Discharge Constriction during Si Nanoparticlel Synthesis in an RF Atmospheric-Pressure LTP

Low-temperature plasma (LTP) reactors have become popular for synthesizing semiconductor nanoparticles. In recent years, there has been significant interest in how crystalline nanoparticles can form in these reactors. LTPs offer highly energetic electrons, while maintaining overall low temperatures for the ions and neutral species. Some groups have measured the plasma properties during synthesis of crystalline nanoparticles and used these diagnostics to predict the requirements for formation of crystalline nanomaterials. For example, prior work by Kramer et al. (J. Phys. D, 2015) suggested that LTP synthesis of crystalline silicon nanoparticles (SiNPs) at atmospheric pressure requires a higher ion density and/or atomic hydrogen density as compared to the low-pressure operating regime. Here, we present our results on crystalline SiNP synthesis using a mm-scale RF LTP at atmospheric pressure. Visual inspection of the plasma indicates that it diffusely fills the reactor volume, similar to low-pressure reactors. However, high-speed imaging shows that the plasma is in fact a fluctuating filamentary discharge, indicating that species densities in regions of the reaction zone may be much higher than predicted based on a diffuse full-volume discharge. In this work we present our results on how crystalline SiNPs are formed in an atmospheric pressure RF-driven LTP, including analysis of the relationship between the formation of a filament in the discharge and the resulting SiNP properties.