- Email: [email protected]
Ag surface-type cell
ARTICLE IN PRESS Physica E 41 (2008) 18– 22
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Humidity-dependent characteristics of methyl-red thin film-based Ag/methyl-red/Ag surface-type cell Zubair Ahmad , M.H. Sayyad, M. Saleem, Khasan S. Karimov, Mutabar Shah Faculty of Engineering Sciences (FES), Ghulam Ishaq Khan Institute of Engineering Sciences and Technology, Topi 23640, District Swabi, NWFP, Pakistan
a r t i c l e in f o
a b s t r a c t
Article history: Received 8 April 2008 Received in revised form 21 May 2008 Accepted 21 May 2008 Available online 14 June 2008
This paper describes the experimental results for humidity-dependent resistive and capacitive response of methyl-red thin films in a Ag/methyl-red/Ag surface-type cell. A 300-nm-thick film of methyl-red was deposited from 10 wt% solution in benzene by a spin coater at an angular speed of 2000 revolutions per minute (RPM) on a glass substrate with preliminary deposited metal electrodes to form the Ag/methyl-red/Ag surface-type cell. The length and width of the gap between the electrodes were 50 mm and 15 mm, respectively. The resistance of the film reduced from 37 to 17 MO with an elevation of relative humidity level over the whole humidity range. It was also observed that under the effect of humidity, the capacitance of the methyl-red thin film increased by 12 times. The capacitive/resistive sensor has a quasi-linear function with relative humidity in the range of 30–95% and has a small hysteresis. The response and recovery time of the sensor was about 10 s for the capacitive sensor. The humidity-dependent resistive and capacitive properties of this sensor make it promising for use in a humidity meter. & 2008 Elsevier B.V. All rights reserved.
PACS: 07.07.Df 61.82.Pv 61.82.Fk Keywords: Methyl-red Capacitive and resistive humidity sensor Response and recovery time Hysteresis
1. Introduction Rapid advancements in semiconductor technology and thin-film growth techniques have made it possible to fabricate highly accurate humidity sensors at an economical cost. Numerous humidity sensors based on different organic thin films have been reported [1–4]. Study on organic materials showed their high sensitivity to ambient conditions [5–7]. Therefore, investigation of these materials is very promising for the development of various types of sensors because of low hysteresis [8], chemical stability and compatibility with the IC process [9]. However, no single sensor can meet all these requirements. At high humidity level, significant difficulties have been reported in organic humidity-sensing materials, such as the organic material layer is liable to swell, shrink or peel off from the substrate [6]. The main requirements that a good humidity sensing material should meet are a broad sensing range, short response and recovery time, small hysteresis, durability and low cost.
Corresponding author. Tel.: +92 938 271858x2479; fax: +92 938 271890.
E-mail addresses: [email protected], [email protected] (Z. Ahmad). 1386-9477/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2008.05.018
There are various types of humidity sensors. Each type offers its distinct advantages. Capacitive humidity sensors are widely used in weather-monitoring applications. These sensors consist of a substrate on which a thin film of sensing material is deposited between electrodes. The relation between the dielectric constant and the relative humidity of a capacitive humidity sensor is almost directly proportional. The dielectric constant changes due to water vapors [10]. These sensors provide a wide relative humidity range and condensation tolerance. The properties of such types of sensors depend upon the area of the electrodes and the gap between the electrodes [11], whereas resistive humidity sensors measure the change in the electrical resistance of a sensing element. In this type of sensor the change in resistance is inversely proportional to relative humidity, because the water vapors dissociate the ionic functional groups, resulting in a decrease in electrical resistance. The advantages of this type of sensors are the high sensitivity, they are replaceable, useable in remote locations, and are cost effective, but these sensors are significantly temperature dependent, hence either the temperature must be constant or temperature correction must be incorporated [5]. Thermal conductivity sensors are better in corrosive environments and at high temperatures. The main applications of humidity sensors are measurement of humidity in gases, solids and liquids. Humidity sensors are also
ARTICLE IN PRESS Z. Ahmad et al. / Physica E 41 (2008) 18–22
widely applied in various types of combined humidity and temperature sensors for weather-monitoring instruments, whether handheld, benchtop or mounted. Currently, the most resistive humidity sensors have LiCl as the sensing element [12,13], in which a mixture of lithium chloride and carbon is used between the conducting electrodes. The operating principle of this device is based on ionic conductivity [10]. This device is very sensitive to temperature, several efforts have been made to improve this device by using the conductance technique [14]. Ceramic materials such as Al2O3 are also used in many commercial humidity sensors [15] due to the wellestablished etching technology and temperature stability, but are very sensitive to dust and smoke, and their response time is very long. Some organic compounds show high response to humidity but these compounds dissolve in water [7,16]. Organic materials that are insoluble in water such as cellulose acetate butyrate and polyimide have been used as humidity sensors [16]. Methyl-red is also insoluble in water, so it seems reasonable to investigate methyl-red as a humidity sensor. Methyl-red is a pH indicator dye in the form of a dark-red crystalline powder that turns red in acidic solutions. Methyl-red is an organic semiconductor and has potential application in electronic devices. The heterojunctions of methyl-red deposited from solution [17,18] showed good rectification behavior. The molecular structure of methyl-red is shown in Fig. 1. In the present work, we have prepared a methyl-red thin film by a spin-coating technique. A surface-type humidity sensor has been fabricated and variation of capacitance and resistance as a function of relative humidity has been studied. The humiditysensing properties of methyl-red thin films including sensitivity, hysteresis, and response and recovery time have also been studied. The aim of this research is to develop a reliable, accurate and cheap humidity-sensing element for the assessment of relative humidity of the surrounding environment using an organic compound.
