The nucleation and growth of nanoparticles in the gas phase using atmospheric-pressure plasma systems is an important approach to synthesizing novel dimensionally-controlled materials. Here, we investigated the effect of the nanoparticles on a typical type of continuous-flow, substrate-free plasma at atmospheric pressure to understand their potential contribution to electron density changes. A tandem plasma system was set up consisting of one plasma reactor that produced carbonaceous nanoparticles from mixtures of argon and hexane, and another identical plasma reactor where the as-grown particles were injected and non-intrusive electrical and optical measurements were performed. The electron densities obtained from conductivity measurements and a plasma fluid model were found to decrease in the presence of nanoparticles. However, control experiments revealed that the main source of the electron depletion was residual vapor or small molecule products (nanoclusters) and not the particles themselves. These results were validated by constant number Monte Carlo simulations which showed that at the experimentally-measured conditions, the nanoparticles were not of sufficiently high enough concentration to reduce the electron density; however, if residual vapor molecules and clusters are ionizable, they remain in sufficient concentration to deplete electron densities when compared to pristine plasma densities. Our study shows that at atmospheric pressure, because of their typically larger electron density values, particle-producing plasmas are distinct from those at low pressure, and nanoparticle formation does not have the same impact while molecular-scale species may be a more important consideration.
Abuyazid, NH., Chen, X., Mariotti, D., Maguire, P., Hogan, C., & Sankaran, RM. (2020). Understanding the depletion of electrons in dusty plasmas at atmospheric pressure. Plasma Sources Science and Technology, 29(7). https://doi.org/10.1088/1361-6595/ab9cc3