Carbon nanotube diameter tuning using hydrogen amount and temperature on SiO2/Si substrates

Carbon nanotubes (CNTs) were grown on thin iron (Fe) films on SiO2/Si substrates by chemical vapor deposition (CVD) at four different hydrogen (H-2)/methane (CH4) ratios at temperatures ranging from 925 to 1000A degrees C. The effects of temperature and the amount of hydrogen gas on the mean diameter at increasing temperature were examined. We demonstrated that the mean diameter and its distribution depend not only on temperature but also on the H-2 amount. We showed that increasing H-2 amount strongly affects the structure of CNTs, especially at high growth temperature; the mean diameter at 1000A degrees C reduced from about 383 to 34 nm by increasing H-2 amount from 24 to 50 sccm. We observed that at high temperature growth the mean diameter was decreasing very fast initially with increasing H-2 amount suggesting the dominance of H-2 over the growth temperature. A decrease in the slope of diameter vs. H-2 amount with further increment in H-2 amount implied that the temperature was, then, deciding the CNT diameter through catalyst particle coarsening. The statistical analysis presented implies that the H-2 amount has to be adjusted according to the growth temperature for given CH4 amount to keep CNT diameter under control, and the large diameter distributions at high temperature and high H-2 amount can be associated with the large variation in the catalyst particle sizes.