19
using a mask. The thickness of the electrodes was 100 nm. The length of the gap was 15 mm. The 10 wt% solution of methyl-red was prepared in benzene. The solution was stirred for 2 h at room temperature to make it homogeneous. A thin film of methyl-red was deposited by a spin coater with an angular rotation of 2000 revolutions per minute (RPM) with an approximate thickness of 300 nm. The fabricated device was kept at 50 1C for 1 h to let the moisture evaporate from the film. The cross-sectional view of the fabricated device is shown in Fig. 2. Measurements were carried out in a self-made humidity measurement setup, which has been developed in our laboratory using conventional digital instruments. The CEM DT-8860 digital multimeter was used for in situ relative humidity measurements. All the measurements were taken at room temperature.
3. Results and discussion
2. Experimental
Absolute humidity is a measure of the actual amount of water vapors in the air, regardless of the environmental temperature. Conventional humidity sensors determine relative environmental humidity using the capacitive technique. The element of these sensors is made of a thin-film capacitor on substrates. The organic material is used as a dielectric, which absorbs or releases water molecules proportional to the relative environmental humidity, and thus changes the capacitance of the capacitor. The capacitive humidity sensors typically show a nonlinear response as a function of relative humidity [10,19]. The capacitance depends on the polarization [20], dielectric permittivity constant of the sensing material, gap between the electrodes and electrode geometry [11]. Polarization is caused by dipole, ionic and electronic polarizability. Dipoles and ions form due to the charge transfer complexes and dissociation of molecules, whereas electronic polarization arises due to relative displacement of electrons. The relation between dielectric constant and capacitance can be described by equation [10] n CS w ¼ C0 d
Methyl-red 2-[4-(dimethylamino)phenylazo] benzonic acid with the molecular formula (CH3)2NC6H4N ¼ NC6H4COOH purchased from Sigma Aldrich was used without further purification for the fabrication of the Ag/methyl-red/Ag humidity sensor. Glass substrate was cleaned for 10 min using distilled water in an ultrasonic cleaner and dried. Then the substrate was plasma cleaned for 5 min. The substrate was preliminary patterned with silver surface-type electrodes with a gap of 50 mm. These electrodes were deposited by the thermal evaporation of silver
where ed and ew are the permittivities of the dielectric at dry and wet states, respectively. The factor n is related to dielectric (morphology). Fig. 3 shows the capacitance–relative humidity relation for a Ag/methyl-red/Ag surface-type humidity sensor. The capacitance of the device increases from 100 to 1200 pF at relative humidity ranging from 30% to 95%. These results can be explained by the same approach as discussed above, because the humidity affects the concentration of ions, dipoles, electrons and holes, and may increase the polarization in methyl-red. The increase in capacitance with relative humidity may also be due to the porous
OH Methyl-red film O
Ag H3C
Ag
N N
N
H3C
Glass substrate Fig. 1. Molecular structure of methyl-red.
Fig. 2. Cross-sectional view of the Ag/methyl-red/Ag humidity sensor.
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Z. Ahmad et al. / Physica E 41 (2008) 18–22
Humidification
1200
Desiccation
Capacitance (pF)
1000 800 600 400 200 0 20
30
40
50 60 70 Relative Humidity (%)
80
90
100
Fig. 3. Effect of relative humidity on the capacitance of the Ag/methyl-red/Ag humidity sensor.
