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Erbium bis[phthalocyaninato] complex LB film gas sensor
Materials Letters 57 (2003) 2395 – 2398 www.elsevier.com/locate/matlet
Erbium bis[phthalocyaninato] complex LB film gas sensor D. Xie a,*, W. Pan a, Y.D. Jiang b, Y.R. Li b a
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China b Department of Materials Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, PR China Received 7 December 2001; received in revised form 19 August 2002; accepted 1 October 2002
Abstract A new kind of sandwich-like erbium bis[2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyaninato] complex Er[Pc*]2 (Pc* = Pc(OC8H17)8) was used as film-forming material. Pure Er[Pc*]2 and mixture of Er[Pc*]2 and stearic acid (SA) deposited from both pure water and 10 4 M Cd2 + subphases are investigated. It is found that a mixture of 1:6 Er[Pc*]2/SA forms an excellent material for the fabrication of gas-sensing LB film by studying the film-forming characteristic. A new gas sensor has been fabricated by incorporating the multilayer Er[Pc*]2 LB film into the gate electrode of a MOSFET, forming an array of charge flow transistor (CFT). On the application of a gate voltage (VGS) greater than the threshold voltage (VTH), a delay was observed in the response of the drain current. It is due to the time taken for the resistive gas-sensing film to charge up to VGS. This delay characteristic was found to depend on the concentration of NO2. Results are presented showing that the device can detect reversibly concentration of NO2 gas down to 5 ppm at room temperature. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Gas sensor; Charge flow transistor; MOSFET; Nitrogen dioxide; LB films; Bis[phthalocyaninato] complex
A charge flow transistor (CFT) is a special metal oxide semiconductor field effect transistor (MOSFET) in which a portion of the gate electrode is replaced by an organic film (Fig. 1). The charge flow transistor was first reported by Senturia et al. [1]. The major advantage of using CFT in sensor applications, instead of using a conventional interdigitated electrode pair structure, is the level of current measured. Because the organic films used are typically highly resistive, the current measured using the electrode pair device is in the nanoampere to picoampere range, requiring complex detection and good shielding to avoid excessive
* Corresponding author. E-mail address: [email protected] (D. Xie).
problems with noise, while the current measured using CFT device is in the microampere to milliampere range. Other advantages include the reduction in the device size, the possibility of integrating the detection circuitry with sensors and so on. On applying gatesource voltage, VGS, greater than the FET threshold voltage, the metallic sections of the gate electrode charge rapidly to the applied voltage. Due to the high resistance of the organic film, there is a delay before the film is uniformly charged to the applied voltage. Therefore, a complete conducting source-drain channel is not formed until the threshold voltage is exceeded along the whole film, resulting in a delay between applying VGS and the drain current reaching its saturation value. The time required for this charging process depends on the sheet resistance of the film, the
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01242-9
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Fig. 1. A schematic cross-section of Er[Pc*]2/CS mixed LB film based CFT sensor.
width of the gap in the gate electrode and the dielectric constant and thickness of the SiO2 insulator. When being exposed to different gases, the film resistance will vary, hence, the time taken for the drain current to saturate will change, which suggests that the CFT device can be used as an effective gas sensor [1 –3]. The Langmuir –Blodgett (LB) technique is a promising means to develop highly ordered conducting organic thin films. Because such ultrathin films have high ratios of surface area to bulk volume, the use of organic gas-sensitive materials and LB deposition technique will show a great potential for improving the performance of gas sensors. It can be expected to obtain an efficient and quick response gas sensor by using LB technique with good molecular packing and with its gas-sensitive molecular groups aligned near the surface of the LB film [4– 6]. In order to fabricate stable gas sensors, careful selection of the film-forming materials is required. It is known that metal-substituted phthalocyanines are extremely sensitive to oxidizing gas. There are many reports about mono-phthalocyanine gas sensors [7,8]. However, there has been no systematical research on the gas sensitivity of the novel phthalocyanine material-substituted bis[phthalocyaninato] rare earth double deckers, which show great potential for application in molecular electronics, gas sensors, electrochromic and molecular magnetic devices [9,10]. There are few reports on substituted bis[phthalocyaninato] rare earth complex based CFT gas sensor by LB technique. In this paper, we report the characteristics of substituted erbium bis[phthalocyaninato] complex Er[Pc*]2 (Pc* = Pc(OC8H17)8) and octadecanol (SA) on different subphases, as well as the gas-sensing property to NO2 gas of CFT-based gas sensor.
