Progress in reactions, momentum transfer, and energy transfer processes for nanoparticles in processing non-thermal plasmas

Abstract

Nanoparticles can form and grow from vapor phase precursors within processing non-thermal plasmas (NTPs). In physical and chemical vapor deposition NTPs, such particles can act as contaminants, and measures need to be taken to either avoid their formation, or to prevent their deposition onto product thin films. NTPs can also be used to intentionally synthesize nanomaterials at industrially scalable levels. In both instances, nanoparticle behavior and the effects nanoparticles may have on the plasma depend upon particle interactions with the surrounding plasma species and neutral gas. Understanding and predicting the behavior of nanoparticles in NTPs requires the development of models for collision limited reactions, momentum transfer, and energy transfer between particles, electron, ions, photons, and neutral gas. As NTPs can be operated at a wide range of pressures, these transport processes occur over a wide range of collisionalities, and are also strongly influenced by both short range and long range potential interactions. The purpose of this review is to compile state-of-the-art knowledge in predicting the behavior of nanoparticles in plasmas with an emphasis on charging, momentum transfer, and energy transfer processes between particles and the surrounding plasma environment. Model development for nanoparticle reactivity and transport in NTPs lies at the interface of dusty plasma physics and aerosol physics, and efforts are made throughout the review to present, intercompare, and blend approaches from these two, often distinct research communities. The review begins by introducing applications and instances where nanoparticles are encountered in NTPs, and subsequently introduces multidimensional nanoparticle population balance modeling. Solution to population balance modeling highlights the need to develop accurate nanoparticle charging rate models, momentum transfer models, and energy transfer models, which are then discussed in successive chapters. Modeling approaches to examine the evolution of particle size distributions in plasmas are discussed, as are the effects of passage through plasma afterglows. Finally, the review concludes with a discussion of nanoparticle voids and waves which can form in NTPs, and an overview of in-situ and extractive measurement techniques to characterize nanoparticle size distributions, number densities, and charge levels. This review is intended both for the aerosol research community as an introduction to the unique aspects of nanoparticle behavior in non-equilibrium environments, and for the plasma community, introducing models arising from predicting the behavior of aerosols, which can be expanded to predict nanoparticle behavior in NTPs.