Photonic crystals in biology - NanoTR-VI

Poster Session, Thursday, June 17Theme F686 - N1123Investigation of Humidity Sensing Properties of ZnS NanowiresS.Okur 1 , N.Tekgüze1 1 l, A. Erol 2 , N.Üzar 2 , M.Ç. Arıkan 21 Izmir Institute of Technology, Faculty of Science, Department of PhysicsGülbahce Koyu Kampüsü, Urla, Izmir,35430, Turkey2 Istanbul University, Science Faculty, Physics Department, Vezneciler, 34134 Istanbul, TurkeyAbstract— ZnS nanowires synthesized by the VLS (Vapor-Liquid-Solid) method and were investigated by Quartz Crystal Microbalance (QCM)method and electrical measurements. The synthesized nanowires were exposed to relative humidity (RH) between 33% and 100% undercontrolled environment. Our experimental results show that ZnS nanowires have a great potential for humidity sensing applications for roomtemperature operations.Semiconductor nanostructures have attracted great attention asmaterials for sensing gases and humidity due to their superior sensingfeatures such as very high surface to volume ratio, lower cost and easeto fabricate as a sensor compared to bulk or thin films [1]. Sensing andcontrolling of humidity is very important for many manufacturingenvironments such as food, automotive and electronics industries. ZnSnanostructures should be used as humidity and gas sensor due to theirhighly active surface properties.In this work, we explored the humidity sensing capability of ZnSnanostructures using QCM method, at which the measured frequencyshift is directly proportional to the mass change on a quartz crystal [2],and electrical measurements such as voltage-current (I-V), resistance-RH% and capacitance-frequency (C-f) from 33% RH to 100% RH.ZnS nanostructures were synthesized using VLS technique. Fig.1shows the morphology of the synthesized nanostructures is nanowireswith their diameters range from 60 nm to 300nm.decreases almost linearly with increasing RH. This decreasing ofresistance was about four orders. Typical I-V curves of ZnS nanowiressensor from 33% RH to 100% RH are shown in Fig.3b. These I-Vcurves are a straight line, showing ohmic behavior. Molecules ofmoisture interaction with semiconductor surfaces influence surfaceconductivity due to physical and chemical adsorption of watermolecules. Charge exchange occurs between adsorbed species from themoisture and the semiconductor surface. Conductivity of ZnSnanowires sensor increases with increasing relative humidity is relatedto amount of the absorption of moisture molecules on the surface ofZnS nanowires sensor.Resistance (Ohm)10 1110 1010 910 810 7y = 6,9462e+11 * e^(-0,11993x) R= 0,96093B10 12 30 40 50 60 70 80 90 10010 610 5Fig. 1: SEM image of the ZnS nanostructuresIn order to monitor humidity sensing properties, ZnS isultrasonically dispersed in ethanol and solution was applied on thesurface of quartz crystal and between the two gold (Au) electrodes bydrop-casting technique for QCM and electrical measurements,respectively. The dropped solution was dried at room temperature untilethanol was totally evaporated. The quartz crystal and electrodesloaded with ZnS nanowires were exposed to the relative humidity at thesame time. Fig. 2 shows the frequency shift of ZnS loaded QCM crystalunder varying relative humidity (RH) between 45 and 75% for fourhumidity adsorption/desorption cycles. During the adsorption ofmoisture molecules on the sensor surface the frequency shift decreaseswith increasing RH and goes to near saturation values, while frequencyshift decreases during the desorption. This is why ZnS nanowiresposses a large specific surface area, moisture molecules adsorb easilyon the sensor surface and the mass of quartz crystal increases withincreasing RH.dF/Hz0-200-400-600-800-1000dF/Hz Relative Humidity (%)400 200 400 600 800 1000 1200 1400Fig. 2: The frequency responses of an loaded QCM with drop-casted ZnSnanowires (red squares) comparing with relative humidity values of acommercial sensor (blue circles) for 4 humidity adsorption-desorption cyclesbetween 45% and 75% RH.Fig.3a shows the resistance variation of ZnS nanowires dependingon varying relative humidity. The resistance of ZnS nanowiresTime (s)8075706560555045Relative Humidity (%)Current (A)Relative Humidity (%)A0,020,0150,010,0050-0,005-0,01-0,015100 33%-0,02-6 -4 -2 0 2 4 6bVoltage (V)Fig. 3: a) The resistance variation, b) the I-V characterization of ZnS nanowiresunder varying relative humidityIn summary, the QCM and electrical measurements resultsshow that ZnS nanowires can be used for potential humiditysensor application.*Corresponding author:neslihanuzar@istanbul.edu.tr.[1] D.P: Norton, Y.W. Heo, M.P. Ivill, K. Ip, S. J. Pearton, M. F. Chisholm, T.Steiner, Materials Today, 34, 7, (2004)[2] Lukas Schmidt-Mende and Judith L. MacManus-Driscoll, 10, 40-48, 2008.90% %4384% 55%75%55% 84%90% 43%100% 33%6th Nanoscience and Nanotechnology Conference, zmir, 2010 700