UC Berkeley

Colloidal semiconductor nanocrystals, often called quantum dots are small chunks of a bulk semiconducting material. Due to the small size of these materials, quantum mechanical effects modify their properties giving them atomic-like electronic energy levels. If these nanocrystals are brought close enough for the atomic-like states to couple, an artificial molecule or artificial solid may result from the coupling. Thus the assembly would display emergent properties distinct from the isolated nanocrystals. This may allow for the preparation of materials which have properties not displayed in natural materials.

One difficulty is that in order for the coupling to be strong enough to see delocalized states, the nanocrystals must be atomically fused with an inorganic bridge. This attachment is irreversible and thus if the attachment is imperfect, it can trap atomic scale defects such as dislocations at the interface. These atomic scale defects will also interact with carriers potentially preventing the emergence of delocalized states. Currently there isn't a framework for understanding defects in imperfectly attached nanocrystals. As such it is difficult to rationally improve quality in these materials or identify new routes high quality attachment of semiconductor nanocrystals. To this end we aim to better understand the attachment of nanocrystals with a focus on the defects which may form and how to remove those defects.

In this thesis, Chapter 1 outlines the important concepts related to the electronic properties of solids, confined semiconductor physics, nanocrystal synthesis and crystal defects needed to understand the work in this thesis. In Chapter 2, we study the oriented attachment of PbTe using in-situ transmission electron microscopy (TEM) to identify optimal attachment facets to minimize defect formation. In Chapter 3, we develop methods to attach CdSe nanocrystals on their prismatic facets, study the defect which form, and understand their removal mechanisms using in-situ TEM. The insights from these two studies are used in Chapter 4 to design a clean sheet approach to assemble and atomically attach CdSe/CdS nanocrystals into superlattices with long range translational and orientational order. Finally in Chapter 5, we discuss the outlook for preparing attached nanocrystal superlattices for realizing artificial band structures.