在MPCVD法中氫氣/甲烷/氮氣流量對觸媒前驅物輔助成長碳奈米結構及性質的影響

Abstract

在本研究中，CoCrOx 和 CoCrPtOx 被使用來當作觸媒前驅物藉由微波電漿化學氣相沉積系統（MPCVD）合成小管徑碳奈米結構，並在不同的H2/CH4/N2氣體流量下，探討重要製程參數對碳奈米管生成及性質的影響。本實驗先利用射頻物理氣相沉積（RFPVD）在矽晶片上以Ar/O2 的比例為10/30（sccm/sccm）氣氛下，使用CoCrPt合金（Co 57.08 %, Cr 10.97 %, Pt 31.95 %）當靶材來濺鍍合金觸媒，再利用微波電漿化學氣相沉積系統進行氫電漿或氫-氮電漿前處理，取得均勻分布的奈米級觸媒顆粒，接著以相同的微波電漿化學氣相沉積系統合成不同之碳奈米結構。本實驗藉由聚焦離子束（FIB）、場發射-掃描式電子顯微技術（FE-SEM）、穿透式電子顯微技術（TEM）、高解析度穿透式電子顯微技術（HRTEM）、拉曼光譜技術（Raman）、X光電子技術（XPS）及場發射I-V量測技術等來量測不同的製程下之奈米微結構及其性質。由實驗結果可得到下列結論。 在探討碳奈米結構沉積的尺寸，可由文獻報導上得知，在不通入氮氣的情況下藉由微波電漿化學氣相沉積系統以CoCrPtOx觸媒輔助沉積合成管徑約為2.0~3.0 nm之碳奈米結構，其結果接近於在此研究中以兩倍的總氣體流量合成之管徑2.1 nm。在氮氣流量對碳奈米管直徑及管長的曲線顯示，其碳奈米管之成長可大略分為三個階段。在第一階段，側向成長速度遠大於管長垂直成長速度。在第二階段，側向成長停止，管長垂直成長速度不斷加速，導致碳奈米管高度急遽上升至45 □m的最大值。最後在第三階段時，側向成長速度又開始持續增加，而垂直成長速度趨於緩慢。其中氮原子之主要效用基本上是由於其原子量較氫原子大，所以具有較高之轟擊能量。因此，當氮氣流量較低時，由於離子轟擊的效應會使得碳奈米管管長成長速度下降並造成碳奈米管直徑成長速度的大幅提升。而碳奈米管直徑增加的部份原因也是因為觸媒顆粒的粒徑尺寸變大所影響。在氮-氫電漿的前處理實驗中已證實了，氮原子會造成觸媒顆粒粒徑尺寸的增加。在較高的氮氣流量下，增加的離子轟擊也會導致觸媒表面較乾淨且溫度較高，使得成長速度大幅提升。但在高氮氣流量下（> 10 sccm），由於氮氣對於反應氣體所產生的稀釋效應會達到臨界點造成碳物種的供給速率大幅下降。而有趣的是，在上述所提及的第二階段中，反應氣體內的氮原子可能會促使雙壁碳奈米管（DWNTs）的生成。 考慮到沉積時間對碳奈米管管長的影響，由管長對沉積時間的曲線圖顯示管長高度有最大值現象。這種現象是在反應氣體中有氮氣時所發現的。這可能的解釋原因是由於碳原子擴散至觸媒之難度持續增加和持續的氮原子轟擊效應。在總氣體流量數較高腔體中會使得碳奈米管長的成長速率較為快速。 拉曼光譜技術顯示出，當氮氣的氣體流量由0 sccm增加到20 sccm，其拉曼峰值會往較高波數的位置偏移，且IG/ID比值由14.6降至1.1。換言之，碳奈米管的品質由於氮氣的出現而產生了惡化。這亦可以從X射線光子能譜技術分析結果得到證實。氮原子的影響為取代石墨網狀結構中碳原子的位置使一個碳原子和其他物種的原子鍵結或是破壞兩個碳的六元環結構。 場發射I-V量測技術則顯示出，在固定之CH4/H2 比（=10/100 sccm/sccm），氮氣流量從0 sccm增加至20 sccm，起始電場強度會從5.2增強至9.7 V/□m （電流密度為0.01 mA/cm2），而臨界電流密度從10.9變化至4.6 mA/ cm2。

