Fabrication and characterization of hexagonally patterned quasi-1D ZnO nanowire arrays
© Kuo and Lin; licensee Springer. 2014
Received: 22 December 2013
Accepted: 19 January 2014
Published: 12 February 2014
Quasi-one-dimensional (quasi-1D) ZnO nanowire arrays with hexagonal pattern have been successfully synthesized via the vapor transport process without any metal catalyst. By utilizing polystyrene microsphere self-assembled monolayer, sol–gel-derived ZnO thin films were used as the periodic nucleation sites for the growth of ZnO nanowires. High-quality quasi-1D ZnO nanowires were grown from nucleation sites, and the original hexagonal periodicity is well-preserved. According to the experimental results, the vapor transport solid condensation mechanism was proposed, in which the sol–gel-derived ZnO film acting as a seed layer for nucleation. This simple method provides a favorable way to form quasi-1D ZnO nanostructures applicable to diverse fields such as two-dimensional photonic crystal, nanolaser, sensor arrays, and other optoelectronic devices.
KeywordsZinc oxide Sol–gel-derived ZnO thin films Quasi-1D ZnO nanowire arrays
Zinc oxide (ZnO) has attracted much interest for its promising application in piezoelectric nanogenerators, gas sensors, light-emitting diodes, field-emission displays, and solar cells. Owing to its wide band-gap (3.37 eV at room temperature) and large exciton bonding energy of approximately 60 meV, ZnO has been recognized as an excellent candidate for short wavelength optoelectronic devices. Furthermore, ZnO nanostructures have many promising applications, such as lasers, light-emitting devices, and field emitters. Accordingly, a low-dimensional ZnO nanostructure might be used in novel nanodevices. Quasi-one-dimensional (quasi-1D) ZnO is one of the most important functional nanostructures, exhibiting transparent conductivity, piezoelectricity, and near-ultraviolet (UV) emission [1–3].
The growth of ZnO nanowires with precise control of their alignment, distribution, and aspect ratio is highly desirable for their potential applications in sensor arrays, high-efficiency photonic devices, near-UV lasers, and for assembling complex three-dimensional nanoscale systems [4–10]. A straightforward approach for this purpose is to fabricate metal nanoparticles, which are used as catalyst templates for the subsequent vapor–liquid-solid (VLS) growth of patterned nanowires . In the past few years, numbers of approaches have been proposed to obtain nanoscale metal catalysts for the fabrication of patterned ZnO nanowire arrays, such as electron beam lithography (EBL), soft-photolithography, and mask lithography by porous alumina, self-assembled micro- or nanospheres [12–17]. EBL is known as a relatively complicated and costly method, thus unsuitable for large-scale fabrication. In contrast, imprint and nanosphere lithography (NSL) tend to be more promising as they are less costly techniques with a much higher throughput. Recently, several groups have reported the large-scale fabrication of ZnO nanowires using NSL technique [15–17]. However, the ZnO nanowires in these reports are either not nanopatterned or not truly vertically aligned. The limitation might result from the interconnection of the printed Au, un-optimized growth conditions and/or imperfect lattice matching between substrates and ZnO nanowires [15–17]. These drawbacks might hinder the consideration of such nanowire arrays from device applications. In addition, the VLS process is the most widely used technique for growing aligned ZnO, in which gold is the most frequently chosen metal catalyst [18–20]. However, as limited by the clean room requirements for silicon technology, gold is not the choice of metal for integrating with silicon. Therefore, it is important to explore a catalyst-free technique for ZnO nanowire growth.
In this paper, we report the catalyst-free synthesis of hexagonally patterned quasi-one-dimensional (quasi-1D) ZnO nanowire arrays with the assistance of NSL. The technique demonstrates an effective and economical bottom-up process for ZnO 1D nanostructures for applications as two-dimensional photonic crystals, sensor arrays, nanolaser arrays, and optoelectronic devices.
The morphologies and crystal structures of the resulting ZnO materials were characterized using field-emission scanning electron microscope (SEM) (Hitachi S-4300, Hitachi Co., Tokyo, Japan) and X-ray diffractometer (XRD) (BEDE Scientific Inc., Centennial, CO, USA). The optical property was studied by photoluminescence (PL) measurement (Jobin Yvon Triax320, Horiba Ltd., Minami-ku, Kyoto, Japan). The 325-nm line of a He-Cd laser was used as an excitation light source for the PL measurement.
Results and discussions
Based on the above experimental results, we found that the ZnO thin films with c-axis preferred orientation will provide nuclei sites for the further growth of the nanowires through self-catalyst process . According to the low energy principle, the  plane is the fastest growing crystallographic plane . Therefore, ZnO nanowires are high c-axis orientation. In addition, density control of ZnO nanowire arrays is a valuable concern in the research of field-emitter and photovoltaic devices. In this study, the annealed sol–gel-derived ZnO thin films were used as substrates to fabricate ZnO nanowire arrays. Compared to those unannealed ZnO thin films, the density of nanowire arrays becomes larger and more homogeneous. Recently, Liao et al. also proposed that the residual stresses in the thin film and the density of the nanowire array are in inverse proportion, and will have potential applications in modifying the density of ZnO nanowire arrays . The intensity ratio of the NBE to the DL emission in honeycomb-like nanowires is larger than sol–gel-derived films, which indicates there are more oxygen vacancies for the sample grown at low temperature. This result indicates the proposed simple method is cost-effective approach to fabricated quasi-1D ZnO nanostructures with high-quality optical property.
In summary, we have fabricated hexagonally patterned quasi-1D ZnO nanowire arrays through simple chemical methods. Instead of using metal catalyst, sol–gel-derived ZnO thin film was used as the periodic nucleation sites for nanowire growth with the aid of a PS nanosphere SAM. Structural and optical measurements demonstrate that the quasi-1D nanowires possess high quality. By observation of the process of ZnO nanowire growth, a vapor transport solid condensation mechanism was proposed, in which the role of ZnO thin film was to provide nucleation sites for nanowire growth. The technique is a self-catalyzed process that is entirely bottom-up and can be effectively scaled up to the fabrication of ZnO photonic crystal devices.
Selected area electron diffraction
Transmission electron microscope
This work was supported by the Green Technology Research Center of Chang Gung University and the National Science Council (NSC) of Taiwan under contract nos. NSC100-2815-C-155-013-E, NSC100-2112-M-182-004, and NSC101-2112-M-182-003-MY3.
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