Particle trapping of non-thermal plasmas for nanoparticle synthesis*

Low pressure nonthermal plasmas in cylindrical geometry with strong axial gas flow are widely used for nanoparticle synthesis. In these plasmas, large densities of nanoparticles and their small sizes result in the average nanoparticle charge often being less than one elementary charge. Therefore, the effect of nanoparticle trapping in these plasmas has not previously been considered. Here, we demonstrate the importance of particle trapping for various plasma configurations in argon:silane plasmas for silicon nanoparticle synthesis. The existence of trapped silicon particles is evidenced by turning off the silane precursor flow which stops the particle growth. Particles remain trapped in the argon plasma because the gas drag force acting on particles stopped in their growth is insufficient to overcome the electrostatic trapping force. Particles are only released once the plasma is turned off. The size distribution of trapped particles is compared to that of particles exiting the reactor in steady-state operation. The trapping position is determined from the known gas velocity and the measured time delay between turning off the plasma and collecting particles at the reactor exit. Particle trapping in an inductively coupled plasma configuration is compared with simulation results of a 2D multi-fluid plasma model coupled with a 3D kinetic model. Gas drag forces and electrostatic forces acting on particles are estimated to elucidate the trapping mechanism.