Chapter II. Theoretical Background of Organic Thin Film Transistor
II.6 Research Motivation …
Based on the importance of ammonia sensing, we developed a selectivity and high sensitivity OTFT as ammonia sensor, which does not respond to carbon dioxide, ethanol, formaldehyde or methane [Figure II.5 and Figure II.6]. We propose that this ammonia sensor can be applied to breath ammonia detection. Expiratory ammonia includes water vapor. Ammonia is highly
bound with water molecules to form ammonium. We consider water molecules an important factor affecting ammonia sensing. In this thesis, we discuss how the concentrations of water molecules affect ammonia sensing.
Figure II.1. Chemical structure of pentacene.
Table II.1. Reviewed of OTFT sensors [26].
Active layer Recognition element Analytes
Phthalocyanines Oxygen, iodine, bromine, NO2, ozone, alcohols, ketones, thiols, nitriles, esters, ring compounds, lactic acid, pyruvic acid
Pentacene Water vapor, 1-pentanol, aqueous
analytes,
Oligothiophenes Alcohols, ketones, thiols, nitriles, esters, ring compounds, lactic acid, glucose
Polythiophenes Alkyl or alkoxy side chains Ammonia, water vapor, chloroform, alcohols, ketones, thiols, nitriles,
Figure II.2. Fabrication of OTFT sensors.
Figure II.3. Surface morphology of pentacene based PMMA and PVP sensor. .The roughness of surface of PMMA and PVP are 0.31 and 0.3 nm, respectively. Grain size of pentacene based PMMA and PVP are both around 1.2 μm [Zan, 2009].
Figure II.4. Schematic diagram of mechanisms of ammonia sensing. Ammonia gas molecules diffused to OTFT sensor, there are three proposed mechanisms: a). electron doping, induced electron holes were trapped by electron of nitrogen of ammonia. b). charge trapping, electron holes were attracted to neutralize the negative charges induced by ammonium. c). dielectric layer interaction, ammonium ion molecules interact with dielectric layer. The interaction occupied the electron holes of carrier channel in interface.
0
Figure II.5. Kinds odors dependent OTFT responses. Carbon dioxide (CO2), alcohol (C2H5OH), methane (CH4), and acetone (CH3COCH3) [Zan, 2009].
0 1 2 3 4 5
Figure II.6. Concentration dependent ammonia sensing response by time measurement of OTFT-1200 (a) Threshold voltage shift and (b) mobility variations are shown when devices were exposed to NH3 with concentrations varied from 0 ppm to 5 ppm [Zan, 2009].
Chapter III. Materials and Methods
III.1. Chemicals
Nitrogen and ammonia gas are purchased from 新復發 and 洽隆, respectively.
III.2. Facilities
4200 semiconductor characterization system and Model 2636 Dual-Channel System Source Meter Instrument (Low Current) were purchased from Keithley. Gas chamber was manufactured by EVERBEING (奕葉). PC-540 Four-channel MFC readout power supply and PC-615 vacuum gauge controller were purchased from PROTEK. Mass flow controller 5850E was purchased from BROOKS.
III.3. Sensing-system
Three major components were organized to sensing-system [Figure III.1]: gaseous controller, gas chamber and semiconductor characteristics analyzer. The PC-540 MFC [Figure III.2] was introduced to gas chamber via mass flow controller [Figure III.3].Total volume of chamber is around 50 L [Figure III.4]. Inside pressure of chamber was monitored by vacuum gauge [Figure III.5]. Chamber system was equipped with probes and device station (probe station). The probes were connected to and regulated by semiconductor characteristics analyzer (Keithley 4200 and 2636A) [Figure III.6 and 7].
III.4. The Measurement of Electrical Characteristics of OTFT
Electric properties, the gate potential and source/drain bias voltage, of OTFT devices were monitored by using probe station and semiconductor analyzer (Keithley 2636A and 4200).
