Sun et al. Nanoscale Research Letters 2011, 6:556 http://www.nanoscalereslett.com/content/6/1/556 N A N O E X P R E S S Open Access Improved characteristics of near-band-edge and deep-level emissions from ZnO nanorod arrays by atomic-layer-deposited Al O and ZnO shell layers 2 3 Wen-Cheng Sun1,2, Yu-Cheng Yeh1,2, Chung-Ting Ko1, Hau He Jr2* and Miin-Jang Chen1* Abstract We report on the characteristics of near-band-edge (NBE) emission and deep-level band from ZnO/Al2O3 and ZnO/ ZnO core-shell nanorod arrays (NRAs). Vertically aligned ZnO NRAs were synthesized by an aqueous chemical method, and the Al2O3 and ZnO shell layers were prepared by the highly conformal atomic layer deposition technique. Photoluminescence measurements revealed that the deep-level band was suppressed and the NBE emission was significantly enhanced after the deposition of Al2O3 and ZnO shells, which are attributed to the decrease in oxygen interstitials at the surface and the reduction in surface band bending of ZnO core, respectively. The shift of deep-level emissions from the ZnO/ZnO core-shell NRAs was observed for the first time. Owing to the presence of the ZnO shell layer, the yellow band associated with the oxygen interstitials inside the ZnO core would be prevailed over by the green luminescence, which originates from the recombination of the electrons in the conduction band with the holes trapped by the oxygen vacancies in the ZnO shell. PACS 68.65.Ac; 71.35.-y; 78.45.+h; 78.55.-m; 78.55.Et; 78.67.Hc; 81.16.Be; 85.60.Jb. Introduction Because of large surface-to-volume ratio and spatial con- finement of carriers, researches on one-dimensional (1D) nanostructures have attracted great interest [1-3], and remarkable progress has been achieved in various electro- nic, photonic, and sensing devices [3-7]. Novel synthetic approaches to the fabrication of high-quality semiconduc- tor nanotubes have been reviewed by Yan et al. [8]. Zinc oxide (ZnO) has been regarded as one of the most promis- ing materials for 1D nanostructures due to its distin- guished characteristics such as direct and wide band gap (approximately 3.37 eV), large excitonic binding energy (up to 60 meV), and high piezoelectricity [9-11]. The synthesis of well-aligned ZnO nanorod arrays (NRAs) is crucially important for the practical applications such as field emitters [12], nanogenerators [13], solar cells [14], and nanolasers [15]. One of the popular techniques for fabricating ZnO NRAs is to use Au as catalyst on a lattice- * Correspondence: jhhe@cc.ee.ntu.edu.tw; mjchen@ntu.edu.tw 1Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan 2Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan Full list of author information is available at the end of the article matched substrate [16]. Since the optical properties of ZnO NRAs are strongly dependent on surface conditions [17-20] and natural defect states [21-24], a large variety of surface modifications on ZnO NRAs have been carried out by depositing a shell layer. For instance, the enhance- ment of photoluminescence (PL) has been observed in ZnO/Er2O3 and ZnO/MgZnO core-shell NRAs [25,26]. The enhanced surface-excitonic emission together with the suppression in deep-level emission has also been reported in ZnO/amorphous-Al2O3 core-shell nanowires [27]. Apart from the enhancement of light emission, strong photoconductivity [28], photocatalytic activity [29], and quantum confinement [30] have been observed in var- ious 1D ZnO nanostructures. In this paper, vertically aligned ZnO NRAs were synthe- sized using an aqueous chemical method, which is benefi- cial for low reaction temperature, low cost, catalyst-free synthesis, and large-scale production. The growth of ZnO NRAs was assisted by a ZnO seed layer prepared by atomic layer deposition (ALD). The self-limiting and layer-by-layer growth of ALD contribute to many advan- tages such as easy and accurate thickness control, confor- mal step coverage, high uniformity over a large area, low defect density, good reproducibility, and low deposition © 2011 Sun et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.