Process development to synthesize single-walled carbon nanotubes arrays by ECR-CVD
|摘要:||本研究以電子迴旋共振化學氣相沉積法(ECR-CVD)以及氧氣電漿(O-Plasma)前處理， Fe、Co和Ni為觸媒，甲烷(CH4)和氫氣(H2)為反應氣體，改進在Si基材上以觸媒輔助合成單壁碳奈米管的製程。首先，分別以純金屬Fe、Co和Ni(皆為99.99%)為靶材，在Ar氣氛下，利用物理氣相沉積法(PVD)分別於矽晶片上鍍觸媒薄膜，接著於ECR-CVD中進行氧電漿前處理，使觸媒薄膜形成平均分散的觸媒顆粒，接著於ECR-CVD系統中沉積碳奈米結構。同時，將氧化鎳於SiO2/Si的基板形成圖案，進行選擇性沉積的實驗。成長碳奈米管時，測試藉由試片堆疊方式的影響來改變電漿直接轟擊下對試片的保護。 各階段的實驗結果分別利用掃描式電子顯微技術(SEM),高解析度穿透式電子顯微技術(HRTEM), Ｘ射線光電子能譜技術(XPS)和拉曼光譜技術(Raman spectra)對結構以及性質進行分析。從結果可以得到以下的結論。
在實驗結果中，實驗結果顯示出適當的沉積時間(~ 50 min)和偏壓(-150 V)可獲得最高單壁碳奈管密度(鈷觸媒管的直徑~ 1.13 nm，鎳觸媒管直徑0.87 ~ 1.21 nm)，並且前處理後觸媒顆粒的直徑遠大碳奈米管的直徑。此結果也指出，碳奈米管主要為網狀單壁碳奈米管。對CNTs沉積影響而言，氧氣電漿前處理形成的氧化表面基本上是提供許多小凸起，每個凸起均可用於形成單壁碳奈米管。在沉積單壁碳奈米管時，氫氣電漿先將氧化觸媒顆粒上凸起還原成奈米等級的純金屬顆粒，接著碳原子擴散至觸媒中，再由觸媒中高溫部分沉澱至低溫部分並且在原來的凸起形成單壁碳奈米管。換句話說，最適當的時間提供足夠的時間活化所有凸起以成長單壁碳奈管，而最適當的偏壓則提供足夠的能量還原所有被氧化的凸起。此外，鐵、鈷和鎳觸媒輔助所形成的碳奈米管之管徑為依序：Fe Co＞Ni。
Processes to synthesize the catalyst-assisted single walled carbon nanotubes (SWNTs) on Si wafer were developed by oxygen-plasma pretreatment and electron cyclotron resonance chemical vapor deposition (ECR-CVD), using CH4 and H2 as source gases, Co, Ni and Fe as catalysts. The catalysts were first sputtered on Si wafer by physical vapor deposition (PVD) method with pure metal (Ni 99.99%, Co 99.99% and Fe 99.99%) as targets under Ar atmosphere, and then followed by O-Plasma pretreatment to obtain the well-distributed catalyst particles. The carbon nanostructures were then deposited on the pretreated specimens by ECR-CVD. One pattern made of Ni-oxide-catalyst and SiO2/Si substrate was also conducted to examine selective deposition of CNTs. Effects of stacking sequence of specimens to vary the specimen protection from direct plasma bombardment on carbon nanotubes (CNTs) growth were examined. The structures and properties after each step were characterized by SEM, HRTEM, XPS and Raman spectroscopy. From the experimental results, the following conclusions can be drawn. To grow SWNTs, the results show that there are existence of optimum deposition time (~ 50 min) and optimum bias (-150 V) to obtain the SWNTs with the highest tube number density (diameter ~ 1.13 nm for Co catalyst, and 0.87 ~ 1.21 nm for Ni catalyst), where the pretreated particle sizes are much greater than the tube diameters. The results also indicate that the tubes are mainly the rooy-growth SWNTs net works. In terms of effects on CNTs deposition, the O-plasma pretreatment is basically to provide the oxidized surfaces with many smaller extrusion sites to grow SWNTs at each site. The SWNTs deposition can be imagined that each extrusion site is first reduced by H-plasma to become nano-sized pure metal particles during initial CNTs deposition period, and then carbon atoms will diffuse in the pure metal catalysts from the higher temperature side and to precipitate to the lower temperature side of the catalyst to form SWNTs at the original extrusion sites of the oxidized particles. In other words, the optimum time is basically to have enough time to activate all extrusion sites to grow SWNTs, and the optimum substrate bias is basically to give enough energy to activate the reduction of all oxide extrusion sites. Furthermore, the order of tube diameters for different catalysts is Fe and Co > Ni. The results also indicate that the pattern made of Ni-oxide and SiO2/Si substrate can be successfully used to grow CNTs pattern for potential applications in device fabrication, where SWNTs can merely grow on the positions with Ni-oxide.
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