal the distance between two In-O layers is not identical. In the image, we mark the stacks with different thickness with A, B, and C, respectively. It is obvious that their stacking sequence along the c axis is random, and there is no evidence of a long periodicity. Therefore, this nanowire cannot be described by a precisely defined unit cell as expected for the In2O3-ZnO compounds in conventional results.19 For the compounds In2O3(ZnO)m (m ) integer) with a distance d between two adjacent

Synthesis and Characterization of ZnO:In Nanowires

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Figure 3. HRTEM image of an individual nanowire with longitudinal superlattice structure.

Figure 2. (a) TEM image of a longitudinal superlattice nanowire. (b) HRTEM image of the nanowire; a superlattice structure along the axis of nanowire can be clearly identified. (c) Enlarged image from the box in (b). In-O layers and In-doped ZnO layers (In/Zn-O layers) are illustrated in the image. In/Zn-O layers with different thickness are denoted by A, B, and C. (d) Corresponding SAED pattern of the nanowire (for ease in distinguishing the diffraction structure, all of the SAED patterns shown in this paper are dealt in the anti-phase). (e) Enlarged image of (d) showing a string of diffraction spots between two main spots, corresponding to the superlattice structure. (f) EDS spectra of the nanowire.

In-O layers, a linear relationship between d and m has been proposed:19

d ) 6.349 + 2.602m Å

(1)

From the high-resolution images, we can measure that the average value of d is about 53 Å. Next, m is calculated by eq 1 as ∼18. Thus, the average composition of the nanowire is estimated as In2O3(ZnO)18. Meanwhile, EDS measurement is also performed to detect the composition of the nanowire [see Figure 2f], which gives a close value of the molar ratio Zn:In ≈ 10:1. Moreover, a zigzag-modulated structure is also observed in the nanowire. This kind of structure has been observed in the compounds of In2O3(ZnO)m and InFeO3(ZnO)m and is attributed to the ordered arrangement of In atoms within Zn-O layers.27,28 It is suggested that the formation of the modulated structure is related to minimization of the structural stress that is caused by the introduction of the impurity atoms. From the TEM image [see Figure 2c], it is found that the zigzag shapes in the nanowire have nonuniform periodicities, either in the different In/Zn-O layers or in the same In/Zn-O layers, as well as the angles of them with the In-O layer are not identical,

which range from about 30° to 60°. Also, in some areas, the modulated structures are even too obscure to observe. These results are different from previous research,27 in which the periodicities of zigzag shapes in different In/Zn-O layers were almost equal and the angles of them with respect to In-O layer were also assigned to about 60°. These differences might be due to the relative low growth temperature of the nanowires (800-1000 °C) as compared to that in the solid-state reaction method (>1200 °C).27 It is expected that long times and high temperature annealing produce an orderly modulated structure in the nanowires. Figure 3 shows the lattice image of a nanowire with longitudinal superlattice structure. The distance between two adjacent In-O layers is about 24 Å, and there are about eight In/Zn-O layers interposed between In-O layers. Further investigation demonstrates that the spacing between the In plane and its nearest Zn planes is 0.31 nm, which is significant larger than the Zn (0002) interplanar spacing of 0.26 nm. This result is consistent with a previous report.21 Besides the nanowires with a longitudinal superlattice structure, nanowires with transverse superlattice structure have also been observed in the product. As shown in Figure 4, in this type of nanowire, layers do not stack along the growth direction (longitudinal direction) of the nanowire but along the transverse direction. Although significant changes have happened to the structures of the nanowire, the stacking direction of these layers is still parallel to the ZnO [0001] direction, which is confirmed by the high-resolution lattice image [see Figure 4b] and the corresponding SAED pattern [see Figure 4c]. Based on the analysis of TEM images and SAED pattern, a structure model is illustrated in Figure 4b, and it is similar to that of the longitudinal superlattice nanowire in essence. From the HRTEM image, the distance of adjacent In-O layers is about 19 Å, and then m ≈ 5 can be calculated via eq 1. Thus, the average composition of the nanowire obtained from the structure analysis is about In2O3(ZnO)5. Meanwhile, EDS measurement also gives a close value of Zn:In ≈ 2.1:1 [see Figure 4e]. The bombardment of electron beam to the nanowire gives one an opportunity to further study its structure. Figure 4d shows the HRTEM image taken from the nanowire that has been exposed to electron beam for a few minutes. It is seen that the bombardment and the strong thermal effect caused by electron beam significantly influence the structure of the nanowire. The edge of the nanowire becomes very rough because of the escape of the atoms from the nanowire surface. Yet, on the other hand, the In-O layers are easier to identify in this image. The bombardment seems to induce a more significant influence on transverse superlattice nanowire than on longitudinal superlattice nanowire, which might be due to the different crystal structures of these two kinds of nanowires. For the nanowires with

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Figure 5. Photoluminescence spectra of as-grown product measured at 13 K (black line). For comparison, the spectra of the undoped ZnO nanowires are also shown (dashed line).

