Coagulation of nanoparticles in a plasma.

Coagulation of nanoparticles in a low-pressure radio frequency plasma was studied by means of a detailed numerical model for the spatiotemporal evolution of the nanoparticle-plasma system. Simulation results indicate that the occurrence of coagulation to any significant degree in such systems requires the existence of two effects: first, gas-phase nucleation is not limited to a brief burst, but rather continues in regions that are sufficiently free of nanoparticles; and second, coagulation coefficients for collisions between neutral and negatively charged nanoparticles are enhanced by the image potential induced in the neutral particle. Accounting for these effects, coagulation is predicted to be dominated by coagulation between very small (approximately 1 or 2 nm in diameter) neutral particles and larger negatively charged particles that are trapped in the plasma. Coagulation ceases when the spreading of the nanoparticle cloud across the plasma quenches gas-phase nucleation.