Understanding epitaxial nanocrystal attachment processes across length scales with the aim of designing defect-free interfaces

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

Non-Technical Abstract: The continued development of semiconductor materials with ever increasing quality has allowed for a steady improvement of modern electronic devices over the past half century. Structural defects in semiconductor materials, imperfections where atoms are misplaced in the crystalline material, often limit their performance, and it is desirable to remove such defects. Many of the improvements previously seen relied on a fundamental understanding of the behavior, formation, and removal pathways of different types of imperfections. In this project, we aim to understand defects in nanocrystals, which are tiny pieces of crystalline solids and are known to behave differently than their bulk counterparts. Nanocrystals can be thought of as small analogues of individual LEGO bricks, and similarly they can be attached into larger structures that may have properties that are more interesting or useful than the individual building blocks. Unfortunately, when nanocrystals are attached, they can attach imperfectly which can give rise to the same crystalline defects that limit traditional semiconductors. This project aims to look at these defects with atomic detail using advanced electron microscopy techniques to understand the defects that form when nanocrystals attach, how those defects can be removed, and how to prevent them from forming in the first place. By developing a fundamental understanding of defects in nanocrystals, it may be possible to make nanocrystal assemblies with sufficient material quality (lack of defects) to have properties not observed in any natural material. Furthermore, this project exposes a diverse body of undergraduates to high-level scientific research thought a unique undergraduate research program where ~20 first-year undergraduate students work collaboratively to analyze atomically resolved electron microscopy data under the guidance of graduate student mentors. Technical Abstract: The assembly of epitaxially connected nanocrystals with atomically coherent interfaces and mesoscale order provides a route to materials with interesting emergent properties. However, the irreversible nature of the attachment process often leads to nonequilibrium defect formation at the interfaces, which can limit ultimate materials performance. A suite of in-situ transmission electron microscopy (TEM) experiments are used to understand the process of oriented attachment in colloidal nanocrystals. Specifically, in-situ high resolution TEM experiments probe the fundamentals of dislocation dynamics in imperfectly attached particles. In addition, an automated HRTEM image analysis tool is used to determine faceting, number of step edges, and size of ensembles of single particles with atomic layer precision. This tool is used to develop nanocrystal synthesis protocols for preparing atomically defined nanocrystals which should minimize defect formation during attachment. Finally, alignment and attachment of nanocrystals are imaged in-situ at a liquid-liquid interface in a specially designed liquid phase TEM cell design based on bifunctionalized silicon nitride membranes. In-situ techniques to study dynamics at liquid-liquid interfaces provide insight to many self-assembly processes at these dynamic interfaces. Individually these experiments provide insights into each step of the oriented attachment process, and together they provide a unified picture of assembly processes from the atomic to mesoscale. This project trains graduate students in advanced in-situ materials characterization tools and prepares them to be future scientific mentors. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.