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Item Synthesis of Small Silicon Carbide Nanocrystals in Low Pressure Nonthermal Plasma(North Dakota State University, 2021) Petersen, Reed JeffreyNanoparticles have attracted much attention because of their unusual physical properties. This work represents original, incipient research into small crystalline silicon carbide nanoparticles synthesized in a low-pressure nonthermal plasma reactor. The nonthermal plasma technique offers a route for size-tunable synthesis of high-purity silicon carbide nanocrystals. Even though it has a high sublimation point, silicon carbide is synthesized in crystalline form in a nonthermal plasma reactor since nanoparticles are intensely heated by exothermic surface reactions on a nanoscale level. Using vaporized tetramethylsilane as a precursor and molecular hydrogen as an additive, both silicon carbide and silicon- or carbon-coated silicon carbide were created. Since plasma synthesis is a ligand-free process and charges on particles prevent agglomeration, silicon carbide is soluble in short-chain alcohols.Item The Thiol-ene Encapsulation and Photo-physical Characterization of Colloidal Silicon Nanocrystals Synthesized with Si6H12 Using Non-thermal Plasma Reactor(North Dakota State University, 2021) Sefannaser, Mahmud AyadSilicon nanocrystals (SiNCs) are nanometer-sized semiconducting materials. Their small size endows them with unique photophysical properties. Efficient photoluminescence (PL) from silicon nanocrystal (SiNC) composites has important implications for emerging solar-energy collection technologies, yet a detailed understanding of PL relaxation in non-colloidal SiNCs is still materializing. In this dissertation, we examine the photophysical properties of silicon nanocrystal/off-stoichiometry thiol-ene composites (SiNCs/OSTE hybrids). The dissertation begins with an introduction to the photophysical properties of SiNCs, their photophysical properties, how SiNC/polymer composites are made, the various SiNC preparation techniques, and the most likely application areas for these nanocrystals. A description of experimental methods such as PL spectroscopy and transmission electron microscopy (TEM) follows, and SiNC/OSTE polymer preparation methods are then explained in detail. In the first study, TEM and photophysical characterization were performed on selected polydisperse SiNCs samples. These samples were synthesized in a nonthermal plasma reactor, using Si6H12 as precursor, and functionalized with R (where R is 1-dodecene). These SiNCs were dispersed in mesitylene:1-dodecene (5:1) as a colloid. Optical absorption, quantum efficiency, and PL lifetime of SiNCs were then investigated, as well as the relationship between quantum yield, lifetime, and PL peak. In the second study, we selected samples for size separation via the density gradient ultracentrifugation method (DGU). We successfully applied this technique to separate silicon nanocrystals with sizes from 2 nm to 4 nm from the ensemble samples using an engineered density medium layer stack, and photophysical characterization was performed on the DGU size–separated SiNCs. Lastly, we explored details of PL relaxation in photo-polymerized off-stoichiometric polymer/nanocrystal hybrids. We found time- and air-stable emission from dilute composites with up to 70% QY, and we investigated PL relaxation in the parameter space of nanocrystal size and temperature. In light of previous work, our results reveal similarities between the impacts of crosslinking and cooling to cryogenic temperature, but of which are characterized by a relative reduction in the available of phonons.