Dynamic modeling of carbon nanofiber growth in strong electric fields via plasma-enhanced chemical vapor deposition

Plasma enhanced chemical vapor deposition is an important method in the synthesis of carbon nanofibers which have been widely used in many technologies. Previous work devoted to the theoretical modeling of this process focused only on kinetics, i.e., the steady-state growth rate and its dependence on experimental conditions. This paper develops a dynamic model of a single carbon nanofiber grown in the cathode layer of a weakly ionized C₂H₂ glow discharge plasma. The model takes into account all main processes, including chemical kinetics, heat transfer, and the dynamics of electric field distribution. Specifically, the model considers the effects of a strong electric field on nanofiber growth: the field enhanced neutral particle flux and heat flux toward the catalyst and the increased catalyst temperature as a result of the thermal field emission current (along with its accompanying Nottingham effect). Numerical simulation shows that the increased fluxes caused by a strong electric field are unlikely to lead to a substantial acceleration of nanofiber growth. The growth tends to saturate, up to a complete stop, caused by the catalyst heating, which starts around the same time the field enhanced fluxes become significant. This serves as an alternate termination mechanism of nanofiber growth to the commonly-known catalyst poisoning. The competition and transition of the two mechanisms when changing the characteristic time of catalyst poisoning are shown. The results of this work help to improve the physical understanding of nanofiber growth and lay the foundation for further studies on other types of plasma-assisted nanofabrication.