緩衝層輔助MPCVD法合成水平網狀單壁碳奈米管之製程及其性質

Abstract

本研究利用緩衝層材料輔助微波電漿化學氣相沉積法(MPCVD)，以甲烷(CH4)、乙炔(C2H2)與氫氣(H2)為反應氣體，鎳(Ni)與鐵(Fe)為觸媒，成功地開發出在矽晶片上直接合成水平方向網狀單壁碳奈米管。使用的緩衝層材料包括AlON, AlN, Al2O3, TiO2, TiON, TiN, TiCN and ITO。此製程首先在矽晶片上連續濺鍍10 nm的緩衝層及2~10 nm的觸媒薄膜。接著將鍍好的矽晶片在MPCVD進行氫電漿前處理及碳奈米管沉積。沉積後的碳奈米管藉由掃描電子顯微技術(SEM)、穿透電子顯微技術(HRTEM)、自動變溫控制拉曼光譜技術(Raman spectroscopy)、場發射J-E測量法加以分析探討。從實驗結果可得下列結論。 就緩衝層材質的影響而言，結果顯示，根莖成長機制的水平方向網狀碳奈米管在鋁基緩衝層輔助下較易形成。緩衝層(例如Al基材料)的作用基本上是提供奈米級尺寸的表面凹窩並具有兩個作用：(1) 對於粒徑較小的奈米粒子其作用是提供居留位置防止顆粒的聚集，(2) 對於粒徑較大的奈米粒子是提供一個模版，使奈米粒子在模版表面形成許多突起點作為單壁碳奈米管束的成核點。根據第二個功能可解釋為何觸媒在中間膜厚(~ 5 nm)為最適合成長網狀單壁碳奈米管束的厚度。 對於觸媒的影響而言，Ni 比Fe觸媒需要更多的碳源來成長碳奈米管。這可能是因為Ni的熔點比Fe低且有較高的溶碳量 (0.55 wt.% C)。Ni 在緩衝層材料上成長網狀單壁碳奈米管的趨勢為AlON > Al2O3 >> AlN，相對地，對於鐵而言則為Al2O3 >> AlON = AlN。兩種觸媒間的差異可能是因為熔點的不同，因此形成奈米粒子後的聚集效果也不一樣。 基材溫度對成長碳奈米管也有顯著的影響，當在低溫時(< 570oC)形成的結構為碳膜包覆觸媒的顆粒而不是網狀單壁碳奈米管，這也許是因為碳在形成奈米管的擴散速率比氣相碳源供應碳的速率慢，造成觸媒顆粒快速被毒化。 根莖成長的網狀碳奈米管在改變不同量測溫度至500oC，結果顯示量測拉曼頻譜的RBM特徵訊號有往低頻移動的趨勢，此種頻率下降現象是由於測試升溫時C-C 鍵在高溫下的鬆弛現象所造成。 以Fe為觸媒Al2O3為緩衝層輔助成長的網狀碳奈米管在場發射性質分析中可得到起始電壓為2.7 V/□m (在電流密度0.01 mA/cm2時)，而電流密度3.4 mA/cm2 (在4.9 V/□m的電壓時)。

In this work,the horizontally oriented single-walled carbon nanotubes (SWNTs) networks deposited directly on Si wafer substrate by the buffer layer-assisted microwave plasma chemical vapor deposition (MPCVD) method were successfully developed, using CH4, C2H2 and H2 as the source gases, and Fe and Ni as catalyst materials. The buffer layer materials include AlON, AlN, Al2O3, TiO2, TiON, TiN, TiCN and ITO. The Si substrates were first deposited with 10 nm buffer material by DC reactive sputtering, and then deposited with 2 ~ 10 nm catalyst by E-gun sputtering. The coated substrates were followed by H-plasma pretreatment in MPCVD, and then conducted the CNTs deposition in the same system. The structure and properties of the deposits at each step were characterized by FESEM, TEM, HRTEM, Raman spectroscopy with auto temperature control and field emission I-V measurements. From the experimental results, the following conclusions can be drawn. Regarding effects of buffer layer material on CNTs growth, the results indicate that the root-growth SWNTs net works are much favor to be formed on substrates with Al-base materials as buffer. Effects of buffer layer (such as Al-base materials) are essentially to provide the surface with nano-sized dimples with two functions: (1) to act as accommodation sites for smaller nano particles to prevent them from agglomeration, (2) to act as a template to make the bigger nano particles to become many extrusions on their surfaces to form SWNTs bundles. Base on the buffer second function, it can be used to explain why the catalyst with the intermediate thickness (~ 5 nm) is more favor to form SWNTs bundles networks. Considering effects of catalyst materials, Ni catalyst requires more carbon source than Fe catalyst to form SWNTs networks. This may be due to a lower melting point and a higher carbon solubility (0.55 wt.% C) of Ni than Fe catalyst. Ni catalyst to form SWNTs networks on substrates with different buffer materials are in order of AlON > Al2O3 >> AlN, in contrast, for Fe catalyst in order of Al2O3 >> AlON = AlN. The different behaviors between Ni and Fe catalysts may relate to difference in melting point and so agglomeration effect of the nano particles. For effect of the substrate temperature on CNTs growth, it shows that the catalyst-encapsulated carbon particles instead of SWNTs networks are more favor to be formed at lower temperatures (< 570oC). This may relate to that carbon diffusion rate for forming CNTs is lower than carbon supply rate from gas phases at lower temperatures, which causes the catalyst being quickly poisoned. For the root-growth SWNTs networks nanostructures, Raman analyses show that the RBM signal shifts to the lower frequency side by increasing the measuring temperature to 500oC. The downward shift of Raman peaks is due to C-C bond relaxation at higher testing temperature. For the nanostructures of the Al2O3 buffer-assisted SWNTs networks, the field emission turn-on voltages and current density for Fe as catalyst are 2.7 V/□m (for current density 0.01 mA/cm2) and 3.4 mA/cm2 (for field strength 4.9 V/m) .