ORCID

http://orcid.org/0000-0002-7268-3483

Date of Award

Winter 12-15-2021

Author's School

Graduate School of Arts and Sciences

Author's Department

Chemistry

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

Colloidal cesium lead halide perovskite nanocrystals (CsPbX3, X = Cl, Br or I) are attractive for optoelectronic applications due to their unique chemical and physical properties including high photoluminescence quantum yield, tunable bandgaps, high defect tolerance, and low-cost processibility. A facile method to tune the emission wavelength of perovskite nanocrystals is through anion exchange. For light emission applications, it is desirable to prepare CsPbX3 nanocrystals of uniform size and composition. However, the complex reaction kinetics of this chemical transformation may limit the compositional uniformity and color purity of the CsPbX3 nanocrystals produced by anion exchange when the transformation is scaled up. In this dissertation, we used fluorescence microscopy to monitor reaction trajectories of individual CsPbX3 nanocrystals as they undergo anion exchange, which enables us to quantify heterogeneity in reactivity among hundreds of nanocrystals prepared within the same batch. We observe a varying dependence of the switching time for changes in fluorescence intensity of individual CsPbX3 nanocrystals on the concentration of substitutional halide ions in solution. We found that the switching times at a given halide concentration are similar in both reaction directions for anion exchange between CsPbCl3 and CsPbBr3 nanocrystals. However, the CsPbI3 nanocrystals undergo more abrupt shifts in their emission characteristics as they transform to CsPbBr3, while CsPbBr3 nanocrystals exhibit a smoother transition during their transformation to CsPbI3. We developed kinetic models to simulate anion exchange in these different systems, in which differences in the reaction path are reflected in the way the probability for each exchange event in a particle evolves with the number of previous successful events. We attribute differences in the concentration dependence of fluorescence switching times for different exchange pairs to differences in the relative miscibility of the initial and final structures. Highly miscible systems, such as the CsPbCl3/CsPbBr3 pair exhibit longer switching times compared to immiscible crystal pairs that require a substantial reorganization of the ions. Furthermore, the switching times for miscible systems show a stronger dependence on the concentration of the substitutional ion. However, we observed asymmetric behavior at the single-particle level when the interconversion between CsPbBr3 and CsPbI3 proceeded in opposite directions Compared to the transformation from CsPbBr3 to CsPbI3, the transformation from CsPbI3 to CsPbBr3 exhibits shorter switching times and weaker dependence on the concentration of the substitutional ion. The irreversibility in anion exchange between CsPbBr3 and CsPbI3 nanocrystals is attributed to structural differences between CsPbI3 nanocrystals synthesized by the hot-injection method and those prepared by anion exchange. The distinct transformation kinetics during anion exchange revealed in both anion exchange reactions are important to consider when scaling-up the production of CsPbX3 nanocrystals with high compositional homogeneity and color purity for optoelectronic applications.Optoelectronic devices such as solar cells and light emitting diodes may operate at elevated temperatures up to 100℃, depending on their operating environment. While the crystal structure of the CsPbX3 nanocrystals can be preserved, a loss in photoluminescence has been observed near this temperature. In this dissertation, we used single-particle fluorescence microscopy to study how heating affects the properties of individual CsPbBr3@SiO2 nanocrystals. The growth of SiO2 shells around CsPbBr3 nanocrystals enhanced their stability during characterization and prevented the oriented attachment of CsPbBr3 nanocrystals during annealing. We measured the fluorescence trajectories of individual CsPbBr3@SiO2 nanocrystals before and after annealing at different temperatures. We observed three sub-populations of nanocrystals within a single batch, which behaved differently upon annealing. One population of nanocrystals were stable while the other two either became brighter or darker after annealing. Based on statistical analysis of single-particle fluorescence trajectories, we attributed the difference in behavior to variations in trap states among the different nanocrystals. Furthermore, transient absorption spectroscopy revealed that the ratio of deep traps (i.e., traps with energy levels in middle of the bandgap) decreased after annealing. X-ray photoelectron spectroscopy indicated that defect species associated with deep traps were oxidized during the thermal treatment. This work connects the change in single-particle fluorescence upon annealing with the variation in trap states and suggests that thermal treatment at mild temperatures can be applied to passivate deep trap states in CsPbX3 nanocrystals.

Language

English (en)

Chair and Committee

Bryce F. Sadtler

Committee Members

William E. Buhro

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