The new substituted bis[phthalocyaninato] erbium complex Er[Pc*]2 was synthesized by the method described in Ref. [10]. The molecular structure of Er[Pc*]2 is shown in Fig. 2. Er[Pc*]2 LB films were deposited with WM-1 LB instrument made in Southeast University of Manjing, China. Spreading solution was prepared by dissolving Er[Pc*]2 in chloroform with the concentration of 0.42 mg/ml. In order to produce stable Langmuir films, Er[Pc*]2 was mixed with stearic acid (SA) in different molar ratios of 1:0, 1:1 and 1:6. Two types of subphases were employed: a pure water subphase (pH 6.9) and a 10 4 M Cd2 + subphase (pH 5.7). The monolayer was then compressed at a speed of 3 mm/min and the surface pressure was monitored by Wilhelmy balance. Based on standard MOSFET design, an array of four CFTs has been fabricated as a set of devices. The widths of the gaps in the gate electrode are 0, 10, 20 and 30 Am. The channel length is 100 Am. Fig. 3 shows the plan view of CFT device. The dimension of the CFT device is 10 8 mm2. UV –Vis absorption spectra were measured with Eraic UV1100 spectrophotometer made in Beijing,
Fig. 2. Molecular structure of Er[Pc*]2 (Pc*= Pc(OC8H17)8, R =OC8H17).
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Fig. 3. A plan of electrode arrangements of the four CFT devices: (a) 0-Am gate-hold, (b) 10-Am gate-hold, (c) 20-Am gate-hold, (d) 30-Am gate-hold.
China. After placing the samples in the gas-testing system, the gas-sensing property of CFT sensor was measured. Surface pressure vs. area isotherms recorded for monolayers of pure Er[Pc*]2 and Er[Pc*]2/SA mixture on both pure water and 10 4 M Cd2 + subphases are shown in Fig. 4. It is found that Er[Pc*]2/CS (1:6) mixed monolayer in Cd2 + subphase shows a better film-forming stability (SA in Cd2 + subphase forms stable cadmium stearate, CS). The mixture was transferred to substrates at 36.5 mN/m by the vertical dipping method. Fig. 5 gives the UV – Vis absorption spectra of Er[Pc*]2/CS mixed LB films with various numbers of LB film layers. From the inset of Fig. 5, we can see that the plot of the absorbance at 668.5 nm of the deposited LB films vs. the number of LB film layers
Fig. 4. Surface pressure vs. area isotherms of Er[Pc*]2/CS mixture in the molar ratios (1:0, 1:1, 1:6) on pure water and 10 4 M Cd2 + subphases at the temperature 25 jC.
Fig. 5. The UV – Vis spectra of Er[Pc*]2/CS mixed LB films with various number of layers (inset: plot of the absorbance at 668.5 nm vs. the number of LB film layers).
results in a straight line, respectively, which indicates a constant transfer ratio during sequential dipping of the slide through the film with uniform deposition. To investigate the gas-sensing property, CFT sensor were exposed to NO2 repeatedly at room temperature. Fig. 6 shows the ‘turn-on’ response for 30-Am gatehold CFT sensor with 60-layer Er[Pc*]2/CS (1:6) mixed LB films on exposure to different NO2 concentrations. With the increase of NO2 concentration, the rate of turn-on response of source-drain current (IDS) increases. It can be seen that there is a detectable
Fig. 6. Source-drain current IDS vs. time for 60-layer Er[Pc*]2/CS mixed LB film based CFT sensor with 30-Am gate-hold in various NO2 concentrations.