III.4.1. Id-Vg curve
The Id-Vg was measured for devices selection. In the general Id-Vg measurement, the drain current (Id) was detected at constant bias voltage of drain (Vd = -5V) while sweeping the gate voltage (Vg) was from 20V ~ -40V. When on/off ratio is above 3-4 oder and on current is among 10-7 A, the device was selected for ammonia sensing.
PMMA and PVP device selection was show in Figure III.8.
III.4.2. Id-time measurement
After Id-Vg selection, Vg was chose from the linear region which is the largest variation of Id and the nearest to on current. Id-time measurement, the Id was measured at a constant Vd (-5V) and Vg (-10V or -15V).
III.4.3. I/I0 normalization
Before any gas introduced to chamber, the device was kept in vacuum environment, in the mean time, drain current was measured. We took the vacuum status drain current as a base line. Since, any change of drain current was compared to base line.
III.5. Experimental Design
We considered ammonia solubility in water molecule to form ammonium, which means the more water molecule approach to ammonia; the more ammonium ions are formed. First, we selected three concentrations of water vapor, 0%, 50%, 100% to presume the amount of ammonium ions increased and ammonia sensing ability raised [Figure III.9.]. Second, we chose two materials, PMMA and PVP as dielectric layer of OTFT. As Figure III.10, the contact angle of PVP (51.8°) is smaller than PMMA (61.7°), which referred the material of PVP is more hydrophilic than the PMMA. It means PVP of OTFT more attract water molecules, which could improve ammonia sensing ability.
Figure III.1. Sensing-system.
Figure III.2. PC-540 Four-channel MFC readout power supply.
Figure III.3. Mass flow controller 5850E.
(a) Uncapped gas-sensing chamber (b) Capped gas-sensing chamber
(a) (b)
Figure III.4. Gas chamber.
Figure III.5. PC-615 vacuum gauge controller.
Figure III.6. Model 2636A Dual-Channel System Source Meter Instrument (Low Current).
Figure III.7. 4200 semiconductor characterization system.
Figure III.8. The electric characteristics of PMMA and PVP OTFTs. Drain current (Id) was detected at constant bias voltage of drain (Vd = -5V) while sweeping the gate voltage (Vg) was from 20V ~ -40V. Field effect mobility ( (cm2/Vs) and threshold voltage (Vth (V)) were calculated in the linear regime (Vd=-5V) defined by standard metal-oxide-semiconductor FET model.
Figure III.9. The schematic of water molecule content affect ammonia sensing ability.
Figure III.10. Chemical structure and contact angle of PMMA and PVP [Zan, 2009].
Chapter IV. Results and Discussion
IV.1. Under Nitrogen Gas Environmental as An < 5% Relative Humidity
After a nitrogen gas injection, neither the PMMA nor PVP devices responded to the drain current which represents that nitrogen gas did not effect the device sensing. When the chamber was full of nitrogen gas, we introduced ammonia gas with different concentrations, 0.5 and 2 ppm. In the PMMA sensing, ammonia caused drain current decay with a time of 200 seconds. The decline at 0.5 and 2 ppm of ammonia were 3.5 % (black) and 13.7 % (red), respectively. In PVP, the drain current was decayed to saturate in 100 seconds. The decline at both 0.5 and 2 ppm of ammonia were 6.8% (blue) and 15.5% (green), respectively.
[Figure IV.1]. Without the existence of moisture, PVP was more active to ammonia sensing;
reaction time and saturation status were half-time reduced when compared with PMMA;
drain current reduced by time cost.
Without the existence of moisture, PMMA and PVP are sensitive to ammonia. We propose that in this state, the sensing mechanism might be electron doping. Minakata (1994) studied ammonia sensors formed with thin pentacene films doped with iodine that could detect ppm concentrations of ammonia gas, causing the reduction of conductivity and resistivity increasing linearly with time [35]. Aside from what we propose, the dielectric layer can attract ammonia attachment to a response; the active layer, pentancene, is also responsive to ammonia. Ammonia is a dipolar molecule adsorbed on the active layer of sites in between gain boundaries that might be capable of creating an effective hole trap while adsorbing to the grain boundaries [34]. The presence of polar molecules is known to change the rate of charge transportation in organic materials by increasing the amount of energetic disorder through charge–dipole interactions[36]. Polar molecules behave as acceptor-like deep trap states for the charge carriers moving at the interface between the
organic semiconductor layer and insulator [37, 38].
