Field emission properties of zinc oxide/zinc tungstate (ZnO/ZnWO 4 ) composite nanorods Bohr-Ran Huang ⁎, Tzu-Ching Lin, Kuo-Ting Chu, Ying-Kan Yang, Jun-Cheng Lin Graduate Institute of Electro-Optical Engineering and Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, ROC abstract article info Available online 11 May 2012 Keywords: Zinc oxide/zinc tungsten oxide Composite nanostructure Field emission Schottky contact Zinc oxide/zinc tungstate (ZnO/ZnWO 4 ) nanorods were synthesized as composite nanostructure on the tungsten/ glass substrates by thermal oxidation technique for field emission cathodes. The ZnWO 4 wolframite structure in the ZnO film formed the ZnO/ZnWO 4 composite nanostructure, as indicated by the XRD patterns and Raman spectrum. An ohmic contact was transformed to a Schottky contact between a silver electrode and the ZnO/ZnWO 4 composite nanorods from I–V measurement given that the work function of the ZnO/ZnWO 4 composite nanorods was lower than that of ZnO nanorods. The field emission properties of the ZnO/ZnWO 4 composite nanorods includes a turn-on electric field (E on ) of 2.1 V/μm and a threshold electric field (E th ) of 2.9 V/μm, both much lower than that found in ZnO nanorods without ZnWO 4 nanostructures. The novel ZnO/ ZnWO 4 composite nanorod shows excellent potential for application in field emission cathodes and related devices. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Zinc oxide (ZnO) is a versatile oxide semiconductor and has attracted considerable attention over the past few decades due to its wide direct band gap (3.37 eV), large exciton binding energy (60 eV) at room temperature, high mechanical, chemical, thermal stabilities, piezoelectric characteristics, and biocompatibility. ZnO is believed to have potential applications in ultraviolet lasers [1], field-effect transistors [2], photodetectors [3], and field emission cathodes [4,5]. In recent years, various ZnO nanostructures such as nanopins, nanotubes, nanorods and nanowires have been synthesized by thermal evaporation [4], laser ablation [6], hydrothermal decomposition [5], metal organic chemical vapor deposition [7], and the porous template method [8]. Their characteristics and potential applications have also been studied intensively. ZnWO 4 has the monoclinic wolframite structure which has re- ceived considerable attention due to its applications as an X-ray and γ-scintillator, microwave applications, photocatalytic, and photo- luminescent properties [9,10]. Usually, single-crystal ZnWO 4 is grown by traditional Czochralski technique [11], whereas the powder form of ZnWO 4 is synthesized by solid state method at high temperatures. Field emission is one of the applications of nanostructural material that has great potential in field emissions for commercial application in displays and other electronic devices. In the past few decades, research into field emission properties has mainly focused on carbon-based materials, because of their low work function, high mechanical sta- bility, high conductivity, and high aspect ratio. With structural properties similar to those of carbon nanotube [12], nanostruc- tured ZnO should be a good candidate for field emission cathodes due to the fact that its application allows a relatively high oxygen partial pressure. In this study, we report on the synthesis of the ZnO/ZnWO 4 compos- ite nanorods with a tungsten catalyst on a glass substrate by thermal ox- idation. The lower work function of ZnO/ZnWO 4 composite nanorods is observed with I–V measurement. Comparison of the field emission prop- erties of ZnO nanorods and ZnO/ZnWO 4 composite nanorods, shows that the ZnO/ZnWO 4 composite nanorod is superior due to the lower work function of the composite nanostructure. This work proposes an effective way to enhance the field emission properties of ZnO/ZnWO 4 composite nanorods. 2. Experiment ZnO nanorods and ZnO/ZnWO 4 composite nanorods were synthe- sized on a glass substrate in a horizontal quart tube furnace. Zinc powder (ACROS ORGANICS) of purity 95% was loaded on the front of a quartz boat and served as source material. The synthesis of the ZnO nanorods and ZnO/ZnWO 4 composite nanorods was respectively based on gold and tungsten catalysts, which were deposited on the glass substrates by sputtering. During the experiments, the zinc powders were heated at a rate of 12 °C/min from room temperature with the zinc source carried in by argon at a flow rate of 100 sccm. Once the temperature was raised to 550 °C, oxygen was introduced into the chamber at a flow Surface & Coatings Technology 231 (2013) 289–292 ⁎ Corresponding author. Tel.: + 886 2 2733 3141x3273; fax: + 886 2 2737 6424. E-mail address: huangbr@mail.ntust.edu.tw (B-R. Huang). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.05.006 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat