偏壓效應對奈米碳管的成長與特性之影響

Abstract

本實驗以偏壓輔助微波電漿化學氣相沈積法(Microwave plasma enhanced chemical vapor deposition)成長奈米碳管。探討比較加偏壓對CH4/H2及CH4/CO2兩種反應氣體在奈米碳管的成長以及特性之影響。 結果顯示，以鐵和鈀當催化劑，在CH4/H2反應氣體中皆能長出緻密碳管；然而以CH4/CO2當反應氣體時，以本實驗所設定條件之下，只有鈀能成長出碳管，而以鐵為催化劑只能形成過多缺陷以及不純相摻雜一起的碳膜，不易形成單相碳管。偏壓效應對於碳管的外貌有重大的影響，當施以負偏壓時，負偏壓愈大，碳管管徑隨之增加，反之，當施以正偏壓時，正偏壓愈大，碳管管徑隨之減少。然而無論施以正負偏壓，碳管膜厚並無明顯變化。由TEM分析可知，以鈀當催化劑易成長出魚骨狀結構之碳管，以鐵當催化劑易成長竹節狀結構之碳管，且兩者都為中空多層的石墨結構。利用CH4/CO2當反應氣體，因為能夠提供較多碳源，以致於碳管成長速率(約為4.8μm/min.)比CH4/H2當反應氣體時(約為1.7μm/min.)快。以鐵當催化劑所成長的碳管，由於具有方向性及較小管徑，因此相較於以鈀當催化劑所形成碳管具有較低起始電場以及較高電流。由於施以偏壓能有效改變碳管的管徑，同時利用正偏壓所得較小的管徑在電子場發射應用上具有較佳的性質，如較低起始電場、高電壓下有較高電流。

In this study, two reactive gases ：CH4/H2 and CH4/CO2 are used to grow carbon nanotubes(CNTs) in bias assisted microwave plasma chemical vapor deposition (BAMPCVD) system. The bias effect on the growth and properties of carbon nanotubes is reported. As a result, the high density carbon nanotubes could be synthesized by using CH4-H2 gas mixtures on both Pd and Fe catalysts. In CH4-CO2 gas systems, dense CNTs were also found in Pd Catalyst. However, CNTs with more defects and impurities were grown on Fe catalyst. It was found that the diameter and the quality of CNTs are obviously controlled by the applied bias voltage. Our results show that the diameter of CNTs can be decreased to 45 nm without any amorphous carbon on the surface under +120 V. In addition, carbon nanotubes grown under positive bias possess better field emission characteristics than under negative one. This results from the following reasons (I) smaller diameter and (II) pure surface structure. From the TEM results, the fish-bone like carbon nanotubes are often grown on Pd catalyst; and the bamboo-like carbon nanotubes are usually found on Fe catalyst. Both two different kinds of carbon nanotubes are made of graphite with hollow and multi-wall structure. Due to higher carbon concentration in CH4-CO2 gas systems, the growth rate of carbon nanotubes (~4.8μm/min.) is faster than using CH4-H2 gas mixtures ( ~1.7μm/min.).