Although we had a vacuum (2×10-1 torr), gas chamber, and introduced nitrogen gas, there still may have existed a low concentration of humidity which formed a thin layer water vapor membrane. Another effect of ammonia sensing we propose is ammonia gas physical attachment through pentacene to dielectric surfaces, with forming ammonium interacting with PMMA or PVP, causing a decrease of electric performance(what we propose as dielectric layer interaction).
IV.2. Under Air Containing 50% Relative Humidity
Before ammonia, we introduced air as relative humidity 50±5 % to the chamber. Organic thin film transistor (OTFT) sensors responded to air with water vapor content. In PMMA, the response of air was approximately 10% on average; introduction of air caused a response saturation after 100 seconds. The responses of ammonia in 0.5 and 2 ppm were 8.7
% and 17.3 %, respectively. In PVP, the response of air was 6.5 % on average; instead of PMMA, PVP responded to air with a delay of approximately 100 seconds. The responses of ammonia in 0.5 and 2 ppm were 11.7 % and 19.8 %, respectively [Figure IV.2]. Response rates of ammonia in PMMA and PVP did not yield differences, which may be because the response of PVP was delayed to air and not saturated to the device capacity of air. Therefore, we elongated the PVP device expose time (double) to air until the response was saturated [Figure IV.3]. The results showed the response of air in PVP to be 13 % on average. The time of saturation was approximately 400 seconds, including the initial 100-second delay time. After that, we introduced ammonia of 0.5 and 2 ppm to compare with the results in Figure IV.2. The responses were 31.7 % and 76.1 %, respectively. Ammonia sensing ability
of PVP was increased four times to PMMA and air was unsaturated to the state of PVP.
The ammonia sensing ability of PVP is of higher quality than PMMA when water vapor existence must be saturated. We suggest that with PVP, the dielectric was exposed to air/moisture, and the surface was forming and accumulating a more –OH chemical functional group. That could be attractive to ammonium, which forms by ammonia contacting with water vapor. We discovered that the sensing mechanisms involve weak interactions between the analyte functionalities and the polymers’ side chains[34]. Kim et al. (2008) studied the pyridine group of PVP which interacts strongly with water molecules through hydrogen-bonding. Kim’s results indicate that the surface polarity arose from the functional group in the polymer [39]. When PVP was exposed to ambient air, the surface interacted with water molecules leading PVP to form more –OH groups.
We discovered that water molecules in humid air diffuse into the grain boundary of the polycrystalline semiconductor layer and/or the interface between the semiconductor and dielectric gate, where they created both donor- and acceptor-like traps, leading to significant degradation of device performance [40]. The diffusion of water molecules is intimately related to the density of grain boundaries in the pentacene film because small molecules migrate into the channel region through defects[27]. The results showed that the sensing mechanisms are suitable to charge trapping and dielectric layer interaction.
IV.3. Under >90% Relative Humidity of Water Vapor Environment
We introduced water vapor, relative humidity >90 %, into the chamber before introducing ammonia. In PMMA, the response of water vapor was 20.5 % on average. The responses of ammonia in 0.5 and 2 ppm were 36.9 % and 63.7 %, respectively. In PVP, the response of
water vapor was 28.2 % on average. The responses of ammonia in 0.5 and 2 ppm were 18.8
% and 43.7 %, respectively [Figure IV.2]. Response rates of ammonia were observed in PMMA and PVP. In the response rate and decline rate, PMMA was more sensitive than PVP.
The response rate showed no significance difference between PVP and PMMA with a high ammonia concentration, 2 ppm, but observed PMMA sensing showed a higher quality twice that of PVP at 0.5 ppm. Our results showed that the ammonia sensing ability of PVP was of no higher quality than PMMA in high concentrations of water vapor. The performance was not what we expected in IV.2. The device was exposed to air/moisture and PVP sensing ability was of higher quality than PMMA.
