CRYSTAL GROWTH & DESIGN
Controllable Synthesis of Ordered ZnO Nanodots Arrays by Nanosphere Lithography
2008 VOL. 8, NO. 8 2917–2920
Ling Chen,† Jingyun Huang,*,† Zhizhen Ye,† Haiping He,† Yujia Zeng,† Shuangjiang Wang,‡ and Huizhen Wu‡ State Key Laboratory of Silicon Materials, Zhejiang UniVersity, Hangzhou 310027, P. R. China, and Department of Physics, Zhejiang UniVersity, Hangzhou 310027, P. R. China ReceiVed December 29, 2007; ReVised Manuscript ReceiVed May 12, 2008
ABSTRACT: Controllable ordered hexagonally patterned ZnO nanodots were achieved by polystyrene nanosphere lithography at room temperature. First a self-assembled monolayer of polystyrene spheres was formed as a mask. Then ZnO was deposited through the mask by e-beam vaporization. After the polystyrene spheres were etched away, ordered ZnO nanodots arrays were formed on the substrate. The obtained ZnO nanodots are of acceptable quality with high chemical purity and preferential c-axis orientation. A photoluminescence measurement shows a blue shift in free-exciton emission derived from low-dimensional quantum characteristics. The size of the nanodots can be controlled by varying the deposition time. The ZnO nanodots with growth times of 1, 2, and 3 min exhibit different free-exciton emission at 3.378, 3.356, and 3.314 eV. Therefore this is a technique with precise control of the size and geometry of the dots, which has a great promise for applications in nanoscale optoelectronic devices.
1. Introduction Nanotechnology offers the promise of enabling revolutionary advances in diverse areas ranging from electronics, optoelectronics, and energy to healthcare.1 ZnO, with a direct wide band gap of 3.37 eV, has attracted significant attention as a promising luminescent material for applications in blue and ultraviolet light emitters and detectors. In addition, the large exciton binding energy of 60 meV results in more efficient excitonic emission at room temperature.2 Compared with bulk materials, lowdimensional ZnO nanostructures have superior optical properties because of the significant exciton effects and extraordinary carrier behavior due to quantum confinement effects. Therefore quantum dots, a three-dimensional confined nanostructure, have attracted a great deal of attention for applications in ultraviolet light-emitting diodes, field emitters, or nanolaser devices.3–5 Many methods have been reported to prepare ZnO quantum dots, such as wet chemical techniques,6,7 metal-organic chemical vapor deposition,8 pulse laser deposition, vapor phase transport growth processes9 and so on. But it is very difficult to control the size and distribution of the prepared ZnO quantum dots by these methods. The confined energy levels of these nonuniform quantum dots disperse in a large range, which results in a broadening of luminescence line shape. Therefore, the performance of the prepared quantum dot lasers has difficulty reaching the superiority predicted by theory. The main problem for realizing nanostructure-based laser devices is improving the homogeneity of the ZnO quantum dots and increasing the effective number of quantum dots. In this regard, some groups reported on the control of the position and size of ZnO nanodots using metal-organic chemical vapor deposition with nanopatterning by focused ion beam, as well as other techniques like electron beam lithography, photolithography, or X-ray lithography.10 However, they are mostly limited by time-consuming, expensive preparation and large scale fabrication. Recently, nanosphere lithography11,12 has been used to prepare periodically arranged nanostructures of novel materials, including nanorods, nanotubes, and nanolines. In related reports,13–19 a self* Corresponding author: Fax: 86-571-87952625. E-mail:
[email protected]. † State Key Laboratory of Silicon Materials. ‡ Department of Physics.
assembled single layer of nanospheres is always used as a template for subsequent metal deposition at room temperature, such as Ni and Au, which are the catalyst to grow these nanostructures. Since the growth temperature of many semiconductors is far higher than the template’s holding (
