The role of nanoparticle trapping in the synthesis of highly monodisperse Mie-resonant silicon crystals

Crystalline silicon nanoparticles with optically Mie-resonant properties have garnered significant attention due to their ability to exhibit both electrical and magnetic resonances. In this study, we present a nonthermal plasma process for the production of highly monodisperse particles. By utilizing a continuous-flow plasma reactor, nanoparticles are temporarily trapped, resulting in the synthesis of particles with diameters ranging from 60 to 200 nm and remarkably low standard deviations of the size distribution (less than 5%). Optical extinction measurements demonstrate that colloidal solutions containing these particles exhibit strong magnetic and electric dipole resonances at visible wavelengths. The underlying mechanism responsible for the formation of these highly monodisperse particles is discussed, focusing on the trapping process. The particle trapping occurs in two dimensions, involving axial and radial transport. We propose that the threshold for particle de-trapping is determined by the interplay between electrostatic, thermophoretic, and drag forces, where competition among these forces dictates the size at which particles leave the trap.