Evolution of ZnO nanowires, nanorods, and nanosheets with an oxygen-assisted carbothermal reduction process
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Evolution of ZnO nanowires, nanorods, and nanosheets with an oxygen-assisted carbothermal reduction process Jae-Hwan Park, Young-Jin Choi and Jae-Gwan Park Materials Science and Technology Division, Korea Institute of Science and Technology, PO Box 131, Cheongryang, Seoul 130-650, Korea ABSTRACT A systematic hierarchical evolution of nanowires, nanorods, and nanosheets in ZnO, based on a oxygen-assisted carbothermal reduction process, was presented. Nanowires, nanorods, and nanosheets were synthesized in series with controlling oxygen partial pressure from 10-6 to 10-1. It was impossible to get ZnO nanowires with using Ar gas only and more than 1ppm of oxygen could successfully introduce the ZnO nanowires on the Si substrates at the downstream of the tube. The additional oxygen gas with Ar carrier gas could introduce not only the nanowires but also the combs and sheets. The details of nanocombs and nanosheets also were presented. INTRODUCTION Bottom-up approaches to highly advanced (opto)electronics with using low-dimensional nanoscale building blocks such as quantum dots, nanowires, and nanotubes have received considerable attention to date (1-3). Especially, one-dimensional (1-D) nanostructures have been extensively studied due to their potentials as building blocks for fabricating nanometer-scaled electronic, optoelectronic, electrochemical and sensor devices. Various semiconducting nanowires, nanorods, and nanotubes of single element, oxide, and compound semiconductors have been successfully synthesized (1). However, the feasibility of realizing highly integrated functional devices is still questionable due to the difficulties in the alignment and assembly of the building blocks. In this regard, self-assembled nanostructures of multiple dimensionality and/or hierarchy are highly desirable in terms of realizing mesoscopic assembly of nanodevices. Meantime, ZnO, a wide-bandgap (3.37 eV) semiconductor with large exciton binding energy, have attracted considerable attention due to the potential applications for the optoelectronics. Thus, some complicated nanostructures of ZnO and hierarchical structures have been studied in the recent few years. For example, self-organized nanowire arrays have been fabricated by a simple thermal evaporation and the oxidation of source metal (4-5). Various hierarchical structures with 6-, 4-, and 2-fold symmetry have been grown via carbothermal reduction process at low pressure (6). Although there are a number of reports on the various self-assembled ZnO nanostructures, the fabrication routes and experimental conditions are different at every case and exact growth mechanisms are not clear in most cases. Here, we report a systematic hierarchical evolution of nanowires, nanorods, and nanosheets in ZnO with a novel process. A carbothermal reduction process has been used for synthesizing ZnO nanowires with Ar atmosphere gas. However, in this work, we devised an oxygen-assisted carbothermal reduction process which enables the evolution of nanowires, nanorods, and nanosheets in se
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