ROLE OF PLASMA IN THE GROWTH AND FIELD EMISSION PROPERTIES OF 2D GRAPHENE NANOMATERIAL

Abstract

Graphene is the two dimensional monolayer of densely packed carbon atoms arranged hexagonally in the lattice in two dimensions. Vertically aligned graphene sheets are the potential candidate in the applications of the field emission devices, electronic sensors, and electron emission displays etc. There are various techniques available through which graphene can be synthesized, however, plasma based techniques e.g. plasma enhanced chemical vapour deposition (PECVD) are exclusively used to synthesize vertically oriented graphene sheets at low temperatures. The present thesis aims to elucidate the deep insights of the growth mechanism of the graphene sheet in the reactive plasma environment and consequent field emission characteristics from them. In the present work, the multiscale analytical models describing the growth mechanism of the graphene sheet without catalyst and on the catalystsubstrate surface have been developed separately. The model dealing with the growth of graphene in the plasma without catalyst incorporates the charging of the graphene sheet in the plasma, particle and energy balance of the plasma species (charged and neutral species), and energy balance of the graphene sheet. The simultaneously coupled model equations have been solved for typical glow discharge plasma parameters. It is found that plasma parameters (number densities and temperatures of electrons and ions), presence of dopant species (nitrogen species), and presence of negatively charged species significantly affect the growth characteristics of the graphene sheet, and consequently affects the field emission properties of the graphene sheet. The multistage model dealing with the plasma-assisted catalyzed growth of graphene sheet mainly incorporates two sub-analytical models. One is the plasma sheath model that accounts the excitation of the gaseous sources due to applied plasma power and kinetics of the plasma species and the other one is the surface deposition model that incorporates the adsorption and dissociation of carbon bearing species over the catalyst nanoislands active surface (free surface available for the adsorption of the plasma species) to generates building species

Neha Gupta, Delhi Technological University, Delhi, India vi

(carbon species) via various surface processes, diffusion of building species over the catalyst nanoislands surface, formation of carbon clusters, nucleation and growth of graphene island, and vertical growth of graphene sheet. The solutions of the model equations have been carried out for experimentally determined initial conditions. It is found that the plasma parameters, doping elements (nitrogen and boron), type of gaseous sources, and plasma operating parameters have great influence on the growth characteristics of the graphene sheet and thus, affect the field emission properties of the graphene sheet. In addition, the growth of graphene sheet over the CNT template is also studied. It is considered that graphene sheet growth over the CNT surface is defect guided and density profiles of the defects created over the CNT surface can be controlled by suitably varying the plasma operating conditions and henceforth affect the growth characteristics (i.e., number density and dimensions) of the graphene sheets grown over the CNT surface. A good comparison of the obtained theoretical results with the available experimental observations confirms the adequacy of the present model. The present work of the thesis can be extended to fabricate the thin and long vertically oriented graphene sheets for their potential applications in the field emitters as the field emission characteristics of the graphene sheet depend on its geometrical characteristics, i.e., height and thickness. Moreover, the present work can also be extended to investigate the growth of other carbon based nanostructures.