Coropceanu Igor, Boles Michael A, Talapin Dmitri V
Department of Chemistry and James Franck Institute , University of Chicago , Chicago , Illinois 60637 , United States.
Center for Nanoscale Materials , Argonne National Laboratory , Argonne , Illinois 60439 , United States.
J Am Chem Soc. 2019 Apr 10;141(14):5728-5740. doi: 10.1021/jacs.8b12539. Epub 2019 Mar 29.
The self-assembly of two sizes of spherical nanocrystals has revealed a surprisingly diverse library of structures. To date, at least 15 distinct binary nanocrystal superlattice (BNSL) structures have been identified. The stability of these binary phases cannot be fully explained using the traditional conceptual framework treating the assembly process as entropy-driven crystallization of rigid spherical particles. Such deviation from hard sphere behavior may be explained by the soft and deformable layer of ligands that envelops the nanocrystals, which contributes significantly to the overall size and shape of assembling particles. In this work, we describe a set of experiments designed to elucidate the role of the ligand corona in shaping the thermodynamics and kinetics of BNSL assembly. Using hydrocarbon-capped Au and PbS nanocrystals as a model binary system, we systematically tuned the core radius ( R) and ligand chain length ( L) of particles and subsequently assembled them into binary superlattices. The resulting database of binary structures enabled a detailed analysis of the role of effective nanocrystal size ratio, as well as softness expressed as L/ R, in directing the assembly of binary structures. This catalog of superlattices allowed us to not only study the frequency of different phases but to also systematically measure the geometric parameters of the BNSLs. This analysis allowed us to evaluate new theoretical models treating the cocrystallization of deformable spheres and to formulate new hypotheses about the factors affecting the nucleation and growth of the binary superlattices. Among other insights, our results suggest that the relative abundance of the binary phases observed may be explained not only by considerations of thermodynamic stability, but also by a postulated preordering of the binary fluid into local structures with icosahedral or polytetrahedral symmetry prior to nucleation.
两种尺寸的球形纳米晶体的自组装展现出了令人惊讶的多样结构库。迄今为止，已鉴定出至少15种不同的二元纳米晶体超晶格（BNSL）结构。使用将组装过程视为刚性球形颗粒的熵驱动结晶的传统概念框架，无法完全解释这些二元相的稳定性。这种与硬球行为的偏差可能是由包裹纳米晶体的配体的柔软且可变形层所导致的，该层对组装颗粒的整体尺寸和形状有显著贡献。在这项工作中，我们描述了一组旨在阐明配体晕在塑造BNSL组装的热力学和动力学方面作用的实验。使用碳氢化合物封端的金和硫化铅纳米晶体作为模型二元体系，我们系统地调整了颗粒的核心半径（R）和配体链长度（L），随后将它们组装成二元超晶格。由此得到的二元结构数据库能够详细分析有效纳米晶体尺寸比以及以L/R表示的柔软度在指导二元结构组装中的作用。这个超晶格目录不仅使我们能够研究不同相的频率，还能系统地测量BNSL的几何参数。这种分析使我们能够评估处理可变形球体共结晶的新理论模型，并对影响二元超晶格成核和生长的因素提出新的假设。除了其他见解之外，我们的结果表明，观察到的二元相的相对丰度不仅可以通过热力学稳定性的考虑来解释，还可以通过在成核之前将二元流体假定为具有二十面体或多面体对称性的局部结构的预排序来解释。