40
Humidification Desiccation
Resistancce (MΩ)
35
30
25
20
15 30
40
50 60 70 80 Relative Humidity (%)
90
100
water enhances the ionic conductivity of the methyl-red thin film due to the increase in dielectric constant. Secondly, at the same time, due to the correlation of methyl-red and water molecules, the disassociation of water molecules into ions may occur, which results in the increase in conductivity of the thin film. Thirdly, the polar molecules of water may also increase the conductivity of the thin film due to increase in proton concentration [6]. Fourthly, there may also be some physical phenomena behind this, such as the effect of absorbed water molecules as dipole and impurity [7]. The effect of humidity is relatively less at a low humidity level. This is due to the fact that at low humidity level the coverage of water on the surface is not continuous and only a few water molecules can be absorbed, while in the middle region of the relative humidity level one or several water layers are formed [22]. These layers accelerate the transfer of ions according to the Grotthuss mechanism [23]. At high relative humidity level, there might occur some energetic changes such as disassociation of molecules, due to which its resistance decreases rapidly. The most common problem with humidity sensors based on the principle of absorption is hysteresis. Hysteresis is the percentage difference in the final settling point of relative humidity when approached from above to when it is approached from below. Hysteresis is due to the formation of clusters of absorbed water [16] and depends on pores’ geometry, pores’ size and the changes in geometry of pores relative to the humidity level [19]. Larger pore size reduces the response time but also reduces the sensitivity, so it is difficult to make a fast and sensitive humidity sensor. The hysteresis can be reduced and sensitivity can be improved by an organic multilayer structure [8]. The value of hysteresis is calculated for the sample from the graphs, and its value is found to be about 9%. The dependence of impendence upon the relative humidity of the Ag/methyl-red/Ag humidity sensor at different frequencies is shown in Fig. 5. The value of impendence decreased with increase in frequency at any fixed value of relative humidity. At higher humidity level, the influence of frequency is less than the lower humidity level. The decrease in impendence at higher frequencies is due to the capacitance effect of electrodes [24]. A similar type of behavior for humidity sensors is reported in Refs. [25,26]. Response or recovery time is defined as the time taken by the sensor to achieve 90% of the total impedance change [22].
Fig. 4. Effect of relative humidity on the resistance of the Ag/methyl-red/Ag humidity sensor.
30
10 KHZ
28
100 HZ
26
10 HZ
24 Impedence (MΩ)
nature of the thin film. The dielectric permittivity constant of methyl-red increases with the absorption of water molecules and hence increases in capacitance. At higher relative humidity levels, variation in capacitance becomes less because the pores of the film are being filled with water molecules and sensor sensitivity decreases [21]. This confirms that the permittivity constant of methyl-red may increase with the absorption of water molecules. Fig. 4 shows the dependence of resistance on the humidity of Al/methyl-red/Ag surface-type humidity sensor at room temperature. The results show that the resistance drops with the increase in humidity. The resistance-relative humidity relation is not exactly linear. Therefore a linearization circuit is required to make it linear. There may be several reasons for the enhancement in conductivity of a methyl-red thin film with the elevation of relative humidity level. Firstly, the increase in conductivity of the methyl-red thin film may be due to both electronic and ionic conduction [6]. Ionic conduction exponentially depends on the dielectric constant of the thin-film material. The absorption of
1 KHZ
22 20 18 16 14 12 10 20
30
40
50 60 70 Relative humidity (%)
80
90
100
Fig. 5. Plot of impendence vs. relative humidity at different frequencies.
ARTICLE IN PRESS Z. Ahmad et al. / Physica E 41 (2008) 18–22
The measured capacitance and resistance versus time curve corresponding to humidification and desiccation by changing the humidity rapidly from 30% to 95% and then decrease from 95%
1200
Capacitance (pF)
1000 800 600 400 200 0
10
20
30
40
50
60
70
Time (Sec) Fig. 6. Response and recovery time of the Ag/methyl-red/Ag capacitive humidity sensor.
