UC Berkeley

Colloidal semiconductor and metal nanocyrstals receive attention from studying their physical properties to application in biology, electronics, optics, and catalyst. In addition, colloidal nanocrystals as artificial building blocks construct assemblies in solution phase and on substrate to exploit coupled properties of individual nanocrystals. A facile control of physical properties by tuning size, shape, and composition of nanocrystals can be achieved in solution phase synthetic protocols and now we have a library of literatures about them.

However, most of nanocrystal synthesis and their use are developed empirically, perhaps in a lack of fundamental understanding. Growth kinetics during synthesis, motion and behavior of nanocrystals in liquid and at interfaces include many questions which await mechanistic studies.

This dissertation explores direct observation of nanocrystals in liquid by using liquid cell transmission electron microscopy. Growth of nanocrystals from molecular precursor in solvent requires observation in atomistic resolution of a single particle level. Chapter 1 describes single particle growth trajectories of metal nanocrystals investigated by using graphene liquid cell in transmission electron microscopy. This cell is employed to achieve high-resolution imaging of colloidal platinum nanocrystal growth. The ability to directly image and resolve critical steps at atomistic resolution provides new insights into colloidal nanocrystal growth.

Control of nanocrystal interaction and drying condition of solvent determines the final morphology of nanocrystal self-assembly. Chapter 2 presents direct imaging of nanocrystal motions in solution and real-time formation of two-dimensional nanocrystal superlattices by using liquid phase transmission electron microscopy. Nucleation and growth processes are introduced accompanying with detailed mechanistic steps and single particle trajectories. The important role of solvent fluctuation for self-assembly of nanocrystals is explored in experiment and coarse-grained lattice gas modeling.

Concluding remarks are presented in Chapter 3.