Large-Scale Synthesis of Bicrystalline ZnO Nanowire Arrays by

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Large-Scale Synthesis of Bicrystalline ZnO Nanowire Arrays by Thermal Oxidation of Zinc Film: Growth Mechanism and HighPerformance Field Emission C. X. Zhao,† Y. F. Li,† J. Zhou,‡ L. Y. Li,‡ S. Z. Deng,† N. S. Xu,† and Jun Chen*,† †

State Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Province Key Laboratory of Display Material and Technology, School of Physics and Engineering, Sun Yat-sen University, Guangzhou 510275, People’s Republic of China ‡ Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China ABSTRACT: Understanding the origin crystal nucleation and driving force is critical for the synthesis of one-dimensional nanomaterials with controllable size, morphology, and crystal structure. The growth behavior of ZnO nanowires prepared by thermal oxidation of zinc film is studied. It is found that the grown nanowires have a bicrystalline structure and growth direction significantly different from the commonly observed [0001] direction. On the basis of detailed high resolution morphology and structural analysis, we propose the origin of the initial growth site, as well as the driving force for formation of the bicrystalline structure of aligned ZnO nanowires. The initial zinc film plays an important role in the growth of nanowire. The edge-enhanced oxidation effect of zinc grain initiates the ZnO nanowire nucleation. The strain within the ZnO layer drives and stimulates the nanowire growth. The present study provides insight into the growth mechanism of ZnO nanowires grown from thermal oxidation of zinc film. Field emission measurement results show that the prepared ZnO nanowires have excellent field emission properties. Uniform emission can be obtained and the turn-on field is 7.8 V/μm. The results indicate that the method is advantageous for large-scale synthesis of ZnO nanowires for field emission applications.

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been employed to synthesize ZnO nanowires.29 Among them, the thermal oxidation technique has received much interest due to its good compatibility with silicon-based microfabrication processes. Although low temperature growth of ZnO nanowires from oxidation of zinc metal has been reported previously,2−4,8−19 the mechanism of nanowire growth during oxidation remains unclear. Much effort has been devoted to characterize the growth behavior of ZnO nanostructures. Rackauskas et al.16 reported that the growth of ZnO nanowire was due to the migration of zinc interstitials and vacancies through grain boundary and crystal defects. It has been inferred that grain boundary diffusion is advantageous for initiating the nucleation site of ZnO nanowire growth. Sun et al. proposed that the zinc concentration is vital for the final morphology of ZnO nanostructures.17 Meanwhile, ZnO molecules preferred to deposit on epitaxial substrate with faster kinetics, and aligned ZnO nanowires could be synthesized.18 The general consensus was that the oxide nanowires are grown from the underneath oxide layer with the same phase or crystal structures. Several growth mechanisms have been proposed, including self-catalysis vapor−solid (VLS) mecha-

INTRODUCTION Since the first report of facile synthesis of CuO nanowires on copper substrate by direct heating in air, considerable attention has been paid to oxide nanowires formation by the thermal oxidation method.1 The advantages of this method include technical simplicity and compatibility with microfabrication process, as well as large-scale growth capability.2−5 Until now, it has been reported that CuO, ZnO, Fe2O3, Ga2O3, and Al nanowires can be prepared by this technique.6−25 The oxidation of metals is an old topic. The classical theory of Mott model has been proven to be successful in predicting the oxidation behavior of most metals.26 However, the model usually assumed the uniform and continuous growth of oxide thin film by the transport of ionic species through the oxide layer. However, the underlying mechanism for one-dimensional growth of oxide nanowire is still a matter of debate. Moreover, the driving force for the spontaneous growth of nanowire has not been fully understood. Some effects related to the interface between the crystal grains should be considered, such as the expansion of grain boundary, coalescence of grains, etc. ZnO nanowire is considered as an important material for applications, such as in field emission, catalysis, gas sensor, lightemitting diode, and solar cells,27−29 due to its low cost, environment friendly and unique optical-electric properties. Various methods including physical and chemical methods have © 2013 American Chemical Society

