Growth of Highly Oriented ZnO Nanorods by Chemical Vapor Deposition
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Growth of Highly Oriented ZnO Nanorods by Chemical Vapor Deposition Sai-Chang Liu and Jih-Jen Wu Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan ABSTRACT Highly-oriented ZnO nanorods were grown on the fused silica substrates by a thermal CVD technique using Zinc acetylacetonate (Zn(C5H7O2)2). The substrate was heated to 500oC and the vaporization temperature of Zn(C5H7O2)2 was keep at around 135oC. X-ray diffraction and TEM analyses reveal that the nanorods are preferentially oriented toward the c-axis direction. Photoluminescence (PL) and absorption measurements show a strong emission at around 380nm which corresponds to the near band-edge emission of the bulk ZnO. Moreover, the negligible green emission band in PL spectrum and the absence of E1(LO) mode of the ZnO crystal in Raman spectrum indicate a low concentration of oxygen vacancy in the highly-oriented ZnO nanorods. INTRODUCTION One-dimensional nanometer-sized semiconductor materials, i.e. nanowires and nanorods, have attracted considerable attention due to their unique properties and suitability for fundamental studies of the roles of dimensionality and size in their physical properties as well as for the application to optoelectronic nanodevices [1]. ZnO exhibits a direct band gap of 3.37 eV at room temperature with a large exciton binding energy of 60 meV. The strong exciton binding energy which is much larger than that of GaN(25 meV) as well as the thermal energy at room temperature(25 meV) can ensure an efficient exciton emission at room temperature under low excitation energy [2,3]. In consequence, ZnO is recognized as a promising photonic material in the blue-UV region. Room temperature UV lasing properties have been indeed demonstrated with ZnO epitaxial films, microcrystalline thin films and nanoclusters recently [4,5]. Moreover, room temperature UV lasing in ZnO nanowires has been demonstrated very recently [6]. The synthesis of one-dimensional single crystal ZnO nanostructures has been of growing interest owing to their promising application in nanoscale optoelectronic devices. Single crystalline ZnO nanowires have been synthesized successfully using VLS growth methods [6,7,8] at rather high temperatures for the VLS mechanism proceeding. Besides, single crystalline ZnO nanobelts have also been reported by simply evaporating the ZnO powders at a high temperature of 1400oC [9]. However, high density and well-ordered nanostructures will be needed from practical application V2.4.1
point of view. Here, we present a simple catalyst-free CVD approach for the growth of the highly-oriented ZnO nanorods at a low temperature of around 500oC. EXPERIMENTAL DETAILS ZnO nanorods were grown in a two-temperature-zone furnace (see figure 1). Silicon wafers and fused silica plates were employed as substrates. They were cleaned in an ultrasonic bath of acetone for 20 min. Zinc acetylacetonate hydrate (Zn(C5H7O2)2．xH2O, Lancaster, 98%) placed in a glass cell was loaded into the low temperature zone of the furnace which was controll
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