以CH4-CO2成長多層管壁奈米碳管及其特性探討

Abstract

自1991年飯島〈Iijima〉發現奈米碳管後，吸引許多學者相繼投入奈米碳管之研究以及奈米碳管之應用，目前奈米碳管可應用於平面顯示器之電場發射器、顯微鏡之探針、場效發射器…等微電子元件。這些產品之應用需要高品質之奈米碳管。目前成長奈米碳管之方法很多，其中以化學氣相沉積法最佳，因為其可在較低溫下成長出高品質具方向性之奈米碳管。 在本論文中，採用微波電漿化學氣相沉積法，在矽基板上成長垂直且對準性良好的奈米碳管，以甲烷和二氧化碳混合氣體取代一般製備奈米碳管常用之反應氣體如 氫氣-甲烷，氫氣-乙炔，氫氣-苯…等。結果顯示，以甲烷和二氧化碳混合氣體來製備奈米碳管，可在低溫的成長條件下得高產率､對直性良好的奈米碳管。 此外，為了進一步了解奈米碳管在矽基板上的反應機構，使用電漿放射光譜儀 分析氣相反應中主要的電漿物種，電漿成分明顯影響奈米碳管成長的反應機制，我們發現以CO2 取代氫氣可得品質極佳之奈米碳管。推論其主要原因為在CH4-CO2 混合系統中包含 CO電漿成分，CO 成分的存在可增加電漿物種中C2 的含量，在含大量C2 電漿中，C2根種會促進石墨的沉積，而加強奈米碳管在含觸媒基板上的成長與品質。進而探討在觸媒引導以及CH4-CO2 混合氣體環境氣氛下奈米碳管之成長模式。 分別用不同的金屬觸媒 Fe, Ti ,Fe/Ti 以CH4-CO2 為氣體源來成長奈米碳管，很明顯的沉積出不同型態的碳物質，Fe對CH4-CO2 氣體有極佳之吸附性及脫氫能力､適當的調整參數，以Fe 為觸媒可成長出品質高產率對直性佳之奈米碳管，而Ti則不太適用於成長奈米碳管。 使用Fe為主要的觸媒，探討基板先以H2電漿預處理後對奈米碳管成長之影響，經過H2電漿預處理後明顯觸媒產生燒結現象，而使觸媒顆粒變大，而觸媒顆粒大小控制奈米碳管之管徑。隨H2電漿預處理時間增加奈米碳管之管徑變大。使用未經H2電漿預處理之基板成長之奈米碳管之管徑約 10~20 nm 。分別使用經過H2電漿預處理 1min 至 15 min 之基板成長之奈米碳管之管徑約 30 到 300nm。而使用CH4-CO2 混合氣體成長之不同管徑奈米碳管均有極佳之場效發射性 使用CH4-CO2 混合氣體在適當的控制壓力､功率､以及反應氣體的流量，可在低溫 350℃條件下成長出高產率且對直性極佳之奈米碳管，降低溫度可得到較細管徑之奈米碳管，但同時會減低奈米碳管之成長速率。

Since carbon nanotubes (CNTs ) were discovered, relevant research fever and developments of commercial applications such as hydrogen storage, atomic force microscope probe, microelectronic transistor, electrical field emitter of flat panel display and scanning tunneling microscope tip have been stimulated tremendously. High-quality and well-aligned carbon nanotubes are essential to the potential applications in the field of microelectronic industries. Microwave plasma chemical vapor deposition (MPCVD) has been regarded as the potential method because of high quality and well-aligned carbon nanotubes can grow at low temperature In the thesis, carbon nanotubes were grown vertically and aligned on Fe catalytic nanoparticles deposited on a Si substrate at low temperature by using CH4 and CO2 gas mixtures. This is apparently different from the conventional reaction in gas mixtures of hydrogen and methane, hydrogen and acetylene, and hydrogen and benzene, etc. In microwave plasma deposition of CNTs, many reactions are involved in plasma and on substrate surface. A dynamic form of optical emission spectroscopy was used to detect the species in the plasma. These data show the dominant species in gas phase reaction. The composition of plasma significantly affects the reaction mechanism of carbon nanotubes growth. It is concluded that in the CH4-CO2 gas system can increase the amount of C2. In the C2-rich plasma the higher excited C2 emission intensity is beneficial to graphite deposition, and enhance carbon nanotubes synthesis on catalyst-deposited surface quality. Then a CNTs growth model in catalysts and gas environment of CH4-CO2 gas mixture was investigated Various catalyst, Fe, Ti, and Fe/Ti, were used to synthesize CNTs by CH4-CO2 gas sources. Significant difference of morphology in the carbon deposition was observed among Fe, Ti and Fe/Ti catalyst. By proper adjusting growth parameters a high yield of vertically aligned CNTs can be found in Fe-deposited substrate, but Ti is not suitable as a catalyst in CNTs production. The effects of H2 plasma pretreatment on the CNTs growth were investigated in the view point of CNTs morphology when Fe was used as the catalyst. After the H2 pretreatment, the diameter of CNTs increased significantly as the H2 plasma pretreated time increased because of the catalyst particle sintering to enlarge the catalyst particles. However, the diameter of CNTs was governed by the catalyst particle size. The CNTs diameter distributed in the range about 10~20 nm when Fe-deposited substrate was not pretreated. But the diameter of CNTs changed from 30nm to 300 nm when Fe-deposited substrate was pretreated from 1 min to 15 min. CNTs with various diameter by using MPCVD of CH4-CO2 gas mixture have good field emission properties Vertically aligned CNTs with multi-walled structures were successfully grown at low temperatures below 350oC by MPCVD using a CH4-CO2 gas mixture. The low temperature would be beneficial for reducing the diameter of CNTs but it will also decrease the growth rate on the substrate. Thus the CNTs grown at low temperature by the MPCVD using CH4-CO2 gas mixture have good field emission properties.