Browsing by Subject "quantum dot"
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Item Heterostructured Quantum Dots from Nonthermal Plasmas(2019-07) Hunter, KatharineThis work investigates the production and properties of heterostructured quantum dots (QDs) composed of a group IV semiconductor and an inorganic shell. A nonthermal plasma process for the growth of heterostructured QDs has been developed. The design of this process leverages pioneering work from Mangolini et al. for the nonthermal plasma growth of silicon QDs. The use of plasmas has emerged as a leading technique for the synthesis of group IV NCs because of the unique set of advantages which set this approach apart from other gas-phase approaches. Plasma properties, specifically the temperature and density of neutral and charged species (electrons and ions), dictate the reactive environment within which QDs are shaped. Furthermore, these advantages extend to the growth of core/shell NCs and allow the processing of heterostructured QDs inaccessible by conventional solution-phase processing. First, this work explores germanium (Ge) QDs with silicon (Si) shells deposited epitaxially. This simple QD heterostructure provides crucial insight into the development of a nonthermal plasma process for the synthesis core/shell structures. Furthermore, the epitaxial Ge/Si system allowed for the exploration of strain manipulation of semiconductor bandstructures in core/shell QDs. This strain engineering was extended to germanium (Ge) - tin (Sn) QD heterostructures. This effort leverages the processes developed in for Ge/Si QD epitaxy to explore the ability to tensile strain Ge QDs and induce a direct bandgap. Next the nonthermal plasma growth of an amorphous silicon nitride a-SiNx layer on Si NC as a passivating layer was investigated. In this effort, a second process for shell growth was developed which relies on surface layer modification by plasma enhanced nitridation. Finally, we have demonstrated the application of Reverse Monte Carlo simulations to probe the structure and spatial composition of heterostructured NCs applies to the well-established system of silicon NCs doped with boron and phosphorus. Through the generation and refinement of simulated NCs based on high-energy X-ray diffraction data, elemental distribution and coordination was investigated. This approach may be of importance moving forward for the structural analysis of nanocrystalline materials with increasing compositional and structural complexity.Item Surface functionalization and optical properties of nonthermal plasma-synthesized silicon nanocrystals(2021-03) Li, ZhaohanSilicon 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.