本研究利用電漿輔助硒化製程，製備二硒化錫薄膜以作為氣體感測器應用之反應層，相較於傳統的化學氣相沉積法需要450度以上的溫度，電漿的輔助將能促進硒原子在相對低的溫度下擴散進二氧化錫薄膜合成二硒化錫，顯現了其應用在可撓式電子元件製備上的潛力。
本研究發現合成溫度在250度之低溫電漿輔助硒化合成二硒化錫薄膜對於二氧化氮氣體有高靈敏度，相較於其他氣體有約50倍的差異。為研究最佳的合成條件，本研究發現不同厚度的二硒化錫薄膜內的晶體結構會對於氣體感測能力有直接的影響。研究結果顯示厚度愈薄的硒化錫薄膜，其結構會愈偏向層狀，我們認為層狀結構有利於氣體感測期間的電子傳輸，故有較高的響應，反之，厚度愈厚的硒化錫薄膜則會偏向多晶結構，其中晶界在氣體感測期間會將電子束縛住從而阻礙有效的電子傳輸。所以在我們的研究中，5奈米之二硒化錫薄膜具有最高的二氧化氮響應，在室溫下對濃度為 1 ppm的二氧化氮氣體有263 %的響應。除此之外為了未來應用於實際的偵測元件，本研究也探討如何克服SnSe2的穩定性，發現透過光照能大幅改善二硒化錫薄膜的氣體響應穩定性，針對不同光源與功率，結果顯示使用過短波長與過大功率的光源，會因為氣體脫附速率增加的太大而使氣體響應下降過多，在追求最大感測能力的前提下，我們認為2.5毫瓦之紅光為最佳光源。
最後，我們與儀科中心合作將此二硒化錫氣體感測層結合無線傳輸與顯示之平台，建造出一個具有潛力的戶外氣體感測系統以達到無線監測目的之應用。

In this work, we successfully synthesized Tin diselenide (SnSe2) through a plasma-assisted selenization process as the reactive layer for gas sensor application. Comparison with conventional chemical vapor deposition (CVD) processes which needs a temperature above 450°C, assistance of plasma function facilitates the transformation of Selenium (Se) atoms diffuse into the tin dioxide (SnO2) thin film to synthesize SnSe2 at a relatively low temperature, showing its potential for application in the preparation of flexible device fabrication.
In this study, SnSe2 film is synthesized at 250 °C and exhibits high sensitivity to NO2. Compared to other gases, there is a difference of about 50 times. In order to find out the best synthesis conditions, this study indicated that the crystal structure of SnSe2 thin films with different thicknesses will have a direct impact on the gas sensing ability. The result shows that the layered structure facilitates the electrical pathway for charge carrier transport during the gas sensing reaction, which makes it have a higher gas response. On the contrary, the grain boundaries in the polycrystalline structure act as electron traps, thereby hindering effective electron transport. Therefore, the 5 nm SnSe2 has the highest response, and the response to 1 ppm NO2 is 263 % at room temperature.
In addition, this research also explores how to improve the stability. It is found that photoactivation can greatly improve the gas response stability of SnSe2. The result shows that Using a light source with short wavelength and high power will cause the gas response to decrease dramatically because the gas desorption rate increases too much. The results show that 2.5 mW of red light is the best light source for SnSe2.
Finally, we cooperated with Taiwan Instrument Research Institute (TIRI) to combine SnSe2 with a platform for wireless transmission and display to build a potential outdoor gas sensing system for wireless monitoring applications.

摘要...............................................................I
Abstract .........................................................II
致謝..............................................................III
Contents ........................................................ IV
Table caption .....................................................V
Figure caption ...................................................VI Chapter 1 Introduction ............................................1 1.1 Gas Sensor ....................................................1
1.1.1 Overview of Gas Sensor ......................................1
1.1.2 Mechanism of Gas Sensor .....................................1
1.2 Metal Oxide vs. Two-dimensional (2D) Material .................4
1.2.1 Composition, Crystal phase and Electronic Structure of TMDCs.5
1.2.2 Tin Diselenide ..............................................8
1.3 Manufacturing Process of SnSe2 Thin Films ....................14
1.4 Motivation ...................................................18
Chapter 2 Experiment .............................................20
2.1 Process Flow .................................................20
2.2 Measurement Flow .............................................23
Chapter 3 Results and discussion ................................ 24
3.1 Synthesis and Characterization of SnSe2 ......................24
3.2 Thickness Effect of SnSe2 ....................................29
3.2.1 Gas Sensing Characteristic .................................29
3.2.2 Effect of Structure ........................................32
3.3 Gas Sensor under Photoactivation .............................37
3.4 Circuit integration and application ..........................40
3.4.1 Integration of circuit and wireless transmission system.....40
3.4.2 Application ................................................41
Chapter 4 Summary and future works ...............................43

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