近室壓X光光電子能譜探討酞菁氧鈦對二氧化氮氣體之感測機制

金屬酞菁 (metal phthalocyanines, MPcs) 具備許多獨特優秀的材料性能，如載子易傳輸性、抗水氧性、高溫熱穩定性等，被廣泛應用於有機薄膜電晶體 (organic thin film transistor, OTFT) 形式的氣體感測器。本篇論文主要利用於台灣光源新建置的近室壓X光光電子能譜 (ambient pressure X-ray photoelectron spectroscopy, AP-XPS) 實驗站，探討酞菁氧酞 (titanyl phthalocyanine, TiOPc) 薄膜對二氧化氮 (nitrogen dioxide, NO2) 氣體之感測機制，以釐清表面吸附NO2對有機半導體電子結構的影響。當NO2背景壓力上升至0.025 mbar時，TiOPc各組成元素特徵峰皆往低束縛能處位移0.4 eV，此腔體壓力環境對應OTFT元件感測NO2氣體的偵測極限值，而電子軌域束縛能的減小源自於NO2分子吸附所致的電洞摻雜效應。進一步透過XPS的分峰分析，觀察發現於更高濃度的NO2氣氛下，氮在材料表層結構呈現多種賦存狀態，除了NO2氣體於薄膜表面吸附狀態外，其還有機會走歧化反應路徑產生NO3-以及NO+，其中NO還原物種會鍵結至中心金屬Ti上，吸引其上的價電子雲密度，並改變金屬離子所處的晶場配位環境。另外TiOPc分子在基材上的排列位向於NO2曝氣前後也有顯著變化，藉由角解析X光近緣吸收細微結構 (near edge X-ray adsorption fine structure, NEXAFS) 能譜的分析結果，瞭解原本近似平躺的分子排列結構，在NO2氣體吸附於薄膜上後轉變為凌亂無序的分布狀態。論文最後使用酞菁銅 (copper phthalocyanine, CuPc) 薄膜進行相同的實驗操作，藉以探究不同分子構形與中心金屬種類對NO2分子吸脫附的影響及其在薄膜表面的化學反應，並瞭解CuPc對NO2氣體感測之應用性。經由軟X光臨場光譜技術提供的資訊，我們可以全盤參透NO2氣體與金屬酞菁分子之間的作用機制，且對其特定毒性氣體的偵測原理有更進一步的認識。

關鍵字

酞菁氧鈦；二氧化氮；氣體感測器；近室壓X光光電子能譜

並列摘要

Metal phthalocyanines (MPcs) have been demonstrated to be useful materials for gas sensing application due to their favorable transducer response, high resistance to oxygen and moisture interference, and good thermal stability. They have been used widely for organic thin film transistor (OTFT) gas sensors. In this thesis, we use newly completed ambient pressure X-ray photoelectron spectroscopy (AP-XPS) endstation to investigate the nitrogen dioxide (NO2) sensing mechanism of titanyl phthalocyanine (TiOPc) and derive at how the electronic structure of TiOPc is modified during the gas sensing process. An abrupt lower binding energy shift of 0.4 eV is noted when the NO2 backing pressure is increased above 0.025 mbar, in accord with the detection threshold of 25 ppm determined from OTFT measurements. And the binding energy shift is interpreted as an evidence of hole-doping effect by NO2 on TiOPc film. With XPS curve fitting analysis, a variety of N-containing surface species is characterized under NO2 rich conditions. The surface adsorbed NO2 might undergo a disproportionation process to produce NO3- and NO+. Reduced nitrogenated species could bond to titanium central atom and increase its binding energy due to their electron withdrawing nature. Simultaneously, the formation of the ionic bond modifies the electronic structure of unoccupied orbitals of metal ion through the changed crystal field. In addition, the molecular orientation of TiOPc is found to change after the exposure of NO2. The angle dependent near edge X-ray adsorption fine structure (NEXAFS) data reveal that the aromatic plane of printine TiOPc thin film are oriented preferentially along the substrate surface plane, but turns into random orientation after the dose of NO2. Same experimental approach is extended to the investigation of copper phthalocyanine (CuPc) thin film to address whether different molecular configuration and different central metal atom in CuPc indeed influence the NO2 absorption/reaction behavior and govern its possible application in NO2 sensing. With real time information available from AP-XPS techniques, one can fully explore the the interaction between NO2 gases and TiOPc thin film, and further understanding the principle of the toxic gas detection.

並列關鍵字

Titanyl phthalocyanine ； Nitrogen dioxide ； Gas sensor ； Ambient pressure X-ray photoelectron spectroscopy

參考文獻

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