Precursor-free particle growth in a dusty plasma

In this contribution, submicrometer particles are synthesized in a capacitively coupled $\mathrm{C}_{2} \mathrm{H}_{2} /$ Ar plasma. To monitor particle size distributions, the negatively charged particles are electrostatically accelerated towards a biased substrate in contact with the plasma, enabling the extraction of particles periodically during the growth without interrupting the plasma process. After an initial precursor injection period of 1 minute, particles continue to grow in the plasma for over 10-30 minutes in the apparent absence of vapor phase precursor. Interestingly, as growth continues, particles trapped at the grounded bottom electrode decrease in polydispersity, with geometric standard deviations decreasing from 1.18 to 1.02. We rationalize these results by proposing that material may be released from the smaller particles and then redeposited onto large particles. This is analogous to Ostwald ripening, though it may be distinct in the underlying mechanism; Ostwald ripening is typically driven by the elevated surface concentrations or vapor pressures of smaller particles, leading to their evaporation in systems with lower precursor concentrations than the particle surface concentration. Meanwhile, larger particles, with surface concentrations below the background concentration, are stable and grow. In a nonthermal plasma environment, the preferential heating of smaller particles may lead to their dissociation, and the subsequent growth of larger, highly monodisperse particles, which experience diminished heating. These results suggest that selective, size-dependent heating may play an important role in particle growth dynamics in non-thermal plasmas.