Abbas Abdullah S, Vargo Emma, Jamali Vida, Ercius Peter, Pieters Priscilla F, Brinn Rafaela M, Ben-Moshe Assaf, Cho Min Gee, Xu Ting, Alivisatos A Paul
Department of Materials Science and Engineering, University of California, Berkeley, California 94720, United States.
Department of Chemistry, University of California, Berkeley, California 94720, United States.
ACS Nano. 2022 Jun 28;16(6):9339-9347. doi: 10.1021/acsnano.2c02014. Epub 2022 May 24.
Extensive prior work has shown that colloidal inorganic nanocrystals coated with organic ligand shells can behave as artificial atoms and, as such, form superlattices with different crystal structures and packing densities. Although ordered superlattices present a high degree of long-range positional order, the relative crystallographic orientation of the inorganic nanocrystals with respect to each other tends to be random. Recent works have shown that superlattices can achieve orientational alignment through combinations of nanocrystal faceting and ligand modification, as well as selective metal particle attachment to particular facets. These studies have focused on the assembly of high-symmetry nanocrystals, such as cubes and cuboctahedra. Here, we study the assembly of elliptically faceted CdSe/CdS core/shell nanocrystals with one distinctive crystallographic orientation along the major elliptical axis. We show that the nanocrystals form an unexpectedly well-ordered translational superlattice, with a degree of order comparable to that achieved with higher-symmetry nanocrystals. Additionally, we show that, due to the particles' faceted shape, the superlattice is characterized by an orientational glass phase in which only certain orientations are possible due to entropically frustrated crystallization. In this phase, the nanocrystals do not exhibit a local orientational ordering but rather have distinct orientations that emerge at different locations within the same domain. The distinct orientations are a result of a facet-to-facet lock-in mechanism that occurs during the self-assembly process. These facet-to-facet alignments force the nanocrystals to tilt on different lattice planes forming different projections that we termed apparent polydispersity. Our experimental realization of an orientational glass phase for multifaceted semiconducting nanocrystals can be used to investigate how this phase is formed and how it can be utilized for potential optical, electrical, and thermal transport applications.
先前大量的研究工作表明，包覆有机配体壳层的胶体无机纳米晶体可表现为人工原子，并由此形成具有不同晶体结构和堆积密度的超晶格。尽管有序超晶格呈现出高度的长程位置有序性，但无机纳米晶体彼此之间的相对晶体学取向往往是随机的。最近的研究表明，超晶格可通过纳米晶体刻面和配体修饰的组合，以及选择性地将金属颗粒附着到特定刻面上，来实现取向排列。这些研究主要集中在高对称性纳米晶体（如立方体和立方八面体）的组装上。在此，我们研究了沿椭圆长轴具有一个独特晶体学取向的椭圆刻面CdSe/CdS核壳纳米晶体的组装。我们发现，这些纳米晶体形成了一个出乎意料的有序平移超晶格，其有序程度与高对称性纳米晶体所达到的相当。此外，我们还表明，由于颗粒的刻面形状，该超晶格的特征是存在一个取向玻璃相，在这个相中，由于熵致结晶受阻，只有某些取向是可能的。在这个相中，纳米晶体并不表现出局部取向有序，而是在同一区域内的不同位置出现不同的取向。这些不同的取向是自组装过程中发生的面-面锁定机制的结果。这些面-面排列迫使纳米晶体在不同的晶格平面上倾斜，形成不同的投影，我们将其称为表观多分散性。我们对多面半导体纳米晶体取向玻璃相的实验实现，可用于研究这个相是如何形成的，以及它如何用于潜在的光学、电学和热输运应用。
