Chen Isaiah Y, Cimada daSilva Jessica, Balazs Daniel M, Smeaton Michelle A, Kourkoutis Lena F, Hanrath Tobias, Clancy Paulette
Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, Maryland 21218, United States.
ACS Nano. 2020 Sep 22;14(9):11431-11441. doi: 10.1021/acsnano.0c03800. Epub 2020 Aug 27.
The formation of defect-free two-dimensional nanocrystal (NC) superstructures remains a challenge as persistent defects hinder charge delocalization and related device performance. Understanding defect formation is an important step toward developing strategies to mitigate their formation. However, specific mechanisms of defect formation are difficult to determine, as superlattice phase transformations that occur during fabrication are quite complex and there are a variety of factors influencing the disorder in the final structure. Here, we use Molecular Dynamics (MD) and electron microscopy in concert to investigate the nucleation of the epitaxial attachment of lead chalcogenide (PbX, where X = S, Se) NC assemblies. We use an updated implementation of an existing reactive force field in an MD framework to investigate how initial orientational (mis)alignment of the constituent building blocks impacts the final structure of the epitaxially connected superlattice. This Simple Molecular Reactive Force Field (SMRFF) captures both short-range covalent forces and long-range electrostatic forces and allows us to follow orientational and translational changes of NCs during superlattice transformation. Our simulations reveal how robust the oriented attachment is with regard to the initial configuration of the NCs, measuring its sensitivity to both in-plane and out-of-plane misorientation. We show that oriented attachment nucleates through the initial formation of dimers, which corroborate experimentally observed structures. We present high-resolution structural analysis of dimers at early stages of the superlattice transformation and rationalize their contribution to the formation of defects in the final superlattice. Collectively, the simulations and experiments presented in this paper provide insights into the nucleation of NC oriented attachment, the impact of the initial configuration of NCs on the structural fidelity of the final epitaxially connected superlattice, and the propensity to form commonly observed defects, such as missing bridges and atomic misalignment in the superlattice due to the formation of dimers. We present potential strategies to mitigate the formation of superlattice defects.
无缺陷二维纳米晶体（NC）超结构的形成仍然是一个挑战，因为持续存在的缺陷会阻碍电荷离域和相关器件性能。了解缺陷形成是制定减轻其形成策略的重要一步。然而，缺陷形成的具体机制很难确定，因为制造过程中发生的超晶格相变非常复杂，而且有多种因素影响最终结构的无序性。在这里，我们结合使用分子动力学（MD）和电子显微镜来研究硫族化铅（PbX，其中X = S、Se）NC组件外延附着的成核过程。我们在MD框架中使用现有反应力场的更新实现方式，来研究组成结构单元的初始取向（错）排列如何影响外延连接超晶格的最终结构。这种简单分子反应力场（SMRFF）既捕捉短程共价力又捕捉长程静电力，并使我们能够跟踪超晶格转变过程中NC的取向和平移变化。我们的模拟揭示了NC初始构型对外延附着的稳健程度，测量了其对平面内和平面外取向错误的敏感性。我们表明，外延附着通过二聚体的初始形成而成核，这证实了实验观察到的结构。我们展示了超晶格转变早期二聚体的高分辨率结构分析，并阐明了它们对最终超晶格中缺陷形成的贡献。总体而言，本文提出的模拟和实验提供了对NC外延附着成核、NC初始构型对最终外延连接超晶格结构保真度的影响以及形成常见缺陷（如由于二聚体形成导致超晶格中桥缺失和原子错位）倾向的见解。我们提出了减轻超晶格缺陷形成的潜在策略。
