ARTICLE pubs.acs.org/JPCC
ZnO Nanomaterials Grown with Fe-Based Catalysts Dong-Hau Kuo,*,† Jian-Fong Fang,† R. S. Chen,‡ C. A. Chen,§ and Y. S. Huang§ †
Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei, 10607 Taiwan Graduate Institute of Applied Technology, National Taiwan University of Science and Technology, Taipei 10607, Taiwan § Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan ‡
ABSTRACT: We have demonstrated that slender, uniform, and vertically aligned ZnO nanorods can be grown with the Fe-based catalysts, while Au and ZnO have been widely accepted as the requisites for growing ZnO nanoarrays. The growth of these ZnO nanorods was investigated on the Fe-based catalyst-covered sapphire substrates by a thermal evaporationoxidation method at 700 C for 2 h in the atmospheres of oxygen and nitrogen. In order to develop Fe-related ZnO arrays, catalyst coating techniques of magnetron sputtering and spin-coating were applied. Rate of spincoating and the reduction procedure of coated substrates were explored. Without a reduction procedure, ZnO nanowalls were obtained. At a medium spin rate, ZnO arrays with a diameter of 80100 nm were obtained on the Fe/ZnO bilayer-covered sapphire substrates. Growth rates in the diameter and length directions were investigated. The empirical Avrami equation is applied to formulate the linear growth in the axial length with growth duration, by assuming the growth of each ZnO nanorod from a local cell. The variation of the radial diameter with the square root of growth time is explained. Growth kinetics based upon the vaporsolid mechanism is also explained from the atomistic viewpoint. Growth micromechanisms for growing ZnO nanorods and nanowalls were proposed, which indicate the importance of the catalyst state or the built-in strain energy. Electrical conductivities of 0.25.0 Ω1 and a very shallow defect level of 13 ( 1 meV have been measured for ZnO nanorods via the single-rod measurements.
’ INTRODUCTION Zinc oxide (ZnO) with a wide band gap of 3.37 eV and large exciton binding energy (60 meV) has been an attractive semiconductor. For the potential applications of light-emitting diodes, dye-sensitized solar cells, and nano-piezoelectronics, the fabrication of vertically aligned ZnO nanowires or nanorods has become important.13 Different techniques have been developed for growing onedimensional (1-D) ZnO nanomaterials, which include thermal evaporation, chemical vapor deposition (CVD), solution-phase growth, hydrothermal reactions, and pulse-laser deposition. Basically, the vertical alignment can be achieved with or without catalysts. For the catalyst approach, gold catalyst as a stable and effective nucleation agent needs to be deposited on the (0001)oriented sapphire substrate. The most beautiful displays of uniform and vertically aligned ZnO nanorods with slender diameters of 50150 nm are demonstrated by using the carbothermal reaction of (ZnO þ C) above 850 C on Au catalystpatterned sapphire substrates with the aids of submicrometer-sized polystyrene balls4 and laser-hardening lithography technology.5,6 For the catalyst-free approach, the experiments actually need a ZnO seed layer on the (0001)-oriented sapphire substrates. This seed layer can be deposited on substrates by chemical and physical methods79 or in-situ formed due to the vaporization of the zinc reactants. The in-situ deposition of ZnO templates has r 2011 American Chemical Society
been referred to as the catalyst-free or self-catalytic process. Other single-crystal substrates such as GaN and SiC had been used to grow 1-D ZnO arrays.10 Fan et al. modified the Si substrate with an epitaxial GaN layer in order to grow aligned ZnO nanowires.11 A low-temperature metalorganic CVD method was used to moderate the lattice mismatch between ZnO and Si in order to grow aligned 1-D ZnO.12 The solution-phase growth of 1-D ZnO at
