ROLE OF PLASMA IN THE NUCLEATION AND GROWTH OF CARBON NANOFIBERS

Abstract

Carbon nanofibers (CNFs) are the one dimensional carbon nanostructures in which graphene layers are inclined at some non-zero angle with respect to the growth axis. Vertically aligned carbon nanofibers (VACNFs) are the potential candidate in the applications of field emission devices, electronic sensors, and electron emission displays, etc. There are many synthesis routes available, however, plasma enhanced chemical vapour deposition (PECVD) are exclusively used to synthesize VACNFs at low temperatures. The present thesis aims to elucidate the deep insights of the nucleation and growth mechanism of the VACNFs in the reactive plasma environment through an analytical model developed which mainly incorporates two sub-models. One is the plasma sheath model that accounts the excitation, dissociation and ionization of the gaseous sources due to applied plasma power and kinetics of the plasma species (electrons, positively charged ions, and neutral species); and the other one is the surface deposition model that incorporates the adsorption and dissociation of carbon bearing species (ionic and neutral species of hydrocarbons) over the catalyst nanoparticle active surface (free surface available for the adsorption of the plasma species) to generates building units (carbon species) via various surface processes (surface reaction), surface and bulk diffusion of carbon species over the catalyst nanoparticle surface, precipitation of the carbon species in the form of graphitic shells around the rear of the catalyst, etching of the walls of the CNFs, and vertical growth of the CNFs under the influence of the electric field induced in the plasma sheath. The solutions of the model equations have been carried out for experimentally determined initial conditions and glow discharge parameters. It is found that the plasma parameters, i.e., number density of ions and neutral species in the bulk plasma, insertion of nitrogen as the doping element, insertion of water and oxygen as the etchant gases, and plasma process parameters have great influence on the growth characteristics of the VACNFs. In addition, the plasma pre-treatment of the thin metal catalyst film to generate catalyst nanoparticles have also been studied and it is found that the plasma

Abstract

vi Ravi Gupta, Department of Applied Physics, Delhi Technological University, Delhi, India parameters, plasma compositions and thickness of the metal catalyst film significantly affect the size and the surface concentration of the catalyst nanoparticles which ultimately affect the growth characteristics of the VACNFs. The good comparisons of the analytical results of the present thesis with the available experimental observations confirm the adequacy of the present model. The present work of the thesis can be extended to fabricate the thin and long VACNFs for their potential applications in the field emitters as the field emission characteristics of the VACNFs depend on its geometrical characteristics, i.e., height and tip radius. Moreover, the present work can also be extended to investigate the growth of other carbon based nanostructures.