Synthesis of Well-aligned ZnO Nanowires Using Simple Physical Vapor Deposition without Catalysts or Additives
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Synthesis of Well-aligned ZnO Nanowires using Simple Physical Vapor Deposition without Catalysts or Additives
Lisheng Wang, Xiaozhong Zhang, Songqing Zhao,1 Guoyuan Zhou, Yueliang Zhou,1 and Junjie Qi The Key Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China 1 Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, China ABSTRACT Well-aligned ZnO nanowires were synthesized by simple physical vapor deposition using c-oriented ZnO thin films as substrate without catalysts or additives. The synthesized ZnO nanowires have two typical average diameters: 60 nm in majority and 120 nm in minority. They are about 4μm in length and well aligned along the normal direction of the substrate. Most of the synthesized ZnO nanowires are single crystalline in a hexagonal structure and grow along the [001] direction. The c-oriented ZnO thin films control the growth direction. Photoluminescence spectrum was measured showing a single strong ultraviolet emission (380 nm). Such result indicates that the ZnO nanowire arrays can be applied to excellent optoelectronic devices.
INTRODUCTION ZnO is a wide-band-gap n-type semiconductor, which has direct band gap energy of 3.37 eV and an excitation banding energy of 60 meV1, much larger than those of other semiconductor materials. The near-band emission, transparent conductivity and piezoelectricity make 1D ZnO nanomaterials (especially aligned ZnO nanowires) one of the most important functional semiconductor oxide nanostructures, serving to field emitters, and optoelectronic devices, etc. Since Huang et al.2 reported vapor-phase synthesis of ZnO nanowire arrays using Au catalyst via a vapor-liquid-solid (VLS) process, several methods3,4,5 were reported to synthesize aligned 1D ZnO nanostructures, among which the vapor transport and deposition method is most commonly used. However, such method often uses catalysts such as Au2, Cu6, Sn7 or other additives such as NiO8, Ga9 to assist and control the growth process. Hence, the remains of catalysts or additives will unavoidably influence the purity of the final products. In another respect, some catalyst-free methods have been reported to synthesize 1D ZnO nanostructures via a self-catalysis process of Zn or ZnO10, 11. However, only disarrayed nanostructures can be acquired by these methods. Here, we report a simple physical vapor deposition (PVD) approach to synthesize well-aligned ZnO nanowires on c-oriented ZnO thin films. Such thin films are used as substrates
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to control the growth direction of ZnO nanowires; in addition, no extra catalysts or additives appear in this approach.
EXPERIMENTAL DETAILS In our work, we used zinc powders (~74 μm in diameter and 99.99% in purity) as source material and the whole experiment was performed in a horizontal tube furnace. About 1.00 g zinc powders were spread in an alumina boat and the boat was placed at the center of the furnace tube. A layer of c-oriented ZnO
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