40
to 30% is shown in Figs. 6 and 7, respectively. It was found that the capacitive sensor exhibited quick response compared to the resistive one. These results are obtained in the following way. First, we placed the sample in a sealed glass with 30% relative humidity (for absorption) and placed it in the controlled humidity chamber with a humidity level of 95% and suddenly removed the glass cover. The sample was introduced rapidly into the ambient of humidity-controlled chamber and a similar method was adopted to measure the recovery time. The glass was sealed with a cover of insulating sheet. The response and recovery times were measured to be approximately the same (10 s) for the capacitive sensor, including the equilibration time inside the chamber; therefore, real response and recovery time are much shorter. Repeated cycles between these two relative humidity levels gave almost the same results. The detailed and simplified equivalent circuit diagrams of the surface-type Ag/methyl-red/Ag humidity sensor are shown in Fig. 8(a) and (b), respectively. The equivalent circuit contains two parts, capacitor and resistor. Capacitor represents polarization whereas resistance represents the various conducting species [27]. In a detailed equivalent circuit diagram, there are three basic capacitances due to three kinds of dielectrics: air (Ca), methyl-red (Cs) and glass (Cg); and three resistances: air (Ra), methyl-red (Rs) and glass (Rg). Usually the dielectric permittivity of the organic semiconductor is larger than glass and air. Moreover, the effective distance between the capacitance plates (in the case of Ca and Cg) is larger than Cs. Thus we can assume that CsbCa and Cg. Therefore we can neglect them, and taking into account that Ra and RgbRs, the equivalent circuit can be simplified as shown in Fig. 8(b).
4. Conclusions
35 Resistance (MΩ)
21
30
25
20 0
20
40
60
80
100
120
Time (Sec) Fig. 7. Response humidity sensor.
and
recovery
time
Ca
of
the
Cg
Ag/methyl-red/Ag
Cs
Ra
resistive
Rs
The properties of the surface-type Ag/methyl-red/Ag humidity sensor were investigated. The sensor showed short response and recovery time, good humidity sensitive property and better linearity. The equivalent circuit of the sensor was developed, which reflects the physical processes in methyl-red. Experimental results show that the capacitance of the sensor increased by 12 times, while the resistance decreased by 2.2 times. The change in capacitance is due to the difference between the dielectric constants of methyl-red and water molecule, whereas the conductivity increases due to protons and ions in the thin layer of absorbed water. The response and recovery time was about 10 s between 30% and 95% relative humidity for the capacitive-type sensor. Hence it can be concluded from the experimental results that the methyl-red capacitive sensor is more sensitive and quick in response than the resistive sensor. The main feature of this device is its cost, which is considerably lower than existing such devices. From a technological point of view, the surface-type humidity sensor is much simple and easier to fabricate than the sandwich type.
Rs
Rg Cs
Fig. 8. (a) Detailed equivalent circuit diagram of the Ag/methyl-red/Ag humidity sensor and (b) simplified equivalent circuit diagram of the Ag/methyl-red/Ag humidity sensor.
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Acknowledgments The authors acknowledge the facilitative role of the Higher Education Commission, Pakistan, and appreciate its financial assistance through the Indigenous Ph.D. fellowship program. The authors also wish to thank the Ghulam Ishaq Khan Institute of Engineering Sciences and Technology for its extended support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
M.K. Bernard, J. Am. Ceram. Soc. 74 (1991) 697. G. Harsanyi, Sens. Rev. (2000) 98. G. Harsanyi, Mater. Chem. Phys. 43 (1996) 199. C.W. Lee, H.W. Rhee, M.S. Gong, Sensors Actuators B 73 (2001) 124. S.A. Moiz, Kh.S. Karimov, N.D. Gohar, Eurassian Chem. Tech. J. 6 (2004) 201. C.-W. Lee, H.-S. Park, J.-G. Kima, B.-K. Choi, S.-W. Joo, M.-S. Gong, Sensors Actuators B 109 (2005) 315. K.S. Karimov, I. Qazi, T.A. Khan, P.H. Draper, F.A. Khalid, M. Mahroof-Tahir, Environ. Monit. Assess. (2007). Y. Li, M.J. Yang, Sensors Actuators B 87 (2002) 184. Y.Y. Qiu, C. Azeredo-leme, L.R. Alcacer, J.E. Franca, Sensors Actuators A 92 (2001) 80. Z.M. Rittersma, Sensors Actuators A 96 (2002) 196.
