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

非技术摘要：半导体材料的持续发展和质量的不断提高，使得现代电子器件在过去的半个世纪里得到了稳步的改进。 半导体材料中的结构缺陷，即原子在晶体材料中错位的缺陷，通常限制了它们的性能，并且期望去除这样的缺陷。以前看到的许多改进依赖于对不同类型缺陷的行为、形成和去除途径的基本理解。 在这个项目中，我们的目标是了解纳米晶体中的缺陷，纳米晶体是晶体固体的微小碎片，并且已知其行为与其大块对应物不同。 纳米晶体可以被认为是单个乐高积木的小类似物，同样，它们可以连接到更大的结构中，这些结构可能具有比单个积木更有趣或更有用的特性。 不幸的是，当纳米晶体被附着时，它们可以不完美地附着，这可以引起限制传统半导体的相同晶体缺陷。 该项目旨在使用先进的电子显微镜技术观察这些缺陷的原子细节，以了解纳米晶体附着时形成的缺陷，如何去除这些缺陷，以及如何首先防止它们形成。 通过发展对纳米晶体中缺陷的基本理解，有可能制造具有足够材料质量（缺乏缺陷）的纳米晶体组件，以具有在任何天然材料中观察不到的性质。此外，该项目将不同的本科生群体暴露在高水平的科学研究中，这是一个独特的本科研究项目，其中约20名一年级本科生在研究生导师的指导下合作分析原子分辨电子显微镜数据。 技术摘要：具有原子相干界面和介观顺序的外延连接的纳米晶体的组装提供了一种具有有趣的新兴特性的材料的途径。然而，附着过程的不可逆性通常会导致界面处形成非平衡缺陷，从而限制材料的最终性能。一套原位透射电子显微镜（TEM）的实验被用来了解在胶体纳米晶体的定向附着的过程。 具体而言，原位高分辨率TEM实验探测位错动力学的基本原理，在不完全附着的颗粒。此外，自动化HRTEM图像分析工具用于确定小面，台阶边缘的数量，以及具有原子层精度的单个颗粒集合的尺寸。 该工具用于开发用于制备原子定义的纳米晶体的纳米晶体合成方案，该纳米晶体应在附着期间最小化缺陷形成。 最后，在一个特别设计的液相TEM单元设计的基础上双功能化氮化硅膜的液-液界面处原位成像的纳米晶体的排列和附着。 原位技术，以研究在液-液界面的动态提供洞察力，许多自组装过程在这些动态界面。 这些实验分别提供了对定向附着过程的每一步的见解，它们一起提供了从原子到介观尺度的组装过程的统一图景。 该项目培训研究生先进的原位材料表征工具，并准备他们成为未来的科学导师。该奖项反映了NSF的法定使命，并已被认为是值得通过评估使用基金会的智力价值和更广泛的影响审查标准的支持。

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.