IV.4. Ammonia in Nitrogen Gas
Different from IV.1 performance, we introduced nitrogen gas and ammonia simultaneously.
Nitrogen gas is not sensible to OTFT sensors; therefore, we observed the change of drain current as only an ammonia effect. During the first 200 seconds, responses of ammonia of PMMA were straight declines of drain current and the decreasing rates were 6.9 %, 8.9 %, 11.4 %, 20.1 %, and 15.3 % using ammonia concentrations of 0.5, 1, 2, 5, and 10 ppm, respectively [Figure IV.2]. Instead, responses of PVP during the initial 50 seconds revealed delay phenomena, then received a straight decline of drain current. Within 200 seconds, the decreasing rates of drain current for PVP were 8.9 %, 9.5 %, 6.4 %, 12.2 %, and 18.1 % using ammonia concentrations of 0.5, 1, 2, 5, and 10 ppm, respectively [Figure IV.5]. As long as time treatment was elongated, both PMMA and PVP sensing to ammonia were saturated to device capacity.
Section IV.1 shows the PVP device sensing ability was of higher quality than PMMA.
Although we supposed that PVP with more –OH chemical functional group would be more sensitive to ammonia, instead, the results in ammonia in nitrogen gas showed that there are no differences between PMMA and PVP devices over long time periods, or even that PMMA is slightly more sensitive to ammonia [Figure IV.6]. When nitrogen and ammonia were introduced simultaneously, the devices were not in stable ambient gas, which may influence the dielectric surface polarity.
IV.5. Ammonia in Air
Introducing air and ammonia simultaneously, we observed the responses of PMMA and PVP. During the first 200 seconds, ammonia responses of PMMA were 20.8 %, 32.4 %, 42.0 %, 51.8 %, and 61.3 % (using ammonia concentrations of 0.5, 1, 2, 5, and 10 ppm, respectively) [Figure IV.8]. Responses of PVP were 15.9 %, 19.5 %, 26.1 %, 41.62 %, and 55.2 % (using ammonia concentrations of 0.5, 1, 2, 5, 10 ppm, respectively) [Figure IV.9].
The changing rate of PMMA was of higher quality than PVP. As previously discussed in Section IV.3, PVP has a delay to water vapor that influences ammonia sensing sensitivity.
As long as time was increased, responses of ammonia sensing were saturated both in PMMA and PVP. We compared the changing rate of saturation regions within 500 seconds.
Responses of PMMA using concentrations 0.5, 1, 2, 5, and 10 ppm were 28.9 %, 41.7 %, 55.7 %, 67.2 %, and 76.4 %, respectively; responses of PVP were 33.0 %, 37.5 %, 53.0 %, 70.2 %, and 82.9 %. There were no significant differences between PMMA and PVP [Figure IV.10]. We suggest that PVP was not admitted water vapor completely and ammonia attracted to the device was limited to similar to PMMA.
The results of this section reflect that we supposed the PVP devices were exposed to
ambient air without elongated time leading to the PVP surface contacting with moisture.
Without the saturated surface of chemical functional groups or formation of native oxide to interact with water molecules, the sensing ability was poorer than under atmospheric environments but of much higher quality than ammonia in nitrogen gas.
Figure IV.1. Responses of ammonia after introduced nitrogen gas.
Figure IV.2. Responses of ammonia after introduced air, content water vapor (relative humidity 50±5%).
Figure IV.3. Responses of ammonia after elongation the air introduced time.
Figure IV.4. Responses of ammonia after introduced water vapor, content water vapor (relative humidity >90%).
Figure IV.5. Ammonia in nitrogen gas of PMMA with long time treatment.
Figure IV.6. Ammonia in nitrogen gas of PVP with long time treatment.
Figure IV.7. Ammonia changing rate in 200s and 500s of PMMA and PVP in nitrogen gas.
Figure IV.8. Ammonia in air of PMMA with long time treatment.
Figure IV.9. Ammonia in air of PVP with long time treatment.
