Nanomaterials in Two Dimensions: Mechanistic Insights into the Formation and Synthesis of Superlattices and Nanosheets

Abstract

In the past decades, nanomaterials have become an increasingly important class of materials due to their size-tuneable optical, electronic and magnetic properties. Not only are they of great scientific interest, but their tunability and versatility has also resulted in a wide range of applications ranging from IR photodetectors to solar cells. In this thesis we discuss the synthesis and characterization of colloidal nanocrystals consisting of lead- or cadmium- chalcogenide units (PbX, CdX, X=Se, S). We studied the nanocrystals both as self-standing objects and as larger hierarchical 2D superstructures prepared with a bottom-up approach. Chapter 2 briefly introduces the background necessary to understand the concepts discussed in the rest of the thesis. I give a brief history on the implicit presence and usage of nanocrystals before discussing how we currently understand nanocrystals. First, I discuss the inorganic core of the nanocrystals and the influence of dimensionality on the luminescent properties. Then, I briefly discuss the post-synthetic process of cation exchange with which the crystalline core can be tailored further. From there, I give some insight into our understanding of the surface chemistry of nanocrystals and go on to discuss the interactions between ligand-stabilized nanocrystals in an organic apolar dispersion. Specifically, I go into the interactions between nanocrystals which play a role during self-assembly and oriented attachment. In chapter 3, I give an overview on the literature on oriented attachment up until 2020. Specifically, I focus on oriented attachment in the PbX (X = S, Se, Te) and CdX family (X = S, Se, Te) and the in situ study of this process. In chapter 4, I present a study of the formation of honeycomb superlattices build up from PbSe nanocrystals by self-assembly and oriented attachment. Initially, we studied how to avoid perturbation of the superlattice prior to characterisation before delving into the influence of the reactivity of the liquid substrate on the formation process and the final stages of alignment and attachment of the nanocrystals. The self-assembly pathway was then further investigated by varying the repulsion potentials between the nanocrystals in coarse-grained molecular dynamics simulations. In chapter 5, we studied the synthesis and growth mechanism for 2D PbS nanosheets. With atomically resolved TEM techniques we elucidate the structure of the nanosheets and study the crystallinity of the nanosheets during the growth (synthesis). It allowed us to propose a reaction mechanism for the growth of these nanosheets. Finally, we apply this knowledge to improve the crystallinity of the synthesized PbS sheets. In chapter 6, I show a post-synthetic Cd-for-Pb cation exchange starting with PbS nanosheets, in order to obtain CdS nanosheets. With ex situ TEM analysis we studied the cation exchange in time: Early on, the nanosheets show significant damage; but over time the damage is repaired and the completely exchanged CdS nanosheets even show (trap) luminescence.