In this work, the CoCrOx and CoCrPtOx catalyst precursors were used to synthesize the small sized carbon nanostructures by the microwave plasma chemical vapor deposition (MPCVD) under different H2/CH4/N2 gas flow rates to examine effects of the important process parameters on their formation and properties. The catalyst precursors were first sputtered on Si wafer by physical vapor deposition (RFPVD) with CoCrPt alloy (Co 57.08 %, Cr 10.97 %, Pt 31.95 %) as target under Ar/O2 (=10/30 sccm/sccm) atmosphere, and then followed by H- or H/N-plasma pretreatment in MPCVD system to obtain the well-distributed catalyst nanoparticles. The pretreated specimens were then deposited in the same MPCVD to synthesize different carbon nanostructures. The microstructures and properties at each processing step were characterized by FIB (focused ion beam), FE-SEM, TEM, HRTEM, Raman spectroscopy, XPS, and field emission I-V measurements. From the experimental results, the following conclusions can be drawn. Regarding the sizes of the deposited carbon nanostructures, the reported sizes in the literature for the CoCrPtOx-assisted nanostructures without adding N2 by MPCVD are ranging from 2.0 ~ 3.0 nm in diameter, which is close to the value of 2.1 nm in this work under two times higher in total flow rate. The curves to show effects of N2 flow rate on tube diameter and length indicate that the growth of the tubes can be roughly divided into three stages. In the first stage, the sidewise growth is much greater than the lengthwise vertical growth. In the second stage, the sidewise growth stops and the lengthwise growth rate is greatly enhanced, resulting a great increase in tube length up to 45 □m. In the third stage, the sidewise growth continues, and the rate of tube length growth decreases. Effect of N atoms is essentially to increase the ion bombardment energy due to a heavier mass of N than H atoms. Therefore, at lower flow rate, the enhanced bombardment causes a decrease in growth rate in tube length and a great rate increase in its diameter. An increase in diameter is partially due to an increase in catalyst nanoparticles. Such an increase in particle size due to N atom has been demonstrated in pretreatment experiments. At higher N2 flow rates, the enhanced bombardment may also keep the catalyst surface in a clean state and higher temperature, causing a great increase in growth rate. At higher N2 flow rates (> 10 sccm), the dilute effect of N2 on the reaction gases reaches a critical point, causing too much decrease in supply rate of carbon species. It is interesting to note that addition of N atom in the second stage may promote double-walled CNTs (DWNTs) formation. Regarding the effects of deposition time on the tube length, the curves of tube length versus deposition time show a maximum tube length. This phenomenon is found during present of N2 in the reaction gases. This may be explained by a continuous increase in difficult to diffuse carbon into the catalyst and continuous bombardment effect of N atom. A higher total flow rate gives rise to a much greater growth rate in tube length. The results of Raman spectroscopy show that as the gas flow rate of N2 increases from 0 to 20 sccm, the Raman peaks can shift to a higher wave number and IG/ID ratios changes from 14.6 to 1.1. In other words, the quality of CNTs deteriorates by presence of nitrogen. This is in agreement with XPS analyses. Effect of nitrogen is to replace carbon on the graphite network, inducing one carbon atom to be bonded to other species or destroying two six-member rings. Field emission measurements indicate that the N2 flow rate changes from 0 to 20 sccm at constant CH4/H2 = 10/100 sccm/sccm, the turn-on field strength will vary from 5.2 to 9.7 V/□m (for current density 0.01 mA/cm2) and critical current density from 10.9 to 4.6 mA/ cm2.