Silicon nanocluster anion-argon cation recombination via hybrid continuum-molecular dynamics calculations

Prediction of the charge distribution on particles in an aerosol is critical not only in electrical mobility based characterization methods, but also in understanding the role that charging plays in particle growth in gas phase synthesis reactors. The latter is particularly important in non-thermal plasma synthesis reactors, wherein nanoclusters form and grow from vapor phase precursors in a high electron and high ion density environment. In plasmas, because free electrons are much less massive and much more mobile than positive gas ions, nanocluster charge distributions are biased negative. However, neutral or even positively charged nanoclusters may exist, depending on the rate of nanocluster-gas ion recombination, and nanocluster-nanocluster collisions may greatly contribute to nanocluster growth if not Coulombically suppressed. To better understand the charge distribution on nanoclusters in non-thermal plasma synthesis systems, we applied a recently developed collision rate calculation method, i.e. the continuum-molecular dynamics (C-MD) method, to examine recombination of Si_n^z nanoclusters (n=47,91,266,&494, z=−1,&−2) and Ar⁺ cations at 300 K and pressures from 10^3.5−10⁶ Pa, in argon neutral gas. With collision rate coefficients from the C-MD approach, nanocluster steady-state charge distributions were calculated. C-MD determined recombination rate coefficients are found to be higher than those from the traditionally-used limiting sphere theory approach of Fuchs outside of the continuum limit, leading to charge distributions which, although biased towards negative charge levels, are less biased than predicted by the traditional limiting sphere method, with differences particularly noticeable at lower pressures. Application of C-MD recombination coefficients in steady-state charge distribution calculations shows that the fraction of positively charged nanoclusters in a non-thermal plasma would be negligibly small; however, there is an appreciable fraction of neutral nanoclusters, and hence nanocluster-nanocluster collisions cannot be neglected in modeling particle growth in plasma reactors.