After an introduction on the nanotubes, their properties and their potential applications, this talk will review on the conditions of synthesis, formation and growth mechanisms of carbon SWNTs. Carbon single wall nanotubes (SWNTs) are produced via two main techniques running at either high or medium temperature which all involve a catalyst such as Ni, Co, Fe. The similarities, observed in high resolution electron microscopy, between the samples synthesized from different methods suggest a common growth mechanism. In our model, the formation of the nanotubes proceeds via solvation of carbon into liquid metal particles (about 5 - 15 nm in size), followed by precipitation of carbon excess in the form of nanotubes at temperatures close to their solidification. The conditions of nucleation of nanotubes instead of the formation of a graphene sheet encapsulating the particle are induced by surface instabilities which will be discussed. The role and the nature of the metallic catalyst have been explored by coupling different electron microscopy techniques: high resolution imaging and chemical mapping through energy loss imaging (spectrum-image and image - spectrum modes). The nucleation stage has been studied in detail in the case of the Ni-RE (RE = Y, Ce or La) catalyst and has been shown to be drastically favoured by the addition of a small amount of RE. The role of nickel is clear since it is well known for having the ability to dissolve carbon in the liquid state and to reject it upon solidification, making therefore possible graphitization at very low temperature. The RE has been found to modify the surface catalytic activity of Ni particles by coprecipitating with C at their surface in the form of thin carbide platelets which provide with the nucleation sites of the C-SWNT. This nucleation control is discussed in terms of charge transfer of the RE to C whose effect would be to stabilize pentagons and heptagons necessary to form a SWNT nucleus. This hypothesis will be analysed with the help of tight binding Monte Carlo simulations.

Einladender: Prof. Dr. H. Kohl

Ort: Wilhelm-Klemm-Str. 10, IG I, HS 2

Zeit: Mittwoch, 02. Juli 2003

Kolloquiums-Kaffee ab 16:45 Uhr vor dem Hörsaal

Im Auftrag der Hochschullehrer des Fachbereichs Physik
Prof. Dr. H. Mehrer