Figure 4. (a) TEM image of a transverse superlattice nanowire. (b) HRTEM image of the nanowire. The lattice spacing of 2.48 Å corresponds to the d-spacing of (101h1) crystal planes. In-O layers and In/Zn-O layers are illustrated in the image. (c) Corresponding SAED pattern of the nanowire shown in (a). (d) HRTEM image of the nanowire that suffered from the bombardment of electron beam; the lattice spacing of 2.82 Å corresponds to the d-spacing of (101h0) crystal planes. (e) EDS spectra of the nanowire.

transverse superlattice structure, whose lateral surfaces correspond to the (0002) faces of ZnO, it is noted that the (0002) face will usually be the gliding plane of hexagonal structured crystals, which implies a weaker binding energy between the (0002) faces. This character allows (0002) faces to be more easily stripped from the surface. In contrast, the longitudinal superlattice nanowire has different lateral surfaces of (112h0) faces, and thus it has a better stability than the transverse superlattice nanowire while they are exposed to the same electron beam. The catalyst particles were observed at the end of superlattice nanowires both in FE-SEM and in TEM detection, implying a VLS growth of the superlattice nanowires. A possible growth process of the nanowires in the present work can be described: ZnO, In2O3, and Co2O3 mixed powders were vaporized at elevated temperature, and then the vapors were carried by Ar flow and transported to the position of the substrate. Zn and In vapor reacted with Au and formed little alloy droplets, solid In2O3(ZnO)m precipitated from droplet in the form of nanowires when the In2O3-ZnO system reached the supersaturating state, and continuous feeding of zinc and indium atoms into the liquid droplets sustains the growth of the nanowires. In the experiment, Co2O3 compound was used for vaporizing together with ZnO and In2O3; however, no trace of Co element could be detected in the product by XRD measurement or EDS detection. Although no evidence of Co atoms exists in the nanowires, Co2O3 is still a crucial factor for the formation of ZnO/In-O superlattice nanowires. In the contrasting experiment, ZnO and In2O3 (without Co2O3) mixed powders were vaporized with the same conditions; instead of the superlattice nanowires, only Indoped ZnO nanobelts were obtained, and no superlattice structure was found during TEM detection.29

PL spectra were measured to investigate the optical properties of the product. As shown in Figure 5, the dominant peak is located at 3.327 eV, which has red-shifted strongly to lower energy as well as broadened seriously as compared to the emission of undoped ZnO nanowires. This phenomenon must be due to the heavy doping of indium in ZnO, and a similar result has been obtained from ZnO:In nanobelts.29 Because only a part of the product is superlattice nanowires, the PL spectra in this research cannot directly reflect the optical properties of the superlattice nanowire. Thus, the product with higher purity is necessary for advanced research. Conclusion In conclusion, ZnO:In nanowires with longitudinal and transverse superlattice structures have been successfully synthesized by evaporating ZnO, In2O3, and Co2O3 mixed powders. The superlattice nanowires are characterized by HRTEM and EDS, which demonstrated that they consist of In-O and In/ Zn-O layers stacked alternately perpendicular to the c axis in the wurtzite structure and have approximately In2O3(ZnO)m composition. In addition, a zigzag-modulated structure is also observed in the HRTEM measurement and considered as an ordered arrangement of In atoms within In/Zn-O layers. The transverse superlattice nanowire with the (0002) lateral surface has worse structure stability under electron beam bombardment in TEM measurements due to its structural features. A VLS mechanism is proposed for interpreting the growth of the superlattice nanowires. Furthermore, the PL emission peak has red-shifted as well as broadened seriously because of the heavy doping of In. We are hopeful that a series of superlattice nanowires such as Ga2O3(ZnO)m and InGaO3(ZnO)m can be formed using the same method and that the oxide superlattice nanowires can be very useful in nano-optoelectrics. Acknowledgment. We thank Professor S. Y. Zhang, Mr. Gongpu Li, and Professor F. Q. Li for their assistance with the SEM and TEM experiments. We acknowledge the support of this work by the Natural Science Foundation of China (grant nos. 50121202, 60376008). References and Notes (1) Gudiksen, M. S.; Lauhon, L. J.; Wang, J. F.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617. (2) Park, W. I.; Jun, Y. H.; Jung, S. W.; Yi, G. C. Appl. Phys. Lett. 2003, 82, 964. (3) Duan, X.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241. (4) Rao, C. N. R.; Deepak, F. L.; Gundiah, G.; Govindaraj, A. Prog. Solid State Chem. 2003, 31, 5.

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