An approach to the structural diversity of aligned grown multi-walled carbon nanotubes on catalyst layers

Layers of aligned multi-walled carbon nanotubes were deposited on thin Co layers by bias assisted microwave CVD in the temperature range 973-1273 K and characterized with respect to their structure. The tubular and the bamboo-like structure occurred simultaneously in the entire temperature range. Mechanisms of the structure formation are discussed as well as open questions are addressed. In order to figure out the reasons for the observed structural instabilities, the catalyst particle size distribution on the substrate and in the nanotube tips was investigated. The particle size distribution in the nanotube tips was found to be time dependent. Clearing up this, it could be shown that the different growth rates of the tubular and bamboo-like grown tubes are the cause for this effect. Furthermore, it is stated and experimentally proved that the metastable state of the catalyst particles is responsible for the limited length growth of the tubes and for structure changes occurring during the growth. (C) 2004 Elsevier Ltd. All rights reserved.