[11] J.G. Korvink, L. Chandran, T. Boltshauser, H. Baltes, Sens. Mater. 4 (1993) 323. [12] U. Sinha, Electrical and Electronics Measurement and Instrumentation, Smt. Sumitra Handa, New Delhi, 1992. [13] C.D. Simpson, Industrial Electronics, Prentice-Hall, Englewood Cliffs, NJ, 1996. [14] D.A. Mathews. Review of lithium chloride radiosonde hygrometer, Paper Presented at the Conference on Humidity and Moisture, Washington, DC, 1963. [15] M. Masuda, K. Misho, N. Baba, Appl. Phys. Lett. 63 (1993) 3155. [16] Y. Sakai, Y. Sadaoka, M. Matsuguchi, Sensors Actuators B 35–36 (1996) 85. [17] T. Kilicoglu, M.E. Aydin, Y.S. Ocak, Physica B 388 (2007) 244. [18] M.E. Aydin, A. Turut, Microelectron. Eng. 48 (2007) 2875. [19] M. Bjorkqvist, J. Salonen, J. Paski, E. Laine, Sensors Actuators A 112 (2004) 244. [20] M.A. Omar, Elementary Solid State Physics: Principles and Applications, Person Education, Singapore, 2002. [21] K.S. Shamala, L.C.S. Murthy, M.C. Radhakrishna, K. Narasimha Rao, Sensors Actuators A 135 (2007) 552. [22] Geng Wangchang, Rui wang, Xiaotian Li, Yongcun Zou, Tong Zang, Jinchun Tu, Yuane He, Nan Li, Sensors Actuators B 127 (2007) 323. [23] F.M. Ernsberger, J. Am. Ceram. Soc. 11 (1983) 747. [24] M. Bayhan, N. Kavasoglu, Sensors Actuators B 117 (2006) 261. [25] M.J. Hagon, A.W. Brinkman, T. Hashemi, Appl. Phys. Lett. 72 (1998) 3077. [26] K. Vijayamohanan, G. Pathak, M.P. Vinod, S.S. Pingale, S.F. Patil, Mater. Chem. Phys. 46 (1996) 72. [27] T. Zhang, X. Tian, B. Xu, G. Weidong, L. Sun, S. Xiang, D. Gao, J. Mater. Sci. Lett. 19 (2000) 1419.
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Humidity-dependent characteristics of methyl-red thin film-based Ag/methyl-red/Ag surface-type cell Zubair Ahmad , M.H. Sayyad, M. Saleem, Khasan S. Karimov, Mutabar Shah Faculty of Engineering Sciences (FES), Ghulam Ishaq Khan Institute of Engineering Sciences and Technology, Topi 23640, District Swabi, NWFP, Pakistan
a r t i c l e in f o
a b s t r a c t
Article history: Received 8 April 2008 Received in revised form 21 May 2008 Accepted 21 May 2008 Available online 14 June 2008
This paper describes the experimental results for humidity-dependent resistive and capacitive response of methyl-red thin films in a Ag/methyl-red/Ag surface-type cell. A 300-nm-thick film of methyl-red was deposited from 10 wt% solution in benzene by a spin coater at an angular speed of 2000 revolutions per minute (RPM) on a glass substrate with preliminary deposited metal electrodes to form the Ag/methyl-red/Ag surface-type cell. The length and width of the gap between the electrodes were 50 mm and 15 mm, respectively. The resistance of the film reduced from 37 to 17 MO with an elevation of relative humidity level over the whole humidity range. It was also observed that under the effect of humidity, the capacitance of the methyl-red thin film increased by 12 times. The capacitive/resistive sensor has a quasi-linear function with relative humidity in the range of 30–95% and has a small hysteresis. The response and recovery time of the sensor was about 10 s for the capacitive sensor. The humidity-dependent resistive and capacitive properties of this sensor make it promising for use in a humidity meter. & 2008 Elsevier B.V. All rights reserved.
PACS: 07.07.Df 61.82.Pv 61.82.Fk Keywords: Methyl-red Capacitive and resistive humidity sensor Response and recovery time Hysteresis
1. Introduction Rapid advancements in semiconductor technology and thin-film growth techniques have made it possible to fabricate highly accurate humidity sensors at an economical cost. Numerous humidity sensors based on different organic thin films have been reported [1–4]. Study on organic materials showed their high sensitivity to ambient conditions [5–7]. Therefore, investigation of these materials is very promising for the development of various types of sensors because of low hysteresis [8], chemical stability and compatibility with the IC process [9]. However, no single sensor can meet all these requirements. At high humidity level, significant difficulties have been reported in organic humidity-sensing materials, such as the organic material layer is liable to swell, shrink or peel off from the substrate [6]. The main requirements that a good humidity sensing material should meet are a broad sensing range, short response and recovery time, small hysteresis, durability and low cost.
Corresponding author. Tel.: +92 938 271858x2479; fax: +92 938 271890.
