Growth of carbon nanotubes: effect of Fe diffusion and oxidation

Abstract The diffusion and surface oxidation rates of Fe deposited on Si and barrier layers of Al/SiO2 and Al2O3/SiO2 have been comparatively studied and correlated with the growth of carbon nanotubes (CNTs). Initially, Fe/Si, Fe/Al/SiO2/Si and Fe/Al2O3/SiO2/Si samples were subjected to thermal chemical vapour deposition (CVD) at ∼650°C for ∼30 min to grow the CNTs. Scanning electron microscopy analysis showed that the height of the CNTs on the Fe/Al2O3/SiO2/Si samples was relatively high (∼9.5–11 µm), as compared with the other samples. To investigate this, a few as-prepared samples were thermally annealed at ∼650°C for ∼30 min and characterized by dynamic secondary ion mass spectroscopy (D-SIMS) and X-ray photoelectron spectroscopy (XPS). The D-SIMS results showed that the diffusion depth, x Fe, and magnitude of the diffusivity, D Fe, of the Fe atoms are highest for the Fe/Si sample. This is attributed to vacancy-mediated migration, which leads to the formation of unstable, non-stoichiometric Fe–Si and Fe–O–Si phases. However, for the Fe/Al2O3/SiO2/Si samples, the magnitudes of x Fe and D Fe are found to be the lowest, which indicates steric hindrance to Fe by the Al2O3 layers. The XPS analysis revealed that the surface metallic state, after annealing, is almost unaffected for the Fe/Al2O3/SiO2/Si samples, whereas the majority of the Fe precipitate was observed to be oxidized in the case of the other samples. Acknowledgments This work is supported by the Korea Science and Engineering Foundation (KOSEF) via the Center for Nanotubes and Nanostructured Composites (CNNC) at Sungkyunkwan University. The financial assistance (grant no. RTI04–03–04) provided to the regional technology innovation programme by the Ministry of Commerce, Industry and Energy (MOCIE) is gratefully acknowledged.