Volume 17 Issue 1 - January 21, 2011 PDF
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This article has attracted world-wide attention and interest and has been translated into various languages. These different versions are listed below.
» German, Romania, Czech, Latvia (Translator:Arija Liepkalnietis)
Fabrication of large area ZnO nanostructures toward nanodevices by using hybrid process incorporating magnetron sputtering
Jun-Han Huang and Chuan-Pu Liu*
Department of Materials Science and Engineering, NCKU
NCKU Landmark Project《B015》
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ZnO is one of the most studied wide direct bandgap semiconductors, and has been applied in various areas, including pigments, bio-filters, transport and optoelectronic devices, such as varistor, surface acoustic wave device and transparent conductive oxide electrode. Recently, due to ease of nanostructures synthesis, ZnO has also been well prepared in various low dimensional nanostructures as one of the richest families. These nanostructures may extend further the horizon of ZnO applications in sensor, solar cell, mechanical component, field-effect emitter and nanogenerator. However, realization of real devices still awaits a feasible method for large area growth incorporating nanostructures.

In this report, a novel growth mechanism has been developed to grow oblique array of nanowires by using oblique angle magnetron sputtering. Typical oblique angle deposition (OAD) has been used to grow inclined nanostructures at low energetic conditions with large tilting angle for the incident flux. Due to limited surface diffusion, atoms were only deposited on the radiated area, leading to inclined nanostructure growth with predicted directions.

In contrast to the common OAD system, the ZnO nanostructures grown at higher temperatures and hydrogen/argon mixture ambient, dislocations were introduced into the exposed sides of the nanostructures to release stress and maintain crystal continuity, which made ZnO nanostructures gradually bend into opposite quadrant relative to the incident source, as shown in Figure 1 (a). Moreover, the structural bending degree is directly related to defect density and location, can be controlled by growth parameters. Most importantly, no obvious boundaries such as grain or twin boundaries were found in the entire column, rendering the bending structure is still in single-crystalline.
Figure 1. (a) SEM images of ZnO bent columns by oblique angle sputtering (320℃) and (b) nanowires by subsequent hydrothermal process. (c) reflection spectrum of oblique ZnO nanowire array

Based on abovementioned mechanism of nanostructures bending, as showed in Figure 1 (b), oblique ZnO nanowire arrays have been successfully grown on these bent ZnO columns by subsequent hydrothermal process. The nanowire direction was confined by the neighboring nanowires and the surface curvature of the bent columns as shown in Figure 2. ZnO nanowires exhibit excellent antireflection property from reflection spectra as shown in Figure 1(c). Inherent to sputtering and hydrothermal processes, this growth mechanism has great advantages on production of such devices over large area.
Figure 2. Sketch of nanowire direction limitation on bent columns surface

This novel mechanism enables further design for more complicated three-dimensional nanostructures than before, besides, the single-crystal nanostructures provides template for faster signal transport speed in hierarchical structures. We are on the way to attempt new devices such as nanogenerators and nano-piezodiodes with improved performance.
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