化學氣相沈積法奈米碳管之製作與特性研究

Abstract

本論文最大的成就在於提出一個直接成長多層壁奈米碳管的簡易方法，稱之為熱絲化學氣相沈積法。此合成裝置類似用於成長鑽石的熱電子(或稱熱燈絲)化學氣相沈積法，其不同於鑽石成長之裝置設計在於燈絲部分以Fe-Cr線替代W線，目的在於利用Fe-Cr線加熱時，其溫度接近其融點 (約1520 ℃) 附近所蒸發出來之Fe、Cr元素作為成長碳管之觸媒。其成長方法是以CO2或Ar當作載氣通過酒精送入反應區，即可將各種不同的不規則捲曲碳管及直立碳管直接成長於矽晶片。實驗結果顯示，當CO2載氣的流動方向垂直流向於試片時，試片上會長出直徑60-80 nm的不規則捲曲碳管；當載氣平行流過試片表面時，則生成直徑約為10 nm的直立碳管。不規則捲曲碳管為多層壁且為竹節狀，直立碳管則為多層壁但無竹節出現。綜合兩種碳管的頂端並沒有觸媒出現，推測碳管是屬於底端成長模式。以熱絲法直接成長的直立奈米碳管，起始電場為 1.1 V/□m (10 μA/cm2 時)，場發射電流密度為 0.54 mA/cm2 (2 V/□m 時)。 另外，本論文也嘗試使用微波電漿化學氣相沈積法，以CH4/CO2氣體系，及用基材自我供應觸媒方式來成長奈米碳管，探討比較兩種方式所成長碳管之成長機構與特性差異。熱絲化學氣相沈積法中用Fe-Cr線 (71.4 wt % Fe及22.5 wt % Cr) 做為觸媒來源，在此則用不鏽鋼304 (70 wt % Fe、19 wt % Cr及 9 wt % Ni) 當作基材來成長奈米碳管。結果顯示準直性佳的碳管可直接生成，然而基材中重要的Cr成分，並沒有出現於碳管頂端的觸媒顆粒。故為瞭解Cr對成長奈米碳管的特性，用CH4/H2及CH4/CO2這兩種氣體系在Cr膜上成長碳管。當以CH4/CO2為反應氣體時，實驗中並未有明顯的碳管生成。當以CH4/H2為反應氣體時，則先用施加負偏壓的H2電漿進行Cr膜的前處理，依前處理時間的長短，造成Cr膜不同的表面型態，然後再加入CH4（碳源）進行奈米碳管的成長。實驗證明Cr膜也能成長奈米碳管，但只限制在5分鐘前處理的Cr膜上；意即Cr膜只有在特定的表面型態時，才具有生成奈米碳管的觸媒作用。總結本論文在微波電漿化學氣相沈積法所成長出的碳管，成長過程皆符合頂端成長模式。 比較熱絲化學氣相沈積法與微波電漿化學氣相沈積法，熱絲化學氣相沈積法結合物理及化學氣相沈積法，具有便宜、大面積且連續合成奈米碳管的潛力。同時也不需要在真空的環境下進行。微波電漿化學氣相沈積法則可生成分佈密度較高且較均勻的奈米碳管。場發射性質則以熱絲化學氣相沈積法所長出的直立碳管最佳，推論是由於其直立性、適中的碳管分佈密度、碳管表面覆蓋的非晶質碳，以及極細的碳管管徑所造成的。

A direct hot wire chemical vapor deposition method, which is the main feature of this thesis, is proposed for in situ synthesis of multiwalled carbon nanotubes. This synthesis apparatus is similar to that in the hot filament chemical vapor deposition used to deposit diamonds. However, a Fe-Cr wire is used instead of a W wire in synthesizing diamonds to grow nanotubes. Fe-Cr wires need to be heated to 1200 ℃, approaching the melting point (~1520 ℃) of itself. Evaporated metal atoms, Fe and Cr, can be considered to catalyze the growth of carbon nanotubes. Different morphologies of carbon nanotubes, curved or aligned, were deposited directly on the Si substrate with CO2 or Ar as the carrier gas through alcohol. Indicated in the experimental results are that the vertical flow direction of CO2 to the substrate produced the random network of curved carbon nanotubes with the diameter of 60-80 nm. In contrast, horizontal flow direction of CO2 to the substrate produced the aligned carbon nanotubes of about 10 nm in diameter. The HRTEM observation tells that curved nanotubes were multiwalled and bamboo-like in structure; while vertical nanotubes multiwalled but without bamboo-like structure. In both cases, there was no catalyst in the top end of nanotubes. Therefore, the growth mechanism is suggested to be base-growth model. The turn-on field of the aligned carbon nanotubes was 1.1 V/mm (at 10 μA/cm2), and the emission current density 0.54 mA/cm2 (at 2 V/mm). Meanwhile, microwave plasma chemical vapor deposition by using CH4/CO2 gas mixture was used to grow CNTs directly on substrates, which were self-catalyzed, for comparisons of the growth mechanism and characterizations of carbon nanotubes grown by these two deposition system. In the hot wire chemical vapor deposition, CNTs were grown by Fe-Cr wires (Fe: 71.4 wt %, Cr: 22.5 wt %) that act as catalyst sources. Therefore, typical stainless steel 304 (Fe: 70 wt %, Cr: 19 wt %, Ni: 9 wt %) was used as the substrate to grow CNTs in this system. Experimental results show that well-aligned carbon nanotubes can grow directly on the stainless steel. However, the metal catalysts on the top of the carbon nanotubes contain Fe and Ni, but no Cr. To realize the characterization of Cr, carbon nanotubes were attempted to grow on a Cr film by using CH4/H2 or CH4/CO2 gas mixture. The CH4/CO2 mixture produced no obvious nanotubes under these conditions, but for CH4/H2 source gas, bias-enhanced H2 plasma pretreatment was performed for various periods to modify the surface of the Cr film, then CH4 (carbon source) was added for growth of carbon nanotubes. Experimental results show that carbon nanotubes can be grown, but just in a narrow Cr surface condition. A summation of the studies in microwave plasma chemical vapor deposition suggests the growth mechanism to be tip-growth model. Revealed in the comparison of these two methods is the superior potential of the hot wire chemical vapor deposition for the inexpensive and continuous mass synthesis of nanotubes by combining both physical and chemical vapor depositions. Besides, hot wire chemical vapor deposition is not necessary to operate under vacuum. Microwave plasma chemical vapor deposition can yield CNTs with higher density and uniformity in this stage. In these studies, vertical CNTs grown by hot wire chemical vapor deposition possess the best field emission properties. It may be due to the alignment of MWCNT, medium density, ta-c coated, and the small diameter to enhance their field emission properties.