Plasma-induced crystallization of silicon nanoparticles

While the formation of nanoparticles in nonthermal plasmas is well known, the heating mechanism leading to their crystallization is poorly understood. In this study, we investigate the crystallization of amorphous silicon nanoparticles in nonthermal plasmas using a tandem plasma configuration. Amorphous silicon nanoparticles with diameters of 3, 4 or 5 nm are formed in a low-power nonthermal upstream plasma, and injected directly into a second separate downstream plasma. Crystallization of the amorphous silicon nanoparticles is investigated as a function of the power used to maintain the second plasma. This approach allows for the decoupling of nanoparticle synthesis and heating. The nanoparticle properties and plasma conditions are examined to obtain a comprehensive understanding of nanoparticle heating and crystallization. The particle crystallinity was studied using x-ray diffraction, Raman spectroscopy, and transmission electron microscopy. We discovered a threshold power for complete crystallization of the particles. A combination of comprehensive plasma characterization with a nanoparticle heating model reveals the underlying plasma physics leading to crystallization. Here we found that the nanoparticles reach temperatures as high as 750-850 K in the secondary plasma, which is well above the gas temperature and sufficient for complete nanoparticle crystallization. While we demonstrate this method of predicting nanoparticle temperature using silicon, the approach can be applied broadly to other plasma-synthesized nanomaterials.