Abstract
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 (ID/IG) 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.
Highlights
Synthesis of nanoparticles has been gained significant attention due to remarkably different physical and chemical properties of nanoparticles from their bulk counterpart in spite of possessing the same constituents [1, 2]
Graphene nano-flakes are synthesized by using Radio frequency (RF) inductively coupled plasma (ICP) synthesis system at different growth conditions with in situ investigation of Ar/H2/CH4 plasma by optical emission spectroscopy
The molecular vibrational temperature, a measure of plasma temperature, estimated by analyzing the C2 Swan band emission intensities is found to be decreased with increase in pressure and decrease in plate power
Summary
Synthesis of nanoparticles has been gained significant attention due to remarkably different physical and chemical properties of nanoparticles from their bulk counterpart in spite of possessing the same constituents [1, 2]. Radio frequency (RF) inductively coupled plasma (ICP) synthesis technique is an alternative plasma route for the synthesis of nanoparticles which is better than conventional plasma due to a large volume, high-temperature plasma zone, operating under different atmospheres, permitting synthesis of a great variety of nanoparticles, higher production rate with lower processing cost [5]. It comprises of an induction plasma torch coupled to a RF power supply, a vacuum synthesis chamber, a filtration unit, a gas manifold, a flow control system and a precursor injector. Optical emission spectroscopy (OES) experiment was conducted at different growth conditions using a fiber coupled spectrometer (Ocean Optics) and collecting optics through a view port located at distance of 115 mm from the exhaust of the ICP torch. The Raman scattered signal was recorded using a charge-coupled device (CCD) attached to a spectrometer in the back-scattered geometry
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