Synthesis and humidity sensing analysis of ZnS nanowires Salih Okur a , Neslihan ¨ Uzar b,n , Nesli Tekg ¨ uzel a , Ays -e Erol b , M. C - etin Arıkan b a Izmir Institute of Technology, Faculty of Science, Department of Physics G¨ ulbahce Koyu Kamp¨ us¨ u, Urla, 35430 Izmir, Turkey b Istanbul University, Science Faculty, Physics Department, Vezneciler, 34134 Istanbul, Turkey article info Article history: Received 6 June 2010 Received in revised form 26 July 2010 Accepted 16 August 2010 Available online 19 August 2010 abstract ZnS nanowires synthesized by the vapor–liquid–solid (VLS) method and humidity sensing properties of obtained ZnS nanowires were investigated by quartz crystal microbalance (QCM) method and electrical measurements. The synthesized nanowires were exposed to relative humidity (RH) between 22% and 97% under controlled environment. Our experimental results show that ZnS nanowires have a great potential for humidity sensing applications in room temperature operations. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Semiconductor nanostructures have attracted great attention as materials for sensing gases and humidity due to their superior features such as higher surface-to-volume ratio, lower cost and ease to fabricate as a sensor compared to bulk or thin films [1] and due to high computability with microelectronic processing. Sensing and controlling humidity is very important for many manufacturing environments such as food, automotive, electro- nics and agriculture industries. Therefore, reliable, cheap, sensi- tive, low operation temperature and small-sized humidity sensors are indispensible for daily life. Several sensing techniques such as resistance, capacity, optics, field effect transistor (FET), surface acoustic wave (SAW) and QCM can be used to detect humidity [2]. ZnS is a typical II–VI semiconductor compound with wide direct bandgap energy of 3.68 eV at room temperature and has been widely used for laser, sensor, infrared windows, cathode ray tube and electroluminescence device applications [3,4]. The humidity sensing properties of ZnS nanowires are not studied using QCM as far as we know. Therefore, here we report the fabrication and characterization of ZnS nanowires prepared using vapor–liquid–solid (VLS) technique and humidity sensing capabil- ities using both QCM and electrical measurement techniques. 2. Experimental 2.1. Synthesis of ZnS nanowires The conventional VLS mechanism is based on evaporation of ZnS powder and transfer of ZnS vapor to lower temperature regions using an inert gas as a carrier. The growth parameters strongly depend on the source and substrate temperatures, the thickness of catalyst layer and the flow rate of carrier gas. One terminal of the quartz tube was connected to the vacuum pump and the gas flow system was placed in horizontal tube furnace. The quartz boat containing the source material (diameter 10 mm, 99.99% pure ZnS powder) and the Au coated (50 ˚ A) Si substrate was placed at the other edge of the quartz tube. Prior to the process, the quartz tube was evacuated by the rotary vacuum pump. After closing the rotary pump, the tube was filled with Ar gas for protecting the process medium from oxygen contamina- tion. When the furnace reached the process temperature (1100 1C), the terminal opposite to the gas flow system was opened and kept open for the gas to flow out during the process. Then the ZnS powder and the substrate were transferred from the edge of the tube to 1100 and 750 1C temperature places in the tube, respectively. Evaporated ZnS vapor was carried by 600 cc/min of Ar gas and condensed on the substrate. After 1.5 h process, the quartz boat was taken off rapidly from the quartz tube and the samples were cooled down to room temperature. The Au coated Si substrate was seen to be covered with white wool like layer. Surface morphology, chemical composition and crystal structure of the synthesized structures were examined by SEM, EDS and XRD analyses, respectively. 2.2. Investigation of humidity sensing capabilities of ZnS nanowires using QCM technique QCM has been extensively used to monitor the change in mass loading by measuring the shift of its resonant frequency. The mass change (Dm) on surface of the quartz crystal was calculated using Sauerbrey equation [5] from the frequency change (Df) for thin and homogenous film: Df ¼ 2f 2 0 Dm A ffiffiffiffiffiffi mr p ð1Þ where f 0 is the resonant frequency of the fundamental mode of the QCM crystal, A the area of the gold disk coated onto the crystal, r the density of the crystal and m the shear modulus of quartz. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physe Physica E 1386-9477/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2010.08.015 n Corresponding author. Tel.: + 90 535 427 12 71. E-mail addresses: neslihanuzar@gmail.com, neslihanuzar@yahoo.com (N. ¨ Uzar). Physica E 44 (2012) 1103–1107