Browsing by Author "Welsch, Tory A."
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Item Role of Semiconductor Nanostructures in Photon Upconversion Applications(ACS Applied Optical Materials, 2023-04-28) Cleveland, Jill M.; Welsch, Tory A.; Chase, D. Bruce; Doty, Matthew F.Photon upconversion, a process in which multiple low-energy photons are absorbed and re-emitted as higher-energy photons, has recently received a significant amount of attention due to its potential utility across a wide range of optical applications. Traditionally, two types of materials have been used for photon upconversion applications: lanthanide-doped nanocrystals and triplet–triplet annihilation molecules. While these systems have demonstrated good upconversion efficiencies, they both suffer from some limitations, particularly in spectral utilization. In this review, we will highlight the ways semiconductor nanocrystals have been integrated into existing upconverison platforms to address their limitations and improve their usability for some specific upconversion applications. Additionally, we will discuss the recent development of upconversion platforms based entirely on semiconductor nanostructures. These systems rely on the size-, shape-, and composition-dependent optical properties of semiconductors to design upconverting materials with the necessary electronic structure for a specific application. We discuss the current status of these hybrid and pure semiconductor-based upconverters and suggest future directions for further improving their upconversion performance.Item Separating the Effects of Nonorthogonal Variables on the Hot-Injection Synthesis of Core/Thick-Shell (“Giant”) CdTe/CdS Quantum Dots(Chemistry of Materials, 2022-11-22) Welsch, Tory A.; Cleveland, Jill M.; Chase, D. Bruce; Doty, Matthew F.“Giant” core/thick-shell quantum dot (QD) nanostructures are of interest due to their unusual optical properties and importance as components of more advanced heterostructures tailored to achieve increasingly complex optical functions. However, reliable one-step seeded growth of these structures poses a significant challenge: one must balance multiple competing reaction processes to find the growth regime that realizes spherical shells of both the target size and high crystalline quality. Adjusting synthesis conditions in thicker-shelled reactions is further complicated by multiple nonorthogonal variables that impact the reaction mechanism. These variables include the reaction volume, reaction concentration, and oleic acid (ligand) concentration. Here, we investigate the seeded growth of core/thick-shell CdTe/CdS QDs by adapting a “flash” shelling method. We systematically vary three key reaction parameters (particle concentration, oleic acid:Cd ratio, and Cd–S:CdTe core ratio) over 30+ different thick-shelling reactions to elucidate the separate and intersecting impacts of these parameters on shell growth. Our analysis of the resulting particle quality reveals that the particle concentration of the reaction plays a critical role in the shell growth mechanism. We find the impact of oleic acid to be dependent on the particle concentration for a given shell thickness. We also find that the optimal conditions shift when targeting increasingly thick shells. The results demonstrate the importance of testing and controlling for synthesis variables across a multidimensional parameter space. We develop and present general experimental design criteria to help guide efficient development of new seeded growth reactions that enable reliable synthesis of thick-shelled nanostructures.Item Supramolecular Gelation of Cadmium Oleate in the Synthesis of Nanocrystals for Applications in Photonics and Optoelectronics(ACS Applied Nano Materials, 2024-05-22) Welsch, Tory A.; Cleveland, Jill M.; Thomas, Jessica A.; Schyns, Zoé Odile Georgette; Korley, LaShanda T. J.; Doty, Matthew F.Cadmium oleate is widely used as a cation precursor in the synthesis of cadmium chalcogenide nanocrystal quantum dots (QDs) for a broad range of photonic and optoelectronic applications. Cd oleate can noncovalently assemble to form a supramolecular coordination gel, or metallogel, in solvents commonly used to disperse oleate-capped QDs. The gelation severely impedes the purification of oleate-capped QDs from excess Cd oleate, resulting in a gelled product that cannot be reliably characterized or used in further synthesis reactions. Here, we investigate the Cd oleate gel to gain insights into its viscoelastic properties and behavior under conditions relevant to QD synthesis, purification, and storage. We then examine how to effectively mitigate gelation by adding oleylamine as an additional ligand to disrupt noncovalent assembly. We synthesize PbS/CdS core/shell QDs via cation exchange as a case study to illustrate gelation of the reaction product and further demonstrate how this issue can be resolved through a better understanding of the supramolecular gel.