Role of transition metal catalysts in single-walled carbon nanotube growth in chemical vapor deposition

We characterize the iron and cobalt catalysts for carbon nanotube growth in chemical vapor deposition (CVD) by using electron microscopy. Nanoparticles of iron and cobalt exhibit a melting point drop in the methane ambient. Nanoparticles after nanotube growth are identified as Fe3C and Co3C for iron and cobalt, respectively. Those results indicate that a eutectic compound of metal and carbon is formed in the methane ambient, resulting in the phase separation into graphite (nanotubes) and metal carbide as the carbon uptake in the catalyst melt increases. This supports the vapor-liquid-solid mechanism for nanotube growth by CVD. Iron or cobalt silicide formation causes the poisoning of the catalysts. However, the coexistence of oxygen due to native oxide on the silicon surface or the metal surface causes formation of a SiO2 base, which can prevent silicidation of iron particles.