Optical emission spectroscopic study of Ar/H₂/CH₄ plasma during the production of graphene nano-flakes by induction plasma synthesis

Graphene nano-flakes using CH₄ precursor were synthesized in a radio frequency inductively coupled plasma reactor with in-situ investigation of Ar/H₂/CH₄ plasma by optical emission spectroscopy at fixed H₂ and Ar flow rates of 4 and 75 slpm, respectively, and at different plate powers (12 to 18 kW), pressures (400 to 700 mbar) and CH₄ flow rates (0.3 to 2 slpm). Emissions from C₂ Swan band, C₃, CH and H₂ are observed in the optical emission spectra of Ar/H₂/CH₄ plasma. Plasma temperature estimated analyzing the C₂ Swan band emission intensities is found to be decreased with increasing pressure and decreasing plate power. The decreasing plasma temperature gives rise to increase in production rate due to increase in condensation process. The production rate is observed to be increased from 0 to 0.3 g/h at 18 kW and from 0 to 1 g/h at 15 kW with increase in pressure from 400 to 700 mbar at fixed CH₄ flow rate of 0.7 slpm. Broad band continuum emission appears in the emission spectra at specific growth conditions in which the formation of vapor phase nanoparticles due to condensation of supersaturated vapor is facilitated. The production rate at 12 kW, 700 mbar, and 0.7 slpm of CH₄ flow rate is found to be 1.7 g/h which is more than that at 15 and 18 kW. Thus, the broadband continuum emission dominates the optical emission spectra at 12 kW due to lower temperature and higher production rate, and is attributed to the emission from suspended nanoparticles formed in vapor phase. The synthesized nanoparticles exhibit flake like structures having average length and width about 200 and 100 nm, respectively, irrespective of the growth conditions. Nano-flakes have thickness between 3.7 to 7.5 nm and are composed of 11 to 22 graphene layers depending on the growth conditions. The intensity ratio (I_D/I_G) of D and G band observed in the Raman spectra is less than 0.33 which indicates good quality of the synthesized graphene nano-flakes.