Figure IV.10. Ammonia changing rate in 200s and 500s of PMMA and PVP in air.
Chapter V. Conclusions
When moisture exists, PMMA or PVP are more sensitive to ammonia and interact with water vapor to form ammonium and attract to dielectric layers of OTFT sensors, especially in PVP.
Although PVP has a delay phenomenon of moisture response, when it is admitted to saturation, the response of ammonia can increase four times. However, in high water vapor concentrations, PVP sensing ability was not observed to be of higher quality than PMMA. When carrier gas and ammonia were introduced simultaneously, we did not observe PVP sensing ability to be of higher quality than PMMA. The responding changes showed no significant differences in long time detection. For future applications, we suggest that, prior to ammonia sensing, the device should be exposed to air/moisture until it is saturated for higher responses.
References
1. Timmer, B., W. Olthuis, and A.v.d. Berg, Ammonia sensors and their applications--a review. Sensors and Actuators B: Chemical, 2005. 107(2): p. 666-677.
2. Nelson, D. and M. Cox, Lehninger Principles of Biochemistry, Fourth Edition. 2004:
{W. H. Freeman}.
3. Shimamoto, C., Hirata, I., Katsu, K, Breath and blood ammonia in liver cirrhosis.
Hepato-gastroenterology, 2000. 47(32): p. p. 443-445.
4. DuBois, S., Eng, Sue, et al., Breath Ammonia Testing for Diagnosis of Hepatic Encephalopathy. Digestive Diseases and Sciences, 2005. 50(10): p. 1780-1784.
5. Conkle JP, C.B., Welch BE., Trace composition of human respiratory gas. Arch Environ Health, 1975. 30(6): p. 290-295.
6. Tietz, N.W., Clinical Guide to Laboratory Tests 3ed. 1995: W.B. Saunders 7. Mondzac A, E.G., Seegmiller JE., An enzymatic determination of ammonia in
biological fluids. J Lab Clin Med, 1965. 66(3): p. 526-531.
8. Jacobs, H.A.M. and F.M.F.G. Olthuis, A kinetic determination of ammonia in plasma.
Clinica Chimica Acta, 1973. 43(1): p. 81-86.
9. Kimble, K., J. Walker, D. Finegold, and S. Asher, Progress toward the development of a point-of-care photonic crystal ammonia sensor. Analytical and Bioanalytical
Chemistry, 2006. 385(4): p. 678-685.
10. Rao, G.S.T. and D. Tarakarama Rao, Gas sensitivity of ZnO based thick film sensor to NH3 at room temperature. Sensors and Actuators B: Chemical, 1999. 55(2-3): p.
166-169.
11. Karthigeyan, A., et al., A room temperature HSGFET ammonia sensor based on
iridium oxide thin film. Sensors and Actuators B: Chemical, 2002. 85(1-2): p. 145-153.
12. Li, C., et al., Surface Treatment and Doping Dependence of In2O3 Nanowires as Ammonia Sensors. The Journal of Physical Chemistry B, 2003. 107(45): p.
12451-12455.
13. Wang, Y.-D., et al., Ammonia-sensing characteristics of Pt and SiO2 doped SnO2 materials. Solid-State Electronics, 2001. 45(2): p. 347-350.
14. Xu, C.N., et al., Selective detection of NH3 over NO in combustion exhausts by using Au and MoO3 doubly promoted WO3 element. Sensors and Actuators B: Chemical, 2000. 65(1-3): p. 163-165.
15. Palmqvist, E., et al., DC-resistometric urea sensitive device utilizing a conducting polymer film for the gas-phase detection of ammonia. Biosensors and Bioelectronics, 1995. 10(3-4): p. 283-287.
16. Lähdesmäki, I., A. Lewenstam, and A. Ivaska, A polypyrrole-based amperometric ammonia sensor. Talanta, 1996. 43(1): p. 125-134.
17. Chabukswar, V.V., S. Pethkar, and A.A. Athawale, Acrylic acid doped polyaniline as an ammonia sensor. Sensors and Actuators B: Chemical, 2001. 77(3): p. 657-663.
