Induction Plasma Synthesis of Graphene Nano-flakes with In Situ Investigation of Ar–H2–CH4 Plasma by Optical Emission Spectroscopy

Abstract

Synthesis process of graphene nano-flakes produced in radio-frequency inductively coupled plasma system by axially injecting CH4 into Ar–H2 plasma is investigated at different process conditions such as plate power, pressure and CH4 flow rate. Optical emission characteristics of Ar, Ar–H2, Ar–H2–CH4 and Ar–H2–C2H2 plasmas are sequentially studied to understand different dissociation channels of CH4. Graphene nano-flakes are formed by condensation of C2 species which are generated owing to CH4 decomposition caused by electron impact, dehydrogenation and energy transfer from excited state Ar atoms to C2H2 and/or C2H produced during dissociation. Maximum production rate achieved is ~ 10 g/h at plate power of 22 kW, pressure of 70 kPa and CH4 flow rate of 1.5 slpm at which intensity ratio of D and G band of Raman spectrum is 0.25 in spite of having contribution from edges of nano-flakes to D band intensity indicating good quality. Effect of decrease in cooling rate at the location of nucleation at higher CH4 flow rates ≥ 2 slpm increases average number of graphene layers (#L) which is however reduced with increase in plate power i.e. from #L of 22 to 13 with increase in plate power from 15 to 22 kW at pressure of 70 kPa and CH4 flow rate of 2 slpm.