Radio-frequency plasma system to selectively grow vertical field-aligned carbon nanofibers from a solid carbon source

Vertical field-aligned carbon nanofibers (CNFs), exhibiting a “herring-bone” and a “bamboo-like” structure, were grown at 560 °C using nickel (Ni) as a catalyst and an innovative radio-frequency (RF) plasma-enhanced chemical vapor deposition system. To limit the carbon supply, thereby providing a highly selective growth process with no detrimental parasitic carbon layer formation, a solid graphite sample-holder, RF-polarized, was used as a single carbon source in combination with a pure H2 feed gas. The morphology and the dimensions of the obtained CNFs are investigated with respect to the growth duration. High-resolution transmission electron microscopy analyses typically display a Ni particle at the fiber tip, but this particle is not encapsulated by graphene layers, allowing its easy removal with a chemical acid treatment. Moreover, the particle's upper surface consists of a peculiar polycrystalline area, assumed to be essential for the growth mechanisms and possibly made of nickel carbide. The crucial role played by the average vertical electric field, naturally created in the plasma sheath and responsible for sample-holder and substrate bombardment by cationic species, is highlighted to understand the growth mechanisms of these as-grown oriented CNFs and their progressive base destruction by etching phenomena.