電漿處理之奈米碳管薄膜應用於酸鹼值感測特性研究

Abstract

奈米碳管(Carbon nanotubes)自1991年被發現以來，由於其特殊之材料特性，如高導電性、良好的化學穩定性、最大的楊氏應力係數(Young’s modulus)，以及三維奈米結構形成的高接觸表面積等等，被視為相當具有潛力應用在酸鹼感測上。然而，現有的奈米碳管應用在酸鹼感測器的研究面臨感測特性差、製程難度高、各項污染等問題，限制了它的表現。有鑑於此，本論文使用超音波撒佈(Spraying)的方式製作出均勻、低汙染排放的高品質碳管薄膜，再利用電漿製程做表面處理來提升酸鹼感測特性，以製備出高效能之奈米碳管酸鹼感測器。 本研究首先利用氬氣電漿對碳管薄膜做表面改質，優化處理時間及偏壓功率，成功地製備出具有感測度54.62 mV/pH與線性度0.995之閘極延伸式酸鹼感測器(EGFETs)。藉由掃描式電子顯微鏡(SEM)、X射線光電子能譜(XPS)以及拉曼光譜(Raman spectroscopy)分析材料特性，並與酸鹼感測特性做比較，提出電漿處理之奈米碳管酸鹼感測器可能之感測機制：首先，電漿將碳管鍵結中最弱的π鍵打斷，並在碳管表面形成自由基，與空氣接觸後便和環境中含氧的活性離子鍵結形成官能基。這些官能基扮演酸鹼感測過程中的感測元，電漿處理所形成的大量感測元改善了碳管薄膜的感測特性。有鑑於此，為提供更充足的氧活性自由基，我們進一步將氬氣電漿處理後的奈米碳管薄膜置於低溫(200⁰C)的純氧氣環境中退火，成功地將感測特性從未退火前的54.6 mV/pH提升至56.4 mV/pH。 為了進一步利用氧官能基增強奈米碳管的感測特性，本研究更進一步利用氧氣電漿對碳管薄膜做表面處理。利用離子轟擊碳管表面製造自由基的同時，輝光放電環境提供能量產生更充足之氧活性自由基，因此可以更有效地在碳管表面形成大量氧官能基，進而大幅提升碳管薄膜之酸鹼感測特性，以此方法製備之閘極延伸式酸鹼感測器具有高達56.8 mV/pH 之酸鹼敏感度並同時具有0.9995之高線性度。 最後，本研究成功將酸鹼感測器實現在可撓式基板(聚亞醯胺, polyimide)上，製備具有感測特性高達55.7 mV/pH與線性度0.9996之高效能可撓曲式酸鹼感測器，並且透過撓曲實驗證實，該感測元件受應力撓曲後仍維持良好的酸鹼感測特性，極具潛力應用於未來軟性生醫感測元件中。

With the special material features, such as the high electrical conductivity, high surface-to-volume ratio, strong mechanical strength, and excellent chemical resistivity, the carbon nanotube (CNT) has been regarded as a promising candidate for the applications in the pH sensing. However, in the present CNT-based pH sensors, some significant restrictions, such as the low pH sensitivity, inevitable high-temperature processes, and some contaminations, limiting their applications. In order to conquer these challenges, the high-performance pH sensors based on the extended-gate field-effect transistors (EGFETs) were proposed in this thesis via the low-temperature ultrasonic spraying method and the surface modification of the CNTs by the plasma treatments. In the first part of this thesis, by utilizing the argon plasma treatment on CNTs, high-performance pH-EGFET sensors with the sensitivity of 54.62 mV/pH and the linearity of 0.995 were successfully fabricated by means of optimizing the plasma treatment time and the bias power. A series of material analyses including scanning electron microscopy (SEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) were demonstrated. By comparing the results of the material analyses with the pH sensitivities of the fabricated sensors, a possible pH-sensing mechanism of the plasma-treated CNTFs was proposed. At first, the weakest π bonds of the CNTs were broken up by the ion bombardment of the argon plasma, becoming the free radicals sited on the surface. Then, the free radicals bound with the active oxygen species in the air and became the oxygen-containing functional groups. It was the oxygen-containing functional groups that acted as the sensing sites for the pH sensing. Hence, the increased amounts of these functional groups resulted in the improvement of the pH sensitivity. In view of the importance of the oxygen-containing functional groups, improved ways were proposed. After treated by the argon plasma, the CNTFs were sent into an oxygen furnace at 200oC to generate the functional groups. The sensitivity of such pH-EGFET sensors could increase from 54.6 mV/pH to 56.4 mV/pH. Furthermore, the oxygen-containing functional groups were also produced through the oxygen plasma treatment. The oxygen plasma could play the role in not only the generation of the free radicals but also the formation of the oxygen-containing functional groups, and it was more effective to functionalize the CNTFs. The fabricated pH-EGFET sensors with the oxygen-plasma-treated CNTFs (OPT-CNTFs) exhibited the excellent sensitivity of 56.9 mV/pH with the linearity of 0.9995. Seeing the success on the glass substrates, the OPT-CNTFs were further implemented on the flexible polyimide substrates. The fabricated pH sensors also presented outstanding pH-sensing characteristics of the high sensitivity of 55.7 mV/pH with the linearity of 0.9996. Besides, after bending test, the pH sensors still preserved excellent sensing characteristics, making them promising in the future developments for the flexible biosensors.