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Browsing by Subject "photoluminescence"

Now showing 1 - 3 of 3
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    Silicon quantum dot luminescent solar concentrators
    (2019-08) Hill, Samantha
    Silicon quantum dots (Si QDs) have previously been established as a unique class of quantum-confined materials with potential for a wide variety of optoelectronic applications. In this work, we examine their application to luminescent solar concentrators, or LSCs, for the first time by developing high-quality Si QD / polymer nanocomposites. By encasing Si QDs with in a polymer slab, most of their photoluminescence becomes trapped via total internal reflection and escapes only at the slab edges where solar cells can be placed to harvest the concentrated light. We find that Si QDs are suitable for such LSC devices due to their unique combination of indirect band gap absorption with efficient photoluminescence. The resulting low overlap between the absorption and photoluminescence spectra yields low reabsorption losses in large-area LSCs without the use of rare or toxic elements in the luminophore. We demonstrate effective Si QD LSC prototypes consisting of flexible and rigid bulk nanocomposites as well as films on glass using methacrylate-based polymers. We find the Si QDs maintain their optical properties throughout radically-initiated polymerization processes but are prone to forming light scattering agglomerates in the solid phase. These agglomerates drastically reduce the LSC waveguiding efficiency due to their light scattering properties. We find that light scattering from these nanocomposites increases with Si QD concentration. One approach for improving the dispersion of the Si QDs within solid polymers is to choose surface ligands which mimic the structure of the encasing polymer. We demonstrate this with ester-capped Si QDs compared to alkane-capped Si QDs in poly(methyl methacrylate), or PMMA. Furthermore, we find that fast polymer solidification rates also reduce the formation of light scattering agglomerates. We show ester-Si QD / PMMA films cast from prepolymer solutions have an order of magnitude higher concentration limit before the onset of light scattering compared to their bulk-polymerized counterparts. Overall, this work establishes Si QDs as a promising luminophore for visibly transparent LSCs which may be used in the future for solar harvesting windows and architectural elements or in concert with other LSCs to form more efficient tandem structures.
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    Silicon quantum dots for optical applications
    (2015-08) Wu, Jeslin
    Luminescent silicon quantum dots (SiQDs) are emerging as attractive materials for optoelectronic devices, third generation photovoltaics, and bioimaging. Their applicability in the real world is contingent on their optical properties and long-term environmental stability; and in biological applications, factors such as water solubility and toxicity must also be taken into consideration. The aforementioned properties are highly dependent on the QD's surface chemistry. In this work, SiQDs were engineered for the respective applications using liquid-phase and gas-phase functionalization techniques. Preliminary work in luminescent downshifting for photovoltaic systems are also reported. Highly luminescent SiQDs were fabricated by grafting unsaturated hydrocarbons onto the surface of hydrogen-terminated SiQDs via thermal and photochemical hydrosilylation. An industrially attractive, all gas-phase, nonthermal plasma synthesis, passivation (aided by photochemical reactions), and deposition process was also developed to reduce solvent waste. With photoluminescence quantum yields (PLQYs) nearing 60 %, the alkyl-terminated QDs are attractive materials for optical applications. The functionalized SiQDs also exhibited enhanced thermal stability as compared to their unfunctionalized counterparts, and the photochemically-hydrosilylated QDs further displayed photostability under UV irradiation. These environmentally-stable SiQDs were used as luminescent downshifting layers in photovoltaic systems, which led to enhancements in the blue photoresponse of heterojunction solar cells. Furthermore, the QD films demonstrated antireflective properties, improving the coupling efficiency of sunlight into the cell. For biological applications, oxide, amine, or hydroxyl groups were grafted onto the surface to create water-soluble SiQDs. Luminescent, water-soluble SiQDs were produced in by microplasma treating the QDs in water. Stable QYs exceeding 50 % were obtained. Radical-based and catalytic hydrosilylation reactions were also investigated to engineer individually-dispersed SiQDs in water. The results of this dissertation demonstrate the potential of SiQDs in optical applications. In the future, their application may lead to improvements in the efficiencies of photovoltaic devices and perhaps allow the cells to exceed the Shockley-Queisser limit. In biology, the stability of the SiQDs may allow long-term monitoring of biomolecules and perhaps lead to new discoveries.
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    Surface functionalization and optical properties of nonthermal plasma-synthesized silicon nanocrystals
    (2021-03) Li, Zhaohan
    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.