臺灣博碩士論文加值系統

由微波電漿化學氣象沈積系統所生產的奈米碳管被運用來完成電阻式氣體感測器。奈米碳管在N2流率500 sccm、200℃的環境之下，進行15分鐘的回火過程。之後，奈米碳管暴露在一個充滿N2及真空相互交替的環境裡。當奈米碳管暴露在一個充滿N2的環境裡，其電阻會增加。而未回火奈米碳管對於N2的吸收過程是可逆的，但是已回火奈米碳管卻不是。不過已回火奈米碳管對N2吸收作用的靈敏度卻有較佳的表現。從Raman光譜得知，奈米碳管的ID/IG比值在經過回火之後也有下降，這意即表示藉由回火過程，會形成更多的石墨化結構。因此當回火溫度增加時，經XPS發現碳氧比(O/C)從0.094增加到0.303。由於氧缺陷增加的緣故，奈米碳管對N2的吸收機製由物理吸附轉變成化學吸附。因此，奈米碳管對N2吸收作用的靈敏度在經回火過後有改善的現象。不過，當偏壓增加時，奈米碳管對N2吸收是不可逆的反應將會被改變為可逆。
奈米碳管在C2H2氣體流率為30 sccm、溫度為700 ℃之下，經thermal CVD合成。一束碳管和碳管薄膜被拿來研究，比較那一種結構擁有較好的靈敏度。我們發現一束碳管不但有較佳的靈敏度而且擁有較快的反應時間及恢復時間。他可以歸功於強烈不一致的外型;亦即氣體分子比較容易吸附在兩根碳管間的溝槽及碳管表面，因為這些位置提供較多的鍵結及較低的鍵結力。
一個新穎的三極氣體感測器藉由垂直排列的奈米碳管薄膜加以實行。在室溫下，暴露於一個充滿N2及真空相互交替的環境，氣體感測元件的電阻值亦各自地增加及回復至原值。和較低偏壓相比之下，高偏壓會有較好的靈敏度反應。而元件對閘極施加負電壓下，會有更靈敏的N2感測表現。
總而言之，回火過程在奈米碳管表面上提供更多的氣體鍵結位子，但是在較高的偏壓下，強而有力的鍵結將會被克服。奈米碳管薄膜之自由電洞濃度的改變對於N2氣體偵測扮演著重要角色。

The vertically aligned carbon nanotubes (CNTs) deposited by microwave plasma-enhanced chemical vapor deposition (MPCVD) were utilized as resistive gas sensors. The carbon nanotubes were annealed at 200℃ under N2 flow (500 sccm) for 15 minute. After that, the carbon nanotubes were exposed to an N2 filling and pumping environment. Upon exposure to N2 the electrical resistance of vertically aligned carbon nanotubes was found to increase. It was found that the N2 absorption of unannealed carbon nanotubes was reversible, whereas which of annealing ones was not. However, the sensitivity of the N2 absorption on carbon nanotubes was improved after annealing. From the Raman spectra, the ID/IG ratio of carbon nanotubes also decreased after annealing, indicating that more graphenes were formed by the annealing process. Furthermore, from X-ray photoelectron spectroscopy (XPS), it was observed that the ratio of the oxygen to carbon (O/C) signal intensity increased from 0.094 to 0.303 as the annealing temperature increased. Due to the increase of O-defects, the adsorptive mechanism of N2 on carbon nanotubes was transferred from physisorption to chemisorption. Therefore, the sensitivity of the N2 absorption on carbon nanotubes was improved after annealing. However, as the bias voltage increased, the irreversible response of N2 absorption on annealed carbon nanotubes changed to reversion.
Carbon nanotubes were synthesized by thermal chemical vapor deposition (thermal CVD) at 700℃ under C2H2 gas flow rate of 30 sccm. A bundle CNTs and a mat CNTs were investigated to compare which one possessed better sensitivity. It was found that a bundle CNTs not only had better sensitivity but also possessed faster response and recovery time. It can be ascribed to their greatly different shape, indicating that the gas molecules absorption on the groove and surface sites provided more binding-sites and lower binding-force than interstial sites.
A novel three-terminal gas sensor was fulfilled by utilizing the vertically aligned carbon nanotubes mat. Upon exposure to an N2 filling and pumping environment at room temperature of 25 ℃, the electrical resistance of as-made devices was found to increase and to return back, respectively. The sensitivity increased when applying a high source drain bias voltage, compared to a low bias one. Furthermore, the device became more sensitive for N2 detection by applying a negative gate voltage.
As a consequence, the annealing-process provided more gas-binding sites on the surface of carbon nanotubes but the powerful binding was overcome by larger bias voltage. The groove and surface sites of the tube bundle provided more binding-sites and lower binding-force. The alteration of free holes concentration in the CNTs mat played the major mechanism for the N2 gas detection.

