重力輔助化學氣相沉積成長垂直排列的單壁奈米碳管及其場發射應用

Vertically aligned single-walled carbon nanotubes by gravity-assisted chemical vapor deposition and its field emission application

葉峻銘(Chun-Ming Yeh)

指導教授 ： 黃振昌 ； 甘炯耀

國立清華大學/工學院/材料科學工程學系/博士(2006年)

https://doi.org/10.6843/NTHU.2006.00695

摘要

本篇論文係以重力輔助化學氣相沉積系統成長垂直排列的單壁奈米碳管。在重力輔助化學氣相沉積系統製程中，鈷/矽(100)基板的表面朝下。單壁奈米碳管則會沿著重力方向成長。單壁奈米碳管的長度隨時間增加而增加。我們亦提出經過修改的氣-液-氣成長機制解釋重力對於析出單壁奈米碳管的影響和使其能夠垂直排列。
溫度與甲烷/氫氣比例是兩個重要的因素，能影響垂直排列單壁奈米碳管的品質。垂直排列單壁奈米碳管和其它碳產物的品質利用掃描電子顯微鏡跟拉曼光譜分析。在溫度850和900 ℃時，單壁奈米碳管有很好的品質且垂直獨立站立在基板上。在其他溫度時，奈米碳纖或不規則碳島狀物會出現在基板上。甲烷/氫氣比例對單壁奈米碳管的品質也有很大的影響。在低比例時，沒有碳管形成。甲烷/氫氣比例在範圍160/80至160/40時，適合成長垂直排列的單壁奈米碳管。在高於160/40比例時，多壁碳管將取代單壁碳管。
在成長時間對於單壁奈米碳管品質影響的研究方面。單壁奈米碳管的形狀隨時間不同而不同，其形狀由垂直到彎曲到垂直。高品質的垂直方向單壁奈米碳管在成長時間小於3分鐘時發生。當成長時間5分鐘時，垂直和彎曲的單壁奈米碳管同時出現。單壁奈米碳管長度長於1微米時，它非常容易彎曲因為它有很低的有效彎曲強度。成長時間20分鐘時，非晶碳沉積於單壁奈米碳管之上且讓單壁奈米碳管保持住原本垂直的形狀。而非晶碳沉積於單壁奈米碳管之上的機制也被提出加以解釋。
在場發射應用方面，利用重力輔助化學氣相沉積製程使垂直排列的單壁奈米碳管成長在碳奈米椎上。其中，碳奈米椎包含奈米鈷顆粒在其頂端，是在電漿化學氣相沉積系統中，利用負350伏特偏壓將其製造在鈷/矽(100)基板上。碳奈米椎通常約200奈米高，底部寬度約100奈米，頂端寬度約為10奈米。奈米鈷顆粒直徑約跟奈米椎頂端直徑相當，可以當作催化劑成長單壁奈米碳管。垂直排列的單壁奈米碳管在成長時間1.5分鐘時約150奈米長，相當於成長速率約6微米每小時。垂直排列單壁奈米碳管的直徑計算為1.2到2.1奈米。當垂直排列的單壁奈米碳管成長在碳奈米椎上時，起始電壓從3.9伏特/微米降至0.7伏特/微米且場發射電流密度在5伏特/微米時增加超過200倍。垂直排列單壁奈米碳管/碳奈米椎的複合結構是很好的場發射體。同時也討論了垂直排列單壁奈米碳管/碳奈米椎場發射的穩定性。

關鍵字

奈米碳管 ； 化學氣相沈積 ； 重力 ； 場發射

並列摘要

Vertically aligned single-walled carbon nanotubes (VA-SWCNTs) can be fabricated on the Co/Si(100) substrate in a gravity-assisted chemical vapor deposition process (CVD). The Co/Si(100) substrate is tilted such that its surface normal points downwards in the CVD process. All the SWCNTs are lined up with the direction of gravity. The length of SWCNTs increases with growth time. A modified vapor-liquid-solid mechanism is proposed to explain the role of gravity in the precipitation of SWCNTs and in the line-up behavior of SWCNTs.
Temperature and CH4/H2 ratio of gas flow rates are the two factors that strongly affect the qualities of VA-SWCNTs in gravity-assisted CVD. The qualities of SWCNTs and other carbon products grown by gravity-assisted CVD were characterized by scanning electron microscopy and Raman spectroscopy. At temperatures between 850 and 900 ºC, SWCNTs of very good quality stand alone on the substrate. At other temperatures, nanofibers or irregular islands of carbon are present on the substrate. The CH4/H2 ratio influences the quality of SWCNTs more abruptly than temperature. At low ratio, no carbon nanotube (CNT) is formed. The window of CH4/H2 ratio for the growth of VA-SWCNTs ranges from 160/80 to 160/40. At a ratio of higher than 160/40, MWCNTs replace SWCNTs and become the dominant product.
The effect of growth time on the shapes of SWCNTs grown on Co/Si(100) using the gravity-assisted CVD method has been investigated. A transition of SWCNTs from vertical (1–3 min) to bent (3–10 min) and to vertical (20 min) shape has been observed at different growth time. High quality vertical SWCNTs appear at a growth time less than 3 min, verified by the clear radial breathing mode (RBM) in the Raman spectra and by the high-resolution images taken by transmission electron microscope (TEM). At a growth time of 5 min, vertical and bent SWCNTs co-appear on the substrate. When a SWCNT is longer than ~1 µm, it is easier to bend by its own weight due to the low value of the effective bending stiffness. At a longer growth time such as 20 min, the coating of amorphous carbon (a-C) on a SWCNT becomes dominant which holds the SWCNT straight and keeps the a-C/SWCNT composite structure in a nearly vertical shape. A mechanism has been proposed to explain the coating of a-C on a SWCNT.
VA-SWCNTs have been fabricated on carbon nanocones (CNCs) in a gravity-assisted CVD process. The CNCs with nano-scale Co particles at the top were first grown on the Co/Si(100) substrate biased at 350 V in a plasma enhanced chemical vapor deposition process. The CNCs typically are ~200 nm in height, and their diameters are ~100 nm near the bottom and ~10 nm at the top. The nanoscale Co particles of ~10 nm in diameter act as catalysts which favor for growing VA-SWCNTs out of CNCs at 850 ºC in the gravity-assisted CVD process. The average length and the growth time of VA-SWCNTs are ~150 nm and 1.5 min, equivalent to a growth rate of ~6 μm/h. The diameters of VA-SWCNTs are estimated to be 1.2–2.1 nm. When VA-SWCNTs are fabricated on CNCs, the turn-on voltage is reduced from 3.9 to 0.7 V/μm and the emission current density at the electric field of 5 V/μm is enhanced by a factor more than 200. The composite VA-SWCNT/CNC structure is potentially an excellent field emitter. The emission stability of the VA-SWCNTs/CNCs field emitter is discussed.

並列關鍵字

carbon nanotube ； chemical vapor deposition ； gravity ； field emission

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