Generation of negative-charge carriers in the gas phase and their contribution to the growth of carbon nanotubes during hot-filament chemical vapor deposition

Experiments reported here suggest that charged nanoparticles are generated during the synthesis of carbon nanotubes (CNTs) by hot-filament chemical vapor deposition, and it is these which interact with catalytic metal particles to produce CNTs. During the deposition process, at a filament temperature of 1900 °C, the presence of negative-charge carriers in the form of an electric current measuring ∼2 μA cm−2 is detected at the substrate position. A bias applied to the stainless steel substrate affects both the growth rate and the morphology of the deposited carbon. The masses of CNTs or carbon nanoparticles (CNPs) grown under an applied bias of +25 V are found to be two and ten times larger than those grown under applied biases of 0 and −200 V, respectively. The CNPs in the gas phase have been observed using transmission electron microscopy (TEM) by capturing them through differential pumping through the orifice onto TEM grids placed in a second chamber, in which the vacuum is higher than in the reactor.