13 Figure 2-1. Device configuration of the solvent-resistant OFETs based on cross-linked P3HT-azide copolymers. Detection responses of the sensor with C[8]A for b) Toluene in methanol medium and c) methanol in DI water. Reusability of the sensors for three cycles of detection tests for organic solvents in the liquid phase: a) methanol and b) toluene.
Comparison of detection responses of the low-voltage OFET-based sensors for different liquid-phase analytes. Photo of the flexible sensors fabricated with polymer dielectric (cross-linked PVP) and substrate (ITO-coated PEN). 56 Figure 3-23. a) Height and b) phase images of the solvent-resistant OFET-based sensors with C[8]A on the cross-linked PVP dielectric.
Statistical comparisons of sensitivity results for liquid-phase analytes (□) and calculated induced dipole moments of calix[8]arene/liquid-phase analytes (◇).29 S indicates the average value of ID/IBASE and the bar of error represents one standard deviation. Comparison of electrical characteristics of pristine P3HT and cross-linked P3HT-azide20 before/after washing with chlorobenzene.
Introduction
Organic field-effect transistors (OFETs)
Chemical sensor applications based on OFET platform and limitation of current stage
Research goal
Experiments
- Synthesis of P3HT-azide copolymers 17b
- Fabrication of solvent-resistant OFETs
- Solvent resistance test of OFETs
- Fabrication of liquid-phase sensors based on solvent-resistant OFETs
- Atomic force microscopy (AFM) analysis
- Sensing demonstrations
- Fabrication of flexible & low-voltage P3HT-azide OFET-based sensors 17b
The surface of the Si wafer was modified with self-assembled monolayer (SAM) with n-octadecyltrimethoxysilane (OTS) as follows: Before the treatment of Si wafer, the substrates were cleaned with Piranha solution (3:1 H2SO4: H2O2, v) /v) and successively rinsed with toluene, acetone and isopropyl alcohol (IPA) and dried with dried nitrogen (N2) stream. After exposing the substrate to NH4OH, it was cleaned with toluene, acetone, IPA and dried with dried-N2 stream. For the fabrication of the solvent-resistant OFETs, P3HT azide copolymer containing 20 mol% 3-(azidohexyl)thiophene monomer (azide20) was used as the semiconducting layer and prepared as 2 mg ml-1 solution in chloroform (CF) and for more than 6 hour at 40 oC to dissolve the polymer completely in the solution.
For the electrical characterization of the transistors, a Keithley 4200 semiconductor parameter analyzer was used in a N2-filled glove box. The electrical characteristics of the devices were calculated using equation 1 and equation 2 for saturation and linear regime, respectively. All polymer films were dried in a vacuum oven to completely remove the solvent.
For the morphological analysis of the surface of the devices, a tapping mode AFM (Multimode V, Veeco) was used. The temperature of the drying process was approximately 70% of the boiling point of each solvent. The detection system is designed for the detection of various analytes in the liquid phase, as shown in Figure 2-3.
After the start of operation of the OFET-based sensors, the drain current stabilized around 50 s. To demonstrate the reusability (reversibility) of the sensors, the devices were dried in a vacuum oven for 6 hours from the second period sensor test. The temperature of the drying process was 70% of the boiling temperature of the organic solvents used for the sensitivity tests.
The sensor test procedure after the initial sensitivity test was the same as that of the first sensitivity test. For OFET-based flexible sensors, indium tin oxide (ITO) coated polyethylene naphthalate (PEN) substrates were used. The PVP solution was wrapped twice to form the proper dielectric thickness to reduce gate current leakage.
Results & Discussion
Analysis of cross-linked P3HT-azide copolymers
Electrical characterization of OFETs based on P3HT-azide copolymers
Drain voltage (VDS) for the transistor operation was -100 V. Summary of electrical characteristics of OFETs based on P3HT copolymers.
Resistivity test of cross-linked P3HT-azide OFETs toward organic solvent
Photographic images of the flexible OFETs with cross-linked P3HT-azide10 and pristine P3HT for solvent resistance test.
Electrical characterization and morphological analysis of solvent-resistant OFET-based
Sensing demonstration of solvent-resistant OFET-based sensors
- Sensing test for various liquid-phase analytes
- Sensitivity investigations
- Selectivity of the sensors for different types of organic solvents
- Reusability test
- Sensing demonstration for various pH solutions
- Sensing test with low-voltage & flexible OFET-based sensors
The ID/IBASE slope was also plotted to verify sensor detection. The sensitivity test of sensors without C[8]A layer was also performed with methanol in toluene medium to confirm the C[8]A uptake efficiency (Figure 3-14a). While sensors with C[8]A showed a detection limit as ≈1 vol % for methanol in toluene medium, sensors without C[8]A showed an apparent response at ≈4 vol % methanol in toluene medium.
