Third Day Poster Session, 17 June 2010 - NanoTR-VI

Poster Session, Thursday, June 17
Theme F686 - N1123
Investigation of Humidity Sensing Properties of ZnS Nanowires
S.Okur 1 , N.Tekgüze1 1 l, A. Erol 2 , N.Üzar 2 , M.Ç. Arıkan 2
1 Izmir Institute of Technology, Faculty of Science, Department of Physics
Gülbahce Koyu Kampüsü, Urla, Izmir,35430, Turkey
2 Istanbul University, Science Faculty, Physics Department, Vezneciler, 34134 Istanbul, Turkey
Abstract— 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% under
controlled environment. Our experimental results show that ZnS nanowires have a great potential for humidity sensing applications for room
temperature operations.
Semiconductor nanostructures have attracted great attention as
materials for sensing gases and humidity due to their superior sensing
features such as very high surface to volume ratio, lower cost and ease
to fabricate as a sensor compared to bulk or thin films [1]. Sensing and
controlling of humidity is very important for many manufacturing
environments such as food, automotive and electronics industries. ZnS
nanostructures should be used as humidity and gas sensor due to their
highly active surface properties.
In this work, we explored the humidity sensing capability of ZnS
nanostructures using QCM method, at which the measured frequency
shift 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.1
shows the morphology of the synthesized nanostructures is nanowires
with their diameters range from 60 nm to 300nm.
decreases almost linearly with increasing RH. This decreasing of
resistance was about four orders. Typical I-V curves of ZnS nanowires
sensor from 33% RH to 100% RH are shown in Fig.3b. These I-V
curves are a straight line, showing ohmic behavior. Molecules of
moisture interaction with semiconductor surfaces influence surface
conductivity due to physical and chemical adsorption of water
molecules. Charge exchange occurs between adsorbed species from the
moisture and the semiconductor surface. Conductivity of ZnS
nanowires sensor increases with increasing relative humidity is related
to amount of the absorption of moisture molecules on the surface of
ZnS nanowires sensor.
Resistance (Ohm)
10 11
10 10
10 9
10 8
10 7
y = 6,9462e+11 * e^(-0,11993x) R= 0,96093
B
10 12 30 40 50 60 70 80 90 100
10 6
10 5
Fig. 1: SEM image of the ZnS nanostructures
In order to monitor humidity sensing properties, ZnS is
ultrasonically dispersed in ethanol and solution was applied on the
surface of quartz crystal and between the two gold (Au) electrodes by
drop-casting technique for QCM and electrical measurements,
respectively. The dropped solution was dried at room temperature until
ethanol was totally evaporated. The quartz crystal and electrodes
loaded with ZnS nanowires were exposed to the relative humidity at the
same time. Fig. 2 shows the frequency shift of ZnS loaded QCM crystal
under varying relative humidity (RH) between 45 and 75% for four
humidity adsorption/desorption cycles. During the adsorption of
moisture molecules on the sensor surface the frequency shift decreases
with increasing RH and goes to near saturation values, while frequency
shift decreases during the desorption. This is why ZnS nanowires
posses a large specific surface area, moisture molecules adsorb easily
on the sensor surface and the mass of quartz crystal increases with
increasing RH.
dF/Hz
0
-200
-400
-600
-800
-1000
dF/Hz Relative Humidity (%)
40
0 200 400 600 800 1000 1200 1400
Fig. 2: The frequency responses of an loaded QCM with drop-casted ZnS
nanowires (red squares) comparing with relative humidity values of a
commercial sensor (blue circles) for 4 humidity adsorption-desorption cycles
between 45% and 75% RH.
Fig.3a shows the resistance variation of ZnS nanowires depending
on varying relative humidity. The resistance of ZnS nanowires
Time (s)
80
75
70
65
60
55
50
45
Relative Humidity (%)
Current (A)
Relative Humidity (%)
A
0,02
0,015
0,01
0,005
0
-0,005
-0,01
-0,015
100 33%
-0,02
-6 -4 -2 0 2 4 6
b
Voltage (V)
Fig. 3: a) The resistance variation, b) the I-V characterization of ZnS nanowires
under varying relative humidity
In summary, the QCM and electrical measurements results
show that ZnS nanowires can be used for potential humidity
sensor 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% %43
84% 55%
75%
55% 84%
90% 43%
100% 33%
6th Nanoscience and Nanotechnology Conference, zmir, 2010 700