Study of the influence of gas temperature on nanoparticle structure in a dusty plasma using CARS*

Dusty plasmas have been widely used in fields such as astrophysics for studying cosmic dust behaviors, materials science for synthesis and modification of nanoparticles, as well as in aerosol science and fusion research. Multiple investigations have focused on the nucleation and growth of carbon nanoparticles in non-thermal plasmas, often using acetylene as a carbon precursor. Furthermore, Woodard et al. 2018 have shown that particle structure can be tuned from amorphous to graphitized by varying RF power levels. This is likely due to plasma- induced heating effects, which affect the particle structural evolution while in the plasma (Mangolini et al. 2009). In a continued effort to understand the non- equilibrium nature of low-temperature plasmas and how it affects nanoparticle characteristics, we employ Coherent Anti-Stoke Raman Spectroscopy (CARS) to measure vibrational and rotational temperatures in a nanoparticle-forming plasma. We use the emission from the N2 second positive molecular band to obtain the temperature, and we perform the measurements at various RF plasma powers, reactor pressures and gas compositions. We have observed that there is a correlation between the nanoparticle properties and the vibrational temperatures, at constant RF plasma power and reactor pressure. Ex-situ Scanning Transmission Electron Microscopy - Energy Dispersive Spectroscopy and Raman Spectroscopy also show increasing graphitic behavior of the nanoparticles with increasing RF plasma power and vibrational temperature, suggesting that plasma-induced nanoparticle heating increases rate of graphitization for small particles (~10nm).