One-dimensional protuberant optically active ZnO structure fabricated by oxidizing ZnS nanowires V. Nam Do a, ⁎, N.T. Tuan c, ⁎, D.Q. Trung a , N.D.T. Kien a , N.D. Chien b , P.T. Huy a a Hanoi Advanced School of Science and Technology, Hanoi University of Technology (HUT), Hanoi 10000, Viet Nam b Institute of Engineering Physics, Hanoi University of Technology (HUT), Hanoi 10000, Viet Nam c Department of Physics, College of Science, Can-Tho University, 3/2 Street, Can-Tho, Vietnam abstract article info Article history: Received 19 January 2010 Accepted 10 April 2010 Available online 22 April 2010 Keywords: Protuberant ZnO structure Nanowires ZnO/ZnS heterostructure High crystalline quality ZnS nanowires were fabricated using the thermal evaporation method. They were then oxidized in air at different temperatures to form a one-dimensional protuberant ZnO/ZnS structure. It was argued that the oxidation at low enough temperature can significantly improve the quality of the ZnS nanowires by passivating dangling bonds on the nanowire surface as the absorption of oxygen atoms. This study provides a simple approach for synthesizing optically active ZnO/ZnS heterostructures. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Recently, various morphologies of ZnO/ZnS heterostructures such as the nanorings [1], biaxial nanobelts [2], and saw-like structures [3], have been fabricated and argued to possess anomalous optical properties which cannot be observed for pure ZnO and/or ZnS nanostructures. Such structures have been suggested to have potential applications for electricity generation [4], photovoltaic devices [5], UV lasers and particularly for gas sensors [2]. Interestingly, Li et al. [6] most recently introduced a so-called stalactite-like ZnO/ ZnS composite nanostructures and strongly promoted them for the field emission. Basically, the reason those heterostructures attract attention is because of their surface morphology and microstructural properties. In this letter, we report on a ZnO/ZnS-wire structure which is protuberant, obtained from the oxidation process of ZnS nanowires. We particularly will argue that (i) high crystallinity quality of the ZnS nanowires can be achieved by using the thermal evaporation method, and (ii) the dangling bonds of sulphur and zinc atoms on the ZnS nanowires can be significantly passivated by the oxidation process at appropriate temperature as the adsorption of oxygen atoms. 2. Experimental To our aim, we first of all synthesized ZnS nanowires and then use them as templates for forming ZnO structures. Oxidizing ZnS to form ZnO is a common technique to create ZnO films and/or wires on silicon substrates [7–9]. ZnS nanowires were synthesized using simple thermal evaporation of ZnS nanopowder in a horizontal tube furnace. Home-made nanopowders with average crystalline size of 2–5 nm were placed in an alumina boat and then put into the center of a quartz tube in the tube furnace. Several Si strips which were coated with a layer of Au or Pt film with thickness ranging from 4 nm to 40 nm by using DC sputtering, were placed downstream in the quartz tube subsequently to act as deposition substrates for materials growth. The quartz tube was first purged with high-purity argon for 120 min, prior to heating to eliminate oxygen in the furnace. The system was then heated to 950 °C at a heating rate of 10 °C per minute in a flowing Ar atmosphere (the Ar flow rate was kept at 30 sccm). Further, the temperature of the system was raised to 1150 °C and kept at this temperature for 30 min. During this second heating process, the Ar flow rate was kept at 80–100 sccm (Ar acts as both the protecting medium and the carrying gas). Before the start of the experiment, the temperature profile along the quartz tube was measured by using a moveable thermocouple. After the growth, the obtained ZnS nanowires were annealed at different temperatures from 100 °C to 800 °C for 1 h to oxidize the nanowires in air. The morphology and microstructure of the ZnS and ZnO nanostructures on Si wafers were examined using field emission scanning electron microscopy (FESEM 4800 Hitachi). The structure and crystallinity were characterized and analyzed by X-ray diffraction (XRD SIEMENS D5000). Photoluminescence (PL) spectra were recorded with a JOBIN YVON SPEX FL-3-22 and SPEX 1250 fluorescence spectrophotometer. 3. Results and discussions To prove our first consideration on the crystalline quality of the obtained ZnS nanowires we examined their microstructure. From the XRD patterns (not shown) we can confirm that the obtained ZnS Materials Letters 64 (2010) 1650–1652 ⁎ Corresponding authors. E-mail addresses: vannam.do@gmail.com (V.N. Do), pthuy-hast@mail.hut.edu.vn (P.T. Huy). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.04.021 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet