Highlights

Nanocrystal superlattices: revealing a shape-induced orientation phase within 3D nanocrystal solids

 

Designing nanocrystal (NC) materials aims at obtaining superlattices that mimic the atomic structure of crystalline solids. In such atomic systems, spatially anisotropic orbitals determine the crystalline lattice type. Similarly, in NC systems the building block anisotropy defines the order of the final solid: here, the NC shape governs the final superlattice structure. Yet, in contrast to atomic systems, NC shape-anisotropy induces not only positional, but also orientational order, ranging from full rotational disorder to a stable, fixed alignment of all NCs. This orientational relation is of special interest, as it determines to what extent atomically coherent connections between NCs are possible, thereby enabling complete wave function delocalization within the NC solid.

In addition to predicting the final NC orientation and position structure, the realization of NC materials demands a controllable fabrication process such that the designed order can be produced. Generally, such highly ordered NC superstructures are achieved through solvent evaporation induced self‐assembly on hard substrates. For applications where the 2D nature of this substrates process is limiting, nonsolvent into solvent diffusion, a technique commonly used to grow single crystals of dissolved molecules, is an attractive option. However, the precise influence of self-assembly parameters on the final superlattice outcome remains unknown. In this work, the researchers posed two closely related questions regarding the design of novel free-standing NC materials: (i) how can the NC self-assembly process be controlled to yield highly ordered free-standing supercrystals and (ii) what is the detailed positional and orientational order within the NC solid? A multidisciplinary team of collaborators, including the Austrian Small Angle X-ray Scattering (SAXS) beamline at Elettra, approached this challenge by a combined experimental and computational strategy. First, the precise geometry of single Bi NCs was modeled from experimental SAXS patterns by shape-reconstruction algorithms (see Fig. 1a). Second, in-situscattering experiments were able to capture the full self-assembly process, ranging from single NCs in solution to micrometer sized NC SLs (see Fig. 1b).

Figure 1.  Small angle X-Ray scattering (SAXS) measurements, capturing the transition of Bi NCs from the stable solution (a) to the final NC superlattice (b).

Third, state-of-the-art hard-particle modelling confirmed and extended the experimental results, revealing a unique, unprecedented orientation phase within the NC solid (see Fig. 2). Thus, the researchers provide evidence that orientational and positional order within NC solids can be achieved without the help of surface ligands: the parallel alignment of atomically defined nanocrystal facets offers a new possibility for transferring atomic properties of the individual NCs to the entire supercrystal structure. The findings offer a recipe to push nanocrystal solids toward their full electronic potential and inspire more detailed studies toward orientation phases within colloidal solids.
This publication was selected for the ‘inside front cover’ at the journal “Advanced Materials”.

Figure 2.  Results of the hard-particle Monte Carlo simulations, which, for one, confirm the experimental results regarding the NC position order and, for the other, reveal an unprecedented orientation phase, induced by the NC shape.

Figure 3.  INSIDE FRONT COVER "The self‐assembly of 3D colloidal supercrystals built from faceted 20 nm Bi nanocrystals is studied by in situ synchrotron X‐ray scattering combined with Monte Carlo simulations. The assembly is driven by nonsolvent into solvent diffusion. In article number 1802078, Rainer T. Lechner and co‐workers reveal a unique orientation phase: the nanocrystals' shape induces 6 distinct global orientations within the cubic superlattice enabling parallel facet‐to‐facet alignment.. Adv. Mater. 30/2018, article 180278 inside cover page.

Retrieve article

A Shape­ Induced Orientation Phase within 3D Nanocrystal Solids;
Max Burian, Carina Karner, Maksym Yarema, Wolfgang Heiss, Heinz Amenitsch, Christoph Dellago, Rainer T. Lechner;
Advanced Materials, Vol. 30, 1802078 (2018). doi: 10.1002/adma.201802078

contact e-mail: rainer.lechner@unileoben.ac.at