Morphology-dependent field emission properties and wetting behavior of ZnO nanowire arrays
© Yao et al; licensee Springer. 2011
Received: 2 August 2010
Accepted: 12 January 2011
Published: 12 January 2011
The fabrication of three kinds of ZnO nanowire arrays with different structural parameters over Au-coated silicon (100) by facile thermal evaporation of ZnS precursor is reported, and the growth mechanism are proposed based on structural analysis. Field emission (FE) properties and wetting behavior were revealed to be strongly morphology dependent. The nanowire arrays in small diameter and high aspect ratio exhibited the best FE performance showing a low turn-on field (4.1 V/μm) and a high field-enhancement factor (1745.8). The result also confirmed that keeping large air within the films was an effective way to obtain super water-repellent properties. This study indicates that the preparation of ZnO nanowire arrays in an optimum structural model is crucial to FE efficiency and wetting behavior.
ZnO has been considered as one of the most important electronic and photonic material because of its wide direct bandgap (3.37 eV) and large exciton binding energy (60 meV). Extensive researches have been developed on the growth of quasi one-dimensional (1D) ZnO nanostructures [1, 2] including nanowires, nanotubes, nanobelts, and nanoneedles. Meanwhile, these 1D ZnO nanostructures have been widely applied as room temperature UV detector , transparent conductive electrodes , sensors [1, 5–7], and solar cells . Recently, various inorganic semiconductor nanostructures have been the focus of the researches on the studies of FE properties  and wetting behavior , including the well-aligned 1D ZnO nanostructured arrays which have attracted great attention as promising field emission (FE) sources [1, 11–14] due to their negative electron affinity , chemical stability, tip geometry, or apex structure. A crucial factor to influence FE performance includes the interspacing between individual nanowires or nanorods, and aspect ratio. The manner in which these structural parameters could be controlled during self-organized growth processes has developed into a challenging and technological problem for nanostructure fabrication. Too closely and too densely spaced nanostructures are both not favorable to construct FE nanodevices. On the other hand, another significant application of ZnO related to the geometric effects is the wettability [16, 17], which might bring great advantages in a wide variety of applications in daily life, industry, and agriculture. The vertically aligned nanostructures involving a large amount of trapped air within the films and their high roughness have been proved to be potential for the building of hydrophobic surfaces, various surfaces of ZnO nanostructured arrays showing lotus-like water-repellent properties have been prepared in the past years [16, 18, 19].
However, many previous efforts in the large-scale fabrication of ZnO nanowire or nanorod arrays have been achieved by physical evaporation of the mixture of ZnO and graphite powders, chemical vapor deposition using Zn powder as the source materials, or low-temperature hydrothermal synthesis with the pre-prepared colloidal ZnO nanocrystals as the grown seeds. In this article, a novel fabrication of ZnO nanowire arrays with different structural parameters over Au-coated silicon (100) by facile thermal evaporation of ZnS precursors is reported. The nanowire diameter and growth speed were controlled by changing the thickness of coated Au film layer together with substrate locations. The authors studied the morphology-dependent FE performance, and first revealed that wetting behavior of ZnO nanowire arrays in different void ratios, which confirmed that a large amount of air kept within the films would be an effective way to obtain super water-repellent properties.
The fabrication was performed using a two-end open quartz tube connected to a rotary vacuum pump and a gas inlet through a vacuum coupling. The silicon (100) substrates prepared for samples A, B, and C were sonicated in acetone, washed with de-ionized (DI) water, and dried with nitrogen. Then, Au film layers were deposited on these substrates by ion sputtering from the Au target (99.999%) using an ion sputter coater (Hitachi E-1045, Hitachi Co., Tokyo, Japan.). The target-substrate distance was about 30 mm, and the pressure of sputtering chamber was pumped down to 6 Pa before deposition. The coating rate depending on discharge current was kept at 6 nm/min. The three kinds of above-mentioned substrates were sputtered for 50, 50, and 15 s, respectively. The corresponding thicknesses of Au film layers are about 50, 50, and 15 Å. Growth procedures were conducted by thermal evaporation of commercially available high purity ZnS powder and graphite powder with equal molar ratio, which was placed at the center of the quartz tube furnace. Silicon substrates were placed downstream about 5 cm (samples B and C) and upstream about 5 cm (sample A) away from the source materials to collect the products. Subsequently, we introduced an Ar gas flow of 80 sccm, and a fixed pressure at about 150 Torr was applied. The tube furnace was then heated to 750°C quickly and maintained at this peak for 30 min. After it cooled down naturally to room temperature, all the substrates appeared dark gray indicating the deposition.
