Formation of carbon nanotubes by plasma enhanced chemical vapor deposition: Role of nitrogen and catalyst layer thickness

The growth behavior of carbon nanotubes (CNTs), produced by radio frequency plasma enhanced chemical vapor deposition, is studied here as a function of the ${\mathrm{CH}}_4/{\mathrm{N}}_2$ ratio and Ni catalyst layer thickness. Scanning electron microscopy shows that by employing a suitable deposition (methane) to etching (nitrogen) gas ratio, it is possible to obtain the growth of nanotubes with a limited presence of amorphous carbon on the substrate surface. In particular a progressive transition from random to aligned CNTs is observed when nitrogen is added to the plasma atmosphere. The electronic structure was then investigated by C 1s and valence band photoemission spectroscopy. The results show a shift of the overall spectral to a higher-binding-energy side and a larger density of the states at the Fermi level indicating the formation of metallic aligned tubes with increasing nitrogen fraction in the plasma atmosphere. The electronic structure indicates the insertion of odd-member rings in the graphene network leading to an improvement of the mechanical properties. The thickness of the Ni layer has a strong influence on the CNTs growth showing maximum value for nanotube formation at 20 nm.