The Role of Dimer Formation in the Nucleation of Superlattice Transformations and Its Impact on Disorder

Abstract

The formation of defect-free two-dimensional nanocrystal (NC) superstructures remains a challenge as persistent defects hinder charge delocalization and related device performance. Understanding defect formation is an important step toward developing strategies to mitigate their formation. However, specific mechanisms of defect formation are difficult to determine, as superlattice phase transformations that occur during fabrication are quite complex and there are a variety of factors influencing the disorder in the final structure. Here, we use Molecular Dynamics (MD) and electron microscopy in concert to investigate the nucleation of the epitaxial attachment of lead chalcogenide (PbX, where X = S, Se) NC assemblies. We use an updated implementation of an existing reactive force field in an MD framework to investigate how initial orientational (mis)­alignment of the constituent building blocks impacts the final structure of the epitaxially connected superlattice. This Simple Molecular Reactive Force Field (SMRFF) captures both short-range covalent forces and long-range electrostatic forces and allows us to follow orientational and translational changes of NCs during superlattice transformation. Our simulations reveal how robust the oriented attachment is with regard to the initial configuration of the NCs, measuring its sensitivity to both in-plane and out-of-plane misorientation. We show that oriented attachment nucleates through the initial formation of dimers, which corroborate experimentally observed structures. We present high-resolution structural analysis of dimers at early stages of the superlattice transformation and rationalize their contribution to the formation of defects in the final superlattice. Collectively, the simulations and experiments presented in this paper provide insights into the nucleation of NC oriented attachment, the impact of the initial configuration of NCs on the structural fidelity of the final epitaxially connected superlattice, and the propensity to form commonly observed defects, such as missing bridges and atomic misalignment in the superlattice due to the formation of dimers. We present potential strategies to mitigate the formation of superlattice defects.

Ligand Interactions and Nanoparticle Shapes Guide the Pathways toward Interfacial Self-Assembly

“…With the gradual removal of the {100}-bound ligands, the mutually exposed {100} facets in NN-pairs begin to bond. With the appearance of NP “dimers,” f ep ,1 ≠ 0, there is a corresponding fall in the fraction of free “monomers,” f ep ,0 . Initially, no NPs are found with more than one epitaxial connection, that is, f ep , k = 0 ∀ k ∈ {2,3,4}.…”

Fundamental Processes and Practical Considerations of Lead Chalcogenide Mesocrystals Formed via Self-Assembly and Directed Attachment of Nanocrystals at a Fluid Interface

“…Contamination in the subphase, for example, leftover EDA in the trough or alcohols from cleaning, can also impact the structure of the SL. For example, the addition of a small amount of methanol to the EG subphase (at 100 nM concentration) induces premature necking of PbSe NCs (Figure d), which we recently reported to be an important cause for translational and orientational disorder in connected SLs, such as missing epitaxial bridges and atomic lattice misalignment . In summary, ensuring purity of all chemicals is crucial for successful self-assembly and oriented attachment experiments.…”

“…For example, the addition of a small amount of methanol to the EG subphase (at 100 nM concentration) induces premature necking of PbSe NCs (Figure 3d), which we recently reported to be an important cause for translational and orientational disorder in connected SLs, such as missing epitaxial bridges and atomic lattice misalignment. 93 In summary, ensuring purity of all chemicals is crucial for successful self-assembly and oriented attachment experiments. (ii) Design considerations for the solvent (superphase): A suitable solvent for the superphase must (a) have affinity for the NCs, (b) be immiscible with the subphase, (c) have lower density then the subphase, and (d) result in low interfacial tension between the superphase and the subphase to promote spreading and homogeneous film formation.…”

Emergent properties in supercrystals of atomically precise nanoclusters and colloidal nanocrystals

“…126 While such oriented attachment on the large scale can lead to new materials with potentially very exciting properties, 121,127,128 managing defects in these materials is key toward their full exploitation. 129 In this regard, the formation of dimers has been identified as a central first step, 130 and it has been argued that classical bulk crystal dislocation theory is successful in predicting the result of oriented attachment in an SC with certain defects. 131 This is another demonstration of the many analogies between atomic lattices and SCs.…”