Contents
摘要…………………………………………………………….......…VI
ABSTRACT…………………………………………………………...….VI
誌 謝…………………………………………………………………….VI
CONTENTS………………………………………………………...……VI
Figure Captions………………………………………………..…….…..VI
Table Captions………………………………………………….……...VI
Chapter 1 INTRODUCTION………………………………………….1
Chapter 2 LITERATURE REVIEW………………………………..…4
2.1 Introduction of carbon nanotubes…………………………..….4
2.2 Structure and characteristics of carbon nanotubes………...….6
2.3 Growth mechanism of carbon nanotubes……………….…….11
2.4 Synthesis methods of carbon nanotubes…………………...….13
2.4.1 Arc discharge…………………………………………...….13
2.4.2 Laser vaporization…………………………………..……..13
2.4.3 Thermal chemical vapor deposition…………………….…14
2.4.4 Plasma enhanced chemical vapour deposition………….…15
2.5 Applications and development of carbon nanotubes………....18
2.5.1 Field Emitter for Flat Panel Display……………………….18
2.5.2 Transistors…………………………………………...…….18
2.5.3 Hydrogen Storage Medium………………………………..19
2.5.4 Scanning Probe Microscopy(SPM) Probe…………………19
2.6 Applications of CNTs in gas sensor……………….…………..22
2.6.1 CNTs gas sensors…………………………………………..22
2.6.1.1 Electrical properties of CNTs gas sensors…………….23
2.6.1.2 Gas ionization of CNTs gas sensors…………………..24
2.6.1.3 Variation of CNTs weight of CNTs gas sensors……...28
2.6.2 The theory of gas absorption on carbon nanotubes………..30
Chapter 3 EXPERIMENTAL DETAILS……………………….……39
3.1 Experiment instrument and equipment…………………..…..39
3.1.1 Growth instrument…………………………………………39
3.1.1.1 Sputtering System…………………………………..…39
3.1.1.2 Microwave plasma chemical vapor deposition……….40
3.1.1.3 Thermal chemical vapor deposition……………….….41
3.1.2 Film characterization………………………………...…….46
3.1.2.1 Scanning Electron Microscopy (SEM)………………..46
3.1.2.2 Transmission electron microscopy (TEM)…………....47
3.1.2.3 X-ray Photoelectron Spectroscopy (XPS)…………….47
3.1.2.4 Raman Spectroscopy………………………………….49
3.1.3 measurement equipments………………………………….54
3.2 Experiment processes and measurement design…………..…56
3.2.1 Substrate pretreatment………………………………..……58
3.2.2 catalyst deposition…………………………………………58
3.2.3 CNTs growth………………………………………………59
3.2.3.1 Thermal CVD……………………………..…………..59
3.2.3.2 MPCVD…………………………………….…………60
3.2.3.3 Annealing process of MWNTs………………...………61
3.2.4 Analysis…………………………………………………....63
3.2.4.1 Scanning Electron Microscopy (SEM)………………..63
3.2.4.2 Transmission Electron Microscopy (TEM)……….…..63
3.2.4.3 Raman Spectroscopy……………………………….....63
3.2.4.4 X-ray Photoelectron Spectroscopy (XPS)…………….64
3.2.5 Gas-sensing measurement…………………………………64
3.2.5.1 outgassing…………………………………………..…64
3.2.5.2 N2 detection by using various annealed MWNTs……..64
3.2.5.3.1 Without carrier gas……………………………65
3.2.5.3.2 With carrier gas…………………………….…66
3.2.5.4 N2 detection by using a bundle of MWNTs…………67
3.2.5.5 Three-terminal gas sensor of MWNTs mat………....68
Chapter 4 RESULTS AND DISCUSSION……………………….….73
4.1 MWNTs-based Gas sensor synthesized by MPCVD………....73
4.1.1 The analysis of morphology and composition……………..73
4.1.1.1 MWNTs surface morphology of SEM…………….….73
4.1.1.2 TEM images of MWNTs………………………...….76
4.1.1.3 Raman analyses of un- and 200℃ annealed MWNTs
...................................................................................................78
4.1.1.4 XPS analyses of un- and 200℃ annealed MWNTs…80
4.1.2 N2 detection of un- and annealed MWNTs………….……..83
4.2 MWNTs-based Gas sensor synthesized by thermal CVD
……………………………………………………………………….89
4.2.1 The analysis of morphology and composition……..……89
4.2.1.1 SEM image of MWNTs……………………………..89
4.2.1.2 TEM image of MWNTs……………………….….…93
4.2.1.3 Raman analyses of annealing MWNTs…………..…95
4.2.2 The analysis of N2 absorbed mechanism…………….…..97
4.2.2.1 N2 detection with and without carrier gas…………..97
4.2.2.2 Comparison of MWNTs mat and nanotubes bundle
……………………………………………………………….100
4.2.2.3 Three-terminal gas sensor based on MWNTs mat...109
Chapter 5 SUMMARY AND FUTURE RESEARCH………….…..115
REFERENCE……………………………………………………….….117

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