Plasma Synthesis of Silicon Nanoparticles: Optimization of Yield, Size Distribution, and Crystallinity

Abstract

Silicon nanoparticles (Si NPs) are expected to be high-value added materials for the next generation devices due to their size dependent optical and electronic properties. Previously, we have developed in-flight plasma CVD for synthesis of free-standing, size-tunable Si NPs with mean diameter of smaller than 10 nm using silicon tetrachloride (SiCl₄). As-produced, chlorine-terminated Si NPs are readily dispersed into organic solvent without surface modification, enabling a suspension of Si NPs to form silicon inks. By adapting appropriate solution process with silicon inks, Si NP containing devices can be fabricated without high vacuum dry process, which provides great flexibility for the development of Si NP based devices at low cost. This paper clarifies the operation regime of in-flight plasma CVD to produce desired Si NPs in terms of yield, size distribution, and crystallinity towards Si NP containing photovoltaics. Size distribution and crystallinity are controlled by residence time (or total flow rate) and input power respectively, while the yield is unambiguously determined by SEI, or specific energy input, which is calculated by the power (J/s) over the total flow rate (cm³/s). Moreover, gas analysis by mass spectrometer revealed that Si NP yield increased as SiCl₄ conversion, however, increase in HCl suppresses particle nucleation and amorphous silicon film deposition on the reactor wall became the main silicon loss pathway. Moreover, effect of H₂/SiCl₄ ratio was investigated for better understanding of Si NP formation mechanism.