Methods and processes for synthesizing high quality carbon single-walled nanotubes (SWNTs) are provided. A carbon precursor gas at reduced concentration (pressure) is contacted with a catalyst deposited on a support and at temperature about 10° C. above the SWNT synthesis onset temperature, but below the thermal decomposition temperature of the carbon precursor gas for given growth conditions. The concentration (pressure) of the carbon precursor gas can be controlled by reducing the total pressure of the gas, or by diluting with an inert carrier gas, or both. The methods produce SWNTs with the ratio of G-band to D-band in Raman spectra (IG:ID) of about 5 to about 200.