Cornell High Energy Synchrotron Source, Cornell University , Ithaca, New York 14853, United States.
Sandia National Laboratories, Advanced Materials Laboratory , 1001 University Boulevard SE, Albuquerque, New Mexico 87106, United States.
J Am Chem Soc. 2017 Oct 18;139(41):14476-14482. doi: 10.1021/jacs.7b06908. Epub 2017 Oct 9.
Nanocrystals (NCs) can self-assemble into ordered superlattices with collective properties, but the ability for controlling NC assembly remains poorly understandable toward achievement of desired superlattice. This work regulates several key variables of PbS NC assembly (e.g., NC concentration and solubility, solvent type, evaporation rate, seed mediation and thermal treatment), and thoroughly exploits the nucleation and growth as well as subsequent superlattice transformation of NC assembles and underneath mechanisms. PbS NCs in toluene self-assemble into a single face-centered-cubic (fcc) and body-centered-cubic (bcc) superlattice, respectively, at concentrations ≤17.5 and ≥70 mg/mL, but an intermediate concentration between them causes the coexistence of the two superlattices. Differently, NCs in hexane or chloroform self-assemble into only a single bcc superlattice. Distinct controls of NC assembly in solvent with variable concentrations confirm the NC concentration/solubility mediated nucleation and growth of superlattice, in which an evaporation-induced local gradient of NC concentration causes simultaneous nucleation of the two superlattices. The observation for the dense packing planes of NCs in fast growing fcc rather than bcc reveals the difference of entropic driving forces responsible for the two distinct superlattices. Decelerating the solvent evaporation does not amend the superlattice symmetry, but improves the superlattice crystallinity. In addition to shrinking the superlattice volume, thermal treatment also transforms the bcc to an fcc superlattice at 175 °C. Through a seed-meditated growth, the concentration-dependent superlattice does not change lattice symmetry over the course of continuous growth, whereas the newly nucleated secondary small nuclei through a concentration change have relatively higher surface energy and quickly dissolve in solution, providing additional NC sources for the ripening of the primarily nucleated larger and stable seeds. The observations under multiple controls of assembly parameters not only provide insights into the nucleation and growth as well as transformation of various superlattice polymorphs but also lay foundation for controlled fabrication of desired superlattice with tailored property.
纳米晶体（NCs）可以自组装成具有集体性质的有序超晶格，但控制 NC 组装的能力仍然难以理解，难以实现所需的超晶格。本工作调节了 PbS NC 组装的几个关键变量（例如 NC 浓度和溶解度、溶剂类型、蒸发速率、种子介导和热处理），并彻底利用了 NC 组装的成核和生长以及随后的超晶格转变及其内在机制。在甲苯中，PbS NC 自组装分别形成单一的面心立方（fcc）和体心立方（bcc）超晶格，浓度分别≤17.5 和≥70mg/mL，但在它们之间的中间浓度会导致两种超晶格共存。不同的是，在正己烷或氯仿中的 NC 自组装成仅单一的 bcc 超晶格。在具有不同浓度的溶剂中对 NC 组装的不同控制证实了 NC 浓度/溶解度介导的超晶格成核和生长，其中 NC 浓度的蒸发诱导局部梯度导致两种超晶格的同时成核。在快速生长的 fcc 中而不是 bcc 中观察到 NC 的密集堆积面揭示了导致两种不同超晶格的熵驱动力的差异。减慢溶剂蒸发并不能改善超晶格的对称性，但可以提高超晶格的结晶度。除了缩小超晶格体积外，热处理也在 175°C 下将 bcc 转变为 fcc 超晶格。通过种子介导的生长，在连续生长过程中，浓度依赖性超晶格不会改变晶格对称性，而通过浓度变化新形成的次级小核具有相对较高的表面能，并迅速溶解在溶液中，为原初成核的较大且稳定的种子的成熟提供了额外的 NC 源。在组装参数的多种控制下的观察不仅提供了对各种超晶格多晶型物的成核和生长以及转变的深入了解，而且为具有定制性能的所需超晶格的可控制造奠定了基础。