E-mail addresses: [email protected], [email protected] (Z. Ahmad). 1386-9477/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2008.05.018
There are various types of humidity sensors. Each type offers its distinct advantages. Capacitive humidity sensors are widely used in weather-monitoring applications. These sensors consist of a substrate on which a thin film of sensing material is deposited between electrodes. The relation between the dielectric constant and the relative humidity of a capacitive humidity sensor is almost directly proportional. The dielectric constant changes due to water vapors [10]. These sensors provide a wide relative humidity range and condensation tolerance. The properties of such types of sensors depend upon the area of the electrodes and the gap between the electrodes [11], whereas resistive humidity sensors measure the change in the electrical resistance of a sensing element. In this type of sensor the change in resistance is inversely proportional to relative humidity, because the water vapors dissociate the ionic functional groups, resulting in a decrease in electrical resistance. The advantages of this type of sensors are the high sensitivity, they are replaceable, useable in remote locations, and are cost effective, but these sensors are significantly temperature dependent, hence either the temperature must be constant or temperature correction must be incorporated [5]. Thermal conductivity sensors are better in corrosive environments and at high temperatures. The main applications of humidity sensors are measurement of humidity in gases, solids and liquids. Humidity sensors are also
ARTICLE IN PRESS Z. Ahmad et al. / Physica E 41 (2008) 18–22
widely applied in various types of combined humidity and temperature sensors for weather-monitoring instruments, whether handheld, benchtop or mounted. Currently, the most resistive humidity sensors have LiCl as the sensing element [12,13], in which a mixture of lithium chloride and carbon is used between the conducting electrodes. The operating principle of this device is based on ionic conductivity [10]. This device is very sensitive to temperature, several efforts have been made to improve this device by using the conductance technique [14]. Ceramic materials such as Al2O3 are also used in many commercial humidity sensors [15] due to the wellestablished etching technology and temperature stability, but are very sensitive to dust and smoke, and their response time is very long. Some organic compounds show high response to humidity but these compounds dissolve in water [7,16]. Organic materials that are insoluble in water such as cellulose acetate butyrate and polyimide have been used as humidity sensors [16]. Methyl-red is also insoluble in water, so it seems reasonable to investigate methyl-red as a humidity sensor. Methyl-red is a pH indicator dye in the form of a dark-red crystalline powder that turns red in acidic solutions. Methyl-red is an organic semiconductor and has potential application in electronic devices. The heterojunctions of methyl-red deposited from solution [17,18] showed good rectification behavior. The molecular structure of methyl-red is shown in Fig. 1. In the present work, we have prepared a methyl-red thin film by a spin-coating technique. A surface-type humidity sensor has been fabricated and variation of capacitance and resistance as a function of relative humidity has been studied. The humiditysensing properties of methyl-red thin films including sensitivity, hysteresis, and response and recovery time have also been studied. The aim of this research is to develop a reliable, accurate and cheap humidity-sensing element for the assessment of relative humidity of the surrounding environment using an organic compound.
19
using a mask. The thickness of the electrodes was 100 nm. The length of the gap was 15 mm. The 10 wt% solution of methyl-red was prepared in benzene. The solution was stirred for 2 h at room temperature to make it homogeneous. A thin film of methyl-red was deposited by a spin coater with an angular rotation of 2000 revolutions per minute (RPM) with an approximate thickness of 300 nm. The fabricated device was kept at 50 1C for 1 h to let the moisture evaporate from the film. The cross-sectional view of the fabricated device is shown in Fig. 2. Measurements were carried out in a self-made humidity measurement setup, which has been developed in our laboratory using conventional digital instruments. The CEM DT-8860 digital multimeter was used for in situ relative humidity measurements. All the measurements were taken at room temperature.
