A Growth Mechanism for Vertically Aligned Single-Walled Carbon Nanotubes

One of the difficulties in developing practical nanotube-based devices is the large-scale production of aligned carbon nanotubes (CNTs). Due to their novel electronic and thermal properties, single-walled nanotubes (SWNTs) show great potential for use a variety of applications . The alignment of SWNTs has been reported, but only parallel to the substrate in the presence of strong electric or magnetic fields. However, the use of alcohol in a simple chemical vapor deposition (CVD) method developed by our group has shown to produce dense mats of vertically aligned SWNTs. In this study we investigate the growth mechanism leading to this vertical alignment. Vertically aligned SWNTs were grown at 800 oC and 10 torr using an alcohol CVD process. The CVD reaction was catalyzed by Mo/Co bimetal nanoparticles, which were affixed to quartz substrates by a dip-coat method (0.01 wt% Mo/Co acetate). Subsequent annealing in air at 400 oC oxidized the nanoparticles, preventing catalyst agglomeration in the high-temperature CVD environment. Flowing Ar/H2 (3% H2) during preheating of the reaction chamber reduced the catalyst in order to retrieve catalyst activity before the growth stage. When the CVD chamber reached 800 oC, the Ar/H2 flow was stopped, followed by the introduction of ethanol vapor. The reaction time was varied in order to observe samples at different stages of growth. The samples were characterized with a Hitachi S-4700 field-emission scanning electron microscope (FE-SEM). Figure 1 shows a thick mat of aligned SWNTs on a quartz substrate surface. Further characterization was done by micro-Raman spectroscopy (Fig 2). The G-band peak around 1590 cm and the D-band peak near 1350 cm confirm there is carbon, while the radial breathing mode (RBM) near 200 cm is unique to SWNTs. A high ratio between the G and D band peak intensities (here approx. 30:1) indicates high-purity of SWNTs. Additionally, the sharp peak at 180 cm, indicative of vertical alignment, is clearly present, although the origin of this peak is not yet well understood. The location of the RBM peaks is related to the tube diameters, which fall primarily between 1 and 2 nm. The diameter distribution is not affected by growth time. Aligned growth begins with bundling of individual SWNTs, which initially grow randomly. Figure 3a shows this early stage of alignment, occurring after only 30 seconds of growth. At this stage the SWNT mat is uneven and not well aligned, with long bundles extending far above the top of the mat. After 3 minutes of growth, (Fig. 3b), the SWNT mat has become thicker and more uniform, and is now aligned perpendicular to the substrate surface. The mat thickness at this stage is well over 1 μm. Introducing ethanol for 10 minutes (Fig. 3c) results in a mat of aligned SWNTs that is quite thick (more than 2 μm) and well aligned. The thickness continues to increase as the growth time is extended, being more than 3 μm thick after 30 minutes. However, after 1 hour of growth (Fig. 3d), the mat thickness had decreased to approximately 500 nm. The thickness at this stage is uniform across the substrate, but the bundles are not well aligned. The above results can be explained by the following growth mechanism. It is believed that individual SWNTs are produced by a base-growth mechanism, where the reaction between the carbon source flowing through the chamber and the catalyst particles occurs at the substrate surface. The growth direction is