Charging, Coagulation, and Heating Model of Nanoparticles in a Low-Pressure Plasma Accounting for Ion–Neutral Collisions

Low-pressure silane-argon plasmas allow the production of silicon particles of different sizes and morphologies. A better understanding of the correlations between dusty-plasma properties and particle morphology is very important for understanding and optimizing the particle synthesis. An analytical model predicting the nanoparticle charging, coagulation, and heating in a low-pressure plasma is here presented. The model includes the effect of collisions between ions and neutrals in proximity of the particles. In agreement with experimental evidence for pressures of a few torr, a charge distribution that is less negative than the prediction from the collisionless orbital-motion limited theory is obtained. The reduced charging causes an enhanced ion current to the particle while still preventing coagulation and conserving a monodisperse particle size distribution. Ion-electron recombination at the particle surface, together with other particle heating and cooling mechanisms typical of silane-argon plasmas, are studied in a particle-heating model which predicts the nanoparticle temperature. The effect of plasma parameters on the nanoparticle temperature is discussed, and the predictive power of the model is demonstrated from the appearance of photoluminescent properties in silicon nanoparticles, a property present only in crystalline particles. A correlation between plasma power, ion density, particle temperature, and particle crystallinity is finally developed.