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Fig. 7. Sensitivity plot of CFT sensor (30-Am gate-hold) for turn-on response on exposure to different concentration of NO2 gas.
response for concentration of NO2 as low as 5 ppm. Because phthalocyanine is a kind of p-type organic semiconductor, the gas sensitivity is realized through the charge transfer interaction in which the gas molecule to be sensed acts as a planar k-electron acceptor forming a redox couple, and the positive charge produced is delocalized over the two phthalocyanine macrocycles causing the decrease of the resistance, which will lead to the shortening of the time taken for LB film to charge [11,12]. When NO2 gas concentration and the width of the gate-hold vary, the film resistance of the gate area changes, too, resulting in the variation of the turn-on response. It takes about 8 min for the CFT sensor to recover to 90% of initialization, but complete recovery needs another 8 min. This may be due to the rapid desorption of the NO2 molecules adsorbing on the LB film surface at initial recovery stage. However, during the latter recovery stage, desorption of the NO2 molecules from the LB film surface and diffusion into the film is a complex process [13]. Of course, the interaction process between LB film and the adsorption gas is a dynamic process, and adsorption of NO2 gas molecules occurs during the process of desorption. Through calculating the gradient of the steepest region of the straight-line section of the turn-on response curve, the sensitivity of the CFT sensor was obtained which can be seen from Fig. 7. There is approximately a linear relationship between the increasing rate of drain current and the concentration of NO2 gas. Mixing Er[Pc*]2 with SA in different molar ratios of 1:1 and 1:6 greatly improves the film-forming charac-
teristic. It is especially obvious for 1:6 Er[Pc*]2/SA on 10 4 M Cd2 + subphase. UV – Vis spectra show the transferred materials in the spreading solution in each deposition are approximately equivalent. A new gas sensor has been fabricated by incorporating the multilayer Er[Pc*]2/CS mixed LB film into the gate electrode of a MOSFET, forming an array of CFT device. It is found that 30-Am gate-hold CFT sensor with 60-layer Er[Pc*]2/CS mixed LB film can detect NO2 gas down to 5 ppm. Therefore, detection to different gas with lower concentration can be realized using the turn-on effect of such CFT device. At the same time, it is feasible to achieve the miniaturization and integration of all kinds of sensors integrating with microelectronic fabrication process. Because Er[Pc*]2 has two planar phthalocyanine macrocycles, it is very favorable for charge transfer interaction between certain electrophilic gas molecules and kelectron of the phthalocyanine macrocycle, which shows more remarkable sensitivity than monophthalocyanine. Therefore, substituted bis[phthalocyaninato] rare earth complexes are promising organic materials to develop gas sensors with further improved properties.
References [1] S.D. Senturia, C.M. Sechen, J.A. Wishneusky, J. Appl. Phys. Lett. 30 (1977) 106. [2] P.S. Barker, A.P. Monkman, M.C. Petty, R. Pride, IEE Proc., Circ. Devices Syst. 144 (1997) 111. [3] C.Di. Bartolomeo, P.S. Barker, M.C. Petty, P. Adams, A.P. Monkman, Adv. Mater. Opt. Electron. 2 (1993) 233. [4] M. Ando, Y. Watanabe, T. Iyoda, K. Honda, T. Shimidzu, Thin Solid Films 179 (1989) 225. [5] D.P. Jiang, A.D. Lu, Y.J. Li, X.M. Pang, Y.L. Hua, Thin Solid Films 199 (1991) 173. [6] X. Ding, H. Xu, Thin Solid Films 338 (1999) 286. [7] H.Y. Wang, C.W. Chiang, J.B. Lando, Thin Solid Films 273 (1996) 90. [8] A.V. Nabok, Z.I. Kazantseva, N.V. Lavrik, Int. J. Electron. 78 (1) (1995) 129. [9] J.Z. Jiang, J.P. Wu, W. Liu, J.W. Xie, S.X. Sun, Chem. Rev. 2 (1999) 2. [10] J.Z. Jiang, R.C.W. Liu, T.C.W. Mak, T.W.D. Chan, D.K.P. Ng, Polyhedron 16 (1997) 515. [11] H.Y. Wang, J.B. Lando, Langmuir 10 (1994) 790. [12] X. Vilanova, E. Llobet, J. Brezmes, J. Calderer, X. Correig, Sens. Actuators, B 48 (1998) 425. [13] H.Y. Wang, W.H. Ko, D.A. Batzel, M.E. Kenney, J.B. Lando, Sens. Actuators, B 1 (1990) 138.
