The effect of temperature on the growth of single-walled carbon nanotubes (SWCNTs) was investigated over the range of 725–900 °C. A cold-wall reactor consisting of a heated stage (on which the substrate for SWCNT growth (Al/Fe/Mo) was placed) and a showerhead (from which C2H2 was introduced vertically into the reactor) was used for the growth. The heating was found to play two roles: (1) it generated complex hydrocarbon radicals during the growth process, as well as (2) promoting catalytic nanoparticles on the substrate during the annealing process. The optimum temperature for the highest SWCNT yield was found to be ∼860 °C. For the first time, in situ mass spectroscopy was used to identify the growth precursors generated from thermal pyrolysis of C2H2 within this temperature range. The peak of the radicals found (C6H9, C5H9 and C6H13) and the highest catalyst support particle density (Fe catalyst supported on AlxOy) was correlated to the maximum yield of single walled carbon nanotubes at the optimum growth temperature of ∼860 °C. Bottom gate SWCNT-FETs (single-walled carbon nanotube based field effect transistors) were fabricated showing a high transconductance of ∼0.12 µS and on/off ratio of ∼105 which are both comparable to other state-of-the-art SWCNT-FET.