Browsing by Subject "Quantum Dots"

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    Complex Refractive Index Modeling and Nanoscale Patterning of Solid-State Colloidal Quantum Dots for Nanophotonic Applications
    (2019-05) Dement, Dana
    The small size of photoluminescent, nanocrystal quantum dots (QDs) leads to a variety of unique optical properties that are well-suited to many optoelectronic devices and nanophotonic studies. Here, we demonstrate techniques to further improve the design of solid-state QD structures. A problem for many applications is that predicting the optical behavior of QD solids is difficult because the complex refractive index of QD solids is a composite quantity that is dependent on size, ligand chain length, and the deposition process of the QDs. To address this problem, we show that the intrinsic refractive index of neat CdSe/CdS QDs can be extracted from solution-state absorption data. We then show how this information can be used with effective medium approximations to describe the effective refractive index of QD films associated with a variety of QD sizes and packing fractions. Our predictions are verified experimentally by spectroscopic ellipsometry. With our modeling tool, we can also understand packing variations between QD films and predict the absorption in solid-state QD structures, leading to significant savings in both time and materials. Using the same QD materials, we next address the need for accurate patterning of QD solids at the nanoscale. We have found that direct electron beam lithography is a straightforward patterning process that does not require ligand exchange and results in structures that retain bright photoluminescence. We demonstrate that feature sizes as narrow as 30 nm with many QD layers can be patterned. These structures can withstand sonication in a variety of solvents, show no distortion, and can be placed within 20 nm of their intended location nearly 100% of the time. Combining our nanofabrication technique with the ability to measure the refractive index of the QD pattern, we find that edge effects arising from the finite shape of the QD nanostructure lead to substantial absorption enhancement when compared to an equivalent volume region taken from a continuous QD film. Finally, we explore more complex structures by patterning QD arrays, multilayer QD structures, and QD disks inside plasmonic resonators. We believe that the work presented here lays important groundwork to improve the modeling of QD solids and reveals new ways QDs can be incorporated into devices and nanophotonic designs.
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    Nanophotonic Assemblies and Light Management for Increased Absorption and Photoluminescence
    (2020-10) Quan, Matthew
    This thesis explores three projects involving the interaction of incident light with nanocrystals: absorption enhancement in patterned quantum dot solids (pQDS), time dynamics of plasmonic nanorod assemblies, and the usage of luminescent solar concentrators (LSCs) to enhance upconversion. First, the effect of nanostructure shape on the absorption in pQDS is studied, and then these structures are integrated with plasmonic nanorings and bullseyes to enhance both absorption and photoluminescence (PL). In the next chapter, the time-dependent optical properties of a gold nanorod assembly is modeled following ultrafast laser illumination, accounting for both changes in the electron density and the nanostructure geometry. Finally, Monte Carlo modeling is used to study the effect of LSCs and mirrors on the absorption and upconverted PL from a hydrogel containing the upconverting donor/acceptor pair PtOEP/DPAS.