Control of Silicon Nanoparticle Evaporation and Crystallization During Nonthermal Plasma Synthesis

The synthesis of crystalline silicon nanoparticles (NP) in low temperature plasmas (LTPs) has been established as a leading fabrication technique. LTPs have a unique capability to anneal NPs in-situ and epitaxially grow NPs with the system operating at ambient temperatures. The precise control of NP size and crystallinity is vital for their use in next-generation optoelectronics and energy storage applications. However, the relationship between reactor-scale transport and NP-scale heating mechanism is not well understood. This paper discusses results from a computational investigation of the comparative performance of inductively coupled plasma (ICP) and capacitively coupled plasma (CCP) configurations for NP synthesis using a two-dimensional plasma hydrodynamics model coupled with a three-dimensional kinetic model which tracks NP trajectories and properties. The results show that the reactor geometry fundamentally dictates the spatial NP trapping topology and subsequent thermal history of the particles. The ICP configuration establishes an off-axis, toroidal trapping zone that extends residence times to tens of seconds, enabling growth of crystalline NP into the tens of nanometers. However, intense stochastic heating of the NPs may decrease yield due to the vaporization of sub-nanometer clusters. Conversely, CCP reactors feature on-axis trapping of NPs where strong fluid drag and a shallow potential well reduce residence times, which limits particle sizes to a few nanometers. In spite of this structural size limitation, CCP systems produce one to two orders of magnitude higher yield and high crystallinity NPs across a wider power range due to a more uniform, moderated thermal environment. We identify an evaporative bottleneck in the sub-nanometer regime (< 1 nm in NP radius) where intense thermal fluxes from ion recombination and radical attachment result in rapid cluster evaporation, restricting the population that transitions to stable growth. Ultimately, both configurations exhibit an optimal operational window at low-to-moderate powers and intermediate plasma densities balancing the thermal energy required for NP crystallization with the need to mitigate evaporative yield losses.