Controlling nanomaterial self-assembly for next generation optoelectronic applications

• Main Author: Weidman, Mark Clayton
• Other Authors: William A. Tisdale.
• Format: Others
• Language: English
• Published: Massachusetts Institute of Technology 2017
• Subjects: Chemical Engineering.
• Online Access: http://hdl.handle.net/1721.1/109674

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, February 2017. === "September 28th, 2016." Cataloged from PDF version of thesis. === Includes bibliographical references (pages 127-133). === Semiconductor nanocrystals, also known as quantum dots, are an exciting class of materials because their band gap can be tuned according to the nanocrystal size. In this way, the material band gap can be largely decoupled from its atomic composition - a property unique to this system. The potential applications for semiconductor nanocrystals are wide ranging and include: LEDs, photovoltaics, photon downconversion, photon upconversion, and thermoelectrics. However, their size-dependent band gap can also be a hindrance, as any size variation in the ensemble of nanocrystals introduces energetic disorder and spatial disorder in films. While synthesized as a colloid, for most applications the nanocrystals are deposited as a thin film. The rate of energy transfer between nanocrystals in the film, dictated by the arrangement and distance between neighbors, is therefore a critical parameter affecting device efficiency. As a result, controlling the nanocrystal physical arrangement is crucial to the success of these materials. Despite this, there is a lack of understanding of how to observe and control these processes at the nanoscale. This thesis begins by improving the synthesis of lead sulfide (PbS) nanocrystals to produce narrow size dispersity ensembles with tunable average size by ensuring the reaction is diffusion-limited. We then experimentally determine what parameters (ligand coverage, solvent, size dispersity) most affect the ability of these nanocrystals to self-assemble into highly ordered superlattice structures. We show that superlattices can be produced with a wide variety of surface ligands of differing lengths, either directly from a colloidal suspension or post-deposition and we thoroughly characterize the interparticle spacing as a function of ligand species. Next, we demonstrate an in situ X-ray scattering technique which enables the real-time visualization of nanocrystal self-assembly, with details unprecedented by any other experimental method. This technique led to a better understanding of the colloid to superlattice transition, including the observation of intermediate states and the ability to compare kinetics of different self-assembly aspects. Finally, we present experimental measurements demonstrating that nanocrystal size dispersity and selfassembly are critical to efficient energy transfer in films and that as energetic disorder is minimized through improved synthetic methods, spatial disorder becomes an increasingly important parameter to control. In the final experimental chapter of this thesis, we apply this knowledge to a different material system perovskite nanoplatelets, which have the potential to be useful as an inexpensive, solution-processable emission material. For these 2D materials, we optimize the thickness homogeneity and study the selfassembly of the nanoplatelets into stacked superstructures. We highlight the incredible tunability of this material system accessible through thickness and compositional tuning, which allows absorption and emission to be shifted across the entire visible range. Lastly, we demonstrate the potential of this system for next generation LEDs. === by Mark Clayton Weidman. === Ph. D.