Particle trapping enables the dusty plasma synthesis of highly monodisperse nanocrystals

Dusty plasmas are finding increasing attention for the synthesis of nanoparticles that are difficult or impossible to produce with other synthesis methods. Recent research has shown that temporary particle trapping plays an important role acting as a “size filter” in the synthesis of sub-10 nm nanocrystals in tubular laminar flow plasma reactors. In this process, small particles are trapped by the electrostatic force, which scales linearly with the particle radius, and grow through the surface deposition of precursor until the gas drag force, which scales with the square of the particle radius, overcomes the electrostatic force. This causes particle to leave the plasma after reaching a threshold size at which electrostatic and drag force balance. Here, we demonstrate how the concept of size-filtering through temporary electrostatic trapping can be expanded to the synthesis of highly monodisperse nanoparticles ranging from 30-200 nm with standard deviations of the size distribution of just a few nanometers. We discuss that in this process, particle trapping is indeed two-dimensional, involving both axial and radial transport. We propose that the threshold for particle de-trapping is the result of the competition of the electrostatic, thermophoretic, and drag forces.