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Two types of novel bicrystalline ZnO nanowires have been synthesized by a thermal evaporation method. The morphology and microstructure of the nanowires have been extensively investigated. One type of the nanowires has agg twin boundary extending down its entire length with ð1103Þ twinning plane and the ½5413 zone axis. The other type is those nanowires with twin crystal-single crystal junction. The twin defects in the Sn-Zn alloy droplets in the initial growth process are proposed for interpreting the growth of these two kinds of bicrystalline nanowires.

I. INTRODUCTION

II. EXPERIMENTAL PROCEDURE

Zinc oxide has been recognized as one of the most important semiconductor materials in scientific research and technological applications, owing to its direct wide band gap (3.37 eV) and a large exciton binding energy (60 MeV).1,2 ZnO nanowires have been successfully synthesized by a thermal evaporation method in several groups.3–8 In general, those products contain several kinds of nanowires. In this work, two remarkable types of nanowires with a single twin along the growth axis and the ones with crystal-twin crystal junction have been investigated by a variety of techniques. The observation reveals that those two types of ZnO nanowires have uncommonly observed crystal structure. Although microtwins boundaries have been observed in both silicon and germanium nanowires in several other studies over the course of the last several years,9,10 the twin orientation in vapor-liquid-solid (VLS)-grown ZnO nanowires with a single twin along the growth axis and ones with twin crystal-single crystal junction are obtained successfully. It is well known that the formation of twin crystal is associated with its crystallographic system.11 According to the feature of hexagonal system, it is difficult to form ZnO twin crystal. In addition, as a significant defect structure, twin boundaries could generate positive energy and are expected to have important effects on optical, electronic, mechanical, and chemical properties, and twins can affect their subsequent performance in new technologies.12,13 Therefore, the new types of ZnO nanowires in our work may exhibit high potential for fabricating novel nanoelectronic and optical devices with enhanced performance.

A conventional tube furnace (L4513II-2/QWZ) was used to synthesize the samples. A mixture of commercial ZnO (99.99%) SnO (99.9%) and graphite powders (99.95%) (mass ratio of ZnO/SnO/C = 2:1:1) was placed in a quartz boat as the source material and then positioned in the constant temperature region of 1000  C with Ar (99.999%) at a flow rate of 100 sccm under the ambient pressure for 15 min. Then, the gas supply system was switched off, after being held at 1000  C for 30 min, and a white product in a quartz boat was collected for characterization. A Hitachi (Yokohama, Japan) S-570 scanning electron microscopy (SEM) and a Philip Tecnai-20 fieldemission transmission electron microscopy (Eindhoven, The Netherlands) were applied to characterize the structure and surface morphology