Large-scale fabrication of ordered arrays of microcontainers and the restraint effect on growth of CuO nanowires
© Shao et al; licensee Springer. 2011
Received: 19 August 2010
Accepted: 17 January 2011
Published: 17 January 2011
Technique has been developed to fabricate ordered arrays of microcontainers. We report that ordered microcontainer arrays of Cu can be fabricated on glass substrate by thin film deposition and self-assembly technology. In addition, CuO nanowires are found to grow only in the inner sides of microcontainers, which verifies the stress growth mechanism of CuO nanowires. High-resolution transmission electron microscopy study reveals that CuO nanowires grow along the  direction. Such structure may have potential application in micro-electron sources, which have the self-focused function.
Fabrication of arrays of three-dimensional (3D) micro- or nanostructures is one of the challenging tasks [1, 2]. Much effort has been made to study their fabrication and potential applications such as in biosensor , lithium secondary batteries , and micro- or nanocontainers for reaction. Wang et al.  fabricated large-scale ordered arrays of TiO2 nanobowl by utilizing monolayer self-assembly and atomic layer deposition. Zhang et al.  used colloidal crystals template to fabricate 3D ordered macroporous rare-earth oxides and Li et al.  reviewed similar ways for preparation of various ordered micro- or nanostructured arrays. Srivastava et al.  developed a modified infiltration approach for the fabrication of arrays of cobalt nanobowl. Wang et al.  made free-standing ZnO nanobowls. Kim et al.  investigated formation process of the polypyrrole microcontainers. Zhan et al.  investigated the anomalous infrared transmission of gold films on 2D colloidal crystals. Ye et al.  carried out fabrication, characterization, and optical property study of gold nanobowls. However, most of the above micro- or nanostructures have been achieved by the top-down method.
Here, technique based on self-assembly has been developed. Ordered arrays of microcontainers of copper oxide have been fabricated in large-scale and CuO nanowires have been found to grow only in the inner sides of the microcontainers without use of any catalysts. Moreover, this general and facile method can be applied to fabricate the similar 3D structures using other metals (such as Zn, Cr, Fe, etc.) and/or their oxides microcontainers.
Results and discussion
Different growth mechanism of CuO nanowires has been proposed by different research groups. Jiang et al.  believe that the formation of CuO nanowires by thermal oxidation obeys vapor-solid model (VS), where the growth of CuO nanowires depends on different vapor pressure of CuO. Liu et al.  have proposed a base-up self-diffusion model; namely, the growing process of CuO nanoneedles is controlled by the diffusion of the copper ions from the substrate, which is caused by the local electrical field set up by the oxygen ions at the solid/gas interface. Kaur et al.  and Kummar et al.  have attributed the formation of CuO nanowires to relaxation of accumulating stress. According to the VS mechanism, there exist CuO nanowires on the outside surface of microcontainers in our case. However, we do not observe any CuO nanowires on the outer surface of microcontainers. We believe that the growth of CuO nanowires is due to compressive stress. During the oxidation of Cu microcontainer, oxygen ions will diffuse inside the Cu film. Then a layer of CuO will form on both outer and inner surface of Cu microcontainer, which leads to volume expansion of microcontainer. But the CuO film cannot expand along the surface, because the film is relatively compact. The CuO film can only expand along normal direction of the surface. Due to space limit, CuO film on the inner surface of microcontainer will become concentrated as expansion, while the film on the outer of microcontainer become scattered. Therefore, compressive stress at the inner surface will become greater and greater during oxidation, and finally lead to growth of nanowires. While there is tensile stress at the outer surface, no nanowires can be grown.
In conclusion, we have demonstrated a versatile method to fabricate ordered arrays of metallic or its oxide microcontainers. Growth of CuO nanowire is observed to be retrained by the Cu microcontainers because of compressive stress accumulation. The HRTEM study reveals that CuO nanowires grow along the  direction. A potential application of the microcontainers in practical devices is also simulated. Related experiments for application of 3D metallic/oxide microcontainers, such as using vacuum electron sources, batteries, etc., need to be investigated in future.
The authors gratefully acknowledge the financial support of the project from the National Natural Science Foundation of China (Grant No. U0634002, 50725206), Science and Technology Ministry of China (National Basic Research Program of China: Grant No. 2003CB314701, 2007CB935501 and 2010CB327703; Grant No. 2008AA03A314), the Science and Technology Department of Guangdong Province, the Department of Information Industry of Guangdong Province, and the Science and Technology Department of Guangzhou City.
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