标题: 微波电浆化学气相沉积法合成准直性碳奈米管及碳奈米尖锥之奈米制程与场发射性质
Nanofabrication and field emission of the well-aligned carbon nanotubes and carbon nanocones synthesized by MPCVD
作者: 林奕同
Yi Tung Lin
郭正次
Cheng Tze Kuo
材料科学与工程学系
关键字: 碳奈米管;碳奈米尖锥;微波电浆化学气相沉积;场发射;Carbon nanotubes;Carbon nanocones;Microwave Plasma Chemical Vapor Deposition;Field emission
公开日期: 2003
摘要: 近年来以碳基奈米结构材料所制成的场发射元件上获得举世的注目。特别是碳奈米管(CNTs)及碳奈米尖锥(CNCs),由于它们具备高的深宽比形貌以及奈米级的发射极半径。 影响碳奈米结构场发射性质的因素包括其结构尺寸大小、排列方向、表面形貌、碳的键结、管束成长密度和与基材的附着性..等等,而这些因素都与制程参数有关,因此如何利用制程参数来操控奈米结构是众多重要的课题之一。 本研究以微波电浆化学气相沉积(MPCVD)法合成具准直性碳奈米管与碳奈米尖锥在矽基材上,使用10 nm的钴膜当作触媒,H2、NH3、Ar、C2H2和 CH4当作气体原料。 在每一阶段制程的奈米结构与性质利用扫描式电子显微镜(SEM)、穿透式电子显微镜(TEM)、电子绕射(ED)、拉曼光谱仪(Raman spectroscopy)、欧杰电子能谱仪(AES) 和 以I-V 量测仪进行分析。
氢电浆前处理结果显示高的基材温度相依于大的气氛的压力及微波功率,在条件为9 Torr 的氢气压力及400 W 的微波功率之氢电浆前处理下,有适当的触媒平均粒径和数目密度。 在不同的气体源与外加负偏压所成长出的碳管具准直性,可能是由于电浆自我偏压、负偏压增强电浆鞘能量或是较高的管束密度所造成的。 对于成长时间影响碳管的准直性结果显示,成长时间太长会因为不同触媒毒化速率的不同而造成卷曲状碳管的形成。 在场发射性质方面以无偏压辅助成长10 分钟的准直碳管表现最佳,其开启电场 (Eto) ~ 4.4 V/□m; 起始电场 (Eth) ~ 8.26 V/□m;场发射增强因子□□~□4096,而在10 V/□m 的电场下可达88.7 mA/cm2。但经过多次循环的场发射量测后碳管会从矽基材剥落,推测原因为碳管与基材附着性不佳所以无法承受电场的破坏。 根据不同施加负偏压及碳源浓度影响下成长碳奈米尖锥结果显示,碳奈米尖锥的合成必须在偏压大于 -150 V 且H2/CH4需小于80/5 (sccm/sccm)。准直碳奈米尖锥合成温度皆低于650℃。 尖锥的高度随着负偏压和H2/CH4比例上升而增高。相对的,尖锥的底部宽度及顶端角度随着负偏压和H2/CH4 比例的上升而减小。从SEM结果推测碳奈米尖锥的形貌可能是在电浆环境中碳沉积与离子蚀刻互相竞争所产生的结果。拉曼光谱、AES、 TEM 及ED 分析显示其为一个多晶石墨层、非晶质矽和非晶质碳混合物的尖锥,且在尖锥顶部及底部都有钴触媒的存在。在场发射性质上碳奈米尖锥展显出较碳管优异的性质,其开启电场 (Eto) ~ 5.0 V/□m;起始电场 (Eth) ~ 6.99 V/□m;场发射增强因子□□□□4993,而在10 V/□m 的电场下更可达173.42 mA/cm2。碳奈米尖锥可以承受高电场长时间的场发射量测而且没有电流衰退的情形,显示出碳奈米尖锥对基材的附着性比碳奈米管优异。
Applications of the carbon nanostructured materials to fabricate the field emission (FE) devices are gained greater attention in recent years in the world, especially, the carbon nanotubes (CNTs) and the carbon nanocones (CNCs) due to their high aspect ratios morphologies and nanoscale field emitter radius. There are many factors governing the FE properties of carbon nanostructures, such as size, orientation, morphologies, bonding structure, tube number density and adhesion with the substrate, depending on the process parameters. Therefore, way to manipulate the nanostructures is one of the important topics. In this study, processes were developed to synthesize the well-aligned CNTs and CNCs on Si substrate by microwave plasma chemical vapor deposition (MPCVD), using 10 nm Co film as a catalyst and H2, NH3, Ar, C2H2 and CH4 as source gases. The carbon nanostructure and properties at each processing step were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron diffraction (ED), Raman spectroscopy, Auger electron spectroscopy (AES), and I-V measurements.

On the pretreatment process, the results indicate the greater substrate temperature are depending on the greater gas pressure and microwave power. Under 9 Torr H2 pressure and 400 W microwave power plasma pretreatment condition shows appropriate number density and small average catalyst particle sizes. The CNTs show the aligned orientation which are synthesized by different gas sources and applied negative bias, it may be due to plasma self-bias potential, applied negative bias enhance plasma sheath potential or greater tube number density induced. The result of CNTs synthesized under different growth time reveals wave-like CNTs easily produced on long time growth, it may be induced by each CNT having different catalyst poisoned rate. The well-aligned CNTs which were grown under 10 min without applied bias shows the best FE properties, It’s Eto ~ 4.4 V/□m, Eth ~ 8.26 V/□m, field enhancement factor □□□□4096, and J ~ 88.7 mA/cm2 at 10 V/□m. However, the CNTs after several cycles of I-V measurement will striped from the substrate. It may be due to the bad adhesion between the CNTs and substrate.

The results of different applied negative bias and H2/CH4 ratio on CNCs growth show the well-aligned CNCs preferred to synthesize under above -150 V bias and lower H2/CH4 ratio (< 80/5 sccm/sccm). It is significant to note that the growth temperature of well-aligned CNCs is below 650℃. The Height of CNCs will increase with an increase of applied negative bias and H2/CH4 ratio. In contrary, the bottom diameter and apex angle of CNCs decrease with a decrease of applied negative bias and H2/CH4 ratio. The morphology of CNCs may be produced by the competition between the carbon deposition and ion etching under plasma bulk. The results of Raman spectrum, AES, TEM, and ED analysis indicated the CNC is the cone-shaped mixture of polycrystalline graphite, amorphous Si and amorphous carbon, where the existence of Co catalysts are both in the tip and bottom of the CNC. The CNCs exhibit better FE properties than CNTs, its Eto ~ 5.0 V/□m, Eth ~ 6.99 V/□m, field enhancement factor □□□□4993, and ~ 173.42 mA/cm2 at 10 V/□m. The CNCs can bear long time measurement under high electric field and with stable emission current. It indicates that the CNCs have the better adhesion with the substrate than CNTs.
URI: http://140.113.39.130/cdrfb3/record/nctu/#GT009118510
http://hdl.handle.net/11536/50791
显示于类别:Thesis


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