ORCID

http://orcid.org/https://orcid.org/0000-0003-2180-3948

Date of Award

Spring 5-15-2019

Author's School

School of Engineering & Applied Science

Author's Department

Mechanical Engineering & Materials Science

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

Abstract of the Dissertation

Defect Chemistry and Ion Intercalation During the Growth and Solid-State Transformation of Metal Halide Nanocrystals

Semiconductor metal halides as light-sensitive materials have applications in multiple areas, such as photographic film, antibacterial agents and photocatalysts. One focus of this dissertation is to achieve novel morphologies of ternary silver bromoiodide (AgBr1-xIx, 0

For the silver halide system, we demonstrate that the anion composition of AgBr1-xIx nanocrystals determines their shape through the introduction of twin defects as the nanocrystals are made more iodide-rich. AgBr1-xIx nanocrystals grow as single-phase, solid solutions with the rock salt crystal structure for anions compositions ranging from 0 ≤ x < 0.38. With increasing iodide content the morphology of the nanocrystals evolves from cubic to truncated cubic to hexagonal prismatic. Structural characterization indicates the cubic nanocrystals are bound by {100} facets whereas the hexagonal platelet nanocrystals possess {111} facets as their top and bottom surface. Calculations based on first-principles density functional theory show that iodide substitution in AgBr stabilizes {111} surfaces and that twin defects parallel to these surfaces possess a low formation energy. Our experimental observations and calculations are consistent with a growth model in which the presence of multiple twin defects parallel to a {111} surface enhances lateral growth of the side facets and changes the nanocrystal shape.

To study the reaction kinetics of solid-state conversion in the lead halide system, we use the change in fluorescence brightness to image the transformation of individual lead bromide (PbBr2) nanocrystals to methylammonium lead bromide (CH3NH3PbBr3) via intercalation of CH3NH3Br. Analyzing this reaction one nanocrystal at a time reveals information that is masked when the fluorescence intensity is averaged over many particles. Sharp rises in the intensity of single nanocrystals indicate they transform much faster than the time it takes for the ensemble average to transform. Furthermore, the intensity rises for individual nanocrystals are insensitive to the CH3NH3Br concentration. To explain these observations, we propose a phase transformation model in which the reconstructive transitions necessary to convert a PbBr2 nanocrystal into CH3NH3PbBr3 initially create a high energy barrier for ion intercalation. A critical point in the transformation occurs when the crystal adopts the perovskite phase, at which point the activation energy for further ion intercalation becomes progressively smaller. Monte Carlo simulations that incorporate this change in activation barrier into the likelihood of reaction events reproduce key experimental observations for the intensity trajectories of individual particles. The insights gained from this study may be used to further control the crystallization of CH3NH3PbBr3 and other solution-processed semiconductors.

In this dissertation, we focus on two different systems, silver halide and lead halide perovskite. Even though the systems are different, we find that solid-state immiscibility between different halide compounds plays an important role in both reactions we are studying. In AgBr1-xIx, the structural immiscibility between rock salt AgBr and wurtzite AgI causes the formation of twin boundaries, which change the nanocrystal morphology. For the lead halide system, the sharp transition in fluorescence intensity observed in single nanocrystals is also due to structural immiscibility, which causes the sudden phase transition from PbBr2 to the perovskite phase. This structural immiscibility between different halide compounds plays a critical role for both metal halide systems.

Language

English (en)

Chair

Bryce F. Sadtler

Committee Members

Bryce F. Sadtler, Richard A. Loomis, Rohan Mishra, Shankar Sastry,

Comments

Permanent URL: https://doi.org/7936/pwh6-cf27

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