Defect Chemistry and Ion Intercalation during the Growth and Solid-state Transformation of Metal Halide Nanocrystals

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Friday, January 18, 2019 -
11:00am to 1:00pm
McMillen Laboratory, Rm 311
Bo Yin, IMSE Doctoral Candidate

Semiconductor metal halides are light-sensitive material and have applications in photographic film, as antibacterial agents and photocatalysts.  One focus of this dissertation is to achieve novel morphologies of ternary silver bromoiodide (AgBr1-xIx, 0<x<1) nanocrystals by intentionally introducing defects into the system.  Another goal of this work is to study the reaction kinetics of solid-state transformation in single lead bromide (PbBr2) nanocrystals via fluorescence microscopy. We find that in both of these materials systems, the solid-state immiscibility between different halide compounds plays an important role in their resulting chemistry. 

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.  With increasing iodide content the morphology of the nanocrystals evolves from cubic to truncated cubic to hexagonal prismatic.  Our experimental observations and density functional theory (DFT) 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, 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.  Sharp rises in the intensity of single nanocrystals indicate they transform much faster than the time it takes for the ensemble average to transform.  The insights gained from this study may be used to further control the crystallization of CH3NH3PbBr3 and other solution-processed semiconductors.

 

Dissertation Examination Committee:

Bryce Sadtler, Chair

Richard Loomis          Shankar M.L. Sastry
Rohan Mishra              Srikanth Singamaneni

Faculty, students, and the general public are invited.

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