Driving Forces and Effects of Ligand Exchange in Nanocrystalline Systems

Nanocrystals as functional materials rely heavily on their ability to interface with their immediate environment. This is true across the breadth of material applications, from semiconductor quantum dots to plasmonic metal particles and oxides for catalysis. These interfacial interactions can be just as important in defining material properties as the self-contained particle structure. They may control biocompatibility, directed assembly, resistance to degradation and the can even influence electronic energy levels. Nanoscale interfaces are therefore of great interest in the push to make nanomaterials viable solutions for real-world problems. To understand nanoparticle surfaces and interfaces we must begin by characterizing them at the most fundamental level. Molecular species and bulk interfaces both have techniques that have been well-designed to uncover detailed reaction mechanisms and chemical structures, but the same suite of characterization tools has not been applicable to nanoscale materials.This dissertation will delve into solution-phase and solvent-friendly techniques for characterizing nanocrystal surfaces, the specifics of surface reaction mechanisms and structure in a model system, and the application of surface analysis models to the nanoscale. Chapter 1 will introduce the effects of surface chemistry on nanocrystal functionality with a focus on semiconductor quantum dots. Quantum dot surface passivation and controlled ligand functionality will be discussed in relation to desired nanocrystal properties. We will go into the current state of the literature surrounding post-synthetic modification of nanoscale surfaces and lastly, we will touch upon the ways that ligand exchange reactions can give unique information about the surface of a nanomaterial.Chapter 2 will address some of the previous work done in nanocrystal surface characterization and look further into the surface sensitivity of nanocrystal proton NMR. The basics of nuclear magnetic resonance will be addressed, as will the mechanisms by which nanocrystal binding affects the observed resonance signal. This binding sensitivity will then be explained in the context of directly tracking a surface modification reaction. Chapter 3 will then follow with an introduction to isothermal titration calorimetry and its adaptation from biochemistry to nanoscale organic-inorganic interfaces. We will build on preliminary studies to describe the utility of ITC for understanding the nanocrystal surface.Chapter 4 will describe the design and synthesis of a model CdSe nanocrystal system for developing a better understanding of surface interactions and this system will be further explored in chapters 5 and 6 as the basis for two model classes of surface reaction. Both carboxylate-carboxylate and carboxylate-phosphonate ligand exchanges are thermodynamically characterized and we propose a model combined inter-ligand and ligand binding interactions as the underlying cause of the surface dynamics.Chapter 7 looks deeper into classic analysis methods for surface reactions and their application to quantum dot ligand exchanges. We explore the benefits and limitations of adsorption isotherms and statistical mechanical simulations in corroboration of our surface model. Finally, we derive quantitative estimates of the thermodynamic contributions to ligand exchange driving force in our special case reactions. Chapters 8 and 9 look forward to future surface characterization studies with preliminary results from other surface sensitive measurements and modified nanoparticle systems. In conclusion, we discuss the implications and future directions for this work in the wider field of nanocrystal design and applications