Received: February 27, 2013 Revised: June 4, 2013 Published: June 4, 2013 2897

dx.doi.org/10.1021/cg400318f | Cryst. Growth Des. 2013, 13, 2897−2905

Crystal Growth & Design

Article

nism,2,3,8,12,14,15 mass transfer (or diffusion),5,7,16−19 and vapor− solid (VS) mechanism,1,3,11,13 as well as the stress-driven diffusion mechanism.4,6,9,20−22 The reported experimental observations are varied; the as-grown ZnO nanostructures have single crystalline2,3,11−18 or bicrystal structures.4,5 Some early studies have indicated that metal oxidation usually involves the nucleation and growth of metal oxide islands.30 However, several questions regarding the nucleation and the real driving force for the initiation of ZnO nanowires formation during the oxidation become significant. To date, there has been no straightforward experimental evidence to verify the one-dimensional growth of ZnO nanowires by the thermal oxidation technique. In this paper, we carefully characterized the structure of the ZnO nanowires prepared by thermal oxidation of zinc thin film. The nucleation sites, driving forces for growth, and possible mass transportation mechanism are studied. Field emission characteristics of the prepared ZnO nanowire are also presented. Our study should shed light on the growth mechanism of the thermal oxidation method and is of significance for the controlled growth of ZnO nanowires with uniform morphology and structure.

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EXPERIMENTAL DETAILS

Figure 1. (a) The SEM image of as-deposited zinc thin film before thermal oxidation; inset shows a single zinc grain; (b) diameter distribution of zinc grains; (c, d) typical high resolution SEM images of individual zinc grain.

Zinc oxide nanowires were grown by the thermal oxidation technique. N+-type Si (100) substrates were first ultrasonic cleaned with alcohol, acetone, and deionized water for 15 min, respectively. Then the substrates were dried by blowing N2. Zinc metal thin films (purity 99.9%) were deposited on silicon substrate by the electron beam evaporation method with a thickness of 1.2 μm. The deposition rate was 6 Å/s. The thermal oxidation was carried out in a horizontal quartz tube furnace in air. After the samples of metal zinc film were put into the furnace, the temperature was raised to 500 °C at the rate of 2.5 °C/min. When reaching 500 °C, the temperature was kept constantly for 2 h. Then the furnace was slowly cooled down to room temperature. We defined the holding time for the growth of ZnO nanowires as the constant temperature (500 °C) period. In order to study the oxidation kinetics of nanowire growth, the holding time was varied and set to be 1, 5, 20, 40, 80, and 120 min in separated experiments. The surface and cross-sectional morphologies of deposited film and as-grown ZnO nanowires were examined using scanning electron microscopy (SEM, Zeiss SUPRA-55). Detailed morphology and crystal structure were investigated by high resolution transmission electron microscopy (HRTEM, JEOL2010-HR and FEI Titan G2 60−300). For the growth of ZnO nanowire arrays used for field emission measurement, we used indium−tin-oxide (ITO) coated glass as the substrate. The thickness of the ITO layer is 500 nm, and the sheet resistance of the ITO glass is ∼12 Ω/□. The Zn thin film patterns were first deposited on the ITO electrode by using the lift-off method. Each pattern has a dimension of 25 × 60 μm2. Then, zinc thin film was heated in a tube furnace with the same oxidation procedure mentioned above. The field emission measurement was carried out in two parallel-plate configuration with phosphor coated ITO glass as the anode under a vacuum of 5 × 10−5 Pa. The distance between the phosphor screen and sample is 500 μm. By applying the voltage to the phosphor screen, the field emission current versus applied voltage characteristics (I−V) are recorded, from which the emission current density versus applied field characteristics (J−E) can be derived. In the meantime, the emission image was recorded by a CCD camera.

is about 350 nm. In order to have clear insight into the morphology of the zinc grain, high magnification SEM images of individual grains viewed from the other directions are also shown in Figure 1c,d. It clearly reveals that the disklike zinc particles have hexagonal morphology with crystal steps or facets. As we will discuss later, these steps or facet sites would be beneficial for the growth of nanostructures. Figure 2 shows the representative morphology of as-grown ZnO nanowires (NWs) as examined by SEM. In top-view SEM shown in Figure 2a, it can be seen that the sample surface is covered by dense ZnO nanowires. Figure 2b gives the high magnification SEM image of the nanowires tips. From the top view of SEM images, the average density is estimated to be 3 NWs/μm2. Most of the nanowires have a uniform diameter at the tips, and with a smooth surface along their longitudinal growth direction. Figure 2c is a cross-section view of SEM image of ZnO nanowires. The nanowires have a tapered shape, with a maximum diameter at the base and most of them are relatively aligned with respect to the substrate surface. Detailed statistics of the nanowire morphology are presented in Figure 2d. It can be obtained that the average tip diameter and length of nanowires are ∼15.5 nm and 1.4 μm, respectively. The crystalline nature and detailed structure of nanowires is also examined by TEM and selective-area electron diffraction (SAED). We could not obtain a single set of diffraction spots in the SAED pattern by assuming Zn or ZnO phase, which indicated the existence of a bi- or multicrystal structure in the nanowires. First, we carefully analyzed SAED patterns from multiple nanowires to confirm the crystal structure of assynthesized products. As shown in Figure 3, the SAED ring patterns the Debye−Scherrer concentric rings of (100), (002), (101), (102), and so on, confirming that the ZnO nanowires were in Wurtzite structure (JCPDS# 75-1533). The weak spots in the pattern are attributed to the secondary or multidiffractions of the electron beam from bicrystal sites. We will give further discussion about it later.

