Formation Mechanism of Pentagonal Defects and Bamboo-Like Structures in Carbon Nanotube Growth Mediated by Surface Diffusion

Abstract

The formation of pentagonal defects and bamboo-like structures in carbon nanotube (NT) growth is viewed within the framework of the surface diffusion (SD) growth model. NT open edge stability is considered as a competition between: (i) hexagon formation which depends on the rate at which C units are fed by SD to the edge, and (ii) thermally activated pentagon formation, which causes inward bending of the edge, resulting in end closure and growth termination. The closure of the growing NT is shown to happen whenever a change in the conditions (temperature, C vapor pressure or surface area from which the open end is fed) decreases the SD flux, and the time for the hexagon formation on the edge becomes larger than that of the pentagon formation. An analysis of carbon NT forest growth by chemical vapor deposition on prefabricated metal nanoparticle arrays suggests, that C species are unable to penetrate to the forest bottom whenever the mean free path in gas is much larger than the distance between NTs; instead they collide with NT walls, chemisorbing within the top few microns, diffuse along the NT surface, and feed the growth at NT tips. Wherever a metal nanoparticle is present, at the substrate or on the NT tip, in the postnucleation stage its role in feeding NT growth by C dissolution and bulk diffusion is negligibly small in comparison with the SD of C species on the NT surface. Bulk diffusion of C through the nanoparticle defines the characteristic times of (a) C penetration to the nanoparticle base and (b) surface saturation with C, and plays a major role in the selection of the initial mode of NT nucleation and growth, leading to the formation of straight wall NTs or bamboo-like NT structures.