18. Svehla, G., Vogel's Qualitative Inorganic Analysis. 7 ed. 1996: Prentice Hall.
19. Searle, P.L., The berthelot or indophenol reaction and its use in the analytical chemistry of nitrogen. A review. The Analyst, 1984. 109(5): p. 549-568.
20. Mount, G.H., et al., Measurement of atmospheric ammonia at a dairy using differential optical absorption spectroscopy in the mid-ultraviolet. Atmospheric Environment, 2002. 36(11): p. 1799-1810.
21. Tsumura, A., H. Koezuka, and T. Ando, Macromolecular electronic device:
Field-effect transistor with a polythiophene thin film. Applied Physics Letters, 1986.
49(18): p. 1210-1212.
22. Burroughes, J.H., C.A. Jones, and R.H. Friend, New semiconductor device physics in polymer diodes and transistors. Nature, 1988. 335(6186): p. 137-141.
23. D. J. Gundlach, Y.Y.L., T. N. Jackson, S. F. Nelson, and D. G. Schlom, Pentacene organic thin-film transistors-molecular orderig and mobility. IEEE Electron Device Letters, 1997. 18(3): p. 87-89.
24. D. Knipp, R.A.S., A. Völkel, and J. Ho, Pentacene thin film transistors on inorganic dielectrics: Morphology, structural properties, and electronic transport. JOURNAL OF APPLIED PHYSICS, 2003. 93(1): p. 347-355.
25. Yang, Y.S., et al., Deep-level defect characteristics in pentacene organic thin films.
Applied Physics Letters, 2002. 80(9): p. 1595-1597.
26. Mabeck, J. and G. Malliaras, Chemical and biological sensors based on organic thin-film transistors. Analytical and Bioanalytical Chemistry, 2006. 384(2): p.
343-353.
27. Crone, B., et al., Electronic sensing of vapors with organic transistors. Applied Physics Letters, 2001. 78(15): p. 2229-2231.
28. Someya, T., et al., Integration and Response of Organic Electronics with Aqueous Microfluidics. Langmuir, 2002. 18(13): p. 5299-5302.
29. Dunlap, D.H., P.E. Parris, and V.M. Kenkre, Charge-Dipole Model for the Universal Field Dependence of Mobilities in Molecularly Doped Polymers. Physical Review Letters, 1996. 77(3): p. 542.
30. Gundlach, D.J., et al., Solvent-induced phase transition in thermally evaporated pentacene films. Applied Physics Letters, 1999. 74(22): p. 3302-3304.
31. Someya, T., et al., Vapor sensing with alpha,omega-dihexylquarterthiophene
field-effect transistors: The role of grain boundaries. Applied Physics Letters, 2002.
81(16): p. 3079-3081.
32. Torsi, L., et al., Multi-parameter gas sensors based on organic thin-film-transistors.
Sensors and Actuators B: Chemical, 2000. 67(3): p. 312-316.
33. Sze, S.M. and K.K. Ng, Physics of Semiconductor Devices. 3 ed. 2007: John Wiley &
Sons.
34. Locklin, J. and Z. Bao, Effect of morphology on organic thin film transistor sensors.
Analytical and Bioanalytical Chemistry, 2006. 384(2): p. 336-342.
35. Minakata, T., Fabrication of a gas sensor using pentacene thin films as a detector.
Polymers for Advanced Technologies, 1995. 6(9): p. 607-610.
36. Jeong, J.W., et al., The response characteristics of a gas sensor based on
poly-3-hexylithiophene thin-film transistors. Sensors & Actuators: B. Chemical, 2010.
146(1): p. 40-45.
37. Pacher, P., et al., Chemical Control of Local Doping in Organic Thin-Film Transistors:
From Depletion to Enhancement. Advanced Materials, 2008. 20(16): p. 3143-3148.
38. Etschmaier, H., et al., Continuous tuning of the threshold voltage of organic thin-film
38. Etschmaier, H., et al., Continuous tuning of the threshold voltage of organic thin-film