The morphology and crystal structures were characterized by field emission scanning electron microscope (FE-SEM, Philips Sirion 200) and X-ray diffractometer (Bruker-AXS system) with Cu Kα radiation (λ = 1.5406 Å). The surface chemical composition of these ZnO nanowire arrays was analyzed by XPS (Kratos AXIS Ultra DLD) with a power of 150 W. A monochromatic Al Kα X-ray source (1486.6 eV) was operated in a constant analyzer energy mode. Water contact angle (CA) and sliding angle were measured using an optical contact-angle meter system (Data Physics Instrument GmbH, Germany) at ambient temperature. FE properties were carried out employing a two-parallel-plate configuration in an ultrahigh vacuum chamber (5 × 10- 7 Pa). In brief, samples were stuck onto a stainless-steel sample stage using conducting glue to act as the cathode, while another parallel stainless steel plate served as the anode with a fixed cathode-anode distance of 300 μm. The emission current was monitored via a Keithley 485 picoammeter.
Results and discussions
Structural and compositional characterization of ZnO nanowire arrays
The structural parameters of the three kinds of nanowire arrays
Void ratio (%)
142.1 ± 1°
94.8 ± 1°
154.3 ± 1°
Figure 4c shows the top-view SEM images of Au-coated silicon substrates after annealing at 750°C for 30 min in the absence of source materials, but with the other experimental conditions unchanged. The Au film layer melted into separated nanoparticles with different sizes evenly distributed on the surface of Si substrates, which are about 200-300 nm in diameter for the samples A and B, but only about 40-50 nm for the sample C. It illustrates that thicker Au film layer leads to larger Au nanoparticles during the initiated growth process, in agreement with the previous study . According to the VLS growth mechanism, the nanowire's diameter is defined by the Au nanoparticle's diameter, which was observed by the fact that the sample B with Au film layer about 50 Å has the nanowire with larger diameter than that of the sample C coated with Au film of 15 Å. However, diameters of all these nanowires were observed to be larger than the corresponding Au nanoparticle sizes because of the coarsening effect resulting from the formation of a supersaturated Au-Zn-S alloy liquid droplets. However, the sample A was located upstream, although it has the same Au nanoparticle size formed during the initiated growth as sample B, the captured ZnS vapor would be less than that located in the downstream, leading to insufficiency of zinc vapor so that the growth speed was decreased and the coarsening effect would not be remarkable.
where l, r, and s are the length, radius, and the interspacing of ZnO nanowires, respectively; h is an alterable parameter which can be adjusted to fit the experimental data. It is obvious that the field-enhancement factor β can be decided by the aspect ratio and the interspacing of nanowires. The sample C has the nanowires up to 25 μm in length but only tens of nanometres in diameter; the aspect ratio as high as 312.5 could explain for its excellent FE properties. However, the aspect ratios of the samples A and B are 20 and 41.7, respectively, indicating that β is not linearly increasing with the aspect ratio, which could be attributed to the screening effect. From the experimental results, it can be observed that the E to and β values were all not proportional to their nanowire densities (revealed in Table 1), we could conclude that nanowire density was not the essence in deciding the FE efficiency of nanostructured arrays, and that it was indispensable to consider the aspect ratio including the tip morphology and the relative void ratio.
Three kinds of large scale ZnO nanowire arrays with different aspect ratios and void ratios were fabricated using facile thermal evaporation route using ZnS source materials. Experimental results demonstrated that ZnO nanowire arrays with larger aspect ratio and proper density have better FE properties including lower turn-on field and higher field-enhancement factors. Moreover, a larger void kept within the nanostructured films was proved to be important for preparation of super water-repellent surfaces. This study could be a good platform to elucidate the physical essence of the FE performance and wetting behavior related to the corresponding nanostructured arrays.
field emission scanning electron microscope
This study was supported by the Natural Science Foundation of China (Grant Nos. 10874115 and 10734020), the National Major Basic Research Project of 2010CB933702, Shanghai Nanotechnology Research Project of 0952nm01900, Shanghai Key Basic Research Project of 08JC1411000, and the Research fund for the Doctoral Program of Higher Education of China. The authors sincerely thank Professor D.P Yu and Professor Q. Zhao (the State Key Laboratory for Mesoscopic Physics, and Electron Microscopy Laboratory, School of Physics, Peking University) for their help in FE measurements.
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