Synthesis of fullerenes in a high-frequency arc plasma under elevated helium pressure

We show that in a carbon–helium high-frequency (66 kHz) arc discharge the fullerene yield and composition can be controlled by changing the helium pressure. By emission spectroscopy we could derive the excitation temperature within the arc. We found that the radial temperature gradient of the arc plasma increases with increasing pressure. We further found that with increasing pressure the yield of fullerenes reaches a maximum of 12.6 wt.% at 0.12 MPa pressure and in the range from 0.16 to 0.38 MPa the fullerene content in the carbon soot stays within the limits of 7.2–7.0 wt.%. The content of С70 and higher fullerenes relative to the total amount of extractable fullerenes increases with pressure. Our results suggest that in the carbon–helium plasma fullerenes are mainly formed in a top-down process by the decay and annealing of large, hot precursor carbon molecules.