UC Berkeley

As materials approach the nanoscale, they begin to exhibit size dependent properties. In particular, a larger fraction of atoms that make up the whole of the material are located on the material’s surface. Colloidal nanocrystals are commonly synthesized where their surfaces are terminated by organic molecules, often referred to as ligands. Calorimetric techniques such as isothermal titration calorimetry have been applied to colloidal nanocrystals to study reactions at their surfaces over the past several years, but modeling these reactions has proven challenging. In this work, isothermal titration calorimetry was used to understand reaction mechanisms that increase the luminescence in semiconductor nanocrystals, or quantum dots. Furthermore, the collective effects of the aliphatic carbon chains that make up much of the organic component of colloidal nanocrystals were examined and insights from simulations using a local environment dependent model, the Ising model, will be discussed. With these insights, the structure of the organic ligand shell surrounding the nanocrystal was further probed using traditional X-ray diffraction techniques, which showed ordering of ligands in the organic ligand shell, both as dried powders and in a liquid environment. Finally, dissolution calorimetry experiments measuring negative surface energies of colloidal nanocrystals will be discussed as well as their implications for understanding the thermodynamics of colloidal nanocrystals.

Chapters 1 and 2 will lay the ground work for understanding the structure of colloidal nanocrystals and the main techniques used to study reactions at nanocrystal's surface. Chapter 3 will discuss measurements of reactions at the nanocrystal surface that impact their luminescence, while Chapter 4 will delve into parameters that influence the strength of collective effects between organic ligands bound to nanocrystal surfaces. Chapter 5 will focus on X-ray diffraction measurements of the structure of the organic ligand shell, while Chapter 6 will be devoted to studying negative surface and interface energies of colloidal nanocrystals. Chapter 7 will discuss the future of thermodynamic studies on the surfaces of colloidal nanocrystals.