方向性碳奈米管之製程控制及其鈷觸媒與矽基材界面反應

Abstract

摘 要 為了要開發並有效地控制奈米結構，碳奈米管(CNTs)之性質及成長方向、機制及與基材界面反應之關係研究是必要的。鈷觸媒鍍於矽基材，並利用不同中間層材料，使用微波電漿化學氣相沉積(MPCVD)或電子廻旋共振化學氣相沉積(ECRCVD)，氣體源分別為甲烷(CH4)、氫(H2)及氮(N2)，成功地合成垂直與水平方向之碳奈米管。氧化方式成長SiO2中間層，使用物理化學氣相沉積(PVD)成長 Ti中間層與Co觸媒薄膜於矽基材上。施行氫電漿之前處理，使得鈷觸媒形成均勻分佈之奈米顆粒，以利碳奈米管之後續成長。而合成之奈米結構利用掃描式電子顯微鏡(SEM)、穿透式電子顯微鏡（TEM）、高顯析度穿透式電子顯微鏡、(HRTEM)電子繞射(ED)、X-ray繞射（XRD）、拉曼光譜儀（Raman spectroscopy）、歐傑電子能譜儀(AES)和I-V量測儀來分析其特性。 在利用微波電漿化學氣相沉積法(MPCVD)成長碳奈米管方面，其結果顯示，成長出垂直方向且高密度排列之中空多管壁碳奈米管(MWCNTs)。而直徑大約100 nm之單顆鈷觸媒，成長出多根直徑約10 ~15 nm之多管壁碳奈米管，並將此成長機制稱為tip-root 或 base-root growth成長模式。關於中間層材料之影響，SiO2中間層較傾向形成tip-root growth之鈷觸媒輔助碳奈米管，Ti中間層較傾向形成base-root growth之鈷觸媒輔助碳奈米管。原因為100 nm之 SiO2中間層有效地阻擋鈷觸媒與矽基材之化學鍵結，而成長出tip-root growth碳奈米管，Ti中間層促進Co與矽基材形成矽化物(Silicide)，成為較強之鍵結而成長出base-root growth碳奈米管，並利用SEM、TEM和XTEM結構分析下發現，多管壁碳奈米管成長機制與中間層不同之影響。而成長氣體若從H2改變為N2，將使得多管壁碳奈米管之結構由中空管徑變成竹節狀結構，原因為氮原子取代氫原子而造成。 在利用電子迴旋共振電漿化學氣相沉積法(ECRCVD)成長碳奈米管方面，其結果顯示，若使用Ti foil置於矽基材上面一定距離後，並且加一負偏壓於Ti foil上，發現碳奈米管其成長方向，為平行基材表面成長。TEM分析顯示水平或垂直方向碳奈米管皆為多管壁結構，且二者其成長機制皆屬於Tip growth模式。成長出平行於基材表面之碳奈米管，原因為使用Ti foil置於預成長碳管基材上，並導引成長氣流方向及電場輔助下而形成。I-V量測結果發現，水平方向碳奈米管其場發射性質優於垂直方向。當電流密度為 1 µA/cm2時，水平方向、垂直方向碳管其啟始電場分別為 2 V/µm、3 V/µm。在電場 10 V/µm 時，水平方向碳管電流密度J > 40 mA/cm2，垂直方向為 30 ~ 35 mA/cm2之間。因此可知，水平方向碳奈米管其場發射性質優於垂直方向之碳管。 結果顯示碳奈米管之成長方向、成長機制及其性質，皆可利用不同氣體源，基材偏壓、中間層材料、H2電漿前處理及導引成長氣流來加以操控。

ABSTRACT In order to develop the effective process to control the nanostructure, orientation and properties of carbon nanotubes (CNTs) and to examine the relationships among the CNTs growth and interfacial reactions, The vertically and horizontally oriented CNTs were successfully synthesized on interlayer-coated Si wafer by MPCVD or ECRCVD with CH4, H2 or N2 as source gases and Co as catalyst. The SiO2 interlayer was obtained by wafer oxidation. The Ti interlayer and Co catalyst films were deposited on Si wafer by physical vapor deposition (PVD) method. The interlayer and Co catalyst-coated substrates were then followed by H2 plasma pretreatment to become the well-distributed nano-particles to act as catalyst for CNTs growth. The deposited nano-structures were characterized by SEM, TEM, HRTEM, XRD, Raman spectroscopy, AES and I-V measurements. The results show that the CNTs deposited by MPCVD are dominated by the vertically-oriented and well-aligned dense CNTs. In general, a single catalyst with an average size of ~ 100 nm could grow many MWCNTs of 10 ~ 15 nm in tube diameter, as so called “root-growth model”. About effect of interlayer materials, the SiO2 interlayer prefers to form tip-root growth Co-assisted CNTs, and the Ti interlayer the base-root growth Co-assisted CNTs. This is due to the fact that the 100 nm SiO2 interlayer can effectively block the chemical bonding of Co with the Si substrate, and both Ti and Co can form silicides to promote the bonding with the substrate and so base-root growth of CNTs, which are in agreement with the results of XRD and XTEM analyses. Regarding effect of replacing H2 by N2 gases, the results indicate that the hollow CNTs will become bamboo-like CNTs. It may relate to the stressing effect by replacing C atoms with N atoms in the CNTs lattice. To examine the effect of flow guiding by a biased Ti foil on top of the specimen, the results depict that the horizontally oriented tip-growth CNTs can be obtained by ECR-CVD. The horizontal oriented CNTs show better field emission properties than the vertical oriented CNTs. For horizontal oriented CNTs, turn-on electric field (Eturn-on) defined at J = 1 µA/µm can go down to 2 V/µm, and J value can be greater than 40 mA/cm2 (at 10 V/µm); and in contrast, Eturn-on = 3 V/µm, J = 30 ~ 35 mA/cm2 (at 10 V/µm) for vertically oriented CNTs. This is due to the fact that the field emission of the vertically oriented CNTs is more greatly restricted by the catalysts at the tips, and the defect emission from the body of CNTs is effectively hided; in contrast to field emission from the body instead of tips for the horizontally oriented CNTs. In summary, the results imply that the orientation, growth mechanism, the nanostructures and properties of CNTs can be controlled by manipulating the source gases, the substrate bias, interlayer application, H2 plasma pretreatment and flow guiding.