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Title Page
ABSTRACT OF THE DISSERTATION
Contents
LIST OF ABBREVIATIONS 19
UNIT 21
Chapter 1. INTRODUCTION 23
1.1. Objective 23
1.2. Outline of the Thesis 29
Chapter 2. LITERATURE SURVEY 30
2.1. Carbon Nanotube Based Sensors 30
2.1.1. Semiconducting/metallic carbon nanotubes 30
2.1.2. Semiconducting single-walled carbon nanotube sensors 35
2.2. Plasma Treatment on Carbon Nanotube Networks 40
2.2.1. Functionalization 40
2.2.2. Changes in the electrical properties in carbon nanotube networks 43
Chapter 3. EXPERIMENTAL DETAILS 46
3.1. Design and Fabrication of Carbon Nanotube Gas Sensors 46
3.1.1. Basic structure of a gas sensor 46
3.1.2. Preparation of carbon nanotube networks and sensor fabrication 47
3.1.3. Oxygen plasma treatment on the carbon nanotube networks 52
3.2. Gas Sensors Measurements and Characterization 53
3.2.1. Measurements 53
3.2.2. X-ray photoelectron spectroscopy 55
Chapter 4. RESULTS AND DISSCUSSION 57
4.1. Morphologies of Carbon Nanotube Networks 57
4.2. Gas Sensing Characteristics 59
4.2.1. Contact properties between Pd and SWCNT networks 59
4.2.2. Static characteristics of gas sensors 62
4.2.3. Dynamic characteristics of gas sensors 67
4.2.4. Electrical impedance spectroscopic analysis of gas sensors 71
4.3. Effect of Oxygen Plasma Functionalization on Carbon Nanotube Gas Sensors 87
4.3.1. X-ray photoelectron spectroscopy analysis 87
4.3.2. Sensing performance of the plasma treated gas sensors 93
4.3.3. Effect of oxygen plasma treatment on semiconductor-enriched SWCNT network gas sensors 105
Chapter 5. CONCLUSIONS AND FUTURE WORKS 109
References 111
Achievement 118
Table 3.1. Optimum spray condition for SWCNT film deposition. 48
Table 4.1. Summary of the adsorption and desorption times. 70
Table 4.2. Values of various equivalent circuit parameters obtained after fitting the impedance data of the 99% devices. 74
Table 4.3. XPS results of SWCNTs before and after plasma treatment. 91
Table 4.4. Response and recovery times before and after plasma-treatment of 66% and 90% SWCNT sensors. 102
Fig. 2.1. (A) Schematic honeycomb structure of a graphene sheet. Single-walled carbon nanotubes can be formed by folding the sheet along lattice... 31
Fig. 2.2. Raman spectra of SWCNTs deposited via ac dielectrophoresis compared to a reference sample deposited on Si without the application of an... 34
Fig. 2.3. Metal/semiconductor separation by freeze and squeeze method. 34
Fig. 2.4. CNT-channel chemical gating sensors. 36
Fig. 2.5. CNT/metal Schottky barrier gas sensor. 36
Fig. 2.6. Changes of electrical characteristics of a semiconducting SWNT in chemical environments. 38
Fig. 2.7. Scalable fabrication of the aluminum back-gated separated nanotube RF transistors. 39
Fig. 2.8. (A) Raman spectra of pristine CNTs and plasma functionalized CNTs treated under different plasma conditions and (B) Changes in the relative intensity of D bands compared with the relative intensity of G bands. 42
Fig. 2.9. Curve fitting of the existence of four species on C 1s spectrum 43
Fig. 2.10. Detailed view of the interaction between a (5,5) carbon nanotube and an OH functionalized vacancy with (A) an NH₃ group; and of an O₂ functionalized vacancy with (B) an NH₃ group. 43
Fig. 3.1. Schematic of the SWCNT NH₃ gas sensor geometry. 46
Fig. 3.2. (A) Schematic of the spray-deposition equipment used to deposit SWCNT networks and (B) detail part of the equipment. 48
Fig. 3.3. Solution method for SWCNT deposition. 49
Fig. 3.4. Fabrication procedure of the CNT based gas sensor on a glass substrate. 51
Fig. 3.5. O₂ plasma apparatus for functionalization and etching of the SWCNT films. 52
Fig. 3.6. Schematic diagram of the experimental set-up for measuring response to NH₃. 54
Fig. 4.1. SEM images of (A) 66%, (B) 90%, (C) 99% s-SWCNT networks. 57
Fig. 4.2. AFM images of (A) 66%, (B) 90%, (C) 99% s-SWCNT networks. 57
Fig. 4.3. (A) AFM 3D image and (B) line profile of 66% s-SWCNT network film. 58
Fig. 4.4. The calculated self-consistent electrostatic potential for the Pd-covered (8,0) nanotube 60
Fig. 4.5. Current-voltage characteristics measured before exposing the unenriched 66%, 90% and 99% semiconductor-enriched SWCNT networks to NH₃ gas. 61
Fig. 4.6. I-V (A-C) and calibration (D-F) curves as a function of NH₃ concentration for the unenriched 66%, 90% enriched, and 99% enriched SWCNT-network sensors. 63
Fig. 4.7. Calibration plots of the DC resistance variation versus the NH₃ concentration for same sensors in Fig. 4.6. 64
Fig. 4.8. Comparison of the sensitivity for the unenriched 66%, 90% and 99% enriched SWCNT-network sensors. 66
Fig. 4.9. Responses of the (A) unenriched 60%, (B) 90% and (C) 99% enriched SWCNT-network sensors to different NH₃ concentrations at room temperature. 69
Fig. 4.10. Calibration plots of the AC resistance variation versus the NH₃ concentration for same sensors in Fig. 4.9. 70