Erbium bis[phthalocyaninato] complex LB film gas sensor D. Xie a,*, W. Pan a, Y.D. Jiang b, Y.R. Li b a
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China b Department of Materials Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, PR China Received 7 December 2001; received in revised form 19 August 2002; accepted 1 October 2002
Abstract A new kind of sandwich-like erbium bis[2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyaninato] complex Er[Pc*]2 (Pc* = Pc(OC8H17)8) was used as film-forming material. Pure Er[Pc*]2 and mixture of Er[Pc*]2 and stearic acid (SA) deposited from both pure water and 10 4 M Cd2 + subphases are investigated. It is found that a mixture of 1:6 Er[Pc*]2/SA forms an excellent material for the fabrication of gas-sensing LB film by studying the film-forming characteristic. A new gas sensor has been fabricated by incorporating the multilayer Er[Pc*]2 LB film into the gate electrode of a MOSFET, forming an array of charge flow transistor (CFT). On the application of a gate voltage (VGS) greater than the threshold voltage (VTH), a delay was observed in the response of the drain current. It is due to the time taken for the resistive gas-sensing film to charge up to VGS. This delay characteristic was found to depend on the concentration of NO2. Results are presented showing that the device can detect reversibly concentration of NO2 gas down to 5 ppm at room temperature. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Gas sensor; Charge flow transistor; MOSFET; Nitrogen dioxide; LB films; Bis[phthalocyaninato] complex
A charge flow transistor (CFT) is a special metal oxide semiconductor field effect transistor (MOSFET) in which a portion of the gate electrode is replaced by an organic film (Fig. 1). The charge flow transistor was first reported by Senturia et al. [1]. The major advantage of using CFT in sensor applications, instead of using a conventional interdigitated electrode pair structure, is the level of current measured. Because the organic films used are typically highly resistive, the current measured using the electrode pair device is in the nanoampere to picoampere range, requiring complex detection and good shielding to avoid excessive
* Corresponding author. E-mail address: [email protected] (D. Xie).
problems with noise, while the current measured using CFT device is in the microampere to milliampere range. Other advantages include the reduction in the device size, the possibility of integrating the detection circuitry with sensors and so on. On applying gatesource voltage, VGS, greater than the FET threshold voltage, the metallic sections of the gate electrode charge rapidly to the applied voltage. Due to the high resistance of the organic film, there is a delay before the film is uniformly charged to the applied voltage. Therefore, a complete conducting source-drain channel is not formed until the threshold voltage is exceeded along the whole film, resulting in a delay between applying VGS and the drain current reaching its saturation value. The time required for this charging process depends on the sheet resistance of the film, the
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01242-9
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D. Xie et al. / Materials Letters 57 (2003) 2395–2398
Fig. 1. A schematic cross-section of Er[Pc*]2/CS mixed LB film based CFT sensor.
width of the gap in the gate electrode and the dielectric constant and thickness of the SiO2 insulator. When being exposed to different gases, the film resistance will vary, hence, the time taken for the drain current to saturate will change, which suggests that the CFT device can be used as an effective gas sensor [1 –3]. The Langmuir –Blodgett (LB) technique is a promising means to develop highly ordered conducting organic thin films. Because such ultrathin films have high ratios of surface area to bulk volume, the use of organic gas-sensitive materials and LB deposition technique will show a great potential for improving the performance of gas sensors. It can be expected to obtain an efficient and quick response gas sensor by using LB technique with good molecular packing and with its gas-sensitive molecular groups aligned near the surface of the LB film [4– 6]. In order to fabricate stable gas sensors, careful selection of the film-forming materials is required. It is known that metal-substituted phthalocyanines are extremely sensitive to oxidizing gas. There are many reports about mono-phthalocyanine gas sensors [7,8]. However, there has been no systematical research on the gas sensitivity of the novel phthalocyanine material-substituted bis[phthalocyaninato] rare earth double deckers, which show great potential for application in molecular electronics, gas sensors, electrochromic and molecular magnetic devices [9,10]. There are few reports on substituted bis[phthalocyaninato] rare earth complex based CFT gas sensor by LB technique. In this paper, we report the characteristics of substituted erbium bis[phthalocyaninato] complex Er[Pc*]2 (Pc* = Pc(OC8H17)8) and octadecanol (SA) on different subphases, as well as the gas-sensing property to NO2 gas of CFT-based gas sensor.
