Salzmann Bastiaan B V, van der Sluijs Maaike M, Soligno Giuseppe, Vanmaekelbergh Daniel
Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, P. O. Box 80000, 3508 TA Utrecht, The Netherlands.
Acc Chem Res. 2021 Feb 16;54(4):787-797. doi: 10.1021/acs.accounts.0c00739. Epub 2021 Jan 27.
ConspectusIntuitively, chemists see crystals grow atom-by-atom or molecule-by-molecule, very much like a mason builds a wall, brick by brick. It is much more difficult to grasp that small crystals can meet each other in a liquid or at an interface, start to align their crystal lattices and then grow together to form one single crystal. In analogy, that looks more like prefab building. Yet, this is what happens in many occasions and can, with reason, be considered as an alternative mechanism of crystal growth. Oriented attachment is the process in which crystalline colloidal particles align their atomic lattices and grow together into a single crystal. Hence, two aligned crystals become one larger crystal by epitaxy of two specific facets, one of each crystal. If we simply consider the system of two crystals, the unifying attachment reduces the surface energy and results in an overall lower (free) energy of the system. Oriented attachment often occurs with massive numbers of crystals dispersed in a liquid phase, a sol or crystal suspension. In that case, oriented attachment lowers the total free energy of the crystal suspension, predominantly by removal of the nanocrystal/liquid interface area. Accordingly, we should start by considering colloidal suspensions with crystals as the dispersed phase, i.e., "sols", and discuss the reasons for their thermodynamic (meta)stability and how this stability can be lowered such that oriented attachment can occur as a spontaneous thermodynamic process. Oriented attachment is a process observed both for charge-stabilized crystals in polar solvents and for ligand capped nanocrystal suspensions in nonpolar solvents. In this last system different facets can develop a very different reactivity for oriented attachment. Due to this facet selectivity, crystalline structures with very specific geometries can be grown in one, two, or three dimensions; controlled oriented attachment suddenly becomes a tool for material scientists to grow architectures that cannot be reached by any other means. We will review the work performed with PbSe and CdSe nanocrystals. The entire process, i.e., the assembly of nanocrystals, atomic alignment, and unification by attachment, is a very complex and intriguing process. Researchers have succeeded in monitoring these different steps with in situ wave scattering methods and real-space (S)TEM studies. At the same time coarse-grained molecular dynamics simulations have been used to further study the forces involved in self-assembly and attachment at an interface. We will briefly come back to some of these results in the last sections of this review.
概述
直观地说，化学家们看到晶体逐个原子或逐个分子地生长，这很像泥瓦匠一砖一瓦地砌墙。然而，要理解小晶体如何在液体中或界面处相互相遇，开始对齐它们的晶格，然后一起生长形成一个单晶，就困难得多了。打个比方，这看起来更像是预制建筑。然而，这在很多情况下都会发生，并且有理由被视为晶体生长的一种替代机制。定向附着是指结晶胶体颗粒对齐其原子晶格并生长在一起形成单晶的过程。因此，两个对齐的晶体通过两个特定晶面（每个晶体各一个）的外延生长而变成一个更大的晶体。如果我们仅考虑两个晶体的系统，这种统一的附着会降低表面能，并导致系统的总（自由）能更低。定向附着通常发生在大量晶体分散在液相、溶胶或晶体悬浮液中的情况下。在这种情况下，定向附着主要通过去除纳米晶体/液体界面面积来降低晶体悬浮液的总自由能。因此，我们应该从考虑以晶体为分散相的胶体悬浮液，即“溶胶”开始，并讨论它们热力学（亚）稳定性的原因，以及如何降低这种稳定性，使得定向附着能够作为一个自发的热力学过程发生。定向附着是在极性溶剂中电荷稳定的晶体以及非极性溶剂中配体封端的纳米晶体悬浮液中都能观察到的过程。在最后这个系统中，不同的晶面对于定向附着可能具有非常不同的反应性。由于这种晶面选择性，可以在一维、二维或三维中生长具有非常特定几何形状的晶体结构；可控的定向附着突然成为材料科学家生长用任何其他方法都无法实现的结构的一种工具。我们将回顾用PbSe和CdSe纳米晶体所做的工作。整个过程，即纳米晶体的组装、原子排列以及通过附着实现统一，是一个非常复杂且引人入胜的过程。研究人员已经成功地用原位波散射方法和实空间（S）TEM研究监测了这些不同的步骤。同时，粗粒度分子动力学模拟已被用于进一步研究界面处自组装和附着过程中涉及的力。在本综述的最后部分，我们将简要回顾其中的一些结果。
