Growth of vertically aligned bamboo-like carbon nanotubes from ammonia/methane precursors using a platinum catalyst
Vertically aligned multi-walled carbon nanotubes have been synthesized by microwave plasma enhanced chemical vapor deposition using ammonia/methane source gas mixtures and a platinum thin film catalyst on thermally oxidized silicon substrates. The SiO2 layer served as a diffusion barrier to reduce PtSi formation and aide the formation of well dispersed catalytic nanoparticles. Nevertheless, some degree of substrate–catalyst interaction occurred even with this diffusion barrier present. Carbon nanotube growth using a relatively thick catalyst film was either inhibited or prevented altogether when increasing the duration of plasma pretreatment or by moving the substrate closer to the center of the plasma. This was primarily attributed to increased substrate–catalyst interactions resulting in catalyst poisoning. Decreasing the plasma pretreatment time or moving the substrate away from the center of the plasma improved the growth, increasing the length of the nanotubes. Varying the catalyst film thickness enabled control of the diameter and density of the carbon nanotubes. Transmission electron microscopy revealed a bamboo-like structure with the orientation of internal compartments consistent with a base-growth mode.