The new substituted bis[phthalocyaninato] erbium complex Er[Pc*]2 was synthesized by the method described in Ref. [10]. The molecular structure of Er[Pc*]2 is shown in Fig. 2. Er[Pc*]2 LB films were deposited with WM-1 LB instrument made in Southeast University of Manjing, China. Spreading solution was prepared by dissolving Er[Pc*]2 in chloroform with the concentration of 0.42 mg/ml. In order to produce stable Langmuir films, Er[Pc*]2 was mixed with stearic acid (SA) in different molar ratios of 1:0, 1:1 and 1:6. Two types of subphases were employed: a pure water subphase (pH 6.9) and a 10 4 M Cd2 + subphase (pH 5.7). The monolayer was then compressed at a speed of 3 mm/min and the surface pressure was monitored by Wilhelmy balance. Based on standard MOSFET design, an array of four CFTs has been fabricated as a set of devices. The widths of the gaps in the gate electrode are 0, 10, 20 and 30 Am. The channel length is 100 Am. Fig. 3 shows the plan view of CFT device. The dimension of the CFT device is 10 8 mm2. UV –Vis absorption spectra were measured with Eraic UV1100 spectrophotometer made in Beijing,
Fig. 2. Molecular structure of Er[Pc*]2 (Pc*= Pc(OC8H17)8, R =OC8H17).
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Fig. 3. A plan of electrode arrangements of the four CFT devices: (a) 0-Am gate-hold, (b) 10-Am gate-hold, (c) 20-Am gate-hold, (d) 30-Am gate-hold.
China. After placing the samples in the gas-testing system, the gas-sensing property of CFT sensor was measured. Surface pressure vs. area isotherms recorded for monolayers of pure Er[Pc*]2 and Er[Pc*]2/SA mixture on both pure water and 10 4 M Cd2 + subphases are shown in Fig. 4. It is found that Er[Pc*]2/CS (1:6) mixed monolayer in Cd2 + subphase shows a better film-forming stability (SA in Cd2 + subphase forms stable cadmium stearate, CS). The mixture was transferred to substrates at 36.5 mN/m by the vertical dipping method. Fig. 5 gives the UV – Vis absorption spectra of Er[Pc*]2/CS mixed LB films with various numbers of LB film layers. From the inset of Fig. 5, we can see that the plot of the absorbance at 668.5 nm of the deposited LB films vs. the number of LB film layers
Fig. 4. Surface pressure vs. area isotherms of Er[Pc*]2/CS mixture in the molar ratios (1:0, 1:1, 1:6) on pure water and 10 4 M Cd2 + subphases at the temperature 25 jC.
Fig. 5. The UV – Vis spectra of Er[Pc*]2/CS mixed LB films with various number of layers (inset: plot of the absorbance at 668.5 nm vs. the number of LB film layers).
results in a straight line, respectively, which indicates a constant transfer ratio during sequential dipping of the slide through the film with uniform deposition. To investigate the gas-sensing property, CFT sensor were exposed to NO2 repeatedly at room temperature. Fig. 6 shows the ‘turn-on’ response for 30-Am gatehold CFT sensor with 60-layer Er[Pc*]2/CS (1:6) mixed LB films on exposure to different NO2 concentrations. With the increase of NO2 concentration, the rate of turn-on response of source-drain current (IDS) increases. It can be seen that there is a detectable
Fig. 6. Source-drain current IDS vs. time for 60-layer Er[Pc*]2/CS mixed LB film based CFT sensor with 30-Am gate-hold in various NO2 concentrations.
