Growth termination mechanism of vertically aligned centimeter long carbon nanotube arrays

The mechanism of abrupt growth termination for growing vertically aligned centimeter long CNT arrays in chemical vapor deposition process was studied. We found that the growth length increases linearly with increasing growth time and the CNT growth terminates abruptly for all the experimental conditions applied. We investigated the change of particle size distribution, particle number density and the catalyst composition on the surface of the substrate with the exposure time and temperature in the reactor. For this study X-ray Photoelectron Spectroscopy (XPS), High Resolution Transmission Electron Microscopy (HRTEM) and Atomic Force Microscopy (AFM) were employed. Focused Ion Beam (FIB) was used for preparation of the STEM samples. The experimental results demonstrated that the formed catalyst nanoparticles undergo compositional and morphological changes during the heating step of the substrate, when the temperature of the reactor is rising from ambient to the growth temperature, and during annealing at the set growth temperature. Most of the changes in the catalyst composition and morphology were related to metal diffusion into neighboring substrate layers during heating up to the growth temperature. Based on the results, a new scenario of abrupt termination mechanism for growing centimeter long CNT arrays using an iron-gadolinium binary catalyst was proposed.