I. Introduction
Zinc oxide (ZnO) is a versatile functional material. ZnO can be used in photovoltaic devices [1], light-emitting diodes [2], dye-sensitized solar cells [3], and gas sensors [4]. Other than 2-dimensional (2D) ZnO thin films, it is also possible to grow 1-dimensional (1D) ZnO nanostructures, such as nanowires, nanorods, and nanotubes. It has been shown that one can use various methods such as electrochemical deposition [5], hydrothermal method [6], and chemical vapor deposition [7] to grow these ZnO nanostructures. 1D ZnO nanostructures have attracted great attention recently due to their potential applications in nano-devices. With a band-gap energy of 3.37 eV, 1D ZnO nanostructures could also be used as ultraviolet (UV) photodetectors (PDs).