Huang Xin, Zhu Jinlong, Ge Binghui, Deng Kerong, Wu Xiaotong, Xiao Tianyuan, Jiang Tian, Quan Zewei, Cao Y Charles, Wang Zhongwu
Cornell High Energy Synchrotron Source , Cornell University , Ithaca , New York 14853 , United States.
Center for High Pressure Science and Technology Advanced Research (HPSTAR) , Beijing 100090 , P. R. China.
J Am Chem Soc. 2019 Feb 20;141(7):3198-3206. doi: 10.1021/jacs.8b13082. Epub 2019 Feb 12.
Nanocube (NC) assemblies display complex superlattice behaviors, which require a systematic understanding of their nucleation and growth as well transformation toward construction of a consistent superlattice phase diagram. This work made use of FeO NCs with controlled environments, and assembled NCs into three-dimensional (3D) superlattices of simple cubic (sc), body-centered cubic (bcc), and face-centered cubic (fcc), acute and obtuse rhombohedral (rh) polymorphs, and 2D superlattices of square and hexagon. Controlled experiments and computations of in situ and static small-angle X-ray scattering (SAXS) as well as electron microscopic imaging revealed that the fcc and bcc polymorphs preferred a primary nucleation at the early stage of NC assembly, which started from the high packing planes of fcc(111) and bcc(110), respectively, in both 3D and 2D cases. Upon continuous growth of superlattice grain (or domain), a confinement stress appeared and distorted fcc and bcc into acute and obtuse rh polymorphs, respectively. The variable magnitudes of competitive interactions between configurational and directional entropy determine the primary superlattice polymorph of either fcc or bcc, while emergent enhancement of confinement effect on enlarged grains attributes to late developed superlattice transformations. Differently, the formation of a sc polymorph requires a strong driving force that either emerges simultaneously or is applied externally so that one easy case of the sc formation can be achieved in 2D thin films. Unlike the traditional Bath deformation pathway that involves an intermediate body-centered tetragonal lattice, the observed superlattice transformations in NC assembly underwent a simple rhombohedral distortion, which was driven by a growth-induced in-plane compressive stress. Establishment of a consistent phase diagram of NC-based superlattices and reconstruction of their assembly pathways provide critical insight and a solid base for controlled design and scalable fabrication of nanocube-based functional materials with desired superlattices and collective properties for real-world applications.
纳米立方体（NC）组件表现出复杂的超晶格行为，这需要系统地了解它们的成核和生长过程，以及向构建一致的超晶格相图的转变。这项工作利用了处于可控环境中的FeO纳米立方体，并将纳米立方体组装成简单立方（sc）、体心立方（bcc）和面心立方（fcc）的三维（3D）超晶格、锐角和钝角菱面体（rh）多晶型物，以及正方形和六边形的二维超晶格。原位和静态小角X射线散射（SAXS）以及电子显微镜成像的对照实验和计算表明，fcc和bcc多晶型物在NC组装的早期阶段倾向于一次成核，在3D和2D情况下，分别从fcc(111)和bcc(110)的高堆积平面开始。随着超晶格晶粒（或畴）的持续生长，出现了限制应力，分别将fcc和bcc扭曲成锐角和钝角rh多晶型物。构型熵和方向熵之间竞争相互作用的可变大小决定了fcc或bcc的主要超晶格多晶型物，而对增大晶粒的限制效应的出现增强归因于后期发展的超晶格转变。不同的是，sc多晶型物的形成需要一个同时出现或外部施加的强大驱动力，以便在二维薄膜中实现一种简单的sc形成情况。与涉及中间体心四方晶格的传统巴斯变形途径不同，在NC组装中观察到的超晶格转变经历了由生长诱导的面内压缩应力驱动的简单菱面体畸变。建立基于NC的超晶格的一致相图并重建其组装途径，为具有所需超晶格和集体性质的纳米立方体基功能材料的可控设计和可扩展制造提供了关键见解和坚实基础，以用于实际应用。
