AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Analysis of Si Nanoparticle Growth in Low Pressure Non-thermal Plasmas and Plasma Afterglows via Differential Mobility Analysis and Monte Carlo Simulations
XIAOSHUANG CHEN, Takafumi Seto, Uwe R. Kortshagen, Christopher Hogan Jr., University of Minnesota
Abstract Number: 52 Working Group: Nanoparticles and Materials Synthesis
Abstract Low pressure (< 10 Torr) non-thermal plasma flow tube reactors are frequently utilized for the production of crystalline group 14 semiconductor nanocrystals from vapor phase precursors. Although it is desirable to monitor reactor output and particle growth dynamics, online size distribution determination of non-thermal plasma synthesized nanoparticles is a challenge; the sub-atmospheric operating pressure of most non-thermal plasmas makes it extremely difficult to couple traditional atmospheric pressure aerosol mobility instrumentation to reactors.
In this study, we applied a low pressure differential mobility analyzer (LPDMA) to examine the mobility distributions of Si nanoparticles synthesized via silane reaction in a radiofrequency Argon plasma at 2 Torr. We experimentally evaluated the transfer function of the LPDMA and developed a data inversion routine to determine the collision cross section distribution functions of the charged particles exiting the plasma. The collision cross section is the most direct measure of particle size in the free molecular regime, hence we elected to use it as the measure of nanoparticle size in this study. Inverted collision cross section distribution functions agree well with those inferred from TEM images of particles.
Interestingly, we were able to detect both negatively charged and positively charged particles at the plasma reactor outlet. Particles are expected to be unipolarly charged in the plasma volume, mainly because of the large mass and mobility differences between electrons and Argon ions. We hence subsequently studied nanoparticle aggregation in the afterglow region of the plasma reactor by varying the allowable time for nanoparticle growth between the plasma reactor and the LPDMA, showing that nanoparticles rapidly aggregate after exiting the plasma. Measurements of size distribution evolution in the afterglow were compared to a constant-number Monte Carlo simulation accounting for ion and electron losses, particle-ion and particle-electron collisions, charge ejection from high charged particles, and particle-particle collisions.