Plasma-assisted Synthesis of Carbon Nanomaterial Studied by Spatially-resolved Laser-induced Fluorescence and Optical Emission Spectroscopy

Crystalline nanographite and carbon nanodots have been of great interest for their applicability in photocatalysis, energy conversion, optoelectronics, and biosensing. In this work, we report a scalable synthesis of sp² carbon nanomaterial consisting of crystalline carbon nanodots and nanographite. This chemical vapor deposition (CVD) approach takes a nonthermal CH₄/Ar capacitively coupled plasma actuated by a 13.56 MHz source in a flow-through tubular reactor. Raman spectroscopy confirmed sp² hybridization with the signature D and G bands. TEM image analysis indicated the average particle size to be ~6 nm.



High-resolution optical emission spectroscopy (OES) was used to estimate a gas temperature of ~1200 K. Spatially-resolved planar laser-induced fluorescence (PLIF) was used to capture C₂ species distribution at different locations along the length of the tubular reactor for discharges from different CH₄/Ar ratios. For higher CH₄/Ar ratios, the yield of the sample (collected at the bottom of the tube) is higher and the corresponding C₂ density in the plasma is lower. C₂ density is also found to decrease from 10¹⁸ m^-3 to 10¹⁶ m^-3 down the tube. This can be due to a higher conversion rate of C₂ to nanographite and hence a lower available amount of C₂ to be measured in the plasma. These diagnostics allow for understanding how C_x species evolve from the discharge to form ~6 nm nanoparticles thereby providing for a comprehensive description of the spatiotemporal evolution of graphite nanoparticle growth.