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RESULTS AND DISCUSSION I. Morphology and Structure of Zinc Film and AsGrown ZnO Nanowires. Figure 1a shows the zinc thin film asdeposited by electron beam evaporation. The zinc film is composed of zinc particles. The inset of Figure 1a displays the high magnification image of one zinc particle. The statistical distribution of particle size is shown in Figure 1b, which indicates the as-grown zinc particles have uniform size, and the average size 2898



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Figure 2. Typical SEM images of as-grown ZnO nanowires: (a) low resolution; (b) high resolution; (c) cross-sectional SEM image of ZnO nanowire arrays; (d) size distribution of as-grown ZnO nanowires.

to be mentioned that other growth directions of ZnO nanowire have also been reported before2,9,10,17,18 for ZnO nanowires prepared by the thermal oxidation technique. This probably results from the differences of the as-deposited zinc grain as well as the oxidation parameters. It is interesting to note that the growth feature (such as growth direction, bicrystalline structure) is quite common for other metal oxides grown by a similar thermal oxidation technique, such as Fe2O3,21 CuO,1,6,22 nanostructures. TEM examination of different ZnO nanowires indicates that the bicrystal is a common structure feature in most of the asgrown samples. Analysis of the corresponding SAED patterns of two randomly selected nanowires confirmed that the nanowires were well crystallized ZnO (Figure 5a,b). And the growth direction of nanowires could also be determined to be [112̅0], which is along the a axis of the hexagonal structure. Meanwhile, Figure 5c shows a bright-field TEM image from a single nanowire which is divided by a twin boundary along its growth direction. Figure 5e displays a high resolution image of the boundary which shows the nanowire has bicrystalline structure, and there appears to be hollow pipe in the core of the nanowires. The spacing of 0.28 nm between adjacent lattice planes corresponds to (1010̅ ). The typical corresponding twin SAED pattern is also shown in Figure 5f. The twin direction was determined to be (01̅13). It is well-known that moiré fringes can be formed in TEM images as a result of interference between diffracted beams from overlapping crystals. As shown in Figure 6, the nanowire has lamellae fringes due to superposition of the bicrystals over the twin boundary. By rotating the nanowire, we could find the evolution of the fringe changed from blurred to distinct. Similar moiré fringes have also been reported in ZnO whiskers.9,10

Figure 3. Selective area electron diffraction (SAED) pattern obtained from multiple ZnO nanowires.

Figure 4 shows the typical bright field TEM image and corresponding SAED of one ZnO nanowire taken at different angles. It clearly reveals that the nanowire has tapered morphology with a sharp tip and thick bottom. It is worthy to note that two sets of diffractions spots can be identified, which indicates the nature of the bicrystalline structure within the nanowire. The diffraction pattern can be indexed, with an incident electron beam parallel to [110̅ 2], and the other set of diffraction pattern is the twin diffraction spot with a direction of [2̅203̅]. By rotating the nanowires to proper direction, the growth direction of nanowire could be indexed to be [112̅0]. Similar results of growth direction have been reported before.3,7−12 We note that the bicrystal boundary is not visible in the TEM image. This is because the twin plane is perpendicular to the incident electron beam. In addition, it has 2899



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Figure 4. TEM images of individual ZnO nanowires with little rotation: Bright-field TEM image (top) and its corresponding diffraction patterns (bottom).

Figure 5. (a−c) The bright-field TEM images and corresponding SAED patterns obtained from other nanowires. (d−f) Low and high resolution TEM image of ZnO nanowire with twin boundary structure. The e-beam direction is parallel with the twin plane.

lower than that of either (0001) or (0110̅ ). The certain growth direction along the a axis would not be favored from the energy

It has been reported that ZnO has three fast growth directions: , ⟨0110⟩, and ⟨0001⟩.3 The surface energy of (112̅0) is 2900



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Figure 6. The moiré fringes of ZnO nanowire due to the superimposition of bicrystals (a−c) obtained by in situ rotating the nanowires clockwise with small angle (

Large-Scale Synthesis of Bicrystalline ZnO Nanowire Arrays by

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