經微胞合作形成觸媒以輔助成長碳奈米管之製程開發和性質

Abstract

本研究研發出一個新的製程，目的是希望合成出小尺寸的碳奈米管，也希望因而成長出單壁的碳奈米管。其製程是在微波電漿化學氣相沉積系統(MPCVD)中，以氫氣及甲烷為氣氛，使用鈷、鐵、鎳當作觸媒，以SiO2、a:Si、AAO和a:Si/Si3N4為緩衝層，來進行碳奈米管的成長。而其中的觸媒是利用微胞合作再經過氫電漿前處理所形成分佈均勻的金屬奈米粒子。在MPCVD中，碳奈米管將會成長於前處理完的試片上。而每個步驟中，所形成的奈米結構與性質的特性將會以SEM、TEM、EDX、Raman和Field-emission來量測與分析。由實驗的結果大致可得到以下的結論: 關於含觸媒的微胞製備方面，得到還原劑的種類與濃度是很重要的參數，利用以下的方法可以得到含約2 nm金屬粒子的微胞溶液: toluene (溶劑) + 10 wt% CTABr (界面活性劑) + 0.005M MClx (金屬氯化物) + 5M NaBH4 (還原劑，添加的濃度[BH ]/[Mx+] =3:1)。關於氫電漿前處理的製程，結果顯示本實驗的製程溫度可能太高了，也就是為什麼經過前處理的奈米粒子比微胞中的金屬粒子大很多，但其仍然比傳統PVD法小很多。關於成長碳奈米管的主要參數，包括觸媒種類、緩衝層材質成長壓力、H2/CH4比例和沉積時間。在比較碳奈米管的直徑與氫電漿前處理後的觸媒大小中，其結果鈷觸媒是5~15 nm與19 nm，鐵觸媒數5~20 nm與23 nm，鎳觸媒是10~20 nm與16 nm。換句話說，碳奈米管的管徑與前處理後的觸媒大小沒有明顯的關聯性。 關於製程條件在成長小直徑碳奈米管的角度而言，發現以AAO為緩衝層，且配合以鐵為觸媒時，可以得到較小直徑的碳奈米管(~5 nm左右)，原因是AAO本身擁有奈米級的粗糙度，在觸媒的分散上，有明顯的幫助，使觸媒達到小尺寸，因而成長出小直徑的碳奈米管。在碳奈米管於場發射方面的性質而言，當鐵為觸媒以及a:Si為基材時，在壓力為24 Torr、H2/CH4比例為50/5 sccm/sccm和沉積時間為6 min時，所成長的碳奈米管，其場發射效應最佳，起始電場強度在0.1 mA/cm2時，為 2.0 V/μm。而電流密度超過儀器的極限(35 mA/cm2)。對應最佳的場發射性質與奈米結構，可以發現碳奈米管是呈準直性成長，且成束排列，而碳管的管徑為11~25 nm，長度約為10 μm。

In order to synthesize the small sized carbon nanotubes (CNTs) or single-walled CNTs (SWNTs) on Si wafer, a process was successfully developed by microwave plasma chemical vapor deposition (MPCVD) with CH4 and H2 as source gases; using Co, Ni and Fe as catalysts; SiO2, a:Si, AAO, and a:Si/Si3N4 as buffer layer materials. Where the catalysts were prepared by micelle method, and followed by H-plasma pretreatment to obtain the well-distributed nanoparticles. The CNTs were then deposited on the pretreated specimens in MPCVD system. The deposited nanostructures and properties in every step of the process were characterized by SEM, TEM, EDX, Raman spectroscopy and field emission J-E measurements. From the experimental results, the following conclusions can be drawn: About the catalyst-embedded micelles preparation, the results indicate that the reducing agent and its concentration are two important parameters. The following solution was found to be able to obtain ~2 nm sized catalyst-embedded well-distributed micelles: toluene (solvent) + 10 wt% CTABr (surfactant) + 0.005M MClx (metal chloride) + 5M NaBH4 (reducing agent, concentration of [BH ]/[Mx+] =3:1). Regarding the H-plasma pretreatment process, the results show that the process temperature in this experiment might be too high. This is why that the average particle sizes after pretreatment were much higher than the metal particles in the micelles, though they are much smaller than the conventional PVD methods. On the main process parameters during the CNTs deposition, it includes catalyst and buffer layer materials, pressure, H2/CH4 ratio, and deposition time. By comparing the diameters of CNTs with the particle sizes after H-plasma pretreatment, the results reveal that 5~15 nm with 19 nm for Co, 5~20 nm with 23 nm for Fe, 10~20 nm with 16 nm for Ni catalysts, respectively. In other words, the tube sizes of CNTs are not correlated with the particle sizes after H-plasma pretreatment. Regarding process conditions to obtain the CNTs or SWNTs with smaller sizes, it indicates that AAO buffer layer material in combination with Fe catalyst gives rise to the smallest tube size (~ 5 nm). This may be due to its nano-sized pores on the AAO surface to cause the nano-sized extrusions on the catalyst surface to assist CNTs growth. In terms of field emission, the following process conditions can result in the best properties (Turn-on field strength of 0.1 mA/cm2 = 2.0 V/μm): Fe as catalyst, a:Si as buffer layer material, higher system pressure (24 Torr), proper deposition time (6 min) and lower H2/CH4 ratio (50/5 sccm/sccm), where the current density is beyond the instrument limit, i.e. > 35 mA/cm2. The corresponding nanostructures with the best field emission properties are well-aligned bundles of CNTs with 11 ~ 25 nm in diameter and ~ 10 μm in length.