3. Results and discussion
2. Experimental
Absolute humidity is a measure of the actual amount of water vapors in the air, regardless of the environmental temperature. Conventional humidity sensors determine relative environmental humidity using the capacitive technique. The element of these sensors is made of a thin-film capacitor on substrates. The organic material is used as a dielectric, which absorbs or releases water molecules proportional to the relative environmental humidity, and thus changes the capacitance of the capacitor. The capacitive humidity sensors typically show a nonlinear response as a function of relative humidity [10,19]. The capacitance depends on the polarization [20], dielectric permittivity constant of the sensing material, gap between the electrodes and electrode geometry [11]. Polarization is caused by dipole, ionic and electronic polarizability. Dipoles and ions form due to the charge transfer complexes and dissociation of molecules, whereas electronic polarization arises due to relative displacement of electrons. The relation between dielectric constant and capacitance can be described by equation [10] n CS w ¼ C0 d
Methyl-red 2-[4-(dimethylamino)phenylazo] benzonic acid with the molecular formula (CH3)2NC6H4N ¼ NC6H4COOH purchased from Sigma Aldrich was used without further purification for the fabrication of the Ag/methyl-red/Ag humidity sensor. Glass substrate was cleaned for 10 min using distilled water in an ultrasonic cleaner and dried. Then the substrate was plasma cleaned for 5 min. The substrate was preliminary patterned with silver surface-type electrodes with a gap of 50 mm. These electrodes were deposited by the thermal evaporation of silver
where ed and ew are the permittivities of the dielectric at dry and wet states, respectively. The factor n is related to dielectric (morphology). Fig. 3 shows the capacitance–relative humidity relation for a Ag/methyl-red/Ag surface-type humidity sensor. The capacitance of the device increases from 100 to 1200 pF at relative humidity ranging from 30% to 95%. These results can be explained by the same approach as discussed above, because the humidity affects the concentration of ions, dipoles, electrons and holes, and may increase the polarization in methyl-red. The increase in capacitance with relative humidity may also be due to the porous
OH Methyl-red film O
Ag H3C
Ag
N N
N
H3C
Glass substrate Fig. 1. Molecular structure of methyl-red.
Fig. 2. Cross-sectional view of the Ag/methyl-red/Ag humidity sensor.
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Z. Ahmad et al. / Physica E 41 (2008) 18–22
Humidification
1200
Desiccation
Capacitance (pF)
1000 800 600 400 200 0 20
30
40
50 60 70 Relative Humidity (%)
80
90
100
Fig. 3. Effect of relative humidity on the capacitance of the Ag/methyl-red/Ag humidity sensor.
40
Humidification Desiccation
Resistancce (MΩ)
35
30
25
20
15 30
40
50 60 70 80 Relative Humidity (%)
90
100
water enhances the ionic conductivity of the methyl-red thin film due to the increase in dielectric constant. Secondly, at the same time, due to the correlation of methyl-red and water molecules, the disassociation of water molecules into ions may occur, which results in the increase in conductivity of the thin film. Thirdly, the polar molecules of water may also increase the conductivity of the thin film due to increase in proton concentration [6]. Fourthly, there may also be some physical phenomena behind this, such as the effect of absorbed water molecules as dipole and impurity [7]. The effect of humidity is relatively less at a low humidity level. This is due to the fact that at low humidity level the coverage of water on the surface is not continuous and only a few water molecules can be absorbed, while in the middle region of the relative humidity level one or several water layers are formed [22]. These layers accelerate the transfer of ions according to the Grotthuss mechanism [23]. At high relative humidity level, there might occur some energetic changes such as disassociation of molecules, due to which its resistance decreases rapidly. The most common problem with humidity sensors based on the principle of absorption is hysteresis. Hysteresis is the percentage difference in the final settling point of relative humidity when approached from above to when it is approached from below. Hysteresis is due to the formation of clusters of absorbed water [16] and depends on pores’ geometry, pores’ size and the changes in geometry of pores relative to the humidity level [19]. Larger pore size reduces the response time but also reduces the sensitivity, so it is difficult to make a fast and sensitive humidity sensor. The hysteresis can be reduced and sensitivity can be improved by an organic multilayer structure [8]. The value of hysteresis is calculated for the sample from the graphs, and its value is found to be about 9%. The dependence of impendence upon the relative humidity of the Ag/methyl-red/Ag humidity sensor at different frequencies is shown in Fig. 5. The value of impendence decreased with increase in frequency at any fixed value of relative humidity. At higher humidity level, the influence of frequency is less than the lower humidity level. The decrease in impendence at higher frequencies is due to the capacitance effect of electrodes [24]. A similar type of behavior for humidity sensors is reported in Refs. [25,26]. Response or recovery time is defined as the time taken by the sensor to achieve 90% of the total impedance change [22].
Fig. 4. Effect of relative humidity on the resistance of the Ag/methyl-red/Ag humidity sensor.
