Formation of Carbon-Nanotube Layers on Bulk Nickel: The Effect of Surface Topography Defects

The initial stages of the chemical vapor deposition of carbon onto nickel substrates subjected to different mechanical abrasion treatments are studied by optical, scanning electron, and atomic force microscopies, as well as X-ray diffraction and profilometry. We find that the local accelerated deposition of carbon layers with a highly disordered structure, which occurs at the initial stages of the process, is the underlying cause of nonuniformity in carbon nanotube layers synthesized directly onto a metal surface. Enhanced deposition rates are observed at surface topography defects, including microcavities with a large surface-to-volume ratio. Open microvoids in the subsurface layer left behind by mechanical abrasion treatment also act as foci of the accelerated formation of carbon layers. The cause of this enhanced growth lies in the specific chemistry of the extended pyrolysis of hydrocarbons trapped in microvoids. A high carbon deposition rate results in the formation of carbon-containing materials with a highly disordered structure within such defects. Microscopic asperities and microprotrusions formed as a result of mechanical abrasion are sites of intensive hydrocarbon-induced dusting of the metal. Numerous microslits originate from these defects due to corrosion scattering, which in turn facilitate the emergence of sites for accelerated formation in limited volumes of carbon with a disordered structure.