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

摘要：在过去的半个世纪里，半导体材料的不断发展和质量的不断提高使得现代电子器件得到了稳步的改进。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，通常会限制其性能，因此需要消除这种缺陷。以前看到的许多改进都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的缺陷，纳米晶体是晶体固体的微小碎片，已知其行为不同于它们的大块对偶物。纳米晶体可以被认为是单个乐高积木的小型类似物，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构建块更有趣或更有用的特性。不幸的是，当纳米晶体附着在一起时，它们可能会不完美地附着在一起，从而产生与传统半导体相同的晶体缺陷。该项目旨在利用先进的电子显微镜技术从原子细节上观察这些缺陷，以了解纳米晶体附着时形成的缺陷，如何去除这些缺陷，以及如何从一开始就防止它们形成。通过发展对纳米晶体缺陷的基本理解，有可能使纳米晶体组件具有足够的材料质量（缺乏缺陷），从而具有在任何天然材料中都没有观察到的特性。此外，该项目通过一个独特的本科生研究项目，让不同类型的本科生接触到高水平的科学研究，约20名一年级本科生在研究生导师的指导下协同工作，分析原子分辨电镜数据。技术摘要：具有原子相干界面和中尺度有序的外延连接纳米晶体的组装为具有有趣涌现特性的材料提供了一条途径。然而，附着过程的不可逆性往往导致在界面处形成非平衡缺陷，从而限制了材料的最终性能。采用原位透射电镜（TEM）实验研究了胶体纳米晶体的定向附着过程。具体来说，原位高分辨率透射电镜实验探索了不完全附着颗粒中位错动力学的基本原理。此外，采用自动HRTEM图像分析工具，以原子层精度确定单个粒子的面形、阶边数和集合大小。该工具用于开发纳米晶体合成方案，以制备原子定义的纳米晶体，从而最大限度地减少附着过程中缺陷的形成。最后，在基于双功能化氮化硅膜的特殊设计的液相TEM电池中，在液-液界面处原位成像纳米晶体的排列和附着。研究液-液界面动力学的原位技术为这些动态界面上的许多自组装过程提供了见解。这些实验单独提供了对定向依恋过程的每个步骤的见解，并且它们一起提供了从原子到中尺度的组装过程的统一图像。该项目训练研究生掌握先进的原位材料表征工具，并为他们成为未来的科学导师做好准备。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.