30
10 KHZ
28
100 HZ
26
10 HZ
24 Impedence (MΩ)
nature of the thin film. The dielectric permittivity constant of methyl-red increases with the absorption of water molecules and hence increases in capacitance. At higher relative humidity levels, variation in capacitance becomes less because the pores of the film are being filled with water molecules and sensor sensitivity decreases [21]. This confirms that the permittivity constant of methyl-red may increase with the absorption of water molecules. Fig. 4 shows the dependence of resistance on the humidity of Al/methyl-red/Ag surface-type humidity sensor at room temperature. The results show that the resistance drops with the increase in humidity. The resistance-relative humidity relation is not exactly linear. Therefore a linearization circuit is required to make it linear. There may be several reasons for the enhancement in conductivity of a methyl-red thin film with the elevation of relative humidity level. Firstly, the increase in conductivity of the methyl-red thin film may be due to both electronic and ionic conduction [6]. Ionic conduction exponentially depends on the dielectric constant of the thin-film material. The absorption of
1 KHZ
22 20 18 16 14 12 10 20
30
40
50 60 70 Relative humidity (%)
80
90
100
Fig. 5. Plot of impendence vs. relative humidity at different frequencies.
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The measured capacitance and resistance versus time curve corresponding to humidification and desiccation by changing the humidity rapidly from 30% to 95% and then decrease from 95%
1200
Capacitance (pF)
1000 800 600 400 200 0
10
20
30
40
50
60
70
Time (Sec) Fig. 6. Response and recovery time of the Ag/methyl-red/Ag capacitive humidity sensor.
40
to 30% is shown in Figs. 6 and 7, respectively. It was found that the capacitive sensor exhibited quick response compared to the resistive one. These results are obtained in the following way. First, we placed the sample in a sealed glass with 30% relative humidity (for absorption) and placed it in the controlled humidity chamber with a humidity level of 95% and suddenly removed the glass cover. The sample was introduced rapidly into the ambient of humidity-controlled chamber and a similar method was adopted to measure the recovery time. The glass was sealed with a cover of insulating sheet. The response and recovery times were measured to be approximately the same (10 s) for the capacitive sensor, including the equilibration time inside the chamber; therefore, real response and recovery time are much shorter. Repeated cycles between these two relative humidity levels gave almost the same results. The detailed and simplified equivalent circuit diagrams of the surface-type Ag/methyl-red/Ag humidity sensor are shown in Fig. 8(a) and (b), respectively. The equivalent circuit contains two parts, capacitor and resistor. Capacitor represents polarization whereas resistance represents the various conducting species [27]. In a detailed equivalent circuit diagram, there are three basic capacitances due to three kinds of dielectrics: air (Ca), methyl-red (Cs) and glass (Cg); and three resistances: air (Ra), methyl-red (Rs) and glass (Rg). Usually the dielectric permittivity of the organic semiconductor is larger than glass and air. Moreover, the effective distance between the capacitance plates (in the case of Ca and Cg) is larger than Cs. Thus we can assume that CsbCa and Cg. Therefore we can neglect them, and taking into account that Ra and RgbRs, the equivalent circuit can be simplified as shown in Fig. 8(b).
4. Conclusions
35 Resistance (MΩ)
21
30
25
20 0
20
40
60
80
100
120
Time (Sec) Fig. 7. Response humidity sensor.
and
recovery
time
Ca
of
the
Cg
Ag/methyl-red/Ag
Cs
Ra
resistive
Rs
The properties of the surface-type Ag/methyl-red/Ag humidity sensor were investigated. The sensor showed short response and recovery time, good humidity sensitive property and better linearity. The equivalent circuit of the sensor was developed, which reflects the physical processes in methyl-red. Experimental results show that the capacitance of the sensor increased by 12 times, while the resistance decreased by 2.2 times. The change in capacitance is due to the difference between the dielectric constants of methyl-red and water molecule, whereas the conductivity increases due to protons and ions in the thin layer of absorbed water. The response and recovery time was about 10 s between 30% and 95% relative humidity for the capacitive-type sensor. Hence it can be concluded from the experimental results that the methyl-red capacitive sensor is more sensitive and quick in response than the resistive sensor. The main feature of this device is its cost, which is considerably lower than existing such devices. From a technological point of view, the surface-type humidity sensor is much simple and easier to fabricate than the sandwich type.
Rs
Rg Cs
Fig. 8. (a) Detailed equivalent circuit diagram of the Ag/methyl-red/Ag humidity sensor and (b) simplified equivalent circuit diagram of the Ag/methyl-red/Ag humidity sensor.
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Acknowledgments The authors acknowledge the facilitative role of the Higher Education Commission, Pakistan, and appreciate its financial assistance through the Indigenous Ph.D. fellowship program. The authors also wish to thank the Ghulam Ishaq Khan Institute of Engineering Sciences and Technology for its extended support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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