In addition, a detection demonstration of the sensors for toluene was performed using methanol as the medium (Figure 3-14b). Sensitivity demonstration of the sensors for different concentrations of analytes in the liquid phase: a) Observation result of the sensor without C[8]A for methanol in toluene medium. Detection responses of the sensor with C[8]A for b) Toluene in methanol medium and c) methanol in DI water. vol%) Methanol injection in toluene (without C[8]A).
To confirm the selectivity of the sensors, sensory demonstrations of sensors with and without C[8]A layer were performed for mixed solvent systems (Figure 3-15a). These results clearly support the enhanced selectivity of the sensors due to the introduction of C[8]A. - 46. The reusability of the sensors was investigated with two organic solvents (i.e., methanol and toluene for the positive and negative sensing response, respectively) (Figure 3-16).
After triplicate detection testing of the cross-linked P3HT-azide copolymer sensors, the sensors showed moderate reusability with ≈36.2% and ≈35.8% degradation for methanol and toluene sensing, respectively. In contrast, sensors with the uncrosslinked P3HT-azide copolymer showed a severe degradation of signal intensity by ≈85% after three sensing demonstrations. The reason for the significant degradation of sensors with uncrosslinked P3HT-azide copolymer may be due to solvation of semiconductor films and cracked electrodes due to solvated polymer films (Figure 3-17).
To further investigate the reusability of the sensors, morphological analysis has been performed with AFM (Figure 3-18). The surfaces of the active layer of the sensors with cross-linked P3HT-azide semiconductor layers showed fairly intact morphologies even after three times of sensing tests with methanol (Figure 3-18a,b) and toluene (Figure 3-18c,d), with an average surface roughness of 0.767 nm and 0.587 nm, respectively. These studies support the improved stability of the solvent-resistant OFET-based sensors with cross-linked P3HT-azide copolymer.
The detection results of the low-voltage sensors for various liquid-phase analytes showed identical trends to silicon-based sensors (Figure 3-20). A photo of the flexible sensors is shown in Figure 3-21 and it showed sufficient electrical characteristics (Figure 3-22a,b).
Computational study for sensing mechanism (MD and DFT)
E.; Johnson, O.; Knoll, W.; Bao, Z., Effect of pH and DNA concentration on organic thin-film transistor biosensors. E.; Knoll, W.; Bao, Z., Pentacene-based organic thin-film transistors as a transducer for biochemical sensing in aqueous media. H.; Toney, M.; Locklin, J.; Bao, Z., Crystalline ultrasmooth self-assembled monolayers of alkylsilanes for organic field-effect transistors.
우선, 항상 저를 믿어주시고 전폭적인 지지를 보내주시는 가족들에게 감사의 말씀을 전하고 싶습니다. 우리 가족은 하나님의 은혜 속에 행복하게 살고 있기 때문에 씩씩하게 학업을 이어갈 수 있었던 것 같습니다. 여기서 연구가 무엇인지 알게 되었고, 유기전자공학 연구실의 초석을 다지는 데 기여하고자 하는 마음으로 연구실에서 선배, 후배, 후배들과 함께 우여곡절을 겪으며 마침내 석사 연구를 완수하게 되었습니다. .
제가 석사학위 공부를 마칠 수 있도록 물질적으로, 정신적으로 도움을 주신 많은 분들께 감사의 말씀을 전하고 싶습니다. 유기전자에 대한 지식이 부족한 학부시절부터 연구실에서 일할 수 있도록 허락해주시고, 연구자로서의 자세를 진심으로 지도해주신 오준학 교수님께 감사의 말씀을 전하고 싶습니다. 아직 부족한 부분이 많지만 교수님의 훌륭한 지도 덕분에 연구자로서 점점 더 발전하고 있다고 생각합니다.
앞으로도 열심히 공부하고 연구를 잘해서 교수님께서 자랑스러워하실 수 있는 학생이 되도록 하겠습니다. 연구실 이전이라는 어려운 일을 도와주신 김병수 교수님께도 감사드리며, 바쁜 일정에도 불구하고 석사논문위원으로 참여해주신 고현협 교수님께도 감사드립니다. 또한 전용석 교수님, 박종남 교수님에게도 감사의 말씀을 전하고 싶습니다.
우여곡절도 많았지만 같은 기쁨을 나누며 끈끈한 동료애를 키워준 연구실 선배, 동기, 후배들에게 진심으로 감사의 말씀을 전하고 싶습니다. 덕분에 연구실 생활이 즐거웠고 행복했습니다. 아름누나는 초기 룸매니저를 맡아 연구실의 초석을 다졌고, 늘 밝은 태도로 후배들을 대하고, 어려운 상황에서도 마다하지 않고 리더십을 발휘했다.
