Surface functionalization and optical properties of nonthermal plasma-synthesized silicon nanocrystals

Abstract

Silicon nanocrystals (Si NCs) have been drawing increasing attention over the last few decades due to their earth abundance, biocompatibility, and low toxicity. At the nanoscale, surface chemistry can drastically impact the electronic and optical properties of nanomaterials. Therefore, tailoring the surface of nanocrystals via surface functionalization reactions is crucial in enabling their applications. In this thesis, we develop surface functionalization routes specifically for luminescent solar concentrator and bioimaging applications using scalable and cost-effective methods. Nonthermal plasma synthesis allows for the continuous production of silicon nanocrystals on a large scale. However, post-synthesis steps are necessary for silicon nanocrystals to be suitable for luminescence applications. Therefore, we develop an all-gas-phase synthesis and processing route that integrates nonthermal plasma synthesis, plasma-assisted surface functionalization with alkene ligands, and in-flight annealing within one flow stream. Compared with solution-phase functionalization, the gas-phase functionalization method reduces long reaction times and avoids the use of solvents, which shows potential for large-scale production. The all-gas-phase synthesized and functionalized Si NCs are excellent candidates as emitters for luminescent solar concentrator devices (LSCs). LSC prototypes consisting of Si NCs uniformly embedded in a polystyrene matrix have been successfully fabricated without using additional solvents. After light irradiation, the Si NCs exhibit a photoluminescence quantum yield (PLQY) of above 40\%, comparable to the highest PLQY in Si NCs functionalized by solution-phase methods. Understanding and controlling the energy transfer between Si NCs is of great importance for the design of efficient Si NC-based optoelectronic devices. We demonstrate that energy transfer can be effectively engineered in Si NC films by varying the length and surface coverage of alkyl ligands for Si NC surface functionalization. Using these samples, we are also able to carry out a fundamental study of distance-dependent energy transfer in Si NC solids. Finally, the synthesis of red-emitting and water-soluble Si NCs for bioimaging applications is discussed. Si NCs are promising candidates for biological imaging applications due to their low toxicity and strong biocompatibility. However, the Si NC surfaces are intrinsically hydrophobic, and thus surface functionalization is essential to use them in a biological medium. We demonstrate the successful surface grafting of hydrophilic polyethylene glycol ligand by two distinct reaction schemes. In the first method, we apply the thermal hydrosilylation reaction to synthesize Si NCs that are nearly individually dispersed in water and biological media. In the second method, we develop a two-step surface modification approach coupling gas-phase and liquid-phase methods to synthesize PEGylated acrylic acid grafted Si NCs. Such functionalized Si NCs exhibit efficient red emission in biological media for up to 24 hours.