Growth Mechanism and Controllable Synthesis of Cesium Lead Trihalide Nanocrystals, Nanorods, & Complex Geometries

Abstract

A lack of detailed understanding of the growth mechanisms underlying the synthesis of CsPbX₃ (X=Cl, Br, or I) nanocrystals (NCs) has hindered sophisticated morphological and chemical control. Moreover, the fast kinetics of CsPbX₃ NC growth entails that many synthetic strategies for structural control established in other semiconductor systems do not apply. This dissertation outlines a comprehensive study of the factors that can be manipulated to better control the kinetics of the growing material.

First, I elucidate the growth mechanism of CsPbBr₃ NCs by slowing reaction kinetics. Over the course of reaction, isolated intermediates reveal a sequential transformation of the NC composition. The evolution is dependent on the effective concentration of bromide but is not sensitive to the amount of lead, distinguishing the growth behavior from other semiconductor systems.

With this mechanistic understanding, I demonstrate a slow injection strategy to further manipulate the reaction kinetics of CsPbX₃ growth. In this way, monodisperse, quantum-confined nanorods with controllable size and aspect ratio are obtained. In particular, the crystal lattice can be continuously varied between thermodynamically stable and metastable structures. An expression that describes the structural tailorability is established. Controlled modulation of the bond distances and angles in NC opens opportunities for more sophisticated understanding of structure-property relationships.

Post-synthetic anion-exchange reactions are exploited as an alternative route to finely tune the halide composition and thus the electronic structures of CsPbX₃. I explore the crystallographic and morphological stability of the quantum-confined, metastable CsPbBr₃ nanorods during anion-exchange reactions. The stability of nanorods with respect to each reagent is investigated, and reaction conditions for full exchange without significant degradation are identified.

Semiconductor heterostructures often provide several advantages for their use as functional materials. Building from the method described above, attempts are made to prepare CsPbX₃-based heterostructures with semiconductor or metal nanoparticles as seeds. The surface chemistry of seeds and controlled nucleation of CsPbX₃ are studied, and strategies to exclusively grow CsPbX₃ on the seeds are proposed.

During the syntheses of CsPbX₃, byproducts with hollow nanocubes shape are commonly produced. The byproducts are identified as KBr, and a hollowing mechanism for the architecture is proposed.