Efficiency and sustainability are two primary considerations for the manufacture of modern day electronics. Silicon is one of the most abundant elements on Earth, and silicon nanocrystals (SiNCs) exhibit potential for applications in light emitting devices (LEDs) and photovoltaic devices (PVs) that could, in theory, rival the efficiencies of the more commonly used (but more expensive, toxic, and rare) cadmium-based chalcogenide NCs. Despite these advantages, SiNCs are not commonly used because the surface defects and oxidization that lower the efficiencies of their optical properties (and which plague most NC materials) are more difficult to eliminate due to the covalent nature of the SiNC surface. The covalent surface prohibits the ligand exchange and shell growth processes that have led to the success of these other materials. Here, we hypothesize that surface defects and oxidization of plasma-produced SiNCs can be mitigated via the “capping” of surface defects and reactive sites by injecting vapor-phase species into the plasma to anneal and protect the surfaces of individual SiNCs in-flight.
The nonthermal plasma reactor has been shown to be a versatile and powerful tool for synthesizing high-quality NCs, including some of the highest-efficiency SiNCs, in terms of photoluminescence. To bring the optical performance of these SiNCs to par with the Group II-VI chalcogenide NCs, we explored multiple plasmas in sequence, each with a different role – synthesis, surface defect mitigation, and shell growth – so that individual SiNCs can be perfected in-flight while still maintaining a continuous, high-yield production scheme. For surface defect mitigation, we explored mixtures of hydrogen gas and other inert vapor-phase constituents such as helium in a secondary plasma for reducing defect densities at the SiNC surfaces. Further, we also used an additional plasma in-line with the first to grow thin SiNx layers around the SiNCs, seeking to stabilize them against ambient oxidation. We found that the SiNx layer reduced the formation of oxygen-related defects at the SiNC surfaces as compared to the core SiNCs, and appeared to reduce oxidation-related shifts in the photoluminescence peaks for these NCs. We are also conducing in-situ plasma characterization to understand the reactions and conditions required for high-quality shell growth on SiNCs. Our future work is devoted to using multi-plasma schemes for discovering new reaction pathways towards stable, high-efficiency photoluminescence from SiNCs.