Synthesis, Stability and Dynamic Surface Chemistry of InP-based Core/Shell Quantum Dots
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Date
2020-01-20
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Department
Chemistry & Biochemistry
Program
Chemistry
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Distribution Rights granted to UMBC by the author.
This item may be protected under Title 17 of the U.S. Copyright Law. It is made available by UMBC for non-commercial research and education. For permission to publish or reproduce, please see http://aok.lib.umbc.edu/specoll/repro.php or contact Special Collections at speccoll(at)umbc.edu
This item may be protected under Title 17 of the U.S. Copyright Law. It is made available by UMBC for non-commercial research and education. For permission to publish or reproduce, please see http://aok.lib.umbc.edu/specoll/repro.php or contact Special Collections at speccoll(at)umbc.edu
Abstract
Photoluminescent semiconductor nanocrystals, or quantum dots (QDs) are fast becoming a major source of the world'sbrightest and most color-pure luminescent materials. In addition to numerous applications such as LEDs, photovoltaics and photocatalysts, they possess attributes that make them attractive for bioimaging purposes. To date, CdSe/ZnS is the most mature QD material in terms of synthetic development and optical properties. These properties of high absorptivity (?), narrow peak-width (FWHM) and high photoluminescent quantum yield (?PL) are the critical aspects that make QDs desirable. Despite the excellent optical properties, concern over the use of toxic cadmium has led efforts to pursue alternative materials, like InP/ZnS. Historically, InP has suffered from low ?PL (20-40%), broad emission peaks (50-90nm) and poor synthetic strategies due to toxic and pyrophoric reagent, P(TMS)3. The research contained in this dissertations achieves to bridge the gap between these materials and bring the safer, cadmium-free material closer to replacing CdSe. First, a new method for synthesizing InPZn/ZnS alloyed-core/shell QDs is described using InP magic-sized cluster (MSCs) and zinc stearate. This new method is scalable, reproducible, safer and simpler. This was achieved by eliminating the toxic/pyrophoric P(TMS)3 from the high temperature QD synthesis and providing size control by varying only the zinc stearate concentration. This approach produces color-tunable emission peaks without sacrificing narrow FWHM or ?PL. Experiments testing the core's optical stability, however, show that InP QDs are more susceptible to photooxidation when compared to CdSe cores. In an effort to protect the core from this susceptibility, various shelling strategies were explored. We present experiments describing PL evolution during well controlled shell growth of InP/ZnS, InP/Zn(Se)S and InP/ZnSe/ZnS core/shell QDs with increasing shell thickness. While the effect of each shell component showed different optical effects (peak wavelength and PL intensity) during the synthesis, the final core/shell QD's ?PL showed little dependence on shell composition during surface modification. A significant discovery was that regardless of the shell composition, desorption and readsorption of hydrophobic phosphine ligands greatly affected the QDs' emission intensity. We explored the PL effects of phosphine ligands with various electron withdrawing and sterically bulky groups during ligand exchange. Results indicate a balance between electron density of the phosphine anchoring group and steric bulk which both contribute to alter the QD's PL intensity. In the end, the picture provided of InP-based QDs will help facilitate further development of these safer particles and lead to new strategies to improve their optical properties.