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Fig. 7. Sensitivity plot of CFT sensor (30-Am gate-hold) for turn-on response on exposure to different concentration of NO2 gas.
response for concentration of NO2 as low as 5 ppm. Because phthalocyanine is a kind of p-type organic semiconductor, the gas sensitivity is realized through the charge transfer interaction in which the gas molecule to be sensed acts as a planar k-electron acceptor forming a redox couple, and the positive charge produced is delocalized over the two phthalocyanine macrocycles causing the decrease of the resistance, which will lead to the shortening of the time taken for LB film to charge [11,12]. When NO2 gas concentration and the width of the gate-hold vary, the film resistance of the gate area changes, too, resulting in the variation of the turn-on response. It takes about 8 min for the CFT sensor to recover to 90% of initialization, but complete recovery needs another 8 min. This may be due to the rapid desorption of the NO2 molecules adsorbing on the LB film surface at initial recovery stage. However, during the latter recovery stage, desorption of the NO2 molecules from the LB film surface and diffusion into the film is a complex process [13]. Of course, the interaction process between LB film and the adsorption gas is a dynamic process, and adsorption of NO2 gas molecules occurs during the process of desorption. Through calculating the gradient of the steepest region of the straight-line section of the turn-on response curve, the sensitivity of the CFT sensor was obtained which can be seen from Fig. 7. There is approximately a linear relationship between the increasing rate of drain current and the concentration of NO2 gas. Mixing Er[Pc*]2 with SA in different molar ratios of 1:1 and 1:6 greatly improves the film-forming charac-
teristic. It is especially obvious for 1:6 Er[Pc*]2/SA on 10 4 M Cd2 + subphase. UV – Vis spectra show the transferred materials in the spreading solution in each deposition are approximately equivalent. A new gas sensor has been fabricated by incorporating the multilayer Er[Pc*]2/CS mixed LB film into the gate electrode of a MOSFET, forming an array of CFT device. It is found that 30-Am gate-hold CFT sensor with 60-layer Er[Pc*]2/CS mixed LB film can detect NO2 gas down to 5 ppm. Therefore, detection to different gas with lower concentration can be realized using the turn-on effect of such CFT device. At the same time, it is feasible to achieve the miniaturization and integration of all kinds of sensors integrating with microelectronic fabrication process. Because Er[Pc*]2 has two planar phthalocyanine macrocycles, it is very favorable for charge transfer interaction between certain electrophilic gas molecules and kelectron of the phthalocyanine macrocycle, which shows more remarkable sensitivity than monophthalocyanine. Therefore, substituted bis[phthalocyaninato] rare earth complexes are promising organic materials to develop gas sensors with further improved properties.
References [1] S.D. Senturia, C.M. Sechen, J.A. Wishneusky, J. Appl. Phys. Lett. 30 (1977) 106. [2] P.S. Barker, A.P. Monkman, M.C. Petty, R. Pride, IEE Proc., Circ. Devices Syst. 144 (1997) 111. [3] C.Di. Bartolomeo, P.S. Barker, M.C. Petty, P. Adams, A.P. Monkman, Adv. Mater. Opt. Electron. 2 (1993) 233. [4] M. Ando, Y. Watanabe, T. Iyoda, K. Honda, T. Shimidzu, Thin Solid Films 179 (1989) 225. [5] D.P. Jiang, A.D. Lu, Y.J. Li, X.M. Pang, Y.L. Hua, Thin Solid Films 199 (1991) 173. [6] X. Ding, H. Xu, Thin Solid Films 338 (1999) 286. [7] H.Y. Wang, C.W. Chiang, J.B. Lando, Thin Solid Films 273 (1996) 90. [8] A.V. Nabok, Z.I. Kazantseva, N.V. Lavrik, Int. J. Electron. 78 (1) (1995) 129. [9] J.Z. Jiang, J.P. Wu, W. Liu, J.W. Xie, S.X. Sun, Chem. Rev. 2 (1999) 2. [10] J.Z. Jiang, R.C.W. Liu, T.C.W. Mak, T.W.D. Chan, D.K.P. Ng, Polyhedron 16 (1997) 515. [11] H.Y. Wang, J.B. Lando, Langmuir 10 (1994) 790. [12] X. Vilanova, E. Llobet, J. Brezmes, J. Calderer, X. Correig, Sens. Actuators, B 48 (1998) 425. [13] H.Y. Wang, W.H. Ko, D.A. Batzel, M.E. Kenney, J.B. Lando, Sens. Actuators, B 1 (1990) 138.