Dongyan Zhang

ORCID

http://orcid.org/0000-0002-6072-9149

Date of Award

Spring 5-15-2023

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 (CsPbX3, X = Cl, Br, I) perovskite nanocrystals are being investigated as promising materials for light-emitting devices and fluorescence probes due to their strong absorption and emission (1.55 eV – 3.05 eV) of visible light. Anion exchange is a facile, post-synthetic method to tune the bandgap emission of CsPbX3 nanocrystals, covering most of the spectrum of visible light. However, structural heterogeneities in nanocrystals formed during anion exchange will lead to lower photoluminescence quantum yield and a broader range of emission wavelengths. A single batch of nanocrystals will contain a distribution of sizes, shapes, surface structures, and defect concentrations. These variations will lead to heterogenous reactivity during chemical transformations such as anion exchange. To enhance the brightness and color purity of light-emitting devices that incorporate CsPbX3 nanocrystals, the factors that control heterogeneous reactivity need to be understood. In this dissertation, we performed research on anion exchange between CsPbBr3 and CsPbI3 nanocrystals using in situ single-particle fluorescence imaging. This method can probe differences in the reactivity of hundreds of nanocrystals undergoing anion exchange at the same time. As the emission from each nanocrystal shifts to longer wavelengths during anion exchange, they turn on and appear as bright spots during the fluorescence video, which indicates their transformation. By analyzing different single-particle reaction trajectories, we assign a switching time, which is a measure of the time it takes the nanocrystal to complete anion exchange. For the transformation of CsPbBr3 nanocrystals to CsPbI3, the average switching time of individual nanocrystals is longer than the reverse transformation of CsPbI3 nanocrystals to CsPbBr3. To understand the kinetics during anion exchange, we performed experiments using different concentrations of substitutional halide anions. We observed a stronger concentration dependence in the average switching times for the forward reaction of CsPbBr3 nanocrystals to CsPbI3 than in the reverse reaction (i.e., CsPbI3 to CsPbBr3). We developed different models using Monte Carlo simulations to rationalize the experimental results. We attribute the difference in switching times to a more abrupt change in structure for the transformation of CsPbI3 nanocrystals to CsPbBr3, arising from a larger change in the tilting angle of PbX6 octahedra. These results indicate that CsPbI3 nanocrystals synthesized by the hot injection method have a different structure than CsPbI3 nanocrystals synthesized by anion exchange. We also studied the size dependence of anion exchange from CsPbBr3 nanocrystals to CsPbI3 using single-particle imaging. We observed a longer average reaction time and a stronger dependence of the average switching time on the concentration of substitutional halide anions for smaller nanocrystals (< 8 nm), compared to nanocrystals with a larger size (> 8 nm). Comparison to ensemble fluorescence spectroscopy indicates we are probing a critical stage of the reaction after anion exchange has initiated on the surface of the nanocrystals. We developed different kinetic models using Monte Carlo simulations to understand this size-dependent reactivity. We attribute the size dependence to a larger structural reorganization that must occur in larger nanocrystals during anion exchange, which leads to a more abrupt transformation compared with smaller nanocrystals. Our results indicate that size-dependent miscibility between CsPbBr3 and CsPbI3 at the nanoscale controls the reaction kinetics. Smaller nanocrystals maintain a homogenous structure during the entire transformation. As the nanocrystal size increases, variations in the tilting patterns of PbX6 octahedra in the perovskite crystals lead to different structures for CsPbBr3 and CsPbI3. Thus, a new iodide-rich phase must first nucleate within larger CsPbBr3 nanocrystals, which is then followed by their rapid transformation. Cumulatively, these results indicate that there are intrinsic variations in the reactivity of nanocrystals undergoing the same transformation. This heterogeneity imposes a limit on the narrowness of emission wavelengths for CsPbX3 nanocrystals prepared by anion exchange. Synthetic methods that maintain a high and uniform concentration of substitutional halide anions during anion exchange will minimize variations in the composition and will maximize the color purity of the resulting CsPbX3 nanocrystals for applications in light-emitting devices.

Language

English (en)

Chair and Committee

Bryce B. Sadtler William Buhro

Committee Members

Joseph J. Fournier, Richard R. Loomis, Rohan R. Mishra,

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