Silicon nanocrystals with sizes between 5 and 10 nm have been produced in a non-thermal plasma reactor using silicon tetrachloride as precursor. We demonstrate that high-quality material can be produced with this method and that production rates as high as 140 mg h(-1) can be obtained, with a maximum precursor utilization rate of roughly 50%. Compared to the case in which particles are produced using silane as the main precursor, the gas composition needs to be modified and hydrogen needs to be added to the mixture to enable the nucleation and growth of the powder. The presence of chlorine in the system leads to the production of nanoparticles with a chlorine terminated surface which is significantly less robust against oxidation in air compared to the case of a hydrogen terminated surface. We also observe that significantly higher power input is needed to guarantee the formation of crystalline particles, which is a consequence not only of the different gas-phase composition, but also of the influence of chlorine on the stability of the crystalline structure.

The use of nonthermal plasmas for the processing of nanomaterials remains the focus of ongoing investigations because of the many advantageous properties of this class of processes. Silicon nanoparticles in particular are easily produced using continuous flow nonthermal plasma reactors. This has become a technologically relevant technique especially because silicon is an earth-abundant material with important applications in many energy-related fields. Still, there are significant gaps of knowledge in the dynamics of nucleation, growth, and structural evolution of nanoparticles in continuous flow nonthermal plasma reactors. In this contribution, the authors address these issues by performing in-situ infrared absorption measurements to monitor the rate with which the silicon precursor, silane, is consumed by the plasma. At the same time, the authors extract nanoparticles from the plasma volume at different stages of their growth and monitor their structural evolution. The authors find that particle nucleation occurs rapidly and that as soon as the silane is fully consumed the particles have grown to almost their final size. At the end of this phase, the particles have an amorphous structure. The crystallization then occurs in the plasma on a time scale of 120 ms for the conditions used for this study. By using an experimentally determined crystallization rate for free-standing silicon nanopowders, the authors estimate the particle temperature in the plasma. The authors find that the plasma heats the particle to a temperature of over 1100 K, exceeding estimates based on the reaction between the nanoparticle surface and plasma-generated ions and radicals.

We demonstrate the synthesis of hollow silicon carbide nanoparticles via a two-step process involving the non-thermal plasma synthesis of silicon nanoparticles, followed by their in-flight carbonization, also initiated by a non-thermal plasma. Simple geometric considerations associated with the expansion of the silicon lattice upon carbonization, in combination of the spherical geometry of the system, explain the formation of hollow nanostructures. This is in contrast with previous reports that justify the formation of hollow particles by means of out-diffusion of the core element, i.e., by the Kirkendall nanoscale effect. A theoretical analysis of the diffusion kinetics indicates that interaction with the ionized gas induces significant nanoparticle heating, allowing for the fast transport of carbon into the silicon particle and for the subsequent nucleation of the beta-silicon carbide phase. This work confirms the potential of non-thermal plasma processes for the synthesis of nanostructures composed of high-melting point materials, and suggests that such processes can be tuned to achieve morphological control. V C 2015 AIP Publishing LLC.

We have experimentally determined the crystallization rate of plasma-produced amorphous silicon powder undergoing in-flight thermal annealing, and have found a significant reduction in the activation energy for crystallization compared to amorphous silicon thin films. This finding allows us to shed light onto the mechanism leading to the formation of high quality nanocrystals in non-thermal plasmas.

No abstract