(356a) Analysis of Nanoparticle Growth and Aggregation in Non-Thermal Plasma and Plasma Afterglow By Advanced Ion Mobility Spectrometry and Monte Carlo Simulation

Authors: 
Chen, X., University of Minnesota
Seto, T., Kanazawa University
Kortshagen, U. R., University of Minnesota
Hogan, C. J. Jr., University of Minnesota
Non-thermal plasma synthesis method is an established method for producing nanoparticles in a controllable manner with potential semiconductor, solar cell, and bioimaging applications. Non-thermal plasmas are non-equilibrium systems; the high electron energy in a non-thermal plasma can dissociate vapor phase precursors, initiating the nucleation and growth of nanoparticles. Numerous studies have shown that nanoparticles in non-thermal plasmas are negatively charged due to the large gap of mass and mobility between electrons and ions. The unipolar charging of the nanoparticles also prevents them from coagulation and leads to spherical, monodispersed nanoparticle production.

On the other side, we recently characterized size distribution of Si nanocrystals synthesized in a low-pressure radiofrequency Ar non-thermal plasma by a uniquely developed low pressure differential mobility analyzer (LPDMA). We detected both negatively and positively charged nanocrystals at the plasma reactor outlet, and modest aggregation of nanocrystals was also observed. To further study this phenomenon, we have developed and applied a constant-number Monte Carlo population balance model to simulate nanoparticle charging and growth in the plasma afterglow region. The simulation incorporates the effects of electron desorption from nanoparticles, particle-particle collision, ion-particle collision, electron-particle collision, particle wall deposition and decay of ion and electron concentrations and electron temperature in the afterglow. The simulation results were compared with LPDMA experimental data where a wire mesh at the plasma exit was used to control the extent of nanoparticle aggregation. The result shows that in the plasma afterglow region, since the concentration and energy of electrons and ions decay rapidly, the nanoparticles are neutralized, and can become bipolarly charged through collisions with remaining ions. The change in nanoparticle charging also leads to post plasma aggregation, as we have observed experimentally.