Vertically aligned Zn2SiO4 nanotube/ZnO nanowire heterojunction arrays.

One-dimensional (1D) semiconductor nanostructures are important building blocks for achieving active nanophotonics and nanoelectronics. For example, semiconductor nanowires show great promise for waveguides, nanoelectronic devices, and integrated nanosystems because they can function both as device components for logic, memory, and sensing applications and as interconnects. So far, light-emitting diodes, lasers, photodetectors, and field-effect transistors have been fabricated using semiconductor nanowires. Semiconductor heterostructures 7] have played an important role in modern device physics because of their great importance to fundamental research and practical optoelectronic devices. Unique optical electronic devices, such as single-electron transistorsand resonant tunneling diodes, have been constructed based on such heterojunctions between two semiconductors. However, compared to the significant progress in homogeneous nanowires and nanotube preparation, a general synthetic scheme for 1D nanostructure heterojunctions is still lacking. Recently, axial semiconductor heterojunctions with composition modulations have been prepared based on controlled chemical vapor deposition of GaAs/GaP, InAs/InP, Si/SiGe, and ZnO/ ZnMgO systems. It has been recognized that the vertically aligned growth of 1D nanostructures is a simple but very efficient self-assembly technique for integrating nanowires into nanodevices. It is of great importance that those hetero-nanostructures can be directly fabricated with aligned morphology while keeping their unique structure and functions. In this study, we have for the first time been able to prepare aligned Zn2SiO4 nanotube/ZnO nanowire heterojunction arrays using a template-assisted growth, followed by an annealing process. The formation of the heterojunction is controlled by the annealing temperature and the size of the ZnO nanowire templates. The technique developed in this paper provides a potential technique for fabricating aligned heterojunction arrays. The synthesis strategy of the Zn2SiO4 nanotube/ZnO nanowire heterojunction arrays involved three steps. First, vertical aligned ZnO nanowire arrays were grown on GaN substrates via a vapor–liquid–solid (VLS) process. Second, an amorphous Si layer was deposited on the surface of the ZnO nanowire arrays by plasma-enhanced chemical vapor deposition (PECVD) to form ZnO/Si core/shell nanowire arrays (see Supporting Information). Finally, these arrays were annealed in an argon atmosphere at 800 8C for 10 h, then at 900 8C for 5 h, to obtain Zn2SiO4 nanotube/ZnO nanowire heterojunction arrays. Figure 1a shows a 308-tilted scanning electron microscopy (SEM) image of the vertical aligned ZnO nanowire array template, revealing that the ZnO nanowires had an average length of 8 mm, and the diameters were in the range of 50–500 nm. The typical morphology of the Zn2SiO4 nanotube/ZnO nanowire heterojunction arrays is shown in Figure 1b (308-tilted SEM image). The heterostructure exhibited almost the same average length as the ZnO nanowire template. The bottom region of the heterojunctions was very rough, and typically the top region of the heterojunctions had a larger diameter than the bottom region. Figure 1c shows the X-ray diffraction (XRD) pattern of the ZnO nanowire arrays (lower pattern) and the Zn2SiO4 nanotube/ZnO nanowire heterojunction arrays (upper pattern). Besides the peaks from the substrate, only diffraction from the (0002) and (0004) atomic planes of ZnO are observed from the ZnO nanowire template, indicating a perfect alignment. Compared to this XRD pattern, two new diffraction peaks were observed from the heterostructures. They were indexed as the (011) and (013) atomic planes of the orthorhombic structured Zn2SiO4 (JCPDS card 24-1469: a=0.574 nm, b=1.1504 nm, c=0.8395 nm), respectively. The detailed morphology and the chemical composition of the Zn2SiO4 nanotube/ZnO nanowire heterojunction were characterized by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX). Figure 2a shows a low-magnification TEM image of two [*] J. Liu, Dr. X. Wang, J. Song, Prof. Z. L. Wang School of Materials Science and Engineering Georgia Institute of Technology Atlanta, GA 30332-0245 (USA) Fax: (+1)404-894-8008 E-mail: zhong.wang@mse.gatech. edu