Fig. 4.11. (A) Nyquist plots of impedance spectroscopy on 99% s-SWCNT sensors using the frequency range 0.5 ㎐ - 300 ㎑ with different NH₃... 72
Fig. 4.12. Schematic view of SWCNT network (A) before and (B) after NH₃ exposure, showing the formation of hole depleted regions beneath and in the immediate vicinity of absorbed NH₃ molecules within s-SWCNT. 78
Fig. 4.13. Plots of impedance spectroscopy on 90% s-SWCNT network sensors, which show a pure resistance. 79
Fig. 4.14. Variations of real part (Z', solid lines) and imaginary part (Z", dotted lines) of impedance as a function of frequency for different NH₃ concentrations. 82
Fig. 4.15. Comparison of impedancemetric and dc sensitivity to NH₃ of 99% s- SWCNT network devices. 85
Fig. 4.16. Capacitive responsiveness for 99% SWCNT network devices, showing much lower sensitivity compared to the resistive responsiveness. 85
Fig. 4.17. Variation of ambient capacitance (Camb) between interdigitated Pd electrodes before and after NH₃ gas exposure (41.4 ppm).(이미지참조) 86
Fig. 4.18. XPS spectra of untreated and plasma-treated s-SWCNT network films 89
Fig. 4.19. Responses before and after plasma treatment of the unenriched 66% and 90% enriched SWCNT-network sensors to an NH₃ concentration of 22.6 ppm at room temperature. 94
Fig. 4.20. Comparison of the (A) sensitivity and (B) response time before and after plasma treatment for the unenriched 66% and 90% enriched SWCNT- network sensors. 96
Fig. 4.21. Responses (A) before and (B) after plasma treatment of the 66% s-SWCNT network sensors to different NH₃ concentrations at room temperature. 98
Fig. 4.22. Responses (A) before and (B) after plasma treatment of the 90% s-SWCNT network sensors to different NH₃ concentrations at room temperature. 99
Fig. 4.23. Response and recovery times of pristine and plasma-treated SWCNT sensors with different semiconducting contents 101
Fig. 4.24. Calibration plots of resistance variation vs. NH₃ concentration for 66% s-SWCNT sensors. 103
Fig. 4.25. Calibration plots of resistance variation vs. NH₃ concentration for 90% s-SWCNT sensors. 104
Fig. 4.26. Variations of O and C atomic percentage before and after O₂ plasma treatment. 106
Fig. 4.27. UPS valence band spectra recorded on (A) pristine, (B) 5 s plasma treated and (C) 30 s plasma treated CNTs. 108
초록보기 더보기
Recently, nanostructures, such as carbon nanotubes (CNTs), have begun to attract wide attention in the study of their application to various sensors. The gas sensors based on single-walled carbon nanotubes (SWCNTs) show extreme sensitivity towards changes in their local chemical environment that stems from the susceptibility of their electronic structure to interacting molecules. So far, CNT-based gas sensors have been investigated for the detection of elements such as H₂, N₂, NO₂, and NH₃. For preventing an unexpected accident, such as a gas explosion or suffocation, very sensitive sensors are needed. Therefore, we developed a high-performance gas sensor based on highly semiconductor-enriched SWCNTs. The operating principles of the CNT based gas sensor can be classified into two major categories: the conductivity change of the CNT channels and the variation of the Schottky barrier between the electrode and the CNT film. Although the sensing mechanisms are different, the most effective component of the sensor sensitivity is the semiconductor/metallic tube ratio of CNT films.
Typical SWCNTs consist of 66% semiconducting tubes and 33% metallic tubes. Purification technology has evolved; high purity semiconducting SWCNTs have been developed, and some SWCNTs consist of 99% semiconducting tubes. In this study, we fabricated 99% semiconducting SWCNT gas sensors. For comparison, we used semienriched 90% SWCNTs and typical SWCNT gas sensors that were also fabricated using solution-deposition and spray deposition, respectively. To enhance the sensitivity and the response time of sensors, oxygen plasma treatment was performed on the 66%-SWCNT and 90%-SWCNT gas sensors. We also demonstrated the effect of oxygen plasma treatment on the highly semiconductor-enriched SWCNT device.
The sensor response to NH₃ gas was characterized by a resistance increase at the moment of NH₃ exposure. A general linear response was observed with an increasing NH₃ concentration with a significantly greater responsiveness for the semiconductor-enriched 99% sensor compared with the 66% and 90% sensors. Furthermore, plasma treated 66% and 90% sensors (p-66% and p-90%, respectively) showed a higher performance than NH₃ sensing due to the fact that oxygen functional groups were formed on the CNT film and the electrical structure of SWCNT was changed from metallic to semiconducting[1]. This suggested that a large semiconducting/metallic ratio is the most important factor for achieving a high sensitivity in mixed SWCNT-networked-based gas sensors.
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