Effects of Doping on the Growth of ZnO Nanostructures
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Effects of Doping on the Growth of ZnO Nanostructures Yanfa Yan, P. Liu, M.J. Romero, and M.M. Al-Jassim National Renewable Energy Laboratory, Golden, Colorado 80401 ABSTRACT We report on the effects of doping on the growth of ZnO nanostructures. We find that the incorporation of dopants, such as Si, Li, and Na, usually lead to reduced quality of ZnO nanostructures. Although doping with Li and Na results in the formation of facets on ZnO nanostructures, doping of Si often reduces dramatically the quantity of ZnO nanostructures. The formation of facets on ZnO nanostructures is found to significantly affects the cathodoluminescence emissions of the nanostructures. INTRODUCTION Zinc oxide (ZnO) has long been recognized as a useful material for optically transparent conducting layers in displays and photovoltaic devices [1,2]. Recently, zinc oxide nanomaterials have attracted great attention in both fundamental research and device applications due to their potential applications in optoelectronics. Room-temperature lasing actions of highly oriented nanorod arrays of ZnO have been reported, and field-effect transistors using ZnO nanobelts have been demonstrated [3,4]. The electric and optical properties of semiconductor nanostructures are known to have a sensitive dependence on both their sizes and shapes [5,6]. So far, ZnO nanostructures with various shapes, such as nanowires, nanorod, nanobelts, nanosheets, and nanodisks, have been synthesized [7-10]. Photoluminescence measurements have shown that ZnO nanostructures with different shapes present different optical properties [11]. A variety of approaches have been developed to synthesize ZnO nanostructures, including thermal evaporation [7-10], vapor-liquid-solid [3], and solution-liquid-solid [12,13]. Of these techniques, thermal evaporation is the simplest, yet most effective method. For many device applications, the ZnO nanostructures are required to have good electrical conductivity. Although ZnO can be made conducting by oxygen deficiency, in many cases, ZnO often needs to be doped to obtained desirable conductivity. Here we report the effects of doping on the growth and morphologies of ZnO nanostructures. The dopants studied in this paper are Li, Na, and Si. EXPERIMENTAL The synthesis process was carried out in a quartz tube, which is in atmosphere. The source material was placed in a quartz crucible that was then placed in the middle of the quartz tube. The quartz tube was inserted into a horizontal tube furnace, which was heated at a rate of 20°30°C/min to 1100°C. A quartz plate was placed in the downstream end of the quartz tube. Its temperature, monitored by a thermocouple diode, was about 500°-600°C during growth. After a 30-min evaporation, the quartz tube was taken out of the furnace and cooled to room temperature. Fluffy dark gray products formed on both the quartz plate and nearby areas on the inner wall of the quartz tube. For synthesis of undoped ZnO nanostructures, the source material was pure ZnO powder